EP2188645A1

EP2188645A1 - Method for increasing the location accuracy for unsynchronized radio subscribers

Info

Publication number: EP2188645A1
Application number: EP08786546A
Authority: EP
Inventors: Markus Pichler; Stefan Schwarzer; Claus Seisenberger
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2007-09-13
Filing date: 2008-07-29
Publication date: 2010-05-26
Also published as: CN101802636A; DE102007043649B4; DE102007043649A1; US20100309054A1; WO2009037041A1

Abstract

The invention relates to a method for increasing the location accuracy for unsynchronized radio subscribers, in which phase evaluation is used to ascertain the position of a transmitter which is to be located. The transmitter to be located and a further transmitter, whose location is known, respectively send a sequence of N signals to at least two receivers, wherein the transmission channel to be used for transmitting a signal is varied, in line with the invention, on the basis of a prescribed, symmetrical hopping scheme. The advantageous characteristics of the hopping scheme and the additional application of the TDOA (time difference of arrival) principle mean that highly accurate location is possible.

Description

description

Method for increasing the location accuracy of unsynchronized radio subscribers

To locate an object equipped with a transmitter, TDOA methods (time difference of arrival) are usually used. In these, the object to be located sends out a signal, which is received by several stationary receivers. Using triangulation methods, the position of the object relative to the receivers can be determined via the differences in the time of arrival.

An essential prerequisite for the method is that the transmitter and / or receiver are synchronized in time. If this is not the case, corresponding errors occur during the position determination. Although measures for synchronization are known from the prior art, but associated with considerable financial and material costs. For example, in the Global Positioning System (GPS), satellites are equipped with high-precision atomic clocks. In addition, the satellites can synchronize necessary synchronization data with each other for synchronization.

German patent application DE 10 2006 040 497 A1 describes a method for the time-based positioning (TDOA method) of an unsynchronized radio subscriber. This method uses at least two transmitters and at least two receivers to provide a position estimate for one of the transmitters. The receivers and at least one transmitter (reference transmitter) must have a known location, while the other transmitters are to be located. The number of recipients determines in how many dimensions the location can be performed. Due to the reference transmitter synchronization of the participants of the system is unnecessary.

As has been shown, locating accuracy can be increased by evaluating a signal phase. At the phase It is exploited that when a transmitter transmits signals at different frequency bases, the distance to the transmitter can be deduced from the measurement of the phase angles of these signals at the receiver. The basic prerequisite for this, however, is that the phase relationship when transmitting is known or constant and that transmitter and receiver are synchronized in time. For the received phase positions φ _n at the frequency support points f _n , a signal propagation time τ _R results then: φ _n = 2π • f _n • τ _R

The sought distance R can then be determined with the aid of the relationship R = c • τ _R with the signal propagation velocity c. It should be noted that the phase is unique only in the range between 0 and 2π. Depending on how far apart the frequency bases f _n , there is a more or less wide uniqueness range of the measurement. Absolutely unique is theoretically measured when the frequency bases are at an infinitesimal distance.

Despite the disadvantage of ambiguity, the phase evaluation has the advantage of a potentially much higher accuracy in locating: the total enclosed bandwidth between the lowest and the highest frequency used for the measurement behaves roughly inversely proportional to the mean error due to multipath propagation. In addition, unambiguous but inaccurate timing or time correlation can be used to restore uniqueness, as long as it is more accurate than the uniqueness range of the phase evaluation, or else it is based on the result of a TDOA measurement.

In DE 10 2006 040 497 A1, all transmitters emit signals at different frequency bases, whereby the communication standard IEEE 802.15.4 is used for the signals used for positioning. This requires relatively narrowband signals (about 2 MHz, 3dB bandwidth) in the 2.45 GHz ISM band. If one uses all 16 channels defined in the protocol, results a total included bandwidth of 80 MHz and an unambiguous range D _a = c / (2 • f _d) of 30m (speed of light c, channel spacing f _d). However, since the 16 channels can never be used at the same time but only sequentially, there are several problems:

- Since all transmitters are unsynchronized and thus have different time and frequency offsets, the signals are transmitted at different times and with different frequency errors. Due to both circumstances, the phases of the signals received by the receivers are shifted from the idealized view presented above.

