Abstract:
Systems and techniques relating to wireless signal processing. A described technique includes receiving a signal, having subcarriers, over a wireless channel formed by a number-of-receive-antennas and a number-of-transmit-antennas; determining a signal quality measure of a received signal having subcarriers, the signal quality measure being based on channel gain matrices corresponding respectively to the subcarriers of the received signal, the channel gain matrices having dimensions of the number-of-receive-antennas by the number-of-transmit-antennas; determining a channel quality measure of the received signal that measures a frequency selectivity of the wireless channel; determining a data rate of information transmission over the wireless channel based on the signal quality measure and the channel quality measure, the signal quality measure serving as a primary determinant of the data rate and the channel quality measure serves as a secondary determinant of the data rate; and transmitting information over the wireless channel in accordance with the data rate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of the priority of U.S. application Ser. No. 12/750,531 filed Mar. 30, 2010 and entitled “ADAPTIVELY DETERMINING A DATA RATE OF PACKETIZED INFORMATION TRANSMISSION OVER A WIRELESS CHANNEL” (now U.S. Pat. No. 8,687,510), which is a continuation of and claims the benefit of the priority of U.S. application Ser. No. 10/988,318 filed Nov. 12, 2004 and entitled “ADAPTIVELY DETERMINING A DATA RATE OF PACKETIZED INFORMATION TRANSMISSION OVER A WIRELESS CHANNEL” (now U.S. Pat. No. 7,697,449), which claims the benefit of the priority of: (1) U.S. Provisional Application Ser. No. 60/590,042, filed Jul. 20, 2004 and entitled “A Mechanism to Predict the Physical Layer Transmission Parameters for Use in Current and Future Generations of Wireless LANs”; and (2) U.S. Provisional Application Ser. No. 60/590,138, filed Jul. 21, 2004 and entitled “A Mechanism to Predict the Physical Layer Transmission Parameters for Use in Current and Future Generations of Wireless LANs,” all of which are hereby incorporated by reference. 
    
