The field of the invention is the implementation of information propagation through a propagation channel that can have multiple paths (or echoes).
The invention relates most particularly to monitoring of propagation conditions through such a propagation channel.
The invention has many applications, particularly but not exclusively in the field of broadcasting networks, for example digital video broadcasting networks (particularly according to the DVB-T/T2 (Digital Video Broadcasting—Terrestrial), ISDB-T Integrated Services Digital Broadcasting—Terrestrial), ATSC-3 (Advanced Television Systems Committee) standards, etc.) or digital audio broadcasting networks (particularly according to the DAB standard.
The remainder of this document is particularly concerned with a problem that exists in the field of monitoring of the propagation channel used in digital video broadcasting networks, and particularly in an SFN (Single Frequency Network), faced by the inventors of this patent application. Obviously, the invention is not limited to this particular field of application, but is useful for monitoring any propagation channel with multiple paths (e.g. acoustic, optical transmission, etc.).
The Channel Impulse Response (CIR) of a radioelectric propagation channel is the response of the propagation channel in question to an impulse waveform transmitted by a transmitter at a given instant. Such an impulse is received directly and may also be received in the form of replicas by a receiver after a propagation time in the channel concerned.
For example, if the propagation channel is of the single-path type (e.g. when the transmitter and the receiver are in direct line of sight), an impulse emitted by the transmitter generates a single impulse received by the receiver. On the other hand, if the propagation channel is of the multipath type (e.g. via reflections on surrounding objects), a transmitted impulse will generate a stream of impulses received by the receiver.
Moreover, in an SFN network in which several transmitters transmit the same data at the same time and at the same frequency, the same phenomenon is observed at a receiver placed in a coverage zone of each of the transmitters concerned.
Finally, there can be multiple paths within an SFN network, which results in even more complex CIRs as illustrated on
More particularly, terminal 100 in
As a result, the CIR obtained (
Monitoring equipment can be used to monitor the received CIR at a given point, in order to manage an SFN type network. Alarms for the presence of some paths can then be installed, to make sure that the different network transmitters are still transmitting.
A first problem related to such a monitoring system originates from the fact that the CIR is not necessarily fixed in time. For example, in the case illustrated on
A second problem related to such a monitoring system originates from the fact that the CIR is obtained after demodulation by the receiver of the monitoring device. Such demodulators will attempt to position the CIR within a predefined observation time window, that is optimum in terms of demodulation and decoding.
For example, some demodulators will put the CIR in the observation window concerned based on the weighted centre of gravity of the different CIR peaks (each peak corresponding to a given path in the propagation channel). Such a behaviour is illustrated on the three CIRs in
There is thus a need to stabilise the CIR, for example in order to minimise false alerts from monitoring equipment.
Finally, there is a need for the proposed method to be simple in terms of the hardware implementation.
Thus, according to a first aspect, the invention relates to a method for correcting an impulse response of a multipath propagation channel. Such a method comprises at least one iteration of the following steps:
Thus, the invention discloses a new and inventive solution to correct a multipath propagation channel impulse response (CIR), e.g. a radio frequency propagation channel.
To achieve this, the claimed method proposes to estimate temporal shifts of the CIR concerned (in other words the temporal shift of the temporal support of the CIR) in order to correct it. The CIR is thus stabilised. For example, peaks representative of multiple paths of the propagation channel remain at the same temporal position regardless of the temporal drifts of the CIR over time.
According to one particular embodiment, the estimating a temporal shift includes a calculation of a correlation function between the current impulse response and the reference impulse response. The temporal shift is a function of an extremal value of the correlation function.
Thus, the estimate of the temporal shift is precise.
According to one particular embodiment, the estimating a temporal shift comprises the following sub-steps for at least two peaks in the current impulse response, each corresponding to a path in the propagation channel:
For example, such an estimate can be used to manage CIR configurations in which the use of a correlation can be less reliable, particularly in the case in which here is no variation of temporal positioning of CIR peaks, but a relative variation of the power of the peaks concerned.
According to one particular embodiment, the relevance score is a function of the number of peaks in the current impulse response superposing on a peak corresponding to the reference impulse response after setting up temporal concordance based on the candidate delay.
According to one particular embodiment, if the absolute value of the extremal relevance score is less than a predetermined minimum score, the estimate of a temporal shift also includes a calculation of a correlation function between the current impulse response and the reference impulse response. The temporal shift is a function of an extremal value of the correlation function instead of the candidate delay associated with an extremal relevance score.
Thus, the two complementary methods (i.e. the method based on setting up temporal concordance and the method based on calculating a correlation function) are each used when they give the best results, so that the reliability of the estimate obtained can thus be maximised.
According to one particular embodiment, the correlation function is calculated in the frequency domain.
According to one particular embodiment, the method also includes a step for estimating at least one absolute level of a current peak of the current impulse response. The correcting the current impulse response is also based on said at least one absolute level.
