resources in the DU as well as to reduce bandwidth between the RU and the DU. A typical PRACH processing chain is shown below in Fig. 6-13: The PRACH time domain (baseband) signal contains a sequence of PRACH signa- tures, each prepended by a Cyclic Prefix. The PRACH signal is first filtered to select the target Resource Elements that are dedicated to the PRACH physical channel. This filter- ing can be done in time domain (Finite Impulse Response (FIR) filter) or in frequency domain by using a large FFT and frequency domain subcarrier selection. After convert- ing the relevant PRACH samples to the frequency domain, they are transmitted from RU to DU for correlation with potential root sequences and detection of the transmitted PRACH sequence. Received PRACH Sequence Sequence Signal (baseband) Decimation/ Sequence Sequence Filtering Frequency Domain Correlated Signal X Root Seq X Root Seq Coherent Signal Combining Detection FIGURE 6-13 PRACH processing chain. 230 Open Radio Access Network (O-RAN) Systems Architecture and Design Digital front-end DFE components include (digital) signal conditioning to convert the 3GPP-defined baseband sig- nal to a conditioned signal that compensates for inaccuracies in the analog transmit chain. Functions include: Digital Upconversion (DUC) (also known as channelization), is a group of signal processing blocks which create and condition the composite signal built by multiple individual carriers at defined offsets in the spectrum. DUC operates per antenna per carrier. Its operation depends on the targeted output sample rate and carrier bandwidth. Crest Factor Reduction (CFR) aims to reduce the Peak to Average Power Ratio (PAPR) of the aggregated carrier signal. The NXP implementation operates per antenna on the composite signal of all carriers for this antenna. Digital Predistortion (DPD) is the block which implements a complex nonlinear function that approximates the inverse transfer function H of the power amplifier (PA) baseband transfer function H. It is assumed that the DPD fe