Performance analysis of relay selection schemes in OFDM-based underlay cognitive networks

Cognitive radio (CR) is a promising technology that allows wireless systems to sense the environment and learn from previous experience to improve the communication quality. However, CR needs a flexible and adaptive physical layer in order to perform the required tasks efficiently. An OFDM system is considered to be a good candidate where individual carriers can be switched off for frequencies occupied by a licensed user or interfering sub-carriers. In this paper, an underlay cognitive radio system based on OFDM technique is proposed where secondary users operating in the presence of primary user are adhering to stringent interference and threshold constraint. In order not to violate this interference limit, the underlay network will make the use of relays to transmit signals over the secondary network. This paper proposes new relay or sub-carriers selection schemes. We derive closed form expressions of the probability density function (PDF) of the SNR at the secondary destination, the outage probability, the average bit error probability and the ergodic capacity. Analytical results are validated by simulations for the different relay selection schemes proposed herein.