Alarm device suitable for transmitting an alert message in the presence of frequency interference

Abstract

The invention relates to a method (100) for transmitting an alert message from an alarm device to a cellular network in the presence of frequency interference. The method comprises the following steps: - at a first time (t1), in the absence of interference: obtaining (110) preconfigured radio resources that can be used for transmitting the alert message at at least one future time (tE1); - at least one calibration time (tC1) between the first time (t1) and the at least one future time (tE1): calibrating (120) a clock (12) of the alarm device in order to correct any possible drift of the clock with respect to a clock of the cellular network (20); - when an intrusion is detected by the alarm device: checking (130) whether interference is present; - if interference is present: transmitting (140) the alert message at the at least one future time (tE1) using the preconfigured radio resources.

Description

[0001] Alarm device adapted to emit an alert message in the presence of frequency interference

[0002] Field of invention

[0003] The present invention belongs to the field of alarm devices (devices for detecting intrusion into an area to be protected). More particularly, the invention relates to an alarm device adapted to transmit an alert message to a cellular network in the presence of interference of the cellular network frequencies.

[0004] State of the art

[0005] There are many alarm systems available on the market to protect a building against intrusion. These devices are designed to detect and report unauthorized intrusion into an area to be protected (enclosure, house, apartment, room of a building, etc.). Their purpose is to contribute to the surveillance and protection of buildings, property, and people.

[0006] Alarm systems are generally equipped with infrared or radar sensors to detect an intrusion into the area to be protected. In response to the detection of an intrusion, the device can trigger deterrents (for example, a siren or a smoke generator) and send an alert message to a remote monitoring server.

[0007] The alert message is usually sent over a cellular wireless communication network, such as an LTE network (Long Term Evolution, the fourth generation, or 4G, standard specified by the 3GPP consortium) or a 5G-NR network (5G - New Radio, the fifth generation 3GPP standard).

[0008] To prevent the alarm device from transmitting the alert message, a malicious person can use a frequency jammer. A jammer is a radio transmitter designed to jam, disrupt, or block radiocommunication signals. It generally works by emitting a noise signal at a higher radio power level than the useful signals, on one or more frequency bands. The useful signals are then no longer detected by receivers.

[0009] To overcome this problem, it is known to transmit the alert message via a particular wireless communication network that uses radio transmission techniques that are not very sensitive to frequency jamming. This may in particular be an LPWAN (Low Power Wide Area Network) type network that uses the ultra-narrow band (LJNB) communication technique. Patent EP 3 477 61 1 B1 gives an example of an alarm device comprising a radio communication module configured to send an alert message to an LPWAN network in unidirectional mode when radio jamming is detected.

[0010] However, such an LPWAN network implementing jamming-resistant radio technology is not always available. In such a case, it is necessary to use a cellular network to transmit the alert message. Unlike LPWAN networks with a unidirectional link, cellular networks are bidirectional, this means that a terminal and a base station of the cellular network must be synchronized in time and frequency to communicate. Also, the terminal must receive a message from the base station on a downlink to obtain radio resources to use on an uplink to transmit a message. However, in the case of frequency interference at the terminal, the latter will not be able to receive the resource allocation message for the transmission of the alert message.

[0011] Statement of the invention

[0012] The present invention aims to remedy all or part of the drawbacks of the prior art, in particular those set out above.

[0013] To this end, and according to a first aspect, the present invention proposes a method for transmitting an alert message by an alarm device in the presence of interference in frequency bands of a downlink of the cellular network. The method comprises the following steps:

[0014] - at a first instant, in the absence of jamming: obtaining, from a message transmitted by the cellular network and received by the alarm device, preconfigured radio resources which can be used for transmitting the alert message at at least one future instant,

[0015] - at least one calibration instant between the first instant and said at least one future instant: a calibration of a clock of the alarm device to correct any drift of said clock relative to a clock of the cellular network, - when an intrusion is detected by the alarm device: a check whether jamming is present,

[0016] - if jamming is present: transmission of the alert message at least one future time using the preconfigured radio resources.

[0017] In particular embodiments, the invention may further comprise one or more of the following characteristics, taken individually or in any technically possible combination.

[0018] In particular embodiments, the cellular network is an LTE or 5G-NR network supporting the “PUR” procedure defined by the 3GPP R16 specification; the preconfigured radio resources are defined by a “PUR” configuration received by the alarm device in a connection release message, “RRC Release”, transmitted by a station of the cellular network; the alert message is transmitted by the alarm device on an uplink shared channel, “(N)PUSCH”, independently of a condition relating to the received power level of a reference signal, “(N)RSRP”, or to a time advance, “TA”, for said station.

[0019] In particular embodiments, the cellular network is a 5G-NR network supporting the “CG-SDT Type 1” procedure defined by the 3GPP R17 specification; the preconfigured radio resources are defined by a “CG-SDT Type 1” configuration received by the alarm device in a connection release message, “RRC Release”, transmitted by a station of the cellular network; the alert message is transmitted by the alarm device on an uplink shared channel, “(N)PUSCH”, independently of a condition relating to the received power level of a reference signal, “(N)RSRP”, or to a time advance, “TA”, for said station.

[0020] In particular embodiments, the cellular network is a 5G-NR network supporting the procedure for two-step transmission of a short packet on a random access channel, "2-steps RA-SDT", defined by the 3GPP R17 specification; the preconfigured radio resources are defined by a "PRACH" configuration received by the alarm device in a system information message, "SIB1", transmitted by a station of the cellular network; the alert message is transmitted by the alarm device on a random access channel, "(N)PRACH", independently of a condition relating to the received power level of a reference signal, "(N)RSRP", for said station.

[0021] In particular embodiments, the preconfigured radio resources define several transmission opportunities at several future times, and the alert message is transmitted repeatedly at several of said future times.

[0022] In particular modes of implementation, the calibration step includes:

[0023] - a calculation of a clock time corresponding to a number of clock ticks made by the clock of the alarm device between a time of reception of a first synchronization frame, and a time of reception of a second synchronization frame,

[0024] - a calculation of an offset corresponding to a time difference between the clock time and a reference time corresponding to a theoretical time difference separating the first and second synchronization frames,

[0025] - an update of the alarm device clock based on the offset thus calculated.

