Dynamic Resource Allocation In OFDM-based Cognitive Radio Networks

The current static spectrum management policies have caused a severe dilemma of spectrum scarcity versus under-utilization of licensed spectrum. Consequently, the emerging Cognitive Radio (CR) technology has been widely deemed as a promising candidate to improve the utilization of the precious natural resource-radio spectrum for the next generation of wireless communication. The main idea of CR is to allow the unlicensed users to opportunistically access the licensed spectrum provided that the interference introduced to the licensed users is kept within the tolerable ranges. Due to the inherent flexibility of OFDM technology, OFDM has paved its way for potential application in the physical layer of CR systems. Given the frequency selectivity in wireless communication environment, dynamic resource allocation severs as a pivotal issue to achieve the high performance of CR systems.This thesis is intended to provide comprehensive study of different dynamic re-source allocation schemes in CR systems, with focuses on, particularly, the spectral-efficiency and energy-efficiency in wireless communication networks. With a brief introduction of the landscape of CR technology, we detail the dynamic resource al-location problem for its motivation and principles in CR systems. In terms of pref-erences, the network operator may be inclined toward Spectral-Efficiency (SE) or Energy-Efficiency (EE) metric in practical systems. Accordingly, the Spectral-and Energy-Efficient resource allocation schemes are comprehensively investigated in this thesis, respectively. By and large, the main contribution of this thesis can be summa-rized as follows.1. We developed general models for different types of OFDM-based CR networks to illustrate either spectral-or energy-efficient resource allocation problems. Besides the transmission power constraints and interference threshold of PUs, the quality of service requirements are also included in the modeling process, such as the real-time rate and users’ fairness constraints, providing more practical system models.2. With respect to spectral-efficiency maximization in multiuser CR systems, we developed a two-stage method to deal with the subchannel and power allocation sepa-rately. By jointly considering the channel condition and interference level, an effective procedure of subchannel allocation is firstly performed. In the second step, accord-ing to the structure of the problem, both fast optimal algorithm and low-complexity suboptimal algorithm are proposed.3. The energy-efficient resource allocation problem is addressed by the relaxation method, where the formulated problem is relaxed and then converted into a convex op-timization problem via a series of equivalent transformations. The resource allocation is ultimately determined by the rounding results of relaxed solution and the follow-ing power allocation across the subchannels, which provides a good approximation to the optimal solution. Moreover, the fast barrier method can be applied to solving the relaxed problem and power allocation, significantly reducing the computational over-head.In short, in terms of either spectral-efficient or energy-efficient resource allocation problem, effective and efficient algorithms are proposed respectively for various CR networks. Particularly, compared to other existing algorithms in literature, the signifi-cant lower complexity of our proposals will possibly enlighten the design of resource allocation strategy in the practical CR networks.