Ultra-dense network (UDN) has been considered as one of the advanced technologies in 5G. With the increasing requirements on quality of service (QoS), large numbers of connections and very high area throughput density, various access points (APs) are deployed densely to provide desired services to user terminals. Then how to choose the optimal AP among neighbor candidate APs for accessing to the network is an interesting problem. Meanwhile, some research work has pointed out that the backhaul capability may become the bottleneck for UDN. Therefore, the backhaul links should be considered when making the accessing decision. However, there is few well recognized solution so far. In this paper, we investigate the energy-efficient access scheme in UDN, with joint consideration on backhauling capability. On the basis of the proposed UDN architecture, an access evaluation model is set up with flexible weight adjustment on delay and power consumption of the candidate AP's backhaul links. Then, the resource allocation problem is formulated with joint scheduling on time slot, subchannel and power to optimize system energy efficiency. Simulation results have proven that compared with traditional access algorithm, our proposed scheme has obvious improvement in the performance.
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