HK1023405A - Article tracking system - Google Patents

Description

Article tracking system

no marking

The present invention relates to Radio Frequency Identification (RFID) systems, and more particularly to an RFID system designed to continuously track items and people as they move through a building.

RFID products typically have three components: (1) a tag (identified item); (2) an interrogator (a device that detects the presence of a tag); and (3) a system (typically including a cable system, a computer, and software that links the tag with the interrogator to form a useful means). RFID products are typically designed to detect tags as they pass within range of several fixed or handheld interrogators.

RFID systems are often used as a high-end replacement technology for bar coding. RFID and related systems include passive RFID systems, active RFID systems, infrared ID systems, and Electronic Article Surveillance (EAS) systems.

Tags in a passive RFID system do not have an on-board power source. In such systems the interrogator transmits operating power for the tag. Such systems typically have a detection range of one meter or less, although somewhat longer ranges have been achieved. Typically these systems operate in the 125KHz radio frequency band.

Most passive RFID systems work as follows. The interrogator transmits an electromagnetic field for powering the tag. A coil in the tag is energized by the electromagnetic field, causing the tag's circuitry to "wake up". The tag uses this energy to send an identification signal back to the interrogator.

While most passive RFID systems are read-only (i.e., tags in such systems respond to an interrogation by reading information from their memory and sending this information back to the interrogator), tags used in some passive RFID systems have limited capabilities to receive information and instructions from the interrogator, such as read/write capabilities in smart cards (electronic money) and "electronic presentations" in industrial applications.

Passive RFID tags have been employed in connection with access control, smart cards, vehicle identification (AUI), waste management, item tracking, animal identification, manufacturing control, data processing, and various other purposes.

One of the basic design goals of any RFID system is to distinguish the weak signals emitted from the tags from the much stronger signals emitted by the interrogator. Some strategies to achieve this include:

the frequency is shifted. Circuitry in the tag receives the carrier wave from the interrogator, translates the signal to another frequency, and transmits a modulated reply to the second frequency;

half-duplex operation. The tag is charged by the interrogator. When the charging circuit of the interrogator is open, the tag responds with the stored energy;

modulated backscatter. The tag modulates its antenna effective cross-section to identify itself to the interrogator; and

delayed forwarding. Surface Acoustic Wave (SAW) devices retransmit the interrogator's carrier after a delay. The identification of the tag is indicated by a time change in the delayed acknowledgement.

Active RFID systems require tags that are powered by batteries. The battery enables a long detection range of 3 to 100 meters. These systems can locate tags with greater accuracy than passive RFID systems, typically operating in the 400, 900, or 2440MHz frequency bands. Active tags tend to be able to use the "symbol swap" of the tag with the interrogator to enable multiple tags within range of an interrogator, so that each tag transmits its signal in turn. Communication between tags and interrogators in active RFID systems is also generally faster than with passive tags.

Most active RFID tags respond to an interrogator when polled according to a communication protocol. Some active RFID tags spontaneously "frame" at predetermined intervals (transmit) signals. A marker's chirp signal is detected by an interrogator if the marker is within range of the interrogator.

Infrared Systems (IRIDs), although not RFID systems, attempt to detect and identify the location of moving markers. A typical IRID system includes markers that emit their chirp identification signals at randomized intervals. An infrared reader positioned in the ceiling detects these emissions and reports them to the host. The transmission rate from the tag to the reader is typically about 600 baud. A motion detector in the marker enables the marker to be emitted more frequently in motion. The indicia are typically about the size of the domino.

EAS systems are used for theft protection in retail environments. EAS markers are relatively unreliable, inexpensive, and have limited capabilities. Although they track moving tags, they are not generally considered RFID products because EAS tags are not coded and cannot be distinguished from each other.

A system for tracking a mobile tag includes multiple antenna elements with carrier signals that generate a carrier signal for reception by the tag. The tags respond by transmitting identification codes at randomized intervals, which codes are modulated on a carrier signal. The antenna assembly, which is disposed, for example, in the ceiling, receives the response and sends it to a unit controller, which processes them and uses them to calculate the marker position by approximation and triangulation techniques. The distance of a marker from a particular antenna element is calculated by measuring the round trip signal time. The unit controller transmits the processed data derived from the received signal to a host computer. The host computer collects the data and decomposes them into location measurements. The host computer archives the data into a data repository, such as an SQL server.

The conditional advantages of the present invention are as follows.

One of the advantages of the present invention is that it is designed to maintain constant communication with the tag while covering the complete facility. The system identifies and calculates the location of the marker even in the presence of severe multipath effects.

Another advantage of the present invention is that they employ tags that require low power consumption so that the duration of the tag that is powered is close to the duration of the tag battery itself. Also, a low power mode may be entered when the tag is not in use, thereby further saving energy.

Another advantage of the present invention is that it can be scaled. A small number of widely spaced antenna elements may be used to approximately locate the position of a marker within a facility. The auxiliary antenna assembly can be easily added to the system if more accurate marker positioning is required. But also to add new tags to the system without any reconfiguration of the system.

Another advantage of the present invention is that it mitigates problems caused by tag signal collisions. Because the tag spontaneously wakes up and randomly sends a "chirp" signal, it is unlikely that multiple tags will emit signals simultaneously. Moreover, in some circumstances the system can predict when marker signal collisions occur and can respond accordingly.

Another advantage of the present invention is that the tags can respond to multiple unit controllers simultaneously.

Other features and advantages of the invention will be apparent from the following description, and from the claims.

FIG. 1 illustrates an overview of a system constructed in accordance with the present invention;

FIG. 2 illustrates a plurality of cell controllers used in a multi-story building;

FIG. 3 is a block diagram of a tag RF design according to the present invention;

FIG. 4 is a block diagram of an alternative embodiment of the tag;

FIGS. 5A-5G are graphs of a signal as it passes through various stages of the system;

FIG. 6 is a block diagram of a unit controller RF design;

FIG. 7 is a block diagram of an active antenna assembly of a unit controller;

FIG. 8 is a block diagram of a modulator RF design;

FIG. 9 is a block diagram of a unit controller cable extender assembly;

FIG. 10 is a block diagram of a cell controller;

FIG. 11 illustrates the extraction of marker data from a series of correlations;

12A-C are illustrations of tagged datagrams;

FIG. 13 shows a tag incorporating a delay element; and

figure 14 shows several unit controllers receiving chains running in parallel.

Referring to FIG. 1, an item tracking system 100 includes the following general components;

marking: inexpensive small radio frequency transponder tags 101a-c are attached to the person or object being tracked. The tags 101a-c periodically "wake up" and "chirp" (send) a radio frequency encoded unique identification code (UID). The tags 101a-c are designed such that they range from 15-30m in a typical indoor environment, which is limited primarily by the need to maintain the life of the tag battery and the requirement that the tag 101a and tag battery be small and thin;

a unit controller: the cell controllers 102a-c detect the chirp signals of the markers 101a-c and calculate the distance of these markers 101a-c to the active antenna assemblies 104a-d connected to the cell controllers 102 a-c. Each antenna element preferably has a transmit antenna and a receive antenna. In fig. 1, antenna assemblies connected to the unit controllers 102b and 102c are omitted for simplicity. The unit controller 102a is typically contained in a housing and mounted behind a depending ceiling. The unit controller 102a may receive power via a conventional wall plug or the like. The unit controller 102a is connected to antenna assemblies 104a-d, which cover an area of the indoor facility 110, by coaxial cables 103a-d, respectively. A tag signal 107 transmitted by a tag 101a is received by one or more of the antenna elements 104a-d and processed by a chip, such as a Digital Signal Processing (DSP) chip, in the unit controller 102 a. The information thus processed is used to identify the transmitting tag 101a and the distance between this tag 101a and, for example, each of the receiving antenna assemblies 104 a-d;

a host computer: the unit controllers 102a-c are in data communication with a host computer 105, which collects data and available information from the former and archives the data into an open format database, such as an SQL server; and

the user application program: in a preferred option, the client workstations 102a-c communicate with the host computer 105 over a network, such as the LAN 115. Client applications running on each client workstation 102a-c may access the SQL server and provide data in a manner that is useful to the end user.

