WO2024249781A2

WO2024249781A2 - Systems, methods and apparatus for distributed acoustic beamforming for practical iot applications

Info

Publication number: WO2024249781A2
Application number: PCT/US2024/031898
Authority: WO
Inventors: Mohammad Rostami; Karthikeyan Sundaresan
Original assignee: Georgia Tech Research Institute; Georgia Tech Research Corp
Current assignee: Georgia Tech Research Institute; Georgia Tech Research Corp
Priority date: 2023-05-31
Filing date: 2024-05-31
Publication date: 2024-12-05
Anticipated expiration: 2025-11-30
Also published as: WO2024249781A3

Abstract

An acoustic system may include an array of wirelessly coupled acoustic tags, the array including a master tag configured to receive an exciter signal from a device, and at least one helper tag configured to receive the exciter signal from the device and a relayed signal from the master tag.

Description

SYSTEMS, METHODS AND APPARATUS FOR DISTRIBUTED ACOUSTIC BEAMFORMING FOR PRACTICAL IOT APPLICATIONS

TECHNICAL FIELD

[0001] The present disclosure generally relates to systems, methods and apparatus for distributed acoustic beamforming.

BACKGROUND OF THE INVENTION

[0002] Recently, there has been significant growth of wireless-based internet of things (loT) applications in numerous fields including, for example, smart homes, industrial automation, smart city and transportation, and the like. This has contributed to research advances in low-power radio frequency (RF) based loT solutions that can work with commodity wireless devices employing, for example, WiFi and Bluetooth low energy (BLE) interfaces. Research in enabling ultra-low power, yet reliable communication for loT devices using RF signals has progressed in recent years. However, while RF tags (e.g., RF identification (RFID) tags) may have inter-operability advantages, the coexistence of RF tags with other wireless devices in the surrounding environment may face challenges that hinder their practical deployment such as, for example, (i) operating at low spectral efficiencies (SE) compared to regular data traffic, and bringing down the SE of the entire network; (ii) interference with other wireless devices in the same spectrum, whose transmissions may be significantly more powerful; and (iii) an inherent half-duplex nature that may require more than a single commodity device to be involved in communications with RF tags.

[0003] Research on backscatter has been conducted to enable RF communications at low energy footprints. One class of these works, called commodity backscatter, may rely on existing infrastructure and commodity devices to enable passive transfer of data bits from wireless tags over the same RF spectrum used for existing communications (e.g., Bluetooth, WiFi, LoRa, cellular, etc.). However, these RF-based low-power solutions may face several obstacles in practical deployment such as coexistence, RF spectral efficiency, and deployment configurations. RF tags may share the already crowded and unlicensed industrial, scientific and medical (ISM) bands used by WiFi and Bluetooth devices, which may make it challenging to enable reliable communication for RF tags in the presence of stronger, high traffic devices available in the surrounding environment. RF tags may also occupy the entire RF channel (e.g., a 20 MHz WiFi channel) while only being able to send a few tens to hundreds of kilobits per second (kbps) of data. This may have a large impact on the spectral efficiency of the underlying RF channel that is otherwise used by devices at much higher bit-rates. For example, ten RF tags that transmit at 100 kbps may each incur a total time of 10 milliseconds (ms) to deliver just 100 bits each, which may make it difficult to scale their application. In addition, commodity backscatter designs for RF tags may require the use of two separate commodity devices (e.g., owing to the half-duplex operation of RF tags): one for transmitting the carrier signal to the RF tags, and another for receiving data from the RF tags, which may be on a different frequency to avoid interference. This may make deployment of RF tags challenging in practice.

[0004] Recent years have seen the widespread adoption of voice as a preferred modality of user interaction. Coupled with advances in natural language processing (NLP), this has led to the incorporation of acoustic interfaces (e.g., microphones, speakers, etc.) in a variety of everyday devices such as, for example, security systems, voice assistants, smart appliances, smartphones, and the like.

SUMMARY OF THE INVENTION

[0005] According to some aspects of the present disclosure, an acoustic system may include an array of wirelessly coupled acoustic tags, the array including a master tag configured to receive an exciter signal from a device, and at least one helper tag configured to receive the exciter signal from the device and a relayed signal from the master tag.

[0006] In some embodiments, the relayed signal includes a data signal, and the at least one helper tag is configured to transmit data included in the data signal to the device.

[0007] In some embodiments, the at least one helper tag includes a plurality of helper tags, and the plurality of helper tags are configured to transmit the data to the device using beamforming.

[0008] In some embodiments, the plurality of helper tags are configured to perform temporal beamforming to transmit the data to the device.

[0009] In some embodiments, the master tag is configured to scramble the data with a predetermined sequence to resemble white noise before transmitting the relayed signal to the at least one helper tag.

[0010] In some embodiments, the relayed signal further includes a version of the exciter signal, and the version of the exciter signal is followed by the data signal.

[0011] In some embodiments, a bandwidth of the data signal is greater than a bandwidth of the exciter signal.

[0012] In some embodiments, the master tag is configured to perform a backscatter operation to transmit the relayed signal to the at least one helper tag.

[0013] In some embodiments, the array of wirelessly coupled acoustic tags are coupled through an acoustic interface, and the exciter signal and the relayed signal each include an acoustic signal.

[0014] In some embodiments, the at least one helper tag is configured to determine an angle of departure to the device, based on a time difference of arrival between the exciter signal and the relayed signal. [0015] In some embodiments, the at least one helper tag includes a plurality of helper tags, each of the plurality of helper tags includes a local timer, and the local timer is synchronized to a common time frame of reference, based on the relayed signal.

[0016] In some embodiments, the plurality of helper tags are configured to transmit data included in the relayed signal to the device, and delay their respective transmissions to the device, based on the local timer and a time difference of arrival between the exciter signal and the relayed signal, such that the respective transmissions constructively add at the device.

[0017] In some embodiments, respective distances between the master tag and each of the plurality of helper tags are fixed, and the common time frame of reference is determined based on the respective distances and the relayed signal.

[0018] In some embodiments, a location of the device is unknown to the at least one helper tag prior to receiving the relayed signal.

[0019] In some embodiments, the master tag is configured to delay transmission of the relayed signal to the at least one helper tag such that the at least one helper tag receives the exciter signal before the relayed signal.

[0020] In some embodiments, the device is configured to request data from the array of wirelessly coupled acoustic tags, and the array of wirelessly coupled acoustic tags is configured such that the data requested by the device determines which one of the acoustic tags is the master tag.

[0021] According to some aspects of the present disclosure, an acoustic system may include an array of wirelessly coupled acoustic tags including a master tag and at least one helper tag. The master tag may be configured to receive a first signal from a device that includes an acoustic interface, and transmit a second signal to the at least one helper tag. The at least one helper tag may be configured to receive the first signal from the device and the second signal from the master tag, and locally determine respective times of arrival of the first signal and the second signal.

[0022] In some embodiments, the master tag is configured to transmit the second signal to the at least one helper tag after receiving the first signal from the device.

[0023] In some embodiments, after receiving the first signal from the device, the master tag is configured to wait for a fixed delay to elapse before transmitting the second signal to the at least one helper tag.

[0024] In some embodiments, the at least one helper tag includes a plurality of helper tags, and the plurality of helper tags are configured to perform beamforming toward the device by respectively delaying a time of transmission to the device.

[0025] In some embodiments, each of the plurality of helper tags includes a local timer and is configured to synchronize the local timer to a common time frame of reference, based on the second signal, and determine a respective angle of departure to the device, based on a difference between the respective times of arrival of the first signal and the second signal. [0026] In some embodiments, the plurality of helper tags are configured to independently schedule respective transmissions at the respective angle of departure to the device, based on the local timer, and determine a relative time offset for the respective transmissions, based on the local timer and the difference between the respective times of arrival of the first signal and the second signal, such that the respective transmissions constructively add at the device.

[0027] In some embodiments, the second signal includes a version of the first signal followed by a data signal, and the beamforming toward the device includes transmitting data included in the data signal to the device.

[0028] In some embodiments, the master tag is configured to scramble the data with a predetermined sequence to resemble white noise before transmitting the second signal to the plurality of helper tags. [0029] In some embodiments, a location of the device is unknown to the at least one helper tag prior to receiving the second signal from the master tag.

[0030] In some embodiments, the array of wirelessly coupled acoustic tags are coupled through an acoustic interface, and the first signal and the second signal each include an acoustic signal.

[0031] In some embodiments, the master tag is configured to perform a backscatter operation to transmit the second signal to the at least one helper tag.

