Two-hop MIMO relaying systems using random unitary beamforming in Gaussian broadcast channels

In this paper we propose the multi-user MIMO (MU-MIMO) with the fixed relay station (RS) transmissions scheme where the base station (BS) and the fixed RS are equipped with multiple antennas and the multiple users are equipped with a single antenna. Since the MU-MIMO can improve the sum capacity significantly by transmitting the data streams to multiple users simultaneously without much link gain while the fixed relay station can improve the link gain but loses the spectral efficiency due to the half-duplex transmission, our proposed scheme can compensate each of the drawbacks and can improve both the sum rate and the link gain. We adopt the practical random unitary beamforming (RUB) precoding method for MU-MIMO and the simple amplify-and-forward protocol for the operation of the relay station. Based on our proposed RUB based MU-MIMO with fixed RS, the sumcapacity is upper bounded by the channels between the BS and the RS, not by the channels between the RS and the users. We also show from our extensive simulations that our proposed scheme has lower complexity compartively and provides the performance commensurate with the schemes presented in the literature in terms of the sum-rate for the large user scenario.