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Traditional cooperative communications can improve communication reliability. However, transmissions from multiple relay nodes are challenging in practice. Single transmissions in time-division multiple-access (TDMA) manner cause large transmission delay, but simultaneous transmissions from two or more nodes using frequency-division multiple access (FDMA) and code-division multiple access (CDMA) are associated with the issue of imperfect frequency and timing synchronization. In this work, a novel framework for cooperative communications is proposed to achieve full spatial diversity with low transmission delay and eliminate the issue of imperfect synchronization. This is realized by the use of space-time network codes (STNCs) associated with a novel concept of wireless network cocast. For a network of N client nodes, R relay nodes and a base node, the STNCs provide a diversity order of (R+1) for each symbol with (N+R) time slots, a reduction from 2N time slots in traditional FDMA and CDMA cooperative communications for N being usually greater than R and from N(R+1) time slots in traditional TDMA cooperative communications. The STNCs are also applied in networks, where the client nodes located in a cluster act as relays to help one another to improve their transmission performance. The performance in clustering setting is studied to show the improvement in power saving, range extension, and transmission rate.