HK1117959A - Rate selection with margin sharing - Google Patents

Description

Rate selection with margin sharing

Background

I. Field of the invention

The present invention relates generally to communication, and more specifically to techniques for selecting a data transmission rate in a communication system.

II. background

In a communication system, a transmitter may transmit multiple data streams to a receiver via multiple transmission channels. The transmission channel may be formed in the spatial domain, the frequency domain, the time domain, or a combination thereof. For example, the multiple transmission channels may correspond to different spatial channels in a multiple-input multiple-output (MDVIO) communication system, different frequency subbands in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, or different time slots in a Time Division Multiplexing (TDM) communication system.

The transmission channel may experience different channel conditions (e.g., different fading, multipath, and interference effects) and may achieve different signal-to-noise ratios (SNRs). The SNR of a transmission channel determines its transmission capability, which is typically quantified by a particular data rate that can be reliably transmitted over the transmission channel. If the SNR varies between the various transmission channels, the supported data rates also vary between the various channels. Furthermore, if the channel conditions change over time, the data rate supported by the transmission channel will also change over time.

One of the main challenges in coded communication systems is to select the appropriate rate to use for multiple data streams based on channel conditions. As used herein, "rate" may indicate a particular data rate or information bit rate to be used for a data stream, a particular coding scheme, a particular modulation scheme, and so on. The rate selection should maximize the overall throughput of the multiple transmission channels while meeting certain quality targets, which may be quantified by a target Packet Error Rate (PER).

Accordingly, there is a need in the art for techniques to select appropriate rates for data transmission on multiple transmission channels.

SUMMARY

Techniques for performing rate selection in a margin sharing (margin sharing) manner are described herein. According to one embodiment of the present invention, a method for initially determining SNR estimates for a plurality of data streams is provided. Rates are then selected for the data streams based on the SNR estimates, and such that at least one data stream has a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

According to another embodiment, an apparatus is described that includes a channel estimator and a controller. The channel estimator determines SNR estimates for a plurality of data streams. The controller selects a rate for each data stream based on the SNR estimates and causes at least one data stream to have a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

According to yet another embodiment, an apparatus is described that includes means for determining SNR estimates for a plurality of data streams, and means for selecting a rate for each data stream based on the SNR estimates such that at least one data stream has a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

According to yet another embodiment, a processor-readable medium is provided that stores instructions operable in an apparatus to obtain SNR estimates for a plurality of data streams, select a rate for each data stream based on the SNR estimates, and cause at least one data stream to have a negative SNR margin, each remaining data stream to have a non-negative SNR margin, and the total SNR margin for all data streams to be non-negative.

According to yet another embodiment, a method is provided in which an SNR estimate is initially determined for each of a plurality of transmission channels available for data transmission. A total SNR margin for each of the plurality of rate combinations is then determined based on the SNR estimates for the respective transmission channels. Each rate combination is associated with a particular number of data streams to transmit, a particular rate for each data stream, and a particular overall throughput. A rate combination is selected from among the plurality of rate combinations based on the total SNR margin and the total throughput of the rate combinations.

According to yet another embodiment, an apparatus is described that includes a channel estimator and a controller. The channel estimator determines an SNR estimate for each of a plurality of transmission channels available for data transmission. The controller determines a total SNR margin for each of a plurality of rate combinations based on the SNR estimates for the respective transmission channels and selects a rate combination from among the plurality of rate combinations based on the total SNR margin and the total throughput for the rate combinations.

According to another embodiment, an apparatus is described that includes means for determining an SNR estimate for each of a plurality of transmission channels available for data transmission, means for determining a total SNR margin for each of a plurality of rate combinations based on the SNR estimates for the respective transmission channels, and means for selecting a rate combination from among the plurality of rate combinations based on the total SNR margin and the total throughput for the rate combinations.

Various aspects and embodiments of the invention are described in more detail below.

Brief Description of Drawings

Fig. 1 shows a transmitter and a receiver in a communication system.

Fig. 2 shows a plot of received SNR versus frequency for a transmission channel.

Fig. 3 shows a rate selection process with independent rates per stream.

Fig. 4 shows a process for performing (stream-ordered) headroom sharing for stream ordering.

Fig. 5 shows a process for performing rank-ordered (rank-ordered) headroom sharing.

Fig. 6 illustrates a rate selection process for vector quantizing a set of rates.

Fig. 7 illustrates another rate selection process for vector quantizing a set of rates.

Fig. 8 shows a diagram of a transmitter and a receiver in a MIMO system.

Fig. 9 shows a diagram of a Transmit (TX) data processor at a transmitter.

Detailed Description

The word "exemplary" is used herein to mean "serving as an example, instance, or illustration. Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

The techniques for rate selection with margin sharing described herein may be used for various communication systems with multiple transport channels. For example, the techniques may be used for MIMO systems, OFDM-based systems, TDM systems, MIMO systems utilizing OFDM (i.e., MIMO-OFDM systems), and so on. MIMO systems employ multiple (T) transmit antennas at the transmitter and multiple (R) receive antennas at the receiver for data transmission. The MIMO channel formed by the T transmit antennas and the R receive antennas may be decomposed into S spatial channels, where S ≦ min { T, R }. S transmission channels may be formed from the S spatial channels. An OFDM system effectively partitions the overall system bandwidth into multiple (K) orthogonal subbands, which are also referred to as tones, subcarriers, frequency bins, and frequency channels. Each subband is associated with a respective carrier that may be modulated with data. K transmission channels may be formed from K subbands. The MIMO-OFDM system has S spatial channels for each of the K subbands. Up to S · K transmission channels may be formed from the spatial channels of these subbands in a MIMO-OFDM system. TDM systems may transmit data in frames, where each frame may have multiple (Q) time slots. Q transmission channels may be formed for these Q slots in each frame.

In general, multiple transmission channels may be formed in various ways. For clarity, much of the description below is for a MIMO-OFDM system, and each transmission channel may correspond to a wideband spatial channel (described below). Each transport channel may be used to transmit one data stream.

