Device for transmitting data in a motor vehicle

[0001] The present invention relates to a device for data transmission in a motor vehicle and/or from a motor vehicle in its vicinity, having a first transceiver unit in or on the motor vehicle and a second transceiver unit which is provided in at least one transponder unit whose spatial position relative to the vehicle may be variable or any desired position.

[0002] Such a device is described in IEEE Transactions on Industrial Electronics, Vol. 35, No. 2, May 1988 under the title Keyless entry system with radio card transponder. This known data transmission device transmits a coded query sequence via an induction loop mounted on the vehicle, e.g., in an exterior mirror or in the bumper, to a transponder accommodated in a card the size of a credit card, which then delivers a response sequence via an antenna. The transmission frequencies used here are in the range of a few hundred kHz.

[0003] In general, the mechanical keys customary in the past are currently increasingly being replaced by electronic systems such as remote control systems using infrared or wireless signals for access authorization to vehicles. In other words, electronically encoded keys are being used to an increasing extent for startup authorization.

[0004] Communication systems between vehicles and an infrastructure installation, e.g., for acquisition of road use fees, are also known.

[0005] Also known are radar distance measuring systems which are installed in the vehicle and operate in the gigahertz range and a radar sensor on at least one side of the vehicle for measuring the distance and velocity between the vehicle and an obstacle, e.g., a vehicle driving in front or a parked vehicle.

[0006] The object of the present invention is to utilize in a simple manner a radar transmission channel, which is present in the vehicle for distance measurement, for other data transmission functions which are activated only in conjunction with a data exchange which is deemed valid. Firstly, this should reduce costs in manufacturing these systems in comparison with previous separate radar and data transmission systems, and secondly, the security of the transmission between the vehicle and one or more transponders should increase.

[0007] Therefore, in order to achieve the object formulated above, a wireless connection is established between the vehicle and at least one transponder. A particular simplification combined with a simultaneous increase in security is achieved due to the fact that each transceiver unit has a two-channel data transmission unit whose microwave frequencies for two-channel communication are selected so that their difference yields an intermediate frequency which is processable by using conventional components in a heterodyne receiver of the receiving part of the first and second transceiver.

[0008] The orientation of the transmitting and receiving antennas relative to one another is problematical because the spatial position of the transponder, in particular, the key (i.e. key fob) to the vehicle is variable as desired. For example, the key may be in the driver's pants pocket.

[0009] If a linearly polarized antenna is used in the vehicle sensor and a circularly polarized antenna is used in the transponder to achieve this goal, this avoids the case of two linearly polarized antennas which would produce a theoretically infinitely high attenuation of the transmitted signal if their polarization directions were at right angles.

[0010] Furthermore, one or more antennas having the most spherical possible, i.e., omnidirectional characteristic should be used in the transponder (key).

[0011] The radar sensor of the vehicle used for measuring distance and velocity is expanded by adding a two-channel data transmission system which may be used for local communication with a key for keyless entry applications and also for communication with an installation for acquisition of fees, for telematic applications or other remote control applications.

[0012] The two transceivers are designed so that their oscillator signal (LO signal) is used for downmixing in the case of reception. The transmission frequency and the receiving frequency are such that the band limits of the frequency band used may be upheld due to the their frequency shift. Therefore, frequency stabilization need be used on only one side, i.e., on the motor vehicle side or on the transponder side. Therefore, and due to the fact that conventional components may be used, the entire data transmission system may be implemented inexpensively.

[0013] The data transmission device according to the present invention is explained in greater detail below on the basis of the enclosed drawing using preferred embodiments.

[0014] FIG. 1 shows in the form of a block diagram a first embodiment of the data transmission device according to the present invention in which the transponder side has an AFC circuit which pulls the frequency of an oscillator, which may be a dielectric resonant oscillator (DRO) for example, so that the intermediate frequency is kept constant.

[0015] FIG. 2 shows a second embodiment in the form of a block diagram in which the intermediate frequency, which fluctuates due to the frequency drift of the DRO, is compensated by the fact that a variable LO signal is generated in the receiving branch and downmixes the intermediate frequency to a lower constant intermediate frequency.

[0016] FIG. 3 shows in the form of a table the additional attenuation resulting due to a difference in polarization between the field and the antenna.

[0017] FIG. 4 shows a transmitting/receiving antenna having a directional coupler.

[0018] FIG. 5 shows a block diagram of the part of the data transmission device located in a vehicle, the data transmission device being integrated into an existing automotive radar sensor.

[0019] FIG. 1 shows a first embodiment of a two-channel data transmission device according to the present invention in the form of a block diagram. On the left side of a vertical line indicated with a dash-dot line, the vehicle side having a first transceiver unit 1 is illustrated, and on the right side of the dash-dot line the transponder side having a second transceiver unit 2 is shown. First transceiver unit 1 in or on the vehicle has a dielectric resonant oscillator (DRO) 10 whose oscillator frequency f₁ (first frequency) is modulated by a transmission modulator 11 using a coded sequence, e.g., in the form of an ASK (amplitude shift keying) modulation. Instead of this, it is also possible to use an FSK (frequency shift keying) or PSK (phase shift keying) modulation. In the receiving branch there is a heterodyne receiver having a mixer 12, a low-pass filter 13 and a demodulator 14. A first antenna 15 of first transceiver unit 1 is a transmitting antenna 15 which transmits a signal having frequency f₁. A second antenna 16 is a receiving antenna which receives a signal having frequency f₂ from the transponder. Mixer 12 mixes the received signal having frequency f₂ with transmission frequency f₁ of DRO 10, and intermediate frequency f_IF is formed from difference f₁-f₂ after low-pass filtering 13. The reception signal in intermediate frequency position f_IF is then demodulated by demodulator 14. The two frequencies f₁ and f₂ are selected so that their difference f₁-f₂, i.e., intermediate frequency f_IF, may be processed with inexpensive conventional standard components.

