WO2016100887A2

WO2016100887A2 - Devices and methods for backscatter communication using one or more wireless communication protocols

Info

Publication number: WO2016100887A2
Application number: PCT/US2015/066820
Authority: WO
Inventors: Matthew S. Reynolds; Joshua F. ENSWORTH
Original assignee: University Of Washington
Priority date: 2014-12-19
Filing date: 2015-12-18
Publication date: 2016-06-23
Also published as: WO2016100887A3

Abstract

Examples described herein include devices and methods that may facilitate interoperability between backscatter devices and wireless communication devices. For example, backscatter devices and methods for backscattering are described that provide a transmitted backscattered signal formatted in accordance with a wireless communication protocol (e.g. Bluetooth Low Energy). Such communication may reduce or eliminate any modifications required to wireless communication devices necessary to receive and decode backscattered signals.

Description

DEVICES AND METHODS FOR BACKSCATTER COMMUNICATION USING ONE OR MORE WIRELESS COMMUNICATION PROTOCOLS

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application claims the benefit under 35 U.S.C. 1 19 of earlier-filed provisional applications 62/094,277, filed December 19, 2014 and 62/107,149 filed January 23, 2015. Both earlier-filed provisional applications are hereby incorporated by reference in their entirety for any purpose.

TECHNICAL FIELD

[002] Examples described herein are directed generally to wireless data transmission. In particular, examples are described that transmit data wirelessly by backscattering a signal such that the backscattered signal is compatible with a wireless communication protocol utilized by a receiving device.

BACKGROUND

[003] Wireless communication devices generally transmit information by generating a radiofrequency carrier using a circuit such as an oscillator, and modulating information onto the carrier wave using amplitude modulation, frequency modulation, phase modulation, quadrature amplitude modulation (QAM) or other techniques including a combination of the aforementioned modulation types. Multiple such modulated signals may be combined to form more complex schemes such as orthogonal frequency division multiplexing (OFDM). The carrier is usually a sinusoidal voltage at a radio frequency; that is a frequency at which energy may be propagated in the form of an electromagnetic wave by connecting the sinusoidal voltage to an antenna. The modulation process modifies the amplitude, frequency, and/or phase of the carrier in a time varying manner to convey information. Examples of conventional wireless communication devices include analog communication systems such as analog AM and FM broadcast radio as well as digital communication systems such as the widely used Wi-Fi (e.g. IEEE 802.1 1) and Bluetooth data communication standards as well as digital television (e.g. DTV) and digital broadcast radio standards. [004] Generally, conventional wireless communication devices have radiofrequency carrier generation and the modulation processes carried out in a single device or installation of interconnected devices.

[005] In contrast, backscatter devices generally refer to an alternative communication method where carrier generation and modulation are performed in separate devices. For example, a carrier frequency may be generated in a first device that emits an electromagnetic carrier wave. A second device carries out the modulation process by scattering or reflecting the carrier wave, thus affecting the amplitude, frequency, and/or phase of the carrier emitted by the first device. This can be achieved by modulated scattering; that is by selective reflection of the incident carrier wave by means of a modulator circuit. Backscatter devices, requiring a modulator which may be a simple as a transistor, may be quite simple and low power.

[006] Backscatter communication is widely used in ultra-high frequency RFID systems. By using modulated backscatter to communicate, RFID tags are power efficient compared to alternative approaches using conventional wireless communication schemes. However, RFID tags require a specialized reader or receiver hardware to receive the backscattered signal. RFID readers, for example, are complex devices which include a transmitter circuit, which performs the carrier wave generation process, along with a receiver circuit, which receives the modulated backscatter signal and extracts the data transmitted by the RFID tag. This specialized hardware presents a cost and complexity burden to users of the RFID system, in that RFID readers must be purchased, installed, and maintained on a data communication network to take advantage of the RFID tags.

SUMMARY

[007] Example devices are described herein. An example device may include an antenna configured to receive an incident signal having a carrier frequency. The device may further include a modulator and a symbol generator. The symbol generator may be configured to provide a subcarrier frequency. The symbol generator may further be configured to control the modulator to backscatter the incident signal having the carrier frequency using the subcarrier frequency to provide a backscattered signal to the antenna. The backscattered signal may include a bandpass signal in a predetermined frequency range. [008] In some examples, the predetermined frequency range is a range specified by a wireless communication standard.

[009] In some examples, the predetermined frequency range is a range of an advertising channel specified by a Bluetooth Low Energy specification.

[010] In some examples, the symbol generator may be configured to provide the backscattered signal in part by mixing the subcarrier frequency with the carrier frequency.

[011] In some examples, the symbol generator may be configured to provide the backscattered signal in part by mixing a harmonic of the subcarrier frequency with the carrier frequency.

[012] In some examples, the modulator may include a field effect transistor.

[013] In some examples, the backscattered signal may include a packet. In some examples, the packet may include a preamble, an access address, a payload data unit, and a cyclic redundancy check.

[014] In some examples, the device may further include a frequency source coupled to the symbol generator. The frequency source may be configured to provide the subcarrier frequency. In some examples, the device may include multiple frequency sources coupled to the symbol generator. The symbol generator may be configured to select at least one of the multiple frequency sources for use in providing the backscattered signal. The symbol generator may be configured to select at least one of the multiple frequency sources in accordance with data provided to the symbol generator. In some examples, at least one of the multiple frequency sources is modulated in amplitude, frequency, and/or phase.

[015] In some examples, the subcarrier frequency may be modulated in amplitude, frequency, and/or phase.

[016] In some examples, the backscattered signal may be an orthogonal frequency division multiplex (OFDM) signal.

[017] Examples of methods are described herein. An example method may include receiving an incident signal having a carrier frequency. The method may include backscattering the incident signal to provide a backscattered signal. The backscattering may include modulating, using a backscatter device, impedance presented to at least one antenna in accordance with data to be provided in the backscattered signal, and mixing the carrier frequency with at least one subcarrier provided by the backscatter device.

[018] In some examples, the mixing may result in a bandpass signal having a predetermined frequency range. In some examples, the predetermined frequency range may include a range of a channel in accordance with a wireless communication standard. In some examples, the wireless communication standard comprises Bluetooth Low Energy.

