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WO2007040564A1 - Procede de selection d'antennes et de faisceaux dans des reseaux locaux sans fil a entrees multiples sorties multiples - Google Patents

Procede de selection d'antennes et de faisceaux dans des reseaux locaux sans fil a entrees multiples sorties multiples Download PDF

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Publication number
WO2007040564A1
WO2007040564A1 PCT/US2005/042358 US2005042358W WO2007040564A1 WO 2007040564 A1 WO2007040564 A1 WO 2007040564A1 US 2005042358 W US2005042358 W US 2005042358W WO 2007040564 A1 WO2007040564 A1 WO 2007040564A1
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WO
WIPO (PCT)
Prior art keywords
antennas
field
antenna
training
sounding
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/US2005/042358
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English (en)
Inventor
Daqing Gu
Hongyuan Zhang
Jinyun Zhang
Andreas F. Molisch
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Mitsubishi Electric Research Laboratories Inc
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Mitsubishi Electric Research Laboratories Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from PCT/US2005/035012 external-priority patent/WO2007040515A2/fr
Application filed by Mitsubishi Electric Research Laboratories Inc filed Critical Mitsubishi Electric Research Laboratories Inc
Priority to US12/094,441 priority Critical patent/US8374096B2/en
Priority to JP2008533318A priority patent/JP2009519618A/ja
Priority to EP05852027A priority patent/EP1929654A4/fr
Priority to US12/293,458 priority patent/US8284686B2/en
Publication of WO2007040564A1 publication Critical patent/WO2007040564A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/08—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the receiving station
    • H04B7/0868—Hybrid systems, i.e. switching and combining
    • H04B7/0874—Hybrid systems, i.e. switching and combining using subgroups of receive antennas
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0602—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using antenna switching
    • H04B7/0608—Antenna selection according to transmission parameters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0686—Hybrid systems, i.e. switching and simultaneous transmission
    • H04B7/0691—Hybrid systems, i.e. switching and simultaneous transmission using subgroups of transmit antennas
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H04B7/0686—Hybrid systems, i.e. switching and simultaneous transmission
    • H04B7/0695—Hybrid systems, i.e. switching and simultaneous transmission using beam selection
    • H04B7/06952—Selecting one or more beams from a plurality of beams, e.g. beam training, management or sweeping
    • H04B7/06956—Selecting one or more beams from a plurality of beams, e.g. beam training, management or sweeping using a selection of antenna panels
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03343—Arrangements at the transmitter end
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L2025/03777—Arrangements for removing intersymbol interference characterised by the signalling
    • H04L2025/03802—Signalling on the reverse channel
    • H04L2025/03808—Transmission of equaliser coefficients
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/0224—Channel estimation using sounding signals
    • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/0001—Arrangements for dividing the transmission path
    • H04L5/0014—Three-dimensional division
    • H04L5/0023—Time-frequency-space
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W84/00—Network topologies
    • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
    • H04W84/10—Small scale networks; Flat hierarchical networks
    • H04W84/12—WLAN [Wireless Local Area Networks]

Definitions

  • This invention relates generally to multiple-input, multiple-output wireless local area networks, and more particularly to selecting antennas and beams in such networks.
  • MIMO techniques can significantly increase system capacity in a scattering environment of a wireless network.
  • the use of more antennas increases the hardware complexity and cost because in a typical system, each transmit/receive antenna requires a separate RF chain including a modulator/demodulator, an AD/DA converter, an up/down converter, and a power amplifier.
  • the processing complexity at the baseband also increases with the number of antennas.
  • Antenna/beam selection can reduce the number of RF chains while still taking advantage of the capacity/diversity increase provided by multiple antennas.
  • stations are typically operated at a high signal to noise ratio (SNR), in which diversity plays a key role in protecting the system from a deep fading channel. Furthermore, it is known that the state of a WLAN channel changes slowly. Therefore, it is advantageous to perform antenna/beam selection in a WLAN.
  • SNR signal to noise ratio
  • antenna/beam selection is to select a submatrix from a complete channel matrix or a transformed channel matrix for beam selection, according to some predetermined criterion.
  • the complete channel matrix is estimated by sending training (sounding) frames that enable the antenna selection station to measure the complete channel state information (CSI).
  • CSI channel state information
  • explicit signaling is used in the physical (PHY) or media access (MAC) layer by sending training frame(s) for all the antennas to be selected.
  • PHY physical
  • MAC media access
  • the additional overheads are undesirable due to practical limitations.
  • the slowly varying WLAN channel environment can advocate a more efficient antenna/beam selection training scheme which requires little or no changes in the MAC and PHY layers.
  • the WLAN IEEE 802.1 In standard, incorporated herein by reference, also known as WiFi, proposes to specify a fast link adaptation control (LAC) mechanism defined at the MAC layer for supporting MIMO training requests and exchange of link adaptation information.
  • LAC fast link adaptation control
  • the LAC functionality can be realized either by a single control frame defined as LAC, or the single control frame can be a High Throughput (HT) Control frame, or a HT Control Field can be incorporated into any MAC layer frame.
