EU RAN 5 call for AAL to get to a fully open RAN

The EU MoU Five's latest paper calls for more openness between accelerator options and vRAN functions.

Five EU operators that have been issuing joint Open RAN requirements since January 2021 have called for vendors to implement further hardware and software decoupling to enable their vRAN software to run on either lookaside or inline accelerators, and ideally be ported between the two architectures.

That approach will rely on an O-RAN Aliance interface known as AAL – Accelerator Abstraction Layer – that is still in the early stages of being specified within the Alliance’s Working Group 6, despite having been identified as an required item in the MoU’s original R1 in 2021.

The aim of AAL, as it sounds, is to act as a layer between the network function (vDu, vCU) the cloud layer and underlying hardware. AAL defines hardware accelerator interface functions and protocols including configuration and management functions, procedures and operations. That includes interfacing to such critical functions as Forward Error Correction and hiPHY protocols for Massive MIMO beam forming.

Vendors have been cautious in opening up some of those interfaces. As Nokia said in February 2023 to TMN, “It’s not the place where you go and tear it apart and throw all the balls in the air and reach for the best. It requires laser focus specification to open that interface. And when it is there, we will be there as well, and actually, meanwhile we will support creating that high quality specification.” (See: nokia-says-its-cloudran-can-go-on-anyran/)

At the moment, vendors are choosing between the two accelerator options to implement their stacks.  Nokia, for example, uses inline acceleration within its AnyRAN architecture using Marvell processors. Ericsson has been publicly committed to lookaside acceleration – which it has also termed selected function acceleration. For operators that divergence is sub-optimal because it creates a point of dependency between the vRAN and the underlying hardware.

Within their Open RAN requirements Release 4, the EU Open RAN MoU operators  said they “require technology and vendor agnostic solutions.”

The Release said that operators should be able to make a choice for a specific solution for RAN HW acceleration relying only on best-of-breed technology regarding performance, energy efficiency, hardware footprint, and technology evolution.

“Thus, MoU operators’ short-term assessment is [the] ecosystem should evolve fulfilling these requirements to facilitate and embrace the principles of Open RAN, such as openness and multi-vendor approach.”

Although the original MoU also called for alignment with AAL, Release 4 added new requirements around Open RAN Cloud Infrastructure on the O2 Interface and Acceleration Abstraction Layer (AAL), which were added according to latest O-RAN ALLIANCEWG6 specifications. The Release added HW/SW requirements for decoupling and abstraction at different levels (HW and SW and among different SW levels, e.g.: CaaS) following the O-RAN AAL approach and the AAL interfaces specifications.

Enhanced requirements on O-Cloud platform and energy efficiency were also included in this new release.

The initial document from DT, Vodafone, TIM, Orange and Telefonica called for a united front on backing Open RAN and said the operators will also define roadmaps towards Open RAN adoption. Later that year they laid out their technical requirements, as promised, in Release One.

In February 2023 the five had moved their focus onto areas such as integration and energy efficiency.