## 2 Spectrum Sharing Processes
### Sensing and Monitoring  
Spectrum sensing refers to the ability of a CR entity to observe the characteristics of its radio environment. In other words, every CR entity has to be able to sense, measure, learn and be aware of a number of parameters related to radio channel characteristics, operating environment, spectrum and power availability, network infrastructure, existent radio entities, local policies and other operating restrictions. In short, it is a multi-dimensional process used to identify occupancy in an \(n\)-dimensional space, particularly aiming at identifying \"free spaces\" in the CR domains.  
As seen in Figure 1, spectrum monitoring plays a key role during the spectrum sharing process. Monitoring spectrum means discovering to what extent a frequency band or a channel of interest is occupied by other users. Such occupancy can typically be detected through power densities measured significantly above the noise level, which might interfere with other transmissions.  
The methods of spectrum sensing [12] are typically based on the so-called transmitter detection technique (detecting the presence of a signal transmission), cooperative spectrum sensing (in case of noise uncertainty presence, fading and shadowing), interference-based detection (interference assessment) or using prediction. _Passive_ monitoring [13,14] means that no signals that might interfere with the monitored transmissions are used by the monitoring station(s). It can thus be executed continuously. On the other hand, _active_ monitoring [15] means to produce (possibly interfering) beacon signals and analyze their outcome. Thus, it is preferably used during \"guard\" times, when there is no data traffic that might be affected. Depending on what type of monitoring takes place, it may confirm the existence of other users [16], or even show their identities [17]. Monitoring can be initiated both by a user, as well as by a central entity in order to check for spectrum availability and entity compliance.  
Ahead of the negotiation phase, the targeted spectrum needs to be sensed for availability. For instance, potential primary users must be discovered in order to be considered in the spectrum negotiation between secondary users. Furthermore, newly arriving nodes need also to be identified and integrated into the negotiations to share the same spectrum.  
Once the use of spectrum has been (re-)agreed upon, the role of monitoring becomes evaluative, with compliance to the negotiated sharing rules as the key issue. Once the spectrum sharing plan is in  
Figure 1: Spectrum sharing phases.  
place, correct use of the spectrum needs to be monitored and communicated. A user wanting to take over a channel would typically check whether the channel has been vacated before starting to use it. Failure to access it would eventually be reported. Such failure can, for instance, be triggered by time synchronization issues: a potential guard time is exceeded once the vacating user's clock is lagging behind the waiting user's clock. Another possibility is that the vacating user keeps the channels deliberately, leading to a form of misuse. The control of channel availability is typically performed at regular intervals until the channel is free. The number of subsequent failures might indicate the degree of suspicion of misuse.  
Monitoring has user-related and application-related interfaces. While the requirements of applications and specifications of the generated traffic might provide criteria for the actual division of the spectrum, users may directly express their preferences and in particular their degree of satisfaction with the outcomes of prior spectrum allocation rounds. It is important to note that, apart from user preferences, spectrum sharing mechanisms also consider user equipment capabilities, such as frequencies on which the radio transceivers can operate, transmission power limits or battery availability, among others.