- Since all receivers are unsynchronized and also have different time and frequency offsets, the EintreffZeitpunkte the signals are measured in different time axes. This also applies to the phase angles, since the clocks of the receiver must be used for downmixing the received signals. - Moving a station to be located causes additional errors, since the individual frequencies are used sequentially in time. A movement during a measurement does not lead to a linear increase of the phase with the frequency but to a rise of higher order (quadratic or higher). This usually measures a wrong distance.

The advantages of the phase evaluation can therefore not be used.

However, a number of possibilities are conceivable for synchronizing the transmitters and receivers in such a way that the phase evaluation is applicable:

Synchronization of the transmitters: After that, the transmitters operate in the same time and frequency axis and the receivers can determine the differential phases even if they are unsynchronized (eg GPS). - Synchronization of the receivers: The receivers then use the same frequency and phase position for downmixing the received signals and can determine the difference phases of the un-synchronized transmitters (eg tracking system from Abatec or the LPR-B from Symeo). The mass of tracking solutions uses synchronized receivers.

- Simultaneous occupation of multiple frequency bases: This is possible, for example, with the communication method OFDM, in which a fairly wide received signal consists of many individual carriers. Since the entire frequency range is occupied at the same time, the individual carriers have a phase relation to one another.

The aim of the invention is therefore to provide a method which allows the increase of the positioning accuracy of unsynchronized radio subscribers. This object is achieved by the method specified in the main claim. Advantageous embodiments emerge from the dependent claims.

The proposed location method assumes that it is possible to increase the location accuracy in a system of unsynchronized radio subscribers, for example of a ZigBee or a Bluetooth network, based on the use of a phase evaluation. In particular, such an increase in accuracy is possible even if the frequency bases for transmitting a signal are not occupied simultaneously but sequentially. For this purpose, on the one hand, the TDOA measuring principle from DE 10 2006 040 497 A1 is a necessary prerequisite, a locally known transmitter being used as the reference transmitter for which the time and phase differences of the other transmitters are determined. Furthermore, drifting clocks in the receivers and a possible movement of the stations to be located force us to take further measures. In particular, the phase evaluation provides very good results if the frequency bases are not jumped in any order or chaotic order but after a certain symmetrical hopping scheme. The hopping scheme indicates the order of the frequency bases or channels to be used for transmitting the signals. A channel k _n is described by a center frequency f (k _n ) and a width, and is used for transmitting a signal in the form of an electromagnetic wave. As a rule, for each communication standard, a number of such channels are defined, which mostly belong together in blocks, the center frequencies of the channels of a block having a constant distance f _d . The channel definition in IEEE 802.15.4 (ZigBee's PHY layer), for example, comprises a block of 16 channels whose center frequencies lie between 2405 MHz and 2480 MHz at a channel spacing f _d = 5 MHz.

According to the invention, an object to be located sends out a sequence of N signals S _n . The signals to be transmitted via the channel k _n consist of a carrier signal whose frequency is predetermined by the channel k _n and a data stream modulated thereupon. In this case, for successive signals S _n and S _{n +} I, transmission channels k _n and k _{n +} i are selected according to a predetermined hopping scheme. The hopping scheme is set up according to a special education law, which is characterized in particular by its symmetry.

Based on the following definitions, the Education Act can be drawn up:

Let I be the number of transmitters where I is integer and greater than or equal to 2.

Let N be the number of hops (i.e., N determines the length of the hopping scheme), where N is integer, even and greater than or equal to 4.

Let the channel to be used in Hop n from the transmitter Ti be k ^ ¹ for all i = 0,... I-1 and n = 0,... N-1.

- The transmission time for Hop n of the transmitter Ti is tJJ ¹ for all- Ie i = 0, ... I-1 and n = 0, ... N-1.

