FR3046312A1 - TRANSMISSION CHANNEL EMULATOR - Google Patents

Abstract

A method implemented in an emulator (110) for a propagation channel connected to a device (100), the emulator comprising a processor (143) adapted to perform at least the following steps: • Choose a discrete modeling of the propagation channel on arrival and Doppler frequencies, • Define one or more zones corresponding to a time interval and a Doppler frequency value, and determine a DT propagation channel matrix whose elements are obtained by summing all the complex values paths within a selected time interval and Doppler-frequency value, • Digitize the E (t) signal, • Combine a digitized sample with one line of the DT matrix delay-Doppler-channel coefficient in order to obtain the signal R (t) corresponding to a set of paths propagated in a given space, • Transmitting the signal R (t) to the equipment (100). Emulator for implementing the method according to the invention.

Description

The invention particularly relates to a propagation channel emulator comprising an input for the reception of a signal and a signal output corresponding to the emulation of the signal received at the input, filtered after emulation of the transmission channel.

In many applications, the signals are transmitted via a transmission channel or transmission medium. During this step, propagation phenomena occur and may affect the signal. A communication system transmits data on a medium called a transmission channel. The transmission channel distorts the transmitted signal due, in particular, to free space attenuation, diffraction and scattering, or to the presence of multiple propagation paths whose recompositions are carried out constructively or destructively. Noise of various origins can also impact this transmission. The noise is, for example, generated by equipment connected to or located near the transmission channel.

When designing a communication system or any other system in which there is propagation of signals within a medium, it is sought to establish a model or models of channels representative of the physical phenomena of transmission. Several types of models exist. Statistical empirical models reproduce the characteristic functions of the transmission channel randomly by using a number of statistics observed over a set of measurements. For deterministic models, a fine description of the transmission medium is used.

The publication titled "FPGA-based Wideband Channel Emulator for Evaluation of Wireless Sensor Networks in Industrial Environments", 2014 IEEE Emerging Technology and Automation Factory (ETFA) Ihaki Val, Felix Casado, Pedro Manuel Rodriguez, Aitor Arriola, describes the implementation of 'DWCE digital propagation channel emulator, digital wireless channel emulator. It is based on a real-time architecture (frequency transposition, analog / digital converters, FPGA) with an algorithm allowing the generation of multipath, noise and Doppler spectrum. The main limitations of this work are a long computing time (> 900ns), a low temporal resolution (> 8ns), as well as the number of very limited emulatable macro-paths or the Doppler effect applied on average over the entire profile. delay power. The literature also offers different processing algorithms based on methods of calculating the response of the propagation channel with random selection of the coefficients according to predefined distributions. At first sight, no digital processing algorithm, to be implemented in an emulator and known to the requester, proposes to reproduce the simultaneous response of a real-time propagation channel with discrimination of the Doppler effect. The emulated power-Doppler-delay profile is fixed outside the fading or random fading applied to it. Doppler is applied as a random or medium spectrum. In the most sophisticated cases, dynamic channel models can be emulated. This is to the detriment of certain aspects of performance, such as the minimum delay or the maximum number of emulable macro-paths. These two variables nevertheless play a crucial role in the realism of the emulation and the variety of testable equipments.

The models presented in the prior art, despite the results and the benefits they provide, are generally simplistic and a little removed from reality. The object of the invention relates in particular to a propagation channel emulator allowing a time-varying channel emulation with a large variety of paths and their associated Doppler. The originality of the emulator according to the invention is based on a preconditioning of the data to be supplied to the emulator in order to simplify the propagation channel model from three dimensions to two dimensions, as well as to proceed with its linearization. This also makes it possible to simplify the operations to be performed on the hardware side in order to emulate the channel and thereby reduce the computing power required. On the other hand, the calculations being faster, the emulable minimum delay will be much lower than with conventional methods. Finally, this dynamic method has the advantage of allowing to recreate environments with a large number of macro-paths, while controlling independently and accurately, their delay, Doppler shift and their attenuation. The models of emulated channels are found all the more true to reality with conventional emulators with significant delays, the number of low emulated macrotrafts and the approximate Doppler effect. The invention relates to a method implemented in an emulator for a propagation channel connected to a device, the emulator comprising at least one input receiving a signal E (t) emitted by said device in a direction m which induces a delay ηΔτ with a Doppler frequency d.Af, and an output emitting a signal R (t) after processing, a processor adapted to perform at least the following steps: • Choose a discrete modeling of the propagation channel on the arrival times of the signal and on the Doppler frequencies, • Define one or more zones corresponding to a time interval and a Doppler frequency value and determine a DT propagation channel matrix whose elements are obtained by summing all the complex values of the paths in a range of selected time and frequency-Doppler value, • Digitize signal E (t) transmitted, • Combine a digitized sample with a line of the matrix ice Dr delay matrix-Doppler-coefficient of the channel in order to obtain the signal R (t) corresponding to a set of propagated paths, • To transmit the signal R (t) to the equipment.

