Performance analysis of spatial PPM-based free-space optical communication systems with Gaussian beam

In this paper, we propose to use spatial pulse position modulation (SPPM), which is the combination of optical spatial modulation (SM) and pulse position modulation (PPM), as a powerful technique to improve the performance of free-space optical communication (FSO) systems. We analyze the performance of proposed system taking into account most major effects of FSO channel including geometric spreading (i.e., beam broadening), path loss, and scintillation due to atmospheric turbulence. In addition, background noise and thermal noise are also included in our analysis. The numerical results show the significant improvement in bit error rate (BER) of FSO systems thanks to the use of SPPM. We also found that the location of photodetectors (PDs) and the beam waist at the receiver play important roles in the design of FSO systems using SM and SPPM. Due to the impact of beam broadening, the PDs should be located far from each other otherwise the narrow beam (or small beam waist) should be used.