Abstract:
A decoder that receives a Manchester encoded signal including a start of frame sequence followed by coded data values followed by an end of frame sequence. The decoder also includes a detector that detects the start of frame sequence and the end of frame sequence to determine a beginning and end of the coded data values in the Manchester encoded frame. The decoder includes an integrator with hysteresis that includes an up-down counter and provides an output signal responsive to upper and lower threshold values. The output signal is applied to a discriminator that includes a counter which counts up in response to one value of the output signal of the integrator and is reset in response to the other value of the output signal of the integrator.

Description:
This application is a Divisional Application of U.S. patent application Ser. No. 08/914,940 filed Aug. 20, 1997. 

   FIELD OF THE INVENTION 
   This invention relates to systems for transmitting data and in particular to a system for encoding and decoding data. 
   BACKGROUND OF THE INVENTION 
   As is well known in the art of data transmission and as is described in Simon Haykin,  Communication Systems , pp. 414-15 (2nd ed. 1983), clock data and the data to be transmitted may be encoded using Manchester encoding to produce a signal which includes both the data to be transmitted and the clock data. This encoding scheme is particularly useful when the transmitter and the receiver are not controlled by the same clock. In this case, the receiver may require the clock data associated with the data to be transmitted in order to recover the transmitted data. A receiver may decode a Manchester encoded signal because it includes both the data to be decoded and the associated clock data in a single transmission. Manchester encoding effectively doubles the bandwidth of the signal to be transmitted. 
   In addition to the clock data, a receiver needs to identify the beginning and the end of the transmission from the transmitter. Systems have been developed which identify the beginning of the encoded data by transmitting an illegal Manchester code. An example of the illegal Manchester code is shown in FIG.  14 . Manchester encoded data typically only includes two consecutive high data bits(e.g. “11”). The illegal code includes four consecutive high data bits (e.g. “1111”). As a result, an increased bandwidth transmitter is necessary to transmit the illegal Manchester code. Further, the illegal Manchester code includes three low data bits (e.g. “000”). As a result, the receiver may have difficulty synchronizing its internal clock. Further, the energy of the transmitted signal over a period of time is reduced. As a result, the receiver may increase its gain because of the reduced energy and, thus, reduce its signal to noise ratio. 
   The end of the transmission may not be identified but determined by monitoring the number of received bits. As a result, if two data packets are transmitted at the same time, it may be difficult to determine if a collision has occurred between the transmissions. 
   SUMMARY OF THE INVENTION 
   The present invention provides an encoding scheme and an encoder that Manchester encodes a data value to produce a coded data value and produces a first valid Manchester code encoded in an illegal Manchester sequence as a start of frame code. Also produced is a second valid Manchester code encoded in an illegal Manchester sequence as an end of frame sequence. A transmission packet is produced including the start of frame sequence followed by the coded data value followed by the end of frame sequence. The start of frame sequence is a sequence of “110110” and the end of frame sequence is a sequence of “001000”. 
   The present invention also provides a decoder that receives a Manchester encoded signal including a start of frame sequence followed by coded data values followed by an end of frame sequence. The decoder also includes a detector that detects the start of frame sequence to determine a beginning of the coded data values in the Manchester encoded frame. The detector also detects the end of frame sequence to determine an end of the coded data values in the Manchester encoded frame. 
   By using the exemplary system, an increased bandwidth transmitter is not necessary to transmit the illegal Manchester sequence. Further, the RF receiver  305  may synchronize to the received signal because of the continued transitions between high and low data values in the received signal. Further, the energy of the transmitted signal over a period of time is increased. As a result, the RF receiver  305  may decrease its gain improving the signal to noise ratio. In addition, the size of the message packet  400  may be reduced as compared to the prior art because the number of bits in the start of frame sequence and end of frame sequence is reduced. 
   The present invention further provides a decoder including an integrator. The integrator receives a first signal including a first data value and a second data value different from the first data value. The integrator includes a first counter for increasing a first count value when the first signal includes the first data value and decreasing the first count value when the first signal includes the second data value. The integrator produces a third data value when the first count value is equal to or greater than a first threshold value and a fourth data value when the first count value is equal to or less than a second threshold value. The integrator further produces a second signal including the third data value and the fourth data value. The decoder also includes a discriminator having a second counter that increases a second count value when the second signal includes the third data value and resets the second count value to a predetermined value when the second signal includes the fourth data value. The discriminator also produces a clock synchronization signal when the second count value is equal to or greater than a third threshold value. The decoder also produces a third signal including a fifth data value when the count value is equal to or greater than the first threshold value and a sixth data value when the count value is reset. The first signal is decoded in response to the third signal and the clock synchronization signal. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
       FIG. 1  is a block diagram of a vehicle information system  10  according to an exemplary embodiment of the present invention. 
