MIMO Broadcast Transmission with Outdated Channel State Information

zhangkai98gmails.tsinghua.edu.cn, niuzhsgtsinghua.edu.cn Abstract tial precoding enforcing a zero inter-user interference con-straint. In BD, data streams from different users are multi-The performance of multiuser MIMO downlink sys- plexed in the spatial domain. tems with block diagonalization (BD) relies on the channel The above work is based on the assumption that channel state information (CSI) at the transmitter to a great extent. state information (CSI) is perfectly known at the transmitter. For time division duplex TDD systems, the transmitter esti- However, in many applications, this assumption is not rea-mates the CSI while receiving data at current time slot and sonable, especially if the number of transmit antennas and then uses the CSI to transmit at the next time slot. When the number of users are large, or the channels vary rapidly. the wireless channel is time-varying, the CSIfor transmis- In Frequency Division Duplex (FDD) systems, the CSI at sion is imperfect due to the time delay between the estima- the transmitter is sent by the receiver through a feedback tion of the channel and the transmission of the data and channel, which is usually bandwidth limited. Therefore the severely degrades the system performance. In this paper, CSI at transmitter is usually partially fed back. There is we propose a linear method to suppress the interferences some recent work on multiuser MIMO systems with partial among users and data streams caused by imperfect CSI at CSI feedback [8],[16],[12]. However, this paper focuses o