10Gb/s Two-User Spatial Diversity FSO-CDMA Wiretap Channel Based on Reconfigurable Optical Encoder/Decoders

Due to the effect of atmospheric turbulence, the transmission reliability and physical-layer security of free space optical (FSO) communication system are facing many challenges. In this paper, for the first time, we design and investigate a 10 Gb/s reconfigurable FSO wiretap channel based on spatial diversity and optical code division multiple access (OCDMA). Reconfigurable two-dimensional optical encoder and decoder are constructed by wavelength selective switch and optical delay lines, and reconfigurable one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Under three different turbulence conditions, bit error rates (BER) of legitimate user and eavesdropper are measured. It can be seen that under turbulence effects, BER of legitimate user in spatial- diversity is lower than that of non-diversity system. By the rapid code reconfiguration of legitimate users, the eavesdropper can only use unmatched decoder. Thus, the eavesdropping methods of code interception and brute-force searching can be avoided. Experimental results show that this scheme can ensure the reliable transmission of legitimate users, and the eavesdropper can’t crack the information when the extraction ratio is 1%. Hence, reliability and physical-layer security can be enhanced simultaneously.