The tag 101a does not generate its own radio frequency signal. A direct sequence spread spectrum interrogator signal 106 is transmitted continuously by an antenna element, such as antenna element 104a, at a first frequency, such as 2440 MHz. Tag 106a receives this signal 106, modulates its UID code onto this signal 106, and immediately sends a frequency shifted signal 107 back to, for example, antenna 104a at, for example, 5780 MHz. The distance from the antenna assembly 104a to the tag 101a can then be determined by the unit controller 102a from the round trip transit time taking into account fixed and known delays in the line and electronics. The unit controller 102a can quickly switch between the antenna elements 104a-d to take the distance from the tag 101a to each antenna element 104a-d (which receives the return signal 107) and determine the location of the tag from that information by triangulation techniques.

The system 100 is designed to be scalable, adding unit controllers to existing unit controllers 102a-c and antenna assemblies to existing antenna assemblies 104a-d to improve the accuracy of determining the location of the markers. Fig. 2 shows how a block of unit controllers 102a-c can be implemented in a large multi-story building 110. As shown in FIG. 2, the multiple unit controllers 102a-c feed data to a single host computer 105, typically through a TCP/IP communications network. The use of TCP/IP is not required for system operation and various data protocols and transport mechanisms may be employed. For example, if a local area network is not available, the connection to the host can be through RS485, RS232, RS422, power line modem, or dedicated telephone lines. Alternatively, a dedicated modem designed for application over such a cable may be utilized.

Each unit controller 102a-c may be installed to cover a separate floor 130a-c, respectively, the exact configuration of which may be modified by system management personnel. A floor 130a, the unit controller 102a with its array of antenna assemblies 104a-d is mounted on a ceiling 140 a. The remaining layers 130b-c use the same device configuration. The antenna elements 104a-d are designed to provide good gain in the downward and horizontal directions while the upward gain is low, thus enabling the vertical position (i.e., floor) of a marker 101a to be determined by observing which antenna element 104a-d receives the strongest signal from the marker 101 a. Structurally, the ground plane is located behind each antenna to reflect signals downward. The horizontal position of the marker 101a is then determined roughly by observing which antenna component 104a-d receives a strong signal from the marker 101 a. The horizontal position of the marker 101a relative to the antenna assembly 104a can be more accurately determined by assessing the distance from the marker 101a to each antenna assembly 104a-d based on the combined time of transmission of the interrogation signal 106 and the marker signal 107. Each "unit" consisting of a unit controller 102a and its antenna assemblies 104a-d covers thousands of square feet of floor space. Each unit works independently, so that more units can be added without affecting the operation performance of the existing unit.

If the user wishes to locate the markers in "zones," an antenna may be installed in each zone. A user wishing to track one or more markers 101a-c moving in the hallway may install antenna assemblies 104a-d every 20 meters or so along the hallway 130a-c and calculate the linear position of the marker 101a by measuring the distance from the marker 101a to these antenna assemblies 104 a-d. A user wishing to triangulate the position of a tag 101a must install enough antenna assemblies so that the tag 101a will be within range of at least three antenna assemblies. A typical installation would cover a complete facility 110 with a combination of "zone" and "lobby" areas at a relatively low cost per square foot and update certain areas with enough antenna assemblies over time to triangulate marker positions. Tag RF design

Referring to fig. 3, tag RF circuitry 300 receives signal 106 at tag receiving antenna 301 and transmits tag signal 107 at tag transmitting antenna 312. The function of tag RF circuit 300 is to forward the incoming spread spectrum signal 106 by frequency conversion. A second function of the tag RF circuit 300 is to modulate tag data onto the transmitted tag signal 107 under the control of the microprocessor 308. The information transmitted on the flag signal 107 includes the flag sequence number, datagram header, and flag data 309 in a preferred embodiment of the invention, derived from, for example, a motion indicator or a low power indicator.

The input signal 106 is preferably a direct sequence spread spectrum signal in the 2440MHz band that is phase modulated by the cell controller 102 in two or right angles. Signal 106 is received by tag receiving antenna 301 which collects signal 106 and feeds it into tag RF circuitry 300.

After signal 106 is received by tag receive antenna 301, an RX (receive) band pass filter 302 ensures that the tag only receives signals in the 2440MHz ISM band, while rejecting radar signals, electronic news report signals, and the like. In an embodiment, the filter 302 is implemented as an etched coupled stripline filter embedded in a circuit board. The signal 106 is then amplified by amplifier 303 to ensure that the received signal can be mixed in a mixer 304 without degrading the signal-to-noise ratio (SNR).

The mixer 304 converts or shifts the carrier frequency from 2440MHz to 5780 MHz. The input signal with a center frequency of 2440MHz is mixed with the output of a Phase Locked Oscillator (PLO)305 having a center frequency of 3340 MHz. This results in a sum frequency 5780 along with a difference frequency and various harmonics and sub-harmonics that are removed by the band pass filter 306. In one embodiment, PLO305 is comprised of a Phase Locked Loop (PLL) chip having three inputs: (1) a sampled output from a Voltage Controlled Oscillator (VCO); (2) a reference tone from a 10MHz oscillator; and (3) a frequency programming interface to a microprocessor 308. This produces a pure tone with good phase noise at the 3340MHz marker LO frequency. In an alternative embodiment, the PLO305 outputs a 1670MHz tone, which is then doubled to obtain the desired 3340MHz result.

The next element of tag RF circuit 300 is a two-phase modulator 307 which can either pass the 5780MHz signal unchanged or change the phase of the signal by 180 deg. under the control of microprocessor 308. The modulator 307 is implemented as a single pole double throw RF switch 801 feeding a 180 ° hybrid, as shown in fig. 8. Several forms of modulation may be employed, including on-off keying (OOK) modulation, Binary Phase Shift Keying (BPSK) modulation, multiple phase shift keying (MPK) modulation, and Quadrature Amplitude Modulation (QAM). BPSK is a preferred form of modulation. The output from the modulator 307 is fed to an amplifier 310 and then filtered by a transmitter band pass filter 311, and the output of the filter 311 is transmitted as the marker signal 107 from a transmit antenna 312. Since the amplifier 310 operates at high frequencies, it consumes considerable power, and an alternative embodiment (such as that shown in fig. 4) that does not require such an amplifier 310 would be preferred. A Tx filter 311 implemented as a 5-pole filter is required to ensure that the signature conforms to the FCC part 15 requirements.