According to an alternative embodiment, the processor is adapted to insert in a shift register, at each symbol time, a sample of the signal E (t), to subject the matrix DT to a circular rotation in the direction of the lines and to realize a convolution between the last row of the matrix and the stored samples in order to obtain Ri samples which are then converted into the analog domain before transmission to the equipment under test.

The signal R (t) corresponds, for example, to a sum over all the possible delay values n, of the product of a sample of the signal emitted by the sum over the possible values of Doppler of the impact of a path on the signal following this path, Cn> d. ej-2irdM (t-nAT): with:

where d. Δί. (t) is the Doppler frequency value for a delay ηΔτ,

Cn> d a complex coefficient representing the attenuation related to the propagation in free space, (E (t- ηΔτ)) a sample value of the received signal with a delay ηΔτ.

According to an alternative embodiment, the method comprises a step in which the matrix Dn is precalculated and the Doppler frequencies having a negligible impact on the propagation channel are eliminated.

According to another variant, the time interval is defined for grouping the paths of the signals equal to 1 / Fs, where Fs is the sampling frequency of the emulator, and the signal R (t) is determined in the following manner :

n is a bounded index, i corresponds to the moment when the signal is received.

The size of a time-shift-doppler zone may be chosen as a function of the sampling frequency of the converters and the activation frequency of the channel coefficients in the programmable logic circuit FPGA. The invention also relates to an emulator for a propagation channel connected to a device, the emulator comprising a first frequency transposition module connected to an analog / digital and digital / analog conversion module, which is itself connected to a transmission module. signal processing, at least one input receiving a signal E (t) emitted by said equipment in a direction m which induces a delay ηΔτ with a Doppler frequency d.Af and an output emitting a signal R (t) after processing, a processor signal, a storage memory signals, the emulator is characterized in that the processor is adapted to perform at least the steps of the method according to the invention to generate an emulated signal R (t) transmitted to the equipment. The emulator may comprise a programmable logic circuit or FPGA (Field Programmable Gate Array). The emulator may also include a digital signal processor of the DSP type. Other characteristics and advantages of the present invention will appear better on reading the following description of exemplary embodiments given by way of illustration and in no way limiting, appended figures which represent: • Figure 1, an example of a structure of a digital emulator according to the invention, • Figure 2A, a diagram of an aircraft and the propagation of signals, Figure 2B, a representation of a calculated channel response, Figure 2C, the representation of macro-paths in a delay diagram FIG. 3A, a representation of the macro-paths according to the method according to the invention, FIG. 3B the data packaged in a delay-Doppler-power profile of the macro-paths, FIG. 3C a representation of the simplified channel matrix of propagation, FIG. 3D, a representation of the step of convolution of a sample of the signal with the channel matrix, FIG. 4A, a representation according to the art the combination of the signal samples with the channel matrix, FIG. 4B, the transformation of the channel matrix according to the invention, FIG. 4C, a representation of the steps of combining the signal sample with the channel matrix for obtain the final resulting signal, and • Figure 5, a summary of the steps implemented by the method according to the invention. The following example given by way of illustration and in no way limiting concerns an emulator used by a radar equipment manufacturer.

FIG. 1 schematizes an exemplary structure of a digital emulator according to the invention connected to a system under test. The system under test 100 comprises an output 101 emitting a signal E (t) and an input 102 which receives the signal R (t) processed by the emulator 110. The emulator according to the invention 110 comprises, for example, a first module of frequency translation 120, connected to an analog digital and analog digital conversion module 130, in connection with an FPGA signal processing module 140 in which the algorithm or method according to the invention is implemented.

