Free Running RF Identification System With Increasing Average Inter Transmission Intervals

THIS invention relates to data communication systems, more particularly to interrogation systems such as electronic identification systems.

Radio frequency (RF) electronic identification systems including an interrogator or reader and a plurality of passive transponders are well known in the art. In use, the reader energizes the transponders by transmitting an interrogation signal. Each energized transponder automatically responds with a response signal, normally including an identification code characteristic of the transponder. The response signals are received by the reader and the data is read. The data may be utilized to identify transponders and goods associated with the transponders and/or to count the transponders and/or goods. However, when a plurality of transponders are energized simultaneously, it may well happen that the response signals overlap in time, that the transmissions clash and that the data in the response signals is lost.

Accordingly, systems of the aforementioned kind provide for anti-clash or anti-collision measures or protocols. One way of reducing the effect of collisions is to make use of two-way communications. In a typical system employing two way communications, once the data of a transponder has been read, the reader transmits an acknowledgement signal. This acknowledgement signal causes the transponder just read to enter a sleep mode wherein it no longer responds to the interrogation signal, even while still being energized. This switching of a transponder just read to a sleep mode, reduces the transponder population not yet read and hence reduces the probabilities of collisions. The major disadvantage of this protocol is that it requires a larger communications channel bandwidth. Accordingly, it is not suitable for many narrow band applications.

In so-called free-running systems (these are systems where no acknowledgement signal is transmitted) it has been proposed, for example in SA Patent 95/9519, to configure the transponders such as to retransmit the reply signals at random inter-transmission intervals. This gives a statistical probability that transponders would sometimes transmit at a time when no other transponder is transmitting, thereby enabling the reader eventually to receive a reply signal successfully from each transponder. However, if a large number of transponders are present, the probability of a successful transmission is reduced and the time required to receive the transmissions from all the transmitters becomes larger, i.e. the communication channel becomes congested. If the number of transponders is increased too much, the probability of a successful transmission may become so low, that the time required to receive a transmission from each of the transponders becomes unpractically long (even infinitely long). The communications channel can then be said to be saturated. With these systems and if the maximum inter-transmission interval is made shorter, the communications channel may saturate with a relatively small number of transponders, but on the other hand the time required successfully to read a small number of transponders would be short. Furthermore, if the maximum inter-transmission interval is made longer to accommodate a larger transponder population, the total time to read the transponders (even a small number of transponders), may be too long for many applications. In practice this means that a specific transponder can only be used in an application where its maximum inter-transmission interval is compatible with the number of transponders that would be present and the time required successfully to read the transponders.

Accordingly it is an object of the invention to provide a communication system, a transmitter or transponder and a method of transmitting signals with which the applicant believes the aforementioned disadvantages may at least be alleviated.

According to the invention there is provided a communication system including:

a plurality of transmitters each configured repeatedly to retransmit a signal burst;

a reader for receiving and reading the signal bursts transmitted by the transmitters sequentially;

each transmitter including:

inter-transmission interval generating means for generating random inter-transmission intervals for separating any two consecutive signal bursts; and

control means for the interval generating means automatically to cause an average value of the inter-transmission intervals over a period of time, to vary.

Each transmitter may form part of a passive transponder and the system may further include energizing means for energizing the transponders. The reader and energizing means may be separated or may be combined in a single unit.

Preferably the transmitters are radio frequency transmitters.

Each transmitter may for example include a back-scatter modulator stage for back-scatter modulating the energizing signal with the signal burst.

Each signal burst may include data relating to a unique identification code of the transponder of which the transmitter forms part.

The inter-transmission interval may vary randomly or pseudo-randomly between a minimum value and a maximum value.

The average value may be varied by progressively increasing the maximum value for the inter-transmission interval. The maximum value of the inter-transmission interval may be increased in step-wise manner. In a preferred embodiment, the maximum value of the inter-transmission interval is substantially doubled after each four transmissions of the signal burst.

Also included within the scope of the present invention is a transmitter for repeatedly transmitting a signal burst, the transmitter including:

inter-transmission interval generation means for generating random inter-transmission intervals for separating any two consecutive signal bursts; and

control means for the interval generating means to cause an average value of the inter-transmission intervals over a period of time, to very.

Yet further included within the scope of the present invention is a passive transponder including a transmitter as herein defined and/or described.

The inter-transmission interval generation means may include: a comparator; an n-bit random number generator an output of which is connected to a first input of the comparator; an m-bit first counter an output of which is connected to a second input of the comparator, the first counter being driven by a clock signal and the output of the comparator being connected to enable transmission of a signal burst when the contents of the first counter is equal to the random number generated.

The control means for the interval generating means may include: a second counter for controlling the number of bits of the random number and in the first counter to be compared; and a third counter, an input of the third counter being connected to the output of the comparator, to be incremented with each transmission; an output of the third counter being connected to an input of the second counter, to increment the second counter after a predetermined number of transmissions, thereby to increase the count of the second counter and hence the number of bits to be compared.

Also included within the scope of the present invention is a method of retransmitting a signal burst including the steps of:

generating a sequence of inter-transmission intervals of random length;

repeatedly transmitting the signal burst after successive ones of the sequence of intervals; and

causing an average value of the inter-transmission intervals over a period of time, to vary.

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein:

FIG. 1 is a block diagram of a radio frequency (RF) electronic identification system according to the invention;

FIG. 2 is a block diagram of relevant parts of a transponder forming part of the system;

FIG. 3 is a time diagram illustrating random inter-transmission intervals;

FIG. 4 is a further time diagram, but over a longer period of time, illustrating an increase in the average value of the inter-transmission interval; and

FIG. 5 is a diagram illustrating the statistical distribution of inter-transmission intervals as the average value of the inter-transmission intervals are increased in use and according to the invention.