[0026] In particular embodiments, the clock of the alarm device is a high-precision, low-frequency clock, the alarm device further comprises an additional low-precision, high-frequency clock, and the calibration step comprises:

[0027] - at each clock tick of the low frequency clock, a reset of a clock tick counter of the high frequency clock,

[0028] - a determination of a number of clock pulses of the low-frequency clock between a time of reception of a first synchronization frame, and a time of reception of a second synchronization frame,

[0029] - a calculation of a clock time as a function of the number of clock ticks of the low-frequency clock, and as a function of the number of clock ticks indicated by the clock tick counter of the high-frequency clock respectively at the instant of reception of the first synchronization frame and at the instant of reception of the second synchronization frame,

[0030] - a calculation of an offset corresponding to a difference between the clock time thus calculated, and a reference time corresponding to a theoretical time difference separating the first and second synchronization frames,

[0031] - an update of the low frequency clock based on the offset thus calculated.

[0032] In particular modes of implementation, said at least one calibration instant is defined by the alarm device as a function of a maximum drift duration, said maximum drift duration being predefined as a function of the duration of a cyclic prefix present in a frame of the cellular network and as a function of a precision level of the clock of the alarm device.

[0033] In particular implementations, the step of checking whether interference is present includes:

[0034] - a division of at least one frequency band of the downlink of the cellular network into several frequency sub-bands,

[0035] - a measurement, for each frequency sub-band, of a received radio power level,

[0036] - an evaluation of a jamming detection criterion based on the measurements obtained.

[0037] According to a second aspect, the present invention relates to an alarm device adapted to transmit an alert message to a cellular network in the presence of interference of frequency bands of a downlink of the cellular network. The alarm device comprises a wireless communication module adapted to communicate with the cellular network, a clock, a jamming detection module, and an intrusion detection module. The alarm device is configured to:

[0038] - at a first instant, in the absence of interference: obtaining preconfigured radio resources, from a message transmitted by the cellular network and received by the communication module, said preconfigured radio resources being able to be used by the communication module to transmit the alert message at at least one future instant,

[0039] - at least one calibration instant between the first instant and said at least one future instant: calibrate the clock to correct any possible drift of said clock relative to a clock of the cellular network,

[0040] - when an intrusion is detected by the intrusion detection module: check if jamming is present using the jamming detection module,

[0041] - if jamming is present: transmit the alert message at least one future time using the preconfigured radio resources.

[0042] In particular embodiments, the invention may further comprise one or more of the following features, taken individually or in any technically possible combination.

[0043] In particular embodiments, the communication module is configured to receive the preconfigured radio resources in a "PUR" configuration included in a connection release message, "RRC Release", transmitted by a station of the cellular network. The communication module is configured to transmit the alert message on an uplink shared channel, "(N)PLJSCH", independently of a condition relating to the received power level of a reference signal, "(N)RSRP", or to a time advance, "TA", for said station.

[0044] In particular embodiments, the communication module is configured to receive the preconfigured radio resources in a "CG-SDT Type 1" configuration included in a connection release message, "RRC Release", transmitted by a station of the cellular network. The communication module is configured to transmit the alert message on an uplink shared channel, "(N)PLJSCH", independently of a condition relating to the received power level of a reference signal, "(N)RSRP", or to a time advance, "TA", for said station.

[0045] In particular embodiments, the communication module is configured to receive the preconfigured radio resources in a "PRACH" configuration included in a system information message, "SIB1", transmitted by a station of the cellular network. The communication module is configured to transmit the alert message on a random access channel, "(N)PRACH", independently of a condition relating to the received power level of a reference signal, "(N)RSRP", for said station.

[0046] In particular embodiments, the alarm device is configured to:

[0047] - calculate a clock time corresponding to a number of clock ticks made by the clock between a time of reception of a first synchronization frame, and a time of reception of a second synchronization frame,

[0048] - calculate an offset corresponding to a difference between the clock time and a reference time corresponding to a theoretical time difference separating the first and second synchronization frames,

[0049] - update the clock according to the offset thus calculated.

[0050] In particular embodiments, the clock of the alarm device is a high-precision, low-frequency clock, the alarm device further comprises an additional low-precision, high-frequency clock, and the alarm device is configured to:

[0051] - at each clock tick of the low-frequency clock, reset a clock tick counter of the high-frequency clock, - determine a number of clock ticks of the low-frequency clock between a time of reception of a first synchronization frame, and a time of reception of a second synchronization frame,

[0052] - calculate a clock time as a function of the number of clock ticks of the low-frequency clock, and as a function of the number of clock ticks indicated by the clock tick counter of the high-frequency clock respectively at the time of reception of the first synchronization frame and at the time of reception of the second synchronization frame,

[0053] - calculate an offset corresponding to a difference between the clock time and a reference time corresponding to a theoretical time difference separating the first and second synchronization frames,

[0054] - update the low frequency clock according to the offset thus calculated.

[0055] Presentation of figures

[0056] The invention will be better understood on reading the following description, given by way of non-limiting example, and made with reference to figures 1 to 11 which represent:

[0057] [Fig. 1] a schematic representation of the main steps of an example of implementation of a method according to the invention for transmitting an alert message in the presence of jamming,

[0058] [Fig. 2] another representation of the main steps of an example of implementation of the method according to the invention,

[0059] [Fig. 3] a timing diagram representing several transmission opportunities and several calibration procedures of the alarm device clock,

[0060] [Fig. 4] a schematic representation of an alarm device transmitting an alert message to a remote monitoring server via a cellular network,

[0061] [Fig. 5] a sequence diagram illustrating a first example of implementation of the steps of obtaining preconfigured radio resources, and of transmitting an alert message using these resources,

[0062] [Fig. 6] a sequence diagram illustrating a second example of implementing the steps of obtaining preconfigured radio resources, and transmitting an alert message using these resources,

[0063] [Fig. 7] a schematic representation of a first embodiment of an alarm device according to the invention, [Fig. 8] a schematic representation of a second embodiment of an alarm device according to the invention,

[0064] [Fig. 9] a schematic representation of the calibration of the clock of the alarm device according to the first embodiment illustrated in Figure 7,

[0065] [Fig. 10] a schematic representation of the calibration of the clock of the alarm device according to the second embodiment illustrated in Figure 8,

[0066] [Fig. 1 1] A sequence diagram illustrating a typical four-step random access procedure.

