结合LT码的自适应解调最优解调门限的研究

自适应解调是一种新的传输速率自适应方案，一般都需要结合无率码来使用，以解决解调中的比特删除问题。一种自适应解调方案是计算已调制符号中比特的对数似然比，与预设的解调门限相比较来决定是否解调该比特。解调门限决定了调制的阶数，并对无率码的译码性能有较大影响。文章对结合LT码的自适应解调中解调门限的设置进行研究，找到最优的解调门限。仿真结果表明，在最优门限下，可获得最低的误码率，或者获得最高的传输效率。

Research on optimal demodulation threshold of adaptive demodulation combined with LT codes

Adaptive demodulation is a new rate-adaptive scheme and it always applies with rateless code so as to solve problem of the delete bits. An adaptive demodulation scheme computes the log-likelihood ratio of each bit in the modulated symbol, then compares with the pre-set threshold to determine whether to demodulate this bit or not. The demodulate threshold in this algorithm determines the demodulation order, so the threshold has great influence on the decoding performance. This paper studies the demodulation threshold of this adaptive demodulation algorithm combined with LT code, and finds out the optimal threshold. Simulation results indicate that, at optimal thresholds, system can obtain lowest bit-error-ratio, or best transmission efficiency.