Free-space optical communication has a high communication rate, large capacity, good confidentiality, small size, light weight, low power consumption, and strong anti-interference ability. An instantaneous deployment or setup of a mobile platform communication system is possible. The invention of high-power light sources overcomes the issue of optical signal attenuation in mild weather. Atmospheric turbulence can result in beam drift, beam expansion, light intensity flicker, angle of arrival fluctuation, and other influences, especially at a distance of 1 km or more, which will increase the bit error rate of the link, degrading the communication performance. For an free-space optical communication system, adaptive optical compensation, coherent reception, mode diversity reception, and other technologies have been studied to reduce the impact of atmospheric turbulence. More optical signals can be coupled into the fiber as unrelated signals, and the orthogonality of the mode can conduct diversity reception using multiaperture and multimode fiber for signal acquisition. In this study, we first introduce the turbulent atmospheric channel and the Gamma-Gamma model. We derive the electric field distribution, coupling efficiency, and normalized cutoff frequency of the few-mode fiber receiving under weak conductance. Second, we establish the simulation system of 100 Gbit/s DP-QPSK free-space optical transmission with mode diversity reception and use the LMS-MIMO (multiple input multiple output based on least mean square) equalization algorithm for digital processing. We hope that our experimental results can be helpful for the study of using mode diversity receiver technology in atmospheric turbulence.
In this study, an experimental system of 100 Gbit/s DP-QPSK free-space optical transmission with mode diversity was set up. First, the optical carrier passed through the atmospheric channel and was received using few-mode fiber. Using independent transmission of signals in different modes, the few-mode fiber could transmit more signals. Additionally, the mode multiplexed signals transmitted in the few-mode fiber were damaged by mode coupling, differential group delay, and so on. The equalization of different mode signals at the receiving end was equivalent to a multiple input and multiple output system. The equalization of signals at the receiving end was MIMO equalization. Therefore, the digital single processing (DSP) part adopts a butterfly filter to construct the LMS-MIMO algorithm for dynamic equalization.
The system’s performance was simulated and verified, and the performance of single-mode fiber receiving and multimode fiber spatial diversity receiving at the same transmitting power was compared. First, the LMS algorithm was used to determine the training sequence length of the MIMO algorithm under weak, medium, and strong turbulence receiving conditions of multimode and single-mode fiber. Secondly, under the same turbulence intensity, the bit error rate of single-mode fiber reception and few-mode fiber reception and processing is compared. The experimental results show that few-mode fiber reception can achieve the same level of bit error rate under low optical signal-to-noise ratio compensation condition. Figs. 6-9 show the constellation diagram under the condition of medium turbulence atmosphere; Figs. 10-13 show the constellation diagram under the condition of strong turbulence atmosphere. As can be seen from the figure, single-mode optical fiber reception has a good compensation effect on atmospheric turbulence effect. The constellation map becomes fuzzy with the increase of turbulence intensity due to increased atmospheric scattering intensity, beam offset, and other effects. After balancing with MIMO algorithm, the clear constellation map was basically restored. Further, we compared the performance of single-mode fiber receiving and multimode fiber receiving under weak, medium, and strong turbulence conditions. In the case of weak, medium, and strong turbulence, the optical signal-to-noise ratio (OSNR) compensation costs of the three modes were 1.8 dB, 2.5 dB, and 3.0 dB (Fig. 14-16), respectively, indicating that the LMS-MIMO algorithm can compensate for signal damage well under different turbulence intensities.
In this study, a free-space optical transmission system based on MIMO mode diversity coherent reception is proposed. The experimental setup of 100 Gbit/s DP-QPSK free-space optical transmission with mode diversity is established by simulation, and the performance of the scheme with less mode diversity and the scheme with single-mode diversity is compared. Simulation results show that the system’s performance is improved by 1.8 dB, 2.5 dB, and 3.0 dB, respectively, under weak, medium, and strong turbulence conditions. It shows that the LMS-MIMO algorithm can compensate for signal damage well under different turbulence intensities. This work verifies that the receiving effect of few-mode fiber is better than that of single-mode fiber. To develop and innovate the atmospheric turbulence system, the ideal number of reception modes under various turbulence intensities will be investigated in the future, along with improved MIMO equalization and introducing machine learning methods.
