Abstract:
A method includes multiplying, at a device, a suppressed estimate of a channel by a first matrix to obtain a delay restricted estimate of the channel. The method also includes demodulating a signal received via the channel based at least in part on the delay restricted estimate of the channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from and is a continuation of patent application Ser. No. 13/163,996, filed on Jun. 20, 2011, and entitled “DELAY RESTRICTED CHANNEL ESTIMATION FOR MULTI-CARRIER SYSTEMS,” which is a continuation of patent application Ser. No. 12/467,764, filed on May 18, 2009, now U.S. Pat. No. 7,986,614 and entitled “DELAY RESTRICTED CHANNEL ESTIMATION FOR MULTI-CARRIER SYSTEMS,” which is a continuation of patent application Ser. No. 11/046,192, filed on Jan. 28, 2005, now U.S. Pat. No. 7,551,547, and entitled “DELAY RESTRICTED CHANNEL ESTIMATION FOR MULTI-CARRIER SYSTEMS,” the contents of which are expressly incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to methods and systems for channel estimation of multi-carrier modulation transmissions. 
       BACKGROUND 
       [0003]    In recent years, Orthogonal Frequency Division Multiplexing (OFDM) has attracted attention as a candidate for high-data-rate video and multimedia communications. OFDM belongs to a family of transmission schemes called multi-carrier modulation. Multi-carrier modulation is based on dividing a given high bit-rate data stream into several parallel low bit-rate data streams and modulating each stream on separate sub-carriers. 
         [0004]    The motivation for using multi-carrier modulation is to overcome the problem of inter-symbol interference (ISI). In wireless channels where radio signals from a transmitter traverse multiple paths to a receiver, all the signal energy does not necessarily arrive at the receiver at the same instant of time. This phenomenon of dispersion in a communication channel causes energy from one symbol duration to spill into succeeding symbol durations. 
         [0005]    When a time delay due to dispersion is either a significant fraction of or more than the symbol time duration, the resultant ISI can be detrimental. ISI causes an irreducible error floor that cannot be overcome by simply changing a radio frequency (RF) parameter such as a transmit power, an antenna pattern or a frequency plan. 
         [0006]    In an OFDM system, each sub-carrier can be viewed as a flat fading channel. A single tap equalizer can be used to equalize the transmitted signal in case of coherent demodulation. This requires the receiver to have knowledge of the channel on a per-sub-carrier basis. 
         [0007]    The discrete baseband time representation of a transmitted OFDM signal is: 
         [0000]    
       
         
           
             
               
                 
                   
                     y 
                     
                       n 
                       , 
                       m 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         o 
                       
                       
                         N 
                         - 
                         1 
                       
                     
                      
                     
                       
                         x 
                         
                           k 
                           , 
                           m 
                         
                       
                        
                       
                          
                         
                           
                             j2π 
                              
                             
                               ( 
                               
                                 k 
                                 - 
                                 
                                   N 
                                   2 
                                 
                               
                               ) 
                             
                           
                            
                           
                             n 
                             N 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where x k,m  represents the sub-carrier symbols, N is the number of sub-carriers, k is the modulated symbol index, m is the OFDM symbol index, n is a sub-carrier index, and j represents the square root of −1. 
         [0008]    The discrete version of the OFDM baseband signal shown in equation (1) is identical to the Inverse Discrete Fourier Transform (IDFT) of the sub-carrier symbols x n,m . Thus, OFDM modulation is essentially identical to an IDFT operation which may be performed using an Inverse Fast Fourier Transform (IFFT).  FIG. 1  (prior art) is a block diagram of a baseband representation of an OFDM transmitter. The OFDM transmitter comprises a serial-to-parallel converter  10 , an IFFT processor  12 , and a parallel-to-serial converter  14 . 
         [0009]      FIG. 2  (prior art) is a graph illustrating frequency synchronization of an OFDM signal. The graph illustrates spectral component versus frequency for the case of using four tones. 
         [0010]    The sub-carrier data symbols can be estimated at a receiver by taking the Discrete Fourier Transform (DFT) such as a Fast Fourier Transform (FFT) of a received and equalized OFDM symbol using the following equation. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       x 
                       ^ 
                     
