Antenna system and beamforming method

Abstract

Antenna system and beamforming method The present invention relates to an antenna system (10) comprising: - an array (12) of spatially distinct sources in pairs, each source being capable of emitting a signal decomposed into a plurality of sub-bands distributed on either side of a central band centered on a center frequency of said signal; - a planar array (14) of elementary cells (16), and a digital processing module (18), upstream of said array of sources, configured to determine, by sub-band, a digital control phase of each source, and to digitally compensate, via said digital control phase, both: - the sum, previously calculated, of the phase shifts to be generated between each source and the output of each elementary cell; - the sum, previously calculated, of the path differences to be generated by each phase-shifting element of each elementary cell.Figure for the abbreviation: Figure 1.

Description

Title of the invention: Antenna system and beamforming method

[0001] The present invention relates to an antenna system comprising: an array of Ns spatially distinct pairwise sources, with Ns > 1, each source being capable of emitting a signal decomposed into a plurality of sub-bands distributed on either side of a central band centered on a center frequency of said signal; a planar array of elementary cells, each source being capable of illuminating, at least in part, said array of elementary cells. Each elementary cell comprises at least: a phase-shifting element, capable of shifting said signal by a predetermined phase shift; and a passive transmitter, connected to the output of said phase-shifting element, and capable of transmitting the phase-shifted signal provided by said phase-shifting element.

[0002] The invention also relates to a beamforming method implemented by such an antenna system.

[0003] The present invention relates to the field of wireless communications using an antenna system, and more specifically to phase-controlled antenna arrays.

[0004] Among phase-controlled antenna arrays, one can cite transmission-line phase-controlled antenna arrays, notably including phase control by delay line (according to the usual antenna design terminology), the delay lines being used to temporally delay the signal. The phase response of the delay lines is linear within the bandwidth. This means that the different frequency components of a signal are not phase-shifted identically. This solution is therefore more advantageous for broadband transmissions because it is much less susceptible to beam strabismus (defined below). However, the delay lines are difficult to reconfigure and induce significant line losses.

[0005] In this field, we can also mention phase-controlled antenna arrays (i.e., designed to phase-shift input signals), and antenna arrays based on phase shifters that apply a constant phase shift across the band. Phase shifters are advantageously reconfigurable components, particularly those based on diodes, varactors, etc., and offer a low-cost alternative to delay lines. Such phase shifters are found, in particular, in the lenses of transmitting arrays.

[0006] However, these phase-shifter-based architectures are susceptible to degradation induced by broadband use, such as beam-squinting. Indeed, the response of such phase-shifting elements The frequency response is flat, making it impossible to adjust the phase for different signal frequencies. Generally, the center frequency is chosen to calculate the phase shifts needed to form a beam in the desired direction, and all frequency tones of the signal undergo the same phase shift. Consequently, beams associated with frequency tones sufficiently far from the center frequency become misaligned and point in undesired directions—a phenomenon known as beam strabismus. Note that there is no strabismus on the center beam itself, only on beams outside its center.

[0007] Severe beam strabismus leads to a significant loss of gain, an effect known as beam split.

[0008] A single-beam, multi-source structure has been proposed in the field of transmit-array radio antennas, as described in patent application EP 3 159 965 A1 filed by the Applicant. More specifically, this patent application proposes using several focal sources to correct the illumination law in order to make it more uniform and improve the quality of the central beam of the broadside antenna. This solution indirectly reduces the effects of beam strabismus for transmit-arrays with constant phase shift in the band. Indeed, the beam strabismus effect is related to the array size, and the use of a multi-source structure reduces it, but this can still be improved.

[0009] The aim of the invention is therefore to provide an antenna system that further reduces the effects of off-center beam strabismus. In other words, the present invention aims to increase the reduction of beam strabismus effects present outside the central beam.

[0010] To this end, the invention relates to an antenna system comprising:

[0011] - a set of Ns pairwise spatially distinct source Ns, with Ns>l, each source being capable of emitting a signal decomposed into a plurality of sub-bands distributed on either side of a central band centered on a central frequency of said signal;

[0012] - a planar network of elementary cells,

[0013] each source being capable of illuminating, at least in part, said planar network of elementary cells,

[0014] each elementary cell comprising at least:

[0015] - a phase-shifting element capable of shifting the phase, according to a predetermined phase shift, said signal,

[0016] - a passive transmitter, connected to the output of said phase-shifting element, and suitable for transmit the phase-shifted signal provided by said phase-shifting element;

[0017] said antenna system comprising a digital processing module, upstream of said set of sources, configured to determine, by sub-band, a digital control phase for each source, and to digitally compensate, via said digital control phase, for both:

[0018] - the sum, previously calculated, of the phase shifts that are likely to be generated between each source and output of each elementary cell;

[0019] - the sum, previously calculated, of the differences in paths to be generated by each phase-shifting element.

[0020] In other words, the present invention proposes an antenna system capable of forming a beam in a hybrid manner, via digital processing implemented by the digital processing module upstream of the set of sources, and via analog processing implemented by the planar network of elementary cells.

