Processing an amplitude modulated signal

Abstract

Near field communications (NFC) may be used of radio frequency identification (RFID) tags and may use amplitude modulated (AM) / amplitude shift keyed (ASK) signals, for instance on-off keying (OOK). The invention discloses an AM scheme where the duration / width of the high amplitude pulse is used to encode data. The invention may encode each symbol to a fixed duration (e.g. 8 periods) with one or two low amplitude periods (P) inserted therein (see Fig. 4). Demodulation may need to consider the duration multiple high amplitude segments together (see. Fig. 6). The amplitude modulation scheme of the invention is intended to enable demodulation without the need for clock recovery. The invention may be used to encode NFC-V or ISO / IEC 15693 symbols / commands.

Description

Technical Field

[0001] The present disclosure concerns amplitude modulated signals. More particularly, but not exclusively, this disclosure concerns methods of processing amplitude modulated signals and receiver devices for processing received amplitude modulated signals. Background

[0002] Amplitude modulated, AM, signals are used in a wide variety of applications. One such application is Near Field Communication, NFC, technology. NFC is a set of communication protocols that enables over-the-air communication between two electronic devices (e.g. a transmitter device and a receiver device) over relatively short distances, e.g. 4 centimetres or less. NFC devices may use, or be used in, radio frequency identification, RFID, technology, in which electromagnetic fields are used to identify and track tags. Tags may be attached to or embedded in various objects or surfaces. A tag may comprise a radio receiver and transmitter. When triggered by an electromagnetic signal from a nearby reader, the tag may be configured to transmit digital data, e.g. comprising an identifier of the tag and / or of an object to which the tag is attached, back to the reader.

[0003] At a receiver device (e.g. an NFC tag), an AM signal is received from a transmitter device (e.g. an NFC reader). The AM signal encodes a sequence of symbols which form a command from the transmitter device. Such a signal is typically demodulated at the receiver device and then sampled to identify specific modulation patterns which denote particular symbols encoded in the signal.

[0004] An AM signal, such as that transmitted by an NFC reader to an NFC tag, typically comprises a sequence of high amplitude signal periods and low amplitude signal periods. Some applications, such as NFC Type 5 technology, may operate using 100% amplitude modulation. For a signal having 100% amplitude modulation (also known as On-Off Keying, OOK), low amplitude signal periods have zero amplitude. During such low amplitude signal periods, clock recovery (i.e. the process of extracting timing information from a signal to allow sampling and thus symbol identification) may not be straightforward or even possible, since no signal is transmitted by the transmitter device during such periods.

[0005] The present disclosure seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present disclosure seeks to provide an improved method of processing AM signals. Alternatively or additionally, the present disclosure seeks to provide an improved receiver device for receiving and processing AM signals. Summary

[0006] The present disclosure provides, according to a first aspect, a method of processing an amplitude modulated, AM, signal, the method comprising: receiving an AM signal comprising an encoded sequence of symbols, each symbol in the encoded sequence being from a set of predefined symbols, wherein each predefined symbol in the set of predefined symbols, when encoded, comprises an AM signal portion comprising one or more high amplitude signal periods and a low amplitude signal period, wherein the low amplitude signal period is in a different position in the AM signal portion for different predefined symbols in the set of predefined symbols; processing the received AM signal to measure an interval between two successive low amplitude signal periods in the received AM signal; based at least in part on the measured interval, identifying a symbol of the sequence of symbols encoded in the received AM signal; and performing an action based on the identified symbol.

[0007] According to a second aspect of the disclosure there is also provided a receiver device comprising electronic circuitry configured to: receive an amplitude modulated, AM, signal comprising an encoded sequence of symbols, each symbol in the encoded sequence being from a set of predefined symbols, wherein each predefined symbol in the set of predefined symbols, when encoded, comprises an AM signal portion comprising one or more high amplitude signal periods and a low amplitude signal period, wherein the low amplitude signal period is in a different position in the AM signal portion for different predefined symbols in the set of predefined symbols; process the received AM signal to measure an interval between two successive low amplitude signal periods in the received AM signal; based at least in part on the measured interval, identify a symbol of the sequence of symbols encoded in the received AM signal; and perform an action based on the identified symbol.

[0008] It will of course be appreciated that features described in relation to one aspect of the present disclosure may be incorporated into other aspects of the present disclosure. For example, the method of the disclosure may incorporate any of the features described with reference to the apparatus of the disclosure and vice versa. Description of the Drawings

[0009] Embodiments of the present disclosure will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a schematic view of a receiver device according to the present disclosure; Figure 2 shows a flow diagram illustrating the steps of a method according to the present disclosure; Figure 3 shows a schematic view of a receiver device according to the present disclosure; Figure 4 shows a set of predefined symbols according to the present disclosure; Figure 5 shows a set of commands according to the present disclosure; Figure 6 shows a set of symbol combinations according to the present disclosure; and Figure 7 shows example signal diagrams for a calibration process according to the present disclosure. Detailed Description

[0010] Figure 1 shows a schematic view of a receiver device 100 according to the present disclosure. The receiver device 100 comprises electronic circuitry 110 configured to process amplitude modulated, AM, signals received by the receiver device 100, as will be described below. The electronic circuitry 110 may comprise all or part of an integrated circuit. The receiver device 100 also comprises an antenna 120 for receiving AM signals to be processed by the electronic circuitry 110. This is depicted using a dashed arrow in Figure 1. The antenna 120 may comprise a radio frequency, RF, antenna configured to receive RF signals.

