ynamic se- to statically allocate spectrum for each use, however, lection of channels to utilize or vacate is important. networks can be deployed more rapidly. A CR with Simulations of these cooperating CRs already exist, and spectrum sensing capability and cooperative opportu- additional field demonstrations are expected in the near nistic frequency selection is an enabling technology for future. Another advantageous waveform characteristic is faster deployment and increased spatial reuse. discontinuous spectrum occupancy. This allows a wide- Spectrum access is primarily limited by regulatory band communication system to aggregate available spec- constraints. Recent measurements show that spectrum trum between other existing signals. Careful analysis is occupancy is low when examined as a function of fre- needed to ensure that sufficient guard bands are utilized. quency, time, and space [36]. CRs may sense the local Figure 2.4b-9 shows five suggested alternatives for spectrum utilization either through a dedicated sensor or utilizing spectrum in and around legacy signals. The by using a configured SDR receiver channel. Uses of this characteristics of the legacy signals may be provided to Cognitive radio: The technologies required CHAPTER 2.4b 900,000 Spectrally Aware Channel Activity Statistics 880,000 Usage Policies - Lockouts, Rentals, Unlicensed 860,000 840,000 Regulators Will Define 6781.000 6784.000 6787.000 6790.000 Spectrum Blocks Subject to Time of Day (Seconds) CR Commons Etiquette and Who Is Allowed to Use 7-14% Occupied Spectrum Deployed Waveforms Existing Signals Detailed Signal Parameters Figure 2.4b-8 Spectrum awareness. A CR, or a set of CRs, may be aware of the spectrum and may exploit unoccupied spectrum for its own purposes. the cooperating CRs by federated sensors. An alternative narrowband signals to spread. This spreading will cause method for characterizing the legacy signals is time di- the narrowband signals to appear at a signal level reduced vision sharing of a channel as a sensor and p