基于最大似然检测的自由空间轨道角动量复用光通信研究

Research on free space optical communications by using orbital angular momentum multiplexing and maximum likelihood detection

基于轨道角动量复用的自由空间光通信可以极大地提高通信系统容量,但是由于自由空间信道中存在大气湍流干扰,引起不同轨道角动量间发生串扰,导致通信系统性能下降。文中提出了基于最大似然检测的自由空间轨道角动量复用光通信方案。该方案在接收端使用最大似然检测方法可以有效地降低不同轨道角动量光束所携带的数据间的串扰,降低通信系统误码率,提升通信性能。特别是在大气湍流强度为5×10^-15m^-2/3、信噪比为26dB时,相比于未采用最大似然检测方法的通信方案,该方案的误码率可以下降四个数量级。另一方面,最大似然检测的复杂度会随着调制阶数和子信道的数量的增加而呈指数级的增加,但是针对低阶调制方式和有限数量的子信道,最大似然检测的复杂度是可接受,例如该方案在仿真中采用OOK调制和四种轨道角动量光束进行复用,系统复杂度是可接受的。

Free space optical communications based on orbital angular momentum(OAM) multiplexing can greatly improve the capacity of communication system,but because of atmospheric turbulence interference in free space channel,crosstalk between different OAM will occur,resulting in the performance degradation.In this paper,a free space OAM multiplexing optical communication scheme based on maximum likelihood detection is proposed.The maximum likelihood detection method used in the receiver can effectively reduce the interference of atmospheric turbulence channel.Maximum likelihood detection can effectively eliminate the crosstalk between data carried by OAM beams with different topological charges caused by atmospheric turbulence and achieve optimal performance.Especially when atmospheric turbulence strength is 5×10^-15 m^-2/3 and signal-to-noise ratio(SNR) is 26 dB,the BER of this scheme is four orders of magnitude lower than that of the traditional schemes.The complexity of maximum likelihood detection increases exponentially with the increase of modulation order and the number of sub-channels.However,the complexity of maximum likelihood detection is acceptable for low-order modulation and a limited number of sub-channels,such as the communications system consisting of OOK modulation and four orbital angular momentum beams.