       FIG. 1A  is a diagram of a shuttle including an interrogator  300 A according to an exemplary embodiment of the present invention. 
       FIG. 2  is a block diagram of a vehicle  100  including a tag system  120 B according to an exemplary embodiment of the present invention. 
       FIG. 3A  is a block diagram of the tag system  120 B according to an exemplary embodiment of the present invention. 
       FIG. 3B  is a flow chart diagram useful for illustrating the operation of the processor  205 . 
       FIG. 4  is a diagram illustrating the contents of a message packet  400  transmitted from the tag system  120 B according to an exemplary embodiment of the present invention. 
       FIG. 5  is a timing diagram illustrating Manchester encoding. 
       FIG. 6  is a block diagram of an interrogator  300 B according to an exemplary embodiment of the present invention. 
       FIG. 7  is a timing diagram illustrating the transmission of message packets  400  from different tag systems  120 C- 120 E according to an exemplary embodiment of the present invention. 
       FIG. 8  is a data diagram illustrating the time intervals in which the message packets  400  are transmitted from the tag system  120 B. 
       FIG. 9  is a block diagram of the processor  510  according to an exemplary embodiment of the present invention. 
       FIG. 10  is a timing diagram illustrating the operation of the integrator  505 . 
       FIG. 11  is a timing diagram illustrating the operation of the discriminator  510 . 
       FIG. 12  is a diagram illustrating the start of frame sequence. 
       FIG. 13  is a diagram illustrating the end of frame sequence. 
       FIG. 14  is a diagram showing the format of an illegal Manchester code according to the prior art. 
   

   The entire disclosure of U.S. patent application Ser. No. 08/914,940, filed Aug. 20, 1997, is expressly incorporated by reference herein. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Overview 
   Referring now to the drawing, wherein like reference numerals refer to like elements throughout,  FIG. 1  shows a vehicle information system  10  including a tag system  120 B mounted in a vehicle  100 . The tag system  120 B transmits vehicle-related data acquired from the vehicle  100  to an interrogator  300 B. The vehicle information system  10  utilizes a bus  110  (shown in  FIG. 2 ) in the vehicle  100  with low-cost RF communications in the tag system  120 B and the interrogator  300 B to remotely access the vehicle-related data as the vehicle  100  exits or enters a designated area  20 . The interrogator  300 B provides the vehicle-related data acquired from the tag system  120 B to host computer  320 . 
   The vehicle-related data includes, for example, temperature, fluid levels, oil pressure, odometer, and other data related to the vehicle  100 . The tag system  120 B may also be used to read the status of emissions-related and safety-related parameters without having to directly connect any equipment to the vehicle. 
   The tag system  120 B includes only an RF transmitter  210  (shown in  FIG. 3A ) for transmitting the vehicle-related data. The interrogator  300 B includes only an RF receiver  305  (shown in  FIG. 6 ) for receiving vehicle-related data. As a result, the cost and complexity of the tag system  120 B and the interrogator  300 B may be reduced because the tag system  120 B and the interrogator  300 B each do not include circuitry and software to both transmit and receive data. In an alternative embodiment, the tag system  120 B and the interrogator  300 B may each include a transmitter and receiver for transmitting and receiving data. 
   The tag system  120 B only monitors data transmitted on the bus  110  and, as a result, the control and operation of the bus  110  does not have to be modified to accommodate the tag system  120 B. In this way, the tag system  120 B may be integrated into the vehicle  100  with minimal modification to the vehicle  100 . Thus, the vehicle information system  10  is more likely to be accepted and incorporated into vehicles  100  by vehicle manufacturers. In an alternative embodiment, the tag system  120 B may transmit data on the bus  110 . 
   The vehicle information system  10  may be utilized in a variety of environments to remotely monitor vehicles. For example, the vehicle information system  10  may be used to determine the speed of a vehicle. In this case, the tag system  120 B repeatedly transmits the speed and vehicle identification data of the vehicle  100  as it travels along a road. An interrogator  300 B positioned adjacent to the road receives the transmitted data for subsequent processing. 
   Alternatively, the vehicle information system may be used to monitor trucks as they leave and arrive at a central terminal. In this case, the interrogator  300 B may be located at access points to the central terminal to acquire data transmitted from tag systems  120 B coupled to the trucks. The host computer  320  uses the data acquired from the interrogator  300 B to determine which trucks have entered and exited the central terminal. 
   In another alternative embodiment, the vehicle information system  10  may be used in a vehicle rental system. The operation of the vehicle information system  10  is described below in the context of the vehicle rental system. 
   DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     FIG. 1  shows a vehicle information system  10  including a tag system  120 B mounted in vehicle  100 . The tag may be mounted in an area not visible to the casual observer. Mounting locations could be any area that is not completely enclosed by metallic surfaces. For example, suitable locations include behind the dashboard in the passenger compartment, behind bumpers, or behind non-metallic body parts, or beneath the vehicle. The vehicle  100  may be, for example, a rental vehicle located at an airport. A person renting the vehicle  100  (hereinafter “the renter”) is provided a driver tag system  120 A upon his arrival at the airport. The driver tag system  120 A includes stored data identifying the renter and the vehicle that has been rented. The driver tag system  120 A includes circuitry for transmitting the stored data. 
   As is shown in  FIG. 1A , after receiving the driver tag system  120 A, the renter boards a shuttle  40  to travel to the rental lot where the vehicle  100  is located. The shuttle  40  includes an interrogator  300 A for acquiring the data transmitted from the driver tag system  120 A. The acquired data is transmitted to the host computer  320  which notifies the driver of the shuttle  40  where the vehicle  100  is located in the rental lot. The host computer  320  maintains a database of rental data identifying the renter, the vehicle  100  rented, and other data related to the vehicle  100 . For example, the other data may include the location of the vehicle  100  in the rental lot and the fuel level and the odometer reading of the vehicle  100 . 
   Returning to  FIG. 1 , the renter is dropped by the vehicle  100 , enters the vehicle  100 , and drives to the exit of the rental lot. The tag system  120 B located in the vehicle  100  intermittently transmits message data packets  400  (shown in FIG.  4 ). The interrogator  300 B receives the message packets  400  when the vehicle  100  is in the designated area  20 . The size of the designated area  20  may be adjusted by increasing the strength of the signal transmitted from the tag system  120 B or increasing the sensitivity of the RF receiver  305  (shown in  FIG. 6 ) in the interrogator  300 B. The interrogator  300 B has an antenna (not shown) that is oriented so that one or more message packets  400  transmitted from the tag system  120 B may be received as the vehicle  100  passes through the designated area  20 . The interrogator  300 B may be a hand held device or permanently mounted. For each installation, such as an airport, there is a link via radio or hard wired, from the interrogator  300 B to the host computer  320 . 
   As is shown in  FIG. 4 , the tag system  120 B transmits different types of data in the message packet  400 . The message packet  400  includes vehicle identification data (VID)  415  such as a vehicle identification number (VIN) that uniquely identifies the vehicle  100 . The message packet  400  may also include vehicle-related data such as fuel data  420  indicating the fuel tank level and odometer data  425  indicating the odometer reading. 
   As is shown in  FIG. 2 , the vehicle  100  includes the tag system  120 B. The vehicle  100  includes a bus  110  that is used to transmit data between a fuel tank system  130 , an instrument panel  140 , other modules  150 , and a vehicle computer  160 . The module  150  is an interface with other components, for example, the engine (not shown) in the vehicle  100 . The vehicle computer  160  may be, for example, the body or engine computer in the vehicle  100 . 
   Data is provided to and retrieved from the bus  110  in accordance with a vehicle bus standard such as the Society of Automotive Engineers (SAE) J1850 standard or the Controller Area Network (CAN) standard. The SAE J1850 standard defines an electrical and data protocol for the bus  100  and the components coupled to the bus such as the fuel tank system  130 , the instrument panel  140 , the module  150 , and the vehicle computer  160 . The bus  100  is, for example, a single wire loop. The fuel tank  130 , the instrument panel  140 , the module  150 , and the vehicle computer  160  provide data to or retrieve data from the bus  110 . 
   The inventors have recognized that it may not be advantageous to provide additional components that transmit data on the bus  110 . Vehicle manufactures have designed the bus  110  and the components coupled to the data bus  110  to ensure the reliable transmission of data. The addition of components that transmit data on the data bus  110  may require adjustments in the operation of the other components to ensure reliable transmission of data. 
   The tag system  120 B avoids these problems because the tag system  120 B only monitors data that is transmitted on the data bus  110 . The tag system  120 B may be coupled to the bus  110  by, for example, crimping a connector on the wire forming the bus  110  and connecting the connector to the tag system  120 B. As a result, the operation of the bus  110  does not have to be modified. In this way, the tag system  120 B may be integrated with minimal modification of the vehicle  100 . In addition, vehicle manufactures may be more willing to incorporate the tag system  120 B in the vehicle  100  because it does not require modification of the bus  110  or the components attached to the bus  110 . Further, the installation costs of the tag system  120 B are minimized because only minor modifications may be required to install the tag system  120 B. In an alternative embodiment, the tag system  120 B may transmit data on the bus  110 . 
