Method and system for power limited very low freqency communications

Abstract

Embodiments described herein relate to a VLF data communication system and method that allows a VLF transmitter or Receiver to be operated position below ground and from a co-located power source for a long period of time. Data to be transmitted is encapsulated in a single data packet for transmission, minimizing the on-time of the transmitter and receiver. One or more receivers process the data packet so as to synchronize with the transmitter symbol timing clock and then demodulate the is encapsulated data. The timing information may also be used by the receiver to control external devices. The methods used may be implemented on a low power Micro Controller Unit (MCU) to further reduce power consumption.

Description

METHOD AND SYSTEM FOR POWER LIMITED VERY LOW FREQENCY COMMUNICATIONSFIELD OF THE DISCLOSURE

[0001] The present invention relates to the field of Very Low Frequency (VLF) communications systems.BACKGROUND

[0002] There is a need for the control and monitoring of underground devices, in which a wireless communication link is used to send data to or from the surface to control or monitor devices buried in the Earth. In some applications, the underground device may be operated in a location in which the power supply is an energy storage device that cannot be replaced. These include, for example, the monitoring of sealed nuclear waste repositories and control of underground pipeline and mining equipment. In these applications, once installed, the communication link may need to remain operational for a defined period of time, such as for example up to 10 years.

[0003] References which may be relevant to this include the following US Patents:• 5504785, April 1996, Becker et al.• 15 / 827,297 , Roper et al.• 10,277,335 B2 Roper et al.• 11 ,118912 B2, Roper et al.• US2023 / 0058901 , February 2023, Balls et al.• US2023 / 007043,Mar 2023, Ryan, D

[0004] Other relevant references may include:Mueller et al. “Timing Recovery in Digital Synchronous Receivers” IEEETransactions on Communications, Volume 24 Issue 5, May 1976• Gardner, F “A BPSK / QPSK Timing-Error Detector for Sampled Receivers, IEEE Transactions on Communications, VOL. COM-4, no5 May 1986• Harris, F “Multirate Digital Filters used for Timing Recovery in Digital Receivers”, Conference Record of the Thirty-Fourth Asilomar Conference on Signals, Systems and Computers (Cat. No.00CH37154) October 2000, DOI:10.1109 / ACSSC.2000.910953• Song, X et al. “Fully Digital Asynchronous Symbol Timing Recovery in Digital Receiver” Proceedings ICCEA 2004, 3rdInternational Conference on Computational Electromagnetics and its Applications, November 2004 DOI: 10.1109 / ICCEA.2004.1459356.• Yang et al. “Extension to Gardner Timing Error Detector for QPSK Signals” 2010 international Conference on Wireless Communications and Signal Processing, October 2010 DOI: 10.1109 / WCSP.2010.5633556.

[0005] All of the above patents and references are incorporated herein by reference.SUMMARY

[0006] To pass through any significant amount of overburden rock, concrete or other conductive material, a wireless communication link must operate at a very low frequency VLF, typically in the frequency range 3 to 9 kHz, where the data transfer occurs through magnetic induction.

[0007] In the above applications, data may not be sent very frequently, and so it is therefore possible to reduce the power consumption of the underground wireless device by placing the VLF transmitter or receiver in a low-power standby state between messages. The amount of data in a transmission may also be small, for example the output from a sensor output or control message may be limited to 2 bytes. However, the energy required to send a message depends not only on the amount of data, but also on the overhead required, and in particular the time it takes the receiver to synchronize to the transmitted signal so that it can be demodulated with the minimum error probability. Synchronization of the receiver and the transmitter clocks also allows thecontrol signals from different receivers to be output at a known time. In very general terms, synchronization is achieved in higher frequency wireless systems by estimating the timing error in the receiver clock and using the error to adjust the sampling clock frequency or phase through a feedback loop. The feedback process, which is low-pass may take 20 to 80 symbols to converge. This is typically more symbols than required to transmit the data in the type of application described above, and thus the synchronization process has a major impact on the amount of energy required to operate over a period of many years. There is a thus need for a VLF data communication system and method that minimizes the energy required to send or receive a message in locations where the power supply is an energy storage device that cannot be replaced, and in particular enables the receiver to synchronize to a transmitter in much less time than is required by a feedback loop.

[0008] The energy, E required to be stored in the power source for a VLF transmitter or receiver used for long term monitoring or control of underground devices is approximated byE = Pon x Ton x Nm) Equation 1

[0009] where Ponis the power consumption of the active buried Transmitter or Receiver, Ton is the time for which the device is active and Nmis the total number of messages to be transmitted during the system lifetime. When the messages are not being transmitted, these devices may be placed into a standby state in which the power consumption may, for example, be reduced by 5 to 7 orders of magnitude. Since Nm is determined by operational requirements, the energy requirement E is determined by the power consumption and Ton. It is therefore desirable to make Ton as small as possible, ideally no longer than the time required to transmit the bytes containing the sensor data or control message.

[0010] The Dipole Moment (DM), which determines the magnitude of the magnetic field created by a loop antenna is given by:DM = la x N x A Equation 2

[0011] Where N is the number of turns in the loop and A is its area and la is the root mean squared (rms) antenna current. In practice the loop area A may be limited by the location of the transmitter antenna; in a nuclear repository for example the loop diameter may be no greater than 1 m in some cases. In small spaces, loops with N > 1 may be deployed, but N cannot be made arbitrarily large; for given transmitter voltage, the current in the loop, and hence the Dipole Moment produced by the transmitter is a function the loop reactance which is also proportional to 1 / N2.

