as) United States
`a2) Patent Application Publication co Pub. No.: US 2004/0184398 A1
`(43) Pub. Date: Sep. 23, 2004
`
`Walton et al.
`
`US 20040184398A1
`
`(54) TRANSMISSION MODE SELECTION FOR
`DATA TRANSMISSION IN A
`MULTI-CHANNEL COMMUNICATION
`SYSTEM
`
`(76)
`
`Inventors: Jay Rod Walton, Carlisle, MA (US);
`Irina Medvedev, Somerville, MA (US)
`
`Correspondence Address:
`Qualcomm Incorporated
`Patents Department
`5775 Morehouse Drive
`San Diego, CA 92121-1714 (US)
`
`(21) Appl. No.:
`
`10/394,529
`
`(22) Filed:
`
`Mar. 20, 2003
`
`Publication Classification
`
`(ST)
`
`Ente C1? caccccccsssssssstsassensstsnssntneense H04Q 7/00
`
`(52) US. Ch.
`
`caecessssssssesntstnesseessee 370/203; 370/332
`
`(57)
`
`ABSTRACT
`
`Techniques to select a suitable transmission mode for a data
`transmission in a multi-channel communication system with
`multiple transmission channels having varying SNRs. In one
`method, an SNR estimate is initially obtained for each of
`multiple transmission channels used to transmit a, data
`stream. An average SNR and an unbiased variance are then
`computed for the SNR estimates for the multiple transmis-
`sion channels. A back-off factor is determined, for example,
`based on the SNR variance and a sealing factor. An oper-
`ating SNR for the transmission channels is next computed
`based on the average SNR and the back-off factor. The
`transmission mode is then selectedfor the data stream based
`on the operating SNR. The selected transmission mode is
`associated with a highest required SNR that is less than or
`equal to the operating SNR. The method maybe used for any
`system with multiple transmission channels having varying
`SNRs.
`
`300
`
`312
`
`314
`
`316
`
`318
`
`320
`
`322
`
`
`
`Obtain SNR estimate for
`each transmission channel
`used for the data stream
`
`Compute average SNR
`
`Compute SNR variance
`
`Determine back-off factor
`(e.g., based on the
`average SNR and/or
`the SNR variance)
`
`Compute operating SNR
`based on the average SNR
`and the back-off factor
`
`Determine transmission
`mode for the data stream
`based on the operating SNR
`
`Page 1 of 28
`
`SAMSUNG EXHIBIT 1024
`
`Page 1 of 28
`
`SAMSUNG EXHIBIT 1024
`
`

`

`US 2004/0184398 Al
`
`Spo
`
`10}08;aS
`
`jaye
`
`apo
`
`ro}
`
`Patent Application Publication Sep. 23, 2004 Sheet 1 of 11
`
`Olt
`
`0SJayIUWSUeITL2
`—~4TaAloooy
`
`UOISSILUSUBL|
`JO|]OUOD
`
`OJU]yORQHSse+
`UOISSILUSUEL|UNSBuyesedoSHNS
`snes|BROXYUOeolUNWWOD
`
`UO/SSILUSUBL|
`apolNuoneyndwosy
`
`SUNS||esuueYD
`Jossev0ldyury
`JO,/EWNSS
`
`
`—
`
`0€}
`
`JE|}OUOD
`
`eedXL
`
`JOSS8001q
`
`Byeg
`
`aojnos
`
`uoneinpow3Bulpoa
`
`e1eq
`
`a1ey
`
`$|O4JUODcH
`JoUod
`
`Page 2 of 28
`
`Page 2 of 28
`
`
`
`
`
`
`
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 2 of 11
`
`US 2004/0184398 Al
`
`300
`
`Obtain SNR estimate for each transmission channel
`
`used for the data stream
`
`Compute average SNR
`
`|
`
`Compute SNR variance
`
`Determine back-off factor
`(e.g., based on the
`average SNR and/or
`the SNR variance)
`
`based on the operating SNR
`
`Compute operating SNR
`based on the average SNR
`
`and the back-off factor
`
`Determine transmission
`mode for the data stream
`
`312
`
`314
`
`316
`
`318
`
`320
`
`909
`
`End
`
`FIG. 3
`
`Page 3 of 28
`
`Page 3 of 28
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 3 of 11
`
`US 2004/0184398 Al
`
`FIG.4
`
`Frequency
`
`(ap) UNS
`
`Page 4 of 28
`
`Page 4 of 28
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 4 of 11
`
`US 2004/0184398 Al
`
`Frequency
`
`510b
`
`Yo (Kk)
`
`Vy(K)
`
`510t
`
`Frequency
`
`eee
`
`Frequency
`
`FIG. 5A
`
`Antenna 1
`
`st0a
`
`44K)
`
`azozw
`
`o
`
`Antenna 2
`
`co
`zs
`
`oZ
`
`z
`w”
`
`Antenna N,
`
`oa
`oS
`
`aZ
`
`zn
`
`Page 5 of 28
`
`Page 5 of 28
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 5 of 11
`
`US 2004/0184398 Al
`
`|
`
`>~
`
`KR
`=
`wo
`e
`®D
`
`c<
`
`x
`
`NQ
`
`)
`oe)
`
`eee
`
`x
`
`“rc
`2
`s
`
`® e
`
`a
`52
`ePle cQ
`gy
`
`SL
`
`e
`
`AN
`
`2
`Cc
`<
`
`Antenna1
`
`~~
`
`8
`iD
`
`EquivalentSystem
`
`—_
`
`C (
`<o
`
`gP) HNS
`
`Page 6 of 28
`
`Page 6 of 28
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 6 of 11
`
`US 2004/0184398 Al
`
`Antenna 1
`
`SNR(dB)
`
`kKnk )
`
`Average SNR
`
`
`
`for antenna 1 612a
`
`
`
`Frequency
`
`Antenna 2
`
`SNR(dB)
`
`
` Average SNR
`
`for antenna 2 Yavg,2
`
`
`Frequency
`
`¥y,(kK)
` 612t Y,avg, Ny
`
`Average SNR
`for antenna N-
`
`Frequency
`
`FIG. 6
`
`Antenna N,
`
`
`
`610t
`
`
`SNR(dB)
`
`Page 7 of 28
`
`Page 7 of 28
`
`

