Communication device with antenna attenuation compensation

Abstract

Communication device (2) for a motor vehicle (1), comprising: - a transceiver unit (3) for generating transmission signals (5) at a transmission output (21) of the transceiver unit (3), wherein at least two of the transmission signals (21) differ in their signal power, - a cable (7) coupled to the transmission output (21) for transmitting the transmission signals (5) to an antenna (18), wherein the antenna (18) is connected to an antenna-side end (19) of the cable (7) via a controllable amplifier device (8) for amplifying the transmission signals (7) transmitted via the cable (7), - a power detector device (15) for detecting a respective power value (29) correlated with a power of the transmission signals (5) at a tapping point (28), the tapping point (28) being arranged between the antenna-side end (19) of the cable (7) and the antenna (18), - a control device (13) for controlling the amplifier device (8) as a function of the detected power values ​​(29), wherein the transceiver device (3) is configured to generate at least one of the transmission signals (5) with a predetermined reference power, wherein a reference value (32) describing the reference power is fixedly predetermined in the control device (13), and the control device (13) is configured to select one of the power values ​​(29) as an actual value (29') indicating the power of the at least one transmission signal (5) generated with the reference power at the tapping point (28), and to set a transmission gain value (11) of the amplifier device (8) depending on a difference (33) between the reference value (32) and the actual value (29'), wherein the transceiver device (3) is configured to generate the transmission signals (5) at the transmission output (21) in each case exclusively with a transmission power which is selected from one of at least two predetermined power levels, wherein the reference power corresponds to one of the power levels and wherein the control device (13) is configured to select as the actual value (29') that power value (29) which indicates a power between the power level of the reference power and the next lower power level.

Description

The invention relates to a communication device for a motor vehicle. The communication device comprises a transceiver unit and an antenna for transmitting transmission signals of the transceiver unit. The antenna is coupled to the transceiver unit via a cable. In order to compensate for signal attenuation of the cable in the transmission signals, an amplifier device is provided at the antenna-side end of the cable. The communication device can be provided, for example, for car-to-car communication (vehicle-to-vehicle communication). The invention also includes a motor vehicle and a method for compensating cable attenuation.DE 10 2014 215 578 A1 uses a power stage in a compensator.When arranging the described communication device in a motor vehicle, the problem often arises that the transceiver unit and the antenna have to be arranged at different installation locations. Therefore, a relatively long cable is required to transmit the transmission signals of the transceiver unit to the antenna. However, the signal attenuation of the cable is also noticeable when the transmission signals are transmitted. In particular, in the case of a communication standard which provides for the transmission of the transmission signals with prescribed transmission power, it is then not clear with which power a transmission signal is radiated at all from the antenna after it has been generated with a known signal power at a transmission output of the transceiver unit and transmitted via the cable.The described amplifier device can amplify the transmission signal again at the antenna-side end of the cable in order to compensate for the cable attenuation. However, it must be known for this purpose by which transmission amplification value the transmission signal must be amplified by the amplifier device. One possibility for determining the transmission gain value is to measure the power of the transmission signals once at the transceiver-side end of the cable and once at the antenna-side end of the cable.A disadvantage of this solution is that the measuring circuit on the transceiver side must be wired to the control device for the amplifier device. This means an undesirably high cabling and interconnection outlay when providing the communication device.The invention is based on the object of implementing attenuation compensation for the antenna cable with little circuit complexity in a communication device.The object is achieved by the subject matters of the independent claims. Advantageous further developments of the invention are given by the features of the dependent patent claims.The invention provides a communication device for a motor vehicle, as can be provided for car-to-car communication, for example. In the described manner, a transceiver unit for generating transmission signals is provided at a transmission output of the transceiver unit. Each transmit signal corresponds to a data packet or message. The transmission signals are generated successively in time. In particular, these are transmission signals which are generated with the same carrier frequency. At least two of the transmission signals differ in their signal power. This