[0011] Additionally or alternatively, the receiver device 100 comprises a Near Field Communication, NFC, device. NFC is a set of communication protocols that enable over-the-air communication between electronic devices over relatively short distances. NFC devices may use, or be used in, radio frequency identification, RFID, technology, in which electromagnetic fields are used to identify and track tags. Tags may be attached to or embedded in various objects or surfaces.

[0012] Additionally or alternatively, the receiver device 100 comprises an NFC tag device. An NFC tag device may comprise a radio receiver and transmitter. When triggered by an electromagnetic interrogation signal from a nearby NFC reader device, the tag device may be configured to transmit digital data, e.g. comprising an identifier of the tag and / or of an object to which the tag is attached, back to the reader device. A tag device may be referred to as a “target device” whereas a reader device may be referred to as an “initiator device”. NFC tag devices may have relatively simple and / or small form factors, for example taking the form of stickers, labels, smart posters, access badges, identity documents, key fobs or cards. Additionally or alternatively, the receiver device 100 comprises an RFID tag device that is not an NFC tag device.

[0013] Additionally or alternatively, the receiver device 100 comprises an NFC Type 5 tag device. An NFC Type 5 tag device is a type of NFC tag device that is compatible with the ISO / IEC 15693 standard. NFC Type 5 devices may be configured to operate with 100% amplitude modulated signals. Additionally or alternatively, the receiver device 100 comprises an NFC-V device and / or is configured to process NFC-V signals.

[0014] Additionally or alternatively, the receiver device 100 comprises an unpowered device. That is, the receiver device 100 may not comprise its own power source. An NFC tag device is an example of such an unpowered device. An NFC tag device may instead draw power from an electromagnetic field generated by a nearby NFC reader device. An unpowered device may also be referred to as a ‘passive device’, in that actions are performed by the device only in response to prompting by another device (e.g. a reader device). Additionally or alternatively, the receiver device 100 does not comprise an unpowered device. That is, the receiver device 100 may comprise a powered, or active, device. For example, the receiver device 100 may comprise or be comprised in a mobile phone. The mobile phone may be configured to emulate a smart card or other NFC target device, e.g. to perform transactions. The methods described herein may also be applied to NFC peer-to-peer communication, in which both devices (the “initiator” and the “target”) are powered.

[0015] While examples disclosed herein relate to NFC (e.g. NFC devices, NFC signals, etc.) it will be understood that the presently-described methods may be applied more generally to systems that do not use or are not concerned with NFC technology, but may still use AM signals. As such, in some examples the receiver device 100 does not comprise an NFC device, the received AM signal does not comprise an NFC signal, etc.

[0016] As mentioned above, in the examples shown in Figure 1, the receiver device 100 comprises an antenna 120 configured to receive AM signals from a transmitter device (not shown). The transmitter device may comprise an NFC reader device, for example. It will be understood, however, that in alternative examples the receiver device 100 does not comprise the antenna 120. For example, AM signals may be received at the receiver device 100 from a further device (not shown) comprising an antenna, the further device being configured to receive the AM signals from the transmitter device. The further device may then send the AM signals to the receiver device 100 without the use of an antenna, e.g. via a wired connection between the further device and the receiver device 100. Accordingly, the antenna 120 may be omitted from the receiver device 100 in some examples.

[0017] Additionally or alternatively, the receiver device 100 comprises a flexible integrated circuit. Flexible integrated circuits are generally much thinner and more flexible than conventional integrated circuits, enabling them to be used in a wider range of applications. Flexible integrated circuits may be particularly suitable for use in or as NFC tags, because the thinner and more conformable and pliable physical properties of flexible integrated circuits facilitate integration into objects or surfaces. In accordance with the present disclosure a “flexible integrated circuit” (flexible IC or flexIC) is a type of integrated circuit, IC, that is designed to be flexible and conformable, allowing it to bend, twist, and conform to non-flat or irregular surfaces. Unlike traditional rigid ICs, which are typically made on silicon wafers and are inflexible, flexible ICs, in accordance with the present disclosure, are fabricated on flexible substrates using appropriate materials and thin-film processes. The substrate is typically formed of an appropriate flexible polymer material. Nevertheless, the flexible substrate may be formed from any other materials that provide suitable electrical, chemical, and / or structural properties. The flexible substrate may be formed from a single common material, may be formed from a plurality of different materials, or may be formed from a plurality of different types of the same material (e.g. different polymers). The flexible substrate may, for example, comprise one or more materials selected from the following list of materials: flexible glass, polymer materials, metal oxide materials, resin materials, resist materials, foil materials, paper, insulator coated metals, or any other suitable material.

[0018] Where a polymer based material is used, the substrate may comprise one or more polymers selected from: polyethylene naphthalates, polyethylene terephthalates; polymethyl methacrylates; polycarbonates, polyvinyl alcohols, polyvinyl acetates, polyvinyl pyrrolidones, polyvinyl phenols, polyvinyl chlorides, polystyrenes, polyimides, polyamides (e.g. Nylon); poly(hydroxy ethers), polyurethanes, polycarbonates, polysulfones, parylenes, polyarylates, polyether ether ketones (PEEKs); acrylonitrile butadiene styrene (ABS), 1 Methoxy 2 propyl acetates, Benzocyclobutenes (BCB), polylactic acid (PLA), polyhydroxyalkanoates (PHAs), polybutylene succinate (PBS), polybutylene adipate terephthalate (PBAT), cellulose polymers, or any other suitable polymer material.

