Transmitter, receiver, telecommunications system and associated process

Abstract

Transmitter, receiver, telecommunications system and associated method. The present invention relates to a transmitter (4), adapted given xi + jxq, respectively yi + jyq, a complex symbol from a first, respectively second, data sequence adapted to compose a first, respectively second, symbol to be transmitted in one of the following ways: - the first symbol is xi + jyi and the second symbol is xq + jyq; or - the first symbol is yi + jxq and the second symbol is xi + jyq; or - the first symbol is yq + jxq and the second symbol is yi + jxi; or - the first symbol is xi + jyq and the second symbol is yi + jxq. Figure for the abstract: Figure 2

Description

Title of the invention: Transmitter, receiver, telecommunications system and associated method

[0001] The present invention relates to a transmitter, a receiver, a telecommunications system, and associated methods.

[0002] The development of new telecommunication services requiring high bandwidth, competition from terrestrial networks with the deployment of 400 Gbit / s technology and beyond, as well as the desire to reduce the digital divide by enabling every citizen, wherever they may be, to benefit from the same quality of service, have caused a considerable increase in the transmission capacity needs of satellite operators, requiring the deployment of additional systems.

[0003] Faced with such a demand for capacity, optical technologies offer an alternative to traditional radio frequency (RF) technologies for very high-speed data transmission. In particular, free-space optical communications between the satellite and the ground are a promising solution for the next generation of very high-speed satellites.

[0004] However, the satellite optical channel has several drawbacks. In particular, significant signal degradation is observed due to the composition of atmospheric layers and turbulence. These phenomena cause deep fading, thus interrupting transmission between a transmitter and a receiver for several milliseconds.

[0005] To compensate for this problem, the transmitter is equipped with very high power optical amplifiers (HPOAs) in order to amplify the optical signal before transmission in free space.

[0006] Furthermore, the transmitter uses two polarizations per wavelength to increase spectral efficiency: for a given wavelength, the signal thus comprises two components, one with a polarization axis X and the other with a polarization axis Y, distinct from each other. For short, these components are commonly referred to as the X and Y polarizations of the signal. In this case, due to various technological (technological maturity, spatialization of solutions, etc.) and practical (high amplification gain, optical imperfections, etc.) constraints, the two components of the same wavelength and distinct polarization that make up the optical signal at the considered wavelength—i.e., the two X and Y polarizations—are amplified by two different HPOAs in the transmitter.However, due in particular to technological constraints, inaccuracies in reproducing components, disparities and imperfections in components, and the environment (spatial, etc. (example) etc., it is difficult to obtain the same amplification gain at all times for the two HPOAs contributing to the transmission of the optical signal, which induces a difference in power (and therefore a difference in performance) between the two X, Y polarizations of the final emitted optical signal. Furthermore, this power gain is not predictable and can also vary over time.

[0007] More generally, in the overall architecture (primarily at the optical front end of the transmitter and receiver), the optical signal passes through several pieces of equipment that do not guarantee perfect isolation between the two components: this is how rotations of the polarization axes are observed at different points, which has the effect of creating interference between the two components. For example, optical fibers, polarization beam splitters (PBS) and polarization beam combines (PBC) used in fiber optic telecommunications systems can also cause disparities in the amplitude of the two signal components, and reduce the transmission quality.

[0008] In the context of a coherent transmission, we seek to resolve this mixing of components, at the reception level of the digital signal processing unit, typically in the equalization stage.

[0009] Even if this equalization provides an improvement, the less amplified component will always exhibit degraded performance compared to the other. Thus, for a given signal-to-noise ratio (total optical signal power of the two components relative to the total noise power), an imbalance in quality is observed compared to the ideal case where both amplifiers amplify with the same gain: the less amplified bias will exhibit degraded performance compared to the ideal case. This is detrimental from a system perspective because the link budget is established based on the lower-performing bias.

[0010] There are different types of techniques to compensate for this problem, for example:

[0011] - signal processing algorithms such as the transformation in the space of Stokes, for example: Stokes space is a mathematical representation of the polarization state of an electromagnetic wave; in Stokes space, the polarization state of a wave is represented by a point in a four-dimensional space, where each dimension corresponds to a different polarization parameter; using Stokes coordinates, a matrix transformation is defined, which equalizes the power on the two polarizations;

[0012] - Polarization multiplexing and / or pre-coding: application of pre-coding while distributing the two data streams across the polarizations.

[0013] These techniques have the disadvantage of adding processing complexity and significantly increasing memory requirements for both transmitting and receiving processes (especially at very high data rates). Furthermore, Stokes transform-type algorithms only offer significant gains at very high signal-to-noise ratios, which is not the case for space communications where signal-to-noise ratios are very low due to the large distance between the ground station and the satellite.

[0014] One object of the invention is in particular to improve the transmission of digital information via an optical signal comprising two polarizations, in a simple manner and by limiting the electrical power consumed.