[0067] In these figures, identical references from one figure to another designate identical or similar elements. For reasons of clarity, the elements represented are not necessarily to the same scale, unless otherwise indicated.

[0068] Detailed description of the invention

[0069] Figures 1 and 2 schematically represent the main steps of an example of implementation of a method 100 according to the invention to enable an alarm device to emit an alert message even in the presence of frequency interference.

[0070] Figure 4 represents the transmission of an alert message by an alarm device 10 to a remote monitoring server 40 via a cellular network 20. The alert message is transmitted on an uplink 23 from the alarm device 10 to the cellular network 20.

[0071] The alarm device 10 is for example designed to detect and signal an unauthorized intrusion into an area to be protected. This may be an outdoor area (enclosure, courtyard, garden, etc.) or an indoor area (house, apartment, room of a building, etc.). The remote monitoring server 40 is for example implemented by a remote monitoring operator to trigger actions in response to the alert message (establishing communication with a manager of the protected area in which the intrusion took place, sending a security agent to the area, etc.). Communication between the alarm device 10 and the remote monitoring server 40 is established via the cellular network 20. This cellular network 20 is a wireless communication network which acts as a radio access network (RAN) to a core network 30 (for example the Internet network) to which the remote monitoring server 40 is connected.The cellular network 20 corresponds for example to an LTE network, a 5G-NR network, or a sixth generation (6G) 3GPP network. It may be a cellular network for connected objects (loT for “Internet of Things” in English, “internet of objects” in French), such as for example an LTE-M (or LTE-CAT-M for “LTE category M”) or NB-loT (acronym for “Narrow-Band loT” in French) type network. LTE-M and NB-loT are variations of the LTE and 5G-NR networks for the IoT. As illustrated schematically in FIG. 4, the cellular network 20 comprises base stations 21 connected to infrastructures 22 serving as gateways to the core network 30.

[0072] The alarm device 10 is configured to implement the method 100 according to the invention. In particular, the alarm device 10 is configured to be able to transmit the alert message to the cellular network 20 even in the presence of jamming of the frequency bands of the downlink 24 of the cellular network 20. In FIG. 4, the frequency jamming of the downlink 24 is represented by a cross. This means that in the presence of jamming, the alarm device 10 is no longer capable of receiving a message from the cellular network 20. The frequency jamming is for example implemented by a malicious person to prevent the alarm device from transmitting the alert message. The jamming consists for example in transmitting a noise signal at a radio power level higher than the useful signals, on one or more frequency bands of the downlink 24 of the cellular network 20.Useful signals can then no longer be detected by receivers located near the jammer.

[0073] It should be noted that the jamming primarily affects the downlink 24, and does not significantly impact the uplink 23. In other words, the jamming prevents the alarm device 10 from receiving a message from the cellular network 20, but it does not prevent the cellular network 20 from receiving a message sent by the alarm device 10. However, in order to send a message to the cellular network 20, the alarm device 10 must be able to synchronize with the cellular network 20 and receive radio resources allocated by the cellular network 20. Jamming the downlink 23 therefore generally also prevents the alarm device 10 from sending a message.The invention described in the present application offers a solution to enable the alarm device 10 to transmit an alert message to the cellular network 20 even in the presence of interference of the frequency bands of the downlink 24 of the cellular network 20.

[0074] Figures 7 and 8 schematically represent exemplary embodiments of an alarm device 10 according to the invention. The alarm device 10 comprises in particular a communication module 11, a clock 12, a jamming detection module 13 and an intrusion detection module 14.

[0075] The communication module 11 implements the functions allowing the alarm device 10 to communicate with the cellular network 20. In particular, the communication module 11 comprises software and / or hardware means configured to exchange (send and receive) messages with the cellular network 20. These means are considered to be known to those skilled in the art.

[0076] The clock 12 is used by the communication module to synchronize time and frequency with a clock of the cellular network 20. Indeed, to be able to receive a message from the cellular network 20, or to be able to transmit a message to the cellular network 20, the communication module 11 must be able to precisely determine the start time of a communication frame. In the LTE or 5G-NR case, a communication frame lasts ten milliseconds; each frame comprises ten sub-frames; each sub-frame comprises two slots; each slot comprises seven OFDMA symbols (an acronym for “Orthogonal Frequency Division Multiple Access”, a data multiplexing and coding technique). Each OFDMA symbol comprises radio samples and a cyclic prefix (CP).The cyclic prefix is ​​used as a guard interval (time margin) to maintain the orthogonality and sinusoidal properties of the signal in the case of multipaths. The first cyclic prefix of a communication frame has a duration of approximately 5.2 ps (microseconds); the cyclic prefixes preceding the other six symbols have a duration of approximately 4.7 ps. The time of transmission of a message by the alarm device 10 must generally have a precision lower than the duration of a cyclic prefix for the message to be correctly received by the cellular network 20. The clock 12 corresponds for example to a temperature-compensated quartz oscillator (TCXO, for “Temperature Controlled X (Crystal) Oscillator”).

[0077] The crystal materials and manufacturing processes of the clock 12 of the alarm device 10 and the clock of the base station 21 to which the alarm device 10 is connected are different and this can cause a shift between the clocks. This frequency shift can also be impacted by environmental factors such as temperature or humidity. As will be seen later, it is therefore necessary to correct any drift of the clock 12 of the alarm device 10 relative to the clock of the base station 21 (however, the estimation of the frequency shift allowing this correction cannot conventionally be done in the presence of interference).

[0078] The jamming detection module 13 comprises software and / or hardware means for implementing the functions allowing the alarm device 10 to check whether or not jamming of the frequency bands of the downlink 24 of the cellular network 20 is present. The jamming detection module 13 can in particular interface with the communication module 11 and / or reuse sub-modules of the communication module 11. Nothing would also prevent, in a variant, the jamming detection module from being integrated within the communication module 11.

