US007885228B2
`
`(12) United States Patent
`Walton et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,885,228 B2
`Feb. 8, 2011
`
`(54) TRANSMISSION MODE SELECTION FOR
`DATA TRANSMISSION INA
`MULT-CHANNEL COMMUNICATION
`SYSTEM
`
`(75) Inventors: Jay Rod Walton, Carlisle, MA (US);
`Irina Medvedev, Somerville, MA (US)
`
`(73) Assignee: Qualcomm Incorporated, San Diego,
`CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1451 days.
`
`(*) Notice:
`
`(21) Appl. No.: 10/394,529
`(22) Filed:
`Mar 20, 2003
`(65)
`Prior Publication Data
`US 2004/O184398 A1
`Sep. 23, 2004
`
`(51) Int. Cl.
`(2009.01)
`H0474/00
`(2006.01)
`H04B I7/00
`(2006.01)
`H04B 7/85
`(2006.01)
`H04M I/00
`(2006.01)
`H04K L/10
`(52) U.S. Cl. .................... 370/329; 370/341; 455/226.3:
`455/13.3:455/562.1375/260
`(58) Field of Classification Search ................. 370/332,
`370/329, 334,437, 341; 455/226.3, 13.3,
`455/277.2, 693, 63.1, 562. 1: 375/260
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6,144.711 A * 1 1/2000 Raleigh et al. .............. 375/347
`6.426,971 B1
`7, 2002 Wu et al.
`6.463,290 B1 * 10/2002 Stilp et al. ............... 455,456.1
`6,478,422 B1
`1 1/2002 Hansen
`6,785,341 B2
`8, 2004 Walton et al.
`
`6,821,535 B2 11/2004 Nurmi et al.
`6,927,728 B2 * 8/2005 Vook et al. .................. 342/.377
`
`(Continued)
`FOREIGN PATENT DOCUMENTS
`
`WO
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`
`10, 1997
`
`(Continued)
`OTHER PUBLICATIONS
`International Search Report-PCT/US04/008665. International
`Search Authority-European Patent Office-Oct. 6, 2004.
`(Continued)
`Primary Examiner Rafael Pérez-Gutiérrez
`Assistant Examiner—Allahyar Kasraian
`(74) Attorney, Agent, or Firm Turocy & Watson, LLP
`
`(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 transmission channels.
`A back-off factor is determined, for example, based on the
`SNR variance and a scaling factor. An operating SNR for the
`transmission channels is next computed based on the average
`SNR and the back-off factor. The transmission mode is then
`selected for 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 may be used for any system with multiple trans
`mission channels having varying SNRS.
`
`20 Claims, 11 Drawing Sheets
`
`300
`
`Obtain SNR estimate for 32
`each transmission chaniel
`used for the data stream
`
`Compute average SNR
`
`Compute SNR variance
`
`Determineback-off factor
`(e.g., based on the
`average SNR and/or
`the SNR wariance)
`
`318
`
`Compute operating SNR 320
`based on the average SNR
`and the back-off factor
`
`etermine transmission
`mode for the data streat
`based on the operating SNR
`
`
`
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`US 7,885,228 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
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`2/2006 Ling et al.
`7,006,848 B2
`5/2006 Friedman .................... 375,297
`7,039,125 B2 *
`7,058,367 B1 * 6/2006 Luo et al. ................... 455,101
`7,095,709 B2
`8/2006 Walton et al.
`7,116,652 B2 * 10/2006 Lozano ....................... 370,334
`7,151,809 B2 12/2006 Ketchum et al.
`7,184,713 B2
`2/2007 Kadous et al.
`7,191,381 B2 * 3/2007 Gesbert et al. .............. 714/759
`7,324.429 B2 *
`1/2008 Walton et al. .....
`... 370,203 W
`7,636,573 B2 * 12/2009 Walton et al. ............... 455,454
`2002/0027951 A1* 3/2002 Gormley et al.............. 375,224
`2002/007583.0 A1* 6/2002 Hartman, Jr. ...
`... 370,333
`2002/O127978 A1* 9, 2002 Khatri ........................ 45.5/10.
`2002fO154705 A1 10, 2002 Walton et al.
`2002/0163974 A1* 11/2002 Friedman .................... 375,295
`2003/O125040 A1* 7/2003 Walton et al. ............... 455,454
`2003/0236080 A1* 12/2003 Kadous et al. ........... 455,226,
`2004/0082356 A1
`4/2004 Walton et al.