[0012] For a given VLF communications link, the Dipole Moment is determined by the minimum field strength required at the receiver, BfXand the attenuation through the Earth Atte as given by Equation 3.DM = Brxx AtteEquation 3

[0013] Provided the transmitter is implemented using a power efficient switching topology, such as a class D, the main source of power loss PI is due to current la passing through the antenna resistance and is given by:Pl = R x N x la2Equation 4

[0014] where R is the resistance of a single loop of the antenna. Combining equations [3] and [4], it can be seen that the transmitter current is proportional to the receiver minimum field strength Brx. la = ( Brxx Atte) / (N x 4) Equation 5

[0015] For a given application, Atte is defined by the depth of the buried device and there are limits on the size of N and A. To minimize the energy storage requirement of a buried transmitter, the receiver sensitivity, Brx may be made as small as possible.Coherent demodulation methods, in which the receiver synchronizes to the transmitter in order to reduce the receiver signal level for a given data error probability are generally preferred for this reason.

[0016] A wire loop antenna may also be used at the receiver to convert the modulated magnetic field into an electromotive force (emf) according to Faraday’s Law. The antenna output is proportional to the number of turns on the coil. The receiver coils may use ferrite cores to increase the flux passing through the coil, and because each turn contributes to the emf, N can be made large. The receiver antenna can therefore be designed with sufficient conversion gain that, in absence of a signal from the transmitter, the emf generated from background environmental noise exceeds the internally generated receiver noise.

[0017] For a buried receiver, the power consumption is determined by the complexity of the signal processing required to demodulate the transmitted signal. The signal processing algorithms should therefore be capable of being implemented in a very low power micro-controller unit (MCU). From Equation

[0001] , the energy requirement is directly proportional to the on-time of the buried receiver. The on-time can be significantly longer than the message duration however, due to the need to detect and then synchronize to the transmitter prior to demodulation.

[0018] Synchronization of the receiver to the transmitted data is required for the optimal demodulation of the transmitted symbols, and minimizing the probability of data errors. For a VLF data link where messages are discrete and very short, a high order modulation method, which may require the receiver to synchronize to the transmitted carrier frequency and phase, is not necessary. Instead, a simple modulation method such as Phase Shift Keying (PSK) or Frequency Shift Keying (FSK) may be used in which the receiver synchronizes to the transmitted symbols. Gardner, ibid, teaches a timing error detector (TED) and synchronization method for such digital receivers in which the TED provides the error signal to a phase-locked look (PLL) used to control a voltage-controlled oscillator (VCO), used as the receiver symbol clock. For stability, theloop time constant must be much less than the symbol duration and synchronization is then achieved when the PLL converges after transmission of a long preamble containing 20 or more symbols. For a communication system that operates continuously or transmits long discrete messages, this overhead is not very significant. For a VLF data link where messages are discrete and very short the overhead may require a much larger battery or similar energy storage device to be installed.

[0019] Enhancements to this approach include using a numerically controlled oscillator (NCO) that can be incorporated into a digital processor, and sampling the signal before filtering, in which case the sampling clock phase may be adjusted instead of the VCO. Other known synchronization methods have been developed for very high data rates, where parallel processing may be required, and to provide the ability to adapt to variable symbol rates using polyphase filtering of the sampling clock to track in place of a VCO. In general, these methods are adapted to complex wireless transmission at higher frequencies where synchronization is required to optimize performance in the presence of inter-symbol interference (ISI) as well as Gaussian noise, and require the transmission of a long training sequence before the demodulator SNR is optimized.

[0020] There is therefore a need for a VLF communications system and method which minimizes the energy requirements of a buried transmitter by minimizing the effect of external noise on the receiver and achieving fast synchronization, so that the on-time, and hence power consumption of both the transmitter and receiver are minimized. The method may be suitable for implementation on a low power processor, such as a microcontroller unit (MCU) to further minimize power consumption.BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments will now be described, by way of example only, with reference to the attached figures, wherein in the figures:

[0022] FIG. 1 Illustrates, in a block diagram, an example of an energy efficient VLF data system, in accordance with embodiments of the present disclosure.

[0023] FIG. 2 illustrates, in a flowchart, an example of a method of processing the signal input from the receiver in order to carry out symbol synchronization and demodulation.

[0024] FIG. 3 illustrates, in a spectrum plot, the broadband and power-line harmonic noise output from a VLF receiver antenna.

[0025] FIG. 4 illustrates, in a block diagram, an example of a VLF transmitter in accordance with the embodiments of the present disclosure.

[0026] FIG. 5 illustrates, in a block diagram, an example of a VLF receiver in accordance with the embodiments of the present disclosure.

[0027] FIG. 6 illustrates, as a flow chart, a method of transmitting and receiving data, in accordance with the energy efficient VLF data system.

[0028] FIG. 7A illustrates, in a diagram, an example of the Geortzel filter output time response in accordance with the energy efficient VLF data system.

[0029] FIG. 7B illustrates, in a diagram, the frequency response of the Geortzel filter configured in accordance with the energy efficient VLF data system.

[0030] FIG. 8 illustrates, in a flowchart, a method of obtaining the optimum timing for demodulating the FSK signal in accordance with the energy efficient VLF data system.

[0031] FIG. 9A illustrates, in an exemplary diagram, the construction of a data packet in accordance with the embodiments of the present disclosure.

[0032] FIG. 9B illustrates, in an exemplary diagram, the guard space inserted between consecutive data packets, in accordance with the embodiments of the present disclosure.

[0033] FIG. 10 illustrates, in a block diagram, an example of an energy efficient VLF data system used to communicate sensor data from a nuclear waste storage location to the Earth’s surface.

[0034] FIG. 11 illustrates, in a block diagram, an example of an energy efficient VLF data system used to communicate control messages from the surface to a sealed underground location.

[0035] FIG. 12 illustrates, in an oscilloscope plot, an example of the timing error produced by two separate receivers, in accordance with the embodiments of the present disclosure.

[0036] It is noted that throughout the figures, like features are identified by like reference numerals.DETAILED DESCRIPTION

[0037] Embodiments described herein may provide an energy efficient VLF data system used in applications where the transmitter or receiver is sealed below the Earth with a collocated power supply. Such environments include nuclear waste repositories, mines and buried pipelines.