[0079] The intrusion detection module 14 comprises software and / or hardware means for detecting whether an intrusion has occurred in the area to be protected. These means are considered to be known to those skilled in the art. Conventionally, the intrusion detection module 14 may in particular interface with infrared sensors, radar sensors, and / or door or window opening detectors. These sensors may be integrated within the alarm device 10, but they may also correspond to physical entities distinct from the alarm device 10.When the sensors are physical entities distinct from the alarm device 10, these sensors can communicate with the intrusion detection module 14 using a wireless communication protocol different from that implemented by the cellular network 20, and the communication between the sensors and the intrusion detection module 14 is then not compromised by the jamming of the frequencies of the cellular network 20.

[0080] The alarm device 10 is configured to implement the steps of the method 100 according to the invention to be able to transmit an alert message even in the presence of jamming. The communication module 11, the clock 12, the jamming detection module 13 and the intrusion detection module 14 represent means configured in software and / or hardware to implement this method 100. The alarm device 10 may in particular comprise one or more processors and storage means (electronic memory) in which a computer program product is stored, in the form of a set of program code instructions to be executed to implement the different steps of the method 100. The communication module 11, the jamming detection module 13 and the intrusion detection module 14 may be implemented by a single processor, or by several separate processors.Alternatively or in addition, the alarm device 10 may comprise one or more programmable logic circuits (FPGA, PLD, etc.), and / or one or more specialized integrated circuits (ASIC), and / or a set of discrete electronic components adapted to implement the steps of the method 100.

[0081] As illustrated by figures 1 and 2, the method 100 comprises the following steps: - at a first instant ti, in the absence of jamming: a step 110 of obtaining, from a message transmitted by the cellular network 20 and received by the alarm device 10, preconfigured radio resources that can be used for transmitting the alert message at at least one future transmission instant tEi,

[0082] - at least one calibration instant tci between the first instant ti and the future instant tEi: a calibration step 120 of the clock 12 of the alarm device 10 to correct any possible drift relative to a clock of the cellular network 20,

[0083] - when an intrusion is detected by the alarm device 10 (at the instant of intrusion detection to): a verification step 130 if jamming is present,

[0084] - if jamming is present: a step 140 of transmitting the alert message at the future time tEi using the preconfigured radio resources.

[0085] The preconfigured radio resources are time and frequency radio resources for the uplink 23: they define one or more time slots and one or more frequencies to be used for the transmission of a message by the alarm device 10 to the cellular network 20.

[0086] As will be seen later, these may for example be radio resources of a physical uplink shared channel: PLJSCH channel (acronym for “Physical Uplink Shared Channel”) in the case of LTE or 5G-NR, or NPUSCH channel (acronym for “Narrow-Band Physical Uplink Shared Channel”) in the specific case of LTE-M or NB-loT. These preconfigured radio resources may in particular be obtained via the “PUR” (acronym for “Preconfigured Uplink Resource”) or “CG-SDT Type 1” (for “Configured Grant Small Data Transmission Type 1”) procedures. These procedures will be described in more detail later with reference to FIG. 5. In such a case, the preconfigured radio resources are resources allocated specifically by the cellular network 20 for the alarm device 10 (these radio resources cannot be used by another device).

[0087] According to another example, these may be radio resources of a packet random access channel (PRACH). These preconfigured radio resources correspond, for example, to a PRACH configuration received by the alarm device 10 in a system information message (SIB1, for "System Information Block #1") transmitted by the cellular network 20. These radio resources may in particular be used by the alarm device 10 in a two-step procedure for transmitting a short packet on a random access channel ("2-steps RA-SDT", for "Two-steps Random Access Small Data Transmission i"). This procedure will be described in more detail later with reference to FIG. 6. In such a case, the preconfigured radio resources are resources shared between several users of the cellular network 20.In other words, these radio resources are not exclusively reserved for the alarm device 10, and there may therefore be collisions between several users (one or more users may use these radio resources at the same time as the alarm device 10).

[0088] As illustrated in Figure 3, the preconfigured radio resources can define several transmission opportunities in the future (tEi, tE2, IES). These can be several transmission times configured by the PUR procedure or by the CG-SDT procedure, or they can be several PRACH opportunities.

[0089] The time distance between the first instant ti and a future instant of transmission (for example tEi or tE3) may be greater than a few tens of seconds, for example at least equal to thirty seconds, or even at least equal to one minute. In particular, for the PUR protocol, the time distance between the first instant ti and the first future instant of transmission corresponds to at least eight hyper-frames (one hyper-frame corresponds to 1024 frames; one hyper-frame therefore has a duration of 10.24 seconds). This therefore corresponds to a time distance at least equal to 81.92 seconds.

[0090] It is advantageous to obtain preconfigured radio resources offering close transmission opportunities in time, so that the alert message can be sent as quickly as possible when an intrusion is detected.

[0091] As illustrated in Figures 2 and 3, several calibration times (tci, tea, tes) can be defined, for example with a periodic repetition. As will be seen later, the calibration times can in particular be defined by the alarm device as a function of the precision of the clock 12, as a function of the duration of a cyclic prefix of a frame, and / or as a function of future transmission times (tEi, tE2, IES). The calibration times are defined by the alarm device 10, and they are not specified by the standard implemented by the cellular network 20.

[0092] It should be noted that, as illustrated in FIG. 2, steps 110 (obtaining preconfigured radio resources) and 120 (calibrating clock 12 of alarm device 10) may be iterated several times before an intrusion is detected.

[0093] In particular, and as illustrated in FIG. 2, the step 115 of determining a next calibration time can be repeated each time a calibration 120 of the clock 12 of the alarm device 10 is performed, as long as no intrusion is detected. A new calibration 120 of the clock 12 is performed each time a calibration time is reached.

[0094] Also, and as illustrated in FIG. 2, it is possible for the preconfigured radio resources to be valid only for a predetermined duration. When the end of validity of the preconfigured radio resources is reached, the step 110 of obtaining preconfigured radio resources can then be renewed to obtain new preconfigured radio resources. This renewal of the preconfigured radio resources can take place several times (potentially a large number of times) before an intrusion is detected. For example, the preconfigured radio resources can become invalid if a predefined number of successive transmission occasions are not used.In particular, it is advantageous to periodically repeat the renewal of the preconfigured radio resources so as to permanently retain at least one opportunity for transmitting an alert message in the future (i.e. so as to permanently have at least one future transmission instant configured).

[0095] As illustrated in FIG. 2, in response to detecting an intrusion, the alarm device 10 performs a check 130 whether frequency interference from the downlink 24 is present.

