Patent Publication Number: US-7215976-B2

Title: RFID device, system and method of operation including a hybrid backscatter-based RFID tag protocol compatible with RFID, bluetooth and/or IEEE 802.11x infrastructure

Description:
PRIORITY CLAIM 
   This application claims the benefit of the filing date of Provisional Application Ser. No. 60/340,743, and filed Nov. 30, 2001, assigned to the same assignee as that of the present invention and fully incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   This invention relates to radio frequency communication devices, systems, and program products. More particularly, this invention relates to an RFID system including a hybrid BackScatter-based RFID tag, system and method of operation protocol compatible with Bluetooth and/or IEEE 802.11x infrastructure. 
   2. Description of Prior Art 
   The automatic identification and data capture (AIDC) industry has recently begun to aggressively evaluate active RFID tags for applications now requiring enhanced security and improved efficiency for tracking and locating high-value assets in real-time. Emerging high performance active tag products operate within the 2.4 GHz Industrial, Scientific, and Medical (ISM) band while their passive counterparts have been proposed for operation within disjointed UHF bands that have not yet been harmonized across the world. Nevertheless, we do expect that eventually, most high performance passive RFID tag technologies will operate in the UHF frequencies from 862 MHz to 928 MHz across Europe and North America. Conversely, we also expect that active tag technologies will seek to become protocol compatible with low cost communication system standards that currently operate within the worldwide unlicensed 2.45 GHz ISM band. The 2.4 GHz frequency band is available throughout most parts of the world for low cost short-range wireless communications and has increased in popularity with the proliferation of networking and cordless technologies based on IEEE 802.11x and Bluetooth. A growing number of RFID systems have also been designed for operation within the 2.4 GHz-ISM frequency bands. Doing so leverages design synergies with other ISM band products and provides infrastructure added value, and reducing overall deployment costs. As more wireless technologies seek to use the 2.4 GHz band, co-existence and protocol compatibility issues have become even more pronounced. What is needed in the AIDC industry is an RFID tag, system and method of operation and system protocol compatible with Bluetooth and IEEE 802.11x and incorporating features of a mobile device. 
   Prior art related to RFID systems and devices includes: 
   1. U.S. Pat. No. 6,337,634 entitled “Radio Frequency Data Communication Device”, issued Jan. 8, 2002 describes a radio frequency identification device including an integrated circuit receiver, a transmitter, and a microprocessor. The receiver and transmitter together define an active transponder. The integrated circuit is preferably a monolithic single die integrated circuit including the receiver, the transmitter, and the microprocessor. The transmitter has a plurality of transmitting circuits, the plurality of transmitting circuits configured to selectively realize a plurality of modulated backscatter schemes. The integrated circuit is configured to receive and apply radio frequency signals via an antenna, the antenna and tunable circuitry cooperating in operation by switching the transmitting circuits with respect to the antenna to enable one of the transmitting circuits to realize one of the modulation schemes. Because the device includes an active transponder, instead of a transponder which relies on magnetic coupling for power, the device has a much greater range. 
   2. U.S. Pat. No. 5,790,946, entitled “Wakeup Device for Communication System”, issued Aug. 4, 1998 describes a wake communication system including a first communication device for receiving data and a wake up signal. Operation of the first communication device includes an active mode of operation and a sleep mode of operation for reducing power consumption. Further, the system includes a switch for switching the first communication device to and from sleep mode in response to receiving the wake up signal. The system also includes a second communication device for transmitting data to the first device during its active mode, and for transmitting the wake up signal to the first device during its sleep mode. 
   3. U.S. Pat. No. 6,078,251, entitled “Integrated Multi-Meter and Wireless Communication Link”, issued Jun. 20, 2000 describes an apparatus for object identification including a multi-meter terminal having a plurality of data modules, a triggering device for initiating one of the plurality of data modules to retrieve object identification data from an object. A radio module downloads the retrieved object identification data to a host and a housing contains the data modules, triggering device and the radio module. An RFID interrogator processes multiple tag protocols. The triggering device is responsive to a triggering signal from the host. The multi-meter terminal further includes a user interface and wherein the triggering device is responsive to a triggering signal initiated by the user through the user interface or generated by one of the plurality of data modules or generated manually by a user. 
   None of the prior art discloses an RFID system including a hybrid backscatter-based REID tag protocol compatible with low power communication systems using Bluetooth and/or IEEE 802.11x infrastructure. 
   SUMMARY OF INVENTION 
   A hybrid backscatter-based RFID tag is protocol-compatible with existing 802.11x and/or Bluetooth Standards, as well as RFID tag standards. The tag is linked to a multi-protocol interrogator via an interrogator generated RF Continuous Wave (CW) field. The tag comprises an RF-powered/wake up block and an 802.11x/Bluetooth transceiver block coupled to a protocol processor. Both blocks are serviced by separate antenna systems. The wake-up block is a passive circuit that accumulates energy on a capacitor and provides a threshold based wake-up signal to the processor. The wake-up signal is derived from a division of the capacitor voltage by a digital resistor. Once awakened, the processor activates a “listener” transistor which enables the processor to examine the RFID interrogator signal for RFID or transceiver operation. The “listener” transistor enables or disables both the wake-up block and an Amplitude Shift Keying (ASK) modulation/demodulation function. A regulator/charger block connected to the processor utilizes energy stored on the capacitor to trickle charge a battery whenever RF energy is present, such as each time the tag or a nearby tag is interrogated. The processor is coupled to the wake up block antenna via a backscatter switch, which allows the tag to operate in both passive and semi-passive modes. In operation, the interrogator supplies RF energy to the tag and ASK modulation to command the tag to operate in RFID or transceiver mode. If RFID mode is commanded and the battery is inoperative, the wake-up circuit rectifies the energy and stores it onto the capacitor to power up the tag for RFID operation in a passive mode using backscatter radiation. If the tag battery is operative, the interrogator is able to communicate with the tag in semi-passive RFID mode. The 802.11x/Bluetooth transceiver is activated at the intervals dictated by the sleep function incorporated into the protocol processor via the real-time clocks, or whenever an RFID interrogator asynchronously sends a command to activate the transceiver function before the wake-up interval is due. This can occur, for example, when the sleep intervals are set very long but the tagged item is taken through an RFID portal. Without this capability, the tag may not find a network to report its movement when it does awake. When the 802.11x/Bluetooth transceiver is activated, the processor serves as a host processor to manage the administrative and data transfer functions of the transceiver block. The interrogator can communicate with the tag via an access point in regular or backscatter radiation mode. 
   An aspect of the invention is a mode-determining apparatus in an RFID device, which examines an input signal characteristics from an interrogator to determine whether backscatter or low power communication protocol is required. 
   Another aspect is an RFID tag including a wake up block that accumulates energy on a capacitor which provides a threshold based wake up signal to a protocol processor where the wake up signal is derived from a division of the capacitor voltage by a digital resistor. 
   Another aspect is an RFID tag responsive to an interrogator for switching between backscatter modulation using amplitude shift keying and Bluetooth and/or 802.11x transceiver operation. 
   Another aspect is signal strength threshold apparatus in an RFID device for turning on a wake up apparatus when the device is within a pre-determined distance of a signal source. 
   Another aspect is signal examining apparatus in an RFID device examining the validity of received frames from an ASK demodulator to determine the required RFID communication protocol. 
   Another aspect is an RFID tag having an auxiliary sleep mode using a built in timer that keeps a real time clock enabling the transceiver section to be completely powered down while tracking the network timers and just before awakening powering up the transceivers permitting upload of the time-advanced network timers. 
   Another aspect is an RFID tag including a non-volatile memory which serves as a common repository for system data that can be communicated between RFID, WLAN, and WWAN networks. 
   Another aspect is a tag including a regulated charger block that utilizes energy stored in the tag to trickle charge a battery whenever RF energy is present, such as each time the tag or a nearby tag is interrogated. 
   Another aspect is disabling a wake-up block by turning off a “listener transistor” when RFID operation is not required or a transceiver block is operating. 

