Terrestrial beacons bring wide-area location indoors

Extraordinary though satellite navigation may be, GPS and other satellite-based constellations are limited when there is not a line-of-sight or near-line-of-sight path to at least three (and preferably more) satellites. These systems also do not provide sufficiently accurate and reliable altitude information for most applications, especially indoors. Finally, power consumption is an issue for user equipment.

It has been easy to overlook these limitations as the enormous benefits of GNSS have become pervasive, but the increasing demand especially for indoor geolocation now requires a robust solution designed for the indoors and urban canyons. Support for Terrestrial Beacon System (TBS) location technologies was incorporated in Release 13 of the Third Generation Partnership Project (3GPP). These technologies are complementary to GNSS, and provide a comprehensive solution to these limitations.

One of the TBS in development is the Metropolitan Beacon System (MBS) implementation by NextNav, which is the subject of this article. NextNav is deploying the first MBS network in the United States, using spectrum in the 920–928 MHz band, on licenses that cover about 98 percent of the U.S. urban population.

3GPP is the standards development organization for cellular wireless specifications, and is in part responsible for the popularization of GPS through its standardization in the 3GPP Release ’98 specifications. Release ’98 enabled wireless operators to adopt GPS and bring their economies of scale to GPS positioning.

Release 13 support has similar potential for MBS, enabling support for MBS in any Release 13-compliant LTE network throughout the world. As with the original standardization of GPS in 1999, incorporation of MBS in this release was driven primarily by the need for wireless carriers to provide accurate indoor geolocation for E911 calls.

MBS complements GPS by providing precise geolocation and timing indoors, in urban canyons, and other locations where GPS signals are either unreliable or unavailable. MBS receivers work seamlessly with GPS so they are as transparent to the user as satellite-based systems. MBS can provide floor-level altitude and navigation in indoor environments.

Typical mall experience: green dots show NextNav computed positions relative to ground truth (red line).

How it works

MBS transmitters are similar in many respects to GPS satellites that are deployed terrestrially. Unlike communications systems, MBS is deployed with a view toward minimizing dilution of precision (DOP) so that the signals available at any indoor or outdoor location will meet the unique requirements for accurate geolocation. DOP is an indicator of the three-dimensional positioning accuracy of a radio positioning system’s signals as they are “viewed” by a receiver.

GPS signals are typically 30 dB below the thermal noise floor at the Earth’s surface, and thus GPS receivers require a significant amount of processing resources for acquisition and tracking. Acquisition time can be quite long, up to 12 minutes in the absence of almanac and ephemeris information. Modern commercial implementations with some assistance information is typically closer to 30 seconds.

Mall store accuracy tests depicting indoor tracking performance in suburban mall environment. Dots show MBS-drive information, with no additional data from inertial or other sensors.

Throughout this time the receiver is running at full bore, drawing a considerable amount of current, the bane of any battery-operated device. MBS mitigates these problems because the 30-Watt radiated power of each terrestrially located transmitter combined with a satellite-like link budget provides greater received signal-to-noise ratio.

The result is an acquisition time without assistance information of 6 seconds or less, and 1 second if assistance information is available. The ease of acquiring and tracking MBS signals has significant implications for power draw and power management strategies.

Metropolitan beacon rooftop transmitter.

Although deploying a wireless network of any kind is a complex endeavor, MBS benefits from the ability to cover an area using fewer beacons, thanks to its relatively high RF output power (but much lower than cellular signals) and robust processing gain.

The transmitters typically share space with existing cellular systems on towers and building rooftops and are compact. The antenna is typically a 5-foot, vertically mounted, omnidirectional element.

The system provides for redundancy at both the transmitter and network levels, and the signals are encrypted for security. Like GPS, location can be calculated by the user’s device.

Baseband Change. MBS was designed to be like another constellation on a multi-constellation GNSS processor, and primarily constitutes a firmware change to modern baseband designs. The primary receiver changes are related to the analog components (accommodation for a different frequency band and higher dynamic range).

