How to calculate log likelyhood ratio of received symbols

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  Hi, I wanna try to replace convolutional codes with LDPC codes and using Message Passing Algorithm for channel decoding. But I'm puzzled by how to calculate log likelyhood ratio (LLR) of the received symbols. As I know, Sora uses complex16 to represent a received symbol. Since the received symbol from air has been passed through ADC and Sora chooses some reasonable numbers to denote their values, I'm not so sure whether I can calculate LLR as simulation method. For example, when I do simulation, I can simply get symbol by

      y = h*x+n

  and calculate LLR by

     LLR = 2*h*y/(sigmma^2)

  where sigmma^2 is the variance of the noise.

    I want to ask that how I can calculate LLR on Sora. Wait for your answer. Thanks.

  Friday, June 1, 2012 5:53 AM