[0015] More generally, the aim of the invention is to improve the transmission of digital information between a transmitter and a receiver via two transmission chains through a telecommunication signal comprising two components, in a simple manner and by limiting the electrical power consumed.

[0016] To this end, according to a first aspect, the invention relates to a transmitter adapted to transmit, to a receiver, two sequences of digital input data comprising a first sequence and a second sequence, using two separate transmission chains comprising a first chain and a second transmission chain, the transmitter comprising:

[0017] a transformation block adapted to transform the numerical data of the first sequence, respectively of the second sequence, into complex symbols each comprising an imaginary part and a real part;

[0018] a first transmission processing block adapted to receive as input first complex symbols to be transmitted via the first transmission chain and to implement the transmission of said first complex symbols via said first transmission chain to the receiver;

[0019] a second transmitting processing block adapted to receive as input second complex symbols to be transmitted via the second transmission chain and to implement the transmission of said second complex symbols via said second transmission chain to the receiver;

[0020] Given x; + jxq a complex symbol resulting from the transformation, by the transformation block, of the first sequence and given y; + jyq a complex symbol resulting from the transformation, by the transformation block, of the second sequence,

[0021] said transmitter is characterized in that it is adapted to compose the first complex symbols and second complex symbols to be transmitted by combining the symbols resulting from the first sequence and the second sequence in one of the following ways:

[0022] - the first symbol is x; + jy; and the second symbol is xq + jyq; or

[0023] - the first symbol is y; + jxq and the second symbol is x; + jyq; or

[0024] - the first symbol is yq + jxq and the second symbol is y; + jx;; or

[0025] - the first symbol is x; + jyq and the second symbol is y; + jxq

[0026] The invention thus makes it possible to reduce the impact on the quality of the transmission, Disparities can arise between the two transmission chains, for example, between two polarizations of the optical signal. This method solves the problem simply by exchanging, at both the transmission and reception points, an I or Q component of one of the two signals to be transmitted on one of the transmission chains (for example, the transmission chain corresponding to the X-polarization optical channel) with the I or Q component of the other two signals to be transmitted on the other transmission chain (for example, the transmission chain corresponding to the Y-polarization optical channel). Here, I is the in-phase component, and Q is the quadrature component, of a signal, for example, one with complex modulation on two polarizations (DP-QPSK). Thus, the information to be transmitted is distributed in a balanced way between the two polarizations, which are subjected to different channels (and Signal-to-Noise Ratios, or SNR).

[0027] In embodiments, the transmitter comprises one or more of the following features, taken individually or in all technically possible combinations: - the first transmission chain corresponds to the selective transmission of a polarization component along only one of the distinct axes X, Y of an optical signal at a given wavelength X and the second transmission chain corresponds to the selective transmission of a polarization component along only the other of the axes X, Y of said optical signal at the wavelength X; - The transmitter is adapted to modulate the polarization component of the optical signal along the X-axis according to the first complex symbols and to modulate the polarization component of the optical signal along the Y-axis according to the second complex symbols,

[0028] the first transmission chain comprising a first optical amplifier of gain G1 adapted to selectively amplify the polarization component of the optical signal along the X-axis,

[0029] the second transmission chain comprising a second optical amplifier with gain G2 distinct from G1 and adapted to selectively amplify the polarization component of the optical signal along the Y axis.

[0030] The invention also relates to a receiver adapted to receive the signal emitted by the transmitter according to the first aspect of the invention and to convert it into digital output data.

[0031] In embodiments, the receiver comprises two receiving processing blocks and is adapted to determine, in the first receiving processing block, first complex symbols as a function of the received signal and to determine, in the second receiving processing block, second complex symbols as a function of the received signal, the first and second complex symbols each comprising an imaginary part and a real part;

[0032] said receiver being adapted to compose complex symbols corresponding to a first sequence and complex symbols corresponding to a second sequence, by combining the first and second complex symbols in a manner inverse to the combination carried out in the transmitter;

[0033] said receiver further comprising a transformation block adapted to transform the complex symbols corresponding to the first sequence, respectively corresponding to the second sequence, into a first sequence of digital data, respectively into a second sequence of digital data.

[0034] The invention also relates to a transmission system comprising such a receiver and transmitter according to the first aspect of the invention.

[0035] The invention also relates to a method of transmitting information for transmitting, to a receiver, two sequences of digital input data comprising a first sequence and a second sequence, using two separate transmission chains comprising a first chain and a second transmission chain, said method comprising the following steps implemented by the transmitter:

[0036] - transform the numerical data of the first sequence, respectively of the second sequence, in complex symbols each comprising an imaginary part and a real part;

[0037] - receive as input to a first processing block emitting the first complex symbols to be transmitted via the first transmission chain and to implement the transmission of said first complex symbols via said first transmission chain to the receiver;

[0038] - receive as input to a second processing block emitting second complex symbols to be transmitted via the second transmission chain and to implement the transmission of said second complex symbols via said second transmission chain to the receiver;

[0039] according to which, given x; + jxq a complex symbol resulting from the transformation of the first sequence and given y; + jyq a complex symbol resulting from the transformation of the second sequence

[0040] Said method is characterized in that it is adapted to compose the first complex symbols to be transmitted and second complex symbols to be transmitted by combining the symbols from the first sequence and the second sequence in one of the following ways:

[0041] - the first symbol is x; + jy; and the second symbol is xq + jyq; or

[0042] - the first symbol is y; + jxq and the second symbol is x; + jyq; or

[0043] - the first symbol is yq + jxq and the second symbol is y; + jx;; or

[0044] - the first symbol is x; + jyq and the second symbol is y; + jxq.

