A device and method for electronic communication or sensing through dissipative media using resonant magnetic coupling

Abstract

The present Invention relates to methods and devices for sending and receiving electrical signals through "dissipative" media such as soil, mud, or sea / fresh water to distances extending to several tens of meters. Present methods for sending through such media use very high-power levels ranging to Mega-Watts and employ large equipment. Very Low Frequency (VLF) and Extremely Low Frequency (ELF) communication systems used to communicate with submarines are an example. In contrast, methods and devices disclosed here are powered by general purpose batteries (such as Lithium-Ion batteries) and can be used with consumable items as small as a mobile phone. The invention achieves this functionality by combining near field resonant magnetic coupling with symbol stretching at the transmitting end and matched filtering at the receiving end. While the range of near field magnetic coupling is limited to approximately 1' (30 cm), the combination disclosed in the invention enables to send / receive signals extending to several tens of meters. This new technology (the combination) opens several new applications. At present, there is no systematic methodology to detect victims of natural or man-made disasters such as landslides, earthquakes, and disasters where the victims are buried in media such as soil or mud. The invention can be used to add functionality to consumer devices such as mobile phones or similar so that they can be easily located. Alternatively, a novel device one can easily carry (smaller that the size of a mobile phone) can be introduced, which would be very useful for communities living in countries prone to natural disasters. Scanning for victims can be done fast by mounting the search device on a drone. The methods and devices disclosed can also be used for underwater communication between diver to diver and diver to vessel and the technology is especially suitable for freshwater applications. In addition, this technology can also be used for communication in mines in emergency situations. It is suggested to use in emergency situations due to low data rates.

Description

[0001] A DEVICE AND METHOD FOR ELECTRONIC COMMUNICATION OR SENSING THROUGH DISSIPATIVE MEDIA USING RESONANT MAGNETIC COUPLING

[0002] 1. FIELD OF THE INVENTION

[0003] The methods and devices disclosed here (i.e., the “Invention”) relates to near-field, wireless, electronic communication devices and methods for sending and receiving electrical signals through “dissipative” (i.e., characterized by the dissipation of electromagnetic energy) media such as soil, mud, or sea / fresh water. The Invention also relates to methods and devices that exploit wireless resonant magnetic coupling of spatially separated coil systems, with each coil connected to a capacitor in series or parallel.

[0004] The Invention utilizes resonant magnetic coupling combined with matched filtering to overcome severe energy losses normally observed when sending / receiving signals through a dissipative media using electromagnetic waves. Matched filtering is a linear technique that maximizes the signal-to-noise ratio (SNR) of a signal that is embedded in noise.

[0005] The Invention also relates to Low Frequency (LF) Radio Frequency Identification (RFID) technology. A typical RFID system includes a “tag” comprising a tiny radio transponder, radio receiver, and transmitter. When triggered by an electromagnetic interrogation pulse from a nearby RFID reader device, the tag transmits a signal (i.e., digital data) back to the reader.

[0006] Passive tags are powered by energy from the RFID reader’s interrogating radio waves. Passive tags utilize energy harvested from the environment and are limited to very small transmission distances. Active tags are powered by a battery and thereby transmit greater distances and thus can be read at a greater range from the RFID reader - up to hundreds of meters in air. Only Ultra High Frequency (UHF) RFIDs can transmit at this large a distance. However, UHF RFIDS are not useful for the dissipative media applications of the Invention due to the heavy signal loss when UHF signals propagate through dissipative media like soil or water.

[0007] The Invention employs LF RFID technology, with some unique additions (introduced in this disclosure) to communicate through a conductive but energy-dissipative dissipative medium over a distance of several tens of meters.

[0008] The Invention utilizes weakly-coupled, resonant magnetic coupling in a “Near Field”. The Near Field (as opposed to Far Field) is the region close to a radiating body such as an antenna dominated by either electric or magnetic mutual coupling. The invention is on a connectivity mode that enables short-range wireless connectivity / communication between devices keeping equipment size small and transmitted power levels low. Near Field connections enable communication (i.e., Near Field Communication or “NFC”) between two electronic devices over a typical distance of around 30 centimeters (12 in) or less in air.

[0009] The applications of the Invention include detecting buried consumer devices (e.g., mobile phones, laptop computers), underwater communication, and emergency communication in mining. The term “communication” here refers to both analog and digital data transfer. The methods introduced are based on resonant magnetic coupling.

[0010] Radio Frequency Identification (RFID) technologies use three frequency bands - Low Frequency (LF), High Frequency (HF) and Ultra High Frequency (UHF). Although RFID technology is typically used in through-air applications, in recent years it has been applied in underwater communications. In the Invention, Near Field resonant magnetic coupling combined with matched filtering counters energy losses due to eddy currents, while keeping equipment size small, and the transmitted power levels low. The technical field of the Invention can also be classified as NFC through a dissipative conductive medium.

[0011] The Invention relates to electromagnetic resonators. Such resonators may be coil-based and connected in series or parallel with a capacitor. Radio signal reception with coil-based electromagnetic resonators (i.e., a coil connected to a capacitor) is not a new concept. Such coils connected to capacitors are sometimes referred to as “magnetic antennas” and used for AM (Amplitude Modulation) radio reception. However, in AM reception applications the coils are used in the “Far Field” - a mode of radio-frequency propagation in which the E and M components of an EM (electromagnetic) field are in-phase, and the radiation mode of energy dominates. This in-phase E to M propagation wave relation occurs at a far distance from the transmitter and the coil at the receiver should be placed such that the edge of the coil faces the direction of the incident wave. In contrast, the Invention utilizes axially coupled coils in the Near Field and therefore the coils should be placed such that their axes are in line.

