With industry consensus around the Cat-B ULPI open fronthaul interface standard, the Open RAN ecosystem is ready to scale with massive MIMO support
Open RAN is often described as a movement that reinvents the radio access network (RAN) as a platform for innovation but, at its core, Open RAN is a set of standards developed by the O-RAN Alliance that define interoperability between different hardware and software elements in the RAN. In the 5G era, the open fronthaul interface linking the baseband and radio, especially massive MIMO radios needed to support the performance 5G promises, has come to the fore of industry dialogue.
Ericsson, a major contributor to the O-RAN Alliance, has deployed more than 1 million Open RAN-ready radios, and recently won a major multi-year deal with AT&T that’s all about leveraging Open RAN for network monetization and OPEX reduction. And with recently achieved industry consensus around the Cat-B Uplink Performance Improvement (ULPI) specification, the Open RAN ecosystem seems ready for a rapid scale up and out.
The big issue here is that communications service providers (CSPs) are leading 5G deployments with the use of mid-band spectrum, particularly the 3.5 GHz band, to deliver to customers an optimal balance of coverage and capacity. Massive MIMO radios feature dense transit and receive radio arrays to more effectively serve more traffic from fewer sites. So for the disaggregation of the RAN to happen, there needs to be an open fronthaul specification that allows multiple-vendors to interoperate at a distributed radio site featuring massive MIMO.
These advanced radio systems bring with them significant Layer 1 and 2 computing needs which can be done in a number of parts of the RAN, but performing particular RAN functions in particular places can either enhance or degrade system performance while also influencing the degree of interoperability that’s possible. But, to offer the option, the O-RAN Alliance agreed to split the Cat-B ULPI specification into two variants, Cat-B ULPI-A and Cat-B ULPI-B. The former of those is a lower-layer functional split that Ericsson will introduce in its Cloud RAN portfolio this year.
According to experts at the company, Cat-B ULPI-B “best enables deployments in dense environments such as massive MIMO networks while also providing the highest level of interoperability and minimizing additional fronthaul costs.” The new spec “makes it possible to deploy Open RAN at scale without compromise on performance and at optimal fronthaul costs.”
One of the main selling points of Open RAN is architectural flexibility—radio infrastructure and functions can be flexibly located to suit the needs of a particular deployment. Because most CSPs currently need support for massive MIMO deployments, and given the push towards RAN disaggregation, this was an obvious focus area. The difference in the two variants of the Cat-B ULPI specification is critical. In the Cat-B ULPI-A specification, the equalizer function is hosted in the open radio unit (O-RU) rather than the open distributed unit (O-DU) which is where it’s placed in the Cat-B ULPI-B specification.
Back to Ericsson’s experts: This “solves the original problem of implementing massive MIMO radio[s] in Open RAN with existing fronthaul capacity, while also enabling easy multi-vendor interoperability thanks to a clear separation of concerns between O-RU and O-DU. Indeed, it allows [minimization of] fronthaul bitrate by transmitting only the user data layers between the O-RU and the O-DU.”
As for the $14 billion, five-year Open RAN deal with AT&T, it stands as a major testimony to Ericsson’s commitment to Open RAN that will likely influence the trajectory and pace of global adoption. Speaking of the win, Ericsson CEO Börje Ekholm said, “High-performance and differentiated networks will be the foundation for the next step in digitalization…Through this shift, and with open interfaces and open APIs, the industry will see new performance-based business models, creating new ways for operators to monetize the network.”
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