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Top1. Introduction
Under-utilization of spectrum bandwidths allocated to various licensed services, has given rise to the concept of Cognitive Radio Networks (CRN). It is based on the proposal of using those under-unutilized bandwidths by non-licensed users, through opportunistic spectrum access (Mitola J., 2009). The main advantage of the cognitive radio networks is that they make better use of those bandwidths. If this mechanism is adopted in those frequency ranges in which many of the mobile and cellular communication systems are operating, it will be extremely helpful, because those bands are crowded heavily and hardly any bandwidth is available at government agencies to allot it to new services. In fact, there are some underutilized spectrum frequencies in these frequency ranges, which were originally licensed to terrestrial TV operations, and various applications of military and navigation (Young-June Choi, Shin K.G., 2011). So, cognitive radio networks are vying for better use of these frequencies. In the terminology of cognitive radio networks, the licensed users are called ‘Primary Users’ (PU) and the opportunistic cognitive radio users are called ‘Secondary Users’ (SU).
As sharing of available spectrum among Secondary Users is the most important aspect of cognitive radio networks, several proposals are made by researchers, for this sharing. Based on the way SUs avail the spectrum opportunity, three types of spectrum sharing paradigms are available (Goldsmith, A., Jafar, S.A., Maric, I. & Srinivasa, S., 2009). They are:
- 1.
Underlay Methods: Here PUs and SUs coexist together such that SUs will use less power and hence do not disturb the PUs’ communications.
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Overlay Methods: Here SUs will make use of spectrum holes with the help of messages and code books shared by PUs. In return, SUs need to help in relaying the transmissions of PUs.
- 3.
Interweave Methods: In this paradigm, SUs should continuously sense the spectrum for finding and exploiting the spectrum holes.
Of the above methods, overlay type of spectrum sharing is simple to implement, and hence the preferred one, in many contexts. While sharing the spectrum, allocation of channels to individual users can be made either through central administration or through distributed mechanisms. In the case of central decision making methods, one central system decides all the channel allocations in that region by collecting the data from all the cognitive radios of that region about their measurements and as well as the channel requirements. Measurements here refer to the sensing of spectrum activity by that cognitive radio. Energy detection is the commonly used method of spectrum sensing (Won-Yeol Lee; Akyildiz, I.F., 2008). In the case of distributed decision making methods, each cognitive radio takes the decision on its own by checking its measurements for all channels in its surroundings and then selecting a free channel for its communication. This kind of decision may be required in the cases where the cognitive radios operate in an environment where proper structure of the network doesn’t exist. In the proposed work of this paper, centralized channel allocation is considered.