Abstract:
A method for processing a signal by a receiver, comprises the steps of: receiving the signal by the receiver, calculating one or more symbols based on the received signal; determining a multipath delay spread from the received signal; rebuilding one or more of the calculated symbols as a function of the multipath delay spread; and processing the rebuilt symbols for decoding by the receiver.

Description:
FIELD OF INVENTION 
     This invention relates to methods for signal processing, and, in particular, to methods for inter-symbol-interference (“ISI”) reduction of an orthogonal frequency-division multiplexing (“OFDM”) signal. 
     BACKGROUND 
     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 between transmitted symbols of the signal. 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. 
     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 length can vary to accommodate different multipath environments. Typically, the CP interval length 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. However, the multipath delay can be longer than these set intervals causing unreliable decoding of the signal. 
       FIG. 1  illustrates symbols of an OFDM signal, where each of the symbols has a cyclic prefix. Symbols m−1, m, and m+1 for a single carrier of the OFDM signal can be transmitted sequentially in the time domain. The symbol m comprises a cyclic prefix  10  having N CP  points and a body  12  having N points. The CP  10  is discarded to avoid any ISI to the symbol m from the previous symbol m−1. However, when a multipath delay spread for the signal is greater than the CP  10  length, the ISI will affect the body  12  of the symbol m, causing the data in the symbol m to be unreliable. 
     Therefore, it is desirable to provide new methods and systems for processing a signal to improve the reception of the signal even when the multipath delay spread is greater than the length of a CP for the signal. 
     SUMMARY OF INVENTION 
     An object of this invention is to provide methods and systems for signal processing to reduce ISI for a multipath delay spread that exceeds a cyclic prefix length for a received signal. 
     Another object of this invention is to provide methods and systems for signal processing to identify a multipath delay spread for a received signal. 
     Yet another object of this invention is to provide methods and systems for signal processing to estimate the originally transmitted signal without ISI based upon the received signal. 
     Briefly, the present invention discloses methods and systems for processing a signal by a receiver, comprising the steps of: receiving the signal by the receiver; calculating one or more symbols based on the received signal; determining a multipath delay spread from the received signal; rebuilding one or more of the calculated symbols as a function of the multipath delay spread; and processing the rebuilt symbols for decoding by the receiver. 
     An advantage of this invention is that methods and systems for signal processing are provided to reduce ISI for a multipath delay spread that exceeds a cyclic prefix length for a received signal. 
     Another advantage of this invention is that methods and systems for signal processing are provided to identify a multipath delay spread for a received signal. 
     Yet another advantage of this invention is that methods and systems for signal processing are provided to estimate the originally transmitted signal without ISI based upon the received signal. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  illustrates symbols of an OFDM signal, where each of the symbols has a cyclic prefix. 
         FIG. 2  illustrates timing diagrams for symbols of an OFDM signal having different paths to a receiver for the OFDM signal. 
         FIG. 3  illustrates a method of the present invention for processing a signal to reduce ISI. 
         FIG. 4  illustrates a block diagram for a communications system of the present invention to reduce ISI. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
       FIG. 2  illustrates symbols of an OFDM signal having multiple paths to a receiver causing a multipath delay spread. Due to a multipath environment, a signal can take multiple paths to a receiver causing ISI. The symbols of the received signal can be rebuilt to more closely represent the originally transmitted symbols based upon the received signal. The rebuilding step can eliminate the various components of the received signal that can be due to the multipath environment. Thus, the rebuilt symbols of the signal can more accurately reflect the original transmitted symbols by reducing the effects of ISI. 
     For example, a signal can be transmitted to a receiver, where the signal can take multiple paths to the receiver. The multiple paths can cause a delay in the reception of the signal causing an offset of the signal in the time domain, as illustrated in path  1  and path  2 . Path  1  can be the first path and path  2  can be the last path, where there can be many other paths between the first path and the second path. The signal can comprise symbols m−1 to m+1, where each of the symbols has a CP. However, if a multipath delay spread Ndly for the two multipaths, path  1  and path  2 , is greater than the CP, then ISI will generally occur since the multipaths are constructively added by the receiver. 
     The present invention can generally identify the multipath delay spread Ndly, and then use the multipath delay spread Ndly and the received signal to combat ISI. In particular, the multipath delay spread Ndly is used to remove components of the received signal that are due to the path  2  of the signal, including a segment R 1  of symbol m−1 from the path  2  and a segment R 2  of the symbol m from the path  2 . Once these components are estimated and removed, the transmitted symbol can be estimated by the receiver. 
       FIG. 3  illustrates a method of the present invention for processing a signal to reduce ISI. A signal y(n) in the time domain can be received  40 , where the received signal y(n) may take multiple paths to the receiver. The symbols of the received signal y(n) can be calculated  42 . Generally, symbols Y[m,k] in the frequency domain can be obtained from the received signal y(n) by applying a CP removal and a Fast Fourier Transform (“FFT”) on the received signal y(n). For instance, the symbols Y[m,k] can be found by the following Equation (1):
 
