GB2441373A

GB2441373A - A method of determining location in a mobile device

Info

Publication number: GB2441373A
Application number: GB0617244A
Authority: GB
Inventors: Ian Lasseter Phillips; David Wynne Thomas
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2006-09-04
Filing date: 2006-09-04
Publication date: 2008-03-05
Also published as: GB0617244D0

Abstract

A method of determining location in a mobile device comprises receiving signals (S1, S2, S3, S4) at the mobile device (M1), sent from a plurality of base stations (BSI, BS2, BS3, BS4). Each received signal includes data relating to location and time of transmission of the signal from its respective base station. Location information for the mobile device M1 is derived from the plurality of signals (S1, S2, S3, S4).

Description

I

A N4ETHOD OF DETERMINING LOCATION IN A MOBILE DEVICE This invention relates to a method of determining location in a mobile device.

Mobile positioning is of growing importance in mobile communications systems due, in part, to legislation coming into effect around the world requiring mobile communication systems that support voice calls to allow emergency services to locate a user who makes an emergency call. There is also commercial interest in providing location dependent services, such as finding the nearest café or sending details of specific offers in the vicinity of a user.

Existing mobile communications systems such as global system for mobile communication (GSM), or universal mobile telecommunication system (UMTS), use a location system consisting of a number of specialised receivers. These receivers make timing and received power measurements of specific, known signals transmitted by the mobile terminal. The liming and received power measurements, together with the mobile temiinal's current cell affiliation, are then transferred to a central processor where algorithms are used to make an estimate of the mobile terminal's location. The location information can then be made available to the emergency services or to the user.

For example, a method known as time difference of arrival (TDOA) is used for GSM and UMTS. The mobile systems measure the time difference of arrival of signals either transmitted by the mobile station and received at multiple base stations which have a common timing reference, or signals transmitted by multiple base stations with known timing and received at the mobile station. The position of the mobile station can then be calculated from the received timing differences using, for example, Bancroft' s algorithm.

The disadvantage with this approach is that signals from, or to, the non-serving base stations will be low, relative to interference from other users in the same cell.

This limits the number of signals that can be used in the position estimate. Also, for a mobile to measure its own position requires knowledge of the positions of surrounding base stations. If the radio network makes the measurement, then this problem is dealt with, but there may be privacy concerns. a)

The conventional methods have a number of disadvantages, including that the accuracy of the timing estimate at a given receiver is related to the signal to noise ratio (SNR) of the received signal. Since cellular communications are specifically designed not to transmit large signals into adjacent cells, the signals received by receivers in other cells are weak and thus the timing measurements have large amounts of noise.

Therefore, only a few receivers are able to make good timing estimates, and reducing the number of measurements available to the central processor has a negative impact on the accuracy of the location estimate.

In accordance with the present invention, a method of determining location in a mobile device comprises receiving signals at the mobile device, sent from a plurality of base statkns; wherein each received signal includes data relating to location and time of transmission of the signal from its respective base station; and deriving from the plurality of signals, location infonnation for the mobile device.

The base stations provide effective coverage of a large number of mobiles by broadcasting at high power from the base station' more suitably positioned antennas.

Mobile terminals are therefore able to receive signals from a greater number of base statiorE, than the number of base stations which would be reached if the terminals themselves made the transmissions.

Preferably, a common reference location and time is set for the base stations and the signals sent from each of the base stations contain data, set relative to the comnion reference.

Preferably, location is derived by comparing time of arrival of the signal at each mobile device and the time of transmission and location data of that signal.

Preferably, signals from at least three base stations are rEed to derive location of the mobile device.

Preferably, each base station broadcasts its signal.

Broadcasting the timing and position information allows any receiver within range to make use of the received data to derive its own position.

Preferably, the mobile device transmits the derived location information to the network.

Preferably, the system is an orthogonal frequency division multiplex system (OFDM).

The advantage of using an OFDM system. with a single unique frequency for each transmission from a base station, is that it is possible to transmit a high power signaL whilst limiting the interference, due to the orthogonality of the signals.

Preferably, each signal is sent from each base station on a high power beacon channel.

The mobile device may send position data to the network at regular intervals or only on receipt of an invitation from the network.

Preferably, the base station location data is provided via local satellite receivers at each base station.

Although, any suitable time source could be used, typically, for efficiency, this is often derived from the local satellite receiver, as satellite systems arc fitted to many bases stations.

An example of a method of determining location in a mobile device will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of a typical system for carrying out the method of the present invention: and, Figure 2 illustrates the signal content in more detail.

