FEEDBACK CAN AT MOST DOUBLE GAUSSIAN MULTIPLE ACCESS CHANNEL CAPACITY

The converse for the discrete memoryless multiple-access channel is generalized and is used to derive strong bounds on the total capacity (sum of the rates of all the senders) of an m-user Gaussian multiple-access channel in terms of the input covariance matrix. These bounds are used to show that the total capacity of the channel with feedback is less than twice the total capacity without feedback. The converse for the general multiple access channel is also used to show that for any m-user multiple-access channel, feedback cannot increase the total capacity by more than a factor of m.