WO2016184806A1

WO2016184806A1 - Radio frequency signal transmission in a real-time locating system

Info

Publication number: WO2016184806A1
Application number: PCT/EP2016/060843
Authority: WO
Inventors: Alexis Bisiaux; Arnaud RIGOLLE
Original assignee: Geops Systems
Priority date: 2015-05-18
Filing date: 2016-05-13
Publication date: 2016-11-24
Also published as: FR3036496B1; FR3036496A1

Abstract

In a real-time locating system comprising a plurality of bases suitable for determining arrival times of radio frequency signals emitted by beacons, as well as a central computer suitable for locating the beacons on the basis of said arrival times, each beacon generates a radio frequency signal thus: a preamble symbol, on a first bandwidth (BW₁); at least one synchronization symbol on a second bandwidth (BW₂) at least as wide as the first bandwidth; at least one locating symbol, on a third bandwidth (BW₃) that is wider than the second bandwidth; so that an identifier of said beacon known to said central computer is carried by the set made up of the synchronization symbol(s) and the locating symbol(s).

Description

Radio frequency signal transmission in a real-time location system

The present invention relates to Real-Time Locating Systems (RTLS), used to automatically determine respective positions of objects or people in real time, within an operating perimeter of said system.

Such RTLS real-time location systems thus make it possible to track movements made by these objects or persons. To do this, wireless tags ("wireless tags"), also called tags, are attached to these objects or people, and emit radio frequency (RF) signals. These radio frequency signals are captured by fixed reference points (at least three for a location in a plane, at least four for a location in a three-dimensional space), called bases or base stations ("base stations" in English), delimiting the operating perimeter of said system. When a beacon transmits such an RF signal, the bases receive respective versions whose respective reception times are shifted in time, the time of travel of the RF signal from the beacon to each base being directly proportional to the distance between the tag and said base. These fixed reference points, with their respective known positions, are synchronized on the same reference clock, to make it possible to exploit information of reception instants by RF signal bases coming from a beacon and to deduce the position of said beacon within the operating perimeter of said system . Unlike Global Positioning System (GPS) or GLONASS (Global Navigation Satellite System) satellite tracking systems that only work outdoors, RTLS real-time location systems can be deployed outdoors and indoors.

These RTLS real-time location systems find practical applications in various fields, such as logistics (eg location of pallets of goods in a warehouse), industry (eg tracking of parts along a line of goods). assembly), or hospitals (eg follow-up of specific equipment, follow-up of the nursing staff or certain patients).

For a given transmission power, the radio power collected by a receiver located at a distance R of a transmitter is inversely proportional to the term (fo-Kf, where f is the carrier frequency of the RF signal and has a degree of loss of propagation, the value of which is, depending on the environment, between 2 (free space) and 4 typically, therefore, for a fixed distance R, the power of the received signal decreases with the carrier frequency fo. all the more favorable as the carrier frequency f is low, it is then less expensive (in terms of power consumed) to establish a radio link with a fixed range R in a relatively low frequency band.

On the other hand, the location accuracy is directly related to the bandwidth BW ("bandwidth") of the RF signal, ie the difference between the high and low frequencies of the spectrum of the RF signal. An accuracy of the order of one meter requires, without post-processing, to have a baseband signal sampled with a period of 3.33 ns (duration put by the RF signal to propagate on 1 meter at the speed of the light, ie 3 ^e 8 m / s), ie a sampling frequency of 300 MHz, and a bandwidth of the same order. However, the total available and allocable width (in accordance with the regulations, which may differ in each country) on a given frequency band is generally greater when said band is high in frequency. Location applications aiming for a relatively fine precision (of the order of one meter or less) therefore use, for the most part, fairly high frequency bands, typically greater than 1 GHz. The propagation losses, depending on the minimum square (case of the free space where a = 2) of the frequency, and the location accuracy depending on the occupied bandwidth, show that the choice of the frequency band to be used for the implementation of an RTLS real-time location system responds to a compromise between the range of said system and location accuracy, for a given transmission power. There are therefore two main families of RTLS real-time location systems: low-bandwidth systems, commonly known as narrow-band systems, and high-bandwidth systems, commonly known as wideband systems. The narrow-band systems have interesting properties in terms of simplicity, consumption, and range of communication, but with limitations in terms of location accuracy. Indeed, the band-narrow character of the RF signal used induces time spreading and, after propagation through a multi-path propagation channel, a superposition of multiple replicas delayed and out of phase at the bases. These phenomena favor errors in estimating the position of the beacon. Broadband systems achieve a finer location accuracy (thanks to better temporal resolution), but at a reduced range because they work in higher frequency bands where propagation losses degrade the link budget. In addition, the use of a broadband RF signal induces greater complexity at the receiver (faster sampling), and therefore increased energy consumption.

It is desirable to overcome these various disadvantages of the state of the art.

In particular, it is desirable to provide a solution that makes it possible to reduce the energy consumption of said beacons in such a real-time location system context, and to reduce the energy consumption and processing complexity of said bases.

It is also desirable to make real-time location systems that would be co-located, hermetically relative to one another.

It is also desirable to improve the isolation of radio frequency signals transmitted by such tags in the same real-time location system.

According to one aspect, the invention relates to a beacon intended to be used in a real-time location system comprising a plurality of bases adapted to determine arrival times of a radio frequency signal transmitted by said beacon, and a central computer adapted to locate said beacon from said arrival times. Said beacon is adapted to generate said radio frequency signal by successive symbols as follows: a preamble symbol, of length defined by a first quantity of chips Pi clocked at a first chip frequency Fci over a first bandwidth, consisting of a plurality M successive identical blocks forming a repetition of a first pseudo-random bit sequence code word; at least one synchronization symbol, each synchronization symbol having a length defined by a second quantity of chips; clocked at a second chip frequency Fc ₂ over a second bandwidth at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said symbols of synchronization being representative of a first information of N _b i ₂ bits; and at least one location symbol, each location symbol having a length defined by a third amount of chips P3 clocked at a third chip frequency ¾ over a third bandwidth wider than the second bandwidth, and consisting of a third code word G ,, said location symbol or the set formed by said location symbols being representative of a second information of N _b% 3 bits; such that the N¾2 bits of said first piece of information plus the Nbi3 bits of said second piece of information bear, at least, an identifier of said known tag of said central calculator. Thus, by using a wider bandwidth for the locator symbol (s) than for the preamble symbol and synchronization symbol (s), a target location accuracy can be achieved, while limiting the energy consumption of said beacon, and the energy consumption and processing complexity of said bases.

According to a particular embodiment, said beacon is further adapted to generate said radio frequency signal as follows: for each synchronization symbol k, a first sign bit bs _{2, k} weights said second quantity of chips P ₂ clocked at one second snap frequency FC ₂ by the same binary value. Thus, the amount of information transmittable via the synchronization symbol (s) can be increased, as can the total number of second code words Hj can be reduced to reduce the database processing complexity.

According to a particular embodiment, said beacon is further adapted to generate said radio frequency signal as follows: a first scrambling code S own audit real-time location system is superimposed on all said first code word Hj. Thus, when several real-time location systems are co-located, said systems are hermetic to each other vis-à-vis the synchronization symbols.

According to a particular embodiment, said beacon is further adapted to generate said radio frequency signal as follows: a first secondary scrambling code specific to a subfamily of beacons to which said beacon belongs is superimposed on the first scrambling code S, for each synchronization symbol k, said subfamily being uniquely defined, and for each synchronization symbol of said radio frequency signal, by all the beacons of said real-time location system having the same identifier bits as those previously transmitted through the previous synchronization symbol (s) within said radio frequency signal. Thus, the isolation of the synchronization symbols with respect to a possible concomitant transmission (collision case) of another synchronization symbol from another beacon belonging to said real-time location system is improved.

According to a particular embodiment, said beacon is further adapted to generate said radio frequency signal as follows: for each location symbol k ', a second sign bit bs ₃ , k weight said third quantity of chips P3 clocked at the third chip frequency ¾ by the same binary value. Thus, the amount of information transmissible via the location symbol (s) can be increased, as can the total number of the third G code words, can be reduced to decrease the processing complexity of the bases.

According to a particular embodiment, a second scrambling code S 'specific to said real-time location system is superimposed on any said second code word G _j . Thus, when several real-time location systems are co-located, said systems are hermetic to each other vis-à-vis the location symbols.

According to a particular embodiment, a second secondary scrambling code S'2, k 'specific to a subfamily of beacons to which said beacon belongs is superimposed on the scrambling code S', for each location symbol k ', said subfamily being uniquely defined for each location symbol of said radio frequency signal by all the beacons of said real-time location system having the same identifier bits as those previously mentioned transmitted through the previous location symbol (s) within said radio frequency signal. Thus, the isolation of the location symbols with respect to a possible concomitant emission (collision case) of another location symbol from another beacon belonging to said real-time location system is improved.

According to a particular embodiment, said beacon is, between two alarms of said beacon for generating said radio frequency signal, in standby mode. Thus, the energy consumption reduction of said beacon is further increased.

According to a particular embodiment, said beacon comprises one or more sensors and said first and second information contain, in addition to the identifier of said beacon, additional information representative of measurements made by said sensor (s) (s). Thus, the radio frequency signal used to locate said beacon can also carry sporadic information measurements made by the (s) said (s) sensor (s), which avoids messages and additional signaling (thus reducing the energy consumption of the beacon and bases to process this measurement information).

In another aspect, the invention relates to a base for use in a real-time location system comprising a plurality of such bases adapted to determine arrival times of the radio frequency signals transmitted by beacons, as well as central computer adapted to locate said beacons from said arrival times. Said base is adapted to receive said radiofrequency signals respectively by successive symbols as follows: a preamble symbol, of length defined by a first quantity of chips Pi clocked at a first chip frequency Fci over a first bandwidth, consisting of a plurality M successive identical blocks forming a repetition of a first pseudo-random bit sequence code word; at least one synchronization symbol, each synchronization symbol having a length defined by a second quantity of chips; clocked at a second chip frequency ¾ over a second bandwidth at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said synchronization symbols being representative of a first information of N¾2 bits; and at least one location symbol, each location symbol having a length defined by a third amount of chip P3 clocked at a third chip frequency ¾ on a third bandwidth at less than as wide as the second bandwidth, and consisting of a third code word G ,, said location symbol or the set formed by said location symbols being representative of a second information of Nb _% 3 bits; such that the N¾2 bits of said first information plus the Nb _% 3 bits of said second information carry, at least, an identifier of said known beacon of said central computer. In addition, said base is adapted to: perform a block synchronization on reception of said preamble symbol; perform a symbol synchronization from the block synchronization; recovering the identifier of the transmitting beacon of the radio frequency signal from the synchronization symbol or symbols and the location symbol or symbols, according to the symbol synchronization; and determining the arrival time of said radio frequency signal in a time window defined according to the symbol synchronization.

According to yet another aspect, the invention relates to a real-time location system comprising a plurality of bases as mentioned above which are adapted to determine arrival times of radio frequency signals transmitted by beacons as previously presented, as well as a central computer adapted to locate each beacon from said arrival times.

