Method and arrangement for decoding a convolutionally encoded codeword

Abstract

An arrangement and method for decoding a convolutionally encoded codeword employs a window sliding over the codeword. Path metrics are computed simultaneously forwards and backwards in the sliding window. A decoding result is computed in a synthesis unit on the basis of the path metrics. The sliding window is divided into four parts. Path metrics are computed in the forward direction in the first part of the sliding window and stored in a four-part memory. Path metrics are computed in the backward direction in two other parts of the sliding window in such a way that the input of the computation units comes from the four-part memory; the computed metrics are applied to the synthesis unit; and a decoding result is computed.

1 . A method comprising: 
 decoding a convolutionally encoded codeword by means of a window sliding over the codeword, which method comprises computation of path metrics simultaneously forwards and backwards in the sliding window, the method further comprising: 
 a) using a four-part sliding window comprising four successive parts;  
 b) computing path metrics in the forward direction only in a first part of the four successive parts of the sliding window and storing path metrics in a four-part memory;  
 c) computing path metrics in the backward direction only in two other parts of the sliding window in such a way that an input for a computation unit comes from the four-part memory;  
 d) taking the computed metrics to a synthesis unit and computing a decoding result;  
 e) moving the sliding window one part forwards; and  
 f) repeating the preceding steps a) to e).  
   
   
   
       2 . A method according to  claim 1 , further comprising computing the path metrics in the backward direction in such a way that, in the second part of the sliding window, the computation unit performs a warm-up period for future computation and computing in the fourth part of the sliding window for the path metrics.  
   
   
       3 . A method according to  claim 1 , further comprising the step of computing the path metrics in the backward direction in two parts in such a way that one part performs the warm-up period while the other part is performing an actual computation.  
   
   
       4 . A method according to  claim 1 , further comprising the step of reading the four-part memory always from the same direction, the reading direction being opposite to the storing direction.  
   
   
       5 . A method according to  claim 2 , wherein in the fourth part of the sliding window, the path metrics computed by the computation unit are applied to a synthesis unit.