Hybrid passive optical network–free-space optic-based fronthaul architecture for ultradense small cell network

Abstract. In future radio access network (RAN), many small cells (SCs) will be densely deployed to meet the capacity demand of mobile users. Centralized radio access network (CRAN) is a potential solution to increase the capacity demand of RAN. CRAN breaks the functionality of RAN between remote radio head (RRH) and baseband unit (BBU) where RRH and BBU are preferably connected by an optical link called fronthaul link. However, the deployment of fiber for fronthaul connectivity, at each SC location, is impossible or impractical due to cost or other constraints. As such, passive optical network (PON) and free-space optic (FSO) technologies have emerged as potential candidates for fronthaul transmission when the complete optical fiber-based infrastructure for fronthaul network cannot be deployed alone. We propose a hybrid PON and FSO-based method for SC fronthaul connections that considers three different network constraints, i.e., bandwidth, data rate, and latency. Based on this, we formulate the problem and propose a method to perform cell association, namely minimum sum selection (MSS). The performance is evaluated in terms of the number of SCs connected and the proposed method is compared with two other baselines, namely: minimum rate selection and random selection method. The results show that despite MSS requiring knowledge of all network constraints. It has a better performance at the cost of more computation resources, achieving gains of 7% and 6.5% in cell connections compared to the other two baseline methods.