The signal E (t) emitted by the system under test is transmitted to the frequency transmission module which comprises a first input 121 connected to a first frequency translation chain 122. After transposition in frequency, the transposed signal is transmitted by a signal. output 123 to an input 131 of the converter module 130 and to an analog digital converter 135. The transposed and digitized signal ETN (t) is emitted by an output 136 to an input 141 of the FPGA 140. The transposed and digitized signal is processed by the method according to the invention, then emitted by an output 142 of the FPGA to an input 137 and a digital to analog converter 138. The analog signal is then emitted by an output 139 of the converter to the frequency transposition module via an input 124. The signal processed by the emulator is then retransposed in frequency via a second channel 125 and transmitted via an output 126 to the input of the system. e under test.

The FPGA 140 comprises in particular a processor 143 adapted to perform the steps of the method according to the invention and a memory 144 or buffer for storing the samples e (t) of the signal which are combined with the channel matrix. Samples from the analog-to-digital converter 135, through the input 141 of the FPGA, are synchronized with the digital processing processor 143 through a trimming register (FIFO) 145. The digital processing processor 143 receives the coefficients c, - from the last line of the channel matrix DT which are loaded into the memory 144 (RAM) via a Microblaze ™ processor 146, for example. The processor 146 realizes the circular permutations of DT and updates the memory 144 with the right coefficients. The digital processing processor 143 performs the convolution operation between the samples coming from the input 141 and the coefficients coming from the memory 144. The result of this calculation is sent to a second shift register (FIFO) 147, whose the role is to synchronize the output of the processor 143 with the input of the digital converter 138.

In summary, the signal emitted by the equipment under test is filtered according to the method of the invention, to emulate the propagation channel, and then returned to the equipment. The equipment under test will use the received signal R (t) to carry out measurements specific to its function, for example a height measurement for a radio altimeter, a target detection for a radar, etc.

Without departing from the scope of the invention, the FPGA can be replaced by any type of device comprising at least one memory or buffer and a processor in which the steps of the method according to the invention are implemented, for example a PC computer, a processor DSP digital signal, etc.

FIG. 2A schematizes the signals transmitted, transmitted and retransmitted between an aircraft 200 equipped with a radar 201 and an object on the ground 202. A signal E (t) emitted by the radar propagates via the transmission medium, it is reflected by one of the facets 203 of the floor object 202.

Figure 2B is a representation of non-linearly distributed channel models over time (the y-axis corresponds to the amplitude of a macro-path, the other two axes corresponding to Doppler (Hz) and delays (seconds) , and Figure 2C highlights the irregularity of the frequency and time data (delay-Doppler) which makes their use in a digital processing chain difficult because of the existence of a sampling period fixed.

FIG. 3A illustrates a step of the method which makes it possible to regularize the distribution of the data in frequency and in time. The method according to the invention, in a first step, will precondition the signals by grouping, for example, the paths in linearly spaced delay and Doppler zones. The choice of the size of a zone will be related to the sampling frequency of the analog / digital converters and the frequency of updating the coefficients of the channel in the FPGA as explained below. FIG. 3A schematizes in a diagram (delay, Doppler) the result of this step where several macro-paths 31 Oj have been illustrated within the same delay-shift-Doppler zone 320. The grouping makes it possible to construct a matrix of the delay-doppler-coefficient form of the modeled channel which is adapted to the hardware configuration of the emulator, in particular at its sampling frequency Fs and at the updating frequency Fa of the filter coefficients in the FPGA. On the other hand, the grouping makes it possible to linearly distribute the lagging and Doppler samples, which greatly reduces the complexity of the implementation and the calculation load in the FPGA.

FIG. 3B schematizes the conditioned data in a delay-Doppler-power profile of the macro-paths.

Figure 3C illustrates the simplified matrix of the model. The matrix DT is a diagonal matrix, each of the elements of the channel matrix corresponds to the sum of all the complex values of the paths situated in a time interval (fixed by 1 / Fs, with Fs for example the sampling frequency of the analog / digital and analog digital converters) and a chosen Doppler value.

