WO2020161655A1

WO2020161655A1 - Method for determining the reception time of electromagnetic signals

Info

Publication number: WO2020161655A1
Application number: PCT/IB2020/050944
Authority: WO
Inventors: Fabio Ricciato; Francesco Gringoli
Original assignee: Universita' Degli Studi Di Brescia
Priority date: 2019-02-06
Filing date: 2020-02-06
Publication date: 2020-08-13
Also published as: IT201900001719A1

Abstract

A method for determining the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX), wherein said electromagnetic signal is modulated using a modulation technique (by way of non limiting example, of OFDM type, Orthogonal Frequency Division Multiplexing, of DSSS type, Direct Sequence Spread Spectrum) and is converted into a time sequence of numerical samples, wherein the reception time T_end of said electromagnetic signal is given by the equation T_end = T_last - ΔT', wherein T|ast corresponds to the time said receiver device (RX) takes to collect the last sample of said time sequence of l/Q samples, and wherein ΔΤ is given by the equation ΔT= ΔTa+ ΔTb, wherein ΔT_a is a function of gain again configured during the processing of the signal by said receiver device (RX).

Description

METHOD FOR DETERMINING THE RECEPTION TIME OF ELECTROMAGNETIC SIGNALS

DESCRIPTION TECHNICAL FIELD

The present invention falls within the field of transmitting and receiving electromagnetic signals. In particular, the present invention preferably, but not exclusively, is applied in the case of transmitting and receiving electromagnetic signals generated using an OFDM (Orthogonal Frequency Division Multiplexing) or DSSS (Direct Sequence Spread Spectrum) numerical modulation. In detail, the object of the present invention is a method for determining the reception time of electromagnetic signals of the aforesaid type. In greater detail, the object of the present invention is a method adapted to measure the arrival (reception) time of electromagnetic signals of the aforesaid type in a highly accurate manner.

BACKGROUND ART

The need for determining the reception time of electromagnetic signals with the maximum accuracy possible, is known from the background art. The accuracy with which the reception time of electromagnetic signals is determined indeed plays a fundamental role in various applications such as for example, geolocating applications, synchronizing applications, as well as combined geolocation and synchronizing applications.

Indeed, the accuracy with which the position (absolute or relative to other devices) of a radio device (transmitter and/or receiver) is determined in geolocating applications depends on the accuracy in determining the reception time of an electromagnetic signal, wherein the accuracy with which the clock of a device (transmitter or receiver, see below) is aligned (synchronized with) with the one of another in synchronizing applications depends on the accuracy in determining the arrival time of electromagnetic signals.

With reference to the locating applications, there currently is particular interest in the ones in which it is necessary (or there is a need) to determine the position of a (fixed or mobile) "target" device in a position which is not known from the related distances or pseudo-distances between such a node and other nodes in a known position, called "anchors".

In particular, in the case of N anchors and one (1 ) target device, the determination of the reception time of the N signals emitted by the N anchors translates into the estimation of N target-anchor distances (ranges), the estimation of each of said N distances being as accurate as the determination of the reception time of the respective N signals. Indeed, if the reception times of the N signals are each determined with a common error (ambiguity term, or systematic error), N pseudo distances (which each differ from the actual distance by said common term or error) are measured. The presence of such a common error does not affect the possibility of properly geolocating the target by implementing estimation techniques which are known in literature.

Moreover, it is to be considered that the anchors in certain applications are receivers and the target is a transmitter, wherein smartphones associated with an individual, or also other Wi-Fi devices associated with an object (e.g. a car, a bicycle), are among examples of "receiver anchors", wherein there are Wi-Fi beacons emitted for dialoging with an access point, and a system of Wi-Fi receivers installed in surrounding area determines the position (locating of Wi-Fi source) thereof. The device here is not required to cooperate with the localization process (passive localization of an uncooperative target).

In other applications, the anchors are transmitters and the target is a receiver. Flere, various Access Points are in the environment and broadcast beacons - not necessarily at periodic intervals - and a smartphone (or other device provided with a Wi-Fi receiver) uses the measurements from such anchors to determine its position (auto-locating). Flere, the anchors don't know if and how many devices are exploiting their signals to be auto-located.

Moreover, locating can occur using cellular network signals, LTE signals in particular, in addition to Wi-Fi signals. Flere, the anchors may consists of base radio stations. Also in this case, as in the Wi-Fi case, the auto-locating applications (the target is a receiver) and passive locating applications (the target is a transmitter) are of commercial interest.