`2004/O120411 A1* 6/2004 Walton et al. ............... 375,260
`
`2004/0136349 A1* 7/2004 Walton et al. ............... 370,338
`2004/0252632 A1* 12/2004 Bourdoux et al.
`370/210
`2008/006905 A1
`3/2008 Walton et al. ............... 370,280
`2008/0267098 A1* 10, 2008 Walton et al. ............... 370,280
`2008/0267138 A1* 10, 2008 Walton et al. ...
`370,336
`2010.0067401 A1
`3/2010 Medvedev et al. .......... 370,253
`2010/01 19001 A1* 5, 2010 Walton et al. ............... 375,260
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`WO S. A1 HS
`
`OTHER PUBLICATIONS
`Written Opinion-PCTUS04/008665. International Search Author
`E. East. 2.
`lian Application Serial
`styles Nis
`or Australian Application Seria
`Office Action dateR 27, 2008 for Chinese Application Serial No
`2008003076 sages.
`.O., 5 pages.
`* cited by examiner
`
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`

`

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`U.S. Patent
`
`Feb.8, 2011
`
`Sheet 1 of 11
`
`US 7,885,228 B2
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`UOISSILUSUBI|
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`

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`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 2 of 11
`
`US 7,885,228 B2
`
`300
`
`Obtain SNR estimate for
`each transmission channel
`used for the data stream
`
`312
`
`Compute average SNR
`
`3f4
`
`Compute SNR variance
`
`316
`
`
`
`Determine back-off factor
`(e.g., based on the
`average SNR and/or
`the SNR variance)
`
`38
`
`Compute operating SNR
`based on the average SNR 320
`and the back-Off factor
`
`Determine transmission
`mOde for the data Stream
`based on the operating SNR
`
`322
`
`End
`
`FIG. 3
`
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`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 3 of 11
`
`US 7,885,228 B2
`
`5
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`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 4 of 11
`
`US 7,885,228 B2
`
`Antenna 1
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`FIG. 5A
`
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`

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`U.S. Patent
`U.S. Patent
`
`Feb.8, 2011
`
`Sheet 5 of 11
`
`US 7,885,228 B2
`US 7,885,228 B2
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`IPR2018-01477
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`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 6 of 11
`
`US 7,885,228 B2
`
`Antenna 1
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`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 7 of 11
`
`US 7,885,228 B2
`
`09Z 394
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`U.S. Patent
`
`Feb.8, 2011
`
`Sheet 8 of 11
`
`US 7,885,228 B2
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`IPR2018-01477
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`IPR2018-01477
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`
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`

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`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 9 of 11
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`US 7,885.228 B2
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`
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`

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`IPR2018-01477
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`

`U.S. Patent
`
`Feb. 8, 2011
`
`Sheet 11 of 11
`
`US 7,885,228 B2
`
`Received
`Symbol
`Streams
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`754a
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`
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`US 7,885,228 B2
`
`1.
`TRANSMISSION MODE SELECTION FOR
`DATA TRANSMISSION INA
`MULT-CHANNEL COMMUNICATION
`SYSTEM
`
`BACKGROUND
`
`1. Field
`The present invention relates generally to data communi
`cation, and more specifically to techniques for selecting a
`Suitable transmission mode for a data transmission in a multi
`channel communication system.
`2. Background
`A multi-channel communication system utilizes multiple
`“transmission' channels for data transmission. These trans
`mission channels may be formed in the time domain, fre
`quency domain, spatial domain, or a combination thereof. For
`example, the multiple transmission channels may correspond
`to different time slots in a time division multiplex (TDM)
`communication system, different frequency Subbands in an
`orthogonal frequency division multiplex (OFDM) communi
`cation system, or different spatial channels in a multiple-input
`multiple-output (MIMO) communication system. The TDM,
`OFDM, and MIMO systems are described in further detail
`below.
`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-inter
`ference ratios (SNRs). The SNR of a transmission channel
`determines its transmission capability, which is typically
`quantified by a particular data rate that may be reliably trans
`mitted on the transmission channel. If the SNR varies from
`transmission channel to transmission channel, then the Sup
`ported 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.