In FIG. 1, a radio frequency (RF) electronic identification system 10 is shown. The system includes an interrogator or reader 12 and a plurality of transmitters, in the form of passive transponders 14.1 to 14.n.

In use, the reader 12 transmits an energizing signal 16 towards the transponders 14.1 to 14.n. Each of the transponders is energized in known manner and automatically responds with response signals 18.1 to 18.n, for example by backscatter modulating the energizing signal 16. The response signals 18.1 to 18.n normally include data relating to a unique Identification (ID) code associated with the relevant transponder.

The response signals 18.1 to 18.n are received by a receiver (not shown) of the reader and the data is read by the reader, to identify the transponders and hence articles (not shown) that may be associated with the transponders and/or to count the number of transponders and hence articles.

As shown in FIG. 3, each signal 18.1 to 18.n includes a plurality of repeated similar signal bursts 20.1 to 20.5 separated by inter-transmission intervals 22.1 to 22.4. Each burst includes the ID code data referred to hereinbefore. In SA Patent 95/9519 it is proposed to generate random inter-transmission intervals, to alleviate the problems of signal burst collisions between signals transmitted by different transponders. As stated in the introduction of this specification, this proposal still has severe problems and/or shortcomings.

It is a characteristic of the present invention that the average value A.sub.v1, A.sub.v2, A.sub.v3, A.sub.v4, and A.sub.v5 of the random inter-transmission intervals 22.1 to 22.n over a sequence of successive periods of time 24.1 to 24.5 is automatically varied, preferably increased. This is illustrated in FIG. 4, where A.sub.v1 <A.sub.v2 <A.sub.v3 <A.sub.v4 <A.sub.v5.

In FIG. 2, there is shown a block diagram of relevant parts only, of a transponder 14.1 according to the invention. The transponder 14.1 includes a transmitter stage 30 for transmitting the response signal 18.1 with the signal bursts separated by random inter-transmission intervals. The ID code data is stored in read only memory (ROM) 32.

The transponder further includes random inter-transmission interval generation means 34 including a random number generator 36, an interval counter 38 and a comparator 40. Control means 42 for the generation means 34 includes a transmit counter 44 and a counter 46 controlling a maximum value N.sub.max for the inter-transmission interval. The various components are interconnected as shown in FIG. 2.

The random inter-transmission interval generation means 34 operates as follows. When the transponder is energized by signal 16, a seed value is loaded into the random number generator 36 and a random number is generated. After each transmission burst, 8 new random number is generated. The interval counter 38 Is initially reset to zero. It is incremented with each pulse of clock 48. The comparator 40 compares the random number generated and the count of interval counter 38. When they match, a signal burst including the ID code data is enabled.

The random intervals are varied between a minimum value and a maximum value N.sub.max for the inter-transmission interval. The average value of the inter-transmission interval is controlled by controller 42, by causing the maximum value N.sub.max to be increased periodically, typically after each four transmission bursts, as shown in FIG. 4. Computer simulations have indicated that doubling the value of N.sub.max after every four transmission bursts gives nearly optimal reading times for transponder populations including any number of passive RF ID transponders.

The maximum value is controlled by controlling the number of bits in the random number and the interval counter 38 that are compared. The number of bits that are compared are controlled by counter 46. N.sub.max is given by 2.sup.N -1, where N is the number of bits that must be compared which is equal to the count in counter 46. For example, if the count in counter 46 is four, the value of N.sub.max is 2.sup.4 -1=15. The value of N.sub.max is increased by increasing the number of bits that are compared. This is achieved by periodically incrementing counter 46. The transmit-counter 44 is initially reset to zero and thereafter incremented after each signal burst. When it reaches a predetermined value (typically four), the counter 46 is incremented and the transmit-counter is reset to zero. Incrementing the counter 46 by one, increases the compare bit width by one bit, effectively doubling the value of N.sub.max. For example, if counter 46 is incremented from four to five, the value N.sub.max changes from fifteen (15) to thirty-one (31).

This process continues until the interrogation signal disappears, or until the maximum value for N.sub.max is reached. The maximum value of N.sub.max is determined by the number of bits in the comparator 40, the random number generator 36 and the interval counter 38. If these are all sixteen (16) bits wide, the maximum value of N.sub.max is 65535. Of course in some applications N.sub.max can be restricted to a number smaller than that dictated by the counter bit widths.

In FIG. 5, the statistical distribution of inter-transmission intervals shown as N.sub.max is doubled in step-wise manner as herein described.

The invention thus ensures that a transponder population including only a small number of transponders will be read quickly. With a transponder population including a large number of transponders, the channel will initially be congested, but as the value of N.sub.max is increased, the congestion will be gradually decreased, until all the transponders have been read successfully.

Although a system including passive transponders 14.1 to 14.n is described herein, the system and method according to the invention may also be employed utilizing self powered transmitters or any other group of transmitters which transmit simultaneously. The communication channel between the reader and transmitters may be an RF channel through free space or air as described herein, but it could also be wave guides or cables and the signal may be any electromagnetic signal or even an acoustic signal.

It will be appreciated by those skilled in the art that the invention may be put into effect in a number of different ways. When the invention is implemented as a sub-circuit in a custom chip or in a discrete printed circuit board, a master controller (not shown) will be responsible for incrementing and resetting the various counters. It is also possible to implement the invention as an algorithm in the firmware of a micro-controller (also not shown).

It will further be appreciated that there are many variations in detail on the system, transmitter and method according to the invention without departing from the scope and spirit of the appended claims.