    
     This application is related to U.S. patent application Ser. No. 10/799,548, filed Mar. 11, 2004 and entitled “A MECHANISM TO IMPROVE QUALITY OF CHANNEL ESTIMATES IN OFDM TRANSMISSIONS”, U.S. patent application Ser. No. 10/834,745, filed Apr. 28, 2004 and entitled “A METHOD TO MITIGATE INTERCARRIER INTERFERENCE (ICI) IN OFDM SYSTEMS FOR HIGH DATA RATE TRANSMISSIONS”, and U.S. patent application Ser. No. 10/912,829, filed Aug. 5, 2004 and entitled “MIMO-OFDM RECEIVER PROCESSING”, all of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure describes systems and techniques relating to processing a signal received over a wireless channel, for example, deriving a channel quality measure from an orthogonal frequency division multiplexed (OFDM) signal to aid in adaptively determining a data rate of packetized information transmission over a wireless channel. 
     BACKGROUND 
     Mobile phones, laptops, personal digital assistants (PDAs), base stations and other systems and devices can wirelessly transmit and receive data. Such systems and devices have used orthogonal frequency division multiplexing (OFDM) transmission schemes, such as those defined in the Institute of Electrical and Electronics Engineers (IEEE) 802 wireless communications standards. The IEEE 802 standards include IEEE 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. In an OFDM system, in particular, a data stream is split into multiple substreams, each of which is sent over a different subcarrier frequency (also referred to as a tone or frequency tone). 
     Some wireless communication systems use a single-in-single-out (SISO) transmission approach, where both the transmitter and the receiver use a single antenna. Other wireless communication systems use a multiple-in-multiple-out (MIMO) transmission approach, where multiple transmit antennas and multiple receive antennas to improve data rates and/or link quality. OFDM systems can be implemented as SISO or MIMO communication systems and can provide various advantages, including a relatively simple receiver architecture and potential performance improvements through appropriate coding across OFDM tones. 
     Despite these advantages, OFDM transmissions, like other wireless transmissions, are susceptible to various signal degrading effects, including fading and multipath effects. The efficiency of transmission over wireless channels, in general, is strongly dependent upon the transmitter&#39;s ability to exploit the wireless channel&#39;s characteristics. This can be in the form of information-theory based strategies to achieve near-capacity transmission rates or strategies that exploit some coarse measure of the channel properties to select some transmission parameters like modulation formats and code rates to increase the probability of a successful transmission. For example, in Automatic Repeat-reQuest (ARQ) based packet transmission systems, where a packet error results in the need to retransmit the same information until it is correctly received by the receiving device, the data rate for transmission is typically selected based on a signal quality measure (e.g., a received signal strength indication (RSSI)) and a retry rate. 
     SUMMARY 
     The present disclosure includes systems and techniques relating to processing a signal received over a wireless channel. According to an aspect of the described systems and techniques, a data rate of packetized information transmission over a wireless channel is adaptively determined based on both a signal quality measure of a received signal and a channel quality measure derived from the received signal, the channel quality measure being indicative of frequency selectivity in the wireless channel. Adaptively determining the data rate can involve using the signal quality measure as a first level determinant of the data rate, and using the channel quality measure as a second level determinant of the data rate. Moreover, the wireless channel can be a radio frequency channel, and the received signal can include an orthogonal frequency division multiplexed (OFDM) signal corresponding to multiple frequency tones that make up the radio frequency channel; and the technique can include estimating channel response characteristics of the frequency tones, and deriving the channel quality measure from the estimated channel response characteristics of the frequency tones. 
     According to another aspect, an apparatus includes a channel estimator configured to be responsive to a received OFDM signal corresponding to multiple frequency tones of a wireless channel, and configured to evaluate channel response characteristics of the frequency tones; and a channel state indicator responsive to output of the channel estimator and configured to generate a channel quality measure usable along with a signal quality measure in adaptively determining a data rate of packetized information transmission over the wireless channel, wherein the channel quality measure is generated from the channel response characteristics of the frequency tones and is indicative of frequency selectivity in the wireless channel. 
     The apparatus can include a rate controller that adaptively determines the data rate using the signal quality measure as a first level determinant of the data rate and using the channel quality measure as a second level determinant of the data rate. Additionally, the apparatus can include a transmit section compliant with a wireless communication standard selected from the group consisting of IEEE 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20. 
     The described systems and techniques can result in increased throughput from reduced transmission errors and thus reduced retransmissions of the same information. A transmitter can adaptively determine a maximum transmission rate with acceptable probability of packet error, and the probability of successful transmission on the first attempt can be improved without starting at an unnecessarily low data rate. This can result in improved rate adaptation mechanisms. 
     Details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects and advantages may be apparent from the description and drawings, and from the claims. 
    
    
     
