  Transmitter-side channel state information (CSI) of the legitimate
destination plays a critical role in physical layer secure transmissions.
However, channel training procedure is vulnerable to the pilot spoofing attack
(PSA) or pilot jamming attack (PJA) by an active eavesdropper (Eve), which
inevitably results in severe private information leakage. In this paper, we
propose a random channel training (RCT) based secure downlink transmission
framework for a time division duplex (TDD) multiple antennas base station (BS).
In the proposed RCT scheme, multiple orthogonal pilot sequences (PSs) are
simultaneously allocated to the legitimate user (LU), and the LU randomly
selects one PS from the assigned PS set to transmit. Under either the PSA or
PJA, we provide the detailed steps for the BS to identify the PS transmitted by
the LU, and to simultaneously estimate channels of the LU and Eve. The
probability that the BS makes an incorrect decision on the PS of the LU is
analytically investigated. Finally, closed-form secure beamforming (SB) vectors
are designed and optimized to enhance the secrecy rates during the downlink
transmissions. Numerical results show that the secrecy performance is greatly
improved compared to the conventional channel training scheme wherein only one
PS is assigned to the LU.
