Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B11/00—Transmission systems employing sonic, ultrasonic or infrasonic waves

Definitions

• the present invention relates to digital communications networks, and to communications using acoustic energy.
• ITS Intelligent Transit System
• RF Radio-Frequency
• bandwidth that may be applied within the context of the ITS and other similar transit applications.
• bandwidth should be "clutter-free” and unregulated so as to allow for the consistent commercial use of such bandwidth from jurisdiction to jurisdiction.
• the present invention addresses this need.
• a digital communications network is provided using digital sonic and ultrasonic communications- i.e., communications using acoustic energy instead of RF energy.
• the "acoustic spectrum, " as opposed to the RF spectrum, is uncluttered and unregulated, allowing for unfettered commercial development of equipment for ITS applications as well as a wide variety of other applications, including applications that currently employ RF communications.
• Exemplary applications include electronic toll boothing, controlled entry systems, border crossing systems, etc. Coding and processing techniques are employed that allow acoustic communications, including the communication of digital data, to be reliably transmitted and received even in noisy acoustic environments.
• a digital acoustic communications apparatus includes one or more digital acoustic communications devices comprising a data processor; memory coupled to the data processor and storing digital data; and means for transmitting and/or receiving digital data acoustically; wherein the acoustic digital communications apparatus, during operation, transmits and/or receives digital data acoustically.
• the memory stores at least one of an identifying code word and a command, and the means for transmitting and/or receiving transmits and/or receives at least one of said identifying code word and said command acoustically.
• the means for transmitting and/or receiving may be an acoustic digital communications transmitter operating in the human audible range or may be an acoustic digital communications transmitter operating in the ultrasonic range.
• the means for transmitting and/or receiving may be an acoustic digital communications receiver comprising an analog-to-digital converter, wherein the data processor comprises a digital signal processor coupled to the analog-to-digital converter for filtering a digital representation of a received acoustic signal and for recovering digital data symbols encoded therein.
• the acoustic digital communications receiver may operate in the human audible range or may further comprising a downconverter, whereby the acoustic digital communications receiver operates in the ultrasonic range.
• the means for transmitting and/or receiving may be a digital acoustic transceiver comprising an input sound transducer, an analog-to-digital converter coupled to the input sound transducer, an output sound transducer, and a digital-to-analog converter coupled to the output sound transducer;
• the data processor may be a digital signal processor coupled to the analog-to-digital converter for filtering a digital representation of a received acoustic signal and for recovering digital data symbols encoded therein, and coupled to the memory and to the digital-to-analog converter for transmitting the identifying code word or the command stored in memory acoustically.
• the acoustic digital communications transceiver may operate in the human audible range or in the ultrasonic range.
• a system in accordance with another aspect of the present invention comprises a plurality of digital acoustic communications devices including a plurality of acoustic digital transmitters and at least one acoustic digital receiver for, when one of said acoustic digital transmitters is within range and transmitting digital information, receiving said digital information.
• the system preferably further comprises a computer and at least one wide area network communications link established between the acoustic digital receiver and the computer. More preferably, the system comprises multiple acoustic digital receivers and multiple wide area network communications links, one such link being established between each of a plurality of said acoustic digital receivers and said computer.
• a method of digital communications comprising the steps of generating a carrier signal; modulating the carrier signal in accordance with digital information to produce a modulated signal; and applying the modulated signal to an acoustic transducer to produce a coded acoustic signal.
• the coded acoustic signal is propagated across a distance many times a wavelength of the coded acoustic signal. Further steps include receiving the coded acoustic signal and transducing the coded acoustic signal to produce a modulated signal; and demodulating the modulated signal to produce the digital information.
• Uses of the communications method are many and varied.
• One such use comprising the steps of providing an acoustic digital communications transmitter to be carried with a moving object; providing an acoustic digital communications receiver in proximity to a controlled area; transmitting from the acoustic digital communications transmitter at least one of an identifying code word that identifies the acoustic digital communications transmitter and a command; receiving at the acoustic digital communications receiver the identifying code word or command; and in response to at least one of the identifying code word or command, allowing physical access of the moving object to the controlled area.
• Another such use comprises the steps of providing an acoustic digital communications transmitter to be carried with a moving object; providing an acoustic digital communications receiver within an area to be monitored; transmitting from the acoustic digital communications transmitter an identifying code word that identifies the acoustic digital communications transmitter; receiving at the acoustic digital communications receiver the identifying code word; and when the code word is not received within a predetermined interval of time, producing an alarm indication.
