  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 scaled 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 adaption
and a constant power. An important characteristic of the proposed schemes is
that no negotiation between the users is required. Comparisons with underlay
and interweave approaches to cognitive radio with adaptive transmission
demonstrate the efficiency of the proposed solutions.