[019] In some examples, modulating include modulating the amplitude, frequency, and/or phase of the backscattered signal in a pattern indicative of the data to be provided in the backscattered signal.

[020] In some examples, the data to be provided in the backscattered signal includes a packet having a preamble, an access address, a payload data unit, and a cyclic redundancy check.

[021] In some examples, a method further includes transmitting the backscattered signal.

[022] In some examples, the backscattered signal includes a reading of a sensor associated with a device providing the backscattered signal.

[023] In some examples, the backscattered signal may include an identification of an asset associated with a device providing the backscattered signal.

[024] In some examples, the device providing the backscattered signal includes a tag.

BRIEF DESCRIPTION OF THE DRAWINGS

[025] FIG. 1 is a schematic block diagram of a system including a backscatter device in accordance with examples described herein;

[026] FIG. 2 is a schematic illustration of a backscatter device in accordance with examples described herein; [027] FIG. 3 is a flowchart illustrating a method in accordance with examples described herein; and

[028] FIG. 4 is a schematic illustration of an example packet compatible with the BTLE specification.

DETAILED DESCRIPTION

[029] Certain details are set forth below to provide a sufficient understanding of embodiments of the disclosure. However, it will be clear to one skilled in the art that embodiments of the disclosure may be practiced without various of these particular details. In some instances, well-known device components, circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the described embodiments of the disclosure.

[030] Examples described herein include backscatter devices (e.g. transmitters or transceivers) that utilize backscattered signals to communicate with each other and/or other devices in accordance with established wireless communication protocols. For example, a system may include a backscatter device that is configured to transmit data by modulating a backscattered version of an incident signal and mixing the carrier frequency of the incident signal with a subcarrier frequency such that a resulting backscatter signal includes a bandpass signal having a predetermined frequency range. The predetermined frequency range may, for example, be a frequency range specified by a wireless communication protocol, such as Bluetooth Low Energy (BLE), sometimes called Bluetooth Smart.

[031] FIG. 1 is a schematic block diagram of a system including a backscatter device in accordance with examples described herein. The system may include a signal source 100, which may be configured to provide a signal 130 using antenna 105. The system may include a backscatter device 1 10 which may be configured to receive the signal 130 using the antenna 1 15 and modulate a backscattered version of the signal 1 0 to provide a transmitted backscatter signal 135 using the antenna 1 15. The system may further include a wireless communication device 120 that may receive the transmitted backscatter signal 135 using an antenna 125. The transmitted backscatter signal 135 may be constructed in accordance with established wireless communication protocols, such that the wireless communication device 120 may receive and decode the transmitted backscatter signal 135 without a need for custom programming (e.g., firmware, software) or hardware specific to communication with the backscatter device 1 10.

[032] The signal source 100 may generally be any device that is capable of transmitting a suitable signal 130 for backscatter by the backscatter device 1 10. Generally, the signal 130 may be a radio frequency signal, such as a wireless communication signal. The signal 130 may have a carrier frequency (e.g. a frequency of a carrier wave that may be modulated with an input signal to provide data in the signal 130). The signal 130 may generally be implemented using any signals which may be received and backscattered by backscatter devices described herein. The signal 130 may be implemented using an RF signal including a wireless communication signal.

[033] Examples of signals used to implement the signal 130 include, but are not limited to, television transmission signals, radio transmission signals, cellular communication signals, and Wi-Fi signals. Devices which may be used to implement the signal source 100 include but are not limited to television transmitters, base stations including cellular base stations, AM or FM broadcast stations, digital radio stations, radar, Wi-Fi (e.g. IEEE 802.1 1) access points, Bluetooth devices, mobile devices, telephones (including cellular telephones), computers, routers, appliances, transceivers, tablets, and watches. In some examples the signal source 100 may be terrestrial while in other examples the signal source 100 may be located on a satellite or spacecraft. It should be understood that any externally (e.g. external to the backscatter device 110) generated carrier having at least one frequency component in the frequency range of interest (sometimes referred to as F_carrier) may be employed. In some examples, the signal source 100 may supply at least a portion of the operating power for the backscatter device 110.

[034] The signal 130 may be present in the environment from signal sources already present in an environment, and/or the signal 130 may be provided by a signal source placed in an environment for the purpose of providing a signal to the backscatter device 110. While shown as having one antenna 105 the signal source 100 may be implemented having any number of antennas, including a phased array antenna, or a multiple-input-multiple-output (MI MO) array of antennas.

[035] The signal source 100 may include a frequency source, such as an oscillator or frequency synthesizer, which may supply radio frequency energy to the antenna 105, in some examples via a power amplifier included in the signal source 100. The frequency source may include one or more of a fixed frequency source, a frequency hopping source, or a direct sequence spread spectrum source. It may be powered by batteries, by an AC power source, or by energy harvested from its environment (such as via a solar cell or a thermal or vibrational energy harvester). The signal source 100 (e.g. a transmitter) may be fixed in location or it may be mobile, as in a handheld or vehicle mounted application.

[036] In some examples the signal source 100 may include and/or be co-located with a receiver connected to the same antenna 105 or antenna array. In some examples the signal source 100 may be implemented using an RFID reader.

[037] The backscatter device 1 10 may be implemented, for example, using a tag. In some examples, the backscatter device 110 may be implemented using a device for which low power communication is desirable, such as a tag, sensor node, or the like. Tags implementing the backscatter device 1 10 may be associated with (e.g. placed on and/or proximate to) any of a variety of items to provide information about the items. Such items include, but are not limited to, appliances, food storage containers, inventory items such as personal electronics, and portions of a building. While shown as having one antenna 115, the backscatter device 1 10 may utilize any number of antennas in some examples.

[038] The backscatter device 1 10 may modulate a backscattered version of the signal 130 from the signal source 100 to provide a transmitted backscatter signal 135 encoded with data to the wireless communication device 120. The transmitted backscatter signal 135 may be formatted in accordance with predetermined wireless communication standards, such as but not limited to the Bluetooth Low Energy (also called Bluetooth Smart) standard. There are many different wireless communication standards, each of which may have a specified frequency plan, modulation scheme, and packet data format, among other specified parameters. Data encoded in the transmitted backscatter signal 130 by the backscatter device 100 may, for example, be related to data received from a sensor or an input, or may be related to an identity or parameter of an item with which the backscatter device 1 10 is associated (e.g. temperature in a portion of a building, identity of an inventory item, temperature of a food storage container).

