  This paper considers the security issue of the fifth-generation (5G) wireless
networks with massive connections, where multiple eavesdroppers aim to
intercept the confidential messages through active eavesdropping. To realize
secure massive access, non-orthogonal channel estimation (NOCE) and
non-orthogonal multiple access (NOMA) techniques are combined to enhance the
signal quality at legitimate users, while the inter-user interference is
harnessed to deliberately confuse the eavesdroppers even without exploiting
artificial noise (AN). We first analyze the secrecy performance of the
considered secure massive access system and derive a closed-form expression for
the ergodic secrecy rate. In particular, we reveal the impact of some key
system parameters on the ergodic secrecy rate via asymptotic analysis with
respect to a large number of antennas and a high transmit power at the base
station (BS). Then, to fully exploit the inter-user interference for security
enhancement, we propose to optimize the transmit powers in the stages of
channel estimation and multiple access. Finally, extensive simulation results
validate the effectiveness of the proposed secure massive access scheme.