[0012] The Invention also relates to Wireless Power Transfer (WPT) technology. WPT is based on electromagnetic coupling that occurs when the electromagnetic field (E or H -field component) produced by an electric circuit, such as a coil interacts with another circuit element (e.g., another coil ), and this interaction results in the transfer of electromagnetic energy between the two elements (coils). In the Invention, electromagnetic coupling occurs when the electromagnetic field produced by the coil at one end of the system interacts with the coil at the other end. However, the mode of operation of the Invention is fundamentally different from a typical WPT application since the Invention operates in the weakly-coupled region (coupling factor « 1) and the objective is detecting the induced voltage, whereas WPT applications operate in the strongly coupled region (coupling factor >1) and the objective is transfer of power - i.e., wattage.

[0013] 2. BACKGROUND OF THE INVENTION

[0014] There is a need for methods and devices that can electronically and wirelessly transmit and communicate for relatively short distances through dissipative media. For example, with such capability, persons and devices could be located when buried under mud and debris created by catastrophic land / mudslides or earthquakes. Also, short-distance electronic communication through sea and fresh water could be made more effective.

[0015] Wireless Power Transmission (WPT) using resonant magnetic coupling is a relatively new technology area. These methods and devices transfer electrical power, where the essential factor is wattage, whereas in the invention the focus is on electric signal transmission in which voltage variation at the receiver is the key. Also, methods of wireless electrical power transfer operate in the strongly coupled (coupling coefficient >1) regime, as opposed to the weak coupling (coupling coefficient <1) for signal transfer of the Invention.

[0016] In contrast to the optimization of power transfer in WPT, the Invention optimizes the voltage induced in the Rx coil with respect to the current supplied to the Tx coil (i.e., “trans-resi stance” is optimized. In the Invention trans-resi stance is defined as the ratio between the voltage sensed at the receiver coil and the current in the transmitter coil.

[0017] Currently, there are some methods and devices that approximate wireless signal communication similar to the Invention, but each suffers from technical limitations that inhibit effectiveness - limitations that are alleviated by the inventive features of the Invention. Such similar wireless signal transmission methods include RFID (radio frequency identification) systems, and underwater-RFID. These methods are limited because they do not utilize matched filter processing of long duration signals (explained later).

[0018] In typical RFID applications, the “tag” has either an antenna (if Ultra High Frequency “UHF”) or a coil (if Low Frequency “LF” or Very Low Frequency “VLF”), a chip containing data, and a device for harvesting power from the environment rather than a battery. In contrast, the Invention utilizes battery power at either or both transmitting “Tx” and receiving “Rx” ends. There is a long history of technological efforts to send electronic signals through dissipative media, particularly through seawater. For example, VLF and Extremely Low Frequency (“ELF”) methods and apparatus used in the US Navy’s “Project Sanguine”. These applications feed mega- Watts of high wattage power at radio frequencies to large / bulky antennas. These large antennas contrast with the relatively small resonant coils of the Invention which are fed by small currents on the order of milli-amperes. So, although the Invention and these historical methods both employ low frequencies, the Invention and underwater RFID are unique in their use of low power and resonant coils rather than high power and antennas.

[0019] Comparing the Invention to the prior art, the most similar technology is LF RFID. However, the transmission distance range of state-of-the-art LF RFID technology is limited to only 30 cm in air ( https: / / en.wikipedia.org / wiki / Radio-frequency_identification) whereas the Invention has a

[0020]

[0021] range of tens of meters in a dissipative media. Currently, LF RFIDs are widely used for applications like scanning credit cards, passports etc. In typical in-air applications, matched filtering processing is not used to increase the range. If distance range increase is required, either HF RFID or UHF RFID technologies are used. But these latter two technologies cannot be used in dissipative media (i.e., in the application of the Invention) due to large propagation loss at these frequencies. So, the Invention is a way to increase the range by combining resonant coils and matched filtering. Matched filtering is a method used in the 1940s to increase the range of microwave radars. In radar applications stretching the pulses in time and applying a matched filter at the receiver was later abandoned and replaced by chirp-pulse compression. The Invention utilizes this abandoned approach in a novel application to increase the range. Current applications of LF RFID technology (like scanning credit cards or passports) do not necessitate matched filtering of long duration signals as they are meant to be short range applications.

[0022] Matched filtering is a technique widely used at present in many fields like radar and communications for maximizing signal strength with respect to noise. There are also references in the prior art to the use of matched filtering with RFIDs. However, unlike the prior art, the Invention stretches the width of the transmitted pulse to increase its energy and then applies the matched filter to increase the received signal strength and thereby increase the range.

[0023] Stretching the transmitted signals in an RFID-like method is not used or described in the prior art, which is evident from the fact that the range of LF RFIDs in the prior art is limited to a maximum of 30 cm whereas the range of the Invention is larger by orders of magnitude. There is no technology at present to send / receive electromagnetic signals ranging in the tens of meters through a dissipative medium. The present Invention is directed to overcoming these and other deficiencies in the art.

