Integration Challenges of OTDOA for E911 Indoor Positioning

In the US, mobile phones long ago surpassed landline phones and are now the most common way to contact emergency services. What emergency callers using their mobile device indoors may not realize however; is that a huge gap exists between their expectation of being located and the accuracy of current positioning technologies when used indoors. As stated by FCC Commissioner Jessica Rosenworcel:

“If you call 9-1-1 from a wireless phone outdoors, your phone number and your location are reported—sometimes to within 50 meters, under FCC location standards. But if you call 9-1-1 from a wireless phone indoors, cross your fingers—because FCC location standards do not apply.”

But not for long.

Earlier this year, the FCC proposed new stringent regulations for indoor positioning. As leading carriers transition to LTE, new positioning technologies will be introduced and integrated into hybrid combinations, which are expected to improve indoor location accuracy. However, the integration is not easy and, as large-scale VoLTE rollouts gather momentum, operators are challenged with evaluating the positioning technologies to deliver optimal location performance and ensure VoLTE E911 works from day one.

Basic Architecture for OTDOA PositioningFor indoor environments, one key positioning technology is proving to be Observed Time Difference of Arrival, or OTDOA. Adding OTDOA to existing combinations of multiple positioning technologies requires comprehensive testing to ensure the ability to meet the evolving FCC regulations for E911 and to fully validate compliance with carrier performance requirements. In working with industry experts and leading carriers, Spirent has discovered many potential challenges that must be addressed in order to guarantee successful implementation for OTDOA and VoLTE E911. These include the following:

Accurate neighbor cell information needs to be maintained by the network
Cellular network deployments undergo periodic optimization, e.g., new cell sites may be introduced, some cell sites may be turned off, etc. Operators must maintain periodic optimization of cellular networks in a timely and accurate manner.

Carriers must identify the best PRS configurations for the cells in a given deployment area
Positioning Reference Signal (PRS) transmission requires LTE bandwidth to be utilized. If the PRS periodicity is high, it will impact the bandwidth usage for all the other devices on the network. If the PRS periodicity is low, the device performing PRS measurements will not be able to make accurate measurements. The network topology and the RF environment have an impact on the PRS configurations used by the network to enable OTDOA positioning.

OTDOA performance and measurement can vary between LTE Downlink channels
Network operators utilize multiple LTE bands for their deployment. The same device can have variance in determining the PRS measurements on different LTE downlink channels due to the radio frequencies being used. Network operators not only have to plan the PRS configurations well but also need to choose the appropriate LTE downlink channels for neighbor cells.

The VoLTE E911 challenge is not just at the protocol stack level
It can be an RF performance and isolation challenge for LTE networks that operate in frequencies that can cause interference on the GPS channel. For example, devices operating on LTE band 13 need to ensure that the second harmonic of the LTE signal does not interfere with the GPS signal and hence causing disruption in position determination.

More challenges and guidance are available for download in Spirent’s White Paper entitled Evaluating OTDOA Technology for VoLTE E911 Indoors.

Are you seeing the same challenges? Join the discussion on LinkedIn.

comments powered by Disqus
× Spirent.com uses cookies to enhance and streamline your experience. By continuing to browse our site, you are agreeing to the use of cookies.