[0038] Figure 1 illustrates, in a block diagram, an example of an energy efficient VLF data system 100. The energy efficient VLF data system 100 comprises a VLF transmitter 101 , a VLF magnetic field receiver 102 and a processor 103. VLF transmitter 101 is configured to create a data packet from an external input and modulate a VLF magnetic field with the contents of the packet, such that it propagates through the Earth towards the receiver. In some embodiments themagnetic field may propagate through an Earth half-space, or the transmitter may also be installed below ground so the magnetic field propagates entirely through the Earth. Receiver 102 is configured to convert the incident magnetic field, modulated by the data packet, into a digitally sampled data series which is input to signal processor 103. Processor 103 is configured to detect, synchronize to and demodulate the data in a single data packet.

[0039] In some embodiments transmitter 101 may receive data from an external sensor, and processor 103 may output the demodulated data to a monitoring or storage device, such as a computer. In another embodiment transmitter 101 may receive data from an external control device, such as a computer, and processor 103 may output control message to an external device in order to change its state at a specific time. The VLF transmitter, receiver and processor are described in more detail below.

[0040] Figure 2 illustrates, in a flowchart, an example of method 200 of processing the signal input from the receiver. The method 200 comprises sampling 201 the analog signal from a VLF receiver 102 at a rate that is much higher than the transmitted symbol rate and harmonically related to the data transmission rate. Next, the sampled data is repeatedly filtered 202 at the transmitted modulation frequencies by first filtering a block of data and the then repeating the filter operation by incrementing the starting sample.

[0041] The start of a transmitted data packet is detected 203 by evaluating the filter outputs. Next, the timing of a received symbol is estimated 204. Then this timing estimate is used to demodulate 205 the rest of the data packet, and may also be used to generate 206 a control signal output from the receiver. The steps described in method 200 are described in more detail below.

[0042] In an embodiment, the transmitter 101 of energy efficient VLF data system 100 may be buried below ground with a collocated power source, for example inorder to monitor sensors in a nuclear waste repository. From equation 5, the minimum magnetic field signal that can be demodulated by processor 103 is a key factor in determining the Dipole Moment and hence the power consumption of transmitter 101. Figure 3 is a spectrum plot 300 of the ambient noise output from a VLF receiver connected to a receiver antenna, showing, on the horizontal axis, an exemplary VLF frequency range 301 extending from 4.4 to 5.4 kHz. The vertical axis 302 shows the signal amplitude in dBV. The spectrum plot 300 shows the presence of the harmonics 303 of the 60Hz AC distribution frequency that are 15 to 20dB higher than the broadband component of inter-harmonic noise 304, which has a generally uniform average amplitude across the displayed frequency range 301.

[0043] US 10,277,335 B2 teaches that the noise level in the receiver may be greatly reduced by using an inter-harmonic, multi-carrier, modulation method in which data is modulated onto a number of sub-carriers, each located between the power line harmonics. However, demodulation of multi-carrier and phase-sensitive modulation is computationally intensive and therefore generally results in a higher receiver power consumption than single carrier modulation methods. In an embodiment, in an energy efficient VLF data system 100, single carrier, m-ary FSK may be used to modulate the magnetic field. In processor 103, The m-ary FSK modulation may be efficiently filtered from the harmonic noise by using a Geortzel filter to implement a narrowband filter centered on each FSK frequency. FSK modulation also has the advantage that demodulation is not affected by any amplitude distortion of the signal which means the electrical circuits in transmitter 101 and receiver 102 do not have to have a large linear range, which may reduce the power consumption in these circuits. However, other modulation and filtering techniques, for example based on a higher order modulation such as QPSK could be used, but would require more linear receiver electronics and more complex signal processing for timing recovery and demodulation.

[0044] In an embodiment, the receiver of an energy efficient VLF data system 100 may be sealed below ground with a collocated power source, for example to control the state of corrosion control devices in a buried pipeline. The computational load onprocessor 103 can be further minimized, so that the processor may be implemented on a low power MCU, by modulating the data using only 2 FSK carriers.

[0045] Figure 4 illustrates, in an exemplary block diagram 400 of a VLF Transmitter 401. The transmitter of Figure 4 is however only provided for illustration, and other transmitters could be used with the embodiments of the present disclosure. VLF Transmitter 401 comprises a power supply 402, a micro controller unit (MCU) 403, halfbridges 404 and drivers 405.

[0046] When operated below ground in a sealed location the power supply may have an external DC power supply 406 in the form of a low leakage, high-capacity battery, or other long-term power source. The power supply 402 converts the voltage from the external power source to a stable and regulated supplies for the MCU 403, drivers 405 and half-bridges 404. When active, the MCU 403 may request data from one or more external devices 407, which may, for example, be monitoring devices such as temperature, pressure, humidity and strain gauge sensors, or a control device such as a computer. The MCU 403 formats this data is into a single message for transmission and then generates Pulse Width Modulation (PWM) control signals 408 which synthesize an alternating output current at the desired m-ary FSK frequencies, and where the duration of each FSK symbol may be matched to the bandwidth of the filter (e.g., Geortzel filter) used in the Processor 102. The FSK frequencies, message format, symbol duration and other parameters are stored together with the MCU program code in non-volatile, non- transitory memory, for example a FLASH memory 409.

[0047] The PWM control signals 408 are fed to drivers 405, each of which controls the switching of a half-bridge 404. Each half bridge 404 is arranged to have fast output rise and fall times and is switched at a high frequency, so that the power dissipation in each half-bridge is minimized.

[0048] The output from each half-bridge 404 is connected to the transmit loop antenna 410 whose impedance is defined by the loop inductance, represented by inductor 411and the antenna cable resistance, represented by resistor 412, which is the primary source of the power consumed by transmitter 401.

[0049] When the message has been transmitted, MCU 403 may shut down the outputs from power supply 402 to all other circuits and then enter a low power standby state in which, internally, only one clock and one internal timer 413 remain active. The timer 413 may be set to the required interval between transmissions, which for a system operating autonomously over many years may, for example, be 12 hours or 24 hours. When this time is reached, timer 413 generates an interrupt which sets the MCU 403 back into the active state so that the next message can be sent.