[0032] According to some aspects of the present disclosure, a method of acoustic communication may include receiving, by a plurality of wirelessly coupled acoustic tags, a first signal from a device including an acoustic interface, the plurality of wirelessly coupled acoustic tags including a master tag and at least one helper tag, transmitting, by the master tag, a second signal to the at least one helper tag, and determining, by the at least one helper tag, respective times of arrival of the first signal and the second signal.

[0033] In some embodiments, the method further includes determining, by the at least one helper tag, an angle of departure to the device, based on a difference between the respective times of arrival of the first signal and the second signal.

[0034] In some embodiments, the at least one helper tag includes a plurality of helper tags. [0035] In some embodiments, the method further includes synchronizing the plurality of helper tags to a common time frame of reference, based on the second signal.

[0036] In some embodiments, the method further includes performing, by the plurality of helper tags, beamforming toward the device by respectively delaying a time of transmission to the device.

[0037] In some embodiments, the second signal includes a version of the first signal followed by a data signal.

[0038] In some embodiments, the method further includes whitening, by the master tag, data included in the data signal by scrambling the data with a pre-determined sequence. [0039] In some embodiments, the method further includes selecting, by each of the plurality of helper tags, the time of transmission to the device, based on a respective distance from the master tag and the respective times of arrival of the first signal and the second signal, and transmitting, by the plurality of helper tags, data included in the data signal to the device during the beamforming toward the device.

[0040] Aspects of the present disclosure are not limited to the above. Further aspects of the present disclosure will be understood by one of ordinary skill in the art based on the description hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate example embodiments of the present disclosure and, together with the description, serve to explain principles of the present disclosure.

[0042] FIG. l is a graph illustrating signal -to-noise ratio (SNR) versus distance for a single acoustic tag transmitting at different acoustic frequencies.

[0043] FIG. 2 is a schematic diagram illustrating a distributed acoustic system, according to some embodiments of the present disclosure.

[0044] FIGS. 3 to 6 are schematic diagrams illustrating a sequence of operations in a distributed acoustic system, according to some embodiments of the present disclosure.

[0045] FIG. 7 is a graph illustrating the beam pattern of a distributed acoustic system, according to some embodiments of the present disclosure.

[0046] FIG. 8 is a graph illustrating the scalability of a distributed acoustic system, according to some embodiments of the present disclosure.

[0047] FIG. 9 is a graph illustrating the wideband stability of a distributed acoustic system, according to some embodiments of the present disclosure. [0048] FIG. 10 is a graph illustrating the robustness of a distributed acoustic system relative to aperture size, according to some embodiments of the present disclosure.

[0049] FIG. 11 is a schematic diagram illustrating an example application for a distributed acoustic system, according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

[0050] Various aspects of the present disclosure are directed to systems, methods and apparatus for distributed acoustic beamforming.

[0051] In contrast to the aforementioned limitations of RF tags, acoustic tags may offer a promising alternative for loT applications due to, for example, the acoustic spectrum not interfering with the RF spectrum (since they are in different frequency ranges), the acoustic spectrum being less utilized than the RF spectrum, and commodity acoustic devices often being readily equipped with a speaker-microphone pair to enable full-duplex operation with acoustic tags. Below, several challenges associated with acoustic tags in loT applications are discussed, which the present disclosure aims to address.

[0052] To be compatible with a commodity acoustic device (e.g., a smart device), it may be helpful for acoustic tags to operate in the 0-20 kilohertz (kHz) band, where the device's speaker and microphone exhibit a good frequency response. However, frequencies below 17 kHz may be audible by most people, which is a significant portion of the available spectrum. Another challenge associated with acoustic tags may be operational range. For example, the transmit power of an acoustic tag's speaker may be limited to less than 10 W (i.e., -20 dBm) for an acoustic signal to remain inaudible at a distance of even a few centimeters from the acoustic tag.

[0053] FIG. l is a graph illustrating signal -to-noise ratio (SNR) versus distance for a single acoustic tag transmitting at different acoustic frequencies. In particular, FIG. 1 illustrates the measured acoustic SNR when the acoustic tag is sending a single-tone signal at frequencies of 2 kHz, 7 kHz, and 13 kHz at a -20 dBm transmit power.

[0054] Referring to FIG. 1, if an SNR of greater than 10 dB is desired for a data rate of several tens of kbps, an operational range of only ~ 0.8 meters (m) may be achievable by a single acoustic tag, when all frequencies in the band are leveraged (e.g., when the full 0- 20 kHz frequency band is used for the transmission).

[0055] Employing chirp spread spectrum (CSS) may increase operational range while preserving inaudibility, but this may come at the expense of throughput of just a few bits per second (bps) over the entire 20 kHz frequency band. Bandwidth efficiency and range may also impact the energy efficiency of acoustic tags.

[0056] Acoustic tags may also face challenges related to higher attenuation over the air (OTA), which may make operation beyond a few meters difficult. A resulting by-product may lead to the use of higher power to transmit even a few kbps over a few meters, resulting in lower energy efficiencies.

[0057] Unlike RF, where the wavelengths may fall within a small interval owing to the high carrier frequency, the wavelengths in an acoustic band may vary significantly from, for example, 34 cm to 3.4 cm between 1 kHz and 10 kHz, which may present challenges for phased-array system designs relying on half-wavelength element spacing. The use of additional (e.g., RF) interfaces or wires between acoustic tags for synchronization may reduce the practicality and energy-efficiency of their design. In distributed acoustic systems, data used for beamforming may need to be the same and hence shared across the acoustic tags using the same acoustic interface, which may present design challenges. Also, there may be challenges related to the acoustic tags independently determining how to participate in transmissions to realize the beamforming, along with keeping the total signal power outside of the main beamforming lobe directed at a receiver of a commodity device below the audible threshold for human ears across the entire spectrum of operation. For example, the audible threshold may be 20 dB above the noise level of a silent room.

[0058] In RF communications, an entire data signal with bandwidth BW may be up- converted using a pure tone carrier with frequency fc » BW. Hence, the frequencies of the resulting RF signal may occupy the range (fc ~ BW /2, fc + BW /2). The wavelengths

( .■-■■£■■■ . . may thus correspond to a small interval of F+BVV /2 ’ -Bi /2 _where C is the speed of light. In this case, a phased-array system with its elements spaced a half-wavelength

;. ₌ c apart, where ' F , may work well across all the frequencies in the bandwidth BW. On the other hand, acoustic signals may have wavelengths that vary significantly when they are transmitted at their original frequencies (e.g., 0-20 kHz). Thus, a single phased-array that has each element spaced a half-wavelength

apart may be unable to cater to all the frequencies and effectively leverage the entire 0-20 kHz bandwidth.

[0059] With the proliferation of smart acoustic devices in everyday environments, low power acoustic tags offer a promising alternative to RF for loT applications. However, the operational range, throughput, and/or energy efficiency of acoustic tags may present design challenges for deployment in practical applications.

[0060] Example embodiments of the present disclosure provide various benefits and technical solutions to the foregoing and/or other challenges associated with acoustic tags. A distributed acoustic system according to some embodiments of the present disclosure may enable communication with an exciter device (e.g., a smart device), while leveraging the entire audio spectrum (e.g., 0-20 kHz) for higher bit-rates without sacrificing operational range. In some embodiments, the same acoustic interface employed for beamforming and clock synchronization in the distributed acoustic system may be leveraged for data sharing among an entire distributed array of acoustic tags without relying on alternate interfaces (e.g., RF). After obtaining the data, the acoustic tags may independently determine how to participate in a joint transmission to deliver a desired beamforming effect at the exciter device. The array of acoustic tags in the distributed acoustic system may enable beamforming gain that allows for data decoding at the exciter device, while operating at a low enough transmit power to remain inaudible to human ears at practical distances (e.g., greater than or equal to 30-40 cm) from the array, thereby enabling coexistence in deployment for various loT applications.

[0061] FIG. 2 is a schematic diagram illustrating a distributed acoustic system, according to some embodiments of the present disclosure.

[0062] Referring to FIG. 2, the distributed acoustic system 100 may include a plurality of low power acoustic tags 102 that can be flexibly configured on-demand to form an aperture array capable of distributed beamforming 107. Compared to a single acoustic tag, the array of acoustic tags 102 may increase operational bandwidth, range, and energy efficiency, all while keeping transmissions substantially inaudible to human ears. The array of acoustic tags 102 may yield gains in range, throughput, and power consumption to enable various applications (e.g., loT applications). By configuring a varied number of acoustic tags 102 in different configurations, the distributed acoustic system 100 may adapt the signal-to- noise ratio (SNR) gain of its aperture to cater to different applications and deployment requirements (e.g., throughput, range, power, etc.) as well as the constraints of the object 108 to which they are attached. As used herein, a tag refers to an electronic device that may provide, for example, identification, tracking, and/or data collection services.

