METHOD AND SYSTEM FOR POINT-TO-POINT TRANSMISSION WITH LIMITATION OF COUPLING LOSSES

Description

[0001] The present invention relates to point-to-point radio wave transmissions used particularly in transport networks forming the infrastructure of mobile networks, or in certain private radio networks. Typically, these transmissions are carried out in frequency bands between 6 and 42 GHz.

[0002] With the advent of new uses linked to the democratization of internet access via mobile networks, mobile network operators have a growing need to increase their network capacity. Thus, the data rates offered by point-to-point links have been significantly increased through the use of new techniques, particularly increasingly complex modulation techniques such as MAQ2048 and MAQ4096. However, the data rates of point-to-point links remain limited by the bandwidth of the frequencies allocated to them.

[0003] Furthermore, increasing the bandwidth of transmission channels is a highly complex process because these channels are defined by international regulations and therefore cannot be changed rapidly. In addition, such point-to-point radio transmission links have been in operation for several decades. The radio spectrum has thus already been allocated in the form of channels of a defined bandwidth, and the remaining available radio spectrum consists of narrow and mostly non-adjacent channels.

[0004] There figure 1 This represents a traditional single-channel transmission system. The system comprises an internal unit (IDU) and an external unit (ODU) coupled to an antenna (AT1). The external unit (ODU) should be placed as close as possible to the antenna (AT1), while the internal unit (IDU) can be placed at a distance, for example, in a location where it is easily accessible for maintenance. The internal unit (IDU) includes a modem (M1) that manages the transmission and reception of a radio signal. The digital output signal, on the transmit side, of the modem (M1) is converted by a digital-to-analog converter (D1) into an analog signal, which is filtered by a bandpass filter (BF1). The filtered analog signal is then fed to a first frequency mixer (1) to apply a shift of an intermediate frequency (IFx) generated by a local oscillator (O1) to the analog signal.The modulated IFx carrier is transmitted to the external ODU unit where it is converted to a final carrier frequency RFx+IFx by a second frequency mixer 2, which also receives an intermediate carrier at the RFx frequency from a local oscillator O2. The signal output from frequency mixer 2 is amplified by an amplifier A1 and then filtered by a bandpass filter BF2. The BF2 filter is connected to the AT1 antenna via a three-way duplexer 3, such as a mechanical circulator, to also connect the antenna to an ORC-IRC receiver chain.

[0005] The modem M1, converter D1, and intermediate frequency mixer 1 (IFx) are now commonly integrated into a dedicated modem circuit. The final carrier frequency mixer 2 is also commonly integrated into a dedicated power integrated circuit, with the signal being transmitted from the modem circuit to the power circuit at the IFx frequency.

[0006] The spectrum of the output signal from mixer 2 is shown on the figure 2 This signal has three components: two components centered on the frequencies RFx-IFx and RFx+IFx, respectively, and modulated by the analog signal from the modem M1, and one component at the frequency RFx. The bandpass filter BF2 eliminates one of the two modulated signal images centered on the frequencies RFx-IFx and RFx+IFx, as well as the carrier at the frequency RFx. However, the final frequency RFx+IFx is generally above 6 GHz, and the IFx frequency is below 400 MHz, so that the signal can be transmitted without excessive loss between the internal IDU and the external ODU. Therefore, the BF2 filter must be highly selective. At the relevant frequencies, only mechanical or cavity filters can achieve the required rejection level. However, cavity filters are particularly complex and expensive to manufacture.

[0007] It has also been proposed to couple several channels to increase the transmission rate. For this purpose, as illustrated in Figure 3, several single-channel transmission chains TX1, TX2 are coupled to the antenna AT1. Three-way mechanical circulators C1, C2 are used instead of duplexer 3 and connected in series to the antenna AT1. For a transmission chain TX2, separated from the antenna by several circulators C1, C2, to reach the antenna, the connection between each transmission chain and its circulator must be equipped with a filter F1, F2 having a high reflection coefficient. This filter reflects the signal transmitted by the TX2 chain back to the channel of circulator C1 connected to the antenna. Thus, the signal transmitted by the TX2 chain is passed through circulators C2 and C1 to the filter F1, which reflects the received signal back to the antenna AT1.

[0008] In a symmetrical manner, several single-channel receiving chains can be coupled to the same antenna respectively by a circulator and a selective filter adapted to the corresponding receiving channel.

[0009] Consequently, filters F1 and F2 must not only pass only the signal emitted by the TX1 and TX2 transmission chain to which they are connected, but also exhibit a high reflection coefficient for signals emitted by other transmission chains connected to the antenna. It turns out that the required filters F1 and F2 are even more selective than the BF2 filter located at the output of the transmission chain shown in the diagram. figure 1 , and therefore are even more complex and costly.

[0010] Furthermore, achieving high reflection coefficients is difficult. As a result, the signals to be transmitted undergo significant attenuation as they pass through the circulators. Thus, passing through two circulators in transmit and receive modes can induce a power loss of up to 6 dB, representing three-quarters of the radio power dissipated as heat in the filters and circulators. Moreover, cavity filters are structurally adapted to the frequencies they are filtering, so a change in the transmission channel necessitates replacing the corresponding filters.

[0011] These radio power losses therefore result in a significant increase in the electrical consumption required to achieve a given transmission power.

[0012] The manufacturing technologies for cavity filters also impose a frequency limit. There are virtually no solutions for creating cavity filters that operate at frequencies above 13 GHz, given that the target available frequency bands reach 42 GHz.

