What wireless operators need to consider when choosing a location solution

Feb 22, 2011

Source: Converge! Network Digest

What wireless operators need to consider when choosing a location solution

The mobile geo-location industry emerged in the United States (US) in the late 90's, after the Federal Communications Commission (FCC) mandated that all emergency calls (E911) from mobile devices must be located by wireless operators/carriers with specific location accuracy and reliability requirements. A variety of low, medium and high accuracy performance location solutions emerged as a result of the mandate. However, only the high accuracy solutions were deployed by carriers for meeting the FCC mandates for emergency call applications.

Besides emergency calls (E911/E112/E999 etc.), these high accuracy location solutions are also enabling mission-critical law enforcement applications (e.g. lawful location surveillance (LLS), lawfully authorized electronic surveillance (LAES), tracking etc.). For commercial location-based services (LBS), carriers deploy location solutions based on the specific application's accuracy and reliability (yield and time-to-fix) performance requirements.

This article provides an overview of the different types of commercial location solutions, and the corresponding applications they enable based on performance criteria.

Selecting the Right Location Solution

Location accuracy performance and reliability requirements vary by the type of application they serve and that drives the selection of the appropriate location solution. Other factors, such as battery consumption, also come into play for some applications, such as pervasive, highly penetrated consumer applications or geo-fencing applications.

The US FCC E911 mandate (E911 Phase II) provides the best available performance benchmark for mission-critical and safety-of-life applications. This benchmark - 67% of calls have to be located within 100 meters for non-GPS solutions - can also be used for law enforcement applications that require high location accuracy and reliability. Only three high accuracy location solutions have been deployed to meet the FCC mandate for E911 Phase II purposes: Assisted GPS (A-GPS), Uplink Time Difference of Arrival (U-TDOA) and Polaris Wireless Location Signatures (Polaris WLSTM). A-GPS is a handset-based solution, whereas U-TDOA and Polaris WLS are network-based solutions.

All other low and medium accuracy solutions, such as Cell-ID, are used only for best-effort consumer and enterprise LBS.

High Accuracy Location Solutions

There are only three high accuracy location solutions that have been demonstrated to meet the FCC E911 mandate:

Polaris WLS (Wireless Location Signatures)

Polaris WLS is the only high accuracy, software-based, scalable location solution that requires no additional hardware changes/additions to the mobile device or at the base stations. It uses radio frequency pattern matching (RFPM) to compare mobile measurements (signal strengths, signal-to-interference ratios, time delays, etc.) against a geo-referenced database of the mobile operator's radio environment.

Polaris WLS can locate all calls in Global System for Mobile Communications (GSM), Code Division Multiple Access (CDMA2000), Integrated Digital Enhanced Network (iDEN), Universal Mobile Telecommunications System (UMTS), Worldwide Interoperability for Microwave Access (WiMAX) and 3GPP Long Term Evolution (LTE) networks, and across the range of environments. Polaris WLS works extremely well in non line-of-sight conditions such as dense urban and indoor environments, where GPS based solutions face severe challenges. Since it is independent of line-of-sight conditions, Polaris WLS is highly reliable and is ideal for mission-critical and safety-of-life applications. Given the low battery consumption and geo-fencing capabilities of Polaris WLS, it is the ideal solution for wireless operators interested in the burgeoning mobile marketing opportunity.

U-TDOA (Uplink Time Difference of Arrival)

U-TDOA is a high-cost, hardware-based, non-scalable location solution that determines the device location by using trilateration methods. It uses multiple hardware radio receivers or Location Measurement Units (LMU's) that require manual installation at each base station across an operator's network to trilaterate signals based on measured time delays.

LMU's have only been deployed on 2G GSM networks and would require major hardware upgrades for 3G UMTS networks. U-TDOA has not been standardized for support in 4G LTE or WiMAX. U-TDOA performs well in line-of-sight conditions and its accuracy performance and reliability degrade in non-line-of-sight conditions.