[0019] Where a metal oxide based material is used, the substrate may comprise one or more metal oxides selected from: AI2O3, SiOxNy, SiO2, Si3N4, or any other suitable metal oxide. Where a resin based material is used, the substrate may comprise one or more resins selected from: a UV-curable resin or any other suitable resin. Where a resist based material is used, the substrate may comprise one or more resists selected from: nanoimprint resists, photoresists such as, for example, Bisphenol A novolac epoxy (SU-8) or polyhydroxybenzyl silsesquioxane, or any other suitable resist. Where a foil based material is used the substrate may comprise one or more foils selected from: polymeric foils or any other suitable foil. Where an insulator-coated metal is used, the substrate may comprise one or more insulator-coated metals selected from: insulator coated stainless-steel or any other suitable insulator-coated metal.

[0020] Additionally or alternatively, the receiver device 100 does not comprise a flexible IC. For example, the receiver device 100 may be a silicon-based device.

[0021] Figure 2 shows a flow chart illustrating the steps of a method 200 of processing an AM signal, according to the present disclosure. The method 200 may be performed at least in part by the receiver device 100 described above with reference to Figure 1, and in particular by the electronic circuitry 110 of the receiver device 100. The method 200 provides a versatile solution for processing AM signals that does not require clock recovery (whether using phase locked loops or other methods of clock recovery) and may thus be applied to a wider variety of systems and applications, including low cost and / or low power systems, and / or systems having a relatively small spatial footprint, such as flexible integrated circuits, compared to known methods.

[0022] A first step of the method 200, illustrated by item 210, comprises receiving an AM signal. The AM signal may be received via an antenna 120 of the receiver device 100, or from a further device. The received AM signal may comprise a radio frequency, RF, signal, for example. Additionally or alternatively, the received AM signal comprises an NFC signal, such as an NFC-V signal.

[0023] The received AM signal comprises an encoded sequence of symbols. Each symbol in the encoded sequence is from a set of predefined symbols. Additionally or alternatively, the predefined symbols in the set of predefined symbols are defined according to an NFC standard. For example, the predefined symbols may be defined according to the ISO / IEC 15693 standard and / or the ISO / IEC 14443 standard. Additionally or alternatively, the predefined symbols are not defined according to an NFC standard.

[0024] Each predefined symbol in the set of predefined symbols, when encoded, comprises an AM signal portion comprising one or more high amplitude signal periods and a low amplitude signal period. The low amplitude signal period is in a different position in the AM signal portion for different predefined symbols in the set of predefined symbols. Accordingly, a “symbol” as disclosed herein corresponds to a particular signal pattern defined by the position in the pattern of a low amplitude signal period. An “AM signal portion” as disclosed herein refers to a portion of an AM signal, i.e. a temporal portion, encoding a given symbol. For example, an AM signal may comprise multiple contiguous portions each encoding a respective symbol, such that the overall AM signal encodes a sequence of multiple symbols, constituting a command. A “signal period” as disclosed herein refers to a period, or duration, of an AM signal. A given AM signal portion (corresponding to a symbol) comprises multiple signal periods. A “high amplitude signal period” as disclosed herein refers to a period of an AM signal in which the AM signal has a relatively high amplitude. A “low amplitude signal period” as disclosed herein refers to a period of an AM signal in which the AM signal has a relatively low amplitude. A given AM signal may switch between high amplitude signal periods and low amplitude signal periods, thereby to encode information. That is, each high amplitude signal period may be bounded on each side by a pair of low amplitude signal periods. Similarly, each low amplitude signal period may be bounded by a pair of high amplitude signal periods. Additionally or alternatively, high amplitude signal periods within a given AM signal can have different durations, whereas every low amplitude signal period has the same duration. Alternatively, low amplitude signal periods may have different durations. The high amplitude signal periods of a given AM signal may all have the same amplitude, and the low amplitude signal periods of the given AM signal may all have the same amplitude.

[0025] Additionally or alternatively, the received AM signal comprises a 100% amplitude modulated signal. As such, the low amplitude signal periods of the received AM signal may have zero amplitude, whereas the high amplitude signal periods may have a nominal, or 100%, amplitude. Additionally or alternatively, the low amplitude signal periods may have an amplitude that is greater than zero. For example, the received AM signal may comprise a 5% amplitude modulated signal, a 10% amplitude modulated signal, a 15% amplitude modulated signal, a 20% amplitude modulated signal, a 25% amplitude modulated signal, a 30% amplitude modulated signal, a 35% amplitude modulated signal, a 40% amplitude modulated signal, a 45% amplitude modulated signal, a 50% amplitude modulated signal, a 55% amplitude modulated signal, a 60% amplitude modulated signal, a 65% amplitude modulated signal, a 70% amplitude modulated signal, a 75% amplitude modulated signal, an 80% amplitude modulated signal, an 85% amplitude modulated signal, a 90% amplitude modulated signal, an 95% amplitude modulated signal, etc.