Fig. 1 shows a diagram of a transmitter 110 and a receiver 150 in a multi-channel communication system 100. At transmitter 110, a TX data processor 120 receives traffic data, processes (e.g., encodes, interleaves, and symbol maps) the traffic data according to M rates from a controller 140, and generates M data symbol streams, where M ≧ 1. As used herein, a data symbol is a modulation symbol for traffic/packet data, a pilot symbol is a modulation symbol for pilot (data known a priori by both the transmitter and receiver), a modulation symbol is a complex value corresponding to a point in a signal constellation for a modulation scheme (e.g., M-PSK or M-QAM), and a symbol is an arbitrary complex value. TX spatial processor 130 receives and multiplexes the M data symbol streams with pilot symbols, performs spatial processing on the data and pilot symbols (as applicable), and provides T transmit symbol streams.

A transmitter unit (TMTR)132 processes the T transmit symbol streams and generates T modulated signals, which are transmitted from the T antennas and via a first communication link 148. The communication link 148 distorts the modulated signal with the channel response and further degrades the modulated signal with Additive White Gaussian Noise (AWGN) and possibly with interference from other transmitters.

At receiver 150, the transmitted signals are received by R antennas and the R received signals are provided to a receiver unit (RCVR) 160. Receiver unit 160 conditions and digitizes the R received signals and processes the samples in a manner complementary to the processing performed by transmitter unit 132. Receiver unit 160 provides received pilot symbols to a channel estimator 172 and provides R received data symbol streams to a Receive (RX) spatial processor 170. The channel estimator 172 obtains a channel estimate for the communication link 148 and provides the channel estimate to the RX spatial processor 170. RX spatial processor 170 performs receiver spatial processing (or spatial matched filtering) on the R received data symbol streams with the channel estimates and provides M detected symbol streams, which are estimates of the M data symbol streams transmitted by transmitter 110. RX data processor 180 processes (e.g., symbol demaps, deinterleaves, and decodes) the M detected symbol streams based on the M rates selected for the streams and provides decoded data, which is an estimate of the traffic data transmitted by transmitter 110. RX data processor 180 may further provide the decoding results (e.g., the state and/or decoder metrics for each received packet) to a rate selector 182.

For rate control, the channel estimator 172 may process the received pilot symbols (and possibly the detected data symbols) and determine SNR estimates for the M streams. Rate selector 182 receives the SNR estimates and decoding results, selects an appropriate rate for each stream, and provides the M selected rates for the M streams to controller 190. Controller 190 sends rate information (e.g., the M selected rates) and possibly other information (e.g., an acknowledgement of receipt of the packet) to transmitter 110 via second communication link 152. Controller 140 at transmitter 110 receives the rate information and provides the M rates to TX data processor 120. Fig. 1 shows rate selection being performed by receiver 150. In general, rate selection may be performed by receiver 150, transmitter 110, or both.

For a MIMO-OFDM system, the MIMO channel between the transmitter and the receiver may be formed from a set of K channel response matricesH(k) Characterized in that K is 1. Each channel response matrixH(k) Having dimension R x T and containing each burst corresponding to subband kComplex gain between the transmit antenna and each receive antenna. Each matrixH(k) Comprising S spatial channels, where S is ≦ rn { T, R }. As described below, by decompositionH(k) S orthogonal spatial channels (or eigenmodes) are obtained for each subband k. In any case, up to S wideband spatial channels may be formed for the MIMO channel, where each wideband spatial channel includes one spatial channel for each of the K subbands. For example, each wideband spatial channel may correspond to K subbands for one transmit antenna. As another example, each wideband spatial channel may include one eigenmode for each of the K subbands. Each wideband spatial channel may be used as a transport channel.

The frequency response of each transmission channel m may be represented by h_m(k) Wherein K is 1_m(k) Is the complex channel gain corresponding to subband k of transmission channel m. For simplicity, assume h_m(k) K is constant across subbands. The received SNR for each subband of each transmission channel may be expressed as:

k1, K, and m 1, S (1)

Wherein the content of the first and second substances,

P_m(k) is the transmit power for subband k of transmission channel m;

N₀is the noise variance at the receiver; and is

γ_m(k) Is the received SNR for subband k of transmission channel m.

Equation (1) shows a simple expression of the received SNR. In general, the received SNR expression may include terms corresponding to various factors. For example, as described below, in a MIMO system, the received SNR depends on the spatial processing performed by the transmitter and receiver. For simplicity, the noise variance N₀Is assumed to be constant across the K subbands. The received SNR in equation (1) is given in units of decibels (dB). All SNR calculations described below are also in dB, unless otherwise noted.

Fig. 2 shows an exemplary plot 210 of the received SNR of a transmission channel with frequency selective fading. For a multipath channel, the channel gain h is shown in graph 210_m(k) Varies across K subbands and achieves different received SNRs for different subbands. As shown by line 212, the average SNR of the received SNRs for all subbands may be determined.

The transmitter may transmit one data stream on each of M transport channels, where S ≧ M ≧ 1. The number of data streams to transmit may be selected based on various factors such as channel conditions, achievable overall throughput, and the like. The rates for the M data streams may be selected in various ways. In general, techniques for rate control in a margin sharing manner may be used for (1) systems with independent rates per stream, whereby the rate for each stream may be independently selected, and (2) systems with vector quantized rate sets, whereby only certain combinations of rates are allowed.

Fig. 3 shows a process 300 for selecting rates for M data streams for a system with independent rates per stream. Initially, SNR estimates (also referred to as effective SNRs) are obtained for each of the M data streams (block 310). A rate is then selected for each stream based on the SNR estimate for that stream (block 312). An SNR margin for each data stream is determined based on the SNR estimate for the stream and the required SNR for the rate selected for the stream (block 314). At least one rate for at least one data stream is adjusted upward, if possible, based on the SNR margins for the M data streams such that at least one stream has a negative SNR margin, the remaining streams have non-negative SNR margins, and the total SNR margin for all M streams is non-negative (block 316). The M streams are then transmitted at the rates selected for the streams, where each rate may or may not be adjusted upward by margin sharing (block 318). The blocks in fig. 3 will be described in more detail below.