  • LAC fast link adaptation control
  • HT High Throughput
  • LAC frame contains the following fields: a MAC header 110, a LAC mask 120 for indicating the logical elements carried in the current control frame, a modulation coding scheme (MCS) feedback field 130 for indicating transmitting parameters, and a frame check sequence (FCS) 140 for error detection.
  • the MAC header 110 applies for any MAC layer packet, which includes a frame control 111, duration 112, receive address (RA) 113, and transmit address (TA) 114.
  • the LAC frame is described in detail in IEEE 802.11-O4/O889r7, "TGn Sync Proposal Technical Specification," incorporated herein by reference.
  • the LAC frame supports control of MIMO training requests and exchange of link adaptation information.
  • the LAC frame can be sent by either an initiator station (transmitter) or a recipient station (receiver).
  • FIG 2 shows the LAC mask field 120 in greater detail.
  • the LAC mask field 120 includes the following: RTS (request to send) 121, CTS (clear to send) 122, TRQ (MIMO training request) 123, MRQ (request for MCS feedback) 124, and MFB (MCS feedback) 125.
  • the three bits 126 are reserved.
  • An HT Control Field includes a LAC field which controls the fast link adaptation training process; and two other unused fields which may be dedicated for other HT control features.
  • the LAC field of an HT control field includes: MRQ, MRS (MRQ sequence number), MFS (MFB sequence number), and MFB with 7 bits functioning as the MCS feedback field in the above LAC frame.
  • MRS MRQ sequence number
  • MFS MFS sequence number
  • MFB with 7 bits functioning as the MCS feedback field in the above LAC frame.
  • the same HT Control Field may also form a single HT Control frame by adding in a MAC header, See Figure I, item 110.
  • the HT Control Field is described in detail in IEEE 802.11-20051020 "Link Adaptation Draft Text Alternative 2 r2," incorporated herein by reference.
  • the IEEE 802.1 In standard requires a throughput of 100 megabits per second (Mbps) at the medium access control (MAC) layer service access point (SAP). Based on the channel property in WLAN environment, closed-loop schemes are preferred for increased throughput, including transmit beam forming (TXBF), MCS adaptation, and antenna/beam selection.
  • TXBF transmit beam forming
  • MCS adaptation MCS adaptation
  • antenna/beam selection
  • Each PHY layer packet is composed by two portions: preamble and data.
  • the PHY packet preamble includes training information for channel estimation at the receiver.
  • the number of antennas or spatial streams indicated in the training field can be less than the maximum number provided by the MIMO channel.
  • a sounding packet is a specific PHY layer packet, which contains the training information for all the available data streams in the MIMO channel, no matter how many data streams are used for transmitting the data portion.
  • an alternative category of PHY layer training packet is the one that enforces a MCS set utilizing all the available data streams in the MIMO channel, so that not only the preamble contains the full training information of the MIMO channel, the data portion is also transmitted using all the available data streams.
  • FIG. 3 shows a conventional MIMO training process for MCS adaptation based on LAC frame however, it should be understood that an HT control field can also be used.
  • the initiator also requests its PHY layer to signal a sounding packet.
  • the recipient 302 estimates the MIMO channel and determines an appropriate MCS set for the current channel.
  • the recipient replies to the initiator a LAC frame 320 with MFB set to 1, and the MCS feedback field 130 contains the selected MCS set, or a fame including a HT Control Field with MFS equal to the MRS in the received frame it is currently responding to, and with MFB including a selected MCS set.
  • the recipient 302 can also initiate the MCS training process whenever it has the complete MIMO channel knowledge, by determining the MCS and sending an MFB with MCS feedback directly without any matching MRQ element. This is called unsolicited adaptation.
  • FIG. 4 shows a conventional transmit beam forming (TXBF) training process based on LAC frame.
  • TXBF transmit beam forming
  • the initiator 301 sends out a LAC frame 410 with TRQ set to 1 to the recipient 302.
  • the recipient sends back a sounding packet 420 to the initiator.
  • the initiator estimates the MIMO channel and updates its beam forming steering matrices. Up to now, recipient initiated TXBF training is not defined.
  • some prior art training methods use a single PHY layer training frame (e.g., sounding packet) containing the training information for all the antennas to be selected, and different antenna subsets are subsequently connected to the RF chains for this single training frame. This introduces overhead on existing training frame designs.
  • PHY layer training frame e.g., sounding packet
  • a long sequence of training frames is transmitted from a receive station to a transmit station, and in response the transmit station transmits a short sequence of training frames so that both the transmit and receive station can perform channel estimation and antenna selection, see U.S. Patent Application Number 11/127,006 "Training Frames for MIMO Stations," filed by Andreas Molisch, Jianxuan Du and Daqing Gu on May 11, 2005, incorporated herein by reference.
  • the invention provides a training method for selecting antennas/beams in a MIMO wireless local area network (WLAN).
  • the method operates at the MAC layer.
  • the method according to the invention does not require a modification of the PHY layer.
  • the training method rapidly sends multiple training frames, designated as sounding packets .
