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Testing E911 in LTE networks

Posted: 03 May 2012     Print Version  Bookmark and Share

Keywords:E911  LTE networks  Location Based Services 

North American operators are planning to enable E911 on their LTE networks within the next twelve months. As a result, requirements for mobile devices are evolving to take advantage of new positioning technologies and protocols, which in turn will lead to new test requirements. Additionally, actions by the FCC are pushing improved accuracy and availability of E911 positioning, making it likely that this next wave of LTE-enabled devices will deliver much-needed performance improvements.

Today, more than 70% of 911 calls originate from mobile devices and more than 60% of 911 calls originate indoors. Here's a possible scenario:

Cell phone caller (small child): "My mommy is hurt, she is on the floor and can't talk"
911 Operator: "Where are you?"
Cell phone caller: "Chicago"
911 Operator: "Where in Chicago?"
Cell phone caller: "In my house"
911 Operator: "Where is your house?"
Cell phone caller: "In Chicago"

In a good case, E911 from a mobile phone should locate an indoor user to within 50m, leaving emergency responders with the option of about 27 houses to choose from in the situation illustrated in figure 1. However, the accuracy is often significantly worse. How to save this child's mother?

Figure 1: Impact of location accuracy on E911 Call: A typical E911 call from a mobile phone can pinpoint location within 50m, translating to about 27 houses in this type of neighbourhood.

In the event of an emergency, we have been trained since childhood in the US to dial 911 from the closest available phone which, back when almost every household had plain old telephone service, enabled the emergency operator to look up the exact physical location of the phone line used to originate the call and coordinate a response. Today with the majority of 911 calls originate from mobile devices, pinpointing the exact location of callers is not so simple.

Many technologies have been used to locate mobile phones, including assisted GPS (A-GPS), cell ID (CID), enhanced cell ID (ECID), advanced forward link trilateration (AFLT), downlink observed time difference of arrival (OTDOA), uplink time difference of arrival (UTDOA), and angle of arrival (AoA). They may read like acronym soup, but their effectiveness is fundamental to the outcome of life-or-death emergency 911 calls that originate from mobile phones.

More than 10 years ago, the US Federal Communication Commission (FCC) phased in its E911 Phase II location requirements for US wireless network operators and mobile devices: caller location must be provided to public-safety answering points (PSAPs) with 50m accuracy for 67 per cent of calls and 100m accuracy for 95 per cent of calls. The original requirements included a relaxed location accuracy requirement for network-based technologies, although this is scheduled to be phased out by the end of 2018. The FCC is also investigating ways to further improve the performance of locating 911 callers who use mobile devices. All of this is spurred by a basic problem: despite all the technology, location accuracy is often just not good enough when calls are placed from mobile phones; in some cases usable location information is impossible to obtain, especially when calls are made indoors.

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