Abstract:
In a system of communicating between a master communication device and a plurality of slave communication devices, the master determines if any of the plurality of slave devices exist within a defined geographic area and, if so sends a bit-by-bit command for the slaves to identify themselves via a unique binary identification number. To avoid collision when the slaves respond, the slaves respond to the master&#39;s command within a predetermined period of a time window providing the ID bit by bit. Responses are acknowledged by the master such that only one slave is able to respond with its compute full ID after a bit-by-bit command from the master.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to communication devices. Particularly, the present invention relates to a method and apparatus for determining if a plurality of communication devices exist within a defined geographic area and for identifying the communication devices. 
     2. The Relevant Art 
     There are known in the art of communication devices, systems for locating and identifying objects or tags that exist within a defined area or field of coverage. The systems serve varying purposes, ranging from protecting assets in a store to recognizing for safety reasons when persons who are mentally challenged enter a particular locale. 
     Locating and identifying objects in a field of coverage presents particular difficulties for communication systems where a significant quantity of unknown devices must be located, and where each device requires a significantly lengthy unique identification number. At any given point in such systems, there may be numerous slave communication devices within range of a master communication device, with the slave communication devices moving into and out of the field of coverage either simultaneously or independently. 
     One method to obtain the identification number of the slaves in a master/slave communication system is to sequentially scroll through each possible identification number and interrogate all possible slaves. However, if the possible combination of addresses is great, this process will be extremely cumbersome. For example, in a system having slaves with 24 bit identifiers, the potential number of addresses is 16 million (2 24 ). 
     Alternatively, a slave entering a field, upon detecting that it is within range of the field, could immediately send its identification number to a master. Since multiple tags might enter the field at one time, however, their messages might collide and never reach the master. 
     Other approaches in the art describe various methods for locating and identifying objects in a field of coverage and for handling collision when multiple objects in a field respond to a single remote interrogator. 
     U.S. Pat. No. 5,751,570 issued to Stobbe et al. discloses a method of automatically identifying an unknown number of slave devices in a field. The slave devices transmit their corresponding identities to an interrogator. Particularly, the slave devices are transponders that are activated to send data when they are within the electromagnetic field of a reader. When more than one transponder responds upon being activated, all transponders are put into a dead state by a collision signal transmitted by the reader, thereby ceasing transmission of transponder data. Each transponder thereafter generates a random dead state for itself after receiving the collision signal, with each transponder resuming transmission of its transponder data at the end of the dead state. After receiving the data from a transponder as a result of the deviating dead states, the reader transmits an occupied signal to put the remaining transponders in an idle state in which they no longer transmit transponder data due to the occupied signal. After reception of the complete transponder data from the individual transponder, the reader transmits an acknowledge signal to place the transponder into an idle state. The other transponders are reactivated and the process repeated until all transponders have been identified individually and successively by the reader. Such a system would not be efficient in identifying and locating a significant number of transponders having significantly lengthy identification numbers due to the numerous random period dead states required after collision. 
     U.S. Pat. 5,686,902 issued to Reis et al. discloses a communication device or method wherein an algorithm is used to reduce collision probability between tags. The system provides bit code sequencing techniques during the identification process. Reis requires shorter identification transmission times, as compared to the overall listening time, to reduce the likelihood of collision between tags. If there are a significant number of tags, however, this method requires extremely long listening periods which creates a problem if tags are moving in and out of the field of coverage. Some tags, for example, will have the opportunity to enter the field and leave before the listening period is over. Also, while Reis shortens the listening period as tags are successfully identified, there is still significant inefficiency when there are few tags to be identified and the listening period remains relatively long. 
     U.S. Pat. No. 5,030,807 issued to Landt et al. discloses an RF tag identification system wherein an interrogator unit is able to identify a tag that enters a field of coverage. The interrogation device provides a bit by bit identification scheme. Particularly, an interrogator sends an RF signal to a remote object where the signal includes data intended to be received and stored by the remote object. Upon receipt of the RF signal, one or more remote objects return a backscatter-modulated signal to the interrogator, the backscatter-modulated signal being modulated with data indicating the identity and other data stored in the remote object. The interrogator has the capability to recognize the identity and other data stored in the remote object from the returned backscatter-modulated signal. The interrogator also has the capability to transmit data to the identified remote object. Landt uses time division multiplexing and multiple frequency schemes to separate tag-to-interrogator and interrogator-to-tag communications, and to separate communications where multiple, interrogators communicate in close proximity with one another. Time-division multiplexing, however, involves an automatic “built-in” delay as each tag in a system must respond, one after another, in time during an interrogation. This built-in delay would be inefficient in a system requiring location and identification. of many unknown tags in a field of coverage at one time. Furthermore, using multiple frequency tags is impractical in a system with numerous tags., Also, while Landt focuses on collision avoidance, Landt does not directly provide a solution when there is- an actual collision between two or more tags responding to the same interrogator. 