[0096] In the absence of interference (i.e., in the case where the verification step 130 indicates that there is no interference), the alert message can be transmitted in a conventional manner, for example with a four-step packet random access channel (4-steps PRACH) procedure. This procedure makes it possible in particular to ensure precise synchronization between the alarm device 10 and the cellular network 20 just before the alert message is transmitted. This also makes it possible to transmit the alert message as quickly as possible (no need to reach the future transmission time defined by the preconfigured resources). The conventional four-step random access procedure is described below with reference to FIG. 11.

[0097] On the other hand, if jamming is present (i.e. in the case where the verification step 130 indicates that there is jamming), the alarm device 10 proceeds to a transmission 140 of the alert message at the future instant (IEI in the example of FIG. 1, tE3 in the example of FIG. 3) using the preconfigured radio resources. In this case, it is important to note that this transmission 140 of the alert message is carried out without requiring the reception of a message on the downlink 24 between the instant of detection of the jamming and the instant of transmission of the alert message (the reception of such a message is not possible due to the jamming).The calibration step(s) 120 which have been carried out between the time of obtaining the preconfigured radio resources and the time of transmission of the alert message advantageously make it possible to compensate for the absence of precise synchronization between the alarm device 10 and the cellular network 20 just before transmission of the alert message.

[0098] Figure 5 illustrates the steps of obtaining 110 the preconfigured radio resources and transmitting 140 the alert message when the method 100 is based on the “PUR” procedure or on the CG-SDT Type 1 procedure. The PUR procedure is defined by the 3GPP R16 (Release 16) specification. The CG-SDT Type 1 procedure is defined by the 3GPP R17 (Release 17) specification.

[0099] As illustrated in FIG. 5, when the alarm device 10 is in connected mode (“RRC_CONNECTED” mode), a PUR configuration or a CG-SDT configuration can be obtained in an “RRC Release” message (connection release message) transmitted by the base station 21 of the cellular network 20 to which the alarm device 10 is connected (in response to a request previously transmitted by the alarm device 10). The alarm device 10 then goes into standby mode (“RRCJDLE” mode) in the PUR case, or into inactive mode (“RRCJNACTIVE” mode) in the CG-SDT case. The alarm device 10 then has the possibility of transmitting an alert message at each transmission opportunity defined by the PUR or CG-SDT configuration. The alert message is included in a connection resume message ("RRC Resume" message) transmitted on the uplink shared channel ("PUSCH" channel or "NPUSCH" channel).

[0100] In the PUR case, the request message to request a PUR configuration is described in 3GPP specification 36.331 v16.0.0 in section 5.6.23.2. The alarm device 10 has the possibility to define the desired number and repetition period of PUR transmission opportunities. The PUR configuration is defined in section 6.3.2 (or in section 6.7.3.2 for the loT case).

[0101] The CG-SDT procedure is described for example in section 8.20.2 of the 3GPP 38.401 v17.0.0 specification, or in section 5.27 of the 3GPP 38.321 v17.0.0 specification. The CG-SDT configuration is defined in section 6.3.2 of the 3GPP 38.331 v17.0.0 specification. It defines in particular the number and repetition period of CG-SDT transmission opportunities.

[0102] The 3GPP standard describes conditions to be met regarding the validation of a timing advance (TA) between the terminal and the base station 21. This validation is notably based on a condition relating to a received power level of a reference signal (RSRP for Reference Signal Receive Power, or NRSRP for Narrow-Band Reference Signal Receive Power) for the base station 21. For the PUR procedure, these conditions are notably described in section 5.3.3.1 of the 3GPP 36.331 v16.0.0 specification. For the CG-SDT procedure, they are described in section 5.27.2 of the 3GPP 38.321 v17.0.0 specification.

[0103] Contrary to what is recommended by the standard, in the method 100 according to the invention, the alert message is emitted by the alarm device 10 independently of a condition relating to the received power level of a reference signal (RSRP, or NRSRP), or to a time advance (TA), for the base station 21. Indeed, in the presence of jamming, the reference signal will be masked by the jamming, and the received power level of the reference signal will not be relevant and will not allow the time advance to be validated. It is therefore necessary, in the event of jamming, to disregard the received power level of the reference signal, and to consider that the time advance is still valid. This is a deviation from the specification.

[0104] It should be noted that the CG-SDT Type 2 procedure conditions the transmission of a message on the prior reception of a control message on the downlink 24 (DCI message, acronym for "Downlink Control Indication"). However, in the event of jamming, this DCI message cannot be received on the downlink 24. The CG-SDT Type 2 procedure cannot therefore be used in the event of jamming.

[0105] Figure 6 illustrates the steps of obtaining 110 the preconfigured radio resources and transmitting 140 the alert message when the method 100 is based on the two-step transmission procedure of a short packet on a random access channel “2-steps RA-SDT”. This procedure is defined by the 3GPP R17 specification (Release 17).

[0106] In the example illustrated in Figure 6, when the alarm device 10 is in standby mode (“RRCJDLE” mode), a PRACH configuration can be obtained in a SIB1 message transmitted by the base station 21 of the cellular network 20 to which the alarm device 10 is attached (the SIB1 message is transmitted periodically by the base station 21). The alert message can then be transmitted by the alarm device 10 on a random access channel (PRACH channel, or NPRACH channel in the case of loT) using the radio resources defined in the PRACH configuration.

[0107] The PRACH configuration is defined in 3GPP specification 36.331 v16.0.0 in section 6.3.2. It defines, among other things, the number of PRACH opportunities available, when to transmit, and which frequencies to use.

[0108] The classic four-step PRACH procedure ("4-steps PRACH") is described in Figure 11. In the four-step PRACH procedure, a terminal (UE for "User Equipment") performs a contention resolution to transmit data. To do this, in a first step, the terminal transmits a random access request message ("Msg1", "PRACH Request"). In a second step, the access network responds to this request by transmitting a response message ("Msg2", "PRACH Response") allocating radio resources to the terminal that can be used by its data transmission. The terminal can then transmit its data, during a third step, in a data transmission message ("Msg3", "Data Transmission") on the PLJSCH channel. In a fourth and final step, the network transmits a message confirming the contention resolution ("Msg 4", "Contention Resolution").

