HK1074943B - Rate selection for an ofdm system - Google Patents

Description

Rate selection for OFDM systems

FIELD

The present invention relates generally to data communication, and more specifically to techniques for selecting a rate for a wireless (e.g., OFDM) communication system.

Background

Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may implement Orthogonal Frequency Diversity Multiplexing (OFDM) modulation, which can provide high performance for certain channel environments. In an OFDM system, the system bandwidth is effectively divided into a plurality of (N)_F) Frequency subchannels (which may be referred to as frequency subbands or frequency bins). Each frequency subchannel is associated with a corresponding subcarrier (or frequency tone) upon which data is modulated. In general, data to be transmitted (i.e., information bits) is encoded with a particular coding scheme to produce coded bits, which are then grouped into multi-bit symbols, which are then mapped to modulation symbols according to a particular modulation scheme (e.g., M-PSK or M-QAM). At each time interval depending on the bandwidth of each frequency subchannel, may be at N_FModulation symbols are transmitted on each of the strip-frequency subchannels.

The frequency subchannels of an OFDM system may experience different channel conditions (e.g., different fading and multipath effects) and may achieve different signal-to-noise-and-interference ratios (SNRs). Each transmitted modulation symbol is affected by the frequency response of the communication channel at the particular frequency subchannel via which it is transmitted. The frequency response may vary widely throughout the system bandwidth, depending on the multipath profile characteristics of the communication channel. Thus, the modulation symbols that collectively form a particular data packet may be via N_FThe strip frequency subchannels are individually received with a wide range of SNRs, which then vary accordingly across the packet.

For a multipath channel with an uneven or non-constant frequency response, the number of information bits per modulation symbol (i.e., data rate or information rate) that can be reliably transmitted on each frequency subchannel may vary from subchannel to subchannel. Furthermore, channel conditions typically change over time. As a result, the data rates supported for the frequency subchannels also change over time.

Since the channel conditions experienced by a given receiver are generally not known a priori, it is impractical to transmit data to all receivers at the same transmit power and/or data rate. Fixing these transmission parameters may result in wasted transmit power, use of sub-optimal data rates for some receivers, and unreliable communication for some other receivers, all of which may result in an undesirable reduction in system capacity. The different transmission capabilities of the communication channels of different receivers, coupled with the time-varying and multi-path characteristics of these channels, make it difficult to efficiently encode and modulate data for transmission in an OFDM system.

There is therefore a need in the art for techniques to select an appropriate rate for data transmission in a wireless (e.g., OFDM) communication system having the above-described channel characteristics.

SUMMARY

Aspects of the present invention provide techniques for determining and selecting a rate for data transmission in a wireless (e.g., OFDM) communication system. These techniques may be used to provide improved system performance for OFDM systems operating either in a multipath (non-flat) channel or in a flat channel.

In one aspect, the maximum data rate that can be reliably transmitted by an OFDM system over a given multipath channel is determined based on a metric for an equivalent frequency flat channel (e.g., a channel with a flat frequency response). For a given multipath channel, defined by a particular frequency response and a particular noise variance, an OFDM system can use a particular modulation scheme m (r) to achieve a particular equivalent data rate D_equiv. Equivalent data rate D_equivMay be estimated based on a particular channel capacity function, such as a constrained channel capacity function or some other function. Then using M (r) and further according to a specific function g (D)_equivM (r)) is D_equivDetermining a metric, the metric being an estimate of SNR for an equivalent frequency-flat channel using a modulation scheme M (r) at an equivalent data rate D_equivThe required SNR is reliably transmitted. A threshold SNR is then determined that is required for the equivalent channel to reliably transmit a particular data rate d (r) using a modulation scheme m (r) and a code rate c (r). If the metric is greater than or equal to the threshold SNR, the data rate D (r) is considered to be supported by the multipath channel.

In another aspect, an Incremental Transmission (IT) scheme is provided, preferably used in conjunction with the rate selection of the first aspect, to reduce the number of back-offs and improve system throughput. An IT scheme uses one or more discrete transmissions to send a given data packet, one transmission at a time, up to a particular limit. The first transmission of the packet includes a sufficient amount of data so that the packet can be recovered error-free at the receiver according to expected channel conditions. However, if the first transmission is degraded excessively by the communication channel such that error-free recovery of the packet is not achieved, incremental transmission of an additional amount of data of the packet is performed. The receiver then attempts to recover the packet based on the additional data in the incremental transmission and all the data previously received for the packet. The incremental transmission by the transmitter and the decoding by the receiver may be done one or more times until the packet is recovered error-free or a maximum number of incremental transmissions is reached.

Various aspects and embodiments of the invention are described in further detail below. The present invention also provides methods, receiver units, transmitter units, receiver systems, transmitter systems, and other apparatuses and elements that can implement various aspects, embodiments, and features of the invention, as described in further detail below.

Brief Description of Drawings

The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like elements have like numerals wherein:

FIG. 1A is a simplified block diagram of an OFDM communication system;

FIG. 1B is a diagram illustrating rate selection for a multipath channel using an equivalent channel;

fig. 2 is a flow diagram of an embodiment of a process for selecting a data rate for use in an OFDM system based on a metric t;

FIG. 3 is a block diagram of an embodiment of a transmitter system and a receiver system capable of implementing various aspects and embodiments of the invention;

FIG. 4 is a block diagram of an embodiment of a transmitter unit; and

fig. 5 is a block diagram of an embodiment of a receiver unit.

Detailed Description

The techniques described herein for determining and selecting a rate for data transmission may be used for various wireless communication systems including one or more independent transmission channels, such as multiple-input multiple-output (MIMO) systems. For simplicity, aspects and embodiments of the present invention are described particularly for Orthogonal Frequency Diversity Multiplexing (OFDM) systems, where independent transmission channels are frequency subchannels or segments formed by dividing the overall system bandwidth.

Fig. 1A is a simplified model diagram of an OFDM system. At the transmitter 110, traffic data is provided from a data source 112 at a particular data rate to an encoder/modulator 114, which encodes the data in accordance with one or more coding schemes and modulates the encoded data in accordance with one or more modulation schemes. The modulation may be achieved by: the sets of coded bits are grouped to form multi-bit symbols, and each multi-bit symbol is mapped to a point in a signal constellation corresponding to a particular modulation scheme (e.g., QPSK, M-PSK, or M-QAM) selected for each frequency subchannel used to transmit the symbol. Each mapped signal point corresponds to a modulation symbol.

