by: Monica Alleven
Dynamic spectrum sharing (DSS) garnered a lot of attention this year, underscoring its complexities and a bit of mystery.
In February, T-Mobile President of Technology predicted it would be a tough year for the technology. Flash forward, and rivals AT&T and Verizon are now using DSS for lower band 5G coverage. T-Mobile doesn’t need it as much because it’s got 5G deployed on its 600 MHz spectrum.
Asked about DSS, Verizon CTO Kyle Malady recently said they’re seeing the types of speeds and performance that they expected. Verizon revealed the launch of DSS the same day Apple announced its first 5G phone, the iPhone 12. DSS allows operators to host 4G LTE and 5G services in the same spectrum band.
“We’ve been spending a lot of time on this with our vendor partners with Ericsson, Nokia, Samsung as well as Qualcomm, and our OEMs like Apple. So we’ve been spending a lot of time on it, and it’s working well,” Malady said at a Wells Fargo investor event. “We’ll still be optimizing and working on it as it is new, but it is absolutely meeting our targeted engineering requirements at the moment.”
Bottom line: It’s a work in progress. Signals Research Group (SRG) on December 1 published a report on its DSS study in which the firm said they’re fans of DSS due to the flexibility it provides operators when migrating from LTE to 5G New Radio (NR), but disappointed by the performance it currently delivers.
The analysts also noted that any low band 5G NR deployment, with or without DSS, can’t deliver data speeds that most consumers associate with 5G. T-Mobile is aggressively rolling out 5G using the 2.5 GHz spectrum it acquired with Sprint, and with that, it has the ability to change the 5G landscape, according to SRG.
Mike Thelander, CEO and founder of SRG, said he agrees with Ray when it comes to performance issues related to dynamic spectrum sharing. “DSS definitely has its challenges,” Thelander told Fierce.
The converse of that is if an operator didn’t have DSS at its disposal, it would be stuck and have to turn off LTE in order to turn on 5G, and that’s not a good situation. “DSS is kind of that tool that allows them to make a migration from 4G to 5G data traffic, but definitely today, there’s a penalty to be paid.”
SRG tested DSS on Verizon’s network in two markets: Minneapolis where Ericsson is the vendor and Oklahoma City where Nokia is the vendor. In Plano, Texas, they tested DSS on the AT&T network that uses Ericsson equipment.
AT&T is an example of an operator that uses DSS only when it needs to use it. Gordon Mansfield, AT&T’s VP of Mobility & Access Architecture, said during a recent Fierce 5G virtual event that DSS is a great tool that AT&T and others will continue to use.
In its report, SRG noted there are at least two ways of deploying DSS. In Verizon’s markets where Ericsson is the vendor, it’s using something called Cell Reference Signal (CRS) rate matching. In its Nokia markets, it’s using Multicast Broadcast Single Frequency Network (MBSFN).
According to Thelander, CRS is what operators want because it’s the most dynamic in terms of how they can allocate capacity between LTE and 5G as the traffic shifts. “If you’re going to allocate spectrum or capacity between 4G and 5G, you want it to be as dynamic as the traffic in your network. That’s why the way Ericsson is doing it is what operators want. But it comes with this penalty, right now, due to interference,” he said.
Pros & cons
Earlier this year, T-Mobile’s Ray said one of the major network equipment vendors was “very late” when it came to DSS, and while he never identified the vendor by name, many inferred it was Nokia.
If it was Nokia, it’s not copping to it. In early February, some vendors were more aggressive than others in marketing their DSS solutions, but in practice, no one had a commercial solution ready for the U.S. market; that came later in the year, noted Sandro Tavares, global head of Mobile Networks Marketing for Nokia.
In fact, he said Nokia has solutions available for both MBSFN and CRS rate matching, and Verizon can use CRS rate matching in Nokia markets anytime it decides to go in that direction. The Finnish vendor has DSS trials all over the world, but the deployment of commercial DSS is most advanced in the U.S. market.
“They both have their pros and cons,” Tavares said of MBSFN and CRS rate matching. The iPhone 12 – Apple’s first 5G iPhone – supports both flavors of DSS.
No 5G devices on the market have the ability of canceling the CRS interference, and that’s why there are performance issues on CRS rate matching. CRS cancellation was not implemented on the 5G chipset because there’s no CRS with 5G, but with DSS, it enters the picture, and that’s where the problem happens. That applies to all chipsets regardless of manufacturer, he said.
Putting them side by side, “we see that CRS rate matching usually has 5G performance losses compared to MBSFN,” and if you get to a place with a lot of interference, this loss adds up pretty quick. There are ways to optimize that, such as reducing the CRS signal power on the neighbor cells. But, “it’s a trade off,” he said, and it’s up to the operator to decide what to do. “Our customers have both options available.”
The problem with MBSFN is it’s not as dynamic. “I would call it semi-static, if you will,” Thelander said. There’s a lack of flexibility there in that spectrum may be getting dedicated to 5G when there is no 5G traffic.
In summary, “5G NR DSS with CRS rate matching will likely become the long-term DSS solution of choice for most operators, or at least this statement is true for AT&T and Verizon,” SRG said in its report. “However, the performance issues we observed in our testing will need to be resolved, especially when operators deploy 5G NR DSS in their mid-band spectrum which carries the bulk of the data traffic. 5G NR DSS with MBSFN shouldn’t have interference-related issues, but there are inherent inefficiencies associated with using a semi-static solution to handle the dynamic mix of 5G NR and LTE traffic. Pick your poison!”
T-Mobile has said it will use DSS, but it will do so strategically and it doesn’t need it for a nationwide 5G coverage layer because it’s already got that with 600 MHz.
“My sense is that T-Mobile doesn’t necessarily have as good an LTE network in a lot of markets as the other two operators, so they need 5G to be successful,” Thelander said. “That’s why I think they’re focusing more so on ‘our 5G network is wonderful’ – to some extent, it has to be because I don’t know that their LTE network is as great as it should be.”