Aperture averaging effects on the average spectral efficiency of FSO links over turbulence channel with pointing errors

The average spectral efficiency (ASE) is investigated for the free space optical (FSO) communications employing On-Off keying (OOK), Polarization shift keying (POLSK), and Coherent optical wireless communication (Coherent OWC) systems with and without pointing errors over the Gamma-Gamma (GG) channels. Additionally, the impact of aperture averaging on the ASE is explored. The influence of different turbulence conditions along with varying receiver aperture has been studied and analyzed. For the considered system, the exact average channel capacity (ACC) expressions are derived using Meijer G function. Results reveal that when pointing errors are introduced, there is a significant reduction in the ASE performance. The enhancement in the ASE can be achieved with an increase in the receiver aperture across various turbulence regimes and reducing the beam radius in the presence of pointing errors, but the rate of increment of ASE reduces with a larger diameter and it is saturated finally. The coherent OWC system provides better ASE performance of 49 bits/s/Hz at the average transmitted optical power of 5 dBm with an aperture diameter of 10 cm and 34 bits/s/Hz without and with pointing errors under strong turbulence respectively.