may be because switching between different power stages is performed when generating the transmission signals. This is provided, for example, in the communication protocol for car-to-car communication. For example, the different transmission powers can be predefined in accordance with the IEEE 802.11a or IEEE 802.11h (TPC-Transmitter Power Control). The communication device further comprises a cable coupled to the transmit output for transmitting the transmit signals to an antenna. The cable can be, for example, a coaxial cable. The antenna is connected to an antenna-side end of the cable via a controllable amplifier device (VGA) for amplifying the transmission signals transmitted via the cable.The aim is now to set a transmission amplification value of the amplifier device in such a way that the cable attenuation in the transmission signals is compensated. For this purpose, a power detector device is provided for detecting a respective power value of the transmission signals. The power values are correlated with a power of the transmission signals as the transmission signals have at the tap point. This tap is arranged between the antenna-side end of the cable and the antenna. Each power value thus indicates the power of the transmission signals, as results after transmission via the cable. Depending on whether the tap point of the power detector device is connected in front of the amplifier device or is connected downstream thereof, the influence of the amplifier device is also expressed by the power value. A control device is provided for controlling the amplifier device in dependence on the detected power values.Since the transceiver unit generates at least two of the transmission signals with different signal power, however, the power values themselves are not to be considered as to how far they differ from the desired signal power that the respective transmission signal has exhibited at the signal output. This is initially unknown on the antenna side.In order to be able to decide on the basis of the power values detected at the tap point which value the transmission amplification value of the amplifier device should have, the following is therefore provided according to the invention. The transceiver unit is configured to generate at least one of the transmission signals with a predetermined reference power. Furthermore, a reference value is permanently predefined in the control device, which describes the reference power. It can be stored as a digital value, for example, or defined in an analog comparison circuit, for example, by a reference voltage. The control device is furthermore configured to recognize or select an actual value from the power values. This actual value indicates the power of the at least one transmission signal generated with the reference power at the tap point. In other words, the control device searches from the power values the one from which it is clear on the basis of the transmission situation that it must be a power value which belongs to a signal which has been generated at the signal output of the transceiver device with the reference power. This power value is then treated as the actual value. Depending on a difference between the reference value and the actual value, the control device adjusts the transmission amplification value of the amplifier device. Specifically, the transmission gain value is set such that the difference between the reference value and the actual value is decreased. In other words, the power at the tap point is amplified and is preferably set back to the reference power for signals which have the reference power at the transmission output. The difference can be calculated, for example, as a difference or quotient. In an analog comparison circuit, the difference can be determined, for example, by means of an operational amplifier.The invention has an advantage that the control means can set the transmission gain value for the antenna-side amplifier means based solely on power values of a power detector means also provided at the antenna-side end of the cable. In other words, no additional cabling of the control device to the transceiver-side end of the cable is necessary in order to likewise detect power values there. This saved or no longer required cabling and the second sensor reduce the interconnection outlay when providing the communication device.The invention also includes optional refinements, the features of which result in additional advantages.According to one refinement, the transceiver device is configured to generate the transmission signals at the transmission output in each case exclusively with a transmission power selected from one of at least two predefined power levels. In other words, the transmission power is discretized. The transmission power is therefore only changed by predetermined step values. Naturally, said reference power corresponds to one of the power stages. The control device is configured to select, as the actual value, the power value which indicates a power between the power level of the reference power and the next lower power level. The step size or step height of the power stages is preferably selected in such a way that it is greater than any possible or expected cable attenuation. The development results in the advantage that the transmission amplification value can be determined without interrupting the