[0063] The array of acoustic tags 102 may include a master tag 104 and helper tags 106. In some embodiments, the distributed acoustic system 100 may transform an exciter device 101 and the master tag 104 into a virtual, distributed two-antenna system (DAS) through a backscatter relay. The master tag 104 may receive a first signal 103 (e.g., an exciter signal) from the exciter device 101. The backscatter relay may include the master tag 104 receiving the first signal 103 from the exciter device 101 and re-transmitting the first signal 103 as a relayed signal 105 to the helper tags 106. The distributed acoustic system 100 may configure the acoustic tags 102 such that they jointly perform as a coherent distributed aperture to enable beamforming 107 toward the exciter device 101. As used herein, the first signal 103 may also be referred to as an exciter signal, and the relayed signal 105 may also be referred to as a second signal.

[0064] In some embodiments, the acoustic tags 102 may be wirelessly coupled through an acoustic interface without relying on other interfaces (e.g., RF) to facilitate synchronization. As used herein, an acoustic interface refers to any means of communication or interaction that utilizes sound waves as a medium for transmitting information. In other words, the acoustic tags 102 may be wirelessly coupled using sound waves (e.g., acoustic signals) as a medium for communication without relying on other interfaces (e.g., RF). For example, each of the acoustic tags 102 may include a speaker and microphone to communicate with each other and the exciter device 101 using acoustic signals. The acoustic tags 102 may include the master tag 104 and the helper tags 106. The master tag 104 may receive the first signal 103 from the exciter device 101. The helper tags 106 may also receive the first signal 103 from the exciter device 101 along with the relayed signal 105 from the master tag 104. In some embodiments, the relayed signal 105 may include: (i) a version of the first signal 103, and (ii) a data signal to be beamformed 107 by the helper tags 106 to the exciter device 101. As used herein, the version of the first signal 103 may be the first signal 103 as received from the exciter device 101 by the master tag 104 or may be a modified version of the first signal 103 (e.g., modified by the master tag 104). For example, before relaying the first signal 103 to the helper tags 106, the master tag 104 may add a delay to the first signal 103, append data to the first signal 103, etc. In some embodiments, the master tag 104 may perform a backscatter operation to transmit the relayed signal 105 to the helper tags 106. As used herein, a backscatter operation refers to a method of communication where a device relays an incoming signal to convey information to another device. For example, the master tag 104 may receive the first signal 103 from the exciter device 101 and may retransmit the first signal 103 in the form of the relayed signal 105 to convey information to the helper tags 106. In some embodiments, the master tag 104 may modulate the first signal 103 before retransmitting it.

[0065] The master tag 104 may wirelessly synchronize, share transmission data, and trigger the helper tags 106 to enable their participation in a coherent distributed beamforming transmission 107 from a larger on-demand array. For example, the master tag 104 may provide the trigger for the helper tags 106 to locally estimate their differential time of arrival (ToA) from the master tag 104 and the exciter device 101 (i.e., the distributed two-antenna system). That is, the master tag 104 may provide the trigger for the helper tags 106 to locally estimate a time difference of arrival (TDoA) between the first signal 103 received from the exciter device 101 and the relayed signal 105 received from the master tag 104. As used herein, the exciter device 101 may also be referred to as a commodity device or a smart device. FIG. 2 illustrates that the exciter device 101 is a mobile phone, but the present disclosure is not limited thereto. The exciter device 101 may be any device having an acoustic interface that allows the device to transmit and/or receive acoustic signals. For example, the exciter device 101 may be a device having a speaker and/or a microphone.

[0066] The master tag 104 may include data that the exciter device 101 aims to retrieve. For example, the data may be sensor or product information related to the object 108, but the present disclosure is not limited thereto. The distributed acoustic system 100 may leverage the helper tags 106 in the vicinity of the master tag 104 to transmit the data included in the master tag 104 to the exciter device 101. In some embodiments, the helper tags 106 may transmit the data included in the master tag 104 to the exciter device 101 using distributed beamforming 107 without: (i) explicit knowledge of the location/direction of the exciter device 101, and (ii) wired interconnection between the helper tags 106 or use of alternate interfaces such as RF to facilitate synchronization. Specifically, the helper tags 106 may independently listen to the relayed signal 105 sent by the master tag 104, and independently transmit data included in the relayed signal 105 in an intelligent manner that allows all the signals transmitted by the helper tags 106 to constructively add (e g., through beamforming 107) at the exciter device 101, thereby improving the signal strength at the exciter device 101. FIG. 2 illustrates that the array of wirelessly coupled acoustic tags 102 includes eight helper tags 106 and one master tag 104. However, the present disclosure is not limited thereto, and the array of wirelessly coupled acoustic tags 102 may include any number of acoustic tags to enable distributed beamforming 107. The distributed acoustic system 100 may introduce minimal to no deployment over-head through the easy addition of acoustic tags 102 to the array as needed for increased range of operation and/or SNR gain.

[0067] In some embodiments, any of the acoustic tags 102 may operate as the master tag 104. In other words, any of the acoustic tags 102 may be used on-demand as the master tag 104, while the rest of the acoustic tags 102 may operate as the helper tags 106. In some embodiments, the master tag 104 may rotate among the acoustic tags 102 depending on the data requested by the exciter device 101. For example, each of the acoustic tags 102 may include different data (e.g., different sensor and/or product information), and the data requested by the exciter device 101 may determine which acoustic tag 102 operates as the master tag 104.

[0068] In some embodiments, the acoustic tags 102 may be affixed to an object 108. For example, the acoustic tags 102 may be affixed to the object 108 using an adhesive. In other embodiments, the acoustic tags 102 may be integrated into the object 108. For example, the acoustic tags 102 may be embedded into the object 108 during a process of manufacturing the object 108. [0069] The exciter device 101 may generate the first signal 103 in an inaudible part of the frequency spectrum to humans (e.g., 17-20 kHz), which on reception at the master tag 104 may be relayed wirelessly to the helper tags 106. The first signal 103 may be a narrowband signal and may be relayed by the master tag 104 to synchronize the helper tags 106 in the array. In some embodiments, the distances from each acoustic tag 102 to the exciter device 101 may be unknown, but the separation distance between the master tag 104 and the helper tags 106 may be known and leveraged to synchronize the acoustic tags 102 to a common time frame of reference using the relayed signal 105. In other words, the location of the exciter device 101 may be unknown to the helper tags 106 prior to receiving the relayed signal 105 from the master tag 104. For example, respective distances between the master tag 104 and each of the helper tags 106 may be fixed, and the helper tags 106 may be synchronized to a common time frame of reference using the relayed signal 105 and the fixed distance. The first signal 103 and the relayed signal 105 may each be an acoustic signal. In some embodiments, the distributed acoustic system 100 may assume co-location of a speaker and microphone at the exciter device 101. In other embodiments, the distributed acoustic system 100 may account for a separation between a speaker and microphone in the exciter device 101 by using, for example, inertial sensors.

[0070] The helper tags 106 may measure a TDoA between the first signal 103 and the relayed signal 105, and may use the TDoA to determine a respective angle of departure (AoD) to the exciter device 101, as well as their relative phase offset with respect to the master tag 104. The relayed signal 105 may include a version of the first signal 103 followed by a data signal. A version of the data signal may later be beamformed 107 by the helper tags 106 to the exciter device 101. As used herein, the version of the data signal may be the data signal as received from the master tag 104 by the helper tags 106 or may be a modified version of the data signal (e.g., modified by the helper tags 106). For example, the master tag 104 may generate a data signal, and the relayed signal 105 may include a version of the first signal 103 followed by the data signal (e.g., see the data signal 109 in FIGS. 4 and 5). A version of the data signal may be transmitted to the exciter device 101 by the helper tags 106 during the beamforming 107 (e.g., see the data signal 109 in FIG. 6). The bandwidth of the data signal may be greater than the bandwidth of the first signal 103. For example, the first signal 103 may be a relatively narrowband acoustic signal, and the data signal may be a relatively wideband acoustic signal. In some embodiments, the bandwidth of the data signal may be greater than or equal to 16 kHz and the bandwidth of the first signal 103 may be less than or equal to 1 kHz, but the present disclosure is not limited thereto. In some embodiments, an uppermost frequency of the data signal may be less than or equal to 17 kHz and a lowermost frequency of the first signal 103 may greater than or equal to 17 kHz, but the present disclosure is not limited thereto.

[0071] Each helper tag 106 may include a local timer. The helper tags 106 may listen to the distributed two-antenna system, namely the first signal 103 from the exciter device 101 and the relayed signal 105 from the master tag 104, and may recover each signal's corresponding local ToAs (related to di and do shown in FIG. 2), as well as the data for beamforming 107. By leveraging the ToAs, each helper tag 106 may be able to both synchronize its local timer (using the relayed signal 105) as well as determine its AoD to the exciter device 101, based on the TDoA between the first signal 103 and the relayed signal 105.