[0021] The digital processing module and the planar array of elementary cells form in synergy within said antenna system a "hybrid" two-stage precoder, the two-stage being suitable for implementing in a hybrid manner complementary digital and analog processing.

[0022] Indeed, the digital processing module is designed to determine, by sub-band (i.e. outside the center frequency or even the band centered on the center frequency), a digital control phase of each source, in order to digitally compensate, by sub-band (i.e. outside the center frequency or even the band centered on the center frequency), both the sum, previously calculated, of the analog phase shifts that are to be generated between each source and the output of each elementary cell, and the sum, previously calculated, of the path differences that are to be generated by each phase-shifting element (i.e. by each elementary cell).

[0023] Such compensation advantageously limits the effects induced by broadband transmissions, such as beam strabismus, by digitally manipulating beams associated with frequency tones sufficiently far from the center frequency. Without this digital processing, these beams would be more misaligned and point in undesired directions. Hereafter, broadband refers to a bandwidth with a width greater than 5% of the center frequency of said bandwidth.

[0024] In other words, such compensation is, by definition, sub-band (i.e., and not on the central band centered on the center frequency), configured to act more on tones with frequencies sufficiently far from the center frequency than on those close to the center frequency. Indeed, the further the sub-band is from the center band, the more pronounced the strabismus, whereas for sub-bands close to the center band, the strabismus is negligible, strabismus being a characteristic effect of broadband transmissions (i.e. only associated with sub-bands sufficiently far from the central band).

[0025] Indeed, the phase shifts inherent to be generated between each source and each output of an elementary cell, as well as the path differences, are naturally greater at the edges of the bands (since the transmission is broadband), and the compensation provided by the digital processing makes it possible to attenuate them. The effects of beam misalignment are thus limited.

[0026] Such a hybrid dual-stage precoder is thus suitable for implementing digital frequency processing by sub-band (i.e., distinct from the center band) and by source to supplement the conventional analog processing essentially determined for the center frequency. This digital processing is, moreover, dependent on the said analog processing, as it is complementary. Indeed, the analog processing applies analog control phases calculated at the center frequency to create a beamform in a known and desired direction (0, 0), while, complementaryly, the digital control phases are determined by sub-band (i.e., outside the band centered on the center frequency) and by source.

[0027] According to other advantageous aspects of the invention, the antenna system comprises one or more of the following features, taken individually or in all technically possible combinations:

[0028] - said planar array of elementary cells is a controllable array antenna phase per transmission line, or a transmitting network comprising a lens composed of a plurality of elementary cells, each elementary cell being a reconfigurable component;

[0029] - when said planar network of elementary cells is a transmitting network, by In the sub-band, the digital control phase of each source can be expressed by the following equation:

[0030]

[0031] with:

[0032] / . the predetermined frequency associated with each sub-band of index i el^la length of corresponding wave;

[0033] (do, ^()) the angles defining the predetermined far-field direction of the beam to be emitted by said antenna system;

[0034] (xm, ym) the position of an elementary cell of index m within said planar network of elementary cells;

[0035] the predetermined analog control phase associated with each elementary cell of index m of said planar network of elementary cells;

[0036] the angular operator;

[0037] the illumination law between a source of index 5 said set of sources and an elementary cell of index m of said planar network of elementary cells;

[0038] - the digital processing module is suitable for determining, by sub-band, the digital control phase of each open-loop source using a model of said planar network of elementary cells;

[0039] - when said planar network of elementary cells is a transmitting network, to determine in open loop, by sub-band, the digital control phase of each source, the processing module uses a ray-traveling transmitter network model;

[0040] - said modeling of said planar network of elementary cells is obtained by preliminary measures;

[0041] - said modeling of said planar network of elementary cells is obtained by the HFSS three-dimensional electromagnetic simulation tool;

[0042] - the signal specific to be emitted by each source is an OFDM signal;

[0043] - the digital processing module is suitable for applying a first scheme of modulation and coding to a predetermined center band of said signal proper to be emitted by each source, and to apply to the distinct outer sub-bands of said center band, a second modulation and coding scheme, distinct from said first modulation and coding scheme;

[0044] - said second modulation and coding scheme is weaker than said first modulation and coding scheme;

[0045] - the digital processing module is designed to implement beforehand a channel estimation and feedback from a receiver of a signal, received in the far field and emitted by said antenna system, to determine the illumination law between a source of index 5 of said set of sources and an elementary cell of index m of said planar array of elementary cells.

