O-RAN’s split-RAN concept is a way of designing and deploying radio access networks (RANs) that disaggregates the RAN into multiple functional components. These components can be deployed on different hardware and software platforms and can be interconnected using open interfaces. This disaggregation and openness provide a number of benefits, including:
- Flexibility: Split-RAN networks are more flexible than traditional RANs, which are typically based on monolithic baseband units (BBUs). This flexibility allows network operators to tailor their networks to their specific needs, and to deploy new features and services more quickly.
- Cost: Split-RAN networks can be more cost-effective than traditional RANs, as they allow network operators to use a wider range of hardware and software vendors. This competition can drive down prices.
- Performance: Split-RAN networks can offer better performance than traditional RANs, as they allow network operators to place different RAN functions on the hardware and software platforms that best suit them. For example, real-time functions can be placed on high-performance hardware, while non-real-time functions can be placed on lower-cost hardware.
There are eight (8) known ways to functionally split the RAN, each of which proposes a different way of dividing up the processing of the protocol stack across different hardware components. The most common split is the 7.2x split, which divides the protocol stack into two parts:
- O-RU (O-Radio Unit): The O-RU is responsible for the physical layer (PHY) processing, including RF signal processing and analog-to-digital conversion.
- O-DU (O-Distributed Unit): The O-DU is responsible for the higher-layer processing, including MAC, RLC, and PDCP.
The O-RAN Alliance has defined a multi-vendor fronthaul interface between the O-DU and O-RU based on the 7.2x split. This interface supports the control, user, synchronization (CUS), and management (M) planes.
Other split options include the 7.1 split and the 7.3 split. The 7.1 split places all of the PHY processing in the O-RU, while the 7.3 split places all of the PHY processing in the O-DU. These splits are less common than the 7.2x split, but they may be used in certain cases, such as when deploying networks in rural areas or when using specialized hardware.
O-RAN’s split-RAN option 7.2x splits the physical layer (PHY) into a high-PHY and a low-PHY. This split is designed to achieve a balance between flexibility, cost, and performance.
The high-PHY is responsible for the most computationally intensive PHY functions, such as the fast Fourier transform (FFT), digital beamforming, and precoding.
Below are the listed functions of High-PHY:
- FFT (fast Fourier transform): The FFT is a mathematical algorithm that converts a signal from the time domain to the frequency domain. The time domain is a representation of the signal over time, while the frequency domain is a representation of the signal in terms of its different frequency components.
The FFT is a very efficient algorithm for performing this conversion, and it is used in a wide variety of signal processing applications, including:
- Filtering: The FFT can be used to design filters that can remove specific frequency components from a signal. For example, a filter could be used to remove noise from a signal, or to isolate a particular frequency band.
- Equalization: The FFT can be used to design equalizers that can compensate for the distortion that is caused by the radio channel. This can improve the quality of the received signal.
- Demodulation: The FFT can be used to demodulate certain types of modulated signals, such as OFDM signals.
- Digital beamforming: Digital beamforming is a technique that focuses the transmitted or received signal in a particular direction. This can be used to improve signal quality and reduce interference.
- Digital beamforming works by combining the signals from multiple antennas in a way that reinforces the signal in the desired direction and cancels out the signal in other directions.
- Digital beamforming is used in a variety of applications, including:
- Radar: Digital beamforming is used in radar systems to focus the transmitted signal in the direction of the target. This improves the ability of the radar to detect and track targets.
- Cellular networks: Digital beamforming is used in cellular networks to focus the transmitted and received signals in the direction of the intended user. This improves signal quality and reduces interference.
- Wi-Fi networks: Digital beamforming is used in Wi-Fi networks to improve signal quality and reduce interference.
- Precoding: Precoding is a technique that is used to improve the performance of MIMO systems. MIMO systems use multiple antennas to transmit and receive signals.
- Precoding involves pre-processing the transmitted signal in a way that minimizes interference between the different MIMO streams. This can improve the throughput and reliability of the MIMO system.
Here is an example of how precoding can be used to improve the performance of a cellular network:
Suppose a cellular base station has two antennas and is serving two users, A and B. The base station transmits different data streams to A and B. However, the two data streams will interfere with each other.
Precoding can be used to reduce this interference. The base station can pre-process the transmitted data streams in a way that minimizes the interference between the two streams. This can improve the throughput and reliability of the cellular network for both users A and B.
The low-PHY is responsible for the less computationally intensive PHY functions, such as channel estimation and equalization.
Below are the listed functions of Low-PHY:
- Channel estimation: Channel estimation is the process of estimating the characteristics of the radio channel. This information is used to design equalizers and other signal processing algorithms that can improve the performance of the communication system.
