Investigation On Cooperative Relay Technology In Cognitive Radio Networks

Cognitive radio is now emerging as a promising paradigm to address the contradiction between the rapid development of wireless communications and the scarcity of available radio spectrum resources, which has attracted extensive attention from academic researchers and become a hot research topic in the wireless communication field. Generally speaking, a cognitive radio system consists of the licensed users (also known as primary users) and unlicensed users (referred to as secondary users or cognitive users). Cognitive radio allows an unlicensed secondary user to access an idle licensed spectrum band that is assigned to a primary user but not utilized by that user at a particular time and specific geographic location, which enables the dynamic spectrum sharing between primary and secondary users. In order to design a reliable and efficient cognitive radio system, there are two fundamental issues:1) to devise an accurate spectrum sensing algorithm to detect available spectrum holes as reliably as possible; and2) to design an efficient secondary user transmission mechanism to utilize the detected spectrum holes as efficiently as possible. Therefore, this dissertation proposes solutions to the above-mentioned issues and deeply investigates cooperative relay technology for the cognitive radio system through jointly considering both the spectrum sensing and secondary user transmissions.The main contributions and innovations of this dissertation are summarized as follows.First, the cooperative relay technique is examined for the spectrum sensing and a new selective-relay based cooperative spectrum sensing scheme is proposed. Given a target quality-of-service (QoS) requirement of primary transmissions, the proposed scheme can save dedicated spectrum resources for reporting initial spectrum sensing results to a fusion center, which drives the development of cooperative spectrum sensing from theory to practice. By jointly considering the primary signal detection phase and the initial sensing result reporting phase during the process of cooperative spectrum sensing, the detection performance of the proposed scheme is studied. It is shown that given a target detection probability, a unique optimal primary signal detection overhead exists to minimize the false alarm probability of the cooperative spectrum sensing. Then, the cooperative relay technique is also investigated for cognitive users’data transmission and a best-relay selection based secondary transmission scheme is proposed. An outage analysis is conducted for the traditional non-cooperative and the proposed best-relay based secondary transmission schemes in Rayleigh fading environments. It is shown that with a target primary outage probability constraint, an outage probability floor of the secondary transmissions occurs in high signal-to-noise ratio (SNR) regions, where the outage floor of the proposed secondary transmission scheme is lower than that of the traditional non-cooperative scheme. Besides, a new diversity gain definition is proposed by generalizing the traditional diversity gain to cognitive radio environments with mutual interference between the primary and secondary users, based on which a diversity gain analysis is conducted for cognitive radio transmissions. It is shown that given a primary outage probability, the proposed scheme still achieves the full diversity gain.Next, a sensing-transmission tradeoff is studied in cognitive relay networks, for which two cognitive relay schemes, i.e., FFSS-BRDT (Fixed Fusion Spectrum Sensing and Best Relay Data Transmission) and SFSS-BRDT (Selective Fusion Spectrum Sensing and Best Relay Data Transmission), are proposed and the optimal resource allocation between the spectrum sensing and secondary transmissions is studied accordingly. Under a target primary QoS constraint, the spectrum hole utilization of FFSS-BRDT and SFSS-BRDT schemes is analyzed, showing that the performance of both schemes can be significantly improved in terms of the spectrum hole utilization through the optimal resource allocation between the spectrum sensing and secondary transmissions.Finally, a diversity-multiplexing tradeoff (DMT) of the cognitive relay transmissions is studied. A new DMT definition is proposed for cognitive radio environment, which paves a foundation for the theoretical DMT analysis of cognitive radio transmissions. The traditional DMT definition does not take into account interference factor, which cannot be directly applied in cognitive radio networks where mutual interference between the primary and secondary users exists. With a target primary QoS constraint, a selective cooperation based secondary transmission mechanism is proposed and examined, where two transmission modes are considered, i.e., the relay diversity transmission mode and non-relay direct transmission mode. By using the proposed DMT definition, a DMT analysis is developed for the cognitive radio transmissions considering imperfect spectrum sensing.