Note: Descriptions are shown in the official language in which they were submitted.
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WO 97t17667 PCT/GB96/02742
ENHANCED DETECTION OF MULTIPLE DATA TRANSMISSIONS
BACKGROUND OF THE INVENTION
THIS invention relates to a method of identifying a plurality of transmitters, each of which
transmits data at intervals to a receiver. The invention also relates to an identification
S system comprising a plurality of transmitters and a receiver, and to the ~
themselves. The invention further relates to method and a,u~dlus for improving the
identification systems disclosed in EP 494,114 A and, in particular, EP 585,132 A.
Identification systems are known in which a plurality of transmitters. typicallytransponders. are activated by an interrogation signal and then transmit reply signals,
10 usually cont~ining identification data, to a receiver, which typically forms part of the
interrogator. The signals may be L~ ll,iLLed in many ways, including electromagnetic
energy, e.g. radio frequency (RF), infra red (IR), and coherent light, and sound, e.g.
ultrasound. For example, the tr~n~mi~ion may be achieved by actual emission of RF
energy by the tr~n~mitt~rs, or by the modulation of the reflectivity of an ~ntt?nn~ of the
15 transmitter, resulting in varying amounts of RF energy in the interrogation signal being
reflected or back-scattered from the tr~n~milter ~nt( nn~
In generaL if the tr~ncmic~ions of two transmitters overlap or clash, the tr~n~mi~ions are
lost, since the receiver cannot distinguish the separate tr~n~mi~ions. Thus, the system
must provide for each transmitter to transmit repeatedly until its tr~n~mi~ion takes place
20 in a "quiet" time and is successfully received by the interrogator. GB 2,116,808 A
discloses an identification system in which the individual transponders are programmed to
retransmit data in a pseudo-random manner. Timing signals for the transponders in this
identification system are derived from a crystal oscillator, thereby making the transponders
more expensive to m~nnf~cture.
.
~5 EP 467 036 A describes another identification system which uses a pseudo-random delay
between transponder data tr~n~miccions. In this example. a linear recursive sequence
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WO 97tl7667 PCT/GB96/02742
generator is seeded by the transponder identification address to make the pseudo-random
delay as random as possible.
It is an object of the invention to provide an alternative system for enhancing the detection
of multiple tr~n~miscions in a system of this kind.
5 EP 494,114 A and EP 585,132 A disclose identifications systems in which the
transponders may be programmed with the same data. It is an object of this invention to
provide an improved system for generating the pseudo-random delays b~Lw~~ onder
tr~ncmi~ions.
SUMMARY OF THE INVENTION
10 According to a first aspect of the invention there is provided a method of identifying a
plurality of lld.l~lllillers, each l.,., l~l l l it~ g at intervals to a receiver, co~ Jlising varying the
duration of the intervals between successive tr~n~mi~ ns from each transmitter by
enabling each llcul~,lllill~,. at intervals which are calculated randomly or pseudo-randomly
and are related to an operating frequency of local timing means associated with that
1 5 tr~n~mitt~r.
The intervals are preferably varied between a m7lxi.l.~ duration which is related to a
predetermined number of cycles of the local timing means, and a minimllm interval.
In the above method, the duration of the intervals between tr~n~mi~sions may be calculated
by periodically generating pseudo-random numbers, co. . .l ~, - ;.lg the pseudo-random number
20 with an output of counter means clocked by the local timing means, and enabling the
transmitter when the pseudo-random number and the output of the counter means
correspond.
According to a second aspect of the invention there is provided an identification system
comprising a pluralitv of transmitters each adapted to transmit at intervals and at least one
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receivcr for receiving tr:~n~mi~.sions from the transmitters, each transmitter including means
for generating tr~n.~mi~.sions, means for calculating the duration of the intervals between
successive tr~n.cmi~sions and means for enabling the transmitter at random or pseudo-
random intervals related to an operating frequency of local timing means.
y
S According to third aspect of the invention there is provided a transmitter adapted to
transmit at intervals comprising means for gelle.dlillg tr~n~mi~.cions, means for calc~ ting
the duration of the intervals between s~lcc~ive tr~n~mi~ions and means for enabling the
transmitter at random or pseudo-random intervals related to an op~l~ling frequency of local
timing means.
The local timing means is preferably an oscillator of the tr~n.~mitt~?r, with an unspecified
clock frequency which is subject to a relatively large tolerance, so that the oscillators of
different transmitters tend to run at different frequencies.