According to yet another aspect, the invention relates to a first method implemented by a beacon in a real-time location system comprising a plurality of bases determining arrival times of a radio frequency signal transmitted by said beacon, as well as a central computer locating said beacon from said arrival times. The first method is such that said beacon generates said radiofrequency signal by successive symbols as follows: a preamble symbol, of length defined by a first quantity of chips Pi clocked at a first chip frequency Fci over a first bandwidth, consisting of a plurality of identical M successive blocks forming a repetition of a first pseudo-random bit sequence code word; at least one synchronization symbol, each synchronization symbol having a length defined by a second quantity of chips; clocked at a second chip frequency ¾ over a second bandwidth at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said synchronization symbols being representative of a first information of Nbi2 bits; and at least one location symbol, each location symbol having a length defined by a third quantity of P3 chips clocked at a third chip frequency ¾ on a third bandwidth at least as wide as the second bandwidth, and consisting of a third code word G ,, said location symbol or the set formed by said location symbols being representative a second information of Nb _% 3 bits; such that the N¾2 bits of said first piece of information plus the Nbi3 bits of said second piece of information bear, at least, an identifier of said known tag of said central calculator.

According to yet another aspect, the invention relates to a second method implemented by a base used in a real-time location system comprising a plurality of such bases determining arrival times of radio frequency signals transmitted by beacons, and a central calculator locating said beacons from said arrival times. The second method is such that said base receives said radio frequency signals respectively by successive symbols as follows: a preamble symbol, of length defined by a first quantity of chips Pi clocked at a first chip frequency Fci over a first bandwidth, constituted a plurality of identical successive blocks forming a repetition of a first pseudo-random bit sequence codeword; at least one synchronization symbol, each synchronization symbol having a length defined by a second quantity of chips; clocked at a second chip frequency ¾ over a second bandwidth at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said synchronization symbols being representative of a first information of N¾2 bits; and at least one location symbol, each location symbol having a length defined by a third amount of chips P3 clocked at a third chip frequency ¾ over a third bandwidth at least as wide as the second bandwidth, and being constituted a third code word G ,, said location symbol or the set formed by said location symbols being representative of a second piece of information Nbi3 bits; such that the N¾2 bits of said first piece of information plus the Nbi3 bits of said second piece of information bear, at least, an identifier of said known tag of said central calculator. In addition, the second method is such that said base performs the following steps: performing a block synchronization on receipt of said preamble symbol; perform a symbol synchronization from the block synchronization; recovering the identifier of the beacon transmitting the radio frequency signal from the synchronization symbol (s) and the symbol (s) location, according to the symbol synchronization; and determining the arrival time of said radio frequency signal in a time window defined according to the symbol synchronization.

According to yet another aspect, the invention relates to a method implemented by a real-time location system comprising a plurality of bases implementing the second method previously presented and which determine arrival times of radio frequency signals transmitted by beacons implementing the first method previously presented, as well as a central computer locating each beacon from said arrival times.

The characteristics of the invention mentioned above, as well as others, will appear more clearly on reading the following description of an exemplary embodiment, said description being given in relation to the attached drawings, among which:

FIG. 1 schematically illustrates an RTLS real-time location system in which the invention can be implemented;

FIG. 2 schematically illustrates an exemplary architecture of a beacon RTLS real-time location system;

FIG. 3 schematically illustrates an exemplary architecture of a base RTLS real-time location system;

FIG. 4 schematically illustrates a multi-rate filtering and baseband processing arrangement, within the architecture example of FIG. 3;

FIG. 5 schematically illustrates another multi-rate filtering and baseband processing arrangement, within the architecture example of FIG. 3;

FIG. 6 schematically illustrates a signal transmission algorithm

RF, implemented by each beacon of RTLS real-time location system;

FIG. 7 schematically illustrates an RF signal transmitted by implementation of the algorithm of FIG. 6;

FIG. 8 schematically illustrates an RF signal reception algorithm implemented by each base of the real-time RTLS location system;

FIG. 9A schematically illustrates the real-time location system RTLS, in a configuration in which a beacon is located closer to a first base than a second base; and FIG. 9B schematically illustrates block synchronization detection times and symbol timing detection times in the context of the configuration shown in FIG. 9A.

Fig. 1 schematically illustrates a real-time location system RTLS 100 in which the invention can be implemented.

The real-time location system RTLS 100 comprises at least three bases 120, 121, 122 to locate beacons within a predefined perimeter. The positions of the bases 120, 121, 122 are fixed and known from a central computer 130. The bases 120, 121, 122 are connected to the central computer 130 by means of permanent links or wireless wire. Through these permanent links, the bases 120, 121, 122 can transmit to the central computer 130 information relating to RF signals picked up by the bases 120, 121, 122 and can benefit from a common reference clock. At least one beacon 110 is present in said predefined perimeter and transmits RF signals, as described below in relation to FIGS. 6 and 7. Several bases of the system then capture the RF signals transmitted by the beacon 110, each RF signal to be received by at least three (or four) beacons to enable real-time location of the beacon in a plane. (respectively in three-dimensional space). Each RF signal transmitted by the beacon 110 undergoes different propagation latencies to reach said bases. Thus, for the same RF signal transmitted by the beacon 110: a first version 150 of the RF signal is received by the base 120 with a propagation latency Ll depending on the distance separating the base 120 from said beacon 110 at the time when said RF signal is transmitted by the beacon 110; a second version 151 of the RF signal is received by the base 121 with a propagation delay L2 depending on the distance separating the base 121 from said beacon 110 at the moment when said RF signal is transmitted by the beacon 110; and a third version 152 of the RF signal is received by the base 122 with a propagation delay L3 dependent on the distance separating the base 122 from said beacon 110 at the moment when said RF signal is transmitted by the beacon 110.

Each time an RF signal is received from a beacon of the real-time location system RTLS 100, the base concerned then transfers to the central computer 130 at least the following data:

- instant 't' of the reception of said RF signal according to the reference clock; and identifying 'z' of said beacon (each beacon being distinguished by an identifier of its own, contained in each RF signal transmitted by said beacon, and decoded on reception by each base receiving said RF signal).

The central computer 130 is in charge of centralizing this information from the different bases of the real-time location system RTLS 100, and from deducing the precise position of the identifier tag 'z' by means of a location algorithm ( triangulation, multilateration) using as input variables the differences in TDOA arrival times ("Time Difference Of Arrival") of said RF signal at the different bases having received said RF signal.

Fig. 2 schematically illustrates an exemplary architecture of the beacon 1 10, applicable to any other beacon RTLS 100 real-time localization system. In a preferred embodiment, no beacon RTLS 100 real-time localization system is adapted to receive RF signals (no return path from the bases to the beacons); the beacons are then adapted only to transmit RF signals, as detailed below in relation to FIGS. 6 and 7. FIG. 2 is therefore concerned with presenting a chain of transmission of such RF signals.

According to the example of FIG. 2, the beacon 1 10 comprises a baseband digital processing module 201 which generates a digital signal of complex samples (I + j * Q) thus having a real component (I) and an imaginary component (Q), in the framework of a four-state phase shift digital modulation of QPSK ("Quaternary Phase-Shift Keying") type. The digital signal of complex samples (I + j * Q) to be transmitted on each wakeup of the beacon 1 10 is read by the digital baseband processing module 201 from a non-volatile memory where the samples were previously saved, or preferentially generated over the water by a shift register (thus avoiding memory overhead). This approach makes it possible to reduce digital processing done in baseband. According to the Nyquist-Shannon sampling theorem, the samples of this digital signal are generated with a sampling frequency at least equal to twice the maximum frequency present in this digital signal.

The complex digital signal (J + j * Q) from the digital baseband processing module 201 is then processed by a rate change module 202 made up of interpolator and / or decimator filters, so as to adapt the rate of change. sampling according to the RF signal transmission needs by the beacon The complex digital signal (I + j * Q) from the rate change module 202 is then converted to a complex analog signal (I _a + j * Q _a ) using a pair of analog converters. digital-to-analog converter DAC ("Digital-to-Analog Converter" in English) 203, 204. The analog signal (I _a ) from the analog digital converter DAC 203 is then filtered by an analog low-pass filter 205. Similarly, the signal analog (Q _a ) derived from the digital analog converter DAC 204 is then filtered by an analog low-pass filter 206. A mixer 207 recovers the analog signal filtered by the analog low-pass filter 205 to modulate a carrier frequency fo provided by an oscillator 209. A mixer 208 recovers the analog signal filtered by the analog low-pass filter 206 to modulate the carrier frequency fo provided by the local oscillator 209. The mixers 207, 208 are used in phase quadrature thanks to a module 210 of FIG. Depha wise at 90 °. The signals from the mixers 207, 208 are then summed by an adder module 21 1 in order to create an RF signal on the carrier. The carrier-generated RF signal thus generated is then amplified in power by an amplifier 212 and then filtered by a bandpass filter 213 in order to avoid spectral leakage on frequency bands adjacent to the band. frequency on which the RF signal is expected. The carrier-borne RF signal at the output of the band-pass filter 213 is then transmitted to an antenna 214, in order to be broadcast within the RTLS 100 real-time localization system.

Fig. 3 schematically illustrates an exemplary architecture of the base 120, applicable to any other base RTLS 100 real-time localization system. In a preferred embodiment, no beacon of RTLS 100 real-time localization system is adapted. to receive RF signals (no return path from the bases to the beacons); the bases are then only adapted to receive RF signals, as detailed below in connection with FIG. 8. FIG. 3 is therefore concerned with presenting a reception chain of such RF signals.

The base 120 comprises an antenna 301 via which the base 120 receives RF signals transmitted by beacons of the real-time location system RTLS 100, such as the beacon 1 10. After reception via the antenna 301, the RF signal on the carrier frequency fo is amplified by a low noise amplifier LNA ("Low Noise Amplifier" in English) 302, then filtered by a band-pass filter 303. The RF signal thus filtered is then brought back directly (ie in direct conversion) to a band basic thanks to two mixers 304, 305 used in quadrature phase thanks to a module 307 phase shift at 90 °. The two mixers 304, 305 are powered by a frequency generator 306 oscillating at a frequency equal to the carrier frequency f. The two mixers 304, 305 thus bring the RF signal from the carrier frequency fo to the zero frequency (baseband frequency translation). The real (I) and imaginary (0) components of the base band analog complex signal (I + j * Q) are then filtered by respective low-pass filters 308, 309 and amplified by VGA variable gain amplifiers. ("Variable Gain Amplifier" in English) respectively 310, 31 1, before being digitized by analog-digital converters ADC ("Analog-to-Digital Converter" in English) respectively 312, 313. A multi-filter module The timing 314, as further detailed in relation to Figs 4 and 5, performs filtering and sampling rate conversion on the output signals of the ADC ADCs 312, 313. The multichannel filtering module Rhythm 314 then delivers complex samples (/ + / * 0 in baseband clocked at the rate required by the Nyquist-Shannon sampling theorem, thus allowing the same baseband digital processing module 315 to conduct (potentially in parallel) digital processing on narrow-band RF and RF broadband signals.

Fig. 4 schematically illustrates an arrangement of the multi-rate filtering module 314 and the baseband digital processing module 315 within the base 120, applicable to any other base of the real-time location system RTLS 100.

The multi-rhythm filtering module 314 comprises a cascade of three stages of change of rhythm 401, 402, 403 successive, each of said stages being composed of interpolator filters and / or decimators. The rate-change stage 401 receives, as input, the samples coming from the analog-digital converter ADC 312 or 313, that is to say for one of the components (real (I) or imaginary (0) of the complex samples ( I + j * Q) in baseband The output of stage 401 is connected to the input of the rate change stage 402 and the output of stage 402 is connected to the input of the stage. timing change stage 403. The output of the rate change stage 403 is connected to a preamble symbol processing module 41 1, which is included in the baseband digital processing module 315. The output of the rate change stage 402 is further connected to a synchronization symbol processing module 412, which is also included in the baseband digital processing module 315. The The output of the rate change stage 401 is further connected to a location symbol processing module 413, which is also included in the baseband digital processing module 315.