   
     DESCRIPTION OF THE DRAWING 
     These and other aspects of the invention will be further understood from the following detailed description of a preferred embodiment taken in conjunction with an appended drawing, in which: 
       FIG. 1  is a graph of backscatter range and data rate trade-off in terms of data rate (bits per second) versus Range (meters) in free-space and multi-path conditions for 915 MHz and 2.45 GHz transmission; 
       FIG. 2  is a representation of an RFID system including an interrogator or reader and an RFID device incorporating the principles of the present invention; 
       FIG. 3  is a representation of an active/passive RF tag protocol compatible with RFID, Bluetooth and IEEE 802.11x infrastructures in the system of  FIG. 2 ; 
       FIG. 4  is a representation of an Interrogator transmitting antenna and a Tag/Reader receiver antenna for use in the system of  FIG. 2 ; 
       FIG. 5  is a state diagram for Bluetooth and backscatter RFID protocol compatibility in the system of  FIG. 2 ; 
       FIG. 6  is a representation of an 802.11(b) Request to Send (RTS) command or a Power Save (PS) Poll frame in the operation of the system of  FIG. 5 ; and 
       FIG. 7  is a representation of an 802.11(b) Clear To Send (CTS) command in the operation of the system of  FIG. 5 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   A description of backscatter radiation characteristics is believed appropriate before describing the RFID system including an interrogator or reader and a hybrid backscatter-based RFID tag that can function in an RFID, 802.11x or a Bluetooth infrastructure. 
   When passive and active RFID tags transmit a signal via backscatter radiation, they modulate their antenna impedance in synchronization with a transmitted bit stream to reflect continuous wave (CW) energy from the base station or interrogator. BackScatter-based RFID tags do not independently radiate energy through power conversion from their local energy source. Backscatter radiation results in low modulation index, amplitude modulated (AM) signaling at the interrogator. The rate of change or the change of phase of this AM signal modulation typically encodes transmitted data on a frequency modulated (FM) or phase modulated (PM) sub-carrier respectively. 
   When reflecting a CW signal, the tag antenna characteristics are modified so the tag becomes mostly a poor collector of RF energy. For example, shorting the two terminals of a dipole or switching in an extra capacitor or inductor tap is a simple way of configuring the antenna as a poor collector or reflector. This has the effect of changing the antenna radiation efficiency, which in terms results in a change of its gain and matching efficiency. 
   For battery powered tags utilizing backscatter radiation, rather than active transmission (semi-passive tags), we expect the signal received by the base station will experience typical multi-path fading. We calculate the maximum data rate (R bit ) possible with non-coherent Amplitude Shifted Key (ASK) demodulation of backscatter radiation, as follows: 
   
