# Patent

## Title
Detection of simultaneous double transmission

## Abstract

A measuring device serves for the detection of a simultaneous presence of at least a first signal and a second signal in a measurement signal. The measuring device contains a receiving unit for receiving the measurement signal as a digital measurement signal and a processing unit. This processing unit includes a transformation unit for transforming the digital measurement signal into a measurement signal in the frequency domain. The processing unit further contains a subtraction unit for subtracting the first sideband of the measurement signal in the frequency domain from a second sideband of the measurement signal in the frequency domain. In this context, the subtraction unit generates a residual signal.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a national phase application of PCT Application No. PCT/EP2014/059559, filed May 9, 2014, which claims priority to German Patent Application No. 10 2013 212 067.2, filed on Jun. 25, 2013, the entire contents of which are herein incorporated by reference.
TECHNICAL FIELD
The invention, according to the various embodiments described herein, relates to a measuring device and a measuring method for detecting a simultaneous presence of at least a first signal and a second signal in a measurement signal.
BACKGROUND
Amplitude-modulated signals are still conventional in airborne communications systems. A major problem with the use of such signals is that, in the case of a random simultaneous transmission of signals, for example, by two aircraft, the relatively weaker signal arriving in the receiver remains undetected. This can lead to errors relevant to safety.
A method in which a received signal is distorted in a non-linear manner and then transformed into the frequency domain in order to detect such double transmissions is known. Symmetrical spectral components in the distorted spectrum are then determined and removed. On the basis of still remaining signal components, an inference is then made about the presence of a simultaneously transmitted second signal. For example, the German published patent application DE 10 2011 080 999 A1 discloses a method with non-linear distortion, as described above.
However, this method is disadvantageous because it requires a large computational effort as a result of the non-linear distortion. This is also problematic when using CLIMAX operating mode. It is not capable of distinguishing between several transmitters in the case of the common-wave operation of the CLIMAX system and an actual double transmission worthy of detection.
SUMMARY
Accordingly, there is provided a simple and reliable measuring device and measuring method which are capable of detecting double transmissions reliably.
The measuring device serves to detect a simultaneous presence of at least a first signal and a second signal in a measurement signal, preferably a received signal of a radio device in the context of airborne communications. The measuring device contains a receiving unit for receiving the measurement signal as a digital measurement signal and a processing unit. In this context, the processing unit includes a transformation unit for transforming the digital measurement signal into a measurement signal in the frequency domain. The processing unit further contains a subtraction unit for subtracting a first sideband of the measurement signal in the frequency domain from a second sideband of the measurement signal in the frequency domain. In this context, the two sidebands refer to the same signal component—around the strongest carrier. Accordingly, the subtraction unit generates a residual signal. It is therefore possible to infer the presence of a double transmission very easily on the basis of the residual signal.
In certain embodiments, only the receiving unit processes the measurement signal before digitisation by the receiving unit.
The receiving unit then processes the measurement signal only by means of linear operations, before it digitises the measurement signal to form the digital measurement signal. Since no non-linear operations are necessary, a very economical processing in terms of computational resources can be guaranteed.
The first signal and the second signal are preferably amplitude-modulated signals in each case with a carrier. Accordingly, the transmission method used predominantly in airborne communications can be employed.
In certain embodiments, the subtraction unit is embodied to detect a strongest carrier present in the measurement signal, wherein the sidebands of the measurement signal are separated by the strongest carrier present in the measurement signal. Accordingly, a reliable distinction between the sidebands to be subtracted can be guaranteed.
The subtraction unit can be embodied to reflect one of the sidebands of the measurement signal in the frequency domain to a position of a strongest carrier present in the measurement signal before the subtraction. In this manner, a reliable function of the measuring device is achieved for arbitrary transmission signals.
The processing unit advantageously contains a detection unit for detecting the presence of the second signal in the measurement signal on the basis of remaining signal components of the residual signal. In this manner, an automatic evaluation, which is very simple for the user of the measuring device, can be achieved.
The detection unit can be embodied to determine the presence of the second signal in the measurement signal by means of a threshold-value comparison of remaining signal components or by means of a threshold-value comparison of a signal derived from the signal components remaining in the residual signal. That is to say, a comparison of the derived payload signal with the noise is implemented. A very simple detection can be ensured in this manner.
The detection unit can be embodied to determine the presence of the second signal in the measurement signal by means of a threshold-value comparison of summated powers of remaining signal components in the residual signal. In this manner, the presence of the second signal can be detected with increased accuracy.
The detection unit can be embodied to detect rotor modulations of the first signal and/or of the second signal, and to remove detected rotor modulations of the first signal and/or of the second signal from the first signal or respectively from the second signal. As an alternative, it is embodied to ignore detected rotor modulations of the first signal and/or of the second signal in detecting the presence of the second signal in the measurement signal. Accordingly, errors caused by rotor modulations can be avoided in the detection of a double transmission.
A measuring method according to the invention serves for the detection of a simultaneous presence of at least a first signal and a second signal in a measurement signal. In this context, the measurement signal is received as a digital measurement signal. Following this, the digital measurement signal is transformed into a measurement signal in the frequency domain. This is followed by a subtraction of a first sideband of the measurement signal in the frequency domain from a second sideband of the measurement signal in the frequency domain, wherein, in this context, a residual signal is generated. On the basis of the residual signal, it is very easy to infer the presence of a double transmission.
By preference, the measuring device according to the invention and the measuring method according to the invention can detect double transmissions in the case of a level difference between the two signals from 0 to 20 dB, by particular preference from 0 to 40 dB. Accordingly, even in the case of a very strong and a very weak signal, a reliable detection of a double transmission is possible.
By particular preference, the measuring device and the measuring method are suitable for distinguishing actual double transmissions of different participants from supposed double transmissions in the CLIMAX method.
Additional aspects and technical effects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description wherein embodiments of the present disclosure are described simply by way of illustration of the best mode contemplated to carry out the present disclosure. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention is described by way of example with reference to the drawings in which an advantageous exemplary embodiment is presented. The drawings show:
FIG. 1 a schematic presentation of the problem underlying the present invention;
FIG. 2a a spectral presentation of two simultaneously transmitted signals in the frequency domain;
FIG. 2b a spectral summated presentation of the simultaneously transmitted signals from FIG. 2a; 
FIG. 3 an exemplary embodiment of the measuring device according to the invention in a block-circuit diagram;
FIG. 4 a detail view of the exemplary embodiment of the measuring device according to the invention in a
FIG. 5 a spectral presentation of a signal used by the exemplary embodiment of the measuring device; and
FIG. 6 an exemplary embodiment of the measuring method according to the invention in a flow diagram.



