Pulse or digital communications – Systems using alternating or pulsating current – Antinoise or distortion
Reexamination Certificate
2000-01-10
2002-09-17
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Antinoise or distortion
C375S296000, C375S340000, C375S282000, C340S505000, C342S042000
Reexamination Certificate
active
06452980
ABSTRACT:
TECHNICAL FIELD
The present invention relates, in general, to systems transmitting/receiving data and, more specifically, to a system and method for encoding/decoding of data to reduce coherent signal interference.
BACKGROUND OF THE INVENTION
Use of radio frequency as a data communications link in interrogation/identification (I/I) systems is well known. U.S. Pat. No. 5,491,482 describes coded objects, such as bank credit cards, employee identification (ID) badges, coded tags and the like, that may be read on-the-fly from some feet away by an interrogator/reader (I/R). Portions of the description of the I/I system in the patent are included below.
Referring to
FIG. 1
, there is shown I/I system
10
including one or more I/R units
12
, one or more badges
14
, respective transmit and receive antennas
18
and
19
, and a central computer
22
. I/R units
12
operate at a suitable radio frequency or microwave frequency (e.g. 915 MHz or 5.8 GHz) and transmit microwave (radio frequency) beams
16
. Badges
14
(which uniquely identify individual employees) are internally powered and are interrogated by respective beams
16
transmitted from directional antennas
18
of I/R units
12
positioned at selected locations. Each I/R unit
12
has a receiving antenna
19
which is closely similar to transmitting antenna
18
. I/R units
12
are connected via respective cables
20
to a desktop computer
22
. In the course of being interrogated via microwave beam
16
from I/R unit
12
, a badge or badges
14
reply electronically by reflecting a portion of the same beam
16
back to receiving antenna
19
of I/R unit
12
. Badges
14
thus uniquely identify themselves in accordance with their respectively coded and electronically stored ID numbers.
Each badge may be coded with any one of over 60 billion different numbers. By way of example, five or so different badges
14
may at one time be interrogated and identified (when in range of detection) by a respective I/R unit
12
in less than 20 milliseconds. As soon as badge
14
has been identified, its electronic circuit is put into an inactive or “power down” state, so that badge
14
does not continue to respond to I/R unit
12
for as long as that badge (once it has been identified) remains within range of the respective beam
16
. Once badge
14
is moved out of range of beam
16
, the electronic circuit of badge
14
automatically returns to a quiescent state drawing negligible current from its internal power source. But even in quiescent state, badge
14
has sufficient input sensitivity so that the badge remains able to detect and respond to very low power density levels of beam
16
. By way of example, the power density of beam
16
immediately in front of transmitting antenna
18
of I/R unit
12
is only about 0.3 mW/cm
2
, which is one-tenth the level set by health and safety standards. The power density of beam
16
at the location of badge
14
is substantially lower.
A typical badge includes a badge-integrated-circuit (BIC), an antenna, and a very thin battery placed on a small, insulated PC board. The BIC may be entirely implemented in complementary metal oxide semiconductor (CMOS) technology, as a single IC chip. The thickness of the badge is only slightly greater than the thickness of the battery. For example, the battery may be a lithium battery having a thickness of about 30 mils, a rating of 3 volts and a capacity of 50 mA-hr. The average current drain of the BIC is less then 1 microampere, and the service life of the battery is effectively its shelf life (e.g., four years or more).
Referring now to
FIG. 2
, there is shown a simplified schematic diagram of I/I system
10
. This system includes I/R unit
12
with its beam
16
, transmission antenna
18
, receiving antenna
19
, BIC
30
, antenna
32
and battery
34
. Beam
16
is received by antenna
32
and a RF voltage is applied as an input signal to terminal
42
of BIC
30
. The positive terminal of battery
34
is connected to lead
48
which is coupled to a terminal +VDD and the negative terminal of battery
34
is connected to lead
49
which is coupled to a reference terminal (REF) shown coupled to ground potential. The circuitry of the BIC includes detector/demodulator block
50
, a reset/wake-up block
52
, a control/logic, data memory and data registers block
54
, and modulator
56
.
Incoming coded signals (described in detail in U.S. Pat. No. 5,491,482) on beam
16
are detected and demodulated in block
50
, which is always turned on. Other portions of BIC
30
, when not in range of beam
16
, are turned off. When a “reset” instruction from I/R unit
12
is detected and demodulated by block
50
, block
50
applies a “reset” data word via path
60
to reset/wake-up block
52
, which in turn applies a power-on signal via path
62
to the control/logic, data memory and data registers block
54
. Bit data and clock signals from block
50
are applied, via paths
64
and
66
, to block
54
in response to the instructions and coded words being received by BIC
30
from I/R unit
12
.
By way of example, an identifying number for an employee to which a particular badge
14
is assigned is in the form of six 6-bit words stored in six memory registers (identified as A through F) in block
54
of BIC
30
. To identify this 36-bit number, I/R unit
12
interrogates each badge
14
word by word. BIC
30
, by operation of its modulator block
56
, via path
69
, then replies to I/R unit
12
at appropriate intervals, until badge
14
has completely identified itself. This iterative procedure is described in detail in U.S. Pat. No. 5,491,482.
The I/R unit transmits to the tags at a suitable frequency a stream of binary bits of instruction and data words, and receives responses from each tag. Each of the tags has circuitry for storing, as digital bits, an identifying code number. The circuitry of each tag detects and demodulates the incoming bit stream from the I/R unit, and generates clock and timing signals slaved to the bit stream, thereby framing the incoming digital words. The circuitry has logic for responding internally to the instruction and data words of the bit stream and for responding externally to the I/R unit at selected times such that the code number of a tag is uniquely identified and that tag alone among many communicates solely with the I/R unit when so identified.
Several steps are necessary before a tag is uniquely identified. A first step includes transmitting a bit stream of instruction and data words to each and all tags present to determine the presence of at least one tag. A next step is sequentially sorting through all possible combinations of values of the plurality of coded words stored in each and all tags. A next step is tabulating the matches found between transmitted and stored words of each and all tags and responding by the tag when a match is found. A next step is determining that at least one tag has matches with all of its stored words; and a next step is transmitting instruction and data words to the tags to sort out all possible combinations of matched words in all of the tags which have responded. A last step is responding by the tags one-by-one when each is uniquely identified.
The tag described in U.S. Pat. No. 5,491,482 independently generates an internal clock signal that bears no relationship to the I/R transmitted carrier signal. Other conventional I/I systems, however, generate an internal clock signal from the I/R transmitted carrier signal. For example, each tag (or card) in I/I system
10
may generate its own clock signal
66
from the I/R transmitted carrier signal, by dividing the carrier signal from I/R
12
by a fixed number. When each tag generates its internal clock signal from the interrogator's carrier signal, the tag's internal clock signal is “coherent” with the carrier signal. Since a plurality of tags may concurrently be interrogated by an I/R, the coherent signals may interfere with each other.
The problem of coherent signal interference is explained by reference to FIGS.
3
(
a
)-(
f
Evans Robert
Lin Min-Long
Schepps Jonathan
Zalud Peter
Burke William J.
Chin Stephen
Fan Chieh M.
Sarnoff Corporation
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