   The tag system  120 B determines the status of different components attached to the bus  110  by monitoring the data transmitted from those components on the bus  110 . For example, the fuel tank system  130  includes circuitry for determining the fuel level in the fuel tank as is well known and transmits this data on the bus  110  for subsequent display on a level gauge (not shown) in the instrument panel  140 . The tag system  120 B retrieves the transmitted data from the bus  110  and transmits the fuel level to the interrogator  300 B. As a result, the tag system  120 B may provide the fuel level without using any specialized circuitry for monitoring the fuel level or directly coupling the tag system  120 B to the circuitry that measures the fuel level. 
   The tag system  120 B may also be coupled to different components in the vehicle  100 . As is described in greater detail below, the tag system  120 B may include, for example, circuitry for determining the odometer reading of the automobile by monitoring data provided from a wheel sensor. 
   The tag system  120 B is described in greater detail below with reference to FIG.  3 A. The tag system  120 B includes an interface  200  that is compliant with the SAE J1850 standard and provides an interface between the tag system  120 B and the bus  110 . The interface  200  is coupled to the bus  110  and only retrieves data from the bus  110 . Alternatively, the interface  200  may provide data to the bus  110 . Data that is transmitted on the bus  110  by the components coupled to the bus  110  includes identification data that identifies which component transmitted the data on the bus  110 . For example, the fuel tank system  130  (shown in  FIG. 2 ) transmits a data packet on the bus  110  that indicates that the data was transmitted from the fuel tank  130 . In addition, the data packet may include data indicating the amount of fuel in the fuel tank. Further, the data packet may include data indicating which of the other components coupled to the bus  110  should receive the transmitted data packet. For example, the data packet may include data indicating that the instrument panel  140  should retrieve the transmitted data packet from the fuel tank system  130 . 
   The data acquired from the bus  110  is provided to processor  205  which is, for example, a micro-controller. The interface  200  and the processor  205  may be combined as a single component. One such exemplary combination of the interface  200  and the processor  205  is part number MC68HC05V7 available from Motorola. This particular part is compatible with General Motors automobiles compliant with the SAE J1850 standard. The operation of the processor  205  is described below with reference to FIG.  3 B. 
   As is shown in  FIG. 3B , at step S 100 , the data packets are retrieved from the bus  110  and provided to the processor  205 . At step S 110 , the processor  205  determines if the retrieved data packet is to be further processed. If not, step S 100  is repeated. The processor  205  determines whether the bus data packet is to be further processed by examining the identification data of the data packet. For example, if the data packet is from the fuel tank system  130 , the processor  205  selects this data packet for further processing. The processor  205  does not further process the data packet if it is from, for example, the vehicle computer  160 . Which data packets are to be processed can be determined using a lookup table (LUT) (not shown) provided in the processor  205 . The LUT may include data indicating the data packets to be selected for further processing. For example, the LUT may include data indicting that data packets from the fuel tank system  130  and the instrument panel  140  should be selected for further processing. 
   The processor  205  may also receive data from an analog-to-digital (A/D) converter  215  (shown in  FIG. 3A ) which is coupled directly to a component in the vehicle  100 . For example, the A/D converter  215  may be coupled directly to a sensor  155  that measures the fuel level as is described in the &#39;044 patent and which is not coupled to the bus  110 . In this case, the A/D converter  215  converts the analog signals from the sensor  155  to digital signals for further processing by the processor  205 . The processor  205  determines the fuel level based on the fuel sensor readings. 
   Alternatively, the processor  205  may recover data from other components in the vehicle  100  that do not require conversion by the A/D converter  215 . In other words, the components may provided digital data directly to the tag system. In this case, these components would be coupled directly to the processor  205 . 
   Returning to  FIG. 3B , at step S 120 , the vehicle-related data is retrieved from the data packet. At step S 130 , the processor  205  produces a message packet  400  (shown in FIG.  4 ). At step S 140 , the processor  205  modulates the RF transmitter  210  to transmit the message packet  400 . The RF transmitter  210  is, for example, an AM surface acoustic wave (SAW) transmitter. The modulated signal is produced by the processor  205  turning the transmitter  210  on and off. 
   The RF link between the tag system  120 B and the interrogator  300 B may be implemented using either active or “semi-active” transmission technology. In active transmission systems, the tag system  120 B uses a battery  170  (shown in  FIG. 2 ) to power the entire tag system  120 B. In a semi-active transmission system, the tag system  120 B uses battery power for the tag system  120 B except for the transmitter  210 . Message packets  400  may be transmitted between the tag system  120 B and the interrogator  300 B via passive backscatter where a constant wave is transmitted from the interrogator  300 B, passively modulated, and reflected back from the tag system  120 B to the interrogator  300 B as is known. 
