Precoding for uplink multiuser MIMO with limited feedback

This paper investigates the optimization of precoders for uplink multi-user wireless communication systems, where the transmitters and receiver are equipped with multiple antennas. It is assumed that the centralized receiver has perfect knowledge of each uplink channel and designs the precoders for all users. The precoders are quantized to the vectors of a codebook, and the precoder vector index (PVI) is sent to each user through a limited channel. Since interference exists among users, the traditional single-user optimization-based precoding scheme exhibits poor performance. To improve the multi-user uplink sum-rate performance, a scheme incorporating an iterative multiple user PVI selection algorithm is proposed that iteratively projects one user channel into the null space of the precoded channels of other users, and obtains the PVI from the projected space. Simulation results show significant performance improvement compared to the conventional scheme. We also show that the proposed scheme achieves performance near to that for an exhaustive search, and outperforms the scheme that directly quantizes the precoders calculated from un-quantized joint optimization algorithms.