Survivable BBU Hotel placement in a C-RAN with an Optical WDM Transport

In Centralized Radio Access Networks (C-RANs) Baseband Units (BBUs) are decoupled from Remote Radio Units (RRUs) and placed in BBU Hotels. In this way baseband processing resources can be shared among RRUs, providing opportunities for radio coordination and cost/energy savings. However, the failure of a BBU Hotel can affect a large number of RRUs creating severe outages in the radio network. For this reason, the design of a resilient C-RAN is extremely important. This paper focuses on the survivable BBU Hotel placement problem in C-RANs with an optical wavelength division multiplexing (WDM) transport. We first propose an algorithm that jointly decides (i) the placement of a minimum number of BBU Hotels and (ii) solves the Routing and Wavelength Assignment (RWA) problem for the fronthaul connections, ensuring that each RRU is connected to two different BBU Hotels (i.e., one primary and one backup). Then, we present a strategy for maximizing the sharing of backup BBU ports among RRUs, with the aim of reducing the total cost of the C-RAN while guaranteeing uninterrupted service provisioning in case of single BBU Hotel failure. Simulation results show that the proposed strategy helps reducing the overall C-RAN cost. On the other hand, it becomes also evident that the sharing benefits can be maximized only in the presence of a transport network with enough wavelength resources to handle potential bottlenecks that may occur when BBU Hotels are placed quite far from RRUs.