GB2406021A

GB2406021A - Power efficient method for a mobile terminal to determine its location by processing only parts of the received signal

Info

Publication number: GB2406021A
Application number: GB0321381A
Authority: GB
Inventors: Nikolaos Prelorentzos
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 2003-09-12
Filing date: 2003-09-12
Publication date: 2005-03-16
Also published as: GB0321381D0

Abstract

A mobile terminal that estimates its position within a cell of a serving base station in a CDMA cellular network using signals received from beacons/base stations, saves power by processing only parts of the received signals to obtain the propagation delays of the beacons' signals. The mobile terminal receives beacon signals (e.g. pilot transmissions) in specified periods and correlates the received signals with the parts of the beacons' code sequences expected in the specified periods, i.e. with the portions (chip indexes) of the scrambling codes expected to traverse the cell during the specified periods. The mobile terminal can then obtain the propagation delays of the beacons' signals and these can be used either by the mobile terminal or by the network, to estimate the mobile's location. The specified periods (measurement windows) and the parts of the code sequences to be used by the mobile may be signalled to it by the serving base station.

Description

240602 1 Power saving in a wireless communication terminal This invention

relates to mobile wireless communications and in particular it relates to power saving for a mobile terminal estimating its position A discussion of the prior art is to be found in "Wireless location in CDMA cellular radio systems" by James J. Caffery Jr. published by Kluwer Academic Publishers c 2000 (ISBN 0-7923-7703-6).

There are requirements for determining the locations of mobile terminals operating with cellular radio networks. The Global positioning system (GPS) can provide accurate positions for mobile terminals but requires that a GPS receiver be incorporated in the mobile terminal. A GPS receiver causes extra costs, takes up space within the mobile terminal and in some situations, e.g. for a terminal inside a building, the GPS system does not function.

Alternative radiolocation methods applicable for 3G systems use time of arrival (TOA) and observed time difference of arrival (OTDOA) techniques. A representation of the TOA method is given in figure I where three base stations are transmitting radio signals. The radio transmissions from the base stations are shown as propagating in expanding circles with the mobile terminal 1 at the intersection of the three circles. The instantaneous distance of the mobile terminal from the three base stations is dj, dj and dk respectively. An estimate of the position of the mobile terminal can be computed from measurements of the times of arrival at the mobile terminal of the three base station transmissions. The measurement error margins are shown in figure 1 and the uncertainty in the position of mobile terminal 1 illustrated by the overlap.

Owing to the variation in propagation conditions in cellular networks and in particular the multipath problem, accurate positioning using TOA techniques can be a difficult process. The preferred radiolocation method that is based on time measurement is the OTDOA method that uses the time difference of arrival at the mobile terminal of the signals from a number of base stations. Where the base station transmissions are not synchronized, some means to establish the Real Time Differences (RTD's) must be employed. The range of time differences of arrival of signals from any two base stations describes an hyperbola as illustrated by numeral 5 in figure 2.

In UMTS systems that are based on wideband code division multiple access, discrimination of individual base station signals is also dependent on recognition of their unique pseudo random (PN) codes of 38400 chips (scrambling code) transmitted in a radio frame. The mobile terminal detects common pilot channel (CPTCET) transmissions from a plurality of base stations in its vicinity and measures their times of arrival. An estimate of the location of the mobile terminal is then determined by means of suitable algorithms such as those described in the referenced book of James J. Caffery Jr.

Mobile terminals have a limited power supply and usually depend upon batteries. The process of decoding the base station transmissions associated with location estimation consumes a relatively large amount of power and reduces the availability or performance of the mobile terminal.

The speed of the processing is also important in freeing mobile resources.

It is an object of this invention to reduce the power consumption associated with the mobile location process in a mobile terminal without resorting to any modifications of the normal network transmissions.

In accordance with the invention there is provided a method of saving power for a mobile terminal estimating its position within the cell of a serving base station in a CDMA cellular network in which the mobile terminal receives, in specified periods, beacon signals and in which the received signals are correlated with the parts of the beacons' code sequences expected to traverse the cell during the specified periods, to obtain the propagation delays of the beacons' signals.

In the following example the base stations of a network may be regarded more generally as beacons.

An embodiment of the invention will now be described with reference to the accompanying figures in which: Figure 1 illustrates the time difference of arrival technique.

Figure 2 illustrates the observed time difference of arrival technique.

Figure 3 is a flow diagram showing the steps taken in the mobile station, to make a radiolocation OTDOA measurement, Figure 4 is a flow diagram showing the sequence of operations when the location estimates are determined at the mobile terminal, Figure 5 is a flow diagram showing the sequence of operations when the location estimates are determined at the network.

With reference to figure 2, part of a cellular communications network is illustrated. We shall assume, at first, line of sight reception of all base station signals arriving at the mobile terminal I and consider later the problems associated with multipath distortion. The mobile terminal 1 is in communication with a serving base station 2 and receives also pilot channel signals from base stations (more generally beacons) 3 and 4. The mobile terminal I may receive transmissions from other base stations (not shown) but they will not be considered in this example. The mobile terminal 1 is at a distance d2 from serving base station 2, d3 from base station 3 and d4 from base station 4. The distance d2 is known at the base station 2 and at the mobile terminal 1 by means of standard prior art techniques e.g round trip delay. Measurements of the times of arrival of the transmissions from base stations 3 and 4 at the mobile terminal 1 provide inputs for a standard algorithm to determine the location of mobile terminal 1.

