The second backhaul solution is marked as the distribution solution in Fig. 2. Compared with the central solution in Fig. 1, there is no MBS to collect all backhaul traffic from small cells, and all backhaul traffic is relayed to a specified SBS. In Fig. 2, all SBSs are assumed to be uniformly distributed in a given area. The backhaul traffic of an SBS is relayed by adjacent SBSs using millimeter wave communications. All backhaul traffic from adjacent SBSs will be cooperatively forwarded to a specified SBS that is connected to the core network by FTTC links. There are two logical interfaces, S1 and X2, which are used for backhaul traffic in the distribution solution. Functions of S1 and X2 are the same in the central and distribution solutions. The detailed scenario and logical architecture are illustrated in Figs. 2a and 2b.