`

`US 2004/0184398 Al
`
`Patent Application Publication Sep. 23, 2004 Sheet 7 of 11
`
`yoeqpes4
`
`oldsnyeis
`
`1ayoed
`
`uone|npow9Bulpop
`
`s]jo4uoy
`
`POLgay29GEL022
`
`1Olld
`
`eyedXYrenedsXLeredXL
`
`
`
`
`
`JOSSa001dJOSS8001dJOSS800)1dq
`
`eyedXLyenedsXYeredXu
`
`Ojul082
`
`yoeqpee4
`
`Ojuj
`
`
`
`
`
`asyeurulielBzG/BoeyTonesosegme
`
`Page 8 of 28
`
`
`
`Jossao0ldJossao0ldJOSSO00ldJOSSA001d
`
`
`
`
`
`Page 8 of 28
`
`
`
`
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 8 of 11
`
`US 2004/0184398 Al
`
`BOEZ
`
`joquiAs
`
`jsuueuy
`
`Buiddeyy
`JaAea|JaU|
`Japooug
`
`
`
`JOSSE0014sig
`
`lenedsXLuOHeWOJUT
`
`uoReiNpoW
`
`jo4yUOyD
`
`V8‘Old
`
`jouueUyd
`
`JaAee|J2}U|
`
`|ONUOD
`
`
`
`SWeaI]SSWRAISX0CLSWRA]S.
`
`
`
`
`
`
`
`
`
`joquAS|OQUIASByeq
`
`
`
`ywsues,~~uole|npoRL
`
`Page 9 of 28
`
`Page 9 of 28
`
`
`
`
`
`
`
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 9 of 11
`
`
`
`
`
`US 2004/0184398 Al
`
`jeAoWAay
`
`De8AIB0EHWds0
`
`DeAIeoeY
`
`g8‘Old
`
`
`‘AXd
`
` OAD sjoquiASPSLsjoquiAsHAST
`
`
`
`X@ld
`
`JOYEIBUSL)
`
` O1DAD) [2ezsjoquiAslogzsjoquuAsS
`
`
`
`WdsjOylusuelL
`
`Page 10 of 28
`
`Page 10 of 28
`
`

