Abstract:
A method of controlling downlink transmissions to a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first base station antenna; receiving a second pilot signal from a second base station antenna; and estimating the channel between the base station and subscriber station based on the received first and second pilot signals. The method also comprises determining a set of OFDM symbol processing parameters based on the step of estimating the channel and transmitting the OFDM symbol processing parameters to the base station. The base station uses the OFDM symbol processing parameters to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY 
       [0001]    The present application is related to Prov. Pat. Nos. 60/673,574 and 60/673,674, both entitled “Diversity Transmission in an OFDM Wireless Communication System” and filed Apr. 21, 2005, and to Prov. Pat. No. 60/679,026, entitled “Channel Estimation in a Delay Diversity Wireless Communication System,” and filed May 9, 2005. Prov. Pat. Nos. 60/673,574, 60/673,674, and 60/679,026 are assigned to the assignee of this application. The subject matter disclosed in Prov. Pat. Nos. 60/673,574, 60/673,674, and 60/679,026 is hereby incorporated by reference. This application claims priority under 35 U.S.C. §119(e) to Prov. Pat. Nos. 60/673,574, 60/673,674, and 60/679,026. 
         [0002]    The present application is related to U.S. patent application Ser. No. 11/327,799, entitled “Method And System For Introducing Frequency Selectivity Into Transmissions In An Orthogonal Frequency Division Multiplexing Network”, filed Jan. 6, 2006. Application Ser. No. 11/327,799 is assigned to the assignee of the present application. The subject matter disclosed in application Ser. No. 11/327,799 is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0003]    The present disclosure relates generally to wireless communications and, more specifically, to an apparatus and method for performing channel estimation in an orthogonal frequency division multiplexing (OFDM) network or an orthogonal frequency division multiple access (OFDMA) network. 
       BACKGROUND OF THE INVENTION 
       [0004]    Conventional orthogonal frequency division multiplexing (OFDM) networks and orthogonal frequency division multiple access (OFDMA) network are able to improve the reliability of the channel by spreading and/or coding data traffic and control signals over multiple subcarriers (i.e., tones). However, if the channel is flat, frequency diversity cannot be achieved. In order to overcome this, it is possible to introduce artificial frequency diversity into the transmitted signal. A technique for artificially introducing frequency diversity into an OFDM environment was disclosed in U.S. patent application Ser. No. 11/327,799, filed on Jan. 6, 2006 and incorporated by reference above. In the device disclosed in Ser. No. 11/327,799, multiple copies of the same OFDM symbol are delayed by different delay values, then amplified by the same or different gain values, and then transmitted from different antennas. This artificially introduces frequency-selective fading in the ODFM channel, thereby allowing frequency selectivity to be exploited using frequency-domain scheduling for low-to-medium speed mobile devices or frequency diversity for higher speed mobile devices. 
         [0005]    However, when selecting the symbol processing parameters (i.e., delay values and the gain values) applied to the OFDM symbols, it is important to take into consideration the user channel type and the mobile speed. To accomplish this, channel estimation is performed and the symbol processing parameters are determined based on the channel estimates and mobile speed. Therefore, there is a need for improved apparatuses and methods for performing channel estimation in an OFDM wireless network that artificially introduces frequency diversity by delaying and amplifying multiple copies of the same OFDM symbol and then transmitting the delayed and amplified OFDM symbols from different transmit antennas. 
       SUMMARY OF THE INVENTION 
       [0006]    A method of controlling downlink transmissions to a subscriber station is provided for use in a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first antenna of the base station; receiving a second pilot signal from a second antenna of the base station; estimating the channel between the base station and subscriber station based on the received first and second pilot signals; determining a set of OFDM symbol processing parameters based on the step of estimating the channel, wherein the OFDM symbol processing parameters are usable by the base station to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas; and transmitting the OFDM symbol processing parameter set to the base station. 
         [0007]    According to another embodiment of the present disclosure, a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network is provided. The subscriber station comprises: receive path circuitry capable of receiving a first pilot signal from a first antenna of the base station and receiving a second pilot signal from a second antenna of the base station; and channel estimating circuitry capable of estimating the channel between the base station and subscriber station based on the received first and second pilot signals and capable of determining a set of OFDM symbol processing parameters based on a channel quality estimate. The OFDM symbol processing parameters are usable by the base station to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas and wherein the subscriber station is capable of transmitting the OFDM symbol processing parameters to the base station. 
