U.S. patent application number 11/622092 was filed with the patent office on 2007-05-17 for rfid reader systems detecting pilot tone.
Invention is credited to Scott A. Cooper, Christopher J. Diorio, Aanand Esterberg, Todd E. Humes, Amir Sarajedini, Kurt E. Sundstrom.
Application Number | 20070109129 11/622092 |
Document ID | / |
Family ID | 38040205 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070109129 |
Kind Code |
A1 |
Sundstrom; Kurt E. ; et
al. |
May 17, 2007 |
RFID READER SYSTEMS DETECTING PILOT TONE
Abstract
RFID tags are commanded to generate a pilot tone in their
backscatter. When the backscattered pilot tone is received in the
reader, the pilot tone is used to estimate the tag
period/frequency. Then, the estimate is used to seed and lock a
symbol timing recovery loop, which provides a detected signal to
one or more correlators for detecting the tag preamble. A delayed
version of the received tag signal is compared against a baseline
signal threshold established from the received signal to detect the
pilot tone.
Inventors: |
Sundstrom; Kurt E.;
(Woodinville, WA) ; Cooper; Scott A.; (Seattle,
WA) ; Sarajedini; Amir; (Aliso Viejo, CA) ;
Esterberg; Aanand; (Seattle, WA) ; Humes; Todd
E.; (Shoreline, WA) ; Diorio; Christopher J.;
(Shoreline, WA) |
Correspondence
Address: |
Merchant & Gould P.C.
P.O. Box 2903
Minneapolis
MN
55402-0903
US
|
Family ID: |
38040205 |
Appl. No.: |
11/622092 |
Filed: |
January 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11136948 |
May 24, 2005 |
|
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11622092 |
Jan 11, 2007 |
|
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60760150 |
Jan 18, 2006 |
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Current U.S.
Class: |
340/572.2 ;
235/451; 340/10.1 |
Current CPC
Class: |
G06K 7/0008
20130101 |
Class at
Publication: |
340/572.2 ;
235/451; 340/010.1 |
International
Class: |
G08B 13/14 20060101
G08B013/14 |
Claims
1. A method for a Radio Frequency Identification (RFID) reader
system, comprising: Receiving from an RFID tag a backscattered
signal; estimating a frequency of a pilot tone in the backscattered
signal; recovering a timing of symbols in the backscattered signal
using the estimated frequency; and demodulating the backscattered
signal based on the symbol timing to recover data.
2. The method of claim 1, further comprising: detecting the pilot
tone.
3. The method of claim 2, in which the signal is backscattered from
the RFID tag responsive to a command transmitted by the reader, and
in which the pilot tone is detected only if the command belongs in
a subset of commands.
4. The method of claim 2, in which the pilot tone is detected by:
generating a baseline version of the backscattered signal;
generating a version of the backscattered signal that is delayed
with respect to the backscattered signal; and establishing
detection when the delayed version reaches a detection relationship
with the baseline version.
5. The method of claim 4, in which the detection relationship is
that the delayed version exceed the baseline version.
6. The method of claim 4, in which the baseline version of the tag
signal is generated as a preset portion of a magnitude of the tag
signal.
7. The method of claim 6, in which the preset portion is
substantially equal to 50%.
8. The method of claim 6, in which the baseline version has a
waveform that rises up to a cusp, and then continues at
substantially the same value, and the delayed version is delayed
enough so that detection is established after the cusp.
9. The method of claim 4, in which detection is established only if
both the delayed version and the baseline version exceed a
detection threshold.
10. The method of claim 9, in which the detection threshold is
defined in terms of detected ambient noise.
11. The method of claim 9, in which the detection threshold is
updated.
12. The method of claim 9, in which the backscattered signal is
received responsive to a command signal transmitted by the RFID
reader system, the command signal causes the tag to be silent
during a silent period, and the detection threshold is defined from
a power measurement made during the silent period.
13. The method of claim 1, in which the frequency is estimated by
employing a Discrete Fourier Transform (DFT) of the received pilot
tone.
14. The method of claim 1, in which the frequency is estimated by
employing a Fast Fourier Transform (FFT) of the received pilot
tone.
15. The method of claim 1, in which the frequency is estimated by
correlating the backscattered tag signal with an expected version
of the backscattered signal.
16. The method of claim 1, further comprising: identifying in the
backscattered signal a preamble using the symbol timing, and in
which the backscattered signal is demodulated based on a timing of
the preamble.
17. The method of claim 16, in which the preamble is identified
using a single correlator.
18. The method of claim 16, further comprising: determining from
the preamble timing a Time of Arrival (TOA) for a remainder of the
backscattered signal; and in which the data is recovered from the
remainder.
19. The method of claim 18, further comprising: delaying the
remainder of the backscattered signal the preamble by a delay time;
and adjusting the delay time to compensate for an amount of time
taken to estimate the frequency.
20. A Radio Frequency Identification (RFID) reader system component
operable to receive a backscattered RFID tag signal that includes a
pilot tone, the component comprising: a frequency estimator
operable to estimate a frequency of the pilot tone and to generate
from it a frequency estimation signal; a buffer/delay operable to
generate a delayed version of the backscattered signal; a symbol
timing recovery loop controlled according to the frequency
estimation signal, the symbol timing recovery loop operable to
output, responsive to the delayed version of the backscattered
signal, a TS signal that has recovered a timing of symbols in the
backscattered signal; and a decoder for decoding the TS signal to
recover data.
21. The component of claim 20, further comprising: a detector for
detecting the pilot tone.
22. The component of claim 20, in which the frequency estimator
further detects the pilot tone.
23. The component of claim 20, in which the frequency is estimated
by the frequency estimator by employing a Discrete Fourier
Transform (DFT) of the received pilot tone.
24. The component of claim 20, in which the frequency is estimated
by the frequency estimator by employing a Fast Fourier Transform
(FFT) of the received pilot tone.
25. The component of claim 20, in which the frequency is estimated
by the frequency estimator by correlating the backscattered tag
signal with an expected version of the backscattered signal.
26. The component of claim 20, in which a frequency estimate signal
is generated according to a frequency of the backscattered signal,
and a frequency of the symbol timing recovery loop is locked
responsive to the frequency estimate signal.
27. The component of claim 20, in which a symbol phase estimate
signal is generated according to a symbol phase of the
backscattered signal, and a phase of the symbol timing recovery
loop is controlled according to the symbol phase estimate
signal.
28. The component of claim 20, comprising: at least one correlator
operable to correlate the TS signal with an expected preamble
signal for generating a correlation signal, and in which the TS
signal is demodulated only if the correlation signal exceeds a
threshold.
29. The component of claim 28, in which only one correlator is used
by the reader for correlating the TS signal.
30. The component of claim 20, in which the buffer/delay is
implemented by a variable delay.
31. The component of claim 20, in which the buffer/delay is
implemented by a variable rate buffer.
32. The component of claim 20, in which the delayed version of the
tag signal is controlled to impose a nonzero delay time suitable
for compensating for a delay from the frequency estimator.
33. The component of claim 20, in which the symbol timing recovery
loop comprises: a digital rate converter circuit arranged to:
obtain sample values of a tag response signal waveform at sample
time points; reconstruct signal values at target time points from
the sample values using the frequency estimate signal; and output
the reconstructed signal values as the TS signal; a timing error
detector coupled to the digital rate converter; a loop filter
coupled to the phase detector; and a numerically controlled
oscillator (NCO) coupled to the loop filter and to the digital rate
converter such that the timing error detector, the loop filter, and
the NCO form a feedback loop.
34. The component of claim 33, in which the symbol timing recovery
loop further comprises: a matched filter coupled to an input of the
digital rate converter.
35. The component of claim 33, in which the symbol timing recovery
loop further comprises: a matched filter coupled to between the
digital rate converter and the timing error detector.
36. The component of claim 33, in which the digital rate converter
includes: an interpolating filter arranged to receive the sample
time points, interpolate, and generate offset sample points; a
decimator coupled to the interpolating filter arranged to decimate
a portion of the sample points such that offset sample points
corresponding to the target time points are provided by the digital
rate converter; and a timing processor arranged to provide the
interpolating filter and the decimator with a first and second
control signals based on an output of the NCO.
37. A Radio Frequency Identification (RFID) reader system component
operable to receive a backscattered RFID tag signal that includes a
pilot tone, the component comprising: means for receiving from an
RFID tag a backscattered signal; means for detecting a pilot tone
from the backscattered signal; means for estimating a frequency of
the pilot tone in the backscattered signal; means for recovering a
timing of symbols in the backscattered signal using the estimated
frequency; and means for demodulating the backscattered signal
based on the symbol timing to recover data.