[0046] The invention also relates to a beamforming method implemented by an antenna system as described above,

[0047] said antenna system comprising:

[0048] - a set of Ns pairwise spatially distinct source Ns, with Ns>l, each source being capable of emitting a signal decomposed into a plurality of sub-bands distributed on either side of a central band centered on a central frequency of said signal;

[0049] - a planar network of elementary cells,

[0050] each source being capable of illuminating, at least in part, said planar network of elementary cells,

[0051] each elementary cell comprising at least:

[0052] - a phase-shifting element capable of shifting the phase, according to a predetermined phase shift, said signal,

[0053] - a passive transmitter, connected to the output of said phase-shifting element, and suitable for transmit the phase-shifted signal provided by said phase-shifting element;

[0054] the method comprising:

[0055] - a digital processing, implemented upstream of said set of Ns sources, by a digital processing module of said antenna system, said digital processing comprising at least the determination, by sub-band, of a digital control phase of each source and the digital compensation, via said digital control phase, of both:

[0056] - of the sum, previously calculated, of the phase shifts that are likely to be generated between each source and output of each elementary cell; and

[0057] - of the sum, previously calculated, of the differences in paths specific to be generated by each phase-shifting element; and

[0058] - an analog beamforming process via said planar grating of elementary cells illuminated by said set of Ns sources, successively said digital processing.

[0059] According to other advantageous aspects of the invention, the beam forming process comprises one or more of the following features, taken individually or in all technically possible combinations:

[0060] - said digital processing further comprises a preliminary modeling step said network of elementary cells, said modeling belonging to the group comprising at least:

[0061] - a model using a ray-casting transmitter network model;

[0062] - a model obtained by prior measurements;

[0063] - a model obtained using the electromagnetic simulation tool three-dimensional HFSS;

[0064] - said digital processing further comprises a preliminary estimation step of channel and information retrieval from a receiver of a signal, received in the far field and emitted by said antenna system, to determine the illumination law between a source of index 5 of said set of sources and an elementary cell of index m of said network of elementary cells;

[0065] - said digital processing further comprises a step of applying a first modulation and coding scheme to a predetermined center band of said signal proper to be emitted by each source, and to apply to the distinct outer sub-bands of said center band, a second modulation and coding scheme, distinct from said first modulation and coding scheme.

[0066] The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement the digital processing of the beamforming process as defined above.

[0067] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0068] [Fig-1] [Fig.1] is a schematic representation of an antenna system according to an example of the present invention;

[0069] [Fig.2] [Fig.2] illustrates the phase compensation specific to be implemented via the digital processing proposed according to the present invention;

[0070] [Fig.3] [Fig.3] is a general flowchart of the beam formation process according to an embodiment of the present invention.

[0071] Fig. 1 first schematically illustrates a non-limiting example of an antenna system 10 according to the present invention.

[0072] According to the present invention, such an antenna system 10 comprises first of all a set 12 of Ns spatially distinct sources in pairs, with Ns>1, each source being capable of emitting a signal decomposed into a plurality of sub-bands, distributed on either side of a central band centered on a central frequency of said signal.

[0073] In the example of [Fig. 1], the set 12 of Ns sources is such that Ns = 4. In other words, the set of sources 12 comprises, according to this example, a first, a second, a third, and a fourth source that are spatially distinct in pairs. According to one embodiment, when, for example, the network of elementary cells is a transmitting network, each of the first, second, third, and fourth primary sources of said set 12 comprises a horn antenna.

[0074] The antenna system 10 further comprises a planar network 14 of elementary cells 16.

[0075] In the antenna system 10, the array of sources 12 is arranged on the side of a first face of said planar array 14 of elementary cells 16, and each source of the array 12 is suitable for illuminating, at least in part, said planar array 14 of elementary cells. For example, the first, second, third, and fourth primary sources are adapted to irradiate respectively the first, second, third, and fourth consecutive quadrants of the planar array 14.

[0076] As an optional complement, the signal to be emitted by each source is a signal by orthogonal frequency-division multiplexing.

[0077] Each elementary cell 16 of the planar network 14 comprises at least: a phase-shifting element (not shown) suitable for shifting the phase of said signal according to a predetermined phase shift, and a passive transmitter (not shown), connected to the output of said phase-shifting element, and suitable for transmitting the phase-shifted signal provided by said phase-shifting element.

[0078] According to a first embodiment, said planar array 14 of elementary cells 16 is a phased array antenna with transmission line control. A phased array antenna with transmission line control is understood to be the type of phased array antenna that includes both phased array antennas with delay line control and phased array antennas with constant phase shifters in the frequency band used.

[0079] According to a second embodiment, said planar array 14 of elementary cells 16 is a transmit array comprising a lens composed of a plurality of elementary cells 16, each elementary cell being a reconfigurable component. For such a transmit array, each elementary cell further comprises a passive receiver (not shown) suitable for receiving as input the signal emitted by each source of said array illuminating it, and each phase-shifting element is then connected to the output of said passive receiver. According to this second embodiment, each source of the set of Ns sources is then configured to illuminate all or part of the lens. For example, horn antennas are suitable for use in this role of focal source.

[0080] Furthermore, specifically according to the present invention, said antenna system 10 comprises a digital processing module 18, upstream of said set 12 of sources, configured to determine, by sub-band, a digital control phase of each source of said set 12, and to digitally compensate, via said digital control phase, both: the sum, previously calculated, of the phase shifts to be generated between each source and the output of each elementary cell 16, and the sum, previously calculated, of the path differences to be generated by each phase-shifting element.

