Performance analysis of relay selection schemes in underlay cognitive networks with Decode and Forward relaying

In underlay cognitive networks with regenerative relaying, secondary users operating with primary user are adhering to stringent interference constraint which limits their transmission power and coverage area. In order not to violate this interference limit, underlay network will make the use of relays to transmit signal over the secondary network. The retransmitting relay will be selected among the available secondary users; hence, relay selection becomes more challenging due to strict interference limits. Relay selection is based not only on interference limit, but also on threshold constraint which ensure the satisfactory reception of the signal by the relays and the destination and which is an important criterion since the relaying protocol used is Decode and Forward (DF). This paper proposes three new relay selection schemes based on relay to destination link and relay to primary link qualities. We derive closed form expressions of the probability density function (PDF) of the SNR at the secondary destination, the outage probability and the average bit error probability. Analytical results are validated by simulations and they provide comparative analysis of the different relay selection scheme proposed herein.