  • Each sounding packet conforms to a conventional PHY layer design and is for a different subset of all of the available antennas so that characteristics of the entire channel can be estimated by the receiver of the sounding packets.
  • the receiver of the sounding packets can select either transmit or receive antennas.
  • the entire training method operates at the MAC layer.
  • the sounding packets in addition to training the MIMO channel for selecting antennas/beams, can also include data which makes the method extremely efficient because training and data transfer is performed concurrently. Because the method operates at the MAC layer, the method has less overhead than conventional methods that operate at both the MAC and PHY layers.
  • the sounding and packets can be sequential or alternate. ?
  • the method can be extended for a general closed- loop MIMO system, such as transmit beam forming training processes initiated by the receiver.
  • the antenna/beam selection training method can be combined with transmit beam forming to achieve additional performance improvements.
  • Figure 1 is block diagram of a prior art LAC frame
  • Figure 2 is a block diagram of a prior art LAC mask field
  • Figure 3 is a flow diagram of a prior art MCS training process
  • Figure 4 is a flow diagram of a prior art TXBF training process
  • Figure 5 A is a block diagram of a MIMO system according to an embodiment of the invention
  • Figure 5B is a flow diagram of a method for selecting antennas according to the invention.
  • Figure 5 C is a block diagram of a LAC frame according to an embodiment of the invention.
  • Figure 6 is a block diagram of an ASBFC field of the LAC frame according to an embodiment of the invention.
  • Figure 7 is a flow diagram of the training for transmit antenna/beam selection according to an embodiment of the invention.
  • Figure 8 is a flow diagram of the training for receive antenna/beam selection according to an embodiment of the invention.
  • Figure 9 is a flow diagram of receiver initiated beam forming training according to an embodiment of the invention.
  • Figure 10 is a flow diagram of combined receive antenna/beam selection and TXBF training according to an embodiment of the invention.
  • Figure 11 is a flow diagram of combined transmit antenna/beam selection and TXBF training according to an embodiment of the invention.
  • Figure 12 is block diagram of a HT control field having a LAC field according to an embodiment of the invention
  • Figure 13 is a flow diagram of antenna/beam selection training according to another embodiment of the invention.
  • Figure 14 is another flow diagram of antenna/beam selection training according an embodiment of the invention.
  • FIG. 5A shows a multiple-input, multiple-output (MIMO) wireless local area network (WLAN) 100 according to an embodiment of the invention.
  • the network includes a first station (STA A) 510 and a second station (STA B) 520. Either station can operate in receive or transmit mode. Generally, the station that transmits the data is called the transmit station, and the station that receives the data is called the receive station.
  • a 'set as defined includes one or more elements; the number of elements in a 'subset' is equal to or less than the number of elements in the corresponding set.
  • Each station includes a set of receive (Rx) RF chains 501 and a set of transmit (Tx) RF chains 502 connected to a set of antennas 503 by switches 530.
  • the number of antennas is larger than the number of RF chains. Therefore, a subset of antennas is selected from the set of total available antennas by a method 540 according to an embodiment of the invention during a training phase as described herein.
  • the selection method can be initiated by either the transmitter or the receiver, and the selection can be conducted either at the transmitter or at the receiver.
  • antennas are selected 540 by receiving 562 multiple consecutively transmitted sounding packets 561 in a station.
  • Consecutively transmitting or receiving a predetermined number of packets means that the packets are transmitted one after the other, without any other intervening packets in either direction, and the number of packets is known to both stations before the sounding packets are sent.
  • the sounding packets are transmitted with a relatively short delay.
  • the size of the sounding packets can be minimized.
  • the sounding packets can be limited to a preamble and header, and exclude any data.
  • a channel matrix 564 is estimated 563 from the sounding packers, and a subset of antennas 566 is selected 565 according to the channel matrix.
  • the method can also be used for closed-loop MIMO trainings initiated by a recipient station, e.g., STA B 520.
  • the training process operates entirely at a media access (MAC) layer and is transparent to the physical (PHY) layer to minimize overhead.
  • MAC media access
  • PHY physical
  • the transmitter or initiator station A has a set of N 4 antennas
  • the receive or recipient station B has a set of N B antennas.
  • a relationship between a transmitted signal and a received signal in a flat-fading channel 550 can be expressed as:
  • r B is a N B _ SS x 1 received signal vector
  • s ⁇ is a N A _ SS ⁇ l transmitted signal vector
  • ⁇ is a N ⁇ X N 4 channel matrix
  • a noise vector n has N 3 x 1 entries that are independent and identically distributed (i.i.d.) zero- mean circular complex Gaussian random variables with variance N 0 .
  • F 1 is a N ⁇ x N ⁇ _ ss transmit antenna/beam selection matrix
  • F B is a N B x N 5 55 receive antenna/beam selection matrix.
  • the matrices used for selection are submatrices of an identity matrix for pure antenna selection.
  • the matrices include orthogonally transformed columns of a unitary matrix.
