CA2219381C - Modulated backscatter sensor system - Google Patents

Abstract

In accordance with the present invention, a radio communication system includes an Interrogator for generating and transmitting a radio signal. One or more Tags contained within the radio communication system receive the radio signal. ABackscatter Modulator modulates reflection of the radio signal using a subcarrier signal, thereby forming a reflected modulated signal. The Interrogator receives and demodulates the reflected modulated signal. Based upon the characteristics of the demodulated signal the Interrogator can determine the identity of the Tag, and the relative velocity of the Tag with respect to the Interrogator. The Interrogator can also determine if motion exists in the vicinity of the Interrogator, even no Tag is present, without the need for a separate motion detection system. The characteristics of the demodulated signal, can also be used to determine the characteristics of motion of the Tag, such as the vibrational frequency. Alternate embodiments allow the Interrogator to transmit a first information signal to one or more tags, specifying which Tags should respond using Backscatter Modulator means, so that the characteristics of only particular Tags can be determined. Further alternate embodiments allow the Tag to input analog data, and perform analog to digital conversion of that data. This data may be then transmitted to the Interrogator using Modulated Backscatter. Alternately, this data may be used as input to calculations performed in the Tag in order to analyze the frequency characteristics of the analog input. The Tag can also, based. upon the results of these calculations, identify an abnormal condition and notify the Interrogator of the existence of such a condition.

Description

MODULATED BACKSCATTER SENSOR SYSTEM
1. Field of the Invention This invention relates to wireless communication systems and, more particularly, to a wireless communication system using modulated backscatter technology.

2. Description of the Related Art Radio Frequency Identification (RFm) systems arc used for identification and/or tracking of equipment, inventory, or living things.
ItFiD
systems arc radio communication systems that communicate between a radio 10 transceiver, called an Interrogator, and a number of inexpensive devices called Tags.
In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. After transmitting a message to the Tag (callod the Downlink), the Interrogator then transmits a Continuous-Wave (G'V~ radio signal to the Tag. The Tag then moduliues the CW
signs using modulated backscattering where the antenna is electricall;~
switched, by the modulating signal, from being an absorber of 1ZF radiation to being; a reflector of RF radiation. This modulated backscatter allows communications from the Tag back 5 to the Interrogator (called the IJplink). Conventional Modulated Backscatter (MBS) systems are designed a) to identify an object passing into range of the Interrogator, and b) to store data onto the Tug and then retrieve that data from the Tag at a later time in order to manage inventory or perform some other useful application.
Sensors are used tae monit~ the current state of a device. ,An example of 10 a sensor application is to monitor the temperature, pressure, or other characteristic of a mechanical or biological device. Sensor technology has advanced to the point where inexpensive sensors, such as temperature, pressure, etc. can be aittachcd to microprocessors. However, these sensors must communicate their resets back to a central control unit.
15 In another sensor ~ipplication, we desire to know the relative velocity of a sensor or Tag with respect to a base unit (Interrogator). For example, in an Electronic Toll Collection syseem, it may be important to not only identify the Tag and story or retrieve data from the Tag, but also determine the velocity of the Tag, perhaps to determine if the vehicle is speeding. In a security access aF~plication it 20 would be helpful to identify an object having a Tag, determine the velexity of the Tag, and also to determine if movement is present in the reading field, regardless of whether a Tag is present.
Beyond security, other applications require the ability to nnonitor sensor-outputs. For example, a pump may have a certain vibration "signature" during 25 normal operation, and a different vibration signature during abnormal operation. It is important to ascertain when the pump's vibration signature changes from normal to abnormal.
In some of the described embodiments of this invention, we disclose methods for using MBS RFID systems to perform functions such as, determining the 30 relative velocity of the Tag with respect to the Interrogator, determining if movement is present in the reading field even if not Tag is present, an<i determining the vibration signature of a device, such as but not limited to a pump, t:o which the Tag is attached. In this manner, an inexpensive RFID network, consisting of one or more Interrogators, can (x constructed which: performs RFID functions, sensor 35 functions, motion detection, arid analysis of sensor data functions.

2a Summary of the Invention In accordance with one aspect of the present invention there is provided a modulated backscatter system, comprising: at least one transponder that receives a first transmitted signal and modulates a reflected first transmitted signal using a S subcarrier signal; and at least one interrogator having a transmitter that transmits said first transmitted signal and a receiver that receives said reflected first transmitted signal, said interrogator having a demodulator that obtains a received subcarrier signal from said reflected first transmitted signal, and a subcarrier demodulator that analyzes aaid received subcarrier signal to measure a motion of said transponder.

