Field of the Invention

The present invention relates to an ambient backscatter transmitter tag for backscattering and modulating an ambient FM signal and a receiver for demodulating and decoding the backscattered signal.

Background to the Invention

Microclimate monitoring systems and supply chain management systems require a large number of devices in order to monitor the status of a large number of individual plants or boxes respectively.

Accordingly, there is a demand for the design of low-cost and low-power wireless sensor nodes that are capable of conveying information relating to each of the plants or boxes to a central hub.

Traditional backscatter technology provides a form of communication by modulating reflections of an incident RF signal instead of generating RF waves, thereby reducing the power consumption of devices that utilise backscatter technology (e.g. RFID). However, current RFID tag technology is limited to a low range.

Therefore, an improved way of backscattering incident RF signals is required.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided an ambient backscatter circuit arranged to generate a varying circuit reflection coefficient, said ambient backscatter circuit comprising: a modulation member; a switching device in electrical communication with the modulation member; a first load having a first load reflection coefficient in communication with the switching device; a second load having a second load reflection coefficient in communication with the switching device; and an antenna terminal in communication with the switching device; wherein the modulation member comprises: a baseband processor; and a chirp spread spectrum generator; wherein the baseband processor is configured to generate a signal plan; wherein the chirp spread spectrum generator is configured to generate a modulation signal based on the frequency signal; and wherein the switching device is configured to switch, based on the modulation signal, between: the first load; and the second load such that the load reflection coefficient changes at the antenna terminal according to the modulation signal.

The term “reflection coefficient” will be understood by the skilled addressee as referring to a parameter describing the magnitude of a wave reflected by an impedance discontinuity.

The term “switching device” will be understood by the skilled addressee as referring to any device suitable for switching between the first load and the second load. Examples of said switching device include an RF switch or transistor.

The term “load” will be understood by the skilled addressee as referring to any device suitable for generating an impedance.

The term “baseband processor” will be understood by the skilled addressee as referring to any device suitable for converting input data into a digital signal.

The term “chirp spread spectrum generator” will be understood as a being a device suitable for converting digital signals into analogue chirp signals.

The term “chirp signal” will be understood by the skilled addressee as referring to a signal which changes in frequency as a function of time.

In preferable embodiments, the signal plan comprises: a symbol plan; a chirp bandwidth; a spreading factor; and an initial chirp frequency. Further preferably, the symbol plan comprises a plurality of symbols, wherein each symbol comprises information. In this way, the chirp spread spectrum generator may modulate the switching device according to parameters set by the signal plan. Preferably, the ambient backscatter circuit further comprises an antenna having an antenna reflection coefficient in electrical communication with the antenna terminal; wherein the antenna is configured to: receive an ambient signal; and reflect a portion of the ambient signal based on a reflection coefficient, the reflection coefficient being based on: the antenna reflection coefficient; and the load reflection coefficient, so as to superimpose a packet on to the ambient signal. In this way, the symbols may be superimposed on to the ambient signal in the form of chirp signals, thereby producing a backscattered ambient signal. Advantageously, information may be communicated from by the tag without requiring a dedicated transmitter or emitter. Further advantageously, a communication range may be improved.

In preferable embodiments, the ambient signal is an ambient frequency modulation (FM) signal. Further preferably, the ambient FM signal is a broadcast music signal transmitted from an FM station. In this way, the packet may be superimposed on to the FM signal transmitted by the FM station, thereby removing the need for a dedicated transmitter in the present invention. Advantageously, the present invention may have a lower power consumption, thus reducing the need for a battery. Further advantageously, the FM signals in combination with chirp spread spectrum modulation provides an increased range between the tag and the receiver.

It is also envisaged that the circuit could be arranged to operate with an ambient signal that is a Wi-Fi, Cellular or TV signal.

Preferably, the packet comprises a preamble and a payload. Further preferably, the preamble comprises preamble symbols. In this way, the preamble may indicate the start of the signal. Further preferably, the payload comprises payload symbols, wherein the payload symbols correspond to useful data. Advantageously, the packet may be more easily detected and useful data may more easily be received.

Preferably, the ambient backscatter circuit further comprises a ground plane in electrical communication with the switching device. In this way, the ambient signal may be more accurately reflected.

Preferably, the antenna is one selected from: a differential antenna, wherein the antenna is in further communication with the ground plane; a monopole antenna; a patch antenna; a grounded loop antenna. Advantageously, the antenna of the present invention may not be limited to a particular type.

