While the required reliability of the sensing

process can be expressed in terms of rarely enough overlooking the PU, we assume that the proper QoS for the SU is given by maintaining — with a given confidence level — a minimum

bandwidth availability for the SU in spite of PU dynamics. In this paper we present an overview of approaches which might be used

to achieve these objectives. In addition we point out, that both the sensing process and the secondary link maintenance (necessary to keep the required bandwidth in spite of e configuration due to detected PUs) require a significant spectrum overhead. We identify and elaborate a fundamental tradeoff in using these

overheads in either sensing or link maintenance, and present examples of its optimization.