                     
                       n 
                       , 
                       m 
                     
                   
                   = 
                   
                     
                       1 
                       N 
                     
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                           = 
                           o 
                         
                         
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                           - 
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                           y 
                           
                             k 
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                          
                         
                            
                           
                             
                               - 
                               
                                 j2π 
                                  
                                 
                                   ( 
                                   
                                     n 
                                     - 
                                     
                                       N 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                              
                             
                               k 
                               N 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0011]    In a time dispersive channel such as one which introduces multipath fading, the signal at the receiver can be written as a convolution of the transmitted signal y and the channel impulse response h. Thus, the received signal r n,m  in the discrete time domain can be written as: 
         [0000]    
       
         
           
             
               
                 
                   
                     r 
                     
                       n 
                       , 
                       m 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           o 
                         
                         
                           L 
                           - 
                           1 
                         
                       
                        
                       
                         
                           h 
                           l 
                         
                          
                         
                           y 
                           
                             
                               n 
                               - 
                               l 
                             
                             , 
                             m 
                           
                         
                          
                         
                             
                         
                          
                         n 
                       
                     
                     ≥ 
                     L 
                   
                 
               
               
                 
                   ( 
                   
                     3 
                      
                     a 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     r 
                     
                       n 
                       , 
                       m 
                     
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           
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                             = 
                             o 
                           
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                          
                         
                           
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                             l 
                           
                            
                           
                             y 
                             
                               
                                 n 
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                                 l 
                               
                               , 
                               m 
                             
                           
                         
                       
                        
                       
                           
                       
                       + 
                       
                         
                           ∑ 
                           
                             l 
                             = 
                             
                               n 
                               + 
                               1 
                             
                           
                           
                             L 
                             - 
                             1 
                           
                         
                          
                         
                           
                             h 
                             l 
                           
                            
                           
                             y 
                             
                               
                                 N 
                                 - 
                                 
                                   ( 
                                   
                                     l 
                                     - 
                                     n 
                                   
                                   ) 
                                 
                               
                               , 
                               
                                 m 
                                 - 
                                 l 
                               
                             
                           
                            
                           
                               
                           
                            
                           n 
                         
                       
                     
                     &lt; 
                     L 
                   
                 
               
               
                 
                   ( 
                   
                     3 
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where L is the length of the channel impulse response in units of sample time. 
         [0012]    No ISI is observed in received signal when n=L. However when n&lt;L, the signal from the previous OFDM symbol is corrupted by the received signal. If the cyclic prefix is longer than the channel impulse response, then the effect of the previous OFDM symbol is not seen in the usable part of the signal (i.e. the part of the OFDM symbol after the cyclic prefix). From here onward, it is assumed herein that the cyclic prefix is longer than the channel impulse response, thus ignoring the second term in equation (3b). Under this assumption, the demodulated signal in each sub-carrier is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     s 
                     
                       n 
                       , 
                       m 
                     
                   
                   = 
                   
                     
                       
                         1 
                         N 
                       
                        
                       
                         
                           ∑ 
                           
                             k 
                             = 
                             0 
                           
                           
                             N 
                             - 
                             1 
                           
                         
                          
                         
                           
                             r 
                             
                               k 
                               , 
                               m 
                             
                           
                            
                           
                              
                             
                               
                                 - 
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                                
                               
                                   
                               
                                
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                                
                               
                                 π 
                                  
                                 
                                   ( 
                                   
                                     n 
                                     - 
                                     
                                       N 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
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                                 k 
                                 N 
                               
                             
                           
                         
                       
                     
                     = 
                     
                       
                         
                           1 
                           N 
                         
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                               = 
                               0 
                             
                             
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                               - 
                               1 
                             