The general location of the mobile terminal I is known because of its position within the cell of the serving base station 2. Information regarding the positions of base stations 3 and 4 and information serving base station distance from the mobile terminal 1 could be provided for example by the serving base station 2 or by special location measurement units available in the network. The timing of transmissions from base stations 3 and 4 (the offsets) are known also. A mobile terminal within the cell of the serving base station 2 can therefore, under the direction of the serving base station, set limits on the expected times of arrival of the transmissions from base stations 3 and 4. The mobile terminal can be expected to receive particular portions of the scrambling codes transmitted by base stations 3 and 4 in any specified time intervals.

When mobile location is required, the serving base station 2 directs the mobile terminal 1 to take measurements across all radio channels for specified periods (the measuring periods). Base station 2 also indicates to mobile terminal I those parts (chip indexes) of the scrambling codes from base stations 3 and 4 expected to be received during the measuring period.

The parts of the scrambling codes required for the correlation process may be retrieved from memory or generated on the basis of the indicated chip indexes.

Detection of the signals from base stations 3 and 4 proceeds by correlating the signals received in the measuring periods with the indicated parts of the scrambling codes. The indicated parts of the scrambling codes used in the correlation process are those expected to traverse the cell of the serving base station 2 during the specified period. The maximum and minimum times of arrival at the mobile terminal I of the serving base station 2 signals are taken into account when setting the duration of the measuring period.

The "footprint" of the whole of a 38400 chip frame is 3000 kilometres, assuming a chip duration of 260 nanoseconds and so the "footprint" across an unusually large cell size of radius 20 kilometres equates to 512 chips.

As is always the case with cellular systems, the effects of multi-path and the corresponding delay spread must be considered. The duration of the specified period must be sufficient to receive enough scrambling symbols for a good correlation peak and sufficient to accommodate multipath distortion and timing errors. We take in this example a code length of 5 symbols or 1280 chips, assuming 256 chips per symbol, over a cell of 20 kilometre radius of 512 chips and round off to a total of 2000 chips.

With reference to figure 3, a flow diagram shows the steps taken in the mobile station. The purpose of the process to be described is the estimation of first received path and its distance in chips from the start of the measurement window. The start of the measuring window is then estimated (305). The parameters of the specified period ( measurement window) are set (301) and an estimate of signal to noise (SNR) is derived (302) from the signals received in the window. In this example, the SNR is estimated as the mean of the absolute values of the received signal and is used to set a threshold level for use in detection of the first path. The signals arriving in the measurement window are then correlated (303) with those portions of the scrambling codes expected from the beacons for the duration of the window. After correlating the received signal with the part of the scrambling code for (say) base station 3 the correlation results are normalised with the maximum correlation value and the values that are above a threshold are determined (304). The position of the first of the values above threshold, in chips, is accepted as the first receive path.

At stage 307, a confidence check is applied to the measurement process and acceptable measurement are stored in 308 for use in the location estimation algorithms. The confidence check level could take into account factors such as the difference of the first estimated path from the highest detected correlation peak. Where measurements fail the confidence check 307 those measurements are not stored for use and the process is repeated for a subsequent measurement window.

The same process may be repeated for further measurement periods and the correlated values of each of the measurement windows accumulated. By this means the effect of multipath interference is reduced, the power of the paths increased and the first path position within the received frame becomes easier to identify.

With reference to the flow diagram of figure 4, the sequence of operations when the location estimates are determined at the mobile terminal is shown.

When a location request 401 is generated at the network, control information is sent 402 to the mobile terminal via the serving base station.

The mobile terminal then takes measurements 403 during the measuring periods and processes the measurement data 404. The location estimates are then derived from the processed measurement data at 405 and the estimated position sent to the network 406.

The sequence of operations when the location estimates are determined at the network are shown in the flow diagram of Figure 5. A location request for the mobile terminal is raised at the network 501 and control information sent to the mobile terminal 502. The mobile terminal takes measurements during the measuring periods 503 and processes the measurement data 504.

The processed measurement data are then sent to the network 505 and the mobile position estimates determined by the network 506.

Claims

Claims 1. A method of saving power for a mobile terminal estimating its

position within the cell of a serving base station in a CDMA cellular network in which the mobile terminal receives, in specified periods, beacon signals and wherein the received signals are correlated with the parts of the beacons' code sequences expected to traverse the cell during the specified periods, to obtain the propagation delays of the beacons' signals.
2. A method for power saving as in claim 1 in which beacon signals are standard pilot transmissions from base stations of the cellular network in the vicinity of the serving base station.
3. A method for power saving as in claim I or 2 in which the specified periods and the parts of the beacons' code sequences to be correlated with the received signals are signalled to the mobile terminal from the serving base station.