In this paper, cascaded least mean square-least mean square (LMS-LMS) adaptive equalization scheme is proposed based on fiber coupling system, in order to improve the transmission performance of quadrature phase shift keying (QPSK) modulated optical signals in medium and weak turbulence channels. First, the transmission performance of QPSK optical signal in different atmospheric turbulence channels is simulated and analyzed according to the characteristics of optical fiber coupling system and the characteristics of atmospheric turbulence channel. Secondly, cascaded LMS-LMS equalization scheme is proposed, and its distortion signal is corrected based on the original simulation results to verify the effectiveness and robustness of LMS-LMS. Finally, atmospheric turbulence simulator is used to verify the improvement effect of cascaded LMS-LMS adaptive equalizer under different turbulence conditions. The experimental results show that when the coherence length is r 0 = 1.6 cm and the received optical power is −34 dBm, the system can transfer the information at a rate of 32 Gbps QPSK after using cascaded LMS-LMS adaptive equalizer, and the bit-error ratio (BER) is lower than the minimum limit of forward error correction (FEC, 3.8 × 10 −3 ). Therefore, this work provides parameters for signal processing to suppress signal noise caused by turbulence disturbance and the noise of the device itself, when QPSK optical signal transmission system are used in the actual free space optical communication (FSOC).INDEX TERMS Free space optical communication, quadrature phase shift keying, atmospheric turbulence simulator, cascaded LMS-LMS adaptive equalizer.
I. INTRODUCTIONFree-space optical communication (FSOC) technology uses the laser as a carrier wave for communication, which combines the advantages of radio communication and optical fiber communication. FSOC has the advantages of strong anti-electromagnetic interference ability, high security, high communication speed, and large information capacity. It is characterized by small system size, lightweight, low power consumption, simple construction, and flexibility mobility has significant strategic needs and application value in both military and civilian fields [1]-[3]. FSOC can be used as an emergency communication solution, applied to earthquake relief, emergency, anti-terrorism, public security investigation, and other fields.Specifically, it can provide military confidential information services for multi-arms joint offense and defense, and has outstanding advantages in local warfare, battlefield networking, and information confrontation. In addition, benefiting from the advantages of high bandwidth, fast and convenient transmission, and low cost, FSOC is the best choice for solving the "last 1 kilometer" of information transmission and the transmission of the fifth-generation mobile communication technology (5G) small and micro base stations [3]. Because the traditional microwave satellite communication method is difficult to meet the demand of the highest transmission ba...