hysical layer functions below pre- coding are located in the O-RU; the second type is the Category "B," which additionally hosts the precoding in the O-RU. Standardization of two O-RU categories provides flexibil- ity for a system designer to choose the best option depending on the target use case. Further details can be found in the O-RAN fronthaul specification (ORAN-WG4.CUS.0). Figures 4.5.2 and 4.5.3 show the O-RAN downlink functional split for Category "A" and Category "B" radio unit, respectively. The uplink functional split is similar, as illustrated in Figure 4.5.4, which lists functional blocks residing in O-DU and O-RU. Similar to the downlink, the uplink processing func- tions perform operation to recover the bit stream transmitted by the UE. In the uplink, after the time-domain I/Q data received from the antenna, I/Q data samples go through the FFT and CP removal to be converted into frequency-domain I/Q samples. Through the beam- forming, the radio data are combined into the reduced number of I/Q sample streams. The RE demapping separates UE data and reference signals. The reference signals are sent to the channel estimation function to produce the channel information. The equalization function uses the channel information and received data samples to recover the data sent by UE. The demodulation processing converts complex value I/Q samples into bit blocks. The descram- bling function performs the descrambling operation using the same scrambling sequence as the one applied by the UE. The optional compression/decompression function can be used to reduce the fronthaul bandwidth. Dotted line blocks PDCCH PDSCH are optional. Signal CSI-RS Scrambling Scrambling Scrambling generation generation generation generation generation generation Modulation Modulation Modulation Modulation Modulation Modulation Modulation Modulation Modulation Layer Mapping Precoding Precoding Precoding* Precoding RE Mapping IQ Compression O-RAN FH IQ Decompression Beamforming Beamforming Beamforming Beamforming Beamforming iFFT a