In one embodiment, the data rate is determined by data rate control, the coding scheme is determined by coding control, and the modulation scheme is determined by modulation control, all of which are provided by controller 130 based on feedback information received from receiver 150.

The pilot may also be transmitted to the receiver to help it perform several functions, such as channel estimation, acquisition, frequency and timing synchronization, coherent data demodulation, and so on. In this case, the pilot data is provided to an encoder/modulator 114, which then multiplexes and processes the pilot data with traffic data.

For OFDM, the modulated data (i.e., modulation symbols) is then transformed to the time domain by an Inverse Fast Fourier Transformer (IFFT)116 to provide OFDM symbols, wherein each OFDM symbol corresponds to a symbol to be transmitted during the symbol at N_FN transmitted on frequency sub-channel_FA time representation of a vector of modulation symbols. In contrast to single carrier "time-coded" systems, OFDM systems effectively transmit modulation symbols in the "frequency domain" by transmitting an IFFT of the modulation symbols representing traffic data in the time domain. The OFDM symbols are further processed (not shown in fig. 1 for simplicity) to generate a modulated signal, which is then transmitted over a wireless communication channel to a receiver. As shown in fig. 1A, the frequency response of the communication channel is h (f) and the modulated signal is further degraded due to Additive White Gaussian Noise (AWGN) n (t).

At receiver 150, the transmitted modulated signal is received, conditioned, and digitized to provide data samples. A Fast Fourier Transformer (FFT)160 then receives and transforms the data samples to the frequency domain, and the recovered OFDM symbols are provided to a demodulator/decoder 162 and a channel estimator 164. Demodulator/decoder 162 processes (e.g., demodulates and decodes) the recovered OFDM symbols to provide decoded data and may further provide status for each received packet. Channel estimator 164 processes the recovered OFDM symbols to provide an estimate of one or more characteristics of the communication channel, such as the channel frequency response, the channel noise variance, the signal-to-noise-plus-interference ratio (SNR) of the received symbols, and so forth.

Rate selector 166 receives the estimates from channel estimator 164 and determines an appropriate "rate" that may be used for all or a subset of the frequency subchannels used for data transmission. The rate represents a particular set of values for a set of parameters. For example, the rate may indicate (or be associated with) a particular data rate, a particular coding scheme and/or code rate, a particular modulation scheme, and/or the like to be used for data transmission.

The controller 170 receives the rate from the rate selector 166 and the packet status from the demodulator/decoder 162 and provides appropriate feedback information for transmission back to the transmitter 110. The feedback information may include the rate, the channel estimate provided by channel estimator 164, an Acknowledgement (ACK) or Negative Acknowledgement (NACK) of each received packet, some other information, or any combination thereof. The feedback information is used to improve the efficiency of the system by: data processing at the transmitter is adjusted so that data transmission is performed at the best known setting of power and rate supportable by the communication channel. The feedback information is then sent back to the transmitter 110 and used to adjust the processing (e.g., data rate, coding, and modulation) of the data transmission to the receiver 150.

In the embodiment shown in fig. 1A, receiver 150 performs rate selection and provides the selected rate to transmitter 110. In other embodiments, the rate selection may be performed by the transmitter based on feedback information provided by the receiver, or may be performed jointly by the transmitter and the receiver.

Under appropriate conditions, the recovered OFDM symbol at the output of FFT 160 can be represented as:

formula (1)

Where k is the index of the frequency subchannel of the OFDM system, i.e., k is 0, 1_F-1, wherein N_FIs the number of frequency subchannels;

y (k) is a modulation symbol transmitted on the k-th frequency subchannel, which is derived based on the particular modulation scheme used for the k-th frequency subchannel;

h (k) is the frequency response of the communication channel, expressed in "quantized" form for each frequency subchannel;

n (k) N representing time domain noise_FFFT of a sequence of samples, FFT { N (kt) }, where k is 0, 1_F-1; and

t is the sampling period.

In a single carrier system, the transmitted symbols may all be received at the receiver with almost the same SNR. The relationship between SNR and packet error probability for a "constant SNR" packet is well known in the art. As an approximation, the maximum data rate supported by a single carrier system with a particular achieved SNR can be estimated as the maximum data rate supported by an AWGN channel with the same SNR. The main characteristics of the AWGN channel are: its frequency response is flat or constant over the entire system bandwidth.

However, in an OFDM system, the modulation symbols that make up a packet are transmitted on multiple frequency subchannels. The SNR may vary across the packet depending on the frequency response of the frequency subchannel used to transmit the packet. As system bandwidth increases, and for multipath environments, this problem of "varying SNR" packets is exacerbated.

Thus, a major challenge in OFDM systems is determining the maximum data rate that can be used for data transmission while achieving a particular level of performance, which may be determined by a particular Packet Error Rate (PER), Frame Error Rate (FER),Bit Error Rate (BER) or some other criterion. For example, by maintaining the PER within a small window (e.g., P) around a certain nominal value_e1%), a desired level of performance can be achieved.

In a typical communication system, a specific and discrete set of data rates may be defined, only those data rates being available. Each data rate d (r) may be associated with a particular modulation scheme or constellation m (r), and a particular coding rate c (r). Each data rate further requires a particular SNR (r) at which data transmission at that data rate results in a PER less than or equal to the desired PER P_eMinimum SNR when. The snr (r) assumes that the communication channel is AWGN (i.e., has a flat frequency response over the entire system bandwidth, or H for all k: H (k) ═ H). In general, the communication channel between the transmitter and receiver is not AWGN, but is dispersive or frequency selective (i.e., there is a different amount of attenuation at different sub-bands of the system bandwidth). For such multipath channels, the particular data rate to be used for data transmission may be selected to account for the multipath or frequency selective characteristics of the channel.

