Abstract The performance evaluations and enhancements of wavelength-division-multiplexing (WDM) for the free-space optical (FSO) communication systems using a digital pulse-position modulation (DPPM), M-ary pulse-position modulation (M-ary PPM), and on–offkeying (OOK) are investigated in this paper. In the WDM/DPPM-FSO communication systems, WDM channels suffer from the inter-channel crosstalk (ICC) while FSO channels can be severely affected by the atmospheric turbulence (AT). These impairments and the accumulation of an amplified spontaneous emission (ASE) noise cause reducing of the link efficiency and system performance. To deal with this problem, the use of the M-ary PPM to mitigate the effects of AT and improve the receiver sensitivity. The impact of AT, ICC, scintillation, pointing error, and ASE noise has been taken into account. Here, we have combined the N-hybrid diversity of the spatial modulation (N-SM) with M-ary PPM to improve the BER performance and system efficiency of SM/M-ary PPM-based FSO systems. To reduce the turbulence-induced scintillation and optical power penalty, the use of an aperture averaging has been proposed in this work. Such a system, which could provide a power-efficient, a low cost, excessive flexibility, and reliable or considered a massive solution in the bandwidth provision for future access networks, and together for higher data rate. The SM/PPM offers about 14 dB and 3.0 dB sensitivity improvements more than the OOK non-return-to-zero (NRZ) and PPM approaches for the WDM/FSO link in the absence of AT, respectively. Also, an improvement of the receiver sensitivity at a BER of 1 0 − 12 about 8.0–9.0 dB for a coding level of 5 and FSO link length of 2500 m can be potentially achieved using the SM/PPM technique over OOK − NRZ approach when impaired by the weak turbulence. The proposed system shows a considerable improvement in the receiver sensitivity and power penalty using the SM/PPM technique over the OOK − NRZ approach.

中文翻译：

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M-ary 脉冲位置调制的性能增强波分复用自由空间光学系统受到通道间串扰、指向误差和 ASE 噪声的影响

摘要 使用数字脉冲位置调制 (DPPM)、多路脉冲位置调制 (M-ary PPM) 的自由空间光 (FSO) 通信系统的波分复用 (WDM) 性能评估和改进和开关键控（OOK）在本文中进行了研究。在 WDM/DPPM-FSO 通信系统中，WDM 信道会受到信道间串扰 (ICC) 的影响，而 FSO 信道会受到大气湍流 (AT) 的严重影响。这些损伤和放大的自发辐射 (ASE) 噪声的累积导致链路效率和系统性能的降低。为了解决这个问题，使用M-ary PPM来减轻AT的影响并提高接收器的灵敏度。已经考虑了 AT、ICC、闪烁、指向误差和 ASE 噪声的影响。这里，我们将空间调制的 N 混合分集 (N-SM) 与 M-ary PPM 相结合，以提高基于 SM/M-ary PPM 的 FSO 系统的 BER 性能和系统效率。为了减少湍流引起的闪烁和光功率损失，在这项工作中建议使用孔径平均。这样的系统可以提供节能、低成本、过度的灵活性和可靠或被认为是为未来接入网络提供带宽的大规模解决方案，并共同实现更高的数据速率。在没有 AT 的情况下，SM/PPM 比 WDM/FSO 链路的 OOK 不归零 (NRZ) 和 PPM 方法分别提供大约 14 dB 和 3.0 dB 的灵敏度改进。此外，在 BER 为 1 0 − 12 时接收器灵敏度的改进约为 8.0–9。当受到弱湍流的影响时，使用 SM/PPM 技术通过 OOK - NRZ 方法可以潜在地实现 0 dB，编码级别为 5，FSO 链路长度为 2500 m。与 OOK - NRZ 方法相比，所提出的系统使用 SM/PPM 技术在接收器灵敏度和功率损失方面有相当大的改进。