WO2013081368A1

WO2013081368A1 - Method and apparatus for transmitting reference signal and uplink transmission

Info

Publication number: WO2013081368A1
Application number: PCT/KR2012/010158
Authority: WO
Inventors: Jian Jun Li; Kyoung Min Park
Original assignee: Pantech Co Ltd
Current assignee: Pantech Co Ltd
Priority date: 2011-12-01
Filing date: 2012-11-28
Publication date: 2013-06-06
Anticipated expiration: 2014-06-01
Also published as: KR20130061586A

Abstract

The present invention relates to a method and apparatus for transmitting a reference signal and an uplink transmission method and apparatus using the same. A method of transmitting a reference signal in a Cooperative Multi-Point (CoMP) system in accordance with the present invention includes sending configuration information on the reference signal and sending the reference signal based on the configuration information on the reference signal, wherein the configuration information on the reference signal indicates Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in the transmission of the reference signal.

Description

METHOD AND APPARATUS FOR TRANSMITTING REFERENCE SIGNAL AND UPLINK TRANSMISSION

The present invention relates to wireless communication technology and, more particularly, to a Cooperative Multi-Point (hereinafter referred to as a 'CoMP') operation using closed-loop Multi-Input Multi-Output (MIMO).

In order to increase the performance and communication capacity of a wireless communication system, multi-cell (or multi-transmission/reception (Tx/Rx) point cooperation has been introduced. The multi-cell (or multi-Tx/Rx point) cooperation is also called cooperative multiple point transmission and reception (CoMP).

CoMP includes a beam avoidance scheme in which neighboring cells (or multi-Tx/Rx points) cooperate with one another in order to mitigate interference with the users of a cell (or multi-Tx/Rx point) boundary and a joint transmission scheme in which neighboring cells cooperate with one another in order to send the same data.

In the next-generation wireless communication systems, such as Institute of Electrical and Electronics Engineers (IEEE) 802.16m or a 3^rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A), one of major requirements that that comes to the fore front is to improve the performance of users that are placed in a cell boundary and subject to severe interference from a neighboring cell. In order to solve this problem, CoMP may be taken into consideration. In relation to this CoMP, a variety of scenarios are possible.

Meanwhile, a base station sends a reference signal to a mobile station in order to check a downlink channel state. The mobile station receives the reference signal, performs measurement relating to a channel state based on the transmission state of the reference signal, and feeds back a result of the measurement to the base station. The base station can estimate the state of a downlink channel based on the feedback measurement result. In a CoMP environment, likewise, a reference signal is transmitted in downlink so that a channel state can be estimated. Here, there is a need for a solution regarding how the resources of the reference signal will be used between transmission points that form a CoMP cooperation set.

An object of the present invention is to provide a method and apparatus for controlling uplink transmission power in a CoMP system.

Another object of the present invention is to provide a method and apparatus for estimating an uplink path loss using reference signals that are transmitted by multiple transmission points in a CoMP system.

Yet another object of the present invention is to provide a method and apparatus for configuring reference signals in order to control uplink transmission power based on reference signals transmitted by multiple transmission points in a CoMP system.

Further yet another object of the present invention is to provide a method and apparatus for transmitting configuration information for transmitting reference signals transmitted by multiple transmission points in a CoMP system.

(1) An embodiment of the present invention provides a method of transmitting a reference signal in a CoMP system, including sending configuration information on the reference signal and sending the reference signal based on the configuration information on the reference signal, wherein the configuration information on the reference signal may indicate Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in the transmission of the reference signal.

(2) In (1), the configuration information on the reference signal may indicate a ratio of the EPRE used to transmit the reference signal to EPRE used to transmit a physical downlink shared channel (PDSCH) signal transmitted along with the reference signal, for each of the transmission points that participate in the transmission of the reference signal.

(3) In (1), the EPRE used to transmit the reference signal may be indicated for each antenna port group in which antenna ports of the transmission points that participate in the transmission of the reference signal are grouped.

(4) In (3), the same EPRE may be indicated in antenna port groups that belong to the same transmission point.

(5) In (3), the configuration information on the reference signal may indicate a sequence used to transmit the reference signal for each transmission point having a different cell ID.

(6) In (1), the configuration information on the reference signal may include bitmap information indicating the number of transmission points that participate in the transmission of the reference signal.

(7) In (6), each of bits of the bitmap information may correspond to each of antenna ports that participate in the transmission of the reference signal, and each of the bits has a specific bit value when a transmission point is changed.

(8) In (6), each of bits of the bitmap information may correspond to each of antenna ports that participate in the transmission of the reference signal, and a value of each of the bits is changed in response to a change of a transmission point.

(9) Another embodiment of the present invention provides an uplink transmission method in a CoMP system, including receiving configuration information on reference signals, estimating an uplink path loss using the reference signals received on a downlink physical channel, determining uplink transmission power by incorporating the uplink path loss into the uplink transmission power, and performing uplink transmission using the uplink transmission power, wherein the configuration information on the reference signal may indicate Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in the transmission of the reference signal, and the uplink path loss for each transmission point may be estimated using the reception power of a reference signal transmitted by each of the transmission points and the transmission power of a reference signal indicated in the configuration information for each transmission point.

(10) In (9), determining the uplink transmission power may include determining the uplink transmission power for each transmission point based on uplink path loss estimated for each of the transmission points.

(11) In (9), the configuration information on the reference signal may indicate a ratio of the EPRE used to transmit the reference signal to EPRE used to transmit a physical downlink shared channel (PDSCH) signal transmitted along with the reference signal, for each of transmission points that participate in the transmission of the reference signal.

(12) In (9), the configuration information on the reference signal may indicate a ratio of the EPRE used to transmit the reference signal to EPRE that is transmitted along with the reference signal and used to transmit a physical downlink shared channel (PDSCH), for each of transmission points that participate in the transmission of the reference signal.

(13) In (12), the configuration information on the reference signal may indicate the same EPRE in antenna port groups belonging to the same transmission point.

(14) Yet another embodiment of the present invention provides an apparatus for transmitting a reference signal, including a Radio Frequency (RF) unit configured to transmit and receive pieces of information, memory configured to store the pieces of information, and a processor configured to control the RF unit and the memory, wherein the processor may configure configuration information on the reference signal, and the configuration information on the reference signal may indicate Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in a transmission of the reference signal.

(15) Further yet another embodiment of the present invention provides an uplink transmission apparatus, including an RF unit configured to transmit and receive information, memory configured to store the information, and a processor configured to control the RF unit and the memory, wherein the processor may estimate an uplink path loss for each of transmission points using reception power of a reference signal for each of the transmission points, received on a physical channel, and transmission power of a reference signal for each of the transmission points, and the transmission power of the reference signal for each of the transmission points may be indicated through configuration information on the reference signal.

In accordance with the present invention, a mobile station can effectively control uplink transmission power in a CoMP system.

In accordance with the present invention, a mobile station can control uplink transmission power by taking path loss for each of multiple transmission points into consideration using reference signals transmitted by the multiple transmission points in a CoMP system.

In accordance with the present invention, the configuration of reference signals can be configured so that uplink transmission power can be controlled based on reference signals transmitted by multiple transmission points in a CoMP system.

FIG. 1 is a block diagram showing a wireless communication system to which the present invention is applied.

FIG. 2 schematically shows an example in which a CSI-RS is mapped to a resource element in the case of a normal CP.

FIG. 3 schematically shows an example in which a CSI-RS is mapped to a resource element in the case of an extended CP.

FIG. 4 is a diagram schematically illustrating a method of controlling uplink transmission power.

FIG. 5 schematically shows an example of a transmission point bitmap transmitted by an eNB in a system to which the present invention is applied.

FIG. 6 schematically shows another example of a transmission point bitmap transmitted by an eNB in a system to which the present invention is applied.

FIG. 7 is a flowchart schematically illustrating a downlink transmission operation by the transmission point of a CoMP cooperation set in a system to which the present invention is applied.

FIGS. 8 and 9 are flowcharts schematically illustrating the operation of an MS in a system to which the present invention belongs.

FIG. 10 is a schematic block diagram showing the construction of an eNB in a system to which the present invention is applied.

FIG. 11 is a schematic block diagram showing the construction of an MS in a system to which the present invention is applied.

Hereinafter, in this specification, the contents related to the present invention will be described in detail in connection with exemplary embodiments with reference to the accompanying drawings. It is to be noted that in assigning reference numerals to respective elements in the drawings, the same reference numerals designate the same elements throughout the drawings although the elements are shown in different drawings. Furthermore, in describing the embodiments of the present invention, a detailed description of the known functions and constructions will be omitted if it is deemed to make the gist of the present invention unnecessarily vague.

Furthermore, in this specification, a communication network is described as a target, and tasks performed in the communication network may be performed in a process in which a system (e.g., a base station) managing the communication network controls the communication network and sends data or may be performed in a terminal linked to the communication network.

Referring to FIG. 1, the wireless communication systems 10 are widely deployed in order to provide a variety of communication services, such as voice and packet data. The wireless communication system 10 includes one or more Base Stations (BSs) 11. Each of the eNBs 11 provides communication service to a specific geographical area or frequency domain, and it may be called a site. The site may be classified into a plurality of

areas

15a, 15b, and 15c that may be called sectors. Each of the sectors may have a different ID.

