(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2004/0184398 A1
`
`
` Walton et al. (43) Pub. Date: Sep. 23, 2004
`
`US 20040184398A1
`
`(54) TRANSMISSION MonE 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
`-
`:ZZSDIEI/Ietéflehco/lisgzlgllv$714 (US)
`’
`
`(21) APPL No.:
`
`10394529
`
`(22) Filed:
`L
`'
`
`Mar. 20’ 2003
`
`Publication Classification
`
`(51)
`
`Int. C1.7 ....................................................... H04Q 7/00
`
`(52) US. Cl.
`
`............................................ 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 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 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
`
`based on the operating SNR
`
`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
`
`Page 1 of 28
`
`SAMSUNG EXHIBIT 1024
`
`Page 1 of 28
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`SAMSUNG EXHIBIT 1024
`
`

`

`Patent Application Publication Sep. 23, 2004 Sheet 1 0f 11
`
`US 2004/0184398 A1
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`Page 2 of 28
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`

`

`Patent Application Publication Sep. 23, 2004 Sheet 2 0f 11
`
`US 2004/0184398 A1
`
`300
`
`Obtain SNR estimate for
`
`used for the data stream
`
`each transmission channel
`
`Compute average SNR
`
`7
`
`Compute SNR variance
`
`Determine back-off factor
`
`(e.g., based on the
`average SNR and/or
`the SNFi 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
`
`322
`
`End
`
`FIG. 3
`
`Page 3 of 28
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`Page 3 of 28
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`

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`Patent Application Publication Sep. 23, 2004 Sheet 3 0f 11
`
`US 2004/0184398 A1
`
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`
`Page 4 of 28
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`Page 4 of 28
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`

`

`Patent Application Publication Sep. 23, 2004 Sheet 4 0f 11
`
`US 2004/0184398 A1
`
`Antenna 1
`
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`Antenna 2
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`Page 5 of 28
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`Page 5 of 28
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`

`

`Patent Application Publication Sep. 23, 2004 Sheet 5 0f 11
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`US 2004/0184398 A1
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`Page 6 of 28
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`Page 6 of 28
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`

`

`Patent Application Publication Sep. 23, 2004 Sheet 6 0f 11
`
`US 2004/0184398 A1
`
`Antenna 1
`
`
`
`
`Average SNR
`for antenna 1
`
`
`
`6 12a
`
`Frequency
`
`Antenna 2
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`
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`SNR(dB)
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`FIG. 6
`
`yavg,2
`
`5 a 3
`
`Page 7 of 28
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`Page 7 of 28
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`

`

`Patent Application Publication Sep. 23, 2004 Sheet 7 0f 11
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`US 2004/0184398 A1
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`Patent Application Publication Sep. 23, 2004 Sheet 8 0f 11
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`US 2004/0184398 A1
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`Patent Application Publication Sep. 23, 2004 Sheet 9 0f 11
`
`US 2004/0184398 A1
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`Page 11 of 28
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`

`

`Patent Application Publication Sep. 23, 2004 Sheet 11 0f 11
`
`US 2004/0184398 A1
`
`Selected
`Recovered
`Received
`Symbol
`Symbol
`
`Streams
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`Streams
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`Demodulgtion
`1040b
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`Estimates
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`Stream 1
`
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`Stream 2
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`Demodulagtion
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`Demodulation
`
`Controls
`
`Status
`
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`
`Feedback Info
`
`(e.g., Transmission
`Modes, ACKS)
`
`FIG. 10
`
`Page 12 of 28
`
`Page 12 of 28
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`

`

`US 2004/0184398 A1
`
`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 correspond to different time slots in a time division
`multiplex (TDM) communication system, different
`fre-
`quency subbands in 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 MIMO systems
`are described in further detail below.
`
`[0005] The multiple transmission channels in the multi-
`channel communication system may experience dilferent
`channel conditions (e.g., different fading, multipath, and
`interference effects) and may achieve different signal—to—
`noise-and-interference ratios (SNRs). The SNR of a trans-
`mission channel determines its transmission capability,
`which is typically quantified by a particular data rate that
`may be reliably 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” 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 transmis-
`sion mode selection should be to maximize throughput on
`the multiple transmission channels while meeting certain
`quality objectives, which may be quantified 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 on its SNR (i.e., the transmission mode
`selection 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
`
`this technique has some
`transmission channel. However,
`major drawbacks. First, coding individually for each trans-
`mission channel can 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-channel basis.
`
`[0008] 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 transmission in light of the varying SNRs,
`so that the data transmission 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 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 herein to select the proper
`transmission 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
`[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,
`yavg, is then computed for the SNR estimates for the multiple
`transmission channels as
`
`1 NC
`7an = We; 7”
`
`is the SNR estimate for transmission
`[0012] where yi
`channel i and NC is the number of transmission channels
`used for the data transmission. The variance of the SNR
`
`estimates, Gv2> is also computed as
`
`
`
`for
`is then determined,
`[0013] A back-off factor, Yb”,
`example, based on a function F(yavg,oyz) of the average SNR
`and the SNR variance. For example, the function F(y“‘g,oyz)=
`Kbof may be used for the back-off factor, Where Kb 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
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`