The enabling means may include a pseudo-random number generator arranged to generate
pseudo-random numbers, counter means arranged to count at a rate related to the oscill~tor
frequency, and col-lp~dtor means arranged to compare the outputs of the pseudo-random
number generator and the counter means and to enable the transmitter only when the
outputs correspond.
The transmitters may be radio frequency identification (RF/ID) transponders. In
transponders which embody the "anti-clash protocol" described in EP 494,114 A and EP
585,132 A, it is not necÇ~ry forthe identification code of each transponder to be di~clc;~
the transponders can be identical, allowing them to be m~nllf~ctllred in large quantities,
~ery cheaply. It is an advantage of this invention that when such an anti-clash protocol is
used there is better immunity against clashes. However, reading a plurality of transponders
may be completed more rapidly if unique seeds are used in the pseudo-random number
~, 25 g~enerator.
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WO 97/17667 PCT/GB96/02742
The invention extends to a method of operating a transmitter and the integrated circuits
from whicll the transmitters of the in~ention are customarily constructed.
BRIEF DESCRlPTION OF THE DRAWINGS
Figure 1 is a block sl~h~m~tic diagram of a radio frequency Ll~lal~ollder according to
the invention;
Figure 2 is a block sçhern~tic diagrarn of the transmit controller block in Figure l;
and
Figure 3 is a sçhem~tic diagram showing the timing of the tr~n~mi~ions of two
transponders or tags.
DFscRIpTI(~N OF A~ EMBODIMENT
The purpose of the present invention is to provide a method and a system which enh:~nres
the probability of identification of a number of tr~n.cmitter~7 typically transponders, which
are all transmitting data to a receiver. Although the embodiment described below refers to
passive radio-frequency Ll~la~onders which are activated and powered by an interrogation
signal received from an interrogator. the invention is also applicable to other systems. For
example, the invention may be employed in systems in which a number of self-powered
transmitters must be identified by a receiver, or systems in which a number of transmitters
broadcast using a randomly delayed "back-off and retry" algorithm.
The invention addresses this requirement by having the Lldllalllillers~ or transponders,
transmit at different times, at random or pseudo-random intervals, rather than at regular
intervals. In addition, the degree of randomness of successive tr~n~mi~ions from each
transponder is enhanced by deriving the random or pseudo-random timing from local
timing means, typically an oscillator. of each transponder. The transponders may be of the
tvpe described in EP 494,114 A and EP 585,132 A, the entire contents of these documents
being incorporated herein by reference. The relatively large tolerance in the operating
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frequency of the oscillators of nominally identical transponders increases the randomness
of their tr~n~mic~ions.
Referring now to Figure 1. a passive RF transponder is shown schematically The
s transponder has an antenna 10 which receives energy from an RF interrogation signal
~ d by an interrogator, and a portion of this energy is diverted to charge a capacitor
C which acts as a power supply for the transponder. The transponder has an on-board
oscillator 12 which operates at the same nominal frequency as other nominally identical
transponders. However, due to m~nllf~ct-lring tolerances in the plcr~ d low costintegrated circuit transponders, the output frequency of the oscillator 12 typically has a
m~nllf~cturing tolerance of ~t 25%. The output frequency is also affected by the supply
voltage VDD from the capacitor C, which is in turn affected by the strength of the received
energy from the interrogation signal due to proximity to the interrogator, ~ntenn~
orientation and other factors. Thus each tag has an oscillator whose frequency is subject
to significant uncertainty, being dependent on both m~nl-f~f~turing tolerances and supply
voltage, which is itself dependent on received RF field strength. The frequency of each
oscillator is the.efole ind~l~....h~ate, being unspecified until the tag is in operation.
The heavy black line I indicates the components of the transponder which may be
integrated into a monolithic integrated circuit.
The transponder includes a non-volatile memory element 14, typically an EEPROM. which
stores the transponder's identification code and configuration information which programs
the transponder for different code data l~ "i~ion frequencies (bit rates), m~ximllrn delay
time (Nmax.T), and seeds for a pseudo-random time delay circuit. The transponder further
includes an output driver 16 which in the described embodiment modulates the load applied
to the antenna 10. thus modul~ting its reflectivity, but which could also be an active
transmitter. A control logic circuit 18 controls the operation of the output driver 16 and
f reads data from the memorv element 14 in response to signals from a transmit controller
CirCUil ~0 wllich is shown in greater detail in Figure 2. A power on reset circuit 2'2
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initialises the control logic circuit 18 into a predeterrnined start-up state when a voltage is
applied to the circuit.
