BARCELONA -- Mobile World Congress 2017 -- Vodafone CTO Johann Wibergh has admitted to feeling "worried" about the hype surrounding so-called millimeter wave (or mmWave) technology.

mmWave uses much higher spectrum bands than operators have previously deployed with their mobile networks. Because more spectrum is available in these ranges, the technology is ideally suited to providing the very highest-speed services associated with 5G.

The drawback is that signals travel over relatively short distances in higher spectrum bands, forcing operators to build out more site equipment. Relatively minor obstacles can also become major barriers to signals in these ranges.

"I am personally a bit worried about all the hype on mmWave," said Wibergh during a press conference at this year's Mobile World Congress arranged by the Next Generation Mobile Network (NGMN) Alliance, an association of operators that feeds ideas into network standardization activities.

"It is certainly good to have spectrum available and you will get gigabit speeds, but if that signal even runs into leaves on a tree it doesn't work," said Wibergh. "There are some practical problems on how to build with mmWave -- there might be hotspot areas where you can use it but I think sub-6GHz is the sweet spot for 5G."

The comments provide clues about the way Vodafone Group plc (NYSE: VOD) could deploy 5G services across its European markets from 2020, when the first standardized version of the technology is due to make a commercial appearance.

Vodafone seems likely to rely heavily on so-called mid-band spectrum between 3.5GHz and 3.7GHz for more wide-scale deployments of 5G, with the much higher frequency ranges used in very densely populated or busy areas, like city centers and airports.

Vodafone is not the only European operator to have raised concern this week about the cost implications of higher frequency technologies.

Germany's Deutsche Telekom AG (NYSE: DT), which earlier in the week promised to introduce 5G technology across "100%" of its network footprint, says major industry collaboration will be needed to drive deployment costs down to a manageable level. (See DT CTO: Costs Must Fall or 5G 'Won't Work' and DT Plots 5G Across Entire Footprint.)

Bruno Jacobfeuerborn, Deutsche Telekom's CTO, reckons a cell site based on 28GHz spectrum -- the focus of early 5G efforts by Verizon Wireless in the US -- would have diameter of just 80 meters. A recent Barclays report estimates that deploying a nationwide 28GHz network in the US would cost about €300 billion ($318 billion), he says.

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Such costs are simply not feasible, Jacobfeuerborn told Light Reading on the sidelines of MWC. He hopes that by working with other industry stakeholders to "decouple" the radio access network control plane from the user plane, Deutsche Telekom will in future be able to make use of much lower-cost standardized hardware, shifting critical capabilities into the software domain, and improve the 5G economics.

However, Jacobfeuerborn also talks up the attractions of sub-6GHz spectrum in a 5G context and says that 5G is likely to co-exist with the fast-developing 4G standard for many years to come.

That view is shared by Vijay Perumbeti, the vice president of standards and industry alliances for AT&T Inc. (NYSE: T). "The 5G ecosystem will include 4G for a long time, unlike with 3G where we really want to get rid of it," he said. "There is a lot of life left in 4G."

Speaking at the same NGMN press conference, Alain Maloberti, the senior vice president of Orange Labs -- the research arm of France's Orange (NYSE: FTE) -- said the operator's initial deployments were also likely to focus on sub-GHz spectrum bands, including the 3.5-3.7GHz ranges as well as even lower 700MHz spectrum.

US operators represented on the panel appeared to offer stronger support for higher frequencies, with Sprint Corp. (NYSE: S) CTO Guenther Ottendorfer saying he expects deployments to happen "at both ends of the spectrum scale."

"Fixed wireless access operators will obviously work with high spectrum but sub-6GHz will be necessary for everything with mobile and that is a focus for us," he said.

Those comments came just a day after Sprint said it might push its own 2.5GHz band as a 5G band. (See Sprint Gets Ready for Massive MIMO, Eyes 2.5GHz for 5G.)

Several operators working on the commercialization of 28GHz spectrum, including Verizon, believe a key "use case" for the technology will be serving last-mile connections into homes and businesses, essentially providing a fixed wireless access alternative to fixed-line technologies.

— Iain Morris, , News Editor, Light Reading