Fig. 4 is limited to an arrangement adapted to process samples of a single component (real (I) or imaginary (0) of the complex signal (J + j * Q) in baseband, for the sake of simplicity of FIG. A similar arrangement connecting a cascade of three rate change stages (respectively connected to the preamble symbol processing module 411, the synchronization symbol processing module 412 and the location symbol processing module 413) is also implemented. for the other component of the complex signal (J + j * Q) in baseband.

In a preferred embodiment, this simple arrangement includes, at the rate change level, only decimation stages. This arrangement therefore works only if one goes from one stage to another by decimation of an integer factor, that is to say if sampling frequencies respectively corresponding to the decimation operations carried out by the stages of change of pace 401, 402, 403 are chosen multiple between them. If Fsi is the sampling rate at the output of the rate change stage 403, the sampling frequency at the output of the rate changing stage 402, the sampling frequency at the output of the rate change stage 401, and FS _ADC the output sample rate of the ADC ADCs 312, 313, then these sampling frequencies are related as follows: FS _AD C = D _A * FS3, Fs3 = D _B * Fs2 and Fs2 = D _c * Fsi, where D _A, D _B and D _C are factors of positive integers decimation. If at least one of the factors decimation _Â D, _B or D De equal to the value "1", then there is no corresponding change in rhythm, and the corresponding rate of change of floor can be replaced by a wire.

As detailed below with reference to FIG. 8, the preamble symbol processing module 411 is permanently active. When the preamble symbol processing module 411 recognizes an expected preamble symbol in a received RF signal, the preamble symbol processing module 411 activates the synchronization symbol processing module 412. Similarly, when the processing module of synchronization symbols 412 recognizes one or more synchronization symbols in the received RF signal, the processing module of Synchronization symbols 412 activate the location symbol processing module 413. The synchronization symbol processing module 412 and the location symbol processing module 413 enter a power saving mode when their respective processes are completed. .

Fig. 5 schematically illustrates another example of arrangement of the multi-rate filtering module 314 and the digital baseband processing module 315, that is to say of the part of the reception chain of the base 120 which corresponds to to digital treatments.

According to the arrangement of FIG. 5, the base 120 then comprises, connected by a communication bus 520: a processor or CPU ("Central Processing Unit" in English) 510; a Random Access Memory (RAM) 511; a ROM (Read Only Memory) 512; a storage unit or a storage medium reader, such as a Secure Digital (SD) card reader 513; an interface 514 enabling the base 120 to receive RF signals from beacons present in the RTLS real-time localization system 100, to apply filtering on these RF signals and to carry out sampling. By analogy with the architecture presented in FIG. 3, the interface 514 then corresponds to the analog part of the reception channel of the base 120 (going from the antenna 301 to the analog-digital converters ADC 312, 313). It may be noted that the base 120 further comprises an interface (not shown) for communication with the central computer 130.

The processor 510 is capable of executing instructions loaded into the RAM 511 from the ROM 512, an external memory (not shown), a storage medium (such as an SD card), or a communication network. When the base 120 is turned on, the processor 510 is able to read instructions from RAM 511 and execute them. These instructions form a computer program causing the processor 510 to implement all or part of the multi-rate filtering and the digital baseband processing.

All or part of the multi-rate filtering and the digital baseband processing can thus be implemented in software form by executing a set of instructions by a programmable machine, for example a DSP ("Digital Signal Processor"). or a microcontroller, or be implemented in hardware form by a machine or a dedicated component, for example an FPGA ("Field- Programmable Gâte Array ") or an ASIC (" Application-Specifïc Integrated Circuit ").

Fig. 6 schematically illustrates an RF signal transmission algorithm implemented by each beacon of the RTLS 100 real-time location system. Let us consider, illustratively, that the algorithm of FIG. 6 is implemented by the tag 110.

In a step 600, the beacon 110 wakes up. Indeed, preferably, so as to reduce the energy consumption of the beacon 110 relative to a constant supply of said beacon 110, the beacon 110 wakes sporadically to allow the transmission of an RF signal intended to allow the central computer 130 to obtain an estimate of the geographical position of the beacon 110, and is standby ("stand-by" in English) the rest of the time. As a result, the transmission of RF signals by the beacon 110 is done asynchronously at times unknown bases. In practice, the beacon 110 wakes up at pseudo-random times separated from each other, on average, by a period defined according to the desired location refresh rate, for example a second. This means that the beacon 110 comprises a set (minimized) of continuously powered electronic components, causing the other electronic components of the beacon 110 to switch to the nominal operating mode when the beacon 110 is to be woken up and in the mode of operation. energy saving said other electronic components when the beacon 110 is to be put on standby. Typically, the awakening instants of the beacon 110 are defined by activating a predefined duration time delay when the other electronic components are put in the energy saving mode. It may be noted that, since the awakening instants of the beacon 110 are a priori unknown to the bases, said bases must be permanently capable of receiving any RF signals coming from the beacon 110 (or any other beacon of the beacon system). real-time location RTLS 100).

In a next step 601, the beacon 110 transmits a preamble symbol. The preamble symbol consists of a plurality of identical M successive blocks. The preamble symbol is based on the repetition of a binary sequence (with values in ± 1) pseudo-random, here called code C, and which is generally found under the name "PN code" (for " Nickname Noise code "). The code C is preferably a sequence with a maximum length, called m-sequence. Code C is of length N elements, also called "chips"("chips") in English). A snippet refers to a rectangular pulse of a code, as commonly accepted in code division multiple access (CDMA) systems. A block is then defined as an occurrence of this code C.

When the C code takes the form of an m-sequence, the C code is very easily generated by means of a shift register - composed of an integer number of bits at least equal to [log ₂ (N)] where M represents the rounding of the value x to the higher integer - and has very good autocorrelation properties, making its detection (receiving side) inexpensive by means of a relatively short correlator. The preamble symbol makes it possible to recover the block synchronization on reception, ie to determine at which instant a block actually begins in the flow of the received samples. The same code C being repeated successively M times, the output of a block correlator searching for the code C ideally shows M correlation peaks, then indicating in reception the timing of the blocks and the start time of each of them in the flow of the received samples. Because of the remoteness and fluctuations of the transmission conditions between the beacon 110 and the base 120, a coherent integration of M successive blocks (ie the complete preamble symbol) is necessary to bring out at least one level correlation peak. ambient noise.

Coherent integration is understood to mean a summation carried out on measurements or observations of the RF signal during a duration T during which phase changes of the RF signal are, cumulatively, less than a predefined threshold value ΔΦ _ΜΑΧ . The duration T is called the coherence time of the RF signal. For example, ΔΦ _ΜΑΧ = 67 °.

The preamble symbol, generated at a chip frequency ("chip frequency" in English), is transmitted by the beacon 110 with a bandwidth BWi. The preamble symbol, of length Pj = M * N chips, clocked at the chip frequency Fci, has a duration T _sym bi = Pi / Fci such that T _sym bi≥T _sym b, where T _sym b represents the minimum value of symbol duration to obtain the Gp processing gain by coherent integration necessary to reach the range targeted by the real-time location system RTLS 100. The processing gain Gp is proportional to the product BW * T _sym b, where BW is the width frequency band occupied by the RF signal considered, and T _sym b the coherent integration time on a symbol in reception: Considering, for example, that the processing gain Gp is equal to 41 dB (ie 12,500 in linear) at least to reach a range of 1 km in free space with a signal of BWi bandwidth equal to 2.5 MHz, It follows that a coherent integration over a duration T _sym bi T T _sym b = 5 ms is required in reception.

Considering the formatting performed by analog low-pass filters

205 and 206, the bandwidth BW and fcb frequency variables are linked through a roll-off factor β in the following manner:

BW

For a given emission filter, the drop coefficient β is a constant, typically between 0 and 1, giving a measurement of an excess of bandwidth of said transmission filter considered, ie the occupied bandwidth beyond the minimum bandwidth (called Nyquist bandwidth) equal to Fc: a value of the fallout coefficient β close to "0" makes it possible to reduce the spectral occupancy of the transmitted RF signal (by approaching the bandwidth of Nyquist, at the cost of a long impulse response of the filter), whereas the transmitted RF signal has a spectrum less frequency-concentrated if the value of the coefficient of fall-out β approaches "1", but with the advantage of a short impulse response. It should be noted that for an RTLS real-time localization system 100 whose range is fixed (for example 1 km) and with a constant coefficient of fallout β, the minimum value of integration time symbol T _sym b (for example 5ms) is valid regardless of the value of the BW bandwidth considered: in fact, doubling the value of the chip frequency Fc induces a doubling of the processing gain Gp (the number of chips doubling for the same duration of time). integration). At the same time, the occupied bandwidth BW is also doubled, since the fallout coefficient β is constant.

The order n of the code C is one of the sizing variables of the RTLS real-time localization system 100, and corresponds to a compromise between different criteria relating in particular to the spectral properties of the RF signals transmitted by the beacons, to the detection performances, and the computation complexity required in reception. The length N = 2 "- 1 of the code C must thus:

• Have a sufficiently large length so that the transmitted RF signals have a spectrum as flat as possible in order to reduce disturbances vis-à-vis other users of the frequency band. Indeed, the preamble symbol being periodic, its spectrum is discrete (non-continuous) and composed of N spectral lines: the larger N is, the greater the number of lines is important, each line then being all the less powerful;

• have a length greater than the time spreading of the propagation channel (linked to the band-narrow character of the transmission of the preamble symbol), so as to distinguish in reception the coarse time position of the energy packet in the middle of the noise ;

• be of sufficient length to support several times the range targeted by the real-time location system RTLS 100. This avoids a copy of the RF signal reflected on a distant obstacle can disturb the determination of arrival time of the RF signal being taken, by time folding, for the copy of the RF signal having borrowed the shortest path (direct path). By targeting, for example, a range in free space of 1 km, the length of the code C can be chosen to tolerate a minimum of 10 km range. The conversion of this range to support in a time expressed in chips at the chip frequency Fci is therefore a function of the chip frequency Fci chosen for the preamble symbol;

• Have a sufficiently large length so that the DR dynamic range (dynamic range) of the power of the estimated impulse response is sufficiently high. According to the autocorrelation properties of a m-sequence code C, the dynamic range of the power of the estimated impulse response after block-correlation is equal to N ² , which is the square of the length N of the code C. The higher the order n, the greater the dynamic range DR, making the detection of correlation peaks easier in the presence of noise;

Have a sufficiently small length to moderate a possible phase rotation induced by a frequency offset which can exist between the beacon transmitting the RF signal 110 and the base 120, and thus limit the signal ratio degradation SNR ("Signal to Noise Ratio") noise during the coherent integration over the duration of a block;

• have a sufficiently small length to lighten the detection operations (correlation calculation) in reception. The complexity of the detection operations is in fact linearly related to the sampling frequency of the RF signal, and also to the length N of the C code. Nevertheless, this aspect must be moderated by the fact that an increase in the length N of the code C, for a constant length of preamble symbol Pi = M * N bits, makes it possible to decrease the value of M and consequently the complexity of the processing in reception of synchronization symbols, as transmitted in a step 602 described below.

According to an exemplary embodiment, the code C is of order n = 7, each block then being composed of N = 127 chips clocked at the chip frequency Fcj.

