Extremely high data rate demands, and the spectrum scarcity at the microwave bands, make the millimeter wave (mmWave) band a promising solution to satisfy the high data rate demands in wireless networks. The main advantage of moving to the mmWave spectrum is the availability of large bandwidth. Moreover, due to an order of magnitude smaller wavelength of mmWave signals in compared to the conventional bands, many antenna elements can be incorporated in a small size chip to provide high directivity gain both at the transmitter and the receiver sides.Millimeter wave links experience severe vulnerability to the obstacles compared to the conventional sub-6 GHz networks for two main reasons. First, due to the tiny wavelength, mmWave signals can easily be blocked by obstacles in the environment and this causes severe loss. Second, due to the use of directional communications to compensate for the high path-loss (the distance-dependent component of the attenuation), mmWave links are sensitive to blockages that leads to the high probability of beam misalignment and the frequent updating of beamforming vectors. These issues are more challenging in mobile scenarios, in which mobility of the users and obstacles cause frequent re-execution of the beamforming process. Therefore, the tradeoff between the latency of the beamforming process (which latency increases with the number of the re-execution of the beamforming process) and instantaneous user rate is a significant design challenge in mmWave networks. Moreover, to provide adequate coverage and capacity, the density of the base stations in mmWave networks is usually higher than the conventional sub-6 GHz network. This leads to frequent handovers that make maintaining and establishing the mmWave links more challenging. Motivated by the mentioned challenges, this thesis considers the beamforming and handover problems and proposes lightweight joint beamforming and handover methods to guarantee a certain data rate along user trajectory. Specifically, in the first thread of the thesis, inspired by the fundamental properties of the spacial channel response of mmWave links, we propose a beamforming method in mobile mmWave networks. Our analysis reveals that our proposed method is efficient in terms of signaling and computation complexity, power consumption, and throughput in compared to the benchmark.  In the second thread of the thesis, we focus on the handover problem. We formulate the association problem that maximizes the trajectory rate while guarantees a predefined data rate threshold. We then extend our problem to the multi-user dense scenario that the density of the users is higher than the base stations and consider the resource allocation in the association optimization problem. We apply reinforcement learning in order to approximate the solution of the association problem. In general, the main objective of our proposed method is to maximize the sum rates of all the users and minimize the number of the handovers and reduce the probability of the events in which the users' rate becomes less than a predefined threshold. Simulation results confirm that our proposed handover method provides a reliable connection along a trajectory in compared to the benchmarks. / <p>QC 20210407</p>

Pushing wireless data traffic onto small cells is important for alleviating congestion in the over-loaded macrocellular network. However, the ultimate potential of such load balancing and its effect on overall system performance is not well understood. With the ongoing deployment of multiple classes of access points (APs) with each class differing in transmit power, employed frequency band, and backhaul capacity, the network is evolving into a complex and “organic” heterogeneous network or HetNet. Resorting to system-level simulations for design insights is increasingly prohibitive with such growing network complexity. The goal of this dissertation is to develop realistic yet tractable frameworks to model and analyze load balancing dynamics while incorporating the heterogeneous nature of these networks. First, this dissertation introduces and analyzes a class of user-AP association strategies, called stationary association, and the resulting association cells for HetNets modeled as stationary point processes. A “Feller-paradox”-like relationship is established between the area of the association cell containing the origin and that of a typical association cell. This chapter also provides a foundation for subsequent chapters, as association strategies directly dictate the load distribution across the network. Second, this dissertation proposes a baseline model to characterize downlink rate and signal-to-interference-plus-noise-ratio (SINR) in an M-band K-tier HetNet with a general weighted path loss based association. Each class of APs is modeled as an independent Poisson point process (PPP) and may differ in deployment density, transmit power, bandwidth (resource), and path loss exponent. It is shown that the optimum fraction of traffic offloaded to maximize SINR coverage is not in general the same as the one that maximizes rate coverage. One of the main outcomes is demonstrating the aggressive of- floading required for out-of-band small cells (like WiFi) as compared to those for in-band (like picocells). To achieve aggressive load balancing, the offloaded users often have much lower downlink SINR than they would on the macrocell, particularly in co-channel small cells. This SINR degradation can be partially alleviated through interference avoidance, for example time or frequency resource partitioning, whereby the macrocell turns off in some fraction of such resources. As the third contribution, this dissertation proposes a tractable framework to analyze joint load balancing and resource partitioning in co-channel HetNets. Fourth, this dissertation investigates the impact of uplink load balancing. Power control and spatial interference correlation complicate the mathixematical analysis for the uplink as compared to the downlink. A novel generative model is proposed to characterize the uplink rate distribution as a function of the association and power control parameters, and used to show the optimal amount of channel inversion increases with the path loss variance in the network. In contrast to the downlink, minimum path loss association is shown to be optimal for uplink rate coverage. Fifth, this dissertation develops a model for characterizing rate distribution in self-backhauled millimeter wave (mmWave) cellular networks and thus generalizes the earlier multi-band offloading framework to the co-existence of current ultra high frequency (UHF) HetNets and mmWave networks. MmWave cellular systems will require high gain directional antennas and dense AP deployments. The analysis shows that in sharp contrast to the interferencelimited nature of UHF cellular networks, mmWave networks are usually noiselimited. As a desirable side effect, high gain antennas yield interference isolation, providing an opportunity to incorporate self-backhauling. For load balancing, the large bandwidth at mmWave makes offloading users, with reliable mmWave links, optimal for rate. / text