  Massive MIMO is one of the salient techniques for achieving high spectral
efficiency in next generation wireless networks. Recently, a combined strategy
of the massive MIMO and the artificial noise (AN), namely, {\it AN assisted
massive MIMO (ANAM)} has recently been actively investigated for security
enhancement. However, most of previous studies on the ANAM have been built upon
the full channel state information (CSI) assumption at the eavesdropper (ED),
which may be too pessimistic to provide meaningful information on the security
since the channel uncertainty of the ED may degrade its decoding ability. In
this paper, we provide more sophisticated investigation on the performance of
the ANAM system assuming that the CSI of the ED channels are unknown to both
the BS and the ED or partially known to the ED. We measure the secrecy in terms
of both the leakage rate to the ED and the secrecy rate to the legitimate
users, and characterize their upper and lower bounds in the high SNR regime as
a function of the number of ED antennas, the number of data and AN signal
dimensions, and coherence time. Finally, from numerical results, we demonstrate
the accuracy of our analysis and highlight the security potential of the ANAM
system against the passive eavesdropping attack.
