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：我们考虑受激光相对强度噪声（RIN）影响的强度调制与直接检测光通信。从连续时间波形模型出发，推导出等效离散时间信道模型。由于RIN的存在，最终信道模型呈现具有记忆的信号相关噪声特性。与文献中常见的假设不同，该噪声项的条件方差与所关注符号呈多项式依赖关系。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速率。采用失配解码方法，利用无记忆解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时，GMI随星座图规模增大而趋于饱和，因此密集星座图不能带来增益。我们还证明了这种饱和源于符号对GMI的非对称非消失贡献。