WO2002082848A1

WO2002082848A1 - Method for improving the accuracy of position determination in a digital mobile radio network

Info

Publication number: WO2002082848A1
Application number: PCT/EP2002/002479
Authority: WO
Inventors: Franz Schreib
Original assignee: Siemens Aktiengesellschaft
Priority date: 2001-04-04
Filing date: 2002-03-06
Publication date: 2002-10-17

Abstract

The invention relates to a method for improving the accuracy of position determination of a mobile station in a digital mobile radio network, particularly in a mobile radio network that is operated according to the GSM standard, during which timing advance measurements are conducted for bursts that are structured bit by bit. The exact distance between the mobile station and the base station is estimated from the run of the graph of a transformed and interpolated correlation function between individual signal values and synchronization bits.

Description

Method for improving the accuracy of position determination in a digital mobile radio network

The present invention relates to a method for improving the accuracy of the position determination of a mobile station in a digital mobile radio network, and in particular to a method for improving the accuracy of the position determination of a mobile station in a digital mobile radio network according to the GSM standard, in which information is structured in bits Data packets, so-called "bursts" are exchanged. Such "bursts" are e.g. described in the GSM specification (GSM 05.02 Section 5.2.2).

The most accurate localization possible of mobile radio devices that are located in a reception and transmission area covered by a digital mobile radio network is becoming increasingly important.

For example, the mobile radio network operators want to localize their customers as precisely as possible in the mobile radio network in order to be able to send individually adapted advertising instructions to a mobile station's location where the user is staying. These advertising notices can e.g. Indicates the nearest restaurant, bookshop or similar.

From the legislature's side, the mobile network operators are e.g. made the request to determine as precisely as possible the position of a user of a mobile station who requests an emergency call service via his mobile station. Abuse is to be prevented and accident victims should be pinpointed.

For all of these applications, the accuracies achieved so far for locating a mobile station in a mobile radio network are not sufficient. Various known methods for determining the position of a mobile station in a digital mobile radio network are described, for example, in "Positioning GSM Telephones" by C. Dräne et. al. in IEEE Communications Magazine, April 1998, p. 46 ff. In particular, so-called "timing advance" measurements ("advance time" measurements) are also explained, as will be explained in more detail below.

In digital mobile radio networks, and in particular in GSM systems, the data transmission between a base station and a mobile station takes place by means of so-called "bursts", which are data packets of a specified length and a specified internal structure.

In current mobile radio network specifications, in particular in the GSM standard, time slots of a predetermined length are provided in the data transmission protocol for exchanging a burst between a sending station and a receiving station, within which a sending station can send a burst and a receiving station connected to it can receive this burst ,

In order to utilize the capacities of a base station, in the reception and transmission area of which there can be a large number of mobile stations., .Optimal., The base station is not permanently ready to receive for each mobile station, but can actually only receive a burst arriving from a specific mobile station during certain time slots receive. This means that a mobile station located at a certain distance from a base station has to send a burst while the base station is not yet ready to receive this burst. Rather, the base station has to be switched from the mobile station to the base station during the running time of the burst, so that it is then ready to receive when the burst arrives at the base station. 1 shows the relationship between the timing advance Δt and the distance Ax between a mobile station and a base station, c is the speed of light (approx. 3 * 10 ⁸ m / s). The factor 1/2 must be taken into account, since the mobile station already sees the synchronization burst shifted by the running time between the mobile station and the base station. If the mobile station sends a RÄCH signal (RÄCH = "Random Access Channel"), this is again delayed by the running time between the mobile station and the base station. 10 The timing advance is therefore twice the running time.

Timing advance measurements have a resolution that corresponds to the duration of a GSM symbol,

15 ie 3.77 μs. GSM 05.04 Section 2.1.1 contains more detailed specifications on the "symbols" used in the GSM standard and their properties. The duration of a symbol thus corresponds to a distance of approx. 3 * 10 ⁸ m / s »3.77 μs = 555 m between the base station and the mobile station.

