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An overview of LTe PosiTioningFebruary 2012
Rev. A 02/12
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An overview of LTe Positioning
SPIRENT WhITE PAPER • i
CoNTENTS
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
LTE Positioning Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
Cell ID and Enhanced Cell ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Assisted Global Navigation Satellite Systems . . . . . . . . . . . . . . . . . . . . . 4
observed Time Difference of Arrival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Positioning Architecture in LTE Networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
LTE Positioning Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Control Plane Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
User Plane Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Area Event Triggering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Emergency Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Support for Multi-Location Technologies. . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
An overview of LTE Positioning
1 • SPIRENT WhITE PAPER
INTRoDUCTIoN
Demand for mobile services is exploding and one of the
fastest growing segments is Location Based Services (LBS),
primarily driven by two major requirements: emergency
services and commercial applications. For emergency
services, the most significant driver is the FCC’s E911 mandate
in the US, which requires location (with certain accuracy
limits) of emergency callers to be provided. A wide variety
of commercial applications, such as maps and location-
based advertising, also need fast and accurate positioning
performance. In response to these needs, second and third
generation networks (WCDMA, GSM, CDMA) have added support for several positioning
technologies, which vary in their accuracy and Time to First Fix (TTFF) performance.
They range from simple network-based schemes to complex trilateration and satellite-
based solutions.
With the rollout of LTE comes a new focus on enabling E911 and LBS on these 4G
networks, while providing a seamless transition between LTE and 2G/3G positioning
services. Current LTE standards support three independent handset based positioning
techniques: Assisted Global Navigation Satellite Systems (A-GNSS), observed Time
Difference of Arrival (oTDoA), and Enhanced Cell ID (ECID). There is new protocol for LTE
called LPP (LTE Positioning Protocol), although SUPL 2.0 (Secure User Plane Location)
remains a key User Plane protocol for enabling LBS and E911 on some networks,
with its support for techniques such as WiFi positioning. Taken together, these latest
positioning techniques promise effective and efficient positioning performance in LTE
networks, although at the cost of increased complexity.
fcc e911 requirement
2D error for a given set of
measurements:
67% < 50m
95% <150m
This white paper provides an overview of positioning techniques, protocols and architecture supported in LTE networks (as of LTE release 9).
LTe PosiTioning
A-GNSS, OTDOA, ECID
LPP SUPL 2.0
An overview of LTE Positioning
SPIRENT WhITE PAPER • 2
LTE PoSITIoNING TEChNoLoGIES
3GPP Release 9 for LTE defines support for three handset based positioning
technologies: ECID, A-GNSS, oTDoA and LPP, a new positioning protocol. The following
sections describe each of these technologies in detail.
Cell ID and Enhanced Cell ID
Cell ID (CID) positioning is a network based technique that can be used to estimate
the position of the UE quickly, but with very low accuracy. In the simplest case, the
position of the UE is estimated to be the position of the base station it is camped on.
Cell ID positioning performance can be improved by measuring certain network
attributes, a technique called Enhanced Cell ID (ECID). In ECID, the
Round Trip Time (RTT) between the base station and the UE is used to
estimate the distance to the UE. In addition, the network can use the
Angle of Arrival (AoA) of signals from the UE to provide directional
information. See Figure 1.
The RTT is determined by analyzing Timing Advance (TA)
measurements, either from the eNodeB or by directly querying the
UE. The eNodeB tracks two types of TA measurements – Type 1
and Type 2. Type 1 is measured by summing the eNodeB and the UE
receive-transmit time differences. Type 2 is measured by the eNodeB
during a UE Random Access procedure.
AoA is measured based on uplink transmissions from the UE and the known
configuration of the eNodeB antenna array. The received UE signal between successive
antenna elements is typically phase-shifted by a measurable value. The degree of
this phase shift depends on the AoA, the antenna element spacing, and the carrier
frequency. By measuring the phase shift and using known eNodeB characteristics, the
AoA can be determined. Typical uplink signals used in this measurement are Sounding
Reference Signals (SRS) or Demodulation Reference Signals (DM-RS).
