Two-Level MPPM-MDPSK Modulation for Free-Space Optical Channels

A hybrid two-level multipulse pulse-position modulation-M-ary differential phase-shift keying (2L-MPPM-MDPSK) technique is proposed which achieves high power and spectral efficiencies. The proposed 2L-MPPM-MDPSK scheme is shown to have the highest spectral efficiency in comparison with other hybrid modulation schemes. Additionally, the proposed technique has the advantage of removing the complex variable delay-line required for the direct-detection of other hybrid MPPM-MDPSK systems. The performance of 2L-MPPM-MDPSK is quantified in free-space optical (FSO) channels modeled with an exponentiated Weibull (EW) fading distribution. Closed-from expressions for both bit-error rate (BER) and outage probability upper-bound are derived and verified using Monte Carlo (MC) simulation. At the same data rate and bandwidth, 2L-MPPM-MDPSK techniques outperform traditional MPPM-MDPSK scheme by approximately 2 dB at a BER of 10−4.