Aiming at the opportunistic spectrum access（OSA） system with periodic channel sensing

sensing activity of secondary users was modeled and analyzed with Markov process.Combining physical layer index with MAC layer index

a cross-layer performance measure termed effective transmission capacity（ETC） was introduced.Under the criterion of maximizing ETC

a cross-layer optimal OSA algorithm was proposed and the achievable average ETC of opportunistic spectrum sharing was deduced.Theoretical analysis and simulation results indicate that the proposed algorithm is better than single-layer optimal schemes for it provides the secondary users with higher ETC.