Systems, methods, and media for multi-antenna, directional backscatter tags

Backscatter tags, comprising: antennas; single pole, multiple throw switches each switch (S) having pole terminal connected to one of the antennas, and each having first throw terminal (TT) connected to first side (FS) of inductor (I) for S, second TT connected to FS of capacitor (C) for S, third TT connected to fixed voltage level (FVL), fourth TT that is floating, wherein second side (SS) of I and SS of C are connected to the FVL; power combiner (PC) having inputs connected to fifth TT of each of two of the switches; power detector (PD) having an input connected to output of PC; analog to digital converter (ADC) having input connected to output of PD; hardware processor coupled to output of the ADC and coupled to control terminal of each of the switches.

BACKGROUND

Backscatter tags (e.g., RFID tags) are widely used for a variety of applications, such as security applications, tracking merchandise and people, and storing information. The range of the tags is limited because the tags frequently do not have their own power source (and only get power from a received RF signal) or only have a limited power source.

It is desirable to increase the range of backscatter tags.

SUMMARY

Systems, methods, and media for multi-antenna, directional backscatter tags are provided. In some embodiments, systems for multi-antenna, directional backscatter tags are provided, the systems comprising: a plurality of antennas; a plurality of single pole, multiple throw switches each having a pole terminal connected to a corresponding one of the plurality of antennas, and each having a first throw terminal connected to a first side of an inductor for that switch, a second throw terminal connected to a first side of a capacitor for that switch, a third throw terminal connected to a fixed voltage level, a fourth throw terminal that is floating, and a fifth throw terminal, wherein a second side of the inductor and a second side of the capacitor are connected to the fixed voltage level; a power combiner having inputs connected to the fifth throw terminal of each of two of the plurality of single pole, multiple throw switches and having an output; a power detector having an input connected to the output of the power combiner and having an output; an analog to digital converter having an input connected to the output of the power detector and having an output; a hardware processor coupled to the output of the analog to digital converter and coupled to a control terminal of each of the plurality of single pole, multiple throw switches.

DETAILED DESCRIPTION

In accordance with some embodiments, multi-antenna backscatter tags that backscatter directional beams to tag readers are provided. In some embodiments, using a 3×3 antenna array in a backscatter tag (as shown inFIG. 1) can provide up to a 19 dB sensitivity enhancement and up to a 3× increase in range between the tag and its reader compared to the same characteristics for single antenna tags.

In some embodiments, a compact phase conjugation technique for backscatter modulation can be used with a backscatter tag having a two-dimensional antenna array in order to achieve improved directivity of the backscatter signal in the direction of arrival of the signal being backscattered.

In some embodiments, for each antenna of multi-antenna backscatter tag, an incoming RF signal can be reflected back by the backscatter antenna with different phase lags depending on its load. For example, in some embodiments, the load can be formed using open, short, inductive, and/or capacitive loads. In some embodiments, each antenna of a multi-antenna backscatter tag can switch between different arrangements of these loads to create a directional backscattered signal. These loads can also be used to implement quadrature phase shift keying (QPSK) modulation of the RF signal in some embodiments. QPSK modulation can be useful to convey a serial number of the tag, data stored in the tag, etc.

To understand the principle of operation, assume that a linear array of three antennas with an antenna spacing of d is being used. For an incident signal at an angle θi, when operating in the far field, incident waves with wavelength λ received by two adjacent antennas have a phase difference of ΔΦ=2π·d·sin(θi)/λ. To reflect the signal back in the direction of arrival, the reflected signal must have a phase difference of −ΔΦ. Thus, the loads of adjacent antennas need to provide an extra phase difference of ΦA=−2ΔΦ. If the phase lag of the middle antenna is Φr=0 degrees, the other two antennas require a phase lag of ΦAand −ΦA.

Using different arrangements of open, short, inductive, and capacitive loads, the phase gradient ΦAcan be 0 degrees, 90 degrees, 180 degrees, and 270 degrees as shown inFIG. 2. For these values of ΦA, the corresponding angle of incidence (conjugating angle) at which the backscatter tag achieves maximum directivity of the reflected signal in the direction of arrival is plotted inFIG. 3for increasing spacing between the antennas. By using an antenna spacing of 0.53λ, evenly spaced conjugating angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees can be obtained in some embodiments.