The rate for each data stream may be selected based on the received SNR of the transmission channel used to transmit the data stream. An embodiment of selecting a rate for each data stream (for blocks 310 and 312) is described below. For this embodiment, the received SNRs for all subbands for each transmission channel are first determined, e.g., based on the received pilot symbols.

The average SNR per data stream/transmission channel may be calculated as:

formula (2)

Wherein gamma is_avg，mIs the average SNR for data stream m in dB.

The variance of the received SNR for each data stream can be calculated as:

formula (3)

Wherein sigma_snr，m ²SNR variance of data stream m.

The effective SNR for each data stream can be calculated as:

SNR_eff(m)＝γ_avg，m-γ_bo，mm1., M formula (4) wherein,

γ_bo，mis the back-off factor for data stream m; and is

SNR_eff(m) is the effective SNR (or SNR estimate) for data stream m.

Compensation factor gamma_bo，mTo account for various factors such as variability of received SNR across the transmission channel. The back-off factor may be a function of the average SNR and the SNR variance, orFor example, the compensation factor may be defined as:wherein K_boIs a constant. The compensation factor may also be defined based on system specific factors such as diversity, coding scheme, interleaving scheme, packet size, etc. for the data streams.

The system may support a particular set of rates. Each supported rate is associated with a particular minimum SNR required to achieve a desired level of performance (e.g., 1% PER for non-fading, AWGN channels). The lookup table may store the supported rates and the required SNR for each supported rate. The effective SNR for each data stream can be compared against the required SNR for the supported rates. A supported rate is then selected for the data stream having a highest data rate and a desired SNR less than or equal to the effective SNR

Table 1 lists an exemplary set of 14 rates supported by the system, which are given rate indices of 0 through 13. Each rate is associated with a particular spectral efficiency, a particular code rate, a particular modulation scheme, and a particular desired SNR. Spectral efficiency refers to the data rate (or information bit rate) normalized by the system bandwidth and is given in bits per hertz per second (bps/Hz). The code rate and modulation scheme corresponding to each rate in table 1 are designed for a particular system. The null rate with index 0 has a zero data rate (or no data transmission). For each non-null rate where the data rate is non-zero, the required SNR is based on the particular system design (i.e., code rate, interleaving scheme, modulation scheme, etc. used by the system for that rate) and is obtained for the SWGN channel. As is known in the art, the desired SNR can be obtained by calculation, computer simulation, empirical measurements, and the like. For each non-null rate, the SNR gap (SNR gap) is the difference between the SNR required for that rate and the SNR required for the next higher rate. Since the rate with index 13 is the highest supported rate, its SNR gap is set to infinity or some other large value.

TABLE 1

Rate indexing	Spectral efficiency (bps/Hz)	Code rate	Silk blending scheme	Required SNR (dB)	SNR gap (dB)
						0	0 0	-	-	-	-
1	0.25	1/4	BPSK	-1.8	3.0
						2	0.5	1/2	BPSK	1.2	3.0
3	1.0	1/2	QPSK	4.2	2.6
						4	1.5	3/4	QPSK	6.8	3.3
5	2.0	1/2	16QAM	10.1	1.6
						6	2.5	5/8	16QAM	11.7	1.5
7	3.0	3/4	16QAM	13.2	3.0
						8	3.5	7/12	64QAM	16.2	1.2
9	4.0	2/3	64QAM	1.74	1.4
						10	4.5	3/4	64QAM	18.8	1.2
11	5.0	5/6	64QAM	20.0	4.2
						12	6.0	3/4	256QAM	24.2	2.1
13	7.0	7/8	256QAM	26.3	∝

Initially M rates may be selected for the M data streams as described above and denoted as R_mWherein M is 1. In the following description, the rate R_mAnd rate index R_mMay be used interchangeably. The required SNR for the rate initially selected for each data stream is less than or equal to the effective SNR for that stream. Thus, each data stream has a non-negative SNR margin, which can be expressed as:

SNR_margin(m)＝SNR_eff(m)-SNR_req(R_m) M1., M formula (5)

Wherein the content of the first and second substances,

R_mis the rate initially selected for the data stream;

SNR_req(R_m) Is the rate R_mThe required SNR; and is

SNR_margin(m) is the SNR margin for data stream m.

The total SNR margin for all M data streams may be expressed as:

formula (6)

The amount of SNR margin that may be transferred from any one stream to the other stream(s) may be limited to a predetermined maximum value, e.g.This can be done by comparing the SNR of each stream m in equation (6)_margm(m) is limited to SNR_margm ^maxThe method is realized. SNR_margm ^maxMay be fixed for all streams or may be a function of rate, code rate, modulation scheme, stream index, etc., which may be different for each stream. Limiting SNR_margin(M) the variation in SNR margin for the M streams may be reduced. The total SNR margin may also be limited to another predetermined maximum value, e.g.The amount of SNR that can be reallocated to any one stream is therefore limited to SNR_{total_margm} ^maxThis ensures that no stream is transmitted at a rate where the required SNR is excessively higher than the effective SNR of the stream. In general, the amount of SNR that can be reallocated to any one stream may be limited to SNR_re-allo ^maxThis may be equal to or less than SNR_{total_margm} ^maxTo a suitably selected value. SNR_re-allo ^maxMay be a fixed value or a function of rate, code rate, modulation scheme, stream index, etc.

As described below, the transmitter may encode traffic data using a single base code to generate code bits, then parse the code bits into M streams, and further process (e.g., puncture and symbol map) the code bits for each stream according to a rate selected for that stream. The receiver may perform complementary processing, reassemble the detected symbols corresponding to the M streams, and decode the reassembled detected symbols. The sequence/packet of detected symbols to be decoded at the receiver may be made up of multiple groups of detected symbols generated at different rates. The result of each decoded bit is typically affected by the received SNR of adjacent and nearby detected symbols. If the detected symbols for the M streams are decoded together, the SNR margins for these streams may be shared among the streams to achieve higher overall throughput. Margin sharing reallocates the total SNR margin with the goal of achieving a higher rate on at least one stream. Several embodiments of margin sharing are described below.