[0024] 3 SUMMARY OF THE INVENTION

[0025] The Invention combines magnetic resonant coupling with symbol-stretching in the time domain, including embodiments of matched filtering, frequency hop spread spectrum (FHSS) or chirp spread spectrum (CSS) to overcome severe losses experienced in sending / receiving signals through a dissipative media using electromagnetic waves.

[0026] The Invention is a Near-Field communication device and a method to send / receive electrical signals / messages through “dissipative” (i.e., characterized by the dissipation of electrical / electromagnetic energy) media such as soil or sea / fresh water. Signals received in dissipative media applications are very weak and could be below the noise floor. These weak signals can be enhanced by stretching the signal in the time domain, thereby increasing the signal energy. The signal stretched in the time domain is processed with a matched filter at the Rx receiver to counter the energy losses due to eddy currents in a dissipative medium while keeping the equipment size small and transmitted power low. Since any matched filter can be implemented as a correlation between the signal and the mirror image of the conjugate of the signal, the matched filter can also be realized as a correlation process.

[0027] We present four different embodiments of the Invention, resonant coils and symbol stretching in the time-domain combined with: 1) matched filtering at a single fixed frequency; or 2) with matched filtering at multiple fixed frequencies; or 3) with frequency hop spread spectrum (FHSS) techniques, or 4) with chirp spread spectrum (CSS) techniques. The preferred embodiments are 1, 2, and 3 as the practical implementation of the 4thembodiment is challenging at present with current technology. These embodiments are described in detail in the next section (DETAILED DESCRIPTION OF THE INVENTION).

[0028] In summary, there is no method or device in the prior art useful to send / receive signals ranging to tens of meters through a dissipative medium using low power electromagnetic signals. The closest application is a UHF safety device developed for applications in snow. But this device cannot be used in the applications covered by the Invention, as the signals at UHF experience heavy losses when propagated through media like soil or mud. The key inventive feature of the invention is the combination of resonant magnetic coupling and symbol -stretching in the time-domain in the near field with matched filtering of long duration signals. In addition, in contrast to optimizing power transfer, the invention functions by optimizing the voltage induced in the receiving (“Rx”) coil with respect to the current supplied to the transmitting (“Tx”) coil (i.e., by optimizing trans-resi stance). Moreover, in embodiments that use multiple resonant frequencies, we also use a circuit topology that greatly minimizes equipment size.

[0029] 4. DETAILED DESCRIPTION OF THE INVENTION

[0030] In the Invention, symbol-stretching in the time-domain combined with matched filtering plays the key role. Therefore, matched filtering is described below, followed by symbol-stretching. The concept of optimizing trans-resi stance as opposed to power transfer efficiency and its relevance to the invention is also described. This is followed by a description of the four embodiments of the invention, namely magnetic resonant coupling combined with: 1) matched filtering at a single fixed resonant frequency; 2) matched filtering at multiple fixed resonant frequencies; 3) matched filtering with frequency hop spread spectrum (FHSS); and 4) matched filtering with chirp spread spectrum (CSS).

[0031] 4.1 Matched filter

[0032] Signals encountered in communication systems are generally weak and corrupted by noise. In the case of the Invention a replica of the signal is available at the receiver, and as a result, even very weak signals can be detected with optimal results using what is called a matched filter. It is well established theory that for signals buried in so called white noise (noise with a flat spectrum), the impulse response of the optimal filter (in this case, the matched filter) is given by:

[0033] h_opt(t) = K p(t_d − t)

[0034] is the transmitted signal, K is an arbitrary constant, and is a time delay (see

[0035]

[0036] Fig. 1). As shown in the figure, the input signal p t — has a limited time duration and t0is

[0037]

[0038] the starting time. It follows from theory that y(t) to root mean square (r.m.s.) value of noise ratio maximizes at time t_0 + t_d. Here, t_d is an arbitrary time delay added to make the impulse response of the filter causal. Moreover, the value of the output increases with the energy in the signal. Generally, the energy of a signal can be increased by increasing its duration (i.e., by stretching the pulse), if a replica of the transmitted signal is available at the receiver. We use this approach in the Invention to detect very weak signals. In addition to improving the ability to detect weak signals, increasing the time duration has another advantage in the Invention. Increasing the duration of the signal narrows the bandwidth and this is ideal in the invention as it utilizes “impulse like” narrow bandwidths (see Fig. 4).

[0039] In most applications, increasing the duration of a signal is avoided as it results in slow signal transmission speeds, for example, slow data rates in digital communication. However, matched filters are widely used in current applications due to another reason, and this is explained next under the topic “Realization of the matched filter as a correlation”.

[0040]

[0041] (b) Impulse response of the matched filter

[0042] Fig. 1: Matched filtering and the impulse response of a matched filter.

[0043] 4.2 Symbol Stretching in the Time-Domain

[0044] It is well known that the output of a matched filter is proportional to the energy in the input signal. The energy of a signal can be increased by stretching the durations where the signal is not zero. For example, consider the symbol illustrated in Fig 3 which is a sinewave modulated pulse. In this case, the energy in the symbol can be increased by stretching the pulse (duration where the signal is not zero), that is by increasing the width r (see Fig. 3).

