T-Mobile will expand its LTE network with the same 5GHz spectrum used by Wi-Fi starting next year. The deployment would help the company boost data throughput but work only over short distances, similar to Wi-Fi hotspots.

LTE over Wi-Fi airwaves is unusual in a couple of ways: T-Mobile will share unlicensed airwaves with other networks, rather than using spectrum to which it has an exclusive license, as wireless carriers typically do. And by using 5GHz for cellular, T-Mobile is going in a direction opposite of the coveted low-band frequencies below 1GHz that travel longer distances and more effectively penetrate building walls.

T-Mobile has acknowledged that its network has trouble over long distances and indoors because it has less low-band spectrum than AT&T and Verizon Wireless, hence T-Mobile's reliance on Wi-Fi calling to cover the gaps. But deploying LTE over 5GHz doesn't preclude further purchases of low-band licenses, which T-Mobile will likely do in an auction of 600MHz airwaves expected to occur in early 2016.

LTE over unlicensed airwaves is called Licensed Assisted Access. "We expect T-Mo will utilize License[d] Assisted Access (LAA) extensively on the 500MHz of 5GHz spectrum with handsets becoming available at the end of 2015," Macquarie Research wrote in a short update this week after speaking with T-Mobile CTO Neville Ray. "Ray mentioned Wi-Fi will be complementary with LAA."

T-Mobile confirmed its plans in a statement to FierceWirelessTech.

"Unlicensed LTE is emerging as a promising technology and it is complementary to Wi-Fi and compatible with VoLTE [Voice over LTE]," the T-Mobile statement read. "This new form of LTE, once matured, will enable utilization of the 5GHz unlicensed bands. As Neville has spoken to previously, we are always looking at different technologies to bring additional benefits to our customers and this is more of the same."

We asked T-Mobile where it will deploy the unlicensed LTE technology and how it will avoid interference with Wi-Fi networks, but the company said, "we don’t have anything more to share at this time."

Higher data rates, simpler network management

Getting more low-band spectrum probably remains T-Mobile's most pressing need, but LAA could be an alternative to so-called "carrier-grade Wi-Fi" that carriers use to supplement cellular coverage in small, densely populated areas. Keeping users on the cellular network would simplify matters, as they wouldn't have to connect to Wi-Fi hotspots, and the data subscribers use would continue to count against monthly limits, increasing the need to upgrade to more expensive data plans.

"The use of LTE on unlicensed bands has been considered for public indoor cells or outdoor hot spots, generally places where there is coverage from licensed band LTE operation but where additional capacity would be beneficial," says a white paper by Nokia. The white paper notes that LTE over unlicensed airwaves could simplify network management by giving the operator just one network to maintain. Another benefit according to Nokia is "better spectrum efficiency than the current technologies in use with the 5GHz band. Since LTE radio technology is based on state of the art technology, it can achieve both high data rates and at the same time high spectral efficiency, also in the unlicensed band. As well as higher capacity, LTE technology offers better coverage, especially when combined with the use of licensed band operation."

While it appears T-Mobile may be the first major carrier to deploy LAA, there's plenty of discussion in the industry about how to make it work.

A September blog post by Cisco, which sells equipment for carrier-grade Wi-Fi products, contends that LAA is not ready for prime time. Cisco CTO Paul Mankiewich wrote:

Hype is interesting, but let’s take stock of where the industry is today. The main goal for LTE in unlicensed spectrum is to leverage the relatively large amount of unlicensed spectrum in order to provide a better mobile service. Given the transmit power restrictions imposed on unlicensed bands, it is expected to be primarily deployed in small cells. No standards exist for LTE in unlicensed spectrum. However, the mobile industry is discussing certain technical proposals. These describe an LTE carrier in unlicensed spectrum combined with a LTE carrier in a licensed spectrum using either carrier aggregation (CA) or supplemental downlink (SDL) where all control channels are on the licensed spectrum LTE carrier. Specifically, the industry proposals do not describe a 'stand-alone' LTE in unlicensed spectrum system where there is no licensed LTE carrier. Operators will not be able to leverage someone else’s unlicensed LTE hotspot, they will need to deploy their own (unlike with Wi-Fi). It cannot be overstated that LTE in unlicensed spectrum is meant as supplementary technology to LTE (which is by definition operable only in licensed spectrum).

Some interference with Wi-Fi may be inevitable, but vendors must do their best to minimize it, the Nokia white paper said:

There is always an effect when interference is created by additional transmitters, regardless of whether those additional transmitters are using LTE or Wi-Fi or some other technology. Thanks to advanced radio features, sharing of channels by LTE networks results in a smaller overall capacity loss than would be the case with two Wi-Fi networks... When placed on the same 20MHz channel without specific considerations other than the regulatory requirements, Wi-Fi suffers more as LTE is more resistant to co-channel interference. Both networks experience degradation due to increased interference, but the LTE network can still maintain good performance. When adding an additional fairness algorithm on the LTE side, it is possible to reduce the effects on the Wi-Fi network such that the degradation it experiences is similar to that caused by another Wi-Fi network. This means it does not really matter if the network causing interference is another Wi-Fi or LTE network... The higher capacity of LTE technology will also reduce the number of nodes needed compared to a Wi-Fi network serving the same traffic.

Wireless engineer and consultant Steven Crowley is optimistic about LAA. "Unlicensed LTE at 5GHz, in the US, has to meet the same Part 15 power restrictions as does Wi-Fi," Crowley told Ars. "With both, lower power means shorter range. The use cases are similar—homes, offices, outdoor lampposts. Compared with Wi-Fi, however, unlicensed LTE should provide about twice the throughput. One reason being that the control signaling is done on licensed LTE. The unlicensed LTE part is just data, without the overhead needed for Wi-Fi."