Downlink Assisted Uplink Zero-Forcing for TDD Multiuser MIMO Systems

Coordinated linear transmitter-receiver processing by block diagonalization (BD) with beam selection is a straightforward method to utilize all degrees of freedom available in multiuser multiple-input multiple-output (MIMO) networks in order to increase system capacity. By applying channel state information (CSI) in the transmitter, the BD criterion offers zero-forcing between the downlink data streams of different users. This paper proposes practical uplink MIMO schemes for time division duplex (TDD) systems to co-exist with downlink TX-RX zero-forcing so that the locally available CSI of the BD channel is used by the terminals in the uplink transmission. It is shown that the precoded pilot symbols are sufficient in both uplink and downlink to satisfy the needs of both transmission and reception. The achievable rate of the system is evaluated in conjunction with linear receivers in a time-varying fading channel and with channel estimation. According to the results the proposed uplink transmission strategy provides increased rates compared to user selection as well as non-precoded transmission, without being sensitive to CSI uncertainty.