As wireless operators like AT&T, T-Mobile and Sprint prepare to roll out 5G across the country, a key question in their strategy is what spectrum bands they plan to use for their service.

And, at least for those three operators, the answer is a resounding "sub-6GHz."

Specifically, Sprint has said it will use its 2.5GHz spectrum for its mobile 5G buildout, while T-Mobile said said it will use 600MHz for nationwide coverage, alongside some 28GHz and 39GHz spectrum in urban areas. AT&T's initial mobile 5G launch used the operator's 39GHz spectrum, but AT&T said it plans to reach its goal of nationwide 5G coverage by 2020 using spectrum below 6GHz (though it didn't specify exactly which bands it might use that are below 6GHz). (See AT&T's New Nationwide, Mobile 5G Timeline.)

For those who understand spectrum propagation, this all makes total sense. After all, transmissions in spectrum below 6GHz are generally able to travel several miles, and can usually penetrate buildings or other structures.

Thus, as these three operators move toward 5G using bands below 6GHz, they will be able to do so mostly by using the same general network design they used for 4G, because the spectrum they're planning to use for 5G propagates in much the same way as the spectrum they're currently using for 4G. Meaning, AT&T, T-Mobile and Sprint probably will be able to use most of the same towers in generally the same locations for 4G as for 5G. All they will need to do is attach new 5G antennas to those towers.

The network deployment scenario for 5G in bands above 20GHz -- dubbed the millimeter-wave bands -- is quite different. Transmissions in millimeter-wave bands can only travel a few thousand feet at best, due to the propagation characteristics of that spectrum, and they generally can't penetrate into buildings and other structures. Thus, if a carrier wanted to cover an entire city with millimeter-wave 5G, they might need to install hundreds or even thousands of new 5G transmitters in order to provide coverage throughout that whole city. Meaning, they couldn't simply reuse their existing towers -- instead, they would have to build lots and lots of new transmission sites in between the towers they're already using for 4G.

And this is why Verizon's pledge to double down on mobile 5G in millimeter-wave spectrum is so surprising.

You're invited to attend Light Reading’s Big 5G Event! Formerly the Big Communications Event and 5G North America, Big 5G is where telecom's brightest minds deliver the critical insight needed to piece together the 5G puzzle. We'll see you May 6-8 in Denver -- communications service providers get in free!

Although Verizon has not yet disclosed its specific mobile 5G launch plans, here's what Verizon's Nicki Palmer, the operator's chief network officer, told Light Reading about Verizon and its spectrum strategy for mobile 5G: "We believe that our initial 5G offering will be called 5G Ultra Wideband. And Ultra Wideband to us means it's on millimeter wave, whether it's 28GHz or 39GHz, and we have a lot of that across the United States. And that to us provides a path to the real promise of 5G," she said.

Palmer continued: "Because if you don't have 800, 900 almost 1GHz of capacity in those wavelengths, if you don't have that, then you're not really getting the speed and throughput. Now, can you do different things with that? Sure. But, initially, 5G needs to be different than 4G. And that's why Ultra Wideband is our path of choice. And that's why our spectrum is in the millimeter-wave range. Because it will provide an absolutely different experience. Not just for the home -- where we're getting 300 Mbit/s and above, close to 1 Gig -- but also for mobility and smartphones and unique applications for consumers. So that's where we're starting, and we feel really good about it. And no comments further than that."

It's that last part of Palmer's statement -- about millimeter wave not only for the home but also for mobility -- that's really important here. Verizon in October of last year launched its 5G Home fixed wireless Internet service in four cities, using roughly 400MHz of its 28GHz spectrum holdings. The offering beams 5G signals from a nearby tower (usually 500-1,000 feet away) to a receiver on the outside or inside of a user's home or office, and that receiver then connects to a WiFi router. The WiFi router then broadcasts a WiFi connection that provides speeds of at least 300 Mbit/s but in some cases up to 1Gbit/s.

What Palmer is saying is that Verizon plans to implement the same spectrum and technology for its mobile 5G service that it's currently using for its fixed 5G Home service.

That could be a tall order, at least according to recent findings from analyst Earl Lum of EJL Wireless Research. Lum recently mapped Verizon's 5G Home network in Sacramento and found that the hundreds of 28GHz transmitters Verizon installed there still only covered roughly 10% of the city. (See Analyst Maps Verizon 5G in Sacramento, Finds 'Pretty Sparse' Coverage.)

Now, because Verizon has not disclosed its full mobile 5G launch plans, it's difficult to assess the operator's strategy based on a few sentences from one of its top network executives. For example, AT&T has said that it will use the "5G+" label for transmissions in the operator's millimeter-wave spectrum, retaining the "5G E" label for transmissions on spectrum below 6GHz. Verizon could employ a similar strategy by retaining the "Ultra Wideband" label for its millimeter-wave transmissions and a different label for 5G transmissions in spectrum below 6GHz.

And it would make sense for Verizon to deploy 5G in some of its spectrum below 6GHz, partly because 5G is more spectrally efficient than 4G and also because the operator has promised to sell 5G smartphones from Samsung and Motorola. (See Verizon Plans to Offer Samsung 5G Phone in H1 2019.)

Further, Verizon hasn't really signaled a dramatic increase in spending on its network that a nationwide deployment of mobile 5G in millimeter-wave spectrum would imply. Verizon spent $17.2 billion in capital expenditures in 2017, $16.7 billion in 2018, and in 2019 it expects to spend around $17 billion to $18 billion. "Remember, the majority of investments in the 5G network is coming with the fiber networking we've been doing, the passive asset we're doing, the intelligent edge network design that we've been working on for years, and then, of course, you have the equipment coming at the end of it. So, we are actually -- been for several years investing into be prepared for the 5G, and this is including the capex guidance for this year for obvious reasons," Verizon's CEO Hans Vestberg said last month on the company's quarterly conference call with analysts, according to a Seeking Alpha transcript of the event.

Indeed, Verizon for years has been positioning itself to make a splash, not just with 5G but with 5G on millimeter-wave spectrum. After all, Verizon won a bidding war with AT&T in 2017 over Straight Path and its millimeter-wave spectrum licenses, paying $3.1 billion for the company's holdings. Add that to the $1.8 billion Verizon paid for XO in 2017 in part to acquire that company's millimeter-wave spectrum holdings, and it's clear that Verizon made a long-range bet on the combination of 5G and millimeter-wave spectrum. (See Verizon Completes XO Fiber Buy; 5G Stage Set.)

How that bet will ultimately play out -- and how exactly Verizon plans to deploy mobile 5G services into millimeter-wave spectrum in a wide-scale way -- remains to be seen.

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