On the performance of a novel multi-hop relay-assisted hybrid FSO / RF communication system with receive diversity

One of the main problems in mobile communication systems is long-range links in weather conditions that could disrupt Radio Frequency (RF) connection. This problem could be solved by use of diversity or consuming more power or adding processing complexity. However, a mobile device has limited size and a small battery, and does not afford multiple antennas, or higher power consumption, or more complexity. So it is better to implement these techniques on the receiver, which is the base station. In this paper, a novel multi-hop hybrid Free Space Optical (FSO) / RF system is presented as a solution for the proposed scenario (long-range links in bad weather condition). In the proposed structure, a mobile user is connected to the source Base Station with receive diversity, and the source Base Station is connected to the destination Base Station via a multi-hop hybrid parallel FSO / RF link. In order to have a favorable performance with the lowest possible complexity, selection combining as well as demodulate and forward relaying are used at all hops of the proposed structure. The proposed structure has advantages of FSO, RF, and relay-assisted systems at the same time. 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 demodulate and forward protocol, taking into account a wide range of atmospheric turbulence with the effect of pointing error, use of selection combining at each relay. In order to show the efficiency of the proposed structure, Bit Error Rate and Outage Probability of the proposed structure are investigated in a wide range of atmospheric turbulence from moderate to saturate. Considering these criteria, closed-form exact and asymptotic expressions are derived and verified by MATLAB simulations. Results indicate that the proposed structure shows independent performance at moderate to strong atmospheric turbulence regimes. Hence, it is not required to adaptively adjust system parameters in order to have a constant performance. Accordingly, this structure is economically affordable and particularly suitable for mobile communications that should deal with frequent changes in atmospheric turbulence in urban by a limited complexity and power consumption.