Abstract:
A method for minimizing power consumption in a wireless device which utilizes backscatter transmission in half-duplex mode, wherein a switching device is interposed between an antenna and a transmitter-receiver, and the switching device is capable of causing the antenna load impedance characteristic to be either a short, a value which substantially matches the antenna impedance, or an open, depending on the portion of the half-duplex mode.

Description:
RELATED APPLICATIONS 
       [0001]    The present invention is a continuation of Ser. No. 11/019,494, filed on Dec. 20, 2004, which in turn claims the benefit of priority from the following United States provisional applications: U.S. patent Ser. No. 60/530,819 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using Amplified Backscatter”; U.S. patent Ser. No. 60/530,818 filed Dec. 18, 2003 entitled ‘Wireless Display Tag (WDT) Using an Active Transmitter”; U.S. patent Ser. No. 60/530,817 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using an Active Receiver; U.S. patent Ser. No. 601530,816 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using an Active Transmitter and Diode Receiver”; U.S. patent Ser. No. 60/530,795 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Using Active and Backscatter Transceivers”; U.S. patent Ser. No. 60/530,790 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Unit”: U.S. patent Ser. No. 60/530,783 filed Dec. 18, 2003 entitled “RF Backscatter Transmission with Zero DC-Power Consumption”; U.S. patent Ser. No. 60/530,823 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) Initialization; U.S. patent Ser. No. 60/530,784 filed Dec. 18, 2003 entitled “Wireless Display Tag (WDT) with Environmental Sensors”: U.S. patent Ser. No. 60/530,782 filed Dec. 18, 2003 entitled “High Readability Display for a Wireless Display Tag (WDT)” This application is also related to the following US utility applications filed simultaneously herewith: U.S. patent Ser. No. 11/019,660 Filed Dec. 20, 2004 entitled “Error Free Method for Wireless Display Tag (WDT) Initialization”; U.S. patent Ser. No. 11/019,976 filed Dec. 20, 2004 entitled “Wireless Display Tag (WDT) Using Active Backscatter and Transceivers”; U.S. patent Ser. No. 11/019,978 filed Dec. 20, 2004 entitled “Wireless Display Tag (WDT) Unit”; U.S. patent Ser. No. 11/019,916 filed Dec. 20, 2004 entitled “Multi User Wireless Display Tag (WDT) Infrastructure and Methods”; and U.S. patent Ser. No. 11/019,705 filed Dec. 20, 2004 entitled “Low Power Wireless Display Tag (WDT) Systems and Methods”. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Backscatter transmission is a radio technique whereby signals are sent with typically lower power consumption than comparative techniques. The system requires a Radio Frequency (RF) source, an antenna, a receiver, and a transmitter. Most radio systems include a transmitter and a receiver, both of which are coupled to a logic circuit. The source sends a radio wave over the air using the transmitter. The radio wave propagates from the transmitter&#39;s antenna to the receiver&#39;s antenna. The impedance terminating the receiver/transmitter&#39;s antenna can be in one of three general states: open, short, or the same impedance as the antenna&#39;s characteristic impedance. When the impedance characteristic of the antenna matches the characteristic input impedance of the antenna load, then the impedance is considered to be “the same” as the terms is used herein. 
         [0003]    Referring now to  FIG. 1 , an antenna  10  is shown having a termination impedance characteristic that is representative of an open circuit or high impedance. Accordingly, the signal, having a specific electromagnetic wave property, propagates without change. 
         [0004]    Referring now to  FIG. 2 , the antenna  10  is shown employed in a system  20  that has a characteristic termination impedance equal to the characteristic impedance of the antenna  10 . Accordingly, the power reflected from the antenna is equal to the power absorbed. The characteristic impedance is created electronically by allowing a controlled current to flow through a diode  22 . The impedance is then set to the desired value in response to the amount of direct current. Z o , the characteristic impedance, is set by the diode current as set forth in equation (1): 
         [0000]    
       
         
           
             
               Z 
               o 
             
             = 
             
               
                 1 
                 
                   g 
                   m 
                 
               
               = 
               
                 
                   KT 
                   q 
                 
                 
                   I 
                   DC 
                 
               
             
           
         
       
       
         
           
             K=Boltzman&#39;s constant 
             T=temperature in degrees K 
             q=electronic charge 
           
         
       
