ABSTRACT
CONTENTS 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2 Network Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 Cognitive Radio Network Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.2.2 PR ON/OFF Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Spectrum Sharing Problem Statement and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.4 Issues in Designing Spectrum Sharing Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.4.1 Interference Management and Co-Existence Issue . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.1.1 Self-Coexistence Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.1.2 Providing Performance Guarantees to PR Users . . . . . . . . . . . . . . 23
2.4.2 Distributed Coordination Issue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.5 Tradeoffs in Selecting the Operating Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.6 State-of-the-Art Spectrum Sharing Protocols in CRNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.6.1 Number of Radio Transceivers and Assigned Channels . . . . . . . . . . . . . . . . . . . . 25 2.6.2 Reaction to PR Appearance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.6.3 Spectrum Sharing Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.6.4 Guard-Band Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.7 Complementary Approaches and Optimizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.7.1 Resource Virtualization in CRNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.7.2 Full Duplex Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.7.3 Beamforming Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.7.4 Software Defined Radios and Variable Spectrum-Width . . . . . . . . . . . . . . . . . . . . 28 2.7.5 Cross-Layer Design Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.7.6 Discontinuous-OFDM Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.7.7 Spectrum Sharing for MIMO-Based Ad Hoc CRNs . . . . . . . . . . . . . . . . . . . . . . . . 29 2.7.8 Cooperative CR Communication (Virtual MIMO) . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.7.9 Network Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.8 Summary and Open Research Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
One major challenge in the design of opportunistic cognitive radio networks (CRNs) is how to simultaneously provide efficient spectrum utilization (network throughput) while protecting the performance of licensed legacy primary radio networks (PRNs). Efficient medium access control and careful spectrum assignment have a great potential to achieve this goal. In this chapter, we investigate the issue of spectrum assignment in CRNs and examine various opportunistic spectrum access approaches proposed in the literature. We provide insight into the efficiency of such approaches and their ability to attain their design objectives. We discuss the factors that impact the selection of the appropriate operating channel(s), including the important interaction between the cognitive linkquality conditions and the time-varying nature of PRNs. Protocols that consider such interaction are described. We argue that using best quality channels does not achieve the maximum possible throughput in CRNs (does not provide the best spectrum utilization). The impact of guard bands on the design of opportunistic spectrum access protocols is also investigated. Various complementary techniques and optimization methods are underlined and discussed, including the utilization of variable-width spectrum assignment, resource virtualization, full-duplex capability, cross-layer design, beamforming and multiple input multiple output (MIMO) technology, cooperative communication, network coding, discontinuous-OFDM (orthogonal frequency division multiplexing) technology, and software defined radios. Finally, we highlight several directions for future research in this field.