Further embodiments exist wherein the antenna is any antenna suitable for reflecting the ambient signal.

Preferably, the modulation signal comprises chirp signals having a frequency range of 2 kHz to 200 kHz. In this way, the bandwidth of the chirp signal may overlap both mono and stereo audio signals.

Preferably, the chirp bandwidth is less than or equal to 200 kHz. Advantageously, the low frequency chirp signals may require less power to be generated. Further advantageously, the chirp signals may be generated using low frequencies.

In further embodiments, the modulation signal comprises chirp signals having a stereo frequency range of 23 kHz to 53 kHz of an FM channel band. In this way, the chirp signal frequency may be centred on a stereo audio frequency. Further preferably, the modulation signal further comprises a carrier tone. In this way, the signal may comprise an indication that the signal is within the stereo frequency range. Further preferably, the chirp bandwidth is less than or equal to 15 kHz. In this way, the chirp signals span a single stereo channel. Advantageously, a conventional FM stereo receiver may tune to a stereo mode corresponding to the chirp signals in order to receive the packet.

Preferably, the modulation signal is a square wave chirp signal. In this way, the circuit may require only two loads. Advantageously, the mechanical complexity of the circuit may be reduced and the average power consumption of the circuit may be reduced.

In further embodiments, the modulation signal is a sine wave chirp signal. In this way, the circuit may require more than two loads in order to generate sine wave chirp signals. Advantageously, the sine wave chirp signal may include fewer harmonics than the square wave chirp signal. Further advantageously, a receiver may therefore deal with a less noisy signal. Preferably, the ambient backscatter circuit further comprises a voltage level shifter in electrical communication with the modulation member and the switching device. In this way, if the modulator circuit cannot provide enough voltage for the switching device, the voltage level shifter may shift the modulator circuit voltage to a level high enough for the modulator circuit.

In accordance with a second aspect of the present invention, there is proved an ambient backscatter transmitter tag arranged to superimpose a packet on to an ambient signal, the tag comprising: a modulation member; a switching device in electrical communication with the modulation member; a first load having a first load reflection coefficient in communication with the switching device; a second load having a second load reflection coefficient in communication with the switching device; an antenna terminal in communication with the switching device; and an antenna having an antenna reflection coefficient in electrical communication with the antenna terminal; wherein the modulation member comprises: a baseband processor; and a chirp spread spectrum generator; wherein the baseband processor is configured to generate a signal plan; wherein the chirp spread spectrum generator is configured to generate a modulation signal based on the frequency signal; and wherein the switching device is configured to switch, based on the modulation signal, between: the first load; and the second load such that the load reflection coefficient changes at the antenna terminal according to the modulation signal; and wherein the antenna is configured to: receive an ambient signal; and reflect a portion of the ambient signal based on a reflection coefficient, the reflection coefficient being based on: the antenna reflection coefficient; and the load reflection coefficient, so as to superimpose a packet on to the ambient signal. In this way, an ambient FM signal may be reflected by the antenna and the modulation signal comprising the chirp signals may be superimposed on to the ambient FM signal such that useful information may be transmitted via the tag. Advantageously, the tag requires no battery source.

In accordance with a third aspect of the present invention, there is provided a demodulation system arranged to demodulate and decode an input signal to extract information, the demodulation system comprising: a baseband conversion member; a preamble detector circuit in electrical communication with the baseband conversion member; a synchronization circuit in electrical communication with the preamble detector circuit; a demodulation circuit in electrical communication with the synchronization circuit; and a decoding circuit in electrical communication with the demodulation member; wherein the baseband conversion member is configured to convert the input packet to a baseband packet; wherein the preamble detector circuit is configured to extract a preamble from the baseband packet; wherein the synchronization circuit is configured to determine an initial packet frequency based on the preamble; wherein the demodulation circuit is configured to extract a payload from the baseband packet; and wherein the decoding circuit is configured to decode the payload, thereby extracting information from the input signal.

In accordance with a fourth aspect of the present invention, there is provided an ambient backscatter receiver system arranged to receive, demodulate and decode a backscattered ambient signal, the receiver system comprising: a receiver; a receiver antenna in electrical communication with the receiver; and a demodulation system in electrical communication with the receiver; wherein the receiver antenna is configured to receive the backscattered ambient signal; and wherein the receiver is configured to convert the backscattered ambient signal into the input signal. In this way, a system is provided capable of: receiving a backscattered ambient signal comprising a packet as described in the first and second embodiments of the present invention; demodulating said ambient signal; and decoding the packet, thereby extracting useful data.