communication during ongoing operation of the communication device.A development provides that the transceiver device is configured to generate a calibration signal with the reference power as a transmission signal as a function of a trigger signal. The control device is configured to select the actual value as a function of the trigger signal. In other words, the trigger signal triggers a calibration phase of the communication device. This results in the advantage that the time at which the transmission signal with the reference power, i.e. the calibration signal, is generated, is known. It is therefore not necessary to wait until the transceiver device generates a transmission signal with the reference power as part of a communication. For example, the trigger signal can consist in the communication device being put into operation, i.e. transitioning from a non-energized state to an energized state.A further development provides that the reference power corresponds to a maximum or a minimum transmission power. For example, in the stepped power described, the reference power may correspond to the highest or the lowest power level. If the reference power corresponds to the maximum transmission power, the transmission device is configured to select a maximum value of the detected power values as the actual value. If the reference power corresponds to the minimum transmission power, the transmission device is correspondingly configured to select a minimum value or minimum value of the detected power values as the actual value. In other words, histogram analysis of the detected power values is performed, and the maximum value or the minimum value is selected as the actual value. This results in the advantage that the transceiver device can also use stepped signal power, the power levels of which are smaller than the possible or expected attenuation values of the cable.A further development also makes it possible to compensate for the cable attenuation even in the case of signal reception. In this refinement, the amplifier device is configured to amplify received signals received via the antenna as a function of a received amplification value and then to forward the amplified received signals to the cable. The amplified received signals then pass via the cable to the transceiver device. The control device is configured to set the reception gain value depending on said difference between the reference value and the actual value. This results in the advantage that no additional cable attenuation has to be determined with respect to the received signals.In order to exchange both transmission signals and reception signals between the antenna-side end of the cable and the amplifier device, it can be provided that the amplifier device is connected to the cable via a multiplexer. Such a multiplexer is a switch circuit which alternately connects an amplifier input of the amplifier device to the antenna-side end of the cable and an amplifier output of the amplifier device to the antenna-side end of the cable. The transceiver device is designed to connect the multiplexer by means of a switching signal between a transmission position (cable is coupled to the amplifier input) and a reception position (cable is coupled to the amplifier output of the amplifier device). The switching signal can be transmitted, for example, as a signal via the cable itself, which signal has a lower frequency than the transmission signals. The control device is configured to distinguish between power values of the transmission signals and power values of the reception signals on the basis of the switching signal, which power values can both be detected by the power detection device, since the latter itself does not distinguish between transmission signals and reception signals. The control device is furthermore configured to take into account only power values of the transmission signals in order to determine the said difference. This results in the advantage that the transmission amplification value and the reception amplification value are not adulterated by the power of the reception signals.In the described manner, it is preferably provided that the control device takes into account power values detected exclusively between the antenna-side end of the cable and the antenna for ascertaining the difference. The control device therefore does not have to be coupled to a power detector at the transmission output of the transceiver via an additional measuring circuit.In particular, it is provided that the control device is electrically coupled to the transceiver device exclusively via the cable itself. The only connection that is necessary between the transceiver device and the antenna arrangement (antenna, amplifier device and control device) is thus the cable itself. This minimizes the interconnection outlay. Nevertheless, the antenna arrangement is capable of self-calibrating or parameterizing to the effect that the attenuation of the cable is compensated. The antenna arrangement is thus self-calibrating or self-parameterizing.In order to generate said power values at the tap point, a refinement provides that the power detector device is designed as a diode-based envelope demodulator. As a result, the power detector device can be realized with particularly little circuit complexity.A development provides that the communication device is configured to