On the factor-of-two bound for Gaussian multiple-access channels with feedback

It was shown by Pombra and Cover (1994) that the maximum achievable throughput (sum of rates of all users) of a Gaussian multiple-access channel with feedback is at most twice that achievable without feedback. We prove a somewhat stronger result which establishes the factor-of-two bound not only for the total throughput but for the entire capacity region as well. Specifically, we show that the capacity region of a Gaussian multiple-access channel with feedback is contained within twice the capacity region without feedback.