[0045] 4.2.1 Realization of the Matched Filter as a Correlation

[0046] clt follows from linear systems theory, that the matched filtered output y (t) is given by the convolution integral as shown below. y(t) = ∫r(τ)h_opt(t − τ)dτ

[0047] where r(t) is the received signal, i.e., Ap(t) + n(t) (A is the attenuation through the channel), h(t) is the impulse response and, n(t) is noise. With some manipulation, the above result can also be expressed in the following form which is more suitable for practical implementation.

[0048] y(t) = ∫r(t)p(t)dt

[0049] o

[0050] The above is simply the cross-correlation between the received signal r(t) and a replica of the transmitted signal, p(t). If we consider a digital communication signal for example, several bits to be transmitted are grouped and each group is represented by a so called “symbol”. So, what we transmit as signals are these symbols. The correlation integral above is a measure of similarity between the received symbol r(t) and the replica p(t) which represents a particular symbol. If the modulation method uses M number of symbols, r(t) is correlated against all possible replicas (pᵢ(t) where i = 1, 2, ···M) and the one that matches best (i.e., one that produces the highest output y(t)) is decided as the transmitted symbol. In most applications, matched filtering or its implementation as a correlation is used to identify symbols this way. Generally, matched filtering is not used to increase signal strength as it adversely affects the data rate. However, in the Invention our primary concern is not the data rate but detectability. Therefore, we stretch the duration of a symbol with the objective of increasing the energy of a symbol. This is exactly the approach used in early days (dating back to 1940s) of microwave radar. This approach was later abandoned and replaced by chirped FM pulse compression as the latter produced high range resolution in addition to the capability of detecting weak signals.

[0051] The fact that weak signals resulting in resonant magnetic coupling through a dissipative media can be processed using matched filters to a detectable level is not something obvious. This needs a systematic study taking all factors into account such as the dissipative phenomena, range, size of equipment etc. The inventor has studied this problem both through theoretical analysis and electromagnetic simulations and the results have indicated that signals as deep as 50 m (150 m) can be detected. This study is the basis behind the Invention. Results obtained by electromagnetic simulations for detecting a mobile phone size coil buried 35m deep carrying an average current of 50 mA, is presented as experimental evidence at the end of this document.

[0052] At present, matched filters are employed in some RFID applications. However, in these applications they use matched filters to identify symbols as explained in the previous paragraph. In contrast, in the Invention the symbol / signal duration is stretched to increase the energy of the transmitted symbol / signal. The impact of the Invention method (i.e., stretching the symbol and then applying a matched filter) should be apparent from the fact that the Invention has a range of several tens of meters in dissipative media whereas the range of a LF RFID in air at present is limited to 30 cm. 4.3 Optimizing the Induced Voltage verses Power Transfer Efficiency

[0053] Fig. 2 (a) shows the equivalent circuit of two magnetically coupled resonant circuits. Applying Kirchhoff’s voltage law to both primary and secondary circuits, it can be shown that at resonance the input impedance of the primary circuit, Z is given by:

[0054]

[0055] In an application like WPT, the coils are tightly coupled (strong coupling) and the mutual inductance Mis large. As a result, the input impedance Z is large. In these applications, power transfer to the load (Z is maximized by conjugately matching the source impedance Z to Z using a matching circuit as shown in Fig 2 (b). Usually, in practice, Z and Z values are real (i.e., resistive).

[0056] In the Invention, the coupling is very weak and the mutual inductance M is nearly zero. As a result, Z ≈ R₁ where R is the resistance of the primary coil. This is usually in the range of milli Ohms. As a result, in the Invention it is almost impossible to design matching circuits to maximize power transfer as the load concerned here

[0057]

[0058] is almost a short circuit. Therefore, in the Invention aa different approach is used. This is shown in Fig. 2 (c). As sinusoidal waveforms are used in the Invention, assume

[0059] i = I₀cos(ωt). From Faraday’s law it follows that the electro-motive force (e.m.f.) induced in the secondary coil is:

[0060] (ωt).

[0061]

[0062] Moreover, from circuit theory it follows that the voltage

[0063]

[0064] across the capacitor (see Fig. 2 (c)) at resonance is given by:

[0065] V_out = Q₂(e.m.f.)₂

[0066] where Q2is the quality factor of the secondary circuit. In the Invention, the coils are designed to produce large quality factors ranging from hundreds to thousands. As a result, the voltage induced appears multiplied by this factor which is a great advantage. Moreover, the induced voltage can be increased by increasing the primary current I In contrast to power transfer and some reported RFID applications, this approach does not require any matching circuits which are almost impossible to design.

[0067]

[0068] (b| Very weak coupling case t

[0069] Fig.2: Optimizing trans-resistance vs. power transfer efficiency.

[0070] 4.3 Facts Common to All Four Embodiments

[0071] This section describes some aspects common to all four embodiments of the Invention mentioned earlier. Firstly, it should be noted that in all four embodiments, the device must have both a transmitter (“Tx”) and a receiver (“Rx”). These are referred to as transmitter and receiver subsystems in the figures to follow (Figs. 5, 6, 7, and 9). To further clarify, consider the device used by the rescue team in the case of the mobile phone detection application. The device should not only be able to transmit a signal, but it should be able to detect what is sent by the mobile phone. Same applies to the mobile phone. It should be able to pick the signal sent by the search team and then respond. So, at each end (Tx and Rx) the devices should have both a transmitter and a receiver.

