Performance enhancement of the average spectral efficiency using an aperture averaging and spatial-coherence diversity based on the modified-PPM modulation for MISO FSO links

Abstract

The performance evaluation and enhancement of the proposed model for a free-space optical (FSO) communication links using the modified pulse-position modulation (MPPM) and spatial pulse-position modulation (SPPM) is analyzed in this paper. The average spectral efficiency (ASE) is investigated for the FSO communication using MPPM, SPPM, and spatial-coherence diversity in a coherent optical wireless communication (coherent OWC) systems. We proposed the hybrid MPPM/SPPM as a new modulation to improve the ASE performance of pulse-position modulation. To reduce the atmospheric turbulence (AT) effects and scintillation, the use of an aperture averaging (AA) technique has been proposed in our work. Here, we analyze the pointing error (PE) effects on the performance of M−ary MPPM based on a multiple-input single-output (MISO) of the repetition codes (RCs) and FSO systems using SPPM. The enhancement in the ASE can be accomplished with an increase in the receiver aperture and decreasing the beam radius in the presence of PEs. The ASE performances of 53 bits/s/Hz and 37 bits/s/Hz are achieved with the average transmitted optical power of 10 dBm for an aperture diameter of 10 cm over the effect of without and with PEs for coherent OWC technique, respectively. The simulation results obtained for the bit-error-rate (BER) and ASE against various AT conditions are compared with the diversity techniques and proved that both of the hybrid SPPM and MPPM/MISO−RCs based on the AA and spatial diversity offers better performance in the average channel capacity (ACC). Numerical results show that the ASE performance can be improved using the spatial-coherence diversity in coherent OWC technique. The spatial-coherence diversity in a coherent OWC system significantly outperforms on-off keying and MPPM system even in the presence of PEs under strong turbulence. Efficient estimation of the signal-to-noise ratio also allows accurate quantification of the BER and ACC for adequate quality of service and network link resource allocation.