WO2025177184A1 - Methods for determination and indication of reference downlink reference signal (dl-rs) for inter-transmission-reception point (inter-trp) delay, frequency, and phase difference report - Google Patents

Abstract

A method, network node and user equipment (UE) for determination and indication of reference downlink reference signal (DL-RS) for inter-transmission/reception point (TRP) delay, frequency and phase difference reporting are disclosed. According to one aspect, a method in a UE includes, for each DL-RS of a first set of DL-RSs that excludes a second set of reference DL-RSs, measuring at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the second set of reference DL-RSs. The method includes configuring a channel state information (CSI) report indicating the at least one reference DL-RS and indicating each DL-RS of the first set of DL-RSs, the CSI report further including the at least one measurement quantity. The method also includes transmitting to the network node the CSI report.

Description

METHODS FOR DETERMINATION AND INDICATION OF REFERENCE DOWNLINK REFERENCE SIGNAL (DL-RS) FOR INTER-TRANSMISSION- RECEPTION POINT (INTER-TRP) DELAY, FREQUENCY, AND PHASE DIFFERENCE REPORT FIELD The present disclosure relates to wireless communications, and in particular, to determination and indication of reference downlink reference signals (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting. BACKGROUND The Third Generation Partnership Project (3GPP) has developed and is developing standards for Fourth Generation (4G) (also referred to as Long Term Evolution (LTE)) and Fifth Generation (5G) (also referred to as New Radio (NR)) wireless communication systems. Such systems provide, among other features, broadband communication between network nodes, such as base stations, and mobile user equipments (UE), as well as communication between network nodes and between UEs. The 3GPP is also developing standards for Sixth Generation (6G) wireless communication networks. The next generation mobile wireless communication system (5G) or new radio (NR), will support a diverse set of use cases and a diverse set of deployment scenarios. This includes deployment at both low frequencies (100s of MHz), similar to LTE today, and very high frequencies (mm waves in the tens of GHz). Similar to LTE, NR will use OFDM (Orthogonal Frequency Division Multiplexing) in the downlink (DL) (i.e., from a network node, gNB, eNB, or base station, to a user equipment or UE). In the uplink (UL) (i.e., from UE to gNB), both OFDM and discrete Fourier transform (DFT)-spread OFDM (DFT-S-OFDM), also known as single carrier frequency division multiple access (SC-FDMA) in LTE, will be supported. The basic NR physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where a resource block (RB) in a 14-symbol slot is shown. A resource block corresponds to 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. Different subcarrier spacing values are supported in NR. The supported subcarrier _{spacing values (also referred to as different numerologies) are given by ∆^ = ^15 × 2^^^^^ where ^ is a non-negative integer and may be one of {0,1,2,3,4}. ∆^ = 15^^^ (e.g., ^ = 0) is the basic (or reference) subcarrier spacing that is also used in LTE.} ^ is also referred to as the numerology. In the time domain, DL and UL transmissions in NR will be organized into equally-sized subframes of 1ms each similar to LTE. A subframe is further divided into multiple slots of equal duration. The slot length is dependent on the subcarrier spacing or ^ numerology and is given by ^^{^} ms. Each slot has 14 OFDM symbols for normal Cyclic Prefix (CP). It is understood that data scheduling in NR may be performed on a slot basis. An example is shown in FIG. 2 with a 14-symbol slot, where the first two symbols contain the physical downlink control channel (PDCCH) and the rest contains the physical shared data channel (PDSCH). For convenience, subframe is used throughout the following sections. DL transmissions may be dynamically scheduled, i.e., in each slot the network node (i.e., gNB) transmits downlink control information (DCI) about which UE data is to be transmitted to and which resource blocks in the current DL slot the data is transmitted on. This control signaling is typically transmitted in the first one or two OFDM symbols in each slot in NR. The control information is carried on the PDCCH and data is carried on the PDSCH. A UE first detects and decodes PDCCH and, if a PDCCH is decoded successfully, it then decodes the corresponding PDSCH based on the decoded control information in the PDCCH. UL data transmission may also be dynamically scheduled using PDCCH. Similar to downlink, a UE first decodes uplink grants in PDCCH and then transmits data over the Physical Uplink Shared Channel (PUSCH) based the decoded control information in the uplink grant such as modulation order, coding rate, uplink resource allocation, and etc. Tracking Reference Signal (TRS) Similar to LTE, a Channel state Information Reference Signal (CSI-RS) was introduced in NR for channel measurement in the downlink (DL). A CSI-RS is transmitted over an antenna port (either a physical or virtual antenna) on certain resource elements (REs) for a UE to measure the DL channel associated with the antenna port. CSI- RS for this purpose is also referred to as Non-Zero Power (NZP) CSI-RS. The supported number of antenna ports or CSI-RS ports in NR are {1, 2, 4, 8, 12, 16, 24, 32}. A Tracking Reference Signal (TRS) is a special NZP CSI-RS with one port and is used for time and frequency tracking in the DL. FIG. 3 is an example of a TRS resource configuration in a physical resource block (PRB) and 2 slots. A UE may be configured with one or more periodic TRSs, or one or more periodic TRSs and aperiodic TRSs in NR. A periodic TRS has a periodicity and a slot offset. The periodicity may one of 2^{^}^_^ slots where ^_^ =10, 20, 40, or 80. A TRS occupies multiple resource blocks (RBs). When a NZP CSI-RS resource set contains “trs- info”, then the NZP CSI-RS resource set is for TRS. Quasi Co-location Demodulation reference signals (DM-RS) are used for coherent demodulation of PDSCH. A PDSCH may be associated with one or multiple DMRS antenna ports or simply DMRS ports, each associated with a spatial layer or a multi-input-multiple-output (MIMO) layer. Multiple layers may be multiplexed in a same time and frequency resource, where different data are carried in different layers. The DMRS ports used for a PDSCH transmission are indicated in DCI scheduling of the PDSCH. Several signals may be transmitted from different antenna ports. These signals may have the same large-scale properties, for instance in terms of Doppler shift/spread, average delay spread, or average delay, when measured at a UE receiver. These antenna ports are then said to be quasi co-located (QCL). If the UE knows that first and second antenna ports are QCL with respect to a certain channel property (e.g., Doppler spread), the UE may obtain the channel property of the first antenna port (e.g., DM-RS) from the second antenna port (e.g., TRS). The reference signal (e.g., TRS) associated with the second antenna port is known as the QCL source RS and the reference signal (e.g., DM-RS) associated with the first antenna port is known as the QCL target RS. The supported QCL types in NR are: • 'QCL-TypeA': {Doppler shift, Doppler spread, average delay, delay spread}; • 'QCL-TypeB': {Doppler shift, Doppler spread}; • 'QCL-TypeC': {Doppler shift, average delay}; • 'QCL-TypeD': {Spatial Rx parameter}. QCL relations are specified by transmission configuration indicator (TCI) states. A TCI state contains one or two source RS and the associated QCL types. In case two QCL types are configured, one is QCL type-D. A UE may be configured by radio resource control (RRC) signaling with a list of TCI states. For PDSCH, one or two TCI states from the list may be activated for each of up to 8 TCI codepoints by a medium access control (MAC) control element (CE) command. Up to 8 TCI states may be activated. One of the TCI codepoints is indicated in downlink control information (DCI) scheduling a PDSCH. UE performs PDSCH reception according to the TCI state(s) indicated in the TCI codepoint. Table 1 is a summary of possible source RS and target RS in NR. SSB refers synchronization signal and broadcast channel, and CSI-RS (BM) refers to CSI-RS for beam management in FR2. Table 1: Target and source RS supported in NR. CORESET and CORESETPoolIndex For multi-DCI based multi-TRP operation, Each TRP is associated to a pool of control resource sets (CORESETs) for monitoring DCI transmitted from the TRP. A CORESET pool is identified by a CORESET pool index configured for the CORESET. For the two DCIs in FIG. 4, they are transmitted in CORESETs belonging to different CORESET pools (i.e., with CORESETPoolIndex 0 and 1, respectively). For multi-DCI based multi-TRP operation, up to eight TCI states may be activated by MAC CE for each of the TRPs. The activation is done per TRP. A coreset pool index is contained in the MAC CE to indicate for which TRP the TCI states are activated. One of the activated TCI states may be dynamically indicated in DCI. CSI framework in NR In NR, a UE may be configured with one or multiple Channel State Information (CSI) report configurations for DL CSI feedback by the UE. A CSI report may contain one or more of: • Channel rank indicator (RI); • Antenna precoding matrix indicator (PMI); • Channel quality indicator (CQI); • DL reference signal received power (RSRP) or signal to interference and noise ratio (SINR); • CSI reference signal (CSI-RS) resource indicator (CRI). Each CSI report configuration is associated with a bandwidth part (BWP) and contains all necessary information required for a CSI report, including: • a CSI resource configuration for channel measurement; • reporting type, i.e., aperiodic CSI (on the physical uplink shared channel (PUSCH)), periodic CSI (on the physical uplink control channel (PUCCH)) or semi-persistent CSI (on PUCCH, and DCI activated on PUSCH); and • report quantity specifying what to be reported, such as RI, PMI, CQI, RSRP, etc. A UE may be configured with one or multiple CSI resource configurations for channel measurement. Each CSI resource configuration for channel measurement may contain one or more NZP CSI-RS resource sets. Each NZP CSI-RS resource set, may further contain one or more NZP CSI-RS resources. A NZP CSI-RS resource may be periodic, semi-persistent, or aperiodic. Periodic CSI starts after it has been configured by RRC and is reported on PUCCH. The associated NZP CSI-RS resource(s) are also periodic. Aperiodic CSI is reported on PUSCH and is activated by a CSI request bit field in DCI. The associated NZP CSI-RS resource(s) may be either periodic, semi-persistent, or aperiodic. The linkage between a code point of the CSI request field and a CSI report configuration is via an aperiodic CSI trigger state. A UE is configured by higher layer with a list of aperiodic CSI trigger states, where each of the trigger states contains an associated CSI report configuration. The CSI request field is used to indicate one of the aperiodic CSI trigger states and thus, one CSI report configuration. If there are more than one NZP CSI-RS resource set and/or more than one CSI interference measurement (CSI-IM) resource set are associated with a CSI report configuration, only one NZP CSI-RS resource set is selected in the aperiodic CSI trigger state. Thus, each aperiodic CSI report is based on a single NZP CSI-RS resource set. CQI and PMI may be reported per subband or wideband. In case of wideband CQI or PMI, the CQI or PMI is for the whole bandwidth configured for CSI report. In case of subband QCI or PMI, the CQI or PMI is reported for each subband. The subband size in NR may be from 4 RBs to 32 RBs, depending on the size of the BWP as shown Table 2. Table 2: Configurable subband sizes PDSCH transmission from Multiple TRPs In NR 3GPP Technical Release 16 (3GPP Rel-16), non-coherent joint PDSCH transmission from two transmission and reception points (TRPs) was introduced in which a subset of MIMO layers of a PDCCH to a UE are transmitted from a first TRP and the rest of layers of the PDSCH are transmitted from a second TRP in the same time and frequency resource. Different layers are separated and received at the UE with a MIMO capable receiver. An example is shown in FIG. 5, where a PDSCH with two layers are scheduled with the first layer transmitted from TRP1 and the second layer from TRP2. This is signaled in the corresponding DCI by indicating a TCI codepoint associated with first and second TCI states and DMRS ports x and y in two CDM groups. DMRS port x in the first CDM group is associated with the first TCI state and DMRS port y in the second CDM group is associated with the second TCI state. The first TCI state may contain TRS1 as the QCL source RS and the second TCI state may contain TRS1 as the QCL source RS. Coherent Joint transmission of PDSCH over Multiple TRPs In NR 3GPP Technical Release 18 (3GPP Rel-18), coherent joint downlink transmission (CJT) from multiple TRPs is supported by extending the 3GPP Rel-16 enhanced type II codebook and 3GPP Rel-17 further enhanced type II port selection codebook across multiple TRPs. The 3GPP Rel-16 enhanced type II codebook is specified in clause 5.2.2.2.5 of 3GPP Technical Standard (TS) 38.214 V18.0.0, and the enhanced type II codebook for CJT is specified in clause 5.2.2.2.8 of 3GPP TS 38.214 V18.0.0. The 3GPP Rel-17 enhanced type II port selection codebook is specified in clause 5.2.2.2.7 of 3GPP TS 38.214 V18.0.0, and the enhanced type II port selection codebook for CJT is specified in clause 5.2.2.2.9 of 3GPP TS 38.214 V18.0.0. In CJT, all layers are transmitted from the multiple TRPs used for CJT. An example with two layers and two TRPs is shown in FIG. 6, where data symbols of the two layers are transmitted from two TRPs by applying two different precoding matrices at TRP1 and TRP2. The two precoders are designed such that for each layer, the signals received from the two TRPs are phase aligned at the UE and thus, are coherently combined. There are a number of challenges in supporting CJT. First, propagation delays between different TRPs and a UE may be quite different. These large delay differences would result in a large frequency selective composite channel, i.e., the channel amplitude and phase vary rapidly across frequency. In existing NR CSI feedback, a precoding matrix per subband is reported. The subband size may vary between 2 RBs to 32RBs as specified in 3GPP Technical Standard (TS) 38.214. Table 3 shows phase variation within a subband for different subband sizes with one microsecond (1us) delay difference between two TRPs. It may be seen that even with 2RB subband size, the phase variation exceeds 130 degrees. For constructive combining of two signals, their phase difference should be less than 90 degrees. Therefore, with current subband size and per subband CSI feedback, signals from multiple TRPs cannot be coherently combined with even 1us delay difference. Table 3: Example showing phase variation over a subband for a 1us delay difference for 15 kHz subcarrier spacing. Second, even though a same nominal transmit frequency may be used at multiple TRPs, due to local oscillator stability, there will be some actual transmit frequency difference between the multiple TRPs. In 3GPP RAN4, the maximum transmit frequency error for a base station is specified in 3GPP TS 38.104. The requirements are shown in Table 2.. For the most stringent +/-0.05 ppm requirement, there will be some residual frequency errors. These frequency errors means that the phase of a signal will change over time. Table 4 gives 3GPP minimum requirements for transmit frequency error. Table 4 Delay difference and frequency difference pre-compensation for CJT over Multiple TRPs FIG. 7 is an example of transmission of a signal ^^{^}^^{^} from two TRPs. ^^{^}^^{^} is multiplied by two co-phasing/pre-compensation coefficients ^_^ and ^_^ at the two TRPs before being transmitted to the UE. The effective propagation channels from the two TRPs to the UE, including transmitter and receiver circuitries and antenna patterns associated with the two TRPs, are denoted by and ℎ_^, respectively. and ^_^ are the transmit frequencies and ^_^ and ^_^ are the random initial phases at the two TRPs. ^ is the propagation delay (including possible timing offsets) difference between the two TRPs. The composite signal at the UE may be expressed as; For narrow-band signal and when the delay ^ is small, the signal envelope doesn’t _{change much, i.e., ^^^ − ^^ ≈ ^^^^. Thus, (eq. 1) may be revised as:} (eq.3) To coherently combine the signals from the two TRPs, the following co- phasing/pre-compensation coefficients may be used: ^ _{'^ ^ = ^} ^{〖^∠. "#〗 ^} (eq.4b) where ∠^/^ denotes the angle of a complex variable /. The resulting composite signal, when the above co-phasing/pre-compensation coefficients in eq.4a-4b are applied, is then: ^^{∗^}^^{^} = ^|ℎ_^| + |ℎ_^|^^^{^}^^{^}^^{^^^^ !