Preferably, the receiver system is arranged to determine and tune into FM signals comprising a strongest received power. Advantageously, the communication distance may be increased.

In some embodiments, the receiver is a software based receiver. Advantageously, the complexity of the receiver system may be reduced.

In further embodiments, the receiver is an FM stereo receiver comprising: a first output channel; a second output channel; and an interface circuit in communication with the first output channel and the second output channel; wherein the first output channel is configured to output a first signal; wherein the second output channel is configured to output a second signal; and wherein the interface circuit is configured to combine the first signal and the second signal in to packet merged input signal. Further preferably, the first output channel corresponds to a left portion of a received stereo signal and the second output channel corresponds to a right portion of the received stereo signal. In this way, left and right audio signals are converted in to the merged input signal.

Preferably, the receiver antenna is one selected from the range; a monopole antenna; a dipole antenna; a patch antenna; a grounded loop antenna; a loop antenna. Advantageously, the receiver antenna is not limited to a particular type of antenna.

In accordance with a fifth aspect of the present invention, there is provided a method for generating ambient backscatter signals, the method comprising: receiving, via a peripheral component, a data bitstream; generating, via a baseband processor, a signal plan; converting, via a chirp spread spectrum generator, the signal plan to a modulation signal; driving, via the chirp spread spectrum generator, a switching device; switching, via the switching device, between a first load and a second load; modifying, via the first load and the second load, a reflection coefficient; and reflecting, via an antenna, an ambient signal; wherein the signal plan is generated based on the data bitstream; wherein the switching device is modulated according to the modulation signal; and wherein reflecting the ambient signal superimposes a packet on to the ambient signal. In this way, the packet, which comprises important information received via the data bitstream, may be transmitted without needing a dedicated transmitter.

Preferably, the reflection coefficient is based on: a first load reflection coefficient; a second load reflection coefficient; and an antenna reflection coefficient. In this way, the magnitude of the ambient signal reflected may change depending on the load in electrical communication with the antenna at a particular time.

Preferably, the packet comprises a preamble and a payload. Further preferably, the packet comprises a plurality of symbols represented by chirp signals.

In accordance with a sixth aspect of the present invention, there is provide a method for receiving, demodulating and decoding a packet, the method comprising: receiving, via a receiver antenna, an ambient backscatter signal; converting, via a receiver circuit, the ambient backscatter signal to an input packet; converting, via a baseband conversion member, the input packet to a baseband signal; extracting, via a preamble detector circuit, a preamble from the ambient backscatter signal; determining, via a synchronization circuit, an initial chirp frequency based on the preamble; extracting, via a demodulation circuit, a payload from the ambient backscatter signal; and converting, via a decoding circuit, the payload into bits, thereby converting the ambient backscatter signal into useful information.

Preferably, converting the ambient backscatter signal to an input signal further comprises: outputting, via a first output channel of an FM stereo receiver, a first signal; outputting, via a second output channel of the FM stereo receiver, a second signal; and combining, via an interface circuit, the first signal and the second signal into the input signal.

Detailed Description

Specific embodiments will now be described by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic view of an ambient FM signal backscattering tag in accordance with the second aspect of the present invention;

FIG. 2 shows a schematic view of a method for backscattering an ambient signal according to the fifth aspect of the present invention;

FIG. 3 shows a square wave chirp signal in the time domain;

FIG. 4 shows a backscattered ambient signal in a stereo frequency range;

FIG. 5 shows an example result showing the frequency components of a backscattered ambient signal;

FIG. 6A shows a frequency spectrum of a frequency up-chirp signal and the frequency spectrum of a modulated chirp;

FIG. 6B shows a frequency spectrum of a conjugate version of the up-chirp depicted in FIG. 6A; FIG. 6C shows a frequency spectrum of the up-chirp conjugate of FIG. 6B multiplied by the up-chirp of FIG. 6A;

FIG. 7A shows a schematic view of an ambient backscatter receiver system comprising a conventional stereo receiver in accordance with the fourth aspect of the present invention;

FIG. 7B shows a schematic view of an ambient backscatter receiver system comprising a software based receiver in accordance with the fourth aspect of the present invention;

FIG. 7C shows a schematic view of a mobile smartphone comprising the ambient backscatter receiver system of FIG.7A;

FIG. 8 shows a schematic view of an ambient FM signal backscattering system; and

FIG. 9 shows a schematic view of a method for demodulating and decoding a received signal in accordance with the sixth aspect of the present invention.