     
       
         
           
             
               
                 
                   
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   Table 1 summarizes the parameters of Equation 1 and their values based on a typical backscatter RFID system design. 
   
     
       
         
             
           
             
               TABLE 1 
             
           
          
             
                 
             
             
               Parameters for calculating the range and throughout of semi-passive tags 
             
          
         
         
             
             
             
             
          
             
                 
               Value 
               Units 
               Definition 
             
             
                 
                 
             
          
         
         
             
             
             
             
          
             
               P s   
               100 
               Milli-Watts 
               CW power radiated by the 
             
             
                 
                 
                 
               interrogator. 
             
             
               λ 
                 
               Meters 
               Wavelength of the RF carrier. 
             
             
               Ψ T   
               1.475 
                 
               The base station transmitter antenna 
             
             
                 
                 
                 
               realized gain. 
             
             
               ψ r   
               1.426 
                 
               The backscatter antenna reflectivity 
             
             
                 
                 
                 
               factor. 
             
             
               R 0   
               4 
               Meters 
               Breakpoint distance when free-space 
             
             
                 
                 
                 
               path loss transitions to a higher 
             
             
                 
                 
                 
               loss index. 
             
             
               N B   
               4 
                 
               Multi-path propagation loss index 
             
             
                 
                 
                 
               typically observed in a cluttered 
             
             
                 
                 
                 
               warehouse. 
             
             
               p e   
               10 −6   
                 
               Maximum acceptable probability of 
             
             
                 
                 
                 
               bit error (BER). 
             
             
               T 0   
               298 
               Degrees Kelvin 
               Reference temperature for noise 
             
             
                 
                 
                 
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               k B   
               1.38 × 10 −23   
               Joules/Kelvin 
               Boltzman&#39;s constant. 
             
