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The Ultimate Guide to Open RAN: Deep Dive Into RU, DU, CU

The Ultimate Guide to Open RAN: Deep Dive Into RU, DU, CU Image Credit: pranodhm/Bigstockphoto.com

The choice of how to split New Radio (NR) functions in the architecture depends on some factors related to radio network deployment scenarios, constraints and intended supported use cases. Three key ones are: 1. A need to support specific QoS per offered services (e.g. low latency, high throughput for urban areas) and real/non-real time applications. 2. Support of specific user density and load demand per given geographical area. 3. Available transport networks with different performance levels, from ideal to non-ideal.

Mobile operators need the flexibility to pick and choose different splits based on the same COTS-based hardware and network components by using different software implementations. Different protocol layers will reside in different components based on fronthaul availability and deployment scenarios. This approach will reduce the cost of operations and TCO for mobile operators.

Higher functional splits are more desirable for capacity use cases in dense urban areas while lower functional splits will be the optimum solutions for coverage use cases. So, while lower functional splits utilize less than perfect fronthauls, there is a greater dependence on fronthaul performance for higher functional splits.

Source: Parallel Wireless

To take full benefit of split architecture that can deliver interoperability, ability to select best-of-breed components and scalability, any solution needs to support 2G, 3G, 4G, 5G baseband functions. For the best latency support requirement, baseband functions decoupled from hardware should be deployed on NFVI or as containers. An MNO can use any VM requirements and/or any orchestration to enable these functional splits.

The future evolution of RAN will be toward dynamic functional splits. While the OpenRAN Controller (aggregator) acts as a mediator between the RAN and core network, the functionality of the RAN will be distributed between DUs and CUs as it is defined in 5G, and this software can be co-located with the aggregator. In different scenarios, these elements can collapse together and create a single physical entity with different virtual functionalities. We will cover Open RAN Controller, Near-real-time Radio Intelligent Controller (Near-RT RIC) and Non-real-time RIC (Non-RT RIC) in our upcoming installments.

Different RAN functional splits work for different use cases. One split might not fit it all. A solution that can support many of technologies including not just 4G and 5G, but also 2G and 3G, is the most attractive to MNOs as it will simplify network management and the overall TCO.

RAN Functional Split 6

The Small Cell Forum (SCF) nFAPI (network FAPI) interface in split 6 is enabling the Open RAN ecosystem in its own way by allowing any small cell CU/DU to connect to any small cell radio unit or S-RU. With 5G FAPI, competition and innovation are encouraged among suppliers of small cell platform hardware, platform software and application software by providing a common API around which suppliers of each component can compete. These interfaces will help network architects by allowing them to mix distributed and central units from different vendors. By doing this, SCF provides an interchangeability of parts ensuring the system vendors can take advantage of the latest innovations in silicon and software with minimum barriers to entry, and the least amount of custom re-engineering.

Source: Small Cell Forum

RAN Functional Split 7

In case of requirements for more delay-sensitive service, based on appropriate fronthaul availability, the MAC-PHY split will be the preferred solution. Option 7 Split architecture is where the DU handles the RRC/PDCP/RLC/MAC and higher PHY functions, whereas the RU handles the lower PHY and RF functions. CU functionality may be embedded with the DU on the same server, or it can be pushed up the network as a virtualized aggregation entity, along with an OpenRAN Controller or aggregator. Option 7 allows operators to take advantage of sharing or pooling gains while maintaining the lowest processing utilizations on both the DU and RU - leading to a very cost-effective solution with a low TCO and an ideal option for a distributed RAN deployment, including Massive MIMOs.

Source: Parallel Wireless

For 5G, this is still being discussed as a potentially valid option in some use cases. Higher splits, as in 7.x, will be the best approach going forward for deploying future mobile networks in different deployment scenarios. It is ideal for 4G and 5G and can support traffic in dense urban areas.

RAN Functional Split 8

Split 8 is based on the industry standard CPRI interface and has been around for a while. With traffic split 8, all functions (from PHY to RRC layers) except for RF are handled by the DU, while the RF layer is located in the radio. But why is this split gaining attention now?