[039] Backscatter communication generally includes modulating the reflection of an incident signal at an antenna, rather than generating the signal itself. The signal 130 used by the backscatter device 1 10 may include a signal having a carrier frequency that is provided by the signal source 100 for another purpose, such as a television broadcast or cellular communication between a base station and a mobile device. In some examples, the transmitted backscattered signal 135 may be encoded with data using a modulation scheme. To generate the backscattered signal, the backscatter device 1 10 may modulate the impedance of one or more antennas, such as the antenna 1 15, to alternate between two or more discrete states, e.g., including in some embodiments reflecting and not-reflecting. The reflecting state of the antenna 1 15 may provide a reflection of the signal 130, and the non- reflecting state may not reflect the signal 130. Thus, the backscatter device 1 10 may indicate either a '0' or a Ί ' bit by switching the state of the antenna 1 15 between the reflecting and non-reflecting states.

[040] Switching the state of the antenna 115 of the backscatter device 110 may include adjusting an impedance of a load attached to the terminals of the antenna 1 15. The magnitude and/or phase of the scattered signal from the antenna 1 15 is typically determined by the difference in the impedance values of the load attached to the terminals of the antenna 1 15. By modulating the electrical impedance presented to the antenna 1 15, the magnitude and/or phase of incident energy that is scattered is modulated, thus enabling information to be transmitted. For example, in a first state, the antenna 1 15 may have a first impedance (e.g., a short circuit) to a reference node and may reflect the signal 130 to provide a transmitted backscattered signal 135 that has a first signal magnitude and phase. In a second state, the antenna 115 may have a second impedance (e.g., an open circuit) to the reference node, and may reflect the signal 130 to provide a backscattered signal 135 that has a second signal magnitude and phase. The first magnitude may be greater or less than the second magnitude. This yields an amplitude shift keying (ASK) backscattered signal in some examples. In some examples, the backscattered signal may differ primarily in phase between the first state and the second state. This yields a phase shift keying (PSK) backscattered signal. It should be understood that more than two magnitude states may be employed, thus yielding a pulse amplitude modulated (PAM) backscattered signal. It should further be understood that more than two phase states, such as M states, may be employed, thus yielding an M-ary PSK backscattered signal. In some examples, the impedances of the loads attached to the terminals of the antenna are chosen to affect both the magnitude and the phase of the backscattered signals in each of several states. In such embodiments, a quadrature amplitude modulation (QAM) backscattered signal may be produced. [041] By opening and closing the modulating switch in a time varying pattern, the scattering or reflectivity will be time varying, and thus information may be conveyed by the scattered or reflected signal. In some embodiments, the modulating switch is opened and closed once for each transmitted symbol. The rate of this time varying pattern may then be referred to as the symbol rate of the backscattered signal. The symbol rate is the rate at which the modulator changes its impedance state to convey different pieces of information (e.g. groups of one or more bits). It should be understood that circuits or structures other than a switch may be used to change the impedance state of the load connected to the antenna 1 15. Such devices as a PIN diode, a varactor diode, a field effect transistor, a bipolar transistor, or circuit combinations of these elements may also be used to change the impedance state of the load connected to antenna 115.

[042] The backscatter device 1 10 may include a modulator that may function to modulate the backscatter of the signal 130, e.g. to switch an impedance of the load attached to antenna 1 15 from a non-reflecting to a reflecting state. The backscatter device 1 10 may also provide a subcarrier frequency. In some examples, the subcarrier frequency may be provided, for example, by an oscillator. The switching or modulating action of the backscatter device 1 10 may mix the subcarrier frequency with the carrier frequency of the signal 130 to adjust a frequency component of the transmitted backscatter signal 135. In this manner, the transmitted backscatter signal 135 may include a bandpass signal component having a predetermined frequency range, for example a frequency range specified by a wireless communication standard.

[043] Examples of backscatter devices described herein, including the backscatter device 1 10 of FIG. 1, may have parameters selected to produce frequency components corresponding to at least one band-pass signal in the frequency spectrum of the scattered or reflected signal. These frequency components may be select to be compatible with a bandpass signal expected by a wireless communication device (e.g. the wireless communication device 120 of FIG. 1) such that the wireless communication device will accept and properly decode the transmitted backscattered signal. The transmitted backscattered signal may contain other frequency components that are outside of the desired band-pass signal but these components may be out-of-band with respect to the communication signal and thus discarded by the wireless communication device 120. [044] The wireless communication device 120 may accordingly receive the transmitted backscatter signal 135 at the antenna 125. While one antenna 125 is shown, multiple antennas may also be used. The wireless communication device 120 may be implemented using any device capable of wireless communication, including but not limited to, a cellular telephone, computer, server, router, laptop, tablet, wearable device, watch, appliance, automobile, or airplane. The wireless communication device 120 may be configured to (e.g. include hardware and/or firmware and software for) communicate using a particular protocol for a wireless communication signal (e.g. Bluetooth Low Energy, Bluetooth Smart, Wi-Fi, CDMA, TDMA). The backscatter device 1 10 may provide a transmitted backscatter signal 135 formatted in accordance with the wireless communication protocol expected by the wireless communication device 120. In this manner, no further software, firmware, or hardware may be required for the wireless communication device 120 to receive and decode the transmitted backscatter signal 135 than is required for the wireless communication device 120 to receive and decode received signals from other sources that are formatted in accordance with the wireless communication protocol.

[045] The wireless communication device 120 may employ a frequency shift keying (FSK) or Gaussian frequency shift keying (GFSK) standard having at least one or more specified frequency deviations, one or more specified channel center frequencies, and one or more specified symbol rates. In some examples, the aforementioned FSK or GFSK standard is that of the Bluetooth Low Energy specification as defined by the Bluetooth Special Interest Group (SIG). Accordingly, in some examples the backscatter device 1 10 may provide a transmitted backscattered signal 135 compatible with the FSK or GFSK standard employed by the wireless communication device 120.