Thus, the level (e.g. the amplitude or power) of the peaks present in the CIR is also corrected in addition to the temporal drifts. For example, when the CIR concerned is obtained at the output from the demodulator of a receiver, a compensation is obtained by level variations introduced by a combination of effects of the AGC (Automatic Gain Control) system and effects of the demodulator when it is temporally stuck on a higher-level path.
According to one particular embodiment, the estimating the absolute value of the current peak includes the following sub-steps:
According to one particular embodiment, the step of obtaining comprises elimination of peaks with an absolute level less than a predetermined threshold in a primary impulse response of the propagation channel delivering the current impulse response.
Thus, only paths of interest are processed.
According to one particular embodiment, for a current iteration, the reference impulse response is a corrected impulse response obtained in a preceding iteration.
According to one particular embodiment, for a current iteration, the reference impulse response is an impulse response of the propagation channel selected at a given instant.
According to one particular embodiment, the method also comprises a step to display the corrected impulse response on a screen of propagation channel monitoring equipment.
The invention also relates to a computer program comprising program code instructions for executing the steps of the method for correcting an impulse response of a multipath propagation channel as described above.
The invention also relates to a device for correcting an impulse response of a multipath propagation channel able to implement the method of correcting an impulse response of a multipath propagation channel according to the invention (according to any one of the different embodiments mentioned above).
Thus, the characteristics and advantages of this device are the same as those for the method of correcting an impulse response of a multipath propagation channel described above. Consequently, they are not described in further detail.
Other characteristics and advantages of the invention will become clear after reading the following description, given as a simple illustrative and non-limitative example, and the appended drawings among which:
Identical elements and steps are designated by the same reference signs in all figures in this document.
We will now describe the main steps of a method for correcting a CIR using an embodiment of the invention representative of the general principal of the invention, with reference to
During a step E300, a current CIR of a multipath channel (e.g. a radio frequency, acoustic, propagation channel, etc.) is obtained. Such a CIR represents the response of the propagation channel to an impulse wave shape transmitted by at least one transmitter at a given time.
During a step E310, a temporal shift between the current CIR obtained during step E300 and a reference CIR of the propagation channel is estimated. In this case, temporal shift means the shift in time between temporal supports of the current CIR and the reference CIR. For example, the reference CIR is an impulse response of the propagation channel selected at a given instant (i.e. an “initial” reference CIR).
During a step E330, the current CIR is corrected based on the temporal shift estimated during implementation of step E300. A corrected CIR is then delivered.
It will be noted that these different steps can be implemented in the form of at least one iteration.
Thus, the CIR of the propagation channel is stabilised. For example, peaks representative of multiple paths of the propagation channel remain at the same temporal position regardless of the temporal drifts of the CIR over time.
We will now describe the main steps of the method for correcting a CIR according to different embodiments of the invention, with reference to
More particularly, during a step E301, a primary CIR of the propagation channel is obtained. For example, such a primary CIR is delivered by a demodulator of a receiver of equipment for monitoring an SFN network.
During step E300 for obtaining the current CIR, the peaks of the primary CIR with an absolute level less than a predetermined threshold can thus be deleted.
Thus, the CIR is shaped such that only peaks corresponding to paths of interest, also called significant echoes, in the propagation channel are kept in the current CIR.
In other embodiments not illustrated, step E301 is not implemented and the current CIR includes all peaks representative of all paths of the propagation channel.
In some embodiments, the primary CIR and/or the current CIR and/or the reference CIR are obtained from a signal that has propagated through the propagation channel. For example, this may be an OFDM (Orthogonal Frequency-Division Multiplexing) modulated signal as used in digital broadcasting networks (e.g. DVB-T/T2, ISDB-T, ATSC-3, DAB, etc.). In some embodiments, the signal concerned was captured via a single antenna, as is classically the case for SFN network monitoring equipment.
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For example, the number of peaks present in the current CIR is counted. If this number is less than a minimum number (e.g. the minimum number is chosen to be equal to 3), the method according to the invention is not used. Similarly, if this number is more than a maximum number (e.g. the maximum number is chosen to be equal to 64), the method according to the invention is not used to avoid a calculation overload.
When it is decided that the current CIR is not eligible for the correction method according to the invention, a known method of repositioning in a predefined temporal window is applied to the current CIR during a step E304. For example, it could be a known method based on the weighted centre of gravity of the different peaks of the CIR based on the strongest path as described above with reference to
In other embodiments not illustrated, step E302 is not used and the current CIR is systematically processed using the correction method according to the invention.
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In other embodiments not illustrated, step E303 is not used and the current CIR is systematically processed using the correction method according to the invention.
In the embodiment illustrated on
According to one example embodiment, during step E310a, the temporal shift between the current CIR and the reference CIR is estimated by setting a temporal concordance of the current CIR and the reference CIR in question. Furthermore, the temporal shift thus estimated is associated with a relevance score.