   The RF link may provide either one-way, the tag system  120 B to the interrogator  300 , or two way communication. A one way link provides a monitoring function where the tag system  120 B reports current condition of all monitored parameters of the vehicle  100  as described above. A two-way link allows the interrogator  300 B to send messages to the tag system  120 B to either command the tag system  120 B to monitor certain parameters, or to pass parameters to systems in the vehicle  100 . In the latter case, the link may provide the means to remotely perform functions such as lock/unlock doors and monitor/adjust emission control sensors and systems. 
   As is shown in  FIG. 5 , the RF transmitter  210  is modulated using Manchester encoding. As is well known in the art of data transmission and as is described in Simon Haykin,  Communication Systems , pp. 414-15 (2nd ed. 1983), clock data and the data to be transmitted may be encoded using Manchester encoding to produce a signal which includes both the message packet  400  and the clock data. The width 2t of the data pulses to be encoded is twice the width t of the Manchester encoded data. As a result, the Manchester encoding effectively doubles the bandwidth of the signal to be transmitted. As is described in greater detail below, the integrator  300 B recovers both the clock data and the data from the Manchester encoded signal. 
   As is shown in  FIG. 4 , the processor  205  can transmit one or more message packets  400  including data for different components in the vehicle  100  (shown in FIG.  1 ). The message packet  400  includes packet type data  405  that indicates whether the message packet  400  is a single message packet or one of a sequence of related message packets. The packet sequence data  410  indicates the position of the message packet  400  in the sequence of related message packets. Related message packets  400  are provided to reduce the size of the message packets. The size of the message packet  400  is decreased to provide for transmissions from a number of different tag systems  120 B. 
   In order to the reduce the complexity and cost of the tag systems, the number of frequencies for transmission may be limited and may be, for example, one frequency. Thus, in environments where there are a number of vehicles  100  including tag systems  120 B, the packet size is reduced to minimize the time for transmitting the message packet  400 . In this way, the likelihood of more than packet  400  being transmitted at one time is reduced. By separating large data transmissions into multiple packets  400  interference is reduced. 
   A sequence of message packets  400  may be used to provide data relating to the vehicle  100  that include more bits than are provided in a single message packet  400 . The packet type data  405  and the packet sequence data  410  are, for example, each one (1) byte. The message packet  400  may also include vehicle identification data (VID) which may be a unique sequence of numbers, letters, or symbols used to identify a particular vehicle  100 . For example, the VID  415  may be the vehicle identification number (VIN). The VID  415  is, for example, thirteen (13) bytes. A compression algorithm may be used in order to reduce the number of bits in the VID  415 . 
   Data related to the vehicle  100  is also provided in the message packet  400 . For example, the message packet includes fuel data  420  and odometer data  425 . The fuel data  420  indicates the amount of remaining fuel in the fuel tank of the vehicle  100 . The odometer data indicates the current mileage the vehicle  100  has traveled. The fuel data  420  and the odometer data  425  each are, for example, two (2) bytes. Extra data  430  may be contained in the message packet  400  for providing additional data regarding the vehicle  100 . The extra data  430  may include, for example, data indicating the status of the engine or whether the vehicle  100  has been in an collision. For the case where the vehicle  100  has been in a collision, the tag system  120 B receives data from one or more sensors (e.g. accelerometer (not shown)) in the vehicle  100  that detect impacts to the vehicle  100 . The extra data  430  may also include information regarding the rental of the vehicle  100 . The message packet  400  also includes an error correction code  435  which is, for example, two (2) bytes. The message packet  400  may also transmit only VID  415 . In this case, the tag system may not be coupled to the bus  110 . 
   The message packet  400  is transmitted from the tag system  120 B in a range of zero (0) to X seconds where X is, for example, one half (½). Further, the message packets  400  are transmitted one (1) to two (2) percent of the time. The message packets  400  are also transmitted during one of Y time slots. For example, consider  FIG. 7  which illustrates the message packets  400  transmitted from three different tag systems  120 C,  120 D, and  120 E which are the same as the tag system  120 B. 
   As is describe above, a collision between the message packets  400  is minimized by reducing the size of the message packets  400 . As is shown at time T1, however, two of the message packets  400  may be transmitted at the same time from two different tag systems  120 C and  120 D. In this case, the interrogator  300 B receives segments of two message packets and, as a result, determines that the received transmission is invalid. The operation of the interrogator  300 B when receiving message packets  400  is described in greater detail below. If the message packets  400  where transmitted at a constant time interval from each tag system  120 C and  120 D, then the message packets  400  from each tag system  120 C and  120 D would continually be transmitted at the same time. 