[0026] Additionally or alternatively, the received AM signal comprises an on-off keying, OOK, signal. OOK is a type of amplitude-shift keying, ASK, modulation, that represents digital data as the presence or absence of a carrier wave. That is, during transmission of logic zero, no carrier is transmitted. This corresponds to a low amplitude signal period in which the amplitude is zero. OOK may be referred to as “100% ASK”. Additionally or alternatively, the received AM signal comprises an ASK signal other than an OOK signal. For example, logic zero may be represented by a carrier having an amplitude that is non-zero but still reduced relative to the carrier amplitude for logic one. The received AM signal may comprise a 5% ASK signal, a 10% ASK signal, a 15% ASK signal, a 20% ASK signal, a 25% ASK signal, a 30% ASK signal, a 35% ASK signal, a 40% ASK signal, a 45% ASK signal, a 50% ASK signal, a 55% ASK signal, a 60% ASK signal, a 65% ASK signal, a 70% ASK signal, a 75% amplitude modulate signal, an 80% ASK signal, an 85% ASK signal, a 90% ASK signal, an 95% ASK signal, etc.

[0027] Additionally or alternatively, the received AM signal comprises an NFC signal. Additionally or alternatively, the AM signal is received from an NFC reader device. For example, the received AM signal may comprise an interrogation signal sent by an NFC reader device to an NFC tag device in order to identify the NFC tag device and / or an object attached thereto.

[0028] A second step, illustrated by item 220, of method 200 comprises processing the received AM signal to measure an interval between two successive low amplitude signal periods in the received AM signal. An “interval” as disclosed herein refers to a duration, or period, of the AM signal between two events. Measuring the interval between two successive low amplitude signal periods may be equivalent to measuring the duration of a given high amplitude signal period, i.e. how long a given high amplitude signal period is, since a given high amplitude signal period is bounded on each side by a low amplitude signal period.

[0029] Additionally or alternatively, processing the received AM signal comprises demodulating the received AM signal to obtain a demodulated signal. Low amplitude signal periods of the received AM signal may then correspond to pulses in the demodulated signal. Accordingly, measuring the interval between successive low amplitude signal periods in the received AM signal may comprise measuring the interval (e.g. the time) between pulses in the demodulated signal. Additionally or alternatively, the received AM signal is not demodulated prior to measuring the interval.

[0030] Additionally or alternatively, the processing performed at item 220 comprises measuring the interval using a latch circuit. A latch circuit may be configured to store a counter value representing the duration of a given high amplitude signal period, thereby enabling a measurement of the interval between successive low amplitude signal periods. The latch circuit may be reset by a low amplitude signal period (which may, optionally, correspond to a pulse in a demodulated signal obtained by demodulating the received AM signal). A latch circuit is an example of an asynchronous counter. Other asynchronous counters or synchronous counters may be used to measure the interval in alternative examples.

[0031] A third step, illustrated by item 230, of method 200 comprises, based at least in part on the measured interval, identifying a symbol of the sequence of symbols encoded in the received AM signal. The identity of the symbol may depend on the measured interval, such that different values of the measured interval translate to different symbols. For example, a counter value obtained at item 220 (e.g. via a latch circuit) corresponding to the interval between successive low amplitude signal periods may translate to a particular symbol. Since each symbol is one of a set of predefined symbols, where each predefined symbol may be encoded as an AM signal portion having a low amplitude signal period at a different position, a given symbol can be identified based on a measured interval between successive low amplitude signal periods in the received AM signal. That is, the position of the low amplitude signal period in a given AM signal portion may be used to identify the symbol corresponding to that AM signal portion.

[0032] As such, symbols encoded in the received AM signal may be identified without having to sample pulses in a demodulated signal obtained by demodulating the received AM signal. Clock recovery is thus not an issue and is not required in the presently-described methods. The AM signal can be decoded even if clock information is lost, e.g. even if the signal is 100% amplitude modulated.

[0033] One known method of clock recovery involves the use of phase locked loops, PLLs. PLLs are electronic circuits which constantly adjust voltage to match the frequency of an input signal, and can be used to recover an interrupted signal (such as an amplitude modulated signal having periods of zero amplitude). PLLs may be used in silicon technology. PLL-based methods of clock recovery allow the lengths of the periods of 100% modulation (i.e. absence of signal) to be measured, and then the signal may be decoded in the usual manner. This is not the same as measuring intervals between successive low amplitude signal periods and using the measured intervals to deduce symbols, as disclosed herein. PLLs may require a relatively stable input voltage and a relatively large amount of power. In particular, accurate PLLs may require the availability of complementary transistor types and / or very low variations in device characteristics. Moreover, PLLs are typically large and power-intensive circuits. Accordingly, PLLs may be difficult, expensive and / or complicated to implement in some systems, e.g. systems where there may be a significant variation in device characteristics within a single integrated circuit, where there is an absence of complementary transistor types, and / or where there is a desire for a relatively low power consumption. One such system may be a flexible integrated circuit. Therefore, the methods described herein provide a versatile and low power solution for decoding AM signals that does not require clock recovery (whether using PLLs or other methods of clock recovery), and may thus be applied to a wider variety of systems and applications, including low cost and / or low power systems, compared to known methods.

[0034] Additionally or alternatively, the identifying is further based on a previously identified symbol of the sequence of symbols encoded in the received AM signal. Additionally or alternatively, the previously identified symbol and the identified symbol (i.e. the symbol identified at item 230) are consecutive symbols in the sequence of symbols encoded in the received AM signal. For example, the previously identified symbol may immediately precede the identified symbol in the sequence. It may be known that a given symbol of the set of predefined symbols is necessarily followed or necessarily not followed by another given symbol of the set of predefined symbols. Accordingly, knowledge of the previously identified symbol may facilitate identification of the current symbol, e.g. by reducing the number of possible symbols that the current symbol might be. This improves the reliability and / or efficiency of symbol identification. Additionally or alternatively, the previously identified symbol is a known or fixed symbol.

