E911 and LTE
LTE is the next generation technology for wireless networks. While there may still be plenty of life left in 3G UMTS and CDMA technologies, the rapid growth in the popularity of smartphones and tablets, along with the data-hungry applications they support (mobile data traffic in the U.S. increased by 172 percent in 2011, according to Cisco Systems), is pushing many operators into a position where they have no choice but to deploy LTE.
With LTE migration well underway in North America, wireless service providers are focusing on a comprehensive plan to upgrade the technologies that enable caller location identification in order to deploy next-generation E911 solutions. This technology upgrade offers two major opportunities to the industry.
First, supporting emergency services over LTE networks helps to free network operators to reallocate valuable 2G and 3G spectrum to LTE (4G), enabling better network efficiency and data capacity. As a result, in the next two or three years we are likely to see many areas of the US where LTE is the only technology deployed by some wireless operators. Second, the new location technologies and protocols being deployed as part of the move to LTE bring opportunities for enhanced location performance, mainly through use of additional technologies such as Wi-Fi and sensor-based positioning that perform better indoors (from where 60% of calls are placed).
Designers need to understand how the different positioning technologies (or combinations of technologies) play into the requirement to support E911 on LTE networks in North America, and the implications for testing the complex new positioning scenarios that can result. However, many of the key points are also applicable to emergency services in other regions, which need to take similar testing and deployment challenges into consideration as part of their migration to LTE.
What Does LTE mean for Wireless E911 services?
The transition to LTE means that operators have to aggressively support E911 location technology on LTE networks. Traditionally, legacy networks are used for E911 while new technology is built out to critical mass. This is the current transitional strategy for many operators in the US, but it will not last for long: LTE-only E911 services will launch in 2013, if not sooner.
To understand the impact of this, we will review briefly the technology used for E911 positioning in LTE networks.
LTE Positioning Technology
LTE positioning includes both handset-based and network-based techniques. No current positioning technology is capable of providing the FCC’s required accuracy level by itself, so a combination of multiple technologies will likely be needed . Network-based technologies, including UTDOA and AoA, rely on network measurements to locate mobile devices, whereas handset-based technologies such as OTDOA, A-GNSS, and ECID rely on mobile devices’ measurements. Although network-based technologies play an important role, the focus of this article is on handset-based technologies, because of the test implications for the mobile devices that support them.
Current LTE standards, defined in 3GPP Release 9, support three handset-based positioning technologies:
• 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.
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.
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 multipath, such as city streets.
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 neighboring 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.
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 signaling 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 Table 1, 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 signaling, 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.
Specialized Network Operator Acceptance Tests
In 2G and 3G network deployments, specialized E911 tests were developed by individual network operators to test performance requirements not covered by the standards-based conformance testing, and these operator-specific acceptance tests will extend to LTE devices. Examples include performance in specific simulated outdoor and indoor environments, exercising of special protocol sequences, combined A-GNSS/OTDOA testing, and assessing the reliability of continuous E911 position fixes.
A-GPS Antenna Performance (Over-The-Air)
Where GPS technology is used, the GPS antenna implementation plays a big role in E911 performance. If antenna performance is poor, the ability to acquire satellites is diminished and location performance is degraded in areas where the sky is partially obstructed. To help ensure an adequate level of antenna performance, CTIA-The Wireless Association® added GPS testing to its Test Plan for Mobile Station Over the Air Performance. Although the current 3.1 version of this test plan specifies a procedure only for testing A-GPS in GSM, WCDMA, and CDMA devices, an upcoming version will extend this to LTE.
As if all the “compliance” tests set up by 3GPP, CTIA, OMA, and the individual network operators wasn’t enough, the majority of LTE Location testing falls into the generic category of “R&D” testing. This is the testing done to improve and accelerate the development of the underlying location technologies used for E911. As shown in Figure 2, R&D testing can fit into several different categories, which apply to all location technologies, especially A-GNSS and OTDOA.
Figure 2 Many layers of R&D testing are required to ensure E911 performance
Not surprisingly, it takes a highly-specialized test solution to perform all of this testing for E911 on LTE networks. A solution of this type can be expected to need the following key capabilities:
• LTE Network Emulator – Must be capable of simulating at least three LTE cells, with each cell MIMO-enabled and capable of supporting OTDOA (PRS Physical Channel). Even more LTE cells may be needed in R&D test scenarios.
• GNSS Satellite Simulator – Must be capable of simulating GPS and GLONASS satellites, and be able to support standardized satellite scenarios for conformance testing. Much more advanced simulation capabilities are likely to be required for operator and R&D testing.
• eSMLC Emulation – Simulation of a key network entity called an eSMLC (enhanced Serving Mobile Location Center) is required. This entity is used to deliver assistance data to devices for A-GNSS and OTDOA and (in some cases) to also perform location calculations. The delivery of assistance data must be very accurately synchronized with the LTE and GNSS signal simulation.
• Automated Test Executive – With the exception of very early R&D, almost all testing is performed through automated test software that allows multiple tests to be run in series while results are collected and reported. Best-in-class test executives provide detailed results and analysis to facilitate debugging of issues after tests have been performed, and are robust enough to run test campaigns over weekends that reliably produce trustworthy results.
Designers need to use a highly-specialized test solution to perform E911 testing on LTE networks.
In conclusion, multiple trends are currently combining to drive a major change in mobile device location performance and the associated testing:
(1) Location identification performance for E911 calls originating from mobile phone needs to improve
(2) Network operators, particularly in the US, are looking to re-allocate 2G and 3G spectrum to LTE as quickly as possible, requiring E911 to work on LTE networks.
(3) New technologies and protocols for LTE positioning offer an opportunity to improve E911 performance
One result of all this is a new set of requirements for industry conformance, operator acceptance, and R&D testing that will eventually help E911 and other emergency services around the world to work better.
 More detailed information can be found in the Spirent White Paper “An Overview of LTE Positioning
 More detailed information can be found in the Spirent White Paper “LTE Positioning Technology For Mobile Devices: Test Challenges and Solutions
About the Authors
is the director for Wireless Location Technology at Spirent and has spent over 12 years creating test and measurement solutions for the wireless industry. Brock is part of a team that has made major contributions to development of the LBS standards in the 3GPP: Spirent filled the editor and rapporteur roles for the TS 51.010 and TS 34.171 A-GPS Terminal Conformance Specifications, as well as the editor role for the Enabler Test Specification for SUPL in the OMA. Brock holds a BSc in electrical engineering from Villanova University.
is the product marketing manager for Spirent’s wireless line of business. Susan has spent 14 year working in the telecommunication sector, including Bellcore and Lucent Technologies. Susan holds an MSc in computer science from Stevens Institute of Technology and a BSc in music and computer engineering from the University of Michigan.