[0045] In embodiments, this method comprises one or more of the following features, taken individually or in all technically possible combinations: - the first transmission chain corresponds to the selective transmission of a polarization component along only one of the distinct axes X, Y of an optical signal at a given wavelength X and the second transmission chain corresponds to the selective transmission of a polarization component along only the other of the axes X, Y of said optical signal at the wavelength X; - the process comprising at least the following steps implemented by a receiver: reception of the signal emitted by the transmitter and conversion into digital output data; - the process comprising at least the following steps implemented by the receiver:

[0046] determine, in the first receiving processing block, the first complex symbols as a function of the received signal; and

[0047] determine, in the second receiving processing block, second complex symbols as a function of the received signal, the first and second complex symbols each comprising an imaginary part and a real part;

[0048] compose complex symbols corresponding to a first sequence and complex symbols corresponding to a second sequence, by combining the first and second complex symbols in a manner inverse to the combination carried out in the transmitter;

[0049] transform the complex symbols corresponding to the first sequence, respectively corresponding to the second sequence, into a first sequence of numerical data, respectively into a second sequence of numerical data.

[0050] 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: - [Fig. 1] [Fig. 1] is a representation of a transmission system in one embodiment of the invention, - [Fig.2] [Fig.2] is a block diagram of the transmission system of the [Fig.l], - [Fig.3] [Fig.3] is a diagram of part of the transmission system the [Fig.2], - [Fig.4] [Fig.4] is a logic diagram of a transmission process information in an embodiment of the invention, the method being implemented by the transmission system of the [Fig.1] - [Fig. 5] [Fig. 5] is a diagram of part of the transmission system [Fig. 2] in one embodiment of the invention, - [Fig. 6] [Fig. 6] is a diagram of part of the receiver of the system transmission of the [Fig.2].

[0051] Fig. 1 represents a transmission system 1 in one embodiment of the invention.

[0052] The transmission system 1 comprises a transmitter 4 and a receiver 6.

[0053] In the example considered, the transmitter 4 is located on the ground, for example in a telecommunications station and the receiver 6 is in an embodiment carried on board an aircraft or a satellite.

[0054] Alternatively, the transmitter 4 is carried on an aircraft or satellite and the receiver 6 is on the ground. Alternatively still, the transmitter 4 and the receiver 6 are both on the ground or carried on satellite-type platforms or other platforms.

[0055] As shown in [Fig. 2], the transmitter 4 includes, in one embodiment, a digital processing module 12. The digital processing module 12 is, for example, implemented as a programmable logic component, such as an FPGA (Field Programmable Gate Array), or as an integrated circuit, such as an ASIC (Application-Specific Integrated Circuit). In an alternative (not shown) variant, the digital processing module 12 is implemented as software, stored in memory and executable by a processor associated with the memory. Similarly, in another alternative (not shown) variant, the digital processing module 12 is implemented using optical analog components.

[0056] The transmitter 4 further comprises an optical modulator 14, the input of which is supplied by the output of the digital processing module 12. The optical modulator 14 is, for example, a dual-polarization Mach-Zehnder interferometer, comprising a laser source (one polarization along an X-axis, one polarization along an axis Y). In the embodiment considered, these polarization axes are, for example, orthogonal to each other.

[0057] The transmitter 4 further includes an amplifier module 18, connected to the optical modulator 14.

[0058] With reference to [Fig. 3], the amplifier module 18 includes a polarization beam splitter 20, also called a PBS. Advantageously, the polarization beam splitter 20 is a passive optical device. The polarization beam splitter 20 is configured to divide an optical signal into two components, one component corresponding to the first polarization X, the other component to the second polarization Y,

[0059] The amplifier module 18 comprises two optical amplifiers 21 and 22. The optical amplifiers 21 and 22 are in an embodiment of very high-power optical amplifiers. The optical amplifier 21, respectively 22, is configured to amplify an optical signal passing through it with a gain G1 and a gain G2, respectively. The gains G1 and G2 are predefined and chosen by the manufacturer of the amplifier module 18. In theory, the gains G1 and G2 are chosen to be equal. However, due, for example, to imperfections in the optical amplifiers 21 and 22, material constraints, or the spatial environment, in practice, the gains G1 and G2 are different and can vary over time. This results in biases with different powers, and therefore different performance. From a system and quality of service perspective, this is undesirable.Here, PDL (polarization dependent loss) refers to the quality disparity between the two polarizations, due to the difference in instantaneous amplification gain between the two HPOAs.