[0072] In some embodiments, the helper tags 106 may determine a time of transmission to the exciter device 101, based on the relayed signal 105. The helper tags 106 may be able to determine a respective AoD to the exciter device 101, based on the relayed signal 105 and the first signal 103. The AoD to the exciter device 101 may be determined by the helper tags 106 based on the TDoA between the first signal 103 received from the exciter device 101 and the relayed signal 105 received from the master tag 104. The helper tags 106 may perform beamforming 107 toward the exciter device 101 by respectively delaying a time of transmission to the exciter device 101 such that their respective transmissions will constructively add at the exciter device 101.

[0073] In some embodiments, the master tag 104 and each of the helper tags 106 may include a ToA detector and a modulator. For example, the ToA detector may be used to locally determine respective ToAs of the first signal 103 and the relayed signal 105. The ToA detector may also be used to determine a TDoA between the first signal 103 and the relayed signal 105. For example, the modulator may be used to encode information onto an acoustic signal before transmission. The modulator may modulate the acoustic signal by varying one or more properties of a sound wave, such as, for example, its amplitude, frequency, or phase, in accordance with the information to be transmitted. The modulator may allow the acoustic tags 102 to embed data (e.g., data bits), such as, for example, identification codes, sensor readings, and/or other information, onto the acoustic signal. [0074] In some embodiments, each of the helper tags 106 may include a data buffer and scheduler. For example, the data buffer may be a temporary storage area or memory buffer used to store data before it is transmitted via acoustic signals. For example, the scheduler may be responsible for managing the timing and sequencing of operations performed by the helper tags 106. The scheduler may optimize the use of, for example, power, memory, and processing capacity of the helper tags 106 to ensure efficient operation. In some embodiments, the scheduler may be implemented as part of the helper tag’s 106 firmware and/or software.

[0075] Leveraging the lower time resolution of acoustic signals along with a TdoA approach may enable the distributed acoustic system 100 to achieve tight time synchronization and adopt time-based beamforming 107 (i.e., temporal beamforming). For example, the helper tags 106 may appropriately delay their respective transmissions relative to the master tag 104 to ensure alignment and coherent reception of the signals at the exciter device 101. The implicit estimation of the AoD to the exciter device 101 may allow the helper tags 106 to independently time their transmissions to enable distributed temporal beamforming 107 from the array of acoustic tags 102. For example, the AoD to the exciter device 101 may allow the helper tags 106 to independently schedule their data transmissions at appropriate times based on their synchronized timers for beamforming 107 at the exciter device 101, while implicitly accounting for the relative delay in receiving the data from the master tag 104 along with the respective path delays to the exciter device 101. The helper tags 106 may independently schedule their respective transmissions at the respective AoD to the exciter device 101, based on their respective synchronized local timers. The helper tags 106 may determine a relative time offset for their respective transmissions, based on their respective synchronized local timers, such that their respective transmissions constructively add during beamforming 107 toward the exciter device 101.

[0076] In contrast to phase-based beamforming systems that use wire-synchronized arrays, the distributed acoustic system 100 may perform temporal beamforming 107, whereby each of the helper tags 106 independently identifies their appropriate time of transmission, accounting for both the direction (e.g., AoD) to the exciter device 101 and their relative path lengths, such that all the helper tags' 106 transmissions may arrive at the exciter device 101 coherently to deliver beamforming 107 gains. With temporal beamforming 107, the distributed acoustic system 100 may avoid challenges inherent to acoustic aperture design related to varying wavelengths that are faced by phased array beamforming systems.

[0077] The master tag 104 may append its data (e.g., sensor or product information) onto the relayed signal 105, which may allow the helper tags 106 to recover the data reliably using the first signal 103 as a preamble. The helper tags 106 may incorporate appropriate relative delays to compensate for data reception latency, along with their respective TDoAs between the first signal 103 and the relayed signal 105, and schedule their wideband (e.g., over acoustic band 0-20 kHz) higher data rate transmissions so as to align temporally at the exciter device 101. For example, the master tag 104 may append wideband data on the first signal 103 received from the exciter device 101, and the relayed signal 105 may include the first signal 103 received from the exciter device 101 and the wideband data. The helper tags 106 may receive the wideband data and may use the first signal 103 as a preamble to receiving the wideband data.

[0078] Beamforming 107 may aid with inaudibility by allowing for operation at a much lower transmit power and minimizing energy propagation in undesired directions. The distributed acoustic system 100 may incorporate elements of acoustic data distribution and spatially-selective signal whitening to deliver data to the exciter device 101, while maintaining user experience through inaudibility in undesired directions. In some embodiments, to aid in inaudibility outside of the exciter device 101 and to reduce the impact of sidelobes, the distributed acoustic system 100 may perform signal whitening. For example, the data signal may be encoded (scrambled) by a pre-determined sequence at each acoustic tag 102 so as to appear as independent white noise sequences outside of a receiver of the exciter device 101. This may result in a N fold SNR gain and N range gain at the exciter device 101 (for an V-tag array of helper tags 106), and a 10-fold interference-to-noise ratio (INR) drop everywhere else. For example, before transmitting data to the helper tags 106 via the relayed signal 105, the master tag 104 may scramble the data with a pre-determined sequence to resemble independent and identically distributed (i.i.d.) white noise, which may then be used by all the helper tags 106 in their respective transmissions for beamforming 107. Data whitening may help keep the SNR of the data transmitted by the helper tags 106 to the exciter device 101 sufficiently low in all directions outside of the main lobe for beamforming 107, thereby ensuring that the data transmitted by each helper tag 106 is statistically independent in locations outside the desired direction of the exciter device 101.

[0079] The acoustic power emitted by the helper tags 106 may be concentrated in the direction of the exciter device 101, while being much lower in other directions. This may allow the distributed acoustic system 100 to leverage the entire frequency band of the audio spectrum (e.g., 0-20 kHz), while remaining substantially inaudible to the rest of the surrounding environment.

[0080] Beamforming 107 may significantly improve SNR, thereby extending the communication range of the distributed acoustic system 100. This may enable the acoustic tags 102 to communicate with the exciter device 101 at larger distances. Improved SNR may allow for higher order modulations in data transmissions. Higher order modulations in data transmissions coupled with the ability to leverage the entire audio spectrum may contribute to an improved throughput for the distributed acoustic system 100. Since the power consumption of the acoustic tags 102 may be largely determined by the transmit power, beamforming 107 may allow for operation at a reduced transmit power, which may contribute to an extended battery life for the acoustic tags 102.

[0081] In some embodiments, the master tag 104 may synchronize the helper tags 106 to a common time frame of reference using the relayed signal 105 but may not participate in the beamforming 107, as shown in FIG. 2. However, the present disclosure is not limited thereto. In other embodiments, the master tag 104 may be optimized to be synchronized to the common time frame of reference and may participate in the beamforming 107 with the helper tags 106.

[0082] In some embodiments, the distributed acoustic system 100 may use a power normalization scheme for preserving inaudibility where the total transmit power (e.g., - 20dBm) is split across the helper tags 106 (i.e., a power split). In other embodiments, the helper tags 106 may send at their original fixed power (e.g., -20dBm) regardless of the number of helper tags 106 in the array, which may be useful, for example, when inaudibility of the distributed acoustic system 100 is less of a concern. When the helper tags 106 send at their original fixed power, the operational range of the distributed acoustic system 100 may be extended further. In some embodiments, the distributed acoustic system 100 may consume power at a rate that is, for example, up to 65 times less that that of a phased-array system.

[0083] The distributed acoustic system 100 may perform two-dimensional (2D) beamforming 107 that is achieved through a one-dimensional (ID) array of acoustic tags 102, as shown in FIG. 2. However, the present disclosure is not limited thereto. In other embodiments, the distributed acoustic system 100 may include, for example, a 2D array of acoustic tags 102 to enable three-dimensional (3D) beamforming.

[0084] In some embodiments, the acoustic tags 102 may be implemented using various hardware and/or software configurations. For example, the acoustic tags 102 may include a processing unit, such as a microcontroller (MCU), microprocessor (MPU), digital signal processor (DSP), system-on-chip (SoC), field-programmable gate array (FPGA), and/or application-specific integrated circuit (ASIC) that controls operations of the acoustic tags 102. The processing unit may be coupled to one or more acoustic transducers (e.g., speaker(s), microphone(s), etc.) for generating and receiving acoustic signals. The acoustic tags 102 may include memory for storing data, firmware, and/or software instructions, as well as communication interfaces for transmitting and receiving data to and from external devices along with parameters for operation of the distributed acoustic system 100 (e.g., a number of acoustic tags 102 included in the array). The acoustic tags 102 may include peripheral components, such as, for example, digital-to-analog converters (DACs), anal og-to-digi tai converters (ADCs), baseband clocks, sensors, power sources, and/or interface circuits. It will be understood that the above-described implementations are merely illustrative examples, and the acoustic tags 102 may be implemented using alternative configurations and components without departing from the scope of the present disclosure.

