Abstract:
A novel bi-modal remote identification system is described. In at least one embodiment the system includes a base unit, a mobile unit, and a processor. The base unit and mobile unit utilize both radio frequency and ultrasound wireless technologies for remotely identifying the location of assets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/871,344, filed Dec. 21, 2006, U.S. Provisional Application Ser. No. 60/871,356, filed Dec. 21, 2006, U.S. Provisional Application Ser. No. 60/864,628, filed Nov. 7, 2006, U.S. Provisional Application Ser. No. 60/864,626, filed Nov. 7, 2006, and co-pending Non-Provisional Patent Application titled “Digital Intercom Based Data Management System”, attorney docket number FT-34057, and filed on Nov. 7, 2007, each application is fully incorporated by reference herein. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The U.S. Government has certain rights in this invention as provided for by the terms of Grant No. R44 AG019528 awarded by the National Institutes of Health. 
     
    
     FIELD OF INVENTION 
       [0003]    The present invention relates to identification systems, and more particularly relates to bi-modal remote identification systems employing radio frequency identification. 
       BACKGROUND OF THE INVENTION 
       [0004]    Identification systems often use technologies to remotely identify assets. Depending upon the system, the assets may actually be people, consumer goods, or manufacturing goods. Based upon the value of the assets a particular system often employs various types of technology. 
         [0005]    Radio frequency identification (RFID) is one technology often employed for remotely identifying assets. RFID systems can be either ‘active’, ‘passive’, or a combination of both. RFID systems often employ a transmitter and a receiver, where the transmitter device is connected to the asset. Active RFID systems often employ a transmitter device that requires a battery and are significantly more expensive than corresponding passive RFID transmitters. Active RFID transmitters however, often allow a greater transmission range than passive transmitters. 
         [0006]    However, passive RFID transmitters can be advantageous and often utilized due to a low per unit cost. Passive RFID transmitters also often require expensive detectors and a significantly reduced detection range, which is typically between several inches and several feet. 
         [0007]    Though active RFID systems provide the advantage of greater detection range, this can also pose a significant problem. The stronger transmission signal often passes through solid objects, including doors and walls. When assets need to be located within a particular or confined area the stronger transmission can cause the asset to be detected by detectors outside of the area, but still within the transmission range of the transmitter. This may cause the asset location to appear ambiguous, defeating the purpose of the identification system. 
         [0008]    Another problem with active RFID systems is that of power consumption. In order for the devices to be small and lightweight, the batteries must be small. Given the power consumption of typical active RFID transmitters, the battery life expectancy ranges from only a few months to a year. This is the case even when utilizing good power management techniques. 
         [0009]    Generally speaking, a remote asset identification system performs better if the system includes a location and detection system with a relatively high location resolution. In other words, the instances in which the asset identification system provides value to the user are often increased if the asset identification system is able to determine the location of assets, users, equipment, etc., with high resolution. Current tracking/location systems used for valuable assets often are also based on infrared (IR) or radio frequency (RF) technologies in which the location of the fixed receiver determines the location of the tagged object. Utilizing this strategy, to increase the locating resolution (e.g., to move from being able to determine that the user is next to an asset associated location), additional receivers with limited range must be employed. 
         [0010]    It would be advantageous for an improved system and method for remotely detecting assets to employ a cost effective RF system that could accurately detect the location of assets. More particularly, it would be advantageous if the improved system and method in at least some embodiments allowed for the accurate detection of assets through use of RF and alternative wireless technologies. It would be further advantageous if the alternative wireless technology was ultrasound based and the assets were patients within a healthcare facility. It would be further advantageous to increase the battery life expectancy of an active RFID device. 
         [0011]    Additionally, it would be advantageous if in at least some embodiments of the improved system and methods would detect movement of patients and provide a means for tracking patients within a healthcare facility. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an illustrative block diagram example of a multimodal remote identification system in accordance with at least one embodiment of the invention. 
           [0013]      FIG. 2  is a block diagram of the mobile device in accordance with at least one embodiment of the invention. 
           [0014]      FIG. 3  is flow chart of an illustrative example of a method for locating an asset in accordance with at least one embodiment of the invention. 
           [0015]      FIG. 4  is a flow chart of an illustrative example of a method for conserving energy consumption for the mobile device of  FIG. 2  in accordance with at least one embodiment of the invention. 
           [0016]      FIGS. 5A and 5B  are block diagrams of an alternative embodiment of the multimodal remote identification system in accordance with at least one embodiment of the invention. 
