Method and apparatus for radio communication of encrypted PPM signals

Abstract

Method and device for communication of encrypted PPM signals of a radio communication system, in particular a short-range radio, the method and device comprising the step of PPM encoding an item of information to be transmitted, wherein each symbol in the symbol alphabet for forming the information item to be transmitted is assigned a respective predefined temporal position of one pulse in the original PPM pulse sequence relative to the previous pulse. In particular in the physical layer, there is encryption or a step of encryption, in which at least one of the pulses of the RF-transmitted sequence is time shifted with respect to its predefined temporal position in the original sequence according to a corresponding offset ([delta] a, [delta] b, [delta] c, [delta] 3, [delta] 7, [delta] 16, [delta] '7) that is known only to the authorized receiver. The offset ([delta] a, [delta] b, [delta] c, [delta] 3, [delta] 7, [delta] 16, [delta] '7) is such that the time-shifted pulse is not located in any predefined time position (T0 + n [delta] T) provided in PPM coding relative to its preceding pulse.

Description

[0001] The present invention relates to a method and a device for radio communication of encrypted PPM signals, in particular for telemetry applications. In particular, the present invention relates to a method for transmitting, a method for receiving, a transmitter and a receiver of these signals.

[0002] Using the PPM signal encrypted according to the invention, values ​​of measurements (e.g. performed locally) of physical quantities collected using adapted sensors on a structure (e.g. a rotating, oscillating or vibrating structure), such as values ​​of pressure, temperature, humidity, deformation, vibrations etc. can be transmitted.

[0003] A specific but not exclusive use of the invention is the telemetry of vehicles, for example the device is used on the tire to transmit by radio (Tire Pressure Monitoring System - TPMS) its internal pressure or other physical quantity or parameter measured on the tire or on the wheel carrying the tire. However, the invention is not limited to the field of vehicles or tires, but is suitable for use in any context in which it is necessary to transmit the measurement of a specific physical quantity or parameter performed locally by low-energy radio and / or short-range radio.

[0004] In the prior art, devices for transmitting the results of measurements of physical values ​​such as temperature, pressure, humidity, deformation, etc. are known, which devices are able to convert these measurements performed locally into a data string which is then transmitted by radio.

[0005] In order to significantly reduce the energy consumption of these transmitting devices, a transmitter device is known, for example from US 10,284,120, which comprises a microcontroller configured to receive a signal related to at least one locally measured value and to control a pulse generator in order to generate a corresponding signal modulating the position of a pulse (pulse position modulation, PPM), which pulse is then transmitted by radio. The pulse generator according to this known solution comprises an oscillator and a power amplifier for amplifying the pulses output from the oscillator and for emitting a radio frequency PPM signal. The microcontroller is configured to activate only the oscillator during a first time period for each pulse of the PPM signal to be generated, then also activate only the amplifier during a second time period after the start of the first time period, and to deactivate both the oscillator and the amplifier at the end of the second time period.

[0006] The PPM encoding of the information to be transmitted comprising the value of a local measurement requires that a corresponding predefined time position of a PPM pulse in the transmitted PPM pulse sequence relative to the previous pulse be assigned to each M-bit (e.g. 4-bit) codeword in the alphabet of 2^M codewords (e.g. 16 codewords, where M=4) used to constitute the information item to be transmitted.

[0007] This coding technique is not without its disadvantages, which include the risk of eavesdropping on the radio signal, and the consequent possibility of the message being decoded, which is easy once the correspondence between relative distances and binary symbols is determined from analysis of the intercepted pulse sequence.

[0008] Furthermore, it is possible that only one of several receivers suitable for decoding a PPM signal may be the intended recipient of a message broadcast using that signal, and thus the signal cannot be decoded by other receivers. Also, a receiver may no longer be authorized to decode some or all PPM signals of a given transmitter after a period of time.

[0009] US 2017 / 0150347 A1 discloses applying a key-modified time slot index to a PPM normal baseband signal so as to incrementally change the time slot index to which each pulse of the normal baseband signal is assigned.

[0010] It is an object of the present invention to provide a method and a device for radio communication of PPM signals which are able to improve the known art in one or more of the above-mentioned aspects.

[0011] Within this aim, an object of the present invention is to provide a method and a device for radio communication, in particular short-range radio communication, of PPM signals, which method and device are suitable for increasing the security of radio communication while preventing or significantly reducing the risk of unauthorized decoding of the transmitted PPM signals.

[0012] Another object of the invention is to prevent or reduce the risk of unauthorized decoding of the transmitted PPM signals while keeping the energy consumption low.

