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Microsoft resurrects TV white spaces as a solution for US rural broadband

Over seven years of research and lobbying have gone into the effort to harness the white spaces in sub-1 GHz TV spectrum, for wireless broadband. Most deployments, however, have been in remote areas in emerging economies, but Microsoft is now seeking to trigger adoption in the US.

Microsoft was joined by Google, Dell and others in pushing the TV white spaces (TVWS) technology, and the US was the first country to open up the spectrum for wireless. However, real world deployments have been limited – even though some of the work on spectrum sharing and geolocation databases laid the groundwork for the 3.5 GHz CBRS system in the US, which has greater potential commercial impact because the spectrum is more plentiful and less crowded.

But Microsoft, building on experience of deployments it has supported in Africa, believes there is potential in its homeland too. Last week, Brad Smith, Microsoft’s chief legal officer, outlined the effort in a speech and accompanying blog post. He said Microsoft is creating a Rural Airband Initiative and will invest in partnerships with telecoms companies to create 12 projects in 12 states over the next 12 months, to connect 2m people.

Microsoft produced figures which said that 34m people in the US do not have access to the internet, and that 23.4m of these are unconnected because they live in rural areas.

If Microsoft takes the lead and gets others investing – operators, ISPs and local authorities – this will make it worthwhile for WiFi chipmakers to adopt the underused 802.11af protocol (an implementation of WiFi which supports low power, long range links in the white spaces). This is one of a handful of TVWS protocols devised by the IEEE standards body, which under the right conditions, using up to 256 QAM modulation, OFDM signaling, and multiple MIMO streams, can deliver as much as 400Mbps per cell.

The PHY layer in 802.11af is based on the same scheme used in 802.11ac, the most recent WiFi version. But because the propagation path loss is far smaller in the sub-1 GHz UHF and VHF bands, and because the FCC has allowed them to be operated at far higher EIRP levels, the range can be as long as 10 miles. If there is more than one channel not being used for broadcast, this allows up to four channels to be bonded. Then 400Mbps can be reached by adding 4×4 MIMO and Multiuser-MIMO – which has already been launched in 802.11ac Wave 2 – plus higher modulation schemes and channel aggregation.

If the major chip companies see that there will be a market for these devices Microsoft believes low cost 802.11af-based devices could begin to enter the US market in about two years’ time. It cites work it has done with Mediatek, for example on a triband 5 GHz/2.4 GHz/TVWS prototype based on 802.11af, which was used in a trial in Scotland, UK in 2015.

Microsoft made it clear that it was not getting into the telecoms business and it wants to act mostly as a stimulant for change. This is common practice for the company, whose overall objective is to boost usage of web services overall, and therefore its cloud services and apps. It has often worked to kickstart a new technology – it was active in the early days of WiFi, and less successfully when it invested in fixed wireless providers during the shortlived LMDS boom in the US at the start of the century.

It has commissioned a white paper from the Boston Consulting Group, which outlines numerous ways of bringing broadband to rural America. Where population density is below two people per square mile, satellite remains the most cost-effective option; but when there are between two and 200 people per square mile, white spaces are the answer. Above 200 people, more conventional wireless broadband (WiFi or cellular) is best.

The entire US rural population in that 2-200 population density sweet spot could be reached for just over $10bn, says Microsoft. Given could plow well over $1bn into this project, perhaps as much as $2bn – although it clearly is looking for other contributors to match its funding.

The aim is to connect those 23.4m US rural inhabitants with broadband inside five years, at a speed of 25Mbps or more. The BCG paper estimates that this would cost $65bn using fiber to the home, or $45bn using satellite only, and between $25bn and 40bn using 4G or 5G fixed wireless.

Microsoft said it would also offer digital skills training for people of all ages as part of this initiative and that it will open up 39 of its patents on a royalty-free basis to stimulate others to join in. It also called for the FCC to do its bit, asking for at least three channels below 700 MHz to be made available for wireless use on an unlicensed basis in every market in the country, with additional TV white spaces available in smaller markets and rural areas.

The Windows giant says it has considerable experience with white spaces spectrum, having deployed 20 TVWS projects in 17 countries that have served 185,000 users.

When Rethink did a report on TVWS in 2012, many of the functions that will now be supported by low power wide area networks (LPWANs) were expected to be placed on white spaces networks, including machine-to-machine and vehicle-to-vehicle communications, eHealth and smart city applications. But technologies like LoRa and Sigfox can deliver those more optimally in unlicensed sub-1 GHz spectrum, and NB-IoT in licensed. That was made clear when an early technology for TVWS, Neul’s Weightless IoT protocol, was redeveloped to support the sub-1 GHz ISM spectrum (Neul is now part of Huawei).

Now the role of TVWS is seen as being mostly for rural consumer broadband, including streaming of video.

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