Assisted GPS

Some applications benefit from very high accuracy that can be achieved by GPS receivers

integrated into mobile terminals. Latency, usability and accuracy of this GPS receiver can be

improved by sending GPS assistance data from the network to the UE. With the assisted GPS

method it is possible to:

reduce the GPS initialisation and acquisition times; the search window can be limited

and the measurement speed increased significantly for improved Time-To-First-Fix

(TTFF),

consume less handset power than the conventional GPS; this is because of rapid start-

up times as the GPS receiver can be in idle mode when it is not needed,

increase the GPS sensitivity and coverage; navigation messages are obtained through

UTRAN, so the GPS can operate in situations when GPS data is disturbed (for example

indoors, in urban environment).

The basic idea in Assisted GPS is to establish a GPS reference network whose receivers have

clear views of the sky, and can operate continuously. The RAN collects the required GPS data

from this reference network to be able to generate the required assistance data elements to the

UE to assist and speed up the:

location calculation function (see Figure LCS – UE based GPS)

or

signal measurement function (see Figure LCS – NW based GPS).

Also at the request of a User Equipment (UE) or network-based application, only the assistance

data from the reference network can be transmitted to the UE to improve performance of the GPS

receiver (see Figure LCS – UE based GPS).

Figure 6: LCS – UE based GPS

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GPS and assistance data

The principle of GPS positioning is very simple. GPS positioning is based on measuring relative

times of arrival (TOA) of signals sent simultaneously from a multiplicity of satellites. The distance

between the satellites and the receiver is solved indirectly from the TOA measurements together

with the exact GPS time.

The exact time is needed to calculate the satellites' positions from the received navigation data,

basically to find where they were at the time the signals left the satellites. The GPS satellites are

Medium Earth Orbit (MEO) satellites that move along their orbits very fast, generally a few

kilometres per second. This way, an error of even a few milliseconds induces considerable errors

in SV positions and consequently in the user's position.

In theory, three TOA measurements would be enough to calculate the receiver's position, and also

the velocity in global coordinates assuming that the exact time was already known. In practice,

low-cost and low-accuracy oscillators are used in receivers as local clocks, so a fourth TOA

measurement is required to correct and estimate the error in local time. The fourth measurement

reformulates the 3D position calculation problem into a four-dimensional position-time problem,

where the time error becomes the fourth dimension.

The 50-Hz navigation message includes data unique to the transmitting satellite and data common

to all satellites. The navigation message contains time information, satellite clock correction data,

ephemeris (that is, precise orbital parameters), almanac (that is, coarse orbital parameters),

health data for all satellites, coefficients for the ionospheric delay model and coefficients to

calculate the Universal Coordinated Time (UTC) from the GPS system time. It takes 12.5 minutes

to receive all the satellite data from the GPS System.

GPS positioning depends on the accurate GPS time, navigation data containing satellite orbital

parameters, and distance measurements. If any of these three elements is missing, it can

completely paralyse the GPS-based positioning. This is easily the case in urban areas or indoors,

where constructions or dense foliage attenuate GPS signals, hindering signal reception, and

navigation data demodulation. Moreover, most of the people using positioning services are living

in these areas, which is inherently unsuitable for GPS.

Figure 8: Attenuation of GPS signals

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Navigation data reception depends greatly on the signal propagation path. In order to recover

positioning, the missing elements (except distance measurements) can be delivered through a

cellular network. The cellular network can be equipped with a reference GPS receiver located in a

place having an unobstructed view to the sky. Using the reference receiver, the network can

receive navigation data, exact time and other data, and can relay them over the cellular air

interface. By giving satellite orbital parameters and exact time through the network, the availability

and latency of the GPS can be improved. Despite that GPS is not meant for indoor or urban

positioning, the Assisted GPS (AGPS) can be harnessed for that purpose by cellular networks.

However, the indoor environment is badly contaminated by reflected multipath components.

The GPS receiver usually fails when losing line of sight visibility to satellites. The AGPS receiver

can overcome much of the problems, but when brought indoors, it becomes totally dependent on

network assistance and network support. If there is no network coverage and no fresh data in the

receiver's memory, positioning cannot be done. There are also practical constraints on how weak

signals can be acquired and tracked by the receiver. There are cases where even network

assistance cannot improve sensitivity enough to enable the UE to make the TOA measurements.

The A-GPS has different values for latency according to its operational status. In autonomous

operation with line of sight conditions, the cold start, that is searching the satellite signals

independently and receiving the necessary navigation data from the satellites, takes about 35

seconds. If valid navigation data is already in memory or received as assistance, the latency is

from 1-2 seconds up to 10 seconds. In very weak signal conditions, acquisition of satellite signals

takes approximately 1.5 seconds per satellite causing a total of 10 seconds latency. The latency of

network assistance is within a few seconds.

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Broadcast of A-GPS Assistance Data

UE-based A-GPS Using External Reference Network and Network Based A-GPS Using External

Reference Network features utilize assistance data sent from Serving Mobile Location Centre

(SMLC) to UE over dedicated connections. Assistance data of A-GPS positioning method can be

broadcasted using SIBs instead of dedicated signaling.

This feature can be activated/deactivated on operator request. Operator is able to activate this

feature for BTS basis. Operator is able to activate broadcasting of SIB15, SIB15.2 and SIB 15.3

separately but to be able to activate SIB15.2 or SIB 15.3 broadcasting of SIB 15 must be

activated.

