| Summary: | 5G will be different from previous cellular generations in the fact that it will enable the cellular industry, besides offering superior broadband services, to conquer vertical industries such as vehicular communication, factory automation, healthcare and many more. Many of these use cases have challenging and quite often contradicting requirements in terms of data rate, latency, power consumption and so on. This suggests that 5G needs to adopt a flexible architecture that can adapt to different devices and traffic requirements. Consequently, a fresh look onto how cellular networks are currently designed and deployed is needed. Historically, cellular networks have relied on the axiomatic role of cells as the cornerstone of the radio access network. Cellular systems have witnessed several recent trends such as the increased heterogeneity in infrastructure and spectrum as well as the rise of different traffic types with different requirements. These trends have called for a shift from the cell-centric architecture approach to a more device-centric architecture where a user or a device should be able to communicate with the network by exchanging information over multiple traffic flows through several sets of heterogeneous nodes. This design concept suggests to drop the rigid cell-centric concept and move to a more flexible design where information is exchanged in the most efficient way possible disregarding in which cell the device is located. This thesis features a comprehensive study of some of the technological enablers of the device-centric 5G architecture vision where we start by motivating the need for this architectural change by presenting the envisioned use cases and requirements of 5G and how the current cellular designs are lacking the flexibility and agility to satisfy the 5G ambition. The main contribution of the thesis is on Downlink and Uplink Decoupling (DUDe) where we pioneered the research on this disruptive 5G architectural design. The traditional way for users to associate to the cellular network is through coupled association where a device associates in both uplink (UL) and downlink (DL) to the same cell. However, the ever increasing density and heterogeneity of cellular networks have rendered the traditional design concepts such as coupled cell association obsolete and highly suboptimal. In simple words, a device connecting in the DL to a high power macro cell from which it receives the highest signal power might want to connect in the UL to a small cell to which the pathloss is lower. Therefore, DUDe solves the UL and DL coverage imbalance problem caused by the different transmit powers from the different tiers. The UL and DL imbalance could also be caused by imbalance in the UL and DL loads, interference and traffic requirements. The concept of DUDe is ground breaking in the sense that it introduces the notion of treating the UL and DL as two separate network entities emphasizing the fact that these two entities have different transmission and traffic requirements. The thesis features a comprehensive simulation study on DUDe using Vodafone’s small cell live network deployment in conjunction with a high resolution 3D ray tracing propagation model as well as user deployments based on traffic measurements to guarantee the most realistic simulation setup. Using this setup, the superiority of DUDe compared to baseline LTE is shown in terms of data rate, outage, channel quality and many more parameters. The evaluation starts by examining the basic form of DUDe where the UL and DL associations are based on pathloss and DL received power respectively which is followed by a more complicated form of DUDe where the UL association takes into account the cell load and the backhaul capacity. An extensive theoretical evaluation of DUDe using tools from stochastic geometry is then presented where cell association and SINR/rate distributions are evaluated in great detail for a sub-6GHz deployment as well as a mixed millimeter wave and sub-6GHz deployment. In addition, the architectural aspect is extensively discussed highlighting the support of DUDe in current 4G networks as well as the changes needed for a native support in future 5G networks. The second aspect covered in this thesis is Device-to-Device (D2D) communications. D2D allows to establish a direct link between devices in the same vicinity to exchange data instead of going through the traditional way through the network infrastructure. D2D is considered to be another important aspect of the device-centric framework as it allows devices to exchange information in the most efficient way possible through a direct communications without the need to abide by the normal cellular way of conveying data. The cell association in a D2D enabled network is studied through an optimization framework considering a decoupled access regime. In addition, novel resource management techniques for D2D communications are presented considering bio-inspired genetic algorithms. Finally, the thesis is concluded by a summary of the findings and takeaways from the conducted research along with some directions for future work.
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