We consider optical communications with intensity modulation and direct detection affected by laser relative intensity noise (RIN). Starting from a continuous-time waveform model, we derive an equivalent discrete-time channel model. As a result of RIN, the resulting channel model exhibits signal-dependent noise with memory. Unlike the commonly-assumed model in the literature, the conditional variance of this noise term has a polynomial dependence on the symbol of interest. Finally, we study achievable information rates for this channel under practically-relevant system parameters. We take a mismatched decoding approach and compute the generalized mutual information (GMI) using a memoryless decoding metric. Our numerical results show that when the memory in the channel is ignored by the receiver, GMI saturates as the constellation size increases, and thus, dense constellations do not offer gains. We also show that this saturation results from nonsymmetric nonvanishing contributions of the symbols to the GMI.

翻译：本文研究受激光相对强度噪声(RIN)影响的强度调制直接检测光通信系统。从连续时间波形模型出发，我们推导出等效离散时间信道模型。由于RIN的存在，所得信道模型呈现出具有记忆特性的信号相关噪声。与文献中通常假设的模型不同，该噪声项的条件方差对当前符号具有多项式依赖关系。最后，我们研究了在实际相关系统参数下该信道的可达信息率。采用失配译码方法，通过无记忆译码度量计算广义互信息(GMI)。数值结果表明：当接收机忽略信道记忆效应时，GMI随星座尺寸增大会趋于饱和，因此密集星座无法带来性能增益。我们还证明这种饱和现象源于符号对GMI的非对称非零贡献。