[0013] Another object of the present invention is to achieve secure radio communication of PPM signals that does not require high processing speeds or high clock speeds.

[0014] Another object of the invention is to enable directly adding or removing authorization for PPM decoding to or from a receiver of a PPM signal.

[0015] Furthermore, the present invention aims to overcome the disadvantages of the background art in a way that replaces any existing solutions.

[0016] Another object of the present invention is to provide a communication method and device with high reliability, easy implementation and low cost.

[0017] This object, as well as these and other objects which will become more apparent hereinafter, is achieved by a transmitting method according to claim 1, by a receiving method according to claim 8, by means of a transmitter according to claim 13 and by means of a receiver according to claim 19, optionally provided with one or more features of the dependent claims.

[0018] Further features and advantages of the present invention will become more apparent from the description of a preferred but not exclusive embodiment of the invention illustrated by way of non-limiting example in the accompanying drawings, in which: - Figure 1 is a block diagram of a transmitter according to the present invention; - Figure 2 is a block diagram of a receiver according to the present invention; - Figure 3A Shows Figure 1 A step-shaped control signal for the transmitter's oscillator and power amplifier; - Figure 3B Shown in Figure 1 The PPM pulse obtained at the outlet of the power amplifier; - Figure 4 A PPM pulse sequence is shown, which may be encoded in 4-bit symbols; - Figure 5A shows the original sequence of PPM pulses processed in PPM encoding; - Figure 5B Shows from Figure 5A The encrypted sequence that starts and is transmitted; - Fig. 6A Another original sequence of PPM pulses processed in PPM encoding is shown, which incorporates the receiver's public key in the preamble; - Figure 6B Shows from Fig. 6A The encrypted sequence that starts and is transmitted; - Figure 7 is an example of a set of shift information items used for encryption; - Figure 8 shows an encryption sequence according to an embodiment of the present invention, wherein the PPM pulses are time-shifted by a certain offset; - Fig. 9 An encryption sequence according to another embodiment of the present invention is shown, wherein the PPM pulses are time-shifted by a certain offset.

[0019] With reference to the accompanying drawings, a device for radio transmission of encrypted PPM signals, in particular for telemetry applications, according to an embodiment of the present invention is generally indicated by reference numeral 1 and comprises a microcontroller 15 and a pulse generating device 18 connected to the microcontroller 15 so as to be controlled by the microcontroller 15. Advantageously, the transmitting device 1 is a short-range device powered by an energy source 12, such as a battery or an energy harvesting source.

[0020] The microcontroller 15 is preferably adapted to receive at input one or more detection signals representing one or more measured values ​​of one or more physical quantities. Furthermore, the microcontroller 15 is adapted to control the pulse generating means 18 so that they generate PPM (Pulse Position Modulation) signals comprising in coded form information corresponding to the measured values ​​received by the microcontroller 15 via the detection signals. However, the transmission and reception method according to the invention can generally be applied to any device for radio communication of PPM pulses, for any application, and not necessarily for telemetry.

[0021] The pulse generating device 18 is connected at the output end to an antenna 19 via an optional impedance adaptation stage for radio transmission of the PPM signal generated by the pulse generating device 18 so that the PPM signal is received by the remote receiver 2 via radio.

[0022] The detection signal originates from a detection device 11, which is connected or can be connected to a microcontroller 15, for example, and is suitable for detecting one or more measurement results of a specific physical quantity (for example, one or more of pressure, temperature, acceleration, vibration, voltage, etc.), and generating a detection signal corresponding to the (converted) value of these measurement results. The detection device 11 can basically consist of at least one transducer ("DM" in the figure) for each physical quantity to be measured. The transducer DM can be connected to an appropriate input terminal of the microcontroller 15 and can be a sensor selected from the group including, for example, the following items: pressure sensor, temperature sensor, vibration sensor, accelerometer, magnetometer, strain gauge, inductive sensor, voltage or current detector, etc.

[0023] The detection means 11 and optionally the RFID tag 17 of the transmitting device 1 may optionally be powered by the microcontroller 15 itself.

[0024] The pulse generating means 18 comprises an oscillator 13, the output of which is connected to the input of a power amplifier ("PA") 14, so that the PA will amplify the radio frequency (or "RF") signal emitted by the oscillator 13 and generate PPM pulses at the output to be radio-transmitted. The gain of the PA 14 may be between 10 and 20. These pulse generating means 18 and their control methods may be known per se, for example from US Pat. No. 10,284,120, optionally as described in US Pat. No. 10,284,120. Figure 3A and Figure 3B Improvements have been made as shown.