} (eq.5) Alternatively, the co-phasing/pre-compensation coefficients may be as follows: ^_{^ = 1 (eq.6a)} The resulting composite signal, when the above co-phasing/pre-compensation coefficients in eq.6a-6b are applied, is then: Note that the above applies also in case of multiple antenna ports are deployed in each of the TRPs. In that case, additional precoding or beamforming is applied to ^^^^, where ^^^^ is data associated with a MIMO layer of PDSCH or DMRS. For a given MIMO layer, the signal received from TRP1 would become is a 3_^ by M channel matrix, 2_^ is a 3_^ by 1 precoding vector associated with the corresponding MIMO layer, 3_^ is the number of antenna ports deployed at TRPs and M is the number of receive antennas at the UE. Similarly, for the given MIMO layer, the signal received from TRP2 would become where ^_^ is a 3_^ by M channel matrix, 2_^ is a 3_^ by 1 precoding vector associated with the corresponding MIMO layer, and 3_^ is the number of antenna ports deployed at TRP2. CJT from multiple TRPs is possible for the case of multiple PDSCH layers. For R PDSCH layers, each TRP will use a corresponding N1 x R precoding matrix wherein each column in the precoding matrix corresponds to one of the R MIMO layers. In the case of R PDSCH layers, the transmitted data ^^^^ will consist of R different symbols (i.e., one symbol corresponding to each of the R PDSCH layers). For CJT, it is envisioned that precoding matrices/vectors and the co-phasing/pre- compensation coefficients are reported by the UE to the network. In order to derive the co-phasing/pre-compensation coefficients ^_^ and ^_^, one or more of the following need to be reported from the UE to the network (via the network node): • transmit frequency associated with a TRP; • transmit frequency difference between two TRPs; • delay associated with a TRP; and/or • delay difference between two TRPs. The reporting of delay(s)/delay difference(s) and/or transmit frequency(ies)/transmit frequency difference(s) from the UE to the network may enable the network to pre-compensate for the delay difference(s) and/or frequency difference(s) between the TRPs such that coherent combining (i.e., as shown in eq. 7) is achieved. Reporting by the UE of propagation delay(s)/propagation delay difference(s) for CJT may include propagation delay differences and/or transmit frequency differences and a subset of TRPs (or TRSs) for which these reporting quantities are reported. The delay, frequency and/or phase differences may be calculated relative to reference DL- RSs. In such a case, the network node and the UE need to have the same understanding about the selected reference DL-RSs. Details of how the reference DL-RS is reported by the UE to the network node is an open problem to be solved. SUMMARY Some embodiments advantageously provide methods, network nodes, and user equipment (UE) for determination and indication of reference downlink reference signal (DL-RS) for inter-transmission/reception point (TRP) delay, frequency and phase difference reporting. Some embodiments provide methods for indication of the reference DL-RS(s) to calculate and report the inter-TRP delay, frequency and/or phase difference(s). In some embodiments, for different CSI quantities, the reference DL-RS(s) may be indicated either jointly or separately from the other DL-RSs. Also, the reference DL-RSs and the CSI quantity measurements may be indicated in a two- or one-part CSI quantity report. In some embodiments, methods of indication in a CSI report of a reference DL- RS and/or a subset of DL-RSs selected by a UE out of a plurality of DL RS configured for the CSI report are provided where the report includes one-part or two parts. In some embodiments, one or more of the following may be performed: • Indicating the reference DL-RS(s) and the subset of DL-RS jointly in the report; • Indicating the reference DL-RS(s) jointly with other report quantities, e.g., with a reserved codepoint for indicating the reference DL-RS(s); • Indicating the reference DL-RS(s) and the subset of DL-RS separately with different bit fields in the report; and/or • Indicating selected DL-RS(s) jointly with other report quantities, e.g., with a reserved codepoint for indicating the selected DL-RS(s), where the reporting quantity(ies) above may be any one or more of: • delay difference(s) associated with the remaining selected DL- RS(s) relative to the reference DL-RS (e.g., the difference between the measured delay(s) associated with the remaining selected DL-RS(s) and the measured delay associated with the reference DL-RS); • frequency difference(s) associated with the remaining selected DL-RS(s) relative to the reference DL-RS (e.g., the difference between the measured frequency(ies) associated with the remaining selected DL-RS(s) and the measured frequency associated with the reference DL-RS); and • phase difference(s) associated with the remaining selected DL- RS(s) relative to the reference DL-RS (e.g., the difference between the measured phase(s) associated with the remaining selected DL-RS(s) and the measured phase associated with the reference DL-RS). Some embodiments provide a way for indication of a subset of DL-RS and/or a reference DL-RS with respect to which different delay, frequency and/or phase difference(s) of the other DL-RSs are calculated that is more efficient than other arrangements. In some embodiments, a method reduces the signaling overhead for the indication of the reference DL-RS(s) and/or the subset of DL-RS. According to one aspect, a method in a UE includes, for each of at least one reporting quantity, measuring each set of downlink reference signals, DL-RSs, of a plurality of sets of DL-RSs distinct from a first set of reference DL-RSs, the at least one reporting quantity being one of a delay difference, a frequency difference and a phase difference relative to the first set of reference DL-RSs. The method includes generating a channel state information, CSI, report indicating the first set of reference DL-RSs for each of the at least one reporting quantity, the CSI report further including the at least one reporting quantity. The method also includes transmitting the CSI report to the network node. According to this aspect, in some embodiments, each set of DL-RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a CSI-RS resource set. In some embodiments, each set of DL- RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a tracking reference signal, TRS. In some embodiments, the indication of the first set of reference DL-RSs for at least one reporting quantity is reported jointly with the at least one reporting quantity. In some embodiments, each set of DL-RSs of a plurality of sets of DL-RSs are transmitted by one among multiple transmission-reception points, TRPs. In some embodiments, the CSI report includes a number of measurements for each of at least one reporting quantity that is not greater than a maximum number configured by the network node. In some embodiments, the at least one reporting quantity that is reported is configured by the network node. According to another aspect, a UE is configured with processing circuitry. The processing circuitry is configured to, for each of at least one reporting quantity, measure each set of downlink reference signals, DL-RSs, of a plurality of sets of DL-RSs distinct from a first set of reference DL-RSs, the at least one reporting quantity being one of a delay difference, a frequency difference and a phase difference relative to the first set of reference DL-RSs. The processing circuitry is also configured to generate a channel state information, CSI, report indicating the first set of reference DL-RSs for each of the at least one reporting quantity, the CSI report further including the at least one reporting quantity. The processing circuitry is also configured to transmit the CSI report to the network node. According to this aspect, in some embodiments, each set of DL-RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a CSI-RS resource set. In some embodiments, each set of DL- RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a tracking reference signal, TRS. In some embodiments, the indication of the first set of reference DL-RSs for at least one reporting quantity is reported jointly with the at least one reporting quantity. In some embodiments, each set of DL-RSs of a plurality of sets of DL-RSs are transmitted by one among multiple transmission-reception points, TRPs. In some embodiments, the CSI report includes a number of measurements for each of at least one reporting quantity that is not greater than a maximum number configured by the network node. In some embodiments, the at least one reporting quantity that is reported is configured by the network node. According to yet another aspect, a method in a network node configured to communicate with a user equipment is provided. The method includes receiving a channel state information, CSI, report indicating a first set of downlink reference signals, DL-RSs, distinct from a second set of reference DL-RSs, the CSI report further including for each DL-RS of the first set of DL-RSs at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the second set of reference DL-RSs. The method includes determining compensating coefficients based at least in part on the at least one measurement quantity and the indicated at least one reference DL-RS of the second set of reference DL-RSs. According to this aspect, in some embodiments, each DL-RS of the first set of DL- RSs and the second set of reference DL-RSs is a CSI-RS. In some embodiments, each DL- RS of the first set of DL-RSs and the second set of reference DL-RSs is a tracking reference signal, TRS. In some embodiments, the method includes indicating to the UE the second set of reference DL-RS by radio resource control signaling, RRC. In some embodiments, the method includes indicating to the UE at least one of the first set of DL- RSs, the second set of reference DL-RSs and at least one measurement quantity to be reported in the CSI report. According to another aspect, a network node configured to communicate with a user equipment, the network node including processing circuitry configured to: receive a channel state information, CSI, report indicating a first set of downlink reference signals, DL-RSs, distinct from a second set of reference DL-RSs, the CSI report further including for each DL-RS of the first set of DL-RSs at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the second set of reference DL-RSs. The processing circuitry is further configured to determine compensating coefficients based at least in part on the at least one measurement quantity and the indicated at least one reference DL-RS of the second set of reference DL-RSs. According to this aspect, in some embodiments, each DL-RS of the first set of DL- RSs and the second set of reference DL-RSs is a CSI-RS. In some embodiments, each DL- RS of the first set of DL-RSs and the second set of reference DL-RSs is a tracking reference signal, TRS. In some embodiments, the processing circuitry is configured to indicate to the UE the second set of reference DL-RS by radio resource control signaling, RRC. In some embodiments, the processing circuitry is configured to indicate to the UE at least one of the first set of DL-RSs, the second set of reference DL-RSs and at least one measurement quantity to be reported in the CSI report. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: FIG. 1 is a time-frequency grid; FIG. 2 is an example of a 14-symbol slot; FIG. 3 is an example of a TRS resource; FIG. 4 is an example of DCI; FIG. 5 is an example of PDSCH with two layers; FIG. 6 is an example of two layers and two TRPS; FIG. 7 is an example of transmission of a signal from two TRPs; FIG. 8 is a schematic diagram of an example network architecture illustrating a communication system connected via an intermediate network to a host computer according to the principles in the present disclosure; FIG. 9 is a block diagram of a host computer communicating via a network node with a user equipment over an at least partially wireless connection according to some embodiments of the present disclosure; FIG. 10 is a diagram illustrating example arrangements and methods implemented in a communication system including a host computer, a network node and a user equipment for executing a client application at a user equipment according to some embodiments of the present disclosure; FIG. 11 is a flowchart of an example process in a network node for determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting. FIG. 12 is a flowchart of an example process in a user equipment for determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting; FIG. 13 is a flowchart of an example process in a network node for determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting. FIG. 14 is a flowchart of an example process in a user equipment for determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting; FIG. 15 is a flowchart of an example process for measurement and reporting of inter-TRP delay, frequency and/or phase differences; FIG. 16 is an example of a jointly encoded single field with 6 bits; and FIG. 17 is an example of using a same single field to indicate a DL-RS for a different report quantity. DETAILED DESCRIPTION Before describing in detail example embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Like numbers refer to like elements throughout the description. As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the concepts described herein. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. In embodiments described herein, the joining term, “in communication with” and the like, may be used to indicate electrical or data communication, which may be accomplished by physical contact, induction, electromagnetic radiation, radio signaling, infrared signaling or optical signaling, for example. One having ordinary skill in the art will appreciate that multiple components may interoperate and modifications and variations are possible of achieving the electrical and data communication. In some embodiments described herein, the term “coupled,” “connected,” and the like, may be used herein to indicate a connection, although not necessarily directly, and may include wired and/or wireless connections. The term “network node” used herein may be any kind of network node included in a radio network which may further include any of base station (BS), radio base station, base transceiver station (BTS), base station controller (BSC), radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB), Node B, multi-standard radio (MSR) radio node such as MSR BS, multi-cell/multicast coordination entity (MCE), integrated access and backhaul (IAB) node, relay node, donor node controlling relay, radio access point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU) Remote Radio Head (RRH), a core network node (e.g., mobile management entity (MME), self-organizing network (SON) node, a coordinating node, positioning node, MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current network), nodes in distributed antenna system (DAS), a spectrum access system (SAS) node, an element management system (EMS), etc. The network node may also include test equipment. The term “radio node” used herein may be used to also denote a user equipment (UE) such as a wireless device (WD) or a radio network node. In some embodiments, the non-limiting terms wireless device (WD) or a user equipment (UE) are used interchangeably. The UE herein may be any type of wireless device capable of communicating with a network node or another UE over radio signals, such as a wireless device (WD). The UE may also be a radio communication device, target device, device to device (D2D) UE, machine type UE or UE capable of machine to machine communication (M2M), low-cost and/or low-complexity UE, a sensor equipped with UE, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE), an Internet of Things (IoT) device, or a Narrowband IoT (NB-IOT) device, etc. Also, in some embodiments the generic term “radio network node” is used. It may be any kind of a radio network node which may include any of base station, radio base station, base transceiver station, base station controller, network controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell/multicast Coordination Entity (MCE), IAB node, relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH). Note that although terminology from one particular wireless system, such as, for example, 3GPP LTE and/or New Radio (NR), may be used in this disclosure, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. Other wireless systems, including without limitation Wide Band Code Division Multiple Access (WCDMA), Worldwide Interoperability for Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile Communications (GSM), may also benefit from exploiting the ideas covered within this disclosure. Note further, that functions described herein as being performed by a user equipment or a network node may be distributed over a plurality of user equipments and/or network nodes. In other words, it is contemplated that the functions of the network node and user equipment described herein are not limited to performance by a single physical device and, in fact, may be distributed among several physical devices. