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.

Cognitive Radio is a promising concept that offers solution to spectral crowding problem by introducing opportunistic usage of the unoccupied licensed frequency bands. Improving the network capacity and reliability of the communication has been recently studied and this through the introduction of cooperative technology in the CR networks. Furthermore, OFDM system is considered to be a good candidate to enhance the flexibility of the CR network, where individual carriers can be switched off for occupied frequencies by the primary user (PU) or for interfering Sub-channels. In this paper, an underlay cognitive network based on OFDM technique is proposed where secondary users (SUs) coexisting with a PU are adhering to stringent interference threshold constraint. Cooperative technology will help improving the CR network performances under PU limitation. This paper proposes new Sub-channel selection schemes which adapt either the transmission power or the sequential order of selection to enhance the system performances. We first provide the exact probability density function (PDF) of the received SNR at the secondary destination. Then, the PDFs are used to derive closed form expressions of the outage probability, the average BER and the ergodic capacity. Analytic results are validated by simulation for the proposed Sub-channels selection schemes.

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Cognitive Radio (CR) aims to improve spectrum utilization by allowing unlicensed users to access the licensed spectrum and to coexist with primary user (PU) while respecting the PU's restrictions. An interesting case to consider in underlay CR is when the channel state informations (CSI) of the links between secondary users (SUs) or between SUs and PU are imperfect. In fact, due to feedback or scheduling delays, in practice, the relay and/or the source node can have outdated channel state information. This outdated information will cause a performance degradation. In this paper, its impact on relay selection schemes and PU are investigated. To evaluate the impact of imperfect CSI on the secondary network, closed form expressions of the bit error rate (BER), the outage probability and the ergodic capacity are derived. Additionally, we investigate how harmful imperfect CSI can be on the PU's side. The theoretical results obtained in this work are confirmed through simulations.