USO0RE41471E
(19) United States (12) Reissued Patent
(10) Patent Number: US (45) Date of Reissued Patent:
Wood, Jr. (54)
METHOD OF ADDRESSING MESSAGES AND COMMUNICATIONS SYSTEM
(75) Inventor:
Clifton W. Wood, Jr., Tulsa, OK (US)
(73) Assignee: Round Rock Research, LLC, Mount Kisco, NY (US) (*)
Notice:
This patent is subject to a terminal dis claimer.
RE41,471 E *Aug. 3, 2010
OTHER PUBLICATIONS
Transaction History of related U.S. Appl. No. 09/026,043, ?led Feb. 19, 1998, entitled “Method of Addressing Mes sages and Communications System,” now US. Pat. No.
6,118,789.
(Continued) Primary ExamineriAjit Patel (74) Attorney, Agent, or FirmiGazdzinski & Associates, PC
(21) App1.No.: 11/862,121
(57)
(22) Filed:
(Under 37 CFR 1.47)
A method of establishing Wireless communications between an interrogator and individual ones of multiple wireless identi?cation devices, the method [comprising utilizing a
Related US. Patent Documents
tree search method to attempt to identify individual ones of the multiple Wireless identi?cation devices so as to be able to
Sep. 26, 2007
Reissue of:
perform communications, without collision, between the
(64) Patent No.: Issued: Appl. No.: Filed: (63)
6,282,186 Aug. 28, 2001 09/556,235 Apr. 24, 2000
interrogator and individual ones of the multiple Wireless identi?cation devices, a search tree being de?ned for the tree
search method, the tree having multiple nodes respectively representing subgroups of the multiple Wireless identi?ca
Continuation of application No. 10/652,573, ?led on Aug. 28, 2003, which is a continuation of application No. 09/026, 050, ?led on Feb. 19, 1998, now Pat. No. 6,061,344.
(51)
ABSTRACT
Int. Cl. H04L 1/00
tion devices, wherein the interrogator transmits a command at a node, requesting that devices within the subgroup repre
sented by the node respond, wherein the interrogator deter mines if a collision occurs in response to the command and,
(2006.01)
if not, repeats the command at the same node. An interroga tor con?gured to transmit a command at a node, requesting
(52)
US. Cl. ..................................................... .. 370/346
that devices within the subgroup represented by the node respond, the interrogator ?lrther being con?gured to deter
(58)
Field of Classi?cation Search ...................... .. None
mine if a collision occurs in response to the command and, if
See application ?le for complete search history.
not, to repeat the command at the same node] includes:
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set of bits, the interrogator to identi?) a first subgroup of a
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(Continued)
one or more received responses from respective ones of the one or more RFID devices; and responsive to receiving one of the one or more received responses without a collision,
FOREIGN PATENT DOCUMENTS EP
779520
retransmitting by the interrogator at least the first signal .
9/1997
133 Claims, 3 Drawing Sheets
(Continued)
2a
76\ RF/D CIRCUITRY
INTERROGA TOR j 7 4 L
26/
1 POWER
SOURCE
78/
US RE41,471 E Page 2
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i
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A A A A
A1 A1 A1 A1 A1 A1 A1 A1
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OTHER PUBLICATIONS _
_
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Messages and Communications System,” now US. Pat. No.
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Transaction History of related U.S. Appl. No. 10/693,696, ?led Oct. 23, 2003, entitled “Method and Apparatus to
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Select Radio Frequency Identi?cation Devices in Accor
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METHOD OF ADDRESSING MESSAGES AND COMMUNICATIONS SYSTEM
nals produced by an RF transmitter circuit. Because active
Matter enclosed in heavy brackets [ ] appears in the original patent but forms no part of this reissue speci?ca tion; matter printed in italics indicates the additions made by reissue.
power via magnetic coupling. Therefore, active transponder
devices have their own power sources, and do not need to be in close proximity to an interrogator or reader to receive
devices tend to be more suitable for applications requiring tracking of a tagged device that may not be in close proxim
ity to an interrogator. For example, active transponder devices tend to be more suitable for inventory control or
tracking.
RELA TED REISSUE APPLICATIONS
Electronic identi?cation systems can also be used for remote payment. For example, when a radio frequency iden
More than one reissue application has been ?led for the reissue of US. Pat. No. 6,282,186. The reissue applications are the initial reissue application Ser. No. 10/652,573 ?led Aug. 28, 2003, a continuation reissue application Ser. No. 11/862,130, ?led Sep. 26, 2007, a continuation reissue
ti?cation device passes an interrogator at a toll booth, the toll
booth can determine the identity of the radio frequency iden ti?cation device, and thus of the owner of the device, and debit an account held by the owner for payment of toll or can receive a credit card number against which the toll can be
application Ser. No. 11/862,124,?led Sep. 26, 2007, and the present continuation reissue application.
charged. Similarly, remote payment is possible for a variety
CROSS REFERENCE TO RELATED APPLICATION 20
This is a [Continuation] continuation application of a
responder station or transponder device which replies to the
reissue application Ser. No. 10/652,573, ?ledAug. 28, 2003,
interrogator.
which is a reissue application ofU.S. patent application Ser.
If the interrogator has prior knowledge of the identi?ca
No. 09/556,235, now US. Pat. No. 6,282,186, which is a
continuation application of US. patent application Ser. No. 09/026,050, ?led Feb. 19, 1998, now US. Pat. No. 6,061, 344 and titled “Method of Addressing Messages and Com
25
sions where the interrogator is attempting to determine 30
This invention relates to communications protocols and to digital data communications. Still more particularly, the invention relates to data communications protocols in medi
causing a collision, and thus causing an erroneous message 35
existence, location or movement of objects, or for remote
automated payment. 40
Communications protocols are used in various applica tions. For example, communications protocols can be used in electronic identi?cation systems. As large numbers of
objects are moved in inventory, product manufacturing, and
In one arbitration scheme or system, described in com
45
650; and 5,365,551, all to Snodgrass et al. and all incorpo rated herein by reference, the interrogator sends a command causing each device of a potentially large number of responding devices to select a random number from a known range and use it as that device’s arbitration number. By
50
?cation system. One presently available electronic identi?cation system utilizes a magnetic coupling system. In some cases, an iden
ti?cation device may be provided with a unique identi?ca tion code in order to distinguish between a number of differ
gets a large number of simultaneous replies, the interrogator may not be able to interpret any of these replies. Thus, arbi tration schemes are employed to permit communications free of collisions.
monly assigned US. Pat. Nos. 5,627,544; 5,583,850; 5,500,
merchandising operations, there is a continuous challenge to accurately monitor the location and ?ow of objects. Additionally, there is a continuing goal to interrogate the location of objects in an inexpensive and streamlined man ner. One way of tracking objects is with an electronic identi
to be received by the interrogator. For example, if the inter rogator sends out a command requesting that all devices within a communications range identify themselves, and
inventory control, object monitoring, determining the
BACKGROUND OF THE INVENTION
which of multiple devices are within communication range. When the interrogator sends a message to a transponder
device requesting a reply, there is a possibility that multiple transponder devices will attempt to respond simultaneously,
ums such as radio communication or the like. The invention
also relates to radio frequency identi?cation devices for
tion number of a device which the interrogator is looking for, it can specify that a response is requested only from the device with that identi?cation number. Sometimes, such information is not available. For example, there are occa
munications System”. TECHNICAL FIELD
of other goods or services. A communication system typically includes two tran sponders: a commander station or interrogator, and a
transmitting requests for identi?cation to various subsets of the full range of arbitration numbers, and checking for an error-free response, the interrogator determines the arbitra tion number of every responder station capable of communi cating at the same time. Therefore, the interrogator is able to
conduct subsequent uninterrupted communication with 55
devices, one at a time, by addressing only one device.
ent devices. Typically, the devices are entirely passive (have
Another arbitration scheme is referred to as the Aloha or
no power supply), which results in a small and portable
slotted Aloha scheme. This scheme is discussed in various references relating to communications, such as Digital Com
package. However, such identi?cation systems are only capable of operation over a relatively short range, limited by the siZe of a magnetic ?eld used to supply power to the devices and to communication with the devices. Another wireless electronic identi?cation system utilizes a large, board level, active transponder device ai?xed to an object to be monitored which receives a signal from an inter
60
rogator. The device receives the signal, then generates and transmits a responsive signal. The interrogation signal and
65
the responsive signal are typically radio-frequency (RF) sig
munications: Fundamentals and Application, Bernard Sklar, published January 1988 by Prentice Hall. In this type of scheme, a device will respond to an interrogator using one of
many time domain slots selected randomly by the device. A problem with the Aloha scheme is that if there are many devices, or potentially many devices in the ?eld (i.e. in com
munications range, capable of responding) then there must be many available slots or many collisions will occur. Hav
ing many available slots slows down replies. If the magni
US RE41,471 E 3
4
tude of the number of devices in a ?eld is unknown, then many slots are needed. This results in the system slowing
transponder Which relies on magnetic coupling for poWer and therefore has a much greater range.
doWn signi?cantly because the reply time equals the number of slots multiplied by the time period required for one reply.