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Some embodiments provide determination and indication of reference downlink reference signal (DL-RS) for inter-transmission/reception point (TRP) delay, frequency and phase difference reporting. Returning now to the drawing figures, in which like elements are referred to by like reference numerals, there is shown in FIG. 8 a schematic diagram of a communication system 10, according to an embodiment, such as a 3GPP-type cellular network that may support standards such as LTE and/or NR (5G), which includes an access network 12, such as a radio access network, and a core network 14. The access network 12 includes a plurality of network nodes 16a, 16b, 16c (referred to collectively as network nodes 16), such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 18a, 18b, 18c (referred to collectively as coverage areas 18). Each network node 16a, 16b, 16c is connectable to the core network 14 over a wired or wireless connection 20. A first user equipment (UE) 22a located in coverage area 18a is configured to wirelessly connect to, or be paged by, the corresponding network node 16a. A second UE 22b in coverage area 18b is wirelessly connectable to the corresponding network node 16b. While a plurality of UEs 22a, 22b (collectively referred to as user equipments 22) are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole UE is in the coverage area or where a sole UE is connecting to the corresponding network node 16. Note that although only two UEs 22 and three network nodes 16 are shown for convenience, the communication system may include many more UEs 22 and network nodes 16. Also, it is contemplated that a UE 22 may be in simultaneous communication and/or configured to separately communicate with more than one network node 16 and more than one type of network node 16. For example, a UE 22 may have dual connectivity with a network node 16 that supports LTE and the same or a different network node 16 that supports NR. As an example, UE 22 may be in communication with an eNB for LTE/E-UTRAN and a gNB for NR/NG-RAN. The communication system 10 may itself be connected to a host computer 24, which may be embodied in the hardware and/or software of a standalone server, a cloud- implemented server, a distributed server or as processing resources in a server farm. The host computer 24 may be under the ownership or control of a service provider, or may be operated by the service provider or on behalf of the service provider. The connections 26, 28 between the communication system 10 and the host computer 24 may extend directly from the core network 14 to the host computer 24 or may extend via an optional intermediate network 30. The intermediate network 30 may be one of, or a combination of more than one of, a public, private or hosted network. The intermediate network 30, if any, may be a backbone network or the Internet. In some embodiments, the intermediate network 30 may include two or more sub-networks (not shown). The communication system of FIG. 8 as a whole enables connectivity between one of the connected UEs 22a, 22b and the host computer 24. The connectivity may be described as an over-the-top (OTT) connection. The host computer 24 and the connected UEs 22a, 22b are configured to communicate data and/or signaling via the OTT connection, using the access network 12, the core network 14, any intermediate network 30 and possible further infrastructure (not shown) as intermediaries. The OTT connection may be transparent in the sense that at least some of the participating communication devices through which the OTT connection passes are unaware of routing of uplink and downlink communications. For example, a network node 16 may not or need not be informed about the past routing of an incoming downlink communication with data originating from a host computer 24 to be forwarded (e.g., handed over) to a connected UE 22a. Similarly, the network node 16 need not be aware of the future routing of an outgoing uplink communication originating from the UE 22a towards the host computer 24. A network node 16 is configured to include a coefficient determination unit 32 which may be configured to determine compensating coefficients based at least on at least one measurement quantity and at least one reference DL-RS of a second set of reference DL-RSs indicated in a CSI report. A user equipment 22 is configured to include a CSI reporting unit 34 which may be configured to configure a channel state information, CSI, report indicating the at least one reference DL-RS and indicating each DL-RS of the first set of DL-RSs, the CSI report further including the at least one measurement quantity. Example implementations, in accordance with an embodiment, of the UE 22, network node 16 and host computer 24 discussed in the preceding paragraphs will now be described with reference to FIG. 2. In a communication system 10, a host computer 24 includes hardware (HW) 38 including a communication interface 40 configured to set up and maintain a wired or wireless connection with an interface of a different communication device of the communication system 10. The host computer 24 further includes processing circuitry 42, which may have storage and/or processing capabilities. The processing circuitry 42 may include a processor 44 and memory 46. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 42 may include integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 44 may be configured to access (e.g., write to and/or read from) memory 46, which may include any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only Memory). Processing circuitry 42 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by host computer 24. Processor 44 corresponds to one or more processors 44 for performing host computer 24 functions described herein. The host computer 24 includes memory 46 that is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software 48 and/or the host application 50 may include instructions that, when executed by the processor 44 and/or processing circuitry 42, causes the processor 44 and/or processing circuitry 42 to perform the processes described herein with respect to host computer 24. The instructions may be software associated with the host computer 24. The software 48 may be executable by the processing circuitry 42. The software 48 includes a host application 50. The host application 50 may be operable to provide a service to a remote user, such as a UE 22 connecting via an OTT connection 52 terminating at the UE 22 and the host computer 24. In providing the service to the remote user, the host application 50 may provide user data which is transmitted using the OTT connection 52. The “user data” may be data and information described herein as implementing the described functionality. In some embodiments, the host computer 24 may be configured for providing control and functionality to a service provider and may be operated by the service provider or on behalf of the service provider. The processing circuitry 42 of the host computer 24 may enable the host computer 24 to observe, monitor, control, transmit to and/or receive from the network node 16 and or the user equipment 22. The communication system 10 further includes a network node 16 provided in a communication system 10 and including hardware 58 enabling it to communicate with the host computer 24 and with the UE 22. The hardware 58 may include a communication interface 60 for setting up and maintaining a wired or wireless connection with an interface of a different communication device of the communication system 10, as well as a radio interface 62 for setting up and maintaining at least a wireless connection 64 with a UE 22 located in a coverage area 18 served by the network node 16. The radio interface 62 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and/or one or more RF transceivers. The communication interface 60 may be configured to facilitate a connection 66 to the host computer 24. The connection 66 may be direct or it may pass through a core network 14 of the communication system 10 and/or through one or more intermediate networks 30 outside the communication system 10. In the embodiment shown, the hardware 58 of the network node 16 further includes processing circuitry 68. The processing circuitry 68 may include a processor 70 and a memory 72. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 68 may include integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 70 may be configured to access (e.g., write to and/or read from) the memory 72, which may include any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only Memory). Thus, the network node 16 further has software 74 stored internally in, for example, memory 72, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the network node 16 via an external connection. The software 74 may be executable by the processing circuitry 68. The processing circuitry 68 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by network node 16. Processor 70 corresponds to one or more processors 70 for performing network node 16 functions described herein. The memory 72 is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software 74 may include instructions that, when executed by the processor 70 and/or processing circuitry 68, causes the processor 70 and/or processing circuitry 68 to perform the processes described herein with respect to network node 16. For example, processing circuitry 68 of the network node 16 may include the coefficient determination unit 32 which may be configured to determine compensating coefficients based at least on at least one measurement quantity and at least one reference DL-RS of a second set of reference DL-RSs indicated in a CSI report. The communication system 10 further includes the UE 22 already referred to. The UE 22 may have hardware 80 that may include a radio interface 82 configured to set up and maintain a wireless connection 64 with a network node 16 serving a coverage area 18 in which the UE 22 is currently located. The radio interface 82 may be formed as or may include, for example, one or more RF transmitters, one or more RF receivers, and/or one or more RF transceivers. The hardware 80 of the UE 22 further includes processing circuitry 84. The processing circuitry 84 may include a processor 86 and memory 88. In particular, in addition to or instead of a processor, such as a central processing unit, and memory, the processing circuitry 84 may include integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 86 may be configured to access (e.g., write to and/or read from) memory 88, which may include any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only Memory). Thus, the UE 22 may further include software 90, which is stored in, for example, memory 88 at the UE 22, or stored in external memory (e.g., database, storage array, network storage device, etc.) accessible by the UE 22. The software 90 may be executable by the processing circuitry 84. The software 90 may include a client application 92. The client application 92 may be operable to provide a service to a human or non-human user via the UE 22, with the support of the host computer 24. In the host computer 24, an executing host application 50 may communicate with the executing client application 92 via the OTT connection 52 terminating at the UE 22 and the host computer 24. In providing the service to the user, the client application 92 may receive request data from the host application 50 and provide user data in response to the request data. The OTT connection 52 may transfer both the request data and the user data. The client application 92 may interact with the user to generate the user data that it provides. The processing circuitry 84 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by UE 22. The processor 86 corresponds to one or more processors 86 for performing UE 22 functions described herein. The UE 22 includes memory 88 that is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software 90 and/or the client application 92 may include instructions that, when executed by the processor 86 and/or processing circuitry 84, causes the processor 86 and/or processing circuitry 84 to perform the processes described herein with respect to UE 22. For example, the processing circuitry 84 of the user equipment 22 may include a CSI reporting unit 34 which may be configured to configure a channel state information, CSI, report indicating the at least one reference DL-RS and indicating each DL-RS of the first set of DL-RSs, the CSI report further including the at least one measurement quantity. In some embodiments, the inner workings of the network node 16, UE 22, and host computer 24 may be as shown in FIG. 9 and independently, the surrounding network topology may be that of FIG. 8. In FIG. 9, the OTT connection 52 has been drawn abstractly to illustrate the communication between the host computer 24 and the user equipment 22 via the network node 16, without explicit reference to any intermediary devices and the precise routing of messages via these devices. Network infrastructure may determine the routing, which it may be configured to hide from the UE 22 or from the service provider operating the host computer 24, or both. While the OTT connection 52 is active, the network infrastructure may further take decisions by which it dynamically changes the routing (e.g., on the basis of load balancing consideration or reconfiguration of the network). The wireless connection 64 between the UE 22 and the network node 16 is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to the UE 22 using the OTT connection 52, in which the wireless connection 64 may form the last segment. More precisely, the teachings of some of these embodiments may improve the data rate, latency, and/or power consumption and thereby provide benefits such as reduced user waiting time, relaxed restriction on file size, better responsiveness, extended battery lifetime, etc. In some embodiments, a measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring the OTT connection 52 between the host computer 24 and UE 22, in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring the OTT connection 52 may be implemented in the software 48 of the host computer 24 or in the software 90 of the UE 22, or both. In embodiments, sensors (not shown) may be deployed in or in association with communication devices through which the OTT connection 52 passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software 48, 90 may compute or estimate the monitored quantities. The reconfiguring of the OTT connection 52 may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not affect the network node 16, and it may be unknown or imperceptible to the network node 16. Some such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary UE signaling facilitating the host computer’s 24 measurements of throughput, propagation times, latency and the like. In some embodiments, the measurements may be implemented in that the software 48, 90 causes messages to be