       DRAWING DESCRIPTIONS 
         FIG. 1  is a block diagram of a communication system that adaptively determines a data rate of packetized information transmission over a wireless channel based on both a signal quality measure of a received signal and a channel quality measure derived from the received signal. 
         FIG. 2  illustrates an example of channel response in a OFDM-SISO wireless communications system. 
         FIG. 3  is a block diagram of an example transceiver that adaptively determines the transmission data rate based on both a signal quality measure of a received signal and a channel quality measure derived from the received signal. 
         FIG. 4  illustrates adaptive rate determination using the signal quality measure as a first level determinant of the data rate and using the channel quality measure as a second level determinant of the data rate. 
         FIG. 5  is a block diagram of example transceivers in a MIMO-OFDM wireless communications system. 
         FIG. 6  is a flowchart illustrating an example process communicating packetized information over a radio frequency channel in an OFDM wireless communication system. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a communication system  100  that adaptively determines a data rate of packetized information transmission over a wireless channel  130  based on both a signal quality measure of a received signal and a channel quality measure derived from the received signal. The system  100  adapts to changes in the wireless channel  130  and adjusts the transmission data rate accordingly. The communication system  100  can be a single-in-single-out (SISO) system or a multiple-in-multiple-out (MIMO) system. Thus, a first transceiver  110  can have one or more antennas  112 , and a second transceiver  120  can have one or more antennas  122 . 
     Additionally, the transceiver  110  includes a transmit section  114  and a receive section  116 , and the transceiver  120  includes a transmit section  124  and a receive section  126 . The transceivers  110 ,  120  are sometimes referred to as transmitters and receivers for convenience, with the understanding that the systems and techniques described are applicable to wireless systems that use dedicated transmitters and receivers as well as transceivers generally. 
     The packetized information transmission involves the transmission of information over the wireless channel  130  in the form of discrete sections of information  135 , often referred to as packets or frames. The system  100  can employ an Automatic Repeat-reQuest (ARQ) protocol. The wireless channel  130  can be a radio frequency channel, and the transmissions over the wireless channel  130  can be orthogonal frequency division multiplexed (OFDM) signals. The transceivers  110 ,  120  can be implemented in a wireless local area network (WLAN) that complies with one or more of the IEEE 802 wireless standards (including IEEE 802.11, 802.11a, 802.11b, 802.11g, 802.11n, 802.16, and 802.20). 
     In general, wireless channels are typically affected by two dominant phenomena (among others) known as fading and multipath effects. These two effects are typically random and time varying, and determine the receiver-signal-to-noise ratio (Rx-SNR). Signal processing techniques for recovering transmitted signals in light of these effects are well known. For example, in 802.11a/g wireless systems, the OFDM modulation mechanism is used, and predefined training symbols are included in the preambles of data frames for use in estimating characteristics of the wireless channel in order to equalize the channel. 
     In an OFDM modulation approach, the channel bandwidth is divided into narrow slices called tones, and quadrature-amplitude modulated (QAM) symbols are transmitted over the tones. A defined structure to a SISO-OFDM transmitted signal enables the received signal on a tone, Y k , where k is the tone index, to be written as:
 
 Y   k   =H   k   X   k   +N   k   (1)
 
where H k  is the complex valued channel gain, X k  is the QAM symbol transmitted over the tone, and N k  is the frequency domain additive Gaussian distributed white noise. Thus, an estimate of the channel gain at tone k can be obtained at the receiver by dividing Y k  by X k  in the training sequence portion of the received signal.
 
     In addition to the SNR, the channel profile (i.e., the values of the H k &#39;s) also plays an important role in the capacity of the wireless channel. To account for this, the system  100  uses a channel quality measure that is indicative of frequency selectivity in the wireless channel. 
       FIG. 2  illustrates an example of channel response in a SISO-OFDM wireless communications system. A graph  200  plots frequency versus the magnitude of the frequency response of the channel (in decibels (dB)). This represents the channel response at a particular point in time. As shown, the wireless channel exhibits multipath fading that can result in multiple notches in the channel, including potentially very serious notches, such as a frequency notch  210 . 
     In an OFDM system, the data stream is split into multiple substreams, each of which is sent over a different subcarrier frequency (frequency tone). In  FIG. 2 , the tones are designated by divisions of the channel across the full frequency range shown, and the tones are referenced by k 0  to k N-1 , where N is the number of tones used. Despite this illustration, it should nonetheless be appreciated that the systems and techniques described here can be applied across all sub-bands of the full channel band or across some subset of the sub-bands; for example, in IEEE 802.11a systems, OFDM symbols can include 64 tones (with 48 active tones) indexed as (−32, −31, . . . , −1, 0, 1, . . . , 30, 31), where 0 is the index of a tone that is not used to transmit information. 
     Measuring the channel gain at the midpoint of tone k, gives a signal-to-noise (SNR) ratio for tone k as follows: 
                     SNR   k     =       (       σ   x   2       σ   n   2       )     ⁢            H   k          2               (   2   )               
where σ x   2  is the variance in signal energy (variance of the transmitted QAM signals), and σ n   2  is the noise variance (variance in additive, white Gaussian-distributed noise). Due to multipathing and fading, the channel can have notches (very small values of |H k | 2 ) in the channel that can seriously degrade the performance of the system. By taking the nature of these notches in the channel into consideration during setting of the data rate for transmission over the wireless channel, system performance can be improved. Thus, a channel quality measure that is indicative of frequency selectivity in the wireless channel, as the wireless channel changes over time, is used in the adaptive determination of the data rate.
 