     U.S. Pat. No. 5,673,037 issued to Cesar et al. discloses a method of selecting RF tags for querying, communicating, and/or identifying by an interrogator. The method uses a bit code type control logic algorithm to accomplish these functions. The tags in Cesar are selected and identified in specific groups based on known criteria. Thus, no method for identifying and locating individual unknown tags in a field of coverage is disclosed. 
     None of the above mentioned prior art systems describe a system or method to effectively handle the situation where numerous unknown slave communication devices enter a field of coverage and thereafter begin providing their respective unique identification numbers simultaneously, thus colliding. 
     Accordingly, it would be desirable to have a communication system and method for locating and identifying a significant number of unknown devices having unique identification numbers within a field of coverage in an efficient and reliable manner. Particularly, it would be desirable for the same system and method to have an anti-collision feature allowing numerous slave communication devices to simply and efficiently provide unique identification numbers to a master communication device. 
     SUMMARY OF THE INVENTION 
     The foregoing desired aspects and advantages are achieved in a system of communicating between a master communication device and at least one slave communication device to determine if there is at least one slave device within a defined geographic area and to identify the slave(s). The system comprises a master communication device that establishes a field of coverage and initiates a request to determine if there are any slave communication devices within the field of coverage. Each slave communication device has a unique binary identification number. After receiving the request, slaves in the defined geographic areas send a response to the master indicating that the. slave is within the defined geographic area. The master after receiving, one or more responses from slaves in the defined geographic area, sends a command to the responding slaves to provide their unique binary identification numbers. After receiving the command, the responding slaves begin providing information representative of bits of their respective unique binary identification numbers to the master. 
     It is another aspect of the present invention for the information representative of bits of the unique binary identification numbers to be provided by responding slaves within first and second time periods of a plurality of time windows. 
     It is a further aspect of the present invention for the information representative of bits of the unique binary identification numbers to be provided by responding slaves within the first time period of the time windows to designate a bit that has a first binary value, and within the second time period of the time windows to designate a bit having a second binary value, where the second predetermined time period starts after the first predetermined time period has ended. 
     It is yet a further aspect of the present invention for the master to send an acknowledgment within a predetermined time period after slave responses received during the first and second time periods of the time windows. 
     Still another aspect of the present invention is where a first slave responds to the master&#39;s command within the first time period of a time window, and a second slave responds to the master&#39;s command within the second time period of the time window, and further where only the first slave receives an acknowledgment from the master, the acknowledgment being sent within a predetermined time period after the response from the first slave during the first time period of the window. 
     Still yet another aspect of the present invention is where a first slave responds to the master within the first or second time period of a time window, and a second slave responds to the master within the same time period of the window, and where both the first and second slaves receive an acknowledgment from the master, the acknowledgment being sent within a predetermined time period after the response from the first and second slaves. 
     Still a further aspect of the present invention is where the first slave, after receiving the acknowledgment from the master, provides information representative of bits of its unique binary identification number during subsequent time windows. 
     Other aspects and advantages of the present invention will become apparent when the description of specific embodiments below are read in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating a field of coverage generated by a master communication device in a master-slave communication system, where the master must locate and identify slaves (tags in the preferred embodiment) that are within the field of coverage. 
     FIG. 2 is a flow chart of the first phase of the exchange between a master and one or more slaves to locate slaves within a field of coverage. 
     FIG. 3 illustrates the second phase of the exchange between a master and two tags, whereby the tags, after being determined as within the master&#39;s field of coverage, provide their unique binary identification numbers bit-by-bit according to a preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention describes a mechanism for a master communication device to locate and identify unknown slave communication devices in a communication system. The slave communication devices are addressed using unique 24 bit identifier numbers. The system assumes a single master and multiple slaves, with the slave identification numbers being unknown to the master. A typical embodiment of a such a system is a RFID (Radio Frequency Identification) tag. system. 