transmitted, in particular empty or ‘dummy’ messages, using the OTT connection 52 while it monitors propagation times, errors, etc. Thus, in some embodiments, the host computer 24 includes processing circuitry 42 configured to provide user data and a communication interface 40 that is configured to forward the user data to a cellular network for transmission to the UE 22. In some embodiments, the cellular network also includes the network node 16 with a radio interface 62. In some embodiments, the network node 16 is configured to, and/or the network node’s 16 processing circuitry 68 is configured to perform the functions and/or methods described herein for preparing/initiating/maintaining/supporting/ending a transmission to the UE 22, and/or preparing/terminating/maintaining/supporting/ending in receipt of a transmission from the UE 22. In some embodiments, the host computer 24 includes processing circuitry 42 and a communication interface 40 that is configured to a communication interface 40 configured to receive user data originating from a transmission from a UE 22 to a network node 16. In some embodiments, the UE 22 is configured to, and/or includes a radio interface 82 and/or processing circuitry 84 configured to perform the functions and/or methods described herein for preparing/initiating/maintaining/supporting/ending a transmission to the network node 16, and/or preparing/terminating/maintaining/supporting/ending in receipt of a transmission from the network node 16. Although FIGS. 8 and 9 show various “units” such as coefficient determination unit 32, and CSI reporting unit 34 as being within a respective processor, it is contemplated that these units may be implemented such that a portion of the unit is stored in a corresponding memory within the processing circuitry. In other words, the units may be implemented in hardware or in a combination of hardware and software within the processing circuitry. FIG. 10 shows a communication diagram of a host computer 24 communicating via a network node 16 with a UE 22 over a partially wireless connection in accordance with some embodiments. Example implementations, in accordance with various embodiments, of the UE 22, network node 16, and host (such as host computer 24) discussed in the preceding paragraphs will now be described with reference to FIG. 10. Embodiments of host computer 24 include hardware, such as a communication interface, processing circuitry, and memory. The host computer 24 also includes software, which is stored in or accessible by the host computer 24 and executable by the processing circuitry. The software includes a host application that may be operable to provide a service to a remote user, such as the UE 22 connecting via an over-the-top (OTT) connection 52 extending between the UE 22 and host computer 24. In providing the service to the remote user, a host application may provide user data which is transmitted using the OTT connection 52. The network node 16 includes hardware enabling it to communicate with the host computer 24 and UE 22. The connection 66 may be direct or pass through a core network (like core network 14 in FIG. 8) and/or one or more other intermediate networks, such as one or more public, private, or hosted networks. For example, an intermediate network may be a backbone network or the Internet. The UE 22 includes hardware and software, which is stored in or accessible by UE 22 and executable by the UE’s processing circuitry. The software includes a client application, such as a web browser or operator-specific “app” that may be operable to provide a service to a human or non-human user via UE 22 with the support of the host computer 24. In the host computer 24, an executing host application may communicate with the executing client application via the OTT connection 52 terminating at the UE 22 and host computer 24. In providing the service to the user, the UE's client application may receive request data from the host's host application and provide user data in response to the request data. The OTT connection 52 may transfer both the request data and the user data. The UE's client application may interact with the user to generate the user data that it provides to the host application through the OTT connection 52. The OTT connection 52 may extend via a connection 66 between the host computer 24 and the network node 16 and via a wireless connection 64 between the network node 16 and the UE 22 to provide the connection between the host computer 24 and the UE 22. The connection 66 and wireless connection 64, over which the OTT connection 52 may be provided, have been drawn abstractly to illustrate the communication between the host computer 24 and the UE 22 via the network node 16, without explicit reference to any intermediary devices and the precise routing of messages via these devices. As an example of transmitting data via the OTT connection 52, in Block S100, the host computer 24 provides user data, which may be performed by executing a host application. In some embodiments, the user data is associated with a particular human user interacting with the UE 22. In other embodiments, the user data is associated with a UE 22 that shares data with the host computer 24 without explicit human interaction. In step Block S102, the host computer 24 initiates a transmission carrying the user data towards the UE 22. The host computer 24 may initiate the transmission responsive to a request transmitted by the UE 22. The request may be caused by human interaction with the UE 22 or by operation of the client application executing on the UE 22. The transmission may pass via the network node 16, in accordance with the teachings of the embodiments described throughout this disclosure. Accordingly, in Block S104, the network node 16 transmits to the UE 22 the user data that was carried in the transmission that the host computer 24 initiated, in accordance with the teachings of the embodiments described throughout this disclosure. In Block S106, the UE 22 receives the user data carried in the transmission, which may be performed by a client application executed on the UE 22 associated with the host application executed by the host computer 24. In some examples, the UE 22 executes a client application which provides user data to the host computer 24. The user data may be provided in reaction or response to the data received from the host computer 24. Accordingly, in Block S108, the UE 22 may provide user data, which may be performed by executing the client application. In providing the user data, the client application may further consider user input received from the user via an input/output interface of the UE 22. Regardless of the specific manner in which the user data was provided, the UE 22 initiates, in Block S110, transmission of the user data towards the host computer 24 via the network node 16. In Block S112, in accordance with the teachings of the embodiments described throughout this disclosure, the network node 16 receives user data from the UE 22 and initiates transmission of the received user data towards the host computer 24. In Block S114, the host computer 24 receives the user data carried in the transmission initiated by the UE 22. One or more of the various embodiments improve the performance of OTT services provided to the UE 22 using the OTT connection 52, in which the wireless connection 64 forms the last segment. More precisely, the teachings of these embodiments may improve the throughput and reduce latency and power consumption. In an example scenario, factory status information may be collected and analyzed by the host computer 24. As another example, the host computer 24 may process audio and video data which may have been retrieved from a UE for use in creating maps. As another example, the host computer 24 may collect and analyze real-time data to assist in controlling vehicle congestion (e.g., controlling traffic lights). As another example, the host computer 24 may store surveillance video uploaded by a UE. As another example, the host computer 24 may store or control access to media content such as video, audio, VR or AR which it may broadcast, multicast or unicast to UEs. As other examples, the host computer 24 may be used for energy pricing, remote control of non-time-critical electrical load to balance power generation needs, location services, presentation services (such as compiling diagrams etc. from data collected from remote devices), or any other function of collecting, retrieving, storing, analyzing and/or transmitting data. In some examples, a measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring the OTT connection 52 between the host computer 24 and UE 22, in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring the OTT connection may be implemented in software and hardware of the host computer 24 and/or UE 22. In some embodiments, sensors (not shown) may be deployed in or in association with other devices through which the OTT connection 52 passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software may compute or estimate the monitored quantities. The reconfiguring of the OTT connection 52 may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not directly alter the operation of the network node 16. Such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary UE signaling that facilitates measurements of throughput, propagation times, latency and the like, by the host computer 24. The measurements may be implemented in that software causes messages to be transmitted, in particular empty or ‘dummy’ messages, using the OTT connection 52 while monitoring propagation times, errors, etc. FIG. 11 is a flowchart of an example process in a network node 16 for determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 68 (including the coefficient determination unit 32), processor 70, radio interface 62 and/or communication interface 60. Network node 16 such as via processing circuitry 68 and/or processor 70 and/or radio interface 62 and/or communication interface 60 is configured to receive from the UE a channel state information, CSI, report indicating at least one reference downlink reference signal, DL-RS, a subset of DL-RSs configured for the CSI report and indicating at least one of a delay difference, a frequency difference and a phase difference between a DL-RS of the subset of DL-RSs and the reference DL-RS (Block S116). The process includes determining compensating coefficients based at least on the one of the delay difference, the frequency difference and the phase difference (Block S118). In some embodiments, the at least one reference DL-RS and the subset of DL-RSs are reported jointly. In some embodiments, the at least one reference DL-RS and the subset of DL-RSs are reported jointly with the at least one of the delay difference, the frequency difference and the phase difference. In some embodiments, the at least one reference DL-RS and the subset of DL-RSs are reported separately with different bit fields in the CSI report. In some embodiments, the at least one reference DL-RS, the subset of DL-RSs and the at least one of a delay difference, a frequency difference and a phase difference are reported separately with different bit fields in the CSI report. FIG. 12 is a flowchart of an example process in a user equipment 22 according to some embodiments of the present disclosure. One or more blocks described herein may be performed by one or more elements of user equipment 22 such as by one or more of processing circuitry 84 (including the reporting unit 34), processor 86, radio interface 82 and/or communication interface 60. User equipment 22 such as via processing circuitry 84 and/or processor 86 and/or radio interface 82 is configured to determine at least one reference downlink reference signal, DL-RS, a subset of DL-RSs configured for a CSI report and at least one of a delay difference, a frequency difference and a phase difference between a DL-RS of the subset of DL-RSs and the reference DL-RS (Block S120). The process includes transmitting to the network node the CSI report indicating the at least one reference DL-RS, the subset of DL-RSs configured for the CSI report and indicating at the least one of the delay difference, the frequency difference and the phase difference (Block S122). In some embodiments, the at least one reference DL-RS and the subset of DL-RSs are reported jointly. In some embodiments, the at least one reference DL-RS and the subset of DL-RSs are reported jointly with the at least one of the delay difference, the frequency difference and the phase difference. In some embodiments, the at least one reference DL-RS and the subset of DL-RSs are reported separately with different bit fields in the CSI report. In some embodiments, the at least one reference DL-RS, the subset of DL-RSs and the at least one of a delay difference, a frequency difference and a phase difference are reported separately with different bit fields in the CSI report. FIG. 13 is a flowchart of an example process in a network node 16 for determination and indication of reference downlink reference signal (DL-RS) for inter- transmission/reception point (TRP) delay, frequency and phase difference reporting. One or more blocks described herein may be performed by one or more elements of network node 16 such as by one or more of processing circuitry 68 (including the coefficient determination unit 32), processor 70, radio interface 62 and/or communication interface 60. Network node 16 such as via processing circuitry 68 and/or processor 70 and/or radio interface 62 and/or communication interface 60 is configured to receive a channel state information, CSI, report indicating a first set of downlink reference signals, DL-RSs, distinct from a second set of reference DL-RSs, the CSI report further including for each DL-RS of the first set of DL-RSs at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the second set of reference DL-RSs (Block S124). The process includes determining compensating coefficients based at least in part on the at least one measurement quantity and the indicated at least one reference DL-RS of the second set of reference DL-RSs (Block S126). In some embodiments, each DL-RS of the first set of DL-RSs and the second set of reference DL-RSs is a CSI-RS. In some embodiments, each DL-RS of the first set of DL- RSs and the second set of reference DL-RSs is a tracking reference signal, TRS. In some embodiments, the method includes indicating to the UE 22 the second set of reference DL- RS by radio resource control signaling, RRC. In some embodiments, the method includes indicating to the UE 22 at least one of the first set of DL-RSs, the second set of reference DL-RSs and at least one measurement quantity to be reported in the CSI report. FIG. 14 is a flowchart of an example process in a user equipment 22 according to some embodiments of the present disclosure. One or more blocks described herein may be performed by one or more elements of user equipment 22 such as by one or more of processing circuitry 84 (including the reporting unit 34), processor 86, radio interface 82 and/or communication interface 60. User equipment 22 such as via processing circuitry 84 and/or processor 86 and/or radio interface 82 is configured to, for each DL-RS of a first set of DL-RSs that excludes a second set of reference DL-RSs, measure at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the first set of reference DL-RSs (Block S128). The process includes generating a CSI report indicating the first set of reference DL-RSs for each of the at least one reporting quantity, the CSI report further including the at least one reporting quantity (Block S130). The process also includes transmitting 16 the CSI report to the network node (Block S132). In some embodiments, each set of DL-RS of the plurality of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a CSI-RS resource set. In some embodiments, each set of DL-RS of the plurality of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a tracking reference signal, TRS. In some embodiments, the indication of the first set of reference DL-RS is reported jointly with the at least one reporting quantity. In some embodiments, each set of DL-RSs of the plurality of sets of DL-RSs are transmitted by one among multiple transmission-reception points, TRPs. In some embodiments, the CSI report includes a number of measurements for each of at least one reporting that is not greater than a maximum number configured by the