Energy Efficient Dynamic Resource Allocation Schemes In OFDM- Based Cognitive Radio Networks

With the ever increasing demand for mobile and wireless applications, radio spectrum becomes more and more crowded. However, investigations show that large portions of spectrum are highly underutilized due to inefficient conventional regulatory policies. As a highly promising technique to improve the spectrum usage efficiency, Cognitive Radio (CR) attracts more and more attention in the past decade. CR technol-ogy allows Secondary Users (SUs) served by a CR network to sense radio spectrum en-vironment and dynamically adjust their transmission parameters to access the licensed spectrum, as long as the interference to the Primary Users (PUs) is kept below a preset threshold. In order to meet the requirements of opportunistic access, the physical layer of a CR system should be very flexible, which necessitates multicarrier methods to op-erate in CR networks. Orthogonal Frequency Division Multiplexing (OFDM) has been widely recognized as a promising air interface for CR systems due to its flexibility in adapting to spectral environments and allocating radio spectrum among SUs, which is the prerequisite for the CR system to acquire high performance.On the other hand, the energy consumption of information and communications technology (ICT) has recently become an economic issue for operators as well as a big challenge for sustainable development. With vast and rapid deployment of fourth generation (4G) networks, as well as the 5G vision of a totally connected world, the energy consumption is also growing at a staggering rate nowadays, which results in a large amount of greenhouse gas and high operation expenditure for wireless service providers. Green communication, which emphasizes on incorporating energy aware-ness in communication systems, is becoming more and more important. Thus, energy-efficient RA has attracted much attention in both industry and academia, especially for the OFDM-based system which is the most promising modulation technique for the future wireless networks.In this thesis, energy efficient dynamic resource allocation schemes in OFDM-based CR networks are investigated. The main contribution of this thesis are as fol-lows.1. We formulate a general energy-efficient RA model which covers essential con-straints for the OFDM-based CR system. Our model can be easily extended to many practical scenarios with necessary modifications.2. Efficient algorithms are proposed to tackle the energy efficient resource allocation problems. We develop an efficient heuristic sub-channel allocation scheme to produce (near) optimal solutions with lower complexity, which yields a tradeoff between energy efficiency and computational complexity. We derive a fast power allocation algorithm for a given sub-channel assignment by exploiting the structure of the problem, which reduce the computation complexity significantly.3. The relation of energy efficiency (EE) and spectral efficiency (SE) is investigated. A multi-objective optimization problem to optimize EE and SE simultaneously is formu-lated. To find a unique globally optimal solution, we proposed a unified EE-SE trade off metric.