  • the equivalent channel matrix after antenna/beam selection is a N B ss x N A _ ss matrix H e? which is a submatrix of the channel matrix H A ⁇ B , or a submatrix of the transformed channel matrix for beam selection.
  • the superscript 'IT means the conjugate transpose, which is used here for selection by the receiver.
  • F ⁇ /F 1 The determination of F ⁇ /F 1 , is typically performed to optimize the capacity of the channel 550 and the signal-to-noise ratio (SNR).
  • SNR signal-to-noise ratio
  • only one side antenna/beam selection is considered, i.e., at least one of F ⁇ andF ⁇ equals the identity matrix, and the corresponding number of RF chains equals the number of antennas.
  • Antenna selection is performed by switching an output signal of a transmit RF chain 502 to a selected transmit antenna, or an input signal of a selected receive antenna to a receive RF chain 501.
  • the selection 540 can be implemented in the RF domain using phase-shifters, switches, and linear combiners, Sudarshan, P., Mehta, N.B., Molisch, A.F., Zhang, J., "Spatial Multiplexing and Channel Statistics-Based RF Pre-Processing for Antenna Selection," Globecom, November 2004, incorporated herein by reference.
  • the stations exchange information about the number of RF chains, the number of antenna elements, and the type of antenna/beam selection.
  • the type of information contained iu the feedback packet e.g., whether it is the index of the antennas to be used, and/or the full (instantaneous) channel state information (CSI), and/or the average channel state information
  • CSI channel state information
  • Figure 5 C shows the structure of a link adaptation control (LAC) frame according to an embodiment of the invention.
  • LAC link adaptation control
  • ASI antenna/beam selection indicator
  • Figure 6 shows the structure of field 600, which includes a command field 610 and a data field 620.
  • the command field 610 is defined according to Table A. Table A
  • the data field 620 carries data for training information, e.g., the number of sounding packets used for antenna/beam selection training.
  • the HT control field 1200 includes a LAC field 1201 according to an embodiment of the invention, which controls the fast link adaptation training process, and two other unused fields 1202-1203, which may be dedicated for other HT control features.
  • the LAC field 1201 is also shown in greater detail. Without considering antertt ⁇ eam selection, the LAC field includes: MRQ 1220, MRS 1230, i.e., MRQ sequence number, MFS 1240, i.e., MFB sequence number, and MFB 1250 with 7 bits functioning as the MCS feedback field in the above LAC frame.
  • the HT control field can be a single HT control frame, i.e., by adding a MAC header.
  • the prior art HT control field does not define the bit combination of "111" in MRS.
  • MFB field as ASBFC by using one reserved bit as ASI, either in the LAC field, or in one of the reserved portions in HT Control Field.
  • ASBFC the structure of MFB/ASBFC is similar as in Figure 6, except for a reduced length (from 4 bits to 3 bits) in the data field.
  • An alternative way is to reduce the command field to 3 bits so that the data field remains 4 bits. In this case, values of 5 ⁇ 7 in Table A act as reserved commands.
  • An alternative solution is to add a new ASBFC field 1260 into the HT Control field, or use one of the unused bytes as ASBFC field.
  • the transmit antenna/beam selection can be initiated either by the transmitter 510 or by the receiver 520. Because the receiver is constantly monitoring the channel during the communication process, it is generally more efficient for the receiver to request the transmitter to start the training process and update the antenna subset or beam steering whenever the receiver measures an unacceptable change in channel quality.
  • the training process rapidly transmits multiple consecutive sounding packets to the receiver, each packet for a different subset of antennas of all of the available antennas. Thus, the receiver can 'learn' or estimate a full complexity channel matrix, and select the subset of antennas or beam steering from the channel matrix. Due to the slowly changing nature of the WLAN channel 550, the full channel matrix can be assumed to be substantially unchanged while transmitting the sounding packets.
  • the inter-packet time interval introduces some distortion on the estimated full channel matrix. Therefore, the interval between the consecutive sounding packets should be relatively short, and the present training scheme is designed based on this requirement.
  • the receiver performs the antenna/beam selection and feeds back the selected antenna subset or beam steering matrix. This is different for the case when TXBF with explicit feedback is implemented together with transmit antenna/beam selection, as described herein.
  • FIG. 7 shows the details of the training method with LAC frame according to an embodiment of the invention.
  • dotted lines indicate optional transfers.
  • the training information for all the available streams in the MIMO channel is covered by each of these PHY layer training frames.
  • This MCS set can be applied directly by the transmitter, or based on the one suggested by the receiver and indicated in the previously received LAC frame, if there is any. After consecutively sending out all the training frames, the transmitter then should switch back to the previous MCS set for transmitting LAC frames. Note, the process without applying sounding packets is not depicted in Figure 7, and in the following text, for the ease of illustration, the term "sounding packet" is used to represent both cases.
  • the receiver estimates the full channel matrix from the received sounding packets, and performs the antenna/beam selection accordingly.
  • the appropriate MCS set is determined based on the selected antennas result.
  • the receiver sends 706 back a packet containing the selection results.
  • the transmitter updates its selected antenna subset or beam steering correspondingly, and applies the new MCS set.