network node 16. In some embodiments, the at least one reporting quantity that is reported is configured by the network node 16. Having described the general process flow of arrangements of the disclosure and having provided examples of hardware and software arrangements for implementing the processes and functions of the disclosure, the sections below provide details and examples of arrangements for determination and indication of reference downlink reference signal (DL-RS) for inter-transmission/reception point (TRP) delay, frequency and phase difference reporting. Although the term TRP may be used in this disclosure, the term TRP may not be captured in 3GPP specifications. Instead, a TRP may be represented by any one of ‘NZP CSI-RS resource set’, ‘NZP CSI-RS resource’, ‘TRS resource set’, ‘TRS resource’, or in general downlink reference signal (DL-RS). The terminologies ‘delay(s)’ and ‘propagation delay(s)’ may be used interchangeably in the disclosure. In 3GPP 6G, other terms than NZP CSI-RS may be used. For example, a new downlink reference signal or downlink synchronization signal may be introduced in 6G which may be used instead of NZP CSI-RS. The 6G downlink reference signals and/or downlink synchronization signals may be transmitted aperiodically, semi-persistently or periodically. The terms “DL-RS”, “NZP CSI-RS” and “NZP CSI-RS resource set” may be used interchangeably in this disclosure. In 6G, CSI reports may be transmitted in MAC messages, which means that the CSI report may be included in a single message and where the message may vary in size. Although reference is made to setup for the cases where the delay difference(s) are reported, the embodiments presented herein are non-limiting and may also be applicable to one or more of the following cases: • the UE 22 reports frequency difference(s) instead of delay difference(s); To arrive at the UE 22 steps for the case when the UE 22 reports frequency difference(s), the terms ‘delay difference(s)’ may be replaced with ‘frequency difference(s)’; • the UE 22 reports phase difference(s) instead of delay difference(s); To arrive at the UE 22 steps for the case when the UE 22 reports phase difference(s), the terms ‘delay difference(s)’ may be replaced with ‘phase difference(s)’; and/or • the UE 22 reports phase gradient difference(s) instead of delay difference(s); To arrive at the UE 22 steps for the case when the UE 22 reports difference(s) of phase changes in a time unit, the terms ‘delay difference(s)’ may be replaced with ‘phase gradient difference(s)’. With DL CJT, the same data/layers may be transmitted from multiple cooperative TRPs and the signals from multiple TRPs may be coherently combined at the UE 22 through proper joint antenna precoding at the TRPs. This may be achieved by CSI feedback where the UE 22 measures the channels associated with the TRPs and reports back a joint precoder across the multiple TRPs, such that the precoded signals from these TRPs are phase-aligned when they reach the UE 22. Alternatively, this may also be achieved by reciprocity-based DL transmission, where the channel is obtained via UL sounding reference signal (SRS) measurement. There are a number of challenges in CJT. For instance, the cooperating TRPs may not be perfectly synchronized in time. Such timing misalignment, together with the propagation delay differences between the different TRPs, may result in a large frequency selective composite channel, i.e., where the channel amplitude and/or phase vary rapidly across frequency. In addition, although the same nominal transmit frequency may be considered for the cooperative TRPs, due to local oscillator stability there may be some actual transmit frequency drifts for different TRPs. In such cases, the UE 22 needs to measure the delay, the frequency and/or the phase differences between the TRPs and report them back to the network node 16, so that the network node 16 may compensate for them. Such a problem is of interest in multi-TRP and D-MIMO networks, which are of interest in both 3GPP Rel-19 and 6G. For instance, the following has been considered in a 3GPP Rel-19 WID for CJT enhancement: In some embodiments, the UE 22 may calculate the delay, frequency and/or phase of each DL-RS and report it back to the network node 16. In one possible and most probable method, the UE 22 may measure the delay, frequency and/or phase differences of the DL-RSs with respect to one or more reference DL-RSs and report the delay, frequency and/or phase differences to the network node 16. This disclosure focusses on the cases using reference DL-RSs. Here, the main point is that the network node 16 and the UE 22 should have the same understanding about the reference DL- RS(s) used for computing/calculating the delay, frequency and/or phase difference, so that the network node 16 may properly use the received report and compensate for the inter-TRP delay, frequency and/or phase differences. Thus, methods are disclosed for reporting of the reference DL-RS(s) from the UE 22 to the network node 16. FIG. 15 shows an example flowchart for measurement and report of the inter- TRP delay, frequency and/or phase differences using reference DL-RSs. The details of steps are as follows: In Step 1 of FIG. 15, the UE 22 receives configuration information from the network node 16 of N > 1 different DL-RSs, e.g., N > 1 different NZP CSI-RS resource sets or N>1 different NZP CSI-RS resources. In some embodiments, each of the N NZP CSI-RS resource sets is a tracking reference signal (TRS), i.e., configured with a parameter ‘trs-Info’. In Step 2 of FIG. 15, the UE 22 receives configuration information (e.g., via RRC configuration) from the network node 16 requesting that the UE 22 report one or more of delay, frequency and phase differences based on measurements on all or a subset of the plurality of DL-RSs. In some embodiments, a new reporting quantity may be configured as part of the CSI reporting configuration (e.g., via higher layer parameter reportQuantity as defined in 3GPP TS 38.331 V18.0.0) in the reporting configuration used for configuring delay difference reporting. The new reporting quantity may be set to a value of either ‘delay difference’, ‘frequency difference’, ‘phase difference’, ‘delay and frequency difference’, or ‘delay and phase difference’. This means the UE 22 is requested to report the requested CSI quantity with respect to one or more reference DL-RSs. In some embodiments, the differences with respect to reference DL-RSs may be reported. In some embodiments, the number or the maximum number of CSI quantities to be reported by the UE 22 is configured to the UE 22 by the network node 16. This number may be configured as part of the reporting configuration used for configuring, e.g., delay difference reporting. In Step 3 of FIG. 15, the UE 22 selects one or more reference DL-RS(s) configured in Step 1 which are to be used for calculating the delay, frequency and/or phase differences. In some embodiments, the same or different reference DL-RS(s) are considered for different CSI quantities, i.e., delay/frequency/phase differences. In some embodiments, the UE 22 selects reference DL-RS based on one or more rules from following: • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the DM-RS ports of PDSCH is quasi co-located with where the QCL Type is Type A and/or Type D; • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the DM-RS ports of PDCCH is quasi co-located with where the QCL Type is Type A and/or Type D; • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the CORESET#0 is quasi co-located with where the QCL Type is Type A and/or Type D; • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the CORESET with lowest CORESET-ID is quasi co-located with where the QCL Type is Type A and/or Type D; • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the CORESET with lowest CORESET-ID associated with CORESETPoolIndex 0 or 1 is quasi co-located with where the QCL Type is Type A and/or Type D; • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the TCI-States corresponding to lowest TCI codepoint activated in MAC CE is quasi co-located with where the QCL Type is Type A and/or Type D; and/or • A DL-RS is selected as reference RS for CSI reporting if the SSB or Reference signal that the DL-RS is quasi co-located with is the same as the SSB or CSI- RS that the first TCI of TCI-States corresponding to lowest TCI codepoint activated in MAC CE is quasi co-located with where the QCL Type is Type A and/or Type D. In some embodiments, the selection is applied per Cell, or per active BWP, or per one or multiple search spaces monitored by the UE 22 when there is only one CSI-Report activated/triggered per Cell. Note that it is possible for the reference DL-RS to be selected based a certain pre- defined rules. One example of such a predefined rule is always selecting the first DL-RS (e.g., the DL-RS with the smallest DL-RS Id among the configured DL-RSs). However, this approach may have drawbacks. For instance, if the first DL-RS is not received with sufficient power at the UE 22, measurements of delay, frequency and phase based on this first DL-RS may be error prone. Hence, using the pre-defined first DL-RS as the reference DL-RS may lead to erroneous delay, frequency, and phase difference reporting. Therefore, the UE 22 may select and report a reference DL-RS. In Step 4 of FIG. 151, the UE 22 selects all or a a subset of the DL-RSs for which the UE 22 is to compute/report delay, frequency and/or phase difference(s) and computes the delay, frequency and/or phase difference(s) with respect to the considered reference DL-RS(s). Note that the DL-RSs are selected from the configured DL-RSs configured in Step 1. For instance, the UE 22 may select a subset of DL-RSs which have a received power higher than a pre-defined threshold. Alternatively, the UE 22 may select a subset of M TRSs with the highest received power. In Step 5 of FIG. 15, the UE 22 reports the delay, frequency and/or phase difference(s) of different DL-RSs relative to the reference DL-RSs to the network node 16. In some embodiments, along with the measurement reports, the UE 22 informs the network node 16 about the selected DL-RS(s) and/or the selected reference DL-RS(s) implicitly or explicitly. In some embodiments, the reference DL-RS is indicated jointly with the indication of the selected DL-RSs for which the UE 22 reports delay, frequency and/or phase difference(s). Consider an example of four DL-RS(s) configured in the first step. An example of jointly encoded single field (or indicator) with 6 bits is shown in FIG. 16. Xi (i=1, 2, 3, 4) are used to indicate the selected DL-RSs for which the UE 22 reports delay, frequency, and/or phase differences. X_i indicates if the i^th DL-RS is selected or not (e.g., a first value of X_i indicates that the i^th DL-RS is selected, and a second value of Xi indicates that the i^th DL-RS is not selected). Then, the two bits [Y2 Y1] indicate which DL-RS is selected as the reference DL-RS. For instance, [Y2 Y1] values of 00, 01, 10, and 11 may respectively indicate that the 1^st, 2^nd, 3^rd, and 4^th DL-RS is selected as the reference DL RS. In some embodiments, the same single field/indicator may be used to indicate the reference DL-RS for different report Quantity. As illustrated in FIG. 17, the first 2 bitfields Y1 and Y2 are used to indicate the reference DL-RS for the first report quantity. Then, the bitfields Z1 and Z2 are used to indicate the reference DL-RS for the second report quantity. Finally, the bitfields X1,…,X4 are used to indicate if a reference DL-RS is selected or not. In some embodiments, the reference DL-RS is indicated separately from the indication of the selected DL-RSs for which the UE 22 reports. In some embodiments, separate fields, e.g., [Y₂ Y₁] and [X₄ X₃ X₂ X₁] may be used to indicate the reference DL- RS and the selected DL-RSs, respectively. In some embodiments, Xi (i=1, 2, 3, 4) indicates if the i^th DL-RS is selected or not (e.g., a first value of Xi indicates that the i^th DL-RS is selected, and a second value of X_i indicates that the i^th DL-RS is not selected). Then, the two bits [Y2 Y1] indicate which DL-RS is selected as the reference DL-RS. For instance, [Y2 Y1] values of 00, 01, 10, and 11 may respectively indicate that the 1^st, 2^nd, 3^rd, and 4^th DL-RS is selected as the reference DL RS. Different one- and two-part reporting methods for the UE 22 may be used to report the measurements, which may affect the way the reference DL-RS(s) are indicated. With a two-part report, the selected sub-set of DL-RSs is indicated in the first part, followed by the reporting quantity for the selected DL-RSs in the second part. The payload size is indicated or derived from in the first part. In the one-part method, the payload size is fixed and the UE 22 reports the reporting quantities of the selected DL-RSs in a single part. The reporting quantities of the selected DL-RSs may be indicated with valid/non-zero values, and those of the non-selected DL-RSs may be reported via invalid/default values. The methods for the indication of the reference DL-RS(s) in the two- and one-part reporting methods are explained as follows. Indication of the reference DL-RS(s) and UE selected DL-RSs in the two-part CSI reporting method: With a two-part reporting method, in a first part of the report, a bitmap indicates which DL-RSs are selected. Given that there are N DL-RSs configured to the UE 22 as part of the CSI reporting configuration configured for, e.g., delay difference(s) reporting, the bitmap may have N bits wherein each bit in the bitmap indicates whether an DL-RS is selected or not. When the n-th bit in the bitmap is set to a first value (e.g., a value of 1), it indicates to the network node 16 (e.g., gNB) that the n-th DL-RS is selected. When the n- th bit in the bitmap is set to a second value (e.g., a value of 0), it indicates to the network that the n-th NZP CSI-RS resource set is not selected. Here, in CSI Part 1, the reference DL-RS (e.g., a reference TRP) may also be reported separately from the indication of the selected DL-RSs. For instance, for N=4 DL- RSs, log_^ 3=2 bits are needed to indicate the reference DL-RS. In some embodiments, a bitmp may indicate that the reference DL-RS is associated with which of the “delay difference,” “frequency difference,” “phase difference,” “delay and phase difference,” or “delay and frequency difference” reporting quantities. Here, the indication of the reference DL-RS and its association with the considered reporting quanty(ies) may be encoded jointly or separately. For instance, with the example above of N=4 DL-RSs, 2 bits are required for indication of the reference DL-RS and 3 bits are required to indicate its associated reporting quantity. With joint encoding of the reference DL-RS and its _{associated report quantity, however, 5 bits are required (to cover all 4 × 5 cases of} indicating the reference DL-RS and its associated reporting quantities). In some embodiments, the selected DL-RSs and the reference DL-RSs may be jointly encoded (e.g., as part of CSI part 1). In one example, the number of ways for selecting ^ out of _{3 DL-RSs and selecting one of the ^ resources as reference is given by 3 × This means that the reference DL-RS may be any of the ^ DL-RSs and the rest ^ − 1 DL-RSs may be selected from the remaining 3 − 1 resource sets. Then, the total number of ways for selecting} any number of DL-RSs and selecting one of them as the reference DL-RS is given by _{= 5 bits. Each codepoint, e.g., 00101, identifies the reference DL-RS as well as the} remaining selected DL-RSs. If DL-RS selection and reference DL-RS indication are encoded separately, then 4+2=6 bits are needed. Note that, as explained before, in some embodiments, more than one reference DL-RS may be selected by the UE 22, which may affect the joint encoding method above, accordingly. In some embodiments, the selected DL-RSs, the selected reference DL-RSs and the report type associated with the selected reference DL-RSs may be jointly encoded. In some embodiments, one or more than one of N DL-RSs are used as reference DL-RS. For example, two or more DL-RSs may be transmitted from the same TRP and/or from TRPs that are co-located and well synchronized. In some embodiments, the bitmap only includes DL- RSs that are not used as reference DL-RSs, i.e., given there are N DL-RSs configured to the UE 22 as part of the CSI reporting configuration configured for delay difference(s) reporting. In