Classical Multiple Access Channel (MAC) coding schemes assume independent transmitted signals, as 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 collaboration and its connection to transmission synchronization. For totally asynchronous transmitters we prove that transmitters collaboration has no performance gain.

When the transmitters are fully synchronous, we show that collaboration gain is upper bounded by a single bit for AWGN channels. We further conjecture that for any additive channel the MAC scheme achieves at least half of the optimal performance.