  • the signaling and training preamble of the PHY layer packets are protected much better than a regular data field in which the MCS set is applied.
  • the preamble can still be accurately decoded. Therefore, antenna selection training is not influenced.
  • the transmitter waits for a short ACK packet, defined at the MAC layer, before transmitting the next data packet. Otherwise, timing out, the transmitter retransmits the previous data packet. While transmitting sounding packets with data for antenna/beam selection training, the transmitter switches to different antenna subsets for each consecutive sounding packet, even though the transmitter may not receive an ACK in response to sending each sounding packet.
  • the retransmission of 'lost' data in the sounding packets does not increase the number of sounding packets.
  • the lost data retransmitted in the next available packet, no matter whether the packet is a sounding or a conventional packet.
  • the antenna/beam selection training process is in parallel with the process of transmitting data using the sounding packets. The two processes are independent of each other. In an alternative implementation, it can be required that the data are retransmitted from the same antenna set, but possibly with a different MCS).
  • the time interval between the consecutive sounding packets should be relatively short. Therefore, the amount of data transmitted in the sounding packets should be relatively small.
  • the determination of the length of data packet is beyond the functionality of MAC layer.
  • a function block that determines the length of the data to be transmitted, taking into account the required overhead of a sounding packet, as well as the MCS scheme on the different channels, so that the total net throughput is optimized.
  • the channel estimation is most accurate, and the time required for the whole training process is reduced. Therefore, there is a tradeoff problem between efficiency and performance.
  • each data stream should contain the independent training sequences with a number equal to the channel rank. If the training packets contain no data, the number should equal to the number of RF chains of the transmitter, so that the required number of training packets is reduced correspondingly. This is another potential advantage of not including data in the training packets.
  • the transmitter knows the ordering of antennas in the subsets of the different sounding packets, while the receiver simply numbers the antenna indexes by the order of the arriving sounding packets. Therefore, the transmitter can translate the antenna selection feedback, and select 540 the corresponding subset of antennas.
  • the formatting and queuing of the selection feedback packet can vary based on different antenna/beam selection processes.
  • One example is, when selecting N A SS antennas/beams from N A antennas, one N A XN A _ SS matrix F 4 is fed back.
  • F 4 contains only 0 and 1 which is a permutation of N A SS columns of the N A X N A identity matrix; while for RF beam steering, F 4 contains phase shift factors.
  • ASI 127 and MFB 125 bits in LAC Mask field 120 cannot be set to 1 simultaneously.
  • the forward link from STA A to STA B in Figure 7
  • the updated antenna subset or RF steering at STA A is also be applied on the reverse link, there is no necessity to do MCS updating on the reverse link before finishing the entire training process.
  • this confliction problem between ASI and MFB does not impact on the system.
  • Figure 8 shows a receive antenna/beam selection training process with LAC frame initiated by the receiver.
  • the receiver receives multiple sounding packets from the transmitter, and switches its RF chains to different antenna subsets on receiving the different sounding packets. This enables the receiver to estimate the entire channel matrix and perform antenna/beam selection.
  • the transmitter Similar as in transmit selection training, for protecting LAC frames and the data in the sounding packets (if there are any) from mismatched MCS sets, the transmitter resets MCS upon receiving the RXASI command. Similarly, for the case where sounding packet cannot be applied, the transmitter enforces a MCS set for the consecutive PHY layer training frames, which utilizes all the available data streams in the MIMO channel, while in each data stream, the most reliable modulation and coding set is deployed. This process is not depicted in Figure 8, and the term "sounding packet" is used to represent both cases.
  • the receiver when the preamble in the sounding packets is correctly detected, even if the data are not, the receiver continuous to estimate the channel and to conduct antenna/beam selection, but no ACK is sent back if data field is not detected correctly.
  • the receiver switches back to the previous antenna subset or steering setting, and does the corresponding MCS determination.
  • the transmitter Upon receiving the first RXASI command, the transmitter starts a clock, if no MCS feedback 805 is received after a time out threshold, which means that the current training process probably failed, the transmitter sends a MRQ to renew the MCS setting.
  • the receiver can always monitor the steered channel quality, e.g., the orthogonality among the steered column vectors in MIMO channel matrix in eigen beam forming schemes, it is more efficient for the receiver to initiate the TXBF training, whenever the receiver detect an unacceptable steering.
  • the method as described herein can perform receiver initiated training, by using the command TXBFI_RX.
  • FIG 9 shows the TXBF training process initiated by the receiver with LAC frame. The training process assumes that the RTS/CTS exchange has been completed.
  • the receiver sends one sounding packet 903.
  • TXBF transmit beam forming
  • the antenna/beam selection training process requires explicit feedback of the selection result, which is usually one matrix
  • TXBF can require either explicit feedback of the channel matrices of all the subcarriers in an MIMO-OFDM system applied in high-throughput WLANs, or implicit feedback, where the transmitter can update its beam forming steering matrix based on what the transmitter estimates from the reverse link, assuming the channels on the forward and reverse links are reciprocal.