For example, as illustrated on
The processing associated with steps E310a, E310b and E310c can be better understood, for example by considering the configuration illustrated on
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In this case, the temporal shift is estimated once again during a step E310b by calculating a correlation function between the current CIR and the reference CIR. Thus, the most appropriate method of estimating the temporal shift is used, depending on the situation. Furthermore, the global calculation load of the proposed method remains under control.
In some variants, such a correlation function is calculated in the temporal domain using a sliding window. More particularly, the reference CIR is temporally shifted by a value n*Ts, where Ts is a sampling period and n varies by one unit during each iteration, before being multiplied temporal sample by temporal sample with the current CIR. The sum of the product of coincident points between these 2 CIRs then gives a value of the correlation function for the temporal value n*Ts considered. Such a direct calculation in the temporal domain can be interpreted as a convolution between CIRs.
Alternatively, in other variants, the correlation function is calculated in the frequency domain. In this case, the convolution encountered in the temporal domain is transformed into a simple term-by-term multiplication in the frequency domain, thereby simplifying implementation of the solution. This approach is based on the calculation of the Fourier transform (e.g. in the form of a DFT (Discrete Fourier Transform) or FFT (Fast Fourier Transform) of current and reference CIRs, and the inverse Fourier transform of the result of the term by term multiplication in question.
In embodiments not illustrated, steps E310c and E310b are not implemented and the temporal shift is still estimated by applying temporal concordance of current and reference CIRs through step E310a, for example to manage CIR configurations in which the use of a correlation may be less reliable.
In embodiments not illustrated, steps E310a and E310c are not used and the temporal offset is still estimated by correlation through the use of step E310b.
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During a step E313, it is decided if the temporal shift estimated in step E310 is coherent (e.g. it is decided if step E310 has not produced an aberrant value or no value at all, etc.). If it is decided that the temporal shift estimated in step E310 is not coherent, the temporal shift estimated during application of this step E310 is not kept during a step E314 and the current CIR is delivered as is during a step E335, i.e. without correction based on the temporal shift.
On the contrary, if it is decided that the temporal shift estimated in step E310 is coherent, one or several absolute levels of one or several corresponding peaks of the current CIR is or are estimated in a step E320. Thus, the level (e.g. the amplitude or power) of peaks present in the CIR is also corrected in addition to the temporal drift.
For example, in the embodiment illustrated on
For example, the change in question is obtained by means of an RSSI (Received Signal Strength Indication) delivered by the demodulator of the monitoring equipment currently monitoring the propagation channel in question. Knowledge of such a change to the absolute value of the total signal that passed through the channel can be useful for example when the current CIR and/or the reference CIR were delivered by demodulators hosting some gain control functions in the reception system, namely the AGC (Automatic Gain Control).
However, in other embodiments not illustrated, step E320c is not used and only changes in the level of peaks are determined in step E320b, for example when no AGC system is present.
In yet other embodiments not illustrated, step E320 to estimate the absolute level of peaks in the current CIR is not implemented and the correction in step E330 is based only on the temporal shift estimated in step E310.
With reference once again to
During a step E323, it is decided if the absolute level of peaks of the current CIR estimated in step E320 is coherent (e.g. it is decided if step E320 has not produced aberrant values or no value at all, etc.). If it is decided that the level in question is not coherent, the level thus estimated is not kept and the global level of the CIR is corrected during a step E324. In this case, the absolute level of peaks in the current CIR estimated in step E320 is not used during correction of the CIR in step E330. On the contrary, when it is decided that the absolute level of peaks of the current CIR estimated in step E320 is coherent, the correction of the CIR in step E330 takes account of the absolute value of peaks in the current CIR estimated in step E320.
During step E335, the CIR corrected during step E330 (based on the temporal shift estimated during application of step E310 and possibly based on absolute level(s) estimated during application of step E323) is shaped to be delivered, for example, to a display device or to a device for monitoring the propagation channel, etc. In some embodiments, the method also comprises a step to display the corrected CIR, for example on a screen of the display device or the propagation channel monitoring equipment, etc. More particularly, a CIR is classically displayed on a graph with time and power axes. However, for a CIR that has not been corrected using this technique, the display is made for example by adjusting the highest power echo to the point with coordinates (0 μs, 0 dB). In other words, all echoes are displayed relative to the most powerful echo (main echo). On the contrary, a CIR corrected using this technique is stable in time and also in level depending on the embodiment considered. In this way, a CIR corrected using this technique can be displayed keeping the temporal positions and absolute power levels, i.e. as delivered during step E335.
In some embodiments, the method also comprises a step of monitoring at least one peak in the corrected OR, said monitoring comprising triggering an alarm in case a level of said at least one peak goes outside a predetermined range of levels. The alarm can include a graphic displayed on the display screen, an audible alarm, generation of a message for transmission through a network, etc. for example.
Furthermore if, as described above, it is decided during application of step E313 that the temporal shift estimated in step E310 is not coherent, the current CIR is delivered as is during step E335, i.e. without correction in step E330.
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Number | Date | Country | Kind |
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1856550 | Jul 2018 | FR | national |