Real-time Beam Combining Algorithm For Atmospheric Laser Communication System

Atmospheric laser communication is a high-speed communication system using the atmosphere as the transmission medium.It has attracted extensive attention because of its large bandwidth,no band license and low power consumption.However,the random turbulence introduced by the atmospheric channel will cause the fluctuation of coupling efficiency at the receiver,resulting in the change of signal-to-noise ratio at the receiver,which will deteriorate the communication performance.High sensitivity coherent reception and diversity reception can effectively suppress the influence of atmospheric turbulence and improve the communication quality.Beam combining algorithm is one of the key technologies in diversity system.Due to the fast phase change of the laser beam in the atmospheric modulation system,the phase recovery algorithm is required to be fast.In this paper,an improved beamforming algorithm for carrier phase recovery based on decision directed least mean square(DDLMS)algorithm is proposed and experimentally verified.The main work of this paper is as follows:1.The beam combining algorithms reported at home and abroad are investigated.The coherent beam combining system usually uses the method of carrier phase recovery first and then DDLMS beam combining(CPE-DDLMS).Then,the off-line experiment in turbulent channel is carried out based on the CPE-DDLMS beam combining method.Regarding the error rate above FEC threshold(1.8e-2)as outage,under the atmospheric coherence length of 4cm and 6cm,the beam combining algorithm can reduce the outage probability by 28%and 55%respectively.In the research process,it is found that CPE-DDLMS beam combining mode will be affected by the periodic sliding problem introduced by CPE,which reduces the beam combining quality and requires a high amount of hardware resources.2.A simplified beam combining method(DDLMS for joint CPE and combining,DDLMS-CC)is proposed,which can not only reduce the influence of periodic sliding,effectively improve the ability of phase recovery under low signal-to-noise ratio,but also reduce the amount of hardware resources required for beam combining at coherent receiving terminals.The simulation analysis of beam combining performance under different SNR is carried out.The results show that when the laser linewidth is below 100kHz,the performance of DDLMS-CC and CPE-DDLMS is the same under high SNR;When SNR is less than 5dB,the frequency of CS in DDLMS-CC beam combining method is lower than that in traditional CPE-DDLMS beam combining method,and the performance is better after beam combining.In addition,100ms strong turbulence is simulated.The simulation results show that the DDLMS-CC beam combining method proposed in this paper has lower interruption probability,indicating that the receiver performance is more stable under turbulence.3.Completed the Verilog code writing of the DDLMS algorithm.The FPGA-based DDLMS-CC real-time coherent combined beam reception experiment was completed,and the sensitivity of the receiver was recorded.Dynamic turbulence is simulated,and the bonding performance under turbulence is analyzed.Experimental results show that the use of DDLMSCC in a 2.5Gbps QPSK system can achieve stable coherent combining under the condition that the received power is changed at 100kHz and43dBm to-53dBm sinusoidal changes.CS occurs after the offline CPE of the first three signals of the bundle,resulting in a high bit error rate,with an average bit error rate of 0,while the average bit error rate after real-time beam combination is reduced to 1.06e-3.Using DDLMS-CC combining allows changes in weights to track receiving power fluctuations at 100kHz and 500kHz.