Device for a radiofrequency communication system, and corresponding system and payload

Abstract

The invention relates to a device for a radiofrequency communication system of a first object, the device comprising at least one transmission chain (5), the chain sequentially comprising at least: a digital modulator (7), a communication interface (9) which is designed, when in use, to communicate by radiofrequency with at least a second object which is remote from the first object, the interface sequentially comprising at least a radiofrequency transmitter (11) and a radiofrequency filter (12). Moreover, the device comprises a digital pre-correction module (8) arranged between the digital modulator (7) and the communication interface (9), the digital pre-correction module comprising at least one finite impulse response filter (13) defined on the basis of at least one characteristic of the radiofrequency filter.

Description

[0001] DEVICE FOR RADIO FREQUENCY COMMUNICATION SYSTEM, CORRESPONDING SYSTEM AND PAYLOAD

[0002] The present invention relates to a device for a radio frequency communication system.

[0003] The present invention also relates to a radio frequency communication system comprising such a device. The present invention also relates to a payload comprising such a communication system.

[0004] BACKGROUND OF THE INVENTION

[0005] In the space sector, it is known to exchange data between ground bases and payloads placed in orbit.

[0006] To this end, the payloads include communication systems, each system comprising at least one device capable of communicating by radio frequency with different bases located on the ground.

[0007] The device typically includes a transmission chain for retransmitting or dumping data (images, compressed data, etc.) to databases. This transmission chain ensures, in particular, that the data is formatted in a way that is compatible with a data propagation channel between the payload and the databases.

[0008] Indeed, given the technical and environmental constraints related to the space domain, the radio frequency signal that will circulate on the propagation channel must undergo at least one pre-conditioning.

[0009] As is well known, the transmission chain typically comprises a digital modulator followed by a radio frequency transmission module, which includes a radio frequency transmitter (usually a transmitter combined with an amplification module) and then a radio frequency filter. Such a filter can exhibit phase linearity imperfections, which significantly degrade the quality of the transmitted signal.

[0010] SUBJECT OF THE INVENTION

[0011] One aim of the invention is to improve the quality of at least one radio frequency signal transmitted from object A to object B.

[0012] SUMMARY OF THE INVENTION

[0013] For this purpose, the invention provides a radio frequency communication system apparatus for a first object, the apparatus comprising at least one transmission chain, said chain comprising at least successively:

[0014] - A digital modulator,

[0015] - A communication interface intended to communicate in service by radio frequency with at least one second object distant from the first object, said interface comprising at least successively a radio frequency transmitter and a radio frequency filter.

[0016] According to the invention, the device includes a digital precorrection module arranged between the digital modulator and the communication interface, the digital precorrection module comprising at least one finite impulse response filter defined from at least one characteristic of the radio frequency filter, in order to act in service on at least one phase of a signal at the output of the radio frequency filter.

[0017] Thus, the finite impulse response (FIR) filter (hereafter referred to as the RIR filter) allows for the digital correction of one or more components of a signal output from the radio frequency (RF) filter (hereafter referred to as the RF filter). Specifically, the RIR filter contributes to correcting one or more phase nonlinearities in the RF filter. In particular, the RIR filter allows for adjustment of the phase response of the radio frequency link at the device's output.

[0018] Digital correction is thus ensured without having to modify the RF filter. The latter can therefore advantageously remain a standard, off-the-shelf filter. This is particularly beneficial in high modulation order contexts, where the characteristics of the RF filter have a greater impact on the output signal. By "high," we mean, for example, greater than 3 bits per symbol.

[0019] The invention thus makes it possible to improve emission performance while being relatively simple to implement.

[0020] Note that we speak of "precorrection" in the sense that the RTF filter is placed before the RF filter, and even before the communication interface, and even before the radio frequency transmitter.

[0021] Subsequently, the terms "upstream," "downstream," "successive," etc., must be extended according to the direction of movement of the data to be transmitted from the invention to at least one object located away from the invention, such as a ground base. Optionally, the first object is a payload and the second object is a ground base.

[0022] Optionally, the characteristic is related to (or is) a phase of the signal at the output of the RF filter.

[0023] Optionally, the characteristic is related to (or is) a derivative of a phase of the signal at the output of the RF filter.

[0024] Optionally, the characteristic represents a phase delay and / or a group delay of the RF filter. Optionally, the finite impulse response filter is of a given order and is configured to approximate an infinite impulse response filter of a lower order. Optionally, the RTF filter is defined by expanding an IIR filter and removing at least some of the expansion elements.