A Mobile Stations (MS) 12 may be fixed or mobile and may be also called another terminology, such as User Equipment (UE), a Mobile Terminal (MT), a User Terminal (UT), a Subscriber Station (SS), a wireless device, a Personal Digital Assistant (PDA), a wireless modem, or a handheld device. The BS 11 refers to a fixed station which communicates with the MSs 12, and it may also be called another terminology, such as an evolved NodeB (eNodeB or eNB), a Base Transceiver System (BTS), an access point, a femto eNodeB, a Home eNodeB (HeNodeB), a relay, or a Remote Radio Head (RRH). Each of the

cells

15a, 15b, and 15c should be interpreted as a comprehensive meaning that indicates some area covered by the eNB 11. The cell has a meaning to cover various coverage areas, such as a mega cell, a macro cell, a micro cell, a pico cell, and a femto cell.

Hereinafter, downlink refers to communication or a communication path from the eNB 11 to the MS 12, and uplink refers to communication or a communication path from the MS 12 to the eNB 11. In downlink, a transmitter may be part of the eNB 11, and a receiver may be part of the MS 12. In uplink, a transmitter may be part of the MS 12, and a receiver may be part of the eNB 11. Multiple access schemes applied to the wireless communication system are not limited. A variety of multiple access schemes, such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), OFDM-FDMA, OFDM-TDMA, and OFDM-CDMA, may be used. The modulation schemes increase the capacity of a communication system by demodulating signals received from the multiple users of the communication system. Uplink transmission and downlink transmission may be performed in accordance with a Time Division Duplex (TDD) scheme using different times or a Frequency Division Duplex (FDD) scheme using different frequencies.

The wireless communication system 10 may be a Coordinated Multi-Point (CoMP) system. The CoMP system refers to a communication system which supports CoMP or a communication system to which CoMP is applied. CoMP is technology for coordinating or combining signals that are transmitted or received by multi-transmission/reception (Tx/Rx) points). CoMP can increase a throughput and provide high quality.

The multi-Tx/Rx point may be defined as any one of a component carrier, a cell, an eNB (e.g., a macro cell, a pico eNodeB, and a femto eNodeB), and a Remote Radio Head (RRH). The multi-Tx/Rx point may be defined as a set of antenna ports. Furthermore, the multi-Tx/Rx point may send information on a set of antenna ports to an MS in the form of Radio Resource Control (RRC) signaling. Accordingly, a plurality of Transmission Points (TPs) within one cell may be defined as a set of antenna ports. An intersection between the sets of antenna ports is always a null set.

Each BS or cells may form multi-Tx/Rx points. For example, the multi-Tx/Rx points may be macro cells that form a homogeneous network. Furthermore, the multi-Tx/Rx point may be RRHs which have a macro cell and high transmission power. Furthermore, the multi-Tx/Rx point may be RRHs which have a macro cell and low transmission power within a macro cell area.

CoMP may be selectively applied to a CoMP system. Mode in which a CoMP system performs communication using CoMP is called CoMP mode, and mode in which a CoMP system performs communication without using CoMP is called normal mode or non-CoMP mode.

The MS 12 may be a CoMP MS. The CoMP MS is an element that forms a CoMP system, and it performs communication with a CoMP cooperation set. Like in a CoMP system, the CoMP MS may operate in CoMP mode or normal mode. Furthermore, the CoMP cooperation set is a set of multi-Tx/Rx points which participate in data transmission directly or indirectly in what time-frequency resources in relation to the CoMP MS.

Direct participation in data transmission or reception means that multi-Tx/Rx points actually transmit data to a CoMP MS or receive data from a CoMP MS in corresponding time-frequency resources. Indirect participation in data transmission or reception means that multi-Tx/Rx points do not actually transmit data to a CoMP MS or receive data from a CoMP MS in corresponding time-frequency resources, but contribute to determining user scheduling/beamforming.

A CoMP MS can receive signals from a CoMP cooperation set at the same time or transmit signals to a CoMP cooperation set at the same time. Here, a CoMP system minimizes the influence of interference between CoMP cooperation sets by taking the channel environment of each cell, forming the CoMP cooperation set, into consideration.

A variety of scenarios are possible when operating a CoMP system. A first CoMP scenario is CoMP that includes a homogeneous network between a plurality of cells within one BS and may also be called intra-site CoMP. A second CoMP scenario is CoMP that includes one macro cell and a homogeneous network for one or more high-power RRHs. Each of a third CoMP scenario and a fourth CoMP scenario is CoMP that includes one macro cell and a heterogeneous network for one or more low-power RRHs within the macro cell area. Here, if the physical cell ID of the RRHs is not identical with the physical cell ID of the macro cell, it corresponds to the third CoMP scenario. If the physical cell ID of the RRHs is not identical with the physical cell ID of the macro cell, it corresponds to the fourth CoMP scenario.

The category of CoMP includes Joint Processing (hereinafter referred to as 'JP') and Coordinated Scheduling/Beamforming (hereinafter referred to as 'CS/CB'), and JP and CS/CB may be mixed.

In the case of JP, data for an MS is available in at least one multi-Tx/Rx point of a CoMP cooperation set in any time-frequency resources. JP includes Joint Transmission (hereinafter referred to as 'JT') and Dynamic Point Selection (hereinafter referred to as 'DPS').

JT means that data is transmitted from multi-Tx/Rx points, belonging to a CoMP cooperation set, to one MS or a plurality of MSs at the same time in time-frequency resources. In the case of JT, multiple cells (multi-Tx/Rx points) which transmit data to one MS perform the transmission using the same time/frequency resources.

In the case of DPS, data is transmitted by one multi-Tx/Rx point of a CoMP cooperation set in time-frequency resources. The Tx/Rx point may be changed every subframe by taking interference into consideration. Transmitted data is available at the same time in a plurality of multi-Tx/Rx points. DPS includes Dynamic Cell Selection (DCS).

In the case of CS, data is transmitted by one multi-Tx/Rx point within a CoMP cooperation set in relation to time-frequency resources. User scheduling is determined by cooperation between the multi-Tx/Rx points of a corresponding CoMP cooperation set.

Furthermore, in the case of CB, user scheduling is determined by cooperation between the multi-Tx/Rx points of a corresponding CoMP cooperation set. In this case, interference occurring between the MSs of neighboring cells can be avoided by Coordinated Beamforming (CB).

The CS/CB may include Semi-Static Point Selection (SSPS) that may be changed by selecting a multi-Tx/Rx point semi-statically.

As described above, JP and CS/CB may be mixed. For example, some multi-Tx/Rx points within a CoMP cooperation set send data to a target MS depending on JP, and other multi-Tx/Rx points within the CoMP cooperation set may perform CS/CB.

A multi-Tx/Rx point to which the present invention is applied may include an eNB, a cell, or an RRH. That is, the eNB or the RRH may become the multi-Tx/Rx point. Meanwhile, a plurality of BSs may become multi-Tx/Rx points, and a plurality of RRHs may become multi-Tx/Rx points. The operation of all BSs or RRHs described in the present invention may be likewise applied to a multi-Tx/Rx point of a different form.

Meanwhile, a Multi-Input Multi-Output (hereinafter referred to as MIMO) system also called a multi-antenna system improves transmission and reception data transmission efficiency using multiple transmission antennas and multiple reception antennas.

In a data transmission/reception process performed in an MIMO system, an eNB can receive data from N users and output K streams to be transmitted at once. In an MIMO system, an eNB can determine an MS and a transfer rate to be transmitted available radio resources using channel information on each MS or channel information transmitted by each MS. For example, a code rate, a Modulation and Coding Scheme (MCS), etc. may be selected by extracting channel information from feedback information.

For the operation of an MIMO system, information fed back from an MS to an eNB may include pieces of control information, such as a Channel Quality Indicator (CQI), Channel State Information (CSI), a Channel Covariance Matrix (CCM), a Precoding Weight (PW), and a Channel Rank (CR).

CSI may include a channel matrix, a channel correlation matrix, a quantized channel matrix, or a quantized channel correlation matrix, and a PMI between a transmitter and a receiver. CQI may include a Signal to Noise Ratio (SNR), a Signal to Interference and Noise Ratio (SINR), or a signal to interference ratio between a transmitter and a receiver.

An MS can estimate a channel, select a precoding matrix that maximizes channel performance based on the estimated channel, and report a Precoding Matrix Indicator (PMI) on the selected precoding matrix. A BS can select a precoding matrix indicated by a feedback PMI from a codebook and use the selected precoding matrix in data transmission.

An MIMO method using a precoding weight depending on a channel state is called a Closed-Loop (CL) MIMO method. An MIMO method using a precoding weight according to a specific rule irrespective of a channel state is called an Open-Loop (OL) MIMO method. In a CL MIMO method, a sender, for example, an eNB handles a channel situation using Channel State Information (CSI) that is transmitted by a receiver, for example, an MS.

Meanwhile, in a wireless communication system, it is necessary to estimate an uplink channel or a downlink channel for the purpose of the transmission/reception of data, the acquisition of system synchronization, and channel information feedback. A process of restoring a transmission signal by compensating for the distortion of a signal due to a sudden change of an environment is called channel estimation. It is also necessary to measure a channel state for a cell to which an MS belongs or another cell. In general, in order to estimate a channel or measure a channel state, a Reference Signal (RS) that is known to both a transmitter and a receiver is used.

A receiver can estimate a channel based on the reference signal of a received signal because it knows information on the reference signal and precisely obtain data transmitted by a transmitter by compensating for a channel value. Assuming that a reference signal transmitted by a transmitter is p, channel information that is experienced by the reference signal during the transmission is h, heat noise occurring in a receiver is n, and a signal received by a receiver is y, it may lead to 'y=h·p+n'. Here, since the reference signal p is already known to a receiver, channel information

can be estimated according to Equation 1 if a Least Square (LS) method is used.

In Equation 1, a channel estimation value

estimated using the reference signal p depends on a value

. Thus, it is necessary to converge

to 0 in order to estimate an exact value h. A channel can be estimated by minimizing the influence of

using a large number of reference signal.

