Spectrum Efficiency and Security in Dynamic Spectrum Sharing
We are in the midst of a major paradigm shift in how we manage the radio spectrum. This paradigm shift in spectrum management from exclusive access to shared access is necessitated by the growth of wireless services and the demand pressure imposed on limited spectrum resources under legacy management regimes. The primary constraint in any spectrum sharing regime is that the incumbent users (IUs) of the spectrum need to be protected from harmful interference caused due to transmissions from secondary users (SUs). Unfortunately, legacy techniques rely on inadequately flexible and overly conservative methods for prescribing interference protection that result in inefficient utilization of the shared spectrum.
In this dissertation, we first propose an analytical approach for characterizing the aggregate interference experienced by the IU when it shares the spectrum with multiple SUs. Proper characterization of aggregate interference helps in defining incumbent protection boundaries, a.k.a. Exclusion Zones (EZs), that are neither overly aggressive to endanger the IU protection requirement, nor overly conservative to limit spectrum utilization efficiency. In particular, our proposed approach addresses the two main limitations of existing methods that use terrain based propagation models for estimating the aggregate interference. First, terrain-based propagation models are computationally intensive and data-hungry making them unsuitable for large real-time spectrum sharing applications such as the spectrum access system (SAS). Second, terrain based propagation models require accurate geo-locations of SUs which might not always be available, such as when SUs are mobile, or when their locations are obfuscated for location privacy concerns.
Our second contribution in this dissertation is the novel concept of Multi-tiered Incumbent Protection Zones (MIPZ) that can be used to prescribe interference protection to the IUs. Based on the aforementioned analytical tool for characterizing the aggregate interference, we facilitate a framework that can be used to replace the legacy notion of static and overly conservative EZs with multi-tiered dynamic EZs. MIPZ is fundamentally different from legacy EZs in that it dynamically adjusts the IU's protection boundary based on the radio environment, network dynamics, and the IU interference protection requirement. Our extensive simulation results show that MIPZ can be used to improve the overall spectrum utilization while ensuring sufficient protection to the IUs.
As our third contribution, we investigate the operational security (OPSEC) issue raised by the emergence of new spectrum access technologies and spectrum utilization paradigms. For instance, although the use of geolocation databases (GDB) is a practical approach for enabling efficient spectrum sharing, it raises a potentially serious OPSEC problem, especially when some of the IUs are federal government entities, including military users. We show that malicious queriers can readily infer the locations of the IUs even if the database's responses to the queries do not directly reveal such information. To address this issue, we propose a perturbation-based optimal obfuscation strategy that can be implemented by the GDB to preserve the location privacy of IUs. The proposed obfuscation strategy is optimal in the sense that it maximizes IUs' location privacy while ensuring that the expected degradation in the SUs' performance due to obfuscated responses does not exceed a threshold.
In summary, this dissertation focuses on investigating techniques that improve the utilization efficiency of the shared spectrum while ensuring adequate protection to the IUs from SU induced interference as well as from potential OPSEC threats. We believe that this study facilitates the regulators and other stakeholders a better understanding of mechanisms that enable improved spectrum utilization efficiency and minimize the associated OPSEC threats, and hence, helps in wider adoption of dynamic spectrum sharing.