Performance Analysis of a Non-Hermitian OFDM Optical DQPSK FSO Link over Atmospheric Turbulent Channel

A free-space optic (FSO) system model considering non-Hermitian orthogonal frequency division multiplexing (OFDM) modulation followed by optical differential quadrature phase shift keying (DQPSK) modulation is proposed. Analysis is presented for an OFDM-DQPSK FSO link considering an optical direct detection receiver based on dual detection optical QPSK demodulator. Analytical approach is developed to find the expressions for output current and signal to noise ratio (SNR) in presence of atmospheric turbulence. The conditional bit error rate (BER) for a given turbulence-induced fading is also derived. Average SNR and BER are obtained considering the turbulence-induced fading to be log-normal distribution. Results are evaluated in terms of average SNR and BER for different link distances, number of OFDM subcarriers, background current and different wavelengths. It is noticed that the proposed OFDM FSO system suffers 4 and 1.4 dB power penalty at a BER of 10-12 due to the effect of atmospheric turbulence and background radiation respectively for 512 number of OFDM subcarrier and link distance of 2,000 m. Comparison of the results is made with simulation results as well as experimental results reported earlier. The proposed OFDM FSO system provides better performance, with an almost 3 dB improvement in receiver sensitivity compared to the experimental results on OFDM-QPSK FSO link reported earlier at a BER of 10-6. Finally, the analytical results are verified by numerical simulation. © 2019 Walter de Gruyter GmbH, Berlin/Boston.