METHOD AND SYSTEM FOR MULTICHANNEL POINT-TO-POINT RADIO TRANSMISSION WITH LIMITED COUPLING LOSS

Abstract

METHOD AND SYSTEM FOR MULTICHANNEL POINT-TO-POINT RADIO TRANSMISSION WITH COUPLING LOSS LIMITATION The invention relates to a data transmission method, comprising steps of: generating from a data stream to be transmitted (Dxi), a plurality of frequency-domain digital sample streams (Xl,j), each digital sample stream modulating a respective subcarrier defining a respective subcarrier channel (CPj), converting the frequency-domain sample streams into time-domain sample streams (xl,j) by an inverse spectral transform (IDFT), combining the time-domain sample streams into a composite stream (xi), frequency-shifting the composite stream, using a digital oscillator (NO1) to an intermediate frequency greater than 1 GHz, converting the shifted composite stream into an analog signal (x(t)), frequency-shifting the analog signal to a transmission frequency greater than 2 GHz,Using an analog oscillator (LO1) at a translation frequency (RF), filter the analog signal to attenuate the translation frequency, and transmit the filtered signal through an antenna (AT). Figure for the abbreviation: Fig. 8,

Description

Title of the invention: METHOD AND SYSTEM FOR MULTICHANNEL POINT-TO-POINT RADIO TRANSMISSION WITH LIMITATION OF COUPLING LOSSES

[0001] The present invention relates to point-to-point transmissions by radio waves used in particular in transport networks forming the infrastructure of mobile networks, or in certain private radio networks. Typically, these transmissions are carried out in frequency bands located 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 the capacity of their networks. Thus, the speeds offered by point-to-point links have been significantly increased thanks to the use of new techniques, in particular new increasingly complex modulation techniques such as MAQ2048 or MAQ4096. However, the speeds of point-to-point links remain limited by the width of the frequency bands allocated to these links.

[0003] Furthermore, increasing the width of transmission channels is a very complex process because these channels are defined by international regulations and therefore cannot evolve quickly. Furthermore, such point-to-point radio transmission links have been in operation for several decades. The radio spectrum has therefore already been allocated in the form of channels of defined width, and the radio spectrum still available therefore consists of narrow and mostly non-adjacent channels.

[0004] Figure 1 shows a traditional single-channel transmission system. This system comprises an internal unit IDU and an external unit ODU coupled to an antenna ATI. The external unit ODU must be placed as close as possible to the antenna ATI, while the internal unit IDU can be placed remotely, for example in a room where it can be easily accessible for maintenance operations. The internal unit IDU comprises a modem M1 managing the transmission and reception of a radio signal. The digital output signal, on the transmission side, of the modem M1 is converted by a digital / analog converter DI into an analog signal which is filtered by a bandpass filter BF1, the filtered analog signal being supplied to a first frequency mixer 1 to apply to the analog signal a translation of an intermediate frequency IFx generated by a local oscillator O1.The modulated IFx carrier is transmitted to the external unit ODU where it is converted to a final carrier frequency RFx+IFx by a second frequency mixer 2 also receiving an intermediate carrier at frequency RFx from a local oscillator 02. The output signal . of the frequency mixer 2 is amplified by an amplifier Al, then filtered by a bandpass filter BF2. The filter BF2 is connected to the ATI antenna via a three-way duplexer 3, such as a mechanical circulator, to connect the antenna also to an ORC-IRC reception chain.

[0005] The modem M1, the converter DI and the frequency mixer 1 of the intermediate frequency IFx are today commonly integrated in a dedicated modem circuit. The frequency mixer 2 towards the final carrier is also commonly integrated in a dedicated integrated power circuit, the signal being transmitted from the modem circuit to the power circuit at the frequency IFx.