And again, it is to be considered that with the miniaturization of the transceiver devices (both Wi-Fi and LTE) and the reduction in the cost thereof, the commercial value of the object locating applications is probably intended to exceed the one of devices associated with persons.

Finally, other interesting applications are the ones in which the exact position is not to be determined, but rather the related distance between the objects is to be determined, for example to keep a drone at a fixed distance from a moving smartphone or to ensure the distance between two smartphones does not exceed a preset threshold (in which case the alarm is triggered).

The methods and/or processes according to the prior art for determining the reception (arrival) time of an electromagnetic signal moreover are not very or not sufficiently accurate, wherein the imprecision (due to the entity of the random component of the measurement error) in determining the reception time of an electromagnetic signal compromises the implementation thereof in applications of the aforesaid type and/or if they are implemented, they make the related applications just as inaccurate, and therefore are not very appreciated by the user. DESCRIPTION OF THE PRESENT INVENTION

The object of the present invention therefore is the one of overcoming, or at least minimizing, the drawbacks encountered in the methods for determining the reception time of electromagnetic signals according to the prior art.

In particular, it is an object of the present invention to provide a method which allows determining the reception time of electromagnetic signals with improved accuracy. In detail, it is an object of the present invention to provide a reliable and accurate method for determining the reception time of electromagnetic signals, wherein the accuracy of the method allows implementing the method itself in various applications, by way of non-limiting example, in geolocating and/or synchronizing applications and/or in mixed geolocating and synchronizing applications, and therefore the accurate geolocating and synchronizing of a device adapted to receive said electromagnetic signals.

The objects of the present invention include the ones of providing a method of the aforesaid type which is easy to implement in a wide range of devices, at contained costs and without requiring substantial modifications to the devices themselves.

In consideration of the above objects and of the increasingly felt need to accurately determine the reception time of electromagnetic signals, the object of the present invention is a method for determining the reception time by a receiver device ( RX) of an electromagnetic signal emitted by a transmitter device (TX) , wherein said electromagnetic signal is modulated (by way of non-limiting example, by means of an OFDM (Orthogonal Frequency Division Multiplexing) modulation or DSSS (Direct Sequence Spread Spectrum) modulation) and is converted into a time sequence of numerical samples, wherein the reception time Tend of said electromagnetic signal is given by the equation:

Tend = T last - DG;

wherein T last corresponds to the time said receiver device (RX) takes to collect the last sample of said time sequence of numerical samples, and wherein AT is given by the equation:

AT=ATa+ATb,

wherein ATa is a function of gain again configured during the processing of the signal by said receiver device (RX);

characterized in that ATb is determined by performing deconvolution operations between sequence P of numerical samples collected by the device (RX) (corresponding to the electromagnetic signal sent by the transmitter device (TX)) and a second sequence P’ of reference numerical samples.

According to an embodiment, said signal comprises a preamble (Pr) and possibly a payload (D), wherein gain again is configured during the processing of said preamble (Pr), and ATa is determined by processing a_gain by a look-up table (LU).

According to an embodiment, said look-up table (LU) is formed starting from the analysis of the amplification circuits of said receiver device (RX) and/or from measurements in a controlled propagation environment between said transmitter device (TX) and said receiver device (RX).

According to an embodiment, said controlled propagation environment is provided by connecting said transmitter device (TX) and said receiver device (RX) by cable or in an anechoic chamber.

According to an embodiment, the numerical samples of said first sequence P are the samples of said at least one symbol of said preamble (Pr).

According to an embodiment, the reference samples of said second sequence P’ are obtained from laboratory measurements in a controlled environment.

According to an embodiment, said laboratory measurements are performed with said transmitter device (TX) and said receiver device (RX) connected by cable or in an anechoic chamber.

According to an embodiment, said receiver device (RX) is a mobile device.

An object of the present invention is also a method for geolocating a device, said method comprising determining the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX), wherein the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX) is determined by a method according to one of the embodiments summarized above.

The object of the present invention also is a method for synchronizing the clock of a receiver device (RX) with that of a transmitter device (TX), said method comprising determining the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX), wherein the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX) is determined by a method according to one of the embodiments summarized above. Further possible embodiments of the present invention are defined by the claims. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is clarified later by means of the following detailed description of the embodiments depicted in the drawings. Moreover, the present invention is not limited to the embodiments described later and depicted in the drawings; contrarily, all those variants of the embodiments described later and depicted in the drawings which are obvious to skilled experts in the technical field fall within the scope of the present invention.