[0025] Optionally, the finite impulse response filter is of an order strictly greater than 2. Optionally, the finite impulse response filter is of an order greater than m + 2, with m a predetermined integer from at least one parameter specific to the digital precorrection module, the parameter preferably being linked to a programmable logic circuit forming at least the finite impulse response filter.

[0026] Optionally, the digital precorrection module includes at least one programmable logic circuit forming at least the finite impulse response filter, the parameter being linked to said circuit.

[0027] Optionally, the parameter is a resolution of calculations of the programmable logic circuit.

[0028] Optionally, the RTF filter is shaped to present an architecture that allows it to process multiple samples in parallel.

[0029] Optionally, the RTF filter is shaped to present a MIMO-type architecture.

[0030] Optionally, the finite impulse response filter is arranged upstream of the other components of the digital precorrection module.

[0031] Optionally, the finite impulse response filter is configured so that its response approximates that of an all-pass filter.

[0032] Optionally, the finite impulse response filter is an asymmetric filter.

[0033] Optionally, the characteristic is representative of a phase delay and / or a group delay of the radio frequency filter.

[0034] Optionally, the finite impulse response filter is configured so that its phase delay and / or group delay is the inverse of that of the radio frequency filter.

[0035] Optionally, the finite impulse response filter is of a given order and is configured to approximate an infinite impulse response filter of a lower order. Alternatively, the finite impulse response filter is of an order greater than m + 2, where m is a predetermined integer derived from at least one parameter specific to the digital precorrection module, the parameter preferably being linked to a programmable logic circuit forming at least the finite impulse response filter.

[0036] Optionally, the device is high-speed.

[0037] Optionally, the radio frequency transmitter transmits a signal of several hundred Megahertz.

[0038] Optionally, the radio frequency transmitter transmits a signal at a frequency over an AF bandwidth, F₀ being between 7500 and 8500 Megahertz (MHz) and / or ΔF being between 200 and 400 MHz.

[0039] The invention also relates to a wireless communication system comprising a device as described above.

[0040] Other features and advantages of the invention will become apparent from the following description of a particular and non-limiting embodiment of the invention.

[0041] BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Reference will be made to the attached drawings, among which:

[0043] [Fig. 1] Figure 1 schematically illustrates a payload according to a particular embodiment of the invention in communication with ground bases;

[0044] [Fig. 2] Figure 2 is a block diagram schematically representing part of a radio frequency communication device incorporated into the payload illustrated in Figure 1;

[0045] [Fig. 3] Figure 3 is a schematic of an FIR filter implemented in the part of the device shown in Figure 2; [Fig. 4] Figure 4 is a graph illustrating the group delay of a radio frequency filter implemented in the part of the device shown in Figure 2 for a given frequency range, that of the FIR filter illustrated in Figure 3, and the cumulative group delay of both filters. DETAILED DESCRIPTION OF THE INVENTION

[0046] With reference to figures 1 to 3, according to a particular non-limiting application of a particular embodiment of the invention, a payload 1, which is in orbit, can communicate by radio frequency with one or more bases 2 located on the ground.

[0047] Payload 1 is for example a satellite such as a minisatellite, a microsatellite or a nanosatellite.

[0048] Payload 1, for example, is in orbit around the Earth.

[0049] Furthermore, payload 1 includes a radio frequency communication system 3 comprising a receiver for receiving signals from at least one base 2 (the link between base 2 and payload 1 being then called the "uplink") and a transmitter for transmitting signals from payload 1 to at least one base 2 (the link between the payload and the base being then called the "downlink"). It is this transmitter that will be of interest here, and which will be referred to simply as "device 4" for the remainder of this application.

[0050] The downlink is, for example, in the X-band. Device 4 is thus configured to operate in the X-band. Device 4 is, for example, an image telemetry transmitter, and in particular an X-band image telemetry transmitter. Device 4 is, for example, an RF transmitter of the TMI (Image Telemetry) type.

[0051] Device 4 is compatible with different transmission modes and for example at least one of the following transmission modes: CDMA, OFDMA, TDMA, FDMA, etc.

[0052] For example, device 4 is compatible with different modulation schemes and for example with at least one of the following modulation schemes: DVBS, DVBS2, DVBS2x, SCCC, QPSK, 8PSK, 16APSK, 32APSK, etc.

[0053] The downlink, for example, is designed with a specific scheme called "MODCOD" (short for "Modulation and Coding"). Device 4 can therefore be compatible with the modulation schemes defined by the DVBS2 standard.