38. The component of claim 37, in which the means for detecting the
pilot tone comprises: means for generating a baseline version of
the backscattered signal; means for generating a version of the
backscattered signal that is delayed with respect to the
backscattered signal; and means for establishing detection when the
delayed version reaches a detection relationship with the baseline
version.
39. The component of claim 37, further comprising: means for
identifying in the backscattered signal a preamble using the symbol
timing, in which the backscattered signal is demodulated based on a
timing of the preamble.
40. The component of claim 37, further comprising: means for
determining from the preamble timing a Time Of Arrival (TOA) for a
remainder of the backscattered signal, in which the data is
recovered from the remainder.
41. The component of claim 40, further comprising: means for
determining from the preamble timing a Time Of Arrival (TOA) for a
remainder of the backscattered signal, in which the data is
recovered from the remainder.
42. The component of claim 41, further comprising: means for
delaying the remainder of the backscattered signal the preamble by
a delay time; and means for adjusting the delay time to compensate
for an amount of time taken to estimate the frequency.
Description
RELATED APPLICATIONS
[0001] This utility application claims the benefit of U.S.
Provisional Application Ser. No. 60/760,150 filed on Jan. 18, 2006,
which is hereby claimed under 35 U.S.C. .sctn.119(e). The
provisional application is incorporated herein by reference.
[0002] This utility patent application is a continuation-in-part
(CIP) of U.S. patent application Ser. No. 11/136,948 (IMPJ-0098)
filed on Aug. 19, 2004. The benefit of the earlier filing data of
the parent applications is hereby claimed under 35 U.S.C.
.sctn.120. The parent application is incorporated herein by
reference.
[0003] This application may be found to be related to the following
application, which is incorporated herein by reference: Application
titled "RFID READER SYSTEMS AIDED BY RF POWER MEASUREMENT", by
inventors Scott A. Cooper and Christopher J. Diorio filed with the
USPTO on the same day as the present application, and due to be
assigned to the same assignee (Attorney docket #
50133.51USU1/IMPJ-0178).
[0004] This application may also be found to be related to the
following application, which is incorporated herein by reference:
Application titled "RFID READER SYSTEMS WITH DIGITAL RATE
CONVERSION", by inventor Kurt E. Sundstrom filed with the USPTO on
the same day as the present application, and due to be assigned to
the same assignee (Attorney docket # 50133.61USUI/IMPJ-0198).
[0005] All three related application are incorporated herein by
reference.
BACKGROUND
[0006] Radio Frequency IDentification (RFID) systems typically
include RFID tags and RFID readers (the latter are also known as
RFID reader/writers or RFID interrogators). RFID systems can be
used in many ways for locating and identifying objects to which the
tags are attached. RFID systems are particularly useful in
product-related and service-related industries for tracking larger
numbers of objects being processed, inventoried, or handled. In
such cases, an RFID tag is usually attached to an individual item,
or to its package.
[0007] In principle, RFID techniques entail using an RFID reader to
interrogate one or more RFID tags. The reader transmitting a Radio
Frequency (RF) wave performs the interrogation. A tag that senses
the interrogating RF wave responds by transmitting back another RF
wave. The tag generates the transmitted back RF wave either
originally, or by reflecting back a portion of the interrogating RF
wave in a process known as backscatter. Backscatter may take place
in a number of ways.
[0008] The reflected-back RF wave may further encode data stored
internally in the tag, such as a number. The response is
demodulated and decoded by the reader, which thereby identifies,
counts, or otherwise interacts with the associated item. The
decoded data can denote a serial number, a price, a data, a
destination, other attribute(s), any combination of attributes, and
so on.
[0009] An RFID tag typically includes an antenna system, a power
management section, a radio section, and frequently a logical
section, a memory, or both. In earlier RFID tags, the power
management section included an energy storage device, such as a
battery. RFID tags with an energy storage device are known as
active tags. Advances in semiconductor technology have miniaturized
the electronics so much that an RFID tag can be powered solely by
the RF signal it receives. Such RFID tags do not include an energy
storage device, and are called passive tags.
[0010] In a typical reader-tag communication, the reader and the
tag clocks are not synchronous in frequency or phase. Therefore,
the reader has to recover the timing information from the received
tag signal. To accomplish that the reader needs to estimate and
track symbol rate (frequency) and symbol phase (temporal
boundaries).
[0011] There is a wide variance of when tag responses can be
received for detection. Moreover, the tag and reader clocks are not
synchronized for their period and/or phase. The response is due
after many cycles of these clocks. So, even small discrepancies in
their synchronization may accumulate resulting in potentially large
variances of time and frequency. Therefore, not only are the tag
and reader clocks not synchronized in an RFID communication, but
the frequency errors may be much larger than in traditional
wireless communications systems.
SUMMARY
[0012] The Summary is provided to introduce a selection of concepts
in a simplified from that are further described below in the
Detailed Description. This Summary is not intended to identify key
features or essential features of the claimed subject matter, nor
is it intended to be used as an aid in determining the scope of the
claimed subject matter.
[0013] Embodiments are directed to commanding an RFID tag to
generate a pilot tone in its backscatter. When the backscatter
pilot tone is received in the reader, the pilot tone may be used to
estimate the tag period/frequency. Then, the estimate may be used
to seed and lock a symbol timing recovery loop, which provides a
detected signal to a correlation for detecting the tag preamble.
According to some embodiments, a delayed version of the received
tag signal may be compared against a baseline signal threshold
established from the received signal, and detection of the pilot
tone determined based on the comparison.
[0014] This and other features and advantages of the invention will
be better understood in view of the Detailed Description and the
Drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Non-limiting and non-exhaustive embodiments are described
with reference to the following drawings.
[0016] FIG. 1 is a block diagram of components of an RFID
system.
[0017] FIG. 2 is a diagram showing components of a passive RFID
tag, such as a tag that can be used in the system of FIG. 1.
[0018] FIG. 3 is a conceptual diagram for explaining a half-duplex
mode of communication between the components of the RFID system of
FIG. 1.
[0019] FIG. 4 is a block diagram of a whole RFID reader system
according to embodiments.
[0020] FIG. 5 is a block diagram illustrating an overall
architecture of a RFID reader system according to embodiments.
[0021] FIG. 6 is a block diagram of a receiver block of the RFID
reader of FIG. 5 according to an embodiment;
[0022] FIG. 7 is a block diagram of a receiver block of the RFID
reader of FIG. 5 according to another embodiment;
[0023] FIG. 8 shows an example of a waveform of an FM0
preamble;
[0024] FIG. 9 shows an example of a waveform of a Miller
preamble;
[0025] FIG. 10 is a block diagram of a different implementation of
a component suitable for detecting and acquiring the waveforms of
FIG. 8 and FIG. 9;
[0026] FIG. 11 is a table showing how a Query command can be
constructed to cause a tag to send a pilot tone with its
response;
[0027] FIG. 12 is a block diagram showing a tag response that
includes the pilot tone commanded as in FIG. 11, which in turn
reveals the tag's backscatter link frequency (BLF);
[0028] FIG. 13 shows an example of a waveform of the FM0 preamble
of FIG. 8, further preceded by the pilot tone of FIG. 12;
[0029] FIG. 14 shows an example of a waveform of the Miller
preamble of FIG. 9, further preceded by the pilot tone of FIG.
12;
[0030] FIG. 15 is a hybrid conceptual time and block diagram, for
illustrating embodiments the prior art may need to accommodate also
the pilot tone of FIG. 12;
[0031] FIG. 16 is a block diagram of a signal-processing block of
the RFID reader of FIG. 5 using pilot-tone detection according to
embodiments;
[0032] FIG. 17 is a hybrid conceptual time and operation diagram,
for illustrating operations according to embodiments;
[0033] FIG. 18 is a block diagram of a correlators block of the
diagram of FIG. 16 according to an embodiment;
[0034] FIG. 19 is a block diagram of a correlators block of the
diagram of FIG. 16 according to its preferred embodiment;
[0035] FIG. 20 is a block diagram of a Pilot Tone
Detector/Frequency Estimator of the diagram of FIG. 16 according to
an embodiment;
[0036] FIG. 21 is a more detailed diagram of a pilot tone detection
circuit of FIG. 20 according to an embodiment;
[0037] FIG. 22 is a flowchart of a process for detecting a pilot
tone according to embodiments;
[0038] FIG. 23A shows waveforms of signals according to an
embodiment of the method of FIG. 22;
[0039] FIG. 23B shows how the waveforms of FIG. 23A can be used to
establish detection;
[0040] FIG. 24 is a block diagram of a buffer/delay block of the
diagram of FIG. 16 according to an embodiment;
[0041] FIG. 25 is a block diagram of a buffer/delay block of the
diagram of FIG. 16 according to an embodiment;
[0042] FIG. 26 is a block diagram of a symbol timing recovery loop
block of the diagram of FIG. 16 according to an embodiment;
[0043] FIG. 27 is a block diagram of a symbol timing recovery loop
block of the diagram of FIG. 16 according to other embodiments;
[0044] FIG. 28 is a more detailed block diagram for implementing
the symbol timing recovery loop block of the diagram of FIG. 27
according to embodiments; and
[0045] FIG. 29 is a flowchart of a process for using a pilot tone
in processing RFID tag signals according to embodiments.
