Feedback Gains for Gaussian Massive Multiple-Access Channels

Feedback is shown to increase the sum-rate capacity of K-user Gaussian multiple-access channels by at most a factor of approximately 1.54, improving Thomas' doubling bound (1987). The new bound is the best possible in the sense that it can be approached as closely as desired for a massive number of users. Moreover, feedback provides unbounded power gain in K for a fixed transmit power per user.