[0025] The oscillator 13 of the pulse generating device 18 preferably comprises a Colpitts oscillator in a "common base" configuration and coupled to a SAW (surface acoustic wave) resonator, which allows the oscillator 13 to achieve a stable radio frequency value (e.g. 434.35 MHz). The oscillator 13 is an oscillator of a type known per se, for example from US Pat. No. 8,134,416 B2, incorporated herein by reference. As an alternative, the oscillator 13 may be a Hartley oscillator in a "common emitter" configuration and coupled to a SAW resonator.

[0026] The microcontroller 15 may be a microcontroller with a low clock speed, for example of the order of 1 MHz, and is programmed to generate at its corresponding outputs a first signal MOD1 for driving the power amplifier 14 and a second signal MOD2 for driving the oscillator 13. The signals MOD1 and MOD2 consist essentially of rectangular pulses of voltage or current (concurrent with each other but of different durations), i.e. of signals that take only low values ​​(zero) and high values. Each PPM pulse to be transmitted corresponds to a single rectangular pulse of the first signal MOD1 and a single rectangular pulse of the second signal MOD2, the single rectangular pulse of the second signal MOD2 being substantially concurrent with the pulse of MOD1 and having a greater duration. The microcontroller 15 is configured to activate the oscillator 13 (corresponding to a high level of MOD2) only with MOD2 during a first time period t2 for each pulse of the PPM signal to be generated, then to activate the amplifier 14 also only with MOD1 during a second time period t1, while MOD2 remains at a high level, and to deactivate both the oscillator 13 and the amplifier 14 at the end of the second time period t1, or as Figure 3A As shown, in the third time period t3, the first MOD1 is deactivated and only MOD2 is kept at a high level.

[0027] According to a preferred embodiment of the invention, the microcontroller 15 comprises encoding means for encoding the information received from the detection means 11 into an original sequence of PPM pulses or a sequence of relative distances between pulses to be radio-transmitted.

[0028] The receiver 2 comprises an antenna 21 connected to a demodulator 22, followed by means 23-24 for reconstructing information 25 contained in an encrypted PPM signal received by the antenna 21. The demodulator 22 may be known per se (e.g. from US 9,270,309) and may comprise in cascade a first stage for low-noise amplification of the modulated radio frequency signal originating from the antenna 21, a SAW bandpass filter around a predetermined frequency, a logarithmic amplifier adapted to amplify the signal from the SAW filter, and a peak detector of the output signal from the logarithmic amplifier. The above-mentioned means for reconstructing the information are adapted to decode the PPM signal based on the PPM pulse sequence preferably output from the logarithmic amplifier and optionally on a signal output from the peak detector indicating the strength of the PPM signal received by the antenna.

[0029] The device for reconstructing information according to the invention can be formed by a microcontroller and comprises, in addition to decoding means 24 known per se, decryption means 23 according to the invention.

[0030] The remote receiver 2 may be located at a short distance from the transmitting device 1, for example on the same vehicle on which the one or more transducers DM are mounted.

[0031] In the PPM coding performed by the coding means of the microcontroller 15, each M-bit symbol of the alphabet of 2^M binary symbols used to encode and constitute the information to be transmitted (for example, the information contained in the signal originating from the detection means 11) is assigned a corresponding predefined time position of one pulse in the original sequence of PPM pulses relative to the previous pulse, that is, a predefined relative time distance (T0, T1, T2, ...) is assigned to each pair of consecutive pulses of the original sequence. The "original" sequence is understood to mean a sequence of pulses (indicated by 3 or 5 in the figure) which, once transmitted by radio, can be decoded by the receiver without the need for a key and knowing only the corresponding points between the symbols and the predefined relative time distances of the scheme applied by the transmitter 1 to perform the PPM coding. The sequence of analog pulses output from the pulse generating means 18 corresponds in terms of relative time distances T0, T1, ... to the sequence of "rectangular" pulses appropriately generated by the microcontroller 15 through the signals MOD1 and MOD2 in order to switch the pulse generator 18 on / off.

[0032] In the embodiment shown here, the symbol may be a 4-bit (M=4) symbol, and thus the 16 symbols of the alphabet may be encoded with 16 different predefined time positions T0, T1, ..., T15 of one pulse 322a, 322b, 322c, ... relative to the immediately preceding pulse 321a, 321b, 321c, .... Thus, in this particular case, a pair of pulses represents a half byte or "nibble".

