In this paper, a Decode and Forward (DF) synchronized cooperative communication scheme, obtaining full rate, is proposed. The channel used is a pure partially-coherent relay channel (PCRC), where one hop is coherent and the other is non-coherent, unlike the previously proposed schemes, where channel phase knowledge is known over the two hops. In the first hop, Minimum-Decoding complexity technique is applied on Quasi-Orthogonal Space-Time Block Code (QOSTBC) to acquire linear decoding complexity, using non-linear transmitter. In the second hop, a reduced-complexity Sphere Decoder (SD) is applied attaining the exact Maximum-Likelihood (ML) performance with lower complexity. The complexity of this SD is relatively insensitive to the primary choice of sphere radius. Relay selection technique is applied in the second hop. The first hop transmitter has four antennas, while the second hop receiver uses a single antennas, receiving from the top four selected signals. This scheme achieves a full-rate, full-diversity, ML performance, and low decoding complexity.
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