With reference to FIG. 1, there is shown a schematic view of an ambient FM signal backscattering tag 100 according to the second aspect of the present invention.

The tag 100 comprises: a modulation member 102; a switch 104; a voltage level shifter 105; a first load 106; a second load 108; and an antenna 122. The tag 100 is mounted on a substrate (not shown).

The modulation member 102 is in electrical communication with a peripheral component 103 and, via signal output 117, the voltage level shifter 105. The voltage level shifter 105 is in further electrical communication with a control signal 119 of the switch 104. The switch 104 is configured to toggle between electrical communication with the first load 106 and the second load 108. Further, the switch 104 is in electrical communication with the antenna 122. The antenna 122 is in further electrical communication with the ground plane 112. The modulation member 102 comprises a baseband processor (BBP) 110 and a chirp spread spectrum generator (CSSG) 112. The first load 106 comprises a first reflection coefficient and a first impedance, the second load 108 comprises a second reflection coefficient and a second impedance, and the antenna 122 comprises an antenna reflection coefficient and an antenna impedance.

The following in use method steps are envisaged with reference to FIG. 2.

At step 202, the BBP 110 receives a bitstream based on a payload of useful information received from the peripheral component 103.

At step 204, the BBP 110 encodes a digital signal plan comprising: a carrier tone;; a symbol plan based on the bitstream; a spreading factor (SF); a bandwidth (BW); an error correction code rate (R) and an initial chirp frequency (Fstart). The BBP 110 then transmits the signal plan to the CSSG 112.

In the present example the BW is 10 kHz and the Fstart is 40 kHz. Accordingly, the payload is encoded on to a stereo FM signal, as depicted on FIG. 4. FIG. 4 shows a chirp signal 404 in a stereo frequency range (23 kHz to 53 kHz) comprising shows a left portion 402 and a right portion 404. FIG. 4 also shows a mono frequency range 405 and a carrier tone 401.

The payload is encoded in code words by the BBP 110 using a Hamming (7,4) code. R is equal to a total number of bits in a code word (i.e. the code word length) divided by the Hamming block length. Further, the payload is scrambled into new code words using an interleaving operation and the code words are encoded according to a Gray code into Gray code words.

At step 206, the CSSG 112 generates a modulation signal based on the signal plan, comprising a preamble and the payload represented by a plurality of square wave chirp spread spectrum signals, with a 50% duty cycle, as a function of the signal plan. The modulation signal also comprises a carrier tone of 19 kHz. The modulation signal is a real signal. The modulation signal is transmitted, via the signal output 117, to the voltage level shifter 105. A square wave chirp signal is envisaged in FIG. 3, which depicts a square wave chirp signal over a time period T comprising an amplitude A in the time domain.

The chirp spread spectrum signals generated by the CSSG 112 comprise a plurality of chirp signals, each of which comprises multiple bits represented by a respective symbol defined by the BBP 110.

The chirp is defined as a sinusoidal signal of frequency increase or decrease over time. Applying a Fast Fourier Transform (FFT) to the chirp signal, leads to a time delay in the chirp signal translating to a frequency shift at the output of the FFT. The symbols are therefore encoded as cyclic time shifts in the chirp signals, represented in frequency shifts at the output of the FFT. A chirp signal comprising a frequency that increases linearly with time is called an up-chirp and is depicted in FIG. 6A as up-chirp 602. A chirp signal comprising a frequency that decreases linearly with time is called a down-chirp and is depicted in FIG. 6B as down-chirp 610.

In the present invention, the BW, the SF and the Fstart are used to define the chirp signals. The chirp signal comprises a frequency sweep equal to the BW. The frequency of the up-chirp increases between the Fstart and the Fstart + BW. The frequency of the down-chirp decreases between the Fstart and the Fstart + BW. Each symbol can be a cyclic shifted version of the up-chirp or the down-chirp. The SF represents a number of bits of the bitstream received by the BBP 110 that can be encoded in a symbol.

A further quantity 2 ^SF , represents the number of possible chirp frequencies, wherein the symbols can be cyclically shifted from 1 to 2 ^SF positions. In the present example, the SF is 7. Accordingly, each chirp signal can encode 7 bits and has 128 cyclic shifts. The chirp signal can also consist of 128 different frequencies.

