Experimental investigation of polarisation modulated free space optical communication with direct detection in a turbulence channel

Binary polarisation shift keying (BPOLSK) has been proposed to mitigate the atmospheric turbulence-induced fading in free space optical (FSO) communication systems. In this study, the Q-factors obtained for the BPOLSK-FSO system are verified in conjunction with theoretical results to confirm the validity of the proposed scheme. The analytical bit error rate (BER) for the BPOLSK and non-return-to-zero on-off keying (NRZ-OOK) schemes are presented. The authors show that the BPOLSK scheme with direct detection offers improved BER performances compared to NRZ-OOK in the presence of weak turbulence, which is inferred from the experimental Q-factor and theoretical BER. For a turbulence variance σ²₁ of 0.003 and the transmitted optical power of -16.8-dBm, values for Q-factor are ~11 and ~8.5 for BPOLSK and NRZ-OOK schemes, respectively. The authors show that the predicted signal-to-noise ratio (SNR) for BPOLSK and NRZ-OOK schemes are ~13.5 and ~15~dB, respectively, for a BER of 10^-6 and σ²₁ of 0.01. When σ²₁ increases to 0.1, ~8~dB lower values of SNR is required for BPOLSK compared with NRZ-OOK.