BRIEF DESCRIPTION OF THE DRAWINGS
An electronic identi?cation system Which can be used as a
Preferred embodiments of the invention are described
radio frequency identi?cation device, arbitration schemes,
beloW With reference to the folloWing accompanying draW
and various applications for such devices are described in
ings.
detail in commonly assigned US. [patent application Ser. No. 08/705,043, ?led Aug. 29, 1996, and] Pat. No. 6,130,
FIG. 1 is a high level circuit schematic shoWing an inter
rogator and a radio frequency identi?cation device embody ing the invention.
602, which is incorporated herein by reference. SUMMARY OF THE INVENTION
The invention provides a Wireless identi?cation device
con?gured to provide a signal to identify the device in response to an interrogation signal. In one aspect, a method includes: transmitting by an
interrogator a ?rst signal including a ?rst set of bits, the interrogator to identify a first subgroup of a group ofpos sible random numbers; communicating by each of one or more RFID devices a first response
20
the one or more RFID
devices has generated a random number that is included in the first subgroup; receiving by the interrogator one or more received responses from respective ones of the one or more RFID devices; and responsive to receiving one of the one or more received responses without a collision, retransmitting
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
by the interrogator at least the?rst signal. One aspect of the invention provides a method of estab lishing Wireless communications betWeen an interrogator and individual ones of multiple Wireless identi?cation devices. The method comprises utiliZing a tree search method to attempt to identify individual ones of the multiple
This disclosure of the invention is submitted in further ance of the constitutional purposes of the US. Patent LaWs 30
“to promote the progress of science and useful arts” (Article
1, Section 8). FIG. 1 illustrates a Wireless identi?cation device 12 in accordance With one embodiment of the invention. In the
Wireless identi?cation devices so as to be able to perform
communications, Without collision, betWeen the interrogator and individual ones of the multiple Wireless identi?cation devices. A search tree is de?ned for the tree search method.
FIG. 2 is a front vieW of a housing, in the form of a badge or card, supporting the circuit of FIG. 1 according to one embodiment the invention. FIG. 3 is a front vieW of a housing supporting the circuit of FIG. 1 according to another embodiment of the invention. FIG. 4 is a diagram illustrating a tree splitting sort method for establishing communication With a radio frequency iden ti?cation device in a ?eld of a plurality of such devices. FIG. 5 is a diagram illustrating a modi?ed tree splitting sort method for establishing communication With a radio frequency identi?cation device in a ?eld of a plurality of such devices.
35
illustrated embodiment, the Wireless identi?cation device is a radio frequency data communication device 12, and includes RFID circuitry 16. The device 12 further includes at
The tree has multiple nodes respectively representing sub
least one antenna 14 connected to the circuitry 16 for Wire
groups of the multiple Wireless identi?cation devices. The interrogator transmits a command at a node, requesting that
less or radio frequency transmission and reception by the circuitry 16. In the illustrated embodiment, the RFID cir cuitry is de?ned by an integrated circuit as described in the
devices Within the subgroup represented by the node
40
above-incorporated [patent application Ser. No. 08/705,043,
respond. The interrogator determines if a collision occurs in response to the command and, if not, repeats the command at
?led Aug. 29, 1996] US. Pat. No. 6,130,602. Other embodi
the same node.
ments are possible. A poWer source or supply 18 is con
Another aspect of the invention provides a communica tions system comprising an interrogator, and a plurality of Wireless identi?cation devices con?gured to communicate With the interrogator in a Wireless fashion. The interrogator
45
nected to the integrated circuit 16 to supply poWer to the integrated circuit 16. In one embodiment, the poWer source
18 comprises a battery. The device 12 transmits and receives radio frequency
is con?gured to employ tree searching to attempt to identify
communications to and from an interrogator 26. An exem
individual ones of the multiple Wireless identi?cation devices, so as to be able to perform communications Without collision, betWeen the interrogator and individual ones of the
plary interrogator is described in commonly assigned US. [patent application Ser. No. 08/907,689, ?led Aug. 8, 1997]
50
Pat. No. 6,289,209 and incorporated herein by reference.
multiple Wireless identi?cation devices. The interrogator is con?gured to folloW a search tree, the tree having multiple
Preferably, the interrogator 26 includes an antenna 28, as
Well as dedicated transmitting and receiving circuitry, simi
nodes respectively representing subgroups of the multiple Wireless identi?cation devices. The interrogator is con?g
lar to that implemented on the integrated circuit 16. 55
Generally, the interrogator 26 transmits an interrogation
ured to transmit a command at a node, requesting that
signal or command 27 via the antenna 28. The device 12
devices Within the subgroup represented by the node respond. The interrogator is further con?gured to determine
receives the incoming interrogation signal via its antenna 14.
if a collision occurs in response to the command and, if not, to repeat the command at the same node.
Upon receiving the signal 27, the device 12 responds by 60
generating and transmitting a responsive signal or reply 29. The responsive signal 29 typically includes information that
One aspect of the invention provides a radio frequency identi?cation device comprising an integrated circuit includ
uniquely identi?es, or labels the particular device 12 that is
ing a receiver, a transmitter, and a microprocessor. In one
Which the device 12 is associated. Although only one device 12 is shoWn in FIG. 1, typically there Will be multiple
transmitting, so as to identify any object or person With
embodiment, the integrated circuit is a monolithic single die
single metal layer integrated circuit including the receiver,
65
devices 12 that correspond With the interrogator 16, and the
the transmitter, and the microprocessor. The device of this
particular devices 12 that are in communication With the
embodiment includes an active transponder, instead of a
interrogator 26 Will typically change over time. In the illus
US RE41,471 E 5
6
trated embodiment in FIG. 1, there is no communication
ing the identi?cation numbers of the devices 12 in the ?eld.
between multiple devices 12. Instead, the devices 12 respec
After this is accomplished, point-to-point communication
tively communicate With the interrogator 26. Multiple
can proceed as desired by the interrogator 26. Generally speaking, RFID systems are a type of multi
devices 12 can be used in the same ?eld of an interrogator 26
access communication system. The distance betWeen the
(i.e., Within communications range of an interrogator 26).
interrogator 26 and devices 12 Within the ?eld is typically
The radio frequency data communication device 12 can be
fairly short (e.g., several meters), so packet transmission time is determined primarily by packet siZe and baud rate. Propagation delays are negligible. In such systems, there is a potential for a large number of transmitting devices 12 and
included in any appropriate housing or packaging. Various methods of manufacturing housings are described in com
monly assigned U.S. [patent application Ser. No. 08/800, 037, ?led Feb. 13, 1997, and] Pat. No. 5,988,510 which is
there is a need for the interrogator 26 to Work in a changing environment, Where different devices 12 are sWapped in and
incorporated herein by reference. FIG. 2 shoWs but one embodiment in the form of a card or
out frequently (e.g., as inventory is added or removed). In
badge 19 including a housing 11 of plastic or other suitable material supporting the device 12 and the poWer supply 18. In one embodiment, the front face of the badge has visual identi?cation features such as interrogators, text, informa
such systems, the inventors have determined that the use of random access methods Work effectively for contention
resolution (i.e., for dealing With collisions betWeen devices 12 attempting to respond to the interrogator 26 at the same
tion found on identi?cation or credit cards, etc.
time).