           [0017]      FIG. 6  is a flow chart of a method for locating an asset for the embodiment in  FIG. 5  in accordance with at least one embodiment of the invention. 
           [0018]      FIG. 7  is an exemplary electrical block diagram of the mobile device in accordance with at least one embodiment of the invention. 
           [0019]      FIG. 8  is an exemplary electrical block diagram of the base unit in accordance with at least one embodiment of the invention. 
           [0020]      FIG. 9  is a block diagram of an alternative embodiment of the system in a healthcare facility in accordance with at least one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Referring to  FIGS. 1-2 , an illustrative example of the system  10  is shown. The system  10  includes a base unit  12 , a mobile device  14 , and a controller  16 . The base unit  12  includes a radio frequency (RF) receiver  18  and an ultrasound transmitter  20 . The RF receiver  18  receives RF signals and the ultrasound transmitter  20  sends ultrasound signals in alternate time frames. Alternatively, the controller  16  can be integral with the base unit  12 . 
         [0022]    The base unit  12  is connected to a substantial power supply. The base unit  12  is stationary, which can include integration with a wall, ceiling, floor, or some other non-movable fixture within a room or defined area within a building. An exemplary placement would be integrated within the wall of room in a healthcare facility. Power supplied to the base unit  12  is substantial and can be internal to the building and/or a portable battery (not shown). Integrated with the device  12  is the receiver  18 , which acts as a listening device for RF signals transmitted from the mobile device  14 . The transmitter  20  sends out a strong and non-directional ultrasound pulse  30 . The pulse  30  is transmitted in a range of about 25 kHz to about 100 kHz. The pulse  30  is transmitted more preferably in a range of about 40 kHz to about 60 kHz. Alternatively, the pulse  30  is transmitted in a non-directional range from about 20 kHz to about 50 kHz. In alternative embodiment the transmitter  20  is replaced with an array of directional ultrasound transmitters (not shown). 
         [0023]    Mobile device  14  includes an RF transmitter  22 , a ultrasound receiver  24 , a microcontroller  26 , and a power source  28 . The transmitter  22  sends an RF signal  32  in a frequency range of about 300 MHz to about 900 MHz. The signal  32  is a multi-directional signal transmitted in the high frequency (HF) or ultra high frequency (UHF) range. Frequency of the signal  32  is determined by a system  10  user, and can be dynamically altered based upon the needs of the system  10  user. 
         [0024]    The microcontroller  26  determines and dynamically alters the time frequency transmission of the RF signal  32  and Bit Pattern. Signals  32  are sent in a range of about 1 per second to about 1 per minute. The microcontroller  26  selects the transmission time frequency based upon a predefined set of conditions. The conditions can include the implementation location of the system  10 , the assets being tracked and the location of the base units  16  within the system  10 . After an ultrasound pulse  30  is received and detected the microcontroller  26  sends a signal to the RF transmitter  22  indicating that an RF signal  32  is to be transmitted. Based upon a particular condition, the microcontroller  26  causes the device  14  to go into a sleep mode, the sleep mode being defined as a period of time where the device  14  does not transmit any RF signals  32 . The duration of the sleep mode is determined by the microcontroller  26  and based upon the condition that dictated the sleep mode entry. 
         [0025]    The power source  28  is a compact battery having an estimated life cycle in a range of about a few months to a year. Based upon the particular needs of the system  10  the battery  28  can be selected to have a life cycle less than a few months or more than a year. The battery  28  is selected from a variety of known manufactures and technologies. By example, the battery  28  is a lithium battery alternatively used in medical technology applications. An example of such a lithium battery is a Panasonic (Secaucus, N.J.) BR3032 which has a capacity of 500 mA hours. 
         [0026]    The device  14  is affixed to a mobile asset (not shown) such that the location of the device  14  is indicative of the asset location. Device  14  can be attached to the asset in a variety of ways, which include an adhesive, hook and loop arrangement, or other known and suitable ways for attachment. 
         [0027]    In at least one embodiment of the invention, a unit sequence code is generated by the microcontroller  26  and is transmitted as part of the RF signal  32 . The sequence code is a data suffix which is part of the RF signal  32 . The suffix is a two-bit binary code (00, 01, 10, or 11) sent by the RF transmitter  20  that correlates to the code of a single or plurality of base units  12 . A base unit  12  can have a hardwired suffix code associated with it, or alternatively the suffix code can be programmable and changed by the controller  16 . In an alternative embodiment, the ultrasound sequence code is a 1-bit, a 3-bit or greater than 3-bit code. 