[0013] A two-carrier or two-channel system has also been proposed, comprising a modem that generates two carriers which are modulated separately and transmitted to the external unit at a first intermediate frequency. The external unit performs analog-to-digital conversion and digital filtering to isolate the two carriers. The external unit then performs frequency shifting of these two carriers to frequencies that combine the initial position of each carrier relative to the first intermediate frequency and the second intermediate frequency used to shift the two carriers to the final frequency.

[0014] This system must perform several analog-to-digital and inverse conversions, generating a significant number of unwanted images that must then be filtered. The positions of the output carriers from the modem around the first and second intermediate frequencies impose strict constraints on the characteristics of the filter that must process the images from both channels and eliminate the unwanted images. Furthermore, the complexity of this system increases exponentially if the number of channels is increased. In practice, this number can hardly be extended beyond two.

[0015] An OFDM (Orthogonal Frequency-Division Multiplexing) subcarrier modulation technique has also been proposed, enabling multi-channel transmission by grouping subcarriers. However, the spectral efficiency of such a technique decreases rapidly as the channel width decreases.

[0016] US 2017 / 034317 A1 (KENNEY THOMAS J [US] ET AL) February 2, 2017 (2017-02-02) D1 describes an OFDM system with logic that analyzes a data stream into two or more frequency segments having a total bandwidth greater than 160 megahertz, whether contiguous or non-contiguous. The OFDM signal includes a guard interval and a zero direct component, DC.

[0017] US 2012 / 182948 A1 (HUANG XIAOJING [AU] ET AL) July 19, 2012 (2012-07-19) proposes group sub-bands, from a set of radio transmission frequency channels assigned to a terminal, in a plurality of aggregated sub-bands, and allocates data to at least some of said aggregated sub-bands. The proposed solution is a multi-channel system, except for the guard band. Unused channels carry zero.

[0018] It is therefore desirable to be able to increase the data rate of point-to-point multichannel radio links by using available frequency bands. It is also desirable to avoid the need for selective, and therefore costly, mechanical filters. Furthermore, it may be desirable to implement a multichannel transmission system in which the number and bandwidth of the channels can be modified.

[0019] Embodiments relate to a data transmission method, comprising steps of: generating from a data stream to be transmitted, a plurality of frequency-domain digital sample streams, each digital sample stream containing a portion of the data stream to be transmitted and modulating a respective subcarrier defining a respective subcarrier channel, the set of subcarrier channels covering a frequency band; converting the frequency-domain digital sample streams into time-domain digital sample streams by an inverse spectral transform; combining the time-domain digital sample streams into a composite time-domain digital sample stream; frequency-shifting the composite time-domain digital sample stream using a digital oscillator at an intermediate frequency above 1 GHz; and converting the shifted composite sample stream into an analog signal.To frequency-shift the analog signal to a final transmission frequency greater than 2 GHz, using an analog oscillator with a shift frequency, to filter the analog signal to attenuate the shift frequency, and to transmit the shifted analog signal via an antenna.

[0020] According to one embodiment, the process includes a step of filtering each stream of time-domain digital samples by a respective digital filter before generating the composite stream of time-domain digital samples.

[0021] According to one embodiment, the composite stream of time-domain digital samples is transmitted via an optical link before being translated to the intermediate frequency.

[0022] According to one embodiment, the subcarrier channels are grouped into transmission channels dividing a frequency band, one of the transmission channels grouping together subcarrier channels not used to transmit data streams.

[0023] According to one embodiment, the process includes steps consisting of: generating in parallel several composite streams of time-series digital samples, and translating each of the composite streams of time-series digital samples, to a respective intermediate frequency using a respective digital oscillator, each of the translated composite streams of time-series digital samples being transmitted in analog form in a respective frequency band.

[0024] According to one embodiment, the data stream to be emitted undergoes scrambling processing so that bits of the stream have a random distribution, coding processing to introduce redundancy or error correction data into the stream, interleaving processing forming a stream of data blocks, and modulation processing to transform the stream of data blocks into a stream of frequency samples in the form of complex numbers.

[0025] According to one embodiment, several scrambling and coding processes are performed in parallel to increase the transmission rate of the data stream transmitted to the interleaving process.

[0026] Embodiments also relate to a data reception method comprising steps of: receiving an analog signal by an antenna, translating the received analog signal to an intermediate frequency above 1 GHz and below an analog signal transmission frequency above 2 GHz, using an analog oscillator at a translation frequency, filtering the analog signal to attenuate the translation frequency, converting the analog signal into a composite stream of digital samples at the intermediate frequency, translating the composite stream of time-domain digital samples to a baseband using a digital oscillator at the intermediate frequency, and generating, from the baseband composite stream of digital samples, using a spectral transform, a plurality of frequency-domain digital sample streams.Each stream of frequency-based digital samples is associated with a subcarrier defining a respective subcarrier channel, and a data stream is generated from the streams of frequency-based digital samples.

[0027] Embodiments also relate to a transmission device configured to implement one or the other of the previously defined processes.

[0028] According to one embodiment, the device includes a modem for generating the composite stream of time-domain digital samples, a converter for translating the composite stream of time-domain digital samples to the intermediate frequency and converting the composite stream of samples translated to the intermediate frequency into analog signals, and an analog stage connected to an antenna for generating the translated analog signals.

[0029] According to one embodiment, the modem is connected to the converter via an optical link.