                           
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                                 = 
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                              
                             
                               
                                 h 
                                 l 
                               
                                
                               
                                 y 
                                 
                                   
                                     k 
                                     - 
                                     l 
                                   
                                   , 
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                                
                               
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                                     - 
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                                    
                                   
                                       
                                   
                                    
                                   2 
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                                     π 
                                      
                                     
                                       ( 
                                       
                                         n 
                                         - 
                                         
                                           N 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                    
                                   
                                     k 
                                     N 
                                   
                                 
                               
                             
                           
                         
                       
                       = 
                       
                         
                           λ 
                           n 
                         
                          
                         
                           x 
                           
                             n 
                             , 
                             m 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where λ n  is the DFT of the channel impulse response. 
         [0013]    Even though the channel is time dispersive, the effect of the channel can be visualized in the frequency domain (i.e. after the DFT) by a single multiplicative constant λ n  for each of the sub-carriers. This results because a convolution operation in the time domain translates to a simple multiplication in the frequency domain. A single-tap equalization using a zero-forcing equalizer can be used to estimate the sub-carrier signal using the following equation. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       x 
                       ^ 
                     
                     
                       n 
                       , 
                       m 
                     
                   
                   = 
                   
                     
                       s 
                       
                         n 
                         , 
                         m 
                       
                     
                     
                       λ 
                       
                         n 
                         , 
                         m 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0014]    However, in order to use equation (5), the receiver needs to know the channel response in the frequency domain (i.e. the values of λ n ). 
         [0015]    The channel response may be estimated either by embedding pilot symbols within data stream or by using preambles. The known pilot or preamble symbols are used to estimate the channel on the given sub-carriers. To estimate the channel in other sub-carriers, a subsequent channel interpolation can be performed. 
         [0016]    Existing channel estimation techniques include zero forcing and linear minimum mean square error (LMMSE). Zero-forcing channel estimation can be performed over known pilot and/or preamble symbols by dividing the received symbol by the expected symbol. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       λ 
                       ^ 
                     
                     n 
                   
                   = 
                   
                     
                       s 
                       
                         n 
                         , 
                         m 
                       
                     
                     
                       x 
                       
                         n 
                         , 
                         m 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0017]    A disadvantage of a zero forcing estimate is its unreliability in low signal-to-noise ratio (SNR) conditions. For example, a zero-forcing-estimated channel response can be significantly inaccurate if some of the sub-carriers experience a deep fade. 
         [0018]    The LMMSE channel estimator is designed to minimize the mean square error between an estimated channel response and an actual channel response. For convenience in formulating the LMMSE estimator, the relationship between received and transmitted symbols in the sub-carriers carrying the pilot/preamble is represented in the following vector form: 
         [0000]    
       
         
           
             
               
                 
                   
                     S 
                     = 
                     
                       
                         P 
                          
                         
                             
                         
                          
                         Λ 
                       
                       + 
                       W 
                     
                   
                   , 
                   
                     
                       or 
                        
                       
                         
 
                       
                       [ 
                       
                         
                           
                             
                               s 
                               0 
                             
                           
                         
                         
                           
                             
                               s 
                               1 
                             
                           
                         
                         
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               s 
                               
                                 M 
                                 - 
                                 1 
                               
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   p 
                                   0 
                                 
                               
                               
                                 0 
                               
                               
                                 … 
                               
                               
                                 0 
                               
                             
                             
                               
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                                 0 
                               
                               
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                                 … 
                               
                               
                                 
                                   p 
                                   
                                     M 
                                     - 
                                     1 
                                   
                                 
                               
                             
                           
                           ] 
                         
                          
                         
                           [ 
                           
                             
                               
                                 
                                   λ 
                                   0 
                                 
                               
                             
                             
                               
                                 
                                   λ 
                                   1 
                                 
                               
                             
                             
                               
                                 ⋮ 
                               
                             
                             
                               
                                 
                                   λ 
                                   
                                     M 
                                     - 
                                     1 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       + 
                       