[0072] 4.4 Modulation method

[0073] As the Invention is based on narrow bandwidths resulting from resonance, preferred modulation methods should use narrow bandwidths. Examples of such methods include amplitude shift keying (ASK) and phase shift keying (PSK). Fig. 3 shows the transmitted waveform corresponding to ON-OFF Keying (OOK), a special case of ASK. With OOK, each symbol will represent either a digital ‘ 1 ’ or ‘0’. With PSK, more bits can be represented by a symbol. For example, if three bits are grouped to represent a signal, there will be 8 symbols (23) and 8-PSK modulation would be a good choice. In the Invention, in addition to selecting symbols this way, the symbol duration is also increased (stretched) to increase the energy of a signal to facilitate detection of weak signals.

[0074] 4.5 Half-duplex and Full-duplex Modes

[0075] In the Invention, the transmission and reception are done on a time-sharing basis. This is referred to as half-duplex mode1. Fig.3 shows the transmitted signal in the half-duplex mode. In the half-duplex mode, signals are received during the silent internals between transmissions. In the full-duplex mode, transmissions and reception can take place simultaneously, i.e., the communication channel is two-way. In the case of half-duplex, the communication channel is one-way. In the following description, the half-duplex mode is referred to for simplicity.

[0076] Extension to the full-duplex case is straightforward. For example, if there are two (or more) frequencies available (as in the case of multiple resonant frequencies), one set of frequencies can be used to transmit from say device A to B and another set to transmit from B to A.

[0077] 4.5 Different Embodiments

[0078] 4.5.1 Embodiment 1: Matched filtering with a single resonant frequency

[0079] The waveform of the transmitted signal is shown in Fig. 3. At both the transmitter and receiver ends only a single series resonant circuit comprising a capacitor and an inductor is used. This results in a single

[0080] 1Although it is tempting to extend to a full-duplex mode, it is not practically feasible. The problem is suppressing the unnecessary coupling of the transmitted signal to the receiver to a level below received signal strength which could well be in nano-volt or pico-volt range. fixed resonant frequency as shown in Fig. 4 (a). Moreover, the half-duplex case is considered for simplicity. However, arguments made are equally valid for the case of full-duplex transmission.

[0081] Fig. 5 shows the block diagram of the unit used at either the rescue team end or at the buried target end for the single resonant case. A unit comprises a both receiver and a transmitter. An important feature here is that a switch (SW-0) is used to share a single coil between transmission and reception modes. The inductance of the coil is L. Both the transmitter and the receiver circuits are tuned to the same resonant frequency f so that,

[0082]

[0083] The resistor R1 controls the current in the Tx coil. The components used in the circuit are described in Table 1. The switch SW-1 is only if OOK is used as the method of modulation. For PSK, this component is not required. At the receiver, the voltage across the capacitor is tapped (see Fig. 5), converted from analog to digital, and then passed through a matched filter. As mentioned in the previous section, the matched filter can also be realized as a correlation process. The matched filter gives an output value proportional to the energy in the signal. The energy in the signal can be increased by simply stretching the signal, i.e., by increasing the pulse width r (see Fig. 3). In a preferred embodiment of the invention, we stretch the pulse (use long duration pules) combined with a matched filter to overcome the losses encountered in propagation through a dissipative media. Increasing the duration of symbols has another advantage here. Unlike in normal communication applications where the spectrum available is flat, the spectrum in this case is impulse like (see Fig. 4 (a)) as the reliance is on resonance. Stretching the signal makes the spectrum of the signal narrow and by selecting a suitably long symbol duration, the spectrum of the signal can be easily accommodated within the available narrow spectral band.

[0084]

[0085] Table 1

[0086] 4.5.2 Embodiment 2: Matched filtering with multiple resonant frequencies

[0087] Fig. 6 shows the block diagram of the device used in the multi-resonant case. Similar to the single-resonant case, the device has both a transmitter and receiver. The switch SW 0 is for the half-duplex mode operation. An important feature of the device is that although it operates at multiple resonant frequencies f, f, ••• f it uses only a single coil. To the best of the inventor’s knowledge, this contrasts with present multi-frequency applications in WPT and underwater RFID where resonance at multiple frequencies is obtained through the use of multiple coils. Use of multiple coils needs space and is not suitable for compact designs. In the invention resonance at multiple frequencies is obtained by using a single coil connected to multiple resonant circuits made of inductors and capacitors. This can be seen in different embodiments shown in Fig. 6 and Fig. 7. These inductors and capacitors can be very small surface mount components, an ideal choice for compact designs required by applications such as buried mobile phone detection. These resonant circuits have high quality factors (Q-factors) and they resonate at different frequencies thus preventing any interaction between them. Although not essential, amplifiers Amp la, Amp 2a, etc., further suppress any interaction between different resonant circuits. This is because amplifiers permit signal flow only in one direction.

[0088] The resonant frequencies are set by selecting inductor and capacitor values according to the following equations.

[0089]

[0090] Resistor values R1 to RN are used to set the currents in the coil when the circuit is in the transmission mode. In Fig. 6, we use a multi-channel A / D converter and multiple matched filters. This is mainly for the purpose of illustrating the concept. As we work with a linear system, it is also possible to sum the inputs to amplifier stages Amp lb to Amp Nb using an analog adder circuit and then use a single A / D converter followed by a single matched filtering stage to obtain the same results.