[0030] According to one embodiment, the device comprises a plurality of modems connected to a respective input of the converter.

[0031] According to one embodiment, the device comprises a parallel / serial conversion circuit including a plurality of inputs connected respectively to the modems, and a serial / parallel conversion circuit connected to the parallel / serial conversion circuit by an optical fiber link and including an output per modem connected to the input of a respective frequency conversion circuit to translate the composite stream of time-series digital samples provided by a respective modem, to a respective intermediate frequency.

[0032] According to one embodiment, each modem is implemented by a program-controlled processor or a programmable circuit. The invention is defined by independent claims 1 and 6.

[0033] Examples of embodiments of the invention will be described below, by way of non-limiting example, in relation to the accompanying figures, among which: there figure 1 described previously, represents a traditional single-channel transmission system, the figure 2 described previously, represents a frequency spectrum of a signal to be emitted by the transmission system of the figure 1 , there figure 3 described previously, illustrates a device for generating a signal distributed across multiple transmission channels, the figure 4 schematically represents the transmission section of a multi-channel digital transmission modem, according to one embodiment, the figure 5 schematically represents an example of the spectrum of the signal produced by the transmission chain, the figure 6 schematically represents another example of the spectrum of the signal produced by the transmission chain, the figure 7 schematically represents the receiving section of a multi-channel digital transmission modem, according to one embodiment, the figure 8 schematically represents an external unit of a multi-channel transmission system, according to one embodiment, the figure 9 schematically represents an example of the spectrum of a signal emitted by the multi-channel transmission system, according to one embodiment, the figure 10 schematically represents an example of a filter template for a multi-channel transmission system in relation to transmission channels implemented per transmission chain, the figure 11 schematically represents another embodiment of a digital transmission chain in a multi-channel transmission system, the figures 12A, 12B, 12C [Fig. 12 ] schematically represent examples of internal signal spectra of the multi-channel transmitter of the figure 11 , there figure 13 schematically represents an embodiment of a conversion unit in the transmission chain of the multi-channel transmission system of the figure 11 , there figure 14 schematically represents the transmission chain of a multi-channel transmission system according to another embodiment.

[0034] There figure 4 This represents the MTX transmission portion of an MDM digital modem in a digital data transmission system. The MTX transmission portion may be of the OFDM (Orthogonal Frequency-Division Multiplexing) type. In one embodiment, the MTX transmission portion is configured to perform multi-channel transmission within an allocated frequency band. The MTX transmission portion receives digital data to be transmitted (Dx i) from a physical interface, for example, an Ethernet interface, which may be connected to an external switch providing the data to be transmitted. The MTX transmission portion includes SCB scrambling modules, CD coding modules, INTL interleaving modules, MOD modules, an SP serialization circuit, an IDFT frequency-domain-to-time-domain transformation circuit, and an ADD summing modulator.The SCB scrambling module is configured to process the received binary data stream so that the bits in the stream have a predefined random distribution, preventing long sequences of bits with the same value. The CD coding module introduces redundancy or error correction data into the scrambled string.

[0035] The INTL interleaving module receives the output signals from the CD encoder and interleaves them to form data blocks. The MOD modulator receives these data blocks from the INTL interleaving module and modulates them, producing frequency samples Xi as complex numbers distributed according to a multi-state modulation constellation. The applied modulation can be, for example, quadrature amplitude modulation (QAM) or phase-shift keying (PSK) such as binary PSK (BPSK) or quadrature PSK (QPSK). The SP serialization circuit distributes the modulated frequency samples Xi from the MOD modulator into vectors XI = (Xi,0, ..., XI,N-1) of size N, where N is greater than 1.

[0036] The IDFT transformation circuit performs a transformation from the frequency domain to the time domain by transforming the frequency vectors XI = (XI,0, ..., XI,N-1) into time-domain digital symbols xI = (xI,0, ..., XI,N-1) composed of N time samples xI,0, ..., XI,N-1. This transformation is achieved by applying an inverse spectral transform of order N to the modulated samples XI,0, ..., XI,N-1 provided by the MOD modulator. The inverse spectral transform can be, for example, an inverse fast Fourier transform or an inverse wavelet transform. The resulting digital signal x(t), consisting of the xI symbols, exhibits an amplitude spectrum as illustrated in the diagram. figure 5 . Each sample x I,j produced corresponds to a modulated subcarrier j, the bandwidth of the frequency allocated to the transmission being divided into N subcarrier channels CP j each corresponding to a subcarrier.

[0037] The highest frequency in the allocated frequency band can correspond to the highest frequency of the subcarrier channel located at the highest frequencies, and the lowest frequency in the allocated frequency band can correspond to the lowest frequency of the subcarrier channel located at the lowest frequencies. In this way, the higher the number N of subcarrier channels, the smaller the subcarrier channel width, and the higher the operating frequency of the MDM modem, which also depends on the total bandwidth of the processed frequency band.

[0038] In one embodiment, subcarrier channels are grouped into transmission channels. In the field of microwave transmissions, international regulations mandate predefined transmission channel widths. Depending on the country and the transmission frequency (6 GHz or 38 GHz, for example), transmission channel widths are specified. Typically, transmission channel widths are 3.5 MHz, 7 MHz, 14 MHz, 28 MHz, 40 MHz, 56 MHz, or 112 MHz. International telecommunication standards (defined by the ITU) impose emission standards to limit interference generated outside each transmission channel. The narrower the transmission channels, the lower the out-of-channel energy level must be.