The tag RF circuit 300 shown in fig. 3 is used to explain the general functionality of the tags 101a-c in a practical and self-explanatory embodiment. Those skilled in the art will be able to combine multiple functions into a single component to save power and take full advantage of the available components, or to implement the same functions in a custom ASIC. Fig. 4 illustrates an alternative embodiment 400 that performs the same basic functions as shown in fig. 3, but with fewer components and less power. The main difference between the circuit 400 of fig. 4 and the circuit 300 of fig. 3 is that the modulator 404 of fig. 4 is placed before the mixer 406 in order to reduce the number of component parts (e.g., to eliminate the amplifier 310) and to save energy.

Instead of the mixer 304 (fig. 3) or the delay element 1505 (fig. 13), other transfer discriminators may be used for switching in other ways. For example, a tag such as tag 101a may be retransmitted using backscatter, by shifting frequency by mixing, by shifting frequency by taking harmonics, by shifting frequency by taking subharmonics, or by signal delay (e.g., by a SAW device).

Not shown in fig. 4, but desirable for the tag RF circuitry is the use of a common crystal reference for both PLO407 and clock timing in microprocessor 405. Accurate timing is an important, if not critical, characteristic of the system, so that the unit controller 102a-c can predict when a marker 101a will emit a marker signal 107. The use of the same crystal reference in the PLO407 and microprocessor 405 clock timing enables the unit controller 102a to accurately target the signal source (as will be explained later) by measuring the phase shift in the received signal and synchronize its clock timing accordingly.

Not shown in fig. 4, but desirable for some applications is an embodiment in which the transmit antenna 409 and receive wire 401 are combined into a single element, which utilizes a duplexer having a single antenna structure.

The manner in which the tags 101a-c are powered depends on the application. (note that fig. 3 and 4 omit the tag power supply). Typically, tag 101a will be battery powered, and the RF stage will turn the power on and off under the control of microprocessor 405. In a preferred embodiment, the microprocessor 405 enters a low power state where it simply waits until such time as the flag 101a is again powered up (power up). In another alternative embodiment, all of the tag circuits 400 are periodically turned on and off under analog control using the RC time constant in the circuit 400 as a timing source.

With the tag RF circuit 300 or 400 of fig. 3 or 4, if the tag 101a is within range of two of the unit controllers 102a-c and these unit controllers transmit spurious noise with low correlation characteristics, then the tag 101a will correctly retransmit both signals simultaneously.

The tags 101a-c require a time period on the order of one millisecond to charge and discharge. Typically during these brief periods, tags 101a-c will not be stable enough to be used, but will still transmit RF into the radio frequency channel via transmit antenna 409. For high performance applications, where the rf bandwidth is limited, a microprocessor controlled switch to tag transmit chain may be added to eliminate such spurious emissions.

The tag RF circuits 300, 400 shown in fig. 3 and 4 may be used in conjunction with different frequency pairs. The general scheme described above is applicable to any two allowed FCC spread spectrum segments. For example, the following combinations are permissible for unlicensed radio frequencies under FCC regulation section 15.247:

915MHz converted to 2440 MHz;

915MHz converted to 5780 MHz;

2440MHz converted to 915 MHz;

5780MHz converted to 915 MHz;

is converted to 5780MHz of 2440 MHz.

But does not require spread spectrum operation; two licensed narrow frequency bands may be utilized. But spread spectrum operation in the 2440 and 5780MHz bands is assumed for the remainder of this discussion. Marking with time delay

The tag RF circuits 300, 400 shown in fig. 3 and 4 employ frequency division multiple access, i.e., the tag circuits 300, 400 receive and transmit signals at different frequencies. An alternative embodiment 1500 employs time division multiple access, as shown in FIG. 13. For purposes of illustration, assume that marking circuit 1500 shown in FIG. 13 takes as input a signal at a frequency such as 915MHz at receive antenna 1501 and transmits this same signal at the same frequency through transmit antenna 1508 after a one microsecond delay. Assume that a cell controller, such as cell controller 102a, transmits a burst of interrogation signals 106 every 2 microseconds. A tag, such as tag 101a, takes this signal as input via receiving antenna 1501. The signal then passes through the elements 1502-1504 as shown in FIGS. 3 and 4. A delay element 1505 is then used to delay one microsecond. This signal then passes through a transmit bandpass filter 1507 and is transmitted by the transmit antenna. A SAW device may be used as delay element 1505. During this delay, the cell controller stops transmitting and the reflection of the interrogation signal 106 in this environment gradually drops to a minimum level. This half-duplex scheme allows single frequency operation, although the bandwidth is lower than when a full-duplex frequency-shifting scheme is used, as in the frequency variation signature, the delay-based signature can modulate the reply signal with a 180 ° phase shift. In other respects, the marker design 1500 shown in FIG. 13a is similar to those shown in FIGS. 3 and 4. Unit controller RF design

Fig. 6 shows the rf stages of a cell controller 102 a. The structure of an antenna assembly, such as the antenna assembly 104, is shown in fig. 7. Collectively, the unit controller 102a and its remote antenna elements 104a-d modulate a baseband square wave input onto a 2440MHz carrier, filter the resulting 2440MHz signal to accommodate FCC transmit requests, transmit the filtered 2440MHz signal through a selected antenna element, receive a returned 5780MHz tag reply through the same antenna element, extract the I (in-phase) and Q (quadrature) components of the demodulated baseband signal, and digitize the result for subsequent processing.

Fig. 10 shows the main components of the unit controller digital subsystem 650. In summary, the digital subsystem 650 provides a baseband input signal 601 and receives the demodulated response 107 from a tag 102a after a few nanoseconds. As noted above, the microprocessor 1001 may change the operating conditions of the radio frequency system by: (a) modifying the baseband input signal 601; (b) modifying chip rate, pseudo-noise sequence length, and/or pseudo-noise sequence code; (c) modifying the transmitting frequency 610 of the radio frequency transmitter 1002 and the receiving frequency of the radio frequency receiver 1003 within a narrow range; (d) modifying the transmission gain of the radio frequency transmitter 1002 and the reception gain of the radio frequency receiver 1003; and (e) converting the antenna elements 104 a-d.

The demodulated response 107 from the indicia 102a is split by the radio receiver 1003 into I (in-phase) and Q (quadrature) components and digitized by the digitizer 636. An integer DSP processor 1004, such as TMS320C54, compresses the output of digitizer 436 to perform correlation operations at high speed. If Binary Phase Shift Keying (BPSK) modulation is used at the transmitter, the I and Q channels are correlated separately and combined. For Quadrature Phase Shift Keying (QPSK) modulation, each channel must be correlated twice, once for each sequence. The relevant data from the integer DSP1004 is processed by a microprocessor 1001, such as a Pentium processor. To reduce cost and improve performance, lower power X86 processors and a floating point DSP processor such as a TMS320C30 may be used. Communication between microprocessor 1001 and host computer 105 is accomplished using a TCP/IP protocol, preferably Ethernet.

The data input to the transmit chain is a baseband input signal 601 that is a pseudonoise spreading sequence. The length of the sequence and the code encoded in the sequence are set by a unit controller microprocessor 1001 and can be changed according to signal processing requirements. Typically 31 or 127 bits, giving compression gains of about 15dB and 20dB, respectively. The 2440MHz and 5780MHz bands may support a 40MHz baseband input signal 601, while the cell controller 102a is designed to make it possible to take advantage of this full bandwidth.