generate the transmission signals according to the car-to-car communication protocol. In other words, the communication device is designed according to the IEEE 802.11 standard. This is thus a Wi-Fi communication device.The invention also includes a motor vehicle. The motor vehicle according to the invention has an embodiment of the communication device according to the invention. The motor vehicle according to the invention has the advantage that the transceiver unit and the antenna can be arranged at different locations and can be connected to a cable and subsequently the signal attenuation caused by the cable is compensated by the communication device in a self-calibration or self-parameterization without additional measurement lines having to be provided for this purpose between the transceiver device and the antenna-side end of the cable.The invention also includes a method for compensating cable attenuation in a communication device. In the method, a transceiver unit generates transmission signals at a transmission output of the transceiver unit, wherein at least two of the transmission signals differ in their signal power, as can be caused, for example, by the described TPC (Transceiver Power Control). A cable coupled to the transmit output transmits the transmit signals from the transmit output to an antenna. In this case, a controllable amplifier device for amplifying the transmission signals transmitted via the cable is connected between an antenna-side end of the cable and the antenna. Furthermore, a power detector device detects respective power values correlated with the power of the transmission signals at a tap point which is arranged between the antenna-side end of the cable and the antenna. A control device controls the amplifier device as a function of the detected power values. In order to be able to read the cable attenuation from the power values and to be able to set the transmission amplification value in the amplifier device in accordance with the invention in order to compensate for the cable attenuation, it is provided that the transceiver device generates at least one of the transmission signals with a predetermined reference power and a reference value describing the reference power is fixedly predefined in the control device. The control device selects an actual value from the power values of the power detector device, which indicates the power of the at least one transmission signal generated with the reference power at the tap point. Depending on a difference between the reference value and the actual value, the transmission amplification value of the amplification device is adjusted by the control device.The method according to the invention also has the advantage that no power measurement is necessary at the transceiver-side end of the cable in order to determine the cable attenuation and thus the transmission amplification value.The invention also includes developments of the method according to the invention, which have features as have already been described in connection with the developments of the communication device according to the invention. For this reason, the corresponding developments of the method according to the invention are not described again here.An embodiment of the invention is described below. For this purpose, the single FIGURE (FIG.) shows a schematic representation of an embodiment of the motor vehicle according to the invention.The exemplary embodiment explained below is a preferred embodiment of the invention. In the exemplary embodiment, the described components of the embodiment each represent individual features of the invention that are to be considered independently of one another and that develop the invention in each case also independently of one another and are therefore also to be considered as part of the invention individually or in a combination other than the combination shown. Furthermore, the described embodiment can also be supplemented by further features of the invention that have already been described.The figure shows a motor vehicle 1, which can be, for example, a motor vehicle, in particular a truck or passenger car. For a vehicle-to-vehicle communication or car-to-car communication or C2C communication, the motor vehicle 1 has a communication device 2. The communication device 2 can have a transceiver unit 3 and an antenna arrangement 4. The transceiver device 3 can be designed in a manner known per se. For transmitting transmission signals 5 and reception signals 6 between the transceiver unit 3 and the antenna arrangement 4, a cable 7 is provided for electrically connecting the transceiver unit 3 and the antenna arrangement 4. The cable 7 may be, for example, a coaxial cable. The cable 7 can be, for example, longer than 50 centimeters, in particular longer than 1 meter.When transmitting the transmission signals 5 and the reception signals 6, the cable 7 attenuates the signals, that is, a power or amplitude of the transmission signals 5 and the reception signals 6 is reduced when transmitting via the cable 7. To compensate for the cable attenuation of the cable 7, the antenna arrangement 4 has an amplifier device 8. The amplifier device 8 can have a transmit amplifier 9 and a receive amplifier 10. The transmission signals 5 can be amplified by means of the transmission amplifier 9 as a function of a transmission amplification