Influence of CSI Feedback Errors on Capacity of Linear Multi-User MIMO Systems

In this paper, we discuss the influence of feedback channel errors on throughput of linear multi-user multiple-input multiple-output (MIMO) systems employing vector quantization (VQ) algorithms for encoding channel state information (CSI). We consider an approach where the receiver estimates the downlink channel and chooses one of the predefined channel characterization codewords whose indices are transmitted back to the transmitter. In practice, the feedback channel will introduce errors in transmission of the indices and we evaluate the resulting channel capacity loss for different system setups. We demonstrate the performance of two types of systems, one using time division multiplexing of individual users and another transmitting to multiple users. We evaluate two basic error mitigation approaches: the first one with user expurgation using error-detection codes and the second one, based on specially designed VQ indexing. We show that the proposed schemes are very robust to even relatively high bit error rates in the feedback link and that proper VQ indexing may suffice for actual system design, even if the feedback channel is not very reliable.