Turbo码的高效调制方案

A High Spectral Efficiency Modulation Scheme for Turbo-Codes

Turbo码由于其接近香农限的特性而倍受关注 ,但标准的Turbo码是低码率码 ,其频带效率低 ,为了利用Turbo码低误码率的特点 ,同时又能提高其频带利用率 ,一个简单的方法是让其与高效率的高阶调制方案M QAM相结合 .本文在高斯信道和瑞利平坦衰落信道条件下对对数域最大后验概率 (log MAP)解码算法进行了研究和修改 ,使之适合这 2种信道和高阶调制方案 .通过采用一种实用方法 ,使得编、解码系统具有通用性 ,同时推导出精确计算高斯信道下调制码元对数似然值 (LLR)的计算公式 ,并与近似算法进行了比较 .仿真结果表明 ,在 2种信道中 ,该方案具有很高的编码增益 。

Turbo code has won much attention in recent years for the property of near Shannon limit. However, the bandwidth efficiency of Turbo code is low because of its low code rate. In order to use the very low bit error rate (BER) and improve the bandwidth efficiency of Turbo code system, a method by combining the Turbo code with high order modulation scheme such as 16QAM is introduced in this paper. Through careful studying the log-MAP algorithm, some modifications are made to fit in with Gaussian channel, flat Rayleigh fading channel and the MQAM modulation technique. The Turbo codes system is universal because a pragmatic approach is utilized and it is pivotal for VLSI realization. A precise formula of calculating the LLRs is deduced in this paper and a comparison is made. It is shown that the new scheme is of high coding gain and the bandwidth is improved.