  In this paper, we investigate the effects of an additional trusted relay node
on the secrecy of multiple-access wiretap channel (MAC-WT) by considering the
model of multiple-access relay wiretap channel (MARC-WT). More specifically,
first, we investigate the discrete memoryless MARC-WT. Three inner bounds (with
respect to decode-forward (DF), noise-forward (NF) and compress-forward (CF)
strategies) on the secrecy capacity region are provided. Second, we investigate
the degraded discrete memoryless MARC-WT, and present an outer bound on the
secrecy capacity region of this degraded model. Finally, we investigate the
Gaussian MARC-WT, and find that the NF and CF strategies help to enhance
Tekin-Yener's achievable secrecy rate region of Gaussian MAC-WT. Moreover, we
find that if the noise variance of the transmitters-relay channel is smaller
than that of the transmitters-receiver channel, the DF strategy may also
enhance Tekin-Yener's achievable secrecy rate region of Gaussian MAC-WT, and it
may perform even better than the NF and CF strategies.