[0081] As an optional complement, as detailed later in relation to Figure 2, when said network of M elementary cells is a transmitting network, by sub-band, the digital control phase of each source as [ f. ] is suitable to be expressed by the following equation:

[0082] «,[ / .] = (sin( ]

[0083] with:

[0084] f. the predetermined frequency associated with each sub-band of index i and the corresponding wavelength;

[0085] (^o the angles defining the direction of emission (i.e. the direction of the beam) predetermined in the far fields of said antenna system;

[0086] ym) the position of an elementary cell of index m within said network of elementary cells;

[0087] 0 the predetermined analog control phase associated with each cell elementary of said network of elementary cells;

[0088] the angular operator;

[0089] A^ / J the illumination law between a source of index 5 of said set of sources and an elementary cell of index m of said network of elementary cells.

[0090] Such an expression taking into account the law of illumination makes it possible to compensate for the fact that the illumination of a given source is stronger at the center than at the edges.

[0091] Fig. 1 also illustrates more precisely an example of the structure of the digital processing module 18.

[0092] According to this example, the digital processing module 18 includes first of all a tool 20 for determining, by sub-band, the digital control phase as of each source s.

[0093] As an optional complement, as represented in dotted lines, said digital processing module 18 includes a tool 22 for determining said illumination law A^ / J between a source of index 5 of said set of sources and an elementary cell of index m of said network of elementary cells.

[0094] To do this, according to a first variant, the tool 22 of the digital processing module 18 is suitable for determining, by sub-band, the digital control phase of each source ds [ / .], in open loop using a modeling of said elementary cell network.

[0095] More specifically, according to a first option of this first variant, when said network of elementary cells is a transmitting network, to determine in open loop, by sub-band, the digital control phase of each source, the tool 22 uses a ray-traveling transmitting network model.

[0096] According to a second option of this first variant, said modeling of said elementary cell network is obtained by prior measurements.

[0097] According to a third option of this first variant, said modeling of said elementary cell network is obtained by the three-dimensional electromagnetic simulation tool HFSS.

[0098] According to a second embodiment, the tool 22 of the digital processing module 18 is adapted to implement beforehand a channel estimation and a feedback from a receiver of a signal, received in the far field and emitted by said antenna system, to determine the illumination law between a source of index s said set of sources and an elementary cell of index m of said planar network of elementary cells.

[0099] According to another optional complement, as represented in dotted lines, said digital processing module 18 includes a tool 24 for applying MCS (Modulation and Coding Scheme) modulation and coding schemes suitable for applying a first modulation and coding scheme to a predetermined center band of said signal suitable for being emitted by each source, and for applying to the outer sub-bands distinct from said center band, a second modulation and coding scheme, distinct from said first modulation and coding scheme.

[0100] Optionally, according to this other optional addition, said second modulation and coding scheme is weaker than said first MCS modulation and coding scheme. This option makes said antenna system 10 more robust to the degraded signal-to-noise ratio on said extreme sub-bands. Indeed, the digital processing proposed according to the present invention is, by definition, designed to act more on the band edges. In other words, the antenna gains of the subcarriers at the band edges are reduced due to beam strabismus, so that the associated signal-to-noise ratios are also lower, and this option aims to make the link more robust to Gaussian noise by allowing the use of a weaker MCS modulation and coding scheme on these subcarriers.

[0101] In the example of [Fig.1], the digital processing module includes an information processing unit 26 formed for example of a memory 28 and a processor 30 associated with the memory 26.

[0102] In the example of Figure 1, the tool 20 for determining, by sub-band, the phase as of digital control of each source s, as well as, as an optional complement, the tool 22 for determining said illumination law Am^ / J between a source of index 5 of said set of sources and an elementary cell of index m of said network of elementary cells and the tool 24 for applying modulation and coding schemes, are each implemented in the form of software, or a software block, executable by the processor.The memory of the digital processing module is then capable of storing software for determining, by sub-band, the digital control phase of each source s, and optionally, software for determining the illumination law Aw£ / ;] between a source of index 5 in the source set and an elementary cell of index m in the elementary cell network, and software for applying modulation and coding schemes. The processor is then capable of executing each of the following software programs: the software for determining, by sub-band, the digital control phase of each source s, and optionally, the software for determining the illumination law. illumination between a source of index 5 said set of sources and an elementary cell of index m of said network of elementary cells and the application software for modulation and coding schemes.

[0103] In an alternative not shown, the tool 20 for determining, by sub-band, the digital control phase as of each source s, as well as, optionally, the tool 22 for determining said illumination law A^ / J between a source of index 5 of said set of sources and an elementary cell of index m of said network of elementary cells, and the tool 24 for applying modulation and coding schemes, are each implemented in the form of a programmable logic component, such as an FPGA (Field Programmable Gate Array), or an integrated circuit, such as an ASIC (Application-Specific Integrated Circuit).