     
         [0008]    Referring now to  FIG. 3 , the antenna  10  is employed in a system  30  having a characteristic terminating impedance representative of a short or low impedance. Accordingly, the power reflected from the antenna  10  is approximately four times the reflected power value when connected to a system having a characteristic impedance that is the same as the antenna&#39;s characteristic impedance. The short is created with a significant amount of current flowing from IDC through the diode  32 . The exact value of the short can be described and determined using equation (1) above. 
         [0009]    A radio that uses the current art of backscatter requires that direct current be used to create the characteristic impedance and the short circuit. Such systems use power that shortens the battery life and generated a great deal of heat, which becomes a problem in design trends that dictate smaller and more compact components. Compact designs typically call for smaller batteries and reduced heat generation. Thus, what is needed is a system and method that minimizes, or even eliminates, current consumption in order to maximize battery life and reduce heat generation. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, a system and method are disclosed that minimize and even eliminate direct current demands and consumption in order to maximize battery life and reduce heat generation. This invention varies the load impedance on the antenna by electronically connecting either fixed impedances or impedances created using a FET. This is in contrast to the prior art where the impedance was created by changing current value in a device. 
         [0011]    An advantage of the present invention is that the system has low power consumption and, hence, low heat generation. Thus, the system is capable of operating with minimum drain on the system battery. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a prior art figure of an antenna with an open or high termination impedance characteristic; 
           [0013]      FIG. 2  shows a prior art figure of an antenna with a matching or characteristic termination impedance; 
           [0014]      FIG. 3  shows a prior art figure of an antenna with a short or low termination impedance characteristic; 
           [0015]      FIG. 4  shows a radio communication system in accordance with the present invention; 
           [0016]      FIG. 5   a  shows a block diagram representation of a radio transceiver in accordance with the present invention; 
           [0017]      FIG. 5   b  shows a graph of radio transmission vs. time in accordance with the present invention; 
           [0018]      FIG. 5   c  is a flow chart for a radio communication system in accordance with the present invention; 
           [0019]      FIG. 6  shows a transmitter portion of the radio transceiver of  FIG. 5  with an open or a high, or low impedance characteristic in accordance with the present invention; 
           [0020]      FIG. 7  shows a transmitter portion of the radio transceiver of  FIG. 5  with a matching characteristic or open impedance in accordance with the present invention; 
           [0021]      FIG. 8  shows a transmitter portion of the radio transceiver of  FIG. 5  with a matching impedance characteristic or open impedance in accordance with the present invention; 
           [0022]      FIG. 9  shows a transmitter portion of the radio transceiver of  FIG. 5  with a combined implementation of characteristic impedance, short or open impedance in accordance with the present invention; 
           [0023]      FIG. 10  shows a transmitter portion of the radio transceiver of  FIG. 5  with a combined implementation of characteristic impedance, short, or open impedance in accordance with the present invention; 
           [0024]      FIG. 11  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement-mode CMOS with a short or low impedance characteristic or open impedance in accordance with the present invention; 
           [0025]      FIG. 12  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement-mode CMOS with a short or low impedance characteristic or open impedance in accordance with the present invention; 
           [0026]      FIG. 13  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement-mode CMOS with an open or matched characteristic impedance characteristic in accordance with the present invention; 
           [0027]      FIG. 14  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement-mode CMOS with an open or matched characteristic impedance characteristic in accordance with the present invention; 
           [0028]      FIG. 15  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement-mode CMOS with an open, shorted, or matched characteristic impedance characteristic in accordance with the present invention; 
           [0029]      FIG. 16  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement-mode CMOS with an open, shorted, or matched characteristic impedance characteristic in accordance with the present invention; 
           [0030]      FIG. 17  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement and depletion mode CMOS with a short or low impedance characteristic, or open impedance in accordance with the present invention; 
           [0031]      FIG. 18  shows a transmitter portion of the radio transceiver of  FIG. 5  having a enhancement and depletion mode CMOS with a matching or open impedance characteristic in accordance with the present invention; 
