Optimal power allocation for fading channels in cognitive radio networks

Abstract

With the rapid development of wireless services and applications, the currently deployed radio spectrum is becoming more and more crowded. How to accommodate more wireless services and applications within the limited radio spectrum becomes a big challenge faced by modern society. Cognitive radio (CR) is proposed as a promising technology to tackle this challenge by introducing the secondary (unlicensed) users to opportunistically or concurrently access the spectrum allocated to primary (licensed) users. Currently, there are two prevalent CR models: the opportunistic spectrum access model and the spectrum sharing model. In the opportunistic spectrum access model, secondary users (SUs) are allowed to access the spectrum only if the primary users (PUs) are detected to be inactive. In the spectrum sharing model, the SUs are allowed to coexist with the PUs as long as the interference from SUs do not degrade the quality of service (QoS) of PUs to an unacceptable level.

This thesis studies a number of topics in CR networks under the framework of the spectrum sharing model. First, we investigate the ergodic, delay-limited, and outage capacity of a single SU point-to-point channel under various fading models. The optimal power allocation strategies to achieve these capacities are derived under different combinations of peak and average transmit/interference power constraints. Then, we extend the obtained results to the multi-SU scenario. Specifically, the outage capacity regions for a M-SU cognitive multiple access channel (C-MAC) network is characterized. The optimal resource allocation schemes to achieve the boundary points of the defined outage capacity regions are obtained. It is rigorously proved that the optimal decoding strategy is the successive decoding strategy.

Though applying the interference power constraint to protect the PU is simple and effective, the resultant capacities of the secondary networks are not high. With the aim to improve the capacities of fading CR networks, new PU protection techniques are studied in this thesis. Start from the single-user single-carrier scenario, we propose the PU outage constraint. This new type of constraint protects the PU by limiting the maximum transmission outage probability of the PU to be below a desired target. The optimal power allocation strategies for the SU to maximize its ergodic/outage capacity are derived under the proposed PU outage constraint. It is shown that the obtained power allocation strategies can achieve substantial capacity gains for the SU over the conventional schemes obtained under the interference power constraint, with the same resultant PU outage probability. Then, we consider a more challenging scenario: the multi-carrier scenario. The rate loss constraint, in the form of an upper bound on the maximum rate loss of each PU due to the CR transmission, is proposed to protect PUs for an OFDM-based spectrum sharing network. It is shown that the cognitive system can achieve a significant rate gain under the proposed rate loss constraint as compared to that under the interference power constraint.

Finally, a new spectrum sharing model, called sensing-based spectrum sharing is proposed for fading CR networks. In this model, SU first listens to the spectrum allocated to the PU to detect the state of PU, and then adapts its transit power based on the sensing results. If the PU is inactive, SU allocates the transmit power based on its own benefit. However, if the PU is active, the interference power constraint is imposed to protect the PU. Under this new model, the optimal sensing time and power allocation strategies to achieve the ergodic capacity are studied. It is shown that SU can achieve a significant capacity gain under the proposed model over that under either the opportunistic spectrum access or the conventional spectrum sharing model.