             
               f r   
               100 
                 
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   Equation 1 can be intuitively interpreted as follows: 
   (a) Bit rate (R bit ) is linearly proportional to realized or net power gain of both the transmitting (Ψ T ) and reflecting (ψ r ) antenna systems. 
   (b) Bit rate is inversely proportional to the signal propagation losses at a given distance based on the channel propagation (R 0 , N b ) characteristics. 
   (c) Given a maximum desirable bit error probability (p e ), the bit rate is inversely proportional to the base station receiver noise factor (f r ) or equivalently its noise figure. Hence we can conclude that given no other changes, we can improve the bit rate or throughput of the communication system by the same factor that we can reduce the receiver noise figure. Therefore, we can achieve higher system throughput (R bit ) at a given desirable maximum operating distance r, maximum regulated output power (P s ), minimum realizable antenna system gain (Ψ T ψ r ), and the maximum desirable bit error rate (p e ) simply by reducing the receiver noise figure (f r ). However, reducing a receiver&#39;s noise figure does come at the expense of higher burst power consumption. Likewise, we must also increase the base station receiver bandwidth in order to accommodate the higher modulation speed without degrading its noise figure. Doing so also implies increasing the receiver power consumption. 
   (d) The maximum possible bit rate for a given communication&#39;s distance r improves with the fourth power of the carrier wavelength λ. The implication here is that given the greater signal collection capability of antennas at lower frequencies, and barring interference, we can use more bandwidth in order to increase data rate without sacrificing bit error rate. Unfortunately, greater bandwidth is not always available within the lower frequency UHF bands, for example, compared with higher frequency microwave bands. 
   Given the same amount of information to transmit, an improvement in bit rate results in shorter packet transmission times. This means that the tag will access the airwaves less often and for shorter time slots. Additionally, shorter packets are statistically less susceptible to interference from other equipment utilizing the same frequency band, therefore resulting in fewer collisions and correspondingly, fewer requests for re-transmission. Hence, higher bit rate will result in a decrease in the average power consumption of the tag per information unit communicated since its transceiver will tend to utilize the airwaves less often. 
     FIG. 1  describes semi-passive backscatter performance in terms of Data Rate versus Range in meters for 915 MHz free space transmissions (A) with Multi-path (B) and 2.45 GHz free space transmissions (C) with multi-path (D). Given tag interrogators operating under reduced power transmission levels of 100 mW, which is identical to that of most 802.11x wireless LAN systems, we calculate the range and data rates for typical backscatter semi-passive tags from Eq. 1. The results in  FIG. 1  show that within the 915 MHz UHF frequency band, and at a distance of about 10 meters from the base station, data rates between ten kilobits and one megabit per second are possible depending on the multi-path conditions and receiver sensitivity. In contrast, data rates between 200 bits per second and 30 kilobits per second are possible at ten meters when operating in a 2.45 GHz frequency band. 
   In order to match the data rates of the 802.11x and Bluetooth systems, the tags must operate within four meters of the transmitter. This has important implications for Bluetooth or 802.11x compliant RFID tags operating in the backscatter mode within the 2.45 GHz frequency band. For maximum throughput efficiency, these types of tags should be designed with adaptive rate scaling so that neither range nor data rates will be sacrificed when propagation conditions are favorable. 
   Since better receiver sensitivity and higher Signal-to-Noise Ratio (SNR) are possible with active transmission, we should expect orders of magnitude improvement in the range and throughput over backscatter based systems. However, we give up the low power consumption, and hence long battery life (or smaller, lighter, cheaper battery) characteristics that backscatter radiation would provide. Battery life is generally the most important feature amongst similar capability battery powered RFID tags. 
     FIG. 2  shows a hybrid backscatter-based RFID Interrogator/Reader and RFID tag system  300  incorporating the principles of the present invention. The system protocol is compatible with RFID, Bluetooth and/or IEEE 802.11x infrastructures. A multi-protocol interrogator  302  energizes, demodulates and decodes Continuous Wave (CW) Radio Frequency (RF) signals  304  via a tuned circuit  306  operating in the 915 MHz or 2.45 GHz frequency bands. Interrogators or Readers are described in more detail in the text “ RFID Handbook: Radio Frequency Identification Fundamentals and Applications”  by K. Finkenzeller and R. Waddington, published by John Wiley &amp; Sons, New York, N.Y. (IBN 0471988510) January 2000. The interrogator is coupled to an Access Point (AP)  308  for initiating and receiving Bluetooth or 802.11(b) transmissions  310  via an antenna  312  operating in the 2.45 GHz frequency range. Access Points are well known in the low power communication art, and need not be described herein. Both the interrogator and the AP are linked to a passive/active RFID tag  314  via a tuned antenna  316  adapted for backscatter radiation or tag generated transmissions  318 . 