                         [ 
                         
                           
                             
                               
                                 w 
                                 0 
                               
                             
                           
                           
                             
                               
                                 w 
                                 1 
                               
                             
                           
                           
                             
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                                 w 
                                 
                                   M 
                                   - 
                                   1 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where s i  and p i  are the received and transmitted symbols in the sub-carriers carrying the pilot/preamble, and w i  represents noise. The noise can be assumed to be additive, Gaussian white noise. 
         [0019]    The LMMSE estimate {circumflex over (Λ)} of the channel impulse response is determined by the following equation: 
         [0000]      {circumflex over (Λ)}= AS=APA+AW   (8)
 
         [0000]    where A is an estimation matrix. 
         [0020]    The estimation matrix A is determined by the following equation: 
         [0000]        A=R   λ   [R   λ+(   P   H   P ) −1 Σ] −1   P   −1 .  (9)
 
         [0000]    where R λ  is the covariance matrix of the channel impulse response and Σ is the covariance matrix of the noise vector and usually is a diagonal matrix. 
         [0021]    In the absence of noise, the covariance matrix Σ is equal to a zero matrix. If the fading in the different sub-carriers is independent, the covariance matrix R λ  of the channel impulse response is an identity matrix. Under both of these two conditions, the LMMSE estimate is identical to the zero forcing estimate. 
         [0022]    A drawback of the LMMSE channel estimation scheme is its requiring knowledge of the correlation between the fades of different sub-carriers to form the covariance matrix R λ . In most practical systems, this information is not known at the receiver a priori, thus making the LMMSE estimator impractical. 
         [0023]    Channel estimation and equalization are fundamental components of wireless communication systems, especially those that have been designed to work in a non-line-of-sight condition. In multi-carrier systems such as OFDM, equalization is relatively straightforward, but sophisticated channel estimation and channel interpolation techniques are presently required. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention is pointed out with particularity in the appended claims. However, other features are described in the following detailed description in conjunction with the accompanying drawings in which: 
           [0025]      FIG. 1  (prior art) is a block diagram of a baseband representation of an OFDM transmitter; 
           [0026]      FIG. 2  (prior art) is a graph illustrating frequency synchronization of an OFDM signal; 
           [0027]      FIG. 3  is a flow chart summarizing an embodiment of a method of delay-restricted channel estimation; 
           [0028]      FIGS. 4 and 5  show symbol/sub-carrier combinations in pilot and preambles that can be used for delay-restricted channel estimation; 
           [0029]      FIG. 6  is a block diagram of an embodiment of a communication system having a delay-restricted channel estimator; and 
           [0030]      FIG. 7  illustrates graphs of throughput versus signal-to-noise ratio for simulations of the delay restricted channel estimation method. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    In a particular embodiment, a method includes performing a channel estimation to obtain an estimate of a channel based on a received signal. The method also includes suppressing a subset of elements of the estimate of the channel to obtain a suppressed estimate of the channel. The method further includes multiplying the suppressed estimate of the channel by a matrix to obtain a delay restricted estimate of the channel. 
         [0032]    In another particular embodiment, an apparatus includes a channel estimator configured to compute a channel estimate based on a received signal. The apparatus also includes a suppressor to suppress a subset of elements of the channel estimate to obtain a suppressed channel estimate of the channel. The suppressor is further configured to suppress a particular element by setting the particular element to zero. The apparatus further includes a multiplier configured to multiply the suppressed channel estimate by a matrix to obtain a delay restricted estimate of the channel. 
         [0033]    In another particular embodiment, a computer readable storage medium includes instructions that, when executed by a processor, cause the processor to perform a channel estimation to obtain an estimate of a channel based on a received signal. The instructions, when executed by the processor, also cause the processor to suppress a subset of elements of the estimate of the channel to obtain a suppressed estimate of the channel. The instructions, when executed by the processor, further cause the processor to multiply the suppressed estimate of the channel by a matrix to obtain a delay restricted estimate of the channel. The matrix is a unitary matrix obtained from a singular value decomposition of a Fourier transform matrix. 
         [0034]    Existing frequency domain channel estimation schemes for OFDM do not incorporate information that the channel impulse response exists only over a finite number of samples in the time domain because the delay spread of the environment is finite and much smaller than the OFDM symbol duration. This assumption can be made since the delay spread of most environments is of the order of a few microseconds, e.g. 15-20 microseconds in some environments. Thus, in the time domain, a span of channel impulse response is essentially limited. 
         [0035]    Described herein are embodiments of delay restricted channel estimation methods and systems that take into account a finite spread of the channel impulse response in the time domain to improve the accuracy of channel estimation in the frequency domain. The new methods can be used for preamble-based or pilot-based channel estimation. 
         [0036]    The disclosed methods provide channel estimates that are less susceptible to noise. As a benefit, the disclosed methods also allow for estimation of a multi-carrier noise variance estimation. Simulation results show a 2 dB improvement in the link performance by implementing a particular disclosed channel estimation method when compared to traditional channel estimation schemes such as zero forcing and LMMSE. 
         [0037]    The delay restricted channel estimation can be based on a zero-forcing estimate, an LMMSE estimate, or an alternative estimate with an additional constraint that the impulse response of the channel exists only over a finite and known interval of time. 
         [0038]    A vector Λ comprising the values λ n  from equation (4) is related to a DFT matrix F and a channel impulse response vector H by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     Λ 
                     = 
                     FH 
                   