         [0008]    According to yet another embodiment of the present disclosure, a base station is provided for use in an orthogonal frequency division multiplexing (OFDM) network. The base station comprises: 1) receive path circuitry capable of receiving an uplink signal from a subscriber station, estimating the channel between the base station and subscriber station based on the received uplink signal, and determining a set of OFDM symbol processing parameters based on a channel quality estimate; and 2) transmit path circuitry capable of using the OFDM symbol processing parameters to control the relative gains and the relative delays of processed OFDM symbols transmitted from a first antenna and a second antenna of the base station. The base station is capable of transmitting the OFDM symbol processing parameters to the subscriber station. The OFDM symbol processing parameters are based on the multipath characteristics and the frequency selectivity characteristics of the channel. 
         [0009]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the term “each” means every one of at least a subset of the identified items; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0011]      FIG. 1  illustrates an exemplary orthogonal frequency division multiplexing (OFDM) wireless network that is capable of performing channel estimation according to the principles of the present disclosure; 
           [0012]      FIG. 2A  is a high-level diagram of the orthogonal frequency division multiplexing (OFDM) transmit path in a base station according to one embodiment of the disclosure; 
           [0013]      FIG. 2B  is a high-level diagram of the orthogonal frequency division multiplexing (OFDM) receive path in a subscriber station according to one embodiment of the disclosure; 
           [0014]      FIG. 3  illustrates the OFDM symbol processing block in the base station in greater detail according to an exemplary embodiment of the present disclosure; 
           [0015]      FIG. 4A  illustrates data traffic transmitted in the downlink from a base station to a subscriber station according to an exemplary embodiment of the present disclosure; 
           [0016]      FIG. 4B  is a flow diagram illustrating the determination of the user channel type based on the measurements on the preamble according to an exemplary embodiment of the disclosure; 
           [0017]      FIG. 5  is a message flow diagram illustrating the transmission of OFDM symbols from a base station to a subscriber station according to the principles of the disclosure; 
           [0018]      FIG. 6  is a flow diagram illustrating the processing of pilot signals and OFDM data symbols according to an exemplary embodiment of the present disclosure; 
           [0019]      FIG. 7  is a message flow diagram illustrating the transmission of OFDM symbols from a base station to a subscriber station according to another embodiment of the disclosure; and 
           [0020]      FIG. 8  is a message flow diagram illustrating the transmission of OFDM symbols from a base station to a subscriber station according to another embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIGS. 1 through 8 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless network. 
         [0022]    The present disclosure is directed to apparatuses and algorithms for channel estimation and channel quality estimation for demodulation and data rate selection in an orthogonal frequency division multiplexing (OFDM) wireless network that uses delayed diversity. Such a delayed diversity wireless network was disclosed previously U.S. patent application Ser. No. 11/327,799, incorporated by reference above. The present disclosure uses a number of factors, including user channel type and mobile speed, to select OFDM symbol processing parameters (i.e., delays D 1 , D 2 , . . . , DP and gains g 0 , g 1 , . . . , g p ) for OFDM symbols transmitted from up to P antennas (i.e., ANT 1  to ANTP). Therefore, different OFDM symbol processing parameters may be used to transmit to different mobile devices that are scheduled simultaneously, depending upon their channel types. 
         [0023]    It is noted that the scope of the present disclosure is not limited to orthogonal frequency division multiplexing (OFDM) wireless networks. The present disclosure is also applicable to orthogonal frequency division multiple access (OFDMA) wireless networks. However, for simplicity and brevity, the embodiments described below are directed to OFDM wireless networks, except where otherwise noted or where the context indicates otherwise. 
         [0024]    For relatively low-speed mobile devices, it is usually possible to track changes in the channel, thereby allowing channel sensitive scheduling to improve performance. Thus, the OFDM symbol processing parameters may be selected in such a way that relatively large coherence bandwidth results. That is, a relatively larger number of subcarriers experience similar fading. This goal may be achieved by keeping the delays for OFDM symbols from different antennas relatively small. A mobile device may then be scheduled on a subband consisting of contiguous subcarriers. 
