Final Report Summary - S3ISE (Spectrum Sharing Systems for Improving Spectral Efficiency) Cognitive radio is an intelligent wireless system that is aware of its surrounding and adapts its operating parameters with the objective of efficient utilization of the radio spectrum. Spectrum sharing systems are usually considered as an instance of cognitive radio systems in which under certain conditions, a Secondary Service is able to access a frequency band formally allocated to the Primary Service by adopting cognitive scenarios. Spectrum sharing is capable of addressing the radio spectrum scarcity issue.In spectrum sharing, the under-utilized spectrum as the available radio resource which can be utilized by the secondary service varies over time. The main challenge in addition to detecting the underutilised parts of the spectrum is devising efficient radio resource allocation techniques which do not require signalling between the two systems due to practical reasons. We started with addressing the inter-system signalling and developed an analytical framework for a general multi-carrier system where we very limited information about the channel variations. We then developed techniques which achieve similar performance comparing to the state-of-the-art but with a much lower signalling overhead and computational complexity. We then devised a technique for indirect signalling between the primary and secondary systems and showed that the proposed method does not make the primary system unstable and at the same time provide higher capacity per unit of radio spectrum. This means less expensive radio spectrum costs for the operators, thus lower communications costs for the mobile users.We then extended our work by integrating of sensing into radio resource allocation and proposed a method to find optimal sensing parameters in the secondary systems. This provides a theoretical foundation for benchmarking spectrum sensing techniques. We further proposed new high performance spectrum sensing mechanisms based on deployment of spectrum sensors in the network. Build upon the above research we then designed a mixed underlay overlay access system with minimum signalling based on a distributed algorithm for evaluating activity index in a coexisted multi-system setting. We showed that the proposed method significantly improves the spectrum sharing system throughput efficiency and tackles the need for inter-system signalling.We further proposed techniques and measure which provides a quantitative base for negotiation of various parties using the same part of the radio spectrum. The proposed method is capable of increasing competition in acquiring and sharing radio spectrum which ultimately reduces the mobile communication costs.