some embodiments, two out of N DL-RSs are indicated as reference resource sets. Then the bitmap may have N-2 bits wherein each bit in the bitmap indicates whether an DL-RS (among the DL-RSs not used as reference DL-RS) is selected or not. This may reduce the number of bits required in the bitmap. In the second part of the two-part CSI report, in some embodiments, one delay, frequency and/or phase difference value for each DL-RS indicated as selected in the first part is included in the report, wherein the delay, frequency and/or phase difference is with respect to the selected reference DL-RS(s). In some embodiments, the reference DL-RS is implicitly signaled in Part 2, where a DL- RS with zero delay offset, frequency, and/or phase offsets is the reference DL-RS (or, in general, a default/pre-defined value). Alternatively, for k selected DL-RSs, ceil(log2(k)) bits are used in Part 2 to indicate the reference DL-RS and report quantities associated with the remaining k-1 DL-RSs are reported in Part 2. For example, if k=3, 2bits are needed to indicate the reference DL-RS, i.e., ‘00’ for the 1^st of the k DL-RSs, ‘01’ for the 2^nd of the k DL-RSs, and ‘10’ for the 3^rd of the k DL-RSs, and ‘11’ is reserved. In some embodiments, a bit map of k bits is used with the most significant bit (MSB) associated to the 1^st of the k DL-RSs, the LSB bit associated to the last of the k DL-RSs. Indication of the reference DL-RS(s) and UE selected DL-RSs in the one-part CSI reporting method: With one-part CSI report, for each selected DL-RS, the single part CSI report includes one valid and/or non-zero delay difference value with respect to the reference DL-RS, where the valid non-zero delay difference value may be quantized to X bits. A valid and non-zero delay difference value is a codepoint in the X bit field that represents a non-zero delay difference value that may be pre-defined in a table in 3GPP specifications. Here, in some embodiments, the reference DL-RS is indicated by a codepoint in the X bit filed of reporting numbers. For instance, 000…0 may be used to indicate the reference DL-RS. For instance, consider a setup with 6 DL-RS and an X=3 bit table of quantized values of delay differences. The valid non-zero delay differences are given by codepoints 001, 010, 011, 100, 101, 110 and 111. The codepoint 000 is reserved to indicate the reference DL- RS. Then, for instance, the report [010, 001, 001, 110, 111,000] indicates different measured delay differences for DL-RSs 1-5 and that DL-RS 6 is selected as the reference TRP. Joint encoding of reference DL-RS with one or more of selected DL-RSs, and delay/frequency/phase difference(s) Assuming N DL-RSs are configured, consider an example of joint encoding between reference DL-RS and selected DL-RSs. In this case, the joint encoding may include an indication of reference DL-RS. For the reference DL-RS, no reporting of delay/frequency/phase is needed. For the remaining N-1 DL-RSs, the above methods with a reserved codepoint may be reused to indicate which among N-1 DL-RSs are selected and which ones are not selected. In some embodiments, for N configured DL-RSs, ceil(log2(N)) bits are used to indicate the reference DL-RS, and report quantities (e.g., delay/frequency/phase differences) of N-1 remaining DL-RSs are included in the report. For example, if N=3, 2 bits are needed to indicate the reference DL-RS, i.e., ‘00’ for the 1^st DL-RS, ‘01’ for the 2^nd DL- RS, ‘10’ for the 3^rd DL-RS, and ‘11’ is reserved. In some embodiments, a bit map of (N-1) bits is used to indicate the selected DL-RS excluding the reference DL-RS, with the MSB bit associated to the 1^st DL-RS (excluding the reference DL-RS), and the lease significant bit (LSB) associated to the last DL-RS (excluding the reference DL-RS). In this embodiment, ceil(log2(N)) bits are used to indicate the reference DL-RS, (N-1) bits are used to indicate the selected DL-RSs. Report quantities are only reported for the number of selected DL-RSs indicated by the (N-1) bits. In this way, the network node 16 and the UE 22 may have the same understanding about the selected reference DL-RS for inter-TRP delay, frequency and/or phase difference measurement and report. This enables proper compensation of the delay, frequency and/or phase differences between the TRPs, resulting in efficient CJT. Moreover, the overhead of measurement reporting may be reduced, depending on the considered method of reference DL-RS indication. Some embodiments address issues raised in 3GPP Rel-19 as well as in 6G. Alternative Embodiments for Indication of the reference DL-RS(s): In some embodiments of implicit indication, along with the delay, frequency and/or phase difference(s) report, the UE 22 sends auxiliary measurement reports. This enables the network node 16 to understand the selected reference DL-RS. For instance, along with the reported delay, frequency and/or phase difference(s) report, the UE 22 may report the received powers of the DL-RS(s). Then the network node 16 understands that, e.g., the DL-RS with the highest received power has been selected as the reference DL- RS. In some embodiments of implicit indication, the network node 16 may understand the UE’s selected reference DL-RS implicitly based on the reported values of the delay, frequency and/or phase difference(s). For instance, the UE 22 may indicate the delay, frequency and/or phase difference(s) of the reference DL-RS with a specific value (e.g., zero). Then, the network node 16 understands that the rest of reported values are relative to the selected reference DL-RS. In some embodiments, along with the delay, frequency and/or phase difference(s) report, the UE 22 may inform the network node 16 about the considered criteria for selecting the reference DL-RS. Some Further Examples Some embodiments may include one or more of the following: Example A1. A network node configured to communicate with a user equipment (UE), the network node configured to, and/or comprising a radio interface and/or comprising processing circuitry configured to: receive from the UE a channel state information, CSI, report indicating at least one reference downlink reference signal, DL-RS, a subset of DL-RSs configured for the CSI report and indicating at least one of a delay difference, a frequency difference and a phase difference between a DL-RS of the subset of DL-RSs and the reference DL-RS; and determine compensating coefficients based at least on the one of the delay difference, the frequency difference and the phase difference. Example A2. The network node of Example A1, wherein the at least one reference DL-RS and the subset of DL-RSs are reported jointly. Example A3. The network node of Example A2, wherein the at least one reference DL-RS and the subset of DL-RSs are reported jointly with the at least one of the delay difference, the frequency difference and the phase difference. Example A4. The network node of Example A1, wherein the at least one reference DL-RS and the subset of DL-RSs are reported separately with different bit fields in the CSI report. Example A5. The network node of Example A4, wherein the at least one reference DL-RS, the subset of DL-RSs and the at least one of a delay difference, a frequency difference and a phase difference are reported separately with different bit fields in the CSI report. Example B1. A method implemented in a network node, the method comprising: receiving from the UE a channel state information, CSI, report indicating at least one reference downlink reference signal, DL-RS, a subset of DL-RSs configured for the CSI report and indicating at least one of a delay difference, a frequency difference and a phase difference between a DL-RS of the subset of DL-RSs and the reference DL-RS; and determining compensating coefficients based at least on the one of the delay difference, the frequency difference and the phase difference. Example B2. The method of Example B1, wherein the at least one reference DL- RS and the subset of DL-RSs are reported jointly. Example B3. The method of Example B2, wherein the at least one reference DL- RS and the subset of DL-RSs are reported jointly with the at least one of the delay difference, the frequency difference and the phase difference. Example B4. The method of Example B1, wherein the at least one reference DL- RS and the subset of DL-RSs are reported separately with different bit fields in the CSI report. Example B5. The method of Example B4, wherein the at least one reference DL- RS, the subset of DL-RSs and the at least one of a delay difference, a frequency difference and a phase difference are reported separately with different bit fields in the CSI report. Example C1. A user equipment (UE) configured to communicate with a network node, the UE configured to, and/or comprising a radio interface and/or processing circuitry configured to: configure a channel state information, CSI, report indicating at least one reference downlink reference signal, DL-RS, and a subset of DL-RSs configured for a CSI report and indicating at least one of a delay difference, a frequency difference and a phase difference between a DL-RS of the subset of DL-RSs and the reference DL-RS; and transmit to the network node the CSI report indicating the at least one reference DL-RS, the subset of DL-RSs configured for the CSI report and indicating at the least one of the delay difference, the frequency difference and the phase difference. Example C2. The UE of Example C1, wherein the at least one reference DL-RS and the subset of DL-RSs are reported jointly. Example C3. The UE of Example C2, wherein the at least one reference DL-RS and the subset of DL-RSs are reported jointly with the at least one of the delay difference, the frequency difference and the phase difference. Example C4. The UE of Example C1, wherein the at least one reference DL-RS and the subset of DL-RSs are reported separately with different bit fields in the CSI report. Example C5. The UE of Example C4, wherein the at least one reference DL-RS, the subset of DL-RSs and the at least one of a delay difference, a frequency difference and a phase difference are reported separately with different bit fields in the CSI report. Example D1. A method implemented in a user equipment (UE), the method comprising: configuring a channel state information, CSI, report indicating at least one reference downlink reference signal, DL-RS and a subset of DL-RSs configured for a CSI report and indicating at least one of a delay difference, a frequency difference and a phase difference between a DL-RS of the subset of DL-RSs and the reference DL-RS; and transmitting to the network node the CSI report indicating the at least one reference DL-RS, the subset of DL-RSs configured for the CSI report and indicating at the least one of the delay difference, the frequency difference and the phase difference. Example D2. The method of Example D1, wherein the at least one reference DL- RS and the subset of DL-RSs are reported jointly. Example D3. The method of Example D2, wherein the at least one reference DL- RS and the subset of DL-RSs are reported jointly with the at least one of the delay difference, the frequency difference and the phase difference. Example D4. The method of Example D1, wherein the at least one reference DL- RS and the subset of DL-RSs are reported separately with different bit fields in the CSI report. Example D5. The method of Example D4, wherein the at least one reference DL- RS, the subset of DL-RSs and the at least one of a delay difference, a frequency difference and a phase difference are reported separately with different bit fields in the CSI report. STANDARDIZING THE PROPOSED SOLUTIONS The following provides non-limiting examples of how certain aspects of the proposed solutions could be implemented within the framework of a specific communication standard. In particular, the following provides non-limiting examples of how the proposed solutions could be implemented within the framework of a 3GPP TSG RAN standard. The changes described by the following are merely intended to illustrate how certain aspects of the proposed solutions could be implemented in a particular standard. However, the proposed solutions could also be implemented in other suitable manners, both in the 3GPP Specification and in other specifications or standards. Enhancement for CJT In Rel-18, CSI feedback for CJT was introduced assuming ideal time and frequency synchronization between different TRPs. In most practical deployments, there exists certain level of frequency and time differences between TRPs due to both time/frequency synchronization errors in the TRPs and/or channel differences (e.g., different propagation delays and/or Doppler frequencies) associated to different TRPs. These time and frequency differences can cause large performance degradation for CJT if they are not taken into account. Table and Table show an example of CJT performance with and without compensation of the time and frequency difference, respectively, between three TRPs. It can be observed that there is a high-performance loss if the time and frequency differences are not compensated. Table A Impact of timing error and pre-compensation on system level performance. Carrier frequency=3.5GHz, SCS=30kHz, subband size = 8 PRB, timing error = B_{C, C. F, GHIJK} Table B Impact of frequency drift and pre-compensation on system level performance. Carrier frequency=3.5GHz, SCS=30kHz, reporting period = 10ms, frequency drift = [0, 50, 100] ppb Measurement resources In NR, a UE performs time and frequency tracking based on TRS. For multi-TRP transmission, it is logical to have different TRSs transmitted from different TRPs for time and frequency tracking of each TRP. Hence, measurements for both time difference and frequency difference can be based on TRSs. Since TRS is typically cell/beam specific and can be transmitted on a different antenna port from PDSCH, TRS is not suitable for measuring phase differences between TRPs. CSI-RS seems to be best suited for the purpose. Also, because TRS is used for time and frequency difference measurement and reporting, it makes sense that the phase differences are reported separately from time and frequency difference, i.e., they are separately configured. Proposal 1: In Rel-19, support the following measurement resources: -> TRS is used for both time difference and frequency difference measurements -> NZP CSI-RS is used for phase difference measurements Reporting configuration In general, not all the reporting quantities (i.e., delay offset, frequency offset and phase offset) may always need to be reported by the UE. Particularly, depending on whether CJT is to be deployed in FDD/TDD and on deployment scenarios such as collocated vs non- collocated TRPs, there may be a need for only a subset of such reporting quantities. In our view, the following subsets need to be considered as reporting quantities in a single CSI report: • time difference only: this is relevant in scenarios where the network may estimate frequency difference proprietarily while the network doesn’t estimate the time difference, or in some scenarios where the time difference needs to be reported less frequently than for frequency difference reporting and reporting both each time would result in more feedback overhead; • frequency difference only: this may be relevant to collocated TRPs where a common baseband timing is used for all TRPs while there are still frequency differences due to separate radios used for the TRPs, or in some scenarios where the frequency difference needs to be reported more frequently than for time difference reporting • time difference and frequency offset together: this is the most likely case for FDD or TDD where the multiple TRPs are non-collocated. • phase difference only: this is relevant to TDD where the phase difference is measured on precoded CSI-RS and thus, a separate report is needed Hence, in Rel-19, it is important to flexibly configure the UE to measure and report only a subset of such delay, frequency and/or phase differences. Proposal 2: In Rel-19, support flexible configuration of which subset of reporting quantities to report. The following subsets can be considered as reporting quantities: time difference only frequency offset only frequency offset and time difference only phase difference only For time, frequency, and phase difference reporting purposes, one TRP can be selected as a reference TRP and the time, frequency, and phase difference between each TRP