Cognitive Radio Sensor Networks

2. Background

Traditionally, wireless networks follow fixed spectrum assignment policy, regulated by the government. The spectrum is assigned to users, who pay for the assigned spectrum for specific time and frequency. These users are called licensed users (Primary Users or PUs). This policy worked well in the past but now, with the passage of time, most of the spectrum has been assigned to licensed users and very less spectrum is remained. On the other hand, with the advancement in technology, there is a drastic increase in the development of new devices (e.g., smartphones, PDAs, tablets etc.) which operate on unlicensed spectrum. Therefore, the unlicensed spectrum has become overcrowded. Due to these two issues, a problem of spectrum scarcity arises. However, according to the survey, licensed users do not use their assigned spectrum all the time. Hence there is much sparsity in the utilization of licensed (fixed assigned) spectrum bands which resulted in underutilization of licensed spectrum. It is observed that the utilization of licensed spectrum ranges from 15%-85% with high variance in time and location (Akyildiz et al., 2006). Thus, in order to solve the problem of spectrum scarcity, the Federal Communications Commission (FCC, 2003), a regulatory authority in USA has allowed the usage of licensed spectrum by unlicensed devices; subject to the condition that licensed user should not be interfered. Consequently, CRNs are proposed to solve the problem of spectrum scarcity. They exploit Dynamic Spectrum Access (DSA) technique in order to access multiple licensed spectrum bands dynamically. CRNs are composed of CR devices, which opportunistically utilize white spaces in licensed channels. White spaces are the spectrum which is not in use of licensed user for specific time and frequency. Also, CR devices are intelligent devices which can change their parameters based on the interaction with their operating environment (Akyildiz et al., 2009).

On the other hand, a WSN is composed of resource constraint sensor nodes which have limited resources of energy and power. They are deployed densely in the observing environment. WSNs follow fixed spectrum assignment policy; therefore they also suffer from spectrum scarcity problem. Thus, WSNs can exploit CR technology in order to solve the problem of spectrum scarcity. This integration of WSNs and CR technology will improve network performance, spectrum utilization and resource utilization (communication and processing) in WSNs. Sensor nodes in WSNs, equipped with CR technology form Cognitive Radio Sensor Network (CRSN) (Akan et al., 2009).