For an N×N array, assuming the required horizontal and vertical phase gradients are ΦAand ΦB, the required phases for the (i, j)thantenna in the array are:
Φref(i,j)=(i−(n+1)/2)ΦA+(j−(n+1)=2)ΦB,
where i and j are indices to the antennas in the antenna array of the tag and n is equal to N (i.e., the number of antennas in each dimension of the two-dimensional array. The tag needs to determine the gradients ΦAand ΦBusing a direction of arrival estimation). An N×N array can provide N2times the link enhancement (e.g., up to 19 dB for a 3×3 array as shown inFIG. 4), thus N times the range. As shown inFIG. 5, for ΦB=0, the backscatter tag directivity is plotted at θi=0 degrees (portion (a) ofFIG. 5), 30 degrees (portion (b) ofFIG. 5), 45 degrees (portion (c) ofFIG. 5), 60 degrees (portion (d) ofFIG. 5) using the corresponding ΦAfor a 3×3 array. The directivity improves by 9 dB for the conjugating angles. For ΦB=0, the maximum achievable array factor is plotted with respect to the angle of incidence θiinFIG. 4. Limiting the loads to an open load, a shorted load, an inductive load, and a capacitive load leads to a quantization error in ΦAand a degradation of up to 2 dB in the array gain.

By changing the reference phase Φr, while maintaining the phase gradients ΦAand ΦB, QPSK modulation can be implemented with directional backscattering. For a baseband signal mapped to a QPSK phase Φm(t), the required phase for the (i, j)thantenna is:
Φ(i,j,t)=Φref(i,j)+Φm(t).

For an incoming wave at an angle of incidence θi, any two adjacent antennas in a 3×1 linear array receive signals V1(t)=A·sin(ω·t) and V2(t)=A·sin(ω·t+Φ), where Φ=2π·d·sin(θi)/λ, A is the maximum amplitude of the signals, ω is the angular frequency of the signal, and t is time. V1(t)+V2(t)=2·A·sin(ω·t)cos(Φ/2). Adding the signals in-phase (FIGS. 6A-6C) and detecting the power received gives P=2·A2·cos2(Φ/2), θican then be calculated by θi=sin−1(λ·cos−1(√{square root over (P/(2A2)))}/(2πd)). An example of the sum of the two signals normalized with the received power on one antenna is plotted as a first received signal strength indicator (RSSI1) inFIG. 7. However, RSSI1is symmetric with respect to θi=0. The sign of θican be evaluated using the derivative of RSSI1, measured by connecting an inductor as shown inFIGS. 6B and 6Cand calculating the difference in the received signal strengths (RSSI2and RSSI3). An example of RSSI2−RSSI3is plotted inFIG. 7. Direction of arrival can be successfully estimated over a range of θifrom −60 degrees to +60 degrees. For |θi|>60 degrees, RSSI1is less than −13 dB and the required phase gradient, ΦA=0 degrees.

Turning toFIG. 8, a schematic800of an example of a tag architecture in accordance with some embodiments is provided. As shown, the tag can include an array of antennas802(e.g., such as a 3×3 array as illustrated). Any suitable number and any suitable type of antennas can be used in the array in some embodiments. For example, a 3×3 antenna array using patch antennas sized 3 cm by 3.8 cm with a 6.5 cm center-to-center spacing on a 31 mil-thick ISOLA 370HR substrate can be used for a 2.45 GHz signal in some embodiments.

Each antenna can be connected to a pole terminal of a single pole, multiple throw switch804. This switch can be used to switch the antenna load for load modulation and determine the direction of arrival. Any suitable switch can be used as switch804in some embodiments, and the switch can have any suitable number of throw terminals. For example, in some embodiments, an SP5T switch (SKY13415) available from SKYWORKS SOLUTIONS, INC. of Woburn, Mass. can be used as switch804. Moreover, in some embodiments, a combination of switches can be used to realize switch804.

As shown in the figure, the throw terminals of the switches can be connected to inductors806, capacitors808, a fixed voltage (e.g., such as a ground voltage), and inputs to power combiners in some embodiments. Any suitable inductor(s), capacitor(s), and fixed voltage(s) can be used in some embodiments. For example, in some embodiments, an inductor806can be an SMD 3nH 0201 inductor available from MURATA MANUFACTURING COMPANY, LTD. of Kyoto, Japan. As another example, in some embodiments, a capacitor808can be a 1.3 pF 0402 capacitor available from AVX Corporation of Greenville, S.C., USA.