Figure 1: ECID positioning
An overview of LTE Positioning
3 • SPIRENT WhITE PAPER
As stated earlier, CID positioning has very low accuracy, typically equating to the size
of the cell the UE is camped on (which may be in the order of kilometres). ECID is able
to provide better accuracy in comparison to CID; the main sources of error in ECID are
receive timing uncertainty (which affects the RTT calculation) and multipath reflections.
summAry of cid/ecid PosiTioning
PrinciPLe
Use knowledge of the serving cell, Round Trip Time and Angle of Arrival of the uplink signal to position the UE
Key use cAses
Quick, coarse fix as an input to other, more accurate positioning technologies Fall back methods in case A-GNSS/oTDoA are unavailable
AccurAcy
Typically 150m or coarser
ECID is able to provide better accuracy in comparison to CID; the main sources of error in ECID are receive timing uncertainty (which affects the RTT calculation) and multipath reflections.
An overview of LTE Positioning
SPIRENT WhITE PAPER • 4
Assisted Global Navigation Satellite Systems (A-GNSS)
GNSS refers collectively to multiple satellite systems, such as GPS and GLoNASS.
With conventional standalone GNSS, the GNSS receiver in the mobile device is solely
responsible for receiving satellite signals and computing its location. The receiver needs
to acquire satellite signals through a search process; it must lock onto at least four
satellites in order to compute a 3-D position. The acquisition process can be demanding
in terms of battery and processing power, and TTFF can be long.
The performance of standalone GNSS can be significantly improved by
a technique called Assisted GNSS. See Figure 2. In a typical A-GNSS
implementation, the standalone GNSS facilities of the phone are augmented
by data provided by the network, termed “Assistance Data”, which includes
information the mobile GNSS receiver can use to accelerate the process of
satellite signal acquisition. The final position can be calculated by either the
UE or the network and shared with third parties (such as emergency PSAPs1).
A-GNSS speeds up positioning performance, improves receiver sensitivity
and helps to conserve battery power. A-GNSS works well outdoors and in
scenarios where a reasonably good view of the sky is available. Performance
is generally poor in environments with high obscuration and multipath, such
as indoors and in dense urban settings.
Currently, two global systems are fully operational – GPS and GLoNASS. Although
mobile receivers have traditionally supported positioning using A-GPS alone, it
is possible to use both satellite systems simultaneously to acquire a position.
The advantage of this technique is to effectively increase the number of satellites
available for signal acquisition, and it can improve performance in high-obscuration
environments like cities. Assistance data can be provided by the LTE network for both
GPS and GLoNASS satellites (as well as Galileo and QZSS when these systems are fully
operational).
summAry of A-gnss PosiTioning
PrinciPLe
Use standalone GNSS with help from the LTE network to speed up the position calculation process
Key use cAses
highly accurate, technology of choice for positioning
AccurAcy
Typically 10 – 50m
Figure 2: A-GNSS positioning
1 PSAP - Public Safety Answering Point.
An overview of LTE Positioning
5 • SPIRENT WhITE PAPER
observed Time Difference of Arrival (oTDoA)
oTDoA techniques are similar in principle to the GNSS position calculation
methodology. The UE measures time differences in downlink signals from two or
more base stations. Using the known position of the base stations and these time
differences, it is then possible to calculate the position of the UE. Generally, the signals
used for oTDoA are cell Reference Signals (RS). See Figure 3.
In LTE, the measured time difference between the RS from the serving cell and one or
more neighboring cells is known as Reference Signal Time Difference (RSTD). In order
to calculate the position of the UE, the network needs the positions of the eNodeB
transmit antennas and the transmission timing of each cell (which can be challenging if
the eNodeBs are asynchronous).
one of the biggest challenges faced by LTE oTDoA is the requirement to measure
neighboring cell RS accurately enough for positioning. To overcome this problem,
special positioning sub frames have been defined in Release 9 called Positioning
Reference Signals (PRS). See Figure 4. These special reference signals can assist in the
measurement of neighboring cell signals by increasing RS energy.