         [0025]    For relatively high-speed mobile devices, channel quality variations cannot be tracked accurately, so that frequency-diversity may be helpful. Thus, the OFDM symbol processing parameters are selected in such a way that relatively small coherence bandwidth results. That is, potentially independent fading may occur from subcarrier to subcarrier. This goal may be achieved by having relatively large delays for OFDM symbols transmitted from different antennas. 
         [0026]    The symbol processing parameters may also be selected based on the degree of frequency-selectivity already present in the channel. For example, if a channel already has a lot of multipath effects and is, therefore, frequency selective, there may be little or no need for additional frequency selectivity. The OFDM symbol processing parameters may be selected on a user-by-user basis because different mobile devices experience different channel types. 
         [0027]      FIG. 1  illustrates exemplary orthogonal frequency division multiplexing (OFDM) wireless network  100 , which is capable of performing channel estimation according to the principles of the present disclosure. In the illustrated embodiment, wireless network  100  includes base station (BS)  101 , base station (BS)  102 , base station (BS)  103 , and other similar base stations (not shown). Base station  101  is in communication with base station  102  and base station  103 . Base station  101  is also in communication with Internet  130  or a similar IP-based network (not shown). 
         [0028]    Base station  102  provides wireless broadband access (via base station  101 ) to Internet  130  to a first plurality of subscriber stations within coverage area  120  of base station  102 . The first plurality of subscriber stations includes subscriber station  111 , which may be located in a small business (SD), subscriber station  112 , which may be located in an enterprise (E), subscriber station  113 , which may be located in a WiFi hotspot (HS), subscriber station  114 , which may be located in a first residence (R), subscriber station  115 , which may be located in a second residence (R), and subscriber station  116 , which may be a mobile device (M), such as a cell phone, a wireless laptop, a wireless PDA, or the like. 
         [0029]    Base station  103  provides wireless broadband access (via base station  101 ) to Internet  130  to a second plurality of subscriber stations within coverage area  125  of base station  103 . The second plurality of subscriber stations includes subscriber station  115  and subscriber station  116 . In an exemplary embodiment, base stations  101 - 103  may communicate with each other and with subscriber stations  111 - 116  using OFDM or OFDMA techniques. 
         [0030]    Base station  101  may be in communication with either a greater number or a lesser number of base stations. Furthermore, while only six subscriber stations are depicted in  FIG. 1 , it is understood that wireless network  100  may provide wireless broadband access to additional subscriber stations. It is noted that subscriber station  115  and subscriber station  116  are located on the edges of both coverage area  120  and coverage area  125 . Subscriber station  115  and subscriber station  116  each communicate with both base station  102  and base station  103  and may be said to be operating in handoff mode, as known to those of skill in the art. 
         [0031]    Subscriber stations  111 - 116  may access voice, data, video, video conferencing, and/or other broadband services via Internet  130 . In an exemplary embodiment, one or more of subscriber stations  111 - 116  may be associated with an access point (AP) of a WiFi WLAN. Subscriber station  116  may be any of a number of mobile devices, including a wireless-enabled laptop computer, personal data assistant, notebook, handheld device, or other wireless-enabled device. Subscriber stations  114  and  115  may be, for example, a wireless-enabled personal computer (PC), a laptop computer, a gateway, or another device. 
         [0032]      FIG. 2A  is a high-level diagram of the transmit path in orthogonal frequency division multiplexing (OFDM) transmitter  200  according to an exemplary embodiment of the disclosure.  FIG. 2D  is a high-level diagram of the receive path in orthogonal frequency division multiplexing (OFDM) receiver  260  according to an exemplary embodiment of the disclosure. OFDM transmitter  200  comprises quadrature amplitude modulation (QAM) modulator  205 , serial-to-parallel (S-to-P) block  210 , Inverse Fast Fourier Transform (IFFT) block  215 , parallel-to-serial (P-to-S) block  220 , add cyclic prefix block  225 , and OFDM symbol processing block  230 . OFDM receiver  250  comprises remove cyclic prefix block  260 , serial-to-parallel (S-to-P) block  265 , Fast Fourier Transform (FFT) block  270 , parallel-to-serial (P-to-S) block  275 , quadrature amplitude modulation (QAM) demodulator  280 , and channel estimation block  285 . 