 Y[m,k ]=FFT( y[Ncp+ 1+( N+Ncp )*( m− 1):( Ncp+N )* m ])  Equation (1)
 
where Ncp is the sample points of a cyclic prefix, N is the total number of sample points of a body of a symbol, and m is the symbol number. Channel estimation can also be performed to obtain the channel response H[m,k].
 
     Next, the symbols Y[m,k] in the frequency domain and the channel response H[m,k] can be used to estimate the symbols S est [m,k], e.g., according to Equation (2). Then, a slicer function (or other hard decision device) can be applied to the results of the symbol estimation S est [m,k] to obtain hard decision symbols S dec [m,k], e.g., in Equation (3). The hard decision symbols S dec [m,k] can be the calculated symbols for this step.
 
 S   est   [m,k]=Y[m,k]/H[m,k]   Equation (2)
 
 S   dec   [m,k ]=slicer( S   est   [m,k ])  Equation (3)
 
     A multipath delay spread  44  for the signal can also be calculated by using 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 (4). Next, the CIR is used to calculate the CIRP of the channel, e.g., according to Equation (5). 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 (6).
 
 h=ifft ( H )  Equation (4)
 
where H is the channel frequency response, h is the channel impulse response, and “ifft” or “IFFT” is the Inverse Fast Fourier Transform function.
 
                            h   ⁡     [   n   ]            2     =       h   ⁡     [     mod   ⁢           ⁢     (       n   -     N   2       ,   N     )       ]       *       h   ⁡     [     mod   ⁢             ⁢             (       n   -     N   2       ,   N     )     ]       *               Equation   ⁢           ⁢     (   5   )                 
where N&gt;=n&gt;=0, mod is the modulo operator, and N is the total number of samples of the symbol.
 
     
       
         
           
             
               
                 
                   
                     
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     A first path position for the calculated CIRP and a last path position for the calculated CIRP can be determined, see Equations (7) and (8). Finally, the number of samples between the last path position and the first path position can be the multipath delay spread, see Equation (9). 
     
       
         
           
             
               
                 
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                   firstPathPos   =       arg   ⁢           ⁢       max   n     ⁢              h   1     ⁡     [   n   ]            2         &gt;   0             Equation   ⁢           ⁢     (   8   )                   N dly=lastpathpos−firstpathpos  Equation (9)
 
     Next, the calculated symbols S dec [m,k] can be rebuilt  46  to reduce ISI using the calculated multipath delay spread Ndly from step  44 . Transmitter side functions can be applied to the calculated symbols S dec [m,k] to derive an estimated received signal function in the time domain, y R [m,p]. The estimated received signal function in the time domain y R [m,p] can then be used to rebuild the symbols without ISI by eliminating various components caused by the multipath delay spread Ndly. The rebuilt symbols can then be processed  48  further for decoding. 
     Symbols can be rebuilt by first applying an IFFT function to the calculated symbols S dec [m,k] to obtain equations for the transmitted symbols, e.g., in Equations (10)-(12).
 