20

However, a spatial resolution of approx. 555 m is not sufficient for the requirements that the American federal authority FCC specifies for position location services (see e.g. Christopher Dräne, Malcolm Macnaughtan, and Craig Scott, "Positioning GSM

25. Telephones ", IEEE Communications magazine, April 1998 or

Sirin Tekinay, Ed Chao and Robert Richton "Performance Benchmarking for Wireless Location Systems", IEEE Communications magazine, April 1998.

The object of the present invention is therefore to provide improved methods for locating an active mobile station in a digital mobile radio network (in particular in a GSM system) in which timing advance measurements are implemented.

35

This is achieved according to the invention by a method according to independent method claim 1. Furthermore, a com Computer program product provided with which a method according to the invention is represented in a machine-readable form and which can be downloaded to base stations or mobile stations of a digital mobile radio network in order to carry out a method according to the invention.

The dependent claims relate to preferred embodiments of the present invention.

The improvement in the accuracy of the methods proposed according to the invention is based on signal evaluation and calculation methods which take advantage of the fact that individual bursts transmit energy in a structured manner. According to the invention, this is used in such a way that synchronization bits broadcast on the transmitter side for the premature time measurement are evaluated on the receiver side, and a correlation is carried out between received signals and individual synchronization bits, wherein characteristic correlation coefficients are evaluated, which provide a further estimate for the actual distance between the transmitting and the receiving

The station.

In this case, in the case of a GSM mobile radio system, software resources which have already been implemented in it can be used to carry out timing. Advance. Algorithm must be used, such as. this is expressed by claims 3 and 4.

The advantages and features of the present invention also result from the exemplary embodiments explained below in conjunction with the drawings.

Show it:

1 shows the relationship between the delay time ("timing advance") and the distance between a mobile station and a base station; 2 shows the structure of an "access burst" in the GSM standard;

3 shows a block diagram of an exemplary embodiment of a position determination algorithm on which a method according to the invention is based (“CONTEST”).

Algorithm) in a GSM system;

4 shows the course of an interpolated correlation function for the RACH channel as a function of a correlation index, as it is when using the

"CONTEST" algorithm must occur; and

5 shows the course of an interpolated energy sum function as a function of a correlation index, as occurs when the "CONTEST" algorithm is used.

The present invention is explained using the GSM standard as an example. However, it is readily apparent to the person skilled in the art that the method according to the invention can also be used in digital mobile radio systems operated according to other mobile radio standards, provided that individual data packets (bursts) structured with synchronization bits are used to carry out timing advance measurements. It is also not absolutely necessary that the timing advance measurements are only carried out in "uplink" mode (from the mobile station to a base station). In principle, mobile radio systems are also conceivable in which timing advance measurements are carried out in the "downlink" - Operation (from the base station to a mobile station) can be performed.

Fig. 2 shows an example of the structure of a so-called "access burst" as it is in the GSM standard 05.02. is specified in section 5.2.7. The access burst consists of 87 bits, namely 7 tail bits ("end bits"), 41 bit training sequence ("synchronization sequence"), 36 bit information and 3 tail bits

("End bits"). It is only used in "uplink" mode (ie from the mobile station to the base station) and in RÄCH. Next, an algorithm is described as it can be used in a method according to the invention for operating a GSM system in order to determine the location of a mobile station. The algorithm is to be described with reference to FIGS. 2 to 5 for the RÄCH channel (random access channel) in a GSM system. But it ^does' also applies to "normal bursts" in GSM systems apply (see. GSM 05.02 Section 5.2.3).

The 41 synchronization bits indicated in FIG. 2 in an access burst are, in the exemplary embodiment explained below, essential for the implementation of a location determination algorithm according to the invention.

Fig. 3 shows a block diagram of a

Embodiment for a position determination algorithm on which a method according to the invention is based ("CONTEST" algorithm = Correlation Energy Threshold Method = correlation energy threshold method) in a GSM system in which a timing advance measurement for exchanging a timing advance value is carried out between the mobile station and the base station.