In general, a reference signal is transmitted in the form of a sequence. A specific sequence may be used as the RS sequence without special limitations. A PSK-based computer generated sequence based on Phase Shift Keying (PSKP) may be used as the RS sequence. PSK may include, for example, Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). Alternatively, a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence may be used as the RS sequence. The CAZAC sequence may include, for example, a Zadoff-Chu (ZC)-based sequence, a ZC sequence with cyclic extension, and a ZC sequence with truncation. Alternatively, a pseudo-random (PN) sequence may be used as the RS sequence. The PN sequence may include, for example, an m-sequence, a computer-generated sequence, a gold sequence, and a Kasami sequence. Alternatively, a cyclically shifted sequence may be used as the RS sequence.

A downlink RS includes a Cell-specific Reference Signal (CRS), an MBSFN RS, a UE-specific RS, a Positioning RS (PRS), and Channel State Information-RS (CSI-RS).

In a multi-antenna system, a resource element used in the RS of one antenna is not used in the RS of another antenna in order not to give interference between antennas. For example, one RS for one antenna may be transmitted.

A CSI-RS from among downlink RSs may be used to estimate CSI. The CSI-RS is disposed in the frequency domain or the time domain. An MS may report a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), and a Rank Indicator (RI) as pieces of CSI through the estimation of a channel state using a CSI-RS at need.

Table 1 schematically shows an example in which configuration information on a CSI-RS (CSI-RS-Config) is defined. The configuration information on a CSI-RS is an information element used to specify a CSI-RS configuration and is individually transmitted to an MS which uses the CSI-RS.

Table 1

Referring to Table 1, a CSI-RS is configured through parameters, such as antennaPortsCount, subframeConfig, resourceConfig, p-c, etc.

A CSI-RS may be transmitted in one or more antenna ports. In Table 1, the parameter antennaPortsCount indicates the number of antenna ports that are used to send a CSI-RS. 'an1' indicates that the number of antenna ports is 1, 'an2' indicates that the number of antenna ports is 2, 'an4' indicates that the number of antenna ports is 4, and 'an8' indicates that the number of antenna ports is 8.

The parameter p-C indicates a ratio of CSI-RS Energy Per Resource Element (EPRE) to PDSCH EPRE when an MS derives a CSI feedback. The parameter p-C has a value having a range of [-8, 15] dB and increases or decrease at an interval of 1 dB.

The parameter p-C-BS indicates that a multi-Tx/Rx point is an eNB, and it is p-C regarding a CSI-RS that is transmitted by an eNB. The parameter p-C-RRH indicates that a multi-Tx/Rx point is an RRH, and it is p-C regarding a CSI-RS that is transmitted by an RRH.

The parameter subframeConfig indicates timing on which the CSI-RS is transmitted. For example, the parameter subframeConfig may indicate a subframe on which the CSI-RS is transmitted.

Furthermore, the parameter resourceConfig indicates the pattern of the CSI-RS. The CSI-RS may have a specific pattern depending on antenna ports.

FIG. 2 schematically shows an example in which a CSI-RS is mapped to a resource element in the case of a normal CP. The mapping of the CSI-RS shown in FIG. 2 is an example regarding a CSI configuration 0 for a normal CP. In FIG. 2, R_p indicates a resource element that is used in CSI-RS transmission in an antenna port P.

FIG. 3 schematically shows an example in which a CSI-RS is mapped to a resource element in the case of an extended CP. The mapping of a CSI-RS shown in FIG. 3 relates to a CSI configuration 0 for an extended CP. As shown in FIGS. 2 and 3, a CSI-RS may be mapped to a resource element in a specific pattern depending on a transmitted antenna port.

Table 2 shows an example of information on a PDSCH configuration. Table 2 shows a case where information included in an RRC message, for example, a System Information Block (SIB2) used to specify a common or UE-specific PDSCH configuration, is an element of PDSCH configuration information.

Table 2

In Table 2, PDSCH configuration information elements include a field PDSCH-ConfigCommon and a field PDSCH-ConfigDedicated. 'p-a' is a UE-specific parameter, and 'p-b' is a cell-specific parameter. referenceSignalPower indicates downlink RS transmission power, and it is provided in dBm. The Energy Per Resource Element (EPRE) of a downlink RS can be derived from referenceSignalPower. Here, the downlink RS transmission power is defined as a linear average for the power contribution of all resource elements that carry a CRS or CSI-RS within an operating system bandwidth. In the case of CoMP mode, it is assumed that Energy Per Resource Element (EPRE) values regarding all multi-Tx/Rx points are the same.

Table 3 shows another example of the PDSCH configuration information. Table 3 shows a case where Energy Per Resource Element (EPRE) values regarding an eNB and an RRH are differently set in CoMP mode.

Table 3

Referring to Table 3, in a field PDSCH-ConfigCommon, different Energy Per Resource Element (EPRE) values are set in an eNB and an RRH. Here, the Energy Per Resource Element (EPRE) values of the eNB and the RRH are different, but the RRHs have the same EPRE value.

Table 4 shows yet another example of the PDSCH configuration information. Table 4 shows a case where Energy Per Resource Element (EPRE) values for multi-Tx/Rx points are differently set.

Table 4

Referring to Table 4, in a field PDSCH-ConfigCommon, different EPRE values are set in an eNB, an RRH1, and an RRH2.

In a CoMP system, a plurality of cells or multi-Tx/Rx points can transmit a reference signal, for example, a CSI-RS to an MS. In a CoMP system, a reference signal sequence may be determined in a cell-specific manner. In particular, in the fourth scenario, that is a CoMP environment in which multi-Tx/Rx points (e.g., RRHs) forming a cooperation set along with a specific Tx/Rx point (e.g., a macro cell) have the same cell ID, the same reference signal sequence is used to generate a reference signal within one macro cell. This means that all the multi-Tx/Rx points (e.g., RRHs) belonging to the same cooperation set as the macro cell send a reference signal using the same reference signal sequence.

Referring to FIG. 4, an MS derives a downlink reference signal, that is, the Energy Per Resource Element (EPRE) of a CSI-RS, from the parameter referenceSignalPower or derives the Energy Per Resource Element (EPRE) of the CSI-RS from the value p-C in the field PDSCH-ConfigCommon and calculates Reference Signal Received Power (RSRP) at step S410.

The RSRP may be defined as a linear average for the power contributions of all resource elements which carry the CSI-RS within a considered measurement frequency bandwidth. Here, the RSRP may be defined using a CRS instead of the CSI-RS. Furthermore, the CRS is defined in relation to antenna ports 0 to 3 and the CSI-RS is defined in relation to antenna ports 15 to 22. Accordingly, R₁₅ means a CSI-RS placed in the antenna port 15 (refer to FIGS. 2 and 3).

The RSRP may be calculated according to the following procedure. The MS obtains measurement samples through filtering in a physical layer level and filters the measurement samples in a higher layer level as in the following equation.

In Equation 2, M_n is the most recent measurement sample, F_n is a measurement value that will be reported in a measurement report, F_n-1 is a measurement value that has been reported in a previous measurement report, 'a' is 1/2^(k/4), and 'k' is a filter coefficient used for filtering.

Each of the measurement samples is a measurement value for each subframe and is a parameter necessary to derive the RSRP or a Reference Signal Received Quality (RSRQ). In some embodiments, the measurement sample may refer to a measurement value for a subframe that is selected according to a measurement rule defined in a wireless system, from among measurement values for all the subframes received by the MS. The measurement sample may be obtained in the physical layer of the MS, and the filtering may be performed in a higher layer of the MS, for example, a Radio Resource Control (RRC) layer.

The measurement samples may be consecutively obtained every subframe, but may be discontinuously obtained within a range in which the capacity of an MS or a condition defined in a system is satisfied. That is, after obtaining one measurement sample, another measurement sample may be obtained after an interval of a specific time. In this case, the measurement samples are not obtained from some subframes. The interval may be periodic or aperiodic.

Meanwhile, the RSRQ may be defined as the ratio between the RSRP and a Received Signal Strength Indicator (RSSI) as in Equation 3.

In Equation 3, N is the number of resource elements of the carrier RSSI measurement bandwidth of a wireless access network. In Equation 3, measurement for a numerator and a denominator is performed on a set of identical resource blocks. The RSSI includes a linear average for all reception powers. All the reception powers are monitored only within OFDM symbols including reference symbols within the measurement bandwidth and are values obtained over the resource blocks. When the MS receives signaling indicative of RSRQ measurement is received in a higher layer, RSSI measurement in a subframe in which the RSRQ measurement has been indicated is performed on all OFDM symbols within a corresponding subframe.

The MS calculates a path loss (PL) estimated value between multi-Tx/Rx points and the MS from the EPRE value of the CSI-RS and the RSRP at step S415. The PL estimation value can be calculated according to Equation 4 below.

Referring to Equation 4, PL_C is a downlink PL estimation value for a serving cell C that is calculated by the MS, and it has a dB unit. referenceSignalPower is the EPRE value of a downlink reference signal provided by a higher layer, and it has a dBm unit. The serving cell C selected as a reference serving cell and a link between the EPRE value 'referenceSignalPower' and the RSRP used to calculate the PL estimation value PL_C are determined based on path loss reference link information 'path lossReferenceLinking', that is, a higher layer parameter.

The reference serving cell configured based on the path loss reference link information may become the downlink Secondary CC (downlink SCC) of a (corresponding) secondary serving cell (SCell) that has set up SIB2 connection with a primary serving cell (PCell) or an Uplink Component Carrier (UL CC).