[0109] In case of downlink frequency interference, the terminal cannot receive messages from the cellular network. The classic four-step PRACH procedure is therefore not possible.

[0110] In the two-step procedure “2-steps RA-SDT”, and as illustrated in Figure 6, the terminal transmits its data directly in the first message, “Msg A”, at the same time as the PRACH Request. Thus, after receiving the PRACH configuration in a SIB1 message, the terminal no longer needs to receive another message from the cellular network on the downlink to be able to transmit its data. The message “Msg A” corresponds to the combination of the messages “Msg 1” and “Msg 3” of the classic four-step procedure. The cellular network then transmits a response message “Msg B” which corresponds to the combination of the messages “Msg 2” and “Msg 4” of the classic four-step procedure (this message “Msg B” is not shown in Figure 6 because it is not received by the alarm device 10 in the event of jamming).

[0111] The 3GPP standard describes conditions to be met relating to the received power level of the reference signal (RSRP, or NRSRP). In particular, the 3GPP standard recommends that the two-step random access procedure can only be used if the received power level of the reference signal is greater than a predefined threshold. Contrary to what is recommended by the standard, in the method 100 according to the invention, the alert message is emitted by the alarm device 10 independently of a condition relating to the received power level of the reference signal for the base station 21. Indeed, in the presence of interference, the reference signal will be masked by the interference, and the received power level of the reference signal is not relevant.

[0112] It is possible that the “Msg A” message containing the alert message is not correctly received by the cellular network 20 because, as explained previously, the PRACH radio resources are not exclusively reserved for the alarm device 10 (there may be collisions with one or more other users who may be using these radio resources at the same time as the alarm device 10). To optimize the chances that the alert message is correctly received by the cellular network 20 (and then routed to the remote monitoring server 40), it is advantageous to repeat the alert message several times, at several PRACH transmission opportunities (several future transmission times). For example, the “Msg A” message containing the alert message is repeated four times, at four different PRACH opportunities.

[0113] It should be noted that the 3GPP standard introduced the PUR, CG-SDT, and RA-SDT procedures to limit the energy consumption of terminals and to optimize the use of radio resources. The 3GPP standard does not recommend the use of these procedures to overcome the presence of downlink frequency interference. The use of these procedures to allow a terminal to transmit even in the presence of frequency interference is therefore particularly unexpected.

[0114] The step 130 of verifying whether interference is present may in particular comprise a division of at least one frequency band of the downlink 24 of the cellular network 20 into several frequency sub-bands, and a measurement, for each frequency sub-band, of a received radio power level (RSSI for “Received Signal Strength Indication”). A jamming detection criterion may then be evaluated based on the measurements obtained. For example, the jamming detection criterion is satisfied if the RSSI of at least one sub-band is greater than a threshold, or if the average of the RSSIs of the different sub-bands is greater than a threshold.

[0115] The width of a frequency band of the downlink 24 of the cellular network 20 may in fact correspond to several tens of megahertz, for example 30 MHz, and it is advantageous to divide it into narrower sub-bands, for example 1 MHz sub-bands, or even 180 kHz sub-bands, in order to obtain individual RSSIs representative of the different sub-bands. If only a few sub-bands have an abnormally high power level, it is possible that this is simply due to network overload. On the other hand, if a very large proportion of the sub-bands have an abnormally high power level, it is likely that this is due to interference.

[0116] Figure 7 shows a first example embodiment of an alarm device 10 according to the invention, in which the clock 12 is a 32 MHz TCXO having an accuracy of 0.1 parts per million (ppm). This means that, in unfavorable situations, the clock drift can cause an additional time error of 0.1 ps every second. It was seen previously that, in the PUR case, the time distance between the time ti of obtaining the preconfigured radio resources and the first future time of transmission tEi is at least 81.92 seconds, which corresponds in the worst case to a time error of approximately 8.2 ps due to the clock drift. This error being greater than the duration of the cyclic prefix of an OFDMA slot, it could make it impossible for the cellular network 20 to correctly decode a message transmitted by the alarm device 10.This is the reason why one or more calibration steps 120 must be carried out between the instant ti of obtaining the preconfigured radio resources and the instant t. Ri of transmitting the alert message, in order to maintain precise synchronization between the alarm device 10 and the cellular network 20 (or in other words to correct the drift of the clock 12).

[0117] In particular, it is possible to define a maximum allowed drift duration depending on the duration of a cyclic prefix and on the accuracy level of the clock 12. This maximum duration corresponds, for example, to the duration after which the time error due to the drift will become greater than the duration of a cyclic prefix if we consider the worst case. In the example considered, the maximum drift duration would be approximately 47 seconds if we want to ensure a time error of less than 4.7 ps. This means that a new 120 calibration of the clock 12 must be carried out at least every 47 seconds. For example, we can consider carrying out a new 120 calibration every 40 seconds.

[0118] As illustrated in Figures 1 and 9, the calibration step 120 may include:

[0119] - a calculation 121 of a clock time T H corresponding to a number N Tof clock ticks made by the clock 12 of the alarm device 10 between an instant t0 of reception of a first synchronization frame 31, and an instant t R 2 reception of a second synchronization frame 32,

[0120] - a calculation 122 of an offset A T corresponding to a time difference between the clock time T H and a reference time T Re f corresponding to a theoretical time difference separating the first and second synchronization frames,

[0121] - an update 123 of the clock 12 of the alarm device 10 according to the offset thus calculated.

[0122] The synchronization frames can notably correspond to primary synchronization signal frames (PSS for “Primary Synchronization Signal”, or NPSS for “Narrow-Band Primary Synchronization Signal” in the case of IoT).

[0123] For example, and as illustrated in FIG. 1, the first (N)PSS frame 31 is received prior to the calibration time tci (for example at a time close to the time ti of obtaining the preconfigured radio resources, or at a previous calibration time). The second (N)PSS frame 32 is received at a time close to the calibration time tci (for example, it is the first (N)PSS frame received after tci)-

[0124] The reference time T Re f corresponds to the number of system frames (SFN for "System Frame Number") separating the two synchronization frames multiplied by the duration of a frame. The clock time T H is equal to the number N T of clock ticks multiplied by the duration of a clock tick. The duration of a clock tick is equal to the inverse of the frequency F T of the TCXO. The offset A T corresponds to a difference between T Re t and T H .