 {circumflex over (x)}   R   [m, 1 +Ncp: N+Ncp]=ifft ( S   dec   [m, 1 : N ])  Equation (10)
 
 {circumflex over (x)}   R   [m,k]={tilde over (x)}   R   [m,N+k],k= 1,2 , . . . ,Ncp   Equation (11)
 
 {circumflex over (x)}   R   [m,k]= 0 ,k≦ 0 or  k&gt;N+Ncp   Equation (12)
 
     Using the above equations for the transmitted symbols, an estimated received signal in the time domain y R [m,p] can be derived as follows:
 
 y   R   [m,p]=Σ   tau=1   Ndly   h (τ)* {circumflex over (x)}   R ( m,p −τ), p= 1,2 , . . . ,N+Ncp+Ndly   Equation (13)
 
     The estimated received signal function in the time domain y R [m,p] can be used to delete the various components of the multipath for the originally received signal. For instance, the estimated received signal function in the time domain y R [m,p] for the symbol m can comprise a component for the unaffected multipath segment of the symbol m, i.e., the sample points starting after the Ndly sample points to N sample points of the body and another component for the affected multipath segment of the symbol m, i.e., from k=1 to Ndly−Ncp, which may no longer be reliable due to ISI. 
     The unaffected multipath segment portion of the symbols can be denoted as follows:
 
 y   new   [m,k]=y [( m− 1)*( Ncp+N )+ Ncp+k]   Equation (14)
 
where k is equal to Ndly-Ncp+1, . . . , N.
 
     The affected multipath segment portion of the symbols can be denoted as follows: 
                       y   new     ⁡     [     m   ,   k     ]       =     (           y   ⁡     [         (     m   -   1     )     *     (     Ncp   +   N     )       +   Ncp   +   k     ]                 +     y   ⁡     [       m   *     (     Ncp   +   N     )       +   k     ]                     -       y   R     ⁡     [       m   +   1     ,   k     ]         -       y   R     ⁡     [       m   -   1     ,     N   +   Ncp   +   Ncp   +   k       ]               )             Equation   ⁢           ⁢     (   15   )                 
where k is equal to 1, 2, . . . , Ndly−Ncp.
 
     The y new [m,k] can then be processed as the received signal by receiver. For instance, a FFT function can be applied to the y new [m,k] to obtain the frequency domain function of the signal Y new . Next, channel estimation can be applied to Y new , and further demapping and decoding can be performed to process the rebuilt symbols without ISI. 
       FIG. 4  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  102 , e.g., over-the-air wireless channel. The transmission is received by a receiver  104  for processing and decoding. 
     The receiver  104  can comprise a digital front end block  120 , a CP processing block  122 , a first FFT block  124 , a first channel estimator  126 , a rebuild channel and data block  128 , a y new  calculation block  130 , a second FFT block  132 , a second channel estimator  134 , and a demapper and decoder block  136 . In terms of implementation, the first FFT block  124  and the second FFT block  132  can be a single functional block for providing the functionality as further described for each of the FFT blocks  124  and  132 . Additionally, the channel estimator  126  and the new channel estimator  134  can be a single functional block for providing the functionality as further described for each of the channel estimators  126  and  134 . 
     The received analog transmission can be processed by the digital front end block  120  for outputting a digital signal y(n) with a certain sampling rate that is ready for baseband processing. The digital signal y(n) is outputted to the CP processing block  122 . The CP processing block  122  can remove the CP from the digital signal y(n), and outputs the digital signal y(n) without the CP to the FFT block  124  to apply an N point FFT function on the digital signal y(n). The FFT function converts the time domain signal y(n) to a frequency domain signal Y[m,k]. The signal Y[m,k] is outputted to the channel estimator  126 , which performs channel estimation on the signal Y[m,k] to generate a channel frequency response H[m,k]. The channel frequency response H[m,k] is then outputted to the rebuild channel and data block  128 . 
     The rebuild channel and data block  128  generates the estimated received signal in the time domain y R [m,p], which is outputted to the y new [m,k] calculation block  130  for generating the estimated signal y new [m,k]. The rebuild channel and data block  128  can use the signal Y[m,k] and the channel response H[m,k] to determine the symbols S dec [m,k] for the received signal and also calculate the multipath delay spread Ndly. The rebuild channel and data block  128  can have a slicer function to determine the symbols S dec [m,k]. Furthermore, the rebuild channel and data block  128  can implement the above Equations [2]-[13]. 
     The y new [m,k] calculation block  130  can also receive the received signal y(n) to generate the estimated signal y new [m,k]. The estimated signal y new [m,k] is inputted to the FFT block  132  for further processing to decode the data in the signal. The FFT block  132  applies a FFT to the y new [m,k] to obtain Y new , a frequency domain signal of the time domain signal y new [m,k]. Next, the signal Y new  is further processed by the new channel estimator  134  and the demapper and decoder  136  to decode the received signal. 
     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.