[0085] The distributed acoustic system 100 may provide benefits to the operational range, throughput, and energy efficiency of the acoustic tags 102, which may be helpful for enabling acoustic communication between the acoustic tags 102 and the exciter device 101, while also ensuring minimal to no interference to the surrounding environment (e.g., through inaudibility). Accordingly, the distributed acoustic system 100 may include the array of acoustic tags 102 that can be flexibly configured on-demand to enable temporal beamforming 107 that allows for data decoding at the exciter device 101, while operating at a low enough transmit power to remain substantially inaudible to human ears, thereby allowing for deployment in various applications (e.g., loT applications).

[0086] FIGS. 3 to 6 are schematic diagrams illustrating a sequence of operations in a distributed acoustic system, according to some embodiments of the present disclosure. In particular, FIG. 3 is a schematic diagram illustrating an excitation stage of the distributed acoustic system 100. FIG. 4 is a schematic diagram illustrating a relaying stage of the distributed acoustic system 100 that includes a backscatter operation and data sharing.

FIG. 5 is a schematic diagram illustrating a synchronization and scheduling stage of the distributed acoustic system 100. FIG. 6 is a schematic diagram illustrating a temporal beamforming stage of the distributed acoustic system 100.

[0087] Referring to FIGS. 2 to 6, the exciter device 101 may initiate a distributed beamforming operation by sending the first signal 103 (e.g., a narrowband signal, which may be similar to a preamble) in the inaudible part of the acoustic spectrum to the acoustic tags 102 (see FIG. 3). Upon receiving the first signal 103, the master tag 104 may relay the first signal 103, creating a virtual relay channel and resulting in an on- demand two-antenna system for the helper tags 106 (see FIG. 4). For example, the master tag 104 may perform a backscatter operation to relay the first signal 103 to the helper tags 106, which may result in the exciter device 101 and the master tag 104 forming a virtual distributed two-antenna system for the helper tags 106. For example, the helper tags 106 may receive the first signal 103 from the two-antenna system (e.g., from the exciter device 101) and the relayed signal 105 from the two-antenna system (e.g., from the master tag 104). To ensure that the helper tags 106 can detect the ToA of both the first signal 103 received from the exciter device 101 and the relayed signal 105 received from the master tag 104, the master tag 104 may add a delay TBS) prior to transmitting the relayed signal 105 (i.e., may delay transmission of the relayed signal 105). In other words, after receiving the first signal 103 from the exciter device 101, the master tag 104 may wait for a fixed delay

to elapse before transmitting the relayed signal 105 to the helper tags 106, which may help ensure that the helper tags 106 fully receive the first signal 103 from the exciter device 101 before receiving the relayed signal 105 from the master tag 104.

[0088] In some embodiments, the relayed signal 105 may include: (i) a version of the first signal 103, and (ii) a data signal 109. As used herein, the version of the first signal

103 may be the first signal 103 as received from the exciter device 101 by the master tag

104 or may be a modified version of the first signal 103 (e.g., modified by the master tag 104). As used herein, the version of the first signal 103 may also be referred to as a backscatter signal (BS), and the data signal 109 may also be referred to as a payload signal (payload). As shown in FIGS. 4 and 5, the relayed signal 105 may include the version of the first signal 103 and the data signal 109, and the signal that is on the lefthand side is the signal that is prior in time (i.e., the signal that is transmitted first by the master tag 104). In other words, in FIGS. 4 and 5, a time axis is considered to increase from left to right for signal transmission. The version of the first signal 103 may arrive at the helper tags 106 before the data signal 109 when the relayed signal 105 is transmitted by the master tag 104.

[0089] At each of the helper tags 106 (represented by z in the following notations), the version of the first signal 103 (represented by B in the following notations) transmitted by d +i.d the master tag 104 may arrive at -6, after a delay of ~ , and may follow the first signal 103 (represented by E in the following notations) transmitted by the exciter device f — d_t)+i.d dj

101, which may arrive at ^£’ after a delay of v (see FIG. 3), i.e., v ~ v (owing to triangle inequality). The relayed signal 105 and the first signal 103 may each be considered by the helper tags 106 to be preamble signals. Here, v is the speed of sound in

B air (^ 340 m/s). Accordingly, - £. To detect the correlation peak for both signals accurately, the beginning of the version of the first signal 103 (represented by B) transmitted by the master tag 104 may arrive at the helper tags 106 after the end of the first signal 103 (represented by E) transmitted by the exciter device 101, so as not to overlap these signals in the time domain. Thus, the master tag 104 may wait for ^TB - 7 , before relaying the version of the first signal 103 to the helper tags 106, where T is the duration of the first signal 103 transmitted by the exciter device 101. The minimum ToA difference between the two signals may equal the duration of the first signal 103 transmitted by the exciter device 101, which may allow each signal to be reliably detected at each of the helper tags 106 through preamble correlation.

[0090] To enable successful temporal beamforming 107, local timers of all the acoustic tags 102 may be synchronized. Even a small amount of time offset may result in degradation in the beamforming 107 gain at the receiver of the exciter device 101. In some embodiments, the distributed acoustic system 100 may enable tight synchronization among a set of distributed acoustic tags 102 while relying solely on the relayed signal 105 from the master tag 104. This may allow the distributed acoustic system 100 to perform synchronization before every transmission, thereby resetting timer drifts between transmissions (see FIGS. 5 and 6). For example, each of the helper tags 106 may include a local timer. To determine a time of transmission to the exciter device 101, the local timer may be synchronized to a common time frame of reference, based on the relayed signal 105 (see FIG. 5). The helper tags 106 may delay their respective transmissions to the exciter device 101, based on the local timer and a time difference of arrival (TDoA) between the first signal 103 and the relayed signal 105, such that the respective transmissions constructively add during beamforming 107 toward the exciter device 101 (see FIGS. 2 and 6).

[0091] To better illustrate how the distributed acoustic system 100 synchronizes its acoustic tags 102, notations may be established for global and local times. The time at which the first signal 103 is transmitted over the air from, for example, a speaker of the exciter device 101 may be considered to be the global time reference, i.e., the global time t = 0 (see FIG. 3). The local timer of every helper tag 106 in the array may have a distinct time offset 8^l with respect to this global reference, i.e., the local time offset when the global time is zero. Without synchronization, the

may have arbitrary values across the acoustic tags 102. Thus, if the local and global time values are denoted at the i- f fi ~ f * ( t* ~ t th helper tag 106 by t and .9, then ' 9 , where 9 and may be the same for all helper tags 106. Since every helper tag 106 may rely on its own local timer for scheduling its transmit signal, these t^l may introduce timing errors and thus may not be suitable to use for temporal beamforming 107. The distributed acoustic system 100 may reset all the local timers simultaneously so that all acoustic tags 102 have the same offset with respect to global time. ^r-9 may indicate the global time at which the timer is reset

~

to zero, i.e., ^{f r}-9. The acoustic tags 102 may all have the same

value so that all the l " are reset simultaneously. In contrast to a wired array where all the timers can share a common timer or be reset by a central unit, the distributed acoustic system 100 may preserve the distributed nature of the acoustic tags 102 by employing a common over-the-air signal, whose ToA may serve as the timing reference for all the acoustic tags 102

[0092] The ToA of the first signal 103 at an acoustic tag 102 (represented by i in the following notations) may be

is the distance from the exciter device 101 to the acoustic tag 102 (see FIG. 3). If all the di ^s were known prior to synchronization, then it may be sufficient to reset the acoustic tag's 102 timer at ^r ?

, where C is a constant greater than the maximum

In other words, the z-th acoustic tag's 102 local timer may be reset at ^c ~ ~ seconds following the detection of the first signal 103. However, since di S _may take different values that are dependent on the location of the exciter device 101, which is unknown to the acoustic tags 102, the ToA of the first signal 103 may not be used to reset the local timer.