Figure 3: oTDoA positioning
One of the biggest challenges faced by LTE OTDOA is the requirement to measure neighboring cell RS accurately enough for positioning. To overcome this problem, special positioning sub frames have been defined in Release 9 called Positioning Reference Signals.
An overview of LTE Positioning
SPIRENT WhITE PAPER • 6
The PRS is periodically transmitted along with the cell specific RS in groups of
consecutive downlink sub frames. In a fully synchronized network, these positioning
sub frames overlap, allowing for reduced inter-cell interference. In the case that the PRS
patterns in two neighboring cells overlap, the network may mute the transmissions to
improve signal acquisition. The network can also provide Assistance Data to the UE to
aid its acquisition of the PRS. This data usually consists of relative eNodeB transmit
timing differences (in the case of a synchronous networks), search window length, and
expected PRS patterns of surrounding cells.
In LTE, oTDoA and A-GNSS may be used together in a “hybrid” mode. Since the
fundamental positioning calculation approach is the same, a combination of satellites
and base station locations can be used in the position calculation function. In this
technique, the UE measures the RSTD for at least one pair of cells and satellite signals,
and returns the measurements to the network, which is responsible for analyzing the
measurements and calculating a position. This hybrid mode can be expected to provide
better accuracy than oTDoA positioning alone, and is a key enabler for improving
positioning accuracy in challenging environments.
summAry of oTdoA PosiTioning
PrinciPLe
Use time difference of arrival of special Positioning Reference Signals (PRS) from 2 or more LTE base stations
Key use cAses
Fallback technology when GNSS is not available Positioning indoors and environments without clear sky visibility
AccurAcy
50-200m (based on simulation)
Figure 4: Structure of the PRS
An overview of LTE Positioning
7 • SPIRENT WhITE PAPER
PoSITIoNING ARChITECTURE IN LTE NETWoRkS
Positioning information exchange between the UE and the LTE network is enabled by
the LTE positioning protocol. LPP is similar to protocols such as RRC, RRLP, and IS-801
already deployed in 2G and 3G networks2. LPP is used both in Control Plane and User
Plane (enabled by SUPL 2.0). The key entity in the core network that handles positioning
is the Evolved Serving Mobile Location Center (E-SMLC). The E-SMLC is responsible for
provision of accurate assistance data and calculation of position.
SUPL 2.0 can be deployed across 2G, 3G and 4G networks to provide one common user
plane protocol. In initial LTE deployments, it is possible to use SUPL 2.0 with RRLP over
LTE, which helps in enabling user plane positioning before implementing LPP. So in
summary, positioning in LTE networks can be accomplished in one of three ways.
2 Note that RRLP only supports A-GNSS; delivery of LTE ECID and oTDoA information is not supported. however, SUPL 2.0 has native support for sending information about the serving LTE and neighboring cells.
Time difference of ArrivAL TechnoLogies in 2g/3g services – An overview
cdmA AfLT
In AFLT, CDMA pilot signals are used for measuring the time difference of arrival. CDMA base stations are synchronized with GPS time, which eliminates timing offsets between base stations and optimizes hybrid AFLT + A-GNSS positioning.
gsm e-oTd
In E-oTD, the UE measures the time difference of arrival at its receiver of burst signals from different BTS’s. A Location Measurement Unit (LMU) is used to synchronize BTS timing.
wcdmA oTdoA-iPdL
oTDoA in WCDMA is characterized by Idle Periods in Down Link (IPDL) to allow the UE to listen to neighboring cell signals which otherwise are subject to interference from the stronger serving cell signal.
