Modeling the Effect of Non-Linear Distortion in a Centralized RAN with Analog Optical Fronthaul

The Centralized Radio Access Network (C-RAN) architecture has potential to achieve the target goals that have been set for 5G. However, as the number of radio signals to be transported from the Centralized Base Station (CBS) to the distributed Remote Antenna Units (RAUs) grows, the use of contemporary digital fronthaul technologies (such as CPRI) complicates. One simple way to address this problem consists in multiplexing the baseband signals intended to the different RAUs in the frequency domain and, after that, use the resulting analog signal to modulate the intensity of the optical carrier. This way, there is no bandwidth expansion and the processing delay is kept low; however, as the ideal fronthaul assumption does not hold any more in this case, the impairments that the analog optical fronthaul introduces should be taken into account when performing the digital signal processing in transmission. In this paper, we focus on the non-linear distortion that the external Mach-Zehnder (optical) modulator introduces and, through it, estimate the End-to-End (E2E) performance when different intensity modulation indexes (or clipping ratios) are used. As expected, moderate levels of non-linear distortion are beneficial if properly selected according to the C-RAN configuration.