verter/analog-to- digital converter) and analog components After DAC a reconstruction filter (low-pass filter in the case of zero-IF interface systems) is needed to remove DAC aliasing images. The remainder signal processing steps include a modulator to bring (in our example) the baseband frequency to RF followed by filtering and amplification stages including the final stage: the PA. The reverse flow includes low-noise amplification, filtering, demodulation, and baseband antialiasing filtering to remove ADC aliasing images. 84 Open Radio Access Network (O-RAN) Systems Architecture and Design Analog gain control Given the high cost associated with the ADC, we want to make sure that we make best possi- ble use of the available dynamic range in this component. Analog gain control to one of the amplifier components before the ADC is required for this reason: avoid saturation (overflow) and underflow, providing highest possible effective number of bits. Gain setting can either be manual, automatic, or combination of both. Manual means that the gain is configured in a quasiopen loop: statically set by the CP of the system. In a very static environment (think: fixed wireless access or point-to-point links), this can be good enough. Automatic gain control is a closed-loop approach where receiver power is measured, and the gain is tuned accordingly. An algorithm is put in place to converge quickly enough to the appropriate value but does not overshoot/oscillate. In 3GPP/OFDM system, gain change should be applied on symbol boundaries to make. Dynamic user scheduling can cause received power fluctuation across slots. By implica- tion, a safety margin is set. Time and frequency synchronization Wireless 3GPP systems need to be both frequency- and time-synchronous across physical units (DU and RU), which implies a requirement for a board-level clock that is kept synchro- nous across the network. Time and frequency alignment are achieved through (a combina- tion of): Synchronous Ethernet. GNSS/GPS. IEEE1588 (PTP). Radio interf