     A measure of the SNR average across all the tones is a general measure of the ability of the wireless channel to support a certain data rate. Moreover, the geometric mean of the SNRs across all the OFDM tones is generally indicative of frequency selectivity in the wireless channel. Thus, the channel quality measure used can be the geometric mean as given by:
 
GeomSNR=(Π k SNR k ) 1/N   (3)
 
where N is the number of tones and Π is the product operation. When expressed in the dB scale:
 
                     GeomSNR     d   ⁢           ⁢   B       =         (     10   /   N     )     ⁢       ∑   k     ⁢       log   10     ⁡     (     SNR   k     )           =       {       (     10   /   N     )     ⁢       ∑   k     ⁢       log   10     ⁡     (            H   k          2     )           }     +     {     10   ⁢           ⁢       log   10     ⁡     (       σ   x   2       σ   n   2       )         }                 (   4   )               
The geometric SNR has been found to be strongly indicative of the capacity of the channel. An alternative channel quality measure is to extract only the channel component of the geometric SNR (treating the second term above, {10 log 10 (σ x   2 /σ n   2 )}, as a constant bias that can be disregarded), thus resulting in a frequency selectivity measure (FSM) in a SISO system as follows:
 
                     FSM     SISO   -     d   ⁢           ⁢   B         =       (     10   /   N     )     ⁢       ∑   k     ⁢       log   10     ⁡     (            H   k          2     )                   (   5   )               
Simulations of a wireless communication system have shown that this channel quality measure is a good indication of the geometric SNR and the capacity of the wireless channel, and can be reliably used for rate selection.
 