[0060] Figure 3 illustrates the effects of gains on the X and Y polarizations during transmission. Rotations (symbolically represented in Figure 3 by arrow F) of random polarization are introduced by the various optical components used in the transmitter and receiver (e.g., single-mode fiber optic cable (SMF), front-end FSO Tx and Rx, etc.) and by the propagation channel 44, which causes a mixing of the two polarizations. In the case of coherent modulation, adaptive equalization algorithms such as the Constant Modulus Algorithm (CMA) are commonly used to separate the two polarizations.

[0061] The amplifier module 18 further includes a polarization beam combiner 24, also called a PBC. In one embodiment, the PBC 24 is a passive optical device. In one embodiment, the PBC 24 combines the two components into a single optical signal.

[0062] In one embodiment, the optical modulator 14 and the amplifier 18 are connected to each other by polarization-maintaining fibers 23, also known as PMF fibers. In another embodiment, the fibers used are instead single-mode fibers, known as SMF fibers.

[0063] Advantageously, the emitter 4 comprises an optical head 26, also called an air interface module, and also known by its English name, "optical front end" or OFE. The optical head 26 advantageously comprises one or more of the following devices: an optical device, a pointing device, a collimation device, a coupling device, or devices for compensating or pre-compensating for atmospheric turbulence. These devices are active or passive, and are, for example, formed from arrangements of lenses and / or mirrors.

[0064] With reference to [Fig.2], the receiver 6 includes in one embodiment an optical head 32. This optical head 32 is in one embodiment configured to receive an optical signal and to focus it into an optical fiber 34 which connects in one embodiment the optical head 32 and an amplifier module 36, also included in the receiver 6. The amplifier module 36 is in one embodiment a low-noise amplifier.

[0065] The receiver 6 includes in one embodiment an optical demodulator 38 and a digital processing module 42. In another embodiment, the amplifier module 36 and the optical demodulator 38 are also connected to each other by an optical fiber 34.

[0066] The optical demodulator 38 is configured to convert an optical signal into an electrical signal representative of the optical signal. In one embodiment, the digital processing module 12 and the optical modulator 14, on the one hand, and the optical demodulator 38 and the digital processing module 42, on the other hand, are compatible with each other. For example, the digital processing module 12 and the optical modulator 14 are configured to generate an optical signal using coherent modulation, and the optical demodulator 38 and the digital processing module 42 are configured to perform operations that demodulate a coherent optical signal.

[0067] The digital processing module 42 is, for example, implemented as a programmable logic component, such as an FPGA (Field Programmable Gate Array), or as an integrated circuit, such as an ASIC (Application-Specific Integrated Circuit). In an alternative (not shown) variant, the digital processing module 42 is implemented as software, stored in memory and executable by a processor associated with that memory. Similarly, in another alternative (not shown) variant, the digital processing module 42 is implemented using analog optical components.

[0068] An information processing method will now be explained, with reference to Figures 3, 4, 5 and 6. This method is implemented by the transmission system 1. It aims to transmit information from the transmitter 4 to the receiver 6 via two components of an optical signal at wavelength X. The polarization axes of these components X, Y are distinct and as indicated above, in the embodiment considered by way of example, the X axis is orthogonal to Y.

[0069] The digital processing module 12 receives, in one embodiment, digital input data De during a reception step 102. The digital input data De is distributed on the one hand into a first set of bits, named bits x, and on the other hand into a second set of bits, named bits y.

[0070] The digital processing module 12 processes the input digital data De during a conversion step into complex symbols 104. In one embodiment, during step 104, the digital processing module 12 also performs oversampling and / or formatting operations, etc. This makes it possible, for example, to improve the transmission of digital information and to limit errors in the signal reproduced in the receiver 6.

[0071] For example, with reference to [Fig. 5] schematically representing the digital processing module 12 and the modulator 14 in one embodiment, in the digital processing module 12, the bits x are received as input to a block 122x which determines complex symbols based on these bits x and outputs these complex symbols. Such a symbol is written x; + jxq> ; it comprises the real part x; (or I component for In-Phase Component) and the imaginary part xq (or Q component for Quadrature-Phase Component).

[0072] In one embodiment among these symbols, there are some whose real part is non-zero and there are some whose imaginary part is non-zero.

[0073] Similarly, the bits y are received as input to a block 122y which determines complex symbols based on these bits y and outputs these complex symbols. Such a symbol is written y; + jyq> ; These symbols thus have a real part y; and an imaginary part yq (and among these symbols, there are some whose real part is non-zero and there are some whose imaginary part is non-zero).

[0074] The determination of the symbols from the bits x (same for the bits y) is carried out by the block 122x (same for the block 122y) for example by QPSK modulation (in English Quadrature Phase Shift Keying), or any complex modulation (QAM, M-PSK, PCS ...) transposing digital data into complex symbols.

[0075] Then in a step 106, the digital processing module 12 determines modified complex symbols by permuting one of the imaginary and real parts of the symbols from block 122x with one of the imaginary and real parts of the complex symbols provided by block 122y.

