Analysis and performance improvement of 60 GHz mm-wave based hybrid RoF and RoFSO system under atmospheric turbulence using FFE + DFE electronic equalizer

Abstract

The mm-wave-based hybrid radio over fiber (RoF) and radio over free space optic (RoFSO) systems are emerging as a solution for high bandwidth demand by end users. Applications such as the expansion of 5G cellular networks, high definition (4 K/8 K) video streaming, Internet of things (IoTs), autonomous vehicles, unmanned aerial vehicles (UAVs), and, upcoming 6G wireless networks have pushed the limits of RF technology. Solutions based on only RF technologies are creating and will bound to create bottlenecks in access networks. In such a scenario hybrid RoF + RoFSO-based communication systems are appearing as a solution to achieve the desired goals in providing high-quality multimedia services in upcoming years. RoF (for reach extension) + RoFSO (for access networks) based hybrid backhaul and front haul not only provide low-cost installation, license-free FSO wavelengths, and lesser cost per bit but also provide an opportunity to install demand-based access networks in just-in-time scenarios such as disaster management, crowded festivals or sports events and also in difficult terrain. Besides all such benefits, adverse atmospheric conditions such as atmospheric turbulence, rain, fog, snow, and beam divergence deteriorate the link's reliability, causing huge costs to service providers. In this paper, we have analyzed with the help of numerical simulations, the effects of varied atmospheric turbulence and accompanying weather conditions (beam divergence and FSO channel attenuation), on the performance of the RoF + RoFSO system. Many techniques are part of existing or recent literature to counter the effects of atmospheric turbulence such as aperture averaging, diversity techniques, adaptive beam forming, adaptive optics, power equalization, and different coding techniques. The effects introduced by varied atmospheric turbulence such as dispersion, fading, amplitude, and phase fluctuations are non-deterministic effects, and an equalizer-based mitigation or compensation technique may serve a better purpose than other methods. This paper discusses the increase in launch power as one of the methods for countering distortions due to the FSO channel and also the limits of this method. This paper also discusses its main focus area, the equalization of signal impairments caused by high and very high atmospheric turbulence using an electronic equalizer based on the combination of feed-forward equalization (FFE) and decision feedback equalization (DFE), where least mean square (LMS) algorithm is used for updating the weights of filter taps. The paper discusses certain new incorporations such as the need for finding the optimum operating point of the LMS algorithm in terms of step size and also the optimum number of forward taps of FFE, for maximizing the performance improvement of the considered system. The paper also highlights the contribution of ODSB-PCS in improving the performance of the considered system under very high atmospheric turbulence.