       FIG. 3  is a block diagram of an example transceiver  300  that adaptively determines the transmission data rate based on both a signal quality measure of a received signal  310  and a channel quality measure derived from the received signal  310 . The transceiver  300  includes a physical (PHY) layer and a media access control (MAC) layer. The MAC and PHY components can be integrated into a single device or constructed in separate devices, which can then be operationally connected to form the transceiver  300 . 
     A channel estimator  330  is configured to be responsive to the signal  310  received over a wireless channel through an antenna  320 . The received signal  310  can be an OFDM signal corresponding to multiple frequency tones of the wireless channel, and the channel estimator  330  can also be configured to evaluate channel response characteristics of the frequency tones, such as from predefined training symbols in the received signal  310 . 
     A channel state indicator  310  is responsive to output of the channel estimator  330  and can be configured to generate a channel quality measure from the channel response characteristics of the frequency tones, where the channel quality measure is indicative of frequency selectivity in the wireless channel. The channel quality measure can be a geometric mean of SNRs across the frequency tones or, specifically, a frequency selectivity measure, as described above. Moreover, the channel estimator  330  and the channel state indicator  340  can be integrated into the PHY as shown or can be constructed in a separate device that is configured to be operationally connected with the PHY. 
     The channel quality measure can be used along with a signal quality measure in adaptively determining a data rate of packetized information transmission over the wireless channel. The MAC can include a rate controller  350  that adaptively determines the data rate using the signal quality measure as a first level determinant of the data rate and using the channel quality measure as a second level determinant of the data rate. For example, the rate controller  350  can select an original rate based on the signal quality measure and one or more signal quality thresholds, and the rate controller  350  can then adjust the original rate if the signal quality measure falls within a tolerance range of a signal quality threshold and if the channel quality measure indicates an adjustment is needed. 
     It is to be appreciated that various divisions of processing tasks into components and various placements of components in the transceiver  300  are possible. In general though, the channel quality measurement can be implemented in the PHY, and the usage of that information (e.g., for determining desired data rate) can be implemented in the PHY or the MAC as desired. 
       FIG. 4  illustrates adaptive rate determination  400  using the signal quality measure as a first level determinant of the data rate and using the channel quality measure as a second level determinant of the data rate. The signal quality measure can be one or more indications of signal quality generally, such as an overall SNR for the received signal or an RSSI. In the example shown, the signal quality measure is compared with two signal quality thresholds  410 ,  420  to select a data rate from three possible data rates: zero, one, and two. Thus, when the signal quality measure is in a first range, data rate zero is selected; when the signal quality measure is in a second range, data rate one is selected; and when the signal quality measure is in a third range, data rate two is selected. 
     However, this original selection is only a provisional selection, since the signal quality measure likely includes uncertainty that causes the rate determination to be potentially sub-optimal when the signal quality measure falls within a tolerance range  430 . When the signal quality measure falls within the tolerance range  430  of one of the thresholds  410  and  420 , then the channel quality measure is used as a second level determinant of the data rate. 
     The channel quality measure serves as a check on the accuracy of the signal quality measure and can thus be used to adjust the data rate determined originally based on the signal quality measure. This adjustment can be designed to go in both directions (e.g., decrease an original rate when the channel quality measure falls below a threshold and increase an original rate when the channel quality measure falls above a threshold), or this adjustment can be designed to go in only one direction (e.g., decrease an original rate when the channel quality measure falls below a channel quality threshold). The later case can be illustrated in  FIG. 4  by moving the signal quality thresholds  410 ,  420  to the left until they line up with the lower values of the tolerance ranges  430  (i.e., the signal quality threshold can be set such that falling below the threshold specifies a certain data rate, but falling above the threshold by only a little results in the channel quality measure being taken into consideration). 
     The available data rates can be many more than three data rates, and the selection of a data rate can be performed using a lookup table  450 , which includes columns for signal quality values, channel quality values, and the corresponding data rates to choose. For example, the following sequence logic can be employed in various wireless systems, including an 802.11g wireless system, to use the channel quality measure defined in equation (5) to pick the PHY layer data rate that can be reliably transmitted (i.e., at an acceptable packet error rate):
         1. Read the RSSI; based on the RSSI value, read the RSSI→PHY date rate lookup table to determine the data rate R (the lookup table can be interpreted as “pick data rate R if RSSI&gt;δ(R)”).   2. If |RSSI−δ(R)|&lt;ε AND FSM SISO-dB &gt;t, then pick R corresponding to the condition RSSI&gt;δ(R), else pick the next lower rate.       

     The lookup table  450  can be included in the MAC in the transceiver, and the signal quality measure can be an RSSI that is a direct current (DC) measure of radio frequency (RF) signal strength at an input to an intermediate frequency (IF) amplification stage of the transceiver. 
     In general, when confidence can be placed solely in the signal quality measure, this value is used to determine the data rate, and otherwise, both the signal quality measure and the channel quality measure are used to determine the data rate. This approach to choosing a data rate can improve the probability of successful packet transmission on the first attempt, and can improve throughput by reducing the probability of retries. Simulations run using the above sequence logic in an 802.11g wireless system have shown considerable improvements in performance, with throughput increasing by as much as 2-3 megabits per second (Mbps), which may be a conservative estimate as the simulations showed a significant drop in packet error events. 
     The channel quality measures described above for SISO can be generalized to MIMO devices, which operate using M t  transmit antennas and M r  receive antennas.  FIG. 5  is a block diagram of example transceivers  510 ,  520  in a MIMO-OFDM wireless communications system  500 . The PHY in the transceiver  510  provides quality measures  550  to the MAC, which in turn provides transmission parameters  560  and data  570  to the PHY. 
     In a MIMO system, data is transmitted over a wireless channel  530  through multiple antennas. While in SISO-OFDM based systems, the wireless channel  530  manifests in the frequency domain as a complex gain factor H k , in MIMO-OFDM based systems, the wireless channel  530  manifests as a matrix H k  of dimension M t ×M r  whose elements are complex numbers. Such a system is capable of simultaneously supporting a transmission of a maximum M t  independent data streams. 
     A measure analogous to the frequency selectivity measure used in SISO systems can be derived for MIMO systems as well. For every tone k, a measure called reciprocal noise enhancement factor (RNEF) can be defined as follows: 
                     RNEF     n   ,   k       =     1   /       (       [       H   k   Hm     ⁢     H   k       ]       -   1       )       n   ,   n                 (   6   )               
where n is the spatial index, the superscript Hm stands for the Hermitian operation on matrix H k , and A n,n  corresponds to an nth diagonal element of any square matrix A.
 
