The first challenge is how to design a new backhaul network architecture and protocols for ultra dense cell deployment scenarios. As discussed in the last paragraph, small cells and ultra dense deployment will become the main features in future 5G networks. In this case, the number of small cells will obviously increase in the unit area. As a consequence, the corresponding backhaul traffic will increase exponentially at the gateway if the conventional centralized control model is adopted in the 5G backhaul network architecture. The massive backhaul traffic not only creates congestion but also collapses the backhaul network. It looks as though the distributed control model has to be adopted in the 5G backhaul network architecture. However, another challenge then is whether or not existing network protocols can support the massive backhaul traffic of wireless links.