How MU-MIMO could change the FWA game for T-Mobile and Verizon

A recent study highlighted a new flavor of 5G hiding in plain RF sight at T-Mobile that should yield a significant capacity upgrade to its fixed wireless access (FWA) service – and to that of other carriers.

Drawing on field tests done in December, Signals Research Group found that a combination of Sounding Reference Signal (SRS) beamforming and Multi-User Multiple Input Multiple Output (MU-MIMO) yielded throughput gains as high as 75% compared to single-user MIMO, taking average throughput from 901 Mbit/s to 1,576 Mbit/s.

"That is significant," said Michael Thelander, SRG president and founder. "If an operator can get 10% capacity gains for a little effort, they would love it."

The Independence, Minnesota, firm performed this research using OnePlus 10 Pro and Galaxy S22 smartphones as well as a not-yet-shipped FWA receiver from Arcadyan. T-Mobile provided technical support, including guidance about where to find its Ericsson MU-MIMO n41 cell sites in Southern California.

This capacity boost withstood having receivers in close proximity, Thelander emphasized.

"Without MU-MIMO, if a second mobile device started receiving data, basically they would share that speed," he said. With it: "You get your second phone, and your peak data speed does remain the same."

Don't move!

SRG posted a preview of these findings in a December 16 edition of its Signals Flash! newsletter, then published details in a January 10 issue of its Signals Ahead report. That longer publication also revealed hints of T-Mobile testing a new TDD frame structure that increases downlink capacity.

The FWA-relevant portion of the firm's research comes in its finding that the call-and-response protocol of SRS beamforming (Thelander described the dialog between cell site and user equipment to optimize a signal as, "you yell Marco, they come back Polo") worked better with receivers that moved less.

That could mean very good things for a FWA business that has been booming for T-Mobile – the carrier now expects to have 7 to 8 million customers by 2025 and has been considering deploying millimeter wave spectrum to augment FWA capacity in urban areas.

"If you're fixed, the CPE never moves," Thelander said. "And so multi-user MIMO tends to work better."

Plus, when carriers already provide fixed-wireless customer premise equipment to subscribers for free, it's easier for them to push upgrades than it is to coax customers to upgrade handsets with their own money. Thelander said that while "certain iPhone models support it," he wasn't sure about the overall level of MU-MIMO support among current smartphones.

T-Mobile hasn't spoken much about its deployment of MU-MIMO since a 2020 press release about a test of the technology that delivered 5.6 Gbit/s speeds over 2.5GHz 5G – a distinct contrast to its publicity over its midband 5G buildout.

But a spokesperson confirmed that the carrier's current FWA CPE supports MU-MIMO. Older customer equipment does not, however, and the company is not moving to replace that with MU-MIMO-compliant gear.

The T-Mobile publicist also said the carrier would finish upgrading non-Ericsson basestations to support SRS and MU-MIMO by the middle of 2023.

Not just T-Mobile

SRG's December research only covered T-Mobile sites, but earlier testing surfaced Verizon 5G service near Minneapolis with performance characteristics that matched MU-MIMO, as covered in a September 27 Signals Flash! issue.

"I was doing software testing in Minneapolis back in September, and I discovered that Verizon was trying multi-user MIMO on their network," Thelander said in January. "I don't know where they are in terms of rolling it out."

Verizon has been as quiet as T-Mobile in terms of public announcements of MU-MIMO deployment, but a corporate publicist confirmed that the carrier has lit up both beamforming and MU-MIMO.

"Yes, we rolled MU-MIMO and beamforming out to a large majority of our network, and we definitely see service gains from these technologies both in mobility and FWA," spokeswoman Karen Schulz emailed. "We will continue rolling it out this year throughout the rest of our network."

She did not specify which of Verizon's fixed-wireless CPE models supported these upgrades.

Thelander noted some limits of MU-MIMO, saying "You need relatively decent signal conditions to achieve it." But the capacity gains in those places come at vastly lower costs than the traditional tools of carriers: "new cell sites or buying new spectrum, that's the expensive side of the house."

He voiced confidence that this cost-benefit calculation will lead every carrier to deploy these upgrades.

"If they're not doing that or trialing it today, then I can guarantee they will very soon," he said. "It's a simple – simple is in quotes – software upgrade, and as long as you have the backhaul in place, then you've got it."

– Rob Pegoraro, special to Light Reading. Follow him @robpegoraro.