DETAILED DESCRIPTION
Initially, the problem underlying the present invention will be explained with reference to FIGS. 1-2b. Following this, an exemplary embodiment of the measuring device according to the invention and its function will be explained in detail with reference to FIGS. 3-4. Finally, with reference to FIG. 6, the functioning of an exemplary embodiment of the method will be explained. In similar drawings, the presentation and description of identical elements has not been repeated in some cases.
FIG. 1 shows the problem underlying the invention in a schematic manner. A first aircraft 10 transmits a first signal 11 to a receiver 14. At the same time, a second aircraft 12 transmits a second signal 13 to the receiver 14. Because of the significantly shorter distance of the second aircraft 12 from the receiver 14, the second signal 13 arrives in the receiver 14 with a significantly higher level than the first signal 11. Because of the amplitude modulation and the use of the nominally identical transmission frequency, the first signal 11 disappears within the relatively stronger second signal 13 and cannot be detected without further measures by the receiver 14. Accordingly, a message transmitted from the aircraft 10 to the receiver 14 does not arrive. Of course, the measuring device according to the invention and the measuring method according to the invention can also be used in the case of different configurations of participants. For example, a double transmission of an aircraft and a ground station is also conceivable in the case of reception by an aircraft or a ground station. Expressed in general terms, each of the participants presented can be replaced by an aircraft or a ground station.
FIG. 2a presents the situation illustrated in FIG. 1 in the frequency domain. Here, the left sideband 20b, the right sideband 21b and the carrier 22b correspond to the first signal 11 from FIG. 1. The left sideband 20a, the right sideband 21a and the carrier 22a here correspond to the second signal 13 from FIG. 1. It is clearly evident here that the components corresponding to the first signal 11 include a significantly lower level than the components corresponding to the second signal 13. That is to say, the components of the first signal 11 disappear into the second signal 13.
FIG. 2b shows a total signal arriving in the receiver 14 from FIG. 1 in a spectral view. A slight asymmetry of the total signal presented in FIG. 2b is obtained as a result of slight frequency differences between the two signals. A left sideband and a right sideband 31 can be detected around a recognisable central carrier 32. The contributions of the first signal 11 from FIG. 1 are hardly detectable here. Only in the lower left region of the left sideband 30 and of the right sideband 31 and at the bottom of the carrier 32, minor differences are detectable by comparison with a single signal. However, it is not possible to infer a double transmission directly from these regions. Of course, the entire received signal may also contain only one payload signal, for example, the first signal 11 or the second signal 13 and noise. Within the scope of the double-transmission detection, this case must be distinguished from the case described above.
FIG. 3 shows an exemplary embodiment of the measuring device according to the invention in a block circuit diagram. Together, the first signal 11 from FIG. 1 and the second signal 13 from FIG. 1 form a measurement signal 50. The measuring device 2 contains a receiving unit 3, a digital processing unit 7, an output unit 8 and a control unit 9. The receiving unit 3 further contains an antenna 4, an analog-processing unit 5 and an analog-digital converter 6. In this context, the antenna 4 is connected to the analog-processing unit 5, which is connected in turn to the analog-digital converter 6. This, in turn, is connected to the digital processing unit 7. In this context, the analog-processing unit 5, the digital processing unit 7 and the output unit 8 are further connected to a control unit 9. Here, only the components of the measuring device relevant for the invention have been shown. Conventional receiving technology, as used in an airborne radio device, has not been shown in detail here.
The measurement signal 50, which contains at least one signal, for example, from an aircraft—but which can, in the case of a double transmission, alternatively contain a first and a second signal—is received by the antenna 4 and transmitted to the analog-processing unit 5 as a received signal 51. The latter performs analog-processing steps, such as a filtering and an amplification, and outputs a processed measurement signal 52 to the analog-digital converter 6. The latter digitises the signal and transmits it to the digital processing unit 7 as a digital measurement signal 53. The latter performs a detection of the double transmission and communicates the result to the control unit 9. In the case of a double transmission, the control unit 9 controls the output unit 9 in order to indicate the double transmission, for example, with an acoustic warning signal or a visual warning signal. The analog-processing unit 5, the digital processing unit 7 and the output unit 8 are therefore controlled by the control unit 9. The detailed functioning of the digital processing unit 7 will be explained in greater detail with reference to FIG. 4.