[0006] The spectrum of the output signal from the mixer 2 is shown in Figure 2. This signal comprises three components, namely two components centered respectively on the frequencies RFx-IFx and RFx+IFx and modulated by the analog signal from the modem M1, and one component at the frequency RFx. The bandpass filter BF2 makes it possible to eliminate one of the two images of the modulated signal 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 greater than 6 GHz and the frequency IFx is less than 400 MHz so that the signal can be transmitted without excessive losses between the internal unit IDU and the external unit ODU. The filter BF2 must therefore be very selective. At the frequencies concerned, only mechanical filters or cavity filters make it possible to 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 ATI. For this purpose, three-way mechanical circulators C1, C2 are used instead of the duplexer 3, and connected in series to the antenna ATI. In order 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, to reflect the signal emitted by the chain TX2 towards the channel of the circulator C1 connected to the antenna. Thus, the signal emitted by the chain TX2 is transmitted by the circulators C2 and C1 towards the filter F1, which reflects the received signal towards the antenna ATI.

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

[0009] It follows that the filters F1, F2 must not only be passable only for the signal emitted by the transmission chain TX1, TX2 to which it is connected, but also have a high reflection coefficient for the signals emitted by the other transmission chains connected to the antenna. It turns out that the required filters Fl, F2 are even more selective than the BF2 filter placed at the output of the transmission chain shown in Figure 1, and are therefore even more complex and expensive.

[0010] Furthermore, it is difficult to obtain high reflection coefficients. As a result, the signals to be transmitted undergo strong attenuations when they pass through the circulators. Thus, passing through two circulators in transmission and reception 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. Furthermore, the cavity filters are structurally adapted to the frequencies to be filtered, so that a modification of the transmission channel requires the replacement of 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] Cavity filter manufacturing technologies also impose a frequency limit. There is practically no solution for producing cavity filters operating at frequencies above 13 GHz, knowing that the available frequency bands targeted reach 42 GHz.

[0013] A two-carrier or two-channel system has also been proposed, comprising a modem generating 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 a frequency translation of these two carriers to place them at frequencies combining the initial position of each carrier relative to the first intermediate frequency and the second intermediate frequency used to translate the two carriers to the final frequency.

[0014] This system must perform several conversions, analog / digital and inverse, which generates a large number of parasitic images which must then be filtered. The positions of the carriers at the modem output around the first intermediate frequency and the second intermediate frequency impose strong constraints on the characteristics of the filter which must process the images of the two channels and eliminate the parasitic images. In addition, the complexity of this system increases exponentially if the number of channels is to be increased. In practice, this number can hardly be extended beyond two.

[0015] A subcarrier modulation technique OFDM ("Orthogonal Frequency-Division Multiplexing") has also been proposed, making it possible to carry out multi-channel transmission by grouping subcarriers. However, the efficiency The spectral power of such a technique decreases rapidly if the channel width is reduced.

[0016] It is therefore desirable to be able to increase the throughput of point-to-point multichannel radio links by using the available frequency bands. It is also desirable to avoid having to use selective and therefore expensive mechanical filters. It may also be desirable to produce a multichannel transmission system in which the number and width of the channels can be modified.

[0017] Embodiments relate to a method for transmitting data, comprising steps consisting in: generating from a data stream to be transmitted, a plurality of streams of frequency digital samples, each stream of digital samples 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 streams of frequency digital samples into streams of time digital samples, by an inverse spectral transform, combining the streams of time digital samples into a composite stream of time digital samples, frequency-translating the composite stream of time digital samples, using a digital oscillator at an intermediate frequency greater than 1 GHz, converting the translated composite stream of samples into an analog signal,frequency-translating the analog signal to a final transmission frequency above 2 GHz, using an analog oscillator at a translation frequency, filtering the analog signal to attenuate the translation frequency, and transmitting the translated analog signal through an antenna.

[0018] According to one embodiment, the method comprises a step of filtering each stream of digital time samples by a respective digital filter before generating the composite stream of digital time samples.