value 11. By means of the reception amplifier 10, the reception signals 6 can be amplified as a function of a reception amplification value 12.The transmit gain value 11 and the receive gain value 12 can be adjusted by a control device 13 of the antenna arrangement 4. For this purpose, a control output 14 of the control device 13 is coupled to the amplifier device 8. The control device 13 can be realized, for example, on the basis of a microcontroller. The control device 13 can also be realized as an integrated circuit or as a discrete circuit.For determining or detecting the power of the transmission signals 5, a power detector device 15 is provided in the antenna arrangement 4. The power detector device 15 can have a diode 16 for envelope demodulation. In the example illustrated in the figure, the power detector device 15 is connected to an antenna line 17 which couples an antenna 18 of the antenna arrangement 4 to the amplifier device 8. The antenna 18 transmits the transmission signals 5 into the air and receives the reception signals 6 from the air.The cable 7 can be connected to the antenna arrangement 4 via a coupler 19 in a manner known per se. The coupler 19 represents an antenna-side end of the cable 7. The cable 7 can be coupled to the transceiver device 3 via a further coupler 20. The coupler 20 represents a transceiver-side end of the cable 7. By means of the couplers 19, 20 it can be provided to transmit via the cable 7 in addition to the transmission signals 5 and the reception signals 6 also a supply voltage or DC voltage DC and a control signal LF which is low-frequency in relation to the transmission signals 5 and the reception signals.The transceiver device 3 generates the transmission signals 5 at a transmission output 21 in a manner known per se and receives the reception signals 6 at a reception input 22, In order to couple the converter 20 alternately to the transmission output 21 for transmitting the transmission signals 5 and the reception input 22 for receiving the reception signals 6, a controllable switch or a controllable switch or a controllable multiplexer 23 is connected between the converter 20 on the one hand and the transmission output 21 and the reception input 22 on the other hand. The multiplexer 23 is controlled, for example, by the transceiver device 3. Furthermore, the converter 19 is connected via a controllable multiplexer 24 to an amplifier input 25 of the transmit amplifier 9 and to an amplifier output 26 of the receive amplifier 10. The transmit amplifier 9 and the receive amplifier 10 on the one hand and the antenna line 17 on the other hand are connected via a further controllable multiplexer 27. The multiplexers 23, 24, 27 can be designed in a manner known per se.A tap 28 of the power detector device 17 for detecting the power of the received signals 5 in the antenna arrangement 4 is preferably arranged between the multiplexer 27 and the antenna 18. In other words, the tap point 28 is located on the antenna line 17. the multiplexers 24 and 27 can also be switched or controlled by the transceiver device 3. For switching, the communication signal LF may include or contain a switching signal for switching the multiplexers 24 and 27.The antenna arrangement 4 only has to be coupled to the transceiver device 3 via the cable 7. No additional measuring lines are necessary. Also, only one power detecting means 15 is necessary to set the transmission gain value 11 and the reception gain value 12 by the control means 13. For this purpose, the control device 13 receives power values 29 from the power detector device 15 at a signal input 30, The power values 29 can be, for example, amplitude values of an analog signal. Depending on the power values 29, the controller 13 generates the transmission gain value 11 and the reception gain value 12.The gain control by means of the power detector device 15, the control device 13 and the amplifier device 8 thus takes place exclusively within the antenna arrangement 4, that is to say exclusively at the antenna-side end of the cable 7. the control device 13 detects power values 29 of the transmission signals 5, that is to say the strength of the transmitted data packets, for this purpose. The power values 29 are therefore detected only on that side of the cable 7 on which attenuation compensation 31 of the cable attenuation of the cable 7 by the amplifier device 8 is also carried out.The control device 13 compares the power values 29 with a reference value 32. the reference value 32 represents a desired power or reference power, which for example indicates the transmission power that some, specific transmission signals 5 are to have at the tap point 28. Via the difference, for example the difference, the power control is carried out in the variable amplifier (VGA) in the form of the transmission amplifier 9. In other words, the transmission gain value 11 is set depending on the difference 33.In order to be able to estimate the cable attenuation without a power detector on the transceiver side, the power to be transmitted must be known at the tap 28. The same applies to the time of the transmission signals 5. otherwise a power value 29 of a reception signal 6 would be used as a basis. The first point (known performance) can be achieved via the following technical solutions:A first solution provides that the transmission power of the