I have $2$ users with different distance from the receiver and each user's message is spread with a different waveform. The $2$ waveforms are orthogonals.
With a perfect synchronization, I don't have proble to find the messages. But with synchronization I use preambles to get the beginning of the messages. And I cannot find the beginning of the signals because the synchronization is ineffective.
Where is the problem with preamble synchronization?