Impact of Fiber Nonlinearity on 5G Backhauling via Mixed FSO/Fiber Network

The inherently high bandwidth of fiber and free-space optical (FSO) links make them ideally suited to provide broadband backhaul in fifth-generation (5G) mobile networks. However, both fiber and FSO systems suffer from a variety of impairments, which must be properly modeled in order to design the network. In this paper, we present analytical results for mixed FSO/fiber amplify-and-forward backhauling systems, where the impacts of radio-frequency (RF) co-channel interference, FSO pointing errors, and both fiber and FSO modulator nonlinearity are modeled and taken into consideration. Closed-form and asymptotic expressions are derived for the outage probability, the average bit-error rate, and the cumulative distribution function (CDF) of the channel capacity for mixed FSO/fiber backhauling systems. Our results reveal an optimal average-launched power for the fiber, which balances the impact of fiber nonlinear distortion with the receiver noise. In particular, when using the optimal fiber average-launched power, our estimated user capacity CDF results show that the 50th percentile user rates using mm-wave RF access can reach over 1.5 Gb/s in ideal conditions. However, user rates are more sensitive to the FSO backhaul channel characteristics.