Fig. 4 shows a process 316a for performing margin sharing for flow ordering, which is a first embodiment of margin sharing for a system with independent rates per flow. Process 316a may be used for block 316 in fig. 3. For this embodiment, the total SNR margin is reallocated to the M streams in a sequential order based on their effective SNRs. Initially, the M streams are ordered based on their effective SNRs, with the first stream having the highest effective SNR and the last stream having the lowest effective SNR (block 412). The stream index m is initialized to 1 (block 414).

Stream m, which is the stream with the highest effective SNR not considered, is selected (block 416). The SNR required to boost stream m to the next higher rate is determined (block 418) as follows:

wherein the content of the first and second substances,

R_minis the lowest supported rate, which is the rate index 0 in table 1;

R_maxis the highest supported rate, which is the rate index 13 in table 1; and is

SNR_promote(m) is the SNR needed to boost stream m to the next higher rate assuming that the SNR margin on stream m has been removed.

If the effective SNR of stream m is less than-1.8 dB, a null rate R is initially selected for stream m_min. The SNR required to boost stream m to the lowest non-null rate with index 1 is equal to the difference between the SNR required for rate index 1 and the effective SNR for stream m. If the highest supported rate R is initially selected for the stream_maxThen the SNR will be_promote(m) setting to infinity or a large value ensures that the total SNR margin is not sufficient to boost stream m.

It is then determined whether the total SNR margin is greater than or equal to the SNR required to boost stream m to the next higher rate (block 420). If the answer is "yes," the next higher rate is selected for stream m (block 422), and the total SNR margin is updated as follows (block 424):

SNR_{total_margin}＝SNR_{total_margin}-SNR_promote(m) formula (8)

Following block 424, and if the answer to block 420 is "no," a determination is made as to whether all M streams have been considered (block 426). If the answer is "no," the stream index m is incremented (block 428) and the process returns to block 416 to consider the stream with the next lower effective SNR. Otherwise, if all M flows have been considered, the process terminates. Although not shown in fig. 4, the process may also terminate if the total SNR margin is zero or cannot boost the small value of any remaining streams.

For the first embodiment of margin sharing shown in fig. 4, the M streams are ordered from highest to lowest effective SNR and then considered one at a time in sequential order starting with the stream with the highest effective SNR. For the second embodiment of margin sharing, also referred to as reverse flow ordering margin sharing, the M streams are ordered from lowest to highest effective SNR and then considered one at a time in sequential order starting with the stream with the lowest effective SNR. The second embodiment may be implemented as shown in fig. 4, although where the M streams are ordered in increasing order of effective SNR (rather than decreasing order of effective SNR).

Fig. 5 shows a process 316b for performing rank ordered margin sharing, which is a third embodiment of margin sharing for systems with independent rates per stream. Process 316b may also be used for block 316 in fig. 3.

The SNR required to initially select a higher rate for each stream, also referred to as the differential SNR, in block 312 of fig. 3 is determined as follows (block 510):

if initially the space velocity R is selected for stream m_minSNR of_diff(m) is equal to the SNR required to boost stream m to the lowest nonproductive rate. If the highest supported rate R is initially selected for stream m_maxThen the SNR will be_diff(m) setting to infinity or a large value ensures that stream m will be selected by the last for margin sharing. As described above, the amount of SNR that can be reallocated to any one stream may be limited to SNR_re-allo ^max. In this case, if the differential SNR of any stream is greater than the SNR_re-allo ^maxThe differential SNR can be set to infinity so that the stream will not be boosted.

The M streams are then ordered based on their differential SNRs, with the first stream having the lowest differential SNR and the last stream having the highest differential SNR (block 512). The stream index m is initialized to 1 (block 514).

Stream m, which is the stream with the lowest differential SNR that is not considered, is selected (block 516). The SNR required to boost stream m to the next higher rate is then determined as shown in equation (7) (block 518).Initially selecting rate R for stream m_mAnd the SNR margin for stream m is included in the total SNR margin. Thus, SNR is required_promote(m) not SNR_diff(m) to select the next higher rate R for stream m_m+1. It is then determined whether the total SNR margin is greater than or equal to the SNR required to boost stream m to the next higher rate (block 520). If the answer is "yes," the next higher rate is selected for stream m (block 522) and the total SNR margin is updated as shown in equation (8) (block 524).

Following block 524, and if the answer to block 520 is "no," a determination is made as to whether all M streams have been considered (block 526). If the answer is "no," the stream index m is incremented (block 528), and the process returns to block 516 to consider the stream with the next lower differential SNR. Otherwise, if all M flows have been considered, the process terminates. The process may also terminate if the total SNR margin is zero or a small value (not shown in fig. 5). Steps 514 through 528 may also be repeated any number of times until all available SNR margin is exhausted, or each stream has been boosted a maximum number of times, or no more streams may be boosted, or some other exit criteria is met. This third embodiment boosts the streams in an orderly manner such that (1) the streams that require the least amount of SNR margin to boost are boosted first, and (2) the streams that require the most amount of SNR margin are boosted last. This embodiment may improve performance and may allow more flows to be promoted.

In a fourth embodiment of margin sharing for a system with independent rates per stream, the SNR required to boost each stream to the next higher rate is initially calculated as shown in equation (7). The M streams are then ordered based on their boosted SNRs, with the first stream having the lowest boosted SNR and the last stream having the highest boosted SNR. The M streams are then considered one at a time in sequential order starting with the stream with the lowest boosted SNR. This fourth embodiment attempts to boost the stream with less boosted SNR first, which may allow for boosting of more streams.

The above-described headroom sharing embodiments are directed to embodiments in which the rate for each stream may be individually adjustedAnd (4) selecting a system. This allows the total SNR margin to be reallocated to any stream. If the total SNR margin allows, the rate for each stream is adjusted to the next higher rate index R_m+1。

The rate for one stream may also be boosted up by more than one rate index. In one embodiment, the rate for each stream may be boosted as high as possible based on the total SNR margin. For example, instead of calculating SNR for stream m_promote(m), a stream m may be selected having less than SNR_eff(m)+SNR_{total_margin}The highest rate of the required SNR. In another embodiment, the rate for each stream may be boosted by up to Q rate indices, where generally Q ≧ 1. Thus, the embodiments shown in fig. 4 and 5 above are for the case where Q is 1.