In Figure 2, the transmit controller circuit 20 is shown in greater detail. This circuit
includes a sequencer 24 which receives the clock signal from the oscillator 12 and derives
5 from it a frequency, which may be lower, to generate a "memory read" signal which is
applied to the conkol logic circuit 18, together with the clock signal. The "memory read"
signal is a continuous sequence of pulses at a con~L~.L frequency and with a period T. Each
of these pulses causes the control logic block 18 to instruct the memory element 14, via a
"read out command" signal, to output the identification code stored therein sequentially into
10 the control logic circuit. However, the code is not passed to the output driver 16 by the
control logic circuit for tr~n~mi~cion unless the transmit controller 20 also outputs a
"transmit enable" signal to the control logic circuit 18 ~im~lltzlneously with the respective
"memory read" pulse. This occurs only occasionally, at pseudo-random time intervals, as
described below.
15 The output of the sequencer circuit 24 is also fed to a code cycle counter 26 which is
therefore incremented each time the identification code output sequence from the memory
element 14 is started. The code cycle counter is never reset, but counts up to its maximum
count, after which it returns to zero and counts up again. A pseudo-random number
generator circuit 28 ge~ aLt;s a pseudo-random number from time to time, and both the
20 output of the pseudo-random number generator 28 and the current output of the code cycle
counter 26 are fed to a co~ dLor 30. The colllphl~lor 30 gives an output whenever the
two numbers being conlp~ed are equal, which output is the "transmit enable" signal
referred to above. The transmit enable signal also triggers the pseudo-random number
~Tenerator to generate a new pseudo-random number. When the "transmit enable" signal
25 goes high at the same time as the "memory read" signal, the code which has been read from
tlle memory element 14 by the control logic circuit 18 is output to the output driver 16 and
transmitted.
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Referring to Figure 3, the tr~n~mi~sions of two transponders or tags, Tag 1 and Tag 2 are
compared. Tag 1 has an oscillator running somewhat faster than Tag 2. Both have the same
Nmax pro~rammed into them, but at the particular time shown in the diagram, Tag I will
remain in a passive (non-transmitting) mode for a period of Nl .T, where Nl varies with
y/ S each tr~n~mi~ion/quiet cycle, (0<=N<=Nmax), while Tag 2 remains passive for N2.T.
Note that T for Tag 1 is not the same as for T for Tag 2, but subject to the same variations
as the frequency referred to above. Furthermore, TI, T2, T3.. TN for any tag are not
exactly equal owing to variations with time in the supply voltage for the particular tag.
There are therefore frequency variations between tags, and b~ di~,..,.lL times with the
10 same tag. The anti-clash system used in this invention achieves a con~ ahly better
immunity against clashes than if the tag clock signals were derived from the RF carrier
frequency (by frequency division, for example), as is the case in some other RF
identification systems, and is relatively inexpensive to implement.
The value of Nmax is cl~terminP~l by the number of bits being compared by the c~ r
15 30. In the case of an 8 bit co.ll~ison, Nmax=255. Nmax is configured at the time of
pro~",. " " " i "g or m~mlf~rtnre of the transponders. It has been ~ n~l ly ~let~rmin~d
that for a particular figure of Nmax there is a practical limit to the number of transponders
or tags which can be read simultaneously, in the same RF interrogation field. If the number
of transponders in the same interrogation field is conlp~ed with the time taken for all
20 transponders to be identified successfully, that time is roughly proportional to the number
of transponders, due to clashes between transponder tr~n~mi~sion~, until the number of
transponders approaches Nmax/2. If the number of transponders is increased beyond this
point, the time taken to identify the tags increases rapidly towards a condition where no
transponders are identified at all, as all tr~n~mi~ions result in cl~ch~s
25 Because of the various advantageous aspects of the design of the above described
transponders, a large degree of randomness is obtained in the tr~n~mi~ion of data from
each transponder. notwith~t~n~ling that the transponders are programmed with the same
value of Nrnax and are essentiallv identical. This means that the only value which may
need to be adjusted from one transponder to the next is the transponder identification code.
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It will be appreciated by those skilled in the art that the invention may be put into ef'fect in
a number of different systems. In systems in which the variation of local timing means is
not an inherent feature. such a variation may be included into the system design. For
example. a number of different crystal oscillator circuits which run at a various speeds may
S be used. Each transmitter may be provided with a different oscillator circuit although the
provision of unique frequencies for a set tr~n~mi~ters is not ~Scenti~l Alternatively,
transmitters may be able to dynamically alter the frequency of the local timing means, by
e.g. switching between oscillator circuits. In the same spirit, local timing means in which
the frequency is dependent on external factors e.g. l~ ,.dlule, incident light, may provide
10 the necessary variation in frequency.
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