`

`US 2004/0184398 Al
`
`
`
`(sHoy‘sapoy|
`
`V6‘Did
`
`Patent Application Publication Sep. 23, 2004 Sheet 10 of 11
`
`
`ByeqpepovegIp18po99jeuueYyDjoquiAs!Is!LfHAQY
`
`
`
`
`
`
`Bedpepooag“ay,]*POPSjeuueydjoquts[*jhy,|HASH
`
`
`Gg!BuiddNweedsJ9AB9|1B}UI8Gulddewaq
`
`....JOSS8d01d/$$:1|lrenedsxy]!.14:IIo!Iod!|WealsJOARSLB]LIOGBulddeweg
`
`
`uolssiusuel|‘6°9)LVPRrZZ
`
`oju|yoeqpae419||O1JUOD
`
`—" PSlsAOIeypeaiaoey
`
`
`I1ot!;E9E60c6|BgGL|bpc/
`L--=---------iAxP9l
`
`\|IIIoIS9€64CGZ\tpGZ
`
`
`sueens *c9ZSLUPBOIS
`
`|joqwAsJOqWAS
`
`sniels
`
`MAPKgy
`
`|ONUOD
`
`|O1JU0D
`
`Bulpooeq
`uole|npowag
`
`CLL
`
`sa}euisy
`Joyewnsy
`
`jeuueUy
`
`jouueyy
`
`Page 11 of 28
`
`Page 11 of 28
`
`
`
`
`
`
`

`

`Sot Tt testers rts sss
`
` RX Data
`
`
`Processor
`
` Decoding &
`
`Demodulation
`Controls
`
`
`
`Modified
`Symbol
`Streams
`
`
`Decoding &
`Demodulation
`Controls
`
`ee = —_ ee ee ew ew em em em ew ee KK
`
`Channe!
`Estimates
`
`Decoded Data
`Stream 1
`
`Decoded Data
`Stream 2
`
`Decoded Data
`Stream N,
`
`Patent Application Publication Sep. 23, 2004 Sheet 11 of 11
`
`US 2004/0184398 Al
`
`Received
`Symbol
`
`
`754a
`
`Selected
`Recovered
`Symbol
`St
`ream
`
`900y
`a
`
`
`
` Decoding &
`
`Demodulation
`
`
`Stage N.
`Controls
`
`
`
`
`Decoding &
`
`Demodulation
`Controls
`
`
`
`FIG. 10
`
`Feedback Info
`(e.g., Transmission
`Modes, ACKs)
`
`774
`
`Page 12 of 28
`
`Page 12 of 28
`
`