If a plurality of multi-Tx/Rx points having the same physical cell ID in a CoMP cooperation set transmits the CRS as in the fourth scenario, the CRS, that is, a criterion for the RSRP measurement, is the same in the plurality of multi-Tx/Rx points. Accordingly, the MS cannot distinguish PL estimation values for the multi-Tx/Rx points from each other based on the CRS. However, in terms of the definition of the PL estimation value, the RSRP has to be individually measured for each multi-Tx/Rx point. In particular, since DPS is supported in CoMP mode, the MS can accurately control uplink transmission power only when it knows a PL estimation value for each multi-Tx/Rx point. For example, it is assumed that in a CoMP system, such as that shown in FIG. 1, a PL estimation value in a multi-Tx/Rx point1 is PL1 and a PL estimation value in a multi-Tx/Rx point2 is PL2. A CoMP MS may dynamically perform uplink transmission on any one of the multi-Tx/Rx point1 and the multi-Tx/Rx point2 or on both them based on DPS. Here, if the PL1 is not distinguished from the PL2, the CoMP MS may mistake the PL estimation value for the multi-Tx/Rx point2 as PL1 and erroneously calculate uplink transmission power.

In contrast, if the CSI-RS is used as a criterion for the RSRP measurement as in the steps S410 and S415, the MS can calculate an exact uplink transmission power for each multi-Tx/Rx point because PL estimation values for the multi-Tx/Rx points are distinguished from one another.

In the state in which the MS can derive a different PL estimation value for each multi-Tx/Rx point as described above, whether uplink transmission power will be derived using what PL estimation value has to be determined. For example, the MS may select one of the multi-Tx/Rx points as a target for uplink transmission according to a DPS operation. Here, the MS uses a PL estimation value, calculated based on a signal received from the selected multi-Tx/Rx point, in order to derive the uplink transmission power. That is, the MS may calculate the PL estimation value for the selected multi-Tx/Rx point in an uplink radio link under its determination without additional signaling from a multi-Tx/Rx point, particularly, an eNB and use the calculated PL estimation value to derive the uplink transmission power.

For another example, the MS may use a PL estimation value for a multi-Tx/Rx point, set as a first serving cell, to derive uplink transmission power for a multi-Tx/Rx point selected according to a DPS operation. This corresponds to a case where the MS cannot select one multi-Tx/Rx point as a target for uplink transmission according to the DPS operation in CoMP mode. Accordingly, the MS coordinates the PL estimation value of the multi-Tx/Rx point selected by the DPS operation from the eNB through Transmit Power Control (TPC) signaling. The TPC signaling can be performed through the signals of DCI format 3/3A.

The MS calculates uplink transmission power from the PL estimation value for the serving cell C at step S420. An uplink physical channel includes a Physical Uplink Shared Channel (PUSCH) and a Physical Uplink Control Channel (PUCCH). The uplink transmission power may be differently controlled depending on a transmitted uplink physical channel.

In the case of a PUSCH, uplink transmission power P_PUSCH,C(i) is scaled by the number of antennas, including at least one PUSCH transmission, and the number of antennas configured according to a transmission method. C is a serving cell that will perform uplink transmission and is a subframe number on which the uplink transmission is performed with P_PUSCH,C(i). Furthermore, the entire scaled uplink transmission power is equally divided and allocated to antennas through which the at least one PUSCH transmission is performed.

The PUSCH transmission power is divided into i) a case where a PUSCH and a PUCCH are not transmitted at the same time and ii) a case where a PUSCH and a PUCCH are transmitted at the same time in relation to a specific serving cell C.

In the case of i), the MS calculates uplink transmission power P_PUSCH,C(i), defined according to Equation 5, in a subframe i for the serving cell C.

In the case of ii), the MS calculates uplink transmission power P_PUSCH,C(i), defined according to Equation 6, in a subframe i for the serving cell C.

In Equation 5 and Equation 6, P_CMAX,C(i) is maximum UE transmission power configured for the serving cell C, and

is a value linearly converted from a dB value. Meanwhile, the value

is obtained by linearly converting P_PUCCH(i). M_PUSCH,C(i) is a value obtained by representing the bandwidth of resources to which a PUSCH has been allocated in the subframe i for the serving cell C in the form of the number of resource blocks.

P_{0_PUSCH,C}(i) is the sum of P_{0_NOMINAL_PUSCH,C}(j) and P_{0_UE_PUSCH,C}(j) for the serving cell C. For example, in the case of semi-persistent grant PUSCH (re)transmission, j=0 value. In contrast, in the case of dynamic scheduled grant PUSCH (re)transmission, j=1 value. When j=0 or 1, signaling is performed by a higher layer. Furthermore, in the case of random access response grant PUSCH (re)transmission, j=2 value. Furthermore, in the case of random access response grant PUSCH (re)transmission, P_{0_UE_PUSCH,C}(2)=0 and P_{0_NOMINAL_PUSCH,C}(2)=P_{0_PRE}+Δ_{PREAMBLE_Msg3}. Here, parameters preambleInitialReceivedTargetPower(P_{0_PRE}) and Δ_{PREAMBLE_Msg3} are signalized from a higher layer.

If j=0 or 1, one of α_C∈{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1} values may be selected as α_C for determining P_PUCCH,C(i) according to a 3-bit parameter provided by a higher layer. In the case of j=2, α_C(j)=1.

Δ_TF,C(i) is equal to 10log₁₀((2^BPRE·Ks-1)·β^PUSCH _offset) and is a parameter for incorporating influence due to a Modulation and Coding Scheme (MCS). Here, K_S is a parameter that is provided as deltaMCS-Enabled from a higher layer in related to each serving cell C. In the case of transmission mode 2, that is, mode for transmission diversity, K_S=0 always. If only control information is transmitted through a PUSCH without UL-SCH data, BPRE=O_CQI/N_RE, and

in all cases other than the above case. Here, C is the number of code blocks, and

is the size of the code block.

Furthermore, O_CQI is the number of CQI/PMI bits including the number of CRC bits, and N_RE is the number of determined resource elements. That is, 'N_RE=M_sc ^{PUSCH-initial}·N_symb ^{PUSCH-initial'}.

If only control information is transmitted through a PUSCH without UL-SCH data, β^PUSCH _offset=β^CQI _offset. In other cases, β^PUSCH _offset is always set to 1.

δ_PUSCH,C is a modified value. Furthermore, the modified value is determined with reference to a Transmit Power Control (TPC) command within a DCI format 0 or 4 for the serving cell C or a TPC command within the DCI format 3/3A that is jointly coded with other MSs and transmitted. In the DCI format 3/3A, only MSs to which RNTI values have been allocated can be checked because Cyclic Redundancy Check (CRC) parity bits are scrambled into a TPC-PUSCH-RNTI. Here, if a specific MS is formed of a plurality of serving cells, a different TPC-PUSCH-RNTI value may be allocated to each of the serving cells in order to distinguish the serving cells from one another. In some embodiments, in order to distinguish multi-Tx/Rx points from one another, a different TPC-PUSCH-RNTI value may be allocated to each of the multi-Tx/Rx points.

f_c(i) indicates a PUSCH power control coordination state for a current serving cell C, and it is defined as in Equation 7 below.

Equation 7 corresponds to a case where accumulation has been activated by a higher layer in relation to the serving cell C or a case where the DCI format 0 into which a TPC command has been scrambled by a temporary cell-RNTI (C-RNTI) is included in a PDCCH. Here, δ_PUSCH,C (i-K _PUSCH ) is a TPC command within the DCI format 0/4 or 3/3A within the PDCCH that had been transmitted in a subframe in a subframe #i-K_PUSCH, and f_c(0) is the first value after the accumulation is reset. The value K_PUSCH is 4 in the case of FDD, and the value K_PUSCH is as follows when a TDD configuration is 1 to 6.

Table 5

TDD UL/DL configuration	Subframe Number
TDD UL/DL configuration	0	1	2	3	4	5	6	7	8	9
0	-	-	6	7	4	-	-	6	7	4
1	-	-	6	4		-	-	6	4
2	-	-	4	-	-	-	-	4	-	-
3	-	-	4	4	4	-	-	-	-	-
4	-	-	4	4	-	-	-	-	-	-
5	-	-	4	-	-	-	-	-	-	-
6	-	-	7	7	5	-	-	7	7	-

Referring to Table 5, a part indicated by '-' is a DL subframe, and a part indicated by a number is an UL subframe.

In the TDD UL/DL configuration #0, in the case where there is a PDCCH that schedules PUSCH transmission in a subframe #2 or a subframe #7, if the Least Significant Bit (LSB) value of a 2-bit UL index within the DCI format 0/4 within the PDCCH has been set to '1', the value K_PUSCH is 7. In all other cases, the value K_PUSCH is given as in Table 5. The 2-bit UL index is used to schedule UL subframes which cannot be scheduled according to Table 5.

Meanwhile, the MS attempts the decoding of the PDCCH in all subframes other than a case where discontinous reception (DRX) is operating. This includes the DCI format 0/4 for the C-RNTI of the MS or the DCI format 0 for the SPS C-RNTI of the MS and the PDCCH of the DCI format 3/3A for the TPC-PUSCH-RNTI of the MS.

If the DCI format 0/4 and the DCI format 3/3A for the serving cell C are received in the same subframe at the same time, the MS has to use only the δ_PUSCH,Cof the DCI format 0/4.

A case where δ_PUSCH,C is 0 dB in relation to a specific subframe is a case where there is no TPC command for the serving cell C or a case where DRX is operating or where a corresponding subframe is the UL subframe of a TDD method. When TPC fields within the DCI format 0/3/4 are 0, 1, 2, and 3, respectively, accumulated δ_PUSCH,C dB values are -1, 0, 1, and 3, respectively. If the PDCCH of the DCI format 0 is approved as an SPS activated or released PDCCH, δ_PUSCH,C is 0 dB. When the TPC fields within the DCI format 3A are 0 and 1, accumulated δ_PUSCH,C dB values are -1 and 1.