[0091] Similar to the single resonant frequency case, this case (multiple resonant frequency) also uses long duration (stretched in time) symbols combined with matched filtering to improve detectability. In addition, in the Invention the same signal is sent at multiple frequencies and the summation is taken, which results in a stronger output compared to all other approaches. It is believed that this approach is more suitable for applications demanding a long range such as mining. .5.3 Embodiments 3: Frequency hop spread spectrum combined with resonant coupling and matched filtering

[0092] Fig. 7 shows the block diagram of this embodiment. As in Embodiment 2, aa single coil is used and combined with multiple resonant circuits made of inductors and capacitors to obtain resonance at multiple frequencies. Although it is possible to connect the resonant circuits in parallel as in Embodiment 2, we use a software-controlled switch here. This embodiment (Embodiment 3) has only a single source, but its frequency is hopped in time as shown in Fig. 8 and Fig. 10 (a). For example, in Fig. 8, the user represented in red transmits at frequency fN-±in time duration 7^ and the oscillator switches to frequency f in the time duration T These time durations, T T etc., are stretched times and we use matched filtering at the receiver to increase the signal strength. General applications of FHSS do not rely on stretching signals as it lowers data rates. As it was emphasized in Embodiment 1 onwards, in the Invention, symbol-stretching (aka “pulse-stretching”) is incorporated as a means of strengthening the signal.

[0093] Embodiments 1 and 2 allow communication between a user and a detector coil. As with other applications of FHSS, Embodiment 3 allows multi-user operation. As shown in Fig. 8, multi-user capability is due to sharing the available frequency spectrum (in the case of the invention, sharing the frequencies f, f, ••• f among different users. As the frequency allocation can be made using random algorithms, this also facilitates secure communication and immunity to interference. In summary, Embodiment 3 allows to incorporate all benefits of FHSS technology, namely, multi-user capability (spectrum sharing), secure communication and immunity to interference in the applications covered in the Invention.

[0094] .5.4 Embodiment 4: Chirp spread spectrum combined with matched filtering

[0095] The block diagram of the implementation is shown in Fig. 9. Here the frequency is swept continuously in the time domain as shown in Fig. 10 (c). The time domain waveform of such a signal is shown in Fig. 10 (b). One way of achieving this type of variation is tuning the capacitance Cl (see Fig. 9) to synchronize with the oscillator frequency. The capacitor at the receiver should also be tuned in synchronism with the frequency of the received signal which requires special algorithms. Such algorithms are currently used in techniques such as LoRa modulation. Moreover, LoRa modulation also offer multi-user capability using chirp spread spectrum.

[0096]

[0097] DESCRIPTION

[0098] This section gives the drawings referred to in Section 4, DETAILED DESCRIPTION OF THE INVENTION.

[0099]

[0100] (J

[0101] time T

[0102] Fig.3: Transmitted waveform in the half-duplex case

[0103] Frequency Single resonant frequency.

[0104]

[0105] Frequency (b) Multiple resonant frequencies.

[0106] Fig.4: Spectrum of waveforms, single and multiple resonance cases.

[0107]

[0108] Fig.5: Matched filtering with a single-resonant frequency.

[0109]

[0110] Fig.6: Matched filtering combined with multiple resonance.

[0111]

[0112] Fig.7: Frequency hop spread spectrum (FHSS) with multiple-resonant frequencies.

[0113]

[0114] Fig. 8: Frequency hop spread spectrum frequency allocation.

[0115]

[0116] Fig. 9: Chirp spread spectrum (CSS).

[0117]

[0118] (b) CSS waveform c) CSS waveform

[0119] in the time domain. in the frequency domain.

[0120] Fig. 10: Spectrum of spread spectrum waveforms.

[0121] 6 EXAMPLES OF THE INVENTION

[0122] The examples below are intended to exemplify the practice of embodiments of the disclosure but are by no means intended to limit the scope thereof.

[0123] Example 1: Detecting buried mobile phones

[0124] We demonstrate the feasibility of using the invention for detecting a buried mobile phone using accurate electromagnetic (EM) simulations combined with system modeling. For EM modeling we use ANSYS High Frequency Structure Simulator (HFSS). System modeling is done using the Virtual System Simulator (VSS) of AWR Microwave Office by Cadence Inc.

[0125]

[0126] Fig. 11: Detecting a buried mobile phone.

[0127] Fig. 11 shows a scenario where a buried mobile phone is detected. The detector coil used by the rescue team can be mounted on a drone for fast scanning. The detected signals can then be transmitted to an operator with a handheld controlling device.

[0128] Materials and Methods

[0129] Experimental setup:

[0130] In selecting the coil and component sizes, we paid careful attention to select sizes that can be fitted to a mobile phone, we selected the size of a mobile phone to be 150mm x 70mm to match with most smartphones in the market today. The coil used in the mobile phone is a rectangular coil with a single turn and has dimensions 140mm x 60mm. This leaves a 5mm margin along the border. In any mobile phone, electronic components are mounted using multi-layer printed circuit board technology. In the invention, it is suggested to use one of the layers of the multi-layer printed circuit board to fabricate the coil. In the simulation, we considered a 1 oz. thick copper layer (0.036 mm) to implement the coil. The width of the copper trace used for the coil is 2.0 mm. The detector coil is a single turn loop of mean diameter 700 mm. It has a circular cross-section with a diameter of 70 mm. These dimensions and other parameters used in the simulations are summarized in Table 2. The HFSS simulation models of coils used are shown in Fig. 12.