`

`US 2004/0184398 Al
`
`Sep. 23, 2004
`
`TRANSMISSION MODE SELECTION FOR DATA
`TRANSMISSION IN A MULTI-CHANNEL
`COMMUNICATION SYSTEM
`
`BACKGROUND
`
`[0001]
`
`1. Field
`
`invention relates generally to data
`[0002] The present
`communication, and more specifically to techniques for
`selecting a suitable transmission mode for a data transmis-
`sion in a multi-channel communication system.
`
`[0003]
`
`2. Background
`
`[0004] A multi-channel communication system utilizes
`multiple “transmission” channels for data transmission.
`These transmission channels may be formed in the time
`domain, frequency domain, spatial domain, or a combina-
`tion thereof. For example, the multiple transmission chan-
`nels may correspondto different time slots in a time division
`multiplex (TDM) communication system, different
`fre-
`quency subbandsin an orthogonal frequency division mul-
`tiplex (OFDM) communication system, or different spatial
`channels in a multiple-input multiple-output (MIMO) com-
`munication system. The TDM, OFDM,and MIMOsystems
`are described in further detail below.
`
`[0005] The multiple transmission channels in the multi-
`channel communication system may experience different
`channel conditions (e.g., different fading, multipath, and
`interference effects) and may achieve different signal-to-
`noise-and-interference ratios (SNRs). The SNR ofa trans-
`mission channel determines its transmission capability,
`which is typically quantified by a particular data rate that
`may bereliably transmitted on the transmission channel.If
`the SNR varies from transmission channel to transmission
`channel, then the supported data rate would also vary from
`channel to channel. Moreover, since the channel conditions
`typically vary with time,
`the data rates supported by the
`transmission channels would also vary with time.
`
`[0006] A major challenge in a coded communication sys-
`tem is selecting the appropriate transmission mode(s) to use
`for data transmission based on the channel conditions. As
`used herein, a “transmission mode”mayindicate a particular
`data rate or informationbit rate, a particular coding scheme,
`a particular modulation scheme, or a combination thereof, to
`use for a given data transmission. The goal of the transmis-
`sion mode selection should be to maximize throughput on
`the multiple transmission channels while meeting certain
`quality objectives, which may be quantificd by a particular
`packet error rate (PER).
`
`[0007] One straightforward technique is to select a par-
`ticular transmission mode for each of the multiple transmis-
`sion channels based onits SNR (i.c., the transmission mode
`selection is done on a per transmission channelbasis to “bit
`load” each transmission channel according to its SNR). The
`data for cach transmission channel would then be sent at the
`data rate and with the coding and modulation schemes
`associated with the transmission mode selected for that
`
`this technique has some
`transmission channel. However,
`major drawbacks. 'irst, coding individually for each trans-
`mission channelcan significantly increase the complexity of
`the data processing at both a transmitter and a receiver.
`Second, coding individually for each transmission channel
`may greatly increase coding and decoding delay. Third, a
`
`high feedback rate may be needed to send back information
`(e.g., the SNR or transmission mode) for each transmission
`channel, which is needed by the transmitter to code and
`modulate data on a channel-by-channelbasis.
`
`[0008] Another technique is to use a commontransmission
`mode for all transmission channels. This technique avoids
`the major drawbacksofthe bit-loading technique. However,
`if a data transmission is sent on multiple transmission
`channels with varying SNRs,
`then the SNR would vary
`correspondingly across the received data transmission. The
`challenge is then to select the proper transmission mode to
`usc for the data transmission in light of the varying SNRs,
`so that the data transmission can be reliably received. If the
`data rate for the selected transmission modeis too high, then
`the entire data transmission would be received in error.
`Conversely, if the data rate for the selected transmission
`mode is too low,
`then the transmission capacity of the
`multiple transmission channels is under utilized.
`
`[0009] There is therefore a need in the art for techniques
`to determine a suitable transmission mode for data trans-
`mission on multiple transmission channels having varying
`SNRs.
`
`SUMMARY
`
`[0010] Techniques are provided hereinto select the proper
`transmission modefor a data transmission in a multi-channel
`communication system with multiple transmission channels
`having varying SNRs. A suitable transmission mode may be
`determined for each data stream to be independently pro-
`cessed (e.g., coded and madulated) and transmitted on a
`designated group of transmission channels.
`
`In one specific method of determining a suitable
`[0011]
`transmission mode for a data stream sent on multiple trans-
`mission channels, an SNR estimate (for example, in units of
`dB)is initially obtained for each of the multiple transmission
`channels used to transmit that data stream. An average SNR,
`Yavy 1s then computed for the SNR estimates for the multiple
`transmission channels as
`
`1%
`Yavg = Neos Ye
`
`is the SNR estimate for transmission
`(0012] where y;
`channel i and Ng,is the number of transmission channels
`used for the data transmission. ‘The variance of the SNR
`estimates, oy", is also computed as
`
`
`
`for
`is then determined,
`[0013] A back-off factor, y,,,
`example, based on a function K(y,.,,,0,) of the average SNR
`and the SNRvariance. For example, the function F(y*,o,7)=
`K,-0,? maybe used for the back-off factor, where K, is a
`scaling factor that may be selected based on one or more
`characteristics of the system, such as, for example,
`the
`interleaving, packet size, and/or type of coding scheme used
`
`Page 13 of 28
`
`Page 13 of 28
`
`

