In order to investigate the influence of different system parameters on linewidth tolerance in space coherent optical communication system, we give the ensemble average bit error rate (BER) model of binary phase-shift keying (BPSK) modulation and homodyne detection space coherent optical communication system. The BER model also considers the laser linewidth induced phase noise and pointing error. Based on the BER model, the numerical simulation is conducted to investigate the relationship between linewidth tolerance and different system parameters of divergence angle, receiving aperture, zenith angle, and transmitted optical power. Through our numerical simulation, it is found that the linewidth tolerance will change from several kHz to several hundred kHz. Besides, linewidth tolerance will decrease with the increase of divergence angle and zenith angle. While with the increase of receiving aperture and transmitted optical power, the linewidth tolerance will increase. Also, we find that the linewidth tolerance changes more obviously with the variation of the zenith angle. This paper has a good reference value for the selection of laser linewidth of the space coherent optical communication system under different system parameters.
Quadrature Phase Shift Keying (QPSK) is an important digital signal modulation method, which has the advantages of high spectrum efficiency and strong anti-interference. Coherent laser communication has the advantages of high speed, large capacity, light and small equipment as well as high sensitivity. In order to better introduce the QPSK modulation method into the space downlink coherent laser communication system, we give a bit error rate (BER) of QPSK modulation model with laser linewidth suitable for space downlink coherent laser communication system with QPSK modulation under atmospheric turbulence. Based on the model, we simulate and analyze the effects of atmospheric wind speed, zenith angle, gain of Erbium doped optical fiber amplifier (EDFA) and communication rate on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation. The results show that the linewidth tolerance of the system increases when the zenith angle decreases or the gain of EDFA increases or the wind speed decreases. And the effects of zenith angle, gain of EDFA and communication rate on the linewidth tolerance of the space downlink coherent laser communication system with QPSK modulation are large. The effect of wind speed on the linewidth tolerance of space downlink coherent laser communication system with QPSK modulation is smaller. And the larger the wind speed is, the smaller the effect of increasing wind speed on the linewidth tolerance. For the communication rate, the linewidth tolerance increases and then decreases with the increase of the communication rate. This work can be a reference for the design of space downlink coherent laser communication system with QPSK modulation.
Atmospheric intensity scintillation effect deteriorates the communication quality of space chaos laser communication. To analyze the effect of internal mismatch and internal mismatch induced by intensity scintillation on bit error rate (BER) for space downlink chaos laser communication system, numerical simulation results based on BER formula are conducted and analyzed. Numerical results demonstrate that BER worsens with the effect of intensity scintillation. Although the probability that received optical power causes larger mismatch noises is small, its corresponding larger instantaneous BER deteriorates the ensemble average BER significantly. Results also indicate that internal mismatch deteriorates BER more than external mismatch. These results are beneficial for designing practical space chaos laser communication system with optimal performance.
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