Placement of Base-Band Units (BBUs) over fixed/mobile converged Multi-Stage WDM-PONs

The convergence between fixed access networks and mobile backhauling networks is the basis of the evolution towards future Next-Generation Access Networks. Multi-Stage WDM-PONs are the ideal converged infrastructures for carrying both fixed access and mobile backhauling, because they inherit some features of WDM core/metro networks, e.g., large capacity and high transparency. Moreover, they provide support for the “BBU Hotelling” backhauling solution, which consists in separating base stations' Base-Band Units (BBU) from their Remote Radio Heads (RRH), and grouping them into consolidated “Hotels”, with reduced costs and energy consumption. In this work, we propose and model by an ILP formulation the novel “BBU Placement” network optimization problem, on a converged Multi-Stage WDM-PON. The aim is to decide in which nodes to place BBUs, together with the routing and wavelength assignment of traffic demands, such that the number of Hotels is minimized. We capture the importance of the maximum latency of Digitized Radio-over-Fiber (D-RoF) flows exchanged between each BBU and its RRH, which becomes a constraint on the maximum propagation delay of corresponding routes. Simulation results are obtained by generating some random multi-stage tree instances and solving them via CPLEX. The analysis of the number of Hotels versus network sizes and maximum D-RoF delay validates the proposed model and highlights the strong impact of such parameters on the achievable BBU consolidation.