           [0032]      FIG. 19  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement and depletion mode CMOS with a short or low impedance characteristic, or open impedance in accordance with the present invention; and 
           [0033]      FIG. 20  shows a transmitter portion of the radio transceiver of  FIG. 5  having an enhancement and depletion mode CMOS with a matching or open impedance characteristic, or open impedance in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0034]    Referring now to  FIGS. 4 and 5   a , a system  40  is shown with radio communication occurring between radio  42 , which in some embodiments may be a wireless device adapted to fit within the C-channel of a shelf display, and an access point or wireless terminal  50  in accordance with the teachings of the present invention. Each radio  42  includes a receiver  52  and a transmitter  54 , as shown in  FIG. 5 . As disclosed in U.S. patent Ser. No. 11/019,660 Filed Dec. 20, 2004 entitled “Error Free Method for Wireless Display Tag (WDT) Initialization”; U.S. patent Ser. No. 11/019,976 filed Dec. 20, 2004 entitled “Wireless Display Tag (WDT) Using Active Backscatter and Transceivers”; U.S. patent Ser. No. 11/019,978 filed Dec. 20, 2004 entitled “Wireless Display Tag (WDT) Unit”; U.S. patent Ser. No. 11/019,916 filed Dec. 20, 2004 entitled “Multi User Wireless Display Tag (WDT) Infrastructure and Methods”; and U.S. patent Ser. No. 11/019,705 filed Dec. 20, 2004 entitled “Low Power Wireless Display Tag (WDT) Systems and Methods”; all of which are incorporated herein by reference, the radio can include an active transceiver and coupled with a backscatter transceiver. 
         [0035]    In a half-duplex environment, with respect to the operation of the radio  42 , during the listening stage of the communication cycle, receiver  52  takes the incoming radio information from an antenna  56  and processes the information in a manner that a digital logic unit  58  can utilize. During the transmission stage, as discussed in detail below, the transmitter  54  varies the characteristic impedance of the antenna load that is coupled to the antenna  56  in correspondence to the information that is being transmitted from the radio  42 . 
         [0036]    Referring now to  FIG. 5   b , during the listening stage of the communication, labeled t 1 , the wireless terminal  50  transmits data to the radio  42 . The radio  42  sets the antenna load impedance characteristic to match the impedance of the antenna  56 . During the transmission stage, labeled t 2 , the radio  42  transmits data by varying the antenna load impedance characteristic between a short impedance characteristic and matching impedance characteristic. 
         [0037]    Under ideal conditions, there is no DC current flow into the gate or control node of the FET. In order to simulate a digital transmission the load impedance is switched between short and matching load impedance. On the other hand, in order to operate in an analog environment, then the load impedance can vary in the range between short impedance, matching, and open impedance. In an alternative embodiment, the phase and magnitude of the baseband can be altered instead of or in addition to alteration of the antenna load impedance characteristic. 
         [0038]    Thus, as detailed above, the transmitter  54  takes data or information from the digital logic unit  58  and processes the information so that the information can be sent wirelessly via the antenna  56  using radio waves. The receiver  52  and transmitter  54  are made primarily with analog circuits. In contrast, the digital logic unit  58  is made with digital circuits. 
         [0039]    In the various embodiments that follow, N-channel enhancement mode devices are shown due to the popularity of their use; however, in alternative embodiments, N-channel, P-channel, enhancement, or depletion mode Field Effect Transistors (FETs) can be used. Additionally, CMOS FETs are shown due to their popularity. However, other types of FETs or IgFETs can be used, such as MOSFETs, JFETs, and other types. Different FET technologies can be used besides Silicon, such as GaAs, InGaAs, SOI, plastic transistors, and others. 
         [0040]    In order to achieve the desired impedance levels various systems and methods can be utilized. For example, in one embodiment, the FETs are used as low-impedance switches to switch in and out the desired impedances. In another embodiment, the FET&#39;s channel impedance is designed to be the desired impedance in order to eliminate the resistor. 
         [0041]    Furthermore, in another embodiment, at least one FET can be used as low-impedance switches to switch in and out the desired impedances along with another FET, wherein the channel impedance is designed to be the desired impedance, which would eliminate the resistor. This embodiment can produce either a short or an open characteristic impedance, as desired, by appropriately turning on or off the FET. 
         [0042]    An enhancement-mode NMOS FET is turned on by raising the gate or control voltage above the source voltage by at least v t , which is the threshold voltage for the particular FET. On the other hand, the enhancement-mode NMOS FET is turned off when the voltage difference between the gate and source is less than v t . The same is true for a depletion-mode PMOS. The reverse is true for both depletion-mode NMOS and enhancement-mode PMOS. 