`

`US 2004/0184398 A1
`
`Sep. 23, 2004
`
`for the data stream. An operating SNR, yap, for the trans-
`mission channels is next computed based on the average
`SNR and the back-off factor as y0p=yan—ybu. The transmis-
`sion mode for 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 is utilized for all 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 mode selected for the data stream
`is the supported 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
`transmission channels, such as TDM, OFDM, MIMO, and
`MIMO-OFDM systems (all of which are described below).
`
`selection techniques
`transmission mode
`[0015] The
`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 transmit antennas. The transmission mode
`for this data stream may be selected based on SNR estimates
`for all subbands of all 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 SNR esti-
`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 SNR estimates
`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
`invcntion will bccomc more apparent from thc dctailcd
`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 shows a transmission mode selector;
`
`[0020] FIG. 3 shows a process to determine the transmis-
`sion mode for 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 SNRs of NT transmit
`antennas in a MIMO—OFDM system and the SNR of an
`equivalent system, respectively;
`
`[0023] FIG. 6 shows the average SNRs of the NT transmit
`antennas;
`
`[0024] FIG. 7 shows a base station and a terminal in a
`MIMO-OFDM system;
`
`[0025] FIGS. 8A and 8B show a transmitter subsystem
`and a transmitter unit within the transmitter subsystem,
`respectively;
`
`[0026] FIGS. 9A and SB show a 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 tcchniqucs may be used for TDM systcms,
`OFDM-based systems, MIMO systems, MIMO systems that
`utilize OFDM (i.e., MIMO-OFDM systems), 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 (NTS) time slots that may be assigned
`different indices. NTS transmission channels may be formed
`for the NTS time slots in each frame.
`
`[0031] An OFDM system effectively partitions the overall
`system bandwidth into multiple (NF) orthogonal subbands,
`which may also be referred to as tones, bins, and frequency
`channels. Each subband is associated with a respective
`carrier that may be modulated with data. N F transmission
`channels may be formed for the NF subbands.
`
`[0032] A MIMO system employs multiple (NT) transmit
`antennas and multiple (N19 receive antennas for data trans-
`mission, and is denoted as an (NT, NR) system. A MIMO
`channel formed by the NT transmit and NR receive antennas
`may be decomposed into NS independent channels, with
`Nsémin{NT, NR}. 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 NT transmit and NR 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 NS=N1éNK N1.
`transmission channels may be formed for the NT spatial
`channels.
`
`[0033] A MIMO-OFDM system has NT spatial channels
`for each of NF subbands. A transmission channel may be
`formed for each spatial channel of each subband. NF~NT
`transmission channels would then be available for data
`
`transmission between the NT transmit antennas and NR
`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
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`Page 14 of 28
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`

`

`US 2004/0184398 A1
`
`Sep. 23, 2004
`
`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.
`
`[0035] 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
`demultiplexes the traffic data into ND data streams, where
`ND‘él. 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 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 provided by a con—
`troller 130 and are generated 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 ND
`modulation symbol streams for the ND data streams.
`
`[0037] A transmitter unit (TMTR) 116 then receives and
`processes the ND 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. Apilot may
`also be transmitted to receiver 150 to assist it perform a
`number of 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 NO 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 ND streams of “recovered”
`
`symbols, which are estimates of the ND 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 (i.e., symbol demap-
`ping), deinterleaving, and decoding. RX data processor 162
`may further provide the status of each received data packet.
`
`[0040] 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 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
`SNR estimates from channel estimator 164 and determines
`a suitable transmission mode for each of the ND data
`streams.
`
`[0041] A controller 170 receives the ND 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 ND transmission modes for the ND 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 ND 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 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 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 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 that is flat
`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 perfor-
`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 AWGN transmission channels.
`
`[0044] However, as noted above, the multiple transmis—
`sion channels may experience different channel conditions
`
`Page 15 of 28
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`