and the reference TRP can be calculated and reported. For reliable measurements, the reference TRP can be selected by the UE as the UE has the best view of which TRP can be measured reliably. Proposal 3: In Rel-19, For time, frequency, phase difference reporting, the measurements for each TRP is differential with respect to a reference TRP, wherein the reference TRP is selected by the UE 3.3 Reporting Time Differences Between TRPs Rel-18 CJT is supposed to be work for time arrivals within a CP. However, depending on the reference TRP, the time differences can be within +/- 1CP. Therefore, time difference reporting needs to cover +/-1CP. Proposal 4: For time difference reporting, the reporting range for time differences is +/- one CP. _{As for quantization, uniform quantization can be used. The quantization step size, ∆L,} should be small enough such that the resulting residual phase rotation within a PMI subband is small . A starting point can be the phase quantization step size in Rel-18 type II codebook, i.e., maximum 360^o/16 or 22.5^o phase rotation within a PMI subband due to quantization. The quantization design should also consider the maximum PMI subband size to be supported. Table below shows the maximum phase rotations within a PMI subband of 8RBs for a quantization step from 10ns to 60ns. Since PMI subband is configured for CJT CSI report, how to determine the maximum PMI subband size for this purpose needs to be discussed. One approach is to consider the worst-case scenario among all the possible BWP and R parameter configurations, which could however result in an over design. Another alternative approach is to signal the PMI subband size in the time difference report configuration, this effectively would result in a configurable quantization step. Observation 1: For time difference reporting, the quantization step needs to be determined based on the residual phase rotation due to quantization within a PMI subband for CJT based on Rel-18 type II CJT report. Table C: Maximum residual phase rotation within a PMI subband of 8RBs for different quantization steps. For a given subband size in RBs, the actual subband size in Hz is dependent on the subcarrier spacing (SCS). Therefore, the quantization step size is linked to the subcarrier spacing , i.e., ^ the time domain quantization step size in seconds is proportional to where ^ is the numerology. The quantization step size for SCS=15kHz is twice as large as that of SCS=30kHz. The number of bits is the same for all numerologies. Thus, the time difference reporting range for SCS=15kHz is also twice as large as the range for SCS=30kHz. Observation 2: For time difference reporting, the quantization step size should be dependent on the subcarrier spacing of the associated TRS. The number of bits is the same for different SCS. 3.4 Reporting Frequency Differences Between TRPs The maximum frequency error defined in RAN4 is +/-0.1ppm according to RAN4 specification in Table 6.5.1.2-1 (copied below) in TS38.104, which can be the starting point for determining the range for reporting frequency differences between TRPs. Note that the maximum frequency error for a given +/-0.1ppm is dependent on the carrier frequency operated. The maximum frequency offsets between two TRPs at different FR1 frequencies are shown in Table below. Table 6.5.1.2-1: Frequency error minimum requirement (TS38.104) In one option, the reporting range is determined to cover the worst-case (or largest) carrier frequency. In another option, the reporting range can be dependent on the carrier frequency of the TRS. Table D: Maximum frequency errors for different carrier frequencies in FR1 assuming +/-0.1ppm. Proposal 5: For the range of frequency difference reporting, the RAN4 specification of +/-0.1ppm can be used as a starting point. Observation 3: For frequency difference reporting, the frequency range may depend on the carrier frequency of the associated TRS. As for quantization of the frequency differences for reporting, again uniform quantization can be used. The quantization step size, ∆M, would result in a residual phase rotation over time . For CJT based on Rel-18 CJT CSI feedback, the amount of phase rotation depends on the PMI reporting period. A large CJT CSI reporting period would require a smaller quantization step size. For a given maximum allowed residual phase rotation of 360/16 degrees, Table 6 shows some possible frequency resolutions with different PMI reporting periods. In one option, a fixed step size is used to cover the worst-case scenario of PMI reporting period, for example, 40ms. In another option, the step size can be linked to a PMI reporting period, which could be indicated in the report configuration for frequency differences. Table 6: Frequency resolution vs PMI feedback period for a given maximum phase rotation of 360^o/16=22.5^o in a reporting period. Proposal 6: For frequency difference reporting, uniform quantization is used. Proposal 7: For frequency difference reporting, investigate whether to use a fixed quantization step or a variable step size depending on a PMI reporting period indicated in the report configuration. Reporting Phase Differences Between TRPs In reciprocity-based downlink (DL) transmission, the receive and transmit circuitries at each TRP is typically calibrated such that the same TX-RX gain difference N and TX-RX phase difference P are maintained and the same across all receive and transmit circuitries associated to different physical antennas. The absolute phase P at each TRP is unknown after TRP calibration. However, P is not needed for single TRP transmission if all antennas have been calibrated to the same value P. For CJT based on Rel-18 CJT CSI feedback, the unknown phase P at each TRP is not a problem because it is considered in the reported PMI. The unknown absolute phase P will be different at each TRP after calibration and is thus a problem for reciprocity based CJT because coherent transmission is not possible without _{knowing the absolute phase for each TRP (P^, P^, PQ … ^ or at least, knowing the phase differences (P^ − P^, PQ − … ^ . Hence, one solution is to let the UE to measure and} feedback these phase differences based on DL reference signals. The phase of a receive DL signal from a TRP can also be influenced by the channel in addition to the absolute phase at each TRP. In addition, when there is time differences and/or frequency differences between TRPs, the phase difference between two received DL signals _{is also a function of both time and frequency. The received DL reference signals /^S, ^^ from TRP #1 and ^^S, ^^ from TRP#2 at the ith subcarrier and the kth OFDM symbol can be} expressed as _{where ^W, ^W ^X = 1,2^ are respectively the time delay and carrier frequency of TRP#n, ∆Y = Y^ − Y^, ∆^ is the subcarrier spacing, ^Z is the local oscillator frequency at the UE, The phase of received signal depends also on the RE location ^S, ^^ due to the frequency} and time difference between the two TRPs. Therefore, the effect of the channel and time and/or frequency differences between TRPs _{need to be removed before the TX-RX phase differences (e.g., P^ − P^^ between TRPs due} to TRP circuitries can be measured. Observation 4: The effect of propagation channel and time and/or frequency differences between TRPs needs to be removed for properly measuring phase differences between TRPs To remove the effect of the channel, the DL RS can be precoded by a same precoder as for PDSCH and transmitted on the same antenna ports as PDSCH with the precoder derived based on the uplink channel. Observation 5: Precoded CSI-RS with the same precoder as for PDSCH and on the same antenna ports as PDSCH is needed to remove the channel effect. The effect due to time and frequency differences between TRPs can be removed by either the gNB or the UE, but not both, assuming that the time and frequency differences between TRPs are known at the UE or the gNB. Observation 6: The effect due to time and frequency differences between TRPs can be removed by either the gNB or the UE, but not both. For removing or compensating the effect of time and frequency differences between TRPs, it seems to be natural to do it at the UE side because the time and frequency differences are measured by the UE and the UE can remove the estimated time and frequency differences from the channel measurement based on the CSI-RS at each TRP. Proposal 8: For phase difference reporting, the effect of time and frequency differences between TRPs is removed from the channel measurement at the UE before computing the phase differences. QCL In Rel-18 CJT, when two TCI states are indicated, one of the following two QCL assumptions is assumed by a UE. • CJT scheme A: PDSCH DM-RS port(s) are QCLed with the DL RSs of both indicated TCI-States with respect to QCL-TypeA • CJT scheme B: PDSCH DM-RS port(s) are QCLed with the DL RSs of both indicated TCI-States with respect to QCL-TypeA except for QCL parameters {Doppler shift, Doppler spread} of the second indicated joint TCI state. With time and frequency difference feedback in Rel-19, gNB can also pre-compensate time delay at each TRP. When delay is pre-compensated at a TRP, DMRS of PDSCH is no longer QCLed with a TRS or CSI-RS transmitted from the TRP with respect to average delay, which is not supported by the existing two schemes. Hence to support delay pre- compensation, a new QCL scheme is needed. Proposal 9: A new QCL assumption is needed for supporting delay pre-compensation in Rel-19 CJT. • The following observations were made: Observations for CSI up to 128 ports _{Observation 1 For a ^[, \, K^ = ^G], G], ^^ antenna array deployment, when compared to 32 ports using a ^\^, \G^ = ^G, G]^ port layout, there are significant throughput gains for 64 ports with a ^\^, \G^ = ^_, `^ port layout; further gains are seen for 128 ports with a ^\^, \G^ = ^_, G]^ layout. Observation 2 For a ^[, \, K^ = ^G^, `, ^^ antenna array deployment, when compared to 32 ports legacy NR with a ^\^, \G^ = ^^, `^ port</sub> layout, there are significant throughput gains for 48 ports with a ^<sub>\^, \G^ = ^a, `^ port layout; in this scenario, 64 ports with a ^\^, \G^ = ^_, `^ port layout doesn’t provide additional gains over 48 ports with a ^\^, \G^ = ^a, `^ port layout.} Observation 3 When increasing the number of CSI-RS ports, the gains provided by Rel-16 eType-II codebook are in a similar range as of when increasing the number of CSI-RS ports for the Type-I codebook. Observations for CJT Enhancements Observation 4 For time difference reporting, the quantization step needs to be determined based on the residual phase rotation due to quantization within a PMI subband for CJT based on Rel-18 type II CJT report. Observation 5 For time difference reporting, the quantization step size should be dependent on the subcarrier spacing of the associated TRS. The number of bits is the same for different SCS. Observation 6 For frequency difference reporting, the frequency range may depend on the carrier frequency of the associated TRS. Observation 7 The effect of propagation channel and time and/or frequency differences between TRPs needs to be removed for properly measuring phase differences between TRPs Observation 8 Precoded CSI-RS with the same precoder as for PDSCH and on the same antenna ports as PDSCH is needed to remove the channel effect. Observation 9 The effect due to time and frequency differences between TRPs can be removed by either the gNB or the UE, but not both. • The following are proposed: Proposals for CSI up to 128 ports Proposal 1 Support P_CSI-RS values of 48, 64, 96, and 128 ports for Type-I single- panel CB _{Proposal 2 Support the ^\^, \G^ port layout combinations proposed in Table 1} for Type-I single panel CB Proposal 3 In Rel-19, evaluate further at least the following candidates for Type I codebook refinement: (a) indication of spatial 2D-DFT vectors can be made independent of each other wherein the indicated spatial 2D-DFT vectors can be either layer specific or each indicated spatial 2D-DFT vector can be common to up to 2 layers, and (b) different polarization co-phasing factors for each of the selected beams can be allowed. Proposal 4 In Rel-19, for performance evaluations, use the straightforward e_{xtension of ^\G, \^^ in Rel-15 Type-I Single-Panel Codebook as} the performance baseline. Proposal 5 In Rel-19, consider potential solutions (within Type I codebook design framework) to the problem of co-existence between terrestrial systems and fixed satellite services (e.g., in the 6425–7125 MHz band). Proposal 6 For supporting up to 128 ports with multiple CSI-RS resources, discuss whether port aggregation along both N1 and N2 dimensions is supported and the associated port mappings. _{Proposal 7 Support the same Kbcd − ec values and the set of ^\^, \G^ port} layouts in Table 1 as for the Type-I single panel CB, for at least Rel.16 Type-II CB Proposal 8 For CSI reporting of hybrid beamforming when a single CRI is configured, support the following extensions: For at most 16 ports p_{er resource, support ^ < gh ≤ ` CSI-RS resources and for at most 32 ports per resource support ^ < gh ≤ _ CSI-RS resources} Proposals for CJT Enhancements Proposal 9 In Rel-19, support the following measurement resources: -> TRS is used for both time difference and frequency difference measurements -> NZP CSI-RS is used for phase difference measurements Proposal 10 In Rel-19, support flexible configuration of which subset of reporting quantities to report. The following subsets can be considered as reporting quantities: ^ time difference only ^ frequency offset only ^ frequency offset and time difference only ^ phase difference only Proposal 11 In Rel-19, For time, frequency, phase difference reporting, the measurements for each TRP is differential with respect to a reference TRP, wherein the reference TRP is selected by the UE Proposal 12 For time difference reporting, the reporting range for time differences is +/- one CP. Proposal 13 For the range of frequency difference reporting, the RAN4 specification of +/-0.1ppm can be used as a starting point. Proposal 14 For frequency difference reporting, uniform quantization is used. Proposal 15 For frequency difference reporting, investigate whether to use a fixed quantization step or a variable step size depending on a PMI reporting period indicated in the report configuration. Proposal 16 For phase difference reporting, the effect of time and frequency differences between TRPs is removed from the channel measurement at the UE before computing the phase differences. Proposal 17 A new QCL assumption is needed for supporting delay pre- compensation in Rel-19 CJT. As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, data processing system, computer program product and/or computer storage media storing an executable computer program. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a “circuit” or “module.” Any process, step, action and/or functionality described herein may be performed by, and/or associated to, a corresponding module, which may be implemented in software and/or firmware and/or hardware. Furthermore, the disclosure may take the form of a computer program product on a tangible computer usable storage medium having computer program code embodied in the medium that may be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD-ROMs, electronic storage devices, optical storage devices, or magnetic storage devices. Some embodiments are described herein with reference to flowchart illustrations and/or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, may be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer (to thereby create a special purpose computer), special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer program instructions may also be stored in a computer readable memory or storage medium that may direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. It is to be understood that the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows. Computer program code for carrying out operations of the concepts described herein may be written in an object oriented programming language such as Python, Java® or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the "C" programming language. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user's computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments may be combined in any way and/or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination. It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.