In the chain of individual function blocks 2 to 5 shown at the top in FIG. 3, software resources are used, as are already implemented in base stations, and which are combined in FIG. 3 in a block "Timing Advance Algorithm".

Details of such timing advance algorithms are not specifically specified in the GSM standard, but are usually implemented individually by mobile radio operators in their proprietary specifications for the operation of mobile radio base stations. For the purposes of the present invention, it can be assumed that the "Timing Advance Algo-" function block shown at the top in FIG. rithmus "is representative of the timing advance algorithms installed in practice by mobile operators at the time of registration.

For further explanation it is assumed that there are 156 complex IQ values x. (In-phase quadrature phase values) from an AD

Converter 1 (ADC - analog digital converter) in the receiving unit of a base station (ie in "uplink" mode) that corresponds to an incoming signal from a mobile station that is connected to the base station and its distance relative to the base station should be determined as accurately as possible. The discrete IQ values x _t correspond to the (analog) signal voltage values of an incoming signal at a receiving antenna of a receiving station, in the present example thus a GSM base station.

It is assumed that these complex IQ values x- are output in the baseband of the AD converter 1. The specification 0 <i = 156 results from the GSM 05.02 standard. Section 5.2.2.

The complex values x- are reduced in a function block 2 by approximately 67,708 kHz, which corresponds to a rotation of 90 ° per symbol duration in the complex plane. This operation has already been implemented for the timing advance algorithm, which results in a linear system for the rotated complex values y-.

The following formula describes this operation;

ι * / ^ry. = X ,. ^« Exp (-r—) (1)

Then a correlation of the y is found in function block 3 as an example. with the synchronization sequence of the GSM access burst. In another embodiment, there could also be a correlation of the y with the synchronization sequence of a GSM

Normal bursts take place. For the correlation of the y _t with the synchronization sequence of the GSM access burst, the symbols t- are used, for example, which are derived from the bits defined in the GSM 05.02 version 4.6.0 section 5.2.7, ie

t, = ÄV, ₊₇ »2-l (2)

where 0 <Z <41 (3)

As a general rule for other synchronization sequences, synchronization takes place with bits t-, a number l _max of synchronization bits being present.

When synchronizing with the synchronization bits of an access burst, for example 73 (= 68 + 5, namely 68 extended guard bits according to GSM 05.02 Section 5.2.7 and for example 5 bits for the duration of the mobile radio channel) correlation results c _{m are obtained} in the following way:

with 0 <m <73 (5:

As a general rule for other synchronization sequences, correlation results c _{m are obtained} in the following way:

Instead of 5 symbol durations on the basis of equation (5), a different number of symbols can also be assumed for the duration of the mobile radio channel, so that the following generally applies to the number of correlation results obtained:

0 <m≤m _max (5 '). FIG. 4 shows an example of the correlation coefficients c _m for the case of the correlation of the y obtained according to equation (1) with the synchronization sequence of the GSM access bursts, the discrete values c _m having been interpolated in this representation, so that a continuous Envelope results as a correlation function.

The correlation index m according to equation (4) is plotted on the horizontal axis, and a correlation function derived from the correlation coefficients c _m by interpolation is plotted on the vertical axis. For the interpolation of the correlation function from the discrete standardized correlation coefficients according to equation (4), any interpolation method can be used.

The location of the maximum of the correlation function is a measure of the distance between the mobile station and the base station. The distance between two successive correlation indices m, m + l in FIG. 4 corresponds on a time axis to the duration of a symbol in the GSM standard, that is to say 3.77 μs. If the mobile station moves relative to the base station, the correlation maximum also shifts. A shift in the correlation maximum by the width of an entire symbol corresponds to a change in the distance between the mobile station and base station by approximately 555 m.

