Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2001-10-31
2003-07-08
Lee, Benjamin C. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S505000, C340S573100, C340S928000, C343S720000, C343S895000, C343S866000, C455S277100, C455S041300, C455S059000, C455S062000
Reexamination Certificate
active
06590498
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, wireless communication systems using backscatter technology.
2. Description of the Related Art
RF Tag systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices denoted as Tags. In RF Tag systems, the Interrogator communicates to the Tags using modulated radio signals which activate any Tag in range or may activate a specific Tag within the range. After activating a Tag, the Interrogator may transmit information to it (this is called the Downlink). The Interrogator transmits a Continuous-Wave (CW) radio signal to the Tag; the Tag then modulates the CW signal using modulated backscattering (MBS) in which the Tag is electrically switched by the modulating signal, from being an absorber of RF radiation to a reflector of RF radiation. This modulated backscatter allows communications from the Tag back to the Interrogator (called the Uplink). The Downlink transmission of messages can include information relating to a desired operation of the RF Tag and, for example, the Interrogator is capable of instructing the RF Tag to turn on and/or off on demand. Modulated Backscatter (MBS) systems can be used to manage inventory or perform other useful monitoring application such as reading the state of a sensor.
The operation of an RF Tag system utilizing MBS is now described. In
FIG. 1
, there is shown an overall block diagram of an RF Tag system. An Application Processor
101
communicates over Local Area Network (LAN)
102
to a plurality of Interrogators
103
-
104
. (Although commonly a plurality of Interrogators
103
-
104
connected by a LAN
102
to an Application Processor
101
are used, the inventions described herein are also capable of being configured with only a single Interrogator connected either to a LAN or directly to an Application Processor.) The Interrogators may then each communicate with one or more of the Tags
105
a
-
107
. For example and with further reference to
FIG. 2
, the Interrogator
103
receives an information signal, typically from Applications Processor
101
. The Interrogator
103
takes this information signal and Processor
200
formats a Downlink message (Information Signal
200
a
) to be sent to the Tag. The information signal (
200
a
) may include data 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 such as activation and deactivation, and/or any other information to be used and/or stored by the Tag's processor. With joint reference to
FIGS. 1 and 2
, Radio Signal Source
201
synthesizes a radio signal, Modulator
202
modulates the radio signal using Information Signal
200
a
, and Transmitter
203
transmits this modulated signal via Antenna
204
, illustratively using amplitude modulation, to a Tag. Amplitude modulation is a desirable choice because the Tag can demodulate such a signal with a single, inexpensive nonlinear device (such as a diode). However, many modulation schemes are possible such for example, as Phase Shift Keying (PSK) of the subcarrier (e.g., BPSK, QPSK), more complex modulation schemes (e.g., MSK, GMSK), etc.
In the Tag
105
a
(see FIG.
3
A), the reflecting antenna element
301
a
(e.g. a loop or patch antenna) receives the modulated signal. This signal is demodulated directly to baseband using the Detector/Modulator
302
which, illustratively, may be a single Schottky diode. The result of the diode detector is essentially a demodulation of the incoming signal directly to baseband. The Information Signal
200
a
is then amplified 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 amounts of data are being transferred in frames, then frame synchronization may be implemented, as for example by detecting a predetermined bit 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 clock recovery circuit
304
is sent to a Processor
305
. Processor
305
is typically an inexpensive 4 or 8 bit microprocessor and its associated memory, and it generates an Information Signal
306
from Tag
105
a
back to the Interrogator (e.g.,
103
). Information Signal
306
is sent to Detector/Modulator
302
to modulate the RF signal received by Tag
105
a
to produce a modulated backscatter (i.e. reflected) signal. A Battery
310
or other power supply provides operating power to the circuitry of Tag
105
a
. Power may also be received, for example, by using inductive coupling or microwaves.
Returning to
FIG. 2
, the Interrogator
103
receives the reflected modulated signal through Receive Antenna
206
, amplifies the signal in a Low Noise Amplifier
207
, and demodulates the signal using homodyne detection in a Mixer
208
. In an alternative embodiment, a single antenna may replace Transmit antenna (
204
) and Receive Antenna (
206
), in which case an electronic method of canceling the transmitted signal from that received by the receiver chain is required.
Using the same Radio Signal Source
201
as is used in the transmit chain means that the demodulation to baseband is done using homodyne detection; this has advantages in that it greatly reduces phase noise in the receiver circuits. The Mixer
208
then sends the Demodulated Signal
209
(if Mixer
208
is a Quadrature Mixer, it would send both I (in phase) and Q (quadrature) signals) to the Filter/Amplifier
210
. The resulting filtered signal—typically an Information Signal
213
is sent to a Processor
200
to determine the content of the message.
Generally, RF Tags have a single reflecting antenna. Since the Tag only reflects RF energy instead of generating it, an RF Tag is less expensive to manufacture and requires little battery power when operating. Consequently, an RF Tag provides a low cost arrangement and method of transmitting sensor measurements to a central processing system or operator for evaluation.
The advantages of using RF Tags to transmit information to an Interrogator are accompanied by a disadvantage: since the RF Tag is only a reflector, the signals returned from it are generally weaker than in systems that generate RF energy at both ends of the communications link. For example, in an RF Tag system the signal-to-noise ratio (SNR) of a signal sent from the Tag falls off rapidly (proportionally to r
−4
, where r is the distance between the transmitter and reflector). By comparison, in a communication system having a transmitter at one end and a receiver at the other, the SNR in each direction falls off slower (proportionally to r
−2
) than in the RF Tag system. Thus, the interrogation range of the RF Tag is notably more limited by its distance from the RF transmitting source.
FIG. 4
depicts incoming RF radiation generated by RF Interrogator
103
and directed toward an RF Tag
105
a
having a reflecting antenna element
301
a
, and
FIG. 5
shows the reflectance of the RF Tag
105
a
of FIG.
4
. As is clear from
FIG. 5
, during operation the reflecting antenna
301
a
of RF Tag
105
a
is either in a fully reflecting mode or an essentially non-reflecting mode. For each full period T of the square wave depicted in
FIG. 5
, reflecting antenna
301
a
of RF Tag
105
a
is only in the fully reflecting mode for half of each period T.
SUMMARY OF THE INVENTION
The invention solves the above problems by providing multiple antenna elements positioned for reflecting expected incident radiation to an interrogator and spaced from each other such that the echo signal from each reflecting antenna element is out of phase with the other(s) to produce constructive interference of the ref
Cohen & Pontani, Lieberman & Pavane
Lee Benjamin C.
Lucent Technologies - Inc.
Nguyen Tai T.
LandOfFree
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