Once the length N of the code C has been defined, the quantity M of repeated blocks is derived from the expected processing gain Gp in order to reach the range targeted by the real-time location system RTLS 100. Moreover, in a preferred embodiment, M has a value equal to a power of two to facilitate the implementation.

As calculated above for an embodiment aiming at a range in free space of 1 km with a signal of bandwidth BWi of 2.5 MHz, a coherent integration is required in reception over a duration T _symb i≥ T _symb = 5 ms. For a chip frequency Fci equal for example to 1.625 Mcps (mega "chips" per second), this integration time must be greater than or equal to Tsymb * Fci = 8125 chips clocked at the frequency Fcj chip. For N = 127 chips clocked at the chip frequency Fci per block, a minimum number M = 64 blocks is therefore necessary.

For this exemplary embodiment, the preamble symbol transmitted in step 601 has a duration T _symb i = 5,0018 ms, a length Pi = 8128 chips clocked at the chip frequency Fci = 1.625 Mcps, and is composed of M = 64 successive repetitions of the code C of length N = 127 chips clocked at the frequency fcj Fcj.

M is the minimum integer number of repeated blocks required, preferably in the form of a power of two, so that the duration T _symb i of the preamble symbol is greater than or equal to the duration T _symb . To increase the probability of detection of the preamble symbol in reception, an optional improvement consists in extending the transmission duration by adding a certain number M 'of blocks. The recommended preamble extension values are for example equal to 1/16, 1/8, 1/4 or 1/2 of the value of M.

In the real-time location system RTLS 100, the preamble symbol transmitted in step 601 is exactly the same regardless of the sending beacon: the preamble symbol therefore does not contain any identifier bits specific to the beacon. transmitter, but only a hung sequence known bases, allowing them to detect the presence of an RF signal to allow the central computer 130 to determine the location of said transmitter beacon in the operating perimeter of the time-tracking system. real RTLS 100. In the case of deploying several real-time RTLS location systems in contiguous or overlapping geographical areas, it is advantageous to seek to minimize interference between these systems. Assuming two co-deployed RTLS Sy _A and Sys real-time localization systems using exactly the same preamble symbol structure, an RF signal transmitted by a z _A tag belonging to RTLS real-time location system Sy _A risk therefore to be detected by one or more bases of the real-time location system RTLS Sys: this is not unacceptable insofar as this detection will be classified as false detection (and therefore without continuation) within the system of real-time location RTLS Sys at the time of processing the RF signal synchronization symbol or symbols. This approach results in over-consumption at the base level of RTLS Sys real-time location system for the treatment of false alarms. A first improvement consists in using, for real-time location systems RTLS Sy _A and Sys, C codes (which can be respectively noted CA and CB) of the same length, but of different contents. It is indeed possible to choose CA and CB codes which have limited levels of intercorrelation peaks (eg of the order of only 10% of the autocorrelation peak). For a length N = 127 chips clocked at the chip frequency Fci, there exists for example a set of six m-sequences which have the property of presenting peaks of intercorrelation reduced to 13.3% of the height of the autocorrelation peaks. . A second improvement consists in using, for RTLS Sy _A and Sys real-time localization systems, discrete block lengths. The cross-correlation between the m-sequences of real-time RTLS Sy _A and Sys localization systems is then reduced.

The number NID of bits necessary to represent respective identifiers of the tags is a function of the total number of tags to be located in the real-time location system RTLS 100. For, for example, 1,000 tags, the identifier of each tag requires a representation on NID = 10 bits. The transmission of the complete identifier of the beacon (on NID bits) is performed during steps 602 and 603 described below. Among these NID bits (carrying the identifier of the beacon) to be transmitted to each RF signal intended to allow the central computer 130 to determine the location of the beacon 1 10 in the operating perimeter of the real-time location system RTLS 100, a number of bits N¾2 NNro are to be transmitted during the step 602 described below, through N _sym b2 synchronization symbols {N _sym b2 ≥ 1), after transmission of the preamble symbol in step 601. To title illustrative, it is considered that Nu ₂ is equal to 6 bits: it remains then Nbi3 = N _ID - NW2 = 4 bits of identifier to be transmitted in the subsequent step 603 described below. The distribution of these N _ID bits between the steps 602 and 603 described hereinafter is a compromise of complexity at the base level between the processing costs of the synchronization symbols and the location symbols.

In the next step 602, the beacon 110 transmits at least one synchronization symbol. Each synchronization symbol, of length P ₂ chips clocked at the chip frequency ¾, is transmitted by the beacon 110 using a bandwidth BW2 (Fc2≤BW2≤2 * Fc2) dependent on the waveform of the transmitted RF signal. , set by the drop coefficient β of the analog low pass filters 205 and 206. The synchronization symbol has a duration T _sym b2 = P2 I Fc2, such that T _symb2 ≥ T _sym b since, with coefficient of fall β constant, the symbol duration is the same regardless of the bandwidth used. By defining strictly positive integers b and b ₂ so that bi = D _B * D _c and ô ₂ = ¾, this means that bi is greater than or equal to 0 ₂ and that the bandwidth BW ₂ is greater than or equal to to the bandwidth BWi used for the preamble symbol transmitted in step 601, considering the following relation:

In a preferred embodiment, the decimation factor ¾ is chosen to be a power of "2", so as to be advantageously implemented in the form of a cascade of half-band filters ("Half-Band Filter"). ). According to an exemplary embodiment, D _B = 8 and D _c = 1, hence bi = b ₂ = 8 so as to have a bandwidth BW ₂ = BWi = 2.5 MHz. In addition, the chip frequency c ₂ is set at 1.625 Mcps (of the same value as the chip frequency Fci), hence a coefficient of fallout β equal to 53.85% for the analog low-pass filters 205 and 206. A number P2 = T _sy mb2 * FC2, at least equal to 8,125 chips clocked at the chip frequency c ₂ is then necessary to constitute each synchronization symbol, unit duration T _symb2 then equal to 5.0 ms.

In a particular embodiment, for each k ^th synchronization symbol (except the first in sequence, of index k = 1, for which there is no previous synchronization symbol that can serve as a phase reference), a bit of sign weighting all the chips (clocked at the chip frequency Fci) of said k ^th synchronization symbol by the same binary value (± 1). This makes it possible to transmit one information bit per index synchronization symbol 2 <k <N _sym b2 (except for the first in sequence, of index k = 1), which is easily decoded in reception by comparison of the phase of the result of a symbol correlation with the phase of the preceding synchronization symbol, of index k - 1. Thus, the N _sym b2 synchronization symbols allow the transmission of Nb _S 2 = (N _sym b2 - 1) bits of information through the sign bits {bs _{2 k} } _k ^s ™ ^b2 possibly available (if N _sym b2 ≥ 2). In the particular case where the bandwidth BW2 used for the transmission of the synchronization symbols is equal to the bandwidth BWi used for the transmission of the preamble symbol, the preamble symbol can advantageously be used as a phase reference for the first synchronization symbol in sequence: it is then possible to transmit a sign bit from the first synchronization symbol. Thus, in this particular case, Nb _S 2 = N _sym b 2 information bits are available through the sign bits, at least one is then available because the number N _sym b 2 of symbols

synchronization is at least 1.

In addition to the Nb _S 2 bits transmitted through any sign bits, it remains to transmit in step 602, a total of Ν 2 · = NW2 - N¾ identifier bits through the N _sym b2 * P2 clocked chips at the FC2 chip frequency composing the N _sym b2 synchronization symbols. These N¾2 - bits are preferentially distributed equitably between the N _{sym2 and} b2 synchronization symbols. Thus, without counting the eventual sign bit of the index synchronization symbol k, the number of bits transmitted through each synchronization symbol is equal to:

N _bi2 - N _bs2

Nsymb2

while the value Ν 2 · is equal

^N bi2 '

It follows that the total capacity of the N _sym b2 synchronization symbols (in number of bits transmitted at each transmission of an RF signal) in step 602 is finally equal to:

N _bS 2 + N _bi2 '= N _bs2 + N _symb2 * ≥N _bi2

This total capacity of the N _sym b2 synchronization symbols is necessarily greater than or equal to the number Nb _% 2 of identifier bits to be transmitted at step 602. When the beacon 110 is for example equipped with at least one sensor and that said total capacity of the N _sym b2 sync symbols is greater than the number Nbi2 of identifier bits to be transmitted in step 602, this allows the beacon 110 to transmit to the central computer 130, via the bases of the real-time location system RTLS 100, additional information representative of measurements made by the (s) said sensor (s).

It should be borne in mind, however, that adding one or more synchronization symbols results in an increase in the duration of the RF signals being transmitted by the beacons, which increases their respective energy consumption. This also increases the risk of collision between two competing broadcasts from two separate beacons of RTLS 100 real-time location system, for the same refresh rate of beacon location information. As an illustration, considering that the RF signal to be transmitted by the beacon 1 10 comprises a single synchronization symbol (N _sym b 2 = 1) transmitted in the same bandwidth as the preamble symbol, the number N 2 = 6 bits of identifier to be transmitted during step 602 is reached through Nb _S 2 ⁼ 1 sign bit bs2 and a set of N¾2- = N & 2 - Nfe2 = 5 information bits carried by the P2 frequency-clocked chips Fc2 snippet of the sync symbol. In this example, the total capacity of the N _sym b2 synchronization symbols is equal to the number of identifier bits Nb _% 2 to be transmitted, which therefore does not make it possible, in this example, to transmit additional information to the step 602.

For reliable transmission of these Nbi2-bits of information through the? bits at the chip frequency Fc2 of each synchronization symbol, a set H of N _C2 = 2 ^N bi2 'code words H _£ (H =

^is set to a binary alphabet (± 1), each H code word having a length of? chips at the chip frequency Fc2 and unambiguously associated with a message of length N ^~ u2 'bits representative of at least a part of the identifier of a beacon belonging to RTLS 100 real-time location system. different codewords Hj are distributed in the code space so as to maximize their distances two by two, and thus improve their error correcting power. It should be noted that the size (?) Of the code words H _t is generally very large compared to the length of the information to be transmitted {Ν 2), which allows a very important coding redundancy, and therefore a high power error corrector, even in very noisy conditions.

At the base level, it is necessary to be able to find the information conveyed by the code words of the set H transmitted for each symbol of synchronization so as to determine from which beacon comes a received RF signal. This is preferably achieved by means of a bank of Nc ₂ correlators in parallel, since it is necessary to determine, for each synchronization symbol, which code word H is actually used by the beacon transmitting the received RF signal among the Nc ₂ words of possible code from the set H.

The code words of the set H have the following characteristics:

Good autocorrelation properties, so that, in reception, it is possible to clearly distinguish a single peak during the temporal correlation of the received signal with the code word H actually transmitted (and with this code only, among the Nc ₂ codewords to be tested of the set H), even with a very attenuated received signal and in the presence of noise. Said correlation peak detected in the flow of the received samples has a time index corresponding to the symbol synchronization instant;

Good cross-correlation properties so that different echoes of the same transmission that are received by a base through a multi-path propagation channel interfere with each other as little as possible, and that a synchronization symbol (carrying an H code) _A belonging to the set H) corresponding to a tag z _A can not be confused with a synchronization symbol (carrying a code H _B different from H _A , but also belonging to the set H) corresponding to another tag Z _B.

