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.
This paper presents a two-stage frequency offset estimation (FOE) algorithm to compensate for Doppler frequency shift (DFS) without local oscillator (LO) laser frequency tuning. By increasing the sampling rate to 12 samples per symbol (SPS) and using a small number of pilots, the frequency offset compensation range is increased from ±Rs/8 of the conventional 4-th power FOE to ±12Rs, where Rs is symbol rate. In the offline experiment of a 2.5GBaud QPSK coherent optical communication system, this algorithm compensates simulated DFS, which ranges from -8GHz to 8GHz. The scheme achieves a sensitivity of -50.5dBm at an average bit error rate (BER) of 1E-3. The penalty with or without DFS is less than 1 dB.
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