If the MS reaches P_CMAX,C in relation to the serving cell C, a positive TPC command will not be accumulated. If the MS reaches minimum power, a negative TPC command will not be accumulated.

If the value P_{0_UE_PUSCH,C} is changed by a higher layer in relation to the serving cell C or the MS receives a random access response message in relation to a primary serving cell, the MS will reset the accumulation.

In Equation 7, if the accumulation has been deactivated by a higher layer in relation to the serving cell C, f_c(i) is given as in Equation 8.

In Equation 8, δ _PUSCH,C (i-K _PUSCH ) is transmitted through the DCI format 0/4 within the PDCCH for the serving cell C in a subframe #i-K_PUSCH. The value K_PUSCH is 4 in the case of an FDD method and given as in Table 2 in TDD UL/DL configurations #1 to #6.

If PUSCH transmission for a subframe #2 or a subframe #7 is scheduled in the TDD UL/DL configuration #0 and the LSB of the 2-bit UL index of the DCI format 0/4 within the PDCCH is set to '1', the value K_PUSCH is 7. In other cases, the value K_PUSCH is given as in Table 2.

If the DCI format 0/4 within the PDCCH for the serving cell C is not decoded, DRX occurs, or the subframe #i in a TDD scheme is not an UL subframe, f_c(i) is equal to f_c(i-1).

If the value P_{0_UE_PUSCH,C} is changed by a higher layer and the serving cell C is a primary serving cell or the value P_{0_UE_PUSCH,C} is received by a higher layer and the serving cell C is a secondary serving cell, f_c(0) is 0. In other cases, if the serving cell C is a primary serving cell, f_c(0)=ΔP_rampup+δ_msg2. Here, δ_msg2 is a TPC command indicated by a random access response. The TPC command is placed in DCI within a PDCCH for indicating the position of a PDSCH including an RAR MAC CE, and it has 3 bits. Furthermore, ΔP_rampup is provided by a higher layer and related to total power ramp-up from the first preamble to the last preamble.

The MS sends the PUSCH to an Rx point using the calculated uplink transmission power at step S425.

Furthermore, the MS compensates for path loss using the CSI-RS, uses the CSI-RS to calculate the uplink transmission power, and also feeds back the pieces of information measured based on the CSI-RS to an eNB. The pieces of information (i.e., CSI feedback) fed back to the eNB may include pieces of control information, such as a Channel Quality Indicator (CQI), Channel State Information (CSI), a Channel Covariance Matrix (CCM), Precoding Weight (PW), and a Channel Rank (CR).

CSI may include a channel matrix, a channel correlation matrix, a quantized channel matrix, or a quantized channel correlation matrix, and a PMI between a transmitter and a receiver. CQI may be a Signal to Noise Ratio (SNR), a Signal to Interference and Noise Ratio (SINR), or a signal to interference ratio between a transmitter and a receiver.

The MS may estimate a channel, select a precoding matrix that maximizes channel performance based on the estimated channel, and report an indicator for the selected precoding matrix (i.e., a Precoding Matrix Indicator (PMI)) to the eNB. The eNB may select a precoding matrix, indicated by the feedback PMI, from a codebook and use the selected precoding matrix in data transmission.

An MIMO method of using a precoding weight according to a channel state is called a Closed-Loop (CL) MIMO method, and an MIMO method of using a precoding weight according to a specific rule irrespective of a channel state is called an Open-Loop (OL) MIMO method. In the CL MIMO method, a sender, for example, an eNB handles a channel situation using Channel State Information (CSI) that is transmitted by a receiver, for example, an MS. The CSI, including the PMI, may be transmitted.

Meanwhile, in order to support a CoMP environment, multiple non-zero-power CSI-RS resources have to be configured to an MS through dedicated signaling, such as an RRC message for the purpose of at least CSI feedback only. In this case, if a configuration, such as that shown in Table 1, is used, it is difficult to precisely incorporate the occurrence of different path loss in transmission from each of multi-Tx/Rx points. This leads to a problem in that uplink transmission power cannot be scaled precisely. Furthermore, since the same CSI-RS resources and the same CSI-RS configuration are used for different multi-Tx/Rx points, a problem in which a ratio of a CSI-RS EPRE to a PDSCH EPRE is different for each of the multi-Tx/Rx points is not taken into consideration. Accordingly, there is a problem in that precise CQI feedback is not performed.

The above problems can be solved by adding a field indicative of a CSI-RS EPRE value and a field indicative of a p-C value regarding each of multi-Tx/Rx points within a CoMP cooperation set to a CSI-RS configuration.

A path loss into which an actual channel state has been incorporated can be calculated for each multi-Tx/Rx point by indicating a different CSI-RS EPRE value for each multi-Tx/Rx point. In this case, a different p-c value is set in each multi-Tx/Rx point by indicating a different CSI-RS EPRE value for each multi-Tx/Rx point in relation to given PDSCH EPRE value.

Furthermore, a different p-c value may be indicated for each multi-Tx/Tx point. When a different p-c value is indicated for each multi-Tx/Tx point, path loss into which an actual channel state is incorporated can be calculated. In this case, a different CSI-RS EPRE value can be set in each multi-Tx/Tx point because a different p-c value is indicated for each multi-Tx/Tx point in relation to a given PDSCH EPRE.

In addition, a field indicative of a CSI-RS EPRE value and a field indicative of a p-c value in relation to each multi-Tx/Rx point within a CoMP cooperation set can be added along with a CSI-RS configuration. A different path loss can be calculated for each multi-Tx/Tx point based on the CSI-RS EPRE and the p-c value for each multi-Tx/Rx point, and uplink transmission power can be scaled by incorporating the different path loss into the uplink transmission power.

Not only in the case where different CSI-RS resources are used, but also in the case where transmission points use the same CSI-RS resources, a CSI-RS EPRE or a p-c value may be indicted for each transmission point or both the CSI-RS EPRE and the p-c value may be indicted for each transmission point.

Table 6 schematically shows an example in which multiple transmission points use one CSI-RS resource in a CoMP system to which the present invention is applied.

Table 6

Transmission points	Antenna ports		CSI-RS resources
Macro RRH	15	16	same CSI-RS resources
RRH1	17	18
RRH3	19	20
RRH4	21	22

Even when multiple transmission points use one CSI-RS resource, that is, one CSI-RS pattern, as in Table 6, a field indicative of a CSI-RS EPRE for each transmission point may be included in a CSI configuration. Furthermore, a field indicative of a p-c value for each transmission point may be included in a CSI configuration, and both a field indicative of a CSI-RS EPRE and a field indicative of a p-c value for each transmission point may be included in a CSI configuration as described above. The CSI configuration may be transferred to an MS through higher layer signaling, such as an RRC message, as described above.

Table 7 schematically shows an example of a CSI configuration including a CSI-RS EPRE field and a p-c field in a CoMP system to which the present invention is applied.

Table 7

CSI-RS configuration	Field
Number of antenna ports	antennaPortsCount
CSI-RS resource(CSI_RS pattern)	resourceConfig
CSI-RS transmission timing	subframeConfig
EPRE	referenceSignalPower
p-c	p-C

antennaPortCount indicative of the number of antenna ports, resourceConfig indicative of CSI-RS resources, and subframeConfig indicative of CSI-RS transmission timing in Table 7 are the same as those of Table 1.

referenceSignalPower added to the CSI configuration of Table 7 is a field indicating the EPRE of a CSI-RS, and it indicates a CSI-RS EPRE for at least one antenna port. Furthermore, p-C is also a field indicative of a ratio of a CSI-RS EPRE to a PDSCH EPRE, and it indicates p-C for at least one antenna port.

In an environment in which a PDSCH EPRE for a CSI-RS EPRE may have a different value for each RRH, a different p-c value may be allocated to each of different transmission points using a plurality of p-c values in order to support a plurality of RRHs using the same CSI-RS resources as in Table 6.

Table 8 schematically shows an example in which p-c/EPRE are indicated for each antenna port that transmits a CSI-RS in order to support multiple transmission points which use one CSI-RS resource in a CoMP system to which the present invention is applied. In the example of Table 8, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, are taken into consideration, for convenience of description.

Table 8

EPRE(referenceSignalPower)	p-c	Antenna Port
EPRE1	p-c1	Antenna Port 15
EPRE2	p-c2	Antenna Port 16
EPRE3	p-c3	Antenna Port 17
EPRE4	p-c4	Antenna Port 18
EPRE5	p-c5	Antenna Port 19
EPRE6	p-c6	Antenna Port 20
EPRE7	p-c7	Antenna Port 21
EPRE8	p-c8	Antenna Port 22

The example of Table 8 describes a CSI-RS configuration for each antenna port. In this case, an EPRE value and a p-c value are included in the CSI-RS configuration for each antenna port.

If the example of Table 8 is applied to a CSI-RS configuration in detail, it may lead to Table 9. Table 9 shows an example of a CSI-RS configuration in which the example of Table 8, together with other CSI-RS parameters, is shown.

Table 9

Table 10 schematically shows an example in which p-c/EPRE are indicated for each antenna port group which transmits a CSI-RS in order to support multiple transmission points using one CSI-RS resource in a CoMP system to which the present invention is applied. In the example of Table 10, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, are taken into consideration, for convenience of description.