The peripheral component samples the useful information at a sampling rate of define by the Nyquist Rate, 1/BW samples per second. The length of one symbol Tc is defined as 2 ^SF /BW seconds. The symbol rate is defined as 1/Tc and therefore, the bit rate of the modulation signal is defined as Tc*SF. Accordingly, the bit rate is modified by changing the BW or the SF. In the modulation signal, the payload is represented as a series of symbols, which represent the useful information, in the form of an arbitrarily long sequence of cyclic shifted chirps.

In addition to the payload, the preamble is comprised in the symbol plan. The preamble is transmitted before the payload and comprises: a detection sequence of between two and twenty unmodulated up-chirp or down-chirps; an initialisation sequence of two to four shifted cyclic chirps; and one or more frequency synchronization symbols. The synchronization symbols are conjugate versions of the preamble chirp sequence.

At step 208, the voltage level shifter 105 shifts a voltage of the modulation signal to a voltage level above a voltage threshold if the voltage of the modulation signal does not meet the voltage threshold.

At step 210, the voltage level shifter 105 transmits the modulation signal to switch 104 via the control signal 119.

At step 212, the modulation signal toggles the switch 104 between the first load 106 and the second load 108 at an instantaneous frequency (F _tag ) of the modulation signal. Toggling between the first load 106 and the second load 108 in turn toggles the load 106, 108 which is connected to the antenna 122 via the switch 104.

At step 214, an ambient FM signal 151 is modulated by toggling between the loads 106, 108, since a radar cross section of the antenna is modulated to reflect or absorb an ambient FM signal 151, thereby superimposing a packet of information on top of the ambient FM signal 151 and producing a backscattered ambient FM signal 154.

The antenna reflection coefficient T _j of the antenna 122 when connected to the load i 106, 108 is represented by the following equation: with Z _j denoting the impedance of the respective load 106, 108, Z _a denoting the impedance of the antenna 122 and Z _a ^* denoting the complex conjugate of the antenna impedance. Accordingly, the ambient FM signal is modulated by switching between the first load 106 and the second load 108, resulting in two reflection coefficients, G and G ₂ over time. In the present example, in the G case, the antenna 122 reflects a portion of the ambient FM signal 151 and in the G ₂ case, the antenna 122 absorbs all of the power in the FM signal 151 , thereby causing no reflection to occur.

With reference to FIG. 5, an example frequency spectrum 500 of the backscattered ambient signal 154 is shown.

The ambient FM signal 151 incident upon the antenna 122 comprises a central frequency F _c .

The backscattered ambient signal 154 also comprises the central frequency 502 at frequency F _c . In addition, the instantaneous frequency of the switching device causes subcarriers 504, 506 to appear in the spectrum 500 with frequencies of F _c + F _tag and F _c - F _tag , respectively. Harmonics of the subcarriers 504, 506 may appear in the spectrum 500. In the case that the modulation signal is a square wave with a 50% duty cycle, the spectrum 500 will include odd order harmonics 503.

With reference to FIG. 7A, a schematic view of an ambient backscatter receiver system 700 is shown, comprising a conventional stereo receiver 702 according to the fourth aspect of the present invention.

The receiver system 700 further comprises: a receiver antenna 704; and a receiver circuit 706, both of which are mounted on a substrate (not shown).

The receiver circuit 706 comprises: the conventional stereo receiver 702 in electrical communication with the receiver antenna 704; an interface circuit 710 in electrical communication with the conventional stereo receiver 702; and a decoding and demodulation circuit 712 in electrical communication with the interface circuit 710.

The decoding and demodulation circuit 712 comprises: a multiplier circuit 714; a resampling circuit 716 in electrical communication with the multiplier circuit 714; a preamble detection circuit 718 in electrical communication with the resampling circuit 716; a synchronization circuit 720 in electrical communication with the preamble detection circuit 718; a demodulation circuit 722 in electrical communication with the synchronization circuit 720; and a decoding circuit 724 in electrical communication with the demodulation circuit 712.

The following in use method steps are envisaged with reference to FIG. 8 and FIG. 9.

FIG. 8 depicts a schematic view of an ambient FM signal backscattering system 800 comprising the tag 100, the receiver 700 and an FM station transmitter 802 wherein the ambient FM signal 151 is transmitted by the FM station transmitter 802

At step 902, the conventional stereo receiver 702 receives the backscattered ambient signal 154 and the ambient FM signal 151 via the antenna 704. The carrier tone indicates that the backscattered ambient signal 154 is a stereo signal.

