The capacity of bandlimited direct-detection channels is challenging to compute or approach due to the receiver non-linearity. A generalized vector approximate message passing (GVAMP) detector is designed to achieve high rates at a reasonable level of complexity. The rates increase by using multi-level coding and successive interference cancellation. The methods are applied to fiber-optic channels with intersymbol interference caused by spectrally efficient pulse shapes, chromatic dispersion, and receiver sampling at twice the baud rate. Bipolar modulation operates within 0.26 bits per channel use (bpcu) of the real-alphabet coherent capacity for optically amplified links, reducing the best-known theoretical gap of 1 bpcu. Remarkably, bipolar modulation achieves 6 dB and 3 dB of power gain over unipolar modulation with and without optical amplification, respectively. Simulations with polar-coded modulation confirm the gains. The GVAMP complexity, measured in multiplications per information bit (mpib), is proportional to the number of iterations and to the logarithm of the block length, and is substantially lower than that of other equalizers. For example, a system with 64-ary bipolar modulation and a root-raised cosine pulse with a 1% roll-off factor was simulated over 4 km of optically amplified standard single-mode fiber in the C-band. The GVAMP receiver requires 93 mpib to achieve 5 bpcu at 300 gigabaud.

翻译：由于接收端非线性，限带直接检测信道的容量难以计算或逼近。本文设计了一种广义向量近似消息传递（GVAMP）检测器，以合理的复杂度实现高信息速率。通过采用多级编码和连续干扰消除，该速率得以提升。该方法应用于受码间干扰影响的光纤信道，其中码间干扰由频谱高效脉冲形状、色散以及以两倍波特率采样的接收机导致。在光放大链路中，双极性调制的性能距离实字母集相干容量仅0.26比特每信道使用（bpcu），将已知最佳理论差距缩小了1 bpcu。值得注意的是，在有无光放大的情况下，双极性调制相较于单极性调制分别实现了6 dB和3 dB的功率增益。采用极化码调制的仿真验证了这些增益。GVAMP的复杂度（以每信息比特乘法次数（mpib）衡量）与迭代次数和块长度的对数成正比，且远低于其他均衡器。例如，在C波段4公里光放大标准单模光纤上，仿真了采用64进制双极性调制及滚降因子为1%的根升余弦脉冲的系统。GVAMP接收机在300吉波特下，需要93 mpib才能实现5 bpcu的信息速率。