DETAILED DESCRIPTION
[0046] Various embodiments will be described in detail with
reference to the drawings, where like reference numerals represent
like parts and assemblies throughout the several views. Reference
to various embodiments does not limit the scope of the invention,
which is limited only by the scope of the claims attached hereto.
Additionally, any examples set forth in this specification are not
intended to be limiting and merely set forth some of the many
possible embodiments for the claimed subject matter.
[0047] Throughout the specification and claims, the following terms
take at least the meanings explicitly associated herein, unless the
context clearly dictates otherwise. The meanings identified below
are not intended to limit the terms, but merely provide
illustrative examples for the terms. The meaning of "a," "an," and
"the" includes plural reference, the meaning of "in" includes "in"
and "on". The term "connected" means a direct electrical connection
between the items connected, without any intermediate devices. The
term "coupled" means either a direct electrical connection between
the items connected or an indirect connection through one or more
passive or active intermediary devices. The term "circuit" means
either a single component or a multiplicity of components, either
active and/or passive, that are coupled together to provide a
desired function. The term "signal" means at least one current,
voltage, charge, temperature, data, or other measurable quantity.
The terms "RFID reader" and "RFID tag" are used interchangeably
with the terms "reader" and "tag", respectively, throughout the
text and claims.
[0048] FIG. 1 is a diagram of components of a typical RFID system
100, incorporating aspects of the invention. An RFID reader 110
transmits an interrogating Radio Frequency (RF) wave 112. RFID tag
120 in the vicinity of RFID reader 110 may sense interrogating RF
wave 112, and generate wave 126 in response. RFID reader 110 senses
and interprets wave 126.
[0049] Reader 110 and tag 120 exchange data via wave 112 and wave
126. In a session of such an exchange, each encodes, modulates, and
transmits data to the other, and each receives, demodulates, and
decodes data from the other. The data is modulated onto, and
decoded from, RF waveforms.
[0050] Encoding the data in waveforms can be performed in a number
of different ways. For example, protocols are devised to
communicate in terms of symbols, also called RFID symbols. A symbol
for communicating can be a delimiter, a calibration symbol, and so
on. Further symbols can be implemented for ultimately exchanging
binary data, such as "0" and "1", if that is desired. In turn, when
the waveforms are processed internally by reader 110 and tag 120,
they can be equivalently considered and treated as numbers having
corresponding values, and so on.
[0051] Tag 120 can be a passive tag or an active tag, i.e. having
its own power source. Where tag 120 is a passive tag, it is powered
from wave 112.
[0052] FIG. 2 is a diagram of an RFID tag 220, which can be the
same as tag 120 of FIG. 1. Tag 220 is implemented as a passive tag,
meaning it does not have its own power source. Much of what is
described in this document, however, applies also to active
tags.
[0053] Tag 220 is formed on a substantially planar inlay 222, which
can be made in many ways known in the art. Tag 220 includes an
electrical circuit, which is preferably implemented in an
integrated circuit (IC) 224. IC 224 is arranged on inlay 222.
[0054] Tag 220 also includes an antenna for exchanging wireless
signals with its environment. The antenna is usually flat and
attached to inlay 222. IC 224 is electrically coupled to the
antenna via suitable antenna ports (not shown in FIG. 2).
[0055] The antenna may be made in a number of ways, as is well
known in the art. In the example of FIG. 2, the antenna is made
from two distinct antenna segments 227, which are shown here
forming a dipole. Many other embodiments are possible, using any
number of antenna segments.
[0056] In some embodiments, an antenna can be made with even a
single segment. Different places of the segment can be coupled to
one or more of the antenna ports of IC 224. For example, the
antenna can form a single loop, with its ends coupled to the ports.
When the single segment has more complex shapes, it should be
remembered that at, the frequencies of RFID wireless communication,
even a single segment could behave like multiple segments.
[0057] In operation, a signal is received by the antenna, and
communicated to IC 224. IF 224 both harvests power, and responds if
appropriate, based on the incoming signal and its internal state.
In order to respond by replying, IC 224 modulates the reflectance
of the antenna, which generates the backscatter from a wave
transmitted by the reader. Coupling together and uncoupling the
antenna ports of IC 224 can modulate the reflectance, as can a
variety of other means.
[0058] In the embodiment of FIG. 2, antenna segments 227 are
separate from IC 224. In other embodiments, antenna segments may
alternately be formed on IC 224, and so on.
[0059] The components of the RFID system of FIG. 1 may communicate
with each other in any number of modes. One such mode is called
full duplex. Another such mode is called half-duplex, and is
described below.
[0060] FIG. 3 is a conceptual diagram 300 for explaining the
half-duplex mode of communication between the components of the
RFID system of FIG. 1, especially when tag 120 is implemented as
passive tag 220 of FIG. 2. The explanation is made with reference
to a TIME axis, and also to a human metaphor of "talking" and
"listening". The actual technical implementations for "talking" and
"listening" are now described.
[0061] RFID reader 110 and RFID tag 120 talk and listen to each
other by taking turns. As seen on axis TIME, when reader 110 talks
to tag 120 the communication session is designated as "R.fwdarw.T",
and when tag 120 talks to reader 110 the communication session is
designated as "T.fwdarw.R". Along the TIME axis, a sample
R.fwdarw.T communication session occurs during a time interval 312,
and a following sample T.fwdarw.R communication session occurs
during a time interval 326. Of course interval 312 is typically of
a different duration than interval 326--here the durations are
shown approximately equal only for purposes of illustration.
[0062] According to blocks 332 and 336, RFID reader 110 talks
during interval 312, and listens during interval 326. According to
blocks 342 and 346, RFID tag 120 listens while reader 110 talks
(during interval 312), and talks while reader 110 listens (during
interval 326).
[0063] In terms of actual technical behavior, during interval 312,
reader 110 talks to tag 120 as follows. According to block 352,
reader 110 transmits wave 112, which was first described in FIG. 1.
At the same time, according to block 362, tag 120 receives wave 112
and processes it, to extract data and so one. Meanwhile, according
to block 372, tag 120 does not backscatter with its antenna, and
according to block 382, reader 110 has no wave to receive from tag
120.
[0064] During interval 326, tag 120 talks to reader 110 as follows.
According to block 356, reader 110 transmits a Continuous Wave
(CW), which can be thought of as a carrier signal that ideally
encodes no information. As discussed before, this carrier signal
serves both to be harvested by tag 120 for its own internal power
needs, and also as a wave that tag 120 can backscatter. Indeed,
during interval 326, according to block 366, tag 120 does not
receive a signal for processing. Instead, according to block 376,
tag 120 modulates the CW emitted according to block 356, so as to
generate backscatter wave 126. Concurrently, according to block
386, reader 110 receives backscatter wave 126 and processes it.
[0065] In the above, an RFID reader/interrogator may communicate
with one or more RFID tags in any number of ways. Some such ways
are called protocols. A protocol is a specification that calls for
specific manners of signaling between the reader and the tags.
[0066] One such protocol is called the Specification for RFID Air
Interface--EPC (TM) Radio-Frequency Identity Protocols Class-1
Generation-2 UHF RFID Protocol for Communications at 860 MHz-960
MHz, which is also colloquially known as "the Gen2 Spec". The Gen2
Spec has been ratified by EPCglobal, which is an organization that
maintains a website at: <http://www.epcglobaline.org/> at the
time this document is initially filed with the USPTO.
[0067] It was described above how reader 110 and tag 120
communicate in terms of time. In addition, communications between
reader 110 and tag 120 may be restricted according to frequency.
One such restriction is that the available frequency spectrum may
be partitioned into divisions that are called channels. Different
partitioning manners may be specified by different regulatory
jurisdictions and authorities (e.g. FCC in North America, CEPT in
Europe, etc.).
[0068] The reader 110 typically transmits with a transmission
spectrum that lies within one channel. In some regulatory
jurisdictions the authorities permit aggregating multiple channels
into one or more larger channels, but for all practical purposes an
aggregate channel can again be considered a single, albeit larger,
individual channel.
[0069] Tag 120 can respond with a backscatter that is modulated
directly onto the frequency of the reader's emitted CW, also called
baseband backscatter. Alternatively, Tag 120 can respond with a
backscatter that is modulated onto a frequency, developed by Tag
120, that is different from the reader's emitted CW, and this
modulated tag frequency is then impressed upon the reader's emitted
CW. This second type of backscatter is called subcarrier
backscatter. The subcarrier frequency can be within the reader's
channel, can straddle the boundaries with the adjacent channel, or
can be wholly outside the reader's channel.
