# Research on Polarization Coding Cooperative Communication Scheme for FSO System
## 1 Introduction  
Free-space optical (FSO) communication technology uses light as an information transmission medium, and has several features including large bandwidth, fast speed, and strong anti-interference. In particular, the technology does not depend on fixed-frequency band communication, which saves spectrum resources. However, the laser is easily affected by atmospheric turbulence, resulting in the change of beam arrival angle and the fluctuation of light intensity at the receiving end [1]. To overcome the influence of atmospheric turbulence on the FSO system, cooperative communication technology is applied to the FSO communication system to improve system performance via forming spatial diversity. In reference [2], [PERSON] and [PERSON] analyzed the influence of a relay cooperative communication mode on FSO link performance in an FSO communication system using the gamma-gamma turbulence model and subcarrier intensity modulation direct detection (IM/DD) technology. To specifically analyze the performance of the FSO cooperative communication system, in reference [3], [PERSON] and [PERSON] analyzed the effects of turbulence intensity, modulation mode, and diversity communication scheme on the system outage probability (OP) and average bit-error rate (BER) for a hybrid radio frequency/free-space optical (RF/FSO) aeronautical communication system under three-point cooperation architecture. In reference [4], [PERSON] and [PERSON] analyzed the security outage performance of a hybrid RF/FSO communication system under imperfect channel state information, and deduced the closed expression of the lower bound of security outage probability under fixed gain relay and variable gain relay. In reference [5], a switching scheme was proposed for anFSO/RF hybrid system. The system used selective decoding and a forwarding relay network and derived the expressions of progressive outage probability and symbol error rate with low computational complexity. In reference [6], the performance and capacity of a two-hop asymmetric RF/FSO communication system were analyzed based on the fixed amplification and forwarding (AF) protocol, and the mathematical expressions of the cumulative distribution function, probability density function, and moment generating function of the end-to-end signal-to-noise ratio were derived, which are used to analyze the outage probability and average bit-error rate of the system. In reference [7], [PERSON] et al. studied the performance of asymmetric radio frequency (RF) and free-space light (FSO) double-hop cognitive amplification and forward relay networks, and calculated the closed form and asymptotic expression of the outage probability of the system. Although the above researchers specifically analyzed the outage probability and bit-error rate performance of the cooperative FSO communication system, there is still some room for improvement in the performance of this system type. As a key factor affecting transmission efficiency, an appropriate coding scheme can improve the performance of the FSO cooperative communication system. At present, polarization code, as a coding method that can reach the Shannon limit of binary discrete memoryless channel, is arousing the interest of several industries. Because of its low encoding and decoding complexity and superior performance, it has become the mainstream coding method of channel coding in 5G and has very good development potential. In recent years, with the rapid development of cooperative communication technology, polarization code is widely used in the field of cooperative communication. In reference [8], [PERSON] and [PERSON] proposed a polarization-coded multiple-input multiple-output (MIMO) FSO communication system to combat the fading caused by turbulence, and studied the ergodic ability of the gamma-gamma model atmospheric turbulence channel under the conditions of correlated fading or not.