[0019] According to one embodiment, the composite stream of digital time samples is transmitted by an optical link before being translated to the intermediate frequency.

[0020] According to one embodiment, the subcarrier channels are grouped into transmission channels dividing a frequency band, one of the transmission channels gathering subcarrier channels not used to transmit data streams.

[0021] According to one embodiment, the method comprises steps consisting of: generating in parallel several composite streams of time-domain digital samples, and translating each of the composite streams of time-domain digital samples, to a respective intermediate frequency using a respective digital oscillator, each of the translated composite streams of time-domain digital samples being transmitted in analog form in a respective frequency band.

[0022] According to one embodiment, the data stream to be transmitted 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.

[0023] According to one embodiment, several scrambling and coding processes are carried out in parallel to increase a transmission rate of the data stream transmitted to the interleaving process.

[0024] Embodiments also provide a method for receiving data comprising steps of: receiving an analog signal by an antenna, translating the received analog signal to an intermediate frequency greater than 1 GHz and lower than a transmission frequency of the analog signal greater than 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, generating, from the composite stream of baseband digital samples, using a spectral transform, a plurality of streams of frequency-domain digital samples,each stream of digital frequency samples being associated with a subcarrier defining a respective subcarrier channel, and generating a stream of transmitted data from the streams of digital frequency samples.

[0025] Embodiments also relate to a transmission device configured to implement one or other of the previously defined methods.

[0026] According to one embodiment, the device comprises 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.

[0027] According to one embodiment, the modem is connected to the converter by an optical link.

[0028] According to one embodiment, the device comprises a plurality of modems connected to a respective input of the converter.

[0029] According to one embodiment, the device comprises a parallel / serial conversion circuit comprising a plurality of inputs connected respectively to the modems, and a serial / parallel conversion circuit connected to the conversion circuit parallel / serial via a fiber optic link and comprising a modem output connected to the input of a respective frequency conversion circuit for translating the composite stream of digital time samples provided by a respective modem, to a respective intermediate frequency.

[0030] According to one embodiment, each modem is produced by a processor controlled by a program or a programmable circuit.

[0031] Examples of embodiments of the invention will be described in the following, without limitation, in relation to the attached figures, among which:

[0032] [Fig.l] described above represents a traditional single-channel transmission system,

[0033] [Fig.2] described previously, represents a frequency spectrum of a signal to be transmitted by the transmission system of Fig. 1,

[0034] [Fig. 3] described previously illustrates a device for generating a signal distributed across several transmission channels,

[0035] [Fig.4] schematically represents the transmission part of a multi-channel digital transmission modem, according to one embodiment,

[0036] [Fig.5] schematically represents an example of the spectrum of the signal produced by the transmission chain,

[0037] [Fig.6] schematically represents another example of the spectrum of the signal produced by the transmission chain,

[0038] [Fig.7] schematically represents the reception part of a multi-channel digital transmission modem, according to one embodiment,

[0039] [Fig.8] schematically represents an external unit of a multi-channel transmission system, according to one embodiment,

[0040] [Fig.9] schematically represents an example of a spectrum of a signal emitted by the multi-channel transmission system, according to one embodiment,

[0041] [Fig. 10] schematically represents an example of a template of a filter of the multi-channel transmission system with respect to transmission channels implemented by transmission chain,

[0042] [Fig. 1 1] schematically represents another embodiment of a digital transmission chain of a multi-channel transmission system,

[0043] Figures 12A, 12B, 12C [Fig. 12] schematically represent examples of spectra of internal signals of the multi-channel transmitter of Figure 11,

[0044] [Fig. 13] schematically represents an embodiment of a conversion unit of the transmission chain of the multi-channel transmission system of Fig. 11,

[0045] [Fig. 14] schematically represents the transmission chain of a multi-channel transmission system according to another embodiment.