[0104] When the digital processing module is implemented in the form of one or more software programs, that is, in the form of a computer program, also called a computer program product, it is further capable of being stored on a computer-readable medium, not shown. The computer-readable medium is, for example, a medium capable of storing electronic instructions and being connected to a bus of a computer system. By way of example, the readable medium is an optical disc, a magneto-optical disc, ROM, RAM, any type of non-volatile memory (for example, FLASH or NVRAM), or a magnetic card. A computer program comprising software instructions is then stored on the readable medium.

[0105] Figure 2 illustrates more precisely the operation of an antenna system 10 according to the present invention, in particular when said network of elementary cells is a transmit-array.

[0106] In transmission mode, a baseband signal is generated by a digital modem and transmitted to each of the Ns radio frequency chains respectively associated with each of the Ns source 12.

[0107] Each radio frequency chain is designed to implement successively, in particular, a digital-to-analog conversion, an up-conversion to the carrier frequency, a bandpass filtering, etc.

[0108] The Ns focal sources 12 illuminate the planar network 14 of elementary receiving cells 16 suitable for processing the signal received from said sources, via their respective phase-shifting element, and then re-emitting it, via their passive transmitter, to a remote receiver 32.

[0109] By correctly adjusting the phase shift applied at the level of each elementary cell 16, the wave radiated by the focal sources can thus be transformed into a quasi-plane wave radiated in the direction of interest defined by the angles (G, ¢).

[0110] The phase shifters of each elementary cell also form a network, and according to the example of [Fig.2] the phase shifter elements are each electronically reconfigurable by means of bias lines (from the English "bias Unes").

[0111] As indicated previously, according to a particular variant, a frequency multiplexing technique, such as orthogonal frequency-division multiplexing, known as OFDM (Orthogonal frequency-division multiplexing), is for example used to modulate the signal thus decomposed into a plurality of sub-bands of index i distributed on either side of a central band centered on a center frequency of said signal, and processed digitally according to the present invention by sub-band.

[0112] In other words, we subsequently consider a spectral decomposition of the signal into Nfsous-bands, each associated with a frequency f and corresponding wavelength 2,- such that i belongs to the interval [0 ; Nrl].

[0113] The gain factor AF of such a network of M elementary cells, with M an integer, such that M>1, is suitable to be expressed in the following form:

[0114] 9, «i] = LaIAF-'' ^(sinftfkos^^+sint^^ (1)

[0115] with Am GC the excitation, in amplitude and in phase received by the elementary cell 16 of index m, ym) the position of an elementary cell 16 of index m within said planar network of elementary cells; and fi the predetermined analog control phase associated with each elementary cell of index m of said network of elementary cells.

[0116] Note that the analog control phase is obtained beforehand in a manner known in itself and calculated at the center frequency to achieve beam formation in a known and desired direction.

[0117] And when, as implemented according to the present invention, several focal sources are involved, the excitation received at the level of each elementary cell Am can be decomposed as the sum of the excitations from each focal source: [oiisi 4,4 ]=

[0119] where Ams represents the illumination law (i.e. excitation law) between the focal source 5 and the elementary cell of index m, and as[ / .] is the digital control phase of each source 5 suitable to be determined and applied (in the digital domain) as specifically proposed according to the present invention.

[0120] The two preceding equations (1) and (2) illustrate the fact that the transmitted signal can be decomposed into Ns x M radio links up to the remote receiver 32 of the [Fig.2],

[0121] The phase associated with each of these bonds depends on four elements illustrated in [Fig.2], namely:

[0122] - the digital control phase of each source 5, illustrated in Figure 2, by the term ^“441 for each source of index 5 from the set 12 of Ns sources;

[0123] - the illumination law (i.e., excitation law) A»^ associated with each source 5 specific to illustrate the phase shift occurring during propagation between the focal source 5 and the elementary cell 16 of index m, illustrated in figure 2, by the term g1;

[0124] - fi the predetermined analog control phase, illustrated in Figure 2, by the term gJ^m;

[0125] - the far-field phase, illustrated in Figure 2, by the term corresponding to the phase shift occurring during propagation between the elementary (i.e. unitary) cell of index m and the receiver 32 placed in the far field direction defined by the angles (Oq, (|>o).

[0126] Consequently, the following properties are admitted according to the present invention:

[0127] - the illumination laws Amj are frequency-dependent in phase and in amplitude;

[0128] - the AF network gain factor is consequently also dependent on the frequency which leads to a broadband induced effect such as beam strabismus.

[0129] - fi the predetermined analog control phase is assumed to have a frequency frequency flaf (from the English frequency flaf) according to an implementation constraint at the level of the phase-shifting elements;

[0130] - the digital control phase °'s is also dependent on the frequency, in particular to implement digital compensation applied to an OFDM signal.

[0131] As previously indicated, the digital processing module providing, by sub-band, the digital control phase of each source, and the planar network of elementary cells, providing at the level of each elementary cell, a predetermined analog control phase, form in synergy within said antenna system a "hybrid" two-stage pre-coder, the two-stage being suitable for implementing in a hybrid manner complementary digital and analog processing.