• Still a further use comprises the steps of providing a first acoustic digital communications transceiver to be carried on an object; providing a second acoustic digital communications transceiver at a fixed location; transmitting from one of the first and second acoustic digital communications transceivers a query message; receiving the query message at another of the first and second acoustic digital communications transceivers and transmitting a response message; determining a one-way propagation time between the first and second acoustic digital communications transceivers; and determining a distance between the first and second acoustic digital communications transceivers.
• the location of the object may be intended to remain fixed for a time, in which case the foregoing steps are repeated multiple times; a determination is made whether the location of the object has changed; and if the location of the object has changed, an alarm indication is produced.
• the first acoustic digital communications transceiver may be a mobile acoustic digital communications transceiver carried on a moving object
• the second acoustic digital communications transceiver may be a base acoustic digital communications transceiver, in which case the foregoing steps are repeated multiple times; and a rate of change of location of the object is determined.
• the foregoing steps may be repeated at multiple base acoustic digital transceivers and results communicated from the multiple base acoustic digital transceivers to a common site. In this manner one or both of a location and a heading of the object may be determined.
• Figure 1 is a timing diagram of a Coded Audio Signal transmitted at fixed intervals
• Figure 2 is a block diagram of a Coded Audio Sound Generator
• Figure 3 is a plot of a correlation output for a codeword UW1 in no noise
• Figure 4 is a plot of a correlation output for codeword UW1 in noise, with the noise level set at twice the signal level;
• Figure 5 is a block diagram of a Coded Detector Module System
• Figure 6 is an equivalent functional block diagram of a portion of the Coded Detector Module of Figure 5 realized by the DSP
• Figure 7 is a timing diagram of a Coded Audio Signal transmitted at fixed intervals
• Figure 8 is a diagram of an Audio Command Packet
• Figure 9 is an equivalent functional block diagram of a portion of the Coded Detector Module realized by the DSP
• Figure 10 is a block diagram of an ultrasonic Transit Control Module
• Figure 11 is a block diagram of a Coded Audio Transceiver Module
• Figure 12 is a diagram illustrating a communications sequence allowing a distance ranging operation to be performed using acoustic energy
• Figure 13 is a timing diagram illustrating the timing of query and response messages for purposes of performing a distance ranging calculation
• Figure 14 is a block diagram of an acoustic digital communications network, in particular a network for geolocation;
• Figure 15 is a diagram of a portion of a dedicated short-range communications system.
• Building blocks of the present digital sonic and ultrasonic communications networks include sonic transmitters (sound generators), sonic receivers (detectors) and sonic transceivers (sound generators and detectors). Different embodiments of these building blocks may possess varying degrees of sophistication. Whereas the sound generators are relatively simple in their construction, the sound detectors rely on Digital Signal Processing techniques to achieve accurate detection over moderate distances (on the order of one mile).
• the first embodiment provides the ability to uniquely detect a particular coded sound generator.
• the second embodiment adds to this unique detection capability the further ability of a vehicle operator to have the detector take specific actions.
• an ultrasonic downconverter module is provided, allowing the coded sound detector to operate in the ultrasonic range.
• a Coded Audio Detector Module is DSP-based.
• a vehicle is equipped with a special Coded Sound Generator which issues "codewords" at fixed intervals, or on command of the driver. Between the transmission of these special codewords, the Coded Sound Generator need not emit any sound.
• the DSP-based Coded Audio Detector Module receives the foregoing codewords, decodes the codeword to determine if it is one of a pre-determined set of codewords recognized as being valid, and then issues a signal to a controller if the codeword assigned to that vehicle is valid.
• the Coded Audio Detector Module is used as part of a two-part system.
• the first part is an audio-based transmitter system on each vehicle that is to control or interact with the system.
• the transmitter sends a codeword at specific time intervals, say 5 seconds, using binary FSK modulation of the audio carrier for example.
• Each vehicle is provided with a Coded Sound Generator.
• This Coded Sound Generator may be a simple audio generator the output of which is input to the microphone input of an amplifier. This Coded Sound Generator generates the appropriate codeword.
• a programmed microprocessor 201 is coupled to a Digital to Analog Converter (DAC) 203.