Antennas1and2(Ant1and Ant2) can provide a first pair of antennas and are connected to a first power combiner810. Antennas3and4(Ant3and Ant4) can provide a second pair of antennas and are connected to a second power combiner812. Antennas5and6(Ant5and Ant6) can provide a third pair of antennas and are connected to a third power combiner814. Antennas7and8(Ant7and Ant8) can provide a fourth pair of antennas and are connected to a fourth power combiner816. And antenna9(Ant9) can provide a reference antenna.

As shown, the outputs of the four power combiners (810,812,814, and816) and antenna9can each be connected to a rectifier (e.g., rectifiers818and820). Any suitable power combiners can be used in some embodiments. For example, in some embodiments, a SP-2U2+ available from Mini-Circuits of Brooklyn, N.Y. can be used.

The five rectifiers can be also connected to the inputs of five analog to digital converters (this connection and the analog to digital converters (ADCs822and824) are only shown for the first pair of antennas and the second pair of antennas for clarity of the figure). The ADCs can be used to digitize the output of the power detectors. Any suitable ADCs can be used in some embodiments.

As shown inFIG. 8, the analog to digital converters can be part of a Micro-Controller Unit (MCU)826in some embodiments. The MCU can also include a controller that determines the direction of arrival (DoA) and the required load configuration (for controlling the direction of the backscatter), a baseband modulator (for QPSK modulation), and a local oscillator. Any suitable MCU can be used in some embodiments. For example, in some embodiments, the MCU can be implemented using a Teensy 3.6 MCU available from Paul Stoffregen of Sherwood, Oreg.

FIG. 9provides a more specific example of a schematic of a portion of a tag in accordance with some embodiments. As illustrated,FIG. 9only shows two antennas, two switches (and associated inductors and capacitors), a single power combiner, and a single power detector for purposes of clarity. It should be understood that replicas of these components could be provided for each pair of antennas in a tag having any suitably sized array in some embodiments.

As shown inFIG. 9, each antenna can be connected to a single pole, multiple throw switch, which can be implemented using part number 863-1583-1-ND from Skyworks Solutions, Inc. of Woburn, Mass. in some embodiments. One throw terminal of each switch can be connected to one side of a 3nH inductor (which can be for a 2.4 GHz implementation; any other suitable value can be used in some embodiments), whose other side is connected to a fixed voltage level, such as ground. Another throw terminal of each switch can be connected to the fixed voltage level, such as ground. Still another throw terminal of each switch can be floating. Yet another throw terminal of each switch can be connected to one side of a 1.2 pF capacitor (which can be for a 2.4 GHz implementation; any other suitable value can be used in some embodiments), whose other side is connected to the fixed voltage level, such as ground. Still another throw terminal of each switch can be connected to the input of a power combiner. In some embodiments, the power combiner can be implemented using part number SP-2U2+ from Mini-Circuits of Brooklyn, N.Y. The output of the power combiner can be connected to the input of a power detector, which can be implemented using part number LT5538 from Analog Devices of Norwood, Mass. The output of the power detector can be connected to analog to digital converter input of an MCU. In some embodiments, the MCU can be implemented using a Teensy 3.6 MCU available from Paul Stoffregen of Sherwood, Oreg.

Turning toFIG. 10, an example1000of a process that can be implemented in an MCU in accordance with some embodiments is shown. As illustrated, after process1000begins, at1002the process can configure the switches for each antenna. For example, for Ant1(which is illustrated inFIG. 8as having five sub-switches) to have Γ1=0, S1can be configured to have the top sub-switch open, the second from the top (grounded) sub-switch closed, the inductor sub-switch and the capacitor sub-switch open, and the bottom sub-switch closed, and for Ant2(which is also illustrated inFIG. 8as having five sub-switches) to have Γ2=0, S2can be configured to have the top four sub-switches open and the bottom sub-switch open.

Next, at1004, process1000can measure the received signal strength indication (RSSI) from Ant1as RSSIref,A. Alternatively, in some embodiments, RSSIref,Acan be measured using a dedicated reference antenna, such as the center antenna inFIG. 8, in which case1002and1004can be modified accordingly.