- Channel estimation can be performed in a number of different ways. One common method is to transmit known pilot symbols and then compare the received pilot symbols to the transmitted pilot symbols to estimate the channel characteristics.
Here is an example of how channel estimation can be used in a cellular network:
A cellular base station transmits a known pilot symbol to all of the users in the cell. The users then measure the received pilot symbol and send the measurement back to the base station. The base station uses the measurements from all of the users to estimate the channel characteristics for each user.
The base station can then use this information to design equalizers and other signal processing algorithms that can improve the performance of the cellular network for each user.
- Equalization: Equalization is a technique that is used to compensate for the distortion that is caused by the radio channel. The radio channel can distort the transmitted signal in a number of ways, such as by causing attenuation, delay, and phase distortion.
- Equalizers are signal processing algorithms that can compensate for these distortions. Equalizers work by filtering the received signal in a way that minimizes the distortion.
Here is an example of how equalization can be used in a cellular network:
The radio channel can cause the transmitted signal to be attenuated, delayed, and phase shifted. This distortion can make it difficult to recover the original transmitted signal from the received signal.
The cellular base station can use an equalizer to compensate for this distortion. The equalizer filters the received signal in a way that minimizes the distortion. This allows the base station to recover the original transmitted signal with a high degree of accuracy.
- Modulation: Modulation is the process of converting a digital signal into an analog signal that can be transmitted over the air.
- There are a number of different modulation schemes that can be used. One common modulation scheme is quadrature amplitude modulation (QAM). QAM works by representing each digital symbol as a complex number. The complex number is then mapped to a point on the I/Q plane.
- The I/Q plane is a two-dimensional plane that represents the in-phase and quadrature components of the transmitted signal. The in-phase component is the real part of the signal, and the quadrature component is the imaginary part of the signal.
- The modulated signal is then transmitted over the air. The receiver demodulates the signal to recover the original digital signal.
Here is an example of how modulation can be used in a cellular network:
The cellular base station modulates the digital data stream that it wants to transmit to the users in the cell. The base station uses a modulation scheme such as QAM to modulate the data stream.
The modulated signal is then transmitted over the air to the users. The users demodulate the signal to recover the original digital data stream.
- Demodulation: Demodulation is the process of converting a received analog signal back into a digital signal.
- The demodulation process is the reverse of the modulation process. The receiver demodulates the received signal by filtering the signal in a way that recovers the original digital symbols.
Here is an example of how demodulation can be used in a cellular network:
The cellular users demodulate the received signal to recover the original digital data stream that was transmitted by the base station. The users use a demodulation scheme such as QAM to demodulate the signal.
The demodulated signal is then processed by the users to extract the desired information.
- CP removal: Removes the cyclic prefix from the received signal.
- FFT: Converts the received signal from the time domain to the frequency domain.
- Digital beamforming: Focuses the received signal in the direction of the intended user.
- Prefiltering: Prefilters the received signal for the PRACH channel.
- The rest of the UL PHY processing is performed in the DU. This includes functions such as channel estimation, equalization, and decoding.
- iFFT: Converts the transmitted signal from the frequency domain to the time domain.
- CP addition: Adds a cyclic prefix to the transmitted signal.
- Precoding: Precodes the transmitted signal to improve performance.
- Digital beamforming: Focuses the transmitted signal in the direction of the intended user.
- The rest of the DL PHY processing is performed in the DU. This includes functions such as channel estimation, equalization, and modulation.
- Reduce bandwidth usage and increase virtualization in gNB CU and gNB DU. – Use less bandwidth and save money by running more functions in the centralized unit.
- Enable simple and affordable RRU designs for widespread adoption. – Make it easier and cheaper to deploy more radio units, which will improve coverage and capacity.
- Maintain performance parity with integrated solutions with perfect fronthaul. – Even though the radio unit and centralized unit are now separate, the network should perform just as well as if they were integrated.
- Remove limits on receiver architecture for performance. – Allow network operators to use the best receiver designs for their needs, without being limited by the capabilities of the radio unit.
- Eliminate the need to redesign for NR, unlike LTE. – Make it easy to upgrade to NR without having to redesign the entire network.
- Provide greater scalability with a fixed-rate streaming interface, were transport data rate scales with traffic and bandwidth. – Allow the network to handle more traffic without having to upgrade the transport infrastructure.
- Centralize scheduling for improved efficiency and reduced interference. – Improve network performance and reduce interference by having the centralized unit schedule all downlink transmissions.
- Support advanced signal processing techniques such as UL compression. – Use advanced techniques to improve network performance and reduce the amount of data that needs to be transmitted over the transport interface.