                   , 
                   
                     
                       or 
                        
                       
                         
 
                       
                       [ 
                       
                         
                           
                             
                               λ 
                               0 
                             
                           
                         
                         
                           
                             
                               λ 
                               1 
                             
                           
                         
                         
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               λ 
                               
                                 N 
                                 - 
                                 1 
                               
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   F 
                                   
                                     0 
                                     , 
                                     0 
                                   
                                 
                               
                               
                                 … 
                               
                               
                                 
                                   F 
                                   
                                     0 
                                     , 
                                     
                                       L 
                                       - 
                                       1 
                                     
                                   
                                 
                               
                             
                             
                               
                                 ⋮ 
                               
                               
                                 ⋱ 
                               
                               
                                 ⋮ 
                               
                             
                             
                               
                                 
                                   F 
                                   
                                     
                                       N 
                                       - 
                                       1 
                                     
                                     , 
                                     0 
                                   
                                 
                               
                               
                                 … 
                               
                               
                                 
                                   F 
                                   
                                     
                                       N 
                                       - 
                                       1 
                                     
                                     , 
                                     
                                       L 
                                       - 
                                       1 
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                          
                         
                           [ 
                           
                             
                               
                                 
                                   h 
                                   0 
                                 
                               
                             
                             
                               
                                 ⋮ 
                               
                             
                             
                               
                                 
                                   h 
                                   
                                     L 
                                     - 
                                     1 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0039]    The elements of the DFT matrix F are given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       F 
                       
                         i 
                         , 
                         k 
                       
                     
                     = 
                     
                        
                       
                         
                           - 
                           j 
                         
                          
                         
                             
                         
                          
                         2 
                          
                         
                             
                         
                          
                         
                           π 
                            
                           
                             ( 
                             
                                
                               - 
                               
                                 N 
                                 2 
                               
                             
                             ) 
                           
                         
                          
                         
                           k 
                           N 
                         
                       
                     
                   
                   , 
                   
                     
                       for 
                        
                       
                           
                       
                        
                       k 
                     
                     &lt; 
                     
                       L 
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0040]    The DFT matrix F is an N×L matrix of rank L, where L is the number of taps over which the channel impulse response exists. A singular value decomposition of the DFT matrix F can be performed to determine a unitary transformation U and V such that 
         [0000]        F=U  F V   H               F =U   H   FV   (12)
 