This split is highly effective in 2G and 3G, where traffic rates are much lower (and therefore processing itself is lower, to a certain extent) and can be easily done on an x86 server, while allowing operators to use cost-optimized RUs with minimal logic and processing. The DU and RU should be interoperable with other third party DUs and RUs. The enhancement over the legacy Split-8, is that in order for RUs to run multiple technologies over the same FH interface, they now need to utilize eCPRI instead of the legacy CPRI interface between the RU and DU.

This approach allows for centralized traffic aggregation from the RUs, which in turn enables a seamless migration path from the traditional LTE ecosystem to the NR ecosystem.

Source: Parallel Wireless

The RAN DU sits between the RU and CU and performs real-time L2 functions, baseband processing. In the O-RAN Alliance working group, the DU is proposed to support multiple layers of RUs. To properly handle the digital signal processing and accelerate network traffic, FPGA can be used. But what is important to understand is that hardware acceleration is considered a requirement for 5G but less so in previous technologies like 2G, 3G, and even 4G.

Source: Supermicro

There has also been a focus around hardware accelerators - FPGA and GPU - to accelerate real-time sensitive processing for the lowest layers of the radio baseband for 5G. Ericsson and Nokia are looking at GPU-based acceleration for some vRAN workloads, especially for 5G M-MIMO and for AI. So, there is definitely the need to invest more in different chip technologies to ensure Open RAN can have access to the best DUs available on the market.

Reducing overall TCO will be a priority, and a solution around GP processor architectures to deliver the most efficient and cost-effective compute, storage and network elements will drive the innovation.

Summary

Previous RAN architectures (2G, 3G and 4G) were based on a “monolithic” building blocks, where few interactions happened between logical nodes. Since the earliest phases of the New Radio (NR) study, however, it was felt that splitting up the gNB (the NR logical node) between Central Units (CUs) and Distributed Units (DUs) would bring flexibility. Flexible hardware and software implementations allow scalable, cost-effective network deployments - but only if hardware and software components are interoperable and can be mixed and matched from different vendors. A split architecture (between central and distributed units) allows for coordination for performance features, load management, real-time performance optimization and enables adaptation to various use cases and the QoS that needs to be supported (i.e. gaming, voice, video), which have variable latency tolerance and dependency on transport and different deployment scenarios, like rural or urban, that have different access to transport like fiber.

But what makes any split architecture open and suited for Open RAN? A mobile operator can deploy a fully compliant functional split architecture, but unless the interfaces between RU, DU and CU are open, the RAN itself will not be open - see a diagram below that shows current industry thinking. Based on their experience, Nokia believes that the only valid split is between RU and DU. Time will tell if integration of one vendor’s DU with another vendor’s CU will deliver flexibility and savings.

Source: Nokia

In summary, RAN Functional splits will bring cost savings if interfaces between hardware and software components are open.

Author

Eugina, a female executive and an immigrant, started her telecom career as a secretary and now has gone on to become the CMO of the prominent industry organization, Telecom Infra Project (TIP).

She has over 20+ years of strategic marketing leadership experience, leading marketing and communications for small and Fortune 500 global technology companies like Starent and Cisco.

Previously, she served as the VP of Marketing of the major telecom industry disruptor Parallel Wireless and was instrumental in creating the Open RAN market category.

She is a well sought-after speaker at many technology and telecom events and webinars. She is a well-known telecom writer contributing to publications like The Fast Mode, RCR Wireless, Developing Telecoms and many others.

She is also an inventor, holding 12 patents that include 5G and Open RAN.

She is a founding member of Boston chapter of CHIEF, an organization for women in the C-Suite, to strengthen their leadership, magnify their influence, pave the way to bring others, cross-pollinate power across industries, and effect change from the top-down.

Her passion is to help other women in tech to realize their full potential through mentorships, community engagement, and workshops. Her leadership development book “Unlimited: How to succeed in a workplace that was not designed for you” is due for release in May 2023.

Ms. Jordan resides in Massachusetts with her husband, teenage son, and three rescue dogs. She loves theater and museums. She volunteers for dog rescues and programs that help underprivileged children and women.

Ms. Jordan has a Master’s in Teaching from Moscow Pedagogical University, and studied computer undergrad at CDI College in Toronto, Canada.

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