Thus, each data rate d (r) is associated with a set of parameters that characterize it. These parameters may include the modulation scheme m (r), the code rate c (r), and the required snr (r), as follows:

formula (2)

Where r is the index of the data rate, i.e. r 0, 1_R-1, wherein N_RIs the total number of data rates available for use. Expression (2) states that data rate D (r) can be transmitted using modulation scheme M (r) and coding rate C (r), and further requires SNR (r) in AWGN channel to achieve the desired nominal PER P_e. Can arrange N_RData rate such that D (0)<D(1)<D(2)...<D(N_R-1)。

In accordance with one aspect of the invention, the maximum data rate reliably transmitted over a given multipath channel in an OFDM system is determined based on a metric for an equivalent AWGN channel. If the desired PER P is maintained for the data transmission_eReliable transmission is achieved. Details of this aspect are described below.

Fig. 1B is a diagram illustrating rate selection for a multipath channel using an equivalent channel. For a channel consisting of the channel response H (k) and the noise variance N₀For a defined multipath channel, the OFDM system is able to achieve an equivalent data rate D using a modulation scheme M (k)_equivWhere m (k) may be different for different frequency subchannels. This D_equivMay be based on a particular channel capacity function f [ H (k), N as follows₀，M(k)]To estimate. Since the bandwidth of each individual frequency subchannel is normalized to 1, it is not a function f [ ·]The argument of (2) appears. M (k) may be used and further according to a function g (D)_equivM (k) is D_equivDeriving a metric, the metric being an estimated SNR of the SNR_equivSaid SNR_equivIs the equivalent AWGN channel to the desired PER P_eUsing a modulation scheme M (k) at an equivalent data rate D_equivAs is required for transmission, as is also described below.

For data rate D (k), modulation scheme M (k), and code rate C (k), the AWGN channel would need to have a value of SNR_thOr better SNR to achieve the desired PER P_e. The threshold SNR_thMay be determined by computer simulation or some other means. If metric (or SNR)_equiv) Is equal to or greater than SNR_thThe data rate D (k) is considered to beSupported by an OFDM system of multipath channels. Threshold SNR increases with data rate D (k)_thFor H (k) and N₀The defined given channel condition increases. Therefore, the maximum data rate supported by an OFDM system is limited by channel conditions. Various schemes are provided herein to determine the maximum data rate that can be supported by an OFDM system for a given multipath channel. Some of these schemes are described below.

In a first rate selection scheme, the metric t receives a set of parameters for data transmission over a given multipath channel in an OFDM system, and provides an estimate of the SNR for an AWGN channel equivalent to the multipath channel based on the received parameters. These input parameters to the metric t may include one or more parameters related to the processing of the data transmission (e.g., modulation scheme m (k)) and one or more parameters related to the communication channel (e.g., channel response h (k) and noise variance N)₀). As described above, the modulation scheme m (k) may be associated with a particular data rate d (k). The metric t is an estimate of the SNR of the equivalent AWGN channel (i.e., t ≈ SNR)_equiv). The maximum data rate supported by the multipath channel is then determined as the highest data rate associated with an equivalent SNR that is greater than or equal to the threshold SNR_thSaid SNR_thIs that the AWGN channel uses a coding and modulation scheme related to the data rate to achieve the desired PER P_eAs required.

Various functions may be used for the metric t, some of which are provided below. In one embodiment, the metric ψ is defined as:

formula (3)

In equation (3), the function f [ H (k), N₀，M]It is determined that the modulation scheme M may have a frequency response H (k) and a noise variance N₀The maximum data rate supported on the kth frequency subchannel. Function f [ H (k), N₀，M]May be defined in terms of various channel capacity functions, as described below.

Parameters H (k) and N₀May be mapped to snr (k). If the total transmission power P of the system_totalIs fixed and transmits power to N_FThe allocation of the frequency subchannels is uniform and fixed, and then the SNR of each frequency subchannel can be expressed as:

formula (4)

SNR (k) is the channel response H (k) and the noise variance N, as shown in equation (4)₀A function of (2), bothIs a function f [ H (k), N₀，M]Two parameters of (2).

For all N_FF [. on a strip frequency subchannel]The addition in equation (3) is performed to provide an equivalent data rate D that can be transmitted over the AWGN channel_equiv. Then, function g (D)_equivM) determining an equivalent data rate D for using the modulation scheme M_equivA required SNR in AWGN channel for reliable transmission.

Equation (3) assumes all N in an OFDM system_FThe same modulation scheme M is used for the bar frequency subchannels. This limitation results in simplified processing at the transmitter and receiver in an OFDM system, but sacrifices performance.

If different modulation schemes can be used for different frequency subchannels, the metric t can be defined as:

formula (5)

As shown in equation (5), the modulation scheme m (k) is a function of the index k of the frequency subchannel. The use of different modulation schemes and/or coding rates for different frequency subchannels is also referred to as "bit loading".

Function f [ x ]]The data rate that can be reliably transmitted over the AWGN channel is determined for a set of parameters generally denoted as x, where x may be a function of frequency (i.e., x (k)). In equation (5), the function f [ H (k), N₀，M(k)]Determining a data rate at which a modulation scheme M (k) may be transmitted on a k-th frequency subchannel, wherein x (k) { H (k), N₀M (k), said k-th frequency subchannel having a frequency response H (k) and a noise variance N (k)₀. Then, function g (f [ x (k))]M (k) determines the number of bits to be transmitted from f [ x (k))]The determined data rate and the required SNR in an equivalent AWGN channel. Then for all N_FG (f [ x (k))]M (k)) performs the addition in equation (5) to provide an estimate of the SNR of the equivalent AWGN channel: SNR_equiv。

The function f x may be defined according to various channel capacity functions or some other function or technique]. The absolute capacity of a system is typically given as the theoretical maximum data rate for the channel response h (k) and the noise variance N₀Can be reliably transmitted. The "constrained" capacity of the system depends on the particular modulation scheme or constellation m (k) used for data transmission and is lower than the absolute capacity.

In one embodiment, the function f [ H (k), N₀，M(k)]May be defined according to a constrained channel capacity function and may be expressed as:

formula (6)

Wherein M is_kIn relation to modulation schemes M (k), i.e. modulation schemes M (k) correspond toMeta-constellations (e.g. M)-meta QAM), wherein in the constellation diagramEach of the points may be defined by M_kA number of bits to identify;

a_iand a_jIs that-a point in a meta-constellation;

x is a complex gaussian random variable with zero mean and variance of 1/snr (k); and

e [. cndot.) is the expected value operation, which is employed with respect to variable x in equation (6).

