Abstract
The performance of a free space optics (FSO) transmission suffers from the atmospheric turbulence and the attenuation in foggy environment. By employing relay nodes, the error rate and the coverage area of the FSO communication system can be significantly improved. However, the pointing errors, generated because of the building sway, have the potential to eradicate the benefits of the relay-based FSO communication system. The effect of pointing errors in the presence of Gamma Gamma atmospheric fading together with path loss attenuation is considered in this paper. To counteract the adverse affects of the FSO link, a reliable millimeter-wave radio frequency (MMW RF) link is used as a backup. In this context, this paper proposes a cooperative decode-and-forward (DF)-relaying-based hybrid FSO/RF system with maximal-ratio-combining (MRC) at the destination. The system consists of FSO and RF sub-systems, where FSO sub-system has the priority to transmit and RF sub-system serves as a back up when the FSO sub-system is in outage. The exact and asymptotic outage probability and average symbol error rate (SER) expressions for the proposed system are derived in closed-form and the diversity order is determined. The effect of pointing errors on the system performance is analyzed extensively. The optimum values of transmit beam waist and radius of receiver aperture are determined. The theoretical results, which are validated by Monte-Carlo simulations, show that the proposed cooperative hybrid FSO/RF system drastically improves the system performance compared to single hop (SH) hybrid FSO/RF and cooperative FSO systems especially for large pointing errors scenario.
Highlights
Free space optics (FSO) communication has attracted significant importance to provide gigabit capacity links owing to its unique features: cheap installation cost with faster deployment, unlicensed spectrum, narrow laser beam enabling numerous FSO links
For a realistic study of the FSO system, the effect of pointing errors in the presence of atmospheric turbulence induced fading together with path loss attenuation is considered in this paper
In order to counteract the adverse affects of the FSO links, a backup RF sub-system is used
Summary
Free space optics (FSO) communication has attracted significant importance to provide gigabit capacity links owing to its unique features: cheap installation cost with faster deployment, unlicensed spectrum, narrow laser beam enabling numerous FSO links. Despite of many advantages, FSO supports high data rate only for short-range transmission and its performance is limited by the adverse effects of the atmospheric turbulence induced fading, attenuation due to fog, and pointing errors [1]. FSO and MMW RF channels exhibit complementary characteristics to fog and rain. These complementary characteristics pave the way for hybrid FSO/MMW RF communication, which provides reliable high data rate transmission [2]. Mixed RF-FSO systems, where FSO is used as a last mile access link, use the concept of relaying technique to enhance the coverage and to counteract atmospheric turbulence induced fading and attenuation due to fog, thereby achieving some improvement in the system performance. The RF link is Nakagami-m distributed and the FSO link is
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