In the drawings:

- figure 1 diagrammatically shows a transmitter-receiver system in communication through a channel C;

- figure 2 diagrammatically shows the constitution of a data unit;

- figure 3 diagrammatically shows the modulation of a signal and the conversion thereof into a sequence of "samples";

- figure 4 diagrammatically shows the error or time offset between the last sample of the signal and the sampling moment;

- figure 5 diagrammatically shows the correlation between the gain (of the signal) and the offset;

- figure 6 diagrammatically shows the determination of the offset by means of the analysis of the transmitted samples.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The application of the present invention is particularly advantageous when used for geolocating and/or synchronizing applications, this being the reason whereby the present invention is described later with possible reference to geolocating and/or synchronizing applications.

Moreover, the possible applications of the present invention are not limited to the applications of the aforesaid type alone; contrarily, the present invention is adapted to be implemented for any application in which the exact determination of the reception time of electromagnetic signals is required (or advantageous).

The present invention derives from the consideration according to which the "accuracy" of a measurement (in the specific case of the measurement of the reception time of an electromagnetic signal) refers to the random measurement error component alone, and is independent of possible systematic error components. A highly accurate measurement indeed has a very low variance of the random error (low measurement "noise"), but it may be affected by systematic errors (also called "terms of ambiguity") of arbitrary entity. According to a consideration at the basis of the present invention, the latter may be neglected and/or "resolved" with techniques which are known in literature, by taking advantage of the multiplicity of transmission (many data units transmitted by the same transmitter or by various transmitters) and/or receiving (the same data unit is received by a multiplicity of receivers) events. According to the consideration at the basis of the present invention, it is important during the measurement to eliminate or at least minimize the random components of the measurement error.

As depicted in figure 1 , transmitter TX, which communicates with receiver RX through channel C, transmits a signal s_tx(t ) which encodes the contents of a data unit using a pass-band modulation; by way of non-limiting example, the encoding may be carried out using a numerical modulation, in particular of the Orthogonal Frequency Division Multiplexing (OFDM) or DSSS (Direct Sequence Spread Spectrum) type.

Moreover, as depicted in figure 2, the data unit (called "beacon", "frame", "sequence of OFDM symbols" etc. according to the standard) is formed by a preamble P which consists of a physical level header and transports training information for the correct decoding of the signal to receiver RX and a possible payload D of arbitrary length. The two components P and D of the signal are transmitted later and form the signal s_rx(t ) seen by the receiver from the time instant t_start up to the time instant t_end. The transmitted signal s_tx(t ) is formed by a series of sequences of samples indicated as S_q(t), 0 £ q < Q = L₁ + L₂ which follow each other over time: the first sequences

form preamble P, the remaining sequences L₂, when present, form payload D. Each sequence S_q(t is a function of time and is obtained from transmitter TX by multiplying the frequency system wave carrier F_c by a series of samples P, in the particular but non-exclusive case of complex samples v'(j), 0 £ j < P herein described, i.e. samples provided with actual part (in-phase, "I") and imaginary part (in quadrature, "Q"), in brief also called "l/Q samples". The series v'(j) of l/Q samples is generated at the speed of N_s samples per second. The series of samples v'(j) of the preamble in turn is obtained by applying a particular transform function Fp(.) also known to receiver RX, to a series of M < P values v(j), 0 £ j < M which form the information content to be transmitted to the receiver.

Certain L sequences S_q(t), 0 £ j < L₁ which form preamble P are obtained from known values v(j) and are used at receiver RX to:

i) determine the boundary with the successive sequence, and therefore the start of each following sequence by frequency;

ii) estimate the channel parameters so as to compensate for the effect of the channel itself on the successive sequences;

iii) configure gain a_gain of the amplifier which is to amplify the receiver input signal corresponding to the successive sequences (function commonly called Automatic Gain Control, AGC).

Successive sequences of preamble P are obtained by encoding, in the values v(j), the number of sequences L2 present in the possible following payload D and the transform function FD(.) used to generate the series of the corresponding samples v) from the series of samples v(j) and potentially different from the analog function FP(.) used for the preamble. Finally, the payload, when present, is generated in a similar manner using the transform function FD (.).

In a particular modulation technique (called Orthogonal Frequency Division Multiplexing, OFDM), the transformations F_{P D}(.) operate by concatenating, by means of the Cyclix Prefix Extension Technique i ) M < P values obtained from the discrete Fourier Anti-Transform 3^_1 of a series of M < P values v(j), 0 £ j < M assigned from a suitable constellation at the M sub-carriers in which the spectrum of the signal is divided with ii) the first P - M samples

0 < j < P - M obtained after the transformation 3^-1 according to the diagram in the left-hand part of figures 3.