`A major challenge in a coded communication system is
`selecting the appropriate transmission mode(s) to use for data
`transmission based on the channel conditions. As used herein,
`a “transmission mode' may indicate a particular data rate or
`information bit rate, a particular coding scheme, a particular
`modulation scheme, or a combination thereof, to use for a
`given data transmission. The goal of the transmission mode
`selection should be to maximize throughput on the multiple
`transmission channels while meeting certain quality objec
`tives, which may be quantified by a particular packet error
`rate (PER).
`One straightforward technique is to select a particular
`transmission mode for each of the multiple transmission
`channels based on its SNR (i.e., the transmission mode selec
`tion is done on a per transmission channel basis to “bit load
`each transmission channel according to its SNR). The data for
`each 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 transmission channel.
`However, this technique has some major drawbacks. First,
`coding individually for each transmission channel can sig
`nificantly 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 transmis
`sion mode) for each transmission channel, which is needed by
`the transmitter to code and modulate data on a channel-by
`channel basis.
`
`2
`Another technique is to use a common transmission mode
`for all transmission channels. This technique avoids the major
`drawbacks of the 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 use for the data trans
`mission in light of the varying SNRs, so that the data trans
`mission can be reliably received. If the data rate for the
`selected transmission mode is 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 chan
`nels is under utilized.
`There is therefore a need in the art for techniques to deter
`mine a Suitable transmission mode for data transmission on
`multiple transmission channels having varying SNRs.
`
`SUMMARY
`
`Techniques are provided herein to select the proper trans
`mission mode for 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 modulated) and transmitted on a
`designated group of transmission channels.
`In one specific method of determining a suitable transmis
`sion mode for a data stream sent on multiple transmission
`channels, an SNR estimate (for example, in units of dB) is
`initially obtained for each of the multiple transmission chan
`nels used to transmit that data stream. An average SNR, Y,
`is then computed for the SNR estimates for the multiple
`transmission channels as
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
`
`where Y, is the SNR estimate for transmission channel i and
`N is the number of transmission channels used for the data
`transmission. The variance of the SNR estimates, O,. is also
`computed as
`
`45
`
`50
`
`55
`
`60
`
`65
`
`C
`1
`2.
`O =
`9
`NC - 1 2. (Yi - Yag)
`
`Aback-off factor, Y, is then determined, for example, based
`on a function F(Y.O.) of the average SNR and the SNR
`variance. For example, the function F(YO,)-KO, may
`be 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 for the data stream. An
`operating SNR, Y, for the transmission channels is next
`computed based on the average SNR and the back-off factor
`as Y Y-Y. The transmission mode for the data stream is
`then selected based on the operating SNR, for example, using
`a look-up table of Supported transmission modes and their
`required SNRs. The selected transmission mode is utilized for
`all of the multiple transmission channels used to transmit the
`data stream.
`
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`US 7,885,228 B2
`
`3
`A set of transmission modes may be Supported by the
`system, and the minimum SNR required to achieve a particu
`lar 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 mode selected for the data stream is the Sup
`ported transmission mode with the highest data rate and a
`required SNR that is less than or equal to the operating SNR.
`The method may be used for any system with multiple trans
`mission channels, such as TDM, OFDM, MIMO, and
`MIMO-OFDM systems (all of which are described below).
`The transmission mode selection techniques described
`herein may be used for various transmission schemes in a
`MIMO-OFDM system. For an all-antenna transmission
`scheme, one data stream is transmitted on all Subbands of all
`transmitantennas. The transmission mode for this data stream
`may be selected based on SNR estimates for all subbands of
`all transmit antennas. For aper-antenna transmission Scheme,
`one data stream is transmitted on all Subbands of each trans
`mitantenna. The transmission mode for each data stream may
`be selected based on SNR estimates for all subbands of the
`transmit antenna used for that data stream. For a per-eigen
`mode 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 SNR estimates for all subbands of the wideband
`eigenmode used for that data stream.
`Various aspects and embodiments of the invention are
`described in further detail below.
`
`10
`
`15
`
`25
`
`4
`The transmission mode selection techniques described
`herein may be used for various types of multi-channel com
`munication system having multiple transmission channels
`that may be used for data transmission. For example, these
`techniques may be used for TDM systems, OFDM-based
`systems, MIMO systems, MIMO systems that utilize OFDM
`(i.e., MIMO-OFDM systems), and so on.
