Abstract—This letter considers the multi-antenna cognitive radio (CR) network, which has a single secondary user (SU) and coexists with a primary network of multiple users. Our objective is to maximize the service probability of the SU,

subject to the interference constraints on the primary users (PUs) in the form of probability. Exploiting imperfect channel state information (CSI), with its error modeled by added Gaussian noise, we address the optimization for the beamforming weights at the secondary transmitter. In particular, this letter devises an iterative algorithm that can efficiently obtain the robust optimal

beamforming solution. For the case with one PU, we show that a much simpler algorithm based on a closed-form solution for the antenna weights of a given power can be presented. Numerical results reveal that the optimal solution for the constructed problem provides an effective means to tradeoff the performance between the PUs and the SU, bridging the non-robust and worstcase based systems.

Index Terms—Cognitive radio, interference control, robust

beamforming.

ABSTRACT

            Stratellites (the term comes from combining the words stratosphere and satellite) are high-altitude, unmanned airships designed to operate continuously for weeks, months, or even years at a time. Their main purpose is to hover over one geographical location and function as a relay and/or transmission source for radio signals, including as broadband, mobile phone, and digital television transmissions. The idea is currently being developed by the company Sanswire and others. A stratellite would hover about 20 kilometers up, above the jet stream and most weather, and could service an area about 320 kilometers in diameter, or about 480,000 square kilometers. Lag times would be many times less than that for orbiting satellites carrying the same type of signals, but would cover a larger area than traditional transmission towers.