Fig. 5A-5G show interrogation signal 106 at various stages through cell processor RF circuitry 600. Fig. 5A shows a square wave baseband input to modulator 500. Fig. 5A shows a square ribbon input to modulator 500. Fig. 5B shows this baseband input 510 being digitally correlated. Fig. 5C shows the output of the modulator 602 as viewed by a spectrum analyzer centered at 2440 MHz. FIG. 5D shows a spectrum analyzer plot of the marker signal 107 at a center frequency of 5780 MHz. Fig. 5E shows the demodulated response from the tag 107, split into its I (in-phase) 545 and Q (quadrature) 540 components. Fig. 5F shows the digitally correlated I and Q components 550. Fig. 5G shows the negative value 560 of the second derivative of the correlated waveform combining the I and Q components.

Modulator 602 (fig. 6) modulates baseband input 601 onto a 2440MHz carrier. Various forms of modulation are available and are well known to those skilled in the art. For BPSK modulation, the modulator 602 is implemented as a single pole double throw RF switch 801 feeding a 180 ° hybrid combiner 803, as shown in fig. 8. The modulator 602 is preferably implemented as a QPSK modulator, which is identical to a BPSK modulator in that one channel is offset by 90 ° from the other, and each channel is driven by a different baseband sequence with acceptable cross-correlation properties. Higher order modulation is also possible. The modulation by modulator 602 results in sidelobes that extend hundreds of MHz, which must be filtered out to meet FCC requirements. The 2440MHz band has a nearby band that introduces very strong filtering requirements, for which it is preferable to employ a SAW filter 607 that combines a wide passband with a tight stopband in this illustrative embodiment. The wider passband supports faster trim rates in the baseband input signal 60, but the narrower passband provides the opportunity to take advantage of a wider range of frequency diversity to avoid interference sources and/or support advanced signal processing techniques. The modulator 602 must operate at the same frequency as the IF filter 607 is feasible, typically in the 200 to 400MHz range. A pre-amplifier 606 is required before the SAW IF filter 607, and the output of the filter must be amplified by an amplifier 608.

The transmit IF oscillator 605, like all other RF oscillators in the unit controller circuit 600, is phase locked to a 10MHz crystal source 603, which is distributed to each oscillator through a filter and splitter network 604. The 10MHz source 603 must be within a few KHz of the 10MHz sources on the tags to avoid excessive baseband phase shift.

The output of the IF filter 607 (from amplifier 608) is then mixed by mixer 609 with the output of a phase locked oscillator (PLQ)611 and converted to the carrier frequency of 2440 MHz. The frequency of the PLO611 may be modified within narrow ranges under microprocessor control 610 in order to provide the required frequency diversity to avoid interference sources and/or for various advanced signal processing techniques. The degree of frequency diversity available is related to the technical details of the IF filter 607, with narrower filters allowing lower trimming rates but greater frequency flexibility. A filter that is typically required to remove unwanted harmonics and different frequencies from the output of the mixer 609 is not shown in fig. 6.

Following the mixer 609 is a driver amplifier 612 that boosts the power level of the signal 106 to enable driving of the signal 106 along the cable 103a to the remote antenna assembly 104a and buffers the output of the mixer 609 for a band pass filter 413. An RF band pass filter 613 is used to remove the non-FCC compliant output of the mixer 609. Directional coupler 616 provides a port to examine signal 106 before it is transmitted to remote antenna elements, such as antenna elements 104 a-d.

An attenuator 614 under microprocessor control 615 allows the signal processing software to reduce the output power when it is known that the markers 101 a-d are nearby. This may be useful in situations where it is known that nearby markers are being overdriven by the unit controller, and/or where the signal processing software needs the markers to operate in a more linear range.

The signal is then fed into a duplexer 618 which combines the transmit signal 106 and the receive signal 107 onto a single wire. The duplexer 618 is a high pass/low pass filter combination 619a that attenuates the received signal 107 with respect to the transmitting side and the transmitted signal 106 with respect to the receiving side. Because of the presence of the Tx and Rx bandpass filters 613 and 624, the specifications of this duplexer 618 may be less stringent.

The unit controller RF stage 600 shown in fig. 6 supports one remote antenna assembly 104a-d at a time. In order to support multiple antennas from the same unit controller, the system requires a switch 619 that allows the microprocessor control 620 to quickly switch from one antenna to the next. Switch 619 takes the RF and transmits it to one of n cables, where n is 8 or 16, for example. Switch 619 also provides DC power to the selected line. The RF signal is coupled into the cable through a DC blocking capacitor (not shown) and DC power is coupled to the cable through an RF choke that blocks RF. Thus, the DC and RF travel together through a single coaxial cable to the selected antenna.

The rise time of DC in an antenna is in the 100 microsecond range, limited by the effective resistance and characteristics of the circuitry in the antenna and the capacitance required for operation. To make the antenna switching time in the microsecond range, the DC power to the antenna is pre-loaded before the RF is turned on.

Referring to fig. 7, in an antenna system 700, the combined DC and RF signal arrives through a coaxial cable, such as cable 103a from unit controller 102 a. An offset tee 701 separates the RF signal 710 from the DC signal 712. The DC signal 712 is provided to the Tx/Rx power control logic 702, which in the simplest embodiment is a filter that removes line noise and provides a clean 5 volt supply. The RF output 710 from the bias tee 701 is fed to a duplexer 715, which is the same as duplexer 618 in the unit controller 102 a. The RF output 710 is then amplified by amplifier 703 to a FCC allowed power level and filtered by filter 704 to remove line and amplifier noise to fit FCC regulations. The resulting signal is then directed to transmit antenna 705.

In this embodiment the transmit 705 and receive 706 antennas are patch arrays providing reduced power in the vertical direction and expanded power in the horizontal direction so that power is not wasted in the floor and ceiling and energy radiated upward is minimized. The response 107 at 5780MHZ from the tag 101a is filtered by the filter 707, amplified by the amplifier 708, and sent back to the cable 103a to the unit controller 102 a.

The system is designed to use cables 103a-d of standard length, for example 20 metres. The cable expander assembly 900 connects two lengths of cable and supports an expanded cable length. Referring to fig. 9, the elements of the assembly 900 utilize a DC power supply 910 from the cable 103 to drive low noise amplifiers 903, 904 that provide sufficient gain to drive the next length of cable. Bias tees 906, 907 separate the DC power supply 910 from the RF signal, and duplexers 908, 909 operate to separate the transmit signal 106 from the receive signal 107.

Referring to fig. 6, the signal returning from the antenna assembly 104a to the unit controller 102a is passed through switching elements 621, 619 and duplexer 618 to the unit controller receive RF chain 622. The signal passes through a combination of a preamplifier 623 and a band pass filter 624, the configuration of which is determined depending on the selected component. A digitally controlled receive attenuator 625 under microprocessor control 626 is used to avoid saturating the receive chain when the marker 101a is known to be nearby. This is necessary to avoid losing the relationship between the I and Q components of the received signal 107, to properly correlate, and other signal processing.

The signal then enters the I-Q zero IF demodulator circuit 627 633. As previously noted, the microprocessor RX frequency control 635 must be placed in series with its counterpart in the transmit chain. The resulting signal, as exemplified in fig. 5E, is fed to a digitizer 636 (fig. 10) in preparation for digital signal processing.