[0131]

[0132] Detector coil (Diameter 700 mm)

[0133]

[0134] mo^-tic ph»^

[0135] ( 140mm x 60 mm)

[0136] Fig. 12: ANSYS HFSS models of coils. Two figures are drawn to different scales.

[0137]

[0138] Table 2.

[0139] Soil properties are at 600 kHz and are from a recent publication [1], Results

[0140] As the simulation software we use (ANSYS HFSS) for electromagnetic modeling is primarily based on scattering parameters (S-parameters), we present S-parameter results first. Fig. 13 shows how the magnitude of S21 and S12 varies with frequency and depth. The important conclusion that we can draw from this is that even at greater depths like 35 m a detectable signal can be observed. Fig. 14 (a) and (b) are provided for the sake of completeness. Both Sil and S22 have very low magnitudes, a result one can expect due to the low impedance values of coils. In fact, this is the motivating factors for maximizing the voltage induced in the receiving coil for a given current in the transmitter coil in the invention (i.e., the motivation for maximizing the trans-resistance opposed to maximizing power transfer efficiency).

[0141] The S-parameters obtained from HFSS are then exported to the Virtual System Simulator (VSS) of AWR MWO software. The block diagram shown in Fig. 15 shows the system implemented using VSS. The resonant frequency is mostly decided by the inductor values LI, L2, and the capacitor values Cl and C2. This is because the self-inductance of transmitter and receiver coils are negligible when compared to the values of LI and L2. We use 140 pH for LI and L2 whereas the self-inductance of detector coil and the coil in the mobile phone are 1.029 pH and 0.317 pH respectively. The value of Cl and C2 is 500 pF. These values give resonance at 600 kHz. The magnitude of the voltage source is 1 V, and we use a 10 Q resistor for R1 to set the current in the Tx coil to 100 mA.

[0142] Fig 16 shows that a voltage of 9.95 pV can be observed across the capacitor in the Rx circuit (C2 in Fig. 15) for a 99.84 mA (peak value) current in the Tx coil. This is when the Rx coil is buried 35 m deep. Note that the resonance occurs at 598 kHz, not at 600 kHz. This is due to the inductance of Tx and Rx coils. In practice the resonance frequency can be exactly set to 600 kHz by adding a tuning capacitor. However, for this validation, we continue with resonance at 598 kHz. Fig. 17 shows the waveforms of the current in the transmitted coil, the received signal and the signal after matched filtering. The current in the transmitted coil has a peak value of 100 mA and a 50 % duty cycle which gives an average current of 50 mA. The results indicate that at this depth (i.e., at 35 m), the signal can even be detected without any processing (matched filtering in this case) as the 9.95 pV received signal is well above noise. Fig. 18 shows a situation where the received signal is 50 nV corresponding to a deeper target. From the received waveform, it can be seen that the signal is indistinguishable and buried in noise. However, even in this case the signal can be detected by matched filtering as shown in Fig. 18. This confirms that the invention can be used to detect targets at large depths deeper than 35 m with modest sized equipment and currents.

[0143] Analysis of the Results

[0144] Some important aspects related to the results are summarized in the bullet-points below.

[0145] • This experiment demonstrates results with an average current of 50 mA. This enables to use small components suitable for use in a mobile phone circuit.

[0146] • A mobile phone battery normally has a capacity of about 4000 mAh. This gives an 80 hrs of battery life in the transmission mode. The device proposed in the invention is in the receiver mode by default and switches to the transmit mode only when a signal sent by the rescue team is picked. As the power consumption in the receiver mode is only few milliamperes, the device can operate hundreds of hours, a very desirable feature for a safety device.

[0147] • Although we place the detector coil directly above the buried phone, this is not necessary.

[0148] Simulations show that even if the detector is 5 m away from the point on the surface directly above the buried phone, it can be detected. These simulation results are not included to keep the document short.

[0149] • Performance depends on the electrical conductivity of soil. Depending on the conductivity, the actual performance may be better or poorer than what is demonstrated. In the case of poor performance, there is room to adjust the duty cycle and amplitude of the current in the Tx coil to improve performance.

[0150] • The choice of 600 kHz in this experiment is arbitrary. One can select a higher or a lower frequency. For example, 125 kHz can be used for this, as Instrument Scientific and Medical (ISM) bands can be used without a license.

[0151] • The experiment has demonstrated how a carrier modulated pulse, i.e., an amplitude shift keyed (ASK) signal can be detected. This means that one can easily transmit information like a phone number so that the invention can be used not only to detect buried mobile phones but also to identify them.

[0152] • The detector coil can be easily fabricated by coating copper on a hollow plastic tube. At 600 kHz the thickness of the coating only needs to be 84.2 pm (the skin depth of copper at 600 kHz). This way one can make a lightweight detector coil that can be easily carried by a drone.

[0153]

[0154]

[0155] Variation of |S22|

[0156] Fig. 14: Variation of |S11| (i.e., at the Rx coil) and |S22| (i.e., at the Tx coil) with frequency and depth.

[0157]

[0158] Fig. 15: Block diagram.

[0159] Fi I 0 0.0 00 0.0 0.0

[0160]

[0161] Received signal and the matched filtered waveform

[0162] is i - • -: - -: - -: -, - -: -, t. Depth is 35111.