FIG. 3 illustrates but one alternative housing supporting the device 12. More particularly, FIG. 3 shoWs a miniature
housing 20 encasing the device 12 and poWer supply 18 to de?ne a tag Which can be supported by an object (e. g., hung from an object, af?xed to an object, etc.). Although tWo par ticular types of housings have been disclosed, other forms of housings are employed in alternative embodiments. If the poWer supply 18 is a battery, the battery can take any suitable form. Preferably, the battery type Will be selected depending on Weight, siZe, and life requirements for a par ticular application. In one embodiment, the battery 18 is a thin pro?le button-type cell forming a small, thin energy cell more commonly utiliZed in Watches and small electronic
RFID systems have some characteristics that are different
25
30
from other communications systems. For example, one char acteristic of the illustrated RFID systems is that the devices 12 never communicate Without being prompted by the inter rogator 26. This is in contrast to typical multiaccess systems Where the transmitting units operate more independently. In addition, contention for the communication medium is short lived as compared to the ongoing nature of the problem in other multiaccess systems. For example, in a RFID system, after the devices 12 have been identi?ed, the interrogator can communicate With them in a point-to-point fashion. Thus, arbitration in a RFID system is a transient rather than steady state phenomenon. Further, the capability of a device 12 is
devices requiring a thin pro?le. A conventional button-type
limited by practical restrictions on siZe, poWer, and cost. The
cell has a pair of electrodes, an anode formed by one face and a cathode formed by an opposite face. In an alternative embodiment, the poWer source 18 comprises a series con
lifetime of a device 12 can often be measured in terms of
nected pair of button type cells. In other alternative
35
embodiments, other types of suitable poWer source are
arbitrate a set of devices 12. Another measure is poWer con
employed.
sumed by the devices 12 during the process. This is in con trast to the measures of throughput and packet delay in other
The circuitry 16 further includes a backscatter transmitter
and is con?gured to provide a responsive signal to the inter
rogator 26 by radio frequency. More particularly, the cir
number of transmission before battery poWer is lost. Therefore, one of the most important measures of system performance in RFID arbitration is total time required to
40
types of multiaccess systems. FIG. 4 illustrates one arbitration scheme that can be
cuitry 16 includes a transmitter, a receiver, and memory such
employed for communication betWeen the interrogator and
as is described in Us. [patent application Ser. No. 08/705, 043] Pat. No. 6,130, 602.
devices 12. Generally, the interrogator 26 sends a command causing each device 12 of a potentially large number of
Radio frequency identi?cation has emerged as a viable and affordable alternative to tagging or labeling small to
45
responding devices 12 to select a random number from a knoWn range and use it as that device’s arbitration number.
large quantities of items. The interrogator 26 communicates
By transmitting requests for identi?cation to various subsets
With the devices 12 via an electromagnetic link, such as via an RF link (e.g., at microWave frequencies, in one embodiment), so all transmissions by the interrogator 26 are
of the full range of arbitration numbers, and checking for an error-free response, the interrogator 26 determines the arbi tration number of every responder station capable of com municating at the same time. Therefore, the interrogator 26 is able to conduct subsequent unterrupted communication With devices 12, one at a time, by addressing only one device
50
heard simultaneously by all devices 12 Within range. If the interrogator 26 sends out a command requesting that all devices 12 Within range identify themselves, and gets a
large number of simultaneous replies, the interrogator 26 may not be able to interpret any of these replies. Therefore, arbitration schemes are provided. If the interrogator 26 has prior knoWledge of the identi?
55
12. Three variables are used: an arbitration value (AVALUE), an arbitration mask (AMASK), and a random value ID (RV).
The interrogator sends an Identify command
cation number of a device 12 Which the interrogator 26 is
(IdentifyCmnd) causing each device of a potentially large
looking for, it can specify that a response is requested only
number of responding devices to select a random number
from the device 12 With that identi?cation number. To target a command at a speci?c device 12, (i.e., to initiate point-on point communication), the interrogator 26 must send a num ber identifying a speci?c device 12 along With the command.
60
devices 12. The receiving devices 12 evaluate the folloWing
equation: (AMASK & AVALUE)==(AMASK & RV)
At start-up, or in a neW or changing environment, these iden
ti?cation numbers are not knoWn by the interrogator 26.
Therefore, the interrogator 26 must identify all devices 12 in the ?eld (Within communication range) such as by determin
from a knoWn range and use it as that device’s arbitration
number. The interrogator sends an arbitration value (AVALUE) and an arbitration mask (AMASK) to a set of
65
Wherein “&” is a bitWise AND function, and Wherein “==” is an equality function. If the equation evaluates to “l”
(TRUE), then the device 12 Will reply. If the equation evalu
US RE41,471 E 8
7
The device 12 With the random value of 1010 receives a
ates to “0” (FALSE), then the device 12 Will not reply. By performing this in a structured manner, With the number of bits in the arbitration mask being increased by one each time, eventually a device 12 Will respond With no collisions.
command and evaluates the equation (AMASK & AVALUE)==(AMASK & RV). The left side of the equation is evaluated as (0011 & 0010)=0010. The right side of the equation is evaluated as (0011 & 1010)=0010. The right side
Thus, a binary search tree methodology is employed. An example using actual numbers Will noW be provided using only four bits, for simplicity, reference being made to
With the random value of 1010. Because there are no other
FIG. 4. In one embodiment, sixteen bits are used for AVALUE and AMASK. Other numbers of bits can also be
device 12 With the random value of 1010. There is no
equals the left side, so the equation is true for the device 12
devices 12 in the subtree, a good reply is returned by the
employed depending, for example, on the number of devices
collision, and the interrogator 26 can determine the identity (e.g., an identi?cation number) for the device 12 that does
12 expected to be encountered in a particular application, on desired cost points, etc. Assume, for this example, that there are tWo devices 12 in the ?eld, one With a random value RV) of 1100 (binary), and another With a random value (RV) of 1010 (binary). The
respond.
interrogator is tying to establish communications Without collisions being caused by the tWo devices 12 attempting to communicate at the same time.
The interrogator sets AVALUE to 0000 (or “don’t care” for all bits, as indicated by the character “X” in FIG. 4) and AMASK to 0000. The interrogator transmits a command to
20
By recursion, What is meant is that a function makes a call to itself. In other Words, the function calls itself Within the body of the function. After the called function returns, de-recursion takes place and execution continues at the place just after the function call; ie at the beginning of the state ment after the function call. For instance, consider a function that has four statements (numbered 1,2,3,4) in it, and the second statement is a recur
all devices 12 requesting that they identify themselves. Each
sive call. Assume that the fourth statement is a return state
of the devices 12 evaluate (AMASK & AVALUE ==
ment. The ?rst time through the loop (iteration 1) the func
(AMASK & RV) using the random value RV that the respec
tion executes the statement 2 and (because it is a recursive call) calls itself causing iteration 2 to occur. When iteration 2
tive devices 12 selected. If the equation evaluates to “1”
(TRUE), then the device 12 Will reply. If the equation evalu
25
ates to “0” (FALSE), then the device 12 Will not reply. In the ?rst level of the illustrated tree, AMASK is 0000 and any thing bitWise ANDed With all Zeros results in all Zeros, so both the devices 12 in the ?eld respond, and there is a colli s1on.
execution in iteration 3 of statement 1, assume that the func tion does a return. The information that Was saved on the
stack from iteration 2 is loaded and the function resumes 30
Next, the interro gator sets AMASK to 0001 and AVALUE to 0000 and transmits an Identify command. Both devices 12 in the ?eld have a Zero for their least signi?cant bit, and
execution at statement 3 (in iteration 2), folloWed by the execution of statement 4 Which is also a return statement. Since there are no more statements in the function, the func
tion de-recurses to iteration 1. Iteration 1, had previously recursively called itself in statement 2. Therefore, it noW executes statement 3 (in iteration 1). Following that it
(AMASK & AVALUE)==(AMASK & RV) Will be true for both devices 12. For the device 12 With a random value of
gets to statement 2, it calls itself making iteration 3. During
35
1100, the left side of the equation is evaluated as folloWs
executes a return at statement 4. Recursion is knoWn in the
(0001 & 0000)=0000.
art.
The right side is evaluated as (0001 & 1100)=0000. The left side equals the right side, so the equation is true for the device 12 With the random value of 1100. For the device 12 With a random value of 1010, the left side of the equation is evaluated as (0001 & 0000)=0000. The right side is evalu
40
ated as (0001 & 1010)=0000. The left side equals the right side, so the equation is true for the device 12 With the ran dom value of 1010. Because the equation is true for both
Consider the folloWing code Which can be used to imple ment operation of the method shoWn in FIG. 4 and described above.
Arbitrate(AMASK, AVALUE) collision=IdentifyCmnd(AMASK, AVALUE) if (collision) then
45
devices 12 in the ?eld, both devices 12 in the ?eld respond, and there is another collision. Recursively, the interrogator next sets AMASK to 0011
/* recursive call for left side */ Arbitrate
((AMAS K<<1)+1, AVALUE) /* recursive call for right side */ Arbitrate
With AVALUE still at 0000 and transmits an Identify com
mand. (AMASK & AVALUE)==(AMASK & RV) is evalu
50
ated for both devices 12. For the device 12 With a random
((AMAS K<<1)+1, AVALUE+(AMASK+1)) } /* endif */ }/* return */
value of 1100, the left side of the equation is evaluated as folloWs (0011 & 0000)=0000. The right side is evaluated as
(0011 & 1100)=0000. The left side equals the right side, so the equation is true for the device 12 With the random value of 1100, so this device 12 responds. For the device 12 With a random value of 1010, the left side of the equation is evalu ated as (0011 & 0000)=0000. The right side is evaluated as
55
(0011 & 1010)=0010. The left side does not equal the right side, so the equation is false for the device 12 With the ran dom value of 1010, and this device 12 does not respond; Therefore, there is no collision, and the interrogator can determine the identity (e. g., an identi?cation number) for the device 12 that does respond. De-recursion takes place, and the devices 12 to the right for the same AMASK level are accessed When AVALUE is set at 0010, and AMASK is set to 0011.
