Blame the mountains for Denver’s sluggish mobile data speeds. But the city’s scenic backdrop isn’t the only culprit.

It’s the stuff we can’t see that appears to be dragging us down — the invisible wireless frequencies that create a path for movies, music and other digital data to get to and from smartphones.

There is promise for a faster future. By the end of September, T-Mobile’s revamped network will help subscribers in the Denver area get a much-needed speed boost and may help pull us out of last place in the nation for mobile data speeds.

But the region will always be limited because of who owns what in the wireless spectrum. The more spectrum a mobile company owns, the faster the data speeds should be. And in Denver, ownership is all over the place.

“What I quickly see here with Denver is, wow,” said Brian Goemmer, president of AllNet Insights & Analytics, which tracks who owns what spectrum. (The FCC has a handy guide to spectrum ownership but hasn’t updated it since 2014.)

Goemmer paused after seeing how spectrum ownership is split up in Denver. T-Mobile owns a good chunk of frequency bands in what is known as Advanced Wireless Spectrum, or AWS, which some 4G mobile services run on. But between its chunks of bandwidth, AT&T and Verizon own spectrum. By not owning contiguous bands, none of the carriers can offer the top speeds available in other cities.

Spectrum isn’t easy to get. It’s regulated by the Federal Communications Commission, which auctions wireless bands to the highest bidder. If you can’t win more at auction, you have to horse-trade it with existing owners. T-Mobile won a bunch of AWS spectrum when its proposed tie-up with AT&T came undone in 2012. AWS is essentially what T-Mobile has used to upgrade its national network in recent years.

“What’s a little bit surprising is that Verizon and T-Mobile would have traded, but Verizon won’t trade with T-Mobile in this market because there is no net benefit to Verizon,” Goemmer said. Trading, he said, would “only make T-Mobile stronger.”

The split spectrum over Denver was noticed by others — including Kevin Fitchard, an analyst with OpenSignal, a mobile research firm that tested service in 31 cities and concluded in August. “Denver didn’t score well,” he said.

“For a fast network, you need a lot of spectrum so you can put a bunch of people on your network,” Fitchard said. “And to get fast, you need a contiguous strand. And what we see in Denver is that you don’t have either.”

It got so bad that another research firm that drove around testing mobile service this year ranked Denver at the very bottom of 125 cities for mobile-data speeds. Denver was slower than Flint, Mich. (No. 54), McAllen, Texas (No. 122), and even Colorado Springs (No. 121), according to the RootMetrics report.

The worst provider was T-Mobile. With an embarrassingly slow median speed of 4.4 megabits per second, T-Mobile dragged down the entire region. That doesn’t impact voice and texting. But a low data connection means choppy Netflix videos, interrupted Spotify music streams and even unavailable websites. Compare it with a typical home’s broadband connection of 20 mbps — and even those speeds can’t always prevent buffering.

But, as RootMetrics pointed out, Denver was the lousiest because all the local carriers here seemed to hit a road block. Based on tests conducted in February, Verizon, AT&T and Sprint all posted data speeds below 10 mbps, compared with places such as Atlanta, where Verizon’s median hit 33.3 mbps, or Lansing, Mich., where T-Mobile hit 41.5 Mbps.

Coincidentally, a third research firm also ranked Denver near the bottom. Ookla, which offers the popular Speedtest.net for computer and mobile users to test their internet speeds, found Denver’s mobile internet performance at 12 mbps. That put Denver in second-to-last place out of 45 cities.

Related Articles Denver is dead last in U.S. mobile data speeds

Second report ranks Denver’s mobile data speeds as nearly the slowest in the U.S.

Denver among 5 worst cities of 125 for mobile performance

Sprint and Verizon best for Denver mobile service, T-Mobile improves speeds It’s all relative, said Annette Hamilton, RootMetrics’ director. Denver may have tested poorly overall. But for each carrier, Denver wasn’t the worst. Sprint’s median speeds were 4.1 mbps in Nashville, Tenn., and 3.1 mbps in Tuscon. AT&T’s download speed in Los Angeles slowed to 8.9 mbps. In Colorado Springs, however, data speeds topped with Sprint’s 23.3 mbps to AT&T’s low of 12.3 mbps.

“Looking at network performance in Denver, and more widely across Colorado, network data speeds seem to be a constant challenge,” Hamilton said.

