  Recently, the demand for faster and more reliable data transmission has
brought up complex communications systems. As a result, it has become more
difficult to carry out closed-form solutions that can provide insight about
performance levels. In this paper, different from the existing research, we
study a cognitive radio system that employs hybrid-automatic-repeat-request
(HARQ) protocols under quality-of-service (QoS) constraints. We assume that the
secondary users access the spectrum by utilizing a strategy that is a
combination of underlay and interweave access techniques. Considering that the
secondary users imperfectly perform channel sensing in order to detect the
active primary users and that there is a transmission deadline for each data
packet at the secondary transmitter buffer, we formulate the state-transition
model of the system. Then, we obtain the state-transition probabilities when
HARQ-chase combining is adopted. Subsequently, we provide the packet-loss rate
in the channel and achieve the effective capacity. Finally, we substantiate our
analytical derivations with numerical results.