60
The symbol “<<” represents a bitWise left shift. “<<” means shift left by one place. Thus, 0001<<1 Would be 0010. Note, hoWever, that AMASK is originally called With a value of Zero, and 0000<<1 is still 0000. Therefore, for the ?rst
recursive call, AMASK=(AMASK<<1)+1. So for the ?rst recursive call, the value of AMASK is 0000+0001=0001. For the second call, AMASK=(0001<<)+1=0010+1=0011. For the third recursive call, AMASK=(0011<<1)+1=0110+ 1 =01 1 1 .
65
The routine generates values for AMASK and AVALUE to be used by the interrogator in an Identify command “Iden tifyCmnd.” Note that the routine calls itself if there is a collision. De-recursion occurs When there is no collision. AVALUE and AMASK Would have values such as the fol
US RE41,471 E 9
10
lowing assuming collisions take place all the Way doWn to the bottom of the tree.
5 AVALUE
AMASK
0000 0000 0000 0000 0000 1000 0100 0100 1100
0000 0001 0011 0111 1111* 1111* 0111 1111* 1111*
10
15
This sequence of AMASK, AVALUE binary numbers
AVALUE
AMASK
0000 0000
0000 0001
0001 0000 0010 0001 0011 0000 0100
0001 0011 0011 0011 0011 0111 0111
RoWs in the table for Which the interrogator is successful in receiving a reply Without collision are marked With the
assumes that there are collisions all the Way doWn to the
SymbO1“*”_ FIG. 5 illustrates an embodiment Wherein the interrogator 26 retries on the same node that yielded a good reply. The occurs. RoWs in the table for Which the interrogator is suc cessful in receiving a reply Without collision are marked 20 search tree has a plurality of nodes 51, 52, 53, 54 etc. at
bottom of the tree, at Which point the Identify command sent by the interrogator is ?nally successful so that no collision
respective levels 32, 34, 36, 38, or 40. The siZe of subgroups
With the symbol “*”. Note that if the Identify command Was successful at, for example, the third line in the table then the interrogator Would stop going doWn that branch of the tree and start doWn another, so the sequence Would be as shoWn in the folloWing table.
of random Values decrease In SlZe by half Wlth each node
descended The interrogator performs a tree search, either depth-?rst 25 or breadth-?rst in a manner such as that described in connec
tion With FIG. 4, except that if the interrogator determines that no collision occurred in response to an Identify
command, the interrogator repeats the command at the same node. This takes advantage of an inherent capability of the 30 devices, particularly if the devices use backscatter
AVALUE 0000
AMASK 0000
0000
0001
communication, called self-arbitration. Arbitration times can
88%
885*
be reduced, and battery life for the devices can be increased. 35
This method is referred to as a splitting method. It Works
When a single reply is read by the interrogator, for example, in node 52, the method described in connection With FIG. 4 Would involve proceeding to node 53 and then
sending another Identify command. Because a device 12 in a ?eld of devices 12 can override Weaker devices, this embodi are resolved in turn. The splitting method can also be vieWed ment is modi?ed such that the interrogator retries on the as a type of tree search. Each split moves the method one same node 52 after silencing the device 12 that gave the level deeper in the tree. Either depth-?rst or breadth-?rst 40
by splitting groups of colliding devices 12 into subsets that
good reply. This, after receiving a good reply from node 52,
traversals of the tree can be employed. Depth ?rst traversals
the interrogator remains on node 52 and reissues the Identify command after silencing the device that ?rst responded on
are performed by using recursion, as is employed in the code listed above. Breadth-?rst traversals are accomplished by using a queue instead Of recursion
node 52. Repeating the Identify command on the same node Either depth-?rst or breadth-?rst traversals of the tree can 45 o?én ylelds other good rephes, thus takmg advantage of the be employed. Depth ?rst traversals are performed by using devlces natural ablhty to self'arbltrate
recursion, as is employed in the code listed above. BreadthAVALUE and AMASK WOuld have Values Such as the ?rst traversals are accomplished by using a queue instead of folloWing for a depth-?rst traversal in a situation similar to recursion. The folloWing is an example of code for performthe one described above in connection With FIG. 4. ing a breadth-?rst traversal. 50 AVALUE
AMASK
0000
0000
0000 0000 0000
0001 0011
0000 1000
1111* 1111*
Arbitrate(AMASK, AVALUE) (AMASK,AVALTE)=dequeue( ) collision=IdentifyCmnd(AMASK, AVALUE) if (collision) then
55
TEMP =AMASK+1 NEWEAMASK = (AMASK<<1)+1
1111*
1000
1111*
0100 0100 0100
01 11 11 11* 1111*
endWhile
1100
1111*
}/* return */
1100
1111*
enqueue(NEWiAMASK, AVALUE) enqueue(NEWiAMASK, AVALUE+TEMP) }/* endif*/
60
The symbol “ ! =” means not equal to, AVALUE and 65 RoWs in the table for Which the interrogator is successful AMASK Would have values such as those indicated in the in receiving a reply Without collision are marked With the
folloWing table for such code.
symbol “*”.
US RE41,471 E 11
12 are retransmitted by the colliding users after a random delay. If the colliding users attempted to retransmit Without the
In operation, the interrogator transmits a command at a
node, requesting that devices Within the subgroup repre sented by the node respond. The interrogator determines if a
random delay, they Would collide again. If the user does not receive either an acknoWledgment or a negative acknoWl edgment Within a certain amount of time, the user “times out” and retransmits the message. There is a scheme knoWn as slotted Aloha Which improves
collision occurs in response to the command and, if not, repeats the command at the same node. In one alternative embodiment, the upper bound of the
number of devices in the ?eld (the maximum possible num ber of devices that could communicate With the interrogator) is determined, and the tree search method is started at a level 32, 34, 36, 38 or 40 in the tree depending on the determined upper bound. The level of the search tree on Which to start the tree search is selected based on the determined maxi mum possible number of Wireless identi?cation devices that
could communicate With the interrogator. The tree search is started at a level determined by taking the base tWo loga rithm of the determined maximum possible number. More particularly, the tree search is started at a level determined by taking the base tWo logarithm of the poWer of tWo nearest the determined maximum possible number of devices 12. The level of the tree containing all subgroups of random values is considered level Zero, and loWer levels are numbered 1, 2, 3,
10
the Aloha scheme by requiring a small amount of coordina tion among stations. In the slotted Aloha scheme, a sequence of coordination pulses is broadcast to all stations (devices). As is the case With the pure Aloha scheme, packet lengths are constant. Messages are required to be sent in a slot time
betWeen synchronization pulses, and can be started only at
20
the beginning of a time slot. This reduces the rate of colli sions because only messages transmitted in the same slot can interfere With one another. The retransmission mode of the pure 11 Aloha scheme is modi?ed for slotted Aloha such that if a negative acknoWledgment occurs, the device retransmits after a random delay of an integer number of slot times. Aloha methods are described in a commonly assigned
[patent application (attorney docket MI40-089)] US. Pat.
4, etc. consecutively.
No. 6,275,476, naming Clifton W. Wood, Jr. as an inventor,
Methods involving determining the upper bound on a set of devices and starting at a level in the tree depending on the determined upper bound are described in a commonly
titled “Method of Addressing Messages and Communica 25
tions System,” [?led concurrently hereWith, and] which is incorporated herein by reference.
assigned [patent application (attorney docket MI40-ll8)]
In one alternative embodiment, an Aloha method (such as
US. Pat. No. 6,118,789, naming Clifton W. Wood, Jr. as an
the method described in the commonly assigned patent
inventor, titled “Method of Addressing Messages and Com
munications System,” [?led concurrently hereWith, and] which is incorporated herein by reference.