[0070] A number of jurisdictions require a reader to hop to a new
channel on a regular basis. When a reader hops to a new channel it
may encounter RF energy there that could interfere with
communications.
[0071] Embodiments of the present disclosure can be useful in
different RFID environments, for example, in the deployment of RFID
readers in sparse- or dense-reader environments, in environments
with networked and disconnected readers such as where a hand-held
reader may enter the field of networked readers, in environments
with mobile readers, or in environments with other interface
sources. It will be understood that the present embodiments are not
limited to operation in the above environments, but may provided
improved operation in such environments.
[0072] FIG. 4 is a block diagram of a whole RFID reader system 400
according to embodiments. System 400 includes a local block 420,
and optionally remote components 470. Local block 420 and remote
components 470 can be implemented in any number of ways. It will be
recognized that reader 110 of FIG. 1 is the same as local block
420, if remote components 470 are not provided. Alternatively,
reader 110 can be implemented instead by a system 400, of which
only the local block 420 is shown in FIG. 1.
[0073] Local block 420 is responsible for communicating with the
tags. Local block 420 includes a block 451 of an antenna and a
driver of the antenna for communicating with the tags. Some
readers, like that shown in local block 420, contain a single
antenna and driver. Some readers, like that shown in local block
420, contain a single antenna and driver. Some readers contain
multiple antennas and drivers and a method to switch signals among
them, including sometimes using different antennas for transmitting
and for receiving. And some readers contain multiple antennas and
drivers that can operate simultaneously. A demodulator/decoder
block 453 demodulates and decodes backscattered waves received from
the tags via antenna block 451. Modulator/encoder block 454 encodes
and modulates an RF wave that is to be transmitted to the tags via
antenna block 451.
[0074] Local block 420 additionally includes an optional local
processor 456. Processor 456 may be implemented in any number of
ways known in the art. Such ways include, by way of examples and
not of limitation, digital and/or analog processors such as
microprocessors and digital-signal processors (DSPs); controllers
such as microcontrollers; software running in a machine such as a
general purpose computer; programmable circuits such as Field
Programmable Gate Arrays (FPGAs), Field-Programmable Analog Arrays
(FPAAs), Programmable logic Devices (PLDs), Application Specific
Integrated Circuits (ASIC), any combination of one or more of
these; and so on. In some cases some or all of the decoding
function in block 453, the encoding function in block 454, or both,
may be performed instead by processor 456.
[0075] Local block 420 additionally includes an optional local
memory 457. Memory 457 may be implemented in any number of ways
known in the art. Such ways include, by way of examples and not of
limitation, nonvolatile memories (NVM), read-only memories (ROM),
random access memories (RAM), any combination of one or more of
these and so on. Memory 457, if provided, can include programs for
processor 456 to run, if provided.
[0076] In some embodiments, memory 457 stores data read from tags,
or data to be written to tags, such as Electronic Product Codes
(EPCs), Tag Identifiers (TIDs) and other data. Memory 457 can also
include reference data that is to be compared to the EPC codes,
instructions and/or rules for how to encode commands for the tags,
modes for controlling antenna 451, and so on. In some of these
embodiments, local memory 457 is provided as a database.
[0077] Some components of local block 420 typically treat the data
as analog, such as the antenna/driver block 451. Other components
such as memory 457 typically treat the data as digital. At some
point there is a conversion between analog and digital. Based on
where this conversion occurs, a whole reader may be characterized
as "analog" or "digital", but most readers contain a mix of analog
and digital functionality.
[0078] If remote components 470 are indeed provided, they are
coupled to local block 420 via an electronic communications network
480. Network 480 can be a Local Area Network (LAN), a Metropolitan
Area Network (MAN), a Wide Area Network (WAN), a network of
networks such as the internet, and so on. In turn, local block 420
then includes a local network connection 459 for communicating with
network 480.
[0079] There can be one or more remote component(s) 470. If more
than one, they can be located at the same place with each other, or
in different places. They can access each other and local block 420
via network 480, or via other similar networks, and so on.
Accordingly, remote component(s) 470 can use respective remote
network connections. Only one such remote network connection 479 is
shown, which is similar to local network connection 459, etc.
[0080] Remote component(s) 470 can also include a remote processor
476. Processor 476 can be made in any way known in the art, such as
was described with reference to local processor 456.
[0081] Remote component(s) 470 can also include a remote memory
477. Memory 477 can be made in any way known in the art, such as
was described with reference to local memory 457. Memory 477 may
include a local database, and a different database of a Standards
Organization, such as one that can reference EPCs.
[0082] Of the above-described elements, it is advantageous to
consider a combination of these components, designated as
operational processing block 490. Block 490 includes those that are
provided of the following: local processor 456, remote processor
476, local network connection 459, remote network connection 479,
and by extension an applicable portion of network 480 that links
connection 459 with connection 479. The portion can be dynamically
changeable, etc. In addition, block 490 can receive and decode RF
waves received via antenna 451, and cause antenna 451 to transmit
RF waves according to what it has processed.
[0083] Block 490 includes either local processor 456, or remote
processor 476, or both. If both are provided, remote processor 476
can be made such that it operates in a way complementary with that
of local processor 456. In fact, the two can cooperate. It will be
appreciated that block 490, as defined this way, is in
communication with both local memory 457 and remote memory 477, of
both are present.
[0084] Accordingly, block 490 is location agnostic, in that its
functions can be implemented either by local processor 456, or by
remote processor 476, or by a combination of both. Some of these
functions are preferably implemented by local processor 456, and
some by remote processor 476. Block 490 accesses local memory 457,
or remote memory 477, or both for storing and/or retrieving
data.
[0085] Reader system 400 operates by block 490 generating
communications for RFID tags. These communications are ultimately
transmitted by antenna block 451, with modulator/encoder block 454
encoding and modulating the information on an RF wave. Then data is
received from the tags via antenna block 451, demodulated and
decoded by demodulator/decoder block 453, and processed by
processing block 490.
[0086] The invention additionally includes programs, and methods of
operation of the programs. A program is generally defined as a
group of steps of operations leading to a desired result, due to
the nature of the elements in the steps and their sequence. A
program is usually advantageously implemented as a sequence of
steps or operations for a processor, such as the structures
described above.
[0087] Performing the steps, instructions, or operations of a
program requires manipulation of physical quantities. Usually,
though not necessarily, these quantities may be transferred,
combined, compared, and otherwise manipulated or processed
according to the steps or instructions, and they may also be stored
in a computer-readable medium. These quantities include, for
example, electrical, magnetic, and electromagnetic charges or
particles, states of matter, and in the more general case can
include the states of any physical devices or elements. It is
convenient at times, principally for reasons of common usage, to
refer to information represented by the states of these quantities
as bits, data bits, samples, values, symbols, characters, terms,
numbers, or the like. It should be borne in mind, however, that all
of these and similar terms are associated with the appropriate
physical quantities, and that these terms are merely convenient
labels applied to these physical quantities, individually or in
groups.
[0088] The invention furthermore includes storage media. Such
media, individually or in combination with others, have stored
thereon instructions of a program made according to the invention.
A storage medium according to the invention is a computer-readable
medium, such as a memory, and is read by a processor of the type
mentioned above. If a memory, it can be implemented in a number of
ways, such as Read Only Memory (ROM), Random Access Memory (RAM),
etc., some of which are volatile and some non-volatile.
[0089] Even though it is said that the program may be stored in a
computer-readable medium, it should be clear to a person skilled in
the art that it need not be a single memory, or even a single
machine. Various portions, modules or features of it may reside in
separate memories, or even separate machines. The separate machines
may be connected directly, or through a network such as a local
access network (LAN) or a global network such as the Internet.
[0090] Often, for the sake of convenience only, it is desirable to
implement and describe a program as software. The software can be
unitary, or thought in terms of various interconnected distinct
software modules.
[0091] This detailed description is presented largely in terms of
flowcharts, algorithms, and symbolic representations of operations
on data bits on and/or within at least one medium that allows
computational operations, such as a computer with memory. Indeed,
such descriptions and representations are the type of convenient
labels used by those skilled in programming and/or the data
processing arts to effectively convey the substance of their work
to others skilled in the art. A person skilled in the art of
programming may use these descriptions to readily generate specific
instructions for implementing a program according to the present
invention.
[0092] Embodiments of an RFID reader system can be implemented as a
combination of hardware and software. It is advantageous to
consider such a system as subdivided into components or modules. A
person skilled in the art will recognize that some of these
components or modules can be implemented as hardware, some as
software, some as firmware, and some as a combination. An example
of such a subdivision is now described.
[0093] FIG. 5 is a block diagram illustrating an overall
architecture of a RFID reader system 500 according to embodiments.
