Performance enhancement of M-ary pulse-position modulation for a wavelength division multiplexing free-space optical systems impaired by interchannel crosstalk, pointing error, and ASE noise

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 10−12 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.