Fig. I illustrates an example of the present invention. Each base station (BSI to BS4) transmits a high power signal (S 1 to S4) containing information relating to the base station position and timing relative to an absolute reference. This information may he derived from various sources, for example. the high power broadcast channel may be carrying information about the position and timing of the base station from which it was sent that has been supplied by a global navigation satellite system (GNSS) receiver, such as a global positioning system (GPS) receiver at the base station. The high power signalling channels, or beacon channels, have a lot of energy at a single frequency, so they only interfere with one or two other channels, rather than producing widespread interference.

A mobile terminal Ml receives a number of the high power signals from different base stations, since each signal is transmitted at a higher than average power, so it can he received by a mobile station in another cell, rather than just mobiles within that base station's cell. The mobile M I then uses the information extracted to make an estimate of its own position. The position of the mobile station can be calculated by measuring the time of arrival of the signal from each base station and relating that to the time and position at which the signal was transmitted. Because this information is sent over the broadcast channel, the mobile station is able to estimate its distance froin the base station without further radio network assistance. By receiving three such signals the mobile station is able, using certain assumptions, to estimate its two dimensional position without any height information. It is assumed that the equations are solvable, for example if the three base stations are in a straight line, then the simultaneous equations cannot be solved. It is also assumed that the signal arrived in a direct line from the transmitter. Provided that the transmitters are not in the same plane, more than three signals received at the mobile allows height information to be estimated too.

The format of the signals sent from the base station and sent from the mobile device hack to the network is illustrated in Fig. 2. Fig. 2a shows a broadcast message 1 is sub-divided into a varying number of bits w, x. y, z which contain information relating to base station identifier 2, base station position 3, base station height 4 and base station service 5. Fig. 2b shows a partitioned broadcast message and Fig. 2c shows an OFDM transmitted signal and occurrences of high power beacon symbols 6 over time for each sub-carrier.

Transmitting base-station location information over a high power channel, such as the Flarion Flash-OFDM beacon channel, is particularly effective within OFDM systems. Examples of mobile communication systems using orthogonal frequency division multiplex (OFDM), particularly in the downlink, include third generation partnership project (3GPP) Long Term Evolution (LTE); wireless broadband (WiBro) / wireless interoperability for microwave access (WiMAX -IEEE 802.16); mobile broadband wireless access proposed standard IEEE 802.20; WINNER Project and Flarioiis FLASH-OFDM, although most of these example have been designed with data traflic in mind and so the issue of location has not been considered. However, using a voice over internet protocol (VoIP) service applied to OFDM potentially produces a requirement for mobile positioning.

OFDM has the particular advantages of having very low inter-symbol interference (ISI) and having a high degree of granularity. This allows a single symbol to be transmitted at a much higher than normal power without unduly affecting the performance of adjacent symbols. Receivers in adjacent cells suffer a large amount of interference from the high power signal at a single frequency, but this will only affect a single symbol and can be corrected using strong forward error correction. By using the frequency position of the high power signal to denote sonic information, messages can be received by mobile terminals in adjacent cells.

In the present invention the high power of the transmission to the mobile means that the mobile is able to receive signals from a number of base stations with high signal to noise ratios and is therefore able to make a high quality estimate of its position. This approach increases the number of base stations likely to be received.

when compared with conventional cellular approaches. Furthermore, the antenna tends to be physically higher at the base station than at the mobile, given that size reduction is less of an issue for a base station, so this also improves the likelihood of receipt.

Claims

CLAIMS

1. A method of determining location in a mobile device, the method comprising receiving signals at the mobile device, sent from a plurality of base stations; wherein each received signal includes data relating to location and lime of transmission of the signal from its respective base station; and deriving from the plurality of signals, location information for the mobile device.

2. A method according to claim 1, wherein a common reference location and time is set for the base stations and the signals sent from each of the base stations contain data set relative to the common reference.

3. A method according to claim I or claim 2, wherein location is derived by comparing time of arrival of the signal at each mobile device and the time of transmission and location data of that signal.

4. A method according to any preceding claim, wherein signals from at least three base stations are ied to derive location of the mobile device.

5. A method according to any preceding claim, wherein each base station broadcasts its signal.

6. A method according to any preceding claim, wherein the mobile device transmits the derived location information to the network.

7. A method according to any preceding claim, wherein the system is an orthogonal frequency division multiplex system.

8. A method according to any preceding claim, wherein each signal is sent from each base station on a high power beacon channel.

9. A method according to any preceding claim, wherein the base station location data is provided via local satellite receivers at each base station.