[0046] Figure 4 shows an MTX transmission part of a digital modem MDM of a digital data transmission system. The MTX transmission part can be of OFDM ("Orthogonal Frequency-Division Multiplexing") type. According to one embodiment, the MTX transmission part is configured to carry out a multi-channel transmission in an allocated frequency band. The MTX transmission part receives digital data to be transported Dx; from a physical interface, for example of the Ethernet type, which can be connected to an external switch providing the data to be transported. The MTX transmission part comprises SCB scrambling, CD coding, INTL interleaving, and MOD modulation modules, a SP serialization circuit, an IDFT transformation circuit from the frequency domain to the time domain and an ADD adder.The scrambling module SCB is configured to process the received binary data stream so that the bits in the stream have a predefined random distribution to avoid long sequences of bits of the same value. The coding module CD introduces redundancy or error correction data into the scrambled string.

[0047] The interleaving module INTL receives the output signals from the encoder CD to interleave them with each other and form data blocks. The modulator MOD receives the data blocks at the output of the interleaving module INTL to modulate them by producing frequency samples X; in the form of complex numbers distributed according to a multi-state modulation constellation. The modulation applied can be for example a quadrature amplitude modulation QAM ("Quadrature amplitude modulation"), or a PSK (Phase Shift Keying) type modulation such as BPSK (Binary PSK) or QPSK (Quadrature PSK). The serialization circuit SP distributes the modulated frequency samples X; from the modulator MOD into vectors Xi = (Xi>0, ..., Xi^.i) of size N, N being greater than 1.

[0048] The IDFT transformation circuit performs a transformation from the frequency domain to the time domain by transforming the frequency vectors Xi = (Xi>0, ..., Xi^.i) into digital time symbols Xi = (xi>0, ..., Xi>Ni) formed of N time samples xio, ..., xijN_i. This transformation is performed by applying an inverse spectral transform of order N to the modulated samples Xi>0, ..., XLX । provided by the modulator MOD. The inverse spectral transform may be, for example, an inverse fast Fourier transform or an inverse wavelet transform. The digital signal x(t) consisting of the symbols xi thus produced has an amplitude spectrum as illustrated in FIG. 5. Each sample Xij produced corresponds to a modulated subcarrier j, the width of the frequency band allocated to the transmission being divided into N subcarrier channels CPj, each corresponding to a subcarrier.

[0049] The highest frequency of the allocated frequency band may correspond to the highest frequency of the subcarrier channel located at the highest frequencies, and the lowest frequency of the allocated frequency band may 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 width of the subcarrier channels, and the higher the working frequency of the MDM modem, which also depends on the total width of the processed frequency band.

[0050] According to one embodiment, the subcarrier channels are grouped into transmission channels. In the field of microwave transmissions, international regulations impose predefined transmission channel widths. Depending on the country and the transmission frequency (6 GHz or 38 GHz for example), the transmission channel widths are imposed. Typically, the transmission channel widths are 3.5 MHz, 7 MHz, 14 MHz, 28 MHz, 40 MHz, 56 MHz or 112 MHz. International telecommunications standards (defined by the ITU) impose emission templates to limit interference generated outside each of the transmission channels. The narrower the transmission channels, the lower the off-channel energy level must be.

[0051] It turns out that the value of a last sample XmjN_i of a symbol Xn 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 signal resulting from the concatenation of the symbols conventionally carried out by a serializer or parallel / serial converter, as well as interference between subcarrier channels. To avoid these drawbacks, cyclic prefixes are conventionally inserted between the symbols at the output of the IDFT transformation circuit.Each cyclic prefix consists of samples calculated by applying a window between the samples at the ends of each symbol and a cosine-squared 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.