The constrained channel capacity function shown in equation (6) does not have a closed-form solution. In this way, the function is numerically derived for various modulation schemes and SNR values, the results of which may be stored in one or more tables. Thereafter, the function f [ x ] can be evaluated by accessing the appropriate table with a particular modulation scheme and SNR.

In another embodiment, the function f [ x ] is defined according to the shannon (or theoretical) channel capacity function and can be expressed as:

f(k)＝log₂[1+SNR(k)]formula (7)

Where W is the system bandwidth. As shown in equation (7), the shannon channel capacity is not constrained by any given modulation scheme (i.e., m (k) is not a parameter in equation (7)).

The selection of a particular function for f x to use may depend on various factors, such as the OFDM system design. For a typical system employing one or more particular modulation schemes, the matrix ψ defined in equation (3) has been found to be an accurate estimate of the maximum supported data rate for OFDM systems for AWGN channels as well as multipath channels when used in conjunction with the constrained channel capacity of the function f [ x ] in equation (6).

The function g (f [ x ], m (k)) determines the required SNR in the AWGN channel to support the equivalent data rate determined by the function f [ x ] using the modulation scheme m (k). In one embodiment, the function g (f [ x ], M (k)) is defined as:

g(f[x]，M(k))＝f[x]^-1formula (8)

Due to the function f [ x ]]Depending on the modulation scheme M (k), the function g (f [ x ]) is therefore]M (k)) also depends on the modulation scheme. In an embodiment, a function f x may be derived for each modulation scheme]^-1It can be selected for use and saved to the corresponding table. Then for f [ x ] by accessing a specific table of modulation schemes M (k)]Given value of (c) evaluation function g (f [ x ]]M (k)). Function g (f [ x ]]M (k)) may also be defined using other functions, or derived by other means, which is within the scope of the invention.

Fig. 2 is a flow diagram of an embodiment of a process 200 for selecting a data rate for use in an OFDM system based on a metric t. First, the available data rates (i.e., those supported by the OFDM system) are arranged such that D (0)<D(1)<...<D(N_R-1). The highest available data rate is then selected in step 212 (e.g., by setting a rate variable to the index of the highest data rate). Various parameters, such as the modulation scheme m (rate), associated with the selected data rate d (rate) are then determined in step 214. Depending on the design of the OFDM system, each data rate may be associated with one or more modulation schemes. Each modulation scheme for the selected data rate is then evaluated according to the following steps. For simplicity, it is assumed in the following that only one modulation scheme is associated with each data rate.

The metric t is then evaluated for the particular modulation scheme m (rate) associated with the selected data rate d (rate). This can be achieved by evaluating a function of the metric ψ, as shown in equation (3), as follows:

the metric t represents the estimate of the SNR required in an equivalent AWGN channel in order to reliably transmit an equivalent data rate using the modulation scheme m (rate).

The threshold SNR is then determined in step 218: SNR_th(rate) the threshold SNR is for a desired PER P in an AWGN channel_eThe SNR required to transmit the selected data rate d (rate). Threshold SNR_th(rate) is a function of the modulation scheme m (rate) and the coding rate c (rate) associated with the selected data rate. The threshold SNR may be determined for each of the possible data rates by computer simulation or by some other means and saved for later use.

It is then determined in step 220 whether the metric ψ is greater than or equal toSelecting a threshold SNR associated with a data rate_th(rate). If the metric t is greater than or equal to the SNR_th(rate) which indicates that the SNR achieved by an OFDM system is sufficient to achieve the desired PER P for the data rate D (rate) in a multipath channel_eThen the data rate is selected for use at step 224. Otherwise, the next lower available data rate is selected for evaluation at step 222 (e.g., by decrementing the rate variable by 1, i.e., rate-1). The next lower data rate is then evaluated by returning to step 214. Steps 214 through 222 may be repeated as many times as necessary until the maximum supported data rate is identified and provided in step 222.

The metric t is a monotonic function of the data rate and increases with increasing data rate. The threshold SNR is also a monotonic function that increases with increasing data rate. The embodiment shown in fig. 2 evaluates the available data rates, one at a time, from the maximum available data rate to the minimum available data rate. Selection and threshold SNR_th(rate) associated highest data rate for use, where SNR_th(rate) is less than or equal to the metric ψ.

In another embodiment, the metric ψ may be evaluated for a particular modulation scheme m (r) to derive an estimate of the SNR of the equivalent AWGN channel: SNR_equiv(r) of (A). The maximum data rate D supported by the AWGN channel is then determined (e.g., via a look-up table) for the desired SNR at the equivalent SNR using the modulation scheme m (r)_max(r) of (A). The actual data rate used in the OFDM system for the multipath channel is then selected to be less than or equal to the maximum data rate D supported by the AWGN channel_max(r)。

In the second rate selection scheme, the metric ψ is defined as the post-detection SNR achieved by the single carrier system for the multipath channel after equalization. The post-detection SNR represents the ratio of the total signal power to the noise plus interference after equalization at the receiver. Theoretical values of post-detection SNR achieved in a single carrier system with equalization may be indicative of the performance of an OFDM system and may therefore be used to determine the maximum supported data rate in an OFDM system. Various types of equalizers may be used to process a received signal in a single carrier system to compensate for distortion introduced by a multipath channel in the received signal. Such equalizers may include, for example: minimum mean square error linear equalizer (MMSE-LE), Decision Feedback Equalizer (DFE), etc.

The post-detection SNR of the (infinite length) MMSE-LE can be expressed as:

formula (9a)

Wherein J_minIs given as

Formula (9b)

Wherein X (e)^jωT) Is the folded spectrum of the channel transfer function h (f).

The post-detection SNR of the (infinite length) DFE can be expressed as:

formula (10)

The post-detection SNRs for MMSE-LE and DFE shown in equations (9) and (10) represent theoretical values, respectively. The post-detection SNR of MMSE-LE and DFE is also detailed in this specification below: proakis, entitled "Digital Communications", third edition, 1995, McGraw Hill press, in sections 10-2-2 and 10-3-2, respectively, which is incorporated herein by reference.