LTe PosiTioning meThods
CONTROL PLANE with LPP
SUPL 2.0 with RRLP
SUPL 2.0 with LPP
disAdvAnTAges of oTdoA in gsm/wcdmA
Clock errors, lack of Base Station synchronization, cost of deploying LMUs and heavy signaling overhead discouraged use of these technologies for commercial purposes.
An overview of LTE Positioning
SPIRENT WhITE PAPER • 8
LTE PoSITIoNING PRoToCoL
Positioning over LTE is enabled by LPP, which is designed to support the positioning
methods covered previously. LPP call flows are procedure based, where each procedure
has a single objective (for example, delivery of Assistance Data).
The main functions of LPP are
• to provision the E-SMLC with the positioning capabilities of the UE
• to transport Assistance Data from the E-SMLC to the UE
• to provide the E-SMLC with co-ordinate position information or UE measured signals
• to report errors during the positioning session.
LPP can also be used to support “hybrid” positioning such as oTDoA + A-GNSS.
In the case of network based positioning techniques, the E-SMLC may require
information from the eNodeB (such as receive-transmit time difference measurements
for supporting ECID). A protocol called the LPP-Annex (LPPa) is used to transport this
information.
LPP
OTDOA
ECID
A-GNSS
eXTensions To LPP (LPPe)
LPP was designed to enable the key positioning methods (with enhancements) available on 2G and 3G networks, and provide the minimum set of data necessary for positioning. The oMA has proposed extensions to LPP (LPPe) which can be used to carry more data to improve existing positioning techniques as well enable new methods (such as WLAN positioning). LPPe is primarily considered a User Plane positioning enabler.
An overview of LTE Positioning
9 • SPIRENT WhITE PAPER
CoNTRoL PLANE PoSITIoNING
With Control Plane implementations, most commonly used in emergency services,
positioning messages are exchanged between the network and the UE over the
signaling connection. In LTE, control plane positioning is enabled by the Mobility
Management Entity (MME), which routes LPP messages from the E-SMLC to the UE using
NAS Downlink Transfer Messages. See Figure 5. Control Plane positioning is quick,
reliable and secure.
conTroL PLAne cALL fLows
Network Initiated Location Request (NILR) – Primarily used for emergency positioning. The network instructs the UE to provide a position, and may send unsolicited Assistance Data
Mobile Terminated Location Request (MTLR) – Initiated by the network, this differs from NILR with the addition of privacy features – the user can reject the location request.
Mobile originated Location Request (MoLR) – The positioning session is initiated by the UE, which contacts the MME with the request. The remainder of the call flow is similar to NILR.
Figure 5: Control Plane Positioning
An overview of LTE Positioning
SPIRENT WhITE PAPER • 10
USER PLANE PoSITIoNING
User Plane Positioning over LTE uses the data link to transmit positioning information,
and is enabled by the SUPL protocol. SUPL 2.0 supports positioning over LTE as well as
2G and 3G networks, and provides a common user plane platform for all air interfaces3.
SUPL does not introduce a new method to package and transport Assistance Data,
instead it uses existing control plane protocols (such as RRLP, IS-801 and LPP). See
Figure 6. SUPL uses the data link to transmit positioning information, and is enabled by
an entity called the SUPL Location Platform (SLP). The SLP handles SUPL messaging,
and is typically able to interface with the E-SMLC for obtaining Assistance Data. SUPL
messages are routed over the data link via the LTE P-GW and the S-GW entities. See
Figure 7.
SUPL 2.0 enables a complex feature set that is pertinent to mobile applications,
including area based triggering, periodic reporting and batch reporting. SUPL 2.0
also features support for emergency positioning over the data link, and support for
major positioning technologies (including multi-location technologies such as WiFi
positioning).
The primary positioning enabler in SUPL 2.0 is an underlying control plane protocol (such as RRLP or LPP). This implies that SUPL 2.0 can be used over any network, as long as the SLP and SMLC are able to interface and agree upon a common positioning protocol. This flexibility is very useful in initial LTE roll outs, as it allows operators to enable SUPL 2.0 positioning over an existing control plane protocol such as RRLP.
