With the explosive growth of mobile data demand, there has been an increasinginterest in deploying small cells of higher frequency bands underlying theconventional homogeneous macrocell network, which is usually referred to asheterogeneous cellular networks, to significantly boost the overall networkcapacity. With vast amounts of spectrum available in the millimeter wave(mmWave) band, small cells at mmWave frequencies are able to providemulti-gigabit access data rates, while the wireless backhaul in the mmWave bandis emerging as a cost-effective solution to provide high backhaul capacity toconnect access points (APs) of the small cells. In order to operate the mobilenetwork optimally, it is necessary to jointly design the radio access andbackhaul networks. Meanwhile, direct transmissions between devices should alsobe considered to improve system performance and enhance the user experience. Inthis paper, we propose a joint transmission scheduling scheme for the radioaccess and backhaul of small cells in the mmWave band, termed D2DMAC, where apath selection criterion is designed to enable device-to-device transmissionsfor performance improvement. In D2DMAC, a concurrent transmission schedulingalgorithm is proposed to fully exploit spatial reuse in mmWave networks.Through extensive simulations under various traffic patterns and userdeployments, we demonstrate D2DMAC achieves near-optimal performance in somecases, and outperforms other protocols significantly in terms of delay andthroughput. Furthermore, we also analyze the impact of path selection on theperformance improvement of D2DMAC under different selected parameters.
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