• An output of the DAC 203 is coupled to an amplifier 209, which is coupled to a speaker 211.
• a mode selection switch 213 and a manual signal switch 215 are also provided and are coupled to the microprocessor 201.
• the microprocessor 201 reads the mode selection switch 213 to determine if the operator wants the Coded Sound Generator to be activated continuously at intervals or only upon user command.
• the microprocessor 201 generates synthetic digital waveforms representing the desired codeword. These signals are converted to an analog voltage by the DAC 203 and then input to the amplifier 209.
• the manual signal switch 215 allows the operator to generate codeword signals at will rather than at timed intervals.
• the CADM will only issue a control signal when a coded signal which meets specific conditions is detected. This feature allows for greater security and reliability of operation.
• the CADM receives the codeword using a microphone system and then demodulates the audio codeword. Demodulation is performed using an FSK demodulator.
• the CADM first synchronizes to the incoming bit stream by performing a symbol timing recovery operation on the codeword. Once synchronized, the FSK CADM searches for the codeword. The search may be done by binary correlation with threshold detection, using the stored reference codewords (UW1 to UW7) as a reference. If the codeword is received with more bits matching the stored reference pattern than the threshold value, it will be processed and the desired commands will be interpreted and issued to the controller. If the packet was received with an uncorrectable number of errors, the command will be rejected and no signals will be sent to the controller.
• the 15-bit codeword UW1 was used an example.
• the codeword was sent as a 15-bit sequence using the binary FSK modulation scheme discussed earlier, and was preceded and followed by silence.
• the signal was processed using a binary correlation algorithm, and the correlation output was plotted as a function of time as shown in Figure 3.
• the maximum in the correlation is seen to occur at about sample 101, which marks the location of the codeword in time.
• the maximum of the correlation value is 60, since the 15-bit codeword was sampled at 4 samples per bit. If a threshold value of, say, 56 was taken as a detection threshold, then only correlation outputs in excess of 56 would cause the microphone system to indicate the presence of the codeword.
• a Coded Sound Generator 501 is coupled to a loudspeaker 503.
• the coded signal produced by the Coded Sound Generator and 501 is picked up by a transducer 505 and input to a DSP-based logic board 507.
• Codeword Search may be based on a binary correlation with threshold detection technique, using the pre-stored reference codewords as templates for the binary correlation.
• the software is modified to include a command parser to determine which codeword was received and to then take appropriate action based on the command and data in the command packet.
• An equivalent functional block diagram of the CADM is shown in Figure 6.
• An output signal from a microphone 601 is filtered in a band-pass filter 603.
• the filtered signal is then input to a combination of a discriminator 605, a decimator 607 and a median filter 609.
• An output of the median filter 609 is coupled to a symbol synchronization block 611 , followed by a codeword search block 613.
• An output of the codeword search block is input to a block 615 to control whether a signal is issued to the controller.
• Also input to the block 615 is the output of the decimator 607, indicative of the received signal level.
• the discriminator, decimator, and median filter operations are the same, to ensure the highest sensitivity possible for the CADM based on the excellent signal detection capability inherent in that technique.
• Signal detection is followed by the operations of blocks 611 , 613 and 615, required to decode the codeword and then execute the command associated with that codeword.
• the CADM may be provided with a multiplicity of channels. Different channels are allocated to different approaches to an installation. In the vast majority of cases, a four channel detector system will suffice. Cases with more than four approaches may be dealt with by assigning additional channels.
• the DSP-based Coded Audio Detector Module offers increased functionality, above and beyond that of the first embodiment.
• the CADM is based on the DSP siren detector module of McConnell et al. , with the following exceptions.
• the vehicle which is to activate the control function is equipped with a special Coded Sound Generator which issues coded "frames" at fixed intervals or on command of the driver. Between the transmission of these special coded frames, the Coded Sound Generator need not emit any sounds.
• the DSP-based siren detector is modified to receive the coded frames, decode the frame content, and then issue a control signal if the unique address assigned to that vehicle matches one of a list of addresses that the detector recognizes as being valid.
• the Coded Audio Detector Module is part of a two part system.
• the first part is an audio based transmitter system on each vehicle that is to control or interact with the system.
• the transmitter at specific time intervals, say 5 seconds, sends a packet frame, using binary FSK modulation of the audio carrier for example.
