Iterative distributed channel probing for cognitive radios with power-controlled wireless links

We propose a novel scheme for cognitive radios that enables secondary users (SUs) to verify whether they can use the channel without reducing the signal to interference-noise ratio (SINR) of primary users (PUs) below a pre-specified threshold; i.e., SUs check whether they are admissible or not. Mathematically, to verify admissibility, SUs need to know whether the spectral radius of the channel matrix is lower or greater than a known threshold. In this study, this verification is done with an iterative algorithm that builds upon channel probing techniques. By making SUs probe the channel with controlled power and observe how PUs respond to the additional interference, the algorithm produces two sequences that converge to a value that indicates indirectly whether the spectral radius of the channel matrix is greater or lower than the above-mentioned threshold. The first sequence is monotonically decreasing and the second is monotonically increasing. Admissibility is detected when one of the sequences crosses the known threshold. Simulations show that, in many practical scenarios, only few iterations are required to evaluate admissibility.