Differential circular polarization shift keying with heterodyne detection for free space optical links with turbulence channel

A coherent differential circular polarization shift keying (DCPOLSK) scheme for free space optical (FSO) communications is proposed and analyzed. No polarization coordinate alignment is needed and the complexity is thus reduced. The unconditional bit error rate (BER) expression for DCPOLSK in a weak turbulence has been derived, and its performance is compared with the coherent amplitude shift keying (ASK) and the binary phase shift keying (BPSK) employing a single subcarrier. DCPOLSK scheme is highly insensitive to the phase noise of the phase lock loop (PLL) in contrast to the BPSK scheme. We also show that DCPOLSK requires ~3 dB lower signal-to-noise ratio (SNR) in order to achieve the same BER in a weak turbulence environment compared to the ASK scheme.