Receiver RX processes the signal s_rx(t ) received at the antenna (not depicted but in all receivers) which differs from the transmitted signal s_tx(t) due to the effect of channel C and of the non-ideality of the transmission and receiving systems and attempts to recover the information content contained in the data unit. For this purpose, it processes preamble P and analyzes the sequences therein contained in order to:

i) extract time t_start approximated at the closest value of the clock within the sampling system which is updated at the frequency of N_s ticks per second (the determination of t_start allows the sequences to be processed one after the other from the first sample of each sequence);

ii) estimate the channel parameters and configure the filtered parameters to which the samples of the successive sequences are subjected in order to lessen (reduce) the effect of channel C and of the non-idealities of the transmission and receiving system;

iii) configure the amplification gain a_gain

iv) determine the number of sequences L₂ contained in the payload and the transform function FD(.) used to assign the values v(j ) of the l/Q samples of the sequences in the possible payload.

Then receiver RX processes the possible payload by extracting the values of the l/Q samples by decoding the contents thereof.

The processing of each sequence occurs by sampling, at the frequency of N_s samples per second, the received signal s_rx(t ) brought back to base band through a suitable method (generated with tone at the frequency of the wave carrier).

Having therefore determined, from preamble P , how to delimit and filter the samples belonging to the successive sequences in order to eliminate the effect of channel C and/or of the non-idealities introduced by transmitter TX and/or by receiver RX, each of these sequences is associated with the corresponding series of P values v"(j), 0 £ j < P. By applying the inverse transform function F_p ¹(. ), i.e. F_Q ¹(. ), according to whether the sequence belongs to the preamble or to the possible payload, the series of samples v”'(j), 0 £ j < M £ P which are decoded by extracting the information content, are obtained. In the particular case of OFDM modulation, the transformations /¾(. ) operate by removing the last P - M samples which form the cyclic prefix from the series of samples v" (j), 0 £ j < P and applying the Fourier Transform 3 to the M remaining samples, finally obtaining the samples v"'(j) = 3[v"(/)] (diagram in the right-hand part of figures 3).

Once the processing of the data unit is terminated, therefore the last of the Q = L₁ + L₂ sequences forming it are processed, receiver RX saves the final receiver time t_{end \} in this regard, it extracts the value recorded by the clock counter, which as explained above, is increased to the frequency of N_s ticks per second (equal to the sampling frequency) when it samples the last l/Q sample v”(P - 1) of the Q -th sequence of the data unit.

With respect to the ideal time t_end, receiver RX introduces an error equal to offset DG due to the imperfect alignment between the time in which the last l/Q sample, which is generated by transmitter TX and propagated through channel C, reaches receiver RX and the moment when the signal is sampled to determine the corresponding sample v”(P - 1) of the Q -th sequence, as shown in figure 4.

Said error or offset therefore can be broken down into the sum of two terms AT = DT_a + AT_b, wherein:

the first DT_a depends on gain a_gain configured during the processing of preamble P; the second AT_b is due to the delay between the instant in which sample v'(P - 1) is emitted at transmitter TX, to which the propagation time for reaching the antenna of the receiver is added (point P1 in figure 4), and the instant in which sample v"(P - 1) is collected at the receiver (point P2 in figure 4).

According to one of the peculiarities of the present invention, the determination of offset DT_a, which depends on the different delays due to the circuit stages activated as gain a_gain varies, is carried out through a look-up table: this takes the value of the receiver amplification gain a_gain configured automatically during the processing of preamble P (figure 5), as input.

According to a further peculiarity of the method according to the present invention, the look-up table LU may be constructed from the analysis of the amplification circuits of receiver RX and/or from laboratory measurements taken in a controlled environment, or from a combination of both techniques. "Controlled environment" (here and later) means a propagation environment in which one path alone is present between the transmitter and the receiver, i.e. in which the effect of "multipaths" is neglectable. Such measurements may therefore be taken in a coaxial cable or in an anechoic chamber.

Finally, the offset value AT_b is determined by analyzing one of the sequences belonging to preamble P which transmitted samples v(j), 0 £ j < M are known. For this purpose, the corresponding received samples v"'(j), 0 £ j < M are input into a device D (figure 6) together with a sequence of reference samples v”_gf(j), 0 £ j < M. According to a further peculiarity of the method according to the present invention, the reference sequence is obtained from laboratory measurements taken in a controlled environment, as described above.

Device D removes the uncertainties introduced by the non-idealities due to the receiving and transmission systems and determines offset AT_b by means of deconvolution operations between the sequences v"'(j) and v"_e'_f ).