`ATDM 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 indi
`ces. Ns transmission channels may be formed for the Ns
`time slots in each frame.
`An OFDM system effectively partitions the overall system
`bandwidth into multiple (N) orthogonal subbands, which
`may also be referred to astones, 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.
`A MIMO system employs multiple (N) transmit antennas
`and multiple (N) receive antennas for data transmission, and
`is denoted as an (NN) system. A MIMO channel formed
`by the N7 transmit and N receive antennas may be decom
`posed into Ns independent channels, with Nasmin{N, N}.
`Each of the Ns independent channels may also be referred to
`as a spatial channel or an eigenmode of the MIMO channel.
`The number of spatial channels is determined by a channel
`response matrix H that describes the response between the N.
`transmit and N receive antennas. For simplicity, the follow
`ing description assumes that the channel response matrix His
`full rank, in which case the number of spatial channels is
`given as Ns. NsN. N. transmission channels may be
`formed for the N. spatial channels.
`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 N receive antennas.
`In general, multiple transmission channels may be formed
`in various manners, some examples of which are described
`above. Each transmission channel is associated with a
`received SNR that 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.
`FIG. 1 shows a block diagram of 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 demulti
`plexes the traffic data into N, data streams, where N21.
`Each data stream may be independently processed and trans
`mitted on a respective group of transmission channels. Each
`data stream is associated with a particular transmission mode
`that indicates a set of parameter values for that 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
`modes is 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 pro
`vided by a controller 130 and are generated based on feedback
`information received from receiver 150.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
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`The features, nature, and advantages of the present inven
`tion 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:
`FIG.1 shows a transmitter and a receiverinamulti-channel
`communication system;
`FIG. 2 shows a transmission mode selector;
`FIG.3 shows a process to determine the transmission mode
`for a data stream sent on a group of transmission channels
`with varying SNRs:
`FIG. 4 shows the SNR of an OFDM system with frequency
`selective fading:
`FIGS.5A and 5B show the SNRs of N transmit antennas
`in a MIMO-OFDM system and the SNR of an equivalent
`system, respectively;
`FIG. 6 shows the average SNRs of the N transmit anten
`nas,
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`FIG. 7 shows a base station and a terminal in a MIMO
`OFDM system:
`FIGS. 8A and 8B show a transmitter subsystem and a
`transmitter unit within the transmitter Subsystem, respec
`tively;
`FIGS. 9A and 8B show a receiver subsystem and a receiver
`unit within the receiver subsystem, respectively; and
`FIG. 10 shows a receiver subsystem that performs succes
`sive interference cancellation receiver processing.
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`DETAILED DESCRIPTION
`
`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 embodi
`ments or designs.
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`5
`For each data stream, TX data processor 114 codes, inter
`leaves, 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 sym
`bols. TX data processor 114 provides N, modulation symbol
`streams for the N data streams.
`A transmitter unit (TMTR) 116then receives and processes
`the N modulation symbol streams in a manner specified by
`the system. For example, transmitter unit 116 may perform
`OFDM processing for an OFDM system, spatial processing
`for a MIMO system, or both spatial and OFDM processing for
`a MIMO-OFDM system. A pilot may also be transmitted to
`receiver 150 to assist it perform a number of functions such as
`channel estimation, acquisition, frequency and timing Syn
`chronization, coherent demodulation, 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.
`Each modulated signal is then transmitted from a respec
`tive 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 Gaus
`sian noise (AWGN) having a variance of No and (2) possibly
`interference from other transmission sources.
`At receiver 150, the transmitted signals are 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 pro
`vide a corresponding stream of samples. Receiver unit 160
`further processes the samples in a manner that is complemen
`tary to that performed by transmitter unit 116 to provide N,
`streams of “recovered’ symbols, which are estimates of the
`N, streams of modulation symbols sent by transmitter 110.
`The recovered symbol 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 processor 162 may include demodulation (i.e., symbol
`demapping), deinterleaving, and decoding. RX data proces
`sor 162 may further provide the status of each received data
`packet.
`Receiver unit 160 may also provide “received’ symbols
`(i.e., symbols after OFDM processing but prior to spatial
`processing by receiver unit 160) and/or recovered symbols to
`a channel estimator 164. Channel estimator 164 may then
`process these symbols to obtain an SNR estimate for each
`transmission channel used for data transmission. The SNR
`estimates are typically obtained based on received pilot sym
`bols, but may also be obtained based on received data sym
`50
`bols or a combination of received pilot and data symbols. A
`transmission mode selector 166 receives the SNR estimates
`from channel estimator 164 and determines a suitable trans
`mission mode for each of the N data streams.