[0033] The pulses (321a, 322a, 321b, 322b, 321c, 322c, ...) generated by the pulse generating device 18 for radio transmission preferably have a duration between 1 microsecond and 5 microseconds (e.g. 2 microseconds) and may have a duration of Figure 3B The magnitude of the quasi-Gaussian shape in .

[0034] A coding of 2^M code elements can be defined starting from a minimum time distance T0 (e.g. 52 microseconds) corresponding to the first code element of the alphabet (schematically shown in the figure as hexadecimal code element 0) to a maximum distance T15 (e.g. 97 microseconds for M=4 bits corresponds to hexadecimal code element F), with the spacing ΔT between the minimum and maximum values ​​increasing constantly (e.g. 3 microseconds between one time distance and the next).

[0035] In a first embodiment, the original pulse sequence 3 is formed by a plurality of pairs of pulses 32a, 32b, 32c, ..., wherein the first pulse 321a, 321b, 321c, ... (or "trigger pulse") in each pair is generated periodically over time, for example every IFS microseconds (where IFS, i.e., "intra-frame spacing", is a fixed value, preferably between 200 microseconds and 500 microseconds, for example 305 microseconds). Then, the second pulse in each pair (or "data pulse") is at a time position T0, T1, T2, ... relative to the first pulse, which position is assigned by PPM coding based on the binary symbol to be encoded and is therefore predefined. The duty cycle of the pulse is very low, for example less than 5 / 100 (or about 1 / 100 in the case shown), so that the receiver 2 can clearly distinguish the pulses and the energy consumption is low.

[0036] In a second embodiment, the original pulse sequence 5 can be considered as a series of pulse pairs, but in this case, the second pulse in each pair (e.g., the pulse at position P3 in the pair P2-P3) corresponds to the first pulse in the subsequent pair (P3-P4) in the sequence, and the relative time distance defined by the PPM encoding is the distance between the second pulse (at position P3) and the immediately preceding pulse (at position P2). Also in this case, the duration of each pulse is much smaller than the relative time distance between one pulse and the subsequent or preceding pulse; for example, it is between 2% and 5% of that duration.

[0037] In all preferred embodiments, the pulse sequence is preceded by a preamble 31 or 51a for identifying the start of a frame containing the pulse sequence. The preamble also consists of a pulse whose amplitude and duration are preferably equal to the amplitude and duration of the other pulses in the sequence. For example, the preamble 31 consists of two pulses with a fixed time distance Tp between them, a fixed time distance Ts from the first pulse 321a of the sequence, and a much shorter duration relative to such distance Tp or Ts (for example, a ratio of 1 to 100 or less). In the example of a 2 microsecond pulse sequence, the distance Tp can be 225 microseconds and the distance Ts can be 305 microseconds, i.e. equal to the repetition period of the trigger pulses 321a, 321b, 321c, ...

[0038] The transmitter 1 comprises encryption means 16 (for example programmed in a microcontroller 15) adapted to time-shift at least one of the pulses of the original sequence 3 or 5 (according to a corresponding offset δ which is different from zero and which is known only to the authorized receivers 2, for example because it is stored or is available to these receivers) relative to a predefined time position of the shifted pulse in the original sequence 3 or 5, so that the transmitted pulse sequence comprises at least one time-shifted pulse.

[0039] In this way, the transmitted pulse sequence is unintelligible at an (unauthorized) receiver which is only able to decode the original PPM sequence without being aware of the shift applied to one or more pulses.

[0040] The offset δ is preferably defined so that the new time distance between the shifted pulse of the sequence and the immediately preceding pulse is uniquely different from all time distances provided by the PPM coding. In other words, the time-shifted pulse cannot be found in any predefined time position provided by the PPM coding. For example, if the PPM coding specifies that each symbol of the alphabet corresponds to a pulse at a distance T0+n·ΔT from the previous pulse, where T0 is the minimum time distance between two consecutive pulses (for example, as in the previously mentioned example, the minimum time distance is 52 microseconds), n is an integer, and ΔT is a certain constant spacing (for example 3 microseconds), then for at least one of the time-shifted pulses, the absolute value of the offset δ introduced by the encryption is different from the spacing ΔT and from an integer multiple of the spacing. Therefore, for one or more of the pulses, the shift value applied by the encryption device 16 is outside the value or position allowed by the PPM coding protocol, so as to make it impossible for an unauthorized receiver that knows the PPM coding of the transmitter to read the pulse pair.

[0041] Advantageously, the offset δ can also be defined so that for all pulses of a message of the same symbol of the coding alphabet, the new time distance between the shifted pulse and the immediately preceding pulse in the sequence is the same.