The system may allow only certain combinations of rates, for example, in order to reduce the amount of rate information to be sent back to the transmitter. The set of rate combinations allowed by the system is often referred to as a vector quantization rate set. Table 2 shows an exemplary set of vector quantization rates for a system in which a transmitter may transmit up to 4 data streams. For this rate set, rate Identifiers (IDs) 0 to 13 correspond to the transmission of one data stream and are given in table 1 as rate indices 0 to 13, respectively, rate IDs 14 to 24 correspond to the transmission of two data streams, rate IDs 25 to 35 correspond to the transmission of three data streams, and rate IDs 36 to 43 correspond to the transmission of four data streams. For each rate ID, the number of streams to transmit (Num Str), the rate used for each stream, and the total throughput (OTP)/total spectral efficiency are given in table 2. As an example, for rate ID 31, the total throughput is 12.0bps/Hz, 3 streams are transmitted, stream 1 uses rate 12(256 QAM and code rate 3/4), stream 2 uses rate 9(64 QAM and code rate 2/3), and stream 3 uses rate 5(16 QAM and code rate 1/2).

TABLE 2

Rate control in a margin sharing manner may be performed in various ways in a system having a vector quantization rate set. Several embodiments are described below.

Fig. 6 shows a process 600 for selecting rates for a data stream in a system with a vector quantization rate set according to a first embodiment. Initially, an effective SNR is determined for each transmission channel available for data transmission (block 610). A total SNR margin for each allowed rate combination is determined based on the effective SNRs (block 612). The total SNR margin for a given rate combination with L streams (where S ≧ L ≧ 1) can be determined as follows. First, the SNR margin of each stream m in the rate combination is calculated as shown in equation (5), where the SNR_eff(m) is the effective SNR of the transmission channel for stream m, and SNR_req(R_m) Is the required SNR for the rate specified for stream m by the rate combination. Since the rate for each stream in the rate combination is specified, the SNR margin for each stream may be a positive or negative value. The total SNR margin is equal to the sum of the SNR margins for the L streams in the rate combination, as shown in equation (6). If the SNR margin for any of the streams in the rate combination is below a predetermined minimum value (e.g., -2dB), the total SNR margin for that rate combination may be set to be negative infinity or some negative value so that the rate combination will not be selected for use. If the number of streams to transmit is known, only the rate combinations for that number of streams are evaluated.

The rate combination with the highest overall throughput and non-negative overall SNR margin is identified (block 614). If the rate set has more than one rate combination with the same overall throughput, then multiple rate combinations may be identified in block 614. For example, the rate set shown in Table 2 has five rate combinations with an overall throughput of 12.0 bps/Hz. The identified rate combination with the largest total SNR margin is selected for use (block 616). The data is then transmitted using the selected rate combination (block 618).

Fig. 7 shows a process 700 for selecting rates for data streams in a system with a vector quantization rate set according to a second embodiment. The variable max _ otp represents the maximum total throughput achieved for all data streams and is initialized to zero (block 710). The index e represents the number of streams to transmit and is initialized to one (block 712).

An effective SNR is determined for each of the e transmission channels used to transmit the e data streams (block 714). As described above, the effective SNR calculation may rely on spatial processing performed by the transmitter and receiver for the e streams. For example, as described above for block 612 in fig. 6, the total SNR margin for each rate combination of e streams and the total throughput greater than or equal to max _ otp are determined (block 716). Margin sharing may or may not be applied for each rate combination evaluated in block 716. If margin sharing is applied, any of the margin sharing embodiments described above with independent rates per flow (e.g., flow ordering margin sharing, rank ordering margin sharing, etc.) may be used.

The rate combination with the highest overall throughput and the largest positive overall SNR margin is then selected from the rate combinations of all evaluated e streams (block 718), e.g., as described above for blocks 614 and 616 in fig. 6. The selected rate combination is denoted rc (e), the total throughput for that rate combination is denoted otp (e), and the total SNR margin for that rate combination is denoted margin (e). It is then determined whether the highest overall throughput for the e flows is greater than the current maximum overall throughput, or whether otp (e) > max _ otp (block 720). If the answer is "yes," then the maximum overall throughput is set to the highest overall throughput for the e streams, the rate combination RC (e) is saved in the variable max _ RC, and the total SNR margin for RC (e) is saved in the variable max _ margin (block 724). The process then proceeds to block 726.

If the answer to block 720 is no, a determination is made as to whether (1) the highest overall throughput for the e streams is equal to the current maximum overall throughput, and (2) the total SNR margin for rate combining rc (e) exceeds the current max _ margin by a predetermined amount denoted as Δ margin (block 722). If the rate combining rc (e) for e streams and another rate combining rc (j) for less than e streams can achieve the same overall throughput, the rate combining rc (e) may be selected if the total SNR margin of the rate combining rc (e) is higher by a predetermined amount. Otherwise, the rate combinations rc (j) with fewer streams may be selected to (1) reduce processing at the transmitter and receiver, and (2) enhance protection against crosstalk between streams. If the answer to block 722 is "yes," then in block 724 the rate combining, RC (e), the total throughput for RC (e), and the total SNR margin for RC (e), are saved as max _ RC, max _ otp, and max _ margin, respectively.

If the answer to block 722 is no, and also after block 724, a determination is made as to whether all of the different numbers of streams have been evaluated (block 726). If the answer is "no," index e is incremented (block 728) and the process returns to block 714 to evaluate the next higher number of streams. Otherwise, if all of the different numbers of streams have been evaluated, a rate combination max _ RC is selected for use and the data is transmitted using the selected rate combination (block 730).

Although not shown in fig. 7 for simplicity, process 700 may terminate if the highest overall throughput, otp (e), for the current number of streams does not exceed the maximum overall throughput. For example, if the answer to block 720 is "no," the process may execute blocks 722 and 724 and then terminate.