[0050] In an embodiment, the transmitter 401 may be sealed inside in a water and dust proof enclosure suitable for housing electronic equipment in an outdoor environment; for monitoring nuclear waste, the housing may also incorporate protection against ionizing radiation. In another embodiment, the antenna 410 may be sealed inside the same enclosure as the electronic circuits. Other options for the enclosure are possible.

[0051] Figure 5 illustrates, in an exemplary block diagram 500 the VLF Receiver 501 which comprises a three-axis antenna 502, a three-way switch 504 used to select one of the three orthogonal antennas, an amplifier 507, which may comprise one or more active gain stages and a micro controller unit (MCU) 512. The 3-axis antenna has three differential outputs which are connected to the inputs of switch 504. For clarity only a single connection is shown.

[0052] A surge arrestor 503 in each antenna output protects the switch 504 from an excess input voltage.

[0053] Switch 504 selects one of outputs from the 3-axis antenna 502 at a time, controlled by an antenna selection signal 505 output from MCU 512.

[0054] Amplifier 507 is preceded by a high pass filter 506, that rejects the AC power fundamental frequency of 50 or 60Hz, and other low frequency power line harmonics that may be 60dB higher than those at the operating frequency of the receiver, which may be between 3kHz and 9 kHz. Amplifier 507 is followed by a low pass anti-aliasing filter 508 which outputs the received signal to an Analog to Digital Converter (ADC) 509. Switch 504, amplifier 507 and filters 506 and 508 may be implemented as differential circuits, as shown, in order to attenuate common-mode noise picked up by the receiver. Filters 506 and 508 may be combined into a single band-pass filter, and may be implemented as active filters in one or more of the gain stages of amplifier 507.

[0055] ADC 509 samples the analog input signal and converts it to a digital number at a rate determined by sampling clock 510. Low pass filter 508 may attenuate the output from amplifier 506 by at least 40dB at the frequency of sampling clock 510, which, according the Nyquist criterion must be at least twice the maximum signal frequency, and to facilitate processing 103 may be more than ten times the highest signal frequency and harmonically related to the transmitted symbol rate. Processing 103, which involves synchronization and demodulation may be implemented by the programmable computational core and memory located in MCU 512. The MCU may also incorporate ADC 509. An output 513 from MCU 512 may comprise the demodulated data, or control message or signals derived from the data. The output 513 may be sent to an external computer, or may be used to control one or more external devices 514.

[0056] The Receiver 501 may be powered from an external source 515, such as a battery. The external power is input to a receiver power supply 516, which may provide a low-noise output to supply to the analog components of the receiver. This supply may be switched off by a power-down control signal 517 from MCU 512, to prevent the receiver from consuming power when in a stand-by state.

[0057] In an embodiment, when the direction of the transmit antenna is known and both transmitter and receiver are in fixed locations, the three--axis antenna 502 and switch503 may be replaced by a single receive antenna oriented to receive the maximum magnetic field from the transmitter.

[0058] Figure 6 illustrates, as a flow chart, a method 600 of transmitting and receiving data, in accordance with the energy efficient VLF data system. The method 600 comprises assembling 601 a data packet containing the data to be transmitted and a short preamble, with a symbol duration that is the reciprocal of the AC power line frequency, which may be 50 or 60Hz. Next, the process may comprise modulating 602 a magnetic field with this data packet using, for example, binary FSK modulation. After amplification and sampling 603 by receiver 501, the start of the FSK modulated data packet is then detected 604, for example by means of sliding Geortzel filters configured for the FSK frequencies and implemented in processor 103.

[0059] The process next comprises synchronizing 605 the processor symbol clock to the transmitter symbol clock, based on the response of the Goertzel filters to the message preamble. This timing information is then used to coherently demodulate 606 the remainder of the message.

[0060] After demodulation of the transmitted message, a control message or signal may then be output 607 to an external device, at a fixed time after the start transmission of the data packet.

[0061] In the method of 600, the Geortzel filter output is calculated iteratively from a block of samples of the incoming signal. The sampling rate must be > 2x the maximum input frequency to meet the Nyquist criteria and to simplify computation, harmonically related to the symbol rate. In inter-harmonic modulation, the filter bandwidth and symbol transmission rate are both one half of the interval between power-line harmonics, i.e., in an environment with 50Hz AC power distribution, the filter bandwidth is then 25Hz, and the FSK tones are centered between the 50Hz harmonics at intervals of M x 50Hz. To obtain a high discrimination between the FSK tones, M is preferably > 5.

[0062] The receiver may synchronize to the transmitter with a resolution that is no better than + / - 1 sample, so a high sampling rate is preferred. Sampling at 100kHz, for example, may give a receiver timing error with respect to the transmitter clock that is at least + / - 10ps. The maximum frequency that is available for magnetic induction is 9kHz. For a 50Hz environment a sampling frequency of 100kHz also avoids the need for very sharp anti-aliasing filters and is harmonically related to the symbol rate. In a 60Hz environment the sampling rate may be increased to 120 kHz in order to be harmonically related to the symbol rate. The Goertzel filter output is then obtained using the following calculations for a 50Hz AC environment:Symbol Duration (Ts~) = Block Length = 1 / Symbol Rate Equation 6= 40ms for a 25Hz symbol rate

[0063] The number of samples, N, in the block is given by:N = Block length x Sampling Rate ( / ?) Equation 7= 40,000 for a 100kHz sampling rate (R)

[0064] In a 60Hz environment the symbol rate is 30 Hz and, with a sampling rate of 120kHz, there are also 40,000 samples in each block.

[0065] To calculate the filter output for a given FSK signal with a frequency Ffsk, without the use of complex arithmetic, five constants may be used. These may be precomputed as follows:Equation 8w = (2 x n / N) x k Equation 9 cosine = cos(w) Equation 10 sine = sin(w) Equation 11 c = 2 x cosine Equation 12

[0066] The processing of the block of samples may be performed by calculating the value of three variables from the current sample value Vs:Q = coeff * Ql - Q2 + Vs Equation 13Where; Q1 = value of Q for the previous sampleQ2 = value of Q two samples earlier.