• the physical hardware used to realized the Coded Sound Generator may be the same as previously described in relation to Figure 2.
• the microprocessor reads the mode selection switch to determine if the operator wants the sound generated at intervals or only upon user actuation of a switch, for example.
• the microprocessor generates synthetic digital waveforms representing the packet frame (as opposed to a singular codeword as in the previous embodiment). These signals are converted to an analog voltage by the Digital to Analog Convertor (DAC) and then input to the amplifier.
• DAC Digital to Analog Convertor
• a manual signal switch is also available to allow the operator to generate packet frame signals at will rather than at timed intervals.
• the preamble is typically an alternating binary sequence, such as 1010101010.
• Frame Synch - to provide word alignment to the control, data, and parity portions of the command packet.
• the frame synch is typically a short binary sequence such as a Barker code, Lindner Sequence, Maury-Styles Sequence, etc.
• One suitable frame synch word consists of the binary sequence 0010 0000 0111 0101.
• Header - this field of the frame contains binary address information that is unique to each vehicle in the fleet, as well as a packet type identifier, and control flags.
• the actual binary sequence depends on the values given to the elements of the header field.
• the field is 20 bits in length.
• Data - this field may consist of anywhere from 0 bits to say 256 bits of information. In order to process the commands expeditiously, this field will typically be kept small in practice and may only consist of 8 bits of data. Parity - parity information to be used for error correction and detection in the packet. If a CRC-16 is used, then 16 parity bits are required.
• Postamble a known sequence of data used to allow clearing of buffers in the receiver.
• the postamble would typically be a short sequence of alternating binary l 's and O's, such as 101010.
• the header, data, and parity bits may be exclusive OR'ed with a known but secret pseudorandom Number (PRN) sequence to provide scrambling and a limited degree of security.
• PRN pseudorandom Number
• Other more elaborate encryption schemes may also be applied, if desired.
• the CADM module receives the packet using a microphone system and demodulates the audio packet command. Demodulation is performed using an FSK demodulator.
• the CADM first synchronizes to the incoming bit stream by performing a symbol timing recovery operation on the packet, with the preamble being used to assist this synchronization. Once synchronized, the FSK CADM searches for the frame synch word to achieve frame synchronization, after which it applies the PRN descrambling sequence, then extracts the command, data, and parity fields from the packet.
• the parity bits to perform error correction and/or detection on the control and data fields of the packet. If the packet is received without errors or with a correctable number of errors, it will be processed and the desired commands will be inte ⁇ reted and issued to the controller. Examples of such commands are commands to open a gate, etc. If the packet was received with an uncorrectable number of errors, the command will be rejected and no signals will be sent to the controller.
• the functionality of the CADM as described is implemented using substantially the same hardware platform as the DSP-based siren detector of McConnell et al. (See Figure 5.) Only the DSP software is changed.
• the detection algorithm may be based on the limiter/discriminator approach of McConnell et al. , but includes in addition a low-rate demodulator to perform symbol timing recovery and synch word detection.
• the header and data fields are processed by the CPU for activation of transit control signals, for example.
• the CADM uses the same limiter discriminator operations as described in McConnell et al. to perform FSK demodulation of the FSK signal.
• the software is modified to inco ⁇ orate the following additional functions:
• Symbol Timing Recovery this may be based on a simple early/late-gate symbol synchronizer.
• Error Correction/Detection Coding this may be based on any of a number of well established error correction coding schemes, such as Hamming codes, Golay codes, BCH, etc.
• the software is modified to include a command parser to extract, inte ⁇ ret, and to then take appropriate action based on the command and data in the command packet.
• FIG. 9 An equivalent functional block diagram of the CADM is shown in Figure 9.
• An output signal from a microphone 901 is filtered in a band-pass filter 903.
• the filtered signal is then input to a combination of a discriminator 905, a decimator 907 and a median filter 909.
• An output of the median filter 909 is coupled to a symbol synchronization block 911.
• the block diagram of the enhanced CADM is identical to that of Figure 6.
• the symbol synchronization block 911 is followed by a frame synch block 913 and an optional descrambler 915.
• An output of the descrambler 915 is input to an error correction block 917.
• the error-corrected packet frame is then input to a command parser 919.
• the command is input to a block 920, where the validity of the command is checked. If the command is valid, a block 921 is notified, which controls whether a signal is issued to the controller. Also input to the block 921 is the output of the decimator 907, indicative of the received signal level.