     The frequency selectivity measure can then be written as: 
                     FSM   MIMO     =       [       ∏     n   ,   k       ⁢           ⁢     (     RNEF     n   ,   k       )       ]       (     1   /   nk     )               (   7   )               
where Π n,k  is the product taken over spatial-frequency dimensions. When expressed in decibels, this can be written as:
 
                     FSM     MIMO   -     d   ⁢           ⁢   B         =       (     10   /   nk     )     ⁢       ∑     n   ,   k       ⁢       log   10     ⁡     (     1   /       (       [       H   k   Hm     ⁢     H   k       ]       -   1       )       n   ,   n         )                   (   8   )               
The FSM MIMO-dB  measure can be used in the same way as the FSM SISO-dB  measure, and thus similar system components can be used to generate the channel quality measure in a MIMO-OFDM wireless communication system. Additionally, the signal quality measure can be obtained in a MIMO-OFDM wireless communication system using known methods.
 
     In both a MIMO-OFDM system and a SISO-OFDM system, the details involved in determining the signal quality measure and the channel quality measure should include variations as appropriate for the specific system, as will be understood by those skilled in the art, but the overall process of determining the data rate based on these quality measures remains generally the same in both MIMO and SISO systems. 
       FIG. 6  is a flowchart illustrating an example process of communicating packetized information over a radio frequency channel in an OFDM wireless communication system. This process, and all of the functional operations described in this specification, can be implemented in electronic circuitry, or in computer hardware, firmware, software, or in combinations of them, such as the structural means disclosed in this specification and structural equivalents thereof, including potentially a software program operable to cause one or more machines to perform the operations described. It will be appreciated that the order of operations presented is shown only for the purpose of clarity in this description. No particular order is required for these operations, and some or all of the operations may occur simultaneously in various implementations. 
     Channel response characteristics of the frequency tones are estimated at  610 . This estimation can be performed using predefined training symbols in a received signal, such as those used in 802.11 wireless systems. A channel quality measure, such as described above, is derived from the estimated channel response characteristics of the frequency tones at  620 . 
     A signal quality measure, such as described above, is used as a first level determinant of the data rate at  630 . This can involve selecting an original rate based on a received signal strength indication. The channel quality measure is used as a second level determinant of the data rate at  640 . This can involve comparing the signal quality measure with a value indicating a range of tolerance, and comparing the channel quality measure with a channel quality threshold when the signal quality measure falls within the tolerance range (this can also be thought of as an uncertainty range, in that there is uncertainty as to whether the selected original data rate is appropriate when the signal quality measure falls within this range). 
     The packetized information is transmitted over the wireless channel in accordance with the determined data rate at  650 . This transmission can be performed in a manner appropriate for the type of OFDM system employed. 
     A few embodiments have been described in detail above, and various modifications are possible. For example, the present systems and techniques may be combined with one or more of the systems and techniques described in U.S. patent application Ser. No. 10/799,548, filed Mar. 11, 2004 and entitled “A MECHANISM TO IMPROVE QUALITY OF CHANNEL ESTIMATES IN OFDM TRANSMISSIONS”, U.S. patent application Ser. No. 10/834,745, filed Apr. 28, 2004 and entitled “A METHOD TO MITIGATE INTERCARRIER INTERFERENCE (ICI) IN OFDM SYSTEMS FOR HIGH DATA RATE TRANSMISSIONS”, and U.S. patent application Ser. No. 10/912,829, filed Aug. 5, 2004 and entitled “MIMO-OFDM RECEIVER PROCESSING”. 
     Other embodiments fall within the scope of the following claims.