         [0033]    At least some of the components in  FIGS. 2A and 2B  may be implemented in software while other components may be implemented by configurable hardware or a mixture of software and configurable hardware. In particular, it is noted that the FFT blocks and the IFFT blocks described in  FIGS. 2A and 2B  may be implemented as configurable software algorithms, where the values of FFT and IFFT sizes may be modified according to the implementation. 
         [0034]    QAM modulator  205  receives a stream of input data and modulates the input bits (or symbols) to produce a sequence of frequency-domain modulation symbols. Serial-to-parallel block  210  converts (i.e., de-multiplexes) the serial QAM symbols to parallel data to produce M parallel symbol streams where M is the IFFT/FFT size used in OFDM transmitter  200  and OFDM receiver  250 . IFFT block  215  then performs an IFFT operation on the M parallel symbol streams to produce time-domain output signals. Parallel-to-serial block  220  converts (i.e., multiplexes) the parallel time-domain output symbols from IFFT block  215  to produce a serial time-domain signal. 
         [0035]    Add cyclic prefix block  225  then inserts a cyclic prefix to each OFDM symbol in the time-domain signal. As is well known, the cyclic prefix is generated by copying the last G samples of an N sample OFDM symbol and appending the copied G samples to the front of the OFDM symbol. Finally, OFDM symbol processing block  230  processes the incoming OFDM symbols as described in  FIG. 3  and as described in U.S. patent application Ser. No. 11/327,799. The process OFDM samples at the output of OFDM symbol processing block  230  are then sent to up-conversion circuitry (not shown) prior to being transmitted from multiples transmit antennas. 
         [0036]    The transmitted RF signal arrives at OFDM receiver  250  after passing through the wireless channel and reverse operations to those in OFDM transmitter  200  are performed. Remove cyclic prefix block  260  removes the cyclic prefix to produce the serial time-domain baseband signal. Serial-to-parallel block  265  converts the time-domain baseband signal to parallel time domain signals. FFT block  270  then performs an FFT algorithm to produce M parallel frequency-domain signals. Parallel-to-serial block  275  converts the parallel frequency-domain signals to a sequence of QAM data symbols. QAM demodulator  280  then demodulates the QAM symbols to recover the original input data stream. Channel estimation block  285  also receives the QAM data symbols from parallel-to-serial block  275  and performs channels estimation. As will be described below in greater detail, the channel estimation values are used to determine a parameter set of gain values and delay values that are used in OFDM symbol processing block  230  in OFDM transmitter  200  and are used by QAM demodulator  280  to demodulate the QAM data symbols. 
         [0037]    The exemplary transmit path of OFDM transmitter  200  may be representative of the transmit paths of any one of base stations  101 - 103  or any one of subscriber stations  111 - 116 . Similarly, the exemplary receive path of OFDM receiver  250  may be representative of the transmit paths of any one of base stations  101 - 103  or any one of subscriber stations  111 - 116 . However, since multiple antenna configurations are more common in base stations than in subscriber stations or other mobile devices, for the sake of simplicity and clarity, the descriptions that follow will be directed toward transactions between a base station (e.g., BS  102 ) that implements a transmit path similar to OFDM transmitter  200  and a subscriber station (e.g., SS  116 ) that implements a receive path similar to OFDM receiver  250 . However, such an exemplary embodiment should not be construed to limit the scope of the present disclosure. It will be appreciated by those skilled in the art that in cases where multiple antennas are implemented in a subscriber station, the transmit path and the receiver path of both the base station and the subscriber station may be implemented as in shown in  FIGS. 2A and 2B . 
         [0038]      FIG. 3  illustrates OFDM symbol processing block  230  in greater detail according to an exemplary embodiment of the present disclosure. OFDM symbol processing block  230  comprises P delay elements, including exemplary delay elements  311  and  312 , P+1 amplifiers, including exemplary amplifiers  321 ,  322  and  323 , and P+1 transmit antennas, including exemplary antennas  331 ,  332  and  333 . Delay elements  311  and  312  are arbitrarily labeled “D 1 ” and “DP”, respectively. OFDM symbol processing block  230  receives incoming ODFM symbols and forwards P+1 copies of each OFDM symbol to the P+1 transmit antennas. Each OFDM symbol comprises N+G samples, where N is the number of samples in the original data symbol and G is the number of samples in the cyclic prefix appended to the original symbol. 
