Wi-Fi and unlicensed LTE groups are finding it hard to coexist as mobile phone technology develops

6 min read

Although relatively young, the mobile communications sector seems to have an uncanny propensity to generate major controversies. The latest spat manages to pit one technology – Wi-Fi – against cellular; two networks that have avoided treading on each other’s toes for more than two decades. During this time, each camp has seen huge innovation and enormous growth, with many components and infrastructure suppliers relying on them for their own success.

The flashpoint is an emerging version of LTE. Dubbed LTE-Unlicensed, or LTE-U, it will allow mobile network operators to use, for the first time, the unlicensed 5GHz band and to create 4G networks in Wi-Fi’s traditional turf.

The major groups promoting LTE-U – Qualcomm, Alcatel-Lucent, Ericsson and Samsung – have created the LTE-U Forum. They have been egged on by a number of network operators, led by Verizon and T-Mobile in the US, as well as China Mobile and Japan’s NTT DoCoMo.

The operators have already outlined plans to trial and deploy LTE-U in their small cell networks, though none has launched a service to date.

In the fundamentally opposing corner are Wi-Fi operators, such as Republic Wireless, teamed with cable companies such as Comcast, huge players in the comms sector, such as Cisco and Google, and their increasing vociferous trade and lobbying groups, such the Wi-Fi Alliance and the CTIA.

A further complication is there are two pending official versions of LTE in the unlicensed bands, as well as a few proprietary offerings.

LTE-U, proposed in 2013 by Ericsson and Qualcomm, relies on 3GPP Release 10-12 functionality, including a proprietary contention mechanism such as Qualcomm’s Carrier Sensing Adaptive Transmission (CSAT) to mitigate impact on Wi-Fi.

This will undoubtedly be the first version to become available commercially, but only in countries that do not require implementation of the Listen Before Talk (LBT) protocol – markets such as the US, China, India and Korea.

LBT, which ensures fair sharing of available frequencies, is the mainstay for Wi-Fi’s massive success. It does this by constantly scanning the environment for competing signals. If it detects them, it does not transmit, waiting several milliseconds before trying again and then allocating resources in a fair way.

A version of LTE-U which will overcome this problem is LTE-LAA (for Licensed Assisted Access), which the 3GPP group, working together with proponents of Wi-Fi, expects to standardise for global use in the ongoing Release 13 development. This deploys the LBT mechanism and the signs from the recent meeting of the 3GPP group in Beijing are that it should be finalised late next year. But that is just the start – what follows will be lengthy testing of the protocols and lots of interoperability assessments, so LTE-LAA is unlikely to be implemented before 2017.

Perhaps it was because of this timescale that UK operators declined to share their views on unlicensed LTE with New Electronics. Both versions of unlicensed LTE are fully integrated within LTE networks and act as a secondary channel that relies on carrier aggregation (CA). They are thus hugely valuable to operators as they can manage traffic and services across both the licensed and unlicensed spectrum more efficiently than is the case with basic Wi-Fi offload.

The level of chatter, backbiting, claim and counter claim over the potential of interference between the two camps is not unexpected in view of the huge stakes involved, according to Monica Paolini, president of Senza Fili, a Californian based consultancy that has just released a report into the regulatory, technological and business implications of LTE-U.

Paolini stressed that operators have other options for leveraging license exempt bands. “A more readily available case could be LTE and Wi-Fi Link Aggregation (LWA); in particular for operators who have already invested heavily in Wi-Fi. LWA should achieve most of what LAA does, with the bonus that it would not require a new wireless interface in the 5GHz band,” Paolini told New Electronics.

LWA, largely backed by the group pushing LTE-U, would use LTE for the uplink and both LTE and Wi-Fi for the downlink. The solution would need a software upgrade for devices so they can engage their Wi-Fi radios to handle some LTE traffic, which would be collected at Wi-Fi access points and then sent to an LTE small cell.

Proponents of LWA, such as Ruckus Wireless, stress the solution would not only solve the capacity problem for operators by disguising LTE signals as Wi-Fi to other networks, but it would also get over the coexistence issue, since it would not actually impact the unlicensed band.

Whatever the final solution, the most contentious aspects of the debate concern the extent to which LTE-U could degrade Wi-Fi performance and whether the industry can coalesce around a solution and whether that can be made to stick. This, Paolini suggests, will come down to agreeing protocols and mechanisms for ‘fair’ – maybe even ‘nice’ – coexistence.

“On one level, LTE-U has the potential to be really nasty to Wi-Fi, since it is a technology designed to have exclusive control of the licence-based spectrum in which it operates, using a scheduled transmission model that treats any signal in its channel as interference.

“But, just because they can, will operators use such an aggressive approach and antagonise the huge Wi-Fi community? I very much doubt it and it would be a huge PR disaster if they went down that road,” Paolini continued.

“Both sides are right up to a point, but they are far apart at the moment, not least when it comes to public announcements.”