         [0028]    In an alternative embodiment, the system  10  includes a plurality of mobile devices  14  and at least one base unit  12 . Each of the plurality of devices  14  is associated with a single asset. The controller  16  records movement and directional information for each device and associates the asset and the device  14 . 
         [0029]    Referring to  FIG. 3 , a data exchange  34  is shown in a plurality of steps. The data exchange  34  is a virtual handshake between the base unit  12  and the mobile device  14 . Device  14  or asset data from the handshake  34  is processed by the controller  16  and utilized by the system user for detection and location identification of assets  14 . The system  10  is initiated at step  36 . At step  38  a radio frequency signal and a unit sequence code is transmitted by the mobile device  14 . The radio frequency signal and ultrasound sequence code is received by the base unit  12  at step  40 . The signal and sequence code is further identified at step  42 . The radio frequency signal and ultrasound sequence code is validated at step  44 . If the signal is not valid the system  10  repeats step  40 , but if the signal is determined valid then the ultrasound pulse is transmitted at step  46  by the base unit  12 . If the ultrasound signal is received by the mobile device at step  48  then the ultrasound pulse is identified at step  50 . In the event that the ultrasound pulse is not received, then the system  10  repeats step  38 . Alternatively, a predefined time period is set, such as 10 seconds or 1 minute, before step  38  is repeated. After the pulse is identified at step  50 , the second radio frequency signal and ultrasound sequence code is transmitted at step  52 . The second radio frequency signal and ultrasound sequence code is received by the base unit  12  at step  54 . The signal and sequence code is further identified at step  56 . 
         [0030]    After the signal is identified at step  56  the controller  16  calculates the time delay between the first and second RF signal transmission at step  58  and calculates range information of the device  14  at step  60 . Information generated by the controller  16  is transmitted to a data storage device at step  62 . The data storage device (not shown) is integrated with the controller  16 . Alternatively the data storage device is a network database connected to the controller through a computer network (not shown). After transmitting the asset  14  data the system  10  determines if the handshake  34  is to be repeated at step  64 . Repeating the data exchange occurs by first repeating step  38 . If the exchange  34  is not repeated then the system sequence ends at step  66 , and the unit enters a sleep mode. The controller  16  is capable of determining the distance to the assets  14  within a detection range, based upon the handshake  34  ( FIG. 3 ). 
         [0031]      FIG. 4  represents a set of system steps that conserve the device  14  energy consumption and directly provides enhanced battery  28  life. At step  68  the sequence is initiated. The system  10  undergoes a first handshake at step  70 . A second handshake occurs at step  72 . The controller  16  analyzes the device  14  data generated from the previous handshakes, which are  70  and  72 , at step  74 . The controller  16  determines if the asset  14  moved positions between steps  70  and  72  at step  76 . If the asset  14  moved, the controller  16  transmits the data to a system data storage device (see  FIG. 9 ) at step  78  and then the system performs a third handshake at step  80 . Data analysis of the previous handshakes, now steps  72 ,  74  and  80 , is repeated at step  74 . If the controller  16  determines that the asset  14  did not move at step  76 , then the controller  16  delays the RF signal transmission of the device  14  by a predetermined period of time at step  82 . The predetermined period of time is represented by a time interval value (X). A fourth handshake is represented by step  84 . The controller  16  analyses the data received from the previous handshakes at step  86  and if the asset  14  has moved step  78  is repeated. Movement of the device is determined at step  88 . If the device  14  has not moved, the controller delays RF signal transmission by a predefined time interval (Y) in addition to the previous time interval delay (step  82 ) at step  90 , at this instance it is time interval (X). The more iterations of handshakes that occur without device  14  movement, the longer interval of time between the RF signal transmission. The next iteration of analysis representing a lack of movement for the device  14  presents a time delay interval represented by the equation (Y+(X+Y)). The controller  16  decides to continue positional monitoring of the device  14  at step  92 . If positional monitoring is to continue, step  84  is repeated. Otherwise, positional monitoring is terminated at step  94 . Using this technique, it is possible to increase the battery life by a factor of 10 to 30. For example, if the controller  16  determines the device  14  is stationary, the transmission repetition rate can range from an average of one transmission per second to one every 15 or 30 seconds. 