         [0043]    In alternative embodiments, the FET characteristics are different if the device is operated in triode (linear) mode or saturated mode. In an embodiment where the device is operated in a saturated mode, then the ideal device would have constant-current characteristics. 
         [0044]    Referring now to  FIG. 5   c , the process of determining the communication mode between the radio and terminal begins at step  500 . At step  502 , communication between the radio and the terminal is initiated. At step  504 , if the terminal initiated the communication, then the terminal sends an indicator signal to the radio at step  506 ; if not, then the process moves to step  510 , as discussed below. At step  508  it is determined if the indicator signal transmitted to the radio from the terminal is an indicator to communicate in backscatter mode. If the indicator signal is an indication to communicated in back scatter mode, then at step  510  it is determined if the radio can transmit using backscatter; if not, then the radio selects active mode transmission at step  516 , as discussed below. 
         [0045]    If the radio can transmit using backscatter mode, then at step  512  the radio selects to transmit in backscatter mode. At step  514 , the radio uses backscatter mode to transmit or send information to a nearby device, such as the terminal. At step  520 , if the transmission from the radio is complete, then the process ends at step  522 ; otherwise the process returns to step  510  to determine if the radio can continue to transmit using backscatter. If at step  510  it is determined that the radio can not transmit in backscatter, then at step  516  the radio selects active mode and at step  518  the radio uses active transmission to send information to the terminal. 
         [0046]    With respect to  FIGS. 6 ,  7 ,  8 ,  9 , and  10  that follow, the embodiments contemplate systems deployed in environments wherein the signal has low voltage or small radio signals are present. Thus, the system is operating in the triode mode region of the current-voltage (I-V) characteristics of inherently small-signal operation. In this mode, the channel resistance, which is the small-signal resistance between the source and the drain of the FET is approximately linear. The operation is over two diagonally-opposed quadrants of operation that is defined by a near-linear I-V characteristic response. 
         [0047]    With respect to  FIGS. 11 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17 ,  18 ,  19  and  20 , alternative embodiments are shown with the system operating in an environment wherein the signal has high voltage. Thus, if the FET device is large enough with relatively low resistance, this mode approximates a low impedance characteristic or a short circuit and this is large-signal operation. The operation of the FET and its I-V characteristic curve is non-linear and operates in one quadrant of the I-V characteristic. 
         [0048]    Referring now to  FIG. 6 , a system  60  is an embodiment wherein the antenna load characteristic impedance, which is measured relative to the impedance characteristic of an antenna  62 , can be varied or switched from short to matching to open using a Field Effect Transistor (FET)  64 . In the system  60 , the antenna  62  is coupled to the FET  64 . The FET  64  is coupled to and controlled by control signals from a control unit  66 . When an open impedance characteristic is desired, the control signal is connected to ground, turning off the FET  64 . When a short or low impedance characteristic is desired, then the control signal is set high turning on the FET  64 , thereby shorting the antenna  62  to ground. 
         [0049]    Referring now to  FIG. 7 , a system  70  is shown wherein the characteristic impedance is created with a FET  72  and a resistor  74 . The FET  72  is designed to have a low source-to-drain impedance. The resistor  74  is connected between the source of the FET  72  and ground. The value of the resistor  74  is equal to the characteristic impedance of an antenna  76 . When the characteristic impedance, which is the load characteristic impedance that matches the characteristic impedance of the antenna  76 , is desired the control signal voltage from the control unit  78  is set to high voltage. Otherwise, the control is set to low voltage. 
         [0050]    Referring now to  FIG. 8 , a system  80  is shown with an FET  82  coupled to an antenna  86  and a control unit  88  for generating control signals. The FET&#39;s characteristic impedance can be chosen to be equal to the desired characteristic impedance, which is the same as the impedance of the antenna  86 . Accordingly, when the characteristic impedance is desired, the control signal from a control unit  88  is set high. Otherwise, the control signal from the control unit  88  is set low. 
         [0051]    Referring now to  FIG. 9 , a system  90  is shown with an antenna  92  coupled to an FET  94  and an FET  96 . The FET  94  is coupled to a control unit  95  and the FET  96  is coupled to a control unit  97 . When an open or high impedance characteristic is desired, the control signals from the control units  95  and  97  are low. Alternatively, when a short or low impedance characteristic is desired, the control signal from the control unit  97  is set to high voltage and the control signal from the control unit  95  is set to low voltage. If a characteristic impedance is desired, other than an open or short, high or low characteristic impedance respectively, then the control signal from the control unit  97  is set to low voltage and the control signal from the control unit  95  is set to high voltage. In an alternative embodiment, a digital logic circuit can be implemented if desired using a similar approach. 