[0035] Additionally or alternatively, the measured interval is a first interval, and the identifying is further based on a measurement of a previous interval between two successive low amplitude signal periods in the received AM signal. Additionally or alternatively, the previous interval and the first interval are consecutive intervals between pairs of low amplitude signal periods in the received AM signal. In other words, the previous interval and the first interval correspond to the durations of consecutive high amplitude signal periods in the received AM signal, since each high amplitude signal period is bounded on each side by a low amplitude signal period. Accordingly, the identifying of the symbol may be based on measurements of two consecutive intervals between pairs of low amplitude signal periods (corresponding to the durations of two consecutive high amplitude signal periods). The use of two consecutive intervals (a current interval and a previous interval) improves the accuracy and / or reliability of symbol identification. For example, some symbols or symbol combinations in the set of predefined symbols may not be uniquely identifiable based on a measurement of a single interval, but may be uniquely identifiable based on measurements of two consecutive intervals. Additionally or alternatively, each of the first interval and the previous interval is measured using a respective latch circuit. That is, two latch circuits may be used, each storing a counter for a respective interval.

[0036] Additionally or alternatively, the identifying is further based on a measurement of a further previous interval between two successive low amplitude signal periods in the received AM signal. Additionally or alternatively, the further previous interval and the previous interval (mentioned above) are consecutive intervals between pairs of low amplitude signal periods in the received AM signal. Accordingly, the identifying of the symbol may be based on measurements of three consecutive intervals between pairs of low amplitude signal periods (corresponding to the durations of three consecutive high amplitude signal periods). The use of three consecutive intervals (a current interval and two previous intervals) may guarantee that each symbol in the set of predefined symbols can be uniquely identified.

[0037] Additionally or alternatively, each of the first interval, the previous interval and the further previous interval is measured using a respective latch circuit. That is, three latch circuits may be used, each storing a counter value for a respective interval. The three counter values may then be used to identify a symbol. For example, each symbol in the set of predefined symbols may correspond to one or more different sets of three counter values (corresponding to three consecutive intervals between pairs of low amplitude signal periods), such that the three counter values together uniquely identify a symbol. The three latch circuits may together form a state machine.

[0038] A fourth step, illustrated by item 240, of method 200 comprises performing an action based on the identified symbol.

[0039] Additionally or alternatively, performing the action at item 240 comprises using the identified symbol to identify one or more further symbols encoded in the received AM signal. For example, it may be known that a given symbol is necessarily followed by a known other symbol in a sequence, and therefore the identified symbol may be used to identify subsequent symbols.

[0040] Additionally or alternatively, the sequence of symbols encoded in the received AM signal represents a command, and performing the action comprises determining the command. The command may be determined based on the identified symbol and optionally one or more previously identified symbols in the sequence. Additionally or alternatively, the command comprises an NFC command, such as an NFC-V command. The command may comprise an ISO / IEC 15693 command, i.e. a command from a set of commands defined by the ISO / IEC 15693 standard. Performing the action may comprise identifying the command from the set of predefined commands. The command may comprise a request and / or an instruction. Such a command may be generated by a transmitter device, such as an NFC reader device.

[0041] Additionally or alternatively, performing the action comprises generating a signal in response to the determined command. The signal may be transmitted from the receiver device 100 via the antenna 120, e.g. to an NFC reader device. Such a signal may comprise an identifier of the receiver device 100, a status of the receiver device 100, one or more parameters of the receiver device 100, etc. Additionally or alternatively, performing the action comprises multiple steps, e.g. determining a command represented by the received AM signal, generating a response to the determined command, and transmitting a signal comprising the response.

[0042] Additionally or alternatively, the method 200 comprises a step (not shown) of identifying, in the received AM signal, a predefined calibration sequence of symbols. The method 200 may also comprise a step (not shown) of determining a timing parameter of the received AM signal on the basis of the calibration sequence. In such cases, measuring the interval between the two successive low amplitude signal periods (performed at item 220) is based on the determined timing parameter of the received AM signal. Different AM signals may, in some cases, have different timing parameters, which may affect the measurement of the interval between the two successive low amplitude signal periods. By determining the timing parameter for a particular AM signal using a predefined calibration sequence of symbols, the interval may be measured more accurately and / or reliably.

[0043] Additionally or alternatively, each symbol in the encoded sequence of symbols comprises a plurality of bits, each bit having a fixed bit duration. For example, each symbol may comprise 8 bits, of which at least 1 bit is a low amplitude signal period. Additionally or alternatively, the determined timing parameter of the received AM signal comprises the fixed bit duration. A “bit duration” as disclosed herein refers to the duration of each bit in a given signal. A “bit duration” may also be referred to as a “bit length”. While the bit duration is fixed for a given signal (that is, each bit in a given signal has the same duration), the bit duration may vary between different signals. Accordingly, determining the timing parameter of the received AM signal may comprise determining the bit duration for that signal. The bit duration may correspond to the duration of a low amplitude signal period, since each low amplitude signal period may, in some examples, comprise 1 bit. Since different signals may use different bit durations, this may affect the measurement of the interval between successive low amplitude signal periods. For example, whether the measured interval is determined to be 5 bits or 6 bits long may depend on the duration of each bit. Therefore, by using a calibration sequence to determine the fixed bit duration for the received AM signal, the interval between successive low amplitude signal periods can be measured more accurately and / or reliably. This is described in more detail below. Additionally or alternatively, the timing parameter for the signal comprises a parameter other than the fixed bit duration for the signal. Additionally or alternatively, the bit duration does not vary between different signals.