Not surprisingly, the sharpest criticism regarding the emergence of LTE-U comes from the Wi-Fi Alliance (WFA). The standardisation group’s vice president of technology Greg Ellis is on record as saying: “LTE-U was developed in private, in contrast to LAA and in contrast to recognised standards development methods. The result is that the LTE specification allows wide variances in implementation and configuration choice and therefore the extent to which LTE-U shares spectrum with other technologies can vary widely from one vendor to the next.

“Since such considerations were lacking in the development of LTE-U, they must still be considered before LTE-U equipment is deployed. Further work is necessary to ensure the range of expected LTE-U implementations can coexist fairly with Wi-Fi.”

Other Wi-Fi players, including the cable industry R&D group CableLabs, have been even more outspoken, with some accusing the cellular companies and their components and equipment suppliers of conducting inadequate tests or presenting over-optimistic results.

Some of the sharpest comments come from industry submissions to the US Federal Communications Commission (FCC) after it agreed, earlier this year, to adjudicate on the introduction of LTE-U following a request from WFA. The Alliance demanded stringent tests and rules to govern coexistence between the two technologies, which, it suggested, would need to be run under its auspices.

Many feel the Alliance took a wrong turn in doing this, rather than relying on the bottom up, multi stakeholder approach that has generally worked well in such disputes. The FCC, in turn, was criticised for overstepping its remit by getting involved in the standardisation process for unlicensed LTE. After all, the organisation is supposed to be technology neutral.

Verizon and T-Mobile argued in their response that it would set a dangerous precedent if one technology community is given the right to impose limitations on another. “Allowing an organisation that certifies interoperability for one particular technology to become the gatekeeper for another technology to use unlicensed spectrum would jeopardise the Commission’s entire framework that has made unlicensed spectrum as successful as an open platform for permissionless innovation,” the filing said.

Mike Schabel, vp of small cells with Alcatel Lucent, puts it more succinctly – maybe even bluntly. “Unlicensed spectrum is unlicensed spectrum; everybody can – and should – use it for specific purposes,” he told New Electronics.

“All this political sniping is actually about people trying to protect the interests of companies using unlicensed bands. The bottom line, from a technical point of view, is that this problem is solvable once we can agree on listening mechanisms designed to be co-operative, and we are continuing to work on such solutions with our partners, whether for LTE-U or LTE-LAA.”

Alcatel-Lucent has an LWA based solution dubbed Wi-Fi Boost waiting in the wings. Schabel claimed this is ‘completely compliant with existing standards’ and has been ‘demonstrated with partners such as Qualcomm and Ruckus Wireless’.

While the company will offer different versions of Wi-Fi Boost for different applications, it relies in essence on traffic being transmitted down over Wi-Fi and up over LTE. A more advanced implementation will use dual connectivity for simultaneous Wi-Fi and LTE.

Network software updates are used to blend the two technologies into a unified network and, said Schabel, Wi-Fi Boost will offer a 20% to 30% increase in downlink throughput to a single device, a ten fold boost for uplink at the cell edge and a doubling of Wi-Fi range. “It’s all part of the mix in increasing the throughput users are demanding.”

As discussed, the stakes have become huge, and none more so than for Qualcomm – by far the biggest component supplier involved and the company that has emerged as the principal evangeliser for both flavours of unlicensed LTE.

“We should not lose sight of the fact that most of the major players, including Qualcomm, have an interest in both technologies, so fair coexistence has been a major driver throughout this effort and debate,” Laurent Fournier, director of business development at Qualcomm Europe, told New Electronics.

Indeed, Qualcomm owns Atheros, one of the biggest suppliers of Wi-Fi silicon for devices and infrastructure, so it would make no sense to cannibalise that revenue stream.

Fournier noted Qualcomm has built extensive coexistence test facilities at its San Diego facility to which it has invited all the players in the development of both versions of unlicensed LTE in an attempt to bring some semblance of harmony to the issue. To date, the detractors have declined the invitation.

Tests to date on LTE-U indicate that it does not interfere with Wi-Fi. In a recent briefing looking to ‘set the record straight about coexistence’, Qualcomm claimed that, in some cases, Wi-Fi performance can be improved slightly when a Wi-Fi access point is replaced by LTE-U on a wireless network comprising multiple access points.

A few days later, Qualcomm unveiled its long awaited Snapdragon 820 processor, that it says will incorporate its fastest ever modem for Wi-Fi and LTE, and its first SoC to support LTE-U. Another version is targeted at developers of products supporting LTE-LWA. Qualcomm expects the devices to be used in commercially available small cell gear by the middle of 2016.

The eventual success of unlicensed LTE – of whatever flavour – will depend hugely on the mobile operators’ willingness to commit to large scale deployments of the technology. Despite signals from some US and Asian operators, this is not a given.

But an even bigger problem may be looming – although it maybe an opportunity. In the future, many more devices will feature Wi-Fi connectivity than they will LTE.