`A controller 170 receives the N, transmission 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 N data streams, acknowl
`edgments (ACKs) and negative acknowledgments (NAKs)
`for received data packets, and/or other information. The feed
`back information is then sent to transmitter 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 each
`data stream sent to receiver 150. The feedback information is
`
`6
`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 transmission mode
`selection is performed by receiver 150 and the selected trans
`mission mode for 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 informa
`tion obtained by the transmitter or (2) jointly by both the
`transmitter and receiver.
`An AWGN communication link (e.g., an AWGN channel)
`is characterized by a frequency response that is flat across the
`transmission channels. For an AWGN channel, the transmis
`sion channels achieve similar received SNRs. If a data packet
`is transmitted on a group of transmission channels with simi
`lar 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 performance 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 AWGN transmis
`sion channels.
`However, as noted above, the multiple transmission chan
`nels may experience different channel conditions and achieve
`different received SNRs. If a data packet is transmitted on a
`group of transmission channels with different received SNRs,
`then the SNR would vary correspondingly across the received
`data packet. This problem of “varying SNR packet is exac
`erbated for a wideband communication system and for a
`“multipath’ channel with frequency selective fading (i.e., 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.
`FIG. 2 shows a block diagram of an embodiment of trans
`mission 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).
`Within transmission mode selector 166, an SNR statistics
`computation unit 210 receives SNR estimates for the group of
`transmission channels used for the data stream. The SNR
`estimate for a given transmission channel may be expressed
`aS
`
`y = lolog() for i = 1, ... , NC, (dB)
`
`where i is an index for the transmission channels used for the
`data stream;
`N is the number of transmission channels used for the data
`Stream;
`s, is the complex gain for transmission channel i,
`No is the noise variance for transmission channel i, and
`Y, is the SNR estimate for transmission channel i.
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`7
`The derivation of SNR estimates for several types of multi
`channel communication systems is described below. Unit 210
`computes the average SNR and the unbiased variance of the
`SNR estimates.
`The average SNR may be computed as follows:
`
`Eq. (2)
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`
`where i is an index for the transmission channels used for the
`data stream;
`Y, is the SNR estimate for transmission channel i, and
`Y
`is the average SNR for the Nc transmission channels
`used for the data stream.
`The unbiased variance of the SNR estimates may be com
`puted as follows:
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`C
`1
`or = NC - 1 2. (yi-yag),
`
`Eq. (3)
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`25
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`8
`in Table 1 are specific to the exemplary system design. The
`transmission mode having index 0 is for a null data rate (i.e.,
`no data transmission). For each transmission mode with a
`non-zero data rate, the required SNR is obtained based on the
`specific system design (i.e., the particular coderate, interleav
`ing scheme, modulation scheme, and so on, used by the
`system for that transmission mode) and for an AWGN chan
`nel. The required SNR may be obtained by computation,
`computer simulation, empirical measurements, and so on, as
`is known in the art.
`
`TABLE 1
`
`Transmission
`Mode
`Index
`
`Spectral
`Efficiency
`(bps/Hz)
`
`Code Modulation
`Rate
`Scheme
`
`Required
`SNR
`(dB)
`
`O
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`
`O.O
`O.25
`O.S
`1.O
`1.5
`2.0
`2.5
`3.0
`3.5
`4.0
`4.5
`S.O
`6.O
`7.0
`
`1.f4
`1.2
`1.2
`3f4
`1.2
`5.8
`3f4
`7,12
`2.3
`3f4
`5.6
`3f4
`7/8
`
`BPSK
`BPSK
`QPSK
`QPSK
`16 QAM
`16 QAM
`16 QAM
`64 QAM
`64 QAM
`64 QAM
`64 QAM
`256 QAM
`256 QAM
`
`-1.8
`1.2
`4.2
`6.8
`10.1
`11.7
`13.2
`16.2
`17.4
`18.8
`2O.O
`24.2
`26.3
`
`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. Initially, 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 aver
`age SNR for the group of transmission channels is then com
`puted, 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).
`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 and their required SNRS may
`be consulted for step