ABSTRACT
Contents 16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 16.2 Spectrum Sensing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410
16.2.1 General Model of Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410 16.2.2 Sensing Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411
16.2.2.1 Energy Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 16.2.2.2 Eigenvalue-Based Detection . . . . . . . . . . . . . . . . . . . 412 16.2.2.3 Covariance-Based Detection . . . . . . . . . . . . . . . . . . . 415 16.2.2.4 Feature-Based Detection . . . . . . . . . . . . . . . . . . . . . . . 417
16.2.3 Cooperative Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 16.2.3.1 Optimal Decision Fusion Rule . . . . . . . . . . . . . . . . . 419 16.2.3.2 Logical-OR (LO) Rule. . . . . . . . . . . . . . . . . . . . . . . . . . . 419 16.2.3.3 Logical-AND (LA) Rule . . . . . . . . . . . . . . . . . . . . . . . . . 419 16.2.3.4 Majority Rule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419
16.3 Dynamic Spectrum Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420 16.3.1 Spectrum Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420
16.3.1.1 In-Band and Out-of-Band Sensing . . . . . . . . . . . . . 420 16.3.1.2 Opportunistic Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . 421 16.3.1.3 Other Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
16.3.2 Spectrum Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 16.3.3 Spectrum Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425
16.3.3.1 Scalable Bandwidth Design . . . . . . . . . . . . . . . . . . . . 425 16.3.3.2 QoS Provisioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
16.4 Adaptive Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 16.4.1 Two-Layer Orthogonal Frequency Division
Multiple Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 16.4.2 Adaptive Time Division Duplex (TDD) . . . . . . . . . . . . . . . . . . 427 16.4.3 Adaptive Beamforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 16.4.4 Hidden Incumbent Situation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429
16.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431
16.1 Introduction With the proliferation of wireless communications technology in the last couple of decades, in many countries most of the available spectrum has been allocated. This results in radio spectrum scarcity, which poses a serious problem for the future development of the wireless communications industry. On the other hand, careful studies of the usage pattern reveal that most of the allocated spectrum experiences low utilization. Recent measurements by the FCC show that 70% of the allocated spectrum in the United States is not utilized. Furthermore, the time scale of spectrum occupancy varies from milliseconds to hours.1 This motivates the concept of opportunistic spectrum access that allows secondary networks to borrow unused radio spectrum from primary licensed networks.