[0044] Figure 3 shows schematically a receiver device 300 according to the present disclosure. The receiver device 300 may be similar to and / or have similar functionality to the receiver device 100 described above with reference to Figure 1. The receiver device 300 may be configured to perform at least some of the methods described above.

[0045] In the examples shown in Figure 3, the receiver device 300 receives an AM signal via an antenna 305. The AM signal comprises an RF signal. In this example, the AM signal is a 100% amplitude modulated signal and is received from a reader device (not shown). An RF demodulator 310 demodulates the received AM signal and extracts a data signal, in which low amplitude signal periods of the received AM signal are converted to pulses. A data counter 315 then counts (or measures) intervals between pulses in the demodulated data signal. That is, intervals between pairs of low amplitude signal periods in the received AM signal are measured. A symbol identifier 320 then converts the counter value obtained by the data counter 315 to a symbol, from a set of predefined symbols. The identified symbol is fed to a command interpreter 325 in the form of a 3-bit code that is unique to that symbol. The command interpreter 325 then determines, from a sequence of identified symbols, a command that is encoded in the received AM signal. The command interpreter 325 also queries a laser-programmed read-only memory, LPROM 330 to determine a response for the command. The response is then encoded into an ISO15693 frame and passed to a transmission encoder 335, which generates a signal based on the response. Manchester encoding may be used to encode the response signal. A load modulator 340 produces a load modulated RF response signal which is fed to the antenna 305 for transmission to the reader device.

[0046] The receiver device 300 may comprise more, fewer or different components than those shown in Figure 3. For example, each of the antenna 305, the RF demodulator 310, the command interpreter 320, the LPROM 330, the TX encoder 335 and the load modulator 340 may be omitted in some examples. The data counter 315 and the symbol identifier 320 may together perform a similar function to the electronic circuitry 110 described above with reference to Figures 1 and 2. It will be understood, however, that these two functional blocks in Figure 3 may be combined into a single block in other examples.

[0047] Figure 4 is a table showing a set 400 of predefined symbols according to the present disclosure. As shown in Figure 4, there are six symbols in the set 400 of predefined symbols: E, A, B, C, D and F. Different sequences of symbols from the set 400 may form different commands, and may be encoded in AM signals. Every command may start with the symbol E (also referred to as ‘Start of Frame’, SOF) and end with the symbol F (also referred to as ‘End of Frame’, EOF). In some cases, commands may comprise different arrangements of symbols that do not start with the symbol E and / or do not end with the symbol F. Additionally or alternatively, the set 400 of predefined symbols is defined according to an NFC-related standard, such as ISO15693. Different predefined symbols may be used in other examples.

[0048] Additionally or alternatively, each symbol is made up of 8 bits. A ‘bit’ as used herein refers to a signal period having a fixed duration, i.e. a bit duration. The bit duration may be between 6 microseconds and 12 microseconds. For example, the bit duration (i.e. the fixed duration of each bit in a signal) may be 9.4 microseconds. Each symbol comprises 7 high amplitude bits and 1 low amplitude bit, denoted by ‘P’. An exception is the symbol E, which comprises 2 low amplitude bits. In any case, every symbol includes at least one low amplitude bit. Each symbol in the set 400 of predefined symbols, when encoded, comprises an AM signal portion as set out in the table of Figure 4. For example, the symbol A is encodable as a signal portion comprising 1 high amplitude bit followed by 1 low amplitude bit followed by 6 high amplitude bits; the symbol B is encodable as a signal portion comprising 3 high amplitude bits followed by 1 low amplitude bit followed by 4 high amplitude bits, etc. The low amplitude bit (‘P’) is at a different position in the signal portion for different symbols. For example, P is at the second position (out of 8) for symbol A and is at the fourth position (out of 8) for symbol B.

[0049] Figure 5 is a table showing a set 500 of commands according to embodiments. The set 500 of commands comprises 13 commands, but it will be understood that there may be different numbers of possible commands in other embodiments. The commands may be defined according to an NFC-related standard, such as ISO / IEC 15693. Each of the commands may be sent by an NFC reader device to an NFC tag. Each of the commands comprises a sequence of symbols from the set 400 of predefined symbols described above with reference to Figure 4. Each command starts with the symbol E and ends with the symbol F. For each command, the symbol E is followed by the symbol C. This may be used to determine the bit duration for a given signal, as described in more detail below.

[0050] As mentioned above, each symbol in the set 400 of predefined symbols, when encoded, comprises an 8-bit signal portion, of which at least 1 bit is a low amplitude bit (‘P’). Figure 5 shows how such signal portions may be combined into an overall AM signal for different commands. For example, a command comprising a symbol sequence E-C-B-C-A-B-A-A-A-F may be encoded as: P-4.P.7-P-5-P.9-P-3.P.9.P.5-P-7-P-7.P.8-P-5-P, where each number denotes a high amplitude signal period formed of that number of bits. As can be seen from Figure 5, the longest interval between two low amplitude signal periods is 13 bits (e.g. where the symbol A is followed by the symbol D), and the shortest interval between two low amplitude signal periods is 1 bit (e.g. where the symbol D is followed by the symbol A). A single bit may be equivalent to 2.5 cycles of a 424 kHz clock where the bit duration is 6 microseconds, or 4 cycles of a 424 kHz clock where the bit duration is 9.4 microseconds.