`

`US 2004/0184398 Al
`
`Sep. 23, 2004
`
`for the data stream. An operating SNR,y,,,, for the trans-
`mission channels is next computed based on the average
`SNRand the back-off factor as y,.=Yave7Yuo: The transmis-
`sion modefor the data stream is then selected based on the
`operating SNR,
`for cxamplc, using a look-up table of
`supported transmission modes and their required SNRs. The
`selected transmission mode isutilized forall of the multiple
`transmission channels used to transmit the data stream.
`
`[0014] A set of transmission modes may be supported by
`the system, and the minimum SNR required to achieve a
`particular level of performance (e.g., 1% PER) may be
`determined for each supported transmission mode based on
`an additive white Gaussian noise (AWGN)channel with no
`fading. The transmission modeselected for the data stream
`is the supported transmission modewith the highest data rate
`and a required SNRthatis less than or equal to the operating
`SNR. The method may be used for any system with multiple
`transmission channels, such as TDM, OFDM, MIMO, and
`MIMO-OFDMsystems(all of which are described below).
`
`selection techniques
`transmission mode
`[0015] The
`described herein may be used for various transmission
`schemes in a MIMO-OFDMsystem. For an all-antenna
`transmission scheme, one data stream is transmitted on all
`subbandsofall transmit antennas. The transmission mode
`for this data stream maybe selected based on SNRestimates
`for all subbandsofall transmit antennas. For a per-antenna
`transmission scheme, one data stream is transmitted on all
`subbands of cach transmit antenna. The transmission mode
`for each data stream may be selected based on SNResti-
`mates for all subbands of the transmit antenna used for that
`
`data stream. For a per-eigenmode transmission scheme, one
`data stream is transmitted on all subbands of each wideband
`eigenmode (described below). The transmission mode for
`each data stream may be selected based on SNRestimates
`for all subbands of the wideband eigenmode used for that
`data stream.
`
`[0016] Various aspects and embodiments of the invention
`are described in further detail below.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0017] The features, nature, and advantages of the present
`invention will become more apparent from the detailed
`description set forth below when taken in conjunction with
`the drawings in which like reference characters identify
`correspondingly throughout and wherein:
`
`[0018] FIG. 1 shows a transmitter and a receiver in a
`multi-channel communication system;
`
`[0019]
`
`FIG. 2 showsa transmission mode selector;
`
`[0020] FIG. 3 showsa process to determine the transmis-
`sion modefor a data stream sent on a group of transmission
`channels with varying SNRs;
`
`[0021] FIG. 4 shows the SNR of an OFDM system with
`frequency selective fading;
`
`[0022] FIGS. 5A and 5B show the SNRsof N., transmit
`antennas in a MIMO-OFDM system and the SNR of an
`equivalent system, respectively;
`
`[0023] FIG. 6 shows the average SNRsof the N; transmit
`antennas;
`
`[0024] FIG. 7 shows a base station and a terminal in a
`MIMO-OFDMsystem;
`
`[0025] FIGS. 8A and 8B showa transmitter subsystem
`and a transmitter unit within the transmitter subsystem,
`respectively;
`
`[0026] FIGS. 9A and 8B showa receiver subsystem and
`a receiver unit within the receiver subsystem, respectively;
`and
`
`[0027] FIG. 10 shows a receiver subsystem that performs
`successive interference cancellation receiver processing.
`
`DETAILED DESCRIPTION
`
`[0028] The word “exemplary” is used herein to mean
`“serving aS an example,
`instance, or illustration.” Any
`embodiment or design described herein as “exemplary” is
`not necessarily to be construed as preferred or advantageous
`over other embodiments or designs.
`
`selection techniques
`transmission mode
`[0029] The
`described herein may be used for various types of multi-
`channel communication system having multiple transmis-
`sion channels that may be used for data transmission. For
`example, these techniques may be used for TDM systems,
`OFDM-based systems, MIMO systems, MIMO systemsthat
`utilize OFDM (i.e., MIMO-OFDMsystems), and so on.
`
`[0030] A TDM system may transmit data in frames, each
`of which may be of a particular time duration. Each frame
`may include multiple (N_,) time slots that may be assigned
`different indices. N.-< transmission channels may be formed
`for the N.< time slots in each frame.
`
`[0031] An OFDM system effectivelypartitions the overall
`system bandwidth into multiple (N,) orthogonal subbands,
`which mayalso be referred to as tones, bins, and frequency
`channels. Each subband is associated with a respective
`carrier that may be modulated with data. N,, transmission
`channels may be formed for the N, subbands.
`
`[0032] A MIMOsystem employs multiple (N.,) transmit
`antennas and multiple (N,) receive antennas for data trans-
`mission, and is denoted as an (N;, Ng) system. A MIMO
`channel formed by the N, transmit and N, receive antennas
`may be decomposed into N, independent channels, with
`Ns=min{Ny, Ng}. Each of the Ng independent channels
`mayalso be referred to as a spatial channel or an eigenmode
`of the MIMO channel. The numberof spatial channels is
`determined by a channel response matrix H that describes
`the response between the N; transmit and Nz receive anten-
`nas. For simplicity, the following description assumes that
`the channel response matrix H is full rank, in which case the
`number of spatial channels is given as No=N.,.2Nx. Ni.
`transmission channels may be formed for the N. spatial
`channels.
`
`[0033] A MIMO-OFDM system has N.. spatial channels
`for each of N, subbands. A transmission channel may be
`formed for each spatial channel of each subband. N,N
`transmission channels would then be available for data
`
`transmission between the N; transmit antennas and Np
`receive antennas.
`
`In general, multiple transmission channels may be
`[0034]
`formed in various manners, some examples of which are
`described above. Each transmission channel is associated
`
`Page 14 of 28
`
`Page 14 of 28
`
`

