无线光副载波信号的循环谱分析

Cyclic Spectral Density Analysis of Free Space Optical Sub-Carrier Signals

以无线光通信中的副载波调制技术为背景,研究了光强调制正交相移键控信号的循环自相关和循环谱密度函数,并对实际测得的信号的循环谱和循环自相关函数进行了分析。结果表明,无线光副载波信号经过大气传输,同时受到了加性白噪声和乘性窄带随机噪声的影响,有限强度的大气湍流并未破坏正交相移键控信号的循环平稳特性。影响接收信噪比的主要因素是有用信号功率与低频噪声功率之差,通过加时间窗的方法可以有效抑制低频噪声的影响,从而提高系统接收信噪比。

The background of atmosphere turbulence on free space optical sub-carrier modulation is set. The expressions of the cyclic auto- correlation function and the cyclic spectral density of quadrature phase shift keying （QPSK） signals are derived and the cyclic auto-correlation function and the cyclic spectrum density of series of experimental QPSK data are analysed. The results show that both additive and multiplicative noises in the experimental data are influenced by the atmosphere turbulence. And the turbulence which is not very strong can not disturb cyclostationarity of QPSK signals. The main factor of received signal to noise ratio is the power difference between the signal and low frequency noise. By adding a time window which can restrain low frequency noise, the signal to noise ratio of free space optical subcarrier modulation system can be improved.