[0020] Due to the conventional and inexpensive implementation of the microwave oscillator with a dielectric resonator (DR), there is a certain frequency drift without stabilization measures. For this reason, a safety margin from the band limits is selected for the transmission/receiving frequencies and this yields a certain frequency shift and thus an intermediate frequency f_IF.

[0021] On the transponder side, the transmitting branch of second transceiver unit 2 also has a dielectric resonant oscillator DRO 20 and a modulator 21 which modulates it. Transmission frequency f₂ generated by DRO 20 and modulated by modulator 21 is sent over a power splitter to transmitting antenna 26 of second transceiver unit 2. The reception part of second transceiver unit 2 likewise has a heterodyne receiver which in turn has a mixer 22, a low-pass filter 23 and a demodulator 24. Frequency f₂ of DRO 20 is downmixed in mixer 22 with modulated signal f₁ received by first transceiver unit 1, forming intermediate frequency f_IFf₁-f₂ which is demodulated in demodulator 24 after low-pass filtering in low-pass filter 23.

[0022] The converse case, i.e., when the transmitting branch in second transceiver unit 2 transmits and the receiving branch in the first transceiver unit receives on the vehicle side, functions in the same way. Full duplex operation is thus possible, each of the two transceiver units 1 and 2 possibly being active.

[0023] The only difference between the two transceiver units is that one contains the frequency regulation so that the frequency shift caused by using DROs is compensated, and the intermediate frequency is kept constant. In the embodiment in FIG. 1, second transceiver unit 2 has an AFC circuit 27 on the transponder side, pulling frequency f₂ of DRO 20, so that intermediate frequency f_IF is kept constant. Therefore, DRO 20 of second transceiver unit 2 conforms to the frequency drift of first DRO 10 on the vehicle side.

[0024] On the vehicle side, the embodiment illustrated in FIG. 2 is identical to the embodiment described above and illustrated in FIG. 1. Only the reception part of second transceiver unit 2 in the transponder has been modified in comparison with the embodiment in FIG. 1. Intermediate frequency f_IF which fluctuates due to the frequency drift of DROs 10, 20 is compensated according to FIG. 2 by the fact that it is not regulated but instead it is mixed down with a variable local oscillator signal f_LO to a lower constant intermediate frequency f_IF2. To do so, a local oscillator 28, a mixer 30 and a low-pass filter 29 are used in addition.

[0025] The problem in orientation of transmitting/receiving antennas 15, 16, 27, 26 of the first and second transceiver units is explained below on the basis of FIG. 3.

[0026] Copolarized antennas are normally used in wireless transmission systems on the transmitting and receiving sides and are usually linearly polarized. For example, if two dipoles are used, a maximum signal strength at the receiving dipole (attenuation 0 dB) is obtained with a parallel orientation. If the two dipoles are rotated 90 toward one another, the attenuation is (theoretically) infinitely great. These two cases are illustrated in the table in FIG. 3. However, since there is always some reflection in the vicinity of the antennas, a weak signal is nevertheless received in practice.

[0027] If one antenna is circularly polarized (circularly anticlockwise rotating or circularly clockwise rotating) and the other antenna is linearly polarized, then in the best possible case the signal attenuation amounts to 3 dB, depending on how the antenna is rotated in its plane perpendicular to the direction of the connection.

[0028] Both antennas should not be circularly polarized because if a non-omnidirectional antenna is oriented in the direction opposite that of the other antenna, reflection results in the direction of rotation of the circularly polarized waves being in the opposite direction, and the attenuation being (theoretically) infinitely great.

[0029] In a passive entry system made possible by the data transmission device according to the present invention, the position of the vehicle is assumed to be fixed in space, but there may be any desired orientation of the transponder to the vehicle, so a linear polarization should be used on one side and circular polarization on the other side. Therefore, it is not possible for the case of theoretically infinitely attenuation to occur. According to the present invention, the passive entry system is implemented in the microwave range. Then the high-frequency front end in the vehicle may be provided, for example, with a linearly polarized patch antenna, and an array of one or more circularly polarized patch antennas may be used in the key with the transponder to obtain the best possible omnidirectional characteristic.

[0030] In departure from the two implementations indicated schematically in FIGS. 1 and 2 in which one transmitting antenna each is provided separately from a receiving antenna on both sides of first and second transceiver units 1 and 2, it is also possible to use what is known as a monostatic implementation having a directional coupler. Such an implementation is illustrated in FIG. 4. The receiving branch is connected to the transmitting branch via a directional coupler, and the transmitting/receiving antenna is common to both branches.

[0031] FIG. 5 shows details of a data transmission device implemented on the vehicle side and combined with an existing automotive radar sensor. For the data transmission mode of operation, both high speed switches HSS₁ and HSS₂ are closed by being driven not by snap-off diodes but instead by a direct voltage. The other function blocks of the data transmission device are identical to the components of transceiver unit 1 of FIGS. 1 and 2.

[0032] The required AFC circuit (FIGS. 1 and 2) is integrated into the transponder, i.e., in second transceiver unit 2 in the key fob, in a beacon, etc., because this transceiver unit 2 in the transponder does not have a radar mode in which automatic frequency control AFC would have to be shut down.

[0033] Due to the use of this combined radar/data transmission system, a cost reduction in production is achieved in comparison with previous separate systems, and furthermore, the reliability of transmission between the vehicle and the transponder is increased. Due to the advantageous implementation of the intermediate frequency common to both transceiver units, which may be processed using inexpensive standard components, the proposed data transmission device according to the present invention may be integrated easily and inexpensively into the existing automotive radar system.