[0076] Thus in each modified symbol, either the imaginary part or the real part has been determined as a function of the set of bits x (and not as a function of the set of bits y), the other of said real and imaginary parts has been determined as a function of the set of bits y (and not as a function of the set of bits x).

[0077] It is this step 106 which differs from the prior art processing in which the symbols from the bits x (i.e. real parts and imaginary parts) were transmitted only on the X polarization, while the symbols from the bits y (i.e. real parts and imaginary parts) were transmitted only on the Y polarization.

[0078] Whereas in the example considered here, the digital processing module 12 determines modified complex symbols by permuting the imaginary part xq of the symbols from block 122x with the real part y; of the complex symbols provided by block 122 (cf. dashed box in [Fig.5]).

[0079] Thus, the subsequent digital (DSP) or analog (OFE) stages will process the component pairs (x; ; y;) as a QPSK waveform on the X polarization, therefore with complex symbols x; + jy; and the pair (xq ; yq) as a QPSK xq + jyq for the Y polarization (knowing that subsequently, in reception after the OFE Tx, propagation channel, OFE Rx, and coherent DSP Rx stages, as described later, the xq and y; components will be exchanged again and reordered to find their initial positions). This solution makes it possible to take advantage of the diversity of polarization and quadrature to offer two communication channels with equivalent SNRs in the presence of PDL.

[0080] In one embodiment, a 123x filter applies a raised cosine root filter RRC on the complex symbols x; + jy; and a 123Y filter applies an RRC filter on the complex symbols xq + j.yq.

[0081] In a digital-to-analog conversion block 124x, the real component X; digital, respectively imaginary component y; digital, is converted into the corresponding analog signal by a converter 1241X, respectively 124Q X. The resulting analog signals representing the complex symbol x; + y; are provided as input to the modulator 14 corresponding to the bias channel X.

[0082] In a digital-to-analog conversion block 124Y, the real digital component xq, respectively imaginary digital component yq, is converted into the corresponding analog signal by a converter 1241Y, respectively 124Q Y. The resulting analog signals representing the complex symbol xq + j.yq are provided at the input of the modulator 14 corresponding to the bias channel Y.

[0083] The optical modulator 14 performs a modulation step 108 on these received signals. The modulation step 108 is a so-called dual-polarization modulation step. The modulation step 106 comprises the generation of a modulated optical signal S, at wavelength X, composed of two polarizations Xm (X-axis) and Ym (Y-axis). The polarization Xm is representative of the data x; + jy;. The second polarization Ym is representative of the data xq + jyq.

[0084] The optical signal S is transmitted to the amplifier module 18.

[0085] In an optional embodiment, this transmission is carried out via the polarization-maintaining fiber 23, in order to prevent an unintentional rotation of the polarizations.

[0086] An amplification step 110 is then carried out by the amplifier module 18, in order to generate an amplified optical signal Sa from the optical signal S. The amplification step 110 comprises, in one embodiment, substeps 112 to 116.

[0087] Substep 112 is a separation substep. With reference also to [Fig. 3], the optical signal S received at the input is separated by the polarization beam splitter 20 into two optical components along the two polarization axes X, Y of the splitter 20. The polarization axes of the splitter coincide, in one embodiment, with the polarization axes X and Y of the optical signal S, as shown in [Fig. 3].

[0088] The polarization splitter 20 transmits the optical components to the amplifiers 21 and 22.

[0089] Amplifier 21 amplifies the component Xm (with polarization axis X) with the gain Gl, thus generating an amplified component Xa (with polarization axis X) and amplifier 22 amplifies the component Ym (with polarization axis Y) according to the gain G2, thus generating an amplified component Ya.

[0090] The polarization beam combiner 24 performs substep 116, which is a combination substep. During the combination substep 116, the amplified components Xa and Ya are transmitted to the polarization beam combiner 24, which combines them to form the amplified optical signal Sa. This transmission takes place via an optical fiber or, alternatively, in free space.

[0091] The amplified optical signal Sa is then, in one embodiment, transmitted to the optical head 26, for example via an optical fiber, or alternatively, in free space. The optical head 26 performs an emission step 118 of the amplified optical signal Sa into a propagation medium 44, also called a propagation channel, for the receiver 6. In one embodiment, during the emission step 118, the optical head also performs one or more of the following operations, depending on the devices included in the optical head 26: a pointing, collimation, or compensation operation. or pre-compensation, to improve the quality of the amplified signal Sa, and limit the losses or distortion caused by the amplified signal Sa emitted in the propagation medium 44.

[0092] The propagation medium 44 is, for example, an optical fiber, or the atmosphere in the case of free-space optical transmission, as shown in [Fig. 1].

[0093] Following the propagation of the signal via the medium 44, an optical signal is received by the receiver 6 during a reception step 120, in an embodiment via the optical head 32. This optical signal received by the receiver 6 is called the received optical signal Sr.