         [0032]    In an alternative embodiment (not shown), the mobile device (RF transmitter)  14  sends a signal that excites all ultrasound transmitters within the range of the RF signal. If the ultrasound signal is detected the RF transmitter  14  cycles through a sequence of different codes, resulting in a single RF pulse being sent at a particular time until the system identifies which room or area the mobile device  14  is located. The RF device  14  locks onto a particular code associated with its position, and decreases the pulse repetition rate to conserve battery power, until the mobile device  14  moves to a alternate location or room. The process is repeated after the system detects movement of the device  14 . Alternatively, the RF transmitter  14  can increase the code cycling process in order to determine the correct location. 
         [0033]    The controller  16  tracks the movement and positional information associated with an asset  14  based upon the information received from the handshakes  34 . Assets  14  that remain stationary for periods of time often do not need to transmit RF signals with a high time interval frequency. By delaying the RF signal  32  transmission energy consumption for the active RF device  14  is conserved, which enhances the life of the battery  28 . For example, instead of sending an RF signal  32  approximately once per second, the system can change to one pulse per 15 seconds. After the 15 second interval, the device  14  sends a signal  32 , and if there is a return ultrasound pulse, the controller  16  recognizes the present condition as static, and the 15 second pulse repetition rate would be continued, or increased, based upon predefined criteria set by the system user. Since the power consumption is inversely proportional to the pulse repetition rate, this would provide a very significant power saving feature, that would not be available to an active RFID tag  14 , which does not have a way of identifying if the signal  32  was being received or not. The first time the device  14  did not receive the ultrasound pulse, the controller recognizes the condition is no longer static and that the device  14  has changed locations or is unable to detect the ultrasound pulse  30 . The controller  16  directs the device  14  to resume transmitting a one second pulse repetition rate, or some other transmission time frequency defined by the system user. 
         [0034]    Referring to  FIG. 5A , an alternative embodiment of the system  10  includes base units  96 ,  98 , and  100 , which correspond to rooms  102 ,  104 , and  106  respectively. The present embodiment reflects the ability for radio frequency signals  108 ,  110 , and  112  to travel through a room wall  114 ,  116 , but the inability for ultrasound signals  118 ,  120 , and  122  to travel through the same wall  114 ,  116 . Base units  96 ,  98 , and  100  receive radio frequency signals  108 ,  110 , and  112  and transmit ultrasound signals  120 ,  122 , and  124  in response. The mobile device  14  only receives signal  122 . However, when device  14  sends its RF signal  32  in response to the ultrasound signal from  120 , all the units still receive the second RF signal  32 , which can cause ambiguity. 
         [0035]      FIG. 5B  is an alternative illustrative example of the three room scenario depicted in  FIG. 5A . Each of the base units  96 ,  98 , and  100  are programmed to respond to one of four possible two-bit binary RF signal code suffixes. The base units  96 ,  98 , and  100  will receive RF signals, but will only identify and respond to signals that have the same two-bit code sequence. Base unit  96 ,  98 , and  100  respond to the suffixes “01”, “11”, and “00” respectively. The mobile device  14  is configured to transmit a signal  126  code sequence with the predefined suffix “11”. The base units  96 ,  98 , and  100  can receive the signal  126 , but only base unit  98  can identify the signal  126  and respond by transmitting an ultrasound signal  128 . The mobile device  14  sends a second RF signal  126  which is only identified by base unit  98 , but received by all base units  96 ,  98 , and  100 . The controller  16  identifies that the device  14  is located in room  104 . In an alternative embodiment, placement of at least two base units  12  within a single room (not shown) allows the controller  16  to triangulate the device  14  signals to obtain an exact location of the device  14 . Movement and location of the device  14  is tracked, having an accuracy range from about six (6) inches to about two (2) feet.  FIG. 5B  also represents that base units  96 ,  98 , and  100  do not have to be positioned within the same relative location of the rooms  102 ,  104 , and  106 . 