Post-detection SNR of the MMSE-LE and DFE is also estimated at the receiver from the received signal, as described in U.S. patent application Ser. Nos. 09/826,481 and 09/956,449, both entitled "Method and Apparatus for using Channel State Information in a Wireless communication System", filed on 23/3/2001 and 18/9/2001, respectively, and U.S. patent application Ser. No. 09/854,235, entitled "Method and Apparatus for processing Data in a Multiple-Input Multiple-output (MIMO) communication System using Channel State Information", filed on 11/5/2001, which are assigned to the assignee of the present invention and incorporated herein by reference.

Those post-detection SNRs described by the analytical expressions shown in equations (9) and (10) can be determined for the multipath channel and used as an estimate of the metric ψ (i.e., # SNR ≈ SNR)_mmse-leOr psi ≈ SNR_dfe). Post-detection SNR (e.g., SNR) of an equivalent AWGN channel_mmse-leOr SNR_dfe) Can be associated with a specific set of parameters D (r), M (r), C (r) and P_eDerived threshold SNR_thCompared to determine the data rate that may be used in the OFDM system for the multipath channel.

The metric t may also be defined according to some other function and the equivalent data rate may also be estimated according to some other technique, which is within the scope of the invention.

Selecting a data rate representation for use in an OFDM system based on a metric psi for a desired PER P_ePrediction of data rates that may be supported by a multipath channel. As with any rate prediction scheme, there is inevitably a prediction error. To ensure that a desired PER can be achieved, a prediction error may be estimated and a backoff factor used in determining the data rate that may be supported by the multipath channel. This backoff reduces the throughput of the OFDM system. Thus, it is desirable to keep this back-off as small as possible while still achieving the desired PER.

According to another aspect of the present invention, an Incremental Transmission (IT) scheme is provided that may be advantageously used in conjunction with the rate selection of the first aspect in order to reduce the amount of back-off and improve system throughput. The IT scheme sends a given packet using one or more discontinuous transmissions, one transmission at a time, up to a certain upper limit. The first transmission of a packet includes a sufficient amount of data so that the packet can be recovered without error at the receiver based on expected channel conditions. However, if the first transmission is degraded excessively by the communication channel such that error-free recovery of the packet is not achieved, incremental transmission of additional data amounts is performed for the packet. The receiver then attempts to recover the packet based on the additional data transmitted incrementally and all the data previously received for the packet. The incremental transmission by the transmitter and the decoding by the receiver may be attempted one or more times until the packet is recovered without error or a maximum number of incremental transmissions is reached.

One embodiment of the IT scheme may be implemented as follows. First, the data of the packet is encoded with a lower coding rate (for forward error correction codes) than the coding rate used for packets that are transmitted without any delta. Next, some of the coded bits are truncated and only a subset of all the coded bits are sent for the first transmission of the packet. If the packet is received correctly, the receiver sends back an Acknowledgement (ACK) indicating that the packet was received without error. Alternatively, if the receiver receives the packet with errors, it sends back a Negative Acknowledgement (NACK).

In either case, if the transmitter does not receive an acknowledgement of the packet, or receives a negative acknowledgement, the transmitter sends an incremented packet to the receiver. The incremental packet may include some of the original truncated coded bits that were not sent in the first transmission. The receiver then attempts to decode the packet by using the coded bits sent in both the first transmission and the second transmission. The additional coded bits of the second transmission provide more energy and improve the error correction capability. One or more incremental transmissions may be performed, typically once per transmission, until an acknowledgement is received or no negative acknowledgement is received.

If the system employs incremental transmission, then less backoff may be used to account for rate prediction error and a more progressive rate selection may be made. This results in improved system throughput.

Incremental transmission, and the rate selection described above, also provides an efficient mechanism for determining the maximum data rate supported by a fixed or slowly varying communication channel. Consider a fixed access application in which the multipath profile of the channel varies slowly. In this case, the initial data rate may be selected according to the techniques described above and used for data transmission. If the initial data rate is higher than the channel can support, the IT scheme may send additional coded bits until the packet can be correctly decoded at the receiver. The maximum data rate that the channel can support is then determined based on the total number of coded bits sent in the first transmission and any subsequent incremental transmissions. If the channel changes slowly, the determined data rate may be used until the channel changes, at which point a new data rate may be determined.

Incremental transmission provides a number of advantages. First, the use of incremental transmission allows for progressive data rate selection to improve system throughput. Second, incremental transmission provides a means to remedy prediction errors that inevitably arise for any rate prediction scheme (the frequency and magnitude of the prediction error depends on the amount of backoff employed). Third, incremental transmission provides a mechanism to more accurately determine the maximum supported data rate for fixed or slowly varying channels.

Fig. 3 is a block diagram of one embodiment of a transmitter system 110a and a receiver system 150a, both of which are capable of implementing various aspects and embodiments of the present invention.

At transmitter system 110a, traffic data is provided from a data source 308 at a particular data rate to a Transmit (TX) data processor 310, which formats, interleaves, and codes the data according to a particular coding scheme to provide coded data. The data rate and coding may be provided by data rate control and coding control, respectively, provided by controller 330.

The coded data is then provided to a modulator 320, which also receives pilot data (e.g., data in a known pattern and processed in a known manner, if any). In all or a subset of the frequency subchannels used to transmit traffic data, pilot data is multiplexed with the coded traffic data using Time Division Multiplexing (TDM) or Code Division Multiplexing (CDM). In a particular embodiment, for OFDM, the slave processing by modulator 320 includes: (1) modulate the received data with one or more modulation schemes, (2) transform the modulated data to form OFDM symbols, and (3) append a cyclic prefix to each OFDM symbol to form a corresponding transmission symbol. Modulation is performed according to modulation control provided by controller 330. The modulated data (i.e., transmission symbols) is then provided to a transmitter (TMTR) 322.

Transmitter 322 converts the modulated data into one or more analog signals and further conditions (e.g., amplifies, filters, and quadrature modulates) the analog signals to generate a modulated signal suitable for transmission over a communication channel. The modulated signal is then transmitted via an antenna 324 to a receiver system.