Table 10

EPRE(referenceSignalPower)	p-c	Antenna Ports
EPRE1	p-c1	Antenna Port 15&16
EPRE2	p-c2	Antenna Port 17&18
EPRE3	p-c3	Antenna Port 19&20
EPRE4	p-c4	Antenna Port 21&22

The example of Table 10 describes the CSI-RS configuration in unit of 2 antenna ports. More particularly, all the antenna ports are classified into groups, and each of the groups includes 2 antenna ports and includes an EPRE value and a p-c value. For example, EPRE1 and p-c1 are configured for a group of antenna ports 15 and 16, EPRE2 and p-c2 are configured for a group of antenna ports 17 and 18, EPRE3 and p-c3 are configured for a group of antenna ports 19 and 20, and EPRE4 and p-c4 are configured for a group of antenna ports 21 and 22. If two different groups belong to the same transmission point, the two groups may have different EPRE and p-c values. For example, if antenna ports are divided into two groups, each including two antennas, in relation to an RRH having 4 antenna ports, the two groups may have different EPREs and p-c values.

In the example of Table 10, transmission overhead can be reduced as compared with the example of Table 8 because four EPREs and p-c parameters are transmitted.

If the example of Table 10 is applied to a CSI-RS configuration in detail, it may lead to Table 11. Table 11 shows an example of a CSI-RS configuration in which the example of Table 10 is shown along with other CSI-RS parameters.

Table 11

In order to reduce transmission overhead further, a reference value for EPRE and p-c may be determined in a CSI-RS configuration and a difference value with the reference value may be transmitted so that EPRE and p-c for each antenna port or each antenna port group are indicated. For example, a reference value for specific EPRE and p-c may be separately determined in a CSI-RS configuration, and a difference value with the reference value may be indicated for each antenna port or each antenna port group. In some embodiments, one antenna port or one antenna port group may be set as a reference antenna port or a reference antenna port group in a CSI-RS configuration, and a difference value with the EPRE and p-c of the reference antenna port or the reference antenna port group may be transmitted. In this case, the EPRE and p-c of the reference antenna port or the reference antenna port group may be transmitted as their original values. In some embodiments, the EPRE and p-c of a reference antenna port or a reference antenna port group may be set as a reference value, and a difference between the reference value and the EPRE and p-c of the reference antenna port or the reference antenna port group may indicate a value 0.

Table 12 schematically shows another example in which p-c/EPRE are indicated for each antenna port which transmits a CSI-RS in order to support multiple transmission points using one CSI-RS resource in a CoMP system to which the present invention is applied. In the example of Table 12, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, are taken into consideration, for convenience of description.

Table 12

EPRE(referenceSignalPower)	p-c (Ratio of CSI-RS EPRE to PDSCH EPRE)	Antenna Ports
EPRE1	p-c1	Antenna Port 15&16
ΔEPRE12	Δp-c12	Antenna Port 17&18
ΔEPRE13	Δp-c13	Antenna Port 19&20
ΔEPRE13	Δp-c14	Antenna Port 21&22

Table 12 shows an example in which the transmission points of a CoMP cooperation group are classified into groups each including two transmission points. Referring to Table 12, a first group including antenna ports 15 and 16 is set as a reference group, and EPRE and p-c for the first group indicated original values.

Here, EPRE for another antenna port group may transmit a difference value ΔEPRE with the EPRE for the first group. For example, assuming that EPRE for a first antenna port group is EPRE 1 and EPRE for a second antenna port group is EPRE 2, a difference value ΔEPRE12 of EPRE indicated for the second antenna port group may be equal to 'EPRE2 - EPRE1'.

Furthermore, p-c for another antenna port group transmits a difference value Δp-c with p-c for the first group. For example, assuming that p-c for a first antenna port group is p-c1 and p-c for a second antenna port group is p-c2, a difference value Δp-c12 of p-c indicated for the second antenna port group may be equal to 'p-c2 - p-c1'.

In the example of Table 12, an example in which each antenna port group includes 2 antenna ports has been described, but the present invention is not limited thereto. For example, if EPRE and p-c are configured for each antenna port and each antenna port group includes three or more antenna ports, a method of transmitting a difference value for a reference value may be applied likewise as described above.

In the example of Table 12, the transmission of a difference value using a first antenna port group as a reference has been described, but the present invention is not limited thereto. For example, an antenna port group or antenna port that is subsequent to a second antenna port group or antenna port may be used as a reference. Furthermore, EPRE and p-c for an antenna ports or antenna port group may not be used as a reference, but an additional reference EPRE and p-c may be configured so that a difference value for the reference EPRE and p-c is indicated for all antenna ports or antenna port groups.

Meanwhile, an eNB may transmit a bitmap, indicating that how many of transmission points belonging to a CoMP cooperation set use the same CSI-RS resources (hereinafter referred to as a 'transmission point bitmap', for convenience of description). The transmission point bitmap may be transmitted through higher layer signaling. For example, the transmission point bitmap may be added to an RRC message on which a CSI-RS configuration is transmitted and then transmitted.

Table 13 schematically shows another example in which p-c/EPRE are indicated for each antenna port that transmits a CSI-RS in order to support multiple transmission points using one CSI-RS resource in a CoMP system to which the present invention is applied. In the example of Table 13, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, are taken into consideration, for convenience of description.

Table 13

EPRE	p-c	Antenna Port	RRH
EPRE1	p-c1	15	RRH
EPRE2	p-c2	16
EPRE3	p-c3	17
EPRE4	p-c4	18
EPRE5	p-c5	19	RRH
EPRE6	p-c6	20	RRH
EPRE7	p-c7	21	RRH
EPRE8	p-c8	22	RRH

Table 14 schematically shows yet another example in which p-c/EPRE are indicated for each antenna port group that transmits a CSI-RS in order to support multiple transmission points using one CSI-RS resource in a CoMP system to which the present invention is applied. In the example of Table 14, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, are taken into consideration, for convenience of description.

Table 14

EPRE	p-c	Antenna Ports	RRH
EPRE1	p-c1	15&16	RRH1
EPRE2	p-c2	17&18	RRH1
EPRE3	p-c3	19&20	RRH2
EPRE4	p-c4	21&22	RRH3

In the examples of Table 13 and Table 14, antenna ports 15 to 18 belong to RRH1, antenna ports 19 and 20 belong to RRH2, and antenna ports 21 and 22 belong to RRH3.

As described above, an eNB can inform an MS of the detailed contents of a transmission point (RRH) and antenna ports using one CSI-RS resource using the transmission point bitmap. Furthermore, the transmission point bitmap can be added to an RRC message on which a CSI-RS configuration is transmitted and then transmitted.

FIG. 5 schematically shows an example of a transmission point bitmap transmitted by an eNB in a system to which the present invention is applied. The example of FIG. 5 shows a transmission point bitmap 510 into which the configuration of Table 13 or Table 14 has been incorporated.

In the example of FIG. 5, each of the bits of the transmission point bitmap 510 corresponds to an antenna port. In the transmission point bitmap 510, a bit having a value 0 indicates that there is no change of a transmission point, and a bit having a value 1 indicates that there is a change of a transmission point.

Referring to FIG. 5 and Tables 13 and 14, in the transmission point bitmap 510, when the first bit (i.e., the antenna port 15) has a value 1, it indicates that the transmission point is started from the RRH1 and there is no change of the transmission point until the antenna port 18. Subsequently, the transmission point bitmap 510 indicates that the transmission point is changed into the antenna port 19 and the antenna port 21 in the RRH2 and the RRH3, respectively.

FIG. 6 schematically shows another example of a transmission point bitmap transmitted by an eNB in a system to which the present invention is applied. The example of FIG. 6 also shows a transmission point bitmap 610 into which the configuration of Table 13 or 14 has been incorporated.

Like in FIG. 5, in the example of FIG. 6, each of the bits of the transmission point bitmap 610 corresponds to an antenna port. Unlike in the transmission point bitmap of FIG. 5, however, in the transmission point bitmap 610 of FIG. 6, a previous bit remains intact if there is no change in a transmission point, but a previous bit is changed into a different bit value if there is a change in the transmission point. That is, if a bit value is changed from 0 to 1 or from 1 to 0, it indicates that there is a change a transmission point.

Referring to FIG. 6 and Tables 13 and 14, the transmission point bitmap 610 indicates that the first four antenna ports (i.e., the antenna ports 15 ~ 18) belong to the same transmission point and that each of pairs of subsequent and consecutive antenna ports belongs to the same transmission point.

An eNB can inform an MS of a situation in which transmission is performed through the above-described bitmap.

Meanwhile, in the fourth CoMP scenario of the above-described CoMP scenarios, transmission points (e.g., RRHs) within a CoMP cooperation set have the same physical cell ID. In contrast, in the first to third CoMP scenarios, transmission points within a CoMP cooperation set have different physical cell IDs. Transmission points having different physical cell IDs can transmit CSI-RSs having different patterns.

More particularly, a reference signal sequence

that may be used to generate a CSI-RS may be defined as in Equation 9.

In Equation 9, n_S is the number of slots within a radio frame, and 'l' is the number of OFDM symbols within a slot. Furthermore, N^max,DL _RB indicates a maximum number of downlink resource blocks. In Equation 9, c(i) is a scrambling code and is a pseudo random sequence defined by a length-31 gold sequence. The scrambling code is reset at the start of each OFDM symbol as in Equation 10.

In Equation 10, N_CP has a value 1 in the case of a normal Cyclic Prefix (CP) and has a value 0 in the case of an extended CP. Furthermore, N^cell _ID indicates a physical layer cell ID.

Referring to Equations 9 and 10, transmission points having different cell IDs have different values c_init. Accordingly, since the reference signal sequence of a CSI-RS transmitted by transmission points having different cell IDs is different, the CSI-RS may have a different pattern. That is, transmission points having different cell IDs may transmit CSI-RSs using different CSI-RS resources.

