Device for radio frequency communication system, corresponding system and payload

Abstract

Device for a radio frequency communication system of a first object, the device comprising at least one transmission chain (5), said chain comprising at least successively: A digital modulator (7), A communication interface (9) intended for communicating in operation by radio frequency with at least one second object located at a distance from the first object, said interface comprising at least successively a radio frequency transmitter (11) and a radio frequency filter (12). Furthermore, the device comprises a digital precorrection module (8) arranged between the digital modulator (7) and the communication interface (9), the digital precorrection module comprising at least one finite impulse response filter (13) defined from at least one characteristic of the radio frequency filter. FIGURE IN ABRIDGED DIAGRAM: Fig. 2

Description

Title of the invention: Apparatus for a radio frequency communication system, corresponding system and payload

[0001] The present invention relates to a device for a radio frequency communication system.

[0002] The present invention also relates to a radio frequency communication system comprising such a device.

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

[0004] BACKGROUND OF THE INVENTION

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

[0006] For this purpose, 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 generally includes a transmission chain for retransmitting or dumping data (images, compressed data, etc.) to the 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 which will circulate on the propagation channel must undergo at least one pre-conditioning.

[0009] In a manner known per se, the transmission chain thus generally comprises a digital modulator followed by a radio frequency transmission module which includes in particular a radio frequency transmitter (usually a transmitter associated with an amplification module) followed by a radio frequency filter.

[0010] It turns out that such a filter may exhibit phase linearity imperfections which greatly degrades the quality of the transmitted signal.

[0011] SUBJECT OF THE INVENTION

[0012] One object of the invention is to improve the quality of at least one radio frequency signal transmitted from an object A to an object B. Summary of the invention

[0013] To this end, according to the invention, a device for a radio frequency communication system of a first object is provided, the device comprising at least one transmission chain, said chain comprising at least successively: - A digital modulator, - 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.

[0014] According to the invention, the device comprises 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.

[0015] In this way, the finite impulse response filter (hereafter referred to as the FIR filter) makes it possible to digitally correct one or more components of a signal at the output of the radio frequency filter (hereafter referred to as the RF filter). In particular, the FIR filter contributes to correcting one or more phase nonlinearities of the RF filter. Specifically, the FIR filter allows adjustment of the phase response of the radio frequency link at the output of the device.

[0016] Digital correction is thus ensured without having to modify the RF filter. The latter can therefore advantageously remain a standard commercial filter. This is particularly advantageous in a high modulation order context where the characteristics of the RF filter have a greater impact on the RF filter's output signal. By "high," we mean, for example, greater than 3 bits per symbol.

[0017] The invention thus makes it possible to improve emission performance while being relatively simple to implement.

[0018] Note that we speak of "precorrection" in the sense that the FIR filter is placed before the RF filter, and even before the communication interface, and even before the radio frequency transmitter.

[0019] Subsequently the terms "upstream", "downstream", "successive"... must be extended according to the direction of movement of the data to be transmitted from the invention to at least one object distant from the invention such as for example a base on the ground.

[0020] Optionally, the first object is a payload and the second object is a ground base.

[0021] Optionally, the characteristic is related to (or is) a phase of the signal at the output of the RF filter.

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

[0023] Optionally, the characteristic is representative of a phase delay and / or a group delay of the RF filter.

[0024] 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.

[0025] Optionally, the FIR filter is defined by developing an IIR filter and removing at least part of the development elements.

[0026] Optionally, the finite impulse response filter is of an order strictly greater than 2.

[0027] Optionally, the finite impulse response filter is of an order higher 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.

[0028] 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.

[0029] Optionally, the parameter is a resolution of calculations of the programmable logic circuit.

[0030] Optionally, the FIR filter is shaped to present an architecture enabling it to process several samples in parallel.

[0031] Optionally, the RIF filter is shaped to present a MIMO-type architecture.

[0032] Optionally, the finite impulse response filter is arranged upstream of the other components of the digital precorrection module.

[0033] Optionally, the finite impulse response filter is configured so that its response approximates that of an all-pass filter.

[0034] Optionally, the finite impulse response filter is an asymmetric filter.

[0035] Optionally, the characteristic is representative of a phase delay and / or a radio frequency filter group delay.

[0036] 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.

[0037] 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.

[0038] Optionally, the finite impulse response filter is of an order higher 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.

[0039] Optionally, the device is high-speed.

[0040] The invention also relates to a wireless communication system comprising a device as described above.