[046] The wireless communication device 120 may employ a phase shift keying (PSK) standard. Accordingly, in some examples the backscatter device 1 10 may provide a transmitted backscattered signal 135 compatible with the PSK standard. It should be appreciated that the PSK signal so generated may use two distinct phases to encode a symbol or a bit, or it may alternatively have more than two distinct phases to encode a symbol or a group of bits as in M-ary PSK.

[047] The wireless communication device 120 may employ an amplitude shift keying (ASK) standard. Accordingly, in some examples the backscatter device 110 may provide a transmitted backscattered signal 135 compatible with the ASK standard. It should be appreciated that the ASK signal so generated may use two distinct amplitudes to encode a symbol or a bit, or it may alternatively have more than two distinct amplitudes to encode a symbol or a group of bits as in pulse amplitude modulation (PAM).

[048] The wireless communication device 120 may employ a quadrature amplitude modulation (QAM) standard. Accordingly, in some examples the backscatter device 1 10 may provide a transmitted backscattered signal 135 compatible with the QAM standard. It should be appreciated that the QAM signal may have more than two distinct amplitudes and phase combinations to encode a symbol or a group of bits, as in M-ary QAM.

[049] The wireless communication device 120 may employ an orthogonal frequency division multiplexing (OFDM) standard and/or technique. Accordingly, in some examples the backscatter device 1 10 may provide a transmitted backscattered signal 135 compatible with the OFDM standard and/or technique. This may be achieved by modulating the backscattered signal 135 with more than one subcarrier frequency at the same time. Each subcarrier may in turn be modulated with ASK, PAM, PSK, or QAM to form the OFDM backscattered signal.

[050] While FIG. 1 depicts one backscatter device 1 10, the system may include more than one backscatter device, and multiple backscatter devices may be in communication with the wireless communication device 120 using signals backscattered from the signal source 100. Moreover, while FIG. 1 depicts one signal source 100, in some examples, the system may include more than one signal source.

[051] FIG. 2 is a schematic illustration of a backscatter device in accordance with examples described herein. The backscatter device 200 may be used, for example to implement the backscatter device 1 10 of FIG. 1. The backscatter device 200 includes an antenna 215, a modulator 220, and a symbol generator 230. The modulator 220 may modulate an impedance of the antenna 215 to change the magnitude and/or phase of an incident signal, e.g. the signal 130 of FIG. 1.

[052] The antenna 215 may be used to implement the antenna 1 15 of FIG. 1 in some examples. The antenna 215, during operation, may receive an incident signal having a carrier frequency, such as the signal 130 of FIG. 1. The antenna 215 may further transmit a transmitted backscattered signal, e.g. the signal 135 of FIG. 1, by reflecting and'or absorbing portions of the signal 130 as controlled by the modulator 220 and symbol generator 230. The reflected and/or absorbed portions of the signal 130 may be modulated in combinations of amplitude and phase, and subcarrier frequency and phase as described herein, for example.

[053] The modulator 220 may generally be implemented using any device capable of modulating an impedance of the antenna 215 in accordance with a control signal provided by the symbol generator 230. The modulator 220 is shown in FIG. 2 implemented using a single field effect transistor. The gate of the field effect transistor may be coupled to the symbol generator 230 and receive a control signal from the symbol generator 230 based on the data to be encoded into the backscatter signal. Other devices may be used to implement the modulator 220 in other examples. Such devices as a PIN diode, a varactor diode, a field effect transistor, a bipolar transistor, or circuit combinations of these elements may also be used to change the impedance state of the modulator 220, and thus change the impedance of the load connected to antenna 215.

[054] The symbol generator 230 may provide at least one subcarrier frequency. In some examples, only one subcarrier frequency may be provided by the symbol generator 230. In some examples, multiple subcarrier frequencies may be provided. The symbol generator 230 may provide the subcarrier frequency, for example, by having a frequency source that provides the subcarrier frequency. For example, the symbol generator may have one or more oscillators that may oscillate at the subcarrier frequency or sub-harmonics thereof. In some examples, the symbol generator may have multiple frequency sources coupled to and/or included in the symbol generator and the symbol generator may select one of the multiple frequency sources for use in providing the backscattered signal. The symbol generator may select one of the multiple frequency sources in accordance with data provided to the symbol generator. For example, one of the frequency sources may be used corresponding to a '0' bit and another of the frequency sources may be used corresponding to a Ί ' bit.

[055] The symbol generator 230 may control the modulator 220 to backscatter an incident signal having a carrier frequency (e.g. the signal 130 of FIG. 1) using the subcarrier frequency to provide a backscattered signal at the antenna. By mixing the carrier frequency with the subcarrier frequency or harmonics thereof, the backscattered signal may include a bandpass signal in a predetermined frequency range. The predetermined frequency range may be specified by a combination of the carrier and subcarrier frequencies. [056] In some examples, the backscatter device 200 may use sub-harmonic mixing to permit a carrier at a fraction of a desired band-pass signal frequency to produce energy in the desired communication frequency band. In such embodiments, if the desired communication carrier frequency is at a frequency F_carrier, the signal source (e.g. the signal source 100 of FIG. 1) may be at a sub-harmonic frequency F_carrier/n where n is a harmonic number. For example, an 800 MHz carrier may be used in a sub-harmonic mode to generate backscatter energy in the 2.4 GHz band (in this example, n = 3) due to harmonic mixing in the backscatter device.

[057] In some examples, the predetermined frequency range may be a range specified by a wireless communication protocol (e.g. a wireless communication standard). For example, the wireless communication protocol may be Bluetooth Low Energy and the frequency range may be a range of an advertising channel specified by a Bluetooth Low Energy specification.

[058] Accordingly, the symbol generator 230 may control the modulator to modulate the magnitude and/or phase of an incident signal to generate a backscattered signal. The backscattered signal may encode data, which may be provided to the symbol generator 230. The data may be, e.g. data collected by a sensor or other device in communication with the backscatter device 200. The data may be stored by the backscatter device 200. Examples of the data include, but are not limited to, a temperature of a portion of a building, an identity of an inventory item, and a temperature of a food container. The backscattered signal may be formatted in accordance with a protocol expected by a wireless communication device (e.g. a wireless communication standard). Accordingly, the backscattered signal may include a packet. The symbol generator 230 may control the modulator 220 to provide a packet formatted in accordance with a particular wireless communication protocol. The packet may include a preamble, an access address, a payload data unit, and a cyclic redundancy check.

