Transmission analysis of M-ary phase shift keying multiple-subcarrier modulation signals over radio-on-free-space optical channel with aperture averaging

In this paper, we propose a comparative performance study to characterize the transmission of multiple subcarrier modulation signals with different M-ary phase shift keying (M-ary PSK) schemes over a radio-on-free-space optical system using the aperture averaging (AA) technique. First, we derive modified expressions for carrier-to-noise and distortion ratio (CNDR), outage probability, and bit-error rate (BER) based on the AA technique and take into consideration the laser diode nonlinearity effect. We also show that there is a design trade-off between the receiver lens aperture and the required average CNDR to achieve a given average BER, ensuring substantial scintillation fade reduction. Further, we perform a comparison between different M-ary PSK modulation schemes, i.e., BPSK, QPSK, and 16-PSK, to characterize the achieved performance improvement by using the AA technique. We found out that when higher order PSK modulation techniques are used such as 16-PSK, the fading reduction is far less impressive than that for schemes employing smaller constellation sizes such as BPSK and QPSK. We thus conclude that for more complex modulation techniques no matter how large the receiver aperture size, the accepted BER level cannot be attained only with the use of the AA technique.