An improvement consists in superimposing, for each code word H transmitted by synchronization symbol, a scrambling code S. This scrambling code S is specific to the real-time location system RTLS 100, and all beacons of the time localization system. Real-time RTLS 100 scramble their synchronization symbols using the same scrambling code S, which is then predefined and known from the bases of the real-time location system RTLS 100. When RTLS real-time localization systems are co-located, the scrambling code S differs from one real-time RTLS location system to another, which allows each base to distinguish the RF signals transmitted by a beacon of the real-time RTLS location system to which said signal base belongs RF transmitted by a beacon of another RTLS real-time localization system, thanks to a quasi-zero intercorrelation property between the two scrambling codes. Thus, the distinction between RF signals transmitted by beacons belonging to RTLS co-localized real-time location systems is in reception from the physical layer, which is an advantage in two aspects: impossibility for a location system real-time RTLS to decrypt data intended for another RTLS real-time location system; and no need to implement (on the transmission side, as the receiving side) data encryption features in the layers above the physical layer. The scrambling code S may be a PN type code, such as for example a combination of m-sequences, or a Gold code, or a Kasami code, for their good cross-correlation properties and the relative simplicity of generation.

By application of the scrambling code S, each ^kem (1 <k <N _sy mb2) synchronization symbol transmitted in step 602, carrying a possible bit of sign bs _2, k and a code word H belonging to the set H, is a symbol Bt = Hj * bs _2, k * S, where "*" represents the term-to-term multiplication operation of P2 chips clocked at the chip frequency FC2 and at binary values (± 1). In the same way as without scrambling code, each synchronization symbol transmitted, along P2 bits clocked at the chip frequency FC2, has the good autocorrelation and cross correlation properties previously mentioned.

A further improvement consists in superimposing on the scrambling code S, for each synchronization symbol k, a secondary scrambling code specific to a subfamily of beacons to which the beacon 110 belongs, said subfamily being defined, unambiguously, and for each synchronization symbol k, by the set of tags having the same identifier bits as those previously transmitted through the previous (kl) synchronization symbols within the same RF signal. Thus, the scrambling code S,; used for the first symbol (k = 1) of synchronization is common to all beacons RTLS real-time location system 100, no identifier bit having been transmitted beforehand within the same RF signal; the scrambling code {k> 1) is then different and more and more specific to the tag 110 at each new synchronization symbol k transmitted (1 <k <N _sym b2) within the RF signal. If a number W2 (0 <W2 <N _S ymb2) among the N _sym b2 synchronization symbols carries no identifying bit of said tag (as may be the case, for example, if the system has few tags which then have a transferable identifier in a number X2 of synchronization symbols, and that the number N _sym b2 of synchronization symbols is chosen greater than this number Xi), then the W2 = N _sym b2 - 2 remaining synchronization symbols are scrambled by a secondary scrambling code specific to said tag 110 and itself. This additional improvement is interesting if the number N _sy mb2 of synchronization symbols is greater than the value "1", since this additional improvement makes it possible to improve the isolation of the synchronization symbols with respect to a possible concomitant emission (collision case ) another synchronization symbol from another beacon belonging to the real-time location system RTLS 100, said other beacon using, in the same way, a secondary scrambling code of its own.

In this case, by applying the scrambling code S and the secondary scrambling code each ^kem (1 <k <N _sy mb2) synchronization symbol transmitted in step 602, carrying a possible bit of sign bs _{2, k} and a code word H belonging to the set H, is a symbol Bt = Hj * bs2, k * S * ¾ * · Each synchronization symbol, along with _? chips at the chip frequency ¾, then has the good properties of autocorrelation and cross correlation previously mentioned.

In the next step 603, the beacon 1 10 transmits at least one location symbol. If there remain identifier bits to be transmitted (ie if the complete identifier of the tag 1 10 has not been transmitted within the synchronization symbol or symbols), the tag 1 10 transmits the remainder (NM3> 0 ) identification bits through N _sym b3 locating symbols (N _sym b3≥ 1) successive. Each location symbol, length P ₃ chips clocked at the chip frequency <¾ is transmitted by the beacon 1 10 using a bandwidth BW ₃ (FC3≤BW ₃ ≤ 2 * i¾) depending on the waveform of the transmitted RF signal regulated by the drop coefficient β of the analog low-pass filters 205 and 206. The localization symbol has a duration T _sym b3 = P3 I FC3, such that T _sym b3≥ T _sym b since, with a coefficient β constant fallout, the symbol duration is the same regardless of the bandwidth used. The bandwidth BW ₃ = b * BW used at this stage is greater than the bandwidth BWi used for the preamble symbol transmitted in step 601, and also greater than the bandwidth BW ₂ used for the (s). ) synchronization symbol (s) transmitted in step 602 so as to operate in broadband and thus obtain a better temporal resolution, which subsequently allows the central computer 130 to refine the location of the beacon 1 10.

In the context of the embodiment already mentioned, bi = b ₂ = 8 and BWi = 2.5 MHz are selected, which leads to a BW ₃ bandwidth of 20 MHz for the transmission of location symbols. The frequency chip ¾ = bi * Fci is then equal to 13 MHz, and each location symbol is then constituted of P3 = b2 *? 2 = 65 000 chips at the snap frequency i¾. In this example, each location symbol then has a unit duration T _sym b3 equal to 5.0 ms, ie of the same numerical value as T _sym b2- In a particular embodiment, for each k ^th location symbol (except the first in sequence, of index k = 1, for which there is no preceding location symbol that can serve as a phase reference), a bit of sign bs ₃ , k weights all the chips (clocked at the frequency fragment Fci) of said k ^th location symbol by the same binary value (± 1). This makes it possible to transmit an information bit per location symbol of index 2 <k <N _sym b3 (except for the first in sequence, of index k = 1), which is easily decoded in reception by comparison of the phase of the result of a symbol correlation with the phase of the preceding localization symbol, of index k - 1. Thus, the N _sym b3 locating symbols allow the transmission of N _bS 3 = (N _sym b3 - 1) bits of information through the sign bits {bs _{3 k} } ^ ^s ™ ^b3 possibly available (if N _sym b3≥ 2).

In addition to the N _bS 3 bits transmitted through any sign bits, it remains to transmit in step 603, a total of Λ / = Nb _% 3 - Nb _S 3 bits of identifier through the

N _S ymb3 * P3 snatches clocked at the snap frequency ¾ composing the N _sym b3 location symbols. These Λ / bits are preferentially distributed equitably, between the N _sym b3 locating symbols. Thus, without counting the possible sign bit bs3, k of a location symbol of index k, the number of bits transmitted through each location symbol is equal to:

while the value Λ / is equal to N _sym b3 times this value:

^N bi3 '

It follows that the total capacity of the N _sym b3 locating symbols (in number of bits transmitted at each transmission of an RF signal) during step 603 is finally equal to:

N _bi3 - N _bs3 ]

N _bS 3 + N _bi3 '= N _bs3 + N _symb3 * ≥ N b, i3

Nsymb3

This total capacity of the N _sym b3 locating symbols is necessarily greater than or equal to the number N _b% 3 of identifier bits to be transmitted at step 603. When the beacon 1 is for example equipped with at least one sensor and that total capacity of the N _sym b3 locating symbols is greater than the number Nbi3 of identifier bits to be transmitted at step 603, this allows the beacon 1 10 to transmit to the central computer 130, via the bases of the time localization system real RTLS 100, additional information representative of measurements made by the said sensor (s).

It must be borne in mind, however, that adding one or more location symbols results in an increase in the duration of RF signals being transmitted by the beacons, which increases their respective energy consumption. This also increases the risk of collision between two competing broadcasts from two separate beacons of RTLS 100 real-time location system, for the same refresh rate of beacon location information. As an illustration, considering that the RF signal to be transmitted by the beacon 1 10 has four location symbols (N _sym b3 = 4), and considering that 6 bits (Nbi2) among the 10 bits (NID) of identifier have previously transmitted during step 602, the number Nbi3 = 4 identifier bits remaining to be transmitted in step 603 is exceeded through N _bS 3 = 3 sign bits and a set of

Nbi3 - = 4 bits of information carried by the P3 chips clocked at the chip frequency ¾ of each of the four (N _sym b3) location symbols. In this example, the total capacity of the N _sym b3 locating symbols is thus equal to 7 bits Nb _S 3 + and thus shows a 3-bit excess over the number of identifier bits N _b% 3 to be transmitted: it it is therefore possible to transmit 3 bits of additional information each time an RF signal is transmitted during this step 603. In this example, the RF signal is composed of a preamble symbol with a duration of 5.0018 ms (T _sym bi), a single synchronization symbol (N _sym b2 = 1) with a duration of 5.0 ms (T _sym b2) and four location symbols (N _sym b3 = 4), each with a duration of 5.0 ms (T _sym b3): the duration of transmission of said RF signal is therefore in total equal to 30.0018 ms. The frame format chosen in this example therefore allows the transmission of an additional quantity of 3-bit information during a transmission time of 30.0018 ms, which is equivalent to an additional bit rate of 100 bits per second.

For a reliable transmission of these Λ / bits of information through the P3 chips clocked at the chip frequency ¾ a set G of Nc = 2 ^N ba 'words of

f "1 ^ (73-1

code G _j (G = (G j) is defined on a binary alphabet (± 1), each code word G, having a length of P3 chips clocked at the chip frequency ¾ and unambiguously associated with a message of length Λ / bits representative of at least one part of the identifier of a beacon belonging to the real-time location system RTLS 100. The different code words G _j are distributed in the code space so as to maximize their distances in pairs, and thus improve their power. error corrector. The size (Pi) of these codewords is generally very large compared to the length of the information to be transmitted (Nw), which allows a very important coding redundancy, and therefore a high error-correcting power even in very noisy conditions.

At the base level, it is appropriate to be able to retrieve the information conveyed by the code words of the set G transmitted for each location symbol so as to determine from which tag a received RF signal. This is preferably achieved by means of a bank of Nc3 correlators in parallel, since it is necessary to determine, for each location symbol, which code word G _j is actually used by the transmitter beacon of the received RF signal among the Nc3 words of possible code of the set G.

The code words of the set G have the following characteristics:

Good autocorrelation properties, so that, in reception, it is possible to clearly observe a peak per path of the propagation channel during the time correlation of the received signal with the code word G _j actually transmitted (and with this code only among the Nc3 test code words of the set G), even with a very attenuated received signal and in the presence of noise. Estimating the instant of arrival of the direct path, the sole guarantor of an unbiased location measurement, corresponds to the time index of the first correlation peak detected in the flow of the received samples;

Good cross-correlation properties so that different echoes of the same transmission that are received by a base through a multi-path propagation channel interfere with each other as little as possible, and that a location symbol (carrying a G-code _A belonging to the set G) corresponding to a tag z _A can not be confused with a location symbol (carrying a code G _B different from G _A , but also belonging to the set G) corresponding to another tag zg .

It should be noted that if all beacon identifier bits of RTLS real-time location system 100 can be transmitted within synchronization symbol (s) at step 602, i.e., if Nbi3 = 0, then the set of Nc3 code words G is reduced to one and the same code word G _j used by the set of tags the real-time location system RTLS 100, which reduces, on the receiving side, the processing complexity of the location symbols received.

