In this IF architecture, the baseband device implements (QAM) modulation/de-modula- tion using a digital Numerically Controlled Oscillator (NCO) and digital mixers. The output signal is converted to a final RF frequency with a second (analog) mixer. The advantage of this architecture is that it doesn't require the IQ mismatch compensation from the zero-IF architecture, but this comes at a higher system cost due to increased data converter com- plexity and more difficult (band pass) filtering at IF/RF stages. Direct RF conversion (Fig. 4-21) signal generation with on-chip ADC/DAC. External RF only: In a direct RF sampling architecture, all (QAM) modulation/de-modulation is done in the digital domain, removing the need for a secondary (analog) mixer and associated cost/com- plexity. This comes at a higher system cost associated with high-speed data conversion and associated digital signal processing. Digital-to-analog conversion and analog-to-digital conversion The receiver ADC dynamic range is defined as the difference between the maximum allowed input signal level and the noise plus interference floor level. Thus we need to consider the 160 Open Radio Access Network (O-RAN) Systems Architecture and Design BaseBand IC Analog To RF FIGURE 4-19 Zero-intermediate frequency (Zero-IF) conversion. BaseBand IC Analog To RF FIGURE 4-20 Intermediate frequency (IF) conversion. BaseBand IC To RF FIGURE 4-21 Direct radio frequency (RF) conversion. maximum modulation and coding scheme which requires the maximum possible Signal to Noise Ratio (SNR) in order to decode the signal with the desired rate, say, modulation level, layer count, etc. Assuming a (simple to understand) zero-IF interface: The Peak to Average power ratio of the OFDM signal as discussed elsewhere, let's assume it to be 12 dB. We don't assume any PAPR optimization techniques implemented in the baseband receiver. Gain/offset errors. Gain and offset errors as caused by inaccuracies in internal reference and amplifier offset voltages reduce the usable dynamic ran