In practice, however, it can be seen that the exact position of the correlation maximum in FIG. 4 is often relatively unstable, since the correlation maximum flickers due to multiple-run effects and atmospheric disturbances on the signal transmission path, i.e. the position of the correlation maximum is not stable and is therefore not suitable in practice for determining the position of the mobile station.

The method according to the invention now uses a further transformation of the correspondence shown in FIG. 4, which is preferably brought about by software implemented method steps. relation function in a representation in which there is an increase in the stability of the position of a correlation maximum in relation to atmospheric transmission distortions, taking advantage of the fact that in the function block timing advance algorithm shown in FIG Function block 3 provided implementation of the RÄCH correlation according to equation (4), the energy of adjacent correlation results is added up according to the formula (6) given below.

In the block diagram shown in FIG. 3, an evaluation of the energy of the correlation results is already implemented in two function blocks 4 and 5 in the timing advance algorithm.

It is provided that, for example, be calculated using the 73 correlation results from equation (4) e _n energy values.

This happens, for example, according to the following formula ^e n ~ 2-J ^C n + l ~ l ^C n ₊ ll (6).

/ = 1

Here, k is a relatively small number measured on the total number 41 (generalizing _max ) of the synchronization bits, and c "_+; _j the conjugate complex value to c _{π +;} " _J.

In the present case, let k = 5 be implemented as an example.

With k = 5, a total of 73 - 5 + 1 = 69 energy values e _n are obtained according to equation (6). So the following applies:

0 <n <69 (7).

The values standardized to 1

£ = ^■ (8) e max are called standardized energy values E _n .

In general, the following applies:

0 <n≤n _max (7 * '\)

In a known timing advance algorithm, it is provided in function block 5, for example, that the timing advance value is determined by determining the position with maximum energy e _max .

According to the exemplary embodiment of the present invention considered here by way of example for a GSM mobile radio system, the discrete and standardized energy values E _n determined according to equation (8) are now taken as the basis for a continuous energy sum function f (z) which is obtained by means of interpolation. This is illustrated in FIG. 3 by a branch from the “timing advance algorithm” block into a “location determination algorithm” block provided according to the invention.

In the "location determination algorithm" block, a continuous energy sum function f (z) is first calculated as an envelope for the discrete standardized energy values E _π by interpolation in function block 7.

FIG. 5 shows an example of the course of such a continuous energy sum function / (z), which is based on the (per se discrete - see equation (4)) correlation results in FIG. 4. In Fig. 5, the distance between two successive energy sum indices n, n + 1 again corresponds to the duration of a symbol to be transmitted, i.e. 3.77 μs.

For the interpolation of the energy sum function / (z) from the discrete standardized energy values according to equation (7) can be any interpolation method can be used. The simplest form is linear interpolation.

The goal now is to determine the position of the maximum of the energy sum function f (z), which corresponds to the maximum of the correlation function shown in FIG. 4 as best as possible. Because of the "flickering" of the correlation maximum explained in connection with FIG. 4 and because of the number of, e.g., taken into account in the energy value calculation in connection with equation (6). In the region of its maximum, the energy sum function f (z) in FIG. 5 shows the course of a flattened correlation plateau of height 1 that slowly "moves back and forth over time" in the region of its maximum.

The speed at which the position of the correlation plateau "migrates", depending on the number k of the correlation coefficients that are taken into account to form the energy sum function f (z) in equation (6), is significantly lower than the speed at which the maximum of the according to equation (4 ) or (4M determined correlation coefficient "flickers".

With a very slow back and forth correlation plateau. Of the energy sum function f (z), e.g. the . Estimate the middle of the correlation plateau relatively easily and accept it as an estimate for the maximum of the correlation coefficients determined according to equation (4) or (4).

