  We consider the General Gaussian Multiple Access Wire-Tap Channel (GGMAC-WT).
In this scenario, multiple users communicate with an intended receiver in the
presence of an intelligent and informed eavesdropper. We define two suitable
secrecy measures, termed individual and collective, to reflect the confidence
in the system for this multi-access environment. We determine achievable rates
such that secrecy to some pre-determined degree can be maintained, using
Gaussian codebooks. We also find outer bounds for the case when the
eavesdropper receives a degraded version of the intended receiver's signal. In
the degraded case, Gaussian codewords are shown to achieve the sum capacity for
collective constraints. In addition, a TDMA scheme is shown to also achieve sum
capacity for both sets of constraints. Numerical results showing the new rate
region are presented and compared with the capacity region of the Gaussian
Multiple-Access Channel (GMAC) with no secrecy constraints. We then find the
secrecy sum-rate maximizing power allocations for the transmitters, and show
that a cooperative jamming scheme can be used to increase achievable rates in
this scenario.