It will be appreciated that system 500 is considered subdivided
into modules or components. Each of these modules may be
implemented by itself, or in combination with others. It will be
recognized that some aspects are parallel with those of FIG. 3. In
addition, some of them may be present more than once.
[0094] RFID reader system 500 includes one or more antennas 510,
and an RF Front End 520, for interfacing with antenna(s) 510. These
can be made as described above. In addition, Front End 520
typically includes analog components.
[0095] System 500 also includes a Signal Processing module 530. In
this embodiment, module 530 exchanges waveforms with Front End 520,
such as I and Q waveform pairs. In some embodiments, signal
processing module 530 is implemented by itself in an FPGA.
[0096] System 500 also includes a Physical Driver module 540, which
is also known as Data Link. In this embodiment, module 540
exchanges bits with module 530. Data Link 540 can be the state
associated with framing of data. In one embodiment, module 540 is
implemented by a Digital Signal Processor.
[0097] System 500 additionally includes a Media Access Control
module 550, which is also known as MAC layer. In this embodiment,
module 550 exchanges packets of bits with module 550. MAC layer 550
can be the stage for making decisions for sharing the medium of
wireless communication, which in this case is the air interface.
Sharing can be between reader system 500 and tags, or between
system 500 with another reader, or between tags, or a combination.
In one embodiment, module 550 is implemented by a Digital Signal
Processor.
[0098] System 500 moreover includes an Application Programming
Interface module 560, which is also known as API, Modem API, and
MAPI. In some embodiments, module 560 is itself an interface for a
user.
[0099] System 500 further includes a host processor 570. Processor
570 exchanges signals with MAC layer 550 via module 560. In some
embodiments, host processor 570 is not considered as a separate
module, but one that includes some of the above-mentioned modules
of system 500. A user interface 580 is coupled to processor 570,
and it can be manual, automatic, or both.
[0100] Host processor 570 can include applications for system 500.
In some embodiments, elements of module 560 may be distributed
between processor 570 and MAC layer 550. It will be observed that
the modules of system 500 form something of a chain. Adjacent
modules in the chain can be coupled by the appropriate
instrumentalities for exchanging signals. These instrumentalities
include conductors, buses, interfaces, and so on. These
instrumentalities can be local, e.g. to connect modules that are
physically close to each other, or over a network, for remote
communication.
[0101] The chain is used in opposite directions for receiving and
transmitting. In a receiving mode, wireless waves are received by
antenna(s) 510 as signals, which are in turn processed successively
by the various modules in the chain. Processing can terminate in
any one of the modules. In a transmitting mode, initiation can be
in any one of these modules. That, which is to be transmitted
becomes ultimately signals for antenna(s) 510 to transmit as
wireless waves.
[0102] The architecture of system 500 is presented for purposes of
explanation, and not of limitation. Its particular subdivision into
modules need not be followed for creating embodiments according to
the invention. Furthermore, the features of the invention can be
performed either within a single one of the modules, or by a
combination of them.
[0103] An economy is achieved in the present document in that a
single set of flowcharts is used to describe methods in and of
themselves, along with operations of hardware and/or software. This
is regardless of how each element is implemented.
[0104] FIG. 6 is a block diagram of a receiver block of the RFID
reader of FIG. 5 according to an embodiment.
[0105] Receiver 600 may be a direct down-conversion receiver that
directly converts a received tag response signal to a baseband
signal without an intermediate step of converting to an IF signal.
In some embodiments, receiver 600 may be a zero-IF receiver. In
other embodiments, receiver 600 may be a low-IF receiver in which
an IF signal close to zero frequency is generated. Receiver 600
comprises antenna 628, mixers 602 and 604 to convert the tag
response signal directly to baseband, phase converter 606 to
provide 0 deg and 90 deg phases of a local oscillator signal from
the local oscillator 601 to the mixers, and demodulator 653.
[0106] In some embodiments, receiver 600 may also include one or
more filters to filter undesirable mixing products and interfering
signals, as well as to reduce baseband distortion and noise.
Antenna 628 may comprise a directional or omnidirectional antenna,
including, for example, a dipole antenna, a monopole antenna, a
loop antenna, a microstrip antenna or other type of antenna
suitable for reception and/or transmission of RF signals which may
be processed by receiver 600. In some embodiments, antenna 628 may
be a turned antenna.
[0107] Receiver 600 may down-convert received tag response signals
(RF) to an in-phase (I) channel and a quadrature-phase (Q) channel
(respectively I.sub.DATA and Q.sub.DATA), although the scope of the
invention is not limited in this respect. In these embodiments,
mixers 602 and 604 may be I-channel and Q-channel mixers used to
generate I- and Q-baseband signals based on local oscillator
signals. Local oscillator signals may be separated in phase by
about 90 degrees.
[0108] Although receiver 600 is illustrated as having several
separate functional elements, one or more of the functional
elements may be combined and may be implemented by combinations of
software-configured elements, such as processing elements including
digital signal processors (DSPs), and/or other hardware elements.
For example, some elements may comprise one or more
microprocessors, DSPs, application specific integrated circuits
(ASICs), and combinations of various hardware and logic circuitry
of performing at least the functions described herein. In some
embodiments, at least portions of receiver 600 may be fabricated on
a single monolithic semiconductor substrate, such as a
radio-frequency integrated circuit (RFIC).
[0109] FIG. 7 is a block diagram of a receiver block of the RFID
reader of FIG. 5 according to another embodiment.
[0110] Similarly to receiver 600 of FIG. 6, receiver 700 includes
antenna 628, mixers 602 and 604, phase converter 606, local
oscillator 601, and demodulator 753. Differently from the previous
receiver, however, receiver 700 includes Analog Digital Converters
(ADCs) 710 and 712, which provide digital I.sub.DATA and Q.sub.DATA
to demodulator 753, which is arranged to receive digital
signals.
[0111] FIG. 8 shows an example of a waveform of an FM0 preamble.
There are two common types of RFID modulation, FM0 and Miller
subcarrier. Packets sent with either modulation type include a
preamble portion and a data portion, which may include payload
data, security information, and the like.
[0112] FM0 is currently used in ISO standards. This modulation is
fast but may be more susceptible to interference. Waveform 872
shows an FM0 preamble without a pilot tone. Tag to reader FM0
signaling begins with the preamble 872 shown in the figure. The "v"
shown in the preamble indicates an FM0 violation (i.e. a phase
inversion should have occurred but did not).
[0113] FIG. 9 shows an example of a waveform of a Miller
preamble.
[0114] Miller Subcarrier modulation is slower but less susceptible
to errors in RF noisy environments. This algorithm uses narrow
spectrum for the tags to send back their signal and fits it between
the channels used by the reader. That way the RF signals coming
from the reader do not cover the signals coming back from the tags.
Miller demodulation uses advanced filtering techniques to separate
the tag's response from the reader's transmissions and other noise
compared to FM0.
[0115] Miller-modulated subcarrier sequences contain exactly two,
four, or eight subcarrier cycles per bit, depending on the M value
specified in a Query command that initiated the inventory round
(example subcarrier sequences 992).
[0116] FIG. 10 is a block diagram of a different implementation of
a component suitable for detecting and acquiring the waveforms of
FIG. 8 and FIG. 9.
[0117] One way of detecting the tag's preamble without using a
pilot tone is employing a preamble matched filter block 1012
comprising a plurality of matched filters in combination with a
timebase estimator/synchronizer 1014 comprising a plurality of
magnitude detectors.
[0118] In such a system, incoming data is provided to the series of
preamble matched filters 1022-1 through 1022-N, each with a
different timebase (timebases 1 through N). The matched filters
block incoming signal except for the filter (or filters) with a
close timebase. This gives a large peak when the expected preamble
is filtered through the applicable filter. Each filter's output is
provided to a corresponding magnitude detector 1024-1 through
1024-N.
[0119] One of the matched filter/magnitude detector combinations
gives the largest peak upon detecting the tag preamble. Maximum
magnitude detector 1026 of timebase estimator/synchronizer 1014
detects which magnitude detector captures the expected preamble and
provides the signal from that magnitude detector to the demodulator
with a known frequency, phase, and time of arrival.
[0120] In such systems multiple matched filters operate in parallel
for recovering preamble. The filters are arrayed, so that together
they can cover the wide variance. The large lock range of bit
tracking loop requires a large number of filters and magnitude
detectors to detect the tag preamble increasing cost of
manufacture, power consumption, and complexity of circuits.
[0121] FIG. 11 is a table showing how a Query command can be
constructed to cause a tag to send a pilot tone with its
response.
[0122] In a typical tag design, the tolerance on the tag's
oscillator clock can be as small as .+-.4% or as large as .+-.22%,
depending on the data rate. This can be contrasted with typical
wireless systems where the tolerance can be <<1%. Thus, the
reader does not know when exactly the response will start being
detected due to the wide variance of when the tag response can be
received. Also, the reader does not know the exact length of the
modulated symbols from a tag.
