# Research on Polarization Coding Cooperative Communication Scheme for FSO System
## 1 Introduction
In reference [9], the authors studied the selection method of polarization code frozen bits in the FSO communication system by evaluating the block bit-error rate performance of the system, to improve its performance. In reference [10], [PERSON] proved that polarization code could be applied to the FSO communication system, and its bit-error rate performance was better than low-density parity check (LDPC) code under the same conditions. In reference [11], [PERSON] applied the method of constructing polarization code in the AWGN channel to the FSO communication system, studied the performance of single-chain bit-error rate and coding mode in the turbulent channel, and discussed the application of polarization code in a free-space optical communication MIMO system. In reference [12], [PERSON] explored the effects of the polarization code length, code rate, and decoding width on the performance of a gamma-gamma atmospheric turbulence channel model with weak, medium, and strong turbulence. In reference [13], [PERSON] and [PERSON] proposed an adaptive polarization-coding probability-shaping scheme suitable for free-space optical communication. The system polarization code was used to resist the fading caused by atmospheric turbulence, and its performance was verified. The above researchers applied polarization code to the free-space optical communication system, which improved the communication performance of the system, but there is still some room for improvement. In order to further improve the performance of the free-space optical communication system and solve the problems of interruption events and the high bit-error rate in the atmospheric turbulence channel, the cooperative communication mode and polarization coding can be applied to FSO communication systems at the same time. The main contributions of this paper are as follows: 1. An FSO cooperative communication transmission scheme based on system polarization coding is proposed. This scheme combines the system polarization code with the cooperative communication mode, and introduces a method of constructing polarization code in the cooperative communication system. 2. This method mainly constructs the system polarization code through the channel capacity of each branch in the cooperative communication system, takes the upper limit of the average bit-error rate of the channel as the selection limit of the frozen bit of the system polarization code,and restores the information bit set transmitted by the \(S-R\) link at the destination node according to the relationship between the frozen bit set of the \(S-R\) link and the \(S-D\) link, of which \(S\), \(R\), and \(D\) represent the source node, relay node, and destination node of the cooperative communication system, respectively. 3. In this scheme, the DF (decode and forward) cooperation protocol is used at the relay node, and EGC (equal gain combining) is used at the destination node to process the \(S-R\) link information bit and \(S-D\) link information bit signals, and finally decode and recover the original signal. The results show that the FSO cooperative communication system based on system polarization coding has better outage probability and bit-error rate performance than the FSO communication system.