Figure 3D schematizes a step of convolution, that is to say a representation of the implementation of the digital processing algorithm which considers the samples arriving in the FPGA by combining them with the lines of the matrix with a rotation circular according to the steps described below. The processor will insert in the shift register, at each symbol time, a sample e (t) of the transposed and digitized signal E (t), cause the matrix Dr to undergo a circular rotation in the direction of the lines and to carry out a convolution between the last row of the matrix and the buffer of samples, in order to obtain Ri samples which are then converted into the analog domain before transmission to the equipment under test.

The steps of the method implemented by the emulator according to the invention will now be detailed in relation with FIGS. 4A to 4C and FIG.

A modeling of the discrete channel is chosen as well on the arrival times as on the Doppler frequencies. Let E (t) be the signal emitted by the equipment at time t. This signal is emitted in a direction m which will induce a delay η * Δτ with a Doppler frequency equal to d * Δf (discrete model). The signal will strike a surface element 203 located in the direction m (Figure 2A). We denote by Rf (t) the signal received by this surface element: (1) with δ () the Dirac function, η * Δτ the delay of the signal in a given direction of space, e (t) the transmitted signal at time t, d * Af the Doppler shift frequency related to movements between the aircraft and the ground element.

This means that the facet 203 of the object struck by the signal emitted by the aircraft receives a signal out of phase by the Doppler effect (term in ej2ltd * Aft) and delayed by

It is considered that the attenuation related to the propagation in free space intervenes at the same time as the modification of the signal by its reflection on the considered surface element, the whole being represented by a complex coefficient Cn. Let Ef (t) be the signal reflected by the surface element:

(2) with Cn a coefficient inherent to the chosen channel model, which reflects the attenuation and the phase shift of the signal having borrowed the macro-path considered.

Let Rn (t) be the signal reflected by the facet of the object in the direction associated with this macro-path:

(3)

The signal reflected by the object is delayed again by its propagation and again undergoes the Doppler effect. Equation 3 is simplified by developing:

(4) then:

(5)

Is :

(6)

This amounts to writing the signal received on the radar and after reflection on one of the facets, according to the direction n by the following formula:

(7)

In practice, the received signal R (t) by the radar after propagation in the medium of transmission and diffraction by the object is composed of a set of paths that have propagated in several directions of space, ie a sum of Rn routes.

To obtain the signal R (t), we will sum the paths both late and Doppler according to the equation:

(8) for all the directions of space considered. The set of paths is here distributed temporally on η.Δτ and frequentially on d.Af, corresponding to a discrete representation of the propagation channel.

The signal R (t) is represented by:

(9).

The signal R (t) corresponds to a sum over all the possible delay values n, of the product of a sample of the signal emitted by the sum over the possible Doppler values of cnd. ej-2lTdM (t_nAT) (impact of a path on the signal that borrows it):

(10).

We put the channel matrix corresponding to a delay n:

(11).

Which allows to write:

(12).

The matrix Dn is analogous to a discrete Fourier transform with a near coefficient, which is carried out on the frequency space occupied by the Doppler, the variables η.Δτ and ά.Δί are linearly spaced.

The matrix Dn is a periodic period function

Dn is linearly distributed in the frequency domain, which equates to a periodicity in the time domain and simplifies the calculations.

According to one embodiment, the matrix Dn can be precalculated to limit the calculation times in the emulator. Its size is limited because of its periodicity. It is then possible to eliminate the low Doppler frequencies, which have a negligible effect compared to other higher frequencies, to reduce its size and limit its memory occupancy. This amounts to saying that one can adapt the value of Af to reduce the size of Dn.

From the discrete point of view (in the case of the emulator), this simplification advantageously makes it possible to reduce the computation time by preconditioning the variables.

In the domain of discrete time, equation (6) becomes:

(13) with i corresponding to the instant at which the signal is received. The index n is now limited to preserve the causality of the expression. Indeed, it is impossible to receive in R at the instant i.At a component coming from a signal E at a more recent time. By asking

With Fs the sampling frequency of the emulator, the time interval chosen for the grouping of the paths is equal to At No particular constraint is imposed when choosing the frequency interval Af for the Doppler frequencies.

This stated condition, equation (7) is simplified to become:

(14).

Simplifying with clues to lighten the notation:

(0) sum of n multiplications between a signal sample and an element of the matrix.