[0054] Device 4 is preferably a high-speed transmitting device. "High speed" here means that device 4 is capable of transmitting several Megasymbols per second (MSPS), at least 10 MSPS, preferably at least 100 MSPS, and preferably at least 200 MSPS. For example, device 4 is capable of transmitting between 100 and 250 MSPS.

[0055] For example, device 4 is compatible with at least one New-Space program.

[0056] Device 4 includes, as is known, a transmission chain 5 for retransmitting or dumping data (images, compressed data, etc.) to databases 2. This transmission chain 5 ensures, in particular, that the data is formatted in a way that is compatible with the downlink. Optionally, device 4, and for example its transmission chain 5, includes a data security module 6 for the data to be transmitted. The data security module 6 acts on the data (by encryption, coding, redundancy, etc.) to secure it.

[0057] The transmission chain 5 also includes a digital modulator 7 which receives as input either the data to be transmitted (if the device 4 does not include a data security module 6) or the secured data. In the latter case, the input of the digital modulator 7 is connected to the output of the data security module 6.

[0058] The transmission chain 5 also includes a digital pre-correction module 8 whose input is connected to an output of the digital modulator 7, and a communication interface 9 whose input is connected to the output of the digital pre-correction module 8 and whose output delivers a radio frequency signal 10 to be transmitted via the downlink.

[0059] More specifically, the communication interface 9 includes a radio frequency transmitter 11, the input of which is connected to the output of the digital precorrection module 8, and a radio frequency filter (or RF filter 12) whose input is connected to the output of the radio frequency transmitter 11 and whose output delivers the aforementioned radio frequency signal 10.

[0060] The radio frequency transmitter 11 transmits for example a signal at a frequency Fo over a bandwidth AF so that the frequency band [Fo - AF / 2; Fo + AF / 2] is occupied by the radio frequency spectrum modulated via the device 4.

[0061] For example, radio frequency transmitter 11 operates on signals with a bandwidth of several hundred Megahertz (MHz). For example, Fo is between 7500 and 8500 Megahertz (MHz) and is, for example, between 8025 and 8400 MHz. For example, AF is between 200 and 400 MHz and is, for example, between 300 and 350 MHz. In this case, the frequency band from 8021 MHz to 8345 MHz is occupied by the modulated radio frequency spectrum.

[0062] At least one of the objectives of the RF filter 12 is to reduce potential spectral rises on frequency bands adjacent to the frequency band [Fo - AF / 2; Fo + AF / 2] used by device 4. Thus, if the frequency band 8021 MHz to 8345 MHz is occupied by device 4, particular vigilance must be exercised with the adjacent frequency bands and for example the frequency band [8400 MHz; 8450 MHz], corresponding to the “Deep Space Network” (DSN) program.

[0063] For this purpose, very strong rejection is sometimes necessary at the RF filter 12 level to filter out the aforementioned spectral feedback. However, the use of a radio frequency filter inevitably introduces phase nonlinearities in the filter itself, nonlinearities which can in turn lead to a degradation of the downlink signal quality.

[0064] Therefore, the aforementioned digital precorrection module 8 will at least aim to correct at least some of these phase non-linearities.

[0065] This digital precorrection module 8 can be implemented, for example, by at least one programmable logic circuit and, for example, using at least one field-programmable gate array (FPGA). This programmable logic circuit can be common to at least one element of the transmission chain 5, arranged upstream or downstream of the precorrection module 8. For example, the programmable logic circuit can be common to the digital modulator.

[0066] The digital precorrection module 8 includes at least one finite impulse response filter (FIR filter 13). Preferably, this FIR filter 13 is arranged directly at the output of the digital modulator 7, i.e., directly at the input of the digital precorrection module 8. The FIR filter 13 is thus arranged upstream of the other components of the digital precorrection module 8 and in particular upstream of any distortion, pre-distortion, interpolation and / or additional filter modules of said digital precorrection module 8 such as a square-root-raised-cosine-filter (SRRC).

[0067] The RIF 13 filter, for example, is designed with a specific architecture that allows it to process multiple samples in parallel. For instance, the RIF 13 filter is designed with a MIMO (Multiple-Input Multiple-Output) architecture. The RIF 13 filter is preferably constructed to approximate a recursive filter and / or to exhibit the response of an all-pass filter.

[0068] This allows you to act only on the phase of the FIR filter without affecting its amplitude. The FIR 13 filter therefore does not have feedback.

[0069] The FIR 13 filter is therefore asymmetric, i.e., the list of coefficients defining the filter is not symmetric. The FIR 13 filter is preferably defined based on at least one characteristic of the RF 12 filter.

