Cooperative communications, which refer to the collaborative processing and retransmission of overheard information at stations surrounding a source. This technique takes full advantage of the broadcast nature of the wireless channel. In this paper, the author conducted the first analyze and evaluate the performance of cooperative diversity networks built on physical layers, then propose a crosslayer approach for cooperative wireless communications (from PHY layer to MAC layer). The paper simulated the solution on Matlab program and simulation result showed that the cooperative diversity scheme provides better BER performace than the no cooperative diversity scheme. By analyzing on the theory, the author also showed the significant performance improvement of the cross-layer protocol.