Massive MU-MIMO in FDD: ‘Simply outstanding’ in the uplink

In sum, what to know: -Signals Research Group recently conducted what is almost certainly the first independent testing of 5G massive and Multi-User MIMO in FDD spectrum, in a live network. -The tests were conducted across multiple cells in Verizon’s network in St. Paul, Minnesota, where Ericsson is the equipment vendor. -SRG found that Verizon had performance gains from deploying 32T32R massive MIMO, and the addition of MU-MIMO enhanced the uplink even further. However, downlink improvements were spottier. As mobile network operators seek to squeeze ever more performance and capacity out of their existing spectrum, they have turned to massive Multiple-Input Multiple Output (MIMO) systems, and also to the use of Multi-User MIMO (MU-MIMO), which enables devices to share the same network resources, serving up data transmissions to multiple devices at the same time. MU-MIMO is used in Wi-Fi systems, and in a cellular context, it has largely been deployed in Time Division Duplex (TDD) spectrum. Verizon has already embraced the use of Massive MIMO in TDD spectrum (including some notable speed achievements), but it has also been one of the few operators working to advance massive MIMO systems in FDD spectrum. That work seems to be paying off: Signals Research Group has published the results of recent testing where both Massive MIMO and MU-MIMO were in play in Verizon’s network, and found that the resulting improvement in uplink throughput was “simply outstanding.” For Mike Thelander, founder of SRG, the results were notable because FDD presents a tougher challenge for Massive MIMO and MU-MIMO implementation than TDD spectrum. The technology is also still a rarity in real-world networks. “You can probably count on using the fingers on a couple of hands the number of operators that have commercial traffic running over 5G FDD massive MIMO, and even then we believe the number of massive MIMO FDD cell sites is probably fairly modest, especially outside of China,” he wrote in the SRG report. Why is FDD, as Thelander put it, “an entirely new animal” when it comes to MIMO-related advances? Because while TDD operation uses the same channel for uplink and downlink transmissions (meaning, the network can use channel information from one direction to optimize transmissions in the other), FDD relies on different frequencies for the uplink and the downlink—which makes it more challenging to have a good grasp of channel information. To Thelander, that left an open question as to whether MU-MIMO could be effectively implemented in FDD. To answer that question, he revisited an area in St. Paul, Minnesota where SRG had previously tested the performance differences between 4T4R MIMO and 32T32R MIMO on Verizon’s network. At the time of that testing, however, Verizon hadn’t fully implemented the MU-MIMO capabilities in the uplink and downlink for Massive MIMO. What Thelander found in the latest testing is that performance improvements from moving to Massive MIMO were further enhanced by the new addition of MU-MIMO—although the effects were more consistent in the uplink. “When Verizon upgraded from 4T4R to 32T32R, they got significant, significant gains in performance in both the downlink and the uplink, even before you turn on MU-MIMO,” Thelander said. “Now you turn on MU-MIMO, and in the uplink, it’s another big, big boost in terms of performance.” The tests involved both stationary and walk-testing with two Motorola razr fold 2026 smartphones locked to 5G Standalone operations in Band n2 (1.9 GHz), with capture and analysis tools from Accuver Americas and Keysight Technologies. SRG found that uplink MU-MIMO use “occurred nearly all the time, delivering close to four uplink MIMO layers on a consistent basis from the network perspective, even when the two smartphones were collocated” and serving up “very high double-digit gains on a percentage basis” when compared to Single-User MIMO (SU-MIMO) with 32T32R. In short: Massive MIMO had already outperformed 4T4R MIMO, and MU-MIMO improved the uplink performance even more. The downlink, however, was a different story. On one hand, SRG concluded that its earlier testing had underestimated how well Massive MIMO could perform in the downlink with SU-MIMO alone. But Thelander also characterized the gains from downlink MU-MIMO as “hit or miss, relative to what you already got with the Massive MIMO.” The two UEs shared the same network resources with MU-MIMO (known as UE pairing) on some walk-testing routes and produced “solid double-digit gains in spectral efficiency” over 32T32R with SU-MIMO, according to the SRG report. But on other routes, UE pairing was less consistent, leaving “opportunity for improvement,” as Thelander put it. GET A LITTLE CLOSER SRG’s testing also offered some intriguing perspective on how much physical separation is actually required between devices, for uplink MU-MIMO to be useful. “What was surprising is that when I first tried to test it, I literally had two phones sitting next to each other in my vehicle, and they were using MU-MIMO. … They were sharing the same network resources—literally, 18 inches apart,” Thelander said. “Now, I’ve seen that in TDD. I didn’t expect to see that in FDD.” Generally, the more physical separation between UEs, the better that MU-MIMO would be expected to work: More space between devices means that it’s easier for the network to figure out there are multiple devices to be served. But in this case, there wasn’t much spatial separation between the two phones. Yet within 10 seconds, according to SRG, the two phones were sharing the same network resources. And while Thelander adds the caveat that he was testing in an unloaded cell with only a couple of devices, the smooth MU-MIMO use by two devices so close together bodes well for FDD MU-MIMO on a larger scale. “What that tells me is that … even if you had lots of UEs out there and a lot of them are co-located, MU-MIMO in the uplink should still work reasonably well,” he said. You can access a preview of SRG’s report here.