[0067] Q1 and Q2 are set to zero at the beginning of each block of samples. The magnitude of the filter output, M, is calculated from these variables after the last sample in the block has been processed;M2= Ql2+ Q22- Ql * Q2 * coeff) Equation 14

[0068] These calculations use only real, not complex numbers, requiring a total of (N+2) Real Multiples and (2N+1 ) Real Adds. At low symbol rates this processing does not require a dedicated DSP function and can be performed by a standard low power microcontroller (MCU) in the time interval between samples.

[0069] Because the Block size is equal to the symbol duration, the value of output M depends not only on the magnitude of the input FSK signal but also on the time alignment between the start of the sampling block and the start of the received symbol.

[0070] The filter time domain response is illustrated in Figure 7A which is a graph 700 in which, on Y-axis 701 the output amplitude 702 of a Goertzel filter is plotted against the time offset 703 between the start of the processing block and the start of the symbol. The Block size and Symbol duration are both = T, which may, for example, be 40ms.

[0071] The Geortzel filter time domain output illustrated in graph 700 has the following properties;a) If the time offset is > + / -T, 704, there is no signal present when the block of sampled data is processed and the filter output will be small. b) If the time offset is 0, the signal is present in every sample in the block of data processed by the filter, the filter output will be at a maximum 705. c) If the time offset is < + / -T, for example -T / 2, the signal is present in some of the block of samples processed by the filter, and the filter output will be an intermediate output value 706, proportionate to the number of samples in which the signal is present.

[0072] The filter output amplitude 702 response of the Goertzel filter with block size = symbol duration thus has the triangular shape shown in graph 700, with a peak occurring when the processing start time exactly aligned with the start of the received symbol and the symbol is proceeded and followed by noise. For binary FSK this required symbol pattern may be obtained on both frequencies by transmitting an alternating 0-1 -0-1 training sequence. The time offset between the receiver and transmitter symbol clocks can be determined, if the Goertzel filter processing is started by a receiver clock running at the nominal symbol rate while processing the training sequence at each filter frequency. The correct clock timing may then be found by adjusting the Goertzel filter start time until the maximum output 705 is obtained. All subsequent symbols can then be detected by processing a single block of data using the same time offset to obtain the maximum response from the filter. In an embodiment, the correct clock timing may be obtained by transmitting a series of data packets and adjusting the filter start time iteratively, or by using feedback, so that its output converges towards the maximum. However, this would require many training symbols to be transmitted, resulting in a high energy requirement at both the transmitter and receiver.

[0073] In an embodiment, the length of the preamble may be reduced to, for example, 4 symbols by using a sliding Goertzel DFT filter to continuously process the sampled data. The block size and sampling rate and initial output calculation are the same as for the standard Goertzel filter described above. In the sliding Goertzel DFT, each time anew sample is obtained the output is recalculated by advancing the start of the processed block by one sample. The new output only requires the calculation of one new value for Q (Equation 13), using the value of Vs from the new sample and updating Q1 and Q2 from the previous calculations. The new value of M is then calculated by using the new values of Q, Q1 and Q2. This additional processing requires only 3 Real Multiples and 4 Real Adds and can also be completed in the time between samples. Thus, by continuously incrementing the filter processing sample by sample, the triangular filter time-offset response 702 can be obtained in real time and the peak amplitude and corresponding time offset of the peak response may be obtained with very little additional processing overhead.

[0074] Figure 7B is the measured frequency response 707 for a Goertzel filter configured according to method 600 for a 50Hz environment. The y-axis 708 of the graph shows the filter output amplitude, in dB, and the x-axis 709 shows frequency of the input CW signal. The filter output 710 has a maximum 711 at the target frequency, and exhibits deep nulls at frequency offsets that are multiples of 25Hz. These nulls provide attenuation of all power line harmonics, for example at 3850Hz 712, and also at all other inter-harmonic frequencies, for example 3875Hz 713, some of which may be used for FSK modulation. The frequency response 707 of the Geortzel filter, when configured according to the method 600, thus removes the high-level harmonic noise from the receiver input, which increases the receiver sensitivity, and at the same time facilitates the use of inter-harmonic m-ary FSK modulation.

[0075] Figure 8 is a flow diagram 800 illustrating a method of obtaining the optimum timing for demodulating the FSK signal. To demodulate an FSK signal, a separate filter is required for each frequency. For exemplary purposes, the simplest case of binary FSK with two filters is described. Each Geortzel filter is configured to processes one of the FSK signal frequencies 801 using a block length = 1 / symbol rate. The isolated symbols required for the signal detection are provided by transmitting 802 a 1 -0-1-0 data pattern which is added as a preamble at the beginning of a data packet. One of the Geortzel filters then detects 803 the first symbol of the preamble which initiates thesearch the correct symbol timing. This is obtained by the sliding Goertzel filters processing 804 the next three symbols of the preamble and determining the sample times within each processing block corresponding to the maximum filter outputs.

[0076] The timing accuracy is limited by the sampling rate, and also, in locating the peak filter output 705, the amplitude resolution of the ADC 509. For example, with a 100kHz sampling rate, the timing accuracy can be resolved to within + / -1 sample, or + / -10ps.The timing information is then used to synchronize 805 the receiver symbol timing clock in the processor to the received data. Next the updated symbol timing is used to demodulate 806 the rest of the symbols in the data packet. As the processor symbol clock is synchronized to the transmitter, it may also be used to output 807 a message or signal from the processor at a time that is directly linked to the transmitter data packet timing. In a point to multipoint network, each receiver uses the same process to synchronize to the transmitter and the outputs 807 from the receivers may be simultaneous, within the timing resolution of the receiver.