A-GPS (Assisted GPS)

A-GPS is a handset-based location solution that requires GPS receiver chipsets to be included in the subscriber mobile devices. This solution has been deployed in CDMA2000 and iDEN networks, but not widely in GSM networks due to lack of mass market availability of GPS mobile devices and/or reasonable pricing. Location of the GPS-equipped mobile device is obtained by trilaterating signals received by the device from multiple GPS satellites.

A-GPS works well in direct line-of-sight conditions with the satellites (open sky conditions), such as suburban and rural areas and fails to perform in dense urban (urban canyons) and indoor environments. For law enforcement LLS applications, A-GPS solutions cannot be used because GPS in the phone can be easily disabled by the end user, jammed or spoofed. In addition, GPS does not work well indoors, which is a primary requirement for law enforcement agencies. Because it uses a receiver chip in the handset GPS can have excessive battery consumption when used in demanding applications, such as geo-fencing.

Medium Accuracy Location Solutions

There are two medium accuracy solutions that do not meet the FCC E911 mandate:

Observed Time Difference of Arrival (O-TDOA)

O-TDOA is a handset-based location solution that requires chip and firmware support to trilaterate signals based on measured time delays. For CDMA2000, O-TDOA has been deployed as Advanced Forward Link Trilateration (AFLT) as a fallback for A-GPS when it does not get a location fix. O-TDOA has been standardized for UMTS but has never been deployed commercially. Alternatively, in GSM, O-TDOA was standardized as Enhanced Observed Time Difference (EOTD), but never deployed commercially on a large scale. O-TDOA is currently being standardized for LTE, but it remains to be seen how widely it will be supported in infrastructure and handsets. O-TDOA works best in synchronous networks so would not perform well in asynchronous LTE deployments.

The accuracy performance for O-TDOA degrades in non-line-of-sight conditions and is only slightly better than a solution like Enhanced Cell ID (ECID) discussed below. O-TDOA has not been commercially deployed to date, except for its use as a fallback technology in CDMA2000.

Enhanced Cell ID (ECID)

ECID solution uses cell tower location, distance from cell sector (timing advance) and in some cases signal strengths. ECID provides medium accuracy performance on the order of 1/8th of the cell coverage area. The accuracy provided is marginally better than Cell-ID or Cell-ID+Timing Advance methods discussed in the next section.

ECID works across all air interfaces and is generally used for commercial LBS rather than mission-critical and safety-of-life applications.

Low Accuracy Location Solutions

There are two low accuracy solutions that do not meet the FCC E911 mandate:

Cell ID + Timing Advance (CID +TA)

CID+TA solution uses cell tower location and distance from cell sector (timing advance (TA), round trip time (RTT) or other time delay) to determine mobile location. CID+TA provides low accuracy performance and in the order of 1/4th of the cell coverage area. CID+TA performance is only slightly better than Cell ID and hence is used for some non-demanding commercial applications only.

Cell ID (CID)

CID solution uses cell tower location to determine mobile device location. CID is available as an inherent part of the operator's network.

The accuracy performance is a direct function of the cell coverage area and hence is used for coarse LBS applications, such as local weather.


Accuracy performance, reliability and battery drain requirements vary dramatically by application type. Therefore it is important to match the appropriate location solution directly with the application requirements. Mission-critical lawful location surveillance and safety-of-life applications such as E911 require the best possible accuracy performance and reliability across all environments and all air interfaces that the mobile network operator provides service on.

A-GPS is a technology that has been deployed to meet FCC E911 mandates but it does not serve LLS needs, for reasons discussed above. U-TDOA is also a technology that has been deployed to meet FCC E911 requirements, but it needs radio hardware at cell towers, does not work for all interfaces, and cannot support high volume applications. Polaris WLS, a technology deployed to meet FCC E911 compliance, is the only software-based solution that has the accuracy performance, reliability, scalability and battery consumption to simultaneously serve E911, LLS and LBS needs.

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