Accordingly, when the first to third CoMP scenarios are applied, an eNB may inform an MS of the value c_init of each transmission point. Here, the value c_init is added as one field value within a CSI-RS configuration and may be configured for each antenna port or transmission point (e.g., an RRH).

Table 15 schematically shows an example in which p-c/EPRE and c_init are indicated for each antenna port that transmits a CSI-RS in order to support multiple transmission points having different cell IDs in a CoMP system to which the present invention is applied. In the example of Table 15, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, and a field for c_init are taken into consideration, for convenience of description.

Table 15

EPRE	p-c	c_init	Antenna Port
EPRE1	p-c1	c	_init1	15
EPRE2	p-c2	c_init2	16
EPRE3	p-c3	c_init3	17
EPRE4	p-c4	c_init4	18
EPRE5	p-c5	c	_init5	19
EPRE6	p-c6	c	_init6	20
EPRE7	p-c7	c_init7	21
EPRE8	p-c8	c_init8	22

If the example of Table 15 is applied to a CSI-RS configuration in detail, it may result in Table 16. Table 16 is an example of a CSI-RS configuration in which the example of Table 15 is shown along with CSI-RS parameters.

Table 16

Table 17 schematically shows an example in which p-c/EPRE and c_init are indicated for each antenna port group that transmits a CSI-RS in order to support multiple transmission points having different cell IDs in a CoMP system to which the present invention is applied. In the example of Table 17, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, and a field for c_init are taken into consideration, for convenience of description.

Table 17

EPRE	p-c	c_init	Antenna Port
EPRE1	p-c1	c	_init1	15&16
EPRE2	p-c2	c_init2	17&18
EPRE3	p-c3	c_init3	19&20
EPRE4	p-c4	c_init4	21&22

In the example of Table 17, the transmission points of a CoMP cooperation group are classified into groups each including two transmission points, and EPRE, p-c, and c_init are indicated for each group.

Table 18 schematically shows an example in which p-c/EPRE and c_init are indicated for each antenna port group that transmits a CSI-RS in order to support multiple transmission points having different cell IDs in a CoMP system to which the present invention is applied. In the example of Table 18, only two fields added to a CSI-RS configuration, including a field for EPRE and a field for p-c, and a field for c_init are taken into consideration, for convenience of description.

Table 18

EPRE	p-c	c_init	Antenna Port
EPRE1	p-c1	c	_init1	15&16
ΔEPRE12	Δp-c12	c_init2	17&18
ΔEPRE13	Δp-c13	c_init3	19&20
ΔEPRE14	Δp-c14	c_init4	21&22

In the example of Table 18, a first group including the antenna ports 15 and 16 is used as a reference group, an EPRE value and a p-c value are indicated without changed in relation to the first group, and difference values ΔEPRE and Δp-c with the EPRE and the p-c value for the first group are transmitted regarding EPREs and p-c values for other antenna port groups.

Even when difference values are used in order to transmit an EPRE value and a p-c value, a c_init value may be indicated as an original value for each antenna port group as in the example of Table 18. Unlike in the example of Table 18, however, a c_init value for an antenna port group other than a reference group (e.g., the first antenna port group in the example of Table 15) may be indicated as a value different from the c_init value of the reference group, like in other parameters (i.e., EPRE and p-c).

FIG. 7 is a flowchart schematically illustrating a downlink transmission operation by the transmission point of a CoMP cooperation set in a system to which the present invention is applied. Referring to FIG. 7, an eNB configures a CSI-RS in an environment to which CoMP is applied and sends the configured CSI-RS to an MS at step S710. The eNB may transfer the CSI-RS configuration to the MS through higher layer signaling, such as an RRC message.

The CSI-RS configuration, as described above, may include EPRE/p-c for each antenna port or each antenna port group in addition to antennaPortsCount, subframeConfig, and resourceConfig. Furthermore, if the first CoMP scenario to the third CoMP scenario are applied, the CSI-RS configuration may indicate c_init for each antenna port or each antenna port group. The CSI-RS configuration may also indicate EPRE/p-c for a corresponding antenna port or antenna port group as a value different from a reference value. Furthermore, the eNB may transfer a CSI-RS transmission situation (e.g., RRHs participating in the transmission) to the MS through the bitmap. The CSI-RS configuration and the method of transmitting the CSI-RS configuration in accordance with the present invention have been described above in detail.

Next, the eNB selects an antenna port group at step S720. The eNB selects an antenna port group that will participate in CSI-RS transmission from antenna ports that belong to the transmission points of a CoMP cooperation set. The antenna port group includes at least one antenna port. How many antenna ports are included in the antenna port group or what does the antenna port group include what antenna ports may be determined by the CSI-RS configuration.

Furthermore, different antenna port groups may belong to the same RRH. For example, if an RRH1 includes antenna ports 15 to 18, an antenna port group 1 may include the antenna ports 15 and 16 and an antenna port group 2 may include the antenna ports 17 and 18.

Next, each of the transmission points checks that its antenna port belongs to what antenna port group at step S730. Scheduling between RRHs may be determined through cooperation between the multi-Tx/Rx points (RRHs) of the CoMP cooperation set as described above. The contents of the CSI-RS configuration configured in the eNB may be transferred to the RRHs through wired/wireless connection between the eNB and the RRHs.

Each of the transmission points sends a CSI-RS according to the CSI-RS configuration for an antenna port group to which each of the antenna ports belongs at step S740.

FIG. 8 is a flowchart schematically illustrating the operation of an MS in a system to which the present invention belongs. Referring to FIG. 8, the MS receives information on a CSI-RS configuration form an eNB at step S810. The CSI-RS configuration may be transferred to the MS through higher layer signaling, such as an RRC message. Here, the MS may receive information on a PDSCH configuration along with the information on the CSI-RS configuration. The CSI-RS configuration indicates an EPRE value and a p-c value for each antenna port group including at least one antenna port in relation to antenna ports that participate in downlink transmission.

Contents regarding the information on the CSI-RS configuration and the information on the PDSCH configuration have been described above in detail.

Next, the MS receives information through a downlink physical channel from each of the transmission points of a CoMP cooperation set at step S820. The information received through the downlink physical channel includes a CSI-RS.

The MS may estimate an uplink path loss based on the received CSI-RS and calculate information that forms CSI, such as a PMI, at step S830. The MS can estimate an uplink path los based on the received CSI-RS and values set in the CSI-RS configuration. A detailed method of the MS calculating the uplink path loss and a method of configuring information to be included in a CSI feedback have been described in detail above.

The MS controls uplink transmission power by incorporating the calculated path loss into the control at step S840. The examples of

Equations

5 and 6 may be used as a method of controlling uplink transmission power by incorporating the calculated path loss into the control.

The MS performs uplink transmission using the controlled uplink transmission power at step S850. Here, the MS may send CSI, including pieces of information calculated based on the CSI-RS, to the eNB. The CSI may include PMI information about downlink transmission from each of the transmission points.

Meanwhile, in FIG. 8, assuming that the operation of controlling the uplink transmission power is a first operation of one process and an operation of configuring the CSI feedback information is a second operation of the one process, each of the steps of the MS operation in which the first operation and the second operation are combined is assumed and described, but the present invention is not limited thereto. For example, each of the steps of the first operation and each of the steps of the second operation may be configured in a combination different from that of FIG. 8. In other words, in the MS, the operation of controlling the uplink transmission power using the CSI-RS and the operation of configuring the CSI feedback information may be separately taken into consideration.

FIG. 9 is a flowchart in which the operation of the MS described with reference to FIG. 8 is divided into the control of the uplink transmission power and the configuration of the CSI information.

Referring to FIG. 9, the MS receives CSI-RS configuration as described above with reference to FIG. 8 at step S910.

Next, the MS may measure Reference Signal Received Power (RSRP) based on a CSI-RS received on a physical channel from each of transmission points at step S920. The RSRP may be defined as a linear average for the power contributions of all resource elements that carry the CSI-RS within a considered measurement frequency bandwidth. The MS can calculate an RSRQ as in Equation 3 based on the calculated RSRP.

The MS may estimate a path loss based on the measured RSRQ at step S930. A detailed method of calculating the path loss has been described in connection with Equation 4.

The MS controls uplink transmission power using the calculated path loss at step S940. A detailed method of controlling the uplink transmission power has been described above.

Meanwhile, the MS generates CSI based on the CSI-RS received on a physical channel from each of the transmission points at step S950. The CSI may include a channel matrix, a channel correlation matrix, a quantized channel matrix or a quantized channel correlation matrix, a PMI, etc. between a transmitter and a receiver. CQI may be a Signal to Noise Ratio (SNR), a Signal to Interference and Noise Ratio (SINR), or a signal to interference ratio between a transmitter and a receiver. The MS may estimate a channel, select a precoding matrix that maximizes channel performance, and report a PMI for the selected precoding matrix. Here, a channel state from each of transmission points within a CoMP cooperation set that have participated in CSI-RS transmission may be incorporated into the precoding matrix.

The MS performs uplink transmission using uplink transmission power calculated based on the CSI-RS at step S960. The uplink transmission includes the transmission of CSI feedback information that is configured based on the CSI-RS.

FIG. 10 is a schematic block diagram showing the construction of an eNB in a system to which the present invention is applied. Referring to FIG. 10, the eNB 1000 includes a Radio Frequency (RF) unit 1010, memory 1020, and a processor 1030.

The eNB 1000 transmits and receives pieces of information through the RF unit 1010. The RF unit 1010 includes a plurality of antennas and can support an MIMO operation. The eNB 1000 can be connected to multi-Tx/Rx points (e.g., RRHs), forming a CoMP cooperation set, through the RF unit 1010. Furthermore, the eNB 1000 may also be connected to multi-Tx/Rx points, forming a CoMP cooperation set, over a wired network.