  • a calibration process which typically happens only upon associations, is then required for the enforcement of the reciprocity assumption, which needs to be addressed when combining with antenna/beam selections.
  • explicit feedback is used for TXBF training, it is possible to combine antenna/beam selection with TXBF.
  • the transmitter can calculate its selections (instead of at the receiver as in Figure 7), the beam forming matrices, and the corresponding MCS set, all based on the feedback values.
  • the selection is determined and applied at the receiver while the beam forming matrices are performed at the transmitter.
  • antenna selection and TXBF are also possible to perform antenna selection and TXBF as separate operations, where the selection process is performed as described in the previous sections, while the TXBF is done using an implicit feedback scheme (as described, e.g., in the TGnSync draft specifications), possibly at different intervals compared to the antenna selection.
  • implicit feedback is used for TXBF training, it is also possible for the receiver to perform antenna/beam selection without any significant modification on the training process, because the transmitter side calibration is sufficient for performing TXBF training.
  • the training process in Figure 8 is then correspondingly modified as shown in Figure 10.
  • the receiver only selects antennas without determining the corresponding MCS, and after updating the selection result, the transmitter initiates the TXBF and MCS training processes subsequently.
  • both the calibration process and antenna/beam selection training process is modified correspondingly. For example, there are N S possible antenna subsets at the transmitter
  • the transmitter switches the RF chains to the first subset of antennas and conducts the calibration process.
  • the transmitter then switches the RF chains to the next antenna subset and conducts calibration. This process is repeated until all subsets have been calibrated.
  • the transmitter stores the corresponding calibration correction matrices K 1 ⁇ Ns for further use.
  • the transmitter switches to a new calibration matrix whenever the antenna subset is updated.
  • FIG 11 shows the corresponding training process.
  • the receiver feeds back the selection result 1107. This causes the transmitter to update the antenna subset or the steering, and apply the corresponding calibration matrix.
  • RF-baseband processing has the advantage because the performance gain is independent of frequency- selectivity, while the gain by pure antenna selection tends to be averaged out by frequency-selectivity.
  • the embodiments described herein use the LAC frame and a HT Control Field.
  • the invention can be used for any similarly defined signaling frame in a MAC layer that allows a fast communication between the transmitter and receiver, in which one bit or bit combination is used to indicating an antenna/beam selection training frame, and any suitable field with a length of at least 7 bits in the control frame can be used for exchanging the selection training information.
  • This field can either be a reused field from the one used for MCS feedback, or a newly defined field dedicated only for antenna/beam selection training. In the latter case, the restriction in simultaneous antenna/beam selection training and MCS feedback is not required.
  • the training process can alternate on both sides with marginal performance degradation, compared with selecting on both side simultaneously.
  • Yet another alternative would be to increase the number of training packets, making sure that the total number of spatial streams (taken over all training packets) is sufficient for a complete training of the packet.
  • dummy data i.e., data that do not carry useful information, but rather are values that make sure that the transmit signal during the data transmission has the form of a training field, known at the receiver.
  • Figure 13 shows another embodiment of the invention, regarding the transmit antenna/beam selection training using signaling by an HT control field or other MAC signaling.
  • the basic idea is that the transmitter transmits multiple sounding packets 1301, each of which contains an HT control field in the MAC frame as described above.
  • MRS/ASI 1230 is set to 111
  • the command portion 610 in MFB/ASBFC field 1250 is set to TXASI 5
  • data portion 620 of MFB/ASBFC field 1250 of HT Control field 1200 is used to indicate the remaining number of sounding packets to be sent.
  • the control field can also just indicate that a sounding packet is the first (last) of a sequence of sounding packets intended for antenna selection.
  • the receiver feeds back a packet 1302 containing CSI for each corresponding sounding packet 1301 as each sounding packet is received.
  • the sounding packets and feedback alternate for each subset of antennas.
  • the receiver can feed back the information about which subset of antennas should be used subsequently. This feedback occurs after the last sounding packet has been received.
  • the receiver can also elect to reply 1402 to each of the sounding packets 1401 after all sounding packets are received.
  • extended command portion 610 may be required to differentiate the protocols.
  • the large number of reserved commands guarantees the possibility of the above extensions.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Power Engineering (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Radio Transmission System (AREA)

Abstract

Un procédé informatique consiste à sélectionner des antennes dans réseau local sans fil à entrées multiples sorties multiples composé de plusieurs stations, chaque station comprenant un ensemble d'antennes. Une station reçoit des paquets sonores multiples transmis successivement. Chaque paquet sonore correspond à un sous-ensemble différent de l'ensemble d'antennes. Une matrice de canal est estimée à partir desdits paquets sonores multiples transmis successivement. Une trame comprenant un champ de commande à grande capacité est envoyée pour procéder à une sélection d'antennes, et un sous-ensemble d'antennes est sélectionné en fonction de la matrice de canal.
