  In this paper, cognitive transmission under quality of service (QoS)
constraints is studied. In the cognitive radio channel model, it is assumed
that both the secondary receiver and the secondary transmitter know the channel
fading coefficients perfectly and optimize the power adaptation policy under
given constraints, depending on the channel activity of the primary users,
which is determined by channel sensing performed by the secondary users. The
transmission rates are equal to the instantaneous channel capacity values. A
state transition model with four states is constructed to model 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. The impact upon the effective capacity of several
system parameters, including the channel sensing duration, detection threshold,
detection and false alarm probabilities, and QoS parameters, is investigated.
