Polarisation Shift Keying Modulated Free-Space Optical Communication Systems

The data transmission rate, range, and reliability of free-space optical communication (FSO) systems are affected by a number of atmospheric phenomena, such as rain, haze, fog, snow, and scintillation. Thick fog with over 300 dB/km of attenuation limits the link length to around 100 m. Even under clear air conditions with no atmospheric scattering, the FSO communication link still suffers from fading due to scintillation. Scintillation fade margins are 2 to 5 dB for FSO links of 500 metres or less, which is well below margins for the atmospheric attenuation. For the link range beyond 1 km, scintillation may severely impact the performance of FSO links, thus resulting in the link deterioration, i.e., higher outage probability and ultimately complete link failure. In this work the performance of terrestrial FSO system based on the polarization shift keying modulation (POLSK) scheme under a turbulence channel is being investigated and analysed. The results are theoretically compared with on-off keying (OOK) and phase shift keying (PSK) modulated FSO systems in an atmospheric turbulence channel based on the bit error rate (BER) and the outage probability metrics. Results presented show that the binary POLSK (BPOLSK) offers the highest immunity to the phase noise in the atmospheric turbulence against OOK (with fixed and adaptive threshold levels) modulated FSO systems, primarily because it does not exhibit a BER floor. For BPOLSK under a moderate turbulence regime and for a BER of 10-9 the signal to noise ratio (SNR) requirement is ~ 39.5 dB. For the moderate turbulence regime OOK suffers from higher BER floor level.