[0123] If nothing is done, the reader can have errors in detection
such as false tag responses and missed tag responses. According to
some embodiments, the tag may be commanded to generate a pilot tone
in its backscatter. When the backscattered pilot tone is received
in he reader, the pilot tone may be used to estimate the tag
period/frequency. Then, the estimate may be used to seed and lock
the few or just one symbol timing recovery loop.
[0124] Using the pilot tone to detect a frequency and phase of the
tag signal allows synchronous detection of the tag preamble from an
asynchronous signal with fewer correlators than any parallel
implementation. Indeed, an RFID reader detection circuit using the
pilot tone may be implemented with a single correlator as described
in conjunction with FIG. 10 resulting in manufacturing cost
savings, power consumption reduction, and the ability to reduce an
integrated circuit size (such as an FPGA size) to implement the
circuits. It also reduces the complexity of the reader
circuits.
[0125] According to the Gen2 specification, a TR.sub.ext bit in a
Query command may be used to enable a pilot tone preceding the
tag's backscattered preamble. Other methods may also be used to
command the tag to transmit the pilot tone prior to sending its
data. In the Query command, TR.sub.est=1 chooses the tag to reader
(T.fwdarw.R) preamble to be prepended with a pilot tone.
[0126] As shown in table 1100, an example Query command may include
bits defining a DR value for singulation, a Miller subcarrier
selection bit pair defining what number of cycles per symbol is to
be used, a TR.sub.ext bit defining whether or not a pilot tone is
requested, a session identifier, a cyclical redundancy check bit,
and so on.
[0127] FIG. 12 is a block diagram showing a tag response that
includes the pilot tone commanded as in FIG. 11, which in turn
reveals the tag's backscatter link frequency (BLF).
[0128] As shown in diagram 1270, the tag response following a Query
command with TR.sub.est=1 includes pilot tone 1271 before tag
preamble 1272, which is followed by tag response packet 1273. The
pilot tone is at the period (frequency) of the tag.
[0129] FIG. 13 shows an example of a waveform of the FM0 preamble
of FIG. 8, further preceded by the pilot tone of FIG. 12.
[0130] The FM0 preamble shown in FIG. 12 includes the same preamble
as in FIG. 8 (872) prepended with 12 leading zero bits forming the
pilot tone 1371. According to other specifications a "1" bit value
or a different number of bits may be used as the pilot tone.
[0131] FIG. 14 shows an example of a waveform of the Miller
preamble of FIG. 9, further preceded by the pilot tone of FIG.
12.
[0132] Three example Miller preambled of FIG. 0 for M=2, M=4, and
M=8 are shown in FIG. 14, each prepended with 16 leading zero bits
serving as the pilot tone. Similarly, other bit values or number of
bits may also be chosen as pilot tone for use with Miller
preambles.
[0133] FIG. 15 is a hybrid conceptual time and block diagram, for
illustrating embodiments the prior art may need to also accommodate
the pilot tone of FIG. 12.
[0134] As described in FIG. 12, incoming tag signal with the pilot
tone includes pilot tone 1271 between time points T0 and T1, tag
preamble 1272 between T1 and T2, and tag response packet 1273
starting at T2.
[0135] Processing of the tag signal by the reader involves
frequency, phase, and time of arrival acquisition (1572) from the
pilot tone and tag preamble between T0 and T1. According to some
embodiments, the reader may also process the preamble in one or
more sample rate correlators (1522-1 through 1522-N). Normally, the
correlator performs correlation with an expected preamble. This
gives a large peak when the expected preamble is received, due to
autocorrelation.
[0136] FIG. 16 is a block diagram of a signal-processing block of
the RFID reader of FIG. 5 using pilot-tone detection according to
embodiments.
[0137] In a typical communication session, the RFID reader sends a
command via an RF wave that encodes symbols, which define the
command. Then the reader waits for some time while still
transmitting RF. At the end of the silent period the tag
backscatters a response, which the reader is supposed to detect and
read. The backscatter includes a preamble and data associated with
the command. According to some embodiments, the tag response may
also include a pilot tone before the preamble, if the reader's
command specified it.
[0138] Input signal IN may include combined I- and Q-channels of
the tag response signal. The signal is provided to buffer/delay
block 1644 for generating a delayed version of the received signal,
which is provided to mixer 1646. The delay is set by the frequency
estimator.
[0139] Pilot tone portion of the signal S.sub.Pilot.sub.--.sup.Tone
is also provided to pilot tone detector/frequency estimator 1642.
The tag signal frequency is estimated using any one or combination
of methods including a discrete Fourier transform (DFT), fast
Fourier transform (FFT), or analog clock recovery loop prior to the
impending arrival of the tag's preamble. So, a more accurate
estimate of the tag's backscatter data rate can be made.
[0140] Pilot tone detector/frequency estimator 1642 provides an RF
signal phase estimation (S.sub.RF.sub.--.sup.Ph) to mixer 1646 such
that the I and Q signals can be combined to form S(f.sub.s) that is
provided as input to symbol timing recovery loop 1648.
[0141] A phase of the tag signal may also be estimated by the pilot
tone detector 1642. The estimated frequency and/or phase
(S.sub.Symbol.sub.--.sup.Freq and S.sub.Symbol.sub.--.sup.Ph) are
then provided to symbol timing recovery loop 1648 to seed the
recovery operation.
[0142] Symbol timing recovery loop 1648 estimates a narrowed window
for the time of arrival of the packet, and thus knows when to turn
on the correlator(s) 1636. Once the frequency is established, only
synchronous detection of the preamble is required. Thus, only one
correlator may be sufficient for recovering the preamble.
[0143] Upon exiting the loop, and right before the correlator, the
signal then is frequency locked and/or phase locked. A single
correlator may thus yield the exact Time of Arrival. Correlator
1636 operates synchronously to detect the tag's preamble response
and delivers the preamble to the demodulator.
[0144] The circuits described above may be implemented as analog,
digital, DSP, FPGA circuitry, or any combination thereof.
Furthermore, at least a portion of the functionality may be
implemented as software in dedicated or generic hardware. The
described recovery techniques may be used in both baseband and
subcarrier modes.
[0145] FIG. 17 is a hybrid conceptual time and operation diagram,
for illustrating operations according to embodiments.
[0146] In diagram 1700, the main portions of the incoming signal
(pilot tone 1271, tag preamble 1272, and tag response packet 1273)
are shown against a timeline. Between T0 and T1 that cover the
arrival of pilot tone 1271 and tag preamble 1272, the reader may
perform coarse estimates for RF phase, symbol frequency, and symbol
phase (1770) using Fourier transform (FT 1771) and/or asynchronous
matched filtering (1772). Because this process may take some time
to perform, a delay (1780) may be introduced to the incoming tag
signal (e.g. by a variable buffer/delay block of the reader)
resulting in the delayed pilot tone 1781, delayed tag preamble
1782, and delayed tag response packet 1783.
[0147] Thus, the new packet epoch (time of arrival) gets shifted to
the beginning point of delayed tag response packet 1783. Following
delay 1780, the reader may perform fine estimates on the received
signal (1790) employing symbol time recovery loop (1791) and single
synchronized matched filter (1792). Following fine estimates 1790,
data recovery operations (1793) may be performed on the delayed tag
response packet by the demodulator.
[0148] FIG. 18 is a block diagram of a correlators block of the
diagram of FIG. 16
[0149] By using frequency and phase estimates to seed and lock the
symbol time recovery loop, the detection circuit of the reader can
zoom in on an expected tag preamble within the relatively large
range of possibilities. Therefore, a number of correlators for
detecting the preamble can be substantially reduced. Correlator
block 1836 is such an example using only two matched
filter/magnitude detector sets.
[0150] As described previously in conjunction with FIG. 10,
preamble matched filters 1822-1 and 1822-2 derived from different
timebases 1 and 2 receive signal TS and provide a signal to
corresponding magnitude detectors 1824-1 and 1824-2 in timebase
estimator/synchronizer block 1814. During an operation, one of the
two sets captures the incoming tag preamble and provides a larger
magnitude signal to maximum magnitude detector 1826, which in turn
forwards the detected preamble information to the demodulator.
[0151] FIG. 19 is a block diagram of a correlators block of the
diagram of FIG. 16 according to its preferred embodiment.
[0152] As mentioned previously, due to the lock in on the expected
preamble using the pilot tone detection, correlator block 1936 may
be implemented in a preferred embodiment with only a single set of
matched filter/magnitude detector set.
