For wireless digital communications, digital user signal is usually at first converted into analog domain and then modulated to radio frequency by one or two steps.

Conventional dual up-conversion transmitter usually employs an intermediate frequency (IF) mixer and a radio frequency (RF) mixer to convert baseband user signal to IF and RF in sequence.

Next, minimum mean square error (MMSE) filters are applied to the received signal to recover single user signal wave thus implementing user separation and frequency synchronization at the same time.