Addressing the capacity loss question around Dynamic Spectrum Sharing

Dynamic spectrum sharing is seen as a key technology operators will employ to scale out 5G coverage by using software-based scheduling to transmit LTE and 5G NR signals in the same band at the same time. Current Ericsson is the only major vendor with a commercially available DSS solution, dubbed Ericsson Spectrum Sharing, and the vendor said last month 80% of its customers testing DSS will deploy within 12 months.

Swisscom, for example, use DSS to provide nationwide 5G coverage, which amounts to 90% population coverage. In the U.S., Verizon is currently focusing its 5G capex on deploying millimeter wave small cells but has articulated its plans to use DSS for a coverage expansion sometime this year.

To get a better understanding of how the implementation of DSS will impact test and measurement, RCR Wireless News conducted a Q&A with Andreas Roessler, technology manager for Rohde & Schwarz.

Q: You talked to my colleague Kelly Hill about how DSS will impact test and measurement in October at MWC in Los Angeles and said you were working with other companies to verify and validate the technology. How has that been progressing on both the network and device side?

A: The most significant efforts are, of course, with the various chipset manufacturers around the world implementing DSS into their next generation of modems while using our new mobile radio tester platform, the R&S® CMX500. Second, we worked with a leading company that offers a UE simulator for lab-based 5G NR network testing, to mutually validate the implementation of DSS on both ends. The plan was to demonstrate our collaboration efforts on modem testing and network- testing in two demonstrations at the now-canceled Mobile World Congress 2020. These two individual demos would have been the results of the recent months of testing with our partners, demonstrating the readiness of our R&S® CMX500.

In addition to these fundamentals, we worked with, for example, U.S.-based Tier-1 network operators that announced the rollout of DSS back in 2019. We provided feedback to their DSS-related test plans. The service providers will use these test plans to validate DSS functionality on devices before they are allowed on the network. Our team offers guidance on test case definition, support in implementing these test
cases using the R&S® CMX500 toolchain and then validating them with pre-commercial devices powered by the latest generation of 5G modems.

Q: Can you share your perspective on DSS as not just providing an NR coverage layer but also
helping with the transition from non-standalone to standalone?

A: Today, all 5G NR deployments in FR1 are non-standalone mode based and using typically the 3.5 GHz bands, except for Sprint here in the U.S. Other operators based in the U.S. and worldwide face a significant
challenge if they do not have access to this new mid-band spectrum. To be able to provide a 5G NR coverage layer, service providers will clear range. Typically, this means a frequency band that is nationwide available but has only a small impact on the LTE footprint, such as a 5 megahertz channel at 800 MHz range. In this frequency band, the operator can now roll out 5G NR in standalone mode and provide coverage, but they will face capacity issues as they cannot offer high data rates due to lack of additional spectrum available for 5G. This is where DSS comes into play. Activating DSS on the other LTE carriers in the PCS, the AWS spectrum will allow the operator to aggregate those frequency bands with the newly deployed 5G NR SA carrier. Consequently, while the operator is transitioning their subscriber base from LTE towards 5G NR, more and more 5G devices will be on the network, and this where DSS is vital, with the smart algorithm it will balance the resources and thus enabling a smooth transition from NSA to SA mode.

Q: T-Mobile’s President of Technology Neville Ray said on a February earnings call that DSS is late. So what’re you seeing? Is the need for the technology out of sync with the availability and, if so, what does that mean?

A: From my perspective, I would not call DSS late. It is a combination of several sophisticated features that allow coexistence between LTE and 5G NR using the same frequency band. That means its implementation
takes time, and thorough testing of the functionality is required. But our industry seems to have been in a rush for quite a while now. Let us take the feet from the gas pedal and make sure all the features are
thoroughly standardized, implemented, tested, and validated before they are launched to the network and made available to the consumer. In the case of DSS, it is important to make sure it is working properly in the given framework. Otherwise, you may negatively affect your LTE network. This is something that you don’t wish for as a network operator, as it does not only impact your early adopters using 5G devices but the majority of your subscriber base that is still on 4G LTE.

Q: Ray also said DSS “eats away at net capacity of the shared radio.”  Have you observed what he’s talking about?

A: We need to differentiate here. The way that DSS operates, it will reduce the base capacity on the LTE side by a certain percentage, that is true. But otherwise, the coexistence would not work at all. We also
talk here about single-digit percentages. Let me explain: MSBFN broadcast subframes are enabled to allow 5G NR to transmit its synchronization signal blocks (SSB) and tracking reference signals (TRS). Those are mandatory for the 5G device to initially synchronize to the downlink signal and perform initial access procedure. TRS is required to track time and frequency. Based on the latest network configurations I have seen, this means three subframes within 40 ms, in other words, 7.5%. Those subframes would not be available for LTE, but allow you to have a 5G NR device using the same frequency band. I think it is a fair trade-off given the circumstances.

The dynamic aspect comes into play when using standard LTE subframes that are not allocated as broadcast subframes. First, rate matching around the omnipresent cell-specific reference signals of LTE
is required. Second, the scheduler in the base station and its optimized algorithms allow the balancing between LTE-only devices and 5G NR devices present in that serving cell. The way to schedule resources in
time and frequency is, of course, the intellectual property of the network equipment vendor, so there is no standardized way to do this. But the scheduling is influenced by many parameters: cell load, type of traffic,
and thus the quality of service (QoS), priorities and many more. For DSS, the number of LTE devices and 5G devices which are present in that particular cell plays a vital role. The scheduling of resources is
adjusted, based on these numbers. Therefore, it is in my eyes very tough to predict the impact on capacity, as these scenarios are very dynamic as you can imagine.

Q: Big picture, what are you expecting out of DSS this year? Do you think we’ll see it
commercialized in the US and if we do, will this be a smooth rollout, or do you foresee a bumpy road? I’m particularly thinking about the complexities of field testing and site commissioning.

A: There is already news from December 2019 from Switzerland and Australia that local service providers Swisscom and Telstra had the first roaming data call on their 5G NR networks powered by Ericsson
infrastructure, where DSS was enabled. Pre-commercial devices from Oppo were used, powered by Qualcomm’s X55 modem. Experience shows that about six to nine months after such an announcement, the technology is fully ready and commercialized. Therefore, I expect a roll- out of DSS in networks worldwide starting mid- 2020. How bumpy it will be depends on how much operators care to test this functionality. We are ready and have the R&S®C MX500, the right platform and toolchain available to enable the testing of DSS.

From a network testing and commissioning perspective, you need to have the tools available that allow you to measure LTE and 5G NR in parallel on the same frequency band. Luckily, Rohde & Schwarz also has these solutions in its product portfolio. The R&S® TSME6 universal network scanner can measure 5G NR, LTE, and all other legacy technologies in parallel, thanks to the smart handling concept. It was one of the first scanners that supported MBSFN-based measurement in LTE networks. We now reuse this functionality and combine it with 5G NR. This enables the system performance engineering groups of network operators to optimize LTE and 5G while DSS is activated in both networks.

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