Performance analysis of a hybrid QAM-MPPM technique over turbulence-free and gamma-gamma free-space optical channels

The performance of a hybrid M-ary quadrature amplitude modulation multi-pulse pulse-position modulation (hybrid QAM-MPPM) technique is investigated in both turbulence-free and gamma-gamma freespace optical (FSO) channels. Both the spectral efficiency and asymptotic power efficiency of the hybrid QAM-MPPM are estimated and compared to traditional QAM and MPPM techniques. The bit error rate (BER) of intensity-modulation direct-detection (IM-DD) systems adopting the hybrid technique is investigated over turbulence-free FSO channels. Upper-bound expressions for the average BER and outage probability are derived for FSO systems by adopting a hybrid QAM-MPPM scheme over gamma-gamma turbulent channels. In addition, the performance of a Reed-Solomon coded hybrid QAM-MPPM is considered. The obtained expressions are used to numerically investigate the performance of the hybrid technique. Our results reveal that, under the conditions of comparable data rates, the same bandwidth, and the same energy per bit, FSO systems adopting the hybrid technique outperform those adopting traditional MPPM, QAM, and on-off keying (OOK) techniques by 1.5, 0.4, and 3 dB, respectively, in the case of turbulence-free channels. Moreover, the new technique shows a better BER performance under different turbulence levels when compared with traditional MPPM and QAM techniques in turbulent FSO communication channels. Also, it shows an improvement in outage probability compared to MPPM, QAM, and OOK over gamma-gamma FSO channels.