[0039] It turns out that the value of the last sample xI-1,N-1 of a symbol xI-1 can be very different from that of the first sample xI,0 of the following symbol xi. This discontinuity generates high-frequency components in the spectrum of the time-domain signal resulting from the concatenation of symbols, which is typically performed by a serializer or parallel-to-serial converter, as well as interference between subcarrier channels. To avoid these drawbacks, cyclic prefixes are typically inserted between the symbols at the output of the IDFT transformation circuit.Each cyclic prefix consists of samples calculated by applying a windowing between the samples at the ends of each symbol and a cosine square type function, so that the samples at each end of the symbol and its cyclic prefix cancel each other out, thus limiting the level of the high-frequency components of the resulting signal and therefore the level of energy dissipated outside the corresponding subcarrier channel.

[0040] Cyclic prefixes make transmission more robust in multipath transmissions, which is essential in mobile communications, typically from a base station to a mobile phone. They also reduce the amount of energy transmitted outside each subcarrier channel. This energy can be reduced by increasing the ratio of the duration of each cyclic prefix to the symbol duration, or by reducing the spacing between subcarriers. If the allocated bandwidth is fixed, reducing the spacing between subcarriers is achieved by increasing the Nth order of the spectral transform, which decreases the width of the subcarrier channels.However, the implementation of transmissions in very narrow transmission channels (3.5 MHz or 7 MHz) and therefore the implementation of a high N order of the spectral transform and an extremely constrained transmission template, considerably increases the complexity of the MTX transmission part of the MDM modem.

[0041] Furthermore, increasing the duration of cyclic prefixes significantly reduces the spectral efficiency of the transmission system, because a growing fraction of the transmission time is occupied by the transmission of cyclic prefixes, the portion devoted to the transmission of useful data being reduced accordingly.

[0042] In one embodiment, the MTX transmission section of the MDM modem does not insert cyclic prefixes between the symbols xi, but comprises a set of N digital filters PF0 - PFN-1, each receiving the N samples xI,0, ..., xI,N-1 of each symbol xi, the number of subcarrier channels being fixed. The PF0 - PFN-1 digital filters have a frequency response configured to significantly attenuate emissions outside the N subcarrier channels, and an impulse response to limit discontinuities between the extreme samples xI-1,N-1 and xI,0 of consecutive symbol pairs xI-1, xI. For this purpose, the PF0 - PFN-1 filters are, for example, of the polyphase type, such as the PHYDYAS, IOTA (Isotropic Orthogonal Transform Algorithm), and MMB (Martin-Mirabassi-Bellange) filters.

[0043] Thus, thanks to the suppression of cyclic prefixes and the filtering of subcarriers, spectral efficiency remains constant regardless of the subcarrier channel width. Since the number N of subcarriers is fixed, the subcarrier channel width depends on the bandwidth allocated to the transmission. The suppression of cyclic prefixes also ensures that spectral efficiency remains constant even when the subcarrier channel width is changed.

[0044] Furthermore, the level of out-of-channel emissions can be reduced below the levels required by the transmission templates defined for the channels, even when the channels are wider. By eliminating out-of-channel emissions in this way, the N-order of the inverse Fourier transform can be reduced compared to an OFDM transmission solution, thus simplifying the modem implementation complexity.

[0045] In one embodiment, null samples are assigned to certain components XI,j of the vectors XI = (XI,0 , ..., XI,N-1 ) of size N supplied as input to the IDFT transformation circuit so as to transmit no signal in certain transmission channels. This makes it possible to achieve multichannel transmission, occupying only selected channels, within a frequency band covered by the MDM modem.

[0046] Figure 6 shows an example of the spectrum of the digital signal x(t) produced at the output of the MDM modem. In the example spectrum of the figure 6 , the N channels or subcarriers are distributed into groups C1, C2, ... C8 of four channels forming transmission channels, the group C2 receiving only null samples so as not to transmit in the channels of this group.

[0047] In this way, the MDM modem can perform multichannel transmission, in which the width of the transmission channels is defined by grouping subcarriers per channel and transmitting null samples at the positions of the components of the size N vector that correspond to the channels in which one does not wish to transmit.

[0048] Figure 7 represents the MRX receiver section of the MDM modem, according to one embodiment. The MRX receiver section receives baseband digital signals yi. The MRX receiver section includes N PF'0 -PF'N-1 filters, for example of polyphase type (like those of the MTX transmit section), a DFT transformation circuit, a PS parallel / serial conversion circuit, a DMD demodulation circuit, and DTL deinterlacing, DCD decoding, and DSB descrambling modules.

[0049] Each of the PF'0 - PF'N-1 filters has a template configured to allow signals within its corresponding subcarrier channel to pass through, and to attenuate signals outside that channel. The PF'0 - PF'N-1 filters are, for example, polyphase, like those in the MTX transmission section. Each PF'0 - PF'N-1 filter thus provides a respective component yI,j (j=0, ..., N-1) of a vector yI of size N, corresponding to a subcarrier. The time samples yI,0 - yI,N-1 are transformed from the time domain to the frequency domain by the DFT transformation circuit, for example, by applying an Nth-order spectral transform to the frequency samples YI,0 - YI,N-1. The spectral transform can be, for example, a fast Fourier transform or a wavelet transform.The frequency samples YI,0 - YI,N-1 are grouped by the PS serializer circuit into a binary stream Yi, which is then processed successively by the DMD demodulation circuit, the DTL deinterlacing module, the DCD decoding module, and the DSB descrambling module. The DMD and DTL circuits apply inverse processing to that performed by the INTL and MOD circuits. The DCD decoder decodes the deinterlaced data, performing transmission error corrections using redundancy data if necessary and possible. The DSB descrambling module applies inverse processing to that performed by the SCB scrambling module.

