Feasibility investigation of cognitive radio systems
Chiang, Robin I-Chun · ResearchSpace (University of Auckland) · 2009
Efficient use of the scarce radio spectrum is important for accommodating the rapid growth of wireless communications. This thesis investigates the feasibility of Cognitive Radio (CR) to exploit unoccupied frequencies for an enhanced spectral utilisation. CR refers to a wireless architecture that enables dynamic spectrum access, where unlicensed devices are allowed to operate in temporally/spatially unused licensed channels. Extensive spectral opportunities have been identified from an intensive measurement study, indicating that a significant gain in spectral utilisation may be brought by a CR technology. The main challenge for CR is a robust protection mechanism to guarantee an adequate Licensed Users (LU) system performance at all times. This requires a reliable spectrum sensing technique to accurately identify frequency opportunities; and an autonomous transmit power control algorithm that remains effective in severe fading environments. Cooperation among CR devices to assist the LU identification process is shown to be an imperative CR attribute for maximising the CR performance and the overall spectral utilisation. Cooperation is particularly important for a CR coexisting with a small-coverage LU system, in which an accurate LU detection mechanism is the predominant limiting factor of CR system performance. The consequences of insufficient CR collaborations are an expensive individual CR detector and a large CR-LU separation requirement, yielding a suboptimal gain in spectral utilisation. The effectiveness of cooperative detection largely depends on system application, channel characteristics, and CR detector sensitivity. The feasibility of CR is evaluated via a simulated CR model using the achievable CR system performance and the corresponding operational requirements as performance metrics. The fundamental limit of permissible CR transmit power has been derived from an interference constraint imposed on the CR operation. Assuming the LU accepts a minor performance degradation (an outage probability 99%) with the modest detector specifications commercially available. Above all the CR feasibility is strongly subject to the signal characteristics and the heterogeneous propagation conditions in the CR deployment scenario. An algorithmic approach is developed which demonstrates that a practical CR system independent of LU cooperation may be feasible.