Increasing the accuracy of indoor location using Small Cells

LocationLast month, US FCC regulators announced plans for new rules to accurately report the location of emergency calls made from smartphones inside buildings. Other countries could easily follow suit. I spoke with Rich Lee CEO of iPosi, a GNSS indoor location technology company, to discuss the various technology solutions available to address this issue.

He believes that Small Cells provide one of the best platforms to systematically determine indoor mobile location, not only meeting the proposed FCC 50 metre target but achieving accuracy eventually down to 1 metre using their "toolbox" technologies that combines sensitive GPS receivers with precise timing systems.

Most emergency calls are made from mobile phones, and more occuring indoors

The FCC estimates that over 70% of emergency calls are made from mobile rather than landline phones today. There is already a requirement for mobile phone networks to determine and report the location of an E911 emergency call. Enhanced 911 Phase 1 reports only the location of the serving cellsite, which could be a Small Cell. Phase 2 requires pinpointing the smartphone itself, to within 300 metres, or even down to 50 metres for those with built-in GPS receivers.

Where more cellsites are used, the accuracy achieved can be more much precise. This includes urban areas where macrocells can be found on rooftops every few hundred metres. As more Small Cells become deployed, and the networks become densified, we should expect the accuracy to improve further.

In the future, we might expect more automated calls to be made – such as medical emergencies or serious accidents. Some automobiles are already equipped to make automated emergency calls with accurate location after a crash. For example, BMW offers a feature for their cars which send a text message with location and other details when the airbags go off and are connected to a call centre who can assess and call in help.

Emergency location reporting moving indoors

Up to now, the Public Safety regulators have constrained themselves to outdoor location only. Most callers would be able to report the building or address involved. But whenever this is not possible, FCC tests report representative urban accuracies today of 227 to 449 metres for 67% to 90% of fixes based on today's mobile location technology. With the average building frontage of approximately 50 metres, this means it may not be possible to determine the actual building involved, never mind which floor or apartment number to dispatch the emergency crews to.

The FCC announced late February that it is proposing E911 rules which would mandate reporting indoor location for mobile devices to be accurate indoors to within 50 metres horizontally and 3.5meters vertically. The CTIA are cautious about potentially expensive and demanding requirements.

The major problem at the moment is that smartphone GPS receivers typically can't work indoors because the requisite minimum four incoming GPS satellite signals are too weak.

iPosi's Rich Lee tells me that "the Public Safety folks are realists. They want to encourage real technology to succeed and therefore want to avoid setting unachievable performance or other constraints. The largest known paid location service today is E911. Over $2 billion is collected per annum in the US for this service (approx. 300 million subscribers x 75c/month). It might not be seen as glamorous as other services, but is essential. Few people decline having to pay for E911."

Indoor location using Small Cells

If everyone had their own Small Cell in their apartment or office room with a registered address that would be ideal. Larger Enterprise buildings and public indoor spaces would require some more detailed mapping. There are three stages to this:

  1. Determining the precise 3D location of the serving Small Cell
  2. Using proximity-association, ranging and triangulation techniques to calculate the position of the smartphone
  3. Referencing the 3D location to an in-building map

GNSS typically delivers 3-5 metre accuracy, even indoors, as long as there is enough sensitivity, path distortion and interference mitigations are available to receive GNSS signals imposed by indoor environments. For 3D, this may not be adequate to determine which floor the caller is located – in some cases it could result in registering the floor above or below the mobile and associated Small Cell.

Referencing the location to designated floor numbers may not be easy either--every building is unique. This is further complicated because not every floor in a building has the same height (e.g., the lobby/atrium may be taller); some buildings may not have a 13th floor or other superstitious number; the ground floor may be numbers zero or one.

To resolve, one method to determine the caller's floor height that could be used is barometric pressure difference. Although the pressure itself varies with the weather, if readings are compared between the caller's smartphone and an emergency crew's device, it could help pinpoint which floor they should go to. Many of the latest smartphones do have this type of sensor fitted, and the data could be reported as part of the emergency call. It would also be possible for the Small Cell to be fitted with a barometric sensor.

Technology already in standards is LTE Release 9 includes measuring the downlink arrival time signals take from the cellsites. The location granularity naturally improves as more Small Cells are deployed.

Why not just use Wi-Fi?

Rich says enterprise grade Wi-Fi is available to determine the range via round trip time, and with multiple access points, locating people within buildings to according to supplier claims within 10-20 metre accuracy. For general deployments however, he has heard concerns about the inability to reject ambiguous or incorrect data. He hears from operators that "false positives are worse than just not providing the information."

Google Maps moves indoors

Google have invested a lot of time and technology into indoor location by mapping out the insides of many large buildings, from museums to shopping malls. This allows Google Maps to work indoors and guide you around. Business Insider reports how they've developed detailed mapping of the Smithsonian Museum in Washington including all of the major aircraft exhibits.

They say they've taken advantage of Wi-Fi to determine location, not because it was the best technology or designed for the purpose, but because it was the only available option.

A demonstration and explanation of their approach can be found on this YouTube video (poor sound quality, bit lengthy).

So far, they've claimed to have mapped between 25 to 30,000 venues, mostly larger retail and transport hubs. Using their own maps and simulating location variations of different amounts they cite need for accuracy of 6-7 metres today, and are generally known to seek a goal of 1 metre accuracy. They can determine which floor you are on, and demonstrate the level indication changing as you go up or down the escalator.

Google believes that today's mobile GPS struggle indoors to get below 15 metre fix accuracy and often can't yield positions due to limited signal strength. Today, indoor-calibrated Wi-Fi can with sufficient density outperform the outdoor-oriented Assisted GPS built into smartphones in many situations.

Summary

Improving indoor location accuracy via Small Cells is the next big thing for Emergency calls, an established multi-billion dollar annual business. This same accuracy will also be used for other purposes, from providing aggregate footfall and usage patterns for retail owners to enhancing smartphone apps and enabling more useful indoor services.

Wi-Fi provides a ubiquitous solution today, but as deployed may not be high enough resolution to meet the more demanding premium service requirements.

Mapping the inside of buildings, and linking that to Small Cells of known position, is an important step. Google has already made significant progress here, but there is much more to do.

Technology which can enable reception of GNSS signals in the more challenging indoor locations will greatly assist the realization of that dream.

iPosi is a sponsor of ThinkSmallCell

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