[0050] As an example, the number N of subcarriers can be set at 512, the number of transmission channels in the allocated emission band can be set at 8, 16 or 32 channels.

[0051] Since the MDM modem only deals with digital signals, it can be entirely implemented by a program-controlled microprocessor or implemented by a programmable circuit such as an FPGA ("Field-Programmable Gate Array").

[0052] SCB scrambling and CD coding modules may require significant computing power. If the programmable circuit has insufficient computing power, the SCB and CD modules can be duplicated to process only a portion of the data to be transmitted, with the remaining data processed in parallel by additional duplicated SCB and CD modules.

[0053] There figure 8 This represents an embodiment of a digital conversion stage (NST) and an analog conversion stage (AST) of the transmission system. The digital NST stage comprises a transmit TCNV conversion stage and a receive RCNV conversion stage. The TCNV conversion stage is configured to convert the baseband digital signal xi to be transmitted from the MDM modem output into an analog signal translated by an intermediate frequency IF. To this end, the TCNV conversion stage includes a digital oscillator NO1 connected to a digital frequency mixer (FM), and a digital-to-analog converter (DAC). The FM mixer adds the intermediate frequency IF generated by the NO1 oscillator to the output sample frequency xi of the MDM modem, and the DAC converts the output signal from the FM mixer into an analog signal that can then be processed by the analog AST stage.

[0054] Thus, the modulated signals in the different transmission channels are generated into a single digital time-domain signal. This digital signal is then translated by the intermediate frequency IF, still within the digital domain, before being converted back to the analog domain at the output of the TCNV conversion stage. As a result, all the modulated signals in the different transmission channels outputting the MDM modem can be carried by a single link to the NST digital stage. This eliminates the implementation constraints of traditional analog transmission systems, which require, for each transmitted channel, a modem, a digital-to-analog converter, a coaxial cable carrying the signal at a low intermediate frequency to minimize losses, and an external ODU (Figure 1).

[0055] The analog AST stage comprises a TXC transmit chain, an RXC receive chain, and a DX duplexer coupling the TXC and RXC chains to an AT antenna. The TXC transmit chain receives the analog signal x(t) from the DAC of the TCNV conversion stage. The TXC transmit chain includes a LO1 local oscillator, an FM1 frequency mixer, a PA amplifier, and two bandpass filters, BPF1 and BPF2. The LO1 oscillator generates a signal at the intermediate frequency RF, which it transmits to the FM1 mixer. The analog signal output from the TCNV stage is translated to the final RF+IF frequency by the FM1 mixer, then filtered by the BPF1 bandpass filter before being amplified by the PA amplifier. The amplified signal output from the PA amplifier is filtered by the BPF2 filter before being transmitted to the AT antenna through the DX duplexer.The BPF2 filter's function is to protect the RXC receiving chain from interference caused by the transmitting signal that could "dazzle" or desensitize the RXC receiving chain.

[0056] The RXC receiver chain includes a low-noise LNA amplifier, a frequency mixer FM2, a voltage-controlled local oscillator LO2, and two bandpass filters BPF3 and BPF4. Filter BPF3 receives signals from the AT antenna transmitted by the DX duplexer. The filtered signals at the output of filter BPF3 are amplified by the LNA amplifier and then translated by an intermediate frequency IF' by mixer FM2, which also receives an RF' frequency from oscillator LO2. The translated intermediate frequency signals from mixer FM2 are filtered by bandpass filter BPF4 before being sent to the RCNV conversion stage. Filter BPF3 can have a waveform similar to waveform G2, but only allowing the RXB band to pass (centered on the IF'+RF' frequency). Filter BPF4 can have a waveform G1 to reject signals with frequencies lower than or equal to the intermediate frequency RF'.

[0057] The RCNV converter stage in the receive section receives analog signals y(t) translated to the intermediate frequency, provided by the BPF4 filter. The RCNV converter stage is configured to convert the analog signal y(t) to baseband and generate digital signals yi that can be processed by the MDM modem. To this end, the RCNV converter stage includes an analog-to-digital converter (ADC) connected to a digital frequency mixer FM3, which also receives the intermediate frequency IF' from a digitally controlled digital oscillator NO2. The ADC converts the signal y(t) into digital samples. The frequency mixer FM3 downscales the sample frequencies from the intermediate frequency to baseband. The baseband samples yi are then transmitted to the MRX receiver section of the MDM modem.

[0058] The BPF1 filter allows obtaining a single usable image of the signal output from the FM1 mixer. figure 9 represents the spectrum of the FM1 mixer output signal, as well as the BPF1 filter template. The FM1 mixer output signal has three components: two components centered on the RF-IF and RF+IF frequencies respectively and modulated by the analog signal from the TCNV stage, and one component at the RF frequency. In the example of the figure 9 , the BPF1 filter allows the image of the modulated signal centered on the RF-IF frequency and the component at the RF frequency to be eliminated.

