Abstract:
A method for channel estimation of a signal by a receiver, comprises the steps of: receiving a symbol of the signal, wherein the signal has a cyclic prefix (“CP”); combining a portion of the CP and an end portion of the symbol; and processing the combined symbol for channel estimation by the receiver.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to methods and systems for channel estimation for communication systems, and, in particular, to methods and systems for orthogonal frequency divisional multiplexing (“OFDM”) channel estimation. 
       BACKGROUND 
       [0002]    In a communications system, a transmitter sends data to a receiver through a channel. In the case of a wireless channel, the transmitted waveforms suffer from multipath fading due to reflection, refraction, and diffraction, which ultimately results in inter-symbol-interference (“ISI”) between transmitted symbols. This is particularly problematic for modern broadband wireless communications systems, e.g., OFDM systems, which offer high data rate services. Particularly for such high data rate systems, multipath fading is especially difficult to mitigate. 
         [0003]    Many current communications systems mitigate ISI by using a cyclic prefix (“CP”) for each transmitted symbol. The CP is a copy of the latter portion of a transmitted symbol that is prepended to the transmitted symbol. The CP acts as a buffer region where delayed information for the previous symbol can be stored by the receiver. The receiver has to exclude all the samples from the CP since those samples can be corrupted by the previous symbol. Furthermore, the CP interval can vary to accommodate different multipath environments. Typically, the CP interval is determined by the expected duration of the multipath channel in the operating environment. As such, a DVB-T system has been configured to have four different CP intervals, including ¼, ⅛, 1/16 and 1/32 of the FFT length. 
         [0004]      FIG. 1  illustrates a symbol for an OFDM signal having a cyclic prefix. A symbol  10  can have a CP  12  having Ncp points and a body  14  having N points. The CP  12  is a copy of a latter portion  16  of the body  14  of the symbol  10 . The drawback of using the CP is that a loss in data rate occurs since redundant information is conveyed. Since the CP is a buffer to avoid ISI, the CP is discarded after receiving the symbol  10 . Therefore, it is desirable to provide new methods and systems that can use the CP of a signal to improve the reception of the signal. 
       SUMMARY OF INVENTION 
       [0005]    An object of this invention is to provide methods and systems for channel estimation that can improve sensitivity without sacrificing multipath mitigation. 
         [0006]    Another object of this invention is to provide methods and systems for using a cyclic prefix of a signal to improve channel estimation of the signal. 
         [0007]    Yet another object of this invention is to provide methods and systems to suppress noise of a signal by using a cyclic prefix of the signal. 
         [0008]    Briefly, the present invention discloses methods for channel estimation of a signal by a receiver, comprising the steps of: receiving a symbol of the signal, wherein the signal has a cyclic prefix; combining a portion of the CP and an end portion of the symbol; and processing the combined symbol for channel estimation by the receiver. 
         [0009]    An advantage of this invention is that methods and systems for channel estimation that can improve sensitivity without sacrificing multipath mitigation are provided. 
         [0010]    Another advantage of this invention is that methods and systems for using a cyclic prefix of a signal to improve channel estimation of the signal are provided. 
         [0011]    Yet another advantage of this invention is that methods and systems to suppress noise of a signal by using a cyclic prefix of the signal are provided. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing and other objects, aspects, and advantages of the invention can be better understood from the following detailed description of the preferred embodiment of the invention when taken in conjunction with the accompanying drawings in which: 
           [0013]      FIG. 1  illustrates a symbol for an OFDM signal having a cyclic prefix. 
           [0014]      FIG. 2  illustrates a symbol of the present invention for an OFDM signal having a cyclic prefix. 
           [0015]      FIG. 3  illustrates a method of the present invention for channel estimation of a signal. 
           [0016]      FIG. 4  illustrates a graph of a noise suppressed channel impulse response power versus samples of a symbol for showing a multipath delay spread. 
           [0017]      FIG. 5  illustrates a block diagram for a communications system of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    In the following detailed description of the embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration of specific embodiments in which the present invention may be practiced. 
         [0019]      FIG. 2  illustrates a symbol of the present invention for an OFDM signal having a cyclic prefix. A symbol  40  can have a CP  42  having Ncp sampling points and a body  44  having N sampling points. Thus, Ncp is the length of the cyclic prefix  42  and N is the length of the body  44  of the symbol. The CP  42  is a copy of a latter portion  50  of the body of the symbol. If the multipath channel delay spread is small, then only some of the CP  42  is affected and unreliable due to ISI. This portion can be referred to as the affected portion  46  and can have a length of Ndly points. The remainder of the cyclic prefix  42  is unaffected and presumably reliable. The unaffected portion  48  can be referred to as the unaffected portion  48 , which can also be denoted R1. The unaffected portion  48  can be obtained by subtracting the Ncp points with the Ndly points. Generally, the Ncp points are usually known ahead of time and the Ndly points can be estimated as outlined below. 
         [0020]    Since the unaffected portion  48  is presumed to be reliable, the unaffected portion  48  can be used during channel estimation by a receiver of the respective signal to improve the receiver performance. For instance, the unaffected portion  48  of the CP  42  can be used to strengthen a portion  52  of the symbol  40  by averaging the two sets of samples to get a strengthened portion of the body  44 . This is due in fact that the CP  42  is a duplicate of the latter portion  50 . Thus, the unaffected portion  48  of the CP  42  can be lined up with its counterpart in the latter portion  50 , i.e., the portion  52 , and then averaged to provide a strengthened signal at the portion  52 . 
         [0021]      FIG. 3  illustrates a method of the present invention for channel estimation of a signal. Channel estimation can be performed  80  by obtaining a noise suppressed channel impulse response power (“CIRP”). To obtain the CIRP, a channel impulse response (“CIR”) for the channel is obtained, e.g., according to Equation (1). Next, the CIR is used to calculate the CIRP of the channel, e.g., according to Equation (2). Finally, the CIRP can be filtered by nulling any values for the CIRP lower than a threshold value, giving a |h 1 [n]| 2  function to represent the CIRP, see Equation (3). The threshold value can be determined based on empirical methods. Typically, the threshold value can be set to equal 0.001 multiplied by the sum of all of the CIRP. 
         [0000]      h=ifft(H)   Equation (1)
 