         [0036]      FIG. 6  is a flow chart that represents the system  10  sequence for determining the position of a mobile device  14 . The system  10  is initiated at step  130 . The mobile device  14  moves into room  104  at step  132 . Controller  16  generates a signal code suffix for the mobile device  14  to transmit at step  134 . A radio frequency signal having the code suffix “11” is transmitted by the mobile device at step  136 . The base units  96 ,  98 , and  100  receive the signal at step  138 . The base units  96 ,  98 , and  100  determine if the signal is identified at step  140 . If the signal has not been identified the base units  96  and  100  do nothing at step  142 . If the signal is identified at step  140 , an ultrasound signal  128  is generated by the base unit  98  at step  144 . Receipt of the ultrasound signal  128  is determined at step  146 . If the ultrasound signal  128  is received by the mobile device  14  the pulse is identified at step  148 . The second RF signal  126  containing the code suffix is transmitted by the mobile device  14  at step  150 . The base units  96 ,  98 , and  100  receive the signal  128  at step  152 , and base unit  98  identifies the signal  128  at step  154 . The controller  16  calculates the positional and movement data associated with the mobile device  14  at step  156 . The positional and movement data is transmitted by the controller  16  at step  158 . The system determines if the sequence will be repeated at step  160 . If the sequence is not repeated then it terminates at step  162 , otherwise step  136  is repeated. Steps  136  through  154  represent a handshake between a base unit  98  and a mobile device  14 . 
         [0037]    In an alternative embodiment, the mobile device  14  dynamically cycles through the four two-bit binary code suffixes. By cycling through the available binary suffixes the device  14  is able to adapt to various physical surroundings and present a flexible mode for being detected by the controller  16 . Consequently, the controller  16  would transmit the RFID handshake data, and recognize that the device  14  is located in room  104 . 
         [0038]    Continuing with the present alternative embodiment, the device  14  is located within a room  102 ,  104 ,  106  for which it will remain for an extended time interval. The controller  16  has established that the device  14  is within a room  104  that responds to suffix “11”. The device  14  is directed by the microcontroller  26  to cease cycling through the suffix sequence, once it knows that the “11” has been “answered” by the base unit  98 . The microcontroller  26  dynamically locks the suffix “11” in its code sequence, so that the receiver  12  will respond to each RF transmission of the device  14 . 
         [0039]    In yet another alternative embodiment, the handshake described in  FIG. 6  can be used for actuating a mechanical device. For example, assume that at least one receiver is used to automatically open doors at certain locations, for those individuals who were authorized to use the doors. In this case, the receiver responds to the suffix “10.” Consequently, when the ultrasound pulse is detected in response to a suffix of “10,” the device  14  increases the pulse repetition rate allowing the receiver  12  to better identify when the asset  14  is close to the receiver  12  and ultimately the door. Once the device  14  comes within a predefined distance from the door, the controller  16  will send a signal to open the door. 
         [0040]    In yet another alternative embodiment, the system  10  alternates signal transmissions between a plurality of ultrasound transmitters  20  in a staggered sequence. The transmission of the ultrasound pulses are staggered between units. The system user can implement a situation-specific transmission schedule such that each ultrasound pulse generator  12  is activated with a sequence lasting a given period of time. In the present embodiment the ultrasound transmitter are connected through a network (not shown), and a controller will activate the ultrasound generator in a first room for a period of 1.5 seconds. A second room would be subsequently activated for the second 1.5 seconds, and a third room is activated for a third 1.5 second interval. The three room sequence is continuously rotated. The room interval time can be varied between rooms as well as between cycles. In the present embodiment, the ultrasound generator is active in room one, when the RFID pulse is received, then the pulse would be generated in room one, but the device  14  does not receive the ultrasound signal where it is located in the second room. Following the first room interval, the second room generator interval would be activated. When the second room interval is activated the device would receive the ultrasound pulse generated by the second room and would respond with a second RFID pulse. The system controller identifies which ultrasound generator was active, therefore identifying the room location of the device. 
         [0041]    In yet another alternative embodiment (not shown), more than three rooms can be equipped with base units  12 . A rotating sequence can be applied to units simultaneously where the RF transmitter range does not extend beyond a predefined distance. By example, if there are nine receiving units in the system, but the RF transmitter range will never extend beyond plus or minus one unit, then units “1”, “4”, and “7” can all be active simultaneously. Next, units “2”, “5”, and “8” can be active. Finally, units “3”, “6”, and “9” can be active, and then the sequence would repeat. With respect to the present example, the RFID device  14  is located in room “2”. The ultrasound receivers in room “1” and “3” are not able to receive the ultrasound signal from the unit in room “2”. All of the other rooms would be too far away to receive the answering RF signal. 