Enabling MBS in a smartphone requires a few inexpensive passive components and slight modifications to the antenna. From an RF perspective, NextNav’s MBS operating frequency is sandwiched between bands currently used by wireless carriers, so few if any changes to a standard FR lineup is required.

Tackling cellular first

Most of the billions of mobile phones shipped every year incorporate GPS receivers. Because GPS does not work reliably inside a building, however, mobile devices must fall back to ad hoc positioning methods based on communications infrastructure. This has become increasingly important because mobile wireless devices are used predominately indoors at least 70 percent of the time, according to a study by J. D. Power and Associates. This makes reliable indoor geolocation essential for consumer, commercial and public safety interests.

The MBS architecture was designed to integrate into the GPS ecosystem and integrate organically within modern mobile devices, without the need for separate chips or elaborate reengineering.
The additional benefit of determining altitude along with horizontal position is also significant. Indoors, context is determined as much by the vertical as the horizontal — for example, in a multi-level shopping mall. In emergency-response scenarios, critical seconds or minutes can be shaved off of response time if the floor in which an emergency is occurring can be reliably determined.

Control-plane architecture (LTE) for NextNav E2E.

Power and the IoT. The Internet of Things offers substantial productivity gains. Nevertheless, there have been limitations to the rapid adoption of certain IoT technologies. Among these is a fierce battle among competing low-power wireless communication standards. Lower power operations are the key for many IoT implementations, and location is one area where power savings, especially for wide-area location, are critical.

While MBS is generally designed to complement GPS, in IoT operations it has the potential to replace GPS in some cases due to power savings available from the system. Due to its terrestrial nature, the MBS signal is much stronger than GPS, enabling significant power savings. Many applications are expected to be enabled by such a system, whether for very long-life applications with intermittent position reporting to always-on location (that is, persistent tracking). Location capabilities on wearable devices are also very desirable, but because of power constraints, provision of location through GPS has been difficult to realize.

The general benefits of a terrestrial constellation also apply to non-power-limited applications, especially in urban environments and those where altitude is a critical feature. Driverless cars and unmanned aerial systems, for example, rely on GPS but also need precise 3D location accuracy.

Vertical accuracy performance of mass-market devices.

Another example of vertical accuracy performance of mass-market devices.

Applications in 5G small cells

The fifth generation of carrier wireless, 5G represents another potentially significant application of MBS technology. Achieving 5G’s ambitious goals — standards are expected to be complete by 2019 — will require a massive infrastructure increase, including small base stations, or femtocells, that must be time-synchronized to avoid interfering with each other. A large percentage of these are expected to be deployed indoors.

This means wireless carriers, neutral hosts and other infrastructure operators will need to bring timing synchronization signals inside. This typically requires GPS receivers to be placed on rooftops with the received signal fed to multiple indoor locations by running cables throughout the facility.

To an operator in a metropolitan area with hundreds or even thousands of indoor small cells, this represents a large investment in capital equipment and limits customer-based installation. MBS can provide a timing signal that can be received indoors through the use of a modified multi-constellation GNSS chipset, a low-cost and convenient alternative.

Beyond cellular

The enablement of MBS in 3GPP has drawn attention from those seeking geolocation for a range of other devices. EF Johnson Technologies, a provider of radios and other equipment for public safety applications, demonstrated the integration of MBS in its Viking P25 (Project 25) radios. As P25 radios are the standard for mission-critical voice in the public safety community, the ability to carry MBS information could be a key feature for first responders.

Elder care, monitoring family members, security guards, assets, and hospitality employees: any application that experiences service limitations due to indoor lack of availability is a candidate to augment service with MBS service, or, if power is a very serious issue, simply rely on MBS alone.

Summary

MBS complements GNSS systems by providing indoor coverage, altitude positioning and lower power consumption. By leveraging the existing GNSS ecosystem, low-cost, high-volume receivers can be adopted and service become seamless among satellite and terrestrial systems.

Other indoor PNT technologies

The 2013 CSRIC Trials administered by the FCC also tested technologies from Qualcomm, Polaris Wireless and True Position.

GPS World plans to publish articles about these and other alternative technologies in upcoming issues.