         [0052]    Referring now to  FIG. 10 , a system  100  is shown with an antenna  102  coupled to an FET  104  and an FET  106 . The FET  104  and the FET  106  receive control signals from the control units  105  and  107 , respectively. When an open or high impedance characteristic is desired, the control signals from the both the control units  105  and  107  are low. When a short or low impedance characteristic is desired, the control signal from the control unit  107  is high, and the control signal from the control unit  105  is low. On the other hand, when a characteristic impedance is desired, the control signal from the control unit  107  is low, and the control signal from the control unit  105  is high. In an alternative embodiment, a digital logic circuit can be implemented if desired. 
         [0053]    The previous circuits are less effective with large RF signals when the DC voltage on the antenna is zero volts. The reason is because the MOS current-voltage characteristics change when the devices are “reverse biased” by the antenna voltage going negative. If the RF voltages are small, then there is little undesired effect. However, if the RF signal at the antenna is large, then the undesired effect is noticeable. 
         [0054]    In alternative embodiments, the system includes using negative voltages at the antenna. The alternative circuits are shown and discussed in detail below. The circuits use enhancement mode FETs. However, circuits are also shown that use the enhancement/depletion mode devices. 
         [0055]    Referring now to  FIGS. 11 and 12 , a system  110  includes an antenna  112  coupled to a device  114  and a device  116 . In one embodiment the devices  114  and  116  are standard enhancement-mode devices. When the control signal from a control unit  118  is low, an open impedance characteristic is presented to the antenna  112 . When the control signal from the control unit  118  is high, a short is presented to the antenna  112 . 
         [0056]    When the control signal is low, both the device  114  and the device  116  are off, so that virtually no current flows between the drain and the source of the FET. With a high control signal, device  114  turns on and shorts the antenna  112  to ground; likewise, device  116  turns on. However, a capacitor  115  prevents direct current flow from the drain side of the device  116  to the antenna  112 . In one embodiment, the capacitors is shown in one instance connected between the antenna  112  and the drain of the device  116 ; in an alternative embodiment the capacitor  115  is shown connected between the antenna  112  and the drain of the device  114 . In  FIG. 11 , the antenna is at 0 V DC , while in  FIG. 12  the antenna is at approximately V DD . 
         [0057]    Even though direct current (DC) can not flow through the capacitor  115 , current that results from the radio frequency can flow through capacitor  115 . Accordingly, the capacitance of the capacitor  115  is selected so that the capacitor  115  presents a low-impedance at the operating radio frequency. 
         [0058]    In an alternative embodiment, the system  110  can be used to terminate an antenna coupled to the devices  114  and  116  at the characteristic impedance by sizing the device  114  and the device  116 . 
         [0059]    Referring now to  FIGS. 13 and 14 , a system  130  includes the device  114  and the device  116 , wherein the devices  114  and  116  function as open or short circuits depending on the control signals from the control unit  118  while the resistors  120  and  122  set the characteristic impedance. Alternative embodiments are possible wherein the capacitor  115  is switched from the drain of the device  114  to the drain of the device  116 . 
         [0060]    Referring now to  FIGS. 15 and 16 , the characteristic impedance of the antenna  112  of the system  150  is matched by the correct sizing of the device  114  and the device  116 . As indicated, alternative embodiments are possible wherein the capacitor  115  is switched from the drain of the device  114  to the drain of the device  116 . 
         [0061]    Referring now to  FIGS. 17 ,  18 ,  19 , and  20 , if enhancement and depletion-mode devices are available, then alternative circuits can be used. As indicated above, in a depletion mode device, as the control signal voltage is increased, the depletion mode device gets closer to proximating as open or high impedance characteristic. Thus, the embodiments disclosed herein are similar to those using enhancement mode devices and includes a voltage inverter  170  for inverting the control signal that is sent to the depletion mode device. 
         [0062]    Having fully described various embodiment and various alternatives, those skilled in the art will recognize, given the teachings herein that numerous alternatives and variations exist that do not depart from the invention and it is therefore intended that the invention not be limited by the forgoing description.