## 4 Proposed Approach and Requirements
### Distributed versus Centralized Approach  
In our study, we have adopted a distributed approach for dynamic spectrum access, as opposed to current more widely accepted centralized database approaches. The motivation to adopt a distributed approach rather than a centralized approach in this study is driven by the following observations:  
* Driver: the main driver for a centralized database approach is to protect incumbents (primary users) while allowing opportunistic use of the incumbent's spectrum by secondary users as long as the incumbent does not experience any harmful interference. The main drivers for a distributed receiver-based approach are scalability, by only reacting upon interference events detected at the receiver, and spectrum efficiency, by exploiting spatial reuse of spectrum. * Mechanism: a centralized approach grants access to opportunistic users in a centralized way by consulting a global database that relies on transmission models of transmitters and the definition of exclusion zones (geographical areas within which active opportunistic radio transmitters are not allowed), restriction zones (geographical areas within which active opportunistic radio transmitters are allowed under certain restrictive conditions, limited by equivalent isotropic radiation power [EIRP] or EPFD), and protection zones (geographical areas within which incumbent receivers will not be subject to harmful interference). The distributed approaches that we adopted only take an action when interference is detected at the receiver. As interference only happens at the receiver of a communication link, the receiver is the best place to detect any possible interference, in other words, a receiver can measure the 'ground truth'. A global centralized database, which is built based on transmitter models, cannot give more accurate information on the impact of interference than the local view as measured at the receiver. A model is by definition an approximation of the real world and always falls short of the complexities of reality. It is, for instance, very hard to accurately model multipath signal propagation, out-of-band emission, antenna properties (gain, side-lobe, back-lobe emission), Doppler shift, obstacles, weather conditions, etc. * Spectrum monitoring: both approaches can be augmented with spectrum monitoring information. In the case of centralized approaches, geographically distributed spectrum monitoring devices are employed. A big issue here is accurate radio frequency (RF) calibration and abstracting hardware-specific features and artifacts of the spectrum monitoring device. Ideally, these devices should be capable of measuring the spectrum occupation for different directions around the spectrum monitoring device, increasing the complexity of the device. For comparison, in distributed approaches, when the spectrum is monitored at the receiver of a communication link using the same antenna configuration of the receiver, it will only capture relevant interference signals. Furthermore, there is no need for accurate RF calibration, as threshold levels for harmful interference can be perfectly calibrated through correlation with link statistics. * Rules for spectrum sharing: to compensate for inaccuracies and deficiencies of the models applied in a centralized database, large safety margins are applied, leading to very conservative rules for spectrum sharing and hence largely limiting the benefits thereof. In distributed approaches, resources are dynamically allocated, triggered by interference events, which are detected by monitoring link properties at the receiver. The receiver is not only the best place to detect any possible interference, but also the best place to make decisions on the spectrum reallocation of the interfered communication link. * Reaction time: consulting a centralized database for making decisions on spectrum resource allocation puts serious limitations on the reaction time, compared to local decisions using a fast dedicated CC.