The UEs which are in other state than Cell_DCH read the SIB information primarily from the cell

broadcast. UE stores the GPS assistance data and uses the stored data as long as it is valid. If

the assistance data stored in the UE's memory is valid, the UE can start the positioning

immediately and the delay caused by the dedicated signaling is avoided. In typical cases A-GPS

positioning can be speeded up to two to three seconds.

When this feature is activated RNC shall broadcast assistance data to all UEs in particular area

by sending assistance data in System Information Blocks. The assistance data to be broadcasted

for assisted GPS may contain a subset of or all of the following information: reference time,

reference position, ephemeris and clock corrections, almanac, UTC model, ionospheric model

and RTI data.

When requested by RNC, SAS generates the GPS assistance data and sends it to the RNC over

the Iupc interface. RNC forwards the data to BTS over Iub interface for possible broadcast over

the Uu interface.

This feature requires that SAS supports OnModification reporting.

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Cell coverage-based (Ci+RTT) with geographicalcoordinates

Cell coverage-based positioning is based on the knowledge of the serving cell ID, the

geographical coordinates of the serving cell, the corresponding antenna direction, cell range and

other cell parameters. The information about the serving cell is obtained by, for example, paging,

location area update, cell update, or routing area update.

The accuracy of the cell coverage-based method depends directly on the cell size; in small micro

or pico cells, the accuracy is much better than in large macro cells. The accuracy of the cell-based

method is also improved (as shown in Figure Cell ID + RTT location method) by combining cell-

specific information with the supplementary signal Round Trip Time (RTT), and the UE Rx-Tx Time

Difference (TD) measurements.

Figure 1: Cell ID + RTT location method

The cell coverage-based location result can be provided as the service area identity or as an X-Y-

Z estimate of the geographical coordinates of the UE including the uncertainty of the estimate.

If the LCS client requests the service area identifier (SAI), the serving mobile location centre

(SMLC) maps the cell ID to the SAI, and returns the SAI location of the UE to the core network. If

the geographical coordinates are requested, the LCS calculation algorithms return the estimate of

the UE location in geographical coordinates with altitude, including the uncertainty of the estimate.

RTT and TD measurements

Network architecture and signalling flow

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Network architecture and signalling flow

The network architecture and its elements for the cell coverage-based positioning method are

shown in Figure Network architecture and its elements for cell ID-based positioning method. An

LCS client can request the UE location, for example, from the iGMLC. After validating the location

requestor and the need to locate the UE, the iGMLC performs a request to the MSC. The MSC

does the UE search with, for example, paging and privacy checks and subsequently sends a

location-reporting request to the Serving RNC (SRNC). The SRNC pages the UE if no cell ID is

available, and it also requests the RTT measurement for the UE from the BTS and the Rx-Tx (=TD)

measurements from the UE. The SMLC functionality calculates the UE location and sends the

result to the iGMLC through the core network.

Figure 4: Netw ork architecture and its elements for cell IDbased positioning method

The Figure Cell ID-based positioning method - signalling flow for MT-LR describes the flow of

the signals and messages in the case of a mobile terminated location request procedure (MT-LR).

All requests are defined according to 3GPP standards and RAN responds with the actual

geographical location information.

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Figure 5: Cell ID-based positioning method - signalling f low for MT-LRMac-hs

From the RAN point of view, the location procedure begins when the SRNC receives a location

request from the Iu interface (RANAP: Location Reporting Control message). This message is

forwarded to the integrated SMLC within the RNC. With this message, the SMLC gets vital

information about the location request. The message includes information such as message

priority (emergency/high/normal), time limit for serving this message (low delay/delay tolerant),

required location accuracy and so on.

SMLC then puts the received Location Reporting Control message into the internal LCS queue.

The queue is organised in such a manner that all Location Reporting Control-messages which are

related to emergency calls are served first. When there are no requests related to emergency calls

left in the queue, all messages with high priority are served. When there are no high priority

messages left in the queue, all messages with normal priority are served. SMLC also constantly

monitors the length of the queue and the location requests within it. When SMLC notices that a

request cannot be served in the given time limits because of the location capacity per second and

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the amount and priority of the requests, SMLC rejects the request and sends the RANAP:

Location Report message to the CN with proper cause value.

SMLC takes as many Location Reporting Control-messages as possible from the queue into

processing. The amount is managed with LCS capacity limitations, which is 15 location request

per second including all types of requests. Capacity limitation is valid also when operating in SAS

centric mode.

In the first stage, SMLC initiates RTT and Rx-Tx measurements for all cells in the active set of the

UE. RTT measurements are initiated over the Iub or Iur interfaces (with the NBAP: Dedicated

Measurement Initiation Request or RNSAP: Dedicated Measurement Initiation Request), and Rx-

Tx measurements are initiated over the control plane RRC signalling (with the RRC: Measurement

Control message). If the UE does not support Rx-Tx type 2 measurements, Rx-Tx type 1

measurements are requested. All WCDMA UEs support rx-Tx type 1 measurements. (They are

also used for call setup purposes to compensate propagation delay of DL and UL in the UE.) After

the SMLC receives the measurements from the UE (with the RRC: Measurement Report

message) and from the BTSs (with the NBAP: Dedicated Measurement Initiation or RNSAP:

Dedicated Measurement Initiation Responses), the SMLC calculates the CI+RTT location with

uncertainty with all the available information from the measurements and with the radio network

database information related to the active set cells (WLCSE elements). If all measurements fail,

SMLC uses the radio network database information related to the active set cells to calculate the

CI location for the UE, as backup method.

The location estimate of the UE is sent to the CN through the Iu interface (with the RANAP:

Location Report message). The message contains the location estimate or suitable failure cause.

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Figure 7: LCS – NW based GPS

GPS and assistance data

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