Let the difference between the phase of the data stream and the phase of the carrier signal in channel k _{n of} the transmitter Ti be φ ^Tl (k _n ) for all i = 0, ... I-1 and n = 0, ... N-1 , Based on this, the hopping scheme is created according to the following rules:

a) The hopping schemes are symmetric about their midpoint for all transmitters Ti: C = ^ t _n- . _! V i = 0, ... I - 1 Λ n = 0, ... N / 2 - 1

b) Two or more transmitters must never use the same channel k _{n at the} same time: kl ¹ ≠ k ^τ _n ^J V i, j = 0, ... I - 1 Λ i ≠ j Λ n = 0, ... N - 1

In the event that two or more transmitters use different orthogonal codes (eg DSSS, spreading code) to spectrally spread their data streams (see CDMA), rule b) can be omitted and several transmitters can simultaneously occupy one channel in order to spread the data minimize spectral width. However, disadvantages would have to be expected (near-far problem, insufficient cross-correlation properties of the codes).

c) The quantities of all channels k _n used in the hopping scheme must be identical for all transmitters Ti, ie all transmitters must use the same channels k _n in the course of hoping, no transmitter may omit one or more channels k _n compared to the other transmitters: {k ' ¹ I n = 0, ... N - 1} = {k' ^: | n = 0, ... N - 1} V i, j = 0, ... I - 1

This requirement may be violated if individual transmitters are to be located with less accuracy. Then also a subset of the channels k _n , which uses the stationary transmitter is sufficient. However, the number of matching channels k _n must never be less than 2.

d) The set of all channels k _n used in the hopping scheme forms a linear frequency ramp with constant frequency spacing f _d between the channels k _n (possibly after a sorting and the removal of multiply branched channels): f (k _n ) = f ₀ + n • f _d V n = 0, ... N - 1 fo is the lowest frequency to be used, eg f ₀ = 2405 MHz for IEEE 802.15.4. This rule may not be mandatory. It is also possible to omit a channel without violating the theory. However, this complicates the subsequent evaluation to a considerable extent.

e) The transmission times of a transmitter Ti must have a constant distance over all hops of a hopping scheme:

C ₊₁ - C = C - C ₁ V i = 0, ... I - 1 Λ n = 1, ... N - 2

This constant distance for a transmitter may vary from transmitter to transmitter. The transmission times do not have to meet any further requirements, that is not even after a synchronization between the stations.

f) The relationship between the phase of the data stream and the phase of the carrier signal of each channel (k _n ) must be constant for a transmitter Ti: φ ^Tl (C) = φ ^Tl (C) V i = 0, ^••• I - 1 Λ n = 0, ^••• N - 1 Λ {m IC = Ci This requirement can be met in the transmitters with suitable devices for signal generation (eg integer PLL).

Rules a), e) and f) are mandatory, rules b), c) and d) may be disregarded.

It is conceivable to extend a hopping scheme resulting from the described education law by adding additional channels before, in the middle of or after the hopping scheme, but these are not used for the measurement. Such a hopping scheme is also within the scope of the invention.

Further advantages, features and details of the invention will become apparent from the Ausführungsbei- described below exemplary embodiment and with reference to the drawings. Showing:

FIG. 1 a schematic representation of an arrangement of several radio subscribers for locating one of the subscribers represented,

Figure 2 is a tabular overview of examples of hopping schemes according to the invention.

1 shows a system for locating a transmitter Tl by means of an arrangement of a further transmitter T2 and two receivers El and E2, wherein the system components Tl, T2, El and E2 are unsynchronized. The positions of the receivers El and E2 and the transmitter T2 are known. The arrangement shown allows a one-dimensional location of the transmitter Tl by the distance d _{τl / T2 of} the transmitter Tl to the fixed and known transmitter T2 is determined as described below.

The transmitters Tl and T2 each send a sequence of N signaled len, wherein the signals on channels k _n (n = 0, 1, ... NI) is transmitted. Typically, the channel k _n used for transmission is varied following the inventive hopping scheme. In anticipation of Figure 2, it is assumed that the example no. 2 applies to the embodiment. Accordingly, the transmitter Tl would sequentially transmit on the channels 0, 4, 1, 5, 2, ..., while the transmitter T2 would use a channel order 4, 0, 5, 1, 6, ....

