  Channel state information (CSI) at the transmitter is of importance to the
performance of physical layer security based on multi-antenna networks.
Specifically, CSI is not only beneficial to improve the capacity of the
legitimate channel, but also can be used to degrade the performance of the
eavesdropper channel. Thus, the secrecy rate increases accordingly. This letter
focuses on the quantitative analysis of the ergodic secrecy sum-rate in terms
of feedback amount of the CSI from the legitimate users in multiuser
multi-antenna downlink networks. Furthermore, the asymptotic characteristics of
the ergodic secrecy sum-rate in two extreme cases is investigated in some
detail. Finally, our theoretical claims are confirmed by the numerical results.
