This paper considers iterative decoding of two-level superposition codes used in cooperative broadcasting over wireless networks. The coding scheme consists of two low-density parity-check (LDPC) codes combined using Plotkin\u27s |u|u + v|-construction and provides two levels of error protection. Coding for cooperative broadcasting differs from conventional single-source broadcast coding. Instead of binary addition of subcode codewords performed by a single-source encoder, in superposition coding two modulated subcode sequences, produced by two coordinated and independent sources, are combined at the antenna of the receiver. Expressions are derived for the bit metrics, or log-likelihood ratio values, used by an iterative decoder for two-level superposition coding schemes based on BPSK, QPSK and 4-PAM modulated sequences. It is shown that conventional equal-energy 4-PAM constellations do not work well with two-level superposition coding. A solution is proposed in which 4-PAM constellations of different levels of average energy are used.
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机译:本文考虑了在无线网络上的协作广播中使用的两级叠加码的迭代解码。该编码方案由两个使用Plotkin \ u27s | u | u + v |构造组合的低密度奇偶校验(LDPC)码组成,并提供了两个级别的错误保护。合作广播的编码不同于常规的单源广播编码。代替由单源编码器执行的子码代码字的二进制加法,在叠加编码中,由两个协调且独立的源产生的两个调制子码序列在接收器的天线处组合。得出用于比特度量或对数似然比值的表达式,迭代解码器将这些表达式用于基于BPSK,QPSK和4-PAM调制序列的两级叠加编码方案。结果表明,传统的等能量4-PAM星座图不适用于两级叠加编码。提出了一种使用不同平均能量水平的4-PAM星座的解决方案。
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