  This paper investigates a new spectrum sharing scenario between unmanned
aerial vehicle (UAV) and terrestrial wireless communication systems. We
consider that a cognitive/secondary UAV transmitter communicates with a ground
secondary receiver (SR), in the presence of a number of primary terrestrial
communication links that operate over the same frequency band. We exploit the
UAV's controllable mobility via trajectory design, to improve the cognitive UAV
communication performance while controlling the co-channel interference at each
of the primary receivers (PRs). In particular, we maximize the average
achievable rate from the UAV to the SR over a finite mission/communication
period by jointly optimizing the UAV trajectory and transmit power allocation,
subject to constraints on the UAV's maximum speed, initial/final locations, and
average transmit power, as well as a set of interference temperature (IT)
constraints imposed at each of the PRs for protecting their communications.
However, the joint trajectory and power optimization problem is non-convex and
thus difficult to be solved optimally. To tackle this problem, we propose an
efficient algorithm that ensures to obtain a locally optimal solution by
applying the techniques of alternating optimization and successive convex
approximation (SCA). Numerical results show that our proposed joint UAV
trajectory and power control scheme significantly enhances the achievable rate
of the cognitive UAV communication system, as compared to benchmark schemes.
