Channel Sounding Pilot Overhead Reduction for TDD Multiuser MIMO Systems

Coordinated linear transmit-receive processing with beam selection is a method to utilize all degrees of freedom available in multiuser multiple-input multiple-output (MIMO) cellular systems in order to increase system capacity. By applying channel state information in the transmitter (CSIT), the base station (BS) is able to spatially multiplex downlink data streams for different users. In the time division duplex (TDD) mode, CSIT for the BS is provided by means of uplink CSI sounding pilots. However, antenna-specific uplink pilot streams cause an extensive overhead that restricts the size of the practical user group and the terminal antenna setup that can be handled within the same time-frequency slot. We propose to reduce the required overhead by letting the terminals form pilot beams by transmit precoding, based on the knowledge of the user-specific MIMO channels obtained via a downlink common pilot signal. The achievable rate of the system is evaluated in conjunction with channel estimation and linear receivers. According to the results, the performance loss induced by the incomplete sounding is minor, as the beamforming gain provided by multiple terminal antennas, and the multiuser diversity seen by the BS are retained. When taking into account the CSI estimation error in the BS, the overhead reduction turns out to improve robustness and even increase the average system capacity.