PCT/US2005/042358 2005-09-30 2005-11-21 Procede de selection d'antennes et de faisceaux dans des reseaux locaux sans fil a entrees multiples sorties multiples Ceased WO2007040564A1 (fr)

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US12/094,441 US8374096B2 (en) 2005-09-30 2005-11-21 Method for selecting antennas and beams in MIMO wireless LANs
JP2008533318A JP2009519618A (ja) 2005-09-30 2005-11-21 Ｍｉｍｏワイヤレスｌａｎにおけるアンテナ及びビームを選択する方法
EP05852027A EP1929654A4 (fr) 2005-09-30 2005-11-21 Procede de selection d'antennes et de faisceaux dans des reseaux locaux sans fil a entrees multiples sorties multiples
US12/293,458 US8284686B2 (en) 2005-11-21 2006-03-30 Antenna/beam selection training in MIMO wireless LANS with different sounding frames

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USPCT/US2005/035012 2005-09-30
PCT/US2005/035012 WO2007040515A2 (fr) 2005-09-30 2005-09-30 Signaux d'apprentissage permettant de selectionner des antennes et des faisceaux dans des reseaux locaux sans fil a entrees multiples sorties multiples
PCT/US2005/039651 WO2007040554A2 (fr) 2005-09-30 2005-11-03 Procede de selection d'antennes et de faisceaux dans des reseaux sans fil a entrees multiples sorties multiples
USPCT/US2005/39651 2005-11-03

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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2007108283A1 (fr) 2006-03-23 2007-09-27 Mitsubishi Electric Corporation Procédé pour sélectionner les antennes dans un réseau de communications sans fil à entrées multiples et à sorties multiples
EP2015467A2 (fr) 2007-07-13 2009-01-14 Mitsubishi Electric Corporation Procédé de sélection d'antennes dans un réseau OFDMA sans fil
WO2009060677A3 (fr) * 2007-11-09 2009-06-25 Mitsubishi Electric Corp Procédé et système permettant de sélectionner des antennes dans des réseaux amrof
JP2009273017A (ja) * 2008-05-09 2009-11-19 Canon Inc 通信装置及び制御方法
WO2009099949A3 (fr) * 2008-02-01 2010-03-11 Marvell World Trade Ltd. Stratégies de sondage et d’estimation de canal pour la sélection de l’antenne dans les systèmes mimo
WO2010027864A3 (fr) * 2008-08-26 2011-04-14 Marvell World Trade Ltd. Format d'unité de données de couche physique
WO2011060175A1 (fr) * 2009-11-13 2011-05-19 Interdigital Patent Holdings, Inc. Procédé et appareil assistant des actions de gestion pour des débits très élevés dans des communications sans fil
EP2023504A3 (fr) * 2007-08-06 2011-11-16 Mitsubishi Electric Corporation Procédé et système de sélection d'antennes dans un réseau sans fil
US8176378B2 (en) 2007-09-07 2012-05-08 Broadcom Corporation Method and system for a transmitting antenna selection failure recovery mode
CN103973353A (zh) * 2009-03-31 2014-08-06 马维尔国际贸易有限公司 用于无线通信的探测和引导协议
US8842606B2 (en) 2007-08-31 2014-09-23 Koninklijke Philips N.V. Enhanced multi-user transmission
US9749022B2 (en) 2008-02-01 2017-08-29 Marvell World Trade Ltd. Channel sounding and estimation strategies in MIMO systems

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20050239510A1 (en) * 2004-04-14 2005-10-27 Samsung Electronics Co., Ltd. System and method for reselecting antennas in a cellular mobile communication system using multiple antennas
US20060111148A1 (en) * 2002-10-29 2006-05-25 Mukkavilli Krishna K Low complexity beamformers for multiple transmit and receive antennas
US20060160495A1 (en) * 2005-01-14 2006-07-20 Peter Strong Dual payload and adaptive modulation

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3340488B1 (fr) * 2003-10-01 2020-02-19 Avago Technologies International Sales Pte. Limited Système et procédé de sélection d'antennes

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20060111148A1 (en) * 2002-10-29 2006-05-25 Mukkavilli Krishna K Low complexity beamformers for multiple transmit and receive antennas
US20050239510A1 (en) * 2004-04-14 2005-10-27 Samsung Electronics Co., Ltd. System and method for reselecting antennas in a cellular mobile communication system using multiple antennas
US20060160495A1 (en) * 2005-01-14 2006-07-20 Peter Strong Dual payload and adaptive modulation

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of EP1929654A4 *

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WO2007108283A1 (fr) 2006-03-23 2007-09-27 Mitsubishi Electric Corporation Procédé pour sélectionner les antennes dans un réseau de communications sans fil à entrées multiples et à sorties multiples
EP2015467A2 (fr) 2007-07-13 2009-01-14 Mitsubishi Electric Corporation Procédé de sélection d'antennes dans un réseau OFDMA sans fil