         [0039]    A first copy of each OFDM symbol is applied directly to the input of amplifier  321 , amplified by a gain value, g 0 , and sent to antenna  331 . A second copy of each OFDM symbol is delayed by delay element  311 , applied to the input of amplifier  322 , amplified by a gain value, g 1 , and sent to antenna  332 . Other copies of each OFDM symbol are similarly delayed and amplified according to the number of antennas. By way of example, the P+1 copy of each OFDM symbol is delayed by delay element  312 , applied to the input of amplifier  323 , amplified by a gain value, gP, and sent to antenna  333 . The gain values and the delay values are determined by the values in an OFDM symbol processing parameter set, as described hereafter and in U.S. patent application Ser. No. 11/327,799. The result is that multiple copies of each OFDM are transmitted, wherein each copy of an OFDM symbol is amplified by a selected amount and delayed by a selected amount relative to other OFDM symbol copies. U.S. patent application Ser. No. 11/327,799, incorporated by reference above, describes a number of architectures for OFDM symbol processing block  230  that achieve such a result. In an advantageous embodiment, the delays introduced by OFDM symbol processing block  230  are cyclic delays, as disclosed in U.S. patent application Ser. No. 11/327,799. 
         [0040]      FIG. 4A  illustrates data traffic transmitted in the downlink from base station  102  to subscriber station  116  according to an exemplary embodiment of the present disclosure. An exemplary frame of OFDM data is 10 milliseconds in length and comprises fifteen (15) transmit time intervals (TTIs), namely TTI  1  through TTI  15 , where each one of TTI  1  through TTI  15  is 0.667 milliseconds in duration. Within each of TTI  2  through TTI  15 , there are four OFDM data symbols, where each OFDM data symbol is 0.1667 milliseconds in duration. In the first TTI, namely TTI  1 , there are three OFDM data symbols that are preceded by a pilot preamble symbol. The pilot preamble symbol is used by SS  116  to perform synchronization channel estimation and to determine the OFDM symbol processing parameter set. 
         [0041]      FIG. 4B  is a flow diagram illustrating the determination of the user channel type based on the measurements on the preamble according to an exemplary embodiment of the disclosure. In an OFDM system, a known pilot sequence is transmitted for one or more OFDM symbol durations. Channel estimation block  285  in the receiver (i.e., SS  116 ) detects the known pilot signal, which is then use to perform synchronization (process step  410 ). Channel estimation block  285  uses the detected preamble symbols to determine the degree of multipath effects in the channel and, therefore, the frequency selectivity in the channel between BS  102  and SS  116  (process step  420 ). 
         [0042]    Based on the profile of the channel, channel estimation block  285  (or another processing element or controller in SS  116 ) determines (i.e., calculates) a set of OFDM symbol processing parameters (i.e., gain values and delay values) that may be used in BS  102  to improve reception of OFDM symbols in SS  116  (process step  430 ). SS  116  then feeds back the OFDM symbol processing parameter set to BS  102  in the uplink (process step  440 ). Other factors, such as mobile speed, can also be used in determining (or calculating) the OFDM symbol processing parameters. The channel type may also be determined by using other mechanisms, such as reference in time-frequency. 
         [0043]    In this manner, BS  102  receives an OFDM symbol processing parameter set from each subscriber station. Thereafter, as BS  102  schedules each subscriber station to receive data, BS  102  uses the OFDM symbol processing parameter set for that subscriber station to modify the OFDM symbols transmitted from each antenna for BS  102 . For example, BS  102  may use OFDM Symbol Processing Parameter Set A to transmit OFDM symbols from two or more antennas to SS  116  and may use OFDM Symbol Processing Parameter Set B to simultaneously transmit OFDM symbols from two or more antennas to SS  115 . 
         [0044]      FIG. 5  is a message flow diagram illustrating the transmission of OFDM symbols from base station  102  to subscriber station  116  according to one embodiment of the disclosure. In this example, base station  102  uses two transmit antennas (first antenna ANT 1  and second antenna ANT  2 ) to transmit to SS  116 . SS  116  receives a first pilot signal (Pilot 1 ) from antenna ANT 1  and receives a second pilot signal (Pilot 2 ) from antenna ANT  2 . SS A then determines OFDM Symbol Processing Parameter Set A as described above in  FIGS. 4A and 4B . 