     FIG. 3  describes a semi-passive RFID tag design  400  that is protocol-compatible both with existing RFID tag standards like ISO18000-6 and other communication system standards like Bluetooth or IEEE 802.11x. Although a different radio front-end is required, this approach allows re-use of the same baseband, protocol, and host interface processor technology. Bluetooth and IEEE 802.11x protocol compatible RFID tags are obtained by retaining the frame structure and channel access mechanism plus adding the capability of backscatter radiation and ASK carrier modulation to the RF front end of the tag. By so doing, a tag is constructed with reasonable performance and low energy consumption. 
   In order to enhance power efficiency by significant amounts and greatly increase the tag&#39;s battery life, a multi-protocol interrogator can switch the tag to a backscatter radiation mode and emit CW energy in a frequency-hopping pattern when receiving a response. The tag may maintain protocol compatibility by switching the antenna impedance in synchronism with a Bluetooth/802.11x frame organized bit stream. For reception, the tag can utilize traditional demodulation techniques via the 802.11x/Bluetooth transceiver or it can also switch to a more power efficient and simpler ASK demodulation technique. 
   The hybrid Bluetooth 802.11x/active/passive RF tag  400  is fabricated in or on a substrate or member according to well-known processes and materials as described for example, in Ser. No. 10/014,448, filed Dec. 14, 2001, assigned to the same assignee as that of the present invention and fully incorporated herein by reference 
   In one embodiment, the tag  314  includes an UHF/HF antenna  402  coupled to an RF-Power/Wake Up block  404 . A capacitor  406  serves as a coupling device for a tuned circuit  407  including diodes D 1 , D 2  and a storage capacitor  408 . A threshold setting circuit  409  includes a variable digital resistor R connected across the tuned circuit via a listener switch  410 , and to a protocol processor  414  via a threshold terminal  416  and a listen terminal  418 . The resistor R is also connected to one end of a comparator  411 , the other comparator end being connected to a reference potential  412 . The comparator output is provided to the processor modulator/demodulator terminal  415  that looks for signal transitions in order to wake-up or Amplitude Shift Keyed signals to decode. The processor is coupled to the antenna  402  at a backscatter terminal  420  via a backscatter switch  422  for passive and semi-passive operational modes. A regulator/charger  424  and attached battery  426  are coupled to the processor and to the storage capacitor  408 . The battery provides power to the processor for semi-passive or transceiver operation. 
   A transceiver block  428  includes an 802.11x transceiver serviced by an antenna  430  and a Bluetooth transceiver serviced by an antenna  432 . Further details of the Bluetooth and 802.11x transceivers and accompanying protocols are described in the text “Bluetooth Revealed” by B. A. Miller et al., published by Prentice Hall PTR, Upper Saddle River, N.Y. 07458 2001 (ISBN 0-13-090294-2) and the text “802.11 Wireless Networks, the Definitive Guide” by M. S. Gast, published by O&#39;Reilley and Assoc., Sebastopol, Calif. 95472 2002 which are fully incorporated herein by reference. 
   The transceiver block includes an RF section  434  linked to the transceivers and serviced by a digital signal processor  436  coupled to a microprocessor  438 . A memory  440  services both the 802.11x and Bluetooth transceivers. The transceiver block is linked to the processor via a receive terminal  444 , and a transmit terminal  446 . An interface  448  is coupled to the processor via a control terminal  450 . The processor serves as a host processor to manage the administrative and data transfer functions of the transceiver block via the control terminal  450 . A built in timer  451  keeps a real time clock for tracking network timers while the transceiver are in a powered down state. The processor is connected to its own low frequency crystal to maintain accurate time while consuming less than 1 mW powered during the powered down mode. 
   A non-volatile memory  452  is coupled to the processor and serves both the wake up block  404  and the transceiver block  428 . The shared non-volatile memory  452  stores the tag data and signal recovery programs processing the RF signals for detection of the tag mode of operation, i.e. RFID or transceiver mode. The memory also includes stored programs for implementing 802.11x and Bluetooth transmission with the access point  308  (see  FIG. 2 ). 