In a third embodiment for selecting rates in a system with a vector quantized rate set, the total required SNR is calculated for each rate combination as the sum of the required SNRs for the specified rates for all streams in that rate combination. The total required SNR and total throughput for all rate combinations in the rate set may be stored in a look-up table. For rate selection, the total effective SNR is calculated as the sum of the effective SNRs for all transmission channels available for data transmission. The rate combination with the highest overall throughput and the total required SNR that is less than or equal to the total effective SNR is then selected for use. This embodiment does not limit the amount of SNR margin that can be reallocated to each stream.

Fig. 3 to 5 show exemplary embodiments for performing rate selection in a margin sharing manner for a system having an independent rate per stream. Fig. 6 and 7 illustrate exemplary embodiments for performing rate selection in a margin sharing manner for a system having a vector quantization rate set. Rate selection in a margin sharing manner may also be performed in other manners. The margin sharing allows one or more streams to operate with a negative SNR margin so that a higher overall throughput can be achieved for data transmission.

As described above, the technique of rate selection in a margin sharing manner may be used for various systems and various types of transmission channels. In a MIMO system, different transmission channels may be formed, with the transmitter performing different spatial processing, such as eigensteering (eigensteering), unguided steering, and spatial spreading.

For eigensteering, the channel response matrix for each subbandH(k) Diagonalization can be achieved by eigenvalue decomposition as follows:

R(k)＝H ^H(k)·H(k)＝E(k)·Λ(k)·E ^H(k) formula (10)

WhereinE(k) Is a unitary matrix of the eigenvectors,Λ(k) is a diagonal matrix and "H" denotes a conjugate transpose. The transmitter may use a steering matrixE(k) Data is transmitted on up to S orthogonal spatial channels (or eigenmodes) per subband k. Diagonal matrix per subband kΛ(k) ComprisesH(k) Power gain of the S eigenmodes. Channel response matrix for each sub-bandH(k) Can also be diagonalized by singular value decompositionH(k)＝U(k)·∑(k)·E ^H(k) WhereinU(k) Is a unitary matrix of left singular vectors,E(k) is a unitary matrix of right singular vectors (also a matrix of eigenvectors), and∑(k) is thatH(k) A diagonal matrix of channel gains for the S eigenmodes.

For no steering, the transmitter transmits data without any spatial processing, e.g., one data stream from each transmit antenna. For spatial spreading, the transmitter uses different steering matrices that vary over the frequency bandV(k) To transmit dataSo that the data transmission observes the entirety of the effective channel.

Table 3 shows spatial processing performed by the transmitter to achieve intrinsic steering, no steering, and spatial spreading. In table 3, the subscript "es" denotes intrinsic steering, "ns" denotes no steering, and "ss" denotes spatial spreading. The processing shown in table 3 is for a given subband, and thus, the subband index k has been omitted for clarity.sIs a vector having up to S data symbols to be transmitted on a subband in one symbol period.x _xIs a vector having T transmit symbols to be transmitted from T transmit antennas on one subband in one symbol period for pattern x, where "x" may be "es", "ns", or "ss".H _xIs formed by a data vectorsThe effective channel response matrix observed for mode x.

TABLE 3 transmitter spatial processing

	Intrinsic steering	Without steering	Spatial expansion
				Spatial processing	x es＝E·s	x ns＝s	x ss＝V·s
Active channel	H es＝H·E	H ns＝H	H ss＝H·V

The received symbols obtained by the receiver may be expressed as:

r _x＝H·x _x+n＝H _x·s+nformula (11)

Whereinr _xIs a vector of received symbols for mode x, andnis a noise vector, which is assumed to have a variance of σ_n ²AWGN of (1).

Table 4 shows the steps performed by the receiver to obtain the detected symbols; the detected symbols are estimates of the data symbols transmitted in s. Eigensteering may use full channel state information (full CSI) techniques. Eigensteering, non-steering, and spatial spreading may use Channel Correlation Matrix Inversion (CCMI) and Minimum Mean Square Error (MMSE) techniques. For each technique, the receiver derives a spatial filter matrix for each subband based on the actual or effective channel response matrix for that subbandM. The receiver then performs spatial matched filtering on the received symbols with the spatial filter matrix.

TABLE 4 receiver spatial processing

Table 4 also shows the received SNR for each subband k of transmission channel m. For full CSI techniques, λ_m(k) Is thatΛ(k) The mth diagonal element of (1). For CCMI technique, r_m(k) Is thatThe mth diagonal element of (1). For MMSE technique, q_m(k) Is thatM _mmse(k)·H _x(k) The mth diagonal element of (1).

Fig. 8 shows a block diagram of a transmitter 810 and a receiver 850 in a MIMO system. At transmitter 810, a TX data processor 820 receives traffic data from a data source 812, processes (e.g., formats, encodes, interleaves, and symbol maps) the traffic data, and provides M data symbol streams. TX spatial processor 830 performs spatial processing on the data symbols and pilot symbols (e.g., for intrinsic steering, no steering, or spatial spreading) and provides T streams of transmit symbols to T transmitter units (TMTR)832a through 832T. Each transmitter unit 832 conditions a respective transmit symbol stream and generates a modulated signal. T modulated signals from transmitter units 832a through 832T are transmitted from T antennas 834a through 834T, respectively.

At receiver 850, R antennas 858a through 858R receive the modulated signals from transmitter 810 and each provide a received signal to a respective receiver unit (RCVR) 860. Each receiver unit 860 performs processing complementary to that performed by transmitter unit 832 and provides received symbols. RX spatial processor 870 performs spatial matched filtering on the received symbols from all R receiver units 860 (e.g., to derive a spatial filter matrix using full CSI, CCMI, or MMSE techniques) and provides M detected symbol streams. An RX data processor 880 processes (e.g., symbol demaps, deinterleaves, and decodes) the detected symbols and provides decoded data to a data sink 886.

Channel estimators 838 and 888 perform channel estimation for transmitter 810 and receiver 850, respectively. Controllers 840 and 890 control the operation of various processing units at transmitter 810 and receiver 850, respectively. Memory units 842 and 892 store program codes used by controllers 840 and 890, respectively.