[0041] Other features and advantages of the invention will become apparent from the following description of a particular, non-limiting embodiment of the invention. Brief description of the drawings

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

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

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

[0045] [Fig.3] [Fig.3] is a diagram of an FIR filter implemented in the part of the device shown in [Fig.2];

[0046] [Fig.4] [Fig.4] is a graph illustrating the group delay of a filter radio frequency implanted in the part of the device shown in [Fig.2] for a given frequency range, that of the FIR filter shown in [Fig.3] and that cumulative of said two filters. DETAILED DESCRIPTION OF THE INVENTION

[0047] 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.

[0048] The payload 1 is for example a satellite such as a minisatellite, a microsatellite or a nanosatellite.

[0049] The payload 1 is for example in orbit around the earth.

[0050] Furthermore, the payload 1 comprises a radio frequency communication system 3 having 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.

[0051] 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.

[0052] Device 4 is compatible with different transmission modes and for example at least one of the following transmission modes: CDMA, OFDMA, TDMA, FDMA, etc.

[0053] For example, the 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.

[0054] The downlink is, for example, designed with a specific scheme called "MODCOD" (for the English term "MODulation and CODing"). Device 4 can thus be compatible with the modulation schemes defined by the DVBS2 standard.

[0055] Device 4 is preferably a high-speed transmitting device. "High speed" here means that Device 4 is capable of transmitting several Mega Symbols per Second (MSPS), and at least 10 MSPS, preferably at least 100 MSPS, and preferably at least 200 MSPS. Device 4 is, for example, capable of transmitting between 100 and 250 MSPS.

[0056] For example, device 4 is compatible with at least one New-Space program.

[0057] The apparatus 4 comprises, in a manner known per se, a transmission chain 5 for the retransmission or dumping of data (images, compressed data, ...) to databases 2. This transmission chain 5 ensures in particular a formatting of the data which is compatible with the downlink.

[0058] Optionally, the 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.

[0059] 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.

[0060] The transmission chain 5 also includes a digital precorrection module 8 whose input is connected to an output of the digital modulator 7, as well as a communication interface 9 whose input is connected to the output of the digital precorrection module 8 and whose output delivers a radio frequency signal 10 to be transmitted via the downlink.

[0061] More specifically, the communication interface 9 includes a radio frequency transmitter 11, the input of which is connected to the output of the digital pre-correction module 8, and a radio frequency filter (or RF filter 12) the input of which is connected to the output of the radio frequency transmitter 11 and the output of which delivers the aforementioned radio frequency signal 10.

[0062] The radio frequency transmitter 11 transmits, for example, a signal at a frequency Fo over a bandwidth AF such that the frequency band [Fo_AF / 2 ; F0+AF / 2] is occupied by the radio frequency spectrum modulated via the device 4. 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 the present case, it is therefore the frequency band from 8021 MHz to 8345 MHz that is occupied by the modulated radio frequency spectrum.

[0063] At least one of the objectives of the RF filter 12 is to reduce potential spectral feedback on frequency bands adjacent to the frequency band [Fo - AF / 2 ; Fo + AF / 2] used by the device 4. Thus, if the frequency band 8021 MHz to 8345 MHz is occupied by the device 4, particular attention must be paid to the adjacent frequency bands and for example the frequency band [8400 MHz; 8450 MHz], corresponding to the “Deep Space Network” (DSN) program.

[0064] For this purpose, very strong rejection is sometimes necessary at the RF filter 12 level in order to filter out the aforementioned spectral feedback. However, the use of a radio frequency filter inevitably leads to phase nonlinearities in said filter, nonlinearities which can in turn lead to a degradation of the downlink signal quality.

[0065] Consequently, the aforementioned digital precorrection module 8 will at least aim to correct at least in part these phase non-linearities.

[0066] 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.

[0067] 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 (or SRRC for the English acronym for "square-root-raised-cosine-filter").

[0068] The RIF 13 filter is, for example, shaped to have a particular architecture allowing it to process several samples in parallel. For example, the RIF 13 filter is shaped to have a MIMO (Multiple-Input Multiple-Output) type architecture.

[0069] The RIF 13 filter is preferably constructed so as to approximate a recursive filter and / or the RIF 13 filter is preferably constructed so as to present a response of an all-pass filter.

[0070] This allows us to act only on the phase of the FIR filter without acting on its amplitude.

[0071] The RIF 13 filter therefore does not have any feedback.

[0072] The RIF 13 filter is thus asymmetric, i.e., the list of coefficients defining the filter is not symmetrical.

[0073] The RIF filter 13 is preferably defined from at least one characteristic of the RF filter 12.

[0074] Said characteristic is for example related to (or is) a group propagation time of the RF filter 12.