   In order to avoid this problem, the exemplary processor  205  (shown in  FIG. 3A ) may transmit the message packets in one of sixteen random time slots S 1 -S 16  shown in FIG.  8 . The time slots S 1 -S 16  are selected in response to a random number generated in processor  205 . As a result, the time interval ΔT1, ΔT2, and ΔT3 may vary between each message packet  400  transmitted from the tag system  120 C. Accordingly, although message packets  400  transmitted from tag systems  120 C and  120 D may collide at time T1, subsequent message packets  400  transmitted from the tag systems  120 C and  120 D are not likely to be transmitted at the same time. Thus, numerous tag systems  120 B transmitting at the same frequency may be used in close proximity to each other. As a result, the cost of the tag systems  120 C- 120 E may be reduced because a variety of tag systems  120 C- 120 E transmitting at different frequencies do not have to be provided. Further, the cost of the interrogator  300 B may because it may be designed to receive transmissions at one frequency instead of multiple frequencies. Alternatively, the tag systems  120 C- 120 E may transmit at different frequencies. In this case, the interrogator  300 B would include circuitry to receive transmissions at different frequencies. 
   In an alternative embodiment, the tag system  120 B may monitor the voltage level of the battery  170  (shown in  FIG. 1 ) to determine how often to transmit a message packet  400 . Typically, a charge is applied to the battery  170  when a vehicle  100  is in motion. In contrast, a reduced charge or no charge at all is applied to the battery  170  when the vehicle  100  is idling or turned off. Thus, the tag system  120 B may monitor the difference between the voltage levels to determine whether the vehicle  100  is moving. The tag system  120 B monitors the voltage level using data retrieved from bus  110  or from a direct connection to a sensor (not shown) coupled to the battery  170 . 
   The tag system  120  may increase the transmission rate of the message packets  400  when the vehicle  100  is moving. In this way, a number of message packets  400  may be transmitted as the vehicle  100  passes through the designated area  20 . Thus, the likelihood of the interrogator  300 B receiving a message packet  400  is increased. The tag system  120 B may decrease the transmission rate of the message packets  400  when the vehicle is not moving. In this way, a number of message packets  400  may be transmitted in the designated area  20  while reducing the total number of message packets  400  transmitted in a specific period of time. Thus, the likelihood of a collision between message packets  400  is reduced. 
   Returning to  FIG. 1 , the interrogator  300 B receives data transmitted from the tag system  120 B including the fuel data  420  and the odometer data  425  when the vehicle  100  enters the designated area  20 . The VID  415 , the odometer data  420 , and the fuel data  425  are provided to the host computer  320  for storage and processing. In addition, the interrogator  320  receives the data transmitted from the driver tag system  120 A. 
   The host computer  320  determines whether the vehicle  100  and the renter are “matched” based on the data transmitted from the tag system  120 B and the driver tag system  120 A. The renter and the vehicle  100  are matched if the stored data indicates that the renter has rented the vehicle  100  exiting the rental lot. If there is a match, the host computer provides a signal to access/exit system  30  to allow the vehicle  100  to exit the rental lot. If there is not a match, the renter is instructed to return to the rental lot for assistance. 
   In an alternative embodiment, a user interface  165  may be coupled to the bus  110  or directly to the tag system  120 B. The user interface  165  may be used by the renter to enter an access code, credit card number, or other information which is acquired by the tag system  120 B directly or from the bus  110 . The user interface  165  may be, for example, a key pad, card reader, or other well known device for providing data to a system from an external source. In operation, for example, the user interface  165  may be used to acquire the renter&#39;s credit card number. The tag system  120 B transmits the credit card number to the interrogator  300 B. The credit card number is used to verify that the renter and the vehicle  100  match. If there is a match, the vehicle is allowed to leave the rental lot. If there is no match, the vehicle is not permitted to leave the lot. 
   After exiting the rental lot and upon returning to the rental lot or drop-off point, the interrogator  300 B receives the message packet  400  transmitted from the tag system  120 B. The host computer  320  compares the fuel data  420  and the odometer data  425  from the tag system  120 B to the data stored in the database. The host computer  320  uses the difference between the fuel data  420  from the tag system  120 B when the vehicle was exiting the rental lot to the fuel data  425  from the tag system  120 B when the vehicle  100  is returned to the rental lot to determine if the renter should be charged for gasoline. Similarly, the odometer data  425  is utilized to determine if the renter should be charged for the mileage the vehicle has traveled. A receipt is generated and provided to the renter. The renter then parks the vehicle. Alternatively, the renter may receive the receipt after the vehicle is parked. 
   As is shown in  FIG. 6 , the transmitted message packet  400  (shown in  FIG. 4 ) is received by interrogator  300 B. The interrogator  300 B includes an RF receiver  305  that receives the transmitted message packet  400 . The received message packet  400  is provided to demodulator  310  that demodulates the received message packet  400 . The processor  315  processes the received message packet and converts the data provided in the message packet  400  to a form suitable for use by the host computer  320 . 
