An introduction to the multi-user MIMO downlink
IEEE Communications Magazine
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Closed loop transmit diversity has already been adopted by 3GPP for MIMO HSDPA in the form of TxAA and its dual stream counterpart, D-TxAA. While both these transmission techniques provide performance gains for single user (SU) scenarios, they both introduce multi-user interference in the downlink in multi-user(MU) scenarios. In this paper, we study the extension of these transmission techniques to the multi-user case which entail minimal changes to the existing standard. To this end, we consider the classical MMSE chip equalizer receivers that feed back beamforming weights so as to maximize the receive SINR at each user equipment (UE). Given that the base station (BS) has to use these weights to transmit data to the UEs, we compare practical and realistic strategies that BS can employ in order to maximize downlink capacity. We derive the SINR expression for MMSE chip equalizer receivers for the general case of MU-TxAA which is used at the receivers to select optimum feedback weights. We investigate different multiuser schemes for HSDPA in the downlink (DL), compare their performance and suggest optimal strategies for single and dual stream transmission for both single and multi-antenna receivers and corroborate our arguments with simulation results. We show that for the case of single antenna receivers, scheduling users with same beamforming weights maximizes downlink capacity in TxAA. For the D-TxAA with multiple antennas at receivers (MIMO) we show that SDMA outperforms spatial multiplexing in terms of maximizing DL capacity.