Coherent free-space optical communication system with quadrature phase-shift keying modulation using a digital phase recovery algorithm
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摘要:近年来，大气湍流所引起的信号相位扰动以及光强闪烁对自由空间相干光通信系统性能的影响逐渐成为研究人员关注的焦点。为了提高系统性能，本文对自由空间相干光通信系统进行了研究。在假设大气湍流所引入的光强闪烁以及相位抖动分别服从对数正态以及高斯分布的条件下，本文提出了基于数字相位恢复算法（CPR）的正交相移键控（QPSK）自由空间相干光通信系统，该系统采用了二阶联合的相位恢复算法结构。仿真结果表明：该结构可以极大地降低相位噪声对系统产生的影响，且其误符号率比只采用一阶M次方的相位恢复算法的系统低3个数量级。因此，该系统的提出对于自由空间光通信性能的提升有着较大意义。Abstract:In recent years, the dilemma regarding the limited performance of coherent free-space optical(CFSO) communication with phase modulation caused by the phase fluctuations and intensity scintillations associated with atmospheric turbulence has gradually become the focus of research. In order to improve system performance, a CFSO communication system with quadrature phase-shift keying modulation(QPSK) is studied in this paper. With consideration of log-normal amplitude fluctuations and Gaussian distribution conditions, a two-stage joint carrier phase recovery(CPR) algorithm is proposed. With this scheme, the simulation results show that the phase noise of the system can be greatly reduced and its symbol error rate is three orders lower than that of the single-stage M-th power CPR algorithm. Therefore, the two-stage joint CPR algorithm holds significant improvement in CFSO communication systems.
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图 2一阶M次方的相位恢复算法结构
Figure 2.Structure of the single-stageM-th power phase recovery algorithm
图 4 器线宽为100kHz时的星座图.(a)相位校正前, (b)单阶相位校正, (c)双阶联合相位校正后
Figure 4.Normalized constellation diagram of QPSK signal with laser linewidth of 100 kHz for the transmitter and the local oscillator. (a)Before using carrier phase recovery algorithm, (b)after using single-stage carrier phase recovery algorithm, and (c)after using two-stage joint carrier phase recovery algorithm
图 5 器线宽为100 kHz，频偏为20 MHz时的星座图. (a)相位校正前, (b)单阶相位校正后, (c)双阶联合相位校正后
Figure 5.Normalized constellation diagram of QPSK signal with laser linewidth of 100 kHz and frequency offset of 20 MHz for the transmitter and the local oscillator. (a)Before using carrier phase recovery algorithm, (b)after using single-stage carrier phase recovery algorithm, and (c)after using two-stage joint carrier phase recovery algorithm
图 6双阶校正、单阶校正和无校正系统误符号率和 器线宽的关系
Figure 6.SER versus linewidth for the two-stage, single-stage and without carrier phase recovery algorithm
图 7不同湍流条件下双阶校正和单阶校正系统误符号率和 器线宽的关系
Figure 7.SER versus linewidth which is measured with the two-stage and single-stage carrier phase recovery algorithm forσϕ=0.07,σϕ=0.1,σϕ=0.13
表 1仿真参数
Table 1.Simulation parameters
参数设置 Value 波长 1 550 nm 接收口径D 5 cm 折射率结构常数Cn2 1.5×10-15m-2/3 相干长度r0 24.5 cm 传输距离L 1 000 m 里托夫方差σR 0.17 相位方差σϕ 0.07 rad 
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