30
In one alternative embodiment, a method involving start ing at a level in the tree depending on a determined upper
bound (such as the method described in the commonly
assigned patent application mentioned above) is combined With a method comprising re-trying on the same node that gave a good reply, such as the method shoWn and described in connection With FIG. 5. Another arbitration method that can be employed is referred to as the “Aloha” method. In the Aloha method, every time a device 12 is involved in a collision, it Waits a
40
When a negative acknoWledgment is received, the messages
unique identi?cation numbers are unknoWn. Level skipping reduces the number of collisions, both reducing arbitration time and conserving battery life on a set of devices 12. In one embodiment, every other level is skipped. In alternative embodiments, more than one level is skipped each time. The trade off that must be considered in determining hoW
45
many (if any) levels to skip With each decent doWn the tree is as folloWs. Skipping levels reduces the number of collisions,
thus saving battery poWer in the devices 12. Skipping deeper (skipping more than one level) further reduces the number of collisions. The more levels that are skipped, the greater the 50
reduction in collisions. HoWever, skipping levels results in longer search times because the number of queries (Identify commands) increases. The more levels that are skipped, the longer the search times. Skipping just one level has an
almost negligible effect on search time, but drastically 55
reduces the number of collisions. If more than one level is
skipped, search time increases substantially. Skipping every other level drastically reduces the number of collisions and saves battery poWer Without signi?cantly increasing the
number of queries. 60
Level skipping methods are described in a commonly
assigned [patent application (attorney docket MI40-ll7)] US. Pat. No. 6,072,801, naming Clifton W. Wood, Jr. and Don Hush as inventors, titled “Method of Addressing
ent users Will sometimes overlap in time (collide), causing reception errors in the data in each of the contending mes sages. The errors are detected by the receiver, and the receiver sends a negative acknoWledgment to the users.
by multiple devices 12 responding, reduces the number of subsequent collisions Without adding signi?cantly to the number of no replies. In real-time systems, it is desirable to have quick arbitration sessions on a set of devices 12 Whose
University of HaWaii. In 1971, the University of HaWaii began operation of a system named Aloha. A communication satellite Was used to interconnect several university comput ers by use of a random access protocol. The system operates as folloWs. Users or devices transmit at any time they desire. After transmitting, a user listens for an acknowledgment from the receiver or interrogator. Transmissions from differ
such as the method shoWn and described in connection With FIG. 5. In another embodiment, levels of the search tree are
skipped. Skipping levels in the tree, after a collision caused 35
random period of time before retransmitting. This method can be improved by dividing time into equally siZed slots and forcing transmissions to be aligned With one of these slots. This is referred to as “slotted Aloha.” In operation, the interrogator asks all devices 12 in the ?eld to transmit their identi?cation numbers in the next time slot. If the response is garbled, the interrogator informs the devices 12 that a colli sion has occurred, and the slotted Aloha scheme is put into action. This means that each device 12 in the ?eld responds Within an arbitrary slot determined by a randomly selected value. In other Words, in each successive time slot, the devices 12 decide to transmit their identi?cation number With a certain probability. The Aloha method is based on a system operated by the
application mentioned above) is combined With a method involving re-trying on the same node that gave a good reply,
65
Messages, Method of Establishing Wireless Communications, and Communications System,” [?led con currently hereWith, and] which is incorporated herein by ref erence.
US RE41,471 E 14
13
receiving the communication at multiple devices, devices
In one alternative embodiment, a level skipping method is combined With a method involving re-trying on the same node that gave a good reply, such as the method shoWn and described in connection With FIG. 5. In yet another alternative embodiment, any tWo or more of
receiving the communication respectively determining if the random value chosen by the device falls Within the ?rst speci?ed group and, if so, sending a reply to the
interrogator; and
the methods described in the commonly assigned, concur rently ?led, applications mentioned above are combined. In compliance With the statute, the invention has been
determining using the interrogator if a collision occurred betWeen devices that sent a reply and, if so, creating a
second speci?ed group smaller than the ?rst speci?ed group; and, if not, again transmitting a communication requesting devices having random values Within the ?rst speci?ed group of random values to respond.] [8. A method of addressing messages from an interrogator
described in language more or less speci?c as to structural
and methodical features. It is to be understood, hoWever, that the invention is not limited to the speci?c features shoWn and described, since the means herein disclosed comprise pre ferred forms of putting the invention into effect. The inven tion is, therefore, claimed in any of its forms or modi?ca tions Within the proper scope of the appended claims appropriately interpreted in accordance With the doctrine of
to a selected one or more of a number of communications
devices in accordance With claim 7 Wherein sending a reply
to the interrogator comprises transmitting the unique identi ?cation number of the device sending the reply.]
equivalents. What is claimed is:
[1. A method of establishing Wireless communications betWeen an interrogator and Wireless identi?cation devices, the method comprising utiliZing a tree search technique to establish communications, Without collision, betWeen the interrogator and individual ones of the multiple Wireless identi?cation devices, the method including using a search
tree having multiple nodes respectively representing sub
20
interrogator] 25
groups of the multiple Wireless identi?cation devices, the method further comprising, for a node, transmitting a
comprising, if a collision occurred in response to the ?rst mentioned command, sending a command at a different
node, using the interrogator] [3. A method in accordance With claim 1 Wherein When a subgroup contains both a device that is Within communica tions range of the interrogator, and a device that is not Within communications range of the interrogator, the device that is not Within communications range of the interrogator does
30
devices, the method comprising: establishing unique identi?cation numbers for respective 35
respective devices choose random values indepen dently of random values selected by the other devices; 40
45
transmitting a communication from the transponder requesting devices having random values Within a speci?ed group of a plurality of possible groups of ran dom values to respond, the plurality of possible groups being organiZed in a binary tree de?ned by a plurality
of nodes at respective levels, the speci?ed group being de?ned as being at one of the nodes;
receiving the communication at multiple devices, devices
receiving the communication respectively determining
the command.] [5. A method in accordance With claim 1 Wherein a device
including a receiver, a modulator, and a microprocessor in
devices; causing the devices to select random values, Wherein
Within communications range of the interrogator responds to
in a subgroup changes betWeen being Within communica tions range of the interrogator and not being Within interro gators range, over time.] [6. A method in accordance With claim 1 Wherein the Wireless identi?cation device comprises an integrated circuit
devices in accordance With claim 7 Wherein, after receiving a reply Without collision from a device, the interrogator sends a communication individually addressed to that device.] [11. A method of addressing messages from a transponder to a selected one or more of a number of communications
not respond to the command.] [4. A method in accordance With claim 1 Wherein When a subgroup contains both a device that is Within communica tions range of the interrogator, and a device that is not Within communications range of the interrogator, the device that is
[10. A method of addressing messages from an interroga tor to a selected one or more of a number of communications
command, using the interrogator, requesting that devices Within the subgroup represented by the node respond, deter mining With the interrogator if a collision occurred in response to the command and, if not, repeating the command at the same node.] [2. A method in accordance With claim 1 and further
[9. A method in accordance With claim 7 Wherein one of the ?rst and second speci?ed groups contains both a device that is Within communications range of the interrogator, and a device that is not Within communications range of the interrogator, and Wherein the device that is not Within com munications range of the interrogator does not respond to the
50
if the random value chosen by the device falls Within the speci?ed group and, if so, sending a reply to the transponder; and, if not, not sending a reply; and determining using the transponder if a collision occurred betWeen devices that sent a reply and, if so, creating a
neW, smaller, speci?ed group by descending in the tree; 55
and, if not, transmitting a communication at the same
communication With the receiver and modulator] [7. A method of addressing messages from an interrogator
[12. A method of addressing messages from a transponder
to a selected one or more of a number of communications
to a selected one or more of a number of communications
devices, the method comprising: establishing for respective devices unique identi?cation
node.]
60
numbers; causing the devices to select random values, Wherein
respective devices choose random values indepen dently of random values selected by the other devices; transmitting a communication, from the interrogator, requesting devices having random values Within a ?rst speci?ed group of random values to respond;
devices in accordance With claim 11 Wherein establishing unique identi?cation numbers for respective devices com prises establishing a predetermined number of bits to be used for the unique identi?cation numbers [13. A method of addressing messages from a transponder to a selected one or more of a number of communications
65
devices in accordance With claim 12 and further including establishing a predetermined number of bits to be used for
the random values.]