An improvement consists in scrambling each transmitted location symbol by superimposing a scrambling code S '. This scrambling code S 'is specific to the real-time location system RTLS 100, and all the beacons of the real-time location system RTLS 100 scramble their location symbols using the same scrambling code S', which is then predefined. and known basics of real-time location system RTLS 100. When RTLS real-time location systems are co-located, the scrambling code S 'differs from RTLS real-time location system to another, which allows each base to distinguish the RF signals transmitted by a beacon of RTLS real-time location system to which said base of RF signals transmitted by a beacon of another real-time RTLS location system belongs, thanks to a property virtually zero intercorrelation between the two scrambling codes. Thus, the distinction between RF signals transmitted by beacons belonging to RTLS co-localized real-time location systems is in reception from the physical layer, which is an advantage in two aspects: impossibility for a time location system -Real RTLS to decrypt the data for another RTLS real-time location system; and no need to implement (on the transmission side, as the receiving side) data encryption features in the layers above the physical layer. The scrambling code S 'may be a PN type code, for example a combination of m-sequences, a Gold code or a Kasami code for their good intercorrelation properties and the relative simplicity of generation.

By application of the scrambling code S ', each ^kem (1 <k <N _sy mb3) localization symbol transmitted in step 603, carrying a possible bit of sign bs _{3, k} and a code word G _j belonging to the set G, is a symbol Et = G _j * bs _{3, k} * S ', where "*" represents the term-to-term multiplication operation of P ₃ chips clocked at the chip frequency à and at binary values (± 1). In the same way as without scrambling code, the transmitted location symbol, along P ₃ chips clocked at the chip frequency <¾ has the good properties of autocorrelation and intercorrelation previously mentioned.

A further improvement consists in superimposing on the scrambling code S ', for each location symbol k, a secondary scrambling code S'y specific to a subfamily of tags to which the tag 1 10 belongs, said subset family being defined uniquely and for each location symbol k, by the set of tags having the same identifier bits as those previously transmitted through the set of N _sym b2 synchronization symbols within the same signal RF, as well as previous (k-1) location symbols within the same RF signal. The scrambling code S'y (1 <k <N _S y _m bi) is thus different and more and more specific to the tag 110 with each new location symbol k transmitted {1 <k <N _S y _m bi) within the RF signal. If a number W3 (0 <W3 <Nsymbi) among the N _sym b3 location symbols carries no identifying bit of said beacon, then the remaining W3 location symbols are scrambled by a secondary scrambling code specific to said beacon and to herself. This additional improvement allows a better isolation of the location symbols with respect to a possible concomitant emission (collision case) of another location symbol from another beacon belonging to the real-time location system RTLS 100, the said other tag using, in the same way, a different secondary scrambling code of its own.

In this case, by applying the scrambling codes S 'and S'y, each ^keme (1 <k <Nsy _m bi) location symbol transmitted in step 603, carrying a possible bit of sign ôsy and a word of code G, belonging to the set G, is a symbol E _k = G _j * bs3, _k * S '* S'y. This symbol of location, along P3 chips clocked at the chip frequency <¾ then has the good properties of autocorrelation and intercorrelation previously mentioned.

The fact that the beacon 110 emits several successive location symbols (in this case, N _sym b3> 1) makes it possible, on the reception side, to have several location measurements (one measurement per location symbol) per RF signal transmitted by the beacon 110, and thus to make reliable the calculation of the position of the beacon 110 by the central computer 130. It must however be aware that adding one or more location symbols leads to an increase in the duration of transmission of the RF signal by the beacons , which increases their respective energy consumption. This also increases the risk of collision between two competing broadcasts from two separate beacons of RTLS 100 real-time location system, for the same refresh rate of beacon location information.

In a next step 604, the beacon 110 is preferably placed in standby, so as to reduce the energy consumption of said beacon 1 10, until duty transmit again an RF signal to allow the central computer 130 to update the estimate of the geographical position of the beacon 110 (ie refreshing the location information of the beacon 110).

Fig. 7 schematically illustrates an RF signal transmitted by implementation of the algorithm of FIG. 6. The RF signal shown in FIG. 7 is centered around the null frequency (rather than centered on the carrier frequency fo) for the sake of simplification.

The RF signal is thus represented in three dimensions: the frequency /, the time t and the power spectral density d.

The RF signal starts with a preamble symbol 701 on the duration T _sym bi transmitted with the predefined bandwidth BWi.

The RF signal continues with at least one synchronization symbol 702, each synchronization symbol being transmitted over the duration T _symb2 in the predefined bandwidth BW ₂ . In the example schematically illustrated in FIG. 7, the RF signal comprises a single synchronization symbol 702.

The RF signal terminates with at least one location symbol 703, each location symbol being transmitted over the duration T _sym b i within the predefined bandwidth BW ₃ . In the example schematically illustrated in FIG. 7, the bandwidth BW ₃ is greater than the bandwidth BW ₂ , itself greater than or equal to the bandwidth BWi, and the RF signal comprises a single location symbol 703.

In the example schematically illustrated in FIG. 7, the power used to transmit the preamble symbol 701 equal to the integral along the frequency / power density axis d is represented by the lateral surface (appearing in gray) of the rectangular parallelepiped representing said preamble symbol 701. The same applies to each synchronization symbol 702 and to each location symbol 703. In a particular embodiment, this power is the same to transmit the preamble symbol 701, each synchronization symbol 702, as well as each symbol. localization 703 (the lateral surfaces (grayed) rectangular parallelepipeds representing respectively having the same area).

The symbol times T _sym bi, _sym b2 and T _sym b3 being substantially identical in this FIG. 7, the energy used to transmit each symbol, defined as the integral of the power along the time axis t on the respective duration of each symbol is also constant and corresponds to the volume of each of the three parallelepipeds rectangles represented.

The bandwidth BW ₂ is preferably chosen rather low so that the transmission of each synchronization symbol 702 is done in narrow band to reduce the computation complexity on the reception side. Nevertheless, this bandwidth BW ₂ may be chosen larger than the bandwidth BWi so as to increase the bit rate of information transmitted via each synchronization symbol 702, thereby reducing the number of bits to be transmitted via the symbol or symbols of localization 703 and therefore the processing complexity, in reception, of said location symbols 703. The bandwidth BW ₂ may therefore constitute an adjustment variable for optimizing the calculation complexity distribution, at the base level, between the processing synchronization symbol (s) 702 and the processing of location symbol (s) 703.

In the context of the embodiment already mentioned, the bandwidth BW ₃ used for the transmission of the location symbols is equal to 20 MHz and the chip frequency associated with 13 Mcps. For the preamble and synchronization symbols, the same bandwidth is chosen, narrower by a factor equal to "8" than the BW bandwidth ₃ used for the transmission of the location symbols (ie BWi = BW ₂ = 2.5 MHz), with fcc frequencies Fc ₁ = Fc ₂ = 1.625 Mcps. Using an oversampling factor of two samples per chip for all symbols, the following sampling frequency values result: Fsi = FS2 = 2 * Fci = 3.25 MHz and Fs ₃ = 2 * Fc 3 = 26 MHz. As for the decimation factor D _A to the floor of change of pace 401, its value can be set to "2" (D = _Â 2) to have a oversampling factor at the analog to digital converters ADC 312, 313, thus facilitating the filtering of spectral replicas downstream. The value of sampling frequency of the analog-digital converter ADC 312, 313 is equal to FS _ADC = D * _Â FS3 = 52 MHz in this embodiment.

Fig. 8 schematically illustrates an RF signal receiving algorithm implemented by each base of the RTLS 100 real-time location system. Let us consider, illustratively, that the algorithm of FIG. 8 is implemented by the base 120.

In a step 801, the base 120 permanently activates a preamble symbol detection mechanism 810. Referring to FIG. 4, the base 120 activates the preamble symbol processing module 411; the synchronization symbol processing module 412 and the location symbol processing module 413 are in energy saving mode.

The purpose of this preamble symbol detection mechanism 810 is to start a time synchronization, by determining a block synchronization, ie to determine the start (and therefore the end) times of blocks composing the preamble symbol in a received RF signal. .

In the context of this preamble symbol detection mechanism 810, the base 120 analyzes the samples of the RF signal which are obtained according to the sampling frequency Fsi, and this in a sliding window of size equal to T _sym bi. With reference to FIG. 4, the output of the rate change stage 403 is connected within the preamble symbol processing module 411 to a size buffer for holding samples obtained at the sampling frequency Fsi for a duration of less than T _sym bi. The base 120 thus performs a coherent integration over a duration at least equal to T _sym bi, so as to detect at least one correlation peak between the samples of the sliding window and the code C (known in advance by the base 120 ), the value of said correlation peak being greater than a predefined threshold value Thi.

The value of the predefined Thi threshold is the result of a compromise between:

· Maximize the probability of detection (by lowering the Thi threshold), but by taking the risk of false detections that falsely trigger synchronization symbol processing, which therefore leads to excessive consumption of processing resources; and

• minimize the likelihood of false alarms (by increasing the value of the Thi threshold), but by missing certain preamble symbols actually transmitted by beacons of RTLS 100 real-time location system, and thus miss opportunities to result in a measurement of location of said tags.

Thus, in a step 811, the base 120 checks whether a preamble symbol is detected, ie if at least one correlation peak greater than the predefined Thi threshold value is detected in the sliding window. When at least one such correlation peak is detected, a step 812 is performed; otherwise, the preamble symbol detection mechanism shifts the sliding window of a sample (introduction of a newer sample and removal of the oldest sample) and step 81 1 is repeated with this sliding window shifted by 'a sample. It may be noted that, in reception, the preamble symbol is always searched for with its nominal duration, namely on the number M of blocks, without taking into account a possible extension of the preamble symbol by M 'blocks. The fact that the preamble symbol is longer than M 'blocks allows each base of the real-time location system RTLS 100 to increase the probability of detecting the preamble symbol by concatenating several successive detection attempts on reception windows. offset.

In step 812, the base 120 performs block synchronization, Le. determines the start time (and therefore the end since the size of the code C is known from the base 120) of at least one block composing the preamble symbol whose presence has been detected. Each correlation peak (of value greater than the predefined Thi threshold value) detected in step 811 corresponds to an end-of-block instant within the M blocks (or within the M + M 'blocks if the preamble symbol is extended by M 'blocks) of the preamble symbol. It then remains to raise the uncertainty on the precise temporal position of the first block in sequence, or the last block in sequence, among the blocks composing the preamble symbol. This aspect is processed by a synchronization symbol detection mechanism 820 detailed below.

Then, in a step 813, the preamble symbol detection mechanism 810 activates the synchronization symbol detection mechanism 820. This selective activation of the synchronization symbol detection mechanism 820 reduces the power consumption of the base 120. With reference to FIG. 4, the preamble symbol processing module 411 activates the synchronization symbol processing module 412 (which then exits the energy saving mode). In addition, the preamble symbol processing module 411 provides the timing symbol processing module 412 with information representative of the result of the block synchronization performed at step 812 (i.e., a start or end time of the block). .

The objectives of this synchronization symbol detection mechanism 820 are:

· Terminate the time synchronization phase by determining the symbol synchronization;

• decode all or part of the identifier of the beacon transmitting the received RF signal; and Optionally, to decode certain additional information (eg sensor measurements previously mentioned) transmitted by the beacon transmitting the received RF signal.

In the context of this synchronization symbol detection mechanism 820, the base 120 analyzes the samples of the RF signal which are obtained according to the sampling frequency i¾, and this in a sliding window of size equal to T _sym bi + T _sym b2 from the start or end time of the block resulting from the block synchronization. With reference to FIG. 4, the output of the rate change stage 402 is connected within the preamble symbol processing module 412 to a size buffer for holding samples obtained at the sampling frequency i¾ for a duration of less than T _sym bi + T _sym b2. The size of this buffer memory is at least equal to T _sym bi + T _sym b2, because the actual position of the synchronization symbol which follows the preamble symbol with respect to the start or end time of the block resulting from block synchronization can fluctuate in blocks, depending on the relative position of the beacon transmitting the RF signal relative to the base 120. This aspect is detailed below in relation to FIGS. 9A and 9B.