• both the CADM of the first embodiment and the CADM of the second embodiment may be based on ultrasonic sound energy operating above the threshold of human hearing (i.e. > 20 kHz).
• the module when used to perform detection of ultrasonic control signals in transit applications is referred to herein as a Transmit Control Module, or TCM.
• the TCM detects a digital packet which is binary FSK coded onto an ultrasonic audio carrier at approximately 44 kHz, although multilevel FSK (i.e. 4-level FSK) could also be used.
• the packet structure may be the same as that the CADM of the second embodiment of the invention, described previously in relation to Figure 8.
• the TCM like the CADM previously described, is implemented using a DSP/CPU logic board 1001.
• the DSP/CPU logic board issues control signals to a controller 1003.
• the DSP/CPU logic board is preceded by a conventional audio down-conversion board 1005 which converts the 44 kHz ultrasonic carrier to a nominal carrier frequency of 1 kHz.
• the ultrasonic carrier may be modulated by ⁇ 500 Hz to represent binary 1 or 0.
• the down-converter simply converts the carrier frequency, in this case 44 kHz, down to an intermediate frequency which is suitable for processing by the TCM (300 Hz to 2500 Hz).
• the intermediate frequency is chosen to be 1 kHz.
• a low pass filter (LPF) 1009 follows a mixer 1007 to ensure that the only the difference frequency is processed by the TCM.
• the sound level of the ultrasonic sound is also used in determining if the received command is valid. This feature ensures that only vehicles in close proximity to the TCM can actually issue a command at a specific installation.
• a Coded Audio Transceiver Module results by appropriately combining elements of the Coded Sound Generator of Figure 2 and the Coded Audio Detector Module of Figure 5.
• a DSP-based logic board 1103 is coupled to an input sound transducer 1101 through an A/D converter 1105.
• the DSP-based logic board is also coupled to a Coded Sound Generator consisting of a D/A converter 1107 and an output sound transducer 1109. If the CATM is to operate in the ultrasonic region, a down-converter 1102 and an up-converter 1108 may also be provided. In many instances, the CATM will be coupled to a controller 1111.
• distance ranging In transit applications, it is often desired to know the distance of a vehicle from a particular location (distance ranging). It may further be desired to know the actual location of the vehicle, its speed, heading, etc. In a bus system, for example, passengers at a bus stop would be interested to know how far away the next bus assigned to a particular route is, and its estimated time of arrival at the bus stop. All of the foregoing information may be obtained through the use of a digital sonic or ultrasonic communications network.
• distance ranging is based on the two way messaging concept used in the Transit Detector Module. It requires that an intersection-mounted unit and a vehicle-mounted unit both support a transmit and receive capability. In brief, what the system does is as follows.
• the intersection-mounted unit sends out periodic Query messages which contain a message sequence number. These are short transmissions which are addressed to all vehicle-mounted units (called mobiles).
• a mobile Upon receiving this mobile Query message, a mobile will send a Response message back to the base which contains that vehicle's identity code and the message sequence number to which it is responding. If the mobile always transmits the Response message a fixed time after receiving the Query message, the base unit can calculate the distance of that particular mobile from the intersection. If the mobile responds to a number of sequential Query messages, the distance of the mobile over time can be established, hence enabling the vehicle velocity to be derived.
• T is the time for the Query message to cover the distance between the base and the mobile.
• the time for the Response message to cover the distance between the base and the mobile is simply given by same equation. If TP is used to represent the total processing time of the message by mobile and the base, then the total time TTOTAL for a Query message to be sent by the base and receive a Response message back from the mobile is: T OTAL ⁇ T + T + Tp
• the Query message acts as an ALL CALL message to any vehicle in the vicinity of the base that it wants any vehicle hearing the Query to respond with the message sequence number and the vehicle ID.
• the reason for the message sequence number in the Query and Response messages is to avoid a situation where a distant vehicle hears the Query and sends a response message which arrives after the base has issued a second Query message. In such a situation, the base could falsely inte ⁇ ret the mobile as being closer than it actually is.
• the "mobile" units may be stationary, and it may be desired to know if they actually move when they are not expected to.
• An example of this is in monitoring cargo trailers in storage facilities. These trailers are not expected to move unless a controller or yard manager has issued a release for that particular trailer. If the base detects a trailer as moving when it shouldn't, it may raise an alarm to indicate possible theft of the trailer unit.
