A typical duty cycle of CR, as illustrated in Figure 2, includes detection of available frequency bands also known as white spaces, selecting the best frequency bands, coordinating spectrum access with other users and vacating the frequency when a primary user appears. Such a cognitive cycle is supported by the following functions:

• Spectrum sensing and analysis;

• Spectrum management and handoff;

• Spectrum allocation and sharing

Spectrum Sensing and Analysis: In this phase, CR detects or senses the white space (as illustrated in Figure 3), i.e., a portion of time - frequency band that is not being used by the primary users, and keeps their record for utilization by secondary users. On the other hand, when primary users start using the licensed spectrum again, CRs detect their activity through sensing, and determine the transmitted power of secondary users to avoid the harmful interference to primary users. This sensing has to be done in presence of noise, so that the sensing can not be reliable. It is also important to know the signal level of detected spectrum. Spectrum sensing methods are described later in this chapter.

Spectrum Management and Handoff: After recognizing the spectrum white space by sensing, the CR decides when and which part of spectrum is to be allocated to secondary users for transmission. This decision is difficult because CR system has only knowledge of white space in past and present, but not about the future and sensing information is also not reliable due to presence of noise. Therefore it enables hop among multiple frequency bands according to the time varying channel characteristics to meet various Quality of Service (QoS) requirements. For instance, when a primary user reclaims its frequency band, the secondary user that is using the licensed band can switch its transmission to other available frequencies, according to the channel capacity determined by the noise and interference levels, path loss, channel error rate, holding time etc.

Spectrum Allocation and Sharing: In dynamic spectrum access, a secondary user may share the spectrum resources with primary users, other secondary users, or both. Hence, a good spectrum allocation and sharing mechanism is critical to achieve high spectrum efficiency. Since primary users own the spectrum rights, when secondary users co-exist in a licensed band with primary users, the interference level due to secondary spectrum usage should be limited by a certain threshold. When multiple secondary users share a frequency band, their access should be coordinated to avoid collisions and interference.