   The separation of the RF-Power/Wake-up block  404  and the transceiver block  428  provides a network-bridging device. The shared non-volatile memory  452  serves as a common repository for system data that can be communicated between RFID, WLAN and WWAN networks. For example, the tag&#39;s IP address can be stored in the memory  452 . The RFID interrogator  302  (see  FIG. 2 ) with WWAN, WLAN, or wired-LAN connectivity serves as the network connection point for the tag at the time that it&#39;s being read. Therefore, this is useful in that it allows the interrogator having wide area network capability to proxy for the tag which does not have WWAN connectivity. For example, while the tag is traveling on a truck without WWAN connectivity, a portable or fixed station reader may activate the tag in the RFID mode, retrieve the IP address and communicate over WWAN (e.g. via satellite or cellular) to a network as the tag proxy. Once the desired data is downloaded the interrogator uploads the data to the tag via the local RFID link. Thus, the cost of adding WWAN capability to the tag is avoided but still achieves wide Internet connectivity when necessary. The wide area connectivity is separate from the tag zone small area network connection (WLAN or WPAN) via 802.11x or Bluetooth transceivers. As standards merge, new or updated RFID tag protocols can be automatically downloaded to the protocol processor and the non-volatile memory via the 802.11x or Bluetooth network interfaces. 
   The processor  414  is also coupled to a display  454  indicating the state of tag operation and also serving as an input device for the processor using a touch screen or incorporating a key entrance area. A buzzer/LED  456  may be included in the processor providing an indication when the tag is in an active state. A sensor  458  may be included in the tag to indicate whether RFID or transceiver signals are being received by the tag. The sensor may also provide environmental data such as temperature, pressure, and humidity. An auxiliary sleep battery  459  provides power to the processor when in a powered down state. 
   The wake up block  404  provides the tag with asynchronous wake up. That is, the tag can be completely powered down when not in use and awakened only when it comes within a predetermined distance of the interrogator. We can program this distance via setting signal strength thresholds through communication with the processor. While in a sleep state the wake up block accumulates energy on the capacitor  408  when sufficient RF energy is present from the interrogator. Because of the high quality factor of the receiver circuit  407  centered around select frequencies of the UHF or 2.45 GHz band, the capacitor develops sufficient energy to power up the voltage reference  412  and the comparator circuit  411  to provide a threshold based wake up signal to the processor. The wake up signal is derived from a division of the capacitor voltage by the digital resistor R. Once this derived signal exceeds the reference threshold  412 , the comparator  411  will trigger a digital signal edge, which in turn interrupts the processor so that it can wake up. Once awake, the processor sets a digital resistor threshold value, that turns on the “listener” transistor  418  and recovers the interrogator AM modulated signal, thereby, immediately decoding a command if the impending RF signal is indeed from the RFID interrogator  302  (see  FIG. 2 ). 
   The digital resistor has built in non-volatile memory that stores the last setting by the processor. There are two settings. The first setting establishes the wake up threshold. The second setting establishes a proper level for producing a clean ASK signal for decoding by the demodulator  415 . Once awakened, the processor adjusts the digital resistor value through the ASK setting in order to decode the RFID interrogated signal. The processor may also adjust the digital resistor value adaptively over time as it learns a setting that provides the most error free data recovery. 
   The “listener” transistor enables or disables both the wake up threshold and ASK demodulation functions. The “listener” transistor also allows the processor to examine the RFID signal to determine whether RFID operational mode with or without backscatter or transceiver mode has been commanded by the interrogator. If the processor recognizes an RFID protocol, a response is transmitted in backscatter mode. Otherwise communication is referred to the 802.11x/Bluetooth transceiver for non-RFID mode. The 802.11x/Bluetooth transceivers determine whether or not to transmit in backscatter or active mode based on a capability bit or a flag that is set in the received 802.11x or Bluetooth frames. One of the users definable bits in any standard 802.11x or Bluetooth frame may be used for this feature. 