[0077] In an embodiment, two threshold values, Tn and Ta may be used to determine the demodulator output based on the filter outputs M1 , and M2. For example, for M1 to indicate that a valid symbol has been demodulated, the following conditions must be met:Ml > THnand Ml > M2 x THaEquation 15

[0078] where THn is the noise threshold and THais the amplitude threshold. For M2 to indicate that a valid symbol has been demodulated the same equations apply, but with M1 and M2 reversed;M2 > THnand M2 > Ml X THaEquation 16

[0079] In many applications the direction of the transmitted magnetic field is not known and the receiver determines which of three orthogonal antennas 502 receives the best signal. In some cases, it is therefore necessary to combine antenna selection and FSKsignal detection (for example during signal detection 803) each time a buried RX is powered on after a period in a low power sleep state. In an embodiment, antenna selection and signal detection may be performed simultaneously by switching the input to the receiver between antennas multiple times per symbol. This generates interleaved groups of samples that are processed separately by the sliding Goertzel filters for each antenna. The Goertzel filters accumulate the output from each group of samples to generate a separate peak value in the filter output 702 for each antenna. These peak values are then averaged over a number of symbols, Na, where Nais less than the number of symbols in the transmitted data packets. For example, for a data packet comprising 56 symbols, Namay be set to 48.

[0080] The averaged filter outputs for each antenna may then be compared and the antenna producing the largest averaged output may be selected for processing all subsequent messages, as described with regard to Figure 8. Because the demodulating filters produce a peak signal level estimate for every symbol, the antenna selection process is rapid and may be completed after reception of a single message, minimizing the power consumption of both the VLF Transmitter 101 and the VLF Receiver 102. If the signal is present at a level where it meets the threshold requirements of Equation 15 and 16 on each antenna, the data packet may also be successfully demodulated at the same time, and its contents verified to complete the FSK signal detection.

[0081] If no signal is being transmitted during the signal search, the average filter output for each antenna will be zero. In this case the antenna selection process is repeated, either until the signal is found and the optimum antenna selected, or until a time-out expires and the RX returns to a low power sleep state.

[0082] Figure 9A illustrates, in an exemplary diagram, the construction of a data packet 901 that may be used in an energy efficient VLF data system. The example of Figure 9A comprises four variable data fields, preceded by a preamble 902, that are transmitted serially. The data packet starts with preamble 902 which contains foursymbols creating an alternating 1 ,0, 1 ,0 pattern that is detected by processor 103 and used to synchronize the receiver symbol clock to the transmitted data packet symbol timing, according to method 800. The next field is a sender ID 903 which may also be 4 bits. In a sensor monitoring application this could be used, for example, to identify one of up to 16 sensors 407 connected to the transmitter 401. In a control application this could identify a command for a specific device 514 connected to a receiver 501.

[0083] The next two data fields 904 may each contain a byte of data being sent from transmitter 401. The last data field is used for error detection 905 and may contain a single parity bit, a checksum or a cyclic redundancy checksum (CRC). If the sender ID 903 comprises 4 bits and the error detection uses CRC-4, also comprising 4 bits, the total packet length, including the preamble is 28 bits long. At a transmission rate of 30Hz, the message can be transmitted in 0.933 seconds. If the sensor data is updated once every 12 hours then the transmitter duty cycle is only 0.002%.

[0084] Figure 9B illustrates in a second exemplary diagram the transmission of consecutive data packets 901. This may be required, for example, in a point to multipoint configuration to send different data packets from a transmitter 401 to different receivers 501. Successive data packets must be sent with a guard time 906 that is at least 2 symbols long, so that the output from each Geortzel filter in receiver 501 is set to a low value before the arrival of the preamble in the following data packet.

[0085] In one embodiment, the consecutive data packets are transmitted at a fixed frame rate, for example every 3 seconds, with each data packet 901 transmitted at the beginning of each frame. Where data packets of varying length are transmitted at a fixed frame rate, the guard time 906 may vary depending on the length of each data packet. The guard time remaining after each data packet 901 can be calculated by the receiver by counting the number of symbols demodulated. A high-level noise burst occurring during the guard time may cause false detection of the start of the next data packet, preventing successful timing recovery and demodulation. To reduce the probability of this occurring, noise threshold THa may be increased to a much highervalue during the guard time 906. This prevents the first condition of Equation 15 for demodulation of a symbol from being satisfied. Noise threshold THa typically must be restored to the normal value for signal detection two symbols before the end of the frame to allow the preamble of the next data packet 901 to be detected by the receiver.

[0086] Figure 10 is an exemplary system level diagram 1000 of an energy efficient VLF data system 101 used to communicate sensor data from a sealed underground location to the surface or another location where it can be monitored. For example, VLF Transmitter 401 may be sealed inside an underground repository chamber 1001 containing nuclear waste 1002 in order to report data from one or more sensors 1003. In this application the transmitter may be required to operate for more than 10 years powered from a collocated power source 1004 such as a battery. The VLF magnetic field, represented by an ellipse, 1005 generated by transmitter 401 passes through the antennas of receiver 501 which is located above the Earth’s surface 1006. The receiver detects and demodulates the data in the modulated magnetic field 1005 and may output it to a computer 1007 where it may be observed and stored. In this configuration, the power consumption of the buried equipment is largely determined by the power lost in the transmitter and antenna resistance, and, from Equation 1 , the energy that must be stored in power source 1004 is proportional to Ton, the time required to send a message. The signal processing at processor 103 and the method of Figure 6 used to demodulate the data in a single message using a short preamble minimizes Ton and the energy that must be stored in the collocated energy source 1004.

