MIMO Hybrid FSO/RF System Over Generalized Fading Channels

Free-space optics (FSO) communication is often unreliable, especially for long distances, as it suffers from attenuation due to foggy weather, atmospheric turbulence, and pointing errors. To avoid communication link failure, radio frequency (RF) communication, which is unaffected from these effects, is utilized when FSO communication is in outage. Diversity is known to counteract the adverse fading effects, therefore, this paper presents a transmit aperture/antenna selection (TAS) and selection combining (SC)-based multiple-input-multiple-output (MIMO) hybrid FSO/RF system. The proposed system consists of a MIMO FSO and RF sub-systems. The MIMO FSO sub-system is given higher priority to transmit and receive using the selected best link. The best link of the MIMO RF sub-system is used when the FSO sub-system is in outage. The probability density function (PDF) and cumulative distribution function (CDF) expressions for TAS/SC-based MIMO FSO and RF sub-systems are derived. The derived PDF and CDF expressions are used further to derive the exact and asymptotic expressions for outage probability, average symbol error rate (SER), and capacity of the hybrid system. From the asymptotic SER expressions, the diversity order of the proposed system is determined. The optimum values for the threshold SNR and transmit beam waist, which minimize the average SER, are obtained. Further, the obtained optimal values using analytical expressions are verified using numerical optimization technique. The effect of multiple apertures/antennas, pointing errors, and atmospheric attenuation over the system performance is analyzed extensively by varying the FSO and RF channel parameters. The results show that the proposed hybrid FSO/RF system is more reliable compared to the individual MIMO FSO and RF systems and achieves higher capacity compared to the MIMO RF system.