[0070] This characteristic is for example related to (or is) a group propagation time of the RF filter 12.

[0071] This characteristic is for example viewed over the interval [Fo - AF / 2; Fo + AF / 2].

[0072] The group delay of the RF filter 12 is calculated, for example, from the frequency response of a signal at the output of the RF filter 12 without the presence of the RIF filter 13. This calculation can be performed using any method known to those skilled in the art. The group delay is proportional to the derivative of a phase of a signal at the output of the RF filter 12.

[0073] Therefore, the RIF filter 13 is constructed so that its group delay is inverse of that of the RF filter 12 for at least one frequency belonging to the interval [Fo -AF / 2; Fo + AF / 2] and preferably for at least one sub-interval of frequencies of said interval and preferably for all frequencies of said interval.

[0074] As shown in Figure 4, the FIR filter 13 is constructed such that the sum of the group delays of the RF filter 12 and the FIR filter 13 is constant over at least one frequency sub-interval of the interval [Fo - AF / 2; Fo + AF / 2], and preferably for all frequencies of said interval. By "constant," we mean, for example, that the variation of said sum over time is zero to within ±1 / 10. ème of a symbol's emission time.

[0075] The FIR 13 filter is preferably constructed to approximate an infinite impulse response (IIR) filter. More precisely, the FIR 13 filter is constructed by modifying the structure of an IIR filter. Specifically, the FIR 13 filter is constructed by expanding an IIR filter (such as an all-pass IIR filter) and removing at least some of the expansion elements.

[0076] It is recalled that an all-pass IIR filter obeys the following equation:

[0077] V

[0078]

[0079] ∀ a1∈ ℂ : Y n = a1X n + X n-1 - a1Y n-1 (1) with the following stability condition:

[0080] |ti| < 1

[0081] with X being the values ​​of the input signal to the filter, and

[0082] Y are the values ​​of the signal at the output of the filter.

[0083] With this equation, there is therefore a feedback loop from the output (Yn-i) for the calculation of the next sample (Y n ) which is not desired within the framework of the invention.

[0084] Therefore, equation (1) is developed to eliminate the feedback loop and thus implement it with a FIR filter. To this end, the equation of Y is injected n -i in equation (1) which gives:

[0085] Y

[0086]

[0087] n = + (1 - fliXn-l “ ^Xn-2 + G? K n-2 (2)

[0088] This operation can be done recursively to finally express Y n depending on K n-m-2 for m > 0:

[0089] Y n = a1X n + ∑ m k=0 (-a1) k (1 - a1 2 )X n-k-1 + (-a1) m+1 X n-m-2 + (-a1) m+2 Y n-m-2

[0090]

[0091] (3)

[0092] Since the parameter ai of the equation has a modulus less than 1, the factor of the term Y n -m-2 (the looping component) tends towards 0 when m is high.

[0093] Thus, we define a value m from which we stop the expansion of equation (3) and we eliminate the last two terms of the equation, which gives, V

[0094]

[0095] ∀ a1∈ ℂ : Y n = a1X n + ∑ m k=0 (-a1) k (1 - a1 2 )X n-k-1 (4)

[0096] Equation (4) is thus an approximation of equation (1) without a feedback component.

[0097] It is thus possible to configure an FIR filter that obeys equation (4). In particular, equation (4) corresponds to an FIR filter of order m + 2. The order of the FIR filter is determined here at least by the number of recursion cycles required to make the looping coefficients negligible.

[0098] As already mentioned above, the RIF 13 filter is defined so as to have a group delay inverse to that of the RF 12 filter. However, the group delay of the RIF 13 filter depends on said parameter ai.

[0099] Therefore, we define ai so that the group delay of the RIF 13 filter corrects that of the RF 12 filter.

[0100] For this purpose, we can theoretically calculate ai and / or define ai experimentally by monitoring one or more characteristics of the output signal representative of the quality of said signal such as an amplitude of the error vector.

[0101] As we have seen, the value m is defined such that the term (-ai) m+2 be negligible. To this end, the value m is defined, for example, by taking into account the value of ai and / or at least one parameter (such as the calculation resolution) of at least one programmable logic circuit that will allow the implementation of the RIF 13 filter and / or the intended application. Typically, the value m is defined such that the term (-ai) m+2 is negligible compared to the resolution of calculations of the programmable logic circuit.

