The Radio Unit (RU) purpose is to convert radio signals sent to and from the antenna to a digital signal that can be transmitted over the fronthaul to Distributed Unit (DU). Considering O-RAN Split 7-2, at high level an O-RU has following components.
- Synchronization and Fronthaul Transport
- Lower PHY Layer Baseband Processing
- Digital Front End (DFE)
- RF Front End (RF FE)
Open Radio Unit (O-RU) Reference Architecture is shown in figure below.
- Synchronization and Fronthaul Transport: GPS/PTP (IEEE 1588) modules are used synchronization. For fronthaul connection. The fronthaul connectivity between O-RU and O-DU is eCPRI can be established using Fiber or Ethernet
- Lower PHY Baseband Processing: The lower PHY layer processing can be implemented by using FPGAs or ASICs. It includes functions of FFT/iFFT, CP addition and Removal, PRACH filtering and digital beamforming (Optional).
- Digital Front End (DEF): The digital front end consists of Digital Up Converter (DUC), Digital Down Converter (DDC), Digital Pre-Distortion (DPD) and Crest Factor Reduction (CFR).
- Radio Front End (RFEF): The RF front end is composed of antenna element arrays, bandpass filters, Power Amplifiers (PA), Low Noise Amplifiers (LNA), Digital Analog Converters (DAC), and Analog Digital Converters (ADC)
- Other Subsystems:
- Power Supply is the heart of O-RU as it drives all the subsystem, Power supply can be AC or DC but recommendation says power supply shall be –48 VDC
- In O-RU, LED can be used for local on/off display Status for Fronthaul transport interface, Power supply and Radio transmission
- RJ45 Connection can be provided for local access and debug purpose
O-RU Types/Operation Mode
Based on O-RAN split 7-2, O-RU can also be classified or configured to operate in two following distinct modes. This classification is based upon the functionality resides in O-RU or O-DU.
- O-RU “Category A”
- Precoding function is perform in O-DU allowing the RU design to be simple
- The fronthaul interface will carry spatial streams in the DL which can be larger than layers
- Following figure show Downlink signal processing for Category A
- O-RU “Category B”
- Precoding is perform in O-RU which makes RU design complex
- The fronthaul interface will carry spatial layers which can be less data
- This category allows “Modulation Compression” can be perform in the DL to effectively send only the bits equivalent to the constellation points which helps to reduce the DL throughput much more
- Category B, results in the bandwidth approaching that of alternative Lower Layer Split 7-3
- Following figure show Downlink signal processing for Category B
- O-RAN Specification : WG4 – Open Fronthaul Interfaces Workgroup
- TIP Whitepaper
- O-RAN Fronthaul Spilt Option 7-2x
- O-RAN Fronthaul C/U, Synchronization and Management Planes
- O-RAN Fronthaul Protocols Stack
- O-RAN Fronthaul Delay Management