US RE41,471 E 15
16
[14. A method of addressing messages from an interroga
[19. A method of addressing messages from an interroga
tor to a selected one or more of a number of RFID devices,
tor to a selected one or more of a number of RFID devices in
the method comprising: establishing for respective devices unique identi?cation
accordance With claim 14 and further comprising, after the
interrogator transmits a command requesting devices having random values Within the neW speci?ed group of random
numbers;
values to respond;
causing the devices to select random values, Wherein
devices receiving the command respectively determining
respective devices choose random values indepen dently of random values selected by the other devices; transmitting a command using the interrogator requesting
if their chosen random values fall Within the neW
smaller speci?ed group and, if so, sending a reply to the
interrogator]
devices having random values Within a speci?ed group of a plurality of possible groups of random values to respond, the speci?ed group being equal to or less than the entire set of random values, the plurality of possible groups being organiZed in a binary tree de?ned by a
plurality of nodes at respective levels; receiving the command at multiple RFID devices, RFID devices receiving the command respectively determin ing if their chosen random values fall Within the speci ?ed group and, only if so, sending a reply to the interro gator, Wherein sending a reply to the interro gator comprises transmitting the unique identi?cation num ber of the device sending the reply; determining using the interrogator if a collision occurred betWeen devices that sent a reply and, if so, creating a neW, smaller, speci?ed group using a different level of
the tree, the interrogator transmitting a command requesting devices having random values Within the neW speci?ed group of random values to respond; and, if not, the interrogator re-transmitting a command requesting devices having random values Within the ?rst mentioned speci?ed group of random values to
[20. A method of addressing messages from an interroga tor to a selected one or more of a number of RFID devices in
accordance With claim 19 and further comprising, after the
interrogator transmits a command requesting devices having random values Within the neW speci?ed group of random
values to respond; determining if a collision occurred betWeen devices that sent a reply and, if so, creating a neW speci?ed group 20
25
identi?cation devices, so as to be able to perform communi 30
groups of the multiple Wireless identi?cation devices, the interrogator being con?gured to transmit a command at a
if a reply Without collision is received from a device, the 35
node, requesting that devices Within the subgroup repre sented by the node respond, the interrogator further being con?gured to determine if a collision occurs in response to
the command and, if not, to repeat the command at the same
tor to a selected one or more of a number of RFID devices in
node.]
accordance With claim 14 Wherein the ?rst mentioned speci ?ed group contains both a device that is Within communica tions range of the interrogator, and a device that is not Within
cations Without collision betWeen the interrogator and indi vidual ones of the multiple Wireless identi?cation devices, the interrogator being con?gured to folloW a search tree, the
tree having multiple nodes respectively representing sub
respond; and interrogator subsequently sending a command indi vidually addressed to that device.] [15. A method of addressing messages from an interroga
and repeating the transmitting of the command request ing devices having random values Within a speci?ed group of random values to respond using different speci?ed groups until all of the devices capable of com municating With the interrogator are identi?ed] [21. A communications system comprising an interrogator, and a plurality of Wireless identi?cation devices con?gured to communicate With the interrogator using RF, the interrogator being con?gured to employ tree searching to attempt to identify individual ones of the multiple Wireless
40
[22. A communications system in accordance With claim 21 Wherein the interrogator is con?gured to send a command
communications range of the interrogator, and Wherein the
at a different node if a collision occurs in response to the ?rst
device that is not Within communications range of the inter
mentioned command]
rogator does not respond to the transmitting of the command or the re-transmitting of the command] [16. A method of addressing messages from an interroga
[23. A communications system in accordance With claim 45
is not Within communications range of the interrogator.] [24. A communications system in accordance With claim
tor to a selected one or more of a number of RFID devices in
accordance With claim 14 Wherein the ?rst mentioned speci ?ed group contains both a device that is Within communica tions range of the interrogator, and a device that is not Within
50
communications range of the interrogator, and Wherein the device that is Within communications range of the interroga tor responds to the transmitting of the command and the
21 Wherein a subgroup contains both a device that is Within communications range of the interrogator, and a device that is not Within communications range of the interrogator, and Wherein the device that is Within communications range of
the interrogator responds to the command] [25. A communications system in accordance With claim
re-transmitting of the command] [17. A method of addressing messages from an interroga
21 Wherein a subgroup contains both a device that is Within communications range of the interrogator, and a device that
55
21 Wherein a device in a subgroup is movable relative to the
accordance With claim 14 Wherein a device in the ?rst men
interrogator so as to be capable of changing betWeen being Within communications range of the interrogator and not
tioned speci?ed group is capable of changing betWeen being
being Within communications range.]
tor to a selected one or more of a number of RFID devices in
Within communications range of the interrogator and not being Within communications range of the interrogator over
[26. A communications system in accordance With claim 60
time.]
microprocessor in communication With the receiver and
[18. A method of addressing messages from an interroga
modulator.]
tor to a selected one or more of a number of RFID devices in
accordance With claim 14 Wherein the devices respectively comprise an integrated circuit including a receiver, a modulator, and a microprocessor in communication With the
receiver and modulator.]
21 Wherein the Wireless identi?cation device comprises an integrated circuit including a receiver, a modulator, and a
[27. A system comprising:
65
an interro gator;
a number of communications devices capable of Wireless
communications With the interrogator;
US RE41,471 E 17
18
means for establishing for respective devices unique iden
ti?cation numbers respectively having the ?rst prede
Within communications range of the interrogator, and a device that is not Within communications range of the inter
termined number of bits;
rogator.] [33. A system in accordance With claim 31 Wherein a device in the ?rst mentioned speci?ed group is capable of changing betWeen being Within communications range of the interrogator and not being Within communications range of the interrogator over time.] [34. A system in accordance With claim 31 Wherein the respective devices comprise an integrated circuit including a receiver, a modulator, and a microprocessor in communica tion With the receiver and modulator]
means for causing the devices to select random values, Wherein respective devices choose random values inde
pendently of random values selected by the other
devices; means for causing the interrogator to transmit a command
requesting devices having random values Within a speci?ed group of random values to respond; means for causing devices receiving the command to determine if their chosen random values fall Within the speci?ed group and, if so, to send a reply to the interro gator; and
[35. A system comprising: an interrogator con?gured to communicate to a selected one or more of a number of RFID devices;
means for causing the interrogator to determine if a colli sion occurred betWeen devices that sent a reply and, if so, to create a neW, smaller, speci?ed group; and, if not,
transmit a command requesting devices having random values Within the same speci?ed group of random val ues to respond]
a plurality of RFID devices, respective devices being con ?gured to store a unique identi?cation number, respec tive devices being further con?gured to store a random 20
[28. A system in accordance With claim 27 Wherein send
ing a reply to the interrogator comprises transmitting the unique identi?cation number of the device sending the
reply.]
value; the interrogator being con?gured to transmit a command requesting devices having random values Within a speci?ed group of a plurality of possible groups of ran dom values to respond, the plurality of possible groups being organized in a binary tree de?ned by a plurality
25
of nodes at respective levels, the speci?ed group being
[29. A system in accordance With claim 27 Wherein a
de?ned as being at one of the nodes;
speci?ed group contains both a device that is Within commu nications range of the interrogator, and a device that is not
Within communications range of the interrogator] [30. A system in accordance With claim 27 Wherein the interrogator further includes means for, after receiving a reply Without collision from a device, sending a command individually addressed to that device.]
devices receiving the command respectively being con?g 30
[31. A system comprising: an interrogator con?gured to communicate to a selected
35
one or more of a number of communications devices;
and
a plurality of communications devices; the devices being con?gured to select random values, Wherein respective devices choose random values independently of ran
40
dom values selected by the other devices; the interroga tor being con?gured to transmit a command requesting devices having random values Within a speci?ed group of a plurality of possible groups of random values to
respond, the speci?ed group being less than the entire set of random values, the plurality of possible groups being organiZed in a binary tree de?ned by a plurality of nodes at respective levels, the speci?ed group being
50
55
being con?gured to determine if a collision occurred
[38. A system in accordance With claim 35 Wherein the interrogator is con?gured to determine if a collision respective Identify commands and, if so, to create further neW speci?ed groups and repeat the transmitting of the com mand requesting devices having random values Within a speci?ed group of random values to respond using different
speci?ed groups until all responding devices capable of responding are identi?ed.] 39. A method for performing radio frequency communications, the method comprising: transmitting by an interrogator a first signal, the first sig nal including a first set of bits to identi?) a first sub group of a group ofpossible random numbers;
betWeen devices that sent a reply and, if so, create a
neW, smaller, speci?ed group using a different level of the tree, the interrogator being con?gured to transmit a command requesting devices having random values
[37. A system in accordance With claim 35 Wherein the random values for respective devices are stored in digital form and respectively comprise a predetermined number of
occurred betWeen devices that sent a reply in response to
mine if their chosen random values fall Within the speci?ed group and, only if so, send a reply to the interro gator, Wherein sending a reply to the interro gator
comprises transmitting the unique identi?cation num ber of the device sending the reply; the interrogator
stored in digital form and respectively comprise a predeter mined number of bits.]
