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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 

Current LTE standards, defined in 3GPP Release 9, support three handset-based positioning technologies:

 • ECID
 • A-GNSS. Examples: GPS, GLONASS, Galileo
 • Downlink OTDOA
Two relevant location protocols are also employed in LTE deployments:

 • LTE Positioning Protocol (LPP)
 • Secure User Plane Location Version 2.0 (SUPL 2.0)
These protocols are also able to support other technologies that can be used to improve location performance but are not currently defined by any standard, such as Wi-Fi positioning and sensor-based location (i.e. accelerometers, magnetometers, barometers). Location techniques that make use of multiple technologies are often referred to as "hybrid" positioning.

ECID provides a quick, coarse position fix with accuracy of 50m to 500m and is not capable of meeting E911 performance requirements by itself. However, it is used in conjunction with other more accurate positioning technologies, or as a fallback method when other positioning technologies are not available.

A-GNSS augments the device's GNSS capability with assistance data supplied over the LTE network, allowing the device to more quickly and easily acquire satellite signals. Although A-GNSS is the technology of choice for positioning, providing accuracy within 5-20m, its performance is poor indoors or in areas with high signal obscuration and multi-path, such as city streets.

OTDOA is a handset-based technology that relies on measurement of the time difference of arrival of special Positioning Reference Signals (PRS) from 2 or more neighbouring LTE base stations. This technology is most useful when GNSS is not available because it can provide reasonably accurate positioning (about 25-200m) indoors or in environments with limited visibility of the sky.

A-GNSS and OTDOA can be used together in LTE networks to enable significantly more accurate positioning in challenging environments.

The ultimate combination, which is not currently supported for E911 (although this may eventually change due to pressure from FCC to improve performance), is the fusion of LTE Network, GNSS, Wi-Fi, and Sensor technologies—or simply, hybrid positioning.

Location protocols
With the arrival of LTE comes a new protocol, LPP, which supports positioning technologies by enabling the exchange of positioning and assistance data between the handset and the network.

LPP is intended for use over both the control plane and the user plane and is a critical element in enabling E911 and Location Based Services (LBS) on LTE networks. Control plane positioning is primarily used for emergency services. In these situations, the network instructs the UE to provide a position and may send unsolicited assistance data to the UE over the signalling connection. Control plane positioning offers a quick, reliable and secure method for support of emergency services.

LPP also serves as the primary positioning enabler over the user plane, in conjunction with SUPL 2.0. The latter provides a common user plane protocol with a rich feature set which is critical to enabling LBS on LTE (as well as 2G and 3G) networks. SUPL 2.0 provides an option for handling emergency calls over the user plane, which was not available in previous 2G and 3G deployments. In emergency scenarios, the positioning requests will override user notification and privacy settings and receive priority over all non-emergency SUPL sessions.

For initial deployments of LTE, some operators are choosing to implement LPP over the control plane, while others are choosing the user plane. These implementation differences will require LTE mobile devices to support both methods.

Testing E911 in LTE networks
As shown in the table, new test specifications from 3GPP detail minimum performance and protocol conformance testing for A-GNSS, OTDOA, and ECID using an LTE air interface (3GPP TS 37.571). These test specifications will ultimately become the conformance standard used to certify devices in the United States. However, many other types of tests will ultimately be used to ensure that E911 works well on LTE Networks.

3GPP TS 37.571 Using SUPL 2.0 User Plane: While some operators deploy E911 using LPP control plane, others will use SUPL 2.0 user plane with an LPP payload. Since the 3GPP TS 37.571 tests all use LPP Control Plane signalling, a modified version of these tests will be needed to test SUPL 2.0 emergency call performance. Although it is currently unclear whether this will become an official certification requirement, the operators that plan to deploy E911 over SUPL 2.0 user plane will certainly require devices to pass the modified version of these conformance tests.

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