         [0000]    where  F  is an N×L diagonal matrix containing the singular values of F, and U and V are N×N and L×L dimensional matrices respectively with unitary columns. 
         [0041]    The matrices U and V can be viewed as unitary transformation operators in the frequency and time domains, respectively. In particular, the vector Λ can be pre-multiplied by U H  (i.e. the complex conjugate transpose of U) to form a transformed vector  Λ . The vector H can be pre-multiplied by V H  (i.e. the complex conjugate transpose of V) to form a transformed vector  H . The relationship between  Λ ,  F  and  H  is represented by the following equation: 
         [0000]        Λ =   FH .   (13)
 
         [0042]    The matrix  F  has the following form. 
         [0000]    
       
         
           
             
               
                 
                   
                     F 
                     _ 
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             
                               F 
                               _ 
                             
                             0 
                           
                         
                         
                           … 
                         
                         
                           0 
                         
                       
                       
                         
                           ⋮ 
                         
                         
                           ⋱ 
                         
                         
                           ⋮ 
                         
                       
                       
                         
                           0 
                         
                         
                           … 
                         
                         
                           
                             
                               F 
                               _ 
                             
                             
                               L 
                               - 
                               1 
                             
                           
                         
                       
                       
                         
                           0 
                         
                         
                           … 
                         
                         
                           0 
                         
                       
                       
                         
                           ⋮ 
                         
                         
                           ⋱ 
                         
                         
                           ⋮ 
                         
                       
                       
                         
                           0 
                         
                         
                           … 
                         
                         
                           0 
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
         [0043]    Based on the form of  F , the elements after the first L elements in the vector  Λ  are all zero. 
         [0000]    
       
         
           
             
               
                 
                   
                     Λ 
                     _ 
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             
                               λ 
                               _ 
                             
                             0 
                           
                         
                       
                       
                         
                           ⋮ 
                         
                       
                       
                         
                           
                             
                               λ 
                               _ 
                             
                             
                               L 
                               - 
                               1 
                             
                           
                         
                       
                       
                         
                           0 
                         
                       
                       
                         
                           ⋮ 
                         
                       
                       
                         
                           0 
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0044]    The unitary transformation U is used to modify a channel estimate, such as either the zero forcing or the LMMSE channel estimate, as follows. Once the estimate  Λ  has been evaluated, the elements  λ i    for i≧L are suppressed to zero based on the form of  Λ  given by equation (15). The transformation U is used to transform  Λ  back to Λ in the frequency domain, i.e. U is multiplied by  Λ  to obtain a delay-restricted estimate of Λ. 
         [0045]    To show the derivation, both sides of equation (7) are multiplied by U H  to transform the received symbol vector S, the transmitted symbol matrix P, the Λ vector and the noise vector W. 
         [0000]        U   H   S=U   H   PUU   H   Λ+U   H   W   (16)
 
         [0046]    Thus, equation (16) relates a transformed received symbol vector  S , a transformed transmitted symbol matrix  P , the transformed vector  Λ , and the transformed noise vector  W  according to the following equation. 
         [0000]          S =  PΛ +  W     (17)
 
         [0047]    Since the transformation is unitary, the covariance matrices of the noise vector W and the transformed noise vector  W  are identical. 
         [0048]    The delay-restricted zero-forcing channel estimate can be determined by: (i) determining the transformed vector  Λ  based on the equation: 
         [0000]        Λ =   P     −1     S ;   (18)
 
         [0049]    (ii) suppressing all but the first L elements in  Λ  to zero; and (iii) transforming  Λ  back to  Λ  using the following equation: 
         [0000]      Λ= UΛ   (19)
 
         [0050]    Similarly the delay-restricted LMMSE channel estimate can be determined by: (i) determining the transformed vector  Λ  based on the equation: 
         [0000]        Λ = R     λ     [R     λ +(     P     H     P   ) −1 Σ] −1     P     −1     S ;   (20)
 
         [0051]    (ii) suppressing all but the first L elements in  Λ  to zero; and (iii) transforming  Λ  back to  Λ  using the following equation: 
         [0000]      Λ= U  Λ .   (21)
 
