Delay QoS provisioning in cognitive radio systems using adaptive modulation

The spectrum of licensees can be shared by some secondary users in cognitive radio systems. However, the secondary transmitters are limited by an interference threshold constraint which is imposed by the primary receivers in order to ensure primary users' reliable communication. In this paper, we assume that a number of secondary transmitters are trying to transmit their data to some secondary receivers in the presence of primary transmitters and receivers and, therefore, their inflicted interference at the primary receivers should be less than a specific threshold. We consider the delay quality of service and use the idea of effective capacity to guarantee a desired delay quality of service for every secondary link. By maximizing the sum of effective capacities of the secondary links we propose adaptive modulation schemes to support the desired quality of service. Two different optimal (continuous) and suboptimal (discrete) schemes are proposed and the rate and power allocation is obtained for each scheme. The performance of these two schemes is also compared.