`

`US 2004/0184398 Al
`
`Sep. 23, 2004
`
`with a received SNRthat is indicative of the transmission
`capability of that channel. The received SNR of a given
`transmission channel may be estimated by a receiver, for
`example, based on a pilot sent on that transmission channel.
`
`[0035] FIG. 1 shows a block diagramof a transmitter 110
`and a receiver 150 in a multi-channel communication system
`100. At transmitter 110,traffic data is provided from a data
`source 112 to a transmit (TX) data processor 114, which
`demultiplexes the traffic data into N,, data streams, where
`Np=1. Each data stream may be independently processed
`and transmitted on a respective group of transmission chan-
`nels, Each data stream is associated with a particular trans-
`mission modethat indicates a set of parameter values [orthat
`data stream. For example, the transmission mode for each
`data stream may indicate (or may be associated with) a
`particular data rate or information bit rate, a particular
`coding scheme or code rate,
`a particular
`interleaving
`scheme, a particular modulation scheme, and so on, to use
`for that data stream. For a given transmission mode, the data
`rate may be determined by the coding scheme and the
`modulation scheme associated with that transmission mode.
`An exemplary set of transmission modesis given in Table 1.
`For each data stream, the data rate is determined by a data
`rate control, the coding scheme is determined by a coding
`control, and the modulation scheme is determined by a
`modulation control. These controls are provided by a con-
`troller 130 and are gencrated based on feedback information
`received from receiver 150.
`
`[0036] For each data stream, TX data processor 114 codes,
`interleaves, and modulates the data in accordance with the
`coding, interleaving, and modulation schemes selected for
`that data stream to provide a corresponding stream of
`modulation symbols. TX data processor 114 provides N,
`modulation symbol streams for the N, data streams.
`
`[0037] A transmitter unit (TMTR) 116 then receives and
`processes the N,, modulation symbol streams in a manner
`specificd by the system. For cxamplc, transmittcr unit 116
`may perform OFDM processing for an OFDM system,
`spatial processing for a MIMOsystem,or both spatial and
`OFDMprocessing for a MIMO-OFDMsystem.A pilot may
`also be transmitted to receiver 150 to assist it perform a
`numberof functions such as channel estimation, acquisition,
`frequency and timing synchronization, coherent demodula-
`tion, and so on. In this case, transmitter unit 116 may receive
`and multiplex pilot symbols with the modulation symbols.
`Transmitter unit 116 provides a modulated signal for each
`antenna used for data transmission.
`
`[0038] Each modulated signal is then transmitted from a
`respective transmit antenna over a wireless communication
`link to receiver 150. ‘The communication link distorts the
`modulated signals with a particular channel response and
`further degrades the modulated signals with (1) additive
`white Gaussian noise (AWGN)having a variance of N, and
`(2) possibly interference from other transmission sources.
`
`are
`transmitted signals
`the
`receiver 150,
`[0039] At
`received by each receive antenna, and the received signal
`from each antenna is provided to a receiver unit (RCVR)
`160. Receiver unit 160 conditions and digitizes each
`received signal
`to provide a corresponding stream of
`samples. Receiver unit 160 further processes the samples in
`a manner that
`is complementary to that performed by
`transmitter unit 116 to provide N, streams of “recovered”
`
`symbols, which are estimates of the N,, streams of modu-
`lation symbols sent by transmitter 110. The recovered sym-
`bol streams are then provided to a
`receive (RX) data
`processor 162 and processed to obtain decoded data for the
`transmitted data streams. The processing by RX data pro-
`cessor 162 may include demodulation (Le., symbol demap-
`ping), deinterleaving, and decoding. RX data processor 162
`may further provide the status of each received data packet.
`
`[0040] Receiver unit 160 mayalso provide “received”
`symbols (i.e., symbols after OFDM processing butprior to
`spatial processing by receiver unit 160) and/or recovered
`symbols to a channcl estimator 164, Channcl cstimator 164
`may then process these symbols to obtain an SNR estimate
`for each transmission channel used for data transmission.
`The SNRestimates are typically obtained based on received
`pilot symbols, but may also be obtained based on received
`data symbols or a combination of received pilot and data
`symbols. A transmission mode selector 166 receives the
`SNRestimates from channel estimator 164 and determines
`a suitable transmission mode for each of the N, data
`streams.
`
`transmission
`[0041] A controller 170 receives the N,,
`modes from transmission mode selector 166 and the packet
`status from RX data processor 162 and assembles feedback
`information for transmitter 110. The feedback information
`
`may include the N,, transmission modes for the Nj, data
`streams, acknowledgments (ACKs) and negative acknowl-
`edgments (NAKs) for received data packets, and/or other
`information. The feedback information is then sent to trans-
`mitter 110 and used to adjust the processing of the N,, data
`streams sent to receiver 150. For example, transmitter 110
`may use the feedback information to adjust the data rate, the
`coding scheme, the modulation scheme, or any combination
`thereof, for cach data stream sent
`to receiver 150. The
`feedback informationis used to increase the efficiency of the
`system by allowing data to be transmitted at the best-known
`settings supported by the communication link.
`
`In the embodiment shown in FIG. 1, the transmis-
`[0042]
`sion mode selection is performed by receiver 150 and the
`selected transmission modefor each data stream is sent back
`to transmitter 110. In other embodiments, the transmission
`mode selection may be performed by (1) the transmitter
`based on feedback information provided by the receiver
`and/or other information obtained by the transmitter or (2)
`jointly by both the transmitter and receiver.
`
`[0043] An AWGN communication link (e.g., an AWGN
`channel) is characterized by a frequency response thatisflat
`across the transmission channels. For an AWGN channel,
`the transmission channels achieve similar received SNRs. If
`
`a data packet is transmitted on a group of transmission
`channels with similar received SNRs, then the SNR would
`be approximately constant across the entire data packet. For
`“constant SNR” data packets,
`the relationship between
`required SNR and data rate for a particular level of pertor-
`mance is well known in the art. The desired level of
`performance may be quantified by a particular packet error
`rate (PER), frame error rate (FER), block error rate (BLER),
`bit error rate (BER), or some other measure. A suitable
`transmission mode may readily be selected based on the
`received SNR of the AWGNtransmission channels.
`
`[0044] However, as noted above, the multiple transmis-
`sion channels may experience different channel conditions
`
`Page 15 of 28
`
`Page 15 of 28
`
`