In accordance with an embodiment of the present invention, a radio communication system includes an Interrogator for generating and transmitting a radio signal. One or more Tags or transponders contained within the radio S communication system receive the radio signal. A Backscatter Modulator modulates the reflection of the radio signal using a subcarrier signal, thereby forming a reflected modulated signal. The Interrogator receives and demodulates the reflected modulated signal. Based upon the characteristics of the demodulated signal, the Interrogator can determine the identity of the Tag, and the rzlative velocity of the 10 Tag with respect to the Interrogator. The Interrogator can also determine if motion exists in the vicinity of the Interrogator, even no Tag is present, without the need for a separate motion detection system. The characteristics of the demodulated signal, can also be used to determine the characteristics of motion of the Tag, such as the vibrational frequency. Alternate embodiments allow the Interrogator to transmit a 15 first information signal to one or more tags, specifying which Tags should respond using Backscatter Modulator means, so that the characteristics of only particular Tags can be determined. Further alternate embodiments allow the Tag to input analog data, and perform analog to digital conversion of that data. This data may be then transmitted to the Interrogator using Modulated Backscatter. Altematcly, this 20 data may be used as input to calculations performed in the Tag in order to analyze the frequency characteristics of the analog input. The Tag can also, based upon the results of these calculations, identify an abnormal condition and notify the Interrogator of the existence of such a condition.
Brief Description of the Drawin~t ZS FIG. 1 shows a block diagram of an illustrative Radio Frequency Identification (RFID) system;
FIG. Z shows a block diagram of an illustrative Interrogator Unit used in the RFID system of FIG. 1;
FIG. 3 shows a block diagram of a Tag Unit used in the RF1D system of 30 FIG.1;
FIG. 4 shows a simplified block diagram of a radar system;
FIG. 5 shows a more detailed block diagram of a RFID Interrogator of a radar system;
FIG. 6 shows the relative positions of the signals in frequency space 35 before demodulation;

FIG. 7 shows the relative positions of the signals in frequency space after demodulation;
FIG. 8 shows in more detail the Frequency Selector shown in FIG. 7;
FIG. 9 is a block diagram of a subcarrier demodulator, 5 FIG. 10 shows the relative position of multiple Interrogate~rs with respect to an RFID Tag; .
FIG. 11 shows hove the Tag of FIG. 3 can support analog to digital conversion; and FIG. 12 shows hove the Tag of FIG. 3 can support an analog input port.
Detailed Description An embodiment of this invention provides a method to integrate motion and velocity determination together with conventional RFiD capabiliti~ss such as obtaining the identity of an RFJD Tag. The RF1D Interrogator can dett;rmine, based upon the reflected MBS signal :from a Tag, certain characteristics of thn Tag, such as 15 relative velocity with respect to the Interrogator, and the vibration characteristics of the Tag in the event the Tag is attached to a vibrating object.
MBS Operation We now describe how a RFTD system, utilizing MBS, operates. With reference to FIG. 1, there is shown an overall block diagram of a RF117~
system. An 20 Applications Processor 101 cornmunicates over Local Area Network (1:.AN) 102 to a plurality of Interrogators 103-104. The Interrogators may then each communicate with one ~ more of the Tags 1~D5-10'7. For example and in reference u~ FIG. 2, the Interrogator 103 receives an inl;ormation signal, typically from an Applications Processes 101. The Interrogator 103 takes this information signal and Processor 200 25 formats a Downlink message (l;nformation Signal 200a) to be sent to the Tag. The information signal (200a) may include information such as information specifying which Tag is to respond (each 'tag may have fixed or programmed identification number), instructions for the Tag's processor to execute other infoananon to be used and/or stored by the Tag's processor. With joint reference to FIGS. 1 and 2, Radio 30 Signal Source 201 synthesizes a radio signal, the Modulator 202 modulates the radio signal using Information Signal 200a, and the Transmitter 203 transmits this modulated signal via Antenna 204, illustratively using amplitude modulation, to a Tag. Amplitude modulation i;~ a desirable choice because the Tag can demodulate such a signal with a single, ine:Xpensive nonlinear device (such as a diode).

