Abstract:
An access system ( 2 ) including an electronic key ( 4 ) provided with a transmitter ( 6 ) and including a secured location provided with a receiver ( 10 ), the transmitter ( 6 ) and the receiver ( 10 ) being designed to communicate with one another to exchange authentication data. The transmitter ( 6 ) transmits a signal; the receiver ( 10 ) converts the transmitted signal into spectral data; and, in response to transmission of the authentication data, the access system ( 2 ) grants access to the secured location, provided the spectral data matches the spectral signature of transmitter ( 6 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to an access system, in particular a passive access system for vehicles. 
     BACKGROUND INFORMATION 
     Passive vehicle access system in existence today make use of remote-controlled electronic keys. The keys are equipped with a transmitter, which transmits authentication data to a receiver located in the vehicle when the key is within a predefined range of the receiver. The communication protocol activated between the transmitter and receiver uses a radio frequency (RF) interface to route the transmitted data. The radio frequency (RF) interface has a limited range to ensure that the communications connection is discontinued when the individual possessing the key moves away from the vehicle&#39;s immediate vicinity. 
     Passive access systems are susceptible to tampering by unauthorized individuals, who use a repeater between the vehicle and the key. The repeater uses radio frequency amplifiers to produce the communications connection when the key is not in the vehicle&#39;s immediate vicinity. The present invention proposes a system that will eliminate this problem or at least offer a practical alternative thereto. 
     SUMMARY OF THE INVENTION 
     The present invention introduces an access system, which includes an electronic key that is provided with a transmitter and a secured location for the receiver, the transmitter and receiver being designed to communicate with one another to exchange authentication data. The transmitter transmits a signal; the receiver converts the transmitted signal into spectral data; and, in response to the transmission of the authentication data, the access system grants access to the secured location, provided the spectral data matches the transmitter&#39;s spectral signature. 
     Advantageously, the transmitter can detect the presence of a repeater when the spectral data represents the use of a transmission characteristic of the repeater. 
     The present invention also employs a method for granting access to a secured location, including: 
     receiving a transmitted signal; 
     converting the transmitted signal into spectral data; 
     comparing the spectral data to a spectral signature of a transmitter; and 
     granting access to the secured location in response to receipt of authentication data, provided the spectral data matches the spectral signature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of an embodiment of an access system and transmitted and received signals according to an embodiment of the present invention. 
     FIG. 2 shows a schematic diagram of the received signal strength in relation to the frequency. 
     FIG. 3 is a block diagram of the access system according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A passive access system  2 , as shown in the figures, includes the following: an electronic key  4  having a transmitter  6  and an induction coil antenna  7 , a radio base station  8  having a receiver  10  and an induction coil antenna  12 . Radio base station  8  is kept at a secured location, such as in a vehicle, and controls access to the secured location. When the key is brought within a certain range of antenna  12  of receiver  10 , receiver  10  activates key  4 , thereby causing transmitter  6  to initialize transmission to receiver  10 . Data is transmitted using radio frequency signals, which establish a communications connection between key  4  and radio base station  8 . The data transmitted between key  4  and radio base station  8  is determined by a communication protocol, with which key  4  and radio base station  8  comply, and which includes the transmission of authentication data from key  4  to receiver  10 . Radio base station  8  only grants access to the secured location when the transmitted authentication data matches the authentications stored in radio base station  8 . 
     To establish a communications connection between key  4  and radio base station  8  when key  4  is at a location outside of the predefined range of antenna  12  of the receiver, a radio frequency repeater  16  can be brought between key  4  and radio base station  8 . To establish the communications connection, the repeater uses amplifiers, which must significantly amplify the signals transmitted by system  2 , to span the distance between key  4  and radio base station  8 . The amplifiers of any repeater  16  having a high degree of amplification have a transmission characteristic, which is ideally linear, in practical use, however, it is never linear and reaches a maximum degree of amplification. Therefore, repeater  16  interferes with the signal transmitted by key  4 , and the linearity of repeater  16  determines the magnitude of the signal interference effect. A measurement, known as the two-tone measurement, can be used to measure the linearity of the repeater to determine the third order intercept point of the repeater. The third order intercept point is a theoretical point where third order tones, which are produced by mixing the transmitted fundamental tones, intercept and interfere with the fundamental tones in the sense that the third order signals emitted by the repeater have the same amplitude as the fundamental signals and/or first order signals. The third order intercept point (IP 3 ) of a radio frequency repeater is a characteristic property, which can be determined by measuring the received signal strength of the third order intermodulation tones received by the receiver. 