bits.] 45
de?ned as being at one of the nodes; devices receiving
the command being con?gured to respectively deter
ured to determine if their chosen random values fall Within the speci?ed group and, if so, send a reply to the interrogator; and, if not, not send a reply; and the interrogator being con?gured to determine if a colli sion occurred betWeen devices that sent a reply and, if so, to create a neW, smaller, speci?ed group by descend ing in the tree; and, if not, to transmit a command at the same node.] [36. A system in accordance With claim 35 Wherein the unique identi?cation numbers for respective devices are
60
receiving by one or more RFID devices the first signal; communicating by each ofthe one or more RFID devices
Within the neW speci?ed group of random values to
a first response
respond; and, if not, the interrogator being con?gured
generated a random number that is included in the first
to re-transmit a command requesting devices having random values Within the ?rst mentioned speci?ed group of random values to respond.] [32. A system in accordance With claim 31 Wherein the ?rst mentioned speci?ed group contains both a device that is
the one or more RFID devices has
subgroup; receiving by the interrogator one or more received 65
responses, the one or more received responses being
received from respective ones of the one or more RFID
devices; and
US RE41,471 E 19
20
responsive to receiving one of the one or more received
5]. The system ofclaim 50, wherein the request includes a
responses without a collision, retransmitting by the
selection indicator, the selection indicator identi?ing one or
interrogator at least the first signal. 40. The method of claim 39, wherein the first signal
more RFID devices, the first RFID device being configured to communicate the response only ifthe selection indicator
includes a selection indicator, the selection indicator identi ?/ing one or more RFID devices, the communicating by each ofthe one or more RFID devices only beingperformed ifthe
RFID tag. 52. The system ofclaim 50, wherein the?rst RFID device
corresponds to one or more selection bits stored on the first
is further configured to set an inventoried?ag to a first state to indicate that the first RFID device has responded to the at
selection indicator corresponds to one or more selection bits
stored on each respective RFID device.
least one interrogator
4]. The method ofclaim 39, further comprising setting, by
53. The system ofclaim 50, wherein the?rst RFID device is further configured to communicate the response after receiving a wake-up signal. 54. The system ofclaim 50, wherein the?rst RFID device
the one or more RFID devices communicating the first response, an inventoried ?ag to a first state to indicate that
each respective RFID device has responded to the interroga tor.
is further configured to communicate a response at a time
42. The method ofclaim 39, further comprising transmit ting by the interrogator a wake-up signal, the wake-up sig
slot corresponding to a random number generated by the first RFID device. 55. The system of claim 50, wherein the interrogator is further configured to transmit a sleep command upon receiv ing the reply without a collision. 56. The system of claim 50, wherein the interrogator is further configured to transmit a larger portion of the identi ?er ifthe reply is received with a collision. 57. The system ofclaim 56, wherein a di?'erence between
nal causing an RFID device to transition out ofa sleep state. 43. The method ofclaim 39, wherein the one ofthe one or more received responses without a collision is receivedfrom
a first RFID device, andfurther comprising the interrogator transmitting a command to silence the?rst RFID device. 44. The method ofclaim 39, wherein the one ofthe one or more received responses without a collision is receivedfrom a first RFID device and includes at least one random number
the portion of the identifier and the larger portion of the
generated by the first RFID device, and further comprising
identifier is two or more bits.
transmitting by the interrogator at least one additional com
58. An interrogator comprising:
mand to the first RFID device, the first RFID device being
one or more antennas;
identified in the at least one additional command by an iden ti?er including the at least one random number
a receiver communicatively coupled to at least one ofthe
45. The method ofclaim 39, further comprising transmit ting by the interrogator a wake up signal. 46. The method ofclaim 39, further comprising respon
from one or more radio frequency identification (RFID)
one or more antennas to receive one or more messages
devices; a transmitter communicatively coupled to at least one of
sive to a collision occurring in the receiving by the interro
the one or more antennas to transmit one or more mes
gator one or more received responses, transmitting by the
sages; and a control unit communicatively coupled to the transmitter
interrogator a second signal, the second signal including a second set of bits to identi?) a second subgroup of the group
and the receiver, the control unit configured to imple
ofpossible random numbers, wherein the second subgroup
ment an algorithm to detect at least a single RFID
is a subgroup of the first subgroup, the second set of bits
device in a field of the interrogator, including re-transmitting a first signal responsive to receipt of a
includes the?rst set ofbits plus one or more additional bits,
and the second signal includes the?rst signal. 47. The method ofclaim 46, wherein the second subgroup
40
out a collision, the first signal including a?rst set of bits of at least a first portion ofpossible random num
is determined at least in part by skipping one or more levels of a search tree.
bers that may be generated by the one or more RFID devices, and the first response including at least a sec
48. The method ofclaim 46, wherein the second set ofbits includes the?rst set ofbitsplus two or more ofthe additional bits.
ondportion ofa random number generated by the one or more RFID devices.
49. The method ofclaim 39, wherein the?rst subgroup is
59. The interrogator ofclaim 58, wherein the?rst signal
based at least in part on a maximum number of possible random numbers.
includes a selection indicator, the selection indicator corre sponding to one or more selection bits stored on the one or
50. A system for performing radio frequency communications, the system comprising:
more RFID devices.
60. The interrogator ofclaim 58, wherein the control unit
a first radio frequency identification (RFID) device con
is further configured to transmit a command to silence the
?gured to generate a random number and to communi cate a response, including at least aportion ofthe ran dom number, upon receiving a request that includes an
one or more RFID devices from which the first response was
received without a collision, and the control unit isfurther configured to indicate a number oftime slotsfrom which at least a?rst RFID device is to randomly select a time slot in which to communicate a random value generated by the first RFID device.
indication ofa subset ofpossible random numbers ifthe ?rst RFID device determines that the subset includes
the random number generated by the first RFID device; an antenna positioned in a first region; and an interrogator coupled to the antenna, the interrogator
first response from the one or more RFID devices with
6]. The interrogator ofclaim 58, wherein the control unit 60
isfurther configured to define a second set ofbits ofat least a
configured to transmit a signal comprising aportion of
second portion, greater than the first portion, of possible
an identifier and to receive a reply to the signal from a target RFID device that has generated a random num
random numbers responsive to a collision detected in the receipt of the first response from the one or more RFID devices.
ber having a portion equal to the portion of the
identifier, the interrogator further configured to re-transmit the signal, including at least the portion of the identifier,
the reply is received without a collision.
65
62. The interrogator ofclaim 6], wherein the control unit is further configured to define the second set of bits to be at least two bits greater than the first portion.
US RE41,471 E 21
22
63. The interrogator of claim 58, wherein the possible random numbers de?ne a binary search tree, the ?rst set of
cation (RFID) device in a?eld ofthe interrogator, the ?rst signal comprising a ?rst set of bits and requesting
bits de?ne a level in the search tree, and the control unit is further con?gured to skip one or more intermediate levels in
selected in accordance with at least the ?rst set of bits;
the binary search tree to implement the algorithm. 64. The interrogator ofclaim 58, wherein the control unit
a response?’om one or more RFID devices in the?eld 5
?rst set of bits is equal to a ?rst portion of a random
is further con?gured to transmit a wake-up signal, indicate a
number generated by the RFID device, and, so, modulating an RF?eld, provided by the interrogator, to
number oftime slots, and receive a random value?’om a?rst RFID device in a time slot randomly determined by the?rst RFID device?’om among the number oftime slots. 65. A method comprising:
communicate a reply to the interrogator in accordance
with the algorithm; and receiving, in accordance with the algorithm, a retransmis
providing an interrogator to generate an RF?eld and to
sion of the ?rst signalfrom the interrogator in response to the interrogator receiving the reply without detecting
initiate the implementation ofan algorithm to detect at least a single target RFID device out ofpotentially mul tiple target RFID devices in the RF?eld, the algorithm
including:
a collision. 15
interrogator, wherein in accordance with the ?rst communi 20
bits to identi?) the ?rst subgroup ofpossible random numbers and requesting the target device to respond if the target device has generated a random number included in the subgroup; receiving a response from the target device the target device has generated the random number included in
74. The method ofclaim 73,further comprising communi cating with the interrogator at one ofa plurality ofbit rates 25
76. The method ofclaim 75, further comprising receiving
response from the target device, determining, from 30
algorithm, before the receiving of the retransmission of the 35
?rst signal.
40
78. The method ofclaim 72, wherein the reply comprises a random value generated by the RFID device. 79. The method ofclaim 78, wherein the random number comprises the random value. 80. The method ofclaim 79, wherein the random value is
of the ?rst subgroup and being defined by a second set of bits common to the second subgroup, and
retransmitting the signal. 66. The method ofclaim 65, wherein the signal includes a
the random number
plurality of RFID devices from which a response is being
requested.
