超奈奎斯特传输理论与调制技术相结合, 可有效提高系统的频谱效率。本文将超奈奎斯特理论引入大气激光通信系统, 构建了一种适合于log-normal湍流信道的超奈奎斯特光传输系统, 推导了QPSK调制方式下超奈奎斯特大气光传输系统平均误码率的表达式, 利用蒙特卡洛仿真进一步分析了该系统的误码性能及频谱效率。结果表明: 采用超奈奎斯特技术方案可以较大幅度提升大气光传输系统的频谱效率, 当SNR为18 dB, S.I.为0.4时其频谱效率可以达到1.7 Baud/Hz, 而未采用超奈奎斯特技术时只有1.56 Baud/Hz。另一方面, 大气湍流对超奈奎斯特系统误码性能的影响较明显, 当S.I.为0.4, BER为3.8×10-3时, 信噪比恶化了约1 dB。相对于频谱效率的提升, 误码性能的恶化是能够接受的。因此, 可以将FTN技术引入大气光传输系统来提高系统的频谱效率。

The combination of Faster-than-Nyquist (FTN) theory with modulation technology can effectively improve the spectral efficiency of atmospheric communication systems. In this paper, the FTN theory was introduced to atmospheric optical communications, and an FTN optical transmission system suitable for log-normal turbulence channel was proposed. Moreover, the equation for the Bit Error Rate (BER) of the proposed system was derived and further analyzed using the MonteCarlo method. The results indicate that the FTN scheme can considerably improve the spectral efficiency of atmospheric optical transmission systems. For example, when the signal-to-noise ratio is 18 dB, and the S.I. is 0.4, the spectral efficiency can reach 1.7 Baud/Hz, whereas it is only 1.56 Baud/Hz without the FTN scheme. Regarding the effects of atmospheric turbulence on system BER performance, when the S.I. is 0.4 and BER is 3.8×10-3, the error performance decreases by approximately 1 dB. Compared with the significant improvement in spectral efficiency, the decrease in BER performance is worthwhile.Therefore, FTN technology can be introduced into atmospheric optical transmission systems to improve spectral efficiency.