To perform rate selection in a margin sharing manner, channel estimator 888 estimates the response of the MIMO channel from transmitter 810 to receiver 850 and determines the received SNR of the spatial channels of the MIMO channel. Controller 890 selects rates for the M data streams based on the received SNRs, adjusts one or more of these rates upward in a margin sharing manner, and provides rate information. The rate information is processed by a TX data processor 894 and a TX spatial processor 896, conditioned by a transmitter unit 860, and transmitted via an antenna 858 to the transmitter 810. At transmitter 810, the modulated signals from receiver 850 are received by T antennas 834, conditioned by T receiver units 834, and further processed by an RX spatial processor 844 and an RX data processor 846 to obtain rate information from receiver 850. Controller 840 receives the rate information and provides the selected rate to TX data processor 820.

Fig. 9 shows a block diagram of one embodiment of TX data processor 820 at transmitter 810. Within TX data processor 820, an encoder 910 encodes traffic data according to a coding scheme and generates code bits. The coding scheme may include a convolutional code, a Turbo code, a Low Density Parity Check (LDPC) code, a Cyclic Redundancy Check (CRC) code, a block code, the like, or a combination thereof. In one embodiment, encoder 910 implements a rate 1/2 binary convolutional encoder that generates two code bits for each data bit. Parser 920 receives the code bits from encoder 910 and parses the code bits into M streams.

The M stream processors 930a through 930M receive the M code bit streams from the parser 920. Each stream processor 930 includes a puncturing unit 932, an interleaver 934, and a symbol mapping unit 936. Puncturing unit 932 punctures (or deletes) the required number of code bits in its stream to achieve the selected code rate for the stream. The interleaver 934 interleaves (or reorders) the code bits from the puncturing unit 932 based on an interleaving scheme. A symbol mapping unit 936 maps the interleaved bits according to a selected modulation scheme and provides modulation symbols. The code rate and modulation scheme for each stream is determined by the rate selected for that stream, e.g., as shown in table 1. M stream processors 930a through 930M provide M data symbol streams to TX spatial processor 830.

The techniques for rate selection with margin sharing described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof. For a hardware implementation, the processing units used to perform rate selection with headroom sharing may be implemented within one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.

For a software implementation, rate selection with headroom sharing may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in memory units (e.g., memory units 142 and/or 192 in fig. 1, memory units 842 and/or 892 in fig. 8) and executed by processors (e.g., controllers 140 and/or 190 in fig. 1, controllers 840 and/or 890 in fig. 8). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (36)

1. A method of performing rate selection in a communication system, comprising:

determining a signal-to-noise ratio (SNR) estimate for a plurality of data streams; and

rates are selected for the plurality of data streams based on the SNR estimates, and such that at least one data stream has a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

2. The method of claim 1, wherein selecting the rates for the plurality of data streams comprises:

a rate is selected for each data stream based on an SNR estimate for the data stream,

determining an SNR margin for each data stream based on the SNR estimate for each data stream and the required SNR for the rate selected for that data stream, an

Adjusting at least one rate of the at least one data stream upward based on SNR margins for the plurality of data streams.

3. The method of claim 2, wherein the upward adjustment of at least one rate of at least one data stream comprises:

determining a total SNR margin for the plurality of data streams based on the SNR margins for the plurality of data streams, an

Reallocating the total SNR margin to the at least one data stream to adjust the at least one rate upward.

4. The method of claim 3, wherein reallocating the total SNR margin to the at least one data stream comprises:

selecting one data stream at a time in sequential order, an

If so, the total SNR margin is reallocated to the selected data stream to adjust the rate of the selected data stream upward.

5. The method of claim 4, further comprising:

ordering the plurality of data streams in a sequential order based on the SNR estimates, wherein a first data stream has a highest SNR estimate and a last data stream has a lowest SNR estimate.

6. The method of claim 4, further comprising:

ordering the plurality of data streams in a sequential order based on the SNR estimates, wherein a first data stream has a lowest SNR estimate and a last data stream has a highest SNR estimate.

7. The method of claim 4, further comprising:

ordering the plurality of data streams in a sequential order based on differential SNR, wherein a first data stream has a minimum differential SNR and a last data stream has a maximum differential SNR, wherein the differential SNR for each data stream corresponds to a difference between an SNR required for a next higher rate for the data stream and an SNR estimate for the data stream.

8. The method of claim 4, further comprising:

ordering the plurality of data streams in a sequential order based on boosted SNRs, wherein a first data stream has a smallest boosted SNR and a last data stream has a largest boosted SNR, wherein the boosted SNR for each data stream indicates an additional SNR required to select a next higher rate for the data stream.

9. The method of claim 2, further comprising:

limiting the amount of SNR margin that can be transferred from any one data stream to another.

10. The method of claim 2, further comprising:

limiting the amount of SNR margin that can be reallocated to any one data stream.

11. The method of claim 1, wherein the determining the SNR estimate for the plurality of data streams comprises:

a received SNR is determined for each data stream,

determining an average SNR for each data stream based on the received SNR for the data stream, an

An SNR estimate is determined for each data stream based on the average SNR for the data stream and a back-off factor.

12. An apparatus in a communication system, comprising:

a channel estimator operable to determine a signal-to-noise ratio (SNR) estimate for a plurality of data streams; and

a controller operable to select rates for the plurality of data streams based on the SNR estimates and such that at least one data stream has a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

13. The apparatus of claim 12, wherein the controller is operative to select a rate for each data stream based on the SNR estimate for the data stream, to determine SNR margins for the data streams based on the SNR estimate for each data stream and SNRs required for the rate selected for the data stream, and to adjust at least one rate for the at least one data stream upward based on the SNR margins for the plurality of data streams.

14. The apparatus of claim 13, wherein the controller is operative to determine a total SNR margin for the plurality of data streams based on the SNR margins for the plurality of data streams, and to reallocate the total SNR margin to the at least one data stream to adjust the at least one rate upward.

15. The apparatus of claim 14, wherein the controller is operative to select one data stream at a time to reallocate the total SNR margin and, when sufficient, to reallocate the total SNR margin to the selected data stream to adjust the rate of the selected data stream upward.

