Enhancing spectral efficiency of FSO system using adaptive SIM/M-PSK and SIMO in the presence of atmospheric turbulence and pointing errors

This paper proposes an adaptive transmission modulation (ATM) technique for free-space optical (FSO) links over gamma-gamma turbulence channels.The ATM technique provides efficient utilization of the FSO channel capacity for improving spectral efficiency, by adapting the order of the phase-shift keying modulation scheme, according to the channel conditions and the required bit error rate (BER). To overcome the channel degradation resulting from the turbulence effects as well as the pointing errors (PEs), single-input multiple-output (SIMO) system with maximal ratio combining (MRC) is proposed. Exact closed-form expressions of BER and upper bound of the capacity are derived and verified by Monte Carlo simulations. The numerical results show that the proposed adaptive technique improves the spectral efficiency (SE) five times higher than the nonadaptive technique at the same BER threshold (10(-3)).This improvement is achieved at signal-to-noise ratio (SNR) equals 27 and 42 dB in the case of atmospheric turbulence without and with PE, respectively. Furthermore, this SE could be obtained while the SNR = 30 dB by using (1 x 4) SIMO scheme with MRC and PE and having the same transmitting optical power.