     Referring to FIG. 1, in the preferred embodiment, the system is a wireless two-way radio system  10 . In the system  10 , a master  12 , equipped with a transmitter, and a receiver, generates a radio frequency (RF) field of coverage  14  that defines a particular geographic area by transmitting a constant strength RF signal. Tags  16 ,  18  and  20  (slaves) can enter or leave the field of coverage  14  at any time, simultaneously with other tags or independently. When a tag enters the field of coverage, it responds to the master&#39;s constant RF signal in a fashion detailed further herein. The tags  16 ,  18  and  20  also are each equipped with means for transmitting and receiving. Tags  16  and  18  are shown within the field of coverage  14 . Tag  20  is outside the field of coverage  14 . While a radio frequency field of coverage is used in the preferred embodiment, other means for generating a field of coverage, such as electromagnetic energy, could be used without departing from the scope of the present invention. In such a case, the tags would have to be fitted with appropriate receiving and transmitting means. Also, while the master  12  is fixed to define a fixed field of coverage in the preferred embodiment, a mobile master that defines a non-stationary field of coverage could be substituted without departing from the scope of the present invention. 
     The anti-collision feature of the present invention resolves the problem associated with a plurality of tags entering a field at the same time and provides a mechanism for efficiently handling collision when one or more of these tags simultaneously transmit their identification numbers to the master. This problem is particularly acute where there are a significant number of tags, each requiring a significant length unique binary identification number. In the preferred embodiment, the tags have 24 bit identifiers, meaning that the potential number of tags with unique binary identification numbers is 16 million (2 24 ). A large of number of these tags may move in and out of a field simultaneously, increasing the challenge to reliably and efficiently locate and identify the tags. 
     In the system of the present invention, the entire sequence involved for locating and identifying tags in a field of coverage involves an exchange of information between the master and the tags to determine if the tags are in the field of coverage and, if so, a “bit-by-bit” interrogation of each tag&#39;s unique binary identification number. 
     The first phase of the exchange of information between the master and the tag involves the following exchange illustrated in the flowchart of FIG.  2 . 
     At block  21 , the master transmits an “are you there” message which is detected by any tag which is within the field of coverage. 
     At block  22 , if there is no response to the “are-you-there” message, then the master knows there are no tags in the field. The master will then wait a short period of time before starting the process over at block  21 . If there is a response from any tag, execution continues at block  24 . 
     At block  24 , when the master receives a response, the master knows that one or more tags. have entered the field. Since the master cannot tell if the response came from one or more tags, the master begins a bit-by-bit interrogation to identify the tag(s). 
     In the preferred embodiment the response from any tag in the field is a short transmitted pulse. Upon receiving a transmitted pulse from one or more tags after sending the “are you there message,” the master starts a bit-by-bit interrogation of the tags within the field of coverage to identify the tags. The master begins the “bit-by-bit” interrogation by sending out a “bit-by-bit” command to all tags within the field of coverage. 
     Referring to FIG. 3, the time lines tag  1  TX and tag 2  TX illustrate the timing of pulses transmitted by two tags, tag 1  and tag 2 , entering a field of coverage at the same time. The time line master TX/RX illustrates the timing of pulses transmitted and received by a master. The master TX/RX time line also shows the first three time windows (the bit  23  window, the bit  22  window and the bit  21  window) of the exchange between the master and the two tags. The first time window of any exchange between a master and one or more tags begins at a predetermined time period after the master sends the bit-by-bit command. Subsequent time windows begin at a predetermined time period following acknowledged pulses transmitted by tags during an interrogation. While the invention can be adequately explained using three time windows, it should be noted that in the preferred embodiment twenty-four time windows will be required for a tag to transmit its complete 24 bit unique binary identification number to a master. 
     The time windows each consist of a first time period  26  and a second time period  28 . In the preferred embodiment, a pulse received from a tag during a first time period of a time window designates a binary value of “1” for the corresponding bit position, while a pulse received from a tag during a second time period of a time window designates a binary value of “0” for the corresponding bit position. A master will send an acknowledgment pulse within a predetermined time after receiving a pulse during either the first time period or the second time period of a time window, but not both. 