[0042] With the encryption according to the invention, the relative time distance between two pulses in at least one pair in the original sequence 3 or 5 is actually modified using a criterion or algorithm known only to the authorized receiver, for example because the authorized receiver has a decryption key or the authorized receiver has stored instructions for identifying the encryption criterion used in the received pulse sequence.

[0043] Considering that encryption affects the pulses transmitted physically by radio, the encryption (or cryptography) according to the present invention advantageously takes place at the physical layer of the OSI communication model, and is therefore different from other cryptographic techniques based on the data layer of the OSI model. By bringing cryptography to the physical layer, the present invention acts on the elements (pulses) that constitute the message itself, that is, on the modulation of the pulses that constitute the message. This makes the message unreceivable without the relevant key.

[0044] In the illustrated embodiment, the plurality of pulses of the transmitted sequence are time-shifted by the encryption device 16 relative to respective predefined time positions, wherein the corresponding offset has a value and / or sign that is different for all pulses of the plurality of time-shifted pulses.

[0045] In the first embodiment, the transmitted PPM pulse sequence 4 has the first three pairs of second pulses 422a, 422b and 422c located in positions shifted relative to the positions of the corresponding pulses 322a, 322b and 322c of the original sequence 3, while the positions of the trigger pulses 321a, 321b and 321c remain unchanged.

[0046] The first pair of pulses 42a and the third pair of pulses 42c of the transmitted sequence 4 have a relative time distance between their respective two pulses, which is increased by positive offset values ​​δa and δc respectively relative to the corresponding predefined relative time distances Ta and Tc of the original sequence. In fact, the second pulses 422a and 422c are delayed by δa and δc respectively.

[0047] On the other hand, the second pair of pulses 42b has a relative time distance reduced by δb (considering that δb in the figure has a negative sign, δb is an absolute value), and therefore the second pulse 422b in the pair is advanced by |δb| relative to the predefined time position of Tb microseconds from the previous pulse 321b of the pair 32b of the original sequence 3.

[0048] exist Figure 6B In the second embodiment shown, the pulses in positions P3, P7 and P16 are pulses that are time-shifted in the transmitted sequence 6 relative to their predefined positions in the original sequence 5. In particular, the pulses in positions P3 and P16 are advanced by offsets δ3 and δ16, respectively, while the pulse in position P7 is delayed by offset δ7.

[0049] Advantageously, in the transmitted sequence (e.g. 4 or 6), at least one time-shifted pulse should be located at at least one time position relative to the pulse preceding it that does not correspond to any predefined time position (T0+n·ΔT) provided in the PPM encoding so as to prevent an unauthorized receiver from decoding any codeword from the time-shifted pulse.

[0050] For example, in Figure 8 In an embodiment, one or more of the time-shifted second pulses (e.g., pulse 422a') of the transmitted sequence 4 are at position T0+y, where y is different from n·ΔT and lower than (2^M-1)·ΔT. In a non-limiting example, the position of the time-shifted pulse is T0+7·ΔT in the original sequence and is T0+7·ΔT+δ'7 in the transmitted sequence, where δ'7 is lower than ΔT, is not an integer, and is different from zero.

[0051] exist Fig. 9 In an embodiment, one or more second pulses (422a') in the transmitted sequence 4 are shifted in time at a position T0+Y, where Y is higher than (2^M-1)·ΔT, lower than IFS, and preferably different from n·ΔT.

[0052] The encryption means 16 are advantageously adapted to encrypt at least one shift information item 71, 72, 73 with a symmetric key or with a public key and then to share the at least one shift information item 71, 72, 73 in encrypted form with an authorized receiver 2 in order to be able to decrypt the received PPM signal, i.e. to recover the predefined time positions of the shifted pulses and enable the decrypted PPM signal to be decoded.

[0053] A “shift information item” means a set of data comprising at least the position (P1, P2, P3, ...) of a time-shifted pulse within the original sequence 3, 5 or within the transmitted sequence 4, 6, and the corresponding degree of shift, i.e. the corresponding offset δ (with a positive or negative sign) relative to a predefined time position of the pulse having this position in the original sequence.

[0054] The shift information items 71 , 72 , 73 are stored in the transmitter 1 , in particular in the microcontroller 15 , or they can be generated by the microcontroller 15 using an algorithm.

[0055] The shift information items 71, 72, 73 can advantageously be inserted into the transmitted pulse sequence in encrypted form, for example by PPM encoding the shift information items encrypted with a symmetric key (or with the public key of an authorized receiver) and inserting the PPM pulses thus obtained into a portion (51b) of the preamble of the transmitted pulse sequence (6), for example into the portion 51b after the first two pulses 51a of the preamble at a fixed distance Tp.

