  In this paper, we consider a primary and a cognitive user transmitting over a
wireless fading interference channel. The primary user transmits with a
constant power and utilizes an adaptive modulation and coding (AMC) scheme
satisfying a bit error rate requirement. We propose a link adaptation scheme to
maximize the average spectral efficiency of the cognitive radio, while a
minimum required spectral efficiency for the primary user is provisioned. The
resulting problem is constrained to also satisfy a bit error rate requirement
and a power constraint for the cognitive link. The AMC mode selection and power
control at the cognitive transmitter is optimized based on the modified signal
to noise plus interference ratio feedback of both links. The problem is then
cast as a nonlinear discrete optimization problem for which a fast and
efficient suboptimum solution is presented. We also present a scheme with rate
adaptive and constant power cognitive radio. An important characteristic of the
proposed schemes is that no computation or coordination overhead is imposed on
the primary radio due to the cognitive radio activity. Numerical results and
comparison with the interweave approach to cognitive radio demonstrate the
efficiency of the proposed solutions.