[059] The symbol generator may be implemented using hardware, software, or combinations thereof. In some examples, the symbol generator 230 may be implemented using a microprocessor.

[060] In some embodiments the backscatter device 200 may include a processor (not shown in FIG. 2, but which processor may be in communication with the symbol generator 230). The processor may be, for example, implemented using fixed-function digital logic (such as a finite state machine or FSM) or a microprocessor or microcontroller which implements operations including memory and optional sensor inputs. In such examples the processor may encode a data stream including a unique identifier for the backscatter device 200. Accordingly, the transmitted backscattered signal may include a unique identifier for the backscatter device 200. In some examples, the optional sensor inputs may influence one or more bits of the data stream in such a way as to encode the value of the optional sensor inputs into the data stream by changing the unique identifier that is sent. In such cases the aforementioned data stream may then be fed into the symbol generator as described herein and shown in FIG. 2.

[061] In some examples, the processor formats the unique identifier, the optional sensor input(s), and'or other data that is desired to be sent in the transmitted backscattered signal into a specified packet format, such as but not limited to a Bluetooth Low Energy advertising packet, an IEEE 802.11 beacon frame, an IEEE 802.15.4 beacon frame, or another specified packet format. In such examples the packet format may then form a data stream which may be provided to the symbol generator as described herein. In some examples, such as in the case of the Bluetooth Low Energy advertising packet, information about the channel on which the packet is being sent may be encoded in to the data stream itself. In such examples, the channel number may be derived from the parameters of the carrier frequency and the configuration of the symbol generator as described herein.

[062] The backscatter device 200 may provide a transmitted backscattered signal compatible with an FSK or GFSK standard employed by a receiving wireless communication device. The symbol generator 230 may include or be in communication with a frequency source that may be operated at one of two frequency states, F_mod1 or F_mod2. The selection of the frequency state may be made under the control of data that is input to the symbol generator (e.g. from a sensor or microprocessor). The frequency source may include, for example, a resistance-capacitance (RC) oscillator, an inductance-capacitance (LC) oscillator, a quartz crystal oscillator, a frequency synthesizer, the output of a digital-to-analog converter, the output of a direct digital synthesizer, the output of a clock generator, an arbitrary waveform generator, or any other analog or digital frequency source or combinations thereof. In some examples, the aforementioned frequency source produces a square-wave output, and in some examples the aforementioned frequency source produces a sinusoidal output. In some examples the frequency source produces any waveform having energy at least including the frequency components F_mod1 and F_mod2. In some examples regulatory limits on occupied bandwidth or other properties of the signal may influence the choice of frequency source waveforms.

[063] In some examples, the sum of the frequency of an incident carrier F_carrier (e.g. the carrier received from the signal source 100 of FIG. 1), plus the average of F_mod1 and F_mod2 is provided to be within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. In such examples, x = F_carrier + mean(F_mod1, F_mod2), where x is a frequency within an acceptable range of a channel center frequency signal specification. In such examples, the frequency source in the backscatter device may have any waveform shape. In such examples, the difference between the two frequencies, dl =abs(F_mod1- F_mod2), where abs() denotes the absolute value operator, is provided to be within an acceptable frequency deviation range of a frequency shift keying (FSK) or Gaussian frequency shift keying signal specification.

[064] In some examples, the difference y = F_carrier - mean(F_mod1, F_mod2) is provided to be within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. In such examples, the frequency source may have any waveform shape. In such examples, the difference between the two frequencies, d2 = abs(F_mod1- F_mod2), where abs() denotes the absolute value operator, is provided within an acceptable frequency deviation range of a frequency shift keying (FSK) or Gaussian frequency shift keying signal specification.

[065] In some examples, harmonics of the backscatter signal may be used to form the transmitted backscattered signal. In such examples, the parameters F_carrier, F_mod1, and F_mod2 are provided such that: z = F_carrier ± ( n x mean(F_mod1, F_mod2)), where n is a harmonic number and z is within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. A further constraint on F_mod1 and F_mod2 may be that the frequency difference a= n x abs(F_mod1- F_mod2) is within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of the frequency deviation specification. Thus the spacing between F_mod1 and F_mod2 may be reduced by a factor corresponding to the harmonic number employed, compared to the fundamental-mode where n=l.

[066] In such examples, the frequency source may preferentially have a square wave shape with n being an odd number, but any waveform shape having energy at the harmonics of F_mod1 and F_mod2 are possible. In these embodiments die frequencies the difference between the two frequencies, d2 = n x abs(F_mod1- F_mod2). where abs() denotes the absolute value operator, is provided to be within an acceptable frequency deviation range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a frequency shift keying (FSK) or Gaussian frequency shift keying signal specification.

[067] In some embodiments the frequency source switches nearly instantaneously between F_mod1 and F_mod2 at a rate within an acceptable symbol rate range of a signal specification (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification). In other examples the frequency source transitions smoothly between F_mod1 and F_mod2 over a period of time, such that the transition is completed within an acceptable symbol rate range of a signal specification (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification). In some examples, the smooth transition between F_mod1 and F_mod2 occurs according to a function of time such that the occupied bandwidth of the backscattered signal complies with a regulatory or specification requirement. In some examples, the transition is designed to produce a Gaussian frequency shift keying spectrum. It should be understood that, while some examples herein refer to a binary FSK or GFSK modulation scheme, including two modulator frequencies (F_mod1 and F_mod2), other examples may include more than two modulator frequencies, such as in m-ary FSK, where m refers to a number of frequency states. In such cases multiple modulator frequencies (F_mod1 ...F_mod2 ) may be employed. The analogous constraints on the choice of fundamental-mode and harmonic-mode modulator frequencies would be applied as described herein.