`

`US 2004/0184398 Al
`
`Sep. 23, 2004
`
`and achieve different received SNRs. If a data packet is
`transmitted on a group of transmission channels with dif-
`ferent received SNRs,then the SNR would vary correspond-
`ingly across the received data packet.
`‘This problem of
`“varying SNR” packet is exacerbated for a wideband com-
`munication system and for a “multipath” channel with
`frequency selective fading (i.c., a response that is not flat
`across the transmission channels). The techniques described
`herein address a major challenge for a coded multi-channel
`communication system, which is to determine the maximum
`data rate that may be used for each data stream sent on a
`group of transmission channels with varying SNRs for a
`particular desired level of performance.
`
`[0045] FIG. 2 shows a block diagram of an embodiment
`of transmission mode selector 166, which can determine a
`suitable transmission mode for each of the N,, data streams.
`Each data stream is transmitted on a respective group of
`transmission channels. For simplicity, transmission mode
`selection for one data stream is described below. For the
`following description, SNRs are given in units of decibels
`(dB).
`
`[0046] Within transmission mode selector 166, an SNR
`statistics computation unit 210 receives SNRestimates for
`the group of transmission channels used for the data stream.
`The SNR estimate for a given transmission channel may be
`expressed as:
`
`Isi?
`
`= theaef fori=l, ....Nc, (dB)
`
`Eg (1)
`
`[0047] where i is an index for the transmission channels
`used for the data stream;
`
`[0048] Ng is the number of transmission channels
`used for the data stream;
`
`[0049]
`nel i;
`
`s, 1s the complex gain for transmission chan-
`
`Nois the noise variance for transmission chan-
`[0050]
`nel i; and
`
`[0051]
`neli.
`
`y; is the SNR estimate for transmission chan-
`
`yy... is the average SNR for the N, transmis-
`[0056]
`sion channels used for the data stream.
`
`[0057] The unbiased variance of the SNR estimates may
`be computed as follows:
`
`
`
`Ne
`
`oF =
`
`(i - Yave)s
`
`Eq GB)
`
`[0058] where o,” is the SNR variance.
`[0059] Acomputation unit 212 then uses the average SNR
`and the SNR variance to compute an operating SNR for the
`group of transmission channels used forthe data stream. ‘he
`operating SNR may be computed as follows:
`Yor=Vavg7Ybo> (dB)
`[0060] where y,, is a back-off factor; and
`
`Eq (4)
`
`[0061] Yor, is the operating SNR for the group of
`transmission channels.
`
`[0062] The back-off factor is used to account for fre-
`quencyselectivity of the communication link(i.e., a non-flat
`frequency spectrum that results in variation in the received
`SNRs). The back-off factor may be a function ofthe average
`SNR, the SNR variance, and possibly other factors. The
`back-off factor is described in further detail below.
`
`[0063] The system may be designed to support a set of
`transmission modes. Each supported transmission mode is
`associated with a particular minimum SNR required to
`achieve the desired level of performance, which is deter-
`mined as described below.
`
`[0064] Alook-up table 214 maybe usedto store the set of
`supported transmission modes and the required SNR for
`each of these transmission modes. The operating SNR for
`the group of transmission channels used for the data stream
`is provided to look-up table 214, which then provides the
`transmission mode for the data stream. This transmission
`
`mode is the supported transmission mode with the highest
`data rate and a required SNRthat is less than or equal to the
`operating SNR. Look-up table 214 thus selects the highest
`possible data rate for the data stream based on the operating
`SNR.
`
`[0052] The derivation of SNR estimates for several types
`of multi-channel communication systems
`is described
`below. Unit 210 computes the average SNR and the unbi-
`ased variance of the SNR estimates.
`
`[0053]
`
`‘The average SNR may be computed as follows:
`
`[0065] Table 1 lists an exemplaryset of 14 transmission
`modes supported by the system, which are identified by
`transmission mode indices 0 through 13. Each transmission
`mode is associated with a particular spectral efficiency, a
`particular coderate, a particular modulation scheme,and the
`minimum SNR required to achieve 1% PER for a non-
`fading, AWGNchannel. The spectral efficiencyrefers to the
`data rate (i.e., the information bit rate) normalized bythe
`system bandwidth, and is given in units of bits per second
`Hg (2)
`1 Ne
`avg = =~DVir GB)
`per Hertz (bps/Hz). The code rate and modulation scheme
`Yavg Neou (dB)
`for each transmission mode in Table 1 are specific to the
`exemplary system design. The transmission mode having
`index0is for a null data rate (.e., no data transmission). For
`cach transmission mode with a non-zcro data rate,
`the
`[0054] where i is an index for the transmission channels
`required SNR is obtained based on the specific system
`used for the data stream;
`design (i.e., the particular code rate, interleaving scheme,
`modulation scheme, and so on, used by the system for that
`transmission mode) and for an AWGNchannel. The required
`
`y, is the SNR estimate for transmission chan-
`[0055]
`nel i; and
`
`Page 16 of 28
`
`Page 16 of 28
`
`