EP2015467A3 (fr) * 2007-07-13 2009-05-06 Mitsubishi Electric Corporation Procédé de sélection d'antennes dans un réseau OFDMA sans fil
US7756099B2 (en) 2007-07-13 2010-07-13 Mitsubishi Electric Research Laboratories, Inc. Method and system for selecting antennas adaptively in OFDMA networks
KR100976677B1 (ko) 2007-07-13 2010-08-18 미쓰비시덴키 가부시키가이샤 Ｏｆｄｍａ 무선 네트워크에서의 안테나의 선택 방법
EP2023504A3 (fr) * 2007-08-06 2011-11-16 Mitsubishi Electric Corporation Procédé et système de sélection d'antennes dans un réseau sans fil
US8842606B2 (en) 2007-08-31 2014-09-23 Koninklijke Philips N.V. Enhanced multi-user transmission
TWI487318B (zh) * 2007-09-07 2015-06-01 Broadcom Corp 用於在無線通信系統內傳送資訊的系統和方法
US8176378B2 (en) 2007-09-07 2012-05-08 Broadcom Corporation Method and system for a transmitting antenna selection failure recovery mode
WO2009060677A3 (fr) * 2007-11-09 2009-06-25 Mitsubishi Electric Corp Procédé et système permettant de sélectionner des antennes dans des réseaux amrof
US8498362B2 (en) 2008-02-01 2013-07-30 Marvell World Trade Ltd. Channel sounding and estimation strategies for antenna selection in MIMO systems
WO2009099949A3 (fr) * 2008-02-01 2010-03-11 Marvell World Trade Ltd. Stratégies de sondage et d’estimation de canal pour la sélection de l’antenne dans les systèmes mimo
US9749022B2 (en) 2008-02-01 2017-08-29 Marvell World Trade Ltd. Channel sounding and estimation strategies in MIMO systems
US9178586B2 (en) 2008-02-01 2015-11-03 Marvell World Trade Ltd. Channel sounding and estimation strategies for antenna selection in MIMO systems
JP2009273017A (ja) * 2008-05-09 2009-11-19 Canon Inc 通信装置及び制御方法
CN102160348A (zh) * 2008-08-26 2011-08-17 马维尔国际贸易有限公司 物理层数据单元格式
JP2012501590A (ja) * 2008-08-26 2012-01-19 マーベル ワールド トレード リミテッド 物理層データユニットフォーマット
WO2010027864A3 (fr) * 2008-08-26 2011-04-14 Marvell World Trade Ltd. Format d'unité de données de couche physique
US8477813B2 (en) 2008-08-26 2013-07-02 Marvell World Trade Ltd. Physical layer data unit format
US8867569B2 (en) 2008-08-26 2014-10-21 Marvell World Trade Ltd Physical layer data unit format
CN103973353B (zh) * 2009-03-31 2017-09-29 马维尔国际贸易有限公司 用于无线通信的探测和引导协议
CN103973353A (zh) * 2009-03-31 2014-08-06 马维尔国际贸易有限公司 用于无线通信的探测和引导协议
WO2011060175A1 (fr) * 2009-11-13 2011-05-19 Interdigital Patent Holdings, Inc. Procédé et appareil assistant des actions de gestion pour des débits très élevés dans des communications sans fil
US9042362B2 (en) 2009-11-13 2015-05-26 Interdigital Patent Holdings, Inc. Method and apparatus for supporting management actions for very high throughput in wireless communications
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US9647743B2 (en) 2009-11-13 2017-05-09 Interdigital Patent Holdings, Inc. Method and apparatus for supporting management actions for very high throughput in wireless communications
TWI583216B (zh) * 2009-11-13 2017-05-11 內數位專利控股公司 無線通訊中支援超高流通量管理行動方法及裝置
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EP2782264A1 (fr) * 2009-11-13 2014-09-24 Interdigital Patent Holdings, Inc. Procédé et appareil destinés à supporter des actions de gestion pour très haut débit dans des communications sans fil
US8711821B2 (en) 2009-11-13 2014-04-29 Interdigital Patent Holdings, Inc. Method and apparatus for supporting management actions for very high throughput in wireless communications
US10045385B2 (en) 2009-11-13 2018-08-07 Interdigital Patent Holdings, Inc. Method and apparatus for supporting management actions for very high throughput in wireless communications
EP3461203A1 (fr) * 2009-11-13 2019-03-27 Interdigital Patent Holdings, Inc. Procédé et appareil destinés à supporter des actions de gestion pour très haut débit dans des communications sans fil
US10616937B2 (en) 2009-11-13 2020-04-07 Interdigital Patent Holdings, Inc. Method and apparatus for supporting management actions for very high throughput in wireless communications
EP3764718A1 (fr) * 2009-11-13 2021-01-13 Interdigital Patent Holdings, Inc. Procédé et appareil destinés à prendre en charge des actions de gestion pour très haut débit dans des communications sans fil
US11006469B2 (en) 2009-11-13 2021-05-11 Interdigital Patent Holdings, Inc. Method and apparatus for supporting management actions for very high throughput in wireless communications
EP4152867A1 (fr) * 2009-11-13 2023-03-22 InterDigital Patent Holdings, Inc. Procédé et appareil pour prendre en charge des actions de gestion pour un très haut débit dans des communications sans fil
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