Collaboration in Multiple Access Channels Requires Synchronization

Classical multiple access channel (MAC) coding schemes assume independent transmitted signals, because the given messages are assumed to be independent and the remote transmitters cannot cooperate. However, when the messages are correlated, or in the extreme case of fully cooperative transmitters, a better performance may be achieved. We explore the performance gain of this collaboration and its connection to transmission synchronization. For totally asynchronous transmitters, we prove that transmitters' collaboration has no performance gain. This result is shown for finite-alphabet memoryless MACs and for the additive white Gaussian noise MAC.