At receiver system 150a, the transmitted modulated signal is received by an antenna 352 and provided to a receiver (RCVR) 354. Receiver 354 conditions (e.g., filters, amplifies, and downconverts) the received signal and digitizes the conditioned signal to provide data samples. A demodulator (Demod)360 then processes the data samples to provide demodulated data. For OFDM, the processing by demodulator 360 may include: (1) removing the cyclic prefix previously appended to each OFDM symbol, (2) transforming each recovered OFDM symbol, and (3) demodulating the recovered modulation symbols in accordance with one or more demodulation schemes that are complementary to the one or more modulation schemes used at the transmitter system.

A Receive (RX) data processor 362 then decodes the demodulated data to recover the transmitted traffic data. The processing by demodulator 360 and RX data processor 362 is complementary to that performed by modulator 320 and TX data processor 310, respectively, at transmitter system 110 a.

As shown in FIG. 3, demodulator 360 may derive an estimate of the channel response(k) And provides these estimates to controller 370. RX data processor 362 may also derive and provide the status of each received packet and may further provide one or more other performance metrics indicative of the decoded results. Depending on the type of information received from demodulator 360 and RX data processor 362, controller 370 may determine or select a particular rate for data transmission in accordance with the techniques described above. The feedback information, which may be in the form of a selected rate, channel response estimate, ACK/NACK of received packets, etc., may be provided by controller 370, processed by TX data processor 378, modulated by modulator 380, and conditioned by transmitter 354 and transmitted back to transmitter system 110 a.

At transmitter system 110, the modulated signal from receiver system 150a is received by antenna 324, conditioned by receiver 322, and demodulated by demodulator 340 to recover the feedback information sent by the receiver system. The feedback information is then provided to the controller 330 and used to control the processing of the data transmission to the receiver system. For example, the data rate of the data transmission may be determined based on a selected rate provided by the receiver system, or may be determined based on a channel frequency response from the receiver system. The particular coding and modulation scheme associated with the selected rate is determined and reflected in the coding and modulation control provided to TX data processor 310 and modulator 320. The received ACK/NACK may be used to initiate an incremental transmission (not shown in fig. 3 for simplicity).

Controllers 330 and 370 direct operation at the transmitter and receiver systems, respectively. Memories 330 and 372 provide storage for program codes and data used by controllers 330 and 370, respectively.

Fig. 4 is a block diagram of a transmitter unit 400, which is one embodiment of a transmitter portion of transmitter system 110 a. The transmitter unit 400 includes: (1) a TX data processor 310a that receives and processes traffic data to provide coded data, and (2) a modulator 320a that modulates the coded data to provide modulated data. TX data processor 310a and modulator 320a are one embodiment of TX data processor 310 and modulator 320, respectively, in fig. 3.

In the particular embodiment shown in fig. 4, TX data processor 310a includes an encoder 412, a channel interleaver 414, and a puncturer 416. Encoder 412 receives traffic data and encodes it according to one or more coding schemes to provide coded bits. The encoding improves the reliability of the data transmission. The respective coding schemes may include any combination of CRC coding, convolutional coding, Turbo coding, block coding, and other coding, or no coding at all. Traffic data may be divided into packets (or frames), each of which may be processed and transmitted separately. In one embodiment, for each packet, the data in the packet is used to generate a set of CRC bits that are appended to the data, which are then encoded with a convolutional code or a Turbo code to generate encoded data for the packet.

Channel interleaver 414 then interleaves the coded bits according to a particular interleaving scheme to provide diversity. Interleaving provides time diversity for the coded bits, allows data to be transmitted according to the average SNR of the frequency subchannels used for the data transmission, counters fading, and further eliminates correlation between the coded bits used to form the individual modulation symbols. Interleaving may also provide frequency diversity if the coded bits are transmitted on multiple frequency subchannels.

Puncturer 416 then punctures (i.e., deletes) zero or more interleaved coded bits and provides the desired number of coded bits that have not been punctured to modulator 320 a. Puncturer 416 may also provide punctured coded bits to a buffer 418 that stores the coded bits, should they be needed for later incremental transmission, as described above.

In the particular embodiment depicted in FIG. 4, modulator 320a includes symbol mapping element 422An IFFT 424, and a cyclic prefix generator 426. Symbol mapping element 422 maps the multiplexed pilot data and coded traffic data into modulation symbols for one or more frequency subchannels used for data transmission. One or more modulation schemes may be used for the frequency subchannels, as indicated by modulation control. For each modulation scheme selected for use, modulation may be achieved by grouping sets of received bits to form multi-bit symbols and mapping each multi-bit symbol to a point in a signal constellation corresponding to the selected modulation scheme (e.g., QPSK, M-PSK, M-QAM, or some other scheme). Each mapped signal point corresponds to a modulation symbol. Symbol mapping element 422 then provides (up to N) for each transmission symbol period_FA number of) vectors of modulation symbols, the number of modulation symbols in each vector corresponding to the number of frequency subchannels (up to N) selected for use for the transmission symbol period_FA strip).

The IFFT 424 converts each vector of modulation symbols into its time-domain representation (referred to as an OFDM symbol) using an inverse fast fourier transform. The IFFT 424 is designed to pair any number of frequency subchannels (e.g., 8, 16, 32_F..) the inverse transformation is performed. In an embodiment, for each OFDM symbol, cyclic prefix generator 426 repeats a portion of the OFDM symbol to form a corresponding transmission symbol. The cyclic prefix ensures that the transmission symbol retains its orthogonal properties in the presence of multipath delay spread, thereby improving performance against deleterious path effects. The transmission symbols from cyclic prefix generator 426 are then provided to transmitter 322 (see fig. 3) and processed to generate a modulated signal, which is then transmitted from antenna 324.

Other transmitter unit designs may also be implemented within the scope of the invention. Implementations of encoder 412, channel interleaver 414, puncturer 416, symbol mapping element 422, IFFT 424, and cyclic prefix generator 426 are known in the art and are not described in detail herein.

