edback loop is implemented as part of the receiver chain in what is a TDD system. IQ/Compensation, Gain/DC control to compensate for inefficiencies in external baseband RF converters (in case of a baseband I/Q interface and external up/down conversion) Interpolation before the DAC in the Tx path and decimation after the ADC in the Rx paths is provided to simplify antialiasing filter requirements. Signal aggregation and digital up conversion Assuming that the RU is a wideband unit that transmits multiple wireless communications channels at once, the first step in the processing chain is the combining of these individual signals that can have different characteristics (for example, 2G/GSM, 3G/WCDMA, 4G/LTE and 5G/NR; different amplitudes; different PAPR) into a single combined baseband signal that is processed as one through the next stages in the processing chain (Fig. 6-14). In case of single-carrier operation, this step can be greatly limited or removed altogether. DUC functionalities include channel filter, upsampling, and mixing. The channel filter aims at limiting Out of Band signals and meeting Adjacent Channel Leakage Ratio (ACLR) requirements. Upsampling is done for multi carrier operation to bring all carriers to the com- posite (sum of all carriers) sampling rate allowing them to be summed in the digital domain: mixing. The mixed signal is further filtered and up converted to the sample rate of the data con- verter. This up conversion stage can be implemented before or after CFR. Filtering supports removal of aliasing signals. Given that analog filtering is included to perform the same func- tion, up sampling filters are designed in conjunction of the analog filter so that both provide a good aliasing rejection. System modeling with Matlab or similar tools is required. Chapter 6 User-plane application components IF conversion Baseband Signal in Filter IF conversion to DAC, RF Baseband Signal 2 Filter IF conversion Baseband Signal 1 Filter FIGURE 6-14 Signal aggregator. Delay CORDIC Filter to DPD Baseband