[0093] Instead, the distributed acoustic system 100 may use the relayed signal 105 from the master tag 104 as the timing reference, as shown in FIG. 5. Unlike the first signal 103 transmitted from the exciter device 101 that arrives directly at the helper tags 106, the respective ToAs of the relayed signal 105 at each helper tag 106 may be distributed according to a known pattern. Specifically, the relayed signal 105 may experience three separate delays before arriving at the i-th helper tag 106 (see FIGS. 4 and 5). First, there may be a propagation delay for transmission of the first signal 103 from the exciter do device 101 to the master tag 104, given by ^v . Second, there may be a delay related to the total amount of time,

, that it takes for the master tag 104 to receive the first signal 103 (e.g., received on a microphone included in the master tag 104), buffer the first signal 103 in baseband and re-transmit (i.e., relay) the first signal 103 (e.g., re-transmitted via a speaker included in the master tag 104). Finally, there may be a propagation delay for transmission of the relayed signal 105 from the master tag 104 to the i-th helper tag 106 i.d in the array, given by ^v , where the distance d between the master tag 104 and each of the helper tags 106 in the array is fixed and known. Hence, the ToA of the relayed signal ji _ ⁺ ₊ __

105 at the i-th helper tag 106 may be ^b'^{jl B}, where ^{B t}

, which may be known a priori (i.e., known in advance) and may be estimated locally by the helper tag 106. Leveraging this, it may be sufficient for the i-th helper tag 106 to reset its

(N-i).d local timer >•' seconds after detection of the relayed signal 105, i.e.,

, which may be the same across all the helper tags 106. In other words, all the helper tags 106 may be able to reset their local timers at the same time, allowing for distributed synchronization using only local information and the ToA of the relayed signal 105. [0094] While accurate time synchronization may be needed for efficient temporal beamforming 107, the lower velocity of sound may allow for a coarser granularity of synchronization (e.g., in the order of microseconds) that may be three orders less than that required for RF beamforming. In some embodiments, this may be achieved with low- power commodity clocks.

[0095] When the local timers of all the helper tags 106 are synchronized, the helper tags 106 may then select the appropriate time for sending their respective data signals 109 (see FIG. 6). FIG. 6 illustrates that the data signal 109 transmitted by the helper tag 106 to the exciter device 101 is the same as the data signal 109 received from the master tag 104, but the present disclosure is not limited thereto. In some embodiments, the data signal 109 transmitted by the helper tag 106 to the exciter device 101 may be a modified version of the data signal 109 received from the master tag 104 (e.g., modified by the helper tag 106). It will be understood that the data signal 109 transmitted by the helper tag 106 to the exciter device 101 may be the data signal 109 as received from the master tag 104 by the helper tag 106 or may be a modified version of the data signal 109 (e.g., modified by the helper tag 106).

[0096] For successful temporal beamforming 107, all the data signals 109 may need to arrive at the same time at the receiver of the exciter device 101. Each helper tag 106 may rely only on its local timer to determine when to begin data transmission (represented by

E

D in the following notations). If the i-th helper tag 106 begins transmission at ^D’¹' according to its local time (after resetting to zero), then the global time of its transmission may be offset by the time at which its timer was reset, i.e.,

W Accordingly, the ToA of the transmitted data signal 109 from the i-th helper tag 106 at the exciter device 101 (represented by e in the following notations), namely

may depend on the transmit time and the propagation time over the air, as shown in FIG. 6, where

"^l,y

. being the same across all the acoustic tags 102 after synchronization, to ensure alignment of data signals 109 arriving at the exciter device

101, it may be sufficient to ensure D.f

is the same across all the acoustic tags 102. In other words, the local transmission time may have to implicitly incorporate the fact that helper tags 106 in the array further from the exciter device 101 start transmitting earlier than helper tags 106 closer to the exciter device 101 and vice versa.

[0097] While the ToA of the first signal 103 transmitted by the exciter device 101 may not be directly used for synchronizing the local timers of the helper tags 106, the distributed acoustic system 100 may leverage the virtual two-element distributed antenna system formed by the combination of the exciter device 101 with the master tag 104 and the resulting TDoA of the first signal 103 and the relayed signal 105 at each of the helper

⁺ - tags 106 to distributively schedule their respective transmissions. Using

. is the difference in the ToA of the first signal 103 and the relayed signal 105, each helper tag 106 may locally determine and store the value even before resetting its local timer. As such, each helper tag 106 (represented by i in the following notations) may set its transmission time as

. , ,

alue across all the helper tags 106, i.e., all the transmitted signals may arrive at the same time at the receiver of the exciter device 101, when the i-th helper tag 106 transmits at

based on its determined local TDoA between the first signal 103 and the relayed signal 105 along with compensation for its relative path length to the exciter device 101 in the array (e.g.,

). This may allow each of the helper tags 106 to locally schedule its transmission of the data signal 109, while enabling temporal beamforming 107 at the exciter device 101 (see FIGS. 2 and 6).

[0098] Since the helper tags 106 may not have a priori access to the data signal 109 generated by the master tag 104, the helper tags 106 may need to acquire the data signal 109 before being able to participate in temporal beamforming 107. In some embodiments, the master tag 104 may transmit the data signal 109 (e.g., a wideband signal) immediately following transmission of the version of the first signal 103 (e.g., a narrowband signal) for transmission of the relayed signal 105, as shown in FIG. 4. It may take

for the i-th helper tag 106 to fully receive the data packet included in the data signal 109 from the master tag 104, where

is the time to transmit the data signal 109 (e.g., at a i.d fixed modulation and coding scheme (MCS)) and depends on its size, and is the propagation time. By setting a guard time of ^pkt at each helper tag 106, it may ensure that the last helper tag 106 in the array is able to fully receive and store the data signal 109. Thus, accounting for data sharing, the new local transmission time at each tag may be

. With the additional term being a constant, the new transmission time may preserve the condition for temporal beamforming 107 from all the helper tags 106.

[0099] The distributed acoustic system 100 may ensure that during transmission of the data signal 109, which is an acoustic signal, the data signal 109 is substantially inaudible almost everywhere while being decodable at the exciter device 101. In some embodiments, the distributed acoustic system 100 may accomplish this through a twopronged approach that includes power scaling and signal whitening.

[0100] With respect to power scaling, the distributed acoustic system 100 may enable practical operational ranges through beamforming 107 from multiple acoustic tags 102, which otherwise may not be achievable with a single acoustic tag 102. If the transmit power Pm of a single acoustic tag 102 is low enough (e.g., -20 dBm), it may be inaudible beyond a very short range (e.g., more than 30-40 cm). The distributed acoustic system 100 may scale the transmit power at each of its helper tags 106 as Pm/N, where N is the number of helpers tags 106 in the array, such that the total power at any point outside the beamformed 107 direction may still be limited to that of a single helper tag 106, namely

and thus below the audible threshold. Within the beamformed 107 direction, the SNR gain may increase the audible range by N, which for an array of, for example, eight helper tags 106 may still be under 1 meter. In some embodiments, the distributed acoustic system 100 may enable distributed beamforming 107 to an exciter device 101 while providing a scalable gain (e.g., with an increasing number of helper tags 106). For example, using a linear array of eight helper tags 106, the distributed acoustic system 100 may enable beamforming 107 with data rates as high as 34 kbps (e.g., 7 times gain), coverage as high as 9-18 meters (e.g., 5 times gain), and transmit power as low as 9 W (e.g., 65 times reduction) for inaudibility, as compared to that of an equivalent phased- array system.

[0101] With respect to signal whitening, the transmit beam pattern during beamforming 107 may be directed at the exciter device 101, but nulls in other directions may not be optimized. As such, it may be possible that N signals from an A-tag array of helper tags 106 occasionally add constructively in directions outside of the main lobe during beamforming 107 (owing to side lobes), which may contribute to an audible range of, for example, more than 30-40 cm. To address this, in some embodiments, the distributed acoustic system 100 may employ signal whitening, whereby the data bits of the data signal 109 are scrambled with a predefined sequence such that the resulting modulated signal resembles independent and identically distributed (i.i.d.) white noise. For example, the resulting signal may have a very low correlation (e.g., statistically independent) with a time-shifted version of it. In the distributed acoustic system 100, the signals from different acoustic tags 102 may all be essentially time-shifted versions of the same signal, thereby behaving like N independent noise signals outside of the main lobe direction during beamforming 107. Thus, with signal whitening, the total power outside of the beamformed 107 direction may be restricted to Pm, which may preserve inaudibility. In contrast, within the beamformed 107 direction, the data signals 109 may arrive at the same time, resulting in an aggregate signal that is the sum of A identical signals, as opposed to N statistically independent signals. This may result in a total peak power as high as ^2, Pm/ - ■ Pm, Accordingly, the range may be extended by /V. For example, the range may be doubled by using four inaudible helper tags 106, each operating at P_m/4, with the equivalent total transmit power P_m of a single inaudible helper tag 106 but twice the communication range. Additionally, scrambling may improve the decoding in a noisy environment (e.g., when people are talking).

[0102] Accordingly, the distributed acoustic system 100 may deliver the equivalent benefits of a wired array, but with distributed acoustic tags 102 wirelessly coupled through an acoustic interface. The distributed acoustic system 100 may include the flexible array of low-power acoustic tags 102 that can be configured on-demand to enable temporal beamforming 107, which may be deployed in various applications (e.g., loT applications).