[0102] Example

[0103] If the parameter ai is calibrated to the range of values ​​[0, 0.2] and the resolution of the calculations of the programmable logic circuit is 16 bits, this implies that: - the smallest representable value on said programmable logic circuit is ≈ 3.1 · 10 -5 ,

[0104]

[0105] - with m=6, the factor (-ai) m+2 is increased for (-0.2)8 = 2.56 · 10 -6 , which is therefore negligible in a 16-bit calculation.

[0106] For this example, it is therefore possible to implement equation (4) with m = 6, which corresponds to an 8th order FIR 13 filter.

[0107] Device 4, as described above, allows for preconditioning of the RF signal and, more specifically, compensation for the phase nonlinearities of the RF filter 12 through digital compensation implemented upstream of the RF filter 12. This provides a digital precorrection of these nonlinearities. In particular, it performs digital filtering.

[0108] More specifically, based here on the group delay of the RF filter 12, the RIF filter 13 is defined to correct at least partially one or more defects of the RF filter 12.

[0109] Advantageously, precorrection can be implemented with simple means (such as programmable logic circuits) which are also already qualified for orbital flights.

[0110] Furthermore, the energy consumption of this pre-correction remains modest.

[0111] Furthermore, the pre-correction performed occupies only a small volume in the payload 1.

[0112] Furthermore, the pre-correction performed is stable.

[0113] The pre-correction performed advantageously allows device 4 to work even at high throughput and / or to parallelize its calculations.

[0114] The pre-correction performed allows operation across the entire frequency band of the RF filter, which corresponds to a wide frequency range. The proposed device 4 thus improves the quality of the transmitted signal without significantly increasing its power consumption, mass, or volume compared to existing devices. The finite impulse response filter 13 is defined here based on at least one intrinsic characteristic of the radio frequency filter 12, known in advance (such as its group and / or phase delay). Therefore, a specific component (the radio frequency filter 12) is pre-characterized (for example, by its group and / or phase delay) to allow the application of a pre-calculated inverse correction to the finite impulse response filter 13. The correction is thus fixed and non-iterative.

[0115] The finite impulse response filter 13 thus has a group and / or phase delay which is the inverse of that of the radio frequency filter 12, so that the combined group delay of said two filters is constant.

[0116] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0117] In particular, although here the device is carried on a payload, it can be integrated into any other system, such as a very broadband communication system (like a 5G and / or 6G communication system). The invention is thus integrable into any radio frequency communication system from one object to another.

Claims

DEMANDS 1. Apparatus for a radio frequency communication system of a first object, the apparatus comprising at least one transmission chain (5), said chain comprising at least successively: - A digital modulator (7), - A communication interface (9) intended to communicate in service by radio frequency with at least one second object located away from the first object, said interface comprising at least successively a radio frequency transmitter (11) and a radio frequency filter (12), characterized in that the device comprises a digital precorrection module (8) arranged between the digital modulator and the communication interface, the digital precorrection module comprising at least one finite impulse response filter (13) defined from at least one characteristic of the radio frequency filter.

2. Device according to claim 1, in which the finite impulse response filter (13) is arranged upstream of the other components of the digital precorrection module (8).

3. Apparatus according to any one of the preceding claims, wherein the finite impulse response filter (13) is configured so that its response approximates that of an all-pass filter.

4. Apparatus according to any one of the preceding claims, wherein the finite impulse response filter (13) is an asymmetric filter.

5. Apparatus according to any one of the preceding claims, wherein the characteristic is representative of a phase delay and / or a group delay of the radio frequency filter (12).

6. Device according to claim 5, wherein the finite impulse response filter (13) is configured of so that its phase delay and / or group delay is the inverse of that of the radio frequency filter (12).

7. Apparatus according to any one of the preceding claims, wherein the finite impulse response filter (13) is of a given order and is configured to approximate an infinite impulse response filter of a lower order.

8. Device according to any one of the preceding claims, wherein the finite impulse response filter (13) is of an order higher than m + 2, with m a predetermined integer from at least one parameter specific to the digital precorrection module (8), the parameter being preferably linked to a programmable logic circuit forming at least the finite impulse response filter.

9. Device according to any one of the preceding claims, wherein the device is high-speed.

10. Apparatus according to any one of the preceding claims, in which the radio frequency transmitter (11) transmits a signal of several hundred Megahertz.

11. Apparatus according to any one of the preceding claims, wherein the radio frequency transmitter (11) transmits a signal at a frequency (Fo) over a bandwidth AF, FF₀ being between 7500 and 8500 Megahertz (MHz) and / or ΔF being between 200 and 400 MHz.

12. Wireless communication system comprising a device according to any one of the preceding claims.

Images