     FIG. 9  is a block diagram of the processor  3 . 15 . The processor  315  includes an integrator  505  that integrates the data over time. It has a maximum threshold MAX1 of, for example, 208 counts and a minimum threshold MIN1 of, for example, zero (0) counts. Moreover, it has a built-in hysteresis and a low-high threshold TRH of, for example, 187 counts, and a high-low threshold TRL of, for example, 21 counts. The above exemplary values are for a 9600 baud rate transmission. Every time one of these thresholds is reached, a “start” pulse is generated for synchronization of the subsequent circuitry. The demodulated signal DATA_IN is provided to the integrator  505  which implements an integration operation to reduce the effect of high frequency noise in the demodulated signal DATA_IN. In other words, the integrator  505  implements a low pass filter operation. The integrator  505  includes a counter  507  which counts up when the demodulated signal DATA_IN is high and counts down when the demodulated signal DATA_IN is low. 
   The operation of the integrator  505  is described below with reference to FIG.  10 . At time T1, the counter  507  counts up because the data signal DATA_IN is high. The counter counts up to the maximum value MAX1. At time T4, when the data DATA_IN is low, the counter  507  counts down. The counter does not count lower than the minimum count value MIN1. 
   The integrator  505  utilizes the counter  507  to produce a data signal INT_OUT. The data signal INT_OUT transitions from a low to high state when the count exceeds a threshold value TRH. The threshold value TRH is, for example:
 
 TRH= 0.9 *MAX 1 
 
Thus, at time T2, when the count COUNT1 is equal to or greater than the threshold value TRH, the data signal INT_OUT becomes high. The data signal INT_OUT transitions from a high to low state when the count is equal to a threshold value TRL. The threshold value TRL is, for example:
 
 TRL= 0.1* MAX 1 
 
Thus, at time T5, when the count value COUNT1 is equal to or less than the threshold value TRL, the data signal INT_OUT becomes low. In this way, high frequency components in the data signal DATA_IN are minimized. As a result, high frequency noise in the data signal DATA_IN is suppressed.
 
   The integrator  505  shifts the data signal INT_OUT in time with respect to the demodulated signal. The integrator  505  does not significantly alter the duration of the pulses between the demodulated signal DATA_IN and the data signal INT_OUT. 
   Returning to  FIG. 9 , the processor  315  includes a discriminator  510  that differentiates between ones and zeros in the Manchester encoded data stream. 
   The discriminator includes a re-triggerable counter modulo, for example,  416  with input value sampling at, for example, count  208 . The discriminator  510  discriminates between ones and zeros in the Manchester encoded data in the data signal INT_OUT. The discriminator  510  also synchronizes the internal clock to the clock data in the Manchester encoded data. The operation of the discriminator  510  is described below with reference to FIG.  11 . At time T1, a counter  512  located in discriminator  510  counts up because the data signal INT_OUT is high. The counter  512  counts up to a maximum value MAX2 and does not exceed the maximum value MAX2. The maximum value MAX2 is, for example, substantially equivalent to half the expected amplitude of the pulses in the data signal INT_OUT. 
   At time T2, when the count value COUNT2 is greater than or equal the maximum value MAX2, the signal FRESH transitions from a low to a high state. The transition from the low to high state is used to synchronize the dock to the Manchester encoded data. The signal produced by the counter  512  is provided as a data signal SAMPLE to the decoder  515 . The data signal SAMPLE is either high or low. Thus, at time T1, the data signal SAMPLE is low. At time T2, the data signal SAMPLE is high. The rising edge of the data signal FRESH indicates that a valid data sample is provided in data signal SAMPLE. The counter  512  is set to, for example, zero (0) when the data signal INT_OUT transitions from a high to low state. 
   In response to the data signal FRESH and the data signal SAMPLE, the decoder decodes the Manchester encoded data to retrieve the message data packet  400  (shown in FIG.  4 ). The decoder  515  extracts the data from the message packet  400  and provides it to host computer  320 . Alternatively, the decoder may provide the message packet  400  to the host computer  320  which extracts the data from the message data packet  400 . 
   The decoder  515  receives the Manchester encoded data and converts it to a message packet  400 . In order to convert the Manchester encoded data, the decoder  515  identifies the beginning and the end of the Manchester encoded data corresponding to the message packet. A start of frame sequence is added to the Manchester encoded data by the processor  205  during transmission. The start of frame sequence is shown in FIG.  12 . The start of frame sequence is a series pulses representing ones and zeros added at the beginning of the Manchester encoded message packet  400  (shown in FIG.  4 ). The start of frame sequence is a combination of zeros and ones that would not be produced when the message packet  400  is Manchester encoded. 