[0125] At step 123, to compensate for the drift, the pitch of the TCXO phase-locked loop (PLL) can then be modified according to the offset A T calculated.

[0126] A high-precision, high-frequency clock (such as that of the first embodiment described with reference to Figures 7 and 9) is, however, expensive and energy-intensive. This is why a second embodiment is proposed and described with reference to Figures 8 and 10.

[0127] In this second embodiment, and as illustrated in FIG. 8, the clock 12 is a 32 kHz TCXO having an accuracy of 0.1 ppm (in other words, it is a high-precision, low-frequency clock). The alarm device 10 further comprises a 32 MHz clock 15 with an accuracy of 30 ppm (in other words, it is an additional low-precision, high-frequency clock).

[0128] It should be noted that the frequency and precision characteristics given here for the clocks 12 and 15 of the alarm device 10 are in no way limiting. Other values ​​could be envisaged. In particular, the terms “high precision” and “low precision” are to be considered relatively to each other. This means in particular that the precision of the high-precision clock is greater than that of the low-precision clock (for example, the precision of the high-precision clock is at least 10 times, or even at least 50 times, or even at least 100 times, greater than that of the low-precision clock). The same applies to the terms “high frequency” and “low frequency” (for example, the frequency of the high-frequency clock is at least 100 times, or even at least 500 times, or even at least 1000 times greater than that of the low-frequency clock).

[0129] As illustrated in Figure 10, the calibration step 120 can then comprise: - at each clock tick of the low-frequency clock 12, a resetting 124 of a clock tick counter of the high-frequency clock 15, - a determination 125 of a number N T of clock ticks of the low-frequency clock 12 between an instant tpi of reception of a first synchronization frame 31, and an instant t R2 receiving a second synchronization frame 32,

[0130] - a calculation 125 of a clock time T H depending on the number N T of clock ticks of the 12 low frequency clock, and depending on the numbers NQI and N Q2 of clock ticks indicated by the clock tick counter of the high-frequency clock 15 respectively at the instant tpi of reception of the first synchronization frame 31 and at the instant t R2 reception of the second synchronization frame 32,

[0131] - a calculation 126 of an offset A T corresponding to a difference between the clock time T H , and the reference time T Re f corresponding to the theoretical time difference separating the first and second synchronization frames,

[0132] - an update 127 of the low frequency clock 12 based on the offset thus calculated.

[0133] Clock time T H corresponds to the number of whole cycles of the low-frequency clock 12 carried out between tm and t R2 to which should be added the incomplete clock cycle completed after t Ri and the incomplete clock cycle started before t R2 .

[0134] The duration of the incomplete clock cycle completed after t Ri is calculated based on the number NQI of clock ticks indicated by the clock tick counter of the high-frequency clock 15 at time t Ri. It can be calculated in the form ), expression in which F Q is the frequency of the 15 high clock frequency.

[0135] The duration of the incomplete clock cycle started before t R2 is calculated based on the number N Q2 of clock ticks indicated by the clock tick counter A? of the high-frequency clock 15 at time t R2 . It is equal to — .

[0136] FAQ

[0137] Advantageously, even if the accuracy of the high-frequency clock 15 is low (30 ppm in the example considered), the time error due to the cumulative drift over a clock cycle of the low-frequency clock 12 remains low. The second embodiment described with reference to FIGS. 8 and 10 thus makes it possible to maintain precise synchronization between the alarm device 10 and the cellular network 20 without having to use a high-precision high-frequency clock. This second embodiment is particularly advantageous for alarm devices operating with batteries or a battery. It should be noted that it is also possible to calibrate the high-frequency clock 15 at each clock stroke of the low-frequency clock 12, or each time a predetermined number of clock strokes of the low-frequency clock 12 is reached, in order to limit the drift of the high-frequency clock 15.

Claims

Claims 1. Method (100) for transmitting an alert message by an alarm device (10), to a cellular network (20), in the presence of interference of frequency bands of a downlink of the cellular network (20), said method (100) comprising the following steps: - at a first instant (ti), in the absence of jamming: obtaining (1 10), from a message transmitted by the cellular network (20) and received by the alarm device (10), preconfigured radio resources which can be used for transmitting the alert message at at least one future instant (tEi), - at least one calibration instant (tci) between the first instant (ti) and said at least one future instant (tEi): a calibration (120) of a clock (12) of the alarm device (10) to correct any possible drift of said clock (12) relative to a clock of the cellular network (20), - when an intrusion is detected by the alarm device (10): a check (130) whether jamming is present, - if jamming is present: a transmission (140) of the alert message at least one future instant (tEi) using the preconfigured radio resources.

2. Method (100) according to claim 1 in which: - the cellular network (20) is an LTE or 5G-NR network supporting the “PUR” procedure defined by the 3GPP R16 specification, - the preconfigured radio resources are defined by a “PUR” configuration received by the alarm device (10) in a connection release message, “RRC Release”, transmitted by a station (21) of the cellular network (20), - the alert message is transmitted by the alarm device (10) on an uplink shared channel, "(N)PUSCH", independently of a condition relating to the received power level of a reference signal, "(N)RSRP", or to a time advance, "TA", for said station (21).

3. Method (100) according to claim 1 in which: - the cellular network (20) is a 5G-NR network supporting the “CG-SDT Type 1” procedure defined by the 3GPP R17 specification, - the preconfigured radio resources are defined by a “CG-SDT Type 1” configuration received by the alarm device (10) in a message from connection release, “RRC Release”, issued by a station (21) of the cellular network (20), - the alert message is transmitted by the alarm device (10) on an uplink shared channel, "(N)PLJSCH", independently of a condition relating to the received power level of a reference signal, "(N)RSRP", or to a time advance, "TA", for said station (21).

4. Method (100) according to claim 1 in which: - the cellular network (20) is a 5G-NR network supporting the two-step transmission procedure of a short packet on a random access channel, “2-steps RA-SDT”, defined by the 3GPP R17 specification, - the preconfigured radio resources are defined by a “PRACH” configuration received by the alarm device (10) in a system information message, “SIB1”, transmitted by a station (21) of the cellular network (20), - the alert message is transmitted by the alarm device (10) on a random access channel, “(N)PRACH”, independently of a condition relating to the received power level of a reference signal, “(N)RSRP”, for said station (21).

