Abstract

A hybrid optical modulation approach is described, which layers a continuous wave $M$–ary differential phase-shift keying ($M{\rm DPSK}$) and a two-level ($2L$) multipulse pulse-position modulation (MPPM) intensity-modulated signal for improved spectral efficiency. These $2L$ techniques are a generalization of earlier hybrid MPPM–$M{\rm DPSK}$ techniques and have the added advantage of reducing transmitter and detector complexities over previous hybrid modulation approaches. The spectral and power efficiencies for the proposed $2L$-MPPM–$M{\rm DPSK}$ modulation techniques are formulated and shown to have the highest spectral efficiency in comparison to other hybrid techniques with lower implementation complexity. The performance of the proposed $2L$ hybrid techniques is quantified over free-space optical (FSO) networks as well as fiber networks and verified using Monte Carlo simulation. For FSO channels, the proposed $2L$-MPPM–$M{\rm DPSK}$ technique outperforms the traditional MPPM–$M{\rm DPSK}$ scheme by approximately 2 dB at a bit-error rate (BER) of ${10^{-4}}$ and a spectral efficiency of 2.5 bit/s/Hz. Similarly, in optical fiber, the proposed scheme relaxes the impact of nonlinearity in comparison to traditional MPPM–$M{\rm DPSK}$. Specifically, at a ${\rm BER}{=10^{-3}}$, the $2L$-MPPM–$M{\rm DPSK}$ technique outreaches the MPPM–$M{\rm DPSK}$ by 2000 km at a spectral efficiency of 2.5 bit/s/Hz and an average transmit power of $-{3}\,\,{\rm dBm}$.

© 2019 Optical Society of America

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