   Once the start of frame sequence is detected, the decoder  515  determines the beginning of the Manchester encoded message packet  400  and decodes it. The end of the Manchester encoded message packet  400  may be determined in two ways. If the number of bits of the Manchester encoded message packet  400  is known, the decoder  515  may count the number of bits after receipt of the start of frame sequence. Once the number of counted bits equals the number of bits in the Manchester encoded message packet, the decoder  515  ignores the remaining bits. Alternatively, an end of frame sequence may be added after the Manchester encoded data by the tag system  120 . 
   The end of frame sequence is shown in FIG.  13 . The end of frame sequence is a series pulses representing ones and zeros added at the end of the Manchester encoded message packet  400  (shown in  FIG. 4 ) by the processor  205  during transmission. The end of frame sequence is a combination of zeros and ones that would be not produced when the message packet  400  is Manchester encoded. By using the start of frame sequence and the end of frame sequence, the boundaries of the message packet  400  may be determined by the decoder  515 . Further, if a collision between messages packets occurs, one of the message packets may be detected and recovered by detecting the start of frame sequence. Further, the detection of a start of a second frame sequence indicates that the Manchester encoded data currently being decoded is invalid and should be ignored. In contrast, prior art systems utilize illegal Manchester codes to indicate the start of the encoded data. As a result, if a collision occurs between transmitted message packets  400  in the prior art systems, it may not be possible to recover either message packet  400 . 
   By using the exemplary system, an increased bandwidth transmitter is not necessary to transmit the illegal Manchester sequence. Further, the RF receiver  305  may synchronize to the received signal because of the continued transitions between high and low data values in the received signal. Further, the energy of the transmitted signal over a period of time is increased. As a result, the RF receiver  305  may decrease its gain improving the signal to noise ratio. In addition, the size of the message packet  400  may be reduced as compared to the prior art because the number of bits in the start of frame sequence and end of frame sequence are reduced. 
   Once the decoder  515  retrieves the message packet  400 , the decoder  515  provides the message packet to host computer  320 . Alternatively, the processor  315  (shown in  FIG. 6 ) may include additional circuitry and/or software for performing different operations that are implemented by the host computer  320 . 
   The tag system  120 B is protected from tampering such as being removed from the vehicle  100 . If the tag system  120 B is removed from the vehicle  1100  it could be used to indicate that a vehicle was returned to the rental lot. The tag system  120 B is protected from tampering by storing data such as the vehicle identification data in a volatile memory  730  in the tag system  120 B. In this case, the tag system  120 B is coupled to battery  170  (shown in FIG.  2 ). If the tag system  120 B is removed, the tag system  120 B is disconnected from the battery  170 . Data stored in the volatile memory  730  within the tag system  120 B is lost. For example, the vehicle identification data may be stored in the volatile memory  730 . Upon re-applying battery power, the volatile memory  730  is initialized to indicate that the tag system  120 B had been previously disconnected. 
   The car rental system employing the vehicle information system  10  simplifies and automates the process of renting, returning, and paying for rental vehicles especially in large facilities such as airports. Further, the number of individuals utilized at the rental location may be minimized. For example, the person stationed at the rental area exit may be eliminated and the number of personnel located at the rental and return desks may be reduced. 
   In an alternative embodiment, vehicle  100  may be located in reserved dedicated short-term parking places, for example, at the airport. An interrogator  300 A is located on a walkway to the reserved parking locations. As the renter passes the interrogator  300 A, the interrogator receives the data from the driver tag system  120 A and determines which vehicle  100  is to be provided to the renter. Upon determining which vehicle  100  has been rented to the renter, the interrogator  300 A includes a display and/or speaker system for informing the renter of the location of the vehicle  100 . 
   Another interrogator  300 B next to the vehicle  100  receives the data packet from the driver tag system  120 A and releases the keys for the vehicle  100 . The interrogator  300 B also activates the ignition of the vehicle  100  so that it can be started using the keys. The reserved locations may be positioned around the short and long term parking areas as well as near rail road, cab, and bus connections. 
   When the vehicle is returned to the reserved parking, the interrogator  300 B receives the VID, mileage and fuel level data from the tag system  120 B and relays this information to the central computer as described above. The renter leaves the vehicle  100 , approaches the interrogator  300 B where his tag is read. A key box is opened and the interrogator  300 B disables the vehicle&#39;s ignition. The keys are then placed in the box, the box closed, and a receipt is printed. The host computer  320  also notifies a person to retrieve the vehicle  100 . This embodiment eliminates the necessity of boarding a shuttle when picking up or dropping off the vehicle  100 . 
   Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.