[0087] Figure 11 is an exemplary system level diagram 1100 in which an energy efficient VLF data system comprising transmitter 401 and receiver 501 is used to control one or more external devices 1101, for example, a corrosion prevention device 1101 connected to pipeline 1102. In remote areas, or where the pipeline 1102 passes under a swamp, the electrical equipment may be sealed inside a waterproof underground enclosure 1103, which is typically made of concrete and has no external power source. In this application the transmitter 401 is located above the earth where it may be brought on site when needed and powered by re-chargeable batteries 1104. Thereceiver may also be powered from an external power source 1105, but as it is sealed below ground it must have sufficient energy storage capacity for the operational life of the installation which may be up to 10 years. The transmitter 401 may generate a data packet containing a control command based on data stored internally, or provided from an external source, which modulates a VLF magnetic field, represented by an ellipse, 1106. The modulated VLF magnetic field 1106 passes through concrete enclosure 1103 and the antennas of receiver 501. The receiver detects and demodulates the data in the modulated magnetic field 1106 and may output it to an external device 1101, which may in this example be corrosion control device.

[0088] When active, the power consumption of the VLF receiver 501 is typically much lower than transmitter 401, which may typically be 20W, due to the antenna current required to transmit the signal over a range of 30m. Due to the low computational complexity of the synchronization and demodulation method 801 which can be implemented on a low power MCU, the VLF receiver power consumption may, for example, be 200mW. In continuous operation the receiver may then need 4.8 W-hr. or ~17kJ of energy per day. This would require a very large external power source for operation over any extended length of time. The receiver 501 may therefore be operated in a sleep state in some embodiments, where only a single timer is running in the MCU and the power consumption may be < 5pW. The VLF receiver 501 typically becomes active periodically, for example every 60 minutes, to check if a message is being sent from the transmitter.

[0089] When the control function is needed, the transmitter starts sending repeated messages, separated only by the guard time 906 which may be 4 symbols, or in a 60Hz system, 0.133 seconds. The transmitter, must send messages continuously for a time that is greater than the receiver sleep time., The receiver active time must exceed the transmitter guard time to ensure the transmitter signal is detected. This would be satisfied, for example, by a receiver on time of 0.2 seconds. In this scenario, the standby consumption is 5pW in 1 hour, and when active for 0.2 seconds, twice per hour, an additional 22 pW. The total power consumption is then 27 pW per hour of operation,or ~ 1 E-4 kJ. Over a 10-year period the energy requirement is 8.76kJ which may be provided by a long-life battery. Because the transmit signal may be detected at any point in a data packet, the receiver must stay on to receive a complete second data packet. The on time may then be approximately 2 seconds if the packet structure of Figure 9A is used. In this type of application, the usage may be limited to less than 4 times per year, so the additional energy requirement is negligible.

[0090] At low signal to noise ratios, additive noise may randomly increase the instantaneous magnitude of a wanted FSK signal, moving the timing of the peak of the ideal filter response 702. This creates jitter in the recovered symbol timing, derived from data packet 901 preamble, from data packet to data packet, and between different receivers. Moderate amounts of jitter do not affect data packet demodulation but can degrade the timing accuracy of control signals output from a number of different receivers. In this type of application, when a data packet containing a command to output a control signal is received, instead of using the symbol timing obtained from that one data packet, each receiver may instead use an average symbol timing derived from a number of preceding data packets. In an embodiment, transmission of a data packet may be repeated N times to generate an average symbol timing, with the control signal output at a fixed time after the Nthdata packet.

[0091] In some applications there is a need to output the control signal from a VLF receiver at a known time. This must be after demodulation of the data packet containing the command to do so. Because the receiver is synchronized to the transmitter symbol clock this can arranged by outputting the message or control signal after a fixed delay time from the detection of the timing offset in the preamble. The delay time may be programmed into the MCU or included in the command. The timing accuracy cannot be better than + / - 1 sampling interval, which, for a 100kHz sampling clock is 10ps. This accuracy is further degraded by noise and any frequency drift in the receiver and transmitter clocks during the transmission period. If the transmitter and receiver clocks have an accuracy of + / - 5ppm then the error between them could be as high as 10ppm. Over the duration of a data packet of approximately 1 second duration, the timing errorin the sampling clocks may then be 1 x + / -1 E-5 = + / - 10ps, or one sample. If the output of the control message is delayed by 5 seconds from synchronization, the timing error increases to + / - 60 psec. The same mechanism will create small timing differences between control messages sent out from different receivers in a point to multi-point system .

[0092] The timing error can also be observed by and evaluated by comparing a control output from two different receivers, after demodulating the same message from a transmitter. Figure 12 is a plot 1200 which provides the screen output 1201 from an oscilloscope connected to the control outputs of two different receivers RX_A and RX_B receiving a common data packet from a VLF transmitter, with a signal to noise ratio (SNR) > 20dB. In each receiver, the control output was delayed by 5 seconds after the last symbol in the message was received. Screen output 1201 shows the magnitude of the signals on the Y-axis 1202 with a scale of + / - 5V and time on the X axis 1203 with a scale of + / - 1ms. The measurement was triggered by the output 1204 from RX_A. The output from RX_B 1205 was delayed by 50ps in this test example, which is adequate for many control applications. Due to the high SNR this timing error is at least partly due to the difference in clock timing in the two receivers.

[0093] The embodiments of the devices, systems and methods described herein may be implemented in a combination of both hardware and software. These embodiments may be implemented on programmable computers, each computer including at least one processor, a data storages system (including volatile memory or non-volatile memory or other data storage elements or a combination thereof), and at least one communication interface.

[0094] Program code is applied to input data to perform the functions described herein and to generate output information. The output information is applied to one or more output devices. In some embodiments, the communication interface may be a network communication interface. In embodiments in which elements may be combined, the communication interface may be a softwarecommunication interface, such as those for inter-process communication. In still other embodiments, there may be a combination of communication interfaces implemented as hardware, software, and combinations thereof.

[0095] Embodiments may relate to servers, services, interfaces, portals, platforms, or other systems formed computing devices having at least one processor configured to execute software instructions stored on a computer readable tangible, non-transitory medium. For example, a server can include one or more computers operating as a web server, database server, or other type of computer server in a manner to fulfill described roles, responsibilities, or functions.

[0096] Various example embodiments are described herein. Although each embodiment represents a single combination of inventive elements, all possible combinations of the disclosed elements include the inventive subject matter.