[0094] During the transmission of the amplified optical signal Sa in the propagation medium 44, the amplified optical signal Sa is attenuated and its components Xa and Ya are mixed due, for example, to inhomogeneities in the propagation medium 44, or in the case of the atmosphere, due to turbulence or variations in the composition of the atmospheric layers. Thus, the amplified optical signal Sa as emitted by the transmitter 4 and the optical signal received Sr by the receiver 6 are not identical, as shown in [Fig. 3].

[0095] With reference to [Fig.4], the receiver 6 performs a set 122 of steps converting the received optical signal Sr into digital output data Dscensés to be equal to the input data De. For this purpose, in one embodiment, the receiver 6 performs the following steps 124 to 130.

[0096] With reference to [Fig.2] also, in one embodiment, the optical head 32 focuses the optical signal received Sr during the focusing step 124 and transmits it to the amplification module 36 via the optical fiber 34.

[0097] During the amplification step 126, the amplification module 36 amplifies the received optical signal Sr to form an amplified received optical signal Sa'.

[0098] The amplification module 36 transmits the received amplified optical signal Sa' to the optical demodulator 38.

[0099] In the demodulation step 128, the optical demodulator 38 demodulates the received amplified optical signal Sa': it extracts two demodulated analog signals, which it selectively determines as a function of the component X'a of axis X on the one hand (excluding the component Y'a) and it extracts two demodulated analog signals, which it selectively determines as a function of the component Y'a of axis Y on the other hand.

[0100] This demodulation relies, for example, on an integrated coherent receiver, also called an ICR (from the English "Integrated Coherent Receiver"), comprising a local oscillator, a PBS, a 90° phase shifter, and four photodiodes (one for each component Xi, Xq, Yi, Yq). The ICR allows the conversion of the optical signal into four RF signals, which will be supplied to the 224 converters mentioned below.

[0101] With reference to [Fig.6], one of these two demodulated signals from component X'a, which represents the real part of complex symbols, is then converted to digital (component x;) by a 2241X converter block; the other of these two demodulated signals from component X'a, which represents the imaginary part of complex symbols, is also converted (component yO to digital, by a 224Q _x converter block.

[0102] Similarly, one of the two demodulated signals from the component Y'a, which represents the real part of complex symbols, is then converted (component xq) into digital by a 2241Y converter block; the other of the two demodulated signals from the component Y'a, which represents the imaginary part (component yq) of complex symbols, is also converted into digital by a 224q_y converter block.

[0103] The real and imaginary parts of these complex coefficients respectively from the polarization axes X, Y are provided as input to the digital processing module 42.

[0104] In a step 130, a reciprocal exchange of that made in input between the components of the complex signals in step 106 is carried out, before their provision to the blocks 222x and 222y of transformation of the complex symbols into bits.

[0105] Thus, the complex symbol resulting from the polarization along the X axis being x;+jy; and the complex symbol resulting from the polarization along the Y axis being xq+j yq, the values ​​y; and xq are re-exchanged between them.

[0106] Then, in a bit transformation step 131, block 222x determines a set of bits x based on the complex symbols x;+jxq resulting from the exchange operation, and similarly, block 222y determines a set of bits y based on the complex symbols y;+jyq resulting from the exchange operation, corresponding to the inverse transformation performed in the transmitter. In the example considered, where QPSK modulation was used in the transmitter, the inverse operation of QPSK modulation is therefore implemented.

[0107] The digital output data Ds comprising these sets of bits x and y are delivered at the output of the digital processing block 42, representative of the digital input data De.

[0108] In the case where the received optical signal Sr is a coherent optical signal, the digital processing module 42 implements in one embodiment an adaptive equalization algorithm, such as for example the constant modulus algorithm, or CMA from the English "Constant Modulus Algorithm", a carrier and frame synchronization algorithm, or even decoding algorithms, in order to generate the digital output data Ds.

[0109] Thanks to this exchange between the I, Q components of the two X, Y polarizations compared to the prior art, the power imbalance is compensated digitally without additional algorithmic complexity and two propagation channels with equivalent SNR and performance are guaranteed, even in the presence of PDL.

[0110] The invention ensures identical system performance (BER, mutual information, etc.) on both polarizations and distributes the difference in HPOA gain equally between the two polarizations. Even though the gain of the two HPOAs is different, thus introducing two propagation channels with two different SNRs, once re-exchanged in the receiver, the two sets of bits x and y will have seen an equivalent channel in terms of SNR and therefore performance.

[0111] In the classical approach according to the prior art: - the set of bits x via the X biasing operates on a channel with a signal-to-noise ratio of SNR + PDL / 2; - the set of bits y via the Y biasing operates on a channel with a signal-to-noise ratio of SNR - PDL / 2;

[0112] With the invention: - the bit set x operates on a channel with a signal-to-noise ratio of SNR + PDL / 2 for the x component, and SNR - PDL / 2 for the x component, i.e. a total equivalent signal-to-noise ratio of SNR; - the set of bits y operates on a channel with a signal-to-noise ratio of SNR - PDL / 2 for the component y? and SNR + PDL / 2 for the component y„, i.e. a total equivalent signal-to-noise ratio of SNR.