   The backscatter switch  422  allows the tag to operate in both passive or semi-passive modes. The tag may be operated in passive mode when the on board battery is depleted. The interrogator applies CW RF energy to the tag. The wake up circuit rectifies the energy and stores it onto the capacitor  408 . The regulator/charger  424  is then able to supply stabilized power to the processor. The interrogator supplies ASK modulation to command the processor to switch to the RFID protocol mode. The tag responds by a backscatter modulation. If the battery is operative, the interrogator is able to communicate with the tag in semi-passive RFID mode from greater distances than when the tag is in the passive mode. 
   By separating the RFID wake up block and the transceiver block, the tag achieves power management by saving the 802.11x network timers onto the shared memory and using the sleep mode of a low power processor while completely powering down the 802.11x processor. The low power processor tracks the time and required 802.11x wake up intervals, and will trigger the 802.11x wake up. 
     FIG. 4  discloses antenna models for RFID interrogator transceiver  501  and tag/reader receiver  502 . In the transceiver  501 , an RF voltage source (V s )  503  serves as a power source (P s ) to a matching network  505 , via characteristic impedance  507  of the source  503 . The network  505  matches the signal source  503  and an antenna  507  impedance to reduce power loss. In one embodiment, the antenna  507  may be a looped antenna suitable for generating a magnetic field required to transfer energy to a battery less tag  314  (see  FIG. 2 ). The loop antenna comprises a LC circuit  507 . For a particular frequency, the inductive impedance X A  is equal to the antenna capacitor impedance (not shown) and the antenna will be in resonance. The output power (P IN ) is provided over a load resistor R L  with power loss R R  due to antenna internal resistance. 
   The radiated power (P radiated ) is picked up by the tag/reader antenna  502  as an induced voltage (V oc ) subject to the antenna internal resistance (R r ), load resistance (R L ), and internal inductance (X A ). An antenna output power (P M ) is provided to a matching network  506  which provides a received power (P R ) at a voltage (V R ) to the tag represented by characteristic impedance (Z R ). 
     FIG. 5  describes the operation of the hybrid backscatter-based RFID interrogator in a tag reading process  600 , described in conjunction with  FIGS. 2 and 3 . When the interrogator is ready to receive an RF tag, it may issue a RTS command to the nearby 802.11x access point or mobile device. As shown in  FIG. 6 , the RTS frame  700  is all header. No data is transmitted in the body and the frame check sequence (FCS) follows the header. The frame  700  includes a power save (PS)-poll portion including twenty bytes or 160 bits of which two bytes are for frame control through field  702 . A duration field  704  of two bytes reserves a medium for an entire frame exchange. A receiver address field  706  of six bytes, is the address of the intended recipient of the frame. A transmitter address field  708  of six bytes provides the address of the sender of the frame. A frame check sequence field  710  of four bytes completes the frame. The RTS frame precedes a MAC protocol data unit  712  comprising a physical layer convergence procedure (PLCP) preamble  314 ; a physical layer convergence protocol header  360  and a PLCP service data unit  318 . The function and operation of the MPDU  712  are described in the “ The IEEE  802.11  Wireless Networks, The Definitive Guide ”, supra at pages 173, 185–186. 
   Returning to  FIG. 5 , once the Interrogator receives a clear to send (CTS) response all other 802.11x devices will refrain from using the channel for the requested time slot reservation by setting the appropriate time out on the network allocation vector (NAV) timer. The CTS frame  800  shown in  FIG. 7  includes a frame control field  802  of 2 bits; a duration field  804  of 2 bits; a receiver address  806  of 6 bits identifying the individual MAC address to which the CTS is sent and a frame check sequence field  808  of 4 bits. Further details on the CTS frame are described in the text “ IEEE  802.11  Handbook ”, by Bob O&#39;Hara et al, published by IEEE Press, New York, N.Y. (ISBN 0-7381-1855-9) at pages 45, 46. 
   Continuing in  FIG. 5 , the interrogator commands the RFID tag into a communication mode that suits it best. For example, the interrogator may request the RFID tag to communicate via backscatter radiation using a simpler RFID communication frame format rather than the more complex 802.11x frame format so as to reduce power consumption. This flexibility in active transmission or backscatter radiation is useful in that any Bluetooth enabled device can communicate with the hybrid tag, as well as any traditional RFID Interrogator. Backscatter RFID tags cannot send unsolicited messages unless the Interrogator  602  establishes a CW RF field that can be reflected to its receiver. The multi-mode RFID Interrogator  602  negotiates with other communication devices in the area for collision-free time slots. The negotiation may be added through a common wire network connection to other access points (APs) or by emulating their RF signaling format. 
