  This paper investigates the achievable region and precoder design for
multiple access wiretap (MAC-WT) channels, where each user transmits both
secret and open (i.e., non-confidential) messages. All these messages are
intended for the legitimate receiver (or Bob for brevity) and the eavesdropper
(Eve) is interested only in the secret messages of all users. By allowing users
with zero secret message rate to act as conventional MAC channel users with no
wiretapping, we show that the achievable region of the discrete memoryless (DM)
MAC-WT channel given in [1] can be enlarged. In [1], the achievability was
proven by considering the two-user case, making it possible to prove a key
auxiliary lemma by directly using the Fourier-Motzkin elimination procedure.
However, this approach does not generalize to the case with any number of
users. In this paper, we provide a new region that generally enlarges that in
[1] and provide general achievability proof.