         [0000]    where H is the channel frequency response, h is the channel impulse response, and ifft is the Inverse Fast Fourier Transform function. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                        
                       
                         h 
                          
                         
                           [ 
                           n 
                           ] 
                         
                       
                        
                     
                     2 
                   
                   = 
                   
                     
                       h 
                        
                       
                         [ 
                         
                           mod 
                            
                           
                             ( 
                             
                               
                                 n 
                                 - 
                                 
                                   N 
                                   2 
                                 
                               
                               , 
                               N 
                             
                             ) 
                           
                         
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                     * 
                     
                       
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                               ) 
                             
                           
                           ] 
                         
                       
                       * 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
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         [0000]    where N&gt;=n&gt;=0, mod is the modulo operator, and N is the total number of samples of the symbol. 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                   Equation 
                    
                   
                       
                   
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                     ( 
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         [0022]    The next step is to estimate the multipath delay spread  82 . In order to do this, a first path position for the calculated CIRP and a last path position for the calculated CIRP are determined, see Equations (4) and (5). The number of samples between the last path position and the first path position can be denoted as the Ndly value and found by Equation (6). Thus, Ncp-Ndly provides the samples of the CP data (i.e., the R1 data 48 as shown in  FIG. 2 ) that are presumably not affected by ISI. 
         [0000]    
       
         
           
             
               
                 
                   firstpathpos 
                   = 
                   
                     
                       arg 
                        
                       
                           
                       
                        
                       
                         
                           min 
                           n 
                         
                          
                         
                           
                              
                             
                               
                                 h 
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                           2 
                         
                       
                     
                     &gt; 
                     0 
                   
                 
               
               
                 
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                    
                   
                       
                   
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                     ( 
                     4 
                     ) 
                   
                 
               
             
             
               
                 
                   lastpathpos 
                   = 
                   
                     
                       arg 
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                           n 
                         
                          
                         
                           
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                    
                   
                       
                   
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                     ( 
                     5 
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                   Ndly 
                   = 
                   
                     lastpathpos 
                     - 
                     firstpathpos 
                   
                 
               
               
                 