[0153] Since the frequency and phase information associated with
the preamble is already determined by the symbol time recovery
loop, preamble matched filter 1922 may be used with a single
timebase followed by magnitude detector 1924. Because a single set
of matched filter/magnitude detector is used, no maximum magnitude
detector is needed. The detected preamble information is forwarded
to the demodulator directly by the magnitude detector 1924.
[0154] FIG. 20 is a block diagram of a pilot tone
detector/frequency estimator of the diagram of FIG. 16 according to
an embodiment.
[0155] Pilot tone detector/frequency estimator 2042 includes pilot
tone detection block 2052, which is arranged to receive the pilot
tone and provide RF phase, symbol phase, and symbol frequency
estimate signals S.sub.RF.sub.--.sup.Ph,
S.sub.Symbol.sub.--.sup.Ph, and S.sub.Symbol.sub.--.sup.Freq.
[0156] According to some embodiments, pilot tone detector/frequency
estimator 2042 may include an optional preamble matched filter
block that is arranged to perform the same function employing
matched filters instead of the pilot tone detector/frequency
estimator and provide estimate signals S.sub.RF.sub.--.sup.Ph,
S.sub.Symbol.sub.--.sup.Ph, and S.sub.Symbol.sub.--.sup.Freq. The
pilot tone detection block may be used for coarse estimates, while
the preamble matched filter block is used for fine estimates.
[0157] Estimate signals S.sub.RF.sub.--.sup.Ph,
S.sub.Symbol.sub.--.sup.Ph, and S.sub.Symbol.sub.--.sup.Freq from
either block (or both) may be provided to a selection block 2057
such as a selector, a state machine, and the like. The selection
block 2057 may provide individual estimate signals
S.sub.RF.sub.--.sup.Ph, S.sub.Symbol.sub.--.sup.Ph, and
S.sub.Symbol.sub.--.sup.Freq to other circuits such as
correlators.
[0158] FIG. 21 is a more detailed diagram of a pilot tone detection
circuit of FIG. 20 according to an embodiment.
[0159] Circuit 2152 includes a channelized filter bank 2110 which
receives the incoming signal, divides into spectral components, and
provides to peak detector 2112. The received signal
S.sub.Pilot.sub.--.sup.Tone is also provided to power measurement
block 2102. The measured power is scaled by scaler 2104 and
provided to comparator 2106.
[0160] A delayed version of the detected peak of the filtered input
signal S.sub.Pilot.sub.--.sup.Tone is provided by delay block 2116
to comparators 2106 and 2120. A peak and hold block 2114 receives
the detected peaks of the filtered input signals S.sub.FRQ and
S.sub.IQ from peak detector 2112 providing an input to scaler 2118
as well as a frequency estimate output S.sub.Symbol.sub.--.sup.Freq
and an input to RF phase estimator 2124 for phase estimation.
[0161] An output of scaler 2118 is provided to comparator 2120 for
comparison with the delayed version of the signal from delay block
2116. Both comparator (2106 and 2120) outputs are then combined at
combiner 2108 to provide a signal for symbol phase
S.sub.Symbol.sub.--.sup.Ph. The estimate signals,
S.sub.Symbol.sub.--.sup.Freq and S.sub.Symbol.sub.--.sup.Ph may be
used to seed and lock the few or just one symbol timing recovery
loop.
[0162] According to some embodiments, a method for an RFID reader
system may include receiving from an RFID tag a backscattered
signal, detecting the pilot tone, estimating a frequency of the
pilot tone in the backscattered signal, recovering a timing of
symbols in the backscattered signal using the estimated frequency,
demodulating the backscattered signal based on the symbol timing to
recover data. The signal may be backscattered from the RFID tag
responsive to a command transmitted by the reader, where the pilot
tone is detected only if the command belongs in a subset of
commands.
[0163] The pilot tone may be detected by generating a baseline
version of the backscattered signal, generating a version of the
backscattered signal that is delayed with respect to the
backscattered signal, and establishing detection when the delayed
version reaches a detection relationship with the baseline version.
The detection relationship may be that the delayed version exceeds
the baseline version.
[0164] Moreover, the baseline version of the tag signal may be
generated as a preset portion of a magnitude of the tag signal. The
present portion may be substantially equal to 50%. According to
other embodiments, the baseline version may have a waveform that
rises up to a cusp, and then continues at substantially the same
value, and the delayed version may be delayed enough so that
detection is established after the cusp. Detection may be
established only if both the delayed version and the baseline
version exceed a detection threshold. Furthermore, the detection
threshold may be defined in terms of detected ambient noise and may
be updated.
[0165] According to further embodiments, the backscattered signal
may be received responsive to a command signal transmitted by the
RFID reader system, the command signal may cause the tag to be
silent during a silent period, and the detection threshold is
defined from a power measurement made during the silent period.
[0166] The frequency of the tag signal may be estimated by
employing a Discrete Fourier Transform (DFT) or Fast Fourier
Transform (FFT) of the received pilot tone. The frequency may also
be estimated by correlating the backscattered tag signal with an
expected version of the backscattered signal.
[0167] The method according to embodiments may further include
identifying in the backscattered signal a preamble using the symbol
timing where the backscattered signal is demodulated based on a
timing of the preamble, and determining from the preamble timing a
Time Of Arrival (TOA) for a remainder of the backscattered signal
where the data is recovered from the remainder. The preamble may be
identified using a single correlator.
[0168] In yet further embodiments, the method may include delaying
the remainder of the backscattered signal the preamble by a delay
time and adjusting the delay time to compensate for an amount of
time taken to estimate the frequency.
[0169] The invention also includes methods. Some are methods of
operation of an RFID reader or RFID reader system. Others are
methods for controlling an RFID reader or RFID reader system.
[0170] These methods can be implemented in any number of ways,
including the structures described in this document. One such way
is by machine operations, of devices of the type described in this
document.
[0171] Another optional way is for one or more of the individual
operations of the methods to be performed in conjunction with one
or more human operators performing some. These human operators need
not be collocated with each other, but each can be only with a
machine that performs a portion of the program.
[0172] Some of the methods are now described more particularly
according to embodiments.
[0173] FIG. 22 is a flowchart of a process for detecting a pilot
tone according to embodiments.
[0174] Process 2200 begins at optional operation 2210, where the
tag signal is received. As mentioned previously, tag signal
includes a frequency and a phase that is typically not synchronous
with a frequency and phase of the reader clock.
[0175] According to next operation 2220, a baseline version of the
tag signal is generated as described above in conjunction with FIG.
21.
[0176] According to a next operation 2230, a delayed version of the
tag signal is generated.
[0177] According to a next decision operation 2240, a determination
is made whether the delayed version of the tag signal is larger
than the baseline version. If the baseline version is larger, no
pilot tone has been detected, and processing returns to a calling
process for further operations.
[0178] If the delayed version of the tag signal is larger than the
baseline version, processing continues from decision operation 2240
to optional operation 2250, where the pilot tone is detected and
operations associated with adjusting a frequency and phase of the
reader clock signal may be performed.
[0179] The operations included in process 2200 are for illustration
purposes. Pilot tone detection in an RFID reader by comparing a
delayed version of the tag signal with a baseline version may be
implemented by similar processes with fewer or additional steps, as
well as in different order of operations using the principles
described herein.
[0180] FIG. 23A shows waveforms of signals according to an
embodiment of the method of FIG. 22.
[0181] According to some embodiments, a pilot tone transmitted by
the tag for providing the reader with information about its
response frequency and phase may be detected by the reader
employing two separate criteria. A first criterion may include
detection based on a comparison of the pilot tone with a
noise-based threshold (2310) as shown in diagram 2300.
[0182] The noise-based threshold is set in response to a
measurement performed at a time point before the tag response is
expected. The measurement may be a power measurement and the power
estimate scaled to set a desired False Alarm Rate (FAR) and a
detection probability. Since the threshold is based on an actual
measurement, it adapts to a dynamic RFID environment, which may
include interference noise, different tag power levels, and so
on.
[0183] Once the first detection occurs, an auto-normalized baseline
version of the tag signal 2308 may be established based on the
signal magnitude. The threshold may be peak-held and scaled. A
delayed signal version (2306) may also be derived from the received
signal (2304) by imposing a preset delay 2305. A leading edge of
the received pilot tone 2302 increases with the same gradient as
the delayed version.
[0184] The delayed version 2306 is then compared to the
signal-based threshold (baseline version) 2308 to detect the pilot
tone with an accurate time of arrival estimate.
[0185] FIG. 23B shows how the waveforms of FIG. 23A can be used to
establish detection.
[0186] Diagram 2350 shows pilot tone magnitude over time. Initially
the pilot tone is not detected (2323). The first criterion 2313
(noise-based threshold) initiates the implementation of the second
criterion 2316 (signal-based threshold). If both are satisfied,
pilot tone detection is established (2318).
