  We consider the Gaussian Multiple Access Wire-Tap Channel (GMAC-WT). In this
scenario, multiple users communicate with an intended receiver in the presence
of an intelligent and informed wire-tapper who receives a degraded version of
the signal at the receiver. We define a suitable security measure for this
multi-access environment. We derive an outer bound for the rate region such
that secrecy to some pre-determined degree can be maintained. We also find,
using Gaussian codebooks, an achievable such secrecy region. Gaussian codewords
are shown to achieve the sum capacity outer bound, and the achievable region
concides with the outer bound for Gaussian codewords, giving the capacity
region when inputs are constrained to be Gaussian. We present numerical results
showing the new rate region and compare it with that of the Gaussian
Multiple-Access Channel (GMAC) with no secrecy constraints.