The above embodiments are simplified on the assumption that the unit controller can only transmit and receive from one antenna at a given time. Improved performance can be achieved by selecting transmit and receive antennas that are independent of each other. Software in the unit controller determines which antenna assembly receives the best signal from the tag. For example, if a particular tag, e.g., 101a, is near an antenna, e.g., antenna 104a, then antenna 104 will receive a strong signal from tag 101 a. Cell controller 102a then transmits a signal, such as signal 106, from antenna 104a and receives the repeated response 107 at antennas 104b, 104c, and 104d in turn. This may allow the antennas 104 b-d to receive stronger signals than would be received by the antennas 104 b-d if the signal 106 and the received signal 107 were transmitted from each antenna assembly 104 b-d independently.

The design 1600 shown in FIG. 14 provides for multiple receive chains 1610 a-1610 n to operate in parallel. Each receive chain 1610 a-1610 n includes IQ demodulators, digitizers, and related elements represented as DSPs, e.g., integer DSPs 1620. Implementing each receive chain on a separate card provides scalability. The use of multiple receive antenna elements for the same transmit signal allows the unit controller signal processing software to utilize spatial processing techniques to eliminate multipath effects. These techniques exploit the fact that responses degraded by multipath effects will have different characteristics at each antenna. Bit detection

In an ideal environment, a simple triangular correlation peak can be derived from the received signature signal 107, as shown in FIG. 5B. The distortion introduced into the radio frequency chain, particularly that caused by indoor multipath effects, results in a distorted but still distinct correlation peak, a function of which is shown in fig. 5. For bit detection, the point is to reliably detect the presence of a series of correlations that indicate the operation of the mark. Fig. 11 shows how the tag data is extracted from a series of correlations. In the left half of the 1110 pattern shown in FIG. 111, the markers emit "zeros". This is achieved by arranging the modulator 307 of the tag to pass the interrogator signal 106 unchanged. When the received marker signal 107 is correlated with the transmitted pseudo-noise sequence, substantially the same correlation peak is obtained. Here, three such peaks 1120 a-c are illustrated. In the time of the fourth correlation 1120d, the phase of the inverse modulator is marked 180 °, indicating a "1", as shown in diagram 1110. Since the modulation is changed in the middle of a bit, the fourth correlation data peak 1120d is corrupted and preferably omitted. The fifth and sixth correlation peaks 1120 e-f clearly reflect the 180 phase shift.

The pseudo noise sequence may be changed under microprocessor control of the unit controller. When the presence of a marker is first detected, a relatively short sequence must also be applied, as shown in FIG. 11. Once the bit timing of the marker is determined, it is possible to use longer sequences for improved SNR, which helps in determining the distance.

An important case to consider, not shown in fig. 11, is that the equalization between the in-phase (I) and quadrature (Q) components of the received signal will shift over time. This is because the 10MHz source in the cell controller 102a and tag 101a will typically phase difference KMz. This factor can be calibrated by phase difference between adjacent correlations, and can be detected at baseband by noting changes in the in-phase (I) and/or quadrature (Q) components of the received signal. As noted previously, the same calibration process as this can be used for the reference cell controller to calibrate the mark clock, allowing for precise prediction of the time of a mark chirp signal without the need to precisely measure the timing of the mark bit transitions. Interaction between unit controller and tag

Each marker is an independent unit that is always unknown to the outside. Each tag has a unique identification code (UID) associated with the tag at the time the tag is manufactured. The mark periodically wakes up to convert any 2440MHz input signal 106 to a 5780MHz output signal 107 over a short period of time while modulating its UID and other data onto the signal 107 of its chirp output. This tag does not communicate with other tags. This tag does not explicitly respond to the interrogation signal but merely forwards any input signal 106 in the 2440MHz band, which may or may not include a pseudo-noise sequence from a nearby cell controller antenna assembly 104 a. This arrangement greatly simplifies the design and manufacture of the tag 101 a.

At some time period, two or more tags will be forwarded simultaneously. In many cases, one of the two tags will return a stronger signal than the other tag, and some data will be lost in such a collision. To avoid collisions in a repetitive pattern, the tag "wakes up" at a randomized number and chirps the UID, which can be computed (by the tag and unit controller) from a pseudo-random number generator that incorporates the tagged UID. For example, for a chirp mark of about every 5 seconds, the mark generates pseudo random numbers between 0.0 and 2.0 which add to the minimum delay time of 4.0 seconds, resulting in a series of delay times uniformly distributed between 4.0 and 6.0 seconds.

It is possible to change the seed over time with analog input to the pseudorandom number generator, such as input from an internal clock or delay of an RC circuit, but a purely digital scheme is more advantageous to enable the unit controller 102a to accurately predict a known marker chirp time. A typical pseudo random number generator has the form:

n = rand (seed) formula 1

The resulting N is used as a seed for the next pseudo random number in this sequence of pseudo random numbers. With this type of pseudo-random number generator, it is possible that two tags will utilize the same seed, such that their tag signals repeatedly collide. Moreover, due to the small differences in the tag clocks, all pair markers will eventually drift through this synchronized state within some time. To avoid these situations, it is desirable to incorporate the UID of each tag into the delay time for that tag, as noted above, resulting in a different pseudo-random sequence for each tag, which is:

delay) = f (N, UID) formula 2.

A simple example of such a function is:

n = Xor (Delay, Bit Rotate (UID, AND (N,11112)) formula 3

Referring to equation 3, it is possible to reconstruct the seed from UID, Delay AND (N,1112) by calculating N = Xor (Delay, Bit Rotate (UID, AND (N, 11112)).

Referring to fig. 12a, an embodiment of a tag datagram 1400 includes a header 1401 enabling the unit controller to detect the presence of the tag, followed by an identifier, a preamble 1402, and then a tagged UID 1403. The title 1401 may be zero length. The identifier preamble 1402 may be implemented, for example, as a validity check such as a Cyclic Redundancy Check (CRC). With sufficiently simple delay function and high clock stability, the cell controller can derive a tagged chirp sequence by recording the timing of the chirp signal for a series of datagrams 1400.

Referring to fig. 12b, in another embodiment of marking the datagram 1410, the marking is provided with Delay information 1414 to enable the unit controller to predict the time of transmission of the next and subsequent chirps of the marking of the datagram 1410. In the example of equation 3, this information includes data: delay AND AND (N, 1112).

Referring to figure 12c, in another embodiment of the tag datagram 1420, a shorter header is employed than in the datagrams 1400, 1410 of figures 12a and 12b so that the unit controller does not guarantee sufficient time to detect the presence of the tag before the UID1423 contained in the tag datagram 1420 is transmitted. Attachment to the datagram 1420 is the next chirp's transmit delay 1425, enabling the unit controller to predict the time when the tag will next chirp out of its datagram 1420, even if the unit controller does not have enough time to identify the tag's identity from the chirp of the first received datagram 1420. The unit controller can then detect this next chirp and determine the identity of the tag at that time. Once the mark is identified, the unit controller may copy the pseudo-random number generator of the mark to calculate the full future chirp time by the mark. In the tag datagram 1420 of figure 12c, a series of dedicated synchronization bits 1424 are inserted between the UID1423 and the delay information 1425 to reliably determine when the UID1423 ends; in this case, UID1423 must be defined such that it does not include the synchronization sequence or its inverse.

Fig. 12a, 12b and 12c include optional data segments 1404, 1415, 1426 that enable a tag to transmit data to the unit controller. These segments 1404, 1415, 1426 may include data from within the tag, such as from a motion detector or a low power indicator, or from an external device attached to the tag, such as metabolic information for medical telemetry when the tag is attached to an individual.

An identifier preamble associated with this tag UID precedes the tag UID. This identifier preamble enables the unit controller to quickly verify that a tag will chirp as desired without having to decode the tag's full UID. This enables the unit controller to engage in other operations, such as communicating with different tags in the vicinity of other antennas. This identifier preamble 1402, 1412, 1422 and the flag UID1403, 1413, 1423 are externally set and may be defined to include error correction bits if appropriate.

The tagged UID may be hard-coded into the tag (e.g., as a serial number). The tags may be grouped according to their UIDs, and different groups may be associated with different unit controllers. Each unit controller contains information (received from another source) about which tags are in the set associated with that unit controller. When a unit controller receives a token signal, the unit controller can extract UID information from the token signal to determine whether the token signal was transmitted via a token in the group associated with the unit controller.

In the marked datagrams 1400, 1410, 1420 of fig. 12a-c, the delay information fields 1414, 1425 and datagram segments 1404, 1415, 1426 may also include error correction bits. To simplify processing, data may be reduced to a nibble stream. To determine what value to send for a particular nibble, the flag may look up the nibble's value in a table containing, for example, 8 bits of the value representing this nibble plus error correction information. A single unit controller may process all three types of datagrams 1400, 1410, 1420 shown in figures 12 a-c. The choice of datagram type depends on the application requirements for a particular tag.

The amount of time required for a cell controller to detect the presence of a marker depends on the nature of the cell controller design. For example, a time of 100 microseconds to switch antennas is effective when the cell controller cycles between 16 antennas. To ensure that a tag is identified when it is first received by the unit controller, the tag datagram header must be long enough to give the unit controller time to test all of its antennas. If the performance requirement is in the range of 100 marks per second, an additional 2 or 3 milliseconds can be tolerated in this header. However, for higher performance requirements, or when tag power consumption must be minimized, the performance of the unit controller must be improved or a tag datagram 1420 of the type shown in fig. 12c must be employed.

By predicting the time of transmission from a particular tag, the unit controller can collect tag information from multiple antennas at the party being programmed to better compute tag location by exploiting antenna and/or frequency diversity. If a tag responds exactly when it is expected to respond, the unit controller does not have to detect every bit transmitted in the tag datagram in order to reasonably determine that a signal is being received by a correct tag. A correct identifier preamble that arrives exactly on time is almost certainly from the expected signature. This provides the unit controller with the opportunity to test multiple antennas that may or may not be able to communicate with the tag.

If it is desired to track markers between marker datagram transmissions, the marker groups may be configured to allow shorter transmissions to occur more frequently. For example, if a tag is configured to send its datagrams chirped on average every 10 seconds, it may also be configured to transmit shorter codes more frequently, e.g., every half second. This shorter code can be as short as one bit long and only needs a few microseconds to transmit. Thus, even so many hundreds of transmissions per second consume only a small percentage of the communication channel. The unit controller may predict the exact timing of each such transmission so that the signals are matched to the originating tag according to the time of transmission. The error correction code may be configured such that a long chirp signal from one marker is generally not corrupted by a fast chirp signal from other markers. The unit controller has data forecasting most of such conflicts.

When a tag initially comes within range of a cell controller, collisions between the transmission of datagrams of different tags will occur in an unpredictable manner. The transmission of a marker that has recently entered the area, or a marker that spontaneously increases its transmission rate (e.g., in response to a motion detector or "panic button"), cannot be predicted by the unit controller 102a, and may cause data corruption. But once the marker is identified, previous conflicts can be modeled to discard the problematic data. On the other hand, if the signals from two tags collide, the unit controller may select an antenna so that the antenna will receive a stronger signal from one tag than the antenna will receive from the other tag.

In a more advanced tag design, the unit controller has means for sending information and instructions to the tag during the time the unit controller knows that the tag is in operation. Such instructions may include commands to be passed to devices connected to the tag. The unit controller can download such information, most simply by on-off keying, or for more advanced flags by flipping the pseudo-noise bit sequence to indicate a 1 or 0. Typically, the downlink (download) approach is driven by the cost of the tags and feature requirements, with higher bit downlink transmission rates requiring more expensive receivers that consume more power. In this way, a single cell controller can simultaneously support read-only marks, read/write marks, and high speed read/write marks, the cell controller adapting its behavior based on the features supported in a particular mark.

The timing of the transmission from the tag to the unit controller depends on the item being tagged. The inventory and equipment may be arranged to be fired relatively infrequently, for example once per minute. More frequent transmissions will be required for markers on personnel, for example in a security facility. For a read/write scheme of the marks, the transmission timing may be modified according to a command from the unit controller.

Alternative marker designs allow for varying firing times depending on environmental factors. For example, an automatic detector may be positioned in a marker to reduce the time between transmissions when the marker is in motion. As another example, the indicia is transmitted more frequently and at a higher power when the tag has been tampered with. As another example, a tag may incorporate a slightly modified Electronic Article Surveillance (EAS) device that may cause the tag to transmit its UID more frequently when in range of a standard EAS detector. More generally, if a tag is attached to another electronic device, the emission interval may be modified under the control of that device. Marking power supply

The tags 101a-c transmit low RF power levels for increased portability and uptime. In addition, the marker signal emission 107 is designed to be of only a few milliseconds in duration. Thus, even if a tag forwards its UID every few seconds, careful tag design can bring the tag's battery life close to the battery's own shelf life. For lower power applications, a motion detector may be incorporated into the marker to enable, for example, transmissions to be less frequent when the marker is stationary.

For some cases, battery replacement may be accomplished by installing a battery in an attachment. For example, the reusable tag electronics may be attached to a disposable patient's bracelet, and the battery included in the bracelet. As another example, a battery may be incorporated into the band of the ID bracelet. More generally, the battery may be incorporated into an inexpensive, easily handled portion of the active RFID tag, and the electronics may be in other, more expensive portions.

If the item to which the tag is attached is the power source itself, the tag will tap into the power source. This solution is most practical in situations when the tag can be designed into the device itself (e.g. a handheld computer) or when the device and power source are large (e.g. a forklift). A larger power supply allows for a longer marking range. Estimating marker position

The marker signal 107 is received at a time that is the sum of: (1) the known fixed delay in the cell controller 102a that transmitted the interrogation signal 106 due to its circuitry and wiring to and from its antenna assemblies 104 a-c; (2) a fixed time delay in the antenna assembly 104 and the tag 101 a; and (3) the time required for the interrogator signal 106 and the tag signal 107 to travel through the air.