The signals are received by the receivers El and E2, the transit time of the signal from the transmitter Ti (i = 1, 2) to the receiver Ej (j = 1, 2) being denoted by τ. In the receiver Ej, a phase angle dφ _i: (k _n ) of the signal arriving from the transmitter Ti is determined for each channel k _n . For this purpose, the absolute phase angle φ _i: (k _n ) of the signal transmitted by the transmitter Ti in the channel k _n and received at the receiver Ej is compared with the phase position (f ^ ^{γn of} a synthetic signal stored in the receiver Ej: CUp ₁ . (k _n ) = φ _i: (k _n ) - φ f ⁿ (1)

Due to the symmetry of the hopping scheme, each channel k _{n is used} by a transmitter at least twice. Therefore, in a receiver Ej, the phase positions dφ _i (k _n ) of those signals which were transmitted by one of the transmitters Ti in the same channel k _n are arithmetically averaged. The same procedure is followed by the arrival points described below. This averaging is critical to the result and exploits the advantageous symmetry properties of the hopping scheme.

From the phase positions of the signals of the transmitters Tl and T2 at the receiver Ej, a phase difference Δφ _: (k _n ) = dφ _1: (k _n ) - dφ _2: (k _n ) is again determined for each channel k _n . Under ideal conditions, it could be assumed that the transmitters T1 and T2 transmit in phase, so that Δφ _: (k _n ) would represent the actual phase difference between the signals received at the receiver Ej and thus a measure of the spatial distance between T1 and T2 would. Since, however, this is not the case in particular with unsynchronized transmitters, a contribution dφ ° must be taken into account in the phase difference:

Δφ. (K _n ) = dφ _1: (k _n ) -dφ _2] (k _n ) + dφ ° (2)

However, this contribution dφ ° can advantageously be eliminated by simple subtraction of the phase differences measured at the two receivers:

Δφ ^tot (k _n ) = A (P ₁ (R _n ) -Δφ ₂ (k _n ) = dφ _n (k _n ) -dφ ₂₁ (k _n ) -dφ ₁₂ (k _n ) + dφ ₂₂ (k _n ) (3)

In Δφ ^tot (k _n ), all errors due to frequency offsets of the subscribers and / or a linear component of motion of a transmitter are eliminated, which is largely due to the construction of the hopping scheme. In particular: Δφ ^tot (k _n ) = -4π • f (k _n ) • τ ₀ + φ ₀ (4)

Here, f (k _n ) is the center frequency of the channel k _n and τ _{0 is} the transit time difference of the signals from Tl or T2 to one of the receivers Ej, which in the case of electromagnetic waves corresponds to the light transit time between the transmitters Tl and T2. Finally, φ ₀ is a constant term.

The overdetermined system of equations (4) can be solved numerically, but the ambiguity of the phase information has to be considered. Since the center frequencies f (k _n ) of the channels of the hopping scheme according to the invention have been chosen equidistantly and therefore results in a linear frequency ramp, the phase differences also result in a linear ramp (possibly after an unwrapping operation in which the phase is multiplied by several times) 2π is extended, that results in a linear ramp). The slope of the ramp is proportional to the transit time difference τ ₀ . The constant φ ₀ means a shift of the phase ramp, but it does not affect its slope. Likewise, utilizing the equidistant center frequencies f (k _n ), an inverse discrete Fourier transformation is applied to the complex extended phase exp (iΔφ ^tot (k _n )) within the scope of a further possible solution. In the resulting magnitude spectrum, the sought transit time difference τ _{0 is} at the absolute maximum position. These considerations of possible solutions are only valid without restriction if there are no constructive or destructive multipath propagations which can distort the result more or less.

The result of this determination of the transit time difference τ ₀ is not unique in the entire measuring range. Rather, an ambiguous result is achieved, which is due to the ambiguity of the individual phase measurements. To select the correct uniqueness range, the transit time differences τ ₀ determined via the phase evaluation described above are compared with a travel time difference τ ^0A determined via a TDOA method. In the TDOA method, the arrival times of the signals of the transmitters Ti at the receivers Ej are evaluated in order to be able to deduce the transit time of the signal between the transmitters T1 and T2, from which the distance _{dτ1 / T2 can be} derived. In particular, in the receiver Ej the time interval dτ _i: (k _n ) of the received signal to the stored synthetic signal is again determined for each channel k _n .