[068] The backscatter device 200 may provide a transmitted backscattered signal compatible with phase shift keying (PSK) standard employed by a receiving wireless communication device. The PSK standard may have at least one or more specified phase differences, one or more specified channel center frequencies and one or more specified symbol rates. [069] In examples utilizing PSK, the symbol generator 230 may include a frequency source that may be operated at a frequency F_mod with one of at least two phase states, P_mod1 , Ρ_mod2 through P_{mod_n}- The selection of the phase state is made under the control of a data stream input to the symbol generator. For example, one phase state may be selected corresponding to a '0' bit and another phase state selected corresponding to a ' F bit. In some examples, such as in the binary phase shift keying (BPSK) case, P_mod1 and P_mod2 differ by 180 degrees (pi radians). In other embodiments, such as n-PSK. where n is the number of different phase states, multiple different phases may be employed.

[070] The frequency source may include a resistance-capacitance (RC) oscillator, an inductance-capacitance (LC) quartz crystal oscillator, a frequency synthesizer, the output of a digital-to-analog converter, the output of a direct digital synthesizer, the output of a clock generator, an arbitrary waveform generator, or any other analog or digital frequency source. In some examples, the aforementioned frequency source produces a square-wave output, while in other embodiments the aforementioned frequency source produces a sinusoidal output.

[071] In some examples the frequency source produces any waveform having energy at least including the frequency component F_mod with a phase that can be varied from P_mod1 to

P_mod2 through P_modn-

[072] In some examples, the sum of the frequency of an incident carrier F_carrier, plus the F_mod, is provided within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification, such that x = F_carrier + F_mod- In such embodiments, the frequency source may have any waveform shape. In such examples, the difference between the phase states P_mod1, P_mod2 through P_modn, is selected to be within an acceptable phase shift range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a phase shift keying (PSK) signal specification.

[073] In some embodiments, the difference y = F_carrier F_mod is provided to be within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. In such examples, the frequency source may have any waveform shape. In such examples, the difference between the phase states P_mod1 , P_mod2 through P_modn, is selected to be within an acceptable phase shift range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a phase shift keying (PSK) signal specification.

[074] In some examples, the parameters F_carrier and F_mod are provided such that: z = F_carrier ± ( n x F_mod ), where n is a harmonic number, and z is within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. In such embodiments, the frequency source may preferentially have a square wave shape with n being an odd number, but any waveform shape having energy at the harmonics of F_mod are possible.

[075] In some embodiments the frequency source switches nearly instantaneously between phase states at a rate within an acceptable symbol rate range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a signal specification. In some examples, the frequency source transitions smoothly between phase states over a period of time, such that the transition is completed within an acceptable symbol rate range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a signal specification. In some embodiments, the smooth transition between phase states occurs according to a function of time such that the occupied bandwidth of the backscattered signal complies with a regulatory or specification requirement.

[076] The backscatter device 200 may provide a transmitted backscattered signal compatible with amplitude shift keying (ASK) standard employed by a receiving wireless communication device. The ASK standard may have a specified modulation depth, one or more specified channel center frequencies, and one or more specified symbol rates.

[077] To implement amplitude shift keying of the reflected or scattered signal, two example implementations are described. In examples using 100% modulation depth amplitude shift keying (e.g. sometimes called on-off keying or OOK), one input to the modulator (e.g. modulator 220 of FIG. 2) may be provided. This input may be toggled (e.g. by the symbol generator 230) at a first frequency F_modsc for one symbol period when the symbol to be sent is e.g. a "1 ". The input is held off (e.g. not toggled) for one symbol period when the symbol to be sent is e.g. a "0".

[078] In some examples, at least two inputs to the backscatter modulator are provided to permit amplitude shift keying with other than 100% modulation depth. A first input is used to generate a subcarrier frequency by switching the modulator 220 at a first frequency F_modsc. A second input is used to vary the modulation depth of the subcarrier frequency at the symbol rate. One method for varying the modulation depth is to add a resistor in parallel with the aforementioned modulator 220, such as a resistor with a resistance R in parallel with the switching FET. In such an embodiment, the real part of the impedance presented to the antenna then varies between the real part of the parallel circuit with the transistor off (R //transistor impedance off) and the real part of the parallel circuit with the transistor on (R //transistor impedance_ on). The backscattered subcarrier will then have two different modulation depths when the second input is switched between a " 1 " and a "0" in accordance with the data to be sent.

[079] In examples using ASK, the symbol generator 230 may include a subcarrier frequency source that may be operated at a frequency Fn,^. The frequency source may include a resistance-capacitance (RC) oscillator, an inductance-capacitance (LC) oscillator, a quartz crystal oscillator, a frequency synthesizer, the output of a digital-to-analog converter, the output of a direct digital synthesizer, the output of a clock generator, an arbitrary waveform generator, or any other analog or digital frequency source. In some examples, the aforementioned frequency source produces a square-wave output, while in other embodiments the aforementioned frequency source produces a sinusoidal output. In some examples the frequency source produces any waveform having energy at least including the frequency component F_modsc,

[080] In some examples, the sum of the frequency of an incident carrier F_carrier, plus the F_modsc is provided to be within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification, such that xl = F_carrier + F_modsc- In such examples, the frequency source may have any waveform shape.

[081] In some examples, the difference yl = F_carrier F_modsc is selected to be within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. In such examples, the frequency source may have any waveform shape.

[082] In some examples, the parameters F_carrier and F_modsc are chosen such that: z = F_carrier ± ( n x F_modsc) where n is a harmonic number, and z is within an acceptable range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a channel center frequency signal specification. In such embodiments, the frequency source may preferentially have a square wave shape with n being an odd number, but any waveform shape having energy at the harmonics of F_modsc are possible.

[083] In some examples the modulator switches nearly instantaneously between two modulation depth states at a rate within an acceptable symbol rate range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a signal specification. In other embodiments the frequency source transitions smoothly between modulation depth states over a period of time, such that the transition is completed within an acceptable symbol rate range (e.g. a range at which it may be correctly received by a receiving device communicating in accordance with that specification) of a signal specification. In some embodiments, the smooth transition between modulation depth states occurs according to a function of time such that the occupied bandwidth of the backscattered signal complies with a regulatory or specification requirement.

