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The 5G networks are envisioned to use mmWave bands to provide gigabit-per-second throughput. To extend the coverage of extreme data rates provided by mmWave technologies, we consider two-hop relaying based on D2D communication in an integrated mmWave/sub-6 GHz 5G network. Compared to single-hop multi-cell networks, two-hop D2D relaying in this network will complicate the network management. Relay selection and beam selection should be considered together as relaying in mmWave bands would use directional beamforming transmissions. MmWave/ sub-6 GHz multi-connectivity has to be managed, and resources have to be allocated across frequencies with disparate propagation conditions. In this article, a hierarchical network control framework is considered to address the relay and beam selection, resource allocation, and interference coordination problems. The sub-6 GHz band is responsible for network control and for providing relatively reliable communications, while the mmWave band provides high-throughput enhancement. Opportunistic relay selection and mmWave analog beamforming are used to limit the signaling overhead. We evaluate mmWave/sub-6 GHz multi-connectivity with and without two-hop relaying in urban outdoor scenarios for different site deployment densities. MmWave/sub-6 GHz multi-connectivity with relaying shows considerable promise for reaching consistent user experience with high end-to-end throughput in a cost-effective network deployment.
Deng et al. (Sun,) studied this question.