Operator Survey: Open RAN splits for 5G massive MIMO

Massive MIMO plays an important role in 5G mid-band systems by increasing cell capacity and cell edge performance. This technology requires deep R&D expertise to implement. A particular challenge in open RAN systems is that massive MIMO requires close coordination — and therefore very well-defined interoperability — between the open radio unit (O-RU) and the open distributed unit (O-DU).

To optimize the performance of massive MIMO, there is debate about the optimal split between O-DU and O-RU functions and, in particular, where lower Layer 1 functions should reside. To support massive MIMO, the O-RAN Alliance specifies the RU-DU split over an open fronthaul interface known as "7-2x (Category B)." This interface is well-supported by vendors, and there are now several suppliers of massive MIMO-capable O-DUs and O-RUs and several examples of this multi-vendor architecture in commercial operation. Outside of Japan, where DoCoMo (brownfield) and Rakuten (greenfield) have implemented massive MIMO open RAN networks, commercial multi-vendor deployments are small scale or in the trial or planning phases.

Some argue that better uplink performance can be achieved if part of the Layer 1 function is moved to the O-RU so that the RU can make faster decisions on how best to schedule and process transmissions — particularly uplink transmissions. This is important in mid-band systems because improving uplink performance extends the effective range of the cell, which in turn has implications for cell density, RAN economics and the customer experience. There is not yet an open fronthaul profile for this architecture specified by the O-RAN Alliance. However, there is a new RAN interface called uplink performance improvement (ULPI) in development, with a specification scheduled for November 2023, which will enhance the existing "7-2x" open fronthaul profile.

One of the questions in the new Heavy Reading 2023 Open RAN Operator Survey seeks to understand how important the ULPI profile is for massive MIMO in open RAN deployments. With the proviso that it is a complicated topic, the results shown in the figure below have something for both sides of the debate. Broadly speaking, operator respondents are positive about ULPI and would like to see it specified to improve the performance of massive MIMO. About a third (36%) believe their company will wait for the ULPI specification before they deploy massive MIMO for open RAN.

Close to half of respondents (47%) would like ULPI but will deploy massive MIMO using the existing 7.2b split in the meantime. In Heavy Reading's one-to-one discussions with operators, this is the most popular view. In combination with the 7% that believe 7.2b "meets our needs," the survey therefore shows the majority of respondents plan to move ahead with massive MIMO deployment in open RAN before ULPI is available.

The Heavy Reading 2023 Open RAN Operator Survey can be downloaded in full for free here.

For massive MIMO open RAN deployment, does your organization think there is a need for an updated open fronthaul specification with a new split option for uplink performance improvement (ULPI)?

Figure 1:
(Source: Heavy Reading)

Aside from the technical merits of each RAN split, a critical factor in the operator's decision is determining the timing of when it needs massive MIMO in an open RAN deployment and when compatible equipment will be available. For technology suppliers, ULPI may mean they require updated RU and DU silicon, updated software and multiple rounds of further interoperability testing. The ULPI product ecosystem will follow specification completion, targeted for November 2023.

In conclusion, operators are broadly supportive of UPLI for open RAN, but many will move ahead with massive MIMO in the meantime using existing specifications.

The Heavy Reading 2023 Open RAN Operator Survey can be downloaded in full for free here. This is the fourth in a series of open RAN surveys previously conducted in the fall of 2018, summer of 2020 and fall of 2021.

This blog is sponsored by Ericsson.