Link decision algorithm in underlay cognitive radio networks over a Rayleigh fading channel

A link decision algorithm is proposed for underlay cognitive radio network over a Rayleigh fading channel. The authors assume that a diversity combining technique is available at the secondary receiver. Under the constraints on average interference power (AIC) at the primary receiver (PR) and peak transmit power at the secondary transmitter (ST), for a given bit error ratio (BER) target, the link decision problem is formulated as an optimisation problem. Then, Lagrange dual decomposition is applied to derive the optimal transmit power and the modulation level of the ST to obtain its achievable transmission rate. Based on the optimal transmit power and the modulation level, a practical link decision algorithm which requires integral modulation levels is also proposed. Achievable transmission rates and outage probabilities of the proposed algorithms for various power constraints, BER requirements and different number of diversity branches are analysed. Finally, the influence of the primary user´s transmission activities (including transmit power and non-active probability) on the transmission performance of the secondary link is also discussed. They compare all the aforementioned results with the case when peak interference power constraint (PIC) at the PR is employed instead of the AIC. Simulations reveal that the AIC is indeed superior over the PIC. All these numerical results validate the proposed algorithms and the analysis.