`

`US 2004/0184398 Al
`
`Sep. 23, 2004
`
`SNR maybe obtained by computation, computer simulation,
`empirical measurements, and so on, as is knownintheart.
`
`TABLE1
`
`which is an accurate estimate for an AWGNchannel and a
`not so accurate estimate for a multipath channel. The aver-
`age SNR may be usedto select the transmission modefor the
`data stream sent on these transmission channels.
`‘lhe
`selected transmission mode represents a prediction of the
`Required
`Spectral
`Transmission
`data rate that can be supported by the group of transmission
`
`
`
`Mode Code—ModulationEfficiency SNR
`channels for the desired PER. However, as with any pre-
`Index
`(bps/Hz)
`Rate
`Scheme
`(dB)
`diction scheme, there will inevitably be prediction errors. In
`0
`0.0
`_ _
`_—
`order to ensure that the desired PER can be achieved, a
`1
`0.25
`1/4
`BPSK
`-1.8
`2
`0.5
`1/2
`BPSK
`1.2
`back-off factor may be used. Since the back-off reduces the
`3
`1.0
`1/2
`QPSK
`4.2
`throughputof the system,it is desirable to keep this back-off
`4
`15
`3/4
`QPSK
`6.8
`as small as possible while still ensuring that the desired PER
`5
`2.0
`1/2
`16 QAM
`10.1
`can be achieved.
`6
`25
`5/8
`16 QAM
`11.7
`7
`3.0
`3/4
`16 QAM
`13.2
`8
`3.5
`WAZ
`64 QAM
`16.2
`9
`4.0
`2/3
`64 QAM
`17.4
`10
`45
`3/4
`64 QAM
`18.8
`d1
`5.0
`5/6
`64 QAM
`20.0
`12
`6.0
`3/4
`256 QAM
`24.2
`13
`7.0
`718
`256 QAM
`26.3
`
`[0066] FIG. 3 shows a flow diagram of a process 300 to
`determine the transmission mode for a data stream sent on
`a group of transmission channels with varying SNRs. Ini-
`tially, an SNR estimate for each transmission channel used
`for the data stream is obtained (e.g., based on pilot symbols
`received on the transmission channel) (step 312). The SNR
`estimates for the transmission channels are given in units of
`dB. The average SNRfor the group of transmission channels
`is then computed, as shown in equation (2) (step 314). The
`unbiased variance of the SNR estimates for the transmission
`channels is also computed, as shown in equation (3) (step
`316). A back-off factor is then determined(e.g., based on the
`average SNR, the SNR variance, and/or other factors) (step
`318). The operating SNR for the group of transmission
`channels is then computed based on the average SNR and
`the back-off factor, as shown in equation (4) (step 320).
`[0067] A transmission mode is then determined for the
`data stream based on the operating SNR (step 322). A
`look-up table of supported transmission modes