8]. The method ofclaim 72, further comprising
67. The method ofclaim 65, wherein the algorithmfurther includes transmitting a command to silence the target device
after the determining of the random number generated by
a sleep command?’om the interrogator 77. The method ofclaim 76, wherein the sleep command is received in response to the interrogator receiving the reply without detecting a collision, in accordance with the
from the target device, de?ning a second subgroup of possible random numbers that may be generated by
selection indicator that identi?es a class ofone or more ofa
determined by the interrogator 75. The method ofclaim 72, further comprising receiving a wake up command from the interrogator and, in response, transitioning from a sleep state.
the subgroup;
the target device, the second subgroup being a subset
cation mode the RFID device modulates an RF ?eld gener ated by the RFID device and in accordance with the second communication mode the RFID device modulates an RF
?eld generated by the interrogator
if no collision is detected in the receiving of the
the response, the random number generated by the target device and retransmitting the signal; and if a collision is detected in the receiving of the response
73. The method ofclaim 72,further comprising communi cating with the interrogator in one ofa?rst communication mode and a second communication mode determined by the
defining a ?rst subgroup ofpossible random numbers that may be generated by the target device, the ?rst subgroup being de?ned by a ?rst set of bits common to the ?rst subgroup; transmitting a signal comprising at least the?rst set of
responsive to receiving the ?rst signal, determining the
45
the target device.
receiving, in accordance with the algorithm, a second sig nal?’om the interrogator in response to the interroga tor detecting a collision in the reply, the second signal comprising a second set of bits and requesting a
68. The method ofclaim 65, wherein the algorithmfurther
response from one or more RFID devices in the ?eld
includes transmitting a number oftime slotsfrom which at
selected in accordance with at least the second set of
least a?rst RFID device is to randomly select a time slot in which to communicate a random value generated by the ?rst RFID device.
50
the second set of bits is equal to a second portion of the
69. The method ofclaim 65, wherein the algorithmfurther comprises transmitting a number oftime slots, and receiving
so, random modulating numberthe generated RF?eldbytothe communicate RFID device, a second and, reply to the interrogator in accordance with the
a random valuefrom a?rst RFID device in a time slot ran
domly determined by the?rst RFID device?’om among the number oftime slots. 70. The method ofclaim 65, wherein the second set ofbits includes the ?rst set of bits and the algorithm further com prises transmitting the second set ofbits ifthe collision is detected in the receiving of the response from the target
55
60
portion of the random number comprises the ?rst por tion ofthe random number 82. The method of claim 8], wherein the second reply comprises at least a portion of the random number
7]. The method of claim 70, wherein the algorithm
83. The method ofclaim 82,further comprising communi
accommodates the second set ofbits being two or more bits
cating a random value to the interrogator during a time slot
longer than the?rst set ofbits. with an algorithm to identify a radio frequency identi?
algorithm, wherein the second signal comprises the ?rst signal, the second set of bits comprises the ?rst set ofbits plus at least two additional bits, and the second
device.
72. A method, comprising: receiving a ?rst signalfrom an interrogator in accordance
bits; and responsive to receiving the second signal, determining
65
randomly selected?’om a number oftime slots. 84. The method ofclaim 72,further comprising communi cating a random value to the interrogator during a time slot
randomly selected?’om a number oftime slots.
US RE41,471 E 24
23 85. A system, comprising: a radio frequency identification (RFID) device compris
98. The system of claim 97, wherein the interrogator is configured to generate, as part of the algorithm, the second set of bits including the first set of bits plus at least two
ing a receiver to receive a?rst command including a
additional bits. 99. The system ofclaim 85, wherein the RFID device is configured to communicate a random value during a?rst
portion of an identification number, a random number generator to generate a random number to identify the device, and a transmitter to communicate a reply to the
time slot randomly selectedfrom a?rst number oftime slots.
first command the portion of the identification num ber is equal to a first portion of the random number;
100. The system of claim 99, wherein the transmitter is configured to communicate by modulating an RF?eldpro
and an interrogator configured to implement an algorithm to
vided by a remote device.
10]. The system ofclaim 100, wherein the random value
identify one or more RFID devices in a field of the
identifies the device to the remote device.
interrogator, the algorithm comprising transmitting a from a selected one or more devices, receiving a first response thereto from the selected one or more devices, 15
102. The system ofclaim 10], wherein the RFID device is further configured to communicate the random value to the remote device during a second time slot randomly selected from a second number of time slots, wherein the first number
detecting
oftime slots is di?'erentfrom the second number oftime slots
first signal with a first set of bits to request a response a collision occurred in the first response,
and retransmitting the first signal with at least the first
and is indicated by the remote device.
set ofbits to request a second response from at least one
103. The system ofclaim 102, wherein the algorithmfur ther comprises transmitting a second signalfrom the interro
of the selected one or more devices in response to
detecting no collision in the first response.
20
86. The system of claim 85, further comprising memory
gator in response to detecting a collision in the first response, the second signal comprising a second set of bits
storing a unique identification code to be transmitted by the
and requesting a response from at least one of the one or
system.
more RFID devices in the field selected in accordance with at least the second set of bits, wherein, in accordance with the algorithm, the second set ofbits include at least two bits
87. The system of claim 85, wherein the transmitter is configured to communicate by modulating an RF?eld pro
25
in addition to the?rst set ofbits.
vided by a remote device.
88. The system ofclaim 87, wherein the algorithmfurther comprises transmitting an indication ofthe number ofbits of the first set of bits. 89. The system of claim 85, wherein retransmitting the first signal with at least thefirst set ofbits comprises retrans mitting the?rst signal with no more than the?rst set ofbits. 90. The system of claim 85, wherein the system is con?g ured to communicate at one ofa plurality ofbit rates deter mined by a remote device.
104. An apparatus for wirelessly reading radio frequency
identification (RFID) devices, comprising: 30
a transmitter to transmit a command along with a?rst portion of a set of random numbers to request a response from at least one RFID device that has gener ated a random number in the set; an antenna to provide an RFfield to be modulated by the
device; 35
9]. The system of claim 90, wherein the system is con?g
a receiver to receive the response; and
processing circuitry to perform collision detection, to
ured to operate in a first communication mode during a first
determine the random number using the response, and
period oftime and in a second communication mode during a second period of time, wherein in accordance with the first communication mode the system is configured to modulate
to cause the transmitter to retransmit the command
along with at least the?rstportion ofthe set ofrandom 40
an RF ?eld generated by the remote device and in accor dance with the second communication mode the system is configured to generate and modulate an RFfield.
response.
105. The apparatus ofclaim 104, wherein the transmitter is configured to transmit the command along with an indica
92. The system ofclaim 85, wherein the RFID device is configured to receive a signal to silence the RFID device.
numbers responsive to detecting no collision in the
45
tion ofthe number ofbits on the?rstportion. 106. The apparatus ofclaim 105, wherein the transmitter
93. The system ofclaim 92, wherein the RFID device is
is configured to communicate with the RFID device at one of
configured to receive a wake up command and, in response, to transition from a sleep state.
a plurality of bit rates determined by the apparatus. 107. The apparatus of claim 104, wherein the processing
94. The system ofclaim 85, wherein the algorithmfurther comprises transmitting a signal to silence at least one of the
circuitry is configured to cause the transmitter to retransmit 50
one or more RFID devices in response to the detecting no
collision and before the retransmitting ofthe?rst signal. 95. The system ofclaim 85, wherein the replay comprises
circuitry is configured to cause the transmitter to transmit a
signal addressed to the RFID device responsive to receiving
at least a second portion ofthe random number that is not
part ofthe?rstportion ofthe random number
the command along with no more than the?rstportion ofthe set of random numbers responsive to the detecting. 108. The apparatus of claim 104, wherein the processing
55
the response without collision.
109. The apparatus of claim 108, wherein the signal is
96. The system of claim 95, wherein the interrogator is further configured to use the first response to determine a random value generated by the selected one or more devices
configured to silence the RFID device.
in accordance with the algorithm.
circuitry is configured to cause the transmitter to transmit
97. The system ofclaim 85, wherein the algorithmfurther comprises transmitting a second signal from the interroga
the signal to silence the RFID device before causing the
110. The apparatus of claim 109, wherein the processing transmitter to retransmit the command.
tor in response to detecting a collision in the first response,
1]]. The apparatus ofclaim 104, wherein the transmitter
the second signal comprising a second set of bits and
is configured to transmit a wake up command to transition
the RFID device from a sleep state.
requesting a response from at least one of the one or more
RFID devices in the field selected in accordance with at least the second set of bits, wherein the second set of bits includes the?rst set ofbits plus at least one additional bit.
65
112. The apparatus of claim 104, wherein the response comprises at least a second portion of the random number
that is not part of the first portion.
