We consider the training design and channel estimation in the amplify-and-forward (AF) diamond relay network. Our strategy is\nto transmit the source training in time-multiplexing (TM)mode while each relay node superimposes its own relay training over the\namplified received data signalwithout bandwidth expansion.Theprincipal challenge is to obtain accurate channel state information\n(CSI) of second-hop link due to the multiaccess interference (MAI) and cooperative data interference (CDI). To maintain the\northogonality between data and training, a modified relay-assisted training scheme is proposed to migrate the CDI, where some\nof the cooperative data at the relay are discarded to accommodate relay training. Meanwhile, a couple of optimal zero-correlation\nzone (ZCZ) relay-assisted sequences are designed to avoid MAI. At the destination node, the received signals from the two relay\nnodes are combined to achieve spatial diversity and enhanced data reliability.The simulation results are presented to validate the\nperformance of the proposed schemes.
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