transmission signals 5 at the transmission output 21 has a discretization which is greater than possible values for the cable attenuation. For example, two transmission powers or power stages can be provided, for example +23 dBm and +10 dBm. Due to this rough discretisation, it is possible, in the case of variations in the cable attenuation of up to 10 to 12 dB, to determine the transmission power of the transmission signal 5 at the transmission output 21 on the basis of the power values 29 at the tap point 28. For example, the value +23 dBm can thus be stored as reference value 32. If a power value 29 is then received which is greater than +10 dBm but less than the reference value 32, this power value 29 is an actual value 29' of the power of the transmission signal 5 which has been generated at the transmission output 21 with the reference power, that is to say in the example +23 dBm.A second solution may provide a calibration phase. It is possible, for example, to emit calibration pulses at the start of the communication device 2, when the latter is coupled to a supply voltage, or at predetermined times, i.e. to generate a calibration signal as the transmission signal 5 which has a defined transmission power, for example the +23 dBm. The controller 13 recognizes the calibration phase or the calibration state of the communication device based on, for example, the start or the time points. The power value then determined is then the actual value 29' of the calibration signal at the tap point 28. Then, the variable gain, i.e. the transmission gain value 11 and the reception gain value 12, can likewise be adapted to the effect that the subsequently detected power values 29 result, for example, for the calibration signal of the reference power, for example +23 dBm.A third solution provides that the control device 13 in each case detects the power value 29 for a plurality of transmission signals 5 transmitted one after the other. In other words, transmission packets are observed over a predetermined period of time. The cable attenuation can then be determined heuristically via the maximum and the minimum power observed at the tap point 28, i.e. the maximum power value and / or the minimum power value 29. For example, it can be provided that the transceiver device 3 adjusts the signal power of the transmission signals 5 at the transmission output 21 in a predetermined power interval, for example from +23 dBm to a minimum signal power of -7 dBm. In this case, a discretisation can also be provided, which can provide power stages of the size 0.5 dB, for example. In other words, the magnitude of the power stage is smaller than the maximum cable attenuation to be expected. By observing the transmission packets in a learning phase, i.e. by ascertaining a plurality of power values 29, it is possible to draw conclusions about the maximum and / or the minimum power of the transmission signals 5 at the tap point 28. In other words, the difference 33 then corresponds to the difference between the maximum transmission power on the one hand and the maximum power value 29 on the other hand. Likewise, a difference 33 between the minimum transmission power and the minimum power value 29 can be calculated. The maximum power or the minimum power then represents a reference value 32, respectively. A maximum power value as well as a minimum power value can also be provided for ascertaining the transmission gain value 11 and the reception gain value 12. Then, two reference values 32 are provided.After the learning phase, i.e. after the transmission amplification value 11 and the reception amplification value 12 have been set for the first time, the variable amplification only has to be adapted within the scope of, for example, temperature fluctuations which are very slow. This can be achieved via the discretisation, i.e. for example on the basis of the solution described first, since the temperature fluctuations cause attenuation value changes which are smaller than the discretisation described of for example 0.5 dB.It is also possible to combine the three described solutions by the control device 13 or also to combine a selection from the three solutions. For example, a calibration phase could take place when the communication device 2 is switched on and the heuristic solution could then be applied. The time of each transmission signal 5 can furthermore be transmitted by the transceiver device 3, for example by means of the described communication signal LF. However, no further data transmission is necessary. The signal is for signaling the transmission phases, i.e. if a transmission signal 5 is generated, it can be, for example, the switching signal for switching the multiplexers 24, 27.Overall, this results in a technical simplification of the compensation 31 with a saving of hardware components and system complexity in the communication device 2. This is achieved in that, on the compensator side, that is to say in the antenna arrangement 4, a reference value 32 for a transmission power of a transmission signal 5 is stored in the control device 13, with which at least one transmission signal 5 is generated at the transmission output 21 by the transceiver device 3. This known signal power thus forms a reference power.Overall, the example shows how a simplified compensator interface for a C2C communication can be provided by the invention.