[0132] The design of such a pre-encoder first requires the determination of the predetermined analog control phases fi (i.e., the phase coefficients in the analog domain). More precisely, depending on the analog processing of the pre-encoder, in a manner known per se, to orient the beam in the far-field direction Defined by the angles (Ôq, ¢$), in a manner known per se, the analog control phases must be defined as follows: [° 133] = -Ç(sin(0 o )cos(^ o )x OT + sin(J? o )sin(^ o )j OT )-

[0154] with Z the angular operator, and f the carrier frequency of the signal (from the English carrier frequency, i.e. the center frequency of the signal band), in particular of the OFDM signal, and the corresponding wavelength.

[0155] In addition, according to the digital processing of the precoder, as indicated previously in relation to Figure 1, the digital control phase of each source determined by the digital processing module 18, can be expressed, by sub-band, by the following equation: [° 136 ] «,[ / .] = -

[0157] with:

[0138] f. the predetermined frequency associated with each sub-band of index i eM, the corresponding wavelength;

[0159] 0Q) the angles defining the emission direction (i.e. the beam direction) predetermined in far fields of said antenna system towards a receiving device 32;

[0140] the angular operator;

[0141] the illumination law between a source of index 5 said set of sources and an elementary cell of index m of said network of elementary cells.

[0142] In other words, the digital control phases of each source aim to compensate for the phase shifts transmitted by each focal source and for each frequency f of the subcarriers distributed on either side of the center frequency fc of the signal, in particular an OFDM signal. This makes it possible to limit beam strabismus by digitally acting on the beams associated with frequency tones sufficiently far from the center frequency, which without this digital processing would be more misaligned and would point in undesired directions. [OI43] Indeed, it should be noted that according to the present invention, for each source, (i.e. #s) «s[ fc] = 0 - In other words, for each source, the digital control phase associated with the center frequency of the signal is zero, and the further the sub-band centered on the frequency f. is from the center band centered on the frequency fc, the more pronounced the strabismus is and the more it is corrected according to the present invention, the strabismus evolving in f; / fc, whereas on the contrary when the frequency f; approaches the center frequency fc then °s tends towards zero.

[0144] Furthermore, it should be noted that according to the present invention, equality is considered next: qv[ / ] = «s[ / l which is equivalent to the fact that on the one hand ]= ,•[ / ], that is to say, the same excitation (i.e., illumination) of the lens is implemented by two sources s and s' distinct in pairs, and on the other hand that the direction of the beam is such that: (00, 0()) = (0,0), that is to say at an angle perpendicular to the plane of the planar array of elementary cells (from the English broadside steering), this last condition is according to the present invention assumed to be satisfied by construction because most often the focal sources are equivalent and placed on a uniform grid.

[0145] Thus, according to an embodiment where the sources are placed on a uniform grid (ensuring the same excitation of the lens by each of its sources), for a beam transmitted in this direction (¾ ^0) = (0,0) (i.e., broadside beam) «?[ / ] = «4 / 1 #f (i.e., no performance gain). In other words, no performance gain can be obtained on the central beam.

[0146] It should be noted that when, according to the first variant, said planar network 14 of elementary cells 16 is a phased array antenna with transmission line control (from the English phased array) the phase shifts which are generated between each source and each elementary cell are equal.

[0147] An example of a general embodiment of the operation of the antenna system 10 of [Fig.1] is described subsequently in relation to [Fig.3].

[0148] More specifically, the beamforming method 40 implemented by such an antenna system 10 first comprises a digital processing phase 42 (42), implemented upstream of said set of Ns sources, by a digital processing module of said antenna system, said digital processing comprising at least one D_C determination step 44, by sub-band, of a digital control phase of each source and digital compensation, via said digital control phase, both:

[0149] - of the sum, previously calculated, of the phase shifts that are likely to be generated between each source and output of each elementary cell; and

[0150] - of the sum, previously calculated, of the differences in paths specific to be generated by each phase-shifting element.

[0151] The process 40 also includes an analog processing phase 46 of beam formation via said planar array of elementary cells illuminated by said set of Ns sources, said analog processing 46 being successive to said digital processing 42.

[0152] As an optional addition (represented by dashed lines), said phase 42 of digital processing further comprises a preliminary step 48 of modeling M of said network

[0153]

[0154]

