ROBUST PRECODING FOR MULTI-USER MISO SYSTEMS WITH LIMITED-FEEDBACK CHANNELS

In Multi User Multiple Input Single Output (MU-MISO) sys-tems one centralized transmitter with multiple antennas serves several decentralized single-antenna receivers. Precoding is an attractive way to remove multiuser interferences because it reduces cost and power consumption in the user equip-ment. When implementing precoding, however, the base sta-tion should know the Channel State Information (CSI). In Frequency Division Duplex (FDD), this information is sent from the receivers by means of a feedback channel, whose data rate is often severely limited. In this paper we investigate the utilization of the Karhunen-Loève (KL) transform to com-press the CSI sent through the feedback channel. We model the errors caused by channel estimation, truncation and quan-tization of the KL coefficients and feedback delay. This er-ror modeling is the basic premise to design robust Tomlinson Harashima Precoding (THP) based on a sum Mean Square Error (MSE) criterion. Our results show that robust THP clearly outperforms conventional THP transmitting a mini-mum amount of information from the receivers. 1