[0075] Said characteristic is for example viewed over the interval [Fo - AF / 2 ; Fo + AF / 2],

[0076] The group delay of the RF filter 12 is, for example, calculated from the frequency response of a signal at the output of the RF filter 12 without the presence of the FIR filter 13. This calculation can be performed by 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.

[0077] Therefore, the FIR 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.

[0078] As shown in [Fig. 4], the FIR filter 13 is constructed such that the sum of the group delay of the RF filter 12 and the FIR filter 13 is constant over at least one sub-frequency interval of the interval [Fo - AF / 2; Fo + AF / 2] and preferably for all frequencies of said interval. By "constant," it is meant, for example, that the variation of said sum over time is zero to within ±1 / 10th of a symbol transmission time.

[0079] The FIR 13 filter is preferably constructed to approximate an infinite impulse response (IIR) filter. More specifically, the FIR 13 filter is constructed from a structural modification of an IIR filter. Here, the RIF 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.

[0080] It is recalled that an all-pass IIR filter obeys the following equation:

[0081] V aqeC X€ :€ Yn-=ayXn+Xn.\-aXn-i (1)

[0082] with the following stability condition:

[0083]

[0084] with X being the values ​​of the input signal to the filter, and

[0085] Y the values ​​of the signal at the output of the filter.

[0086] With this equation, there is therefore a feedback loop of the output (Yn.i) for the calculation of the following sample (Yn) which is not desired within the scope of the invention.

[0087] Therefore, equation (1) is developed to be able to remove the looping and thus implement it with an FIR filter.

[0088] To this end, we substitute the equation of Yn4 into equation (1), which gives:

[0089] Yn = a{Xn + (1 - «2) xnA - + ajY„_2 (2)

[0090] This operation can be done recursively to finally express Yn as a function of Yn_m_2 for ni > 0; [00911 Y„ = a ,X„ + ( -a, ) ' ( 1 - «t )

[0092] (3)

[0093] Since the parameter ai of the equation has a modulus less than 1, the factor of the term Yn m 2 (the component of the feedback) tends towards 0 when m is high.

[0094] Thus, we define a value m from which we stop the development of equation (3) and we delete the last two terms of the equation, which gives,

[0095] VX€ :€ Yn = atX,, +

[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 this 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 FIR filter 13 is defined so as to have a group delay inverse to that of the RF filter 12. Now, the group delay of the FIR filter 13 depends on said parameter ab

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

[0100] For this purpose, ai can be calculated theoretically and / or ai can be defined 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 will be understood, the value m is defined such that the term (-ai)m+2 is 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 computational 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 computational resolution of the programmable logic circuit. Examples

[0102] 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-HT5, - with m=6, the factor (-ai)m+2 is bounded for ( -0.2)8 = 2.56 • 10”' which is therefore negligible on a 16-bit calculation.

[0103] For this example, it is therefore possible to implement equation (4) with m = 6, which corresponds to an 8th order FIR 13 filter.

[0104] The device 4 described above allows for preconditioning of the RF signal and, more specifically, compensation of the phase nonlinearities of the RF filter 12 through digital compensation implemented upstream of the RF filter 12. This provides digital precorrection of said nonlinearities. In particular, it performs digital filtering.

[0105] More specifically, based here on the group delay of the RF filter 12, the RIF filter 13 is defined to correct at least in part one or more defects of the RF filter 12.

[0106] Advantageously, precorrection can be implemented with simple means (such as programmable logic circuits) which are also already qualified for orbital flights.

[0107] Moreover, the energy consumption of this precorrection remains modest.

[0108] In addition, the pre-correction performed occupies only a small volume in the payload 1.

[0109] The precorrection performed is also stable.

[0110] The precorrection performed advantageously allows the device 4 to work even at high speed and / or to parallelize its calculations.

[0111] The pre-correction performed makes it possible to work over the entire frequency band of the RF filter, which corresponds to a wide frequency band.

[0112] The device 4 thus proposed makes it possible to improve the quality of the transmitted signal without the device 4 seeing its consumption and / or mass and / or volume increase significantly compared to existing devices.

[0113] 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.

[0114] In particular, although here the device is carried on a payload, the device can be integrated into any other system, such as a very broadband communication system (such as a 5G and / or 6G communication system). The invention is thus integrable into any radio frequency communication system from one object to a second object.

Claims

Demands

1. Device for a radio frequency communication system of a first object, the device 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 a second object distant 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, wherein the finite impulse response filter (13) is arranged upstream of the other components of the digital precorrection module (8).

3. Device 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 such 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. Apparatus according to any one of the preceding claims, wherein the finite impulse response filter (13) is of a higher order than

9.

10. 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. Device according to any one of the preceding claims, wherein the device is high-speed. Wireless communication system comprising a device according to one of the preceding claims.