If every second counts, then, with 5G, every millisecond will count. That's because the move from 4G to 5G will happen dynamically, and it will be counted in milliseconds.

Both AT&T and Verizon confirmed that they're going to migrate traffic from their current 4G LTE networks to their nascent 5G networks by using a technology called "dynamic spectrum sharing." This technology essentially will allow 4G and 5G users to take turns using the exact same chunk of spectrum. Initially, the line of 4G users will be much longer, so they'll take most of the turns. But as the number of 5G users grows, they'll take an increasing number of turns using a given chunk of spectrum.

The result though will be very spectrally efficient, because no matter who is taking their turn, the spectrum will be working rather than lying fallow.

This type of dynamic sharing is a big change from how the wireless industry transitioned from 3G to 4G. In that transition, operators had to carve out a chunk of spectrum that was dedicated to 4G, regardless of how many 4G users they actually had. And, as a result, 3G users were left with a dwindling amount of spectrum as carriers moved customers onto 4G and dedicated a growing amount of spectrum to those customers.

"If you just have a dedicated, static system, you're just going to be inefficient," explained Mike Haberman, VP of Verizon's network engineering, in a recent interview with Light Reading. "The minute I reserve capacity for [5G] NR, I take it away from LTE."

Haberman said that Verizon will use dynamic spectrum sharing to ease 5G customers onto its existing spectrum bands. The operator won't have to use the technology on its millimeter-wave spectrum bands because no other users are on those bands, but the operator will use it on its low- and mid-wave spectrum bands, bands that it's currently using for its LTE services.

Dividing up the time
So how will dynamic spectrum sharing work? "It's divided in time but it's alternating in formats," Haberman explained.

He said that, unlike Verizon's old CDMA technology, LTE is based on the time domain, which means that communications are broken down into 10 ms sections. 5G, meantime, also uses the time domain, though it is broken down into 1 ms sections. Thus, a 4G user might use a given chunk of spectrum for 10 ms, and then a 5G user will sneak into the band for 1 ms to transmit their own communications. That sharing would continue indefinitely -- and it can be dynamically adjusted based on the number of customers on either side of the 4G/5G equation.

"So what's cool there is you can size the number of NR slots to your demand," Haberman said, adding that it's the "ultimate way to do it."

However, Haberman cautioned that operators will need to carefully manage how such sharing is handled in cell site handoffs and in scenarios where carrier aggregation technology is used to glue together transmissions in disparate spectrum bands for faster speeds.

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Regardless, Verizon isn't the only operator planning to utilize this new transition technology.

"Dynamic sharing just allows you to use the same spectrum for both LTE and [5G] NR," AT&T's Igal Elbaz, the operator's senior vice president of wireless technology, told Light Reading in a recent interview. "In the US, it's really important because of the fragmentation of the bands. It takes time until there's enough devices of 5G and enough adoption of 5G. At the same time, a majority of our customers on our network are on LTE. So the ability to be able to dynamically use the same [spectrum] bands for NR and LTE is really important."

AT&T too is deploying 5G into mostly vacant millimeter-wave spectrum bands, but the operator has said that it will use its sub-6GHz spectrum holdings to deploy 5G nationwide by early next year. AT&T will also use dynamic spectrum sharing to ease its spectrum from 4G users to 5G users as it rolls out 5G and adds customers to the network.

Launch timelines remain fuzzy
However, neither Verizon's Haberman nor AT&T's Elbaz would say when they will introduce dynamic spectrum sharing. Indeed, Verizon hasn't provided any details about its 5G launch plans other than to say it will launch mobile 5G services in roughly 30 cities in the coming months. The operator has said that it will eventually expand 5G across all its spectrum bands, but it hasn't said when it might do that.

That relative uncertainty hasn't stopped vendors from trumpeting the elegance of the dynamic transition from 4G to 5G.

"Our new Spectrum Sharing solution enables service providers to intelligently, flexibly and quickly introduce and add 5G within existing 4G carriers. With this Ericsson innovation, our customers can strengthen their 5G offering while continuing to invest in 4G, with the peace of mind that those investments will pay dividends when the time comes to switch on 5G," said Ericsson's Per Narvinger in a joint announcement with Intel boasting of the companies' successful demonstration of spectrum sharing at the recent Mobile World Congress trade show in Barcelona, Spain.

Both Haberman and Elbaz said that dynamic spectrum sharing is a part of the 5G standard and that no one vendor has a lock on the technology. Ericsson said that its spectrum sharing technology likely would be available in the second half of 2019 in North America, depending on individual customer spectrum situations. "Ericsson expects this feature to be a game changer for our customers, allowing cost-effective serving on both 4G LTE and 5G NR on the same spectrum band from the same radio," the vendor said in a statement to Light Reading.

Ericsson, along with Nokia and Samsung, is one of the primary suppliers of 5G equipment in North America, and counts virtually all of the top operators as customers.

Although Ericsson's availability timeframe may trail initial operator 5G launches in the US, the technology is nonetheless heralded by many in the industry as a way to ensure that both 4G networks and 5G networks can concurrently continue to provide speedy services while customers go from from one G to the next.

— Mike Dano, Editorial Director, 5G & Mobile Strategies, Light Reading | @mikeddano