FIG. 4 shows a detail view of the digital processing unit 7 from FIG. 3. The digital-processing unit 7 receives the digital measurement signal 53 from the analog-digital converter 6. The digital-processing unit 7 contains a transformation unit 70, a subtraction unit 71 and a detection unit 72. This transformation unit 70 is connected to the analog-digital converter 6 from FIG. 3. The subtraction unit 71 is connected to the transformation unit 70 and to the detection unit 72. The detection unit 72 is connected to the control unit 9 from FIG. 3. The digital measurement signal is supplied to the transformation unit 70 and transformed by the latter into a measurement signal in the frequency domain 54. For this purpose, a Fast Fourier Transform is used by way of example. The measurement signal in the frequency domain 54 accordingly corresponds to the total signal illustrated in FIG. 2b. The measurement signal in the frequency domain 54 is then supplied to the subtraction unit 71, which subtracts a first sideband of the total signal from a second sideband of the total signal.
Since both of the sidebands of an amplitude-modulated signal are symmetrical relative to the carrier, a complete deletion of the signal is obtained here if only one signal was part of the measurement signal. However, this asymmetry cannot be detected in FIG. 2b without further steps. In particular, FIG. 2b shows an ideal signal without noise. In a real signal, the slight asymmetries of the signal from FIG. 2b would disappear into the relatively stronger signal.
The subtraction unit 71 accordingly first determines the precise carrier position in the frequency range of the measurement signal in the frequency domain 54. Following this, a reflection of one sideband to this determined carrier position is implemented. The reflected sideband is subtracted from the non-reflected sideband. A resulting residual signal 55 in the case of a double transmission is illustrated in FIG. 5.
A residual signal 55 generated in this manner is then transmitted to the detection unit 72, which infers from the signal component present in the residual signal 55 the presence or absence of a double transmission. For example, a threshold-value comparison can be used for this purpose. As an alternative, the residual signal 55 can first be processed, for example, through magnitude formation or power formation. The processed residual signal can then be investigated by means of a threshold-value comparison. If the threshold is exceeded, the presence of a double transmission can be inferred, and this can be communicated to the control unit 9 from FIG. 3. The absence of a double transmission is also communicated to the control unit 9.
With existing systems for detecting double transmissions, rotor modulations, that is, modulations which result from the rotating rotor blades in a propeller-driven aircraft, are often detected as double transmissions. Such rotor modulations generate periodic continuations of the transmitted signal in the frequency domain. In this context, the periodicity is specified by the number of blades of the rotor and the speed of rotation. In order to exclude such rotor modulations and therefore avoid detecting them as double transmissions, the detection unit 72 additionally performs a rotor-modulation compensation. In this case, the detection unit 72 searches in a targeted manner for rotor modulations, which are detected on the basis of the known periodicity, and removes these periodic repetitions of the signal in the frequency domain before the threshold-value comparison is implemented.
FIG. 5 shows the residual signal 55 from FIG. 4 in the case of a double transmission. The residual signal 55 here contains two signal components 40, 41, which correspond to the hardly detectable asymmetries of the total signal in FIG. 2b. After the symmetrical signal components have been removed, these asymmetric signal components can now easily be detected. A threshold value 42 has also been drawn into FIG. 5, on the basis of which, for example, the presence of the double transmission is detected.
FIG. 6 shows an exemplary embodiment of the method according to the invention. In a first step 100, a measurement signal is received. This measurement signal contains at least a first signal, but optionally contains a second signal. In a second step 101, the measurement signal is transformed into the frequency domain. In a third step 102, a strongest carrier within the received signal is detected. In a fourth step 103, the two sidebands which are defined by the two spectral sides of the detected strongest carrier, are subtracted from one another. In this context, one of the sidebands is first reflected to the carrier position. In a fifth step 104, remaining signal components are detected after the subtraction. In a sixth step 105, a double transmission is determined. With regard to the function of the individual steps, reference is made to the corresponding paragraphs relating to FIG. 3 and FIG. 4.
The invention is not restricted to the exemplary embodiment presented. Advantageously, all of the features described or features illustrated in the figures can be combined arbitrarily with one another within the scope of the invention.
In the preceding description, the present disclosure is described with reference to specifically exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present disclosure, as set forth in the claims. The specification and drawings are, accordingly, to be regarded as illustrative and not as restrictive. It is understood that the present disclosure is capable of using various other combinations and embodiments and is capable of any changes or modifications within the scope of the inventive concept as expressed herein.

