  In this paper, cognitive transmission under quality of service (QoS)
constraints is studied. In the cognitive radio channel model, it is assumed
that the secondary transmitter sends the data at two different average power
levels, depending on the activity of the primary users, which is determined by
channel sensing performed by the secondary users. A state-transition model is
constructed for this cognitive transmission channel. Statistical limitations on
the buffer lengths are imposed to take into account the QoS constraints. The
maximum throughput under these statistical QoS constraints is identified by
finding the effective capacity of the cognitive radio channel. This analysis is
conducted for fixed-power/fixed-rate, fixed-power/variable-rate, and
variable-power/variable-rate transmission schemes under different assumptions
on the availability of channel side information (CSI) at the transmitter. The
impact upon the effective capacity of several system parameters, including
channel sensing duration, detection threshold, detection and false alarm
probabilities, QoS parameters, and transmission rates, is investigated. The
performances of fixed-rate and variable-rate transmission methods are compared
in the presence of QoS limitations. It is shown that variable schemes
outperform fixed-rate transmission techniques if the detection probabilities
are high. Performance gains through adapting the power and rate are quantified
and it is shown that these gains diminish as the QoS limitations become more
stringent.
