Multipoint-to-point and point-to-multipoint space-time network coding

Traditional cooperative communications can improve com-munication reliability. However, the simultaneous transmis-sions from multiple relays are challenging in practice due to the issue of imperfect frequency and timing synchronization. In this work, we propose space-time network codes (STNCs) that overcome the issue of imperfect synchronization, reduce the large transmission delay, and still provide full spatial di-versity in multipoint-to-point (M2P) and point-to-multipoint (P2M) transmissions. Relays form single linearly-coded sig-nals from the overheard symbols and transmit them to the destination, where multiuser detection is utilized to detect the desired symbol. For a network of N client nodes, M2P and P2M STNCs result in a diversity order of N with only 2N time slots, a significant reduction from N2 time slots in tradi-tional cooperative communications using TDMA. Index Terms — Space-time network coding, cooperative communications, frequency synchronization, timing synchro-nization 1