[084] The backscatter device 200 may provide a transmitted backscattered signal compatible with orthogonal frequency division multiplexing (OFDM) standards employed by a receiving wireless communication device. Generally, techniques described herein for providing backscattered FSK, PSK, QAM, and ASK signals may be extended to produce OFDM signals.

[085] In examples using OFDM, multiple band-pass signals may be generated by the modulator 220, one such bandpass signal per OFDM subcarrier. This may be implemented by providing multiple modulator frequencies such that their fundamental mode and/or harmonic mode frequency components align with the subcarrier spacing specified for the OFDM standard and/or technique. Each of the OFDM subcarriers may be modulated with e.g. a PSK signal per the description herein for PSK modulation examples. The multiple modulator frequencies may be applied to the same modulator (e.g. transistor). In some examples, a non- linear mixing operation may be implemented using a logic combination of the multiple modulator frequencies such as an exclusive-or (XOR) gate or an OR gate.

[086] In some examples, a linear operation may be employed via an analog power combination of the multiple modulator frequencies provided to the modulator 220.

[087] In some examples, the backscatter device 200 may harvest at least part of its operating power from the environment, for example using an optional RF energy harvesting circuit that may be included in and/or co-located with the backscatter device 200. In some embodiments this energy may be used directly by the backscatter device 200, while in other embodiments this energy may be stored in a reservoir such as but not limited to a capacitor, a supercapacitor, or a battery that is included in and/or co-located with the backscatter device 200. In such examples harvested energy may be accumulated in the reservoir for a period of time and then released to the operating circuitry of the backscatter device 200. This may be either a predetermined period of time, or a period of time corresponding to a time at which the reservoir reaches a particular amount of stored energy.

[088] FIG. 3 is a flowchart illustrating a method in accordance with examples described herein. The method includes receiving a signal having a carrier frequency in block 305, backscattering the signal to provide a backscattered signal in block 310. Block 310 may include modulating impedance of at least one antenna in accordance with data (block 315) and mixing the carrier frequency with a subcarrier (block 320). The method 300 also including transmitting the backscattered signal in block 325.

[089] Block 305 may be implemented, for example, by the backscatter device 200 of FIG. 2 or the backscatter device 110 of FIG. 1 receiving a signal having a carrier frequency (e.g. the signal 130 of FIG. 1) at their antenna. Block 310 may be implemented, for example, by the backscatter device 200 of FIG. 2 or the backscatter device 1 10 of FIG. 1. For example, the symbol generator may receive data and control the modulator of FIG, 2 to modulate impedance of the antenna 215 of FIG. 2 in block 315. This process may involve mixing the subcarrier frequency with the carrier frequency.

[090] The backscattered signal may then be transmitted in block 325 to produce, for example, the transmitted backscattered signal 135 of FIG. 1. [091] The signal having a carrier frequency may be backscattered in block 310 to provide a backscattered signal. This may include mixing the carrier frequency with a subcarrier in block 320. The mixing in block 320 may result in a bandpass signal having a predetermined frequency range. The predetermined frequency range may be a range of an advertising channel in accordance with a wireless communication standard, such as the Bluetooth Low Energy standard.

[092] Backscattering the signal in block 310 may include modulating impedance of at least one antenna in accordance with data to be provided in the backscattered signal. Modulating may include reflecting the signal in a pattern indicative of the data to be provided in the backscattered signal. The data to be provided in the backscattered signal may include a packet having a preamble, an access address, a payload data unit, and a cyclic redundancy check. The data may include, for example, an indication of a temperature associated with a device providing the backscattered signal, or an identification of an asset associated with a device providing the backscattered signal.

[093] Packets that may be provided in accordance with examples described herein include, but are not limited to, Bluetooth Low Energy advertising packets, IEEE 802.11 beacon frames, and IEEE 802.15.4 beacon frames.

EXAMPLE

[094] This detailed example of a system operating in accordance with a Bluetooth Low Energy specification is provided to facilitate understanding, although it is not intended to be limiting, nor to indicate that this was the only detailed example investigated, contemplated, or implemented.

[095] In one example a system provides interoperability between a backscatter device and a wireless communication device having a Bluetooth Low Energy chipset as is commonly found in mobile devices such as tablet computers, such as the APPLE iPAD or SAMSUNG GALAXY tablets or smart phones such as the APPLE IPHONE or SAMSUNG GALAXY series.

[096] The Bluetooth Low Energy (BTLE) specification details a wireless communication scheme using Gaussian frequency shift keying with a channel specification of 40 channels with center frequencies ranging from 2402 MHz to 2480 MHz. The data rate is 1.0 Mbps while the channel spacing is 2.0 MHz. The minimum frequency deviation is 185 kHz.

[097] Three of the 40 channels, channels 37, 38, 39, with center frequencies of 2402 MHz, 2426 MHz, and 2480 MHz respectively, are referred to as advertising channels. A conventional Bluetooth Low Energy device listens on each of the advertising channels in turn to identify nearby BTLE devices.

[098] In one example, the modulator (e.g. modulator 220 of FIG. 2) is implemented using a type BF1 108R field effect transistor manufactured by NXPJnc. The symbol generator (e.g. symbol generator 230 of FIG. 2) is implemented using an Agilent 335008 arbitrary waveform generator. The source and drain terminals of the field effect transistor are connected to a first dipole antenna resonant near 2450 MHz. The signal source (e.g. signal source 100 of FIG. 1) is implemented using an Agilent N5181A signal generator set to a desired carrier frequency F_carrier, with an output power of +15dBm, connected to a second dipole antenna resonant near 2450 MHz. The wireless communication device receiving the communication (e.g. the wireless communication device 120 of FIG. 1) is implemented using an APPLE IPAD running APPLE IOS with the Pally BLE scanner application.