[0056] A symmetric key is preferred because it can be easily stored in all authorised receivers 2 and, optionally, it can have an expiry date so that receivers 2 that are no longer authorised cannot decrypt the PPM signal without a new key.

[0057] Alternatively, the encryption means 16 may employ an asymmetric key encryption algorithm, wherein for encryption a public key of a receiver 2 authorized to decode the PPM signal transmitted by the transmitter is used. The authorized receiver 2 may decrypt the signal using its corresponding private key.

[0058] In a further embodiment of the invention, there may be a set of shift information items 71, 72, 73 (i.e., a plurality of combinations of positions and offsets) which are shared in advance (e.g., at the manufacturing stage) with all authorized receivers. Each combination in the set is associated with an identifier S1, S2, ..., Sn, so that the transmitted PPM signal 4 or 6 may also carry (e.g., PPM encoded in a portion of the preamble 51b) an identifier S1, S2, ..., Sn of the position / offset combination used to encrypt the value associated with the coded symbol in that particular PPM signal. The identifier may also be encrypted with a symmetric key or the public key of the receiver 2 before encoding it with PPM modulation in the preamble 51b portion of the transmitted sequence.

[0059] The decryption means 23 of the receiver 2 are adapted to identify each pulse of the received sequence 4 or 6 (422a, 422b and 422c or P3, P7 and P16) that is time-shifted relative to its predefined time position and to shift each pulse identified as time-shifted by a corresponding offset (δa, δb, δc or δ3, δ7, δ16, δ'7) in order to recover its predefined time position. With the aid of the decryption means 23, the original sequence of pulses 3 or 5 is obtained and adapted to be decoded by the PPM decoding means 24 in order to reconstruct the information 25 originally transmitted.

[0060] For example, the decryption means 23 can identify the identifier S1 , S2 , S3 encoded in a portion of the preamble 51 b and, through the shift information items 71 , 72 , 73 stored in the authorized receiver 2 , the key used to encrypt the received pulse sequence.

[0061] Alternatively, the decryption means 23 are suitable for decrypting a predefined part of the received pulse sequence, for example a part of the preamble 51b or the first part of the pulses of the received sequence following the preamble 31 or 51a, by means of a symmetric key or a private key of the receiver 2, and extracting from this part the shift information item 71, 72 or 73 used to encrypt the sequence, or extracting an identifier S1, S2 or S3 of one of a plurality of shift information items 71, 72, 73 stored in an authorized receiver 2.

[0062] The operation of the present invention is apparent from the foregoing description.

[0063] In order to carry out the transmission of encrypted PPM symbols, a step of PPM encoding of the information item to be transmitted is carried out, wherein each M-bit symbol of the alphabet of 2^M symbols constituting the information item to be transmitted is assigned a respective predefined time position of one of the pulses in the original PPM pulse sequence relative to the preceding pulse. An encryption step is also carried out such that at least one of the pulses of the (RF) transmitted sequence is time-shifted relative to its predefined time position in the original sequence according to a corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) known only to authorized receivers.

[0064] Each offset in the sequence of offsets applied to a given original pulse sequence may optionally be generated by the encryption device 16 convolutionally, ie from a previous offset value in the sequence of offsets.

[0065] Preferably, a plurality of pulses in the transmitted sequence are time-shifted relative to a respective predefined time position, wherein the corresponding offset preferably has a different value and / or sign for all pulses that have been shifted in order to make abusive decoding more difficult.

[0066] At an authorized receiver 2, prior to PPM decoding, each pulse in the received sequence that is time-shifted relative to its predefined time position is identified and each time-shifted pulse is shifted by a corresponding offset known to the receiver so as to restore its predefined time position and obtain the original pulse sequence suitable for decoding using PPM decoding.

[0067] In practice, it has been found that the invention fully achieves the intended aim and objects.

[0068] The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims. Furthermore, all the details may be replaced by other, technically equivalent elements.

[0069] The disclosures in Italian Patent Application No. 102022000015990 from which this application claims priority are incorporated herein by reference.

[0070] In case technical features mentioned in any claim are followed by reference signs, the sole purpose of their inclusion is to improve the intelligibility of the claims and accordingly, these reference signs do not have any limiting effect on the interpretation of each element identified by these reference signs by way of example.