Because (1) and (2) are fixed, attention can now be turned to (3), the interrogator signal 106 and the tag signal 107 travel time through the air. The duration of the pseudo-noise sequence modulated on the 2440MHz carrier signal 106 by the unit controller 102a must be greater than the combined travel time of the signal 106 and the marker signal 107. Techniques for correlating a pseudo-noise sequence are well known in the art. In the absence of multipath effects, the unit controller 102a may derive a simple triangular correlation peak from the received marker signal 107, as shown in fig. 5B. In most indoor environments, however, the actual received signature signal appears more like that shown in fig. 5D-5G. The radio frequency signal in a room is susceptible to multipath effects caused by reflections from various surfaces such as white walls, fluorescent lamps, file cabinets, elevator shafts, steel beams, and the like. When a tag 101a transmits a tag signal 107, the sum of the direct tag signal 107 and the reflected signal is received at the unit controller antenna assembly 104. Various measures may be taken to extract the correlation peak from such information, the particular measure being selected according to the available signal quality, processing power and required performance.

A 40MHz chip rate results in a correlation peak with a rise time of 25 nanoseconds, corresponding to a rise time distance of about 25 feet. Since the marker position is calculated using the round trip time, the accuracy of the single chip allows the marker distance to be calculated within about 12 feet without any prior signal processing.

The approximate position of a marker may be calculated by indicating the time at which the associated signal-to-noise ratio exceeds a predetermined level. Accuracy can be improved by experimenting with a small number of different carrier frequencies and selecting an earliest boosted carrier frequency, such frequency diversity being supported by the radio frequency systems shown in fig. 4-6. This scheme is sensitive to the signal-to-noise ratio of the system.

An alternative solution is to find the peak of the correlation function. To improve the result, the signal delay is measured by taking the inverse of the second derivative of the cross-correlation function and finding the position of its peak, as shown in fig. 5G.

For maximum accuracy, the MUSIC algorithm, well known in the art, can be used, for which there have been reports of accuracy in the 0.01 chip range. MUSIC requires frequency diversity supported by the radio frequency system shown in fig. 6 disclosed herein. This method is based on the decomposition of the eigenvector space of the pseudo-noise correlation matrix of the delay profile (profile) data vector. Frequency diversity is required when information is provided at each distinct frequency to resolve additional multipath components. For markers that are mostly stationary, the required data can be collected and calculations done as a background process. For inventory applications, motion detectors may be incorporated into the markers and then the unit controller notified whenever their position requires a new calculation.

In situations where sufficient data collection time, antenna diversity, frequency diversity, or processing power is not feasible, various heuristic techniques may be utilized to calculate the marker position, even in the presence of severe multipath effects. The positioning of the antennas, which can be applied if desired, can be estimated using techniques well known in the art.

In many environments, while precise accuracy is not required in calculating the marker position, it is still important to calculate the marker position relative to a layer or barrier. Layer-by-layer identification in a building can be achieved by installing a downward radiating antenna in the ceiling (or an upward radiating antenna in the floor), as shown in fig. 2. Similarly, antennas mounted laterally to horizontal partitions may be positioned relative to the partitions. A relatively narrow beamwidth antenna that is less sensitive to multipath effects may be directed toward doorways and the like.

The unit controller antenna 104a may be mounted adjacent to a computer screen with the coverage surface corresponding to the viewing angle of the screen. The software may then be configured to automatically configure the operating system for personnel within range, or blank the screen for security purposes depending on who is or is not within range. Similar concepts may be applied in connection with copiers, microfilm readers, security devices, and the like.

A single antenna assembly may include three individual antennas arranged in a triangle. An indication of the angle of a marker may be determined by using in-phase (I) and quadrature (Q) components of the return signal for phase difference comparison. In high frequency embodiments, such as 2.45 gigahertz, such antennas may be well within inches of each other.

Heuristic techniques can be used to analyze the correlation profile to estimate the correlation onset time, i.e., the time at which the correlation peaks begin to be distinguishable by a "noise" baseline. Frequency diversity can provide various samples and the best of them can be selected. A perfect estimate can be achieved by pattern matching the correlation peak with one of a carefully explored collection of typical correlation profiles. For calibration purposes, markers may be placed at known fixed locations, and markers passing near these locations will likely exhibit similar relative contours. Such fixed markers may also be used to detect the presence of a source of interference (a target that intentionally or unintentionally emits an interfering signal) and provide a real-time test stand for testing various anti-interference techniques.

Antenna diversity is the most important tool for improving the accuracy of marker position calculations. If low accuracy is required, the antennas may be arranged so that only one or two antennas are within range of a given tag. In this case the triangulation data is not sufficient and there is only enough information to detect the presence of a marker at any one time and to estimate the distance of this marker from one or both antennas. The approximate orientation of the marker can be estimated from the signal strength of the antenna designed for this purpose and known to those skilled in the art and indicates that such orientation tends to reflect the strongest signal received, which may include significant multipath components. Conversely, for areas where high accuracy is required, for example, a relatively narrow beamwidth antenna diversity may be installed at the entrance, which together also provides a clear location pattern

Antenna diversity also provides scalability of the system. For other facilities, or parts within a facility, that do not require high accuracy in calculating the marker position, substantially non-directional and/or ceiling mounted antennas may be mounted remotely from each other for lower cost per square foot coverage. For facilities or parts within a facility that require high position accuracy, closely spaced and/or directional antennas can provide high accuracy at a higher cost. Unit controller operation

The design of the overall system, and the fact that the intervals between the transmissions of the marker signals are generated pseudo-randomly, provides the possibility of safe operation. A token dispersed at the entrance of a secure facility can be tracked through the facility and can emit a special code when the token is tampered with. While the marker code may be determined by monitoring the response of the marker, the transmission interval of the marker may be changed according to an algorithm that may be configured to be known only to the marker and the host and not possible to determine directly without corrupting the marker. The indicia may include, for example, an element such as a physical element for reprogramming its code and transmission interval. For example, an optical ID with an incorporated tag may be reprogrammed each time a person with the optical ID passes a security checkpoint, possibly in combination with biometric techniques.

To fully cover a facility, multiple unit controllers covering some overlapping area may be installed. Although each cell controller will operate according to the search and data collection method, rapid movement between antennas, pseudo noise codes, changing trim rates, etc., will all appear as random noise to the other cell controllers. In addition, codes with known cross-correlation properties, such as Gold codes, can be assigned by the host computer to the individual cell controllers, particularly the codes used for search tagging. On the other hand, the unit controller may randomly switch the selection of the pseudo noise code.

For markers on the boundary between two element controllers, each element controller reports the distance of the marker from its antenna assembly. The central host 105 aggregates the data to calculate the location of the marker.

A wide variety of possible pseudo noise codes are available for use by the unit controller. Thus, if one code appears to be receiving interference from other users of the spectrum, the cell controller may select another code. The tag, which is essentially a transponder, need not know the specific code in the application. Also, if another user causes difficulty, the center frequency may be adjusted somewhat.

Other embodiments are within the scope of the following claims. For example, the order in which the steps of the invention are performed can be varied through practice in the art while still achieving desirable results.

Claims (38)

1. A tag for relaying a radio frequency interrogator signal, comprising:

a receiver to receive an interrogator signal at a first frequency;

a mixer to generate a radio frequency transmit signal at a second frequency from the received interrogator signal;

a microprocessor generating a tag specific data signal;

a modulator that modulates the tag data signal on the transmission signal to generate a first tag signal; and

a transmitter transmits a first marker signal at a selected time.

2. A tag for relaying a radio frequency interrogator signal, comprising:

a receiver to receive an interrogator signal at a first frequency;

a delay that generates a radio frequency transmit signal from the received interrogator signal;

a microprocessor generating a tag specific data signal;

a modulator that modulates the tag data signal on the transmission signal to generate a first tag signal; and

a transmitter transmits a first marker signal at a selected time.