[0103] FIG. 7 is a graph illustrating the beam pattern of a distributed acoustic system, according to some embodiments of the present disclosure. In particular, FIG. 7 illustrates example beam patterns when employing a 2-array, a 4-array, and an 8-array of helper tags 106 in the distributed acoustic system 100 to beamform a 16 kHz-wide data signal (e.g., ranging from 0.5 kHz to 16.5 kHz) to an exciter device 101 located 2 meters away at two different AoDs of 0° and 50°. In FIG. 7, the AoD of 0° to the exciter device 101 is illustrated on the left-hand side and the AoD of 50° to the exciter device 101 is illustrated on the right-hand side.

[0104] Referring to FIGS. 2 and 7, as the number of helper tags 106 in the array increases, the beam used for beamforming 107 may become narrower, which increases the SNR gain of the distributed acoustic system 100 at the AoD to the exciter device 101. While a few side lobes may exist, the power of the side lobe peaks may remain at least 10 dB below the main lobe peak (e.g., 15 dB below the main lobe peak for the 8-array of helper tags 106). Accordingly, the distributed acoustic system 100 may perform temporal beamforming 107 toward the exciter device 101 with no significant signal leakage in unwanted directions.

[0105] FIG. 8 is a graph illustrating the scalability of a distributed acoustic system, according to some embodiments of the present disclosure. In particular, FIG. 8 illustrates how beamforming gain can be scaled as more helper tags 106 are added to the array. FIG. 8 illustrates that a number of helper tags 106 in the array ranges from one to eight helper tags 106, but the present disclosure is not limited thereto. In some embodiments, more than eight helper tags 106 may be included in the array.

[0106] Referring to FIGS. 2, 6, and 8, the SNR of the data signal 109 transmitted from the array of helper tags 106 to the exciter device 101 is measured when the exciter device 101 is located at distances of 2, 4, and 8 meters away from the array, and the measurements are averaged over several AoDs at a given distance. As the number of helper tags 106 doubles, the SNR may steadily increase by approximately 3 dB. The largest beamforming gain may be achieved with an 8-array of helper tags 106, which may provide 8-10 dB increased gain in SNR compared to a single helper tag 106 for different distances. Accordingly, the distributed acoustic system 100 may be able to scale its temporal beamforming 107 to larger array sizes of the helper tags 106. In some embodiments, the number of helper tags 106 in the array may be increased beyond eight, which may improve the beamforming 107 gain and/or the operational range of the distributed acoustic system 100 but may also increase the latency and associated overhead in data sharing between the master tag 104 and the helper tags 106.

Accordingly, there may be design tradeoffs associated with increasing the number of helper tags 106 in the array.

[0107] FIG. 9 is a graph illustrating the wideband stability of a distributed acoustic system, according to some embodiments of the present disclosure. In particular, FIG. 9 illustrates the received SNR from an 8-array of helper tags 106 in two cases: (1) when the data signal 109 transmitted by the helper tags 106 to the exciter device 101 is a narrowband signal (e.g., a single tone carrier signal), with 1 kHz bandwidth; and (2) when the data signal 109 transmitted by the helper tags 106 to the exciter device 101 is a wideband signal, with 16 kHz bandwidth. In both cases, the exciter device 101 is at a fixed distance of 1 meter away from the array of helper tags 106.

[0108] Referring to FIGS. 2, 6, and 9, the distributed acoustic system 100 may achieve a total SNR gain of 17 dB for the wideband data signal 109, which may be close to that achieved in the case of the narrowband single tone signal (e.g., < 1 dB loss). The distributed acoustic system 100 may also deliver a substantially uniform SNR gain across the entire frequency band (e.g., 0-20 kHz), when it beamforms 107 the wideband data signal 109. As such, data-carrying subcarriers may span the entire bandwidth to benefit from its SNR gain, which may lead to an increased throughput, particularly when compared to phased-array systems that are highly frequency-selective. In some embodiments, the distributed acoustic system 100 may employ a binary modulation (e.g., on-off keying (OOK)) for its transmissions to the exciter device 101. For example, by using a binary modulation (e.g., OOK) with 1 ms symbol duration and sixteen 1 KHz sub-carriers, the distributed acoustic system 100 may achieve a throughput of 16 kbps (e.g., 1 bps/Hz spectral efficiency). In some embodiments, the distributed acoustic system 100 may employ higher order modulations for transmission on the sub-carriers, which may improve the throughput of the distributed acoustic system 100. For example, the distributed acoustic system 100 may achieve a throughput of at least 30 kbps when the separation distance between acoustic tags 102 in the array is 85 mm or less. In some embodiments, the distributed acoustic system 100 may deliver the data signal 109 to the exciter device 101 with a bit error rate (BER) under .001 at distances of 9 meters or less from the exciter device 101. [0109] FIG. 10 is a graph illustrating the robustness of a distributed acoustic system relative to aperture size, according to some embodiments of the present disclosure. In particular, FIG. 10 illustrates how intertag separation d in the array of acoustic tags 102 may impact the received SNR from a 4-array and 8-array of helper tags 106, with an exciter device 101 located two meters away from the array. FIG. 10 considers three different aperture sizes corresponding to d being 13, 24, and 85 mm. In FIG. 10, the 4- array of helper tags 106 is illustrated on the left-hand side and the 8-array of helper tags 106 is illustrated on the right-hand side.

[0110] Referring to FIGS. 2 and 10, the temporal beamforming 107 performed by the distributed acoustic system 100 may have an SNR range that varies by 2.1 dB or less for the 4-array of helper tags 106 and by 1.9 dB or less for the 8-array of helper tags 106 between the intertag separations d of 13 mm, 24 mm, and 85 mm. Accordingly, the distributed acoustic system 100 may be robust to changes in aperture size (i.e., the distance d between the acoustic tags 102) and may thus be deployed in various different layouts. The temporal beamforming 107 performed by the distributed acoustic system 100 may not depend on frequency, and thus the SNR may vary only slightly for different aperture sizes. This may enable wideband operation for the distributed acoustic system 100 that translates to a higher throughput.

[0111] In some embodiments, the distributed acoustic system 100 may select a default d of 24 mm for separation between its acoustic tags 102 in the array, which may provide a high SNR gain while keeping the array small enough to be compact and easily deployed. However, the present disclosure is not limited thereto, and intertag separations d of less than or greater than 24 mm may also be selected by the distributed acoustic system 100. [0112] FIG. 11 is a schematic diagram illustrating an example application for a distributed acoustic system, according to some embodiments of the present disclosure. [0113] Referring to FIGS. 2 and 11, the distributed acoustic system 100 may be used in an industrial environment. For example, the acoustic tags 102 may be affixed to or embedded with object(s) 108 (e.g., products or packaging) in an industrial environment (e.g., a warehouse). An exciter device 101 may interact with the acoustic tags 102. For example, a user may operate the exciter device 101, or the exciter device 101 may be programmed to operate on its own. In response to the exciter device 101, the acoustic tags 102 may beamform 107 data to the exciter device 101 that includes, for example, identification information, tracking information, sensor or product information, and the like for the object(s) 108. For example, the distributed acoustic system 100 may enable real-time tracking of the object(s) 108 within the industrial environment. Accordingly, the distributed acoustic system 100 may, for example, help businesses streamline their inventory management processes, improve operational efficiencies, and/or enhance the ability to meet customer demands. For example, the distributed acoustic system 100 may be used as part of a smart inventory management system.

[0114] Those of ordinary skill in the art will appreciate that there are various other use cases and applications for the distributed acoustic system 100. Accordingly, the distributed acoustic system 100 is not limited to the use cases and applications set forth herein.

[0115] Aspects of the present disclosure may provide a distributed acoustic system including a plurality of low-power acoustic tags that can be flexibly configured on- demand to create an aperture array capable of distributed beamforming. The distributed acoustic system may include a virtual distributed two-antenna system that serves to both wirelessly synchronize the acoustic tags and enable distributed temporal beamforming from the acoustic tags. For example, the acoustic tags may be wirelessly selfsynchronized to enable the distributed acoustic system to perform time-based beamforming (i.e., temporal beamforming). The beamforming gain of the distributed acoustic system may scale with the number of tags in the array to deliver an increase of multiple fold in, for example, range, throughput, and energy efficiency, thereby bringing acoustic applications to practice (e.g., acoustic loT applications). The acoustic tags may inter-operate with commodity acoustic devices (e.g., smart devices) in the surrounding environment and may serve as a viable alternative to RF (e.g., for loT applications). The distributed acoustic system may be capable of full-duplex operation in an inherently orthogonal spectrum to RF, which may help side-step challenges related to RF applications.

