Hybrid underlay/overlay cognitive radio system with hierarchical modulation in the presence of channel estimation error

Abstract

In this paper, we study the performance of hybrid cognitive radio (CR) system in the presence of channel estimation error (CER) in terms of bit-error-probability (BEP) and outage probability. We assume simultaneous switching between the underlay and overlay modes of CR where in, the secondary user (SU) probabilistically accesses the primary user's (PU) channel in both underlay and overlay modes to utilize the benefits of hybrid CR system. The PU uses two-layer 2/4 - ASK constellation with unequal level of bit protection against the interference imposed by the SU's transmitter. The SU wishes to opportunistically use the PU's channel to transmit its own data while maintaining the performance of the PU's layer with the worst level of protection at a desired value. We derive the SU's maximum allowable transmit power in the overlay mode to guarantee the PU reliable communication then, we discuss about the optimum value of the hybrid switching rate that minimizes the outage probability of the SU along with, deriving the exact BEP and outage probability expressions for the secondary system as well as that of the first and the second layers of the PU system.