A novel combined CSI feedback mechanism to support multi-user MIMO beamforming schemes in TDD-OFDMA systems

In modern Orthogonal Frequency Division Multiple Access (OFDMA) systems Multi-User Multiple-Input Multiple-Output (MU-MIMO) techniques are employed to increase the cell and user throughput without additional bandwidth or transmit power requirements. The increased throughput in the downlink is achieved by simultaneous transmission to multiple mobile stations (MSs) over the same time-frequency resources. MU-MIMO beamforming techniques at the base station (BS) transmitter are used to mitigate interference from one data stream to another. To enable the MU-MIMO beamforming operation in the downlink, channel state information (CSI) feedback from the MS is required. In this paper we propose a novel CSI feedback mechanism which combines two existing CSI feedback schemes to achieve significant system performance improvements in MU-MIMO beamforming for OFDMA systems operating in Time Division Duplex (TDD). The employed CSI reporting schemes are the quantized, codebook-based feedback scheme and the sounding-based feedback scheme. In the first step, the performance of the two feedback schemes is evaluated separately through system-level simulations. It is shown that for accurate calculation of the MU-MIMO beamforming weights, the sounding-based CSI feedback scheme is preferable in deployment scenarios with low noise and interference levels. For the remaining deployment scenarios, the quantized codebook-based reporting mechanism outperforms the sounding-based feedback scheme. In the second step, the proposed combination of the considered feedback mechanisms using a simple feedback selection scheme is evaluated through extensive simulations. The simulation results presented in the paper show that the proposed combined CSI feedback mechanism not only outperforms the existing CSI feedback schemes if used separately but also leads to marginal performance degradation compared to the system performance achieved with prefect CSI knowledge at the BS transmitter.