c_lambda.m

Space-Time Processing for CDMA Mobile Communications.Written for research students and design engine

% ======================================================================
% C_LAMBDA.m
% ==========
% Description: This m-file implements the Log-Likelihood Ratio Calculator
%              (LLRC) to be used in the MAP-decoder.
%
% Usage:  [Lambda] = c_lambda(Alpha0, Alpha1, Beta0, Beta1, Lc)
% Inputs:
%           Alpha0: Forward State-Metrics (FSM) for input bit 0.
%           Alpha1: Forward State-Metrics (FSM) for input bit 1.
%            Beta0: Backward State-Metrics (BSM) for input bit 0.
%            Beta1: Backward State-Metrics (BSM) for input bit 1.
%               Lc: Channel Reliability Value.
% Outputs:
%          Lambda: The Log-Likelihood Ratios (LLRs)
%    Output data format: Lambda(Trellis Depth Indicator).
% ======================================================================
function [Lambda] = C_LAMBDA(Alpha0, Alpha1, Beta0, Beta1, Lc)

% --------------
% Initialisation
% --------------
[Number_Of_States, Trellis_Depth] = size(Alpha0);
Lambda = zeros(1,Trellis_Depth);

% ---------------------------------------------
% M A P: Log-Likelihood Ratio Calculator (LLRC)
% ---------------------------------------------
Scaling_Factor = 1/(2*Lc);
LLR_Limit = 100.0/sqrt(Lc);

for TD=1:Trellis_Depth
   LLR_Input0 = sum(Alpha0(:,TD).*Beta0(:,TD));
   LLR_Input1 = sum(Alpha1(:,TD).*Beta1(:,TD));

   if LLR_Input0 == 0
      Lambda(TD) =  LLR_Limit;
   elseif LLR_Input1 <= 1e-10;
      Lambda(TD) = -LLR_Limit;
   else
      LLR_Ratio = LLR_Input1/LLR_Input0;
      Lambda(TD) =  Scaling_Factor * log(LLR_Ratio);
   end
end
% ======================================================================