[0116] In example embodiments of the present disclosure, systems, methods and apparatus for distributed acoustic beamforming are provided. The distributed nature of the acoustic system may allow for easy construction and deployment of an array of acoustic tags, which may result in improved performance in, for example, throughput, range, and energy efficiency. The distributed acoustic system may be used for various applications such as, for example, over-the-air applications and/or other RF-challenged mediums.

[0117] The present disclosure is described herein with reference to the accompanying drawings and examples, in which embodiments are shown. Additional embodiments may take on many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

[0118] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting thereof. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "includes", "including", "has", "having", and any other variations thereof when used in this specification, specify the presence of the stated features, steps, operations, elements, components, and/or groups but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. As used herein, phrases such as "between X and Y" and "between about X and Y" should be interpreted to include X and Y. As used herein, phrases such as "between about X and Y" mean "between about X and about Y." As used herein, phrases such as "from about X to Y" mean "from about X to about Y."

[0119] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity.

[0120] It will be understood that when an element is referred to as being "on", "attached" to, "connected" to, "coupled" with, "contacting", etc. another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, "directly on", "directly attached" to, "directly connected" to, "directly coupled" with or "directly contacting" another element, there are no intervening elements present. It will also be appreciated by those of ordinary skill in the art that references to a structure or feature that is disposed "adjacent" another feature may have portions that overlap or underlie the adjacent feature.

[0121] Spatially relative terms, such as "under", "below", "lower", "over", "upper", and the like may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is inverted, elements described as "under" or "beneath" other elements or features would then be oriented "over" the other elements or features. Thus, the exemplary term "under" can encompass both an orientation of "over" and "under." The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms "upwardly", "downwardly", "vertical", "horizontal", and the like are used herein for the purpose of explanation only unless specifically indicated otherwise.

[0122] It will be understood that, although the terms "first", "second", etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Thus, a "first" element discussed below could also be termed a "second" element without departing from the teachings of the present disclosure. The sequence of operations (or steps) is not limited to the order presented in the claims, description, or figures unless specifically indicated otherwise.

[0123] The foregoing is illustrative of the present disclosure and is not to be construed as limiting thereof. Although a few example embodiments have been described, those of ordinary skill in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the teachings of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of this present disclosure as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of the present disclosure and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An acoustic system, comprising: an array of wirelessly coupled acoustic tags, the array including a master tag configured to receive an exciter signal from a device, and at least one helper tag configured to receive the exciter signal from the device and a relayed signal from the master tag.

2. The acoustic system of claim 1, wherein the relayed signal comprises a data signal, and the at least one helper tag is configured to transmit data included in the data signal to the device.

3. The acoustic system of claim 2, wherein the at least one helper tag comprises a plurality of helper tags, and wherein the plurality of helper tags are configured to transmit the data to the device using beamforming.

4. The acoustic system of claim 3, wherein the plurality of helper tags are configured to perform temporal beamforming to transmit the data to the device.

5. The acoustic system of claim 2, wherein the master tag is configured to scramble the data with a pre-determined sequence to resemble white noise before transmitting the relayed signal to the at least one helper tag.

6. The acoustic system of claim 2, wherein the relayed signal further comprises a version of the exciter signal, and the version of the exciter signal is followed by the data signal.

7. The acoustic system of claim 2, wherein a bandwidth of the data signal is greater than a bandwidth of the exciter signal.

8. The acoustic system of claim 1, wherein the master tag is configured to perform a backscatter operation to transmit the relayed signal to the at least one helper tag.

9. The acoustic system of claim 1, wherein the array of wirelessly coupled acoustic tags are coupled through an acoustic interface, and wherein the exciter signal and the relayed signal each comprise an acoustic signal.

10. The acoustic system of claim 1, wherein the at least one helper tag is configured to determine an angle of departure to the device, based on a time difference of arrival between the exciter signal and the relayed signal.

11. The acoustic system of claim 1, wherein the at least one helper tag comprises a plurality of helper tags, wherein each of the plurality of helper tags includes a local timer, and wherein the local timer is synchronized to a common time frame of reference, based on the relayed signal.

12. The acoustic system of claim 11, wherein the plurality of helper tags are configured to: transmit data included in the relayed signal to the device; and delay their respective transmissions to the device, based on the local timer and a time difference of arrival between the exciter signal and the relayed signal, such that the respective transmissions constructively add at the device.

13. The acoustic system of claim 11, wherein respective distances between the master tag and each of the plurality of helper tags are fixed, and wherein the common time frame of reference is determined based on the respective distances and the relayed signal.

14. The acoustic system of claim 1, wherein a location of the device is unknown to the at least one helper tag prior to receiving the relayed signal.

15. The acoustic system of claim 1, wherein the master tag is configured to delay transmission of the relayed signal to the at least one helper tag such that the at least one helper tag receives the exciter signal before the relayed signal.

16. The acoustic system of claim 1, wherein the device is configured to request data from the array of wirelessly coupled acoustic tags, and wherein the array of wirelessly coupled acoustic tags is configured such that the data requested by the device determines which one of the acoustic tags is the master tag.

17. An acoustic system, comprising: an array of wirelessly coupled acoustic tags including a master tag and at least one helper tag, wherein the master tag is configured to: receive a first signal from a device that includes an acoustic interface; and transmit a second signal to the at least one helper tag, and wherein the at least one helper tag is configured to: receive the first signal from the device and the second signal from the master tag; and locally determine respective times of arrival of the first signal and the second signal.

18. The acoustic system of claim 17, wherein the master tag is configured to transmit the second signal to the at least one helper tag after receiving the first signal from the device.

19. The acoustic system of claim 18, wherein, after receiving the first signal from the device, the master tag is configured to wait for a fixed delay to elapse before transmitting the second signal to the at least one helper tag.

20. The acoustic system of claim 17, wherein the at least one helper tag comprises a plurality of helper tags, and wherein the plurality of helper tags are configured to perform beamforming toward the device by respectively delaying a time of transmission to the device.

21. The acoustic system of claim 20, wherein each of the plurality of helper tags includes a local timer and is configured to: synchronize the local timer to a common time frame of reference, based on the second signal; and determine a respective angle of departure to the device, based on a difference between the respective times of arrival of the first signal and the second signal.

22. The acoustic system of claim 21, wherein the plurality of helper tags are configured to: independently schedule respective transmissions at the respective angle of departure to the device, based on the local timer; and determine a relative time offset for the respective transmissions, based on the local timer and the difference between the respective times of arrival of the first signal and the second signal, such that the respective transmissions constructively add at the device.

23. The acoustic system of claim 20, wherein the second signal comprises a version of the first signal followed by a data signal, and wherein the beamforming toward the device comprises transmitting data included in the data signal to the device.

24. The acoustic system of claim 23, wherein the master tag is configured to scramble the data with a pre-determined sequence to resemble white noise before transmitting the second signal to the plurality of helper tags.

25. The acoustic system of claim 17, wherein a location of the device is unknown to the at least one helper tag prior to receiving the second signal from the master tag.

26. The acoustic system of claim 17, wherein the array of wirelessly coupled acoustic tags are coupled through an acoustic interface, and wherein the first signal and the second signal each comprise an acoustic signal.

27. The acoustic system of claim 17, wherein the master tag is configured to perform a backscatter operation to transmit the second signal to the at least one helper tag.

28. A method of acoustic communication, the method comprising: receiving, by a plurality of wirelessly coupled acoustic tags, a first signal from a device including an acoustic interface, the plurality of wirelessly coupled acoustic tags including a master tag and at least one helper tag; transmitting, by the master tag, a second signal to the at least one helper tag; and determining, by the at least one helper tag, respective times of arrival of the first signal and the second signal.

29. The method of claim 28, further comprising determining, by the at least one helper tag, an angle of departure to the device, based on a difference between the respective times of arrival of the first signal and the second signal.

30. The method of claim 28, wherein the at least one helper tag comprises a plurality of helper tags.

31. The method of claim 30, further comprising synchronizing the plurality of helper tags to a common time frame of reference, based on the second signal.

32. The method of claim 30, further comprising performing, by the plurality of helper tags, beamforming toward the device by respectively delaying a time of transmission to the device.

33. The method claim 32, wherein the second signal comprises a version of the first signal followed by a data signal.

34. The method of claim 33, further comprising whitening, by the master tag, data included in the data signal by scrambling the data with a pre-determined sequence.

35. The method of claim 33, further comprising: selecting, by each of the plurality of helper tags, the time of transmission to the device, based on a respective distance from the master tag and the respective times of arrival of the first signal and the second signal; and transmitting, by the plurality of helper tags, data included in the data signal to the device during the beamforming toward the device.