         [0045]    Next, SS  116  transmits OFDM Symbol Processing Parameter Set A to BS  102  in signal  505 . Thereafter, BS  102  uses OFDM Symbol Processing Parameter Set A to transmit OFDM data symbols in the downlink back to SS  116 . As noted above, the OFDM symbol processing parameters in Parameter Set A consist of symbol delays and gains from the two antennas. By way of example, in signal  520 , BS  102  transmits from ANT 1  processed OFDM symbols that were processed using Parameter Set A. In signal  525 , BS  102  simultaneously transmits from ANT 2  processed OFDM symbols that were processed using Parameter Set A. 
         [0046]    BS  102  also simultaneously transmits pilot signal  510  (Pilot 1 ) and pilot signal  515  (Pilot 2 ) from the two transmit antennas, ANT  1  and ANT  2 . In the embodiment in  FIG. 5 , Pilot 1  and Pilot 2  are not processed using the parameters in OFDM Symbol Processing Parameter Set A. This is due to the fact that another transmission may be scheduled at the same time for another subscriber station on other OFDM subcarriers using a different set of OFDM symbol processing parameters. The pilot signals must be correctly understood by all the subscriber stations scheduled in the cell, so the pilot signals are not modified using OFDM Symbol Processing Parameter Set A. 
         [0047]      FIG. 6  is a flow diagram illustrating the processing of pilot signals and OFDM data symbols according to an exemplary embodiment of the present disclosure. Because the OFDM symbols in signals  520  and  525  are processed using the values in OFDM Symbol Processing Parameter Set A, signals  520  and  525  are combined during transmission over the radio link in such a way that single OFDM symbols are received in SS  116  from BS  102  (process step  660 ). Since pilot signals  510  and  515  (Pilot 1  and Pilot 2 ) are transmitted on orthogonal subcarriers from antenna ANT 1  and antenna ANT 2 , pilot signals  510  and  515  are received separately at SS  116  (process steps  605  and  610 ). 
         [0048]    In order to get correct channel estimation for demodulation, SS  116  compensates pilot signals  510  and  515  (Pilot 1  and Pilot 2 ) from antennas ATN 1  and ANT 2  using the Parameter Set A received from channel estimation block  285  (process steps  615  and  620 ). Compensated pilot signals  510  and  515  are then combined (process step  630 ) and the overall channel estimate is obtained (process step  640 ). This overall channel estimate is then used by demodulator  280  to demodulate the processed data symbols carried in the OFDM subcarriers (process step  660 ). 
         [0049]      FIG. 7  is a message flow diagram illustrating the transmission of OFDM symbols from base station  102  to subscriber station  116  according to another embodiment of the disclosure. In  FIG. 7 , the OFDM symbol processing parameters are determined in base station (BS)  102 , rather than in subscriber station (SS)  116 . 
         [0050]    BS  102  may determine (or estimate) the OFDM symbol processing parameters in Parameter Set A from a number of different uplink signals  705  transmitted by SS  116 , including pilot signals  705 , preamble signals  705  and/or data signals  705  from SS  116 . 
         [0051]    In this example, since Pilot 1  signal  710  and Pilot 2  signal  715  are not processed using Parameter Set A, BS  101  transmits OFDM Symbol Processing Parameter Set A to SS  116  in control message  720 . SS  116  then uses the OFDM symbol processing parameters as described in  FIGS. 2-6 . BS  102  transmits processed OFDM symbols  725  from ANT 1  and processed symbols  720  from ANT 2  using the gain and delay values in Parameter Set A. SS  116  uses the same gain and delay parameters in control message  720  to compensate the pilots and to perform the overall channel estimation for data demodulation. 
         [0052]      FIG. 8  is a message flow diagram illustrating the transmission of OFDM symbols from base station  102  to subscriber station  116  according to another embodiment of the disclosure. Similar to  FIG. 7 , the OFDM symbol processing parameters in  FIG. 8  are again determined in base station (BS)  102  for the case of two transmit antennas, rather than in subscriber station (SS)  116 . BS  102  may determine (or estimate) the OFDM symbol processing parameters in Parameter Set A from a number of different uplink signals  805  transmitted by SS  116 , including pilot signals  805 , preamble signals  805  and/or data signals  805  from SS  116 . 