In the Tag 105 (see FIG. 3), the Antenna 301 (a loop or patch antenna) receives the modulated signal. "Ibis signal is demodulated, directly to baseband, using the Denector/Modulator 3~D2, which, illustratively, could be a single Schottky diode. The result of the diode detector is essentially a demodulation of the incoming 5 signal directly to baseband. The Information Signal 200a is then amplined, by Amplifier 303, and bit synchronization is recovered in Clock Recovery Circuit 304.
Clock recovery circuits such as circuits that recover a clock from manchester encoded data are well known in the art. If large amoumts of data are being transferred in frames, frame synchronization may be implemented, for example, by 10 detecting a predetermined hit pattern that indicates the start of a frame.
The bit pattern may be detected by clock recovery circuit 304 or processor 305. Bit pattern detection is well known in the art. The resulting information from cloclc recovery circuit 304 is sent w a Processor 305. The Processor 305 is typically any inexpensive 4 or 8 bit microprocessor and it:. associated memory, and it generates an Information 15 Signal 306 from the Tag 105 back to the Interrogator (e.g., 103).
Infonrnation Signal 306 is sent to a Modulator Connrol Circuit 307, which uses the Information Signal 306 to modulate a subcarrier frequency generated by the Frequency Source 308 to produce signal 311. The Frequency Source 308 may be a crystal oscillator separate from the Processor 305, or a signal derived from the output of a crystal oscillator, ac 20 it could be a frequency source derived from signals present inside the Processor 305 - such as a divisor of the fundamental clock frequency of the Processor. The Modulated Subcarrier Signal 311 is used by l7etector/Modulator 302 to modulate the RF signal received from Tag 10:5 to produce a modulated backscatter (i.e., reflected) signal. This is accomplished, fez example, by switching on and off the .Schottky 25 diode of Det:ector/Modulator 302using the Modulated Subcarrier Signal 311, thereby changing the reflectance of Antenna 301. A Battery 310 or other power supply provides power to the circuitry of Tag 105. Power may also be received, for example, by using inductive coupling or microwaves.
It has been found that considerable advantages are present to an MBS
30 design that uses a single frequency subcarrier. Many modulation schomns are possible; Phase Shift Keying (PSK) of the subcarrier (e.g., BPSK, QPS1C), more complex modulation schemes (e.g., MSK, GMSK), etc.
Returning to FIG. 2, the Interrogator 103 receives the reflected modulated signal with the Receive Antenna 206, amplifies the signal with a Low 35 Noise Amplifier 207, and demodulates the signal using homodyne detection in a Mixer 208. (In an alternative e»bodiment, a single antenna may replace Transmit antenna (204) and Receive Antenna (206). In this event, art electronic method of canceling the transmitted signal from that rtceived by the receiver chain is needed;

this could be accomplished by a device such as a Circulator.) Using the same Radio Signal Source 201 as used in the transmit chain means the demodulation to baseband is done using Homodyne detection; this has advantages in that it greatly rcdluces phase noise in the receiver circuits.
The Mixer 5 208 then sends the Demodulated Signal 209 (if Mixer 208 is a Quadratuut Mixer, it would send both I (in phase) arid Q (quadrature) signals) to the Filter/A~mplifier 210.
The resulting filtered signal - then typically an Information Signal 211 carried on a subcarricr - is then demodulated from the subcarrier in the Subcarrier I~modulator 212, which then sends the Information Signal 213 to a Processor 200 to determine 10 the content of the message. Sulxarrier dcmolulation may be implomcmted using a single non-linear device such as diodes or it may be implemented usinli an analog to digital (A/D) converter and a digital signal processor (DSP) for more complex applications. For example, a diode may be used for amplitude modulated subcarriers and the DSP may be used for P~SK modulated subcarriers. The I and Q! channels of 15 Signal 209 can be combined in the Filter/Amplifier 210, or in the Subcarrier Demodulator 212, or they could be combined in the Processor 200. U:~ing the above txhniques as an example, an inexpensive, short-range, bi-directional digital ratdio communications channel is implemented. These techniques are inexpensive as the components consist of (for example) a Schottky diode, an amplifier to boost the 20 signal strength, bit and frame synchronization circuits, as inexpensive 4 or 8 bit microprocessor, subcarrier generation circuits, and a battery. Most of these items arc already manufactured in large quantities for other applications, and thus arc not overly expensive. The circuits mentioned above for bit and frame synchronization and for subcarrier generation may also be implemented in custom logic;
surrounding 25 the microprocessor core; thus, except for a relatively small amount of chip real estate, these functions come almost "for free."
Relative Velocity We first discuss how a MBS system is used to determine the relative velocity between an Interrogat~~r and, for example, a vehicle. For this example, 30 assume that the vehicle is moving in a constant direction and at a constant velocity during the period of time the measurement will be taken. To determine the velocity, an MBS similar to a CW police Doppler radar system is used. A simple Doppler radar system, illustrated in FICi. 4, uses a CW signal (420) transmitted from the Interrogator (410) which is rcfllccted by a moving vehicle (440). The ieflcctcd signal 35 (430) is frequency shifted (0 f, see 430) from the RF carrier (f~, see 42.0) as a result of a Doppler shift from the mowing vehicle. The formula that relates a~ Radar Doppler Shift (~ j) to Relative Velocity (v) is Eq. 1 below. This formula is:

(1) v = ~f*~,/2 where ~, is the wavelength of cF~e RF carrier f~. The reason 1:q. 1 has the factor of "2" is that this is equation is frnr Radar Doppler Shifts, which have two Doppler 5 Shifts.
The fitquency shiia A f is detected in the Interrogator (410;) as follows.
A more detailed block diagram of an Interrogator implementing this method is shown in FIG. 5. The Radio Signal Source (501) generates a CW RF signal, which is then transmitted by the Transmitter (503) using the Transmit Anteruua (504). This 10 signal is called the Transmit Signal (510). The Reflected Signal (520) is received by the Receive Antenna (505) and. amplified by the Low Noise Amplifier (506).
(Note that radar systems can also be implemented using a single TransmitlRexeive Antenna.) The Mixer (507) then mixes the RF Source (502) signal, which comes from the Radio Signal Source (501) to produce signal 508. (The use o;f the same 15 Radio Signal Source (501) as dhe input to the Mixer (507) constitutes I~omodyne Detection.) The difference betvveen f~ and the frequency of the Reflected Signal (520) - i.e., the Doppler shift - is Af . The frequency of D8c of signal .508 is determined by frequency Deta;tor 509, and control processor datermin~cs the relative velocity using the value of D&. 0 f can then be mathematically converted into the 20 relative velocity between the Literrogator and the vehicle, using Eq. 1, since the RF
carrier fitqucncy f~ is known. At this point we note the presence of air ambiguity.
The above procedure can determine the absolute magnitude of the Doppler shift 0 f, however in the absence of other information it cannot determine the sign of O
f; i.e.,~
it cannot determine whether the Interrogator and the vehicle are moving towards 25 each other or moving away from each other. Other data is required to resolve this ambiguity.
One of the classic difficulties of this approach to velocity determination is that the Doppler shift O f can be relatively small. For example, consider an RF
carries at 2.45 GHz, and a velocity of 10 meters/second. The Doppler shift O f is then 30 163 Hz. If one examines the noise spectrum of the output (508) of the Mixer (507), it is common for phase noise to be substantial at this bawband frcquer~cy, especially if inadequate isolation exists between the Transmit Antenna (504) and the Receive.
Antenna (505). Also, since almost everything reflects microwave radiation to some degree, a large amount of reflections are received in a radar system; this is called 35 "clutter." Furthermore, almost any mechanical or electronic device in the radar's field of view not only reflects microwave radiation but also modulates that reflection;
e.g., a motor turning at a ccrtaiin rate will cause modulated reflections at a frequency ~ f~away from the RF carrier. '.Chew modulated reflections will be difficult to distinguish from the Doppler sihifted signatures of objects whose velocity is being measured.
Doppler Shifted Subcarrier We now disclose a~ method by which an Interrogator determines the 5 relative velocity between itself and a cooperative Tag by using a Dopp ler shifted subcarrier. We note that an RFC system can achieve extended range by using a precise frequency subcarrier (f:), digital signal processing, and precise location of the subcarrier with respect to harmonics of the AC pbwer line frcquenc;y. In an embodiment of the invention, a narrowband subcarrier at frequency fs is used.
This 10 narrowband subcarrier may be detected at great distances due to the small noise bandwidth, and the fact that th:; subcarrier is located at a frequency fs .away from the RF carrier frequency f~ such treat the "clutter" noise is greatly reduced, We now consider the effects of Doppler on an RFiD system using a narrowbanai subcarrier signal Assume for simplicity that the RFID Tag is moving towards the Internogato~
15 (a similar analysis holds for the RF1I7 Tag moving away from the Inte:rnogator). Let us use Of as two-way Doppler shift (as used in Eq. 1). The Interrogator (I03) transmits the RF signal at frequency f~ to the Tag (105). The Tag (105) generates the subcarrier frequency fs within frequency source 308 (see FIG. 3). In one embodiment, assume that the Ddodulator Control (307) performs no additional 20 modulation. Thus, the frequency f J is applied to the Dctcctar/Modularor (302), which mixes with the incoming CW frequency at f~. The result of this process are nxeived by the Interrogator 103: a Doppler Shifted Unmodulatcd Reflection (602), at fitquency (f~ + ~ f), and Doppler Shifted Modulated Reflection (604) at frequency (f~ - fs + Of) and Doppler Shifted Modulated Reflection (6(13) at 25 frequency (f~ + f, + 0 f). (It should be noted that amore complex derivation of the received signals yield the same; results.) The relative positions of these signals are shown in FIG. 6. After demodulation through the Mixer (507), the signals (508) appear as shown in FIG. 7. The Doppler Shifted Unmodulated Reflection (602) is the signal discussed above that is processed in a typical radar system; it generally is 30 of the order of a few hundred Hertz and is thus detectable as a low fitduency audible sound. The Doppler Shifted Unmodulated Reflection (602) can be used to determine the relative velocity of an object or objects in the RF field; however, multiple items might be moving in the RF field with different velocities. In this case, multiple Doppler Shifted Unmodulated Reflcction~ (602) with different values of A~
would be 35 present, and it may not be clear which reflection represents the movezr~ent of the Tag.
This is a classic problem in radar to determine which signal representv~ the true target, and which signals are "mutter" from other sources of reflection"