         [0042]    In an alternative embodiment the ultrasound transmitters  20  send out identifying pulses. The mobile device  14  sends a suffix code to the base unit  12  and is capable of receiving data from the ultrasound transmitter  20  as well. The information received could be used to accurately locate the device  14 , thus eliminating ambiguity that can exist from the RF transmissions alone. Two-way communications between the RF transmitting device and the RF detector provides a means for detecting the location and providing movement data for the device  14 . 
         [0043]    An exemplary mobile device  164  diagram is shown in  FIG. 7 . The device  164  includes an ultrasound sensor  166 , a RF transmitter  168 , and an antenna  170 . An exemplary base unit  172  is shown in  FIG. 8 . The base unit  172  includes a RF receiver  174 , a ultrasound transmitter  176 , a microcontroller  178 , and a data transmitter  180 . The microcontroller  178  is used to detect a first RF signal, followed by the generating and transmission of an ultrasound pulse. The microcontroller  178  receives a second RF signal and records the time interval between each transmission. A central processing unit  16  receives the time interval and RF transmission signal data. 
         [0044]    Referring to  FIG. 9 , an alternative embodiment of the system  10  is shown. The system  10  includes a base station  12 , a controller  16 , a mobile device  14 , a network  182 , a database  184  connected to the network  182 , a monitor  186  connected to the network  182 , and a wireless access point  188  connected to the network  182 . A tablet PC  190  is wirelessly connected to the wireless access point  188 . The device  14  transmits RF signals  32  and the base station  12  transmits an ultrasound pulse  30 . Data received from the base unit  12  and the mobile device  14  is transmitted to the controller  16 , which processes the information and transmits data through the network  182  to the database  184 . The data is stored in the database  184  and accessible by the controller  16  and the peripheral devices  186  and  190 . The device  14  is associated with a patient in a healthcare facility. The patient  14  is tracked by the system  10  and patient data is accessed from the graphical user interface (GUI)  186  and the mobile PC  190 . Healthcare facility employees can determine the position and relative movement of a patient  14  by viewing the patient location plotted on a map displayed by the GUI  186 ,  190 . In the event that a patient  14  is not properly located the controller  16  can send an alarm signal to the facility employees informing them of the inappropriate location and/or movement of the patient  14 . The embodiment of  FIG. 9  can alternatively be used in a warehouse for tracking valuable products, or on a production line to follow the progress of an item being assembled as well as monitoring those individuals working on that line. This could potentially allow for the correlation of defects within the assembly process. In yet another alternative embodiment, the devices  14  can be used to accurately track valuable assets in hospitals and care facilities as well as to track medications. In an alternative embodiment, such devices  14 , can be used for security and tracking of individuals in various office settings, including as law firms. 
         [0045]    In yet another alternative embodiment, an operational signal is sent to a device based upon tracking data for the asset device, the device being mechanically or electrically activated based upon the tracking data and asset device security status. The device can be a doorway, the doorway being opened or closed based upon the location of the asset device. Alternatively, the device is selected from the group comprising a sprinkler system, a computer access terminal, moving walkway, a security system activator, and a light activation system. 
         [0046]    In an alternative embodiment of the invention, the mobile device  14  does not automatically send an RF signal, but listens for an ultrasound signal when it is activated. Activation may occur when the mobile device moves from a sleep mode to an active mode of operation. If an ultrasound signal was not received within a predefined period of time, then the device  14  functions as described above. However, if an ultrasound signal is received, then the device  14  function would change to an alternative algorithm operation mode. The alternate mode is based upon a scheme of multiple independent ultrasound transmitters positioned within a particular area or building. Each ultrasound transmitter sends a long (CW) ultrasound signal that is transmitter for a period of time greater than the wake up period for the mobile device (RFID receiver)  14 . The device  14  will detect the CW ultrasound signal. Subsequent to the long pulse transmission, the transmitter is turned off for a period of time sufficient to allow the signal to dissipate. After this predetermined time period the ultrasound transmitter sends out two or more short pulses, which provides a time encoded method of detecting and identifying which ultrasound system was present. The device  14  measures the time period between the pulses (Δt) and in response sends an RF signal having and RFID code, the RFID code includes either a prefix or a suffix code reflecting the encoded time interval. By example, the time interval between the two pulses can be 20 mSec. for a device at location A, the interval encoded for location B can be 40 mSec. and the interval for location C can be 60 mSec. An RFID receiver can therefore be used to identify where the RFID transmit signal is coming from based on the transmitted prefix or suffix. 
         [0047]    While the invention is susceptible to various modifications and alternative forms, illustrative embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.