At step 904, the interface circuit subtracts the left portion 402 from the right portion 404 of the backscattered ambient and outputs a resulting signal 730 to the decoding and demodulation circuit 712.

At step 906, the multiplier circuit 714 and the resampling circuit 716 down-convert the resulting signal 730 to the baseband frequency of the packet wherein the resampling circuit 716 applies a resampling frequency equal to the bandwidth, BW of the chirp signal. The down-converted signal is transmitted to the preamble detection circuit 718 in the form of the packet.

At step 908, the preamble detection circuit 718 detects the detection sequence of the preamble, thereby indicating the initial chirp frequency of the packet.

At step 910, the synchronization circuit 720 drops any packets comprising frequency synchronization symbols that do not match a pre-configured set of synchronization symbols. Provided the packet comprises a frequency synchronization matching the pre-configured set, the packet is transmitted to the demodulation circuit 722. At step 912, the demodulation circuit 722 multiplies the packet by a locally generated chirp. The locally generated chirp is the conjugate version of the unmodulated chirp contained in the preamble and comprises the same SF and BW as the unmodulated chirp.

An example unmodulated up-chirp is illustrated on FIG. 6A. FIG. 6A comprises an up- chirp 602 ad a cyclic shifted chirp 604. An example conjugate version of the unmodulated chirp is illustrated on FIG. 6B. FIG. 6B comprises a first down-chirp 610 and a second down-chirp 612, both being conjugate versions of the up-chirp 602.

At step 914, the demodulation circuit 722 applies a Fast Fourier Transform to the multiplied signal. After multiplication, each symbol resides in on a unique constant frequency and the result is a series of discrete samples corresponding to the time delay in the received chirp. An example of the multiplied signal can be seen in FIG. 6C, which shows a first stable frequency signal 632 and a second stable frequency signal 634. Each symbol resides on a unique stable frequency signal 632, 634.

At step 916, the demodulation circuit 722 normalizes the symbols of the payload relative to the bins of the preamble symbols, thereby producing a correct symbol value.

At step 918, the demodulation circuit 722 converts the correct symbol value to a bit stream.

At step 920, the decoding circuit 724 decodes the bit stream by performing Gray decoding, error detection, error correction and de-interleaving on the bit stream. The decoding circuit 724 then transmits the decoded bit stream for further processing.

With reference to FIG. 7B, a schematic view of an ambient backscatter receiver system 750 is shown, comprising a software based receiver 752 in accordance with the fourth aspect of the present invention is shown.

The receiver system 750 is substantially similar to the receiver system 700 but comprises the software based receiver 752 instead of the conventional stereo receiver 702 and the receiver system 750 does not comprise the interface circuit 710. In use, the receiver system 750 carries out a method substantially similar to the method steps 902 to 920. However, instead of the subtraction 904 step, the software based receiver 752 provides complex samples comprising real and imaginary components to the decoding and demodulation circuit 712.

The software based receiver 752 is tuned to the appropriate FM band automatically by searching for the most powerful FM station in the FM band.

Turning now to FIG. 7C, a mobile smartphone 770 is envisaged comprising the receiver system 700.

The mobile smartphone 770 further comprises a receiver antenna 704. In the present example, wired headphones 704 are used to receive FM radio signals.

In use, a user of the mobile smartphone 770 stands near the tag 100 and the receiver antenna 772 receives both the ambient FM signal 151 and the backscattered ambient FM signal 154.

The receiver system 770 carries out method steps 902 to 920, thereby demodulating and decoding the backscattered ambient FM signal 154.

The above CCSG has been described as producing a square wave chirp spread spectrum signal. The skilled addressee will understand that the CSSG may produce a sinewave chirp spread spectrum signal. In the case that sinewaves are produced, the CSSG may comprise a lower power digital to analogue converter (DAC) in order to convert the digital signal plan to an analogue modulation signal.

The above chirp signals have been described in relation to a stereo mode. The skilled addressee will understand that a mono mode may be used instead. In the case that a mono mode is used, the subtract step is instead an addition step.

The above tag has been described as comprising a transistor. The skilled addressee will understand that a transistor may be used in place of the switch. The above tag has further been described as comprising two loads. The skilled addressee will understand that any number of loads may be used, resulting in any number of corresponding antenna reflection coefficients. The above payload has been described as being encoded using a Hamming code. The skilled addressee will understand that any error correction and detection code may be used in order to detect and correct a number of errors in transmitted data without the need for retransmission.