Therefore, to measure the relative velocity between the Ta;g and the Intenogator, we use the Doppler shifted subcarrier signals; thus we are interested in signals 702 and 703, which are the Doppler Shifted Modulated Reflections at baseband frequencies tfs - ~ f) and (fs + 0 f) respectively. The "bandwidth"
of these 5 two signals, or the distance bet'Ween the center frequency of these signas, is equal to 20 f. It should be noted that if the relative velocity between the Interrogator and the Tag is constant, the signals received will be two tones at frequencies (f,~ -a f) and (fs + 0 f), with no signal between these two tones. Thus, we will refer to dte "bandwidth" of these signals as the distance between the centers of the.~c tones. As above, we note a fundamental ambiguity in the determination of the sign of 0 f Since two identical signals, one located at (fs - a f) and another located at (f'J + D f) are present, it is not possible without additional information to determvne whether the Interrogator and the Tag are; moving towards each other or moving away from each other. Therefore, to deternline the relative velocity between the Tag and an 15 RFID Interrogator similar to the Interrogator of FIG. 2., we filter and amplify the signal 209 through the Filter Amplifier 210. The filter is centered around the subcarrier frequency fs, and would have a bandwidth sufficiently wide to pass the largest Zdf bandwidth signal that is expected. (In practice, if relative velocity is being measured in the same system with traditional RFID communications, the 20 bandwidth of the Filter Amplifier (210) will be wide enough to pass the:
Uplittk signals from Tag to Interrogator, these signals can easily be 100 kHz rnv more in bandwidth, centered around the subcarrier frequency f s.) To detect the bandwidth (20f) of the signal, the Subcart;ier Demodulator (212), which for normal RF117 communications is used to extract the Information Signal (213) from tree 25 demodulated and filtered signal (211), is for this case used to measure the "bandwidth" of the signal present at the subcarrier frequency fs. Once the signal bandwidth 2~f is known, Frq. 1 can be used to calculate the relative velocity v.
To measure the bandwidth of the signal present at the subrarrier frequency fs, several techniques may be used. We note that the frequency fs is 30 generally much larger than the signal bandwidth 2~ f. For example, the subcarrier frequency f J could range from :32 kHz to 1 MHz; while the signal bandwidth 20 f would be 327 Hz (for a velocihr of 10 mettrs/second and an RF carrier frequency of 2.45 GHz). Given the fact that 20 f is much smaller than fs, the Subca~rier Demodulator (212) undersampl.cs the signal, for example at a sample rite of 1-35 kHz, and then processor 510 or a (DSP) within subcarrier demodulator (212) perform a Fourier analysis of the undersampled signal to determine the frequency modes present. The result of tl:~is Fourier transform is a direct measurement of 0 f, since the signals located at (fs -~ Of) and at (fs + ~ f) represent the results of a signal of frequency D f mixed with a signal of frequency f J.
It should be noted, that while we are directly measuring the value of A f, this vallx is not dependent on the frequency f J. The RFTD Tag (103) generates the fnxluency f J, by using an inexpensive crystal. For example, it is common for 5 inexpensive crystals to have frequency accuracy of (t 100 ppm); therefore a 32 KHz crystal would have a fnquene;y accuracy of t 3.2 Hz ~ In the above measurement, we are not concerned with exactly where in the frequency domain the siglals lie, but rather, once the signals have been located, to accurately determine the value of O f.
Therefore, an MB~S RFTD system may operate in several iiifferent 10 modes. The first mode, called the Interrogation Mode, is where the T~~g responds to an raluest from the lintcrrogadx and transmits, using MBS, data back to the Interrogator. In a second mode;, called the Velocity Mode, the InterroF;au~r requests the Tag to respond, not with data, but with a subcarrier tone. Then, u;~ing the uxhnique described above, the RF117 system detcrrnines the relative velocity 15 between the Tag and the Interrogator. Thus, using these two modes, the RFID
Tag is identified, and the relative velocity between the Tag and the Interrogator is determined.
Motion Detection Let us consider a security application in which a person rnoves in single 20 file through an entrance gate. An RFID Tag, operating in the Interrogation Mode, and located on their person, is the mechanism to authorize entrance tanhe gate.
Furthermore, we must assume, that a person cannot pass through the gate without having the proper authorization. One method to accomplish this is to deuyrmine if motion is present in the immediate vicinity of the gate; if motion is detected, and no 25 valid Tag_ is read, then an alarm could be sounded.
The determination of whether motion is present can be accomplished by a relatively minor auddition to the Interrogator hardware. FIG. 8 expands the function of the Frequency Detector (5C19). In one implementation, the output of the Mixer (507) has both I (in-phase) and Q (quadrature) channels. These signals are then be 30 combined in combiner 803 using any one of a number of conventionaa techniques, for example a simple Summer may be used. The resulting signal is then be passed through two different filters; lFilter/Amplifier Z10 and Filter/Amplifier (806).
Filter/Amplifier (806) is a lour-pass filter whose passband is no greau;r than the largest expected Doppler shift The output of Filtcr/Amplificr (806) i,s then 35 processed by an Audio Frequency Detector (807), which determines ~;he Doppler frequency of a moving object in the RF field. Inexpensive implementations of are available due to the widespread use of police and sport radar systems, door s openers, etc. Thus, the Interrogator can determine, based on the output of the Audio Frequency Detector (807), if movement exists in the RF field The signal is also passed through F'tlter/Amplifier 210, whose filter characteristics are designed to pass a signal centered at the expectexi subcarrier frequency fs, and with a b<~ndwidth large enough to pass the modulated ;signal containing the identification data (e.g., a bandwidth of 100 KI3z for a 50 kbps BPSK signal).
This capability allows the Interrogator to add another mode of operation.
The Interrogator can regularly transmit interrogation messages, addressed to all Tags in the RF fisld, requesting those Tags to respond with their identification number.
Simultaneously, the Interrogator detects if movement exists in the RF field.
The sensitivity of the Interrogator to such movement tuned such that the biterrogator only detects movement in the near proximity of the entrance gate. If movement is detected, and no valid Tag is detected, an alarm is sounded.