• Figure 13 illustrates the timing of messages between the base and mobile, and how the base unit measures speed and distance using this technique.
• the time duration T represents the one-way time of travel for a message between the mobile and the base
• TP represents the processing time.
• This example shows the mobile receiving a Query message with the Sequence Number 1.
• the mobile Upon receiving the Query, the mobile responds T + TP seconds later with a Response message containing the Sequence Number of the Query it received plus the MOBILE ID number.
• the base receives this Response T seconds after transmission by the mobile. If the base sends Query messages at frequent rate, say once every 5 seconds for example, then it can estimate the vehicle velocity by dividing the distance of the vehicle at two sequential locations by the time between the queries. This capability may be used to provide management information to determine what the vehicle velocity was at fixed distances from the base. This information is useful in determining if the mobile approaching the location where the base unit is located is: approaching faster than a recommended speed; approaching slower than a recommended speed; accelerating as it approaches the location; decelerating as it approaches the location, etc.
• This technique can be applied using the two-way communications device at audible or inaudible (e.g. , ultrasonic) frequencies, with the only change being that the speed of sound used in determining the distance is chosen to correspond to the speed of sound at the frequency of operation used.
• audible or inaudible e.g. , ultrasonic
• the technique can be applied to vehicles in motion or those that are stationary. However, its application is not limited to vehicles. In general, it is applicable to any object the distance and or speed of which is to be determined as a function of time.
• Other exemplary uses may include, without limitation:
• each base (1412a, b and c) sends Query messages which contain the ID of the base.
• a mobile 1413 sends a Response to every Query it hears from every base unit, the Response including not only the time t but also the ID of the base unit it heard, along with the Sequence Number and Mobile ID.
• Each base unit 1412 calculates the distance and speed pertaining to the MOBILE ID, and sends this information to a central computer 1416 along with the BASE ID of the base that sent the Query and the Time at which the measurement was made.
• the central computer 1416 receives similar information from the other base units.
• the central computer 1416 then has information pertaining to each MOBILE ID which would consist of: the distance to a particular based identified by the BASE ID; and the time the mobile was at that distance. Since the central computer 1416 knows the coordinates of every base
• the system of Figure 14 provides a two-way audio communication system in which one or more vehicles may communicate with a network of fixed or mobile units (which could be one unit) to exchange command, control, and information between the devices.
• the vehicles may be mobile or fixed.
• the communication network is based on an acoustic communication medium and not an electromagnetic one.
• Each network element typically comprises an acoustic transceiver, i.e. , an acoustic transmitter and receiver. It may send commands or requests to mobile/fixed vehicles, or even to other network elements.
• the network element may do a number of things based on these commands/requests, such as: relay this information to some application program running on a computer somewhere in the communications network, and relay a response back to the device over the network element; or convey this command/request from a mobile/fixed vehicle to another mobile/fixed vehicle from the same network element or via another network element elsewhere in the network.
• Communication between the mobile end systems and the network occurs using acoustic energy, which is typically in the range of about 100 Hz to over 100kHz.
• CDPD Cellular Digital Packet Data
• IP Internet Protocol
• Digital acoustic communications may also be used to advantage to provide information services to vehicle occupants.
• vehicles are equipped with CD-ROMs for use in navigation.
• the user is required to ascertain and enter into a computer the vehicle's location, in addition to desired destination.
• the computer will then retrieve and display a map of the locale, showing a selected route to the destination. In strange surroundings, however, ascertaining one's location is not always an easy task.
• a Dedicated Short- Range Communications System (DSRCS) is shown.
• a large number of DSRCSs may be supported by a wide area communications network, e.g. , a typical land-line type of network which supports ITS applications such as fleet management, emergency management, etc.
• GIS Graphical Information System
• the controlled area may be a structure such as a garage, a toll-bridge or toll-road, etc. Alternatively, the controlled area may be a geographic area, as in the case of border crossings between states or countries.
• Acoustic transponders may also be carried on one's person. Exemplary applications of personal acoustic transponders include access control and personnel monitoring. An acoustic transponder may be used to provided access to a locked building, for example. An acoustic transponder may also be used to monitor the whereabouts of children or pets. Numerous other applications of sonic systems as described herein will be apparent to one of ordinary skill in the art.

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