In a step 821, the base 120 performs the symbol synchronization by searching for the presence of all or part (a predefined number of bits N 2) of the identifier of a beacon of the real-time location system RTLS 100, ie determines the start time (and therefore end since the size of the transmitted symbols is known from the base 120) symbols constituting the RF signal. To do this, the base 120 seeks to detect at least one correlation peak between the received samples and a code word H among all the code words H that may be transmitted by a beacon of the real-time location system RTLS 100. {ie those of the set H). The base 120 uses a symbol correlator (of length equal to that of the synchronization symbols) and the block m blocks after the block start or end time resulting from the block synchronization, where m is an index that the base 120 makes. vary successively from 1 to until a correlation peak greater than a predefined threshold value Z¾2 appears. If, after these attempts, no correlation peak greater than the predefined threshold value 7¾ has been found, symbol synchronization fails.

In a particular embodiment, when the preamble symbol has M + M 'blocks, the base 120 successively varies the index m from 1 to M + M 'until a correlation peak greater than the predefined threshold value Z¾2 appears.

The value of the predefined Z seuil2 threshold is the result of a compromise between:

• Maximize the probability of detection (by lowering Thi threshold value), but by taking the risk of false detections that falsely trigger synchronization symbol processing, which leads to excessive consumption of processing resources; and

• to minimize the probability of false alarm (by increasing the Thi threshold value), but by missing certain synchronization symbols actually transmitted by beacons of RTLS 100 real-time localization system, and thus to miss opportunities to result in a measurement of location of said tags.

In a next step 822, the base 120 determines all or part of the identifier of the beacon transmitting the received RF signal. In fact, the synchronization symbol carrying the code word H which, among all the code words H that can be transmitted by a beacon of the real-time location system RTLS 100, has revealed, after any descrambling, the peak correlation greater than the predefined threshold value Z¾2 includes all or part of the bits of the identifier of the beacon transmitting the received RF signal. The descrambling is the opposite operation to that of the scrambling carried out by the beacon transmitting the received RF signal and consists in multiplying, in a piecemeal manner, the symbol received by the scrambling code conjugate used by the beacon transmitting the received RF signal, said code of scrambling (which is possibly associated with a secondary scrambling code) being known from the base 120.

In a next step 823, if the symbol synchronization could be performed, the synchronization symbol detection mechanism 820 activates a locator symbol detection mechanism 830, before being put into a power saving mode. If the symbol synchronization has failed, the synchronization symbol detection mechanism 820 is put in the energy saving mode without activating the locator symbol detection mechanism 830 (not shown in Fig. 8). This selective activation of the location symbol detection mechanism 830 makes it possible to reduce the energy consumption of the base 120. Referring to FIG. 4, the synchronization symbol processing module 412 activates the location symbol processing module 413 (which then exits the energy saving mode). In addition, the symbol processing module of synchronization 412 provides the location symbol processing module 413 with information representative of the result of the symbol timing (i.e., a start time of the location symbol) performed in step 821.

The objectives of this location symbol detection mechanism 830 are:

Precisely determining which beacon transmitting the received RF signal (by ending the decoding of the identifier of said beacon which was started by the synchronization symbol detection mechanism 820), if necessary;

Accurately detect and estimate the instant of arrival of the replica of the RF signal corresponding to the direct path of the RF signal from the transmitting beacon to the base

120, to allow the central computer 130 to estimate the distance between said beacon and the base 120 to locate said beacon after cross-checking (at the central computer 130) with measurements made at other bases; and

Optionally, decoding certain additional information (e.g. the sensor measurements previously mentioned) transmitted by the beacon transmitting the received RF signal.

In the context of this location symbol detection mechanism 830, the base 120 analyzes the samples of the RF signal which are obtained according to the sampling frequency i¾, and this in a sliding time window of duration equal to T _sym b3 + Axbackward opened earlier (of a relative time value called Axbackward parameter) than the estimated start time of the location symbol resulting from the symbol synchronization: the precise start time of the location symbol must indeed be sought earlier than said estimated instant obtained at the end of step 820, taking advantage of the temporal resolution & 2 times finer at step 830 than at step 820: the precise location of the beacon transmitting the RF signal indeed requires that to be able to detect the direct path, the only guarantor of an unbiased measurement, which is also the very first path in time and which can happen very attenuated at the level of the base 120 , and thus not to have been detected in the previous steps 810 and 820 for which the temporal resolution is coarser (the bandwidth being lower). The parameter Azbackward, homogeneous at a time and expressed in seconds, is defined as greater than or equal to a delay spread ATchannei of the propagation channel in the considered environment. With reference to FIG. 4, the output of the rate change stage 401 is connected within the location symbol processing module 413 to a size buffer to contain samples obtained according to the sampling frequency Fs3, for a duration at least equal to T _sym b3 + Axbackward- The size of this buffer memory is at least equal to T _sym b3 + Axbackward, because, following the synchronization symbol and knowing that the sizes of the synchronization and location symbols are known from the base 120, the start and end times of the synchronization and location symbols are known (roughly) of the base 120.

In a step 831, the base 120 determines the remainder of the identifier of the beacon transmitting the received RF signal, if necessary (if all the bits of the identifier have not been transmitted via the symbol (s) ( s) synchronization). To do this, the base 120 seeks to detect at least one correlation peak between the received samples and a code word G, among all the code words G, that can be transmitted by a beacon of the real-time location system. RTLS 100. The base 120 uses a symbol correlator (of length equal to that of the location symbols) and the shim on the start time of the location symbol resulting from the symbol synchronization. The code word G, of all the code words G, capable of being transmitted by a beacon of the real-time location system RTLS 100 which shows a correlation peak, after any descrambling, greater than a predefined threshold value 7¾ is the one retained, which gives the remainder of the identifier of the transmitter beacon of the received RF signal (and / or the additional information (eg sensor measurements (s) previously mentioned) that the beacon transmitting the RF signal wanted to transmit ).

The value of the predefined threshold 7¾ is the result of a compromise between:

• Maximize the probability of detection (by lowering the value of the 7¾ threshold), but by taking the risk of false detections that falsely trigger location symbol processing, which leads to excessive consumption of processing resources; and

• minimize the likelihood of false alarms (by increasing the value of the threshold 7¾), but by missing certain location symbols actually transmitted by RTLS 100 real-time location system beacons, and thus miss opportunities to result in a measurement of location of said tags.

The estimation of the symbol synchronization obtained in step 821 indicates the time of arrival, at the level of the base 120, of the location symbol with a relatively coarse temporal resolution (of the order of 1 / ¾ = 308 ns according to the example of a sampling frequency ¾ equal to 3.25 MHz). Due to the band-narrow nature of the emission of preamble and synchronization symbols, different replicas of a multi-path propagation channel, spaced apart from each other by a delay less than this coarse temporal resolution, typically come together and can not be discriminated: the base 120 can thus, at this stage, only determine approximately the time of arrival (at ± = ± 154 ns according to said

2.Fs ₂

example) of the packet of energy spread in time after transmission through the possibly multi-path propagation channel, which makes it possible to reach at this stage only a coarse distance precision (at ± - ^ - = ± 46 m according to the example of a sampling frequency ¾ equal to 3.25 MHz, and c = 3e8 m / s being the speed of propagation of the RF signal equal to the speed of light). The wide-band character of the emission of the location symbols makes it possible to obtain a better temporal resolution (of the order of i / i¾ = 38.5 ns according to the example of a sampling frequency ¾ equal to 26 MHz, for a bandwidth BW ₃ equal to 20 MHz), and therefore a better location accuracy (at ± - ^ - = ± 5.8 m according to the example of a bandwidth BW ₃ equal to 20 MHz ). Sampling frequencies for preamble and synchronization symbol processing therefore only allow a rough estimate of the response of the possibly multi-path propagation channel, while the sampling frequency for the processing of location symbols allows a more faithful reconstruction of the response of the propagation channel, this time possible to distinguish the possible different time replicas of the possibly multi-path propagation channel thanks to a better temporal resolution.

To make a reliable and unbiased unbiased distance estimation, it is necessary to be able to detect the instant of arrival of the replica of the location symbol corresponding to the direct path of the transmitting beacon to the base 120, and not to not be based on a secondary path (not direct) that would potentially be received with a higher power but that would have traveled a distance longer than the direct path between the transmitting beacon and the base 120, typically by reflection on surrounding objects.

In a next step 832, the base 120 determines which replica of the RF signal corresponds to the direct path of the location symbol via the possibly multi-path propagation channel and determines the time of arrival at the base 120. For this base 120 analyzes the received samples going back in time with respect to the estimated start time of the location symbol resulting from the symbol synchronization in search of the replica of the RF signal corresponding to the direct path, as previously described. For an outdoor environment ("outdoor" in English), we can consider Az _c hannei = 500 ns (corresponding to a distance of 150 m), and for an indoor environment ("indoor" in English), we can for example consider Az _c hannei = 100 ns (corresponding to a distance of 30 m). In addition, the base 120 analyzes the samples received after the estimated start time of the location symbol resulting from the symbol synchronization, in search of the replica of the location symbol corresponding to the direct path: this analysis is done over a period of time. temporal Axf _orwar d equal to half the sampling period ¾ used for the synchronization symbols, ie, according to the example of a sampling frequency ¾ equal to 3.25 MHz, Axf _orwar d = 154 ns ( corresponding to a distance of 46 m). Analysis of samples is thus the total on a time window of width Ax _win d _ow = Axbackward + AXF _orwar of which is thus for example equal to 654 ns in external environment (corresponding to a distance of 195 m). In this time window of width Ax _win _ow around the estimated time of start of location symbol resulting from the symbol synchronization, the base 120 searches for power peaks greater than a predefined threshold value T¾4 in the estimate of the impulse response of the possibly multi-path propagation channel. The first (in sequence) peak of power within this time window of width Ax _win d _0W corresponds to the direct path sought, and its position within the time window of width Ax _w j j _ow defines the precise instant Ti _oc , k, at the temporal resolution 1 / FS3 near (= 38.5 ns in the embodiment already mentioned), starting each location symbol k (1 <k <N _sy mb3) among the N _sym b3 symbols of location of the received RF signal. The estimate ¾ of the instant of arrival of the direct path for the k ^th location symbol at the base 120 is then deduced as follows, knowing the duration and the number of symbols preceding said k ^th location symbol in the RF signal:

¾ Tloc, k ^~ [T _S y _m t, i N _S ymb2 * Tsymb2 (k-1) * T $ ymb2]

The transmitted RF signal being composed of N _sym b3 locating symbols, the base 120 thus performs N _sym b3 instant arrival measurements of the direct path

The value of the predefined threshold T4 is chosen sufficiently low (selectively) to determine which replica of the location symbol corresponds to the direct path of the location symbol considered via the possibly multipath propagation channel, while guaranteeing a reduced probability of false alarm. . In a subsequent step 833, the base 120 transmits to the central computer 130 information representative of the identifier of the beacon transmitting the received RF signal, as well as information representative of the time of arrival at the base 120 of the the replica of each location symbol corresponding to the direct path of the RF signal via the propagation channel (N _sym b3 arrival time information). The base 120 can also transmit to the central computer 130 additional information (eg sensor measurements (s) previously mentioned) transmitted by the transmitting beacon. Thus, by collecting such information from different bases of the real-time location system RTLS 100, the central computer 130 is able to determine the position of the tag identified, thanks to a location algorithm (triangulation, multilateration) using as variables the differences in TDOA arrival times of said one or more location symbols at said different bases.