[0059] In one embodiment, the intermediate frequency (IF) is set at a value greater than 1 GHz. For example, for a transmission band around 18 GHz with a typical width of 1 GHz, the IF is set at least 2 GHz. It is then possible to use only a BPF2 filter, making the BPF1 filter unnecessary, to reject the entire transmission frequency band.

[0060] By setting the intermediate frequency IF to a relatively high value, above 1 GHz, it is possible to implement a relatively inexpensive filter (BPF1). Indeed, choosing a relatively high intermediate frequency IF reduces the selectivity of the BPF1 filter compared to the BF2 filter (Figure 1). Thus, the BPF1 filter can exhibit a selectivity of 20 dB at the RF frequency. The BPF1 filter can therefore be implemented using a technology much less expensive than cavity filters, such as microstrip technology. Furthermore, since the rejection requirement is lower, it is possible to place the BPF1 filter before the PA amplifier, whereas it is necessary in systems of the type of the figure 1 to place the BF2 filter after the amplifier A1.

[0061] There figure 9 also represents a TXB frequency band allocated to transmission, subdivided into transmission channels and centered on the RF+IF frequency, and an RXB frequency band allocated to reception, also subdivided into reception channels, and placed outside the TXB frequency band.

[0062] For example, the intermediate frequency (IF) and final frequency (RF+IF) are 1.5 GHz and 11 GHz, respectively, and the transmission bandwidth (TXB) is 480 MHz, between 10.76 and 11.24 GHz. The RF frequency is therefore set at 9.5 GHz. The BPF1 filter can thus be configured to reject frequencies below 10 GHz to achieve a rejection ratio of 20 dB at the 9.5 GHz RF frequency.

[0063] Because the frequency of the analog output signal from the TCNV conversion stage is relatively high (around the IF frequency), exceeding the frequencies of signals transmissible over several meters of cable, it is desirable to limit the length of the link between the output of the TCNV conversion stage and the analog AST stage. The digital conversion stage (NST) can then be placed in the same external unit (EXU) as the analog AST stage. In contrast, the signals to be transmitted between the MDM modem and the NST conversion stage are digital signals, and therefore less susceptible to noise. These digital signals can thus be transmitted over a relatively longer distance than the signals traveling between the NST conversion stage and the analog AST stage. Therefore, the link between the MDM modem and the TCNV conversion stage can be established using coaxial cable or fiber optic cable.

[0064] Figure 10 shows in more detail the transmission channels C1-C8 and C1'-C8' located in the TXB and RXB frequency bands allocated to transmission and reception, respectively, as well as the G2 template of the BPF2 filter. The BPF2 filter is configured to have a broadband template, allowing the entire TXB frequency band allocated to transmission to pass through, and having sufficient RJ rejection in the RXB frequency band allocated to reception. In particular, the RJ rejection of the BPF2 filter in the RXB band allocated to reception is significantly lower (at least 6 dB) than the amplitude of the minimum signal received by the RXC receiver chain minus the signal-to-noise ratio of the chosen modulation. The properties required for the BPF2 filter can be obtained by a cavity filter. However, it should be noted that the selectivity required for the BPF2 filter is much lower than that required for the BF1 and BF2 filters ( fIgure 3 ) which must allow only one transmission channel to pass and reject adjacent channels.

[0065] Similarly, the intermediate frequency IF' used in the RXC receiver chain can also be set to a value higher than 1 GHz. Again, choosing a relatively high intermediate frequency IF' allows the BPF4 filter to be implemented with relatively low selectivity, for example, 20 dB at the RF' frequency. The BPF4 filter can therefore be implemented using a technology that is much less expensive than cavity filters, such as microstrip technology.

[0066] The transmission system, comprised of the MDM modem and the external EXU unit, enables multi-channel transmission in the microwave bands by configuring the various transmission channels and subcarrier channels via software. It is therefore possible to dynamically modify the configuration of the different channels and subcarriers, as the hardware architecture of the transmission system depends only on the number of subcarriers (defined by the order of the spectral transforms) and the allocated transmission band (RF frequency), and no longer on the channel configuration.

[0067] On the contrary, traditional systems require, by combining the carriers in analog and final frequency, the use of expensive cavity filters which must necessarily be manufactured according to the frequency configuration of the different transmission channels.

[0068] Thus, the transmission system configuration, comprised of MDM and EXU stages, significantly reduces manufacturing costs and drastically shortens the manufacturing and assembly time of a multi-channel system. Indeed, the system can be manufactured and stored, and the carrier configuration, performed solely via software, can be carried out immediately after system installation.

[0069] The transmission system comprised of the MDM and EXU stages also has the advantage of utilizing the full transmission power of the PA amplification stage to output signals across the various transmission channels. In contrast, a significant portion of the power generated in traditional systems is dissipated in cavity filters and circulators. This allows for transmission distances considerably greater than those achievable with traditional systems.

[0070] Furthermore, since it is not necessary to use cavity filters covering a single transmission channel, it becomes possible to use higher frequency bands, particularly in the 15 GHz, 18 GHz, 23 GHz bands and above, knowing that it is possible to make cavity filters with the properties required for BPF1 and BPF2 filters at these frequencies.