Claims

What is claimed is: 1. A method in a user equipment, UE (22), configured to communicate with a network node (16), the method comprising: for each of at least one reporting quantity, measuring (S128) each set of downlink reference signals, DL-RSs, of a plurality of sets of DL-RSs distinct from a first set of reference DL-RSs, the at least one reporting quantity being one of a delay difference, a frequency difference and a phase difference relative to the first set of reference DL-RSs; generating (S130) a channel state information, CSI, report indicating the first set of reference DL-RSs for each of the at least one reporting quantity, the CSI report further including the at least one reporting quantity; and transmitting (S132) the CSI report to the network node (16).

2. The method of Claim 1, wherein each set of DL-RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a CSI-RS resource set.

3. The method of Claim 1, wherein each set of DL-RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a tracking reference signal, TRS.

4. The method of any of Claims 1-3, wherein the indication of the first set of reference DL-RSs for at least one reporting quantity is reported jointly with the at least one reporting quantity.

5. The method of any of Claims 1-4, wherein each set of DL-RSs of a plurality of sets of DL-RSs are transmitted by one among multiple transmission-reception points, TRPs.

6. The method of any of Claims 1-5, wherein the CSI report includes a number of measurements for each of at least one reporting quantity that is not greater than a maximum number configured by the network node (16).

7. The method of any of Claims 1-6, wherein the at least one reporting quantity that is reported is configured by the network node (16).

8. A user equipment, UE (22), configured to communicate with a network node (16), the UE (22) comprising processing circuitry configured to: for each of at least one reporting quantity, measure each set of downlink reference signals, DL-RSs, of a plurality of sets of DL-RSs distinct from a first set of reference DL- RSs, the at least one reporting quantity being one of a delay difference, a frequency difference and a phase difference relative to the first set of reference DL-RSs; generate a channel state information, CSI, report indicating the first set of reference DL-RSs for each of the at least one reporting quantity, the CSI report further including the at least one reporting quantity; and transmit the CSI report to the network node (16).

9. The UE (22) of Claim 8, wherein each set of DL-RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a CSI-RS resource set.

10. The UE (22) of Claim 8, wherein each set of DL-RSs of the plurality of sets of DL-RSs and the first set of reference DL-RSs for each of the at least one reporting quantity is a tracking reference signal, TRS.

11. The UE (22) of any of Claims 8-10, wherein the indication of the first set of reference DL-RSs for at least one reporting quantity is reported jointly with the at least one reporting quantity.

12. The UE (22) of any of Claims 8-11, wherein each set of DL-RSs of a plurality of sets of DL-RSs are transmitted by one among multiple transmission-reception points, TRPs.

13. The UE (22) of any of Claims 8-12 wherein the CSI report includes a number of measurements for each of at least one reporting quantity that is not greater than a maximum number configured by the network node (16).

14. The UE (22) of any of Claims 8-13, wherein the at least one reporting quantity that is reported is configured by the network node (16).

15. A method in a network node (16) configured to communicate with a user equipment, the method comprising: receiving a channel state information, CSI, report indicating a first set of downlink reference signals, DL-RSs, distinct from a second set of reference DL-RSs, the CSI report further including for each DL-RS of the first set of DL-RSs at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the second set of reference DL-RSs; and determining compensating coefficients based at least in part on the at least one measurement quantity and the indicated at least one reference DL-RS of the second set of reference DL-RSs.

16. The method of Claim 15, wherein each DL-RS of the first set of DL-RSs and the second set of reference DL-RSs is a CSI-RS.

17. The method of Claim 15, wherein each DL-RS of the first set of DL-RSs and the second set of reference DL-RSs is a tracking reference signal, TRS.

18. The method of any of Claims 15-17, further comprising indicating to the UE (22) at least one of the first set of DL-RSs, the second set of reference DL-RSs and at least one measurement quantity to be reported in the CSI report.

19. A network node (16) configured to communicate with a user equipment, the network node (16) including processing circuitry configured to: receive (S124) a channel state information, CSI, report indicating a first set of downlink reference signals, DL-RSs, distinct from a second set of reference DL-RSs, the CSI report further including for each DL-RS of the first set of DL-RSs at least one measurement quantity, the at least one measurement quantity being one of a delay difference, a frequency difference and a phase difference relative to at least one reference DL-RS of the second set of reference DL-RSs; and determine (S126) compensating coefficients based at least in part on the at least one measurement quantity and the indicated at least one reference DL-RS of the second set of reference DL-RSs.

20. The network node (16) of Claim 19, wherein each DL-RS of the first set of DL-RSs and the second set of reference DL-RSs is a CSI-RS.

21. The network node (16) of Claim 19, wherein each DL-RS of the first set of DL-RSs and the second set of reference DL-RSs is a tracking reference signal, TRS.

22. The network node (16) of any of Claims 19-21, wherein the processing circuitry is configured to indicate to the UE (22) at least one of the first set of DL-RSs, the second set of reference DL-RSs and at least one measurement quantity to be reported in the CSI report.