Optimal BBU Placement for 5G C-RAN Deployment over WDM Aggregation Networks

5G mobile access targets unprecedented performance, not only in terms of higher data rates per user and lower latency, but also in terms of network intelligence and capillarity. To achieve this, 5G networks will resort to solutions as small cell deployment, multipoint coordination (CoMP, ICIC) and centralized radio access network (C-RAN) with baseband units (BBUs) hotelling. As adopting such techniques requires a high-capacity low-latency access/aggregation network to support backhaul, radio coordination and fronthaul (i.e., digitized baseband signal) traffic, optical access/aggregation networks based on wavelength division multiplexing (WDM) are considered as an outstanding candidate for 5G-transport. By physically separating BBUs from the corresponding cell sites, BBU hotelling promises substantial savings in terms of cost and power consumption. However, this requires to insert additional high bit-rate traffic, i.e., the fronthaul, which also has very strict latency requirements. Therefore, a tradeoff between the number of BBU-hotels (BBU consolidation), the fronthaul latency and network-capacity utilization arises. We introduce the novel BBU-placement optimization problem for C-RAN deployment over a WDM aggregation network and formalize it by integer linear programming. Thus, we evaluate the impact of 1) jointly supporting converged fixed and mobile traffic, 2) different fronthaul-transport options (namely, OTN and Overlay) and 3) joint optimization of BBU and electronic switches placement, on the amount of BBU consolidation achievable on the aggregation network.