[0187] The waveforms and circuits described above illustrate an
example embodiment for using comparison between a baseline version
and a delayed version of tag signal for tag pilot tone detection.
Other circuits and waveforms may also be used without departing
from a scope and spirit of the invention.
[0188] FIG. 24 is a block diagram of a buffer/delay block of the
diagram of FIG. 16 according to an embodiment.
[0189] As described previously in conjunction with FIG. 16, the I
and Q channels of the detected tag response signal may be provided
by the receiver block to a buffer/delay block of a frequency and/or
phase estimator circuit of the reader. The buffer/delay block may
include in one embodiment, a variable rate buffer 2444.
[0190] A rate of the variable rate buffer 2444 may be controlled by
a feedback signal from frequency estimator to compensate for the
time it takes to estimate the frequency. Variable rate buffer 2444
then provides the buffered tag response signal to a mixer for the
symbol timing recovery loop.
[0191] FIG. 25 is a block diagram of a buffer/delay block of the
diagram of FIG. 16 according to an embodiment.
[0192] In another embodiment, a variable delay block 2544 may be
used to receive the I- and Q-channels of the detected tag response
signal and generate a delayed version based on the estimation
control signal from the frequency estimator.
[0193] FIG. 26 is a block diagram of a symbol timing recovery loop
block of the diagram of FIG. 16 according to an embodiment.
[0194] Symbol timing recovery loop block 2648 includes data matches
filter 2650 for receiving signal S(f.sub.s) from a mixer that mixes
an RF estimated signal and a delayed (or buffered) version of the
tag response signal. Data matched filter 2650 provides filtered
signal to digital rate converter 2640, which in turn provides a
derived tag response signal (recovered signal) TS to other circuits
such as one or more correlators of the reader. Digital rate
converter also provides the recovered signal to the symbol timing
recovery loop comprising (in addition to the digital rate
converter) timing error detector 2660, loop filter 2670, and
numerically controlled oscillator (NCO) 2680.
[0195] A phase detector part of the feedback loop comprising the
timing error detector 2660 and the loop filter 2670 enables digital
rate converter 2680 to perform conversion that peaks (corresponding
to correct timing) of the derived signal correspond to the peaks of
the actual tag signal and can be provided to other digital
circuitry within the reader as output.
[0196] NCO 2680 also receives an estimated frequency input
S.sub.Symbol.sub.--.sup.Freq from frequency estimator and provides
feedback phase estimation S.sub.Symbol.sub.--.sup.Ph to digital
rate converter 2640 for adjusting the sample the interpolation and
decimation such that the digital rate converter output samples are
phase and frequency synchronous with the received tag signal.
[0197] In such a system, timing recovery circuits may be
implemented fully in digital domain, and there is no need for an
analog PLL or Nth order non-linearity. Furthermore, ADC clock is
allowed to be a single fixed frequency clock unrelated to the
received signal frequency. ADC, digital processing, and back-end
interfaces may all operate from a single fixed clock. Thus, there
is no need for adjusting ADC sample clock frequency and phase based
on the received signal. The data matched filter may operate
synchronous to the received signal providing optimal
performance.
[0198] FIG. 27 is a block diagram of a symbol timing recovery loop
block of the diagram of FIG. 16 according to other embodiments.
[0199] Similar to the block diagram of FIG. 26, symbol timing
recovery loop block 2748 includes digital rate converter 2740,
which provides a derived tag response signal (recovered signal) to
data matched filter 2750 which in turn provide output signal TS to
other circuits such as one or more correlators of the reader. Data
matched filter 2750 also provides the recovered signal to the
symbol timing recovery loop comprising timing error detector 2760,
loop filter 2770, and numerically controlled oscillator (NCO)
2780.
[0200] Differently from the symbol timing recovery loop block 2648,
data matched filter 2750 is within the timing loop between digital
rate converter 2740 and timing error detector 2760 in the symbol
timing recovery loop block 2748 of FIG. 27. In other embodiments,
the symbol timing recovery loop block may be implemented without
using a data matched filter at all.
[0201] FIG. 28 is a more detailed block diagram 2804 for
implementing the symbol timing recovery loop block of the diagram
of FIG. 27 according to embodiments.
[0202] Digital rate converter 2840 may derive the recovered signal
employing various conversion approaches. According to one
embodiment, digital rate converter 2840 may perform conversion
based on time variant fractional adjustment.
[0203] Digital rate converter 2840 may perform interpolation across
an N point sample set followed by variable rate decimation and
generate derived samples in response to receiving ADC samples of
the tag response signal by fractional delay processing of these
samples.
[0204] Digital rate converter 2840 may include interpolating filter
2844, decimator 2846, and timing processor 2848. Interpolating
filter 2844 receives ADC samples S(f.sub.s) with a frequency
f.sub.s=1/T.sub.s and performs the interpolation. The interpolation
may be an exponential interpolation, Lagrange interpolation, and
the like. Interpolating filter 2844 also receives an input from
timing processor 2848 that includes time variant fractional delay
coefficient .mu..sub.k for adjusting sample peaks.
[0205] Decimator 2846 decimates an output of interpolating filter
2844 based on a control signal from timing processor 2848, which in
turn receives feedback from the NCO of the timing loop. Thus, the
rate converter decimates the interpolated data samples, with the
dynamic decimation rate changing with time, to produce an output
frequency synchronous with the received symbol rate.
[0206] Data matched filter 2850 may be implemented using filter
2850 and decimator 2854. An output of decimator 2854 is the
recovered tag response signal TS.
[0207] In a simple implementation, loop filter 2870 may include an
integrator 2872 that receives and output of the phase detector and
provides a control signal to the NCO of the timing recovery
circuit. The loop filter may include phase lead-lag compensation
and be of any order.
[0208] An NCO is a digital system that synthesizes a range of
frequencies through the accumulation of discrete input phase
increments. The output frequency range is controlled by the size of
the input phase increment and the fixed NCO clock frequency.
[0209] The NCO can also be implemented in a number of ways. One
example embodiment of NCO 2880 includes combiner 2886, which
receives an output of the loop filter, a symbol frequency
estimation signal S.sub.Symbol.sub.--.sup.Freq, and a feedback
signal from digital feedback register 2884. The combiner's output
is provided to amplifier 2882 with a preset gain g3. The NCO's
output is provided to the timing processor of the digital rate
converter such that it can control a decimator for decimating
selected samples and outputting those that correspond to received
sample values synchronous with the received tag signal.
[0210] The circuits described above illustrate an example
embodiment for using pilot tone based timing recovery from tag
signals. Other circuits may also be used without departing from a
scope and spirit of the invention.
[0211] FIG. 29 is a flowchart of a process for using a pilot tone
in processing RFID tag signals according to embodiments.
[0212] Process 2900 begins at optional operation 2910, where the
reader transmits a carrier wave with a command that instructs the
tags to respond with a pilot tone.
[0213] According to a next operation 2920, a backscattered tag
signal is received by the reader.
[0214] According to a next decision operation 2930, a determination
is made whether the command affects a tag memory such as a
Non-Volatile Memory (NVM). If the tag memory is affected,
processing advances to optional operation 2940. If the memory is
not affected processing proceeds to operation 2960.
[0215] At optional operation 2940, a delayed version of the
backscattered signal is generated.
[0216] At a next optional operation 2950, the backscattered pilot
tone in the tag response is detected by comparing the delayed
version of the signal to a baseline established from the received
signal.
[0217] At a next operation 2960, a frequency of the tag response is
estimated. The estimated frequency is used to seed a symbol timing
recovery loop.
[0218] At a next operation 2970, the delayed version of the
received signal is provided to the symbol timing recovery loop and
recovered tag signal TS generated using the recovered symbol
timing.
[0219] At a next operation 2980, the recovered tag signal is passed
through one or more correlators to detect the preamble of the tag
signal.
[0220] At a next operation 2990, the remainder of the received tag
signal is demodulated.
[0221] The operations included in process 2900 are for illustration
purposes. Demodulation of a tag response signal by symbol timing
recovery based on pilot tone detection may be implemented by
similar processes with fewer or additional steps, as well as in
different order of operations using the principles described
herein.
[0222] In this description, numerous details have been set forth in
order to provide a thorough understanding. In other instances,
well-known features have not been described in detail in order to
not obscure unnecessarily the description.
[0223] A person skilled in the art will be able to practice the
embodiments in view of this description, which is to be taken as a
whole. The specific embodiments as disclosed and illustrated herein
are not to be considered in a limiting sense. Indeed, it should be
readily apparent to those skilled in the art that what is described
herein may be modified in numerous ways. Such ways can include
equivalents to what is described herein.
[0224] The following claims define certain combinations and
sub-combinations of elements, features, steps, and/or functions,
which are regarded as novel and non-obvious. Additional claims for
other combinations and sub-combinations may be presented in this or
a related document.
* * * * *
References