         [0052]    A traditional zero-forcing channel estimate does not allow noise variation estimation to be performed. However, by taking advantage of the finite spread of the channel impulse response in the time domain, the delay-restricted zero-forcing channel estimate enables estimation of a noise variance. The estimate of noise Ŵ is determined by subtracting the estimated signal from the received signal. 
         [0000]        Ŵ=r−P{circumflex over (Λ)}   (22)
 
         [0053]    A noise variance σ 2  is determined by an inner product of Ŵ H  and Ŵ. 
         [0000]      σ 2   =             Ŵ   H   Ŵ             (23)
 
         [0054]    This approach to noise variance estimation is unsuitable for the case of a traditional zero-forcing estimate since a zero noise variance will result based on the definition of the zero-forcing estimate. 
         [0055]      FIG. 3  is a flow chart summarizing an embodiment of a method of delay restricted channel estimation. As indicated by block  20 , the method comprises determining the transformation matrix U H . The transformation matrix U H  can be determined by performing a singular value decomposition of the DFT matrix, F, to determine the matrix U, and then determining the complex conjugate transpose of U. 
         [0056]    As indicated by block  22 , the method comprises transforming the transmitted symbol matrix P based on the transformation matrix U H . The transformed transmitted symbol matrix  P  is equal to U H  multiplied by P. 
         [0057]    As indicated by block  24 , the method comprises transforming the received symbol vector S based on the transformation matrix U H . The transformed received symbol vector  S  is equal to U H  multiplied by S. 
         [0058]    As indicated by block  26 , the method comprises determining a transformed vector  Λ  based on the transformed transmitted symbol matrix  P  and the transformed received symbol vector  S . The transformed vector  Λ  is determined using a channel estimation method such as zero-forcing, LMMSE, or an alternative method. For zero-forcing,  Λ  is determined using equation (18); for LMMSE,  Λ  is determined using equation (20). 
         [0059]    As indicated by block  30 , the method comprises suppressing all but the first L elements of the transformed vector  Λ  to zero. Thus, the first L elements (i.e. elements  1 ,  2 , . . . , L) of the transformed vector  Λ  remain the same, and the remaining elements (i.e. elements L+1, L+2, . . . , N) are set to zero. It is noted that the scope of this disclosure includes variations wherein all elements after the L th  element are not suppressed exactly to zero, but substantially to zero. 
         [0060]    As indicated by block  32 , the method comprises inverse transforming the suppressed, transformed vector  Λ  based on the matrix U (which is an inverse transformation of the matrix U H ) to determine a delay-restricted estimate of Λ. The delay-restricted estimate of Λ is equal to U multiplied by  Λ  (see equations (19) and (21)). Although the (L+1) st  to the N th  elements in the transformed vector  Λ  are zero, the corresponding elements in Λ are not necessarily zero. Moreover, it is likely that all N elements of Λ are non-zero. 
         [0061]    The method can be used to estimate the channel impulse response based on either pilots or preambles having symbol/sub-carrier combinations that are known a priori.  FIGS. 4 and 5  show examples of pilots and preambles where symbol/sub-carrier combinations that are known a priori are shaded. 
         [0062]      FIG. 6  is a block diagram of an embodiment of a communication system having a delay-restricted channel estimator  40 . A transmitter  42  modulates a signal using a multi-carrier modulation method such as OFDM. The modulated signal is transmitted over a communication channel  44  to a receiver  46 . The receiver  46  comprises the delay-restricted channel estimator  40 , which performs an embodiment of the method described with reference to  FIG. 3  for known pilots and/or preambles contained in the transmitted signal. The delay-restricted channel estimator  40  outputs the Λ vector to an equalizer  50 . The equalizer  50  equalizes the received signal based on the Λ vector. A demodulator  52  demodulates an equalized signal from the equalizer  50 . 
         [0063]    Simulations of the delay-restricted channel estimation method were performed for an IEEE 802.16(d) system. The IEEE 802.16(d) system is a 256 sub-carrier based OFDM system that has been designed to provide high data rate and high QoS for wireless broadband access. 
         [0064]    A 3GPP-based multipath channel (TS 25.996) was used to simulate the effect of a wireless non-line-of-sight (non-LOS) channel on a baseband signal. At the receiver, realistic channel estimation and frequency synchronization algorithms were used to keep the simulation close to real world performance of such systems. 
         [0065]    TABLE I shows the parameters of the simulations. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE I 
               
