1. 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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2. 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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6. 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 flowfor 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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7. Figure 5: Cell ID-based positioning method - signalling flow 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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