Figure 5 shows schematically the steps implemented by the invention. The matrix of coefficients of the filter is represented in 500, this matrix is diagonal and these elements correspond to the sum of the complex values of the paths situated in a chosen time interval and a frequency-Doppler value. The signal E (t) is sampled at a time t, 501. Then the method combines the samples of the signal 501 obtained at a given moment t with a line of the matrix 502 in order to obtain the signal R (t) with t = 0, and so on for all times t, in the example t = 0 to 4, 503, 504, 505 to obtain the signal Ri (t). The emulator according to the invention is used in particular by equipment manufacturers of radars and radiocommunication products. It can also be implemented for RF radio frequency measuring instruments, and in areas for which knowledge of the impact of the propagation channel on a product is important. The emulator according to the invention offers, in particular, the advantages of reducing the computing time required for digital processing while offering the possibility of associating with each path a Doppler of its own. On the other hand, the updating of the values of the filter makes it possible to vary the impulse response to be emulated and thus makes it possible to faithfully reproduce operating scenarios of a device. The depth of the convolution filter is variable and can be reduced to a few samples when the minimum delay to emulate is short, and increased to several hundred samples for scenarios involving distances and therefore longer delays.

Claims (9)

claims 1 - Method implemented in an emulator (110) for a propagation channel connected to a device (100), the emulator comprising at least one input receiving a signal E (t) transmitted by said device (100) in a direction which induces a delay ηΔτ with a Doppler frequency d.Af, and an output emitting a signal R (t) after processing, a processor (143) adapted to perform at least the following steps: • Choosing a discrete modeling of the propagation channel on the Doppler frequencies, • Define one or more zones (320) corresponding to a time interval and a Doppler frequency value, and determine a DT propagation channel matrix whose elements are obtained by summing all complex values of the paths in a chosen time interval and doppler-frequency value, • Digitize the signal E (t), • Combine a digitized sample of the signal E (t) with a line of the m a matrix Doppler delay-Doppler-channel coefficient in order to obtain the signal R (t) corresponding to a set of propagated paths, • Transmit the signal R (t) to said equipment (100). 2 - Process according to claim 1 characterized in that the processor is adapted to insert in a shift register, at each symbol time, a sample of the signal E (t), to subject the matrix DT a circular rotation in the direction lines and convolution between the last row of the matrix and the sample buffer in order to obtain Ri samples which are then converted into the analog domain before transmission to the equipment under test. 3 - Method according to one of claims 1 to 2 characterized in that the signal R (t) corresponding to a sum over all the possible delay values n of the product of a sample of the signal emitted by the sum over the possible values Doppler Doppler of Af (t_nAT): with: impact of a path on the signal following this path, d. M. (t), the Doppler frequency value for a delay ηΔτ, Cnd, a complex coefficient representing the attenuation related to free space propagation, (E (t - ηΔτ)) a sample value of the signal received with a delay ηΔτ. 4 - Process according to one of claims 1 to 3 characterized in that one precalculates the matrix Dn and eliminates the Doppler frequencies having a negligible impact on the propagation channel. 5 - Method according to one of the preceding claims characterized in that one defines the time interval for grouping the signal paths equal to 1 / Fs, where Fs is the sampling frequency of the emulator, and determines the signal R (t) as follows: n is a bounded index, i corresponds to the moment when the signal is received. 6 - Method according to one of claims 1 to 5 characterized in that one chooses the size of a delay-Doppler zone as a function of the sampling frequency of the converters and the frequency of activation of the channel coefficients in a programmable logic circuit. An emulator (110) for a propagation channel connected to a device (100), the emulator comprising a first frequency translation module (120), connected to an analog / digital and digital / analog conversion module (130) and to a signal processing module (140), at least one input (141) receiving a signal E (t) transmitted by said equipment in a direction which induces a delay ηΔτ with a Doppler frequency d.Af and an output (142) emitting a signal R (t) after processing, a signal processor (143), a memory (144) for storing signals, characterized in that the processor (143) is adapted to perform at least the steps of the method according to the one of claims 1 to 6 to generate an emulated signal R (t) transmitted to said equipment (100). 8 - emulator according to claim 7 characterized in that it comprises a programmable logic circuit or FPGA. 9 - emulator according to claim 7 characterized in that it comprises a digital signal processor type DSP.