Performance Characterization of Spatial Diversity Based Optical Wireless Communication over Atmospheric Turbulence Channels

Optical wireless communication is regarded as the next-generation high-speed technology. It has demonstrated its capability to deliver data faster than any other state-of-the-art wireless communication technique. This technology has drawn attention as a means of implementing reliable high capacity outdoor systems that cannot be implemented by conventional fiber optics. It has emerged recently as an efficient solution to match the larger bandwidth and high data rates requirement of the upcoming wireless communication systems. However, although FSO (free space optics) system has many appealing features, it has rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In the presence of such type of performance impairments, the received signal exhibits random intensity fluctuations, which increase the BER (bit error-rate), where the severe weather conditions can have a detrimental impact on the performance, which may result in an inadequate availability. The MIMO wireless optical procedure, in which the spatial dimensions are used to improve the reliability and spectral efficiency of point-to-point links, provides a promising approach to mitigate turbulence effects due to its powerful performance enhancing capabilities.

In this paper, we investigate a terrestrial atmospheric FSO communication system operating under the influence of strong atmospheric turbulences. Additionally, the MIMO technique with equal gain combining (EGC) is used in this work to enhance the data rate of the proposed system. Atmospheric turbulence impacts are modeled as a lognormal channel with due regard for geometric losses. With the use of NRZ line coding, an FSO highly sensitive receiver using either avalanche photodetector (APD) or PIN is designed and simulated for best system performance. The preference is achieved by using Bessel and Gaussian filters. It has been found that APD receiver using Gaussian filter is suitable for long-range links with APD gain value of 3. Also, the selection of APD gain is critical to the system performance. In addition, the optimal value of APD gain required for best system performance decreases as the size of the MIMO technique increases. The achievable performance improvements including received power levels, BER and Q-factor are also discussed. The results show that the system with optical amplifier at the transmitter gives an optimum performance. In addition the system performance is enhanced in most weather conditions by using an amplified 2×2 MIMO-FSO system with booster amplifier.