Zero-Forcing Precoding with Partially Outdated CSI over Time-Varying MIMO Broadcast Channels

Updating channel state information at the transmitter (CSIT) in time varying channels, is one of the main bottlenecks in wireless communication systems. In MIMO broadcast channels CSIT is required for user scheduling and precoder design. A time varying MIMO broadcast channel with limited-capacity feedback links is considered. An efficient threshold-based feedback technique based on allowing CSIT errors only for certain users is proposed. In the proposed scheme, the frequency of CSI feedback for different users is set to be proportional to their channel strength. Hence, CSI feedback is substantially reduced. Analysis and simulation results show net throughput superiority of the proposed scheme over the case of perfect CSIT. For large numbers of users, it is shown that how increase in the number of users can compensate the sum rate loss due to errors caused by partially outdated channel.