                   Equation 
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                     ( 
                     6 
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         [0023]    The CP can then be used to suppress noise  84  by reusing the R1 data to strengthen the latter portion of the body of the respective symbol for channel estimation. For instance, during channel estimation, the points Ncp-Ndly and the latter portion of a symbol can be averaged to provide a more reliable sample points for the latter portion of a symbol, e.g., see Equation (7). 
         [0000]    
       
         
           
             
               
                 
                   
                     y 
                      
                     
                       [ 
                       n 
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                   = 
                   
                     { 
                     
                       
                         
                           
                             x 
                              
                             
                               [ 
                               
                                 n 
                                 + 
                                 Ncp 
                               
                               ] 
                             
                           
                         
                         
                           
                             n 
                             &lt; 
                             
                               N 
                               - 
                               
                                 ( 
                                 
                                   Ncp 
                                   - 
                                   Ndly 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                               ( 
                               
                                 
                                   x 
                                    
                                   
                                     [ 
                                     
                                       n 
                                       + 
                                       Ncp 
                                     
                                     ] 
                                   
                                 
                                 + 
                                 
                                   x 
                                    
                                   
                                     [ 
                                     
                                       n 
                                       + 
                                       Ncp 
                                       - 
                                       N 
                                     
                                     ] 
                                   
                                 
                               
                               ) 
                             
                             / 
                             2 
                           
                         
                         
                           others 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     7 
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    where n=0,1,2, . . . N−1 and the received signal is represented by a digital signal function x (which is further described in  FIG. 5 ). 
         [0024]      FIG. 4  illustrates a graph of a noise suppressed channel impulse response power versus samples of a symbol for showing a multipath delay spread. In an example, a firstpathpos  90  can be found according to Equation (4) and a lastpathpos  92  can be found according to Equation (5). The firstpathpos  90  and the lastpathpos  92  can be plotted on a graph of a noise suppressed channel impulse response power versus samples of a symbol. The multipath delay spread Ndly can be given in terms of the sampling points by the difference of the lastpathpos  92  and the firstpathpos  90 , see Equation (6). 
         [0025]      FIG. 5  illustrates a block diagram for a communications system of the present invention. A signal is inputted to a transmitter  100  for transmission over a channel  104 , e.g., over-the-air wireless channel. The transmission is received by a receiver  102  for processing and decoding. 
         [0026]    The receiver  102  can comprise a digital front end block  106 , a timing synchronization block  108 , a CP processing block  110 , a Fast Fourier Transform (“FFT”) block  112 , a channel estimator  116 , a Ndly calculation block  114 , and a decoder  118 . The received analog transmission can be processed by the digital front end block  106  for outputting a digital signal x with a certain sampling rate that is ready for baseband processing. The digital signal x is outputted to the CP processing block  110  and the timing synchronization block  108 . The timing synchronization block  108  can estimate timing, frequency, and/or phase errors for the digital signal x and make any corrections as necessary. 
         [0027]    The CP processing block  110  receives the digital signal x and an estimated Ndly value from the Ndly calculation block  114  to remove the CP from the digital signal x. The CP can also be used to strengthen the latter portion of the digital signal x. Particularly, the CP processing block  110  can use the Ndly value to calculate the unaffected portion of the CP from ISI. The unaffected portion of the CP can be averaged with its counterpart at the end of a respective symbol to strengthen the signal. The CP processing block  110  outputs the strengthened signal y to the FFT block  112 . The FFT block y performs a FFT operation on the strengthened signal y to covert the time domain signal to a frequency domain signal Y. The signal Y is outputted to the channel estimator  116  and the decoder  118 . 
         [0028]    The channel estimator  116  performs channel estimation on the signal Y to generate a channel frequency response H, which is outputted to the decoder  118  and the Ndly calculation block  114 . The Ndly calculation block  114  then can calculate the Ndly value based upon the channel frequency response H, as outlined above. The decoder then decodes the signal Y using the channel frequency response H. 
         [0029]    While the present invention has been described with reference to certain preferred embodiments or methods, it is to be understood that the present invention is not limited to such specific embodiments or methods. Rather, it is the inventor&#39;s contention that the invention be understood and construed in its broadest meaning as reflected by the following claims. Thus, these claims are to be understood as incorporating not only the preferred apparatuses, methods, and systems described herein, but all those other and further alterations and modifications as would be apparent to those of ordinary skilled in the art.