     Passive access systems usually transmit data using one single radio frequency tone. To detect the presence of a repeater  16  due to the signal interference, which the repeater causes, access system  2  of the preferred exemplary embodiment transmits two fundamental frequency tones  20  and  22 , as shown in transmission spectrum  25 . The two radio frequency tones  20  and  22  can be used for transmitting data; however, the accuracy of the subsequent two-tone measurement taken by receiver  10 , as described in the following, may only be  5 %. The accuracy of the measurement is ±1% when key  4  transmits tones  20  and  22  with a constant amplitude for the two-tone measurement and then subsequently transmits the authentication data using radio frequency modulation with one or both tones, which represent the carrier signal. 
     In response to the transmission of fundamental tones  20  and  22 , the receiver receives the tones and two third order intermodulation tones  24  and  26 , as shown in the frequency response or spectral response  27  for receiver  10 . Fundamental tones  20  and  22 , as shown in FIG. 2, are stored in adjacent frequency channels C 2  and C 3 , while intermodulation tones  24  and  26 , which are produced by mixing the fundamental tones, have a reduced amplitude and are located in a low frequency channel C 1  and a higher frequency channel C 4 . A received signal strength indicator (RSSI) is produced by most FM receiving terminal semiconductors and can provide a measurement of the amount of energy received in each channel C 1  through C 4 . The RSSI output produced by receiver  10  is a voltage, which is proportional to the in-band energy of the signal received in each measured channel C 1  through C 4 . The RSSI for each channel can therefore be used to determine any variation, which is introduced into the third order modulation tones  24  and  26  by introducing a repeater  16 , as a result of the non-linearity of the amplifiers of repeater  16 . To detect this variation, access system  2  is activated, a normal communications connection first being established within the predefined range between key  4  and radio base station  8 ; the RSSI for each channel C 1  or C 4  being measured; and this measurement being recorded as a spectral signature for transmitter  6  of key  4 . All future transmissions can be measured in a similar manner to determine if any repeater was introduced into the system to alter the amount of received third order intermodulation energy. Furthermore, the difference received in the third order tones can be used to determine a characteristic third order intercept point to identify tampering repeater  16 . The detection of a repeater  16  by radio base station  10  ensures that radio base station  10  denies access to the secured location, even if the authentication data is received as valid. 
     Transmitter  6 , as shown in FIG. 3, includes a switching logic, which transmits two constant sound signals once receiver  10  activates key  4 . The switching logic can include for the tones, two radio frequency oscillators  30  and  32 , respectively, whose outputs are combined in a multiplexer  34  for transmission to antenna  7  of transmitter  6 . Alternatively, the switching logic can include a complex quadrature modulator, which enables the production of two tones separated by more than the channel distance used in receiver  10 . 
     Receiver  10  includes FM receiving terminal  36 , which is connected to antenna  12 , an analog-digital converter  38 , a microcontroller  40 , and a frequency-synthesized local oscillator  42 . Microcontroller  40  is programmed for controlling frequency synthesizer  42  and for processing data received by analog-digital converter  38 . The frequency synthesizer is used for selecting the frequency channels, which are to be processed by FM receiver  36 , which, as discussed above, produces an RSSI output for each of the four channels C 1  through C 4 . The RSSI output for each channel is routed to the analog-digital converter for conversion into a binary word for processing by microcontroller  40 . 
     Microcontroller  40  treats the binary word as spectral data, which represents the received energy in each channel C 1  through C 4 , and then compares the spectral data to a previously stored spectral signature for transmitter  6 . 
     System  2  is actuated in that key  4  is brought within the predefined range of antenna  12 , thereby activating key  4  and causing the transmission of two fundamental tones. The spectral data received by microcontroller  40  is then stored as a spectral signature of transmitter  6  for future comparison for all subsequent communication between key  4  and receiver  10 . 
     Accordingly, key  4  and radio base station  8  then perform the following steps when a communications connection is established: 
     (i) Before transmitting any authentication data, the two fundamental tones in channels C 2  and C 3  are simultaneously transmitted. 
     (ii) Frequency synthesizer  42  selects the four channels C 1  through C 4 , and FM receiver  36  produces an RSSI output for each channel. 
     (iii) Microcontroller  40  receives and processes the spectral data, which is representative of the received signal level for each channel, and the spectral data is compared to the stored spectral signature. 
     (iv) In the event that there is a deviation of more than ±1% between the spectral signature and the spectral data, microcontroller  40  causes radio base station  10  to discontinue the authentication procedure and to prevent access to the secured location. 
     (v) The extent to which the received spectral data deviates from the spectral signature is recorded for subsequent analysis to determine a characteristic third order intercept point so that tampering repeater  16  can be identified. The number of tamperings from repeater  16  can also be stored. 
     (vi) When radio base station  10  then detects an authorized user and grants authorized access, microcontroller  40  causes a warning signal to be produced indicating that tampering was attempted. 
     The warning signal can be in the form of a word code, a warning light, or a sound signal, which is produced at the secured location, meaning the vehicle. 
     A number of modifications to this will become familiar to those skilled in the art without exceeding the scope of the present invention as it is herein described with reference to the attached drawings.