Gray-Space Spectrum Sharing with Cellular Systems and Radars, and Policy Implications
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This dissertation considers gray-space primary-secondary spectrum sharing, in which secondary devices are allowed to transmit when primary transmissions are strong enough that additional interference would be tolerable, rather than when the primary transmissions are weak or absent so the spectrum is considered unused, as occurs in white-space sharing. To avoid causing harmful interference (i.e., interference causing disruptions in services), transmit power of a secondary device is dynamically adjusted. Various novel sharing mechanisms are proposed for two different types of primary system: cellular systems, and rotating radars. Both cases when primary and secondary systems cooperate (cooperative sharing), and when they do not (coexistent sharing) are considered. Based on analyses and extensive Monte Carlo simulations, this dissertation shows that useful secondary transmissions are possible even when the shared spectrum is considered 100% utilized by the primary system under conventional approaches to spectrum management. For example, in spectrum sharing with cellular systems, even when the primary system is 100% utilized, a modest extent of transmissions of around 0.01–0.03 bps/Hz is achievable for secondary transmitter and receiver that are 400 m apart. In spectrum sharing with radars, even in the scenario where radars are the most densely packed, a secondary transmitter can get almost 1.2 bps/Hz on average, when 5% of the transmitters are competing for the shared spectrum. This dissertation also shows the potential of sharing models in which a secondary system has information about a primary system, but does not cooperate in real time; such arrangements are not typically considered today. For sharing with radars, the case in which an OFDMA-based cellular system operates as the secondary spectrum-user in non-contiguous cells is considered, as might occur with a broadband hotspot service, or a cellular system using shared spectrum to supplement its dedicated spectrum. It is found that even with fluctuations and interruptions in secondary transmissions while radars rotate, the shared spectrum could be used efficiently for applications that generate much of the traffic on mobile Internet, including non-interactive video on demand, peer-to-peer file sharing, large file transfers, and web browsing, but not for applications such as real-time transfers of small files, and VoIP. For sharing with cellular systems, the efficiency of cooperative and coexistent sharing is compared based on performance of the secondary system measured as achievable transmissions, and performance of the cellular system measured as power consumption of a mobile device, which may be increased to compensate for additional interference from secondary transmissions. When both achievable secondary transmissions and primary power consumption are of concern, coexistent sharing is found to be as effective as cooperative sharing.