[0051] Additionally or alternatively, a single measured interval between successive low amplitude signal periods may be sufficient to identify a symbol. For example, with reference to Figure 5, a single measured interval of 3 bits may correspond to only symbol A, a single measured interval of 11 bits may correspond to only symbol C, etc. However, in some cases a single measured interval may not be sufficient to uniquely identify each of the symbols in the set of predefined symbols. For example, with reference to Figure 5, an interval of 7 bits may indicate any of symbols A, B or C. Two measured intervals (the current interval and one previous interval) between pairs of low amplitude signal periods may be sufficient to uniquely identify most symbols. However, with reference to Figure 5, successive measured intervals of 7 and 9 bits could correspond to an A-B transition to identify symbol B or a B-C transition to identify symbol C. Therefore, three measured intervals (the current interval and two previous intervals) may be used, and may allow for a unique identification of every symbol.

[0052] Figure 6 shows a set 600 of the possible three-interval combinations of all of the commands shown in Figure 5, and the symbols that such combinations denote. As shown in Figure 6, each three-interval combination can uniquely identify a symbol. For example, the sequence 1 -7-7 can only denote symbol A, the sequence 3-7-9 can only denote symbol B, etc., where each number refers to the duration (in bits) of a respective interval between a pair of low amplitude signal periods.

[0053] An alternative to using three-interval combinations to deduce symbols is to consider a whole string of data bit groups as a pattern and to map these patterns to commands. For example, as shown in Figure 5, each command comprises either 56 high amplitude bits (also referred to as ‘data bits’) or 84 high amplitude bits, not counting the EOF symbol. The high amplitude bits between a pair of low amplitude signal periods may be referred to as a ‘data bit group’. For example, it can be seen that 84-bit commands have either a 3-9 or a 3-7 pattern after the initial 4-7 pattern, whereas 56-bit commands do not. In any case, however, intervals between successive low amplitude signal periods are measured and used to identify symbols (e.g. the symbols making up a command).

[0054] Accordingly, the presently-described methods provide a deduction algorithm for determining a signal encoding by timing the durations of high amplitude signal periods (i.e. intervals between low amplitude signal periods). There may be a limited number of symbol combinations which can comprise a specific signal encoding, e.g. commands, which allows for such a deduction. As consecutive symbols are deduced, this enables a full reader command to be identified.

[0055] Figure 7 shows example signal diagrams 700 for determining a fixed bit duration of a received AM signal, according to the present disclosure. Determining the fixed bit duration may be considered a calibration process, prior to identifying the symbols encoded in the received AM signal. The fixed bit duration is an example of a timing parameter of the received AM signal. The bit duration may vary between 6 microseconds and 14 microseconds for different signals. Additionally or alternatively, such a calibration process is not performed. For example, the bit duration for a given signal may already be known, and / or the encoded symbols may be identified without the bit duration having to be determined. Moreover, if a high frequency counter is used to measure intervals between low amplitude signal periods, a calibration step may not be needed. The bit duration may also be referred to as a “modulation width”, since a low amplitude signal period may have a duration of a single bit and may correspond to a pulse having a particular width in a demodulated signal obtained by demodulating the received AM signal.

[0056] The calibration process may be based on a predefined calibration sequence of symbols in the received AM signal. Additionally or alternatively, the predefined calibration sequence comprises the first two symbols of the received AM signal. In particular, it may be known that every encoded command begins with the sequence E-C, where E and C are symbols from the predefined set of symbols 400 described above with reference to Figure 4. This knowledge (i.e. the presence of a predefined calibration sequence) may be used to determine the bit duration for the received signal. The calibration sequence may comprise other symbols and / or other numbers of symbols.

[0057] As shown in the bottom row of Figure 7, variables x and c may be assigned to a counter state. It should be noted that the assigned variable c in the bottom row of Figure 7 is different from the symbol C described elsewhere (and shown in the second row in Figure 7). The assigned variable c denotes the number of counts (using a 848kHz clock in this example) per each full 106kHz clock. The assigned variable x denotes the number of counts from a falling edge of a DEMOD signal to the next ‘clean’ 106kHz clock period.

[0058] To find x and c, the number of counts between DEMOD toggles is summed. For the symbol E, this results in 3c + x, and for the symbol C it is 6c + x. Then, these values are subtracted from one another. That is, C - E = 6c + x- 3c- x = 3c. To find x, the result is subtracted from the symbol E, i.e. E - 3c = 3c + x-3c= x. To find 3c, instead of having a div / 3 implemented in hardware, a small lookup table can be used.

[0059] Once x and c are known, symbols can be identified despite possible changes in bit duration. In particular, once xand care known, and it is known where the calibration sequence E-C finishes, other symbols can be found by considering the distance from the current symbol to the next.

[0060] To support other possibilities for the calibration sequence such as E + A, E + B etc., the specific computation would be different, but it is still possible to perform the calibration process, since E being 3c + xis fixed and known.

[0061] Whilst the present disclosure has been described and illustrated with reference to particular embodiments or examples, it will be appreciated by those of ordinary skill in the art that the disclosure lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

[0062] Additionally or alternatively, the receiver device 100 comprises or is comprised in an NFC tag device. Additionally or alternatively, the receiver device 100 comprises or is comprised in an NFC reader device. For example, the receiver device 100 may comprise or be comprised in a mobile phone or other computing device capable of receiving NFC signals from an NFC tag device.