5. Method (100) according to claim 4 wherein the preconfigured radio resources define several transmission opportunities at several future times, and the alert message is transmitted repeatedly at several of said future times.

6. Method (100) according to any one of claims 1 to 5 in which the calibration step (120) comprises: - a calculation (121) of a clock time corresponding to a number of clock ticks made by the clock (12) of the alarm device (10) between an instant (tRi) of reception of a first synchronization frame, and an instant (t R 2) reception of a second synchronization frame, - a calculation (122) of an offset corresponding to a time difference between the clock time and a reference time (T Re f) corresponding to a theoretical time difference separating the first and second synchronization frames, an update (123) of the clock (12) of the alarm device (10) according to the offset thus calculated.

7. Method (100) according to any one of claims 1 to 5 wherein the clock (12) of the alarm device (10) is a high precision and low frequency clock, the alarm device further comprises an additional low precision and high frequency clock (15), and the calibration step (120) comprises: - at each clock stroke of the low-frequency clock (12), a reset (124) of a clock stroke counter of the high-frequency clock (15), - a determination (125) of a number of clock ticks of the low-frequency clock (12) between an instant (t«i) of reception of a first synchronization frame, and an instant (t R 2) reception of a second synchronization frame, - a calculation (125) of a clock time as a function of the number of clock ticks of the low-frequency clock (12), and as a function of the number of clock ticks indicated by the clock tick counter of the high-frequency clock (15) respectively at the instant (t«i) of reception of the first synchronization frame and at the instant (t R 2) reception of the second synchronization frame, - a calculation (126) of an offset corresponding to a difference between the clock time thus calculated, and a reference time (T Re f) corresponding to a theoretical time difference separating the first and second synchronization frames, - an update (127) of the low frequency clock (12) according to the offset thus calculated.

8. Method (100) according to any one of claims 1 to 7 wherein said at least one calibration instant (tci) is defined by the alarm device (10) as a function of a maximum drift duration, said maximum drift duration being predefined as a function of the duration of a cyclic prefix present in a frame of the cellular network (20) and as a function of a precision level of the clock (12) of the alarm device (10).

9. Method (100) according to one of claims 1 to 8 in which the step of verifying (130) whether interference is present comprises: - a division of at least one frequency band of the downlink of the cellular network (20) into several frequency sub-bands, - a measurement, for each frequency sub-band, of a received radio power level, - an evaluation of a jamming detection criterion based on the measurements obtained.

10. Alarm device (10), adapted to transmit an alert message to a cellular network (20) in the presence of interference of frequency bands of a downlink of the cellular network (20), said alarm device (10) comprising: - a wireless communication module (11) adapted to communicate with the cellular network, - a clock (12), - a jamming detection module (13), and - an intrusion detection module (14), the alarm device (10) is configured to: - at a first instant (ti), in the absence of interference: obtaining preconfigured radio resources, from a message transmitted by the cellular network (20) and received by the communication module (11), said preconfigured radio resources being able to be used by the communication module (11) to transmit the alert message at at least one future instant (tEi), - at least one calibration instant (tci) between the first instant (ti) and said at least one future instant (tEi): calibrate the clock (12) to correct any possible drift of said clock (12) relative to a clock of the cellular network (20), - when an intrusion is detected by the intrusion detection module (14): check whether jamming is present using the jamming detection module (13), - if jamming is present: transmit the alert message at least one future instant (tEi) using the preconfigured radio resources.

11. Alarm device (10) according to claim 10 in which: - the communication module (11) is configured to receive the radio resources preconfigured in a “PUR” configuration included in a connection release message, “RRC Release”, transmitted by a station (21) of the cellular network (20), - the communication module (1 1 ) is configured to transmit the alert message on an uplink shared channel, “(N)PUSCH”, independently of a condition relating to the received power level of a reference signal, “(N)RSRP”, or to a time advance, “TA”, for said station (21 ).

12. Alarm device (10) according to claim 10 wherein: - the communication module (11) is configured to receive the radio resources preconfigured in a “CG-SDT Type 1” configuration included in a connection release message, “RRC Release”, transmitted by a station (21) of the cellular network (20), - the communication module (1 1 ) is configured to transmit the alert message on an uplink shared channel, “(N)PUSCH”, independently of a condition relating to the received power level of a reference signal, “(N)RSRP”, or to a time advance, “TA”, for said station (21 ).

13. Alarm device (10) according to claim 10 in which: - the communication module (11) is configured to receive the radio resources preconfigured in a “PRACH” configuration included in a system information message, “SIB1”, transmitted by a station (21) of the cellular network (20), - the communication module (11) is configured to transmit the alert message on a random access channel, “(N)PRACH”, independently of a condition relating to the received power level of a reference signal, “(N)RSRP”, for said station (21).

14. An alarm device (10) according to any one of claims 10 to 13 wherein the alarm device (10) is configured to: - calculate a clock time corresponding to a number of clock strokes made by the clock (12) between an instant (toi) of reception of a first synchronization frame, and a time (tR2) of reception of a second synchronization frame, - calculate an offset corresponding to a difference between the clock time and a reference time (tRef) corresponding to a theoretical time difference separating the first and second synchronization frames, - update the clock (12) according to the offset thus calculated.

15. Alarm device (10) according to any one of claims 10 to 13 wherein the clock (12) of the alarm device (10) is a high precision and low frequency clock, the alarm device (10) further comprises an additional low precision and high frequency clock (15), and the alarm device (10) is configured to: - at each clock stroke of the low frequency clock (12), reset a clock stroke counter of the high frequency clock (15), - determining a number of clock pulses of the low-frequency clock (12) between an instant (t«i) of reception of a first synchronization frame, and an instant (tR2) of reception of a second synchronization frame, - calculate a clock time as a function of the number of clock ticks of the low-frequency clock (12), and as a function of the number of clock ticks indicated by the clock tick counter of the high-frequency clock (15) respectively at the instant (t«i) of reception of the first synchronization frame and at the instant (tR2) of reception of the second synchronization frame, - calculate an offset corresponding to a difference between the clock time and a reference time (tRef) corresponding to a theoretical time difference separating the first and second synchronization frames, - update the low frequency clock (12) according to the offset thus calculated.

Images