In addition, the velocity of the Tag with respect to the Ints;rrogator may be determined by the Subcarri<:r Demodulator (212). Since the Doppler shifted, reflected signal would be centered at the Subcarrier frequency, that signal will be far away from the "clutter" effects discussed above; however the RF>D Tag would have a smaller radar cross section linen the object to which it was attached. It should lx possible to determine the relative velocity of the Tag, using for example the undersampling technique outlined above, at least at the same range or greater than that possible using the convcmdonal Doppler shift technique (i.e., using the output of the Audio Frequency Detector (807)).
Therefore, this technique allows the Interrogator, with very little additional hardware, to function as a motion detector as well as an RF~
Interrogator. This obviates the: need far a separate motion detection s~rstem.
Complex Relative Motion - 'librational Analysis In the section above, we have disclosed how to measure the relative velocity of an RFID Tag with respect to an Interrogator. Let us assume an RF117 Tag in motion with respect to the Interrogator, and the direction of that motion is along a direct (i.e., line of sight) path liom the Interrogator to the Tag. We further assume, as a convenience to illustrate the method, that the primary RF propagation path from Interrogator to the Tag is the dLircct path, and that the amplitude of the motion varies with time as sin2nwt. Then, ne velocity (and hence the Doppler Frequency Shift 0 f) is proportional to cos2ttwt. At time t=0, the velocity is at a maximum, and the Doppler Shifted Modulated Ra;flections (702, 703) are at their maximum distance apart. At time t = ~c/ 2, the Doppler Frequency Shift A f is at a minimum, since the . velocity is uro. In this case, the two Doppler Shifted Modulated Signals (702, 703) converge to a single signal, not Doppler shifted, and centered at frequency fs.
Therefore, the Subcarrier Demodulator (212) must first detect the maacimum bandwidth of the signal (2O f)" which occurs when the velocity is at a maximum.
From the measurement 2Af, the maximum velocity of the Tag can ex determined.
5 However, there is additional uaformation contained within this signal, as these signals (702, 703) are constantly in motion, moving from being separated at a maximum distance (for t = 0 avd t = p), and being merged into a single signal (for t =
p~2 and t = 2p). Therefore, the time variability of this signal will give a measurement of the frequency uu at which the RF>D Tag is vibrating. Thus, the 10 Subcarrier Demodulator (212) also measures the frequency w. In summary, we can obtain two measures using this technique; Ar from which the maximum velocity v can be calculated, and the vibration frequency w. From these two parameters, we can determine a description of the RFID Tag's movement. The only t~cmaining parameter of interest, the amplitude of the vibration - can be calculated given the 15 above two parameters and given the assumption that the vibration is sinusoidal The Subcattier Demodulator (212) performs the function; of ejete_r_m__ining tenth AI ane~i the! yihratinn fxny new av~ Tlye :e ~ c~nm~
fmr h~,rL
...~ '~r..v,~ w...» wr problem than the "Simple Rcl;ative Motion" problem above, since the signal is both frequency varying and time varying. One method to determine these ;parameters is 20 now disclosed. FIG. 9 shows the use of DSP (950) and A/D (960) to ~peerform the function of the Subcarrier Dernodulator (212). The output of the F'~ltt~r Amplifier (210) enters the Subcarrier Demodulator (212) and is sampled at a sarnpling rate of 2fs. Far example, if f, is 32 kHz, then 2fJ is 64 kHz. A/D converters that operate-at this sampling rate are readily available because of the popularity of audio CD
25 devices.. For example, a set of K samples are taken and stored in the .,forage of a DSP. The number K should be sufficiently large 100K , since larger values of K
increase the signal to noise ratio of the received signal and thus imprawe the acxuracy of the measurements. After the samples are taken, the DSP :processes the data. (Note that processing ccmld be done at least partially in real time, given a 30 powerful enough DSP). Con~;,eptually, we wish to divide the frequency space near the subcarrier frequency fs into a set of frequency bins, and calculate the signal strength in each bin. The signal we expect to see in each bin is the tinge-averaged signal strength; since the sign;tis have a time-varying value of a f To calculate the signal strength in each bin, a one-dimensional Fixed Fourier Transform (F>~'I~
can be 35 used. DSP algorithms for FF'.C's are readily available. Once the signal strength in each bin is found, we can detc;rmine 0 f. The bin containing the frequency D f is the Last bin with significant signal. strength; the next bin will have much lass signal. Let us call the last bin with significant signal strength as bin J. We thcrefi~re have an estimate of ~ f to an accuracy of the bandwidth (in Hz) of the bin. This;
accuracy in A f cacresponds to a certain errcrc in the velocity v, based upon the above equation relating those two parameters. Now that the bin number j is known, wn determine the frequency tn at which signals appear in bin number j. The set of K samples 5 above can then be re-analyzed, now that we know the maximum value of 0p. We now wish to determine the time: variation of the frequency components within each of the above bins. This determination can be performed by a Two Dimensional Finite Fourier Transform (ZD-FFI~. This type of computation is common in the analysis of vibrations, and 2D-1FFT algorithms such as required here ane readily 10 available. Thus, the results of these computations are the value of 0 f, isom which the velocity v can be calculated, and the frequency of oscillation u>t.
It was assumed above that the oscillation mode was sinuscddal.
However, other vibrational males are not purely sinusoidal. For example, if the direction of motion is not along; the direct path between the Interrogator and the Tag, 15 then a sinusoidal oscillation will not appear as purely sinusoidal when received by the Interrogator. Despite these drawbacks, if the oscillation is periodic and has sufficient mathematical smoothness (e.g., continuous first derivative), then the methods discussal above are still mathematically valid, and FFT algorithms are valid examples of techniques u~ determine the parameters of interest.
20 We note that the RFID Tag could be moving in a direction other than the direct path between the Tag and the Interrogator. We further note that the primary path of RF propagation may not be along the direct path. These problems can be (at least partially) addressed by ph~cing multiple Interrogators in RF range of the Tag, as shown in FIG. 10. The RFID 7.'ag (910) is vibrating in one direction (as indicated);
