Feasibility and Limitations of Secondary Transmission in Cognitive Radio Networks

Abstract

Wireless technology is rapidly proliferating into all aspects of computing and communication. This explosion of wireless applications creates an ever increasing demand for more radio spectrum. However, most usable spectrums have been licensed to primary users, although many studies have shown that these spectrums are significantly underutilized. Given the fact that spectrum is a finite resource, this calls for innovative technology in the radio field. Cognitive radio is an emerging technology that would allow unlicensed users (secondary users) to sense and efficiently utilize an available spectrum. For efficient spectrum utilization, in this thesis, we study the feasibility and limitations of secondary transmission in cognitive radio networks. In Chapter 2 and Chapter 3 we propose an optimal time and power allocation for a collaborative primary-secondary system. In Chapter 2, we assume that there are two source-destination pairs and extend this assumption to broadcasting systems in Chapter 3. We consider applying superposition coding (SC) and interference cancellation (IC) in these systems. The secondary transmitter acts as a relay of the primary signal and is allowed to transmit its own signal only if it is not harmful to the primary transmission. Time and power allocation is optimized and numerical comparison shows that the proposed approach makes a significant improvement in the achievable secondary performance, although the allowable location for the secondary source is limited. In Chapter 4 we consider cognitive relay systems with multiple primary spectrums. The cognitive relays sense presence of available primary channels by detecting energy of received signals from all primary nodes. The best relay among relays that can successfully decode the data from a source as well as find an available primary channel is selected and transmits the data to a destination. The exact and approximated outage probabilities have been obtained. The outage probability becomes low if the number of primary spectrums becomes large. By carefully determining a decision threshold for energy detection, we can achieve full diversity; however, the number of primary nodes should be larger than the number of cognitive relays, and the cognitive relays should perform cooperative spectrum sensing. In Chapter 5 we also consider cognitive relay systems with multiple primary spectrums. Because the scenario that all cognitive relays sense all primary spectrums is not practical due to the requirement of a high sampling rate, we consider that each cognitive relay does not sense all primary channels but senses at most one primary channel according to proposed sensor allocation schemes. The sensor allocation schemes are provided by considering the active probability of each primary channel and the performance of sensing by energy detection. The best relays are selected along with their allocated primary channel, and they simultaneously transmit; a destination combines the received signals using maximal ratio combining (MRC). We have obtained and shown by simulation the outage probability according to the proposed sensor allocation schemes. The proposed sensor allocation schemes have low complexity and make low outage probability. From the above research results for various cognitive radio systems, we can understand feasibility and limitations of secondary transmission and use spectrum efficiently. By this, we expect to show that the increasing demand for radio spectrum of wireless applications can be satisfied.