Claims

Communication device (2) for a motor vehicle (1), comprising: - a transceiver unit (3) for generating transmission signals (5) at a transmission output (21) of the transceiver unit (3), wherein at least two of the transmission signals (21) differ in their signal power, - a cable (7) coupled to the transmission output (21) for transmitting the transmission signals (5) to an antenna (18), wherein the antenna (18) is connected to an antenna-side end (19) of the cable (7) via a controllable amplifier device (8) for amplifying the transmission signals (7) transmitted via the cable (7), - a power detector device (15) for detecting a respective power value (29) correlated with a power of the transmission signals (5) at a tap point (28), wherein the tap point (28) is arranged between the antenna-side end (19) of the cable (7) and the antenna (18), - a control device (13) for controlling the amplifier device (8) as a function of the detected power values (29), wherein the transceiver device (3) is configured to generate at least one of the transmission signals (5) with a predetermined reference power, wherein a reference value (32) describing the reference power is fixedly predefined in the control device (13) and the control device (13) is configured to use the power values (29) to generate one of them as an actual value (29') which indicates the power of the at least one transmission signal (5) generated with the reference power at the tap point (28), selecting a transmission amplification value (11) of the amplifier device (8) and setting it as a function of a difference (33) between the reference value (32) and the actual value (29'), wherein the transceiver device (3) is configured to generate the transmission signals (5) at the transmission output (21) in each case exclusively with a transmission power selected from one of at least two predetermined power levels, wherein the reference power corresponds to one of the power levels and wherein the control device (13) is configured to select, as the actual value (29'), that power value (29) which indicates a power between the power level of the reference power and the next lower power level.Communication device (2) according to one of the preceding claims, wherein the transceiver device (3) is configured to generate a calibration signal with the reference power as a transmission signal (5) as a function of a trigger signal, and wherein the control device (13) is configured to select the actual value (29') as a function of the trigger signal.Communication device (2) according to one of the preceding claims, wherein the reference power corresponds to a maximum or a minimum transmission power and the control device (13) is configured to select a maximum value or a minimum value of the power values (29) as the actual value (29').Communication apparatus (2) according to one of the preceding claims, wherein the amplifier device (8) is configured to amplify received signals (6) received via the antenna (18) as a function of a reception amplification value (12) and then to forward them to the cable (7), and wherein the control device (13) is configured to set the reception amplification value (12) as a function of the difference (33).Communication device (2) according to Claim 4, wherein the amplifier device (8) is connected to the cable (7) via a multiplexer (24), and the transceiver device (3) is designed to switch the multiplexer (27) between a transmission position and a reception position by means of a switching signal (LF), wherein the control device (13) is designed to distinguish between power values (29) of the transmission signals (5) and power values (29) of the reception signals (6) on the basis of the switching signal (LF) and to take into account only power values (29) of the transmission signals (5) in order to determine the difference (33).Communication device (2) according to one of the preceding claims, wherein the control device (13) takes into account power values (29) detected exclusively between the antenna-side end (19) of the cable (7) and the antenna (18) in order to determine the difference (33).Communication apparatus (2) according to one of the preceding claims, wherein the control device (13) is electrically coupled to the transceiver device (3) exclusively via the cable (7).Communication device (2) according to one of the preceding claims, wherein the power detector device (15) is designed as a diode-based envelope demodulator (16).Communication apparatus (2) according to one of the preceding claims, wherein the communication device (2) is configured to generate the transmission signals (5) according to a car-to-car communication protocol.Motor vehicle (1) having a communication device (2) according to one of the preceding claims.Method for compensating (31) cable attenuation in a communication device (2), wherein the method comprises: - a transceiver unit (3) generating transmission signals (5) at a transmission output (21) of the transceiver unit (3), wherein at least two of the transmission signals (5) differ in their signal power, - a cable (7) coupled to the transmission output (21) transmitting the transmission signals (5) to an antenna (18), wherein a controllable amplifier device (8) for amplifying the transmission signals (5) transmitted via the cable (7) is connected between an antenna-side end (19) of the cable (7) and the antenna (18), - a power detector device (15) transmitting respective power values (29) correlated with a power of the transmission signals (5) at a tap point (28), which is arranged between the antenna-side end (19) of the cable (7) and the antenna (18), - a control device (13) controls the amplifier device (8) on the basis of the detected power values (29), wherein the transceiver device (3) generates at least one of the transmission signals (5) with a predetermined reference power, wherein a reference value (32) describing the reference power is fixedly predefined in the control device (13) and the control device (13) selects an actual value (29') from the power values (29) which indicates the power of the at least one transmission signal (5) generated with the reference power at the tap point (28) and sets a transmission amplification value (11) of the amplifier device (8) on the basis of a difference (33) between the reference value (32) and the actual value (29'), wherein the transceiver device (3) generates the transmission signals (5) at the transmission output (21) in each case exclusively with a transmission power which is selected from one of at least two predetermined power stages, wherein the reference power corresponds to one of the power stages, and wherein the control device (13) selects, as the actual value (29'), that power value (29) which indicates a power between the power stage of the reference power and the next lower power stage.

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