Coding and modulation for OFDM and other systems is described in further detail in the following U.S. patent applications: the above-mentioned U.S. patent application serial nos. 09/826,481, 09/956,449, and 09/854,235; U.S. patent application Ser. No. 09/776,075, entitled "Coding Scheme for a Wireless Communication System", filed on 2/1/2001; and U.S. patent application Ser. No. [ attorney docket No. 010254], entitled "Multiple-Access Multiple-Input Multiple-output (MIMO) communication System," filed on 11/6 of 2001, all of which are assigned to the assignee of the present invention and incorporated herein by reference.

An exemplary OFDM System is described in U.S. patent application serial No. 09/532,492, entitled "High Efficiency, High Performance Communication System employing multiple-Carrier Modulation", filed on 30/3/2000, assigned to the assignee of the present invention and incorporated herein by reference. OFDM is also described in a paper entitled "Multicarrier Modulation for DataTransmission: An Idea white Time Has Come," which is written by a.c. bingham, published in the IEEE journal of communications at 1990 month 5, and incorporated herein by reference.

Fig. 5 is a block diagram of an embodiment of a receiver unit 500, the receiver unit 500 being one embodiment of a receiver portion of the receiver system 150a of fig. 3. Signals transmitted from the transmitter system are received by antennas 352 (fig. 3) and provided to receivers 354 (also referred to as front-end processors). Receiver 354 conditions (e.g., filters and amplifies) the received signal, downconverts the conditioned signal to an intermediate frequency or baseband, and digitizes the downconverted signal to provide data samples, which are then provided to a demodulator 360 a.

Within demodulator 360a (fig. 5), the data samples are provided to a cyclic prefix removal element 510, which removes the cyclic prefix included in each transmission symbol to provide a corresponding recovered OFDM symbol. FFT 512 then transforms each recovered OFDM symbol using a fast Fourier transform and is used for data transmission for each transmission symbol period (up to N)_FStripe) frequency subchannel provision (up to N)_FOnes) restored toneA vector of symbols is made. The recovered modulation symbols from FFT 512 are provided to demodulation element 514 and demodulated in accordance with one or more demodulation schemes that are complementary to the one or more modulation schemes used at the transmitter system. The demodulated data from demodulation element 514 is then provided to a RX data processor 362 a.

Within RX data processor 362a, the demodulated data is deinterleaved by a deinterleaver 522 in a manner complementary to that performed at the transmitter system and further decoded by a decoder 524 in a manner complementary to that performed at the transmitter system. For example, a Turbo decoder or a Viterbi decoder may be used for decoder 524 if Turbo or convolutional coding, respectively, is performed at the transmitter unit. The decoded data from decoder 524 represents an estimate of the transmitted data. Decoder 524 may provide the status of each received packet (e.g., received correctly or in error). Decoder 524 may further save the demodulated data of packets that were not decoded correctly so that the data may be combined with data from subsequent delta transmissions and decoded.

As shown in FIG. 5, channel estimator 516 may be designed to estimate the channel frequency responseSum noise varianceAnd provides these estimates to controller 370. The channel response and noise variance may be estimated from the data samples received for the pilot symbols (e.g., from the FFT coefficients of the pilot symbols from FFT 512).

Controller 370 may be designed to implement various aspects and embodiments of rate selection and signaling for incremental transmissions. For rate selection, controller 370 may determine the maximum data rate to use for a given channel condition based on the metric t, as described above. For incremental transmissions, controller 370 may provide an ACK or NACK for each received transmission of a given packet, which may be used to transmit additional portions of the packet at the transmitter system if the packet cannot be properly recovered at the receiver system.

Fig. 1A and 3 show a simple design for the transmitter to send back the rate for data transmission. Other designs may also be implemented and are within the scope of the invention. For example, the channel estimate may be sent to the transmitter (instead of the rate), which then determines the rate for data transmission based on the received channel estimate.

The rate selection and incremental transmission techniques described herein may be implemented with various designs. For example, the channel estimator 516 of fig. 5 used to derive and provide channel estimates may be implemented with various elements within the receiver system. Some or all of the processing for determining the rate may be performed by controller 370 (e.g., using one or more look-up tables stored in memory 372). Other designs for performing rate selection and incremental transmission are also contemplated as being within the scope of the present invention.

The rate selection and incremental transmission techniques 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, certain elements used to implement rate selection and/or incremental transmission 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, some portions of the rate selection and/or incremental transmission may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described above. The software codes may be stored in a memory unit (e.g., memory 332 or 372 in fig. 3) and executed by a processor (e.g., controller 330 or 370). The memory unit may be implemented within the processor or external to the processor, where it may be communicatively coupled to the processor via various means as is known in the art.

The previous description of the preferred 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 the use of the inventive faculty. 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 for determining a data rate for a data transmission over a communication channel in a wireless communication system, the method comprising the steps of:

identifying a set of parameters for the data transmission;

estimating one or more characteristics of the communication channel;

deriving a metric for an equivalent channel based on the set of parameters and the one or more estimated channel characteristics;

determining a threshold signal quality required for the equivalent channel to support a particular data rate; and

indicating whether the communication channel supports the particular data rate based on the metric and the threshold signal quality.

2. The method of claim 1, wherein the set of parameters includes a particular coding scheme and a particular modulation scheme to be used for the data transmission.

3. The method of claim 2, wherein the one or more estimated channel characteristics comprise an estimated frequency response of the communication channel and an estimated noise variance of the communication channel.

4. The method of claim 1, wherein the equivalent channel has a flat frequency response over a system bandwidth.

5. The method of claim 2, wherein the step for deriving a metric comprises: an equivalent data rate of the equivalent channel is determined from a first function, the set of parameters, and the one or more estimated channel characteristics, and the metric is derived from a second function, the equivalent data rate, and the particular modulation scheme.

6. The method of claim 5, wherein the first function is a constrained channel capacity function.

7. The method of claim 5, wherein the second function is an inverse function of the first function.

8. The method of claim 1, wherein the signal quality is quantified by a signal-to-noise-and-interference ratio (SNR).

9. The method of claim 8, wherein the step for deriving a metric comprises: according to a particular equalizer, a post-detection SNR of the communication channel is estimated, and the signal quality estimated for the equivalent channel is the estimated post-detection SNR.

10. The method of claim 2, wherein a single modulation scheme is used for all frequency subchannels used for the data transmission.