Frequently, however, the problem arises in practice that a "snapshot" determined at a certain point in time to determine the position of the plateau of the energy sum function f (z) shown in FIG. 5 does not allow a reliable statement as to where a maximum of the Equation (4) or (4 ') determined correlation coefficients without distortion effects due to the route would ideally be typical. A longer observation of the situation of a slowly "moving" correlation plateau and a temporal averaging could help here. In the case of a mobile station that does not (noticeably) change its distance relative to a base station over a sufficiently long observation period 5, by averaging a number of correlation plateaus determined in successive time intervals, one can form an average for the center of these correlation plateaus, which in turn is an estimate of the Maximum 10 mum serves the correlation coefficient determined according to equation (4) or (4M).

In practice, however, the problem arises that the times required for this are often not available, e.g. because

15 the mobile station moves relatively quickly to the base station, or only a few bursts can be exchanged between a base station and a mobile station due to interference caused by the transmission path, before the connection is broken off or, if necessary, forwarded

20 of the mobile station comes from a dedicated base station to an adjacent base station (so-called "handover").

In other words, the 25th problem often arises in practice that the current position of the correlation plateau of the energy sum function f (z) shown in FIG. 5 is ultimately not a reliable measure for estimating the distance between a mobile station and a base station.

30 On the other hand, it has been shown in practice that the course of the edges of the correlation maximum (in the case of a stationary mobile station) is very stable. In a preferred embodiment of the present invention, this fact is exploited so that the rising or falling

35 edge of the energy sum function f (z) is checked where the energy sum function f (z) has a predetermined threshold value (eg 50%) of the correlation plateau normalized to the value 1.

On the basis of the distortion-free course of an ideal-typical correlation function, which is known from the well-defined properties of the synchronization bits for the ideal case (without interference effects), it can be empirically determined by which "offset distance" Z _QX such a threshold value with Λ: »100% of the" ideal "center away from the correlation plateau.

If one determines the position Z _{QX of} the threshold value on the rising or falling edge of the correlation maximum, where Λ: »100% of the maximum value of the" correlation plateau "is reached, one can take into account a previously empirically determined offset z _ca , _ibm , _e in FIG. 5 determine the position z _pos , which would correspond to an “i-deal typical” correlation maximum shown in the illustration in FIG. 4, that is to say that it would not “flicker” due to distortions caused by transmission links. This position corresponds with the greatest possible probability to the exact position of the mobile station.

According to the invention, by evaluating the position of threshold values on the rising or falling edges of an energy sum function f (z), the exact position of the ..Korrelati-_. onsmaximums of the correlation coefficients c _m estimated using empirical correction factors.

FIG. 5 illustrates how the value 0.5 was chosen as the threshold value on the rising edge of the energy sum function, ie the location of the rising edge is determined where / (z ₀₅ ) = 0.5 applies.

A previously empirically determined calibration constant z _{calibmle must be} subtracted from this value Zo _ι5 (or added to the falling edge) in order to obtain the best possible Chen estimate for _heading the actual position of Korre ^¬ lationsmaximums to obtain, ie in the present example

Zpos ~ θ, 5 Zcalibrate ^• '"'

The distance between the base station and the mobile station can then be calculated in the following way:

Distance = 555 * z p, os (10)

In a generalization of equation (9):

If Z _pos = 0 in equation (9), then Z _calibrate = Z _{0jc results} . That is, the desired calibration _constant Z _{calibaments is} generally obtained by carrying out a measurement when the distance between the mobile station and base station is zero. The position of the plateau of the correlation maximum of the energy sum function f (z) and the position of the threshold value Zg .. can then be used to determine the calibration constant Z _calibmt , which also applies if the distance between the mobile station and the base station is not equal to zero.

Measurement results of the CONTEST algorithm

A version of the CONTEST algorithm was tested in field trials. A comparison was made with position measurement values, that with GPS receivers (Global Positioning System = location bearing by means of location signals transmitted by geostationary satellites). There were very accurate measurements with an accuracy better than 50 m, but there were also measurements with an error of 460 m. The results have to to be analyzed in more detail. The errors could be based on timing errors in the "Layer 1 trace", timing errors in the mobile station or other causes which have not yet been understood. The results show, however, that in principle a mobile station can be localized with an accuracy of 50 m using the CONTEST algorithm.