[0113] The invention facilitates system sizing, which is no longer based on the worst-case scenario (i.e., the weakest polarization). This solution is very simple, inexpensive, and does not require any modification to the optical front end. It offers excellent performance even for PDLs greater than 9 dB.

[0114] To illustrate the performance obtained, a dual-polarization optical communication chain can be implemented with typical digital receiver processing algorithms. The following figures represent the performance of an optical transmission in terms of mutual information (or, equivalently, the bit error rate) as a function of the signal-to-noise ratio, without and with the invention, for 3 and 6 dB PDL.

[0115] By way of comparison, during tests, with a system operating according to the prior art, the performance relating to the transmission of data x or y via the least amplified polarization is lower (by 0.9 and 1.8 dB for 3 and 6 dB of PDL respectively) compared to the average performance of the two (data x on polarization X and data y on polarization Y) in the absence of a suitable compensation mechanism; by implementing the mechanism according to the invention (on When interchanged as indicated in step 106 for transmission and in step 130 for reception, we observe that the transmission performance of data x and data y is balanced (0.3 dB difference regardless of the PDL).

[0116] The invention, via the simple digital mechanism described, allows insensitivity to PDL up to more than 9 dB without increased computational, digital and analog complexity.

[0117] An exchange between components xq and yi5 has been described above, but the invention is also valid for the three other types of exchanges between components x;, xq, y; and yq, which are equally efficient: - Implementation #1: exchange xq and y; (x; + jy; for one polarization and xq + jyq for the other polarization); or - realization n° 2: exchange x; and y; (y; + jxq for one of the polarizations and x; + jyq for the other polarization); - realization n° 3: exchange x; and yq (yq + jxq for one of the polarizations and y; + jxi for the other polarization); - realization n° 4: exchange xq and yq (x; + jyq for one of the polarizations and y; + jxq for the other polarization).

[0118] The type of exchanges to be carried out between components (which may vary over time in embodiments) is the same in the transmitter and the receiver; it is for example defined in a memory of these equipment or coded in the frame information.

[0119] Thus, transmission system 1 improves the quality of data transmission by optical signals by limiting the impact of amplification differences between the two optical components of the optical signal, simply by requiring that the real and imaginary parts of the same symbol not be amplified by the same amplifier (more generally, by requiring that they use separate telecommunication paths). This mechanism does not require the introduction of constellation rotation, digital pre-coding on the Tx side, decoding and equalization on the Rx side, or ML (maximum likelihood) detection.

[0120] In one embodiment, the optical signal comprises several wavelengths. The transmission chain is then modified as follows. The transmitter then comprises a digital processing module and an optical modulator for each wavelength. A corresponding adaptation is performed in the receiver 6.

[0121] The process described above is then implemented by this processing chain for each wavelength.

[0122] In practice, a given wavelength corresponds to an optical signal whose spectral width is, for example, less than Inm.

[0123] The invention has been described in a GEO satellite FSO feeder telecommunications application, but it is applicable to all types of optical communications (LEO FSO feeder, Optical Wireless Communication, RF-FSO hybrid, LiFi, terrestrial fiber telecoms, ...) or radio frequency (RF - Radio Frequency).

[0124] The invention has been presented in the case of an example where the gain imbalance was introduced by separate amplifiers for the biasing, but it is also applicable to all sources of imbalance between the channels regardless of the component (optical, analog, Radio Frequency (RF), or digital, etc.) that is the source: PBS, PBC, SMF / PMF fibers, LNOA, HPMUX, etc.

[0125] The invention has been described above in the case of diversity processing based on the use of two polarizations of an optical signal. It can be implemented for any type of diversity implemented on two wired or wireless, space-based or terrestrial, optical or RF telecommunication chains, i.e., in all cases where it is desired to rebalance the performance between two transmission chains in use. The imaginary or real parts are interchanged between the symbols intended for two communication chains (for example, each corresponding to a distinct wavelength, when the diversity exploited is based on the use of two wavelengths, or each corresponding to a distinct RF polarization when the diversity is based on the polarization of an RF signal, or the telecommunication chains corresponding to different telescopes, or to two distinct orbital angular momentums (OAMs)).

[0126] Any feature described for an embodiment or variant in the foregoing may be implemented for the other embodiments and variants described above, provided that it is technically feasible.

