On the performance of a multi-user multi-hop hybrid FSO/RF communication system

Abstract

One of the main problems in mobile communication systems is long-range impassable links or some specific atmospheric conditions under which Radio Frequency (RF) connection becomes easily disrupted. One way to solve this problem is by consuming more power or adding processing complexity, but a small user mobile phone cannot deserve more complexity or power supply. Another way is to use an access point that amplifies received signal via short-range RF link and forwards it to via long-range Free Space Optical (FSO) link. In this paper, a novel multi-user multi-hop hybrid FSO / RF communication system is presented. This structure consists of two main parts. The first part establishes the connection between the mobile users and the source base station, and the second part establishes the connection between the source and the destination base stations. In the first part, mobile users want to connect to the source base station via a long-range link; therefore, an amplify and forward relay is used for communication establishment. In the second part, the source and the destination base stations are connected via a multi-hop hybrid parallel FSO / RF link with demodulating and forward relaying. The FSO link in moderate to strong and in saturated regimes is assumed at Gamma–Gamma and Negative Exponential atmospheric turbulence, respectively, also the effect of pointing error is considered, and RF link has Rayleigh fading. New closed-form exact and asymptotic expressions are derived for the Outage Probability and Bit Error Rate (BER) of the proposed structure. Derived expressions are verified with MATLAB simulations. The proposed structure has advantages of FSO, RF, relay-assisted, and multi-user systems at the same time. Results indicate that it has low dependence on the number of users and the number of relays. Therefore, it is suitable for areas with varying population and long-range links. This structure offers independent performance without additional power consumption, processing latency, and complexity. The innovations and contributions of this paper, which are first introduced in the multi-hop hybrid FSO / RF hybrid structure, include the novelty of the proposed structure, the presentation of new exact and asymptotic mathematical solutions, use of multi-user scheme in a multi-hop FSO / RF structure, BER analysis, using amplify and forward as well as decode and forward protocols, taking into account a wide range of atmospheric turbulence with the effect of pointing error, using opportunistic selection schemes at each relay.