Sequence v”_gf j), 0 £ j < M (and possibly the look-up table LU) is determined in a laboratory, in a controlled environment, with tests on a device D which is identical at the circuit level to the one which is characterized (receiver RX).

"Circuitally equivalent" device within the scope of the present invention means for example, a cellular phone of the same class or series as the one to be characterized, wherein therefore the method according to the present invention allows the characterization of cellular phones of a same class or series by means of characterization of one only of said cellular phones of the same class or series.

It has therefore been demonstrated by means of the preceding detailed description of the above-described embodiments of the present invention that the present invention allows the preset objects to be achieved while overcoming the drawbacks encountered in the background art.

In particular, a method which allows determining the reception time of electromagnetic signals with improved accuracy is provided by means of the present invention. In detail, a reliable and accurate method for determining the reception time of electromagnetic signals is provided by means of the present invention, wherein the accuracy and reliability of the method allow implementing the method itself in various applications, by way of non-limiting example, in geolocating and/or synchronizing applications and/or in mixed geolocating and synchronizing applications, and therefore the accurate geolocating and synchronizing of a device adapted to receive said electromagnetic signals.

The advantages of the present invention include the ones of providing a method of the aforesaid type which is easy to implement in a wide range of devices, at contained costs and without requiring substantial modifications to the devices themselves.

Although the present invention was clarified above by means of the detailed description of the embodiments depicted in the drawings, the present invention is not limited to the embodiments described above and depicted in the drawings. Contrarily, all those modifications and/or variants of the embodiments described above and depicted in the drawings which are obvious and immediate to skilled experts in the field fall within the scope of the present invention.

For example, although the principle at the basis of the method according to the present invention was described above with particular reference to l/Q samples collected by the receiver device RX, the collection of numerical samples in general is included in the method according to the present invention.

The scope of the present invention is therefore defined by the claims.

Claims

1 . A method for determining the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX), wherein said electromagnetic signal is converted at the receiver device (RX) into a time sequence of numerical samples, wherein the reception time Tend of said electromagnetic signal is given by the equation:

Tend = Tlast - AT

wherein Tiast corresponds to the collection time by said receiver device (RX) of the last sample of said time sequence of numerical samples, and wherein AT is given by the equation:

DT =DT a+DT b,

wherein Ala is a function of the gain again configured during the processing of the signal by said receiver device (RX);

characterized in that ATb is calculated by performing deconvolution operations between the sequence P of numerical samples collected by the receiver device (RX) and a second sequence P’ of reference numerical samples.

2. A method according to claim 1 , characterized in that said signal comprises a preamble (P) and possibly a Payload (D), in that again is configured during the processing of said preamble (P), and in that Ala is determined by processing again by a look-up table (LU).

3. A method according to claim 2, characterized in that said look-up table (LU) is constructed starting from the analysis of the amplification circuits of said receiver device (RX) and/or from measurements in a controlled propagation environment between said transmitter device (TX) and said receiver device (RX).

4. A method according to claim 3, characterized in that said controlled propagation environment is provided by connecting said transmitter device (TX) and said receiver device (RX) by cable or in an anechoic chamber.

5. A method according to one of claims 2 to 4, characterized in that the numerical samples of said first sequence P are the samples contained within said preamble (P)·

6. A method according to one of claims 1 to 5, characterized in that the reference numerical samples of said second sequence P’ are obtained from laboratory measurements in a controlled environment.

7. A method according to claim 6, characterized in that said laboratory measurements are performed with said transmitter device (TX) and said receiver device (RX) connected by cable or in an anechoic chamber.

8. A method according to one of claims 1 to 7, characterized in that said electromagnetic signal is modulated using a numerical or analogic modulation.

9. A method according to claim 8, characterized in that said electromagnetic signal is modulated using an Orthogonal Frequency Division Multiplexing (OFDM) modulation.

10. A method according to claim 8, characterized in that said electromagnetic signal is modulated using a Direct Sequence Spread Spectrum (DSSS) modulation.

1 1 . A method according to one of claims 1 to 10, characterized in that said receiver device (RX) is a mobile device.

12. A method for geolocating a device, said method comprising determining the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX), characterized in that the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX) is determined by a method according to one of claims 1 to 1 1 .

13. A method for synchronizing the clock of a receiver device (RX) with that of a transmitter device (TX), said method comprising determining the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX), characterized in that the reception time by a receiver device (RX) of an electromagnetic signal emitted by a transmitter device (TX) is determined by a method according to one of claims 1 to 1 1 .