Channel capacities and sensing parameters for Secondary Users (SUs) based on Opportunistic Spectrum Access (OSA) technique are studied in this paper. First, characteristics of the Signal-to-Interference-and-Noise Ratio (SINR) of SUs are discussed, from which the expressions about the average SINR are derived under interference and non-interference cases. Then, the influences of SINR on the channel capacity under OSA mode are discussed. After that, the data frame length and the detection frequency are designed from the cross layer design and stochastic characteristics of fading channels points of view. It can be seen from the analysis that the channel occupancy mode of the Primary Users (PUs) has influence on the SINR of SUs. It also has been proved that the scheme which joins the characteristics on the physical layer with occupancy mode of PUs, can reduce the collision probability between SUs and PUs. Meanwhile, it can improve the frequency efficiency of the system on the condition of meeting the required throughput of SUs.