The performance of the ZF-BF system will be degraded unless the channel status information at the transmitter (CSIT) is perfect. In order to provide CSIT to the transmitter, each receiver must feed back CSI estimated at the receiver. Caire, Jindal, Kobayashi, Ravinderan has compared the performance differences of digital feedback and analog feedback schemes[1]:

$ \Delta R^{AF} \leq \log \left( 1 + \frac{1}{\beta} \right) $
$ \Delta R^{DF} \leq \log \left( 1 + \frac{ \frac{P}{N_0} } {\left( 1 + \frac{P}{N_0}\right)^\beta } \right) $

where $ P/N_0 $ is the SNR and $ \beta $ is the symbols per antenna.


  1. G. Caire, N. Jindal, M. Kobayashi, and N. Ravindran, Multiuser MIMO Achievable Rates with Downlink Training and Channel State Feedback, Submitted to IEEE Trans. Information Theory, Nov. 2007. (Revised May 2009)