Claims

claims

1. Method for improving the accuracy of the position determination of a mobile station in a digital mobile radio network, in particular a mobile radio network operated according to the GSM standard, in which lead time measurement methods ("timing advance measurement" method) are implemented, in which synchronization bits for the synchronization of Time slots are used in which a transmitting station transmits symbols to be transmitted and a receiving station receives them, wherein characteristic correlation coefficients and energy coefficients derived therefrom are calculated between signals arriving at a receiving station from a transmitting station and predefined synchronization bits,

characterized,

that the position of the maximum of an energy sum function f (z) derived by interpolation from the energy coefficients and plotted over the duration of the transmitted symbols is evaluated, and the position of this maximum is used as a measure for a further estimate of the actual distance between the transmitting and receiving station becomes.

2. The method according to claim 1,

characterized,

that threshold values are determined on the rising or falling edge of the graph of the energy sum function f (z) plotted over the symbol duration, where the value of the energy sum function f (z) is a predefined percentage (x »100%) of the normalized maximum of the energy sum function f (z), and where a previously empirically determined offset Z _calibrate for the time interval between the maximum the energy _{sum function} f (z) and the threshold value have been determined so that a position shifted by the offset z _{caäbme relative} to the threshold value position ^ in the maximum _corresponds to the energy _{sum function} with the highest possible probability of the ideal-typical position of the maximum of the correlation coefficients of the incoming signals and the synchronization bits , which would ideally assume the maximum of the correlation function without distortions due to the route, and the actual spatial distance between the mobile station and base station is determined from the estimated ideal-typical position of this maximum of the correlation function.

3. The method according to claim 1 or 2,

characterized,

that the following steps are carried out as part of a timing advance algorithm carried out by the base station in a GSM mobile radio system:

a) Providing incoming signal values corresponding to complex values x- in the baseband of an analog-digital converter on the base station receiver.

b) lowering the complex values x- by a frequency value which corresponds to a rotation by 90 ° in the complex plane, as a result of which the x- in corresponding values i * 7t y. = x. • & xp (-j) are transformed; c) performing a correlation with the synchronization bits t - a synchronization sequence of the GSM access bursts or the GSM normal bursts, as a result of which correlation _results c _{m are obtained} in the following way: ₄ ι c _m = 2y _{ffl + /} _ ₁ * t ₁ with 0 <m <_max; ι = ι kd) calculating energy values e _n = ∑c _{n + M} c ^* _{+ M} , where

O κ n≤ m _^ - k + 1, ee) normalize the values to E _n = - -;

"'max f) calculating an enveloping interpolation curve / (z) based on the energy values E _n ;

g) determining a position z ^ of a predetermined threshold value Λ: »100% of a correlation plateau normalized to the value 1 on the rising or falling edge of the interpolation curve;

h) Calculate

z _caiibme with an empirically predetermined calibration _{offset value} z _cα; , _fera , _{e in} order to obtain the best possible estimated value z _pos for a position in the maximum of the energy sum function, which corresponds with the greatest possible probability to the position of the maximum of an ideal-typical maximum, not distorted due to the transmission path, of the correlation function of the incoming signals with the synchronization bits;

i) converting the position of the estimated value z into an estimate of the distance between the mobile station and the base station according to

Distance = 555 m * z _poi .

4. The method according to claim 1 or 2,

d a d u r c h g e k e n n z e i c h n e t that in step d) a correlation with the

Synchronization sequence of the GSM access bursts is carried out using the synchronization bits t-, which are derived as follows from the bits defined in the GSM 05.02 version 4.6.0 section 5.2.7:

t. = 5_V _{/ + 7} * 2-l, where 0 </ <41

5. Computer program product, stored on a computer-readable medium, the computer program product containing machine-readable program means for reading command steps contained in the computer program product for implementing a method according to one of the preceding claims in a base station or a mobile station in a digital mobile radio network.