Optimum Sensing and Opportunistic Channel Access in Cognitive Radio Networks

Abstract

In conventional static spectrum allocation, at any given time, a huge portion of the spectrum is either under-utilized, or unused. To overcome this shortcoming of the existing static spectrum allocation, cognitive radio has been introduced. In cognitive radio networks, there is a set of primary users (PUs), and a set of secondary users (SUs), where the SUs seek for opportunistic channel access in the PUs channels to transmit their data. At a given time, depending on whether a PU has data to transmit or not, the corresponding channel is busy or idle. The SUs carry out sensing to ascertain the availability of the channels, and transmit data, if a channel is found to be idle. The SUs considered are energy-limited devices, and hence, energy must be managed efficiently. With this objective, we propose various algorithms that achieve maximal energy efficiency, or throughput. The SU allocates a substantial portion of its energy for channel state detection, transmission of its data, and for channel switching. Therefore, in this Thesis, we focus on the development of energy-efficient channel sensing, access, and switching algorithms. newlineFirstly, we study the problem of joint channel-sensing and channel-access in CRN. The channel sensing problem that we consider is detecting whether or not there is an unknown signal (with random fading) in noise. We consider time-slotted system in all our problems. For this channel sensing problem, we consider one channel and M SUs. We propose a sequential detection procedure in which each SU takes samples sequentially, one at a time, computes the average energy of the samples and compares with two thresholds. If it is greater than the upper threshold, the channel declared to be busy; otherwise, if it is less than the lower threshold, then the channel declared to be idle. If it falls between the two thresholds, the SU continues to sense until it reaches a maximum number of samples. As soon as an SU detects the idle/busy state of the channel, it broadcasts its local decision to all other SUs.