Performance Analysis of a Decode-and-Forward Based Mixed RF-FSO-VLC System

In this paper, the performance analysis of a cooperative radio frequency-free space optical communication-visible-light-communication (RF-FSO-VLC) system using decode-and-forward (DF) relays is presented. In the proposed model, the RF link acts as the core network, the FSO link is used for providing last mile connectivity for the indoor cell users communicating through VLC environment. The RF link is characterized by Nakagami-m-distributed fading, while the FSO link undergoes double generalized gamma distributed atmospheric turbulence and pointing errors and the VLC link is characterized by the Lambertian pattern of the LED. The signal at each DF relay is decoded , re-encoded and again decoded before sending to the next relay. The signal to the first relay is sent by an RF link. The received signal is then decoded, re-encoded and again decoded to send to the second relay via a FSO link. From the second relay the signal is transmitted to the end users present in the indoor environment via an VLC link . The closed-form expressions of the statistical characteristics of signal-to-noise-ratio of the DF based triple-hop system are evaluated using which various performance metrics, i.e, outage probability, bit error rate (BER), and capacity of the considered system are obtained. The numerical results show that the system's performance gets severely affected by the RF fading, atmospheric turbulence, and pointing errors. The outage performance of the system decreases as field-of-view and the height of the light-emitting-diode (LED) increase. It is also seen from the results that the BER performance of the system is better in case of heterodyne detection as compared to direct-detection. It is also observed that the capacity of the system decreases when the effect of atmospheric turbulence and pointing error is high and it increases when the no. of LEDs increases.