Dynamic Spectrum Sharing

We are finally starting to see a glimpse of some of the 5G specifications being introduced into cellular networks. AT&T announced recently that it was going to be testing dynamic spectrum sharing (DSS) in the network, introducing one of the major benefits of 5G. The company will be testing the technology somewhere in north Texas.

DSS technology allows a cellular carrier to use a block of spectrum for both 4G LTE and 5G customers. Currently, each carrier has set up entire separate spectrum blocks for 5G customers that are only used for 5G. As more people buy 5G phones, those new spectrum blocks will get crowded and could eventually perform worse than 4G.

With DSS, a carrier can use any block of spectrum in the manner being requested by a customer handset. A handset seeking a 4G connection will get a traditional 4G channel of bandwidth. If a handset asks for a 5G connection, then that phone can connect as 5G and can provide whatever 5G features are available at a cell site.

I can’t say for sure because I may have missed announcements, but this is the first introduction of a 5G feature that I have noticed. Until now there has been no true 5G. There have been 4G LTE phones that used traditional bands of spectrum and 5G phones that have used new bands of spectrum – but the 5G phones don’t yet benefit from any of the upgrades due to the 5G specifications that will make 5G superior to 4G. Instead, customers have been reporting a good 5G experience because their phone grabs a relatively unbusy band of new spectrum that isn’t as crowded as 4G has become.

DSS is important to cellular carriers because it will make all of their spectrum available to 5G customers. This will allow for a smooth transition over time where 5G customers can address 5G features from any spectrum band and where 4G LTE customers continue to use those same spectrum bands in the traditional manner.

This is only a trial, and most trials uncover issues that need to be tweaked, so it’s likely to be much later this year or next year before AT&T is ready to roll out DSS in other markets.

We are still waiting for the most important 5G features to be introduced. The most powerful feature is going to be frequency slicing where a cell site can easily tailor the transmission path to meet the demands of a customer. This is the feature that will be able to devote a large data path to a heavy data user. The real benefits of 5G can’t be realized until the 5G features have been widely introduced and also loaded into cellphones.

The timing of getting features into cellphones is one of the biggest challenges for the cellular carriers. For the DSS trial to work there has to be sufficient handsets in north Texas that can use the feature. This means that carriers need to introduce features into handsets in advance of introducing them in the field, and I have to imagine that phones blessed by AT&T already have some version of DSS software included.

Something that the cellular carriers aren’t going to be talking about is that for many years the feature sets in the software for handsets will differ by carrier. It’s not going to be an easy transition to move a 5G phone between AT&T and T-Mobile for a while since they will use 5G differently. Eventually, everybody will provide all of the features, but until they do there will be differences in 5G performance between carriers.

The primary advantage of DSS is that the carriers don’t have to keep turning up new bands of spectrum to keep 5G customers happy with the experience. No matter where a customer is, with DSS a 5G phone will be able to grab spectrum – although if they grab 4G spectrum they end up back in the same crowded pot of traditional spectrum with all of the 4G customers. If carriers decide to give 5G customers any sort of priority in traditional spectrum bands, then we’ll see performance start to degrade over time for people with 4G LTE handsets. That sounds like a good strategy to get people to upgrade phones.

From a customer performance perspective, this is not an exciting announcement. This early 5G feature doesn’t give anybody faster performance. The only real benefit is to the carrier to let them use their full range of spectrum for a 5G customer.

Meanwhile, the 5G marketing machine that has snowed every politician in Washington DC will continue to crow about how great 5G is for America, while at the same time complain how we’re losing the 5G battle with the Chinese. Those are both false narratives because there aren’t yet any customers in the US getting a real 5G experience. Even folks in center cities that are using the fast millimeter wave spectrum don’t have 5G – they just have handsets that use the faster millimeter wave spectrum.

But the introduction of a 5G feature into the wild means the day will come when real 5G will hit the network and customers will begin to see a difference between 4G and 5G. By then, the marketing machine will probably be off talking about 6G.

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One of the first 5G specifications that will be implemented in cell sites is dynamic spectrum sharing (DSS). This technology allows a cellular carrier to use a block of spectrum for both 4G LTE and 5G customers. Without DSS, a carrier could only use 5G in existing blocks of spectrum by subdividing the spectrum and allocating a portion of the spectrum specifically and exclusively to 5G. Since 4G networks are already overloaded today it’s not practical to subdivide existing spectrum.

With DSS, a carrier can use spectrum in the manner being requested by a customer handset. A handset seeking a 4G connection will get a traditional 4G channel of bandwidth. If a handset asks for a 5G connection, then that phone can connect as 5G and can provide whatever 5G features are available at a cell site.

For now, there are almost no 5G features implemented in cell sites, so at first a 4G and 5G connection will be nearly identical. Over time 5G features will be coming. The most powerful 5G feature will be frequency slicing where the cell site provides each customer with the bandwidth they need. If somebody only needs a tiny sliver of bandwidth, then the cell site can operate more efficiently by not wasting a full 4G channel on the customer. If a customer wants to download a large file, the cell site will provide a burst of high bandwidth by assigning more than a typical 4G channel of capacity.

This means that DSS needs to be one of the earliest features introduced because it prepares a cell site for the introduction of other 5G features by allowing 5G to be used across multiple bands of spectrum. A carrier doesn’t have to use DSS if they instead plan to use separate bands of frequency for 5G and 4G.

Ericsson and Qualcomm had their first successful test of DSS in September and they think the technology will be made available to US cellular providers sometime in the second half of 2020.

Like all 5G features, DSS will only work with 5G phones that are enable to use the technology. This means all of the phones being sold as 5G today won’t have the DSS capability. The cellular carriers are handling early 5G deployment by having 5G phones only use new blocks of spectrum. T-Mobile is using 600 MHz spectrum and AT&T is using 850 MHz. All the big carriers are also offering separate millimeter wave spectrum phones. Until DSS is implemented, 5G phones will be routed to use the new blocks of spectrum.

To some degree, the cellular world is gaming the system to make 5G feel better than 4G. For now, callers connecting to one of the largely empty new spectrum bands will have a guaranteed full-bandwidth connection, due mostly to there being few other customers using the spectrum at the same time. As the dedicated 5G spectrum bands get busier, the 5G connection experience will begin to feel more like 4G. The same thing happened when 4G was introduced and for the first 6 months of the new 4G technology, customers reported fast 4G data speeds that still can’t be matched today.

DSS can allow carriers to play the same game with existing bands of spectrum. The first 5G customers to make a DSS connection using the existing 4G spectrum can be given priority routing to make sure they have a high-quality connection.

Most of the 5G features that will make 5G special are still three years or more into the future. For now, the carriers are selling 5G phones that don’t include 5G features. The carriers are instead placing the supposed 5G customers into new spectrum bands to give them a good data experience. Over time, as 5G features are introduced, 5G should have better performance than 4G. What the carriers are not telling the public, though, is that the majority of cellular connections made for the next decade are going to be 4G.

5G will be introduced in fits and starts. Carriers will release a 5G feature, but when first introduced almost no handsets will be able to use it. The first customers with phones enabled for a given feature will have a great experience. Over time, as more people use the new 5G features, the higher traffic volumes will lower the performance seen by the early adapters. We’re going to go through this cycle over and over until a decade from now the handsets sold will include the full set of 5G features. The uneven performance for 5G will baffle customers when they don’t get the same features and experience as others that own slightly newer phones.

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