Millimeter-Wave Communication and Mobile Relaying in 5G Cellular Networks

Future fifth generation (5G) mobile communication systems should fulfill a wide range of technical requirements to cope with the explosive growth of mobile data traffic, massive number of connected devices and emergence of new services. Ultra-fast transmission speeds and consistent user experience are fundamental 5G requirements. To achieve these 5G goals, a combination of different advanced radio transmission technologies, different spectrum bands and different networking methods is necessary. Network densification using millimeter-wave (mmWave) communications, combined with massive multiple-input multiple-output (MIMO) and beamforming techniques, provides a framework to achieve throughputs in the range of Gbps. However, for the operators, the capital and operational expenditures increase significantly as the base station (BS) density increases. In this regard, a cost-effective 5G radio access network (RAN) solution is of great interest. In this thesis, Device-to-Device (D2D) relaying and low-complexity mmWave system architectures are considered to fulfill this objective. The main goal of D2D relaying is to create a new type of network connectivity to carry mobile traffic, so that user experience consistency is improved without a need of ultra-dense BS deployment. The D2D relaying connectivity is extremely useful in mmWave bands as mmWave signals can be easily blocked by various obstacles. D2D relaying should be subject to network control by a RAN controller. We consider a hierarchical control framework to perform D2D discovery, relay selection, mmWave beam selection, resource allocation and interference management. With this framework, D2D relaying in four practical network scenarios is considered, and low-complexity algorithms are developed to manage D2D relaying. In addition to D2D relaying, we also consider developing low-cost mmWave BSs for ultra-dense mmWave network deployments. A low-complexity hybrid architecture with subarrays and low-resolution phase shifters is designed, and a switch network is added into the architecture for the channel estimation purpose. A novel grid-less compressive-sensing channel estimation method based on atomic norm minimization, and a two-stage multi-user MIMO precoding scheme are proposed with this hybrid architecture. Simulations with the 3GPP mmWave channel model show that the considered architecture solution can achieve comparable spectrum efficiency as the performance given by architectures with much higher hardware costs.