[0071] It may be desirable to transmit across several non-contiguous frequency bands. Thus, the figure 11 represents a DTX1 transmission chain of a multi-channel transmission system, according to another embodiment. The DTX1 transmission chain comprises an MDST modem stage and a TCV1 conversion stage replacing the TCNV conversion stage. The MDST modem stage comprises several MDM1, MDM2, ... MDMn modems, which may be identical to the MDM modem described with reference to figures 4 And 7The TCV1 conversion stage includes digitally controlled frequency converters (UFC, UFC1, UFC2, ... UFCn) and a digital-to-analog converter (DAC). Each frequency converter (UFC, UFC1-UFCn) includes a digital frequency mixer connected to a digitally controlled oscillator. Each frequency converter (UFC1-UFCn) is connected to the output of one of the modems (MDM1-MDMn).

[0072] Each MDM1-MDMn modem in the MDST modem stage receives data to be transported (Dx i) in binary form and provides baseband samples in several adjacent channels to its respective UFC1-UFCn converter. Each MDM1-MDMn modem generates samples in its respective set of channels. Each UFC1-UFCn converter raises the frequency of the output samples from the MDM1-MDMn modem to a first intermediate frequency centered within its respective frequency band. The frequency bands thus allocated to the output samples of the modems are spaced apart so that, after translation to the RF frequency, the output signals of each MDM1-MDMn modem are located within the frequency band allocated to that modem. The frequency bands thus allocated to the MDM1-MDMn modems may be non-contiguous, i.e., separated by frequency bands that are not to be used.The samples output from the UFC1-UFCn converters are summed by an ADD adder into a single sample stream, which is then processed by the UFC converter to raise the output frequency of the UFC1-UFCn converters to the intermediate frequency. The DAC converts the UFC converter's output signal into an analog signal, which can then be processed by the AST analog stage.

[0073] THE figures 12A, 12B, 12C represent, respectively, examples of output signal spectra of the UFC1-UFCn converters, with n = 3, of the output signal of the UFC converter, and of the output signal of the FM1 converter of the AST stage, the output signal of the UFC converter being processed by the analog AST stage. The frequency scales (on the x-axis) used respectively in the figures 12A, 12B, 12C are different. On the figure 12A The spectra S1, S2, and S3 of the signals generated respectively by converters UFC1, UFC2, and UFC3 are superimposed, and comprise respectively a frequency band B1 centered on the intermediate frequency IF1 used by converter UFC1, a frequency band B2 centered on the intermediate frequency IF2 used by converter UFC2, and a frequency band B3 centered on the intermediate frequency IF3 used by converter UFC3. The frequency bands B1, B2, and B3 may have different widths and be subdivided into different numbers of channels.

[0074] On the figure 12B which represents the output signal of the UFC converter, the frequency bands B1, B2, B3 are centered respectively on the frequencies IF1+IF, IF2+IF and IF3+IF. On the figure 12C which represents the spectrum of the output signal of the FM1 frequency converter, the frequency bands B1, B2, B3 are centered respectively on the frequencies IF1+IF+RF, IF2+IF+RF and IF3+IF+RF.

[0075] It can be observed that the UFC1 converter can be omitted, the baseband output signal from the MDM1 modem then being supplied directly to the UFC converter. According to another embodiment illustrated by Figure 13, representing a TCV2 conversion stage, the UFC converter is omitted. Thus, the TCV2 conversion stage differs from the TCV1 conversion stage in that it comprises frequency converters UFC1', UFC2', and UFCn' connected to the ADD adder, and in that the ADD adder is connected directly to the DAC. The UFC1'-UFCn' converters are each connected to a respective MDM1-MDMn modem and each supply a digital signal directly in the final output frequency band of the TCV1 conversion stage. In this case, the figure 12A represents the spectra of the signals before being combined by the adder AD1; the frequency translation step illustrated by Figure 12B is omitted, and the center frequencies of the B1, B2, B3 bands on the figure 12C are respectively equal to IF1+RF, IF2+RF and IF3+RF.

[0076] The frequency of the analog output signal from the TCNV, TCV1, TCV2 conversion stage is relatively high. The TCNV, TCV1, TCV2 conversion stage can be placed in an external unit with the AST analog stage to be positioned as close as possible to the AT antenna. However, if the number of modems in the MDST modulation stage is high, the number of links to the conversion stage is also high. Therefore, it may be desirable to limit this number of links. Thus, the figure 14 represents the DTX2 transmission chain of a multi-channel transmission system, according to another embodiment. The transmission chain of the figure 14 It comprises a modulation stage MDS1 and a conversion stage TCV3. The modulation stage MDS1 differs from the MDST stage in that it includes a serialization circuit ENS configured to generate a single serial signal SS into which the output signals of the MDM1-MDMn modems are multiplexed. The conversion stage TCV3 differs from the TCV1 stage in that it includes a deserialization circuit DES connected to the serialization circuit ENS and configured to extract the various signals from the serial signal SS and transmit them to the UFC1-UFCn converters. The signal from the MDMi modem is transmitted to the UFCi converter, i = 1, ..., n. Thus, the modulation stage MDS1 can be placed in an easily accessible location, and the TCV3 stage can be placed with the analog AST stage as close as possible to the AT antenna, with the MDS1 and TCV2 stages being connected by a simple serial SS link.

[0077] In one embodiment, the SS serial link can be established using an optical fiber. Furthermore, the TCV3 conversion stage may not include a UFC1 converter or may only include the UFC1'-UFCn' converters.

[0078] According to one embodiment, the ENS and DES converters are implemented with JES204B components marketed by Texas Instruments ™< comprising a transmitter component on the modem side and a receiver component implementing one or the other of the TCV1, TCV2, TCV3 converters.