[0063] Additionally or alternatively, the received AM signal is a 100% amplitude modulated signal. Additionally or alternatively, the received AM signal is not a 100% amplitude modulated signal. That is, low amplitude signal periods in the received AM signal may have a non-zero amplitude. In this case, clock information in the signal may not be lost (unlike in the case of 100% modulation). However, the presently-disclosed methods may still be used to identify symbols and thus decode commands regardless of whether the clock information is lost. The presently-disclosed methods thus provide a versatile solution that may be employed for both 100% amplitude modulated and <100% amplitude modulated signals.

[0064] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as preferable, advantageous, convenient or the like are optional and 5 do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the disclosure, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A method of processing an amplitude modulated, AM, signal, the method comprising:receiving an AM signal comprising an encoded sequence of symbols, each symbol in the encoded sequence being from a set of predefined symbols, wherein each predefined symbol in the set of predefined symbols, when encoded, comprises an AM signal portion comprising one or more high amplitude signal periods and a low amplitude signal period, wherein the low amplitude signal period is in a different position in the AM signal portion for different predefined symbols in the set of predefined symbols; processing the received AM signal to measure an interval between two successive low amplitude signal periods in the received AM signal;based at least in part on the measured interval, identifying a symbol of the sequence of symbols encoded in the received AM signal; andperforming an action based on the identified symbol.

2. A method according to claim 1, wherein the received AM signal comprises a 100% amplitude modulated signal.

3. A method according to any preceding claim, wherein the received AM signal comprises an On-Off Keying, OOK, signal.

4. A method according to any preceding claim, wherein the identifying is further based on a previously identified symbol of the sequence of symbols encoded in the received AM signal.

5. A method according to claim 4, wherein the previously identified symbol and the identified symbol are consecutive symbols in the sequence of symbols encoded in the received AM signal.

6. A method according to any preceding claim, wherein the processing comprises measuring the interval using a latch circuit.

7. A method according to any preceding claim, wherein the measured interval is a first interval, and wherein the identifying is further based on a measurement of a previous interval between two successive low amplitude signal periods in the received AM signal.

8. A method according to claim 7, wherein the previous interval and the first interval are consecutive intervals between pairs of low amplitude signal periods in the received AM signal.

9. A method according to claim 7 or claim 8, wherein the identifying is further based on a measurement of a further previous interval between two successive low amplitude signal periods in the received AM signal.

10. A method according to claim 9, wherein the further previous interval and the previous interval are consecutive intervals between pairs of low amplitude signal periods in the received AM signal.

11. A method according to claim 9 or claim 10, wherein each of the first interval, the previous interval and further previous interval is measured using a respective latch circuit.

12. A method according to any preceding claim,wherein the sequence of symbols encoded in the received AM signal represents a command, andwherein performing the action comprises determining the command based on the identified symbol and one or more previously identified symbols in the sequence.

13. A method according to claim 12, wherein performing the action comprises generating a signal comprising a response to the determined command.

14. A method according to any preceding claim, wherein the received AM signal comprises a Near Field Communication, NFC, signal.

15. A method according to claim 14, wherein the AM signal is received from an NFC reader device.

16. A method according to any preceding claim, wherein the predefined symbols in the set of predefined symbols are defined according to a Near Field Communication, NFC, standard.

17. A method according to any preceding claim, comprising:identifying, in the received AM signal, a predefined calibration sequence of symbols; anddetermining a timing parameter of the received AM signal on the basis of the calibration sequence, wherein measuring the interval between the two successive low amplitude signal periods is based on the determined timing parameter of the received AM signal.

18. A method according to claim 17, wherein each symbol in the encoded sequence of symbols comprises a plurality of bits, each bit having a fixed bit duration, andwherein the determined timing parameter of the received AM signal comprises the fixed bit duration.

19. A receiver device comprising electronic circuitry configured to:receive an amplitude modulated, AM, signal comprising an encoded sequence of symbols, each symbol in the encoded sequence being from a set of predefined symbols,wherein each predefined symbol in the set of predefined symbols, when encoded, comprises an AM signal portion comprising one or more high amplitude signal periods and a low amplitude signal period, wherein the low amplitude signal period is in a different position in the AM signal portion for different predefined symbols in the set of predefined symbols;process the received AM signal to measure an interval between two successive low amplitude signal periods in the received AM signal;based at least in part on the measured interval, identify a symbol of the sequence of symbols encoded in the received AM signal; andperform an action based on the identified symbol.

20. A receiver device according to claim 19, wherein the receiver device comprises a Near Field Communication, NFC, device.

21. A receiver device according to claim 20, wherein the NFC device comprises an NFC tag device.

22. A receiver device according to claim 21, wherein the NFC tag device comprises an NFC Type 5 tag device.

23. A receiver device according to any of claims 19 to 22, wherein the receiver device comprises a flexible integrated circuit.

24. A receiver device according to any of claims 19 to 23, wherein the receiver device comprises an unpowered device.

25. A receiver device according to any of claims 19 to 24, wherein the receiver device comprises an antenna configured to receive the AM signal from a transmitter device.

26. A receiver device according to claim 25, wherein the transmitter device comprises a Near Field Communication, NFC, reader device, and wherein the received AM signal comprises an NFC signal.

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