[0052] Cyclic prefixes make transmission more robust in the case of multi-path transmission, which is essential in the field of mobile transmissions, typically from a relay station to a mobile phone. They also make it possible to reduce the amount of energy transmitted outside of each subcarrier channel. This amount of energy can be reduced by increasing the ratio between the duration of each cyclic prefix and the duration of the symbol, or by reducing the spacing between the subcarriers. If the width of the allocated frequency band is fixed, the reduction of the spacing between the subcarriers is carried out by an increase in the order N of the spectral transform, which reduces the width of the subcarrier channels. However, carrying out transmissions in very narrow transmission channels (3.5 MHz or 7 MHz) and therefore the implementation of a high order N of the spectral transform and an extremely constrained transmission template, considerably increases the complexity of the MTX transmission part of the MDM modem.

[0053] Furthermore, increasing the duration of the cyclic prefixes significantly reduces the spectral efficiency of the transmission system, due to the fact that an increasing fraction of the transmission time is occupied by the transmission of the cyclic prefixes, the portion devoted to the transmission of useful data being reduced accordingly.

[0054] According to one embodiment, the MTX transmission part of the MDM modem does not insert cyclic prefixes between the symbols xb but comprises a set of N digital filters PF0-PFN i respectively receiving the N samples xi>0, ..., xLX । of each symbol xb, the number of subcarrier channels being fixed. The digital filters PF0-PFN i have a frequency response configured to considerably attenuate the emissions outside the N subcarrier channels respectively, and an impulse response making it possible to limit the discontinuities between the extreme samples XujN4 and Xi>0 of pairs of consecutive symbols xbi, xb. For this purpose, the PFO-PFX । filters are for example of the polyphase type, such as the PHYDYAS, IOTA (Isotropie Orthogonal Transform Algorithm) and MMB (Martin - Mirabassi - Bellange) filters.

[0055] Thus, thanks to the suppression of the cyclic prefixes and the filtering of the subcarriers, the spectral efficiency remains constant regardless of the width of the subcarrier channels. The number N of subcarriers being fixed, the width of the subcarrier channels depends on the width of the frequency band allocated to the transmission. The suppression of the cyclic prefixes also makes it possible to maintain the same spectral efficiency when the width of the subcarrier channels is modified.

[0056] Furthermore, the off-channel transmission level can be reduced below the levels required by the transmission templates defined for the channels, including when the channels are wider. By thus avoiding off-channel transmissions, the order N of the inverse Fourier transform can be reduced compared to an OFDM transmission solution, which makes it possible to simplify the implementation complexity of the modem.

[0057] According to one embodiment, bad samples are assigned to certain components Xij of the vectors Xi = (Xi.o, ..., Xi^.i) of size N provided at the input of the IDFT transformation circuit so as not to transmit any signal in certain transmission channels. It is thus possible to carry out multi-channel transmission, occupying only selected channels, within a frequency band covered by the MDM modem.

[0058] Figure 6 represents an example of the spectrum of the digital signal x(t) produced at the output of the MDM modem. In the example spectrum of Figure 6, the N channels or subcarriers are distributed in groups C1, C2, ... C8 of four channels forming transmission channels, group C2 receiving only numeric samples so as not to transmit in the channels of this group.

[0059] In this way, the MDM modem can perform multi-channel transmission, in which the width of the transmission channels is defined by grouping subcarriers per channel and transmitting nuisance samples at the positions of the components of the vector of size N which correspond to the channels in which it is not desired to transmit.

[0060] Figure 7 shows the MRX reception part of the MDM modem, according to one embodiment. The MRX reception part receives digital signals y; in baseband. The MRX reception part comprises N filters PF'()-PF\4 for example of the polyphase type (like those of the MTX transmission part), a DFT transformation circuit, a parallel / series conversion circuit PS, a DMD demodulation circuit, and DTL deinterlacing, DCD decoding and DSB descrambling modules.

[0061] Each of the filters PF'0-PF'Ni has a template configured to allow the signals located in the respective subcarrier channel corresponding among the N subcarrier channels to pass, and to attenuate the signals located outside this channel. The filters PF'o-PF'Ni are for example of the polyphase type, like those of the MTX transmission part. Each filter PF'0-PF'Ni therefore provