Real-time implementation of selection combining algorithm for mode diversity free space coherent optical receipt

Atmospheric turbulence is a crucial factor affecting the performance of space optical communication systems. The mode diversity is considered an effective turbulence compensation method. In the mode diversity system, coherent combining algorithms are needed to combine the received signals from multiple modes to counteract the effects of turbulence. In this paper, we investigate the performance of 3 mode diversity coherent combining based on the selection combining (SC) algorithm in a 2.5 GBaud QPSK real-time back-to-back transmission system. We simulate turbulence with dynamic changes and calculate the bit error rate (BER) periodically. The experimental results show that within 1 second, the SC algorithm can reduce the frame error rate (FER) by 45% compared to using only single mode reception.