11. The method of claim 2, wherein a plurality of modulation schemes are used for a plurality of frequency subchannels used for the data transmission.

12. The method of claim 1, wherein the wireless communication system is an orthogonal frequency division multiplexing, OFDM, system.

13. A method for determining a rate of data transmission over a communication channel in an orthogonal frequency division multiplexing, OFDM, system, the method comprising the steps of:

identifying a set of parameters for a particular rate, the set of parameters indicating a particular data rate, a particular modulation scheme, and a particular coding scheme;

estimating one or more characteristics of the communication channel;

deriving an equivalent data rate from the first function, the set of parameters and the one or more estimated channel characteristics;

deriving a metric for the equivalent channel from a second function, the equivalent data rate, and the particular modulation scheme;

determining a threshold signal-to-noise-and-interference ratio, SNR, required by the equivalent channel to support the particular data rate with the particular modulation and coding scheme; and

indicating that the communication channel supports the particular rate if the metric is greater than or equal to the threshold SNR.

14. The method of claim 13, wherein the first function is a constrained channel capacity function.

15. The method of claim 13, wherein the first function is a shannon channel capacity function.

16. The method of claim 13, wherein the particular rate is selected from a set of available rates, and each of the one or more available rates is evaluated to determine a highest data rate supported by the communication channel.

17. The method of claim 13, wherein said step for deriving an equivalent data rate and said step for deriving a metric are both accomplished by detecting SNR after estimation for the communication channel after equalization by a particular equalizer.

18. The method of claim 17, wherein the particular equalizer is a minimum mean square error linear equalizer (MMSE-LE) or a Decision Feedback Equalizer (DFE).

19. A method for transmitting data over a communication channel in an orthogonal frequency division multiplexing, OFDM, system, the method comprising:

identifying an initial rate to be used for data transmission on the communication channel;

processing data for transmission over the communication channel in accordance with the initial rate;

transmitting a first portion of the processed data;

receiving an indication that the data transmission was received incorrectly; and

additional portions of the processed data are transmitted.

20. The method of claim 19, wherein the initial rate is determined based on a signal-to-noise-and-interference ratio (SNR) estimated for an equivalent channel.

21. The method of claim 19, wherein the initial rate indicates a particular data rate, a particular modulation scheme, and a particular coding scheme to be used for the data transmission.

22. The method of claim 21, wherein the processing step comprises:

encoding data according to the specific encoding scheme;

puncturing the encoded data according to a particular puncturing scheme; and

the encoded data that is not punctured is modulated according to the particular modulation scheme.

23. The method of claim 22, wherein the first portion comprises the encoded data that was not punctured and the additional portion comprises encoded data that was previously punctured but has not been transmitted.

24. The method of claim 19, further comprising:

the sending of the additional portion is repeated one or more times until an indication is received that the data transmission was correctly received.

25. The method of claim 19, wherein each additional portion to be sent in response to receiving the indication of incorrect reception comprises processed data that was not previously sent.

26. A receiver unit in a wireless communication system, comprising:

a channel estimator for deriving an estimate of one or more characteristics of a communication channel used for data transmission; and

a rate selector for receiving a channel estimate from the channel estimator and a set of parameters indicative of a particular rate of the data transmission; deriving a metric for the equivalent channel; determining a threshold signal quality required for the equivalent channel to support the particular rate; and indicating whether the communication channel supports the particular rate based on the metric and the threshold signal quality.

27. The receiver unit of claim 26, further comprising:

a decoder for providing a status of each received transmission of a particular data packet; and

a controller for providing feedback information consisting of the indication of the particular rate and the packet status.

28. The receiver unit of claim 26, wherein the rate selector is further operative to determine an equivalent data rate for the equivalent channel based on a first function, the set of parameters, and the channel estimate, and to derive the metric for the equivalent channel based on a second function, the equivalent data rate, and a particular modulation scheme associated with the particular rate.

29. The receiver unit of claim 28, wherein the first function is a constrained channel capacity function.

30. The receiver unit of claim 28, further comprising:

a memory for storing one or more tables for the first function.

31. A receiver apparatus in a wireless communication system, comprising:

means for deriving an estimate of one or more characteristics of a communication channel used for data transmission;

means for deriving a metric for an equivalent channel based on the channel estimate and a set of parameters indicating a particular rate for the data transmission;

means for determining a threshold signal quality required for the equivalent channel to support the particular rate; and

means for indicating whether the communication channel supports the particular rate based on the metric and the threshold signal quality.

32. The receiver apparatus of claim 31, further comprising: means for determining an equivalent data rate of the equivalent channel based on a first function, the set of parameters, and the channel estimate, and the metric is

Derived from a second function, the equivalent data rate, and a particular modulation scheme associated with the particular rate.

33. The receiver apparatus of claim 32, further comprising:

means for storing one or more tables for the first function.

34. A transmitter unit in an orthogonal frequency division multiplexing, OFDM, system, comprising:

a controller to identify an initial rate to be used for data transmission over a communication channel, wherein the initial rate represents a particular data rate, a particular modulation scheme, and a particular coding scheme to be used for the data transmission, and to receive an indication that the data transmission was received correctly or incorrectly;

a transmit data processor for encoding data according to the particular encoding scheme;

a modulator for modulating the first portion of the encoded data in accordance with the particular modulation scheme and further modulating additional portions of the encoded data if an indication that the data transmission was received incorrectly is received; and

a transmitter for transmitting the modulated data.

35. The transmitter unit of claim 34, wherein the transmit data processor is further operative to puncture encoded data in accordance with a particular puncturing scheme, and wherein the first portion comprises encoded data that has not been punctured and the additional portion comprises encoded data that has been previously punctured and has not been transmitted.

36. A transmitter apparatus in a wireless communication system, comprising:

means for identifying an initial rate to be used for data transmission on a communication channel, wherein the initial rate represents a particular data rate, a particular modulation scheme, and a particular coding scheme to be used for the data transmission;

means for encoding data according to the particular encoding scheme;

means for modulating a first portion of said encoded data in accordance with said particular modulation scheme;

means for receiving an indication of correct or incorrect receipt of the data transmission at a receiver;

means for modulating an additional portion of said encoded data if an indication that said data transmission was received incorrectly; and

means for transmitting modulated data.