25 this direction of vibration would likely not be detected by Interrogator 1 (920).
However, Interrogator Z (930) would be positioned to detect this vibration mode.
Note that if the Tag (910) were vibrating in multiple directions simultaneously, then valuable data could be obtained from both Interrogators as to different vibration modes. This concept can be exaended to three or more Interrogators 'within RF
range 30 of the Tags. In one embodiment of multiple Interrogators, the system nperates with the Interrogators time synchror~izcd. For example, the Interrogators sn;nultancously transmits the Downlink information, requesting the Tag to respond with its identification number. Each Interrogator, again dme synchronized, would transmit a CW tone for the Tag to respond with its identification number using M;BS. The 35 Interrogators then transmit a Downlink message, again time synchronized, requesting the Tag to respond with a single subcarrier tone at frequency f J.
Each Interrogator the transmits on a. different RF carrier frequency f~, as thiis will allow the signals to be received and decoded by each Interrogator independently of each other Interrogator. In this mariner, each Interrogator will provide an vtdependent assessment of tht relative vibration of the RFID Tag, depending on the orientation of the RFiD Tag with respect to that Interrogator. The overall radio communications system can assimilate the input data from each Interrogator to develoFnan overall 5 assessment of the vibrational modes of the Tag.
Tag Calculations In the above discussion, we took advantage of the characteristics of the modulated backscattercd sign~~l to infer the characteristics of motion caf a device to which a Tag (105) was attached. In this discussion, we disclose how >J~ take 10 advantage of the capabilities of the ltFlD Tag to determine characteristics of motion, such as vibrational analysis, of a device. First, we note today's microprocessors are frequently oquipped with A/D converters on board the integrated circat.
Therefore, the Tag architecture discussed. may be altered by using micropraxesser (1010) in Tag (105). FTG. 11 shows a Microprocessor architecture which allows sensor inputs to be 15 directly sampled. An analog l:nput Port (1020) is then sampled by an A/D
Converter (1030), which i~ an integral part of the Microprocessor (1010). Typir,ally the Analog Input Port (1020) has an input; voltage range from 0 to V~ volts, where V~ is the voltage of the power supply uW he Microprocessor Core (1040) - typi~~ally three volts. The Analog Input Port (1020) is attached to a Sensor whose output is between 20 0 to Va volts. The Tag (105) is first identified by communicating with an interrogator as described above. Then, the Tag is instructed, by infonmation contained within the Information Signal (200x), to begin taking samples of the Sensor input. As discussed above, the sampling rate should be at lea:~t two times the maximum fitquency present rat the sampled signal. The samples are buffered in the 25 Microprocessor Core (1040). In one embodiment, the samples are tru~smitaed to the Interrogator (103), directly as they were sampled, using the modulated backscatter cammuni~cations link discussed above. Once the signals are received and buffered at L. T..~~......~ /1 vlw C~r.nw~"nv n rv~n~ to r..~n 1v. ~n~ v~~ by t~Cina an uac auwaav~rasa~' ~a03), uac aac~yuvaav.y.omy....vf:..~ .,.... w alg~ithm as outlined above.
30 In an alternate ennbodiment, the Tag (105) could begin to perform all or part of the processing for the FFT algorithm. In an FFT algorithm, ttie determination of the FFT expansion cocffici.cnts a k and b,t involve arithmetic calculations; where the trigonometric functions rcxluired can be pre-calculated and/or prat-stoned in a memory device in the Tag (105). Let us assume that a set of sampler are taken and 35 stored in the Tag (105). Then, the Tag (105) can begin the nccessary~
calculations.
This method could be useful in situations where a Tag must take occasional samples, arid then be dormant for a significant part of the time. The fact that the microprocessor ca board the Tag (105) is significantly slower at such calculations than a~DSP in the Interrogator I;103) is not a major drawback. To improve the specrl of these calculations, they could be performed in the Tag (105) in fixedl point arithmetic (since most simple 4. ~ 8 bit microprocessors do not support floating 5 point arithmetic). After the FFT algorithm is completed, the Tag ( lOS;i can transmit the values of the parameters ak and b,~ back to the Interrogator (103).
Let us assume that the RFID system wishes to alter the pwameters of the FFT algorithm. Such alteration is straightforward. The values of the trigonometric functions can be pre-calculated by the Interrogator and transmitted to the Tag (105) 10 by placing those values in the Information Signal (200a). In a similar manner, the:
Tag (105) can be instructed to alter the number of samples taken and tt~e rate at which those samples are taken. Thus, the Tag (105) can be instructed, based on information from the Interrogator (103), to fundamentally alter the typa of analysis performed 15 An Additional Embodiment To illustrate the capabilities of another embodiment of thi:~ invention, Iet us describe how to apply these techniques to monitoring of a human heartbeat.
Conventional texhniques involve the connection of wires to the human, and monitoring electronics connected to the wires. The ItFID Tag as disclosed here 20 contains much of the electronic; necessary to monitor a heartbeat, and teas the advantages of being relatively inexpensive and the system being able tn monitor a number of such devices at the same time.
Let us enhance the Tag ( 105) as shown in FIG. 12. The Analog Input (1130) is connected to the patie:nt's chest in a similar manner to that of an electrical 25 Lead on an electrocardiogram device. This analog signal is amplified by amplifier (1125) with a maximum signal level of V~, and connected to the Analog Tnput Port (1020) of the Microprocessor (1010). The A/D Converter (1030) converts this signal to digital format, where it can he analyzed. As above, in one embodiment, the digitized signals are transmittai back to the Interrogator (103), where m FFT
30 algorithm is executed on a DSI' to determine the frequency modes of dxe heartbeat.
In an alternate embodiment, the Microprocessor (1010) calculates the i:requency modes using the FFT algorithm described above. The data can be returned to the Interrogator in one of several Wrays. The Interrogator (103) could regularly poll all Tags (105) in range of the Interrogator, acid request that the Tags transmit back the 35 results of the FFT algorithm calculations (i.e., the values of the para.me;ters a,t and bk). In this manner, the Interrogator could keep track of the heartbeat. on a regular basis.