The memory 1020 can store information that is necessary to perform communication with the eNB 1000 or to control a network. For example, the memory 1020 may store configuration information on a network. For example, the memory 1020 may store configuration information on a reference signal, such as a CSI-RS configuration, and may store the memory 1020 may store information on a configuration regarding the transmission of a physical channel, such as a PDSCH configuration. Furthermore, the memory 1020 may store information, such as a codebook for operating an MIMO system.

The processor 1030 embodies the functions, processing processes and/or method proposed in the present invention. For example, the processor 1030 can control the general operation of the eNB 1000 and may perform scheduling for a CoMP operation. Furthermore, the processor 1030 is connected to the RF unit 1010 and the memory 1020 and configured to control the operations of the RF unit 1010 and the memory 1020.

The processor 1030 may include a configuration module 1040, a scheduling module 1050, and a control module 1060. The configuration module 1040 can perform necessary configurations in order to perform a network operation and UL/Dl transmission. For example, the configuration module 1040 may perform the configuration of a reference signal, such as a CSI-RS. Furthermore, the configuration module 1040 may perform a configuration for transmitting a downlink physical channel. Parameters set by the configuration module 1040 may be applied in a cell-specific manner or a UE-specific manner. For example, a CSI-RS configuration configured by the configuration module 1040 is applied in a cell-specific manner, and each of RRHs within a cell transmits a CSI-RS according to a CSI-RS configuration. The detailed contents of the CSI-RS configuration performed by the configuration module 1040 in relation to the present invention have been described above.

The scheduling module 1050 performs necessary operations for UL/DL scheduling and/or the Cooperation Scheduling (CS) of a CoMP system. Furthermore, the control module 1060 controls the operations of modules associated therewith.

FIG. 11 is a schematic block diagram showing the construction of an MS in a system to which the present invention is applied. Referring to FIG. 11, the MS 1100 includes an RF unit 1110, memory 1120, and a processor 1130.

The MS 1100 transmits and receives pieces of information through the RF unit 1110. The RF unit 1110 may include a plurality of antennas and can support an MIMO operation and a CoMP operation. The MS 1100 can receive information from multi-Tx/Rx points (e.g., RRHs), forming a CoMP cooperation set, through the RF unit 1110 and transmit information on the multi-Tx/Rx points.

The memory 1120 can store information necessary to perform communication with the MS 1000. For example, the memory 1020 may store various pieces of configuration information. For example, the memory 1020 can store configuration information on a reference signal, such as a CSI-RS configuration, and may store configuration information regarding the transmission of a physical channel, such as a PDSCH configuration. The pieces of information on the configuration may be transmitted by an eNB through higher layer signaling, such as an RRC message.

The processor 1130 embodies the functions, processing processes and/or methods proposed in the present invention. For example, the processor 1130 is connected to the RF unit 1110 and the memory 1120 and configured to control the operations of the RF unit 1110 and the memory 1120.

The processor 1130 may include a path loss (PL) calculation module 1140, an UL power control module 1150, a CSI configuration module 1160, and a control module 1170. The PL calculation module 1140 estimates path loss based on RSRP and EPRE calculated using a received CSI-RS. The UL power control module 1150 controls uplink transmission power by incorporating the path loss, calculated by the PL calculation module 1140, into the control. The CSI configuration module 1160 configures information on CSI to be transmitted to an eNB in the form of a feedback signal based on the received CSI-RS. The information on the CSI may include information indicating a channel state, such as a PMI. The control module 1170 can control the operation of other modules associated therewith.

As described above, in accordance with the present invention, an eNB can classify the antenna ports of transmission points, participating in CSI-RS transmission in a CoMP cooperation set, into antenna port groups and configures EPRE for each antenna port. In response thereto, an MS can estimate path loss for each antenna port group using the EPRE configured for each antenna port group. The EPRE may be configured for each antenna port. In this case, the antenna port group may be considered as including one antenna port. The MS can control uplink transmission power for each antenna port group using the path loss estimated for each antenna port group. In this case, the same transmission power may be configured for antenna port groups that belong to the same multi-Tx/Rx point.

Meanwhile, an example in which both an EPRE value and a p-c value have been configured to be included in a CSI-RS configuration for each antenna port or each antenna port group has been described in the examples of Tables 8 to 18, but it is not necessary to configure both the p-c value and the EPRE in order to control uplink transmission power. As described above in connection with Table 6, only EPREs may be configured for each antenna port or each antenna port group in a CSI-RS, and both EPREs and p-c values may be configured for each antenna port or each antenna port group in a CSI-RS.

If a CSI-RS configuration includes p-c information for each antenna port or each antenna port group, an MS can calculate the EPRE of a downlink physical channel (e.g., a PDSCH) using the configured EPRE and p-c value for each antenna port or each antenna port group. The MS can control the transmission power of an uplink physical channel based on the calculated transmission power of a downlink physical channel (e.g., a PDSCH). Here, the control of the transmission power may be performed by a specific step size within a specific power range. For example, the MS may control the transmission power of the uplink physical channel in a step unit of 1 dB within a range of [-8, 15] dB by taking both the calculated transmission power of the PDSCH and the reception power of the PDSCH into consideration.

In the above exemplary systems, although the methods have been described on the basis of the flowcharts using a series of the steps or blocks, the present invention is not limited to the sequence of the steps, and some of the steps may be performed in order different from that of the remaining steps or may be performed simultaneously with the remaining steps. Furthermore, those skilled in the art will understand that the steps shown in the flowcharts are not exclusive and they may include other steps or one or more steps of the flowchart may be deleted without affecting the scope of the present invention.

The above embodiments include various aspects of examples. Although all possible combinations for describing the various aspects may not be described, those skilled in the art may appreciate that other combinations are possible. Accordingly, the present invention should be construed as including all other replacements, modifications, and changes which fall within the scope of the claims.

Claims

A method of transmitting a reference signal in a Cooperative Multi-Point (CoMP) system, the method comprising:

sending configuration information on the reference signal; and

sending the reference signal based on the configuration information on the reference signal,

wherein the configuration information on the reference signal indicates Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in the transmission of the reference signal.
The method of claim 1, wherein the configuration information on the reference signal indicates a ratio of the EPRE used to transmit the reference signal to EPRE used to transmit a physical downlink shared channel (PDSCH) signal transmitted along with the reference signal, for each of the transmission points that participate in the transmission of the reference signal.
The method of claim 1, wherein, the EPRE used to transmit the reference signal is indicated for each antenna port group in which antenna ports of the transmission points that participate in the transmission of the reference signal are grouped.
The method of claim 3, wherein identical EPRE is indicated in antenna port groups that belong to an identical transmission point.
The method of claim 3, wherein the configuration information on the reference signal indicates a sequence used to transmit the reference signal for each transmission point having a different cell ID.
The method of claim 1, wherein the configuration information on the reference signal comprises bitmap information indicating a number of transmission points that participate in the transmission of the reference signal.
The method of claim 6, wherein:

each of bits of the bitmap information corresponds to each of antenna ports that participate in the transmission of the reference signal, and

each of the bits has a specific bit value when a transmission point is changed.
The method of claim 6, wherein:

each of bits of the bitmap information corresponds to each of antenna ports that participate in the transmission of the reference signal, and

a value of each of the bits is changed in response to a change of a transmission point.
An uplink transmission method in a Cooperative Multi-Point (CoMP) system, comprising:

receiving configuration information on reference signals;

estimating an uplink path loss using the reference signals received on a downlink physical channel;

determining uplink transmission power by incorporating the uplink path loss into the uplink transmission power; and

performing uplink transmission using the uplink transmission power,

wherein the configuration information on the reference signal indicates Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in the transmission of the reference signal, and

the uplink path loss for each transmission point is estimated using reception power of a reference signal transmitted by each of the transmission points and transmission power of a reference signal indicated in the configuration information for each transmission point.
The uplink transmission method of claim 9, wherein determining the uplink transmission power comprises determining the uplink transmission power for each transmission point based on uplink path loss estimated for each of the transmission points.
The uplink transmission method of claim 9, wherein the configuration information on the reference signal indicates a ratio of the EPRE used to transmit the reference signal to EPRE used to transmit a physical downlink shared channel (PDSCH) signal transmitted along with the reference signal, for each of transmission points that participate in the transmission of the reference signal.
The uplink transmission method of claim 9, wherein the configuration information on the reference signal indicates a ratio of the EPRE used to transmit the reference signal to EPRE that is transmitted along with the reference signal and used to transmit a physical downlink shared channel (PDSCH), for each of transmission points that participate in the transmission of the reference signal.
The uplink transmission method of claim 12, wherein the configuration information on the reference signal indicates identical EPRE in antenna port groups belonging to an identical transmission point.
An apparatus for transmitting a reference signal, comprising:

a Radio Frequency (RF) unit configured to transmit and receive pieces of information;

memory configured to store the pieces of information; and

a processor configured to control the RF unit and the memory,

wherein the processor configures configuration information on the reference signal, and

the configuration information on the reference signal indicates Energy Per Resource Element (EPRE), used to transmit the reference signal, for each of transmission points that participate in a transmission of the reference signal.
An uplink transmission apparatus, comprising:

a Radio Frequency (RF) unit configured to transmit and receive information;

memory configured to store the information; and

a processor configured to control the RF unit and the memory,

wherein the processor estimates an uplink path loss for each of transmission points using reception power of a reference signal for each of the transmission points, received on a physical channel, and transmission power of a reference signal for each of the transmission points, and

the transmission power of the reference signal for each of the transmission points is indicated through configuration information on the reference signal.