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

Pombra and Cover (see IEEE Trans. Inform. Theory, vol.40, p.885-92, May 1994) and Thomas (see IEEE Trans. Inform. Theory, vol.33, p.711-16, Sept. 1987) showed 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 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