   The tag will determine which mode to utilize by the listener transistor  418  and processor  414  (see  FIG. 3 ) examining the interrogator signal transmission characteristics and the organization of the header sequence. Once the tag is awakened by the presence of a pre-determined intensity RF signal from the interrogator, the tag leaves its lowest power consumption or dormant state  604  and determines in a test  606  whether Bluetooth, for example, or traditional RFID protocol communication is required. The tag does so by examining the validity of the presently received frame from the Bluetooth and ASK demodulators. If the tag is within a pre-determined distance of the interrogator as set by the digital resistor R (See  FIG. 3 ), the tag will enter the traditional RFID communication mode and will temporarily shut down the Bluetooth transceiver before proceeding with normal backscatter RFID communications in block  608 . Otherwise, standard Bluetooth communications may commence in either the backscatter radiation or active transmission mode. In the former state, the processor  414  ( FIG. 3 ) turns on the switch  452  to disconnect the transceiver section  428  and transmit the tag data stored in the memory  414 . In one embodiment, the processor modifies the characteristics of the antenna  402  by shorting the terminals of the dipole or switching in an extra capacitor or inductor, in accordance with a data stream providing the stored tag information. The RFID interrogator returns to a sleep mode when the tag completes the data transfer mode and/or moves outside the range of the interrogator. 
   If the Bluetooth/802.11x mode is detected in block  606  by the listener transistor  418  and processor  414  ( FIG. 3 ), the processor powers on the transceiver  428 , sets the timer  451  for the network allocation vector and determines whether or not to transmit in the backscatter or Bluetooth mode based on a capability bit or flag that is set in the Bluetooth frame. Bluetooth frames are described in the text “ Bluetooth—Connect Without Cables ”, by J. Bray et al, Second Edition, Prentice Hall PTR Upper Saddle River, N.J. 07458 (ISBN 0-13-066106-6), published 2002 at pages 180–185 which is incorporated herein by reference. 
   After activation, the transceiver  428  enters a standby mode in block  610 . In this state, the device is inactive, no data is being transferred and the radio is not switched on. Thus, the transceiver is unable to detect any access code. 
   The transceiver proceeds to an inquiry state in block  612  and an attempt is made to discover the Access Point  308  ( FIG. 2 ) using the Service Discovery Protocol (SDP). During the inquiry process, the Access Point will supply frequency hop synchronization packets needed to allow the transceiver section  428  to synchronize with the hop sequence of the Access Point  308 . To establish a connection, the transceiver  428  enters a paging state in block  614 . In the paging state, the transceiver transmits paging messages directed at the Access Point which acknowledges the paging message as a slave and the transceiver enters the master response state responding with its Frequency Hopping Sequence (FHS) packet. If the inquiry or paging states are not entered, the transceiver returns to the dormant state, after a timed interval. 
   The transceiver enters into a connection state  616  and switches to the receiver&#39;s clock by applying a relevant offset to its own clock. The Access point moves on to the transceiver frequency hop and timing sequence. During the connection state, various data exchanges can be used to transmit data to the Interrogator via the Access Point in block  618 . The transceiver keeps transmitting periodically, even if there is no data to send. Also, the transceiver may move into a low power state and return to the standby block  610 . When the network allocation vector timer  451  (See  FIG. 3 ) expires, the protocol processor returns the transceiver  428  to the powered-off state. 
   While in the connection state  616 , the transceiver may enter into a hold or park state. A hold state in block  620  causes the transceiver to terminate support for asynchronous connectionless traffic for a defined period of time thereby to free up bandwidth for other operations, such as standing, paging, inquiry or low power slave. After the old time has expired, the transceiver synchronizes to the channel access code and commences to listen for Interrogator traffic. 
   A park mode  622  is entered when the interrogator gives up its active member address and listens to traffic only occasionally. The transceiver is able to enter a low-power sleep mode. Further details of various Bluetooth transceiver states are described in the text “ Bluetooth Connect Without Cables ”, by J. Bray et al, published by Prentice Hall PTR, Upper Saddle River, N.J. (ISBN 0-13-066106-6) at pages 67–70. 
   While the invention has been shown and described in a preferred embodiment, various changes can be made without departing from the spirit and scope of the invention, as defined in the appended claims, in which: