Achievable Transmission Rate of the Secondary User in Cognitive Radio Networks with Hybrid Spectrum Access Strategy

A hybrid spectrum access strategy which is different from traditional underlay or overlay strategy is proposed for cognitive radio networks. In proposed strategy, the secondary users (SU) cooperatively sense the state of the primary user (PU) in a given spectrum band. Based on the sensing results and an additional requirement on the bit error rate, the SU adapts its transmit power and modulation level. Specifically, if the PU is inactive, the SU allocates its power subject to a peak transmit power constraint. Else if the PU is active, the average interference power constraint would be further imposed to protect the primary link. We assume that the maximal ratio combing (MRC) technique is available at the secondary receiver. Imperfect spectrum sensing and imperfect estimations of the channel power gain from the secondary transmitter to primary receiver are also considered. Achievable transmission rate of the SU over Rayleigh fading channel is formulated as an optimization problem and solved by using Lagrange dual decomposition method. Simulation results demonstrate that the transmission rate of the SU with hybrid strategy is significantly improved when compared with the traditional ones.