[0097] Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0098] The term "connected" or "coupled to" may include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements).

[0099] The technical solution of embodiments may be in the form of a software product. The software product may be stored in a non-volatile or non-transitory storage medium, which can be a compact disk read-only memory (CD-ROM), a USB flash disk, or a removable hard disk. The software product includes a number of instructions that enable a computer device (personal computer, server, or network device) to execute the methods provided by the embodiments.

[0100] The embodiments described herein are implemented by physical computer hardware, including computing devices, servers, receivers, transmitters, processors, micro-controllers and micro control units, memory, displays, and networks. The embodiments described herein provide useful physical machines and particularly configured computer hardware arrangements. The embodiments described herein are directed to electronic machines and methods implemented by electronic machines adapted for processing and transforming electromagnetic signals which represent various types of information.

Claims

CLAIMS1 . A Low Energy Very Low Frequency (VLF) Communication System comprising: at least one VLF Transmitter; and a VLF Receiver, wherein a VLF Transmitter from the at least one VLF Transmitter: generates data symbols using M-ary Frequency Shift Keying (FSK) modulation in which a FSK symbol duration is a reciprocal of a frequency of a local Alternating Current (AC) power distribution network; and transmits a transmitted signal; and wherein the VLF Receiver: receives the transmitted signal; samples the transmitted signal and uses a separate sliding Goertzel filter to process a block of sampled signals; concurrently filters each FSK signal frequency; determines a receiver symbol timing offset with respect to the VLF Transmitter; and demodulates the transmitted signal, the VLF receiver performing the sampling, filtering and determining upon receiving a short preamble at a beginning of a single message from the VLF Transmitter.

2. The system of claim 1 where the block of sampled signals processed by each Goertzel filter has the same duration as the transmitted symbol.

3. The system of claim 1 , wherein a sampling rate is 4000 times an FSK symbol rate.

4. The system of claim 1 where the receiver symbol timing offset with respect to the VLF Transmitter is determined by processing a preamble on one or more of the FSK frequencies that comprises an alternating 0, 1 pattern and is at least 4 symbols long.

5. The system of claim 1 where the sliding Goertzel filter repeatedly processes a 4 symbol sequence of the preamble by sampling a block of sampled signals at a receiver sampling rate, and successively offsetting the time of a first sample to determine which first sample timing gives the largest filter output.

6. The system of claim 1 where the first sample timing giving the largest output from the preamble is used to demodulate the rest of the data bits in the message.

7. The system of claim 1 where this first sample timing is also used to determine the receiver symbol timing offset between symbol clocks of the VLF Receiver and VLF Transmitter and output a control message or signal at a time reference to the VLF transmitter clock.

8. The system of claim 7, wherein the timing offset is based on a plurality of preceding packets in the transmitted signal.

9. The system of claim 1 where synchronization and demodulation of the VLF Receiver are completed during reception of a single message from the VLF transmitter.

10. The system of claim 1 where one or both of the VLF receiver or the VLF Transmitter enter a sleep state to reduce energy consumption.11 . The system of claim 1 where the VLF Receiver uses a 3-axis antenna in order to detect the transmitted signal from the VLF Transmitter from any direction.

12. The system of claim 11 , wherein the VLF Receiver is configured to select one axis of the 3-axis antenna concurrently with detection of the transmitted signal based on received signal levels and threshold values.

13. The system of claim 1 where the FSK modulation uses two frequencies, a preamble comprising an alternating 0, 1 pattern that is four symbols long and each sliding Goertzel filter being synchronized independently to a transmitter symbol rate.

14. The system of claim 1 where adjacent transmitted signals are located between harmonics of the AC power distribution network.

15. The system of claim 1 where signal processing performed at the VLF transmitter and VLF Receiver are implemented entirely in firmware or software executed by a low power Micro Controller Unit (MCU).

16. The system of claim 1 , wherein the transmitted signal includes a guard time between successive symbols, and wherein the VLF receiver is further configured to increase a noise threshold for detecting signals during the guard time.

17. A method for minimizing energy consumption of a Transmitter and a Receiver forming a VLF communication link, the method comprising; transmitting a message with a preamble using M-ARY Frequency Shift Keying (FSK) to modulate a magnetic field, where a symbol duration in the message is a reciprocal of a frequency of a local Alternating Current (AC) power distribution network, using a VLF antenna to convert the FSK modulated magnetic field into an electromagnetic field (EMF), which is then sampled by the Receiver and filtered using a sliding Goertzel filter using a block size identical to the symbol duration; repeatedly processing a 4-symbol sequence of the preamble by sampling a block of sampled signals at the receiver sampling rate, successively offsetting the time of the first sample to determine which first sample timing gives the largest output from the Goertzel Filter; and using this first sample timing to determine a symbol clock offset between the Receiver and Transmitter to demodulate a data bits in the remainder of the message.

18. The method of claim 17 where a sampling rate at the Receiver is harmonically related to a FSK symbol rate in the message.

19. The method of claim 18 where the symbol clock offset between the Receiver and the Transmitter is determined by processing a preamble on one or more FSK frequencies and comprises an alternating data pattern of the form 0, 1 , 0,1 .

20. The method of claim 18 where energy consumption at the Receiver and Transmitter are minimized by completing synchronization and demodulation during reception of a single message from the VLF transmitter.21 . The method of claim 17 where at least one of the Receiver or Transmitter may enter a sleep state to reduce energy consumption between message transmissions.

22. The method of claim 17, further comprising using a timing reference to control or obtain data from external, connected devices with messages or control signals that are output simultaneously between multiple receivers that have each synchronized to a single transmitter message.

23. The method of claim 17 where an antenna of the Receiver is a 3-axis antenna.

24. The method of claim 17 where FSK modulation uses two frequencies, a preamble comprises an alternating 0, 1 pattern that is four symbols long and each filter is synchronized to a transmitter symbol rate.

25. The method of claim 17 where frequencies of adjacent transmitted messages are located between harmonics of the local AC power distribution network.

Images