               
                   
               
             
             
               
                 Channel Bandwidth 
                 5 MHz 
               
               
                 Oversampling 
                 7/6 
               
               
                 Modulation 
                 BPSK, QPSK, 16QAM, &amp; 64 QAM 
               
               
                 Coding 
                 Concatenated RS and Convolution Coding 
               
               
                 Wireless Channel 
                 Frequency Selective Fading (based on 3GPP MIMO 
               
               
                   
                 Channel) 
               
               
                 Doppler Spread 
                 2 Hz (1 km/hr @ 2100 MHz) 
               
               
                 Delay Spread 
                 12 μsec 
               
               
                 Transmit Diversity 
                 Yes (2 antennas) 
               
               
                 Receive Diversity 
                 Yes (2 antennas) 
               
               
                 Channel Estimation 
                 Delay restricted and zero-forcing channel estimation 
               
               
                 Noise Variance 
                 Realistic 
               
               
                 Estimation 
               
               
                 Symbol Decoding 
                 Soft 
               
               
                 Synchronization 
                 Ideal 
               
               
                 MAC PDU Length 
                 Variable 
               
               
                 MAC SDU Length 
                 Fixed (49 or 52 bytes) 
               
               
                 Carrier Frequency 
                 2300 MHz 
               
               
                 Bandwidth 
                 5 MHz 
               
               
                   
               
             
          
         
       
     
         [0066]      FIG. 7  are graphs of throughput versus signal-to-noise ratio for simulations of the delay restricted channel estimation method. Graphs  60  and  62  illustrate the performance of delay-restricted and conventional zero-forcing channel estimation, respectively, for QPSK modulation. Graphs  64  and  66  illustrate the performance of delay-restricted and conventional zero-forcing channel estimation, respectively, for 16-QAM modulation. Graphs  70  and  72  illustrate the performance of delay-restricted and conventional zero-forcing channel estimation, respectively, for 64-QAM modulation. 
         [0067]    The graphs show that the delay-restricted channel estimation has improved throughput compared to conventional zero-forcing channel estimation for QPSK, 16-QAM and 64-QAM modulation. The overall performance of the system improves by about 2 dB by using the delay restricted channel estimation algorithm, which implies that the system requires a signal to noise ratio (SNR) 2 dB lower than what would be required if this channel estimation algorithm was not used. A significant portion of the gain comes from more accurate channel estimation and significantly better noise variance estimation. 
         [0068]    The herein-disclosed delay restricted channel estimation method and system can be implemented in a fixed wireless access system. In one embodiment, the fixed wireless access system is based on the WiMAX standard. This implementation would lead to an improvement in coverage and capacity in the fixed wireless access system. 
         [0069]    The herein-disclosed delay-restricted channel estimation method and system can be implemented in mobile radio telephones (e.g. cellular telephones) and/or mobile radio telephone base stations. In one embodiment, the delay-restricted channel estimation method and system are implemented by one or more integrated circuits for mobile telephones (e.g. a mobile telephone chip set). 
         [0070]    Generally, the acts and components described herein can be implemented using a computer system. The computer system is directed by computer program code stored by a computer-readable storage medium to perform the acts described. 
         [0071]    It will be apparent to those skilled in the art that the disclosed embodiments may be modified in numerous ways and may assume many embodiments other than the particular forms specifically set out and described herein. 
         [0072]    The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments that fall within the true scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.