[099] In one example, the signal source (e.g. the signal source 100 of FIG. 1) is implemented as supplying a dedicated carrier source with a frequency of F_carrier ⁼ 2453 MHz. To produce a bandpass signal in the Channel 38 and 39 passbands, modulating frequencies F_mod1 ⁼26.7 MHz and F_mod2 ⁼27.3 MHz. The sum of the carrier frequency plus the mean of the two modulation frequencies is 2453 MHz + mean(26.7 MHz, 27.3 MHz) = 2480 MHz (Channel 39), leveraging the upper sideband modulation, while the difference between the carrier frequency and the mean of the two modulation frequencies is 2453 MHz - mean(26.7 MHz, 27.3 MHz) = 2426 MHz (Channel 38), leveraging the lower sideband modulation. Thus a single carrier frequency may serve multiple channels using the previously mentioned fundamental mode approach. Note that in this implementation the frequency deviation between the two modulation frequencies is 600 kHz which complies with the required minimum frequency deviation specification of 185 kHz.

[0100] To produce a bandpass signal in the Channel 37 passband of 2402 MHz, a second harmonic (n=2) approach may be used. In this case, modulating frequencies F_mod1=25.35 MHz and F_mod2=25.65 MHz. Thus the second harmonic band-pass signal falls at 2453 MHz- 2*mean(25.35 MHz, 25.65 MHz) = 2402 MHz. Note that in this implementation the difference between the two modulation frequencies is only 300kHz to yield a second- harmonic difference of 600kHz which complies with the minimum frequency deviation specification of 185 kHz.

[0101] In this manner, all three advertising channels may be addressed using only a single carrier frequency of 2453 MHz. The modulating frequencies are in the range of 25.35 MHz to 27.3 MHz which is far lower than the carrier frequency. Thus the backscatter device may consume far less power than would be required to generate the carrier frequency. In this example the symbol generator comprises an Agilent 33500B arbitrary waveform generator using a waveform synthesized as a sampled vector using the MATLAB signal processing toolbox. The symbol rate is 1.0 Msps.

[0102] In this example, the symbol generator (e.g. symbol generator 230 of FIG. 2) is produces a packet format compatible with the BTLE specification. FIG. 4 is a schematic illustration of an example packet compatible with the BTLE specification. From the least significant bit to the most significant bit, an example packet includes the preamble OxAA, the access address Ox8E89BED6, the payload data unit Ox43C80844210712140201050909456E73776F727468, and the cyclic redundancy check Ox9BC70A. The MATLAB signal processing toolbox encodes the packet using the data whitening function specified in the BTLE specification. A different data whitening function input is used for communication over each channel. For each bit in the encoded packet, the modulating frequency is set to F_mod1 if the corresponding bit is "0" and F_mod2 if the corresponding bit is "1". The packet shown in FIG. 4 may accordingly be transmitted as, for example, the transmitted backscatter signal 135 of FIG. 1.

[0103] From the foregoing it will be appreciated that although specific embodiments of the technology have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the technology. Also, in some embodiments the microcontroller can be omitted, or the battery can be larger. Further, certain aspects of the new technology described in the context of particular embodiments may be combined or eliminated in other embodiments. Moreover, while advantages associated with certain embodiments of the technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described herein.

Claims

CLAIMS What is claimed is:

1. A device comprising:

an antenna configured to receive an incident signal having a carrier frequency;

a modulator; and

a symbol generator, wherein the symbol generator is configured to provide a subcarrier frequency, and wherein the symbol generator is further configured to control the modulator to backscatter the incident signal having the carrier frequency using the subcarrier frequency to provide a backscattered signal to the antenna, the backscattered signal including a bandpass signal in a predetermined frequency range.

2. The device of claim 1, wherein the predetermined frequency range is a range specified by a wireless communication standard.

3. The device of claim 1, wherein the predetermined frequency range comprises a range of an advertising channel specified by a Bluetooth Low Energy specification.

4. The device of claim 1, wherein the symbol generator is configured to provide the backscattered signal in part by mixing the subcarrier frequency with the carrier frequency.

5. The device of claim 1, wherein the symbol generator is configured to provide the backscattered signal in part by mixing a harmonic of the subcarrier frequency with the carrier frequency.

6. The device of claim 1 , wherein the modulator comprises a field effect transistor.

7. The device of claim 1 , wherein the backscattered signal comprises a packet.

8. The device of claim 7, wherein the packet comprises a preamble, an access address, a payload data unit, and a cyclic redundancy check.

9. The device of claim 1, further comprising a frequency source coupled to the symbol generator, the frequency source configured to provide the subcarrier frequency.

10. The device of claim 1, further comprising multiple frequency sources coupled to the symbol generator, and wherein the symbol generator is configured to select at least one of the multiple frequency sources for use in providing the backscattered signal, wherein the symbol generator is configured to select at least one of the multiple frequency sources in accordance with data provided to the symbol generator.

11. The device of claim 1, wherein the subcarrier frequency is modulated in amplitude, frequency, and/or phase.

12. The device of claim 10, wherein at least one of the multiple frequency sources are modulated in amplitude, frequency, and/or phase.

13. The device of claim 10, wherein the backscattered signal is an orthogonal frequency division multiplex (OFDM) signal.

14. A method comprising:

receiving an incident signal having a carrier frequency;

backscattering the incident signal to provide a backscattered signal, wherein the backscattering comprises:

modulating, using a backscatter device, impedance presented to at least one antenna in accordance with data to be provided in the backscattered signal; and

mixing the carrier frequency with at least one subcarrier provided by the backscatter device.

15. The method of claim 14, wherein the mixing results in a bandpass signal having a predetermined frequency range.

16. The method of claim 15, wherein the predetermined frequency range comprises a range of a channel in accordance with a wireless communication standard.

17. The method of claim 16, wherein the wireless communication standard comprises Bluetooth Low Energy.

18. The method of claim 14, wherein the modulating comprises modulating the amplitude, frequency, and/or phase of the backscattered signal in a pattern indicative of the data to be provided in the backscattered signal.

19. The method of claim 14, wherein the data to be provided in the backscattered signal comprises a packet having a preamble, an access address, a payload data unit, and a cyclic redundancy check.

20. The method of claim 14, further comprising transmitting the backscattered signal.

21. The method of claim 14, wherein the backscattered signal comprises a reading of a sensor associated with a device providing the backscattered signal.

22. The method of claim 14, wherein the backscattered signal comprises an identification of an asset associated with a device providing the backscattered signal.

23. The method of claim 22, wherein the device providing the backscattered signal comprises a tag.