Then, at1006, process1000can re-configure the switches for each antenna. For example, for Ant1to have Γ1=0, S1can be configured to have the top four sub-switches open and the bottom sub-switch closed, and for Ant2to have Γ2=0, S2can be configured to have the top four sub-switches open and the bottom sub-switch closed.

At1008, process1000can then measure RSSIAusing an analog to digital converter coupled to the center antenna ofFIG. 8by a rectifier or power detector, and then calculate RSSInorm(in dB) as RSSIA(in dB) minus RSSIref,A(in dB).

Next, at1010, process1000can evaluate |θi| using:
θi=sin−1(λ·cos−1(√{square root over (RSSInorm/4)})/(2πd)).
where d is the distance between the antennas, λ is the wavelength of the received signal.

Then, at1012, process1000can re-configure the switches for each antenna. For example, for Ant1to have Γ1=1i, S1can be configured to have the top two sub-switches open, the inductor sub-switch closed, the capacitor sub-switch open, and the bottom sub-switch closed, and for Ant2to have Γ2=−1i, S2can be configured to have the top two sub-switches open, the inductor sub-switch open, the capacitor sub-switch closed, and the bottom sub-switch closed. At1012, process1000can then measure RSSI2.

At1014, process1000can then re-configure the switches for each antenna. For example, for Ant1to have Γ1=−1i, S1can be configured to have the top two sub-switches open, the inductor sub-switch open, the capacitor sub-switch closed, and the bottom sub-switch closed, and for Ant2to have Γ2=1i, S2can be configured to have the top two sub-switches open, the inductor sub-switch closed, the capacitor sub-switch open, and the bottom sub-switch closed. At1014, process1000can then measure RSSI3.

Next, at1016, process1000can determine the sign of RSSI2minus RSSI3and infer from that sign the sign of |θi|.

At1020, process1000can repeat steps1002-1018as described above using Ant3, Ant4, S3, and S4instead of Ant1, Ant2, S1, and S2, respectively, for the vertical phase gradient (ΦB). Alternatively, steps similar to steps1002-1018using Ant3, Ant4, S3, and S4instead of Ant1, Ant2, S1, and S2, respectively, can be performed in parallel to steps1002-1018for the vertical phase gradient (ΦB) in some embodiments.

Finally, at1022, the tag can be configured and modulated for data. For example, QPSK modulation can be used to transmit data such as a serial number, data stored in the tag, etc.

In some embodiment, other antennas in an array can also be used in process1000to improve accuracy. For example, each RSSI measurement made for antennas1and2ofFIG. 8can also be made for antennas5and6ofFIG. 8and the average of the two RSSI measurements used. As another example, each RSSI measurement made for antennas3and4ofFIG. 8can also be made for antennas7and8ofFIG. 8and the average of the two RSSI measurements used.

In some embodiments, assuming area constrained applications, using N times the carrier frequency with antenna aperture similar to 1×1 array improves the range √{square root over (N3)} times. In some embodiments, increasing the number of antennas and antenna loads allows higher order modulations with narrower beams for more efficient data transfer or security from unwanted readers.

In some embodiments, direction of arrival estimation performance can be improved by mitigating interference using band select filters placed before each power detector.

Although specific components are described herein, it should be apparent that other components can be used to provide the same or similar functions and/or additional functions in some embodiments. For example, while an MCU is described herein as performing certain processes, it should be apparent that any suitable hardware processor (e.g., microprocessor, controller, digital signal processor, dedicated logic, and/or any other suitable circuitry) can be used to perform some or all of these processes in some embodiments. As another example, in some embodiments, memory and/or storage can be used to store programs, data, and/or any other suitable information in some embodiments. For example, memory and/or storage can include random access memory, read-only memory, flash memory, hard disk storage, optical media, and/or any other suitable memory.

In some embodiments, at least some of the above described blocks of the process ofFIG. 10can be executed or performed in any order or sequence not limited to the order and sequence shown in and described in connection with the figure. Also, some of the above blocks ofFIG. 10can be executed or performed substantially simultaneously where appropriate or in parallel to reduce latency and processing times. Additionally or alternatively, some of the above described blocks of the process ofFIG. 10can be omitted.