Inexpensive modulated backscatter reflector

Details Inexpensive modulated backscatter reflector Inexpensive modulated backscatter reflector

1996-12-31

2001-06-05

Zimmerman, Brian (Department: 2635)

Communications: electrical

Condition responsive indicating system

Specific condition

C340S010340, C340S010100

Reexamination Certificate

active

06243012

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to wireless communication systems and, more particularly, to a wireless communication system using modulated backscatter technology.
BACKGROUND OF THE INVENTION
Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. In a Modulated BackScatter (MBS) RFID system, the Interrogator communicates to the Tags using modulated radio signals. The Tags communicate with the Interrogator by reflecting (backscattering) back to the Interrogator a Continuous Wave (CW) carrier signal originally transmitted by the Interrogator. Communication originating from the Interrogator and transmitted to the Tag is called the Downlink. Communications from the Tag to the Interrogator is called the Uplink. The Tag has an antenna which it uses to receive messages from the Interrogator and to reflect messages back to the Interrogator. After transmitting a message to the Tag, the Interrogator transmits the CW carrier signal to the Tag. The Tag uses the CW carrier signal to respond to the Interrogator message by reflecting or backscattering the CW carrier signal with the modulated subcarrier signal. The CW carrier signal is typically a signal in the microwave frequency range. The reflected or backscattered CW carrier signal is modulated by the Tag with a modulating signal that is generated at the Tag.
In most RFID systems using MBS, the Tag antenna radar cross-section, i.e., the effective reflective area of the antenna, is modulated by alternately biasing ON and OFF a high-quality microwave diode attached to the antenna. When the diode is in its conducting state, i.e., the diode is biased in the ON state, the diode acts as an RF short circuit. When in the OFF state, the diode acts as an RF open circuit. In this manner the impedance or reflectivity of the antenna is modulated which causes the antenna to alternately reflect and absorb the impinging CW carrier signal. Connected to the correct location on the antenna, such as a feedpoint of an antenna, the conducting diode changes a tuned antenna into a detuned antenna causing the antenna to reflect rather than absorb the impinging CW RF signal.
A modulated backscatter RFID tag is sometimes referred to as a passive radio tag in that it reflects and modulates a continuous wave (CW) radio signal with which it is illuminated in an analogous way to a mirror which can be used for communicating by flashing the light from the sun to an observer at a distance.
FIG. 1
illustrates a typical read-only modulated backscatter RFID system comprising an Interrogator
105
and a Tag
101
. Tag
101
comprises a microprocessor
102
, a modulator
103
and an antenna
104
. Within the Interrogator
105
, the Reader RF Unit
108
generates and transmits a CW signal by means of the Transmit Antenna
106
. This signal illuminates the Tag Antenna
104
which reflects (or backscatters) a portion of the incident signal. The reflection coefficient or reflectivity of the antenna, a parameter well known to those in the field of antenna design which indicates an antenna's ability to reflect energy within a certain frequency range, is modulated directly by a modulating signal. The reflection coefficient of the Tag Antenna
104
is switched between being a good absorber of energy to a good reflector in a time-varying pattern (modulating signal) determined by the data stored in the tag memory. The variable reflection coefficient of the Tag Antenna
104
modulates the backscattered RF signal. This backscattered signal is captured by the Interrogator Receive Antenna
107
, amplified and down-converted by the Reader RF Unit
108
and the data recovered by the Demodulator
109
, Digital Signal Processor (DSP
110
if present) and Microprocessor (&mgr;P)
111
.
One of the more important factors that affect directly the overall sensitivity of the RFID System is the efficiency with which the Tag Antenna
104
modulates the reflected RF signal. This, in turn, depends upon the Tag Antenna
104
radar cross-section the impedance matching between the Tag Antenna
104
, the Tag Modulator
103
, and the quality of the Tag Modulator
103
. The Tag Modulator
103
, for optimum system performance, is usually a high-quality microwave semiconductor, often a switching diode. For an inexpensive Tag
101
, this device alone can constitute a major part of the total Tag cost. Thus, in order to significantly reduce the cost of manufacture of the Tag, the cost of Modulator
103
must be reduced while still maintaining acceptable system sensitivity and range. There are RFID system applications in which high sensitivity and long range are less important than low cost. One such class of applications includes tracking airline luggage. Therefore, there is a need to design a relatively inexpensive Tag for such systems.
SUMMARY OF THE INVENTION
In accordance with the present invention an inexpensive Tag design used in an RFID communication system that uses MBS is claimed. The Tag comprises a data storage device, a variable impedance semiconductor gate electrically connected to the data storage device for receiving the stored information and an antenna having a certain reflectivity. The antenna is electrically connected to the variable impedance device which modulates the reflectivity of the antenna in accordance with the received information. In another embodiment, all of the parts of the Tag are incorporated into an integrated circuit which also modulates the reflectivity of the antenna.


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