[0163] ude is 50 nV

[0164]

Claims

A DEVICE AND METHOD FOR ELECTRONIC COMMUNICATION OR SENSING THROUGH DISSIPATIVE MEDIA USING RESONANT MAGNETIC COUPLING WHAT IS CLAIMED IS:

1. A method of Near-Field electromagnetic signal communication comprising weakly-magnetic coupled resonant coils connected to capacitors at transmitter and receiver ends, and with symbol -stretching in the time domain combined with matched filtering.

2. The method of claim 1, further comprising: a resonant circuit comprising a coil, inductor and a capacitor tuned to a fixed frequency f at the transmitter end which is excited by a signal stretched in time at the same frequency f representing a symbol; a series resonant circuit tuned to the same fixed frequency f at the receiver end whereby the voltage across the capacitor in the series resonant circuit is sensed and processed with a matched filter prior to detection.

3. The method of claim 1, further comprising a set of multiple resonant circuits tuned to a set of fixed frequencies f, f, ■■■fNat the transmitter end which are excited by signals at frequencies f, f, representing a symbol, stretched in time; a set ofmultiple series resonant circuits at the receiver tuned to the same fixed frequencies, ’ ’’ ' vwherebYv°ltagesacross the capacitors in the series resonant circuits are sensed and processed with multiple matched filters and the summation of matched filter outputs is used for detection.

4. The method of claim 1, further comprising a set of multiple resonant circuits tuned to a set of fixed frequencies f, f, ■■■fNat the transmitter end which are excited by signals at frequencies f, f, •"fN, stretched in time representing a symbol; a set of multiple series resonant circuits at the receiver tuned to the same frequencies f, ’ '^NwherebY voltages across the capacitors in the series resonant circuits are sensed, and first summed and then processed with a single matched filter prior to detection.

5. The method of claim 1, further comprising a set of multiple resonant circuits tuned to a set of fixed frequencies f, f, ■■■fNat the transmitter end wherein a signal of a frequency randomly picked from the set of fixed frequencies ^, f, "■fN, representing a symbol is stretched in time is used to excite one or all of the resonant circuits at a time; set of multiple series resonant circuits at the receiver tuned to the same fixed frequencies / , f, ■■■fNwhereby the voltage across the capacitor in the series resonant circuit that is tuned to the randomly picked frequency from the set of frequencies / , f, '"fN^ sensed and then processed with a matched filter prior to detection.

6. The method of claim 1, further comprising: a resonant circuit of which the resonant frequency is varied in synchronism with the frequency of the signal representing a symbol which varies either as a linear or a non-linear chirp is used to excite the said resonant circuit; a series resonant circuit tuned in synchronism to the said time varying frequency at the receiver end whereby the voltage across the capacitor in the series resonant circuit is sensed and processed with a matched filter prior to detection.

7. A device of Near-Field electromagnetic signal communication comprising weakly-magnetic coupled resonant coils connected to capacitors at transmitter and receiver ends, and with symbol -stretching in the time domain combined with matched filtering.

8. The device of claim 7, further comprising: a resonant circuit comprising a coil, inductor and a capacitor tuned to a fixed frequency f at the transmitter end which is excited by a signal stretched in time at the same frequency f representing a symbol; a series resonant circuit tuned to the same fixed frequency f at the receiver end whereby the voltage across the capacitor in the series resonant circuit is sensed and processed with a matched filter prior to detection.

9. The device of claim 7, further comprising a set of multiple resonant circuits tuned to a set of fixed frequencies / , f, ■■■fNat the transmitter end which are excited bysignals at frequencies f, f, representing a symbol, stretched in time; a set of multiple series resonant circuits at the receiver tuned to the same fixed frequencies, ’ ’’ ' vwherebYv°ltagesacross the capacitors in the series resonant circuits are sensed and processed with multiple matched filters and the summation of matched filter outputs is used for detection.

10. The device of claim 7, further comprising a set of multiple resonant circuits tuned to a set of fixed frequencies f, f, —f at the transmitter end which are excited by signals at frequencies f, f, •"fN, stretched in time representing a symbol; a set of multiple series resonant circuits at the receiver tuned to the same frequencies f, ’ '^NwherebY voltages across the capacitors in the series resonant circuits are sensed, and first summed and then processed with a single matched filter prior to detection.

11. The device of claim 7, further comprising a set of multiple resonant circuits tuned to a set of fixed frequencies f, f, —f at the transmitter end wherein a signal of a frequency randomly picked from the set of fixed frequencies ^, / , '"fN> representing a symbol is stretched in time is used to excite one or all of the resonant circuits at a time; set of multiple series resonant circuits at the receiver tuned to the same fixed frequencies / , f, ■■■fNwhereby the voltage across the capacitor in the series resonant circuit that is tuned to the randomly picked frequency from the set of frequencies / , f, '"fN^ sensed and then processed with a matched filter prior to detection.

12. The device of claim 7, further comprising: a resonant circuit of which the resonant frequency is varied in synchronism with the frequency of the signal representing a symbol which varies either as a linear or a non-linear chirp is used to excite the said resonant circuit; a series resonant circuit tuned in synchronism to the said time varying frequency at the receiver end whereby the voltage across the capacitor in the series resonant circuit is sensed and processed with a matched filter prior to detection.

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