CA 02219381 1997-10-27 -#

It may become necessary for the Tag (105) to respond very quickly in the event the heartbeat becomes abnormal. Within the FFT algorithm, vibrational modes representing abnormal conditions - such as tachycardia - could ibe easily identified. These abnormal vib~rational modes have recognizable signatures, such as 5 vibration frequencies greater thian those normally seen, etc. When the Interrogator polls the Tags (105) for their input data, this Tag (105) could respond with a message indicating that this Tag (105) must immediately transmit its data to the Interrogator. Methods such as using allow multiple Tags to respond simultaneously.
Such as using a Slotted Aloha protocol this would allow a Tag (105) to respond 10 almost immediately if an abnotinal condition was recognized. Thus, this embodiment of the invention provides an inexpensive device for monitoring vital signals, where a numixr of such devices can be simultaneously monitored, and the communications to the monitrnring devices are performed in a wireless manner.
What has been described is merely illustrative of the application of the 15 principles of the present invention. Other arrangements and methods c:an be implemented by those skilled in the art without departing from the spvat and scope of the present invention.

Claims (5)

1. A modulated backscatter system, comprising:
at least one transponder that receives a first transmitted signal and modulates a reflected first transmitted signal using a subcarrier signal; and at least one interrogator having a transmitter that transmits said first transmitted signal and a receiver that receives said reflected first transmitted signal, said interrogator having a demodulator that obtains a received subcarrier signal from said reflected first transmitted signal, and a subcarrier demodulator that analyzes said received subcarrier signal to measure a motion of said transponder.

2. The modulated backscatter system of claim 1, comprising a first interrogator that transmits said first transmitted signal at a first frequency and a second interrogator that transmits a second transmitted signal at a second frequency, said first and second frequencies being different, and said at least one transponder receiving said second transmitted signal and modulating a reflected second transmitted signal using said subcarrier signal.

3. The modulated backscatter system of claim 1, wherein said demodulator comprises a mixer that mixes said reflected first transmitted signal with another signal to obtain said received subcarrier signal.

4. The modulated backscatter system of claim 1, wherein said demodulator is a homodyne demodulator.

5. The modulated backscatter system of claim 1, wherein said subcarrier demodulator comprises a processor that determine a frequency difference between said received subcarrier signal and said subcarrier signal.