[0079] It will be readily apparent to those skilled in the art that the present invention is susceptible to various embodiments and applications. In particular, the invention is not limited to predicting subcarrier channel filtering processes, both in transmission following inverse spectral transformation and in reception before spectral transformation. Indeed, it can be provided that the modem inserts cyclic prefixes in transmission and removes these cyclic prefixes in reception.

[0080] It is also unnecessary for the connection between the MDM modem or the MDST / MDS1 modem array, on the one hand, and the TCV1 / TCV2 / TCV3 converter, on the other, to be made via fiber optic cable. In fact, using such a connection avoids drops in the quality of transmitted signals, particularly when these devices are far apart. If these devices are installed close to each other, the loss of transmitted signal quality is negligible.

[0081] In the example of figures 8 à 10 The final frequency, or center of the transmission band, is equal to the sum of the intermediate frequencies IF and RF (or RF' and IF'). It goes without saying that the final frequency chosen can be the RF-IF frequency (or RF'-IF'), particularly depending on the intended transmission and reception frequency bands. Thus, the final frequency can be chosen so that none of the RF-IF, RF, and RF+IF images of the frequency shift (FM1) fall within the reception band. Therefore, if the reception band is located at higher frequencies than the TXB transmission band, it is advantageous to choose the RF+IF frequency as the final frequency of the transmission band. If the reception band is located at lower frequencies than the TXB transmission band, it is advantageous to choose the RF-IF frequency as the final frequency of the transmission band.In the latter case, the BPF1 and BPF2 filters described earlier must be adapted to reject the RF frequency and a frequency band around the RF+IF frequency. This provision is also applicable to the embodiments of the . figures 11 à 14 implementing intermediate frequencies RF, IF, IF1, IF2, IF3.

Claims

1. A multichannel data transmission method comprising the steps of: generating from a data stream to be transmitted (Dix), a plurality of frequency-domain digital sample streams (XI,j), each digital sample stream modulating a respective subcarrier defining a respective subcarrier channel (CPj), distributing the subcarrier channels (CPj) over a plurality of consecutive transmission channels (C1-C8) having a width of at least 3.5 MHz, attributing null sample streams to the subcarriers of unused transmission channels (C2), converting the frequency-domain digital sample streams into temporal digital symbol streams (xI,j), using an inverse spectral transform (IDFT), without inserting cyclic prefixes between the symbols, filtering each temporal digital symbol stream (xI,j) by a respective digital filter (PFj), combining the filtered temporal digital symbol streams into a composite temporal digital symbol stream (xi), frequency shifting the composite temporal digital symbol stream using a digital oscillator (NO1) to an intermediate frequency above 1 GHz, converting the shifted composite digital symbol stream into an analog signal (x(t)), frequency-shifting the analog signal to a transmission frequency (RF+IF) above 2 GHz, using an analog oscillator (LO1) at a carrier frequency (RF), filtering the analog signal to attenuate the carrier frequency, and transmitting the shifted analog signal via an antenna (AT).

2. The method according to claim 1, wherein the composite temporal digital symbol stream (xi) is transmitted via an optical link before being shifted to the intermediate frequency (IF).

3. The method according to claim 1, comprising the steps of: generating a plurality of further composite streams of temporal digital symbols in parallel, and shifting each of the further composite streams of temporal digital symbols to a respective intermediate frequency (IF1, IF2, IF3) by means of a respective digital oscillator (UFC1-UFCn), each of the shifted composite streams of temporal digital symbols being transmitted in analog form in a respective frequency band (B1, B2, B3).

4. The method according to claim 1, wherein the data stream (Dxi) to be transmitted undergoes scrambling (SCB) so that bits of the stream have a random distribution, coding (COD) to introduce redundancy or error correction data into the stream, interleaving (INTL) to form a stream of data blocks, and modulation (MOD) to transform the stream of data blocks into a stream of frequency-domain samples (Xi) in the form of complex numbers.

5. The method according to claim 4, wherein the scrambling and coding are performed by a plurality of scrambling modules (SCB) and a plurality of coding modules (COD) in parallel to increase a transmission rate of the data stream transmitted for interleaving (INTL).

6. A transmission device configured to implement the method according to any of claims 1 to 5.

7. A device according to claim 6, comprising: a modem (MDM) for generating the composite temporal digital symbol stream (xi), a converter (TCNV) for shifting the composite temporal digital symbol stream to the intermediate frequency (IF) and converting the composite symbol stream shifted to the intermediate frequency into analog signals (x(t)), and an analog stage (AST) connected to an antenna (AT) for generating the shifted analog signals.

8. The device according to claim 7, wherein the modem (MDM) is connected to the converter by an optical link.

9. The device according to one of claims 7 and 8, wherein the modem belongs to a group of a plurality of modems (MDM1-MDMn) connected to a respective input of the converter (TCV1, TCV2).

10. The device according to claim 9, comprising: a parallel / serial conversion circuit (ENS) comprising a plurality of inputs connected respectively to the modems (MDM1-MDMn), and a serial-to-parallel conversion circuit (DES) connected to the parallel-to-serial conversion circuit by a fiber optic link (SS) and comprising one output per modem connected to the input of a respective frequency conversion circuit (UFC1-UFCn) for shifting the composite temporal digital symbol stream (xi) supplied by a respective modem, to a respective intermediate frequency (IF1, IF2, IF3).

11. The device according to one of claims 7 to 10, wherein each modem (MDM, MDM1-MDMn) is implemented by a processor controlled by a program or by a programmable circuit.