Effect of Pointing Errors on the Performance of Hybrid FSO/RF Networks

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.INDEX TERMS Hybrid FSO/RF systems, decode-and-forward relaying, pointing errors, gamma-gamma distribution, maximal-ratio-combining.

Unified Finite Series Approximation of FSO Performance Over Strong Turbulence Combined With Various Pointing Error Conditions

“…where v in (20) corresponds to α −β in (19) and swapping the integral and summation, (19) can be expressed as (21). Then, to solve the integral in (21), we use the integral identity [28, eq.…”

“…Using the series representation that was applied in (20), the modified Bessel function of the second kind in (25) can be replaced with an infinite series. By swapping the integral and summation, (25) is transformed into (26).…”

“…Then, we apply the series expansion (20) to (30) to transform the modified Bessel function into series form as (31) where…”

“…In [13]- [17], the authors investigated error performance of FSO system with relay and authors in [18], [19] combined multiple-input multiple-output (MIMO) system with FSO to improve performance and derived the error rate. Also, hybrid FSO/RF system has been studied to improve reliability of FSO system and the error rate was derived in [20], [21]. However, the research groups in [6]- [21] assumed that the pointing error model follows Rayleigh distribution in case of strong turbulence and utilized composite PDF derived in [9].…”

“…These results represent only one specific pointing error and are expressed in terms of special function, such as Meijer's G-function; therefore, the channel models are not suited to describing the various types of errors that occur in mobile platform-based environments. Furthermore, most existing studies have focused on intensity modulation and direct detection (IM/DD) technique [7], [9], [10], [14], [17], [20]- [23]. Even though IM/DD with on-off keying (OOK) is usually utilized in optical wireless system, coherent communications have been also considered as alternative detection mode.…”