     If a master receives pulses during the first and second time periods of the same window, the master will only send an acknowledgment pulse within a predetermined period of the pulse received during the first time period. Whenever tags transmit a pulse while responding to a bit-by-bit command, an acknowledgment pulse is expected so that the tags can continue transmitting their unique binary identification numbers. Accordingly, any tags in this case that responded during the first period of the time window will continue transmitting after receiving the expected acknowledgment. On the other hand, any tags that responded during the second time period of the window will enter an idle state, and thus will not transmit further during the current exchange, after failing to receive an expected acknowledgment. 
     FIG. 3 illustrates the second phase of the exchange between a master and two tags. In FIG. 3, it is assumed that tag 1  and tag 2  have both responded to an “are you there” message from the master, and are now responding simultaneously to a bit-by-bit command  30  from the master, thereby colliding. At the bit  23  window, both tag 1  and tag 2  transmit pulses, pulse  34  and  36  respectively, within a second time period  28  of the bit  23  window, designating a binary value of “0” for the bit  23  position of the unique binary identification number of both tags. Within a predetermined time period after the pulses are received by the master, the master sends an acknowledgment pulse  32  indicating that the pulse from the tags was received. Receiving the acknowledgment pulse  32  within a predetermined time period after transmitting the pulses  34  and  36  indicates to tag 1  and tag 2  that they may continue transmitting pulses representative of the bits of their unique binary identification numbers. According to the invention, any number of tags responding with pulses during the second time period of the bit  23  window would receive the acknowledgment pulse  32  and would thereafter continue transmitting pulses representative of the bits of their respective unique binary identification numbers. 
     Tag 2  will abort its exchange with the master when it fails to see an acknowledgment pulse in response to its “0” pulse  38  in the bit  22  window. The early acknowledgment pulse in the bit  22  window indicates that the master has detected another tag, in this case the transmitted pulse  40  from tag 1  in the time, period  26  of bit  22  window. The master then does not send tag 2  the acknowledgment that it expects in response to the pulse  38  that it sent. When tag 2  does not receive an acknowledgment within a predetermined time after sending pulse  38 , tag 2  enters an idle state and will send no more responses to the master during the current “bit-by-bit” exchange. This allows tag 1  to continue its exchange with the master and complete its identification process, as indicated by the transmitted pulse  40  from-tag 1  during the bit  21  window. Regardless of the number of tags responding during a given exchange after a bit-by-bit command, only one tag; in this case tag 1 , will succeed in transmitting its unique 24-bit binary identification number to the master after 24 time windows. The present invention also ensures that one tag will actually succeed during every exchange. after a bit-by-bit command, collision notwithstanding. 
     Tag 2  in FIG. 3 will remain idle until it receives a new “are-you-there” message and a new “bit-by-bit” command while in the field of coverage, and thereafter will begin re-transmitting its unique binary identification number back to the master. Similar to tag 2 , any number of tags responding with pulses during the second time period  28  of the bit  22  window would not receive an expected acknowledgment pulse and would enter an idle state, whereby the respective tags would send no further responses to the master during the current “bit-by-bit” exchange. 
     Tag 1  continues responding to the interrogation until all 24 bits of its unique binary identification number have been communicated to the master. A verification is performed between the master and tag 1  confirming that the correct ID has been transmitted, and if so, tag 1  will consider itself “identified” and will no longer need to respond to subsequent “are-you-there” messages and “bit-by-bit” commands from the master. Accordingly, tag 1  will not interfere while the master proceeds to identify all other tags in the field. To accomplish this, the process starts over with the master issuing new “are you there” messages and new “bit-by-bit” commands until all tags in the field of coverage have been identified. 
     A tag that considers itself “identified” remains in this mode until it fails to see a new “are you-there” message from the master within a specific period of time. If this occurs, the tag knows it is no longer in the field of coverage and then returns to the “un-identified” state. If the tag re-enters the field of coverage it will then respond to subsequent “are you there” messages and “bit-by-bit” commands in the same fashion as described before. 
     Although certain embodiments and examples have been used to describe the present invention, it will be apparent to those of skill in the art that various changes can be made to the embodiments and examples -without departing from the scope or spirit of the invention.