         [0053]    However, unlike  FIG. 7 , Pilot 1  signal  810  from ANT 1  and Pilot 2  signal  815  from ANT 2  are processed using Parameter Set A. In this case, the Pilot 1  signal and the Pilot 2  signal both use the same OFDM subcarriers. In other words, the two pilots are not transmitted on orthogonal subcarriers. Therefore, the Pilot 1  signal and the Pilot 2  signal are received in SS  116  as a single signal that can be directly used for overall channel estimation. The channel estimates are then used for data demodulation. BS  102  also transmits processed OFDM symbols  825  from ANT 1  and processed symbols  820  from ANT 2  using the gain and delay values in Parameter Set A. 
         [0054]    In a scenario where the Pilot 1  signal and the Pilot 2  signal are not compensated, the channel quality estimate is based on the pilot signal strengths SS  116  receives from the two transmit antennas, ANT 1  and ANT 2 . SS  116  compensates the Pilot 1  signal and the Pilot 2  signal using the OFDM symbol processing parameters. This gives an estimate of the expected channel quality when BS  102  transmits OFDM symbols using the OFDM symbol processing parameters for SS  116 . SS  116  then transmits a channel quality estimate (CQE) message back to BS  102 . BS  102  determines an optimum data rate based on the channel quality estimate (CQE) message from SS  116  and then transmits processed OFDM symbols at that data rate. 
         [0055]    In SS  116 , processed OFDM symbols containing data are processed using gain g 0  from ANT 1 , gain g 1  from ANT 2  and delay D 1  from ANT  2 . These operations reverse the operations in OFDM symbol processing block  230  in  FIG. 3 , assuming only transmit antenna  331  (i.e., ANT 1 ) and transmits antenna  332  (i.e., ANT 2 ) are used. In SS  116 , an FFT operation is performed on the received OFDM symbols in order to retrieve the information in the frequency domain. The data and pilot symbols carried on orthogonal subcarriers are separated in the frequency domain. The pilot signals are converted back to the time domain by performing an IFFT operation. In this process, the subcarriers carrying data are set to 0. Also, when an ANT 1  OFDM symbol is generated, the subcarriers carrying ANT 2  OFDM symbols are set to 0. Similarly, when an ANT 2  OFDM symbol is generated, the subcarriers carrying the ANT 1  OFDM symbols are set to 0. 
         [0056]    SS  116  multiplies the pilot OFDM symbols from ANT 1  with gain g 0  and the pilot OFDM symbols from ANT 2  with gain g 1 . The receiver also delays the pilots from ANT 2  with delay D 1 . Again, these operations reverse the operations in OFDM symbol processing block  230  in  FIG. 3 , assuming only transmit antenna  331  (i.e., ANT 1 ) and transmits antenna  332  (i.e., ANT 2 ) are used. The two resulting pilots are then combined to get the overall pilot. An FFT operation is performed on the overall pilot to get the overall channel response in the frequency domain. The channel estimate in the frequency domain is then used for data demodulation in the frequency domain. This additional compensation on the pilot signals allows for estimation of the additional processing done in BS  102  on the OFDM symbols containing data. The effect of the actual radio channel is also reflected in the overall channel estimate because the received pilot signals travel via the radio channel. 
         [0057]    The compensation needs to be done on the pilot symbols only, and not the data symbols, because the data symbols are already processed in BS  102 . In an OFDM system, the pilot and data symbols are carried on OFDM subcarriers. Therefore, the compensation can either be done on the time domain OFDM symbol or directly in the frequency domain. In order to do compensation in the frequency domain, the affect of OFDM symbol delay in the time-domain must be accounted for in the frequency domain. In general, a time delay in the time domain translates into a phase rotation in the frequency domain. Therefore, the OFDM subcarriers carrying the pilot symbols may be appropriately phase rotated in the frequency domain to account for time delays. 
         [0058]    While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The exemplary embodiments disclosed are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. It is intended that the disclosure encompass all alternate forms within the scope of the appended claims along with their full scope of equivalents.