Numeric interpolation postprocessing techniques can be implemented to increase the accuracy of the beacon location estimation within the RTLS 100 real-time location system perimeter. Other post-processing techniques related to an evaluation of the speed of movement and the trajectory of each beacon may further be implemented to increase this estimation accuracy. It is also possible to take advantage of several successive measurements of distance for the same beacon, without the position of said beacon has changed: using the average of these measures reduces the variance due to measurement noise.

Fig. 9A schematically illustrates the real-time location system RTLS 100, in a configuration in which the beacon 1 10 is located closer to the base 121 than to the base 122.

Fig. 9A shows the elements of FIG. 1 by placing the beacon 1 10 much closer to the base 121 than the base 122. When the beacon 1 10 transmits an RF signal, the base 121 receives a first version 15 of the RF signal with a propagation latency Lj ', and the base 122 receives a second version 152 'of the RF signal with a propagation latency L ₂ '. Due to the difference in distance of said bases 121, 122 relative to the beacon 1 10, the propagation latency Li 'is low compared to the propagation latency L ₂ '. In addition, the second version 152 'of the RF signal is received by the base 122 with a power level typically more attenuated than the first version received by the base 121. Thus, it is likely that the base 121 finds quickly (for example after only a few blocks received) block synchronization of the RF signal, because a very small integration gain is needed to bring out the noise signal. Conversely, a much longer integration time (and, to the extreme, close to the duration T _sym bi of the preamble symbol) is probably required to successfully detect the block synchronization at the level of the base 122. This aspect is schematically shown in FIG. 9B.

Fig. 9B schematically illustrates block start times resulting from the block synchronization and timing symbol start times resulting from the symbol synchronization, in the context of the configuration shown in FIG. 9A.

Fig. 9B schematically represents a portion of an RF signal 900 transmitted by beacon 110 in the context of FIG. 9A. The RF 900 signal includes a preamble symbol 901, as previously described, composed of an amount M of blocks 903 repeated.

Fig. 9B also schematically represents a portion of an RF signal 910 corresponding to the version of the RF signal 900 as received by the base 121 in the context of FIG. 9A. The block synchronization is determined by the base 121 at a time 911, corresponding for example to the boundary between the first block and the second block in sequence in the preamble symbol 901. The base 121 then attempts to find the symbol synchronization from this instant 911. By applying a correlation operation on the size of the synchronization symbol from time 911, the base 121 does not find a correspondence with a code word H of the set H. The base 121 retains then such a correlation operation from a time offset in the future of the duration of a block from time 911, and also does not find a correspondence with a code word H of the set H. The correspondence with a code word H of the set H is obtained by applying such a correlation operation from an instant 912 which marks the boundary between the preamble symbol 901 and the synchronization symbol 902. allu M attempts at the base 121 to detect the symbol synchronization after the detection of the block synchronization.

Fig. 9B also schematically represents a portion of an RF signal 920 corresponding to the version of the RF signal 900 as received by the base 122 in the context of FIG. 9A. The block synchronization is determined by the base 122 at a time 921, for example corresponding to the border between Γ penultimate block and the last block in sequence in the preamble symbol 901. The base 122 then tries to find the symbol synchronization from that moment 921. By applying a correlation operation on the size of the synchronization symbol from the instant 921, the base 122 does not find a correspondence with a code word H of the set H. The base 122 then retries such a correlation operation from a moment 922 shifted in the future of the duration of a block with respect to the instant 921, and finds a correspondence with a codeword H of the set H, since the instant 922 marks the boundary between the preamble symbol 901 and the synchronization symbol 902. It thus took two attempts at the base 122 to detect the symbol synchronization after the detection of the block synchronization.

Claims

1) Beacon (110) for use in a real-time location system (100) having a plurality of bases (120, 121, 122) adapted to determine arrival times of a radio frequency signal transmitted by said beacon, and a central computer (130) adapted to locate said beacon from said arrival times, characterized in that said beacon (110) is adapted to generate said radio frequency signal by successive symbols as follows:

a preamble symbol (701) of length defined by a first quantity of chips Pi clocked at a first chip frequency Fci over a first bandwidth (BWi), constituted by a plurality of identical M successive blocks forming a repetition of a first pseudo-random bit sequence code word;

at least one synchronization symbol (702), each synchronization symbol having a length defined by a second quantity of chips; clocked at a second chip frequency ¾ over a second bandwidth {BWi) at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said synchronization symbols being representative of a first information of N _bi2 bits; and

at least one location symbol (703), each location symbol having a length defined by a third quantity of chips P3 clocked at a third chip frequency ¾ over a third bandwidth (BW3) wider than the second bandwidth , and consisting of a third code word G ,, said location symbol or the set formed by said location symbols being representative of a second piece of information Nba bits;

such that the N¾2 bits of said first piece of information plus the Nba bits of said second piece of information bear, at least, an identifier of said known tag of said central calculator. 2) Beacon according to claim 1, characterized in that said beacon is further adapted to generate said radio frequency signal as follows: for each synchronization symbol k, a first sign bit bs ₂ , _k weight said second quantity of P chips ₂ clocked at a second chip frequency Fc ₂ by the same binary value. 3) Beacon according to any one of claims 1 and 2, characterized in that said beacon is further adapted to generate said radio frequency signal as follows: a first scrambling code S specific to said real-time location system is superimposed on everything says first code word H.

4) Beacon according to claim 3, characterized in that said beacon is further adapted to generate said radio frequency signal as follows: a first secondary scrambling code specific to a subfamily of beacons to which said beacon (110) belongs superimposed on the first scrambling code S, for each synchronization symbol k, said subfamily being uniquely defined, and for each synchronization symbol of said radio frequency signal, by all the beacons of said real-time location system ( 100) having the same identifier bits as those previously transmitted through the previous synchronization symbol (s) within said radio frequency signal.

5) Beacon according to any one of claims 1 to 4, characterized in that said beacon is further adapted to generate said radio frequency signal as follows: for each location symbol k ', a second sign bit bs ^ weight said third quantity of chips P3 clocked at the third chip frequency ¾ by the same binary value.

6) Beacon according to any one of claims 1 to 5, characterized in that a second scrambling code S 'specific to said real-time location system is superimposed on any said second code word G _j .

7) Beacon according to claim 6, characterized in that a second secondary scrambling code S'2, k 'specific to a subfamily of beacons to which said beacon (110) belongs is superimposed on the scrambling code S', for each location symbol k ', said subfamily being uniquely defined, and for each location symbol of said radio frequency signal, by all tags of said real-time location system (100) having the same bits of identifying those previously transmitted through the symbol or symbols of synchronization and the previous location symbol (s) within said radio frequency signal.

8) Beacon according to any one of claims 1 to 7, characterized in that said beacon is between two alarms of said beacon for generating said radio frequency signal in standby mode.

9) Beacon according to any one of claims 1 to 8, characterized in that said beacon comprises one or more sensors, and in that said first and second information contain, in addition to the identifier of said beacon, additional information representative of measurements made by the said sensor (s).

10) Base (120) for use in a real-time location system (100) having a plurality of such bases (120, 121, 122) adapted to determine arrival times of radiofrequency signals transmitted by beacons, and a central computer (130) adapted to locate said beacons from said arrival times, characterized in that said base (120) is adapted to receive said radio frequency signals respectively by successive symbols as follows:

at least one location symbol (703), each location symbol having a length defined by a third quantity of chips P3 clocked at a third chip frequency ¾ over a third bandwidth (BW ₃ ) at least as wide as the second bandwidth, consisting of a third code word G _j , said location symbol or the set formed by said location symbols being representative of a second information of Nb _% 3 bits;

such that the N¾2 bits of said first information plus the Nb _% 3 bits of said second information carry, at least, an identifier of said known tag of said central computer,

and in that said base (120) is adapted for:

- performing a block synchronization (812) on receipt of said preamble symbol;

- perform a symbol synchronization (821) from the block synchronization;

recovering (822, 831) the identifier of the beacon transmitting the radio frequency signal from the synchronization symbol or symbols and the location symbol or symbols, as a function of symbol synchronization; and

- Determining the arrival time of said radio frequency signal in a time window defined according to the symbol synchronization.

11) real-time location system (100), characterized in that it comprises a plurality of bases (120, 121, 122) according to claim 10 which are adapted to determine arrival times of radiofrequency signals emitted by beacons according to any one of claims 1 to 9, and a central computer (130) adapted to locate each beacon from said arrival times.

A method implemented by a beacon (110) in a real-time location system (100) having a plurality of bases (120, 121, 122) determining arrival times of a radio frequency signal transmitted by said beacon, and a central computer (130) locating said beacon from said arrival times, characterized in that said beacon (110) generates said radio frequency signal by successive symbols as follows:

at least one synchronization symbol (702), each synchronization symbol having a length defined by a second quantity of chips; clocked at a second chip frequency ¾ over a second bandwidth (BW ₂ ) at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said synchronization symbols being representative of a first information of

at least one location symbol (703), each location symbol having a length defined by a third quantity of chips P ₃ clocked at a third chip frequency ¾ over a third bandwidth (BW ₃ ) at least as wide as the second bandwidth, and consisting of a third code word G _j , said location symbol or the set formed by said location symbols being representative of a second piece of information N _b a bit;

such that the N¾2 bits of said first piece of information plus the N _b a bits of said second piece of information bear, at least, an identifier of said known tag of said central calculator.

A method implemented by a base (120) used in a real-time location system (100) having a plurality of such bases (120, 121, 122) determining arrival times of the radio frequency signals transmitted by beacons, and a central computer (130) locating said beacons from said arrival times, characterized in that said base (120) receives said radio frequency signals respectively by successive symbols as follows:

at least one synchronization symbol (702), each synchronization symbol having a length defined by a second quantity of chips P ₂ clocked at a second chip frequency ¾ over a second bandwidth (BW ₂ ) at least as wide as the first bandwidth, and consisting of a second code word H, said synchronization symbol or the set formed by said synchronization symbols being representative of a first piece of information.

at least one location symbol (703), each location symbol having a length defined by a third quantity of chips P ₃ clocked at a third chip frequency ¾ on a third bandwidth (BW ₃ ) at least as wide as the second bandwidth, and consisting of a third code word G _j , said location symbol or the set formed by said symbols locating being representative of a second Nbi3 bit information;

such that the N¾2 bits of said first piece of information plus the N _b% 3 bits of said second piece of information bear, at least, an identifier of said known tag of said central calculator,

and in that said base (120) performs the following steps:

- performing a block synchronization (812) on receipt of said preamble symbol;

- perform a symbol synchronization (821) from the block synchronization;

- determining (832) the arrival time of said radio frequency signal in a time window defined according to the symbol synchronization.

A method implemented by a real-time location system (100) having a plurality of bases (120, 121, 122) implementing the method according to claim 13 and which determine arrival times of radio frequency signals transmitted by implementing tags. the method of claim 12, and a central computer (130) locating each beacon from said arrival times. 15) Computer program product, characterized in that it comprises a set of instructions causing, when executed by a processor of a beacon of a real-time location system, the method of claim 12 or a set of instructions causing, when executed by a processor of a base of a real-time location system, the method according to claim 13.