16. An apparatus in a communication system, comprising:

means for determining a signal-to-noise ratio (SNR) estimate for a plurality of data streams; and

means for selecting rates for the plurality of data streams based on the SNR estimates, and such that at least one data stream has a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

17. The apparatus of claim 16, wherein the means for selecting rates for a plurality of data streams comprises:

means for selecting a rate for each data stream based on an SNR estimate for the data stream,

means for determining an SNR margin for each data stream based on the SNR estimate for the data stream and the required SNR for the rate selected for the data stream, an

Means for adjusting at least one rate of the at least one data stream upward based on SNR margins for the plurality of data streams.

18. The apparatus of claim 17, wherein the means for adjusting at least one rate of at least one data stream upward comprises:

means for determining a total SNR margin for the plurality of data streams based on the SNR margins for the plurality of data streams, an

Means for reallocating the total SNR margin to the at least one data stream to adjust the at least one rate upward.

19. The apparatus of claim 18, wherein the means for reallocating total SNR margin to at least one data stream comprises:

means for selecting one data stream at a time to reallocate the total SNR margin, and

means for reallocating the total SNR margin to the selected data stream when sufficient to adjust the rate for the selected data stream upward.

20. A processor-readable medium for storing instructions operable in a device to perform acts comprising:

obtaining signal-to-noise ratio (SNR) estimates for a plurality of data streams; and

rates are selected for the plurality of data streams based on the SNR estimates, and such that at least one data stream has a negative SNR margin, each remaining data stream has a non-negative SNR margin, and the total SNR margin for all data streams is non-negative.

21. The processor-readable medium of claim 20, wherein the medium is further for storing instructions operable to:

selecting a rate for each data stream based on the SNR estimate for the data stream;

determining an SNR margin for each data stream based on the SNR estimate for the data stream and the required SNR for the rate selected for the data stream; and

adjusting at least one rate of the at least one data stream upward based on SNR margins for the plurality of data streams.

22. The processor-readable medium of claim 21, wherein the medium is further to store instructions operable to:

determining a total SNR margin for the plurality of data streams based on the SNR margins for the plurality of data streams; and

reallocating the total SNR margin to the at least one data stream to adjust the at least one rate upward.

23. A method of performing rate selection in a communication system, comprising:

determining a signal-to-noise ratio (SNR) estimate for each of a plurality of transmission channels available for data transmission;

determining a total SNR margin for each of a plurality of rate combinations based on SNR estimates for the plurality of transmission channels, wherein each rate combination is associated with a particular number of data streams to transmit, a particular rate for each data stream, and a particular overall throughput; and

selecting a rate combination from the plurality of rate combinations based on a total SNR margin and a total throughput of the plurality of rate combinations.

24. The method of claim 23, wherein the determining the total SNR margin for each rate combination comprises:

determining an SNR margin for each data stream in the rate combining based on an SNR estimate for a transmission channel used for the data stream and a required SNR for the data stream, an

The SNR margins for all data streams in the rate combination are summed to obtain a total SNR margin for the rate combination.

25. The method of claim 23, further comprising:

each rate combination of at least one data stream having an SNR margin below a predetermined value is removed.

26. The method of claim 23, wherein the selecting the rate combination from a plurality of rate combinations comprises:

selecting a rate combination from the plurality of rate combinations having a highest overall throughput.

27. The method of claim 26, wherein the selecting the rate combination from a plurality of rate combinations further comprises:

if there are multiple rate combinations with the highest overall throughput, then the rate combination with fewer data streams is selected.

28. The method of claim 26, wherein the selecting the rate combination from a plurality of rate combinations comprises:

if there are multiple rate combinations with the highest overall throughput, the rate combination with the larger overall SNR margin is selected.

29. The method of claim 23, further comprising:

the plurality of rate combinations are selected for evaluation in a sequential order, the order beginning with the rate combination having the least data streams and ending with the rate combination having the most data streams.

30. The method of claim 29, further comprising:

the rate combinations for a given number of data streams are selected in a sequential order, starting with the rate combination having the lowest overall throughput and ending with the rate combination having the highest overall throughput.

31. An apparatus in a communication system, comprising:

a channel estimator operable to determine a signal-to-noise ratio (SNR) estimate for each of a plurality of transmission channels available for data transmission; and

a controller operable to determine a total SNR margin for each of a plurality of rate combinations based on SNR estimates for the plurality of transmission channels, and to select a rate combination from the plurality of rate combinations based on the total SNR margin and an overall throughput for the plurality of rate combinations, wherein each rate combination is associated with a particular number of data streams to transmit, a particular rate for each data stream, and a particular overall throughput.

32. The apparatus of claim 31, wherein the controller is operative to determine SNR margins for the data streams based on SNR estimates for transmission channels for each data stream in the rate combinations and required SNRs for the data streams, and to sum the SNR margins for all data streams in each rate combination to obtain a total SNR margin for the rate combinations.

33. The apparatus of claim 31, wherein the controller is operative to select a rate combination from the plurality of rate combinations having a highest overall throughput, and to select a rate combination having fewer data streams or a larger overall SNR margin when there are a plurality of rate combinations having the highest overall throughput.

34. An apparatus in a communication system, comprising:

means for determining a signal-to-noise ratio (SNR) estimate for each of a plurality of transmission channels available for data transmission;

means for determining a total SNR margin for each of a plurality of rate combinations based on SNR estimates for the plurality of transmission channels, wherein each rate combination is associated with a particular number of data streams to transmit, a particular rate for each data stream, and a particular overall throughput; and

means for selecting a rate combination from the plurality of rate combinations based on a total SNR margin and a total throughput of the plurality of rate combinations.

35. The apparatus of claim 34, wherein the means for determining the total SNR margin for each rate combination comprises:

means for determining an SNR margin for each data stream in the rate combining based on an SNR estimate for a transmission channel used for the data stream and a required SNR for the data stream, an

Means for summing SNR margins for all data streams in the rate combining to obtain a total SNR margin for the rate combining.

36. The apparatus of claim 34, wherein the means for selecting the rate combination from a plurality of rate combinations comprises:

means for selecting the rate combination with the highest overall throughput from the plurality of rate combinations, an

Means for selecting a rate combination with fewer data streams or a larger total SNR margin when there are multiple rate combinations with the highest overall throughput.