Interference Management in Spectrum Sharing Systems
Most of the radio spectrum is allocated to licensed services for their exclusive use. This has led to a shortage of spectrum for deploying new wireless services. However, measurements show that a large amount of radio spectrum allocated to licensed services is temporally and spatially underutilised. Spectrum sharing between licensed (primary) and unlicensed systems has been proposed for enhancing the spectral utilisation. This thesis investigates spectrum sharing on the highly underutilised television broadcast frequency bands. The Federal Communications Commission (FCC) has analysed spectrum sharing between a broadcast primary system and a single unlicensed device (unlicensed transmitter). This thesis extends the FCC analysis by considering multiple unlicensed devices operating in an ad hoc network. It is important that the interference from the unlicensed system to the primary system is limited to an acceptable level. In this thesis, such interference is limited by imposing geographical constraints on the unlicensed system by defining protection regions (i.e. unlicensed device exclusive regions) around the primary transmitter coverage areas. Analytical tools for determining the required protection distances (that define the size of the protection regions) are presented. It is identified that accurate modelling of the radio channel is extremely important in order to efficiently define protection regions and hence ensure primary system protection. The protection distances are also significantly sensitive to the primary system protection criterion and the density of unlicensed devices, especially when multiple primary transmitters with comparatively (compared to their coverage radius) small separation distances are considered. In this thesis, the capacity of the unlicensed system operating under geographical constraints imposed by the primary system performance requirements is characterised. The maximum density of unlicensed devices allowed to simultaneously contend for the primary channel is determined. For such a density, the outage constraint of a receiving node in the unlicensed ad hoc network is met. It is shown that the maximum capacity is obtained at this density. If the unlicensed system is aware of the locations of the typically passive broadcast primary receivers, then the geographical opportunity for the unlicensed system increases. This is because the unlicensed system can operate inside the primary transmitter coverage area by defining protection regions around primary receivers. Tools for determining such protection regions are presented. It is however identified that, compared to the scenario in which only the primary transmitter locations are known, the improvements in capacity obtained from knowing the primary receiver locations are typically less than 6%. Hence, it may not be significantly advantageous to system planners to invest in detecting the primary receiver locations, which involves a high infrastructure cost due to the required cooperation from the primary system.
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