`US 2004/0184398 A1
`
`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.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.
`
`[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 ND 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 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; = lOlog10[¥], for i: l,
`A0
`
`,NC, (dB)
`
`Eq (11
`
`[0047] where i is an index for the transmission channels
`used for the data stream;
`
`[0048] NC is the number of transmission channels
`used for the data stream;
`
`[0049]
`nel i;
`
`si is the complex gain for transmission chan-
`
`[0050] N0 is the noise variance for transmission chan—
`nel i; and
`
`[0051]
`nel i.
`
`y‘- is the SNR estimate for transmission chan-
`
`[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:
`
`1 NC
`W = — ;. dB‘
`7 g Ncgy (
`J
`
`an)
`
`[0054] where i is an index for the transmission channels
`used for the data stream;
`
`Yr is the SNR estimate for transmission chan—
`[0055]
`nel i; and
`
`yavg is the average SNR for the NC transmis-
`[0056]
`sion channels used for the data stream.
`
`[0057] The unbiased variance of the SNR estimates may
`be computed as follows:
`
`
`
`Iv
`
`Eq (3)
`
`[0058] where of is the SNR variance.
`[0059] A computation unit 212 then uses the average SNR
`and the SNR variance to compute an operating SNR for the
`group of transmission channels used for the data stream. The
`operating SNR may be computed as follows:
`infirm-“(m (dB)
`[0060] where ybo is a back-off factor; and
`
`Eq (4)
`
`yap. is the operating SNR for the group of
`[0061]
`transmissron channels.
`
`[0062] The back-off factor is used to account for fre-
`quency selectivity 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 of the 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 may be used to 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 SNR that is less than or equal to the
`operating SNR. Look-11p table 214 thus selects the highest
`possible data rate for the data stream based on the operating
`SNR.
`
`[0065] Table 1 lists an exemplary set 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 code rate, a particular modulation scheme, and the
`minimum SNR required to achieve 1% PER for a non—
`fading, AWGN channel. The spectral efficiency refers to the
`data rate (i.e., the information bit rate) normalized by the
`system bandwidth, and is given in umts of bits per second
`per Hertz (bps/Hz). The code rate and modulation scheme
`for each transmission mode 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 code rate, interleaving scheme,
`modulation scheme, and so on, used by the system for that
`transmission mode) and for an AWGN channel. The required
`
`Page 16 of 28
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`

`US 2004/0184398 A1
`
`Sep. 23, 2004
`
`SNR may be obtained by computation, computer simulation,
`empirical measurements, and so on, as is known in the art.
`
`TABLE 1
`
`Transmission
`Mode
`Index
`0
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`
`Spectral
`Htficiency
`(bps/Hz)
`0.0
`0.25
`0.5
`1.0
`1.5
`2.0
`2.5
`3.0
`3.5
`4.0
`4.5
`5.0
`6.0
`7.0
`
`(Iode Modulation
`Rate
`Scheme
`— —
`1/4
`BPSK
`1/2
`BPSK
`1/2
`QPSK
`3/4
`QPSK
`1/2
`16 QAM
`5/8
`16 QAM
`3/4
`16 QAM
`7/12
`64 QAM
`2/3
`64 QAM
`3/4
`64 QAM
`5/6
`64 QAM
`3/4
`256 QAM
`7/8
`256 QAM
`
`Required
`SNR
`(dB)
`—
`—1.8
`1.2
`4.2
`6.8
`10.1
`11.7
`13.2
`16.2
`17.4
`18.8
`20.0
`24.2
`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 SNR for 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). Aback-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 and their
`required SNRs may be consulted for step 322. Steps 312
`through 322 may be performed for each of the ND data
`streams to be independently processed.
`
`[0068] For clarity, the transmission mode selection pro-
`cess is now described for a specific example.
`In this
`example, a data stream is transmitted on a group of four
`transmission channels with received SNRs of 2.74, 4.27,
`6.64, and 9.52 dB. The average SNR is computed as
`yavg=5.79 dB, and the SNR variance is computed as of:
`8.75. For this example, the back—off factor is determined
`based on a function ybO=F(yan,oy2)=0.25of and computed
`as yb0=2.19 dB. The operating SNR is then computed as
`Yop=5.79—2.19=3.60 dB.
`[0069] Using the set of supported transmission modes and
`their required SNRs shown in Table 1,
`the transmission
`mode w