Over the past decades, the bandwidth-intensive applications that are previously confined to wired networks are now migrating to wireless networks. This trend has brought unprecedented high demand for wireless bandwidth. The wireless traffic is destined to dominate the Internet traffic in the future, but many of the popular wireless spectrum bands, especially the cellular and ISM bands, are already congested. On the other hand, some other wireless technologies, such as TV bands, often do not fully utilize their spectrum. However, the spectrum allocation is tightly regulated by the authority and adjusting the allocation is extremely difficult. The uneven utilization and the rigid regulation have led to the proposal of heterogeneous wireless networks, including cognitive radio networks (CRN) and heterogeneous cellular networks (HetNet). The CRNs that usually operate on different technologies from the spectrum owner attempt to reuse the idle spectrum (i.e., white space) from the owner, while HetNets attempt to improve spectrum utilization by smallcells. This dissertation addresses some of the challenging problems in these heterogeneous wireless networks.In CRNs, the secondary users (SU) are allowed to access the white spaces opportunistically as long as the SUs do not interfere with the primary users (PU, i.e., the spectrum owner). The CRN provides a promising means to improve spectral efficiency, which also introduces a set of new research challenges. We identify and discuss two problems in CRNs, namely non-contiguous control channel establishment and k-protected routing protocol design. The first problem deals with the need from SUs for a channel to transfer control information. Most existing approaches are channel-hopping (CH) based, which is inapplicable to NC-OFDM. We propose an efficient method for guaranteed NC-OFDM-based control channel establishment by utilizing short pulses on OFDM subcarriers. The results show that the time needed for establishing control channel is lower than that of CH-based approaches. The second problem deals with the interruption to a routing path in a CRN when a PU becomes active again. Existing reactive approaches that try to seek for an alternative route after PU returns suffer from potential long delay and possible interruption if an alternative cannot be found. We propose a k-protected routing protocol that builds routing paths with preassigned backups that are guaranteed to sustain from k returning PUs without being interrupted. Our result shows that the k-protected routing paths are never interrupted even when k PUs return, and have significantly shorter backup activation delays.HetNets formed by smallcells with different sizes of coverage and macrocells have been proposed to satisfy increased bandwidth demand with the limited and crowded wireless spectrum. Since the smallcells and macrocells operate on the same frequency, interference becomes a critical issue. Detecting and mitigating interference are two of the challenges introduced by HetNets. We first study the interference identification problem. Existing interference identification approaches often regard more cells as interferers than necessary. We propose to identify interference by analyzing the received patterns observed by the mobile stations. The result shows that our approach identifies all true interferers and excludes most non-interfering cells. The second research problem in HetNets is to provide effective solutions to mitigate the interference. The interference mitigation approaches in the literature mainly try to avoid interference, such as resource isolation that leads to significantly fewer resources, or power control that sacrifices signal quality and coverage. Instead of conservatively avoiding interference, we propose to mitigate the interference by precanceling the interfering signals from known interferers. With precancellation, the same set of resources can be shared between cells and thus throughput is improved.This dissertation addresses several challenges in heterogeneous wireless networks, including CRNs and HetNets. The proposed non-contiguous control channel protocol and k-protected routing protocol for CRNs can significantly improve the feasibility of CRNs in future wireless network applications. The proposed interference identification and interference precancellation approaches can effectively mitigate the interference and improve the throughput and spectrum utilization in HetNets. This dissertation aims at breaking the barriers for supporting heterogeneous wireless networks to improve the utilization of the precious and limited wireless spectrum.
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