Claims

1. Demands Transmitter (4), adapted to transmit, to a receiver (6), two sequences of digital input data (From) comprising a first sequence and a second sequence, using two separate transmission chains comprising a first chain and a second transmission chain, the transmitter (4) comprising: a transformation block (12) adapted to transform the numerical data of the first sequence, respectively of the second sequence, into complex symbols each comprising an imaginary part and a real part; a first transmission processing block adapted to receive as input the first complex symbols to be transmitted via the first transmission chain and to implement the transmission of said first complex symbols via said first transmission chain to the receiver; a second transmission processing block adapted to receive as input second complex symbols to be transmitted via the second transmission chain and to implement the transmission of said second complex symbols via said second transmission chain to the receiver; given x; + jxq a complex symbol resulting from the transformation, by the transformation block, of the first sequence and given yi + jyq a complex symbol resulting from the transformation, by the transformation block, of the second sequence said transmitter is characterized in that it is adapted to compose the first complex symbols to be transmitted and the second complex symbols to be transmitted by combining the symbols from the first sequence and the second sequence in one of the following ways: - the first symbol is x; + jy; and the second symbol is xq + jyq; Or - the first symbol is y; + jxq and the second symbol is x; + jyq; Or - the first symbol is yq + jxq and the second symbol is y; + jx;; Or - The first symbol is x; + jyq and the second symbol is y; + jxq

2. Transmitter (4) according to claim 1, wherein the first transmission chain corresponds to the selective transmission of a polarization component along only one of the distinct axes X, Y of an optical signal at a given wavelength X and the second transmission chain corresponds to the selective transmission of a polarization component along only the other of the axes X, Y of said optical signal at the wavelength X.

3. Transmitter (4) according to any one of the preceding claims, adapted to modulate the polarization component of the optical signal along the X axis as a function of the first complex symbols and to modulate the polarization component of the optical signal along the Y axis as a function of the second complex symbols, the first transmission chain comprising a first optical amplifier (21) of gain G1 adapted to selectively amplify the polarization component of the optical signal along the X axis, the second transmission chain comprising a second optical amplifier (22) of gain G2 distinct from G1 and adapted to selectively amplify the polarization component of the optical signal along the Y axis.

4. Receiver (6), adapted to receive the signal emitted by the transmitter according to any one of claims 1 to 3, and to convert it into digital output data (Ds).

5. Receiver (6) according to the preceding claim, comprising two receiving processing blocks and adapted to determine, in the first receiving processing block, first complex symbols as a function of the received signal and to determine, in the second receiving processing block, second complex symbols as a function of the received signal, the first and second complex symbols each comprising an imaginary part and a real part; said receiver being adapted to compose complex symbols corresponding to a first sequence and complex symbols corresponding to a second sequence, by combining the first and second complex symbols in a manner inverse to the combination performed in the transmitter (4), said receiver further comprising a transformation block adapted to transform the complex symbols corresponding to

6.

7. the first sequence, respectively corresponding to the second sequence, in a first sequence of digital data, respectively in a second sequence of digital data. Teletransmission system (1) comprising the transmitter (4) according to any one of claims 1 to 3 and the receiver (6) according to any one of claims 4 and 5. A method for transmitting information to a receiver (6) for transmitting two sequences of digital input data (De) comprising a first sequence and a second sequence, using two separate transmission chains comprising a first chain and a second transmission chain, said method comprising the following steps implemented by the transmitter: - transform the numerical data of the first sequence, respectively of the second sequence, into complex symbols each comprising an imaginary part and a real part; - receive as input from a first transmitting processing block the first complex symbols to be transmitted via the first transmission chain and implement the transmission of said first complex symbols via said first transmission chain to the receiver; - receive as input from a second transmitting processing block second complex symbols to be transmitted via the second transmission chain and implement the transmission of said second complex symbols via said second transmission chain to the receiver; according to which, given x; + jxq a complex symbol resulting from the transformation of the first sequence and given y; + jyq a complex symbol resulting from the transformation of the second sequence, said process is characterized in that it is suitable for composing the first complex symbols to be transmitted and the second complex symbols to be transmitted by combining the symbols resulting from the first sequence and the second sequence in one of the following ways: - the first symbol is x; + jy; and the second symbol is xq + jyq; - the first symbol is y; + jxq and the second symbol is x; + jyq; or - the first symbol is yq + jxq and the second symbol is y; + jx;; or - the first symbol is x; + jyq and the second symbol is y; + jxq

8. Information transmission method according to claim 7, according to the first transmission chain corresponds to the selective transmission of a polarization component along only one of the distinct axes X, Y of an optical signal at a given wavelength X and the second transmission chain corresponds to the selective transmission of a polarization component along only the other of the axes X, Y of said optical signal at the wavelength X.

9. Information transmission method according to claim 7 or 8, the method comprising at least the following steps implemented by a receiver: reception of the signal emitted by the transmitter and conversion into digital output data (Ds).

10. Information transmission method according to claim 9, the method comprising at least the following steps implemented by the receiver: - determining, in the first receiving processing block, first complex symbols as a function of the received signal; and - determining, in the second receiving processing block, second complex symbols as a function of the received signal, the first and second complex symbols each comprising an imaginary part and a real part; - composing complex symbols corresponding to a first sequence and complex symbols corresponding to a second sequence, by combining the first and second complex symbols in a manner inverse to the combination carried out in the transmitter (4);- transform the complex symbols corresponding to the first sequence, respectively corresponding to the second sequence, into a first sequence of numerical data, respectively into a second sequence of numerical data.;

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