## Claims

1. A measuring device comprising:
a receiver for simultaneously receiving at least a first signal and a second signal in a measurement signal, the measurement signal received as a digital measurement signal; and
a processor comprising a transformer for transforming the digital measurement signal into a measurement signal in the frequency domain,
wherein the processor further comprises a subtractor for subtracting a first sideband of the measurement signal in the frequency domain from a second sideband of the measurement signal in the frequency domain,
wherein the subtractor generates a residual signal,
wherein the subtractor is embodied to detect a strongest carrier present in the measurement signal, and
wherein the sidebands of the measurement signal are separated by the strongest carrier present in the measurement signal.

2. The measuring device according to claim 1, wherein the receiver processes the measurement signal only by linear operations, before it digitises the measurement signal to form the digital measurement signal.

3. The measuring device according to claim 1, wherein the first signal and the second signal are amplitude-modulated signals in each case with a carrier.

4. A measuring device comprising:
a receiver for simultaneously receiving at least a first signal and a second signal in a measurement signal, the measurement signal received as a digital measurement signal; and
a processor comprising a transformer for transforming the digital measurement signal into a measurement signal in the frequency domain,
wherein the processor further comprises a subtractor for subtracting a first sideband of the measurement signal in the frequency domain from a second sideband of the measurement signal in the frequency domain,
wherein the subtractor generates a residual signal, and
wherein the subtractor is configured to reflect one of the sidebands of the measurement signal in the frequency domain before the subtraction to a position of a strongest carrier present in the measurement signal.

5. A measuring device comprising:
a receiver for simultaneously receiving at least a first signal and a second signal in a measurement signal, the measurement signal received as a digital measurement signal; and
a processor comprising a transformer for transforming the digital measurement signal into a measurement signal in the frequency domain,
wherein the processor further comprises a subtractor for subtracting a first sideband of the measurement signal in the frequency domain from a second sideband of the measurement signal in the frequency domain,
wherein the subtractor generates a residual signal, and
wherein the processor further comprises a detector for detecting the presence of the second signal in the measurement signal on the basis of remaining signal components of the residual signal.

6. The measuring device according to claim 5, wherein the detector is configured to determine the presence of the second signal in the measurement signal by a threshold-value comparison of the remaining signal components in the residual signal or by a threshold-value comparison of summated powers of the remaining signal components in the residual signal.

7. The measuring device according to 5, wherein:
the detector is configured to detect rotor modulations of the first signal or of the second signal, and
to remove the detected rotor modulations of the first signal or of the second signal from the first signal or respectively from the second signal, or
to ignore the detected rotor modulations of the first signal or of the second signal in the detection of the presence of the second signal in the measurement signal.