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[0162] of elementary cells, said modeling belonging to the group comprising at least: - a modeling using a ray-casting transmitter network model; - a model obtained through prior measurements; - a model obtained using the HFSS three-dimensional electromagnetic simulation tool. For example, when a model using a ray-tracing transmitter array model is used, the directivity Ds of the focal sources can be modeled by the following expression: Ds( — UqCO8p( WnKS) with P the order of the source (the directivity of the sources being expressed by raised cosines, the "order" of the source corresponding to the exponent of the cosine) and also y — + 1) un real coefficient guaranteeing that the power radiated in a half-space (the radiation half-space towards the front of the antenna) is unity, and 'hu the angle the elevation relative to the central axis of the focal source svers the elementary cell m relative to the central axis of the source considered. Such a directivity model is suitable for modeling horn antennas (from the English hom antennas) with an order ranging from 4 (10 dBi) to 49 (20 dBi). The directivity Du of each elementary antenna cell (in reception rx and in transmission tx, considering that ^ms = 0q) can also be expressed via the following expression: , . 4 / tAp / ii, / \ with . ( toc / -the surface of the antenna. = Aphy = (“J So, according to this modeling, A^ / J the illumination law between a source of index 5 of said set of sources and an elementary receiving cell of index m of said network of elementary cells, necessary for the determination 42, by band associated with a subcarrier frequency f., of the control phase is suitable to be modeled by a simple direct link (from the English line-of-sight): )c with the angel the elevation relative to the axis central of the focal source, and dmj the distance between the focal source s and the elementary cell of index m. Such modeling therefore makes it possible to determine the illumination law between a source of index 5 of said set of sources and an elementary receiving cell of index m, in open loop, by sub-band, and consequently, the digital control phase of each source as[ / ;], using such a modeling of said planar network of elementary cells.

[0163] In terms of results, a 3dB bandwidth evaluation was implemented, this 3dB bandwidth corresponding to the width of the frequency range, for example centered at a center frequency fc such that f = 140 GHz, providing a beam gain greater than or equal to half the maximum gain obtained for f = fc, the focal length being fixed to optimize the broadside antenna gain, in particular according to the technique indicated in the 2012 article entitled "Multiple feed transmit-array antennas with reduced focal distance" by A. Clemente et al. More precisely, a four-source transmit array, operating at 140 GHz and composed of a lens of 140x140 elementary cells (i.e., an array size of 107 mm per side) is capable of providing a gain of 49dB on the center beam considering a focal length of 47mm.According to these results, the 3dB bandwidth is evaluated as a function of the beamwidth, and the present invention logically does not provide any improvement on the central beam compared to the single-beam multi-source structure proposed in the field of transmit-array radio antennas as described in patent application EP 3 159 965 A1 filed by the Applicant, since, as previously stated, the digital control phase αs for the center frequency f and for each source is zero. However, as the beamwidth increases, the effective bandwidth gain increases, thanks to the digital processing implemented according to the present invention, in synergy with the analog processing associated with a multi-source structure.For example, the beamwidth providing 10% bandwidth is doubled thanks to the hybrid precoding, i.e., digital / analog, implemented according to the present invention. In other words, the effects of beam misalignment are thus limited, because if the 3dB bandwidth increases, the misalignment decreases.

[0164] Conversely, when the preliminary modeling step 48 M is not implemented, open-loop determination is not possible, and in this case the digital processing 42 further includes a prior channel estimation step 50 and a feedback from a receiver of a signal, received in the far field and emitted by said antenna system, to determine the illumination law between a source of index 5 of said set of sources and an elementary cell of index m of said planar array of elementary cells.

[0165] As an optional addition, the digital processing 42 further comprises a step 52 of applying a first modulation and coding scheme to a predetermined center band of said signal specific to be emitted by each source, and of applying to the distinct outer sub-bands of said center band, a second scheme of modulation and coding, distinct from the aforementioned first modulation and coding scheme. This option makes the said antenna system 10 more robust to the degraded signal-to-noise ratio on the said extreme sub-bands.

[0166] A person skilled in the art will understand that the invention is not limited to the embodiments described, nor to the particular examples of the description, the embodiments and variants mentioned above being capable of being combined with each other to generate new embodiments of the invention.

[0167] The present invention thus makes it possible to provide an antenna system with a hybrid two-stage precoder suitable for implementing digital frequency processing (i.e. by sub-band) and by source to supplement the classic analog processing, such digital processing being moreover dependent on said analog processing, because it is complementary.

[0168] Such a precoder is suitable for adapting to all phased networks, namely a phase-controlled array antenna by transmission line (waveguides for example) or a transmit-array network, and this at any frequency.

[0169] The proposed solution involves both frequency compensation and spatial compensation via a plurality of distributed focal sources. It should be noted that the greater the number of sources, the better the compensation according to the present invention. Indeed, the more sources there are, the more digital phases there are, and therefore the more degrees of freedom there are to compensate for, and the more sources there are, the better the spatial resolution for correcting phase shifts.

Claims

Demands

1. Antenna system (10) comprising: - an array (12) of Ns spatially distinct pairwise sources, with Ns>1, each source being capable of emitting a signal decomposed into a plurality of sub-bands distributed on either side of a central band centered on a center frequency of said signal; - a planar array (14) of elementary cells (16), each source being capable of illuminating, at least in part, said planar array of elementary cells, each elementary cell (16) comprising at least: - a phase-shifting element capable of shifting the phase of said signal according to a predetermined phase shift, - a passive transmitter, connected to the output of said phase-shifting element, and capable of transmitting the phase-shifted signal provided by said phase-shifting element;said antenna system (10) being characterized in that it comprises a digital processing module (18), upstream of said set of sources, configured to determine, by sub-band, a digital control phase of each source, and to digitally compensate, via said digital control phase, both: - the sum, previously calculated, of the phase shifts to be generated between each source and the output of each elementary cell; - the sum, previously calculated, of the path differences to be generated by each phase-shifting element.

