Investigation of Costas loop phase deviation effect on BER performance of space downlink coherent communication system with DPSK scheme

The Costas loop is an important technology for synchronous demodulation in space coherent communication. However, due to the limitation of the loop device and processing technology, it is difficult to achieve high-precision locking, which results in the Costas loop phase deviation and further affects the quality of the demodulation signal. In this paper, a closed-form bit error rate (BER) model is derived to evaluate the Costas loop phase deviation effect on BER performance in the space differential phase shift keying (DPSK) modulation and heterodyne detection communication system, considering laser phase noise, atmospheric turbulence, and detector noise. Based on the BER model, analytical solution simulation is conducted. The results show that the BER performance deteriorates varying with the increase of the Costas loop phase deviation. In addition, we research how to better balance the laser linewidth and Costas loop accuracy in the system design to ensure good communication status. We find that the wider the laser linewidth, the more stringent the Costas loop accuracy. Furthermore, the BER performance versus the Costas loop phase deviation under different satellite-to-ground communication system parameter settings including divergence angle, receiving aperture, signal laser power, and data rate are analyzed. This work provides valuable references for space optical coherent communication systems design.