By simple mathematical operations, which are equivalent to the equations (1) to (3) of the method of phase evaluation described above, the sought transit-time difference value Δτ ^tot (k _n ) results for each channel k _n :

Δτ ^tot (k _n ) = dτ _n (k _n ) -dτ ₂₁ (k _n ) -dτ ₁₂ (k _n ) + dτ ₂₂ (k _n ) (5)

The transit time difference values are finally averaged over all channels k _n to determine the sought transit time difference τ ™ ^0A .

The selection of the correct transit time difference τ ₀ is made in such a way that that τ _{0 is} defined as the one which comes closest to the averaged τ ™ ^0A .

The sought position of the transmitter Tl is calculated from the known position of the transmitter T2 and the distance d _{τl / T2} , which according to d _{τl / T2} = τ ₀ • c depends on the transit time τ ₀ and the speed of light c.

The addition of additional receivers would allow extension to two- or three-dimensional location by evaluating correspondingly acquired data using standard techniques such as trilateration. A location of several transmitters could be realized by repeatedly going through the described method.

FIG. 2 shows, by way of example, a selection of hopping schemes that were created using the education law according to the invention. The examples 1 to 9 are each for 2 transmitters Tl and T2 shown, while in Example 10 16 transmitters are provided. The hopping schemes can generally be extended to additional channels. The dashed lines in the individual schemes indicate the symmetry axes.

Example 1 shows the schemes for two transmitters T1 and T2 with N = 32. The channels 1 to 15 are arranged according to the rules a) to d) of the Education Act. However, this is not the only way to arrange these channels (see example 5).

Example 2 shows schemes for N = I 6 where channels 0, 1, 2, ... 7 are used.

In Example 3, N = I 6 is also used, but channels 0, 2, 4 ... 14 are used. This shows that the channel spacing can be arbitrary but must remain constant over the entire hoping scheme.

In Example 4, N = 4. Due to the requirements a) and c) of the Education Act, there can be no hopping scheme with a length less than 4.

Example 5 again shows N = 32, but with a different order of the channels than in Example 1. There are numerous other ways of arranging the channels, which is why the exemplary arrangements shown here are not to be understood as exhaustive.

Example 6 demonstrates for N = I 6 channels 0 to 7 in a chaotic arrangement. In Examples 1 to 5, the channels for the transmitter T1 were jumped in a uniform pattern. In contrast, in Example 6, the channel order for Tl was determined by a random number generator, but without violating the rules a) to d) of the Education Law.

In Example 7, at N = I 6, the channels 0 to 3 from each transmitter are used four times each and not only twice. This generates due to the averaging an additional improvement of the position estimate.

Example 8 shows the channels 0 to 15 for N = 32. In the previous examples, two adjacent hops were always formed in a point-symmetrical manner by interchanging the channels to be used for transmitter T1 and transmitter T2. Example 8 shows an alternative arrangement.

Example 9 corresponds to example 8, but the channel order of transmitter T2 forms a ramp of opposite direction to the channel order of transmitter T1. Such opposite current ramps are possible only with an even number of channels, since otherwise there are two times at which both transmitters use the same channel, whereby rule b) is violated.

Finally, Example 10 shows channels 0 to 15 with N = 32 for 16 transmitters. At any time, all channels are busy. If one of the 16 stations is stationary, the other 15 stations can be located at the same time with this hopping scheme. If rule b) can not be violated, there can never be more channels than channels in a hopping scheme. The minimum number of stations is 2, as there must always be at least one station with a known position.

Claims

claims

1. A method for locating at least one transmitter (Tl) using a further transmitter (T2) and at least two receivers (Ej), wherein

the transmitters (Ti) each transmit a sequence of N signals received by the receivers (Ej),

the N signals are sent on certain channels (k _n ),

in each receiver (Ej) for each channel (k _n ) the phase difference (Δφ _: (k _n )) between the received signals of the transmitters

(Ti) is determined and

- Based on the phase differences (Δφ _: (k _n )) the position of the transmitter to be located (Tl) is determined, characterized in that the channels (k _n ) are selected following a given Hoppingschema following.