IP data connection over any air interface
Figure 6: SUPL 2.0 supports multiple control plane protocols
Figure 7: SUPL 2.0 network architecture
3 For more information, please see the following reference guide “Secure User Plane Location 2.0 Reference Guide” and the two webinars “Unleash the Business Potential of LBS over LTE Using SUPL 2.0” and “SUPL 2.0 Conformance Requirements for LTE” on www.spirent.com.
An overview of LTE Positioning
11 • SPIRENT WhITE PAPER
AREA EVENT TRIGGERING
SUPL 2.0 features the use of geographical ‘triggers’, which enable the UE to report its
position if it enters, leaves, or is within a particular area. Triggering may be enabled
either by the network or by the SET, with the two entities agreeing on trigger criteria.
Area Event triggers enable key mobile applications such as Check-in services, shopping
deals and offers, location based advertising, and child location. The key factor
determining the effectiveness of triggers is how accurate the obtained position is.
EMERGENCy PoSITIoNING
Emergency Positioning in 2G and 3G networks has been processed over control plane,
as user plane protocols did not have the necessary network elements to support such a
requirement. SUPL 2.0 introduces an entity known as the Emergency SLP (E-SLP) which
can co-ordinate with the IP Multimedia Subsystem (IMS) in LTE networks to enable
positioning over an emergency call. The E-SLP functionality can be added to an existing
SLP used by the network. When an emergency call is in process, the IMS coordinates
the call with a Network Initiated Location Request from the E-SLP. Emergency
positioning may override user notification and privacy settings, and receive priority
over all non-emergency SUPL sessions. Emergency sessions are typically initiated by a
Session Initiation Protocol (SIP) Push.
Key Trigger criTeriA
Type of trigger
List of target areas
Start and stop time
Measurement reporting criteria
Number of times to re-use the trigger
An overview of LTE Positioning
SPIRENT WhITE PAPER • 12
SUPPoRT FoR MULTI-LoCATIoN TEChNoLoGIES
one of the goals of SUPL 2.0 is to serve as a single, unifying user plane protocol
independent of air interface. SUPL 2.0 can be used over 2G, 3G and LTE, with full
support for the key positioning techniques and positioning protocols used in these
networks. A key feature of SUPL 2.0 is flexibility in protocol use – for example, RRLP can
be used to transfer assistance data over an LTE air interface.
SUPL 2.0 supports reporting of cell information for all major cellular wireless
technologies as well as wireless LAN access point info. This feature, termed multi
location ID, allows a location server to process many different types of measurements
in order to calculate a more accurate position.
In future, SUPL 3.0 will support extensions to the LPP protocol (LPPe). These extensions
serve to include additional information to enhance existing positioning techniques as
well as to provide a bearer for new positioning methods (such as sensor positioning and
Short Range Node positioning).
SUMMARy
Since the LBS market is growing rapidly in size and scope, enabling high accuracy
positioning both indoors and outdoors, is essential to validate the commercial promise
of the enabling technology, as well as to meet the FCC’s emergency mandate in the US.
2G and 3G networks have used a variety of positioning techniques, such as A-GNSS,
Cell ID and AFLT to satisfy positioning requirements. LTE introduces pivotal technologies
that are not only able to provide adequate positioning performance for emergency and
commercial purposes, but also to seamlessly transition from existing technologies. The
deployment of LPP and SUPL 2.0 enables a diverse set of features, such as geofencing,
emergency positioning over user plane, and multi-location technologies such WiFi
Positioning. however, this advanced feature set comes at the cost of increased
complexity, requiring comprehensive conformance and performance testing to fully
validate the technologies.
LTE introduces new positioning technologies that are complex and will require extensive verification to provide adequate positioning performance for emergency and commercial purposes.