3. A tag for relaying a radio frequency interrogator signal, comprising:

a receiver to receive an interrogator signal at a first frequency;

a signal transmission discriminator for generating from the received interrogator signal a radio frequency transmission signal distinguishable from the interrogator signal at the interrogator;

a microprocessor generating a tag specific data signal;

a modulator for modulating the tag data signal on a transmission signal to generate a first tag signal; and

a transmitter transmits a first marker signal at a selected time.

4. The tag of claim 3, wherein:

the signal emission discriminator is a mixer.

5. The tag of claim 3, wherein:

the signal emission discriminator is a time delay device.

6. The tag of claim 3, wherein:

this selected time is generated pseudo-randomly.

7. The tag of claim 3, wherein:

the difference between the two selected times is a function of the content of the marking data signal.

8. The tag of claim 3, wherein:

the tag data signal uniquely identifies the tag.

9. The tag of claim 3, wherein:

the tag data signal includes a header and a unique tag identifier.

10. The tag of claim 8, wherein:

the tag data signal also includes a validity check.

11. The tag of claim 8, wherein:

the marked data signal additionally comprises error correction bits.

12. The tag of claim 8, wherein:

the tag data signal includes data derived from a target associated with the tag.

13. The tag of claim 8, wherein:

the marker is in a low power state between consecutive transmissions of the first marker signal.

14. The tag of claim 9, wherein:

the microprocessor changes this unique tag identifier in response to user input.

15. The tag of claim 4, wherein:

the microprocessor also changes the selected time in response to user input.

16. The tag of claim 4, wherein the modulator comprises:

a modulator that modulates the transmit signal using on/off keying modulation.

17. The tag of claim 4, wherein the modulator comprises:

a modulator for modulating using binary phase shift keying modulation.

18. The tag of claim 4, wherein the modulator comprises:

a modulator for modulating using multiple phase shift keying modulation.

19. The tag of claim 4, wherein the modulator comprises:

a modulator for modulating using quadrature amplification modulation.

20. The tag of claim 7, further comprising:

the microprocessor causes a second marker signal to be periodically emitted.

21. The tag of claim 4, wherein:

the tag receives and responds to multiple interrogator signals simultaneously.

22. A method for estimating the position of an object, comprising the steps of:

sending an interrogator signal;

receiving the transmitted interrogator signal;

transmitting a marker signal from a marker associated with the target;

receiving the transmitted tag signal at least one receiving antenna, the transmitted tag signal being received at a receiving time at the receiving antenna at the respective receiving antenna;

deriving an identity of a tag from at least one received tag signal at a receiving antenna;

the position of the tag is estimated as a function of the position of the subset of receive antennas receiving the tag signal and the time at which the tag signal is received at the subset of receive antennas.

23. The method of claim 22, wherein receiving the flag signal comprises the steps of:

for each antenna in a set of at least one antenna, determining whether the antenna is receiving a tag signal; and

if the antenna is receiving a tag signal, the antenna is identified as the receiving antenna for the tag signal.

24. The method of claim 22, said step of transmitting further comprising the step of:

modulating a pseudo-noise sequence onto a carrier signal for generating the interrogator signal; and

the pseudo-noise sequence is extended after identifying the antenna as the receiving antenna for the tag signal.

25. A method for estimating the position of an object, comprising the steps of;

transmitting an interrogator signal at a transmission time;

receiving the transmitted interrogator signal;

transmitting a marker signal from a marker associated with the target;

receiving the transmitted tag signal at least one receiving antenna, the transmitted tag signal being received at a receiving time of the receiving antenna at the respective receiving antenna;

deriving an identity of a tag from the tag signal received at the at least one receiving antenna;

determining, at a unit controller, a distance from each receiving antenna to the tag based on the transmission time and the receiving time of the receiving antenna; and

the position of the marker is estimated as a function of the distance from a subset of the receiving antennas to the marker.

26. A method for detecting the presence of a label comprising the steps of:

continuously transmitting, at a cell controller, a first signal at a first frequency;

at the tag, receiving the first signal and forwarding the first signal at a second frequency at the selected time as a second signal;

receiving the second signal at the cell controller; and

in the unit controller, the presence of the mark is detected upon receipt of the second signal.

27. A method for measuring the distance from an antenna to a marker, comprising the steps of:

transmitting, at the antenna, a pseudo-noise sequence on a first signal at a first frequency at a transmit time;

here labeled. Receiving a first signal and forwarding the first signal as a second signal at a second frequency;

receiving, at the antenna, a second signal at a receive time; and

at the unit controller, the distance from the antenna to the tag is determined based on the time of transmission and the time of reception.

28. A cell controller comprising:

an antenna for transmitting a cell controller signal at a first frequency and for receiving a tag signal from a transmitting tag at a second frequency;

a quadrature demodulator for extracting in-phase and quadrature components of the received marker signal; and

the in-phase and quadrature components are digitized for subsequent processing.

29. A cell controller comprising:

circuitry for transmitting a cell controller signal modulated on a carrier signal; and

circuitry for receiving a marker signal transmitted by a marker at a selected time.

30. The cell controller of claim 29, further comprising:

circuitry for determining the identity of the tag from the received tag signal.

31. The cell controller of claim 30, further comprising:

circuitry for determining from the received marker signal the time at which the marker next transmits a marker signal.

32. A system for monitoring the position of an object, comprising:

at least one cell controller unit coupled to the at least one transmit antenna for transmitting a cell controller signal modulated on a carrier signal at a first frequency and at a selected transmit time, and coupled to the at least one receive antenna for receiving a reply tag signal containing a tag datagram at a second frequency and at a receive time;

at least one flag cell having a conversion circuit for converting each received cell controller signal into the flag signal; and

and the at least one calculating unit is connected to each unit controller and used for calculating the position of each marking unit according to the receiving time of at least one receiving antenna.

33. The system of claim 31, wherein: the cell controller may be configured to be connected to a variable number of additional antennas.

34. A system for monitoring the position of a target, wherein:

at least one receiving antenna connected to a unit controller is mounted adjacent to a device;

the unit controller is configured to modify an operating characteristic of the device based on an identity of an answering object proximate to the device.

35. The system of claim 34, wherein:

the receiving antenna is mounted proximate to a computer screen such that the input of the receiving antenna is directed toward the field of view of the computer screen.

36. A system for monitoring the position of an object, comprising:

at least one unit controller unit connected to at least one antenna assembly for continuously transmitting unit controller signals and receiving a flag signal;

at least one tag unit for receiving the unit controller signal, converting the received unit controller signal into a tag signal, and transmitting the tag signal at a selected time; and

at least one computing unit coupled to each of the unit controller units for determining the location of each of the marker units based on the received marker signals and storing information derived from the received marker signals in a computer readable storage medium.

37. The system of claim 36, further comprising:

a marking unit placed at a known fixed position for performing system calibration.

38. A method for calibrating a tag clock of a tag in a system for monitoring the position of a target, comprising:

splitting a marker signal emitted by a marker into in-phase and quadrature components;

repeatedly correlating the in-phase and quadrature components with a pseudo-noise sequence;

determining a phase difference between successive correlations; and

the mark clock is calibrated according to the phase difference.