2. The method according to claim 1, characterized in that for each transmitter (Ti) its own Hoppingsschema is provided, which comprises N entries, wherein - the hopping schemes are symmetrical about its center;

the signals are transmitted at specific transmission times (t _n ) and the transmission times (t _n ) have a constant time interval within a hopping scheme, and the signals are from a carrier signal whose frequency is predetermined by the channel k _n and a data stream modulated thereon, wherein for a transmitter (Ti) the difference (φ (k _n )) between the phase of the data stream and the phase of the carrier signal of each channel (k _n ) is constant.

3. The method according to claim 2, characterized in that never two or more transmitters (Ti) at the same time (t _n ) use the same channel (k _n ).

4. The method according to any one of claims 2 or 3, characterized in that the hopping schemes of all transmitters (Ti) contain the same channels (k _n ).

5. Method according to one of claims 2 to 4, characterized in that a linear frequency ramp can be formed with all the channels (k _n ) used in one of the hopping schemes, the two adjacent channels (k _n ) having a corresponding center frequency have constant frequency spacing.

6. The method according to any one of the preceding claims, characterized in that the positions of the receiver (El, E2) and the further transmitter (T2) are known.

7. The method according to any one of the preceding claims, characterized in that N is an integer, even and greater than or equal to 4.

8. The method according to one or more of the preceding claims, wherein for determining the position of the transmitter to be located (Tl)

at each receiver (Ej) for each channel (k _n ) and for each signal received by one of the transmitters (Ti) a phase position

(dφ _i: (k _n )) is determined,

for each receiver (Ej) for each channel (k _n ), the phase difference (Δφ _: (k _n ) = dφ _1: (k _n ) - dφ _2: (k _n )) of the phase positions is calculated, - from the difference of in the receivers (Ej) determined phase differences (Δφ _: (k _n )) for each channel (k _n ) a total phase difference value Δφ ^tot (k _n ) is calculated according to Δφ ^tot (k _n ) = Δ _Φl (k _n ) - Δφ ₂ (k _n ) and

- via the solution of an overdetermined system of equations Δφ ^tot (k _n ) = -4π • f (k _n ) • τ ₀ + φ ₀ with the center frequency f (k _n ) of the

Channels (k _n ) and a constant φ ₀ a transit time difference (τ ₀ ) is determined.

9. Method according to claim 8, in which - at each receiver (E-,) for each channel (k _n ) and for each signal received from one of the transmitters (T ₁ ) a time of arrival (τ _i: (k _n ) ) is determined - For each receiver (E-,) for each channel (k _n ) from the arrival times (τ _i: (k _n )) a transit time difference

(Δτ _: (k _n ) = τ _1: (k _n ) -τ _2: (k _n )) is calculated,

From the propagation time differences (Δτ _: (k _n )) according to Δτ ^tot (k _n ) = Δτ ^ kJ - Δτ ₂ (k _n ) for each channel (k _n ) a transit time difference value (Δτ ^tot (k _n )) is calculated and

- An average transit time difference value (Δτ ^tot ) is determined from the transit time difference values (Δτ ^tot (k _n )) by averaging over all used channels (k _n ).

10. The method of claim 9, wherein the solution (τ ₀ ) of the overdetermined system of equations

Δφ ^tot (k _n ) = -4π • f (k _n ) • τ ₀ + φ _{0 which} is closest to the average transit time difference value (Δτ ^tot ).

11. The method according to claim 10, _wherein the distance d _{T1 / T2} of the transmitter to be located (Tl) from the other transmitter (Tl) on the equation d _{τl / T2} = τ ₀ • c is determined with the speed of light c.

12. The method according to any one of claims 8 to 11, characterized in that in each receiver (Ej) the phase positions (dφ _i: (k _n )) of those signals are averaged by those of one of the transmitter (Ti) in the same channel ( k _n ) were sent.

13. The method according to any one of claims 8 to 12, characterized in that in each receiver (Ej) the arrival times (τ _i: (k _n )) of those signals are averaged by those of one of the transmitter (Ti) in the same channel ( k _n ) were sent.