This paper investigates the effect of imperfect channel information on the outage performance of underlay cooperative cognitive networks with reactive relay selection under both primary outage constraint and maximum transmit power constraint. Towards this end, we firstly propose a power allocation scheme for secondary users to meet both constraints and take into account channel estimation error. Then, based on this scheme we propose an exact closed-form outage probability expression for secondary networks. This expression facilitates in quickly assessing the performance behaviour of underlay cooperative cognitive networks with reactive relay selection in different aspects and provides useful insights into the system performance. Various results show considerable system performance degradation due to imperfect channel information, a performance trade-off between the secondary network and the primary network, the performance saturation phenomenon in the secondary network, and the significant performance improvement with respect to the increase in the number of relays.
Relay selection is proposed in this paper as an efficient solution to secure information transmission of secondary users against eavesdroppers in energy harvesting cognitive networks. The proposed relay selection method selects a secondary relay among available secondary relays, which are capable of harvesting radio frequency energy in signals of the secondary transmitter and correctly restore secondary message, to curtail signal-to-noise ratio at the wire-tapper. In order to evaluate the security performance of the suggested relay selection method, an exact intercept outage probability formula accounting for peak transmit power confinement, Rayleigh fading, and interference power confinement is firstly derived. Monte-Carlo simulations are then generated to corroborate the proposed formula. Numerous results expose that positions of relays, the number of relays, and parameters of the energy harvesting method significantly influence the security performance while the power confinements on secondary transmitters cause the performance saturation.
This paper provides an analytic performance evaluation of the bit error rate (BER) of underlay decode-and-forward cognitive networks with best relay selection over Rayleigh multipath fading channels. A generalized BER expression valid for arbitrary operational parameters is firstly presented in the form of a single integral, which is then employed for determining the diversity order and coding gain for different best relay selection scenarios. Furthermore, a novel and highly accurate closed-form approximate BER expression is derived for the specific case where relays are located relatively close to each other. The presented results are rather convenient to handle both analytically and numerically, while they are shown to be in good agreement with results from respective computer simulations. In addition, it is shown that as in the case of conventional relaying networks, the behaviour of underlay relaying cognitive networks with best relay selection depends significantly on the number of involved relays.
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