Denver’s mobile networks are also battling other forces, like geography and population explosion. Last year, Denver grew faster than the nation’s 50 most populous cities and was the second-fastest growing city between 2010 to 2015.

“Population growth can certainly have an impact on mobile network performance, since more users mean more stress on a network,” Hamilton said. “Geography, such as the Rocky Mountains, also factors into performance, as does a general increase in data usages by mobile users.”

But opinions differ on the cause of slow internet.

“It’s not topography. It’s not our equipment or anything like that. It’s just the number of people coming into Denver,” said Suzanne Trantow, an AT&T spokeswoman for the Denver area.

A greater number of its users are using AT&T’s network to stream video. “We see more than 60 percent of our traffic is mobile and we’re working as fast as we can to add LTE capacity,” Trantow said.

This month, T-Mobile confirmed it was doing something about the sluggish speeds. T-Mobile said Denver was one of 319 cities where it now offers multiple-input, multiple-output technology, or MIMO, which essentially adds extra antennas at each end — the cell tower and the phone. MIMO creates two paths for data to get to a phone or tower, in case a big rig on the highway blocks off a wireless signal. T-Mobile said that this can double data speeds. But to get the maximum benefit, T-Mobile subscribers also must have a MIMO phone — T-Mobile offers only the Samsung Galaxy S7 and S7 Edge. (To find more, check MIMO chipmaker Qualcomm’s site at dpo.st/mimophones for its Snapdragon 820 chip.)

“The important thing to know is what we’re doing and what we have in front of us is the opportunity to leverage the resources in a way that will significantly improve the customer experience,” said Scott DuBuke, T-Mobile’s Denver-area director of network engineering and operations. “As a result, people will get everything faster. The overall experience will be better.”

DuBuke added that a test he did in Highland Square this month hit 100 mbps, which may be excessive for some. But the more bandwidth, the more a person — or multiple people standing near the same cell tower — can do on a smartphone.

Others have been tweaking their technology too, with positive results.

Back in 2012, Sprint was the worst mobile service provider in Denver. It was so bad, it ranked below Cricket.

“We saw big impacts when calls and data wouldn’t hand off between the old and new network equipment,” said John Votava, a Sprint spokesman for the Denver area. “Calls dropped, data was interrupted.”

Sprint embarked on a major overhaul, ripping out and replacing all of its network equipment under a program called “Network Vision.” The upgrade — which included “hundreds of 2.5 GHz cell sites in the past 18 months” in Denver, Votava said — was completed by the end of 2014. And then Sprint began fine tuning every single cell site to optimize quality of calls and data service.

Sprint is now tied with Verizon for best overall performance in Denver, according to RootMetrics, the same company that ranked Sprint at the bottom four years ago. Sprint’s not-so-secret weapon is that it doesn’t have any AWS spectrum. It competes on the PCS wireless network. By upgrading software, hardware and other technology, companies like Sprint can turn their frequencies into the fastest 4G mobile standard, often referred to as LTE, or Long Term Evolution.

“This could be a very big opportunity for Sprint. Sprint has quite a lot of spectrum in Denver right now. And Sprint has been promising to build an LTE network to end all LTE networks,” said Fitchard, the analyst.

AT&T has the biggest chunk of contiguous AWS spectrum in Denver. The company recently added 10 new cell towers in the area, with the newest offering quadruple the capacity to handle more users.

“We are very focused on Denver and Colorado as a whole,” Trantow said.

Verizon has always ranked among the top carriers for Denver, even if its speeds are slower than in the company’s fastest cities. But Verizon also recently announced what it calls LTE Advanced technology to “bring 50 percent faster peak wireless data speeds” to customers in Colorado.

New technology and towers seem to be helping — and offsetting wireless spectrum impact. In new results released this month by RootMetrics, all four companies in Denver improved. All four hit wireless data speeds above 10 mpbs, with Sprint at the top with 14.9 mbps, and T-Mobile at the low end, with 11.7 mbps. Several other cities nationwide, however, were in the 20 mbps and faster range.

“The best way to look at this is not to compare Colorado’s or Denver’s performance to national performance,” said RootMetrics’ Hamilton. “The better way is to look at metro-to-metro performance, since consumers generally experience mobile network performance at that local level.”