2. Antenna system (10) according to claim 1, wherein said planar array (14) of elementary cells (16) is a transmission line phase-controlled antenna array, or a transmitting array comprising a lens composed of a plurality of elementary cells, each elementary cell being a reconfigurable component.

3. Antenna system (10) according to claim 2, wherein, when said planar array (14) of elementary cells (16) is a transmitting array, by sub-band, the digital control phase of each source «sf / J is suitable to be expressed by the following equation: with: f. the predetermined frequency associated with each sub-band of index i and the corresponding wavelength; (^()) the angles defining the predetermined far-field direction of the beam to be emitted by said antenna system; (x»t, ym) the position of an elementary cell of index m within said planar array of elementary cells; 0 the predetermined analog control phase associated with each elementary cell of index m of said planar array of elementary cells; 21 the angular operator; Ah.s[ / ;] the illumination law between a source of index 5 of said set of sources and an elementary cell of index m of said planar array of elementary cells.

4. Antenna system (10) according to any one of the preceding claims, wherein the digital processing module (18) is suitable for determining, by sub-band, the digital control phase of each source, in open loop using a model of said planar network of elementary cells.

5. Antenna system (10) according to claim 4, wherein, when said planar array of elementary cells is a transmitting array, to determine in open loop, by sub-band, the digital control phase of each source as[ / f], the processing module uses a ray-tracing transmitting array model.

6. Antenna system (10) according to claim 4, wherein said modeling of said planar array of elementary cells is obtained by prior measurements.

7. Antenna system (10) according to claim 4, wherein said modeling of said planar array of elementary cells is obtained by the HFSS three-dimensional electromagnetic simulation tool.

8. Antenna system (10) according to any one of the preceding claims wherein the signal to be emitted by each source is an OFDM signal.

9. Antenna system (10) according to any one of the preceding claims, wherein the digital processing module is adapted to apply a first modulation and coding scheme to a predetermined center band of said signal adapted to be emitted by each source, and to apply to the outer sub-bands distinct from said center band, a second modulation and coding scheme, distinct from said first modulation and coding scheme.

10. Antenna system (10) according to claim 9, wherein said second modulation and coding scheme is weaker than said first modulation and coding scheme.

11. Antenna system (10) according to any one of the preceding claims 1 to 3, wherein the digital processing module is adapted to implement in advance a channel estimation and a feedback from a receiver of a signal, received in the far field and emitted by said antenna system, to determine the illumination law between a source of index 5 of said set of sources and an elementary cell of index m of said planar array of elementary cells.

12. A beamforming method (40) implemented by an antenna system (10) according to any one of the preceding claims, said antenna system (10) comprising: - an array (12) of Ns spatially distinct pairwise sources, with Ns > 1, each source being capable of emitting a signal decomposed into a plurality of sub-bands distributed on either side of a central band centered on a center frequency of said signal; - a planar array (14) of elementary cells (16), each source being capable of illuminating, at least in part, said planar array of elementary cells, each elementary cell (16) comprising at least: - a phase-shifting element capable of shifting said signal by a predetermined phase shift, - a passive transmitter, connected to the output of said phase-shifting element, and capable of transmitting the phase-shifted signal provided by said phase-shifting element; the method (40) comprising: - a digital processing (42), implemented upstream of said set of Ns sources, by a digital processing module of said antenna system, said digital processing comprising at least the determination (44), by sub-band, of a digital control phase of each source and the digital compensation, via said digital control phase, of both: - the sum, previously calculated, of the phase shifts to be generated between each source and the output of each elementary cell; and - the sum, previously calculated, of the path differences to be generated by each phase-shifting element; and - an analog beamforming processing (46) via said planar array of elementary cells illuminated by said set of Ns sources, subsequent to said digital processing.

13. Method (40) according to claim 12, wherein said digital processing (42) further comprises a preliminary modeling step (48) of said network of elementary cells, said modeling belonging to the group comprising at least: - a modeling using a ray-tracing transmitter network model; - a modeling obtained by preliminary measurements; - a modeling obtained by the HFSS three-dimensional electromagnetic simulation tool.

14. A method (40) according to claim 12, wherein said digital processing (42) further comprises a preliminary channel estimation step (50) and feedback from a receiver of a signal, received in the far field and emitted by said antenna system, to determine the illumination law between a source of index 5 of said source array and an elementary cell of index m of said elementary cell array

15. A method (40) according to any one of claims 12 to 14, wherein said digital processing (42) further comprises a step (52) of applying a first modulation and coding scheme to a predetermined center band of said signal suitable for emission by each source, and of applying to the distinct outer sub-bands of said center band, a second modulation and coding scheme, distinct from said first modulation and coding scheme.

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