Claims

1. A method for transmitting an encrypted PPM signal for a radio communication system, in particular for short-range radio, comprising a step of PPM encoding an information item to be transmitted, wherein each symbol in a symbol alphabet used to form the information item to be transmitted is assigned a respective predefined time position (T0+nΔT) of a pulse in an original PPM pulse sequence (3, 5) relative to a preceding pulse, characterized in that The method comprises an encryption step in which at least one of the pulses of the transmitted sequence (4, 6) is time-shifted relative to a predefined time position of the at least one pulse in the original sequence (3, 5) according to a corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) known only to an authorized receiver (2), wherein the offset (δa, δb, δc, δ3, δ7, δ16, δ'7) is such that the time-shifted pulse is not located in any predefined time position (T0+nΔT) provided in the PPM coding relative to a pulse preceding the time-shifted pulse.

2. The method according to claim 1, wherein: A plurality of said pulses in said transmitted sequence (4, 6) are time-shifted relative to said respective predefined time positions, wherein said corresponding offsets (δa, δb, δc, δ3, δ7, δ16, δ'7) preferably have different values ​​and / or signs for all pulses of said plurality of time-shifted pulses.

3. The method according to one or more of the preceding claims, wherein: The encryption step comprises encrypting at least one shift information item (71, 72, 73) or an identifier (S1, S2, S3) of a shift information item taken from a set of predetermined shift information items (71, 72, 73) known to an authorized receiver (2) with a symmetric key or a public key, each shift information item comprising at least the position (P1, P2, ...) of the time-shifted pulse in the original sequence (3, 5) or in the transmitted sequence (4, 6) and the corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) relative to the predetermined time position.

4. The method according to claim 3, wherein: One of the encrypted shift information item, the encrypted identifier or the unencrypted identifier is transmitted together with the encrypted PPM signal.

5. The method according to claim 4, wherein: The transmitted encrypted PPM signal comprises a preamble and the transmitted pulse sequence, the preamble containing the encrypted shift information item or the encrypted or unencrypted identifier.

6. The method according to one or more of the preceding claims, wherein: The transmitted pulse sequence (4, 6) comprises a plurality of temporally consecutive pulse pairs, each pair comprising a first pulse corresponding to the preceding pulse and a second pulse corresponding to the time-shifted pulse in at least one of the pairs.

7. The method according to claim 6, wherein: The first pulses (321a, 321b, 321c) of the pulse pairs have a substantially constant spacing (IFS) in the sequence, or the second pulse (P3) of each pair (P2-P3) corresponds to the first pulse of a subsequent pair (P3-P4) in the sequence (5, 6).

8. A method for receiving an encrypted PPM signal for a radio communication system, in particular a short-range radio, the method comprising the step of PPM decoding the encrypted PPM signal received by a receiver (2), wherein: Obtaining, from the corresponding time position of a pulse in a received sequence of pulses of a received PPM signal relative to the preceding pulse, a corresponding symbol of the alphabet of symbols used in the encoding for constituting an information item to be transmitted with said PPM signal, characterised in that said method comprises a decryption step, wherein prior to PPM decoding: - identifying each pulse (422a, 422b, 422c, P3, P7, P16) in the received sequence (4, 6) that is time-shifted relative to a predefined time position; - shifting each time-shifted pulse by a corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) known to the receiver (2) in order to recover the predefined time position of each time-shifted pulse and obtain the original pulse sequence (3, 5) suitable for decoding with the PPM decoding, wherein the offset (δa, δb, δc, δ3, δ7, δ16, δ'7) is such that the time-shifted pulse is not located in any predefined time position (T0+nΔT) provided in the PPM encoding relative to the pulse preceding the time-shifted pulse.

9. The method according to claim 8, wherein: The offsets (δa, δb, δc, δ3, δ7, δ16, δ'7) have different values ​​and / or signs for all time-shifted pulses.

10. The method according to one or more of claims 8-9, wherein: The receiver comprises a set of shift information items (71, 72, 73) or predetermined shift information items (71, 72, 73) known to an authorized receiver (2) and identifiable by a corresponding identifier (S1, S2, S3), each shift information item comprising at least the position (P1, P2, ...) of the time-shifted pulse in the original sequence (3, 5) or received sequence (4, 6) and a corresponding offset relative to the predefined time position.

11. The method according to claim 10, wherein: The shift information item or the identifier is received together with the PPM signal in encrypted form with a symmetric key or with a public key of the receiver, so that the shift information item or the identifier can be decrypted with the symmetric key or with a private key of the receiver, respectively.

12. The method according to claim 11, wherein: The received PPM signal (6) comprises a preamble (51b) and the received pulse sequence, the preamble (51b) containing the shift information item (71, 72, 73) in encrypted form or the identifier (S1, S2, S3) in encrypted or unencrypted form.

13. A transmitter (1) of a radio communication system, in particular a short-range radio, for encrypting PPM signals, the transmitter comprising PPM encoding means (15) for encoding an information item to be transmitted, the PPM encoding means being adapted to assign to each symbol in a symbol alphabet for constituting the information item to be transmitted a respective predefined time position (T0+nΔT) of a pulse in an original pulse sequence (3, 5) relative to a preceding pulse, characterised in that The transmitter comprises an encryption device (16) adapted to time-shift at least one of the pulses of the original sequence (3, 5) relative to its predefined time position in the original sequence (3, 5) according to a corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) known only to an authorized receiver (2), so that the transmitted pulse sequence (4, 6) comprises the at least one time-shifted pulse (422a, 422b, 422c, P3, P7, P16), wherein the offset (δa, δb, δc, δ3, δ7, δ16, δ'7) is such that the time-shifted pulse is not located in any predefined time position (T0+nΔT) provided in the PPM coding relative to a pulse preceding the time-shifted pulse.

14. The transmitter (1) according to claim 13, wherein A plurality of said pulses in said transmitted sequence (4, 6) are time-shifted by said encryption device (16) relative to respective predefined time positions, wherein said corresponding offsets (δa, δb, δc, δ3, δ7, δ16, δ'7) preferably have different values ​​and / or signs for all pulses of said plurality of time-shifted pulses.

15. Transmitter (1) according to one or more of the preceding claims, wherein The encryption means (16) are adapted to encrypt at least one shift information item (71, 72, 73) or an identifier (S1, S2, S3) of a shift information item taken from a set of predetermined shift information items (71, 72, 73) known to an authorized receiver (2) using a symmetric key or a public key, each shift information item comprising at least the position (P1, P2, ...) of the time-shifted pulse in the original sequence or the transmitted sequence and the corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) relative to the predefined time position.

16. The transmitter (1) according to claim 15, wherein One of the encrypted shift information item, the encrypted identifier or the unencrypted identifier is transmitted together with the encrypted PPM signal.

17. The transmitter according to claim 16, wherein: The transmitted encrypted PPM signal (4, 6) comprises a preamble (51b) and the transmitted pulse sequence, the preamble (51b) containing the encrypted shift information item or the encrypted or unencrypted identifier.

18. The transmitter according to one or more of claims 15 to 17, wherein: The encryption device (16) is suitable for generating the symmetric key or the public key for encryption.

19. A receiver (2) for a radio communication system, in particular a short-range radio, for encrypted PPM signals, the receiver comprising a PPM decoding device (24) for decoding received encrypted PPM signals (4, 6), wherein: The corresponding symbols of the alphabet of symbols used in the coding to compose the information item to be transmitted together with the PPM signal are obtained from the corresponding time position of a pulse in the received pulse sequence of the received PPM signal relative to the previous pulse, characterized in that the receiver comprises decryption means (23) adapted to: - identifying each pulse (422a, 422b, 422c, P3, P7, P16) in the received sequence (4, 6) that is time-shifted relative to a predefined time position; - shifting each pulse identified as time-shifted by a corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) known to the receiver (2) in order to recover the predefined time position of each pulse identified as time-shifted and obtain the original pulse sequence (3, 5) suitable for decoding by the PPM decoding device (24), wherein the offset (δa, δb, δc, δ3, δ7, δ16, δ'7) is such that the time-shifted pulse is not located in any predefined time position (T0+nΔT) provided in the PPM encoding relative to the pulse preceding the time-shifted pulse.

20. The receiver (2) according to claim 19, wherein The offsets (δa, δb, δc, δ3, δ7, δ16, δ'7) have different values ​​and / or signs for all time-shifted pulses.

21. The receiver (2) according to one or more of claims 19-20, wherein: A set of shift information items (71, 72, 73) or predetermined shift information items identifiable by respective identifiers (S1, S2, S3) are stored, each shift information item comprising at least a position (P1, P2, ...) of said time-shifted pulse in said original sequence (3, 5) or received sequence (4, 6) and a corresponding offset (δa, δb, δc, δ3, δ7, δ16, δ'7) relative to said predefined time position.

22. The receiver (2) according to claim 21, wherein The shift information item or the identifier is received together with the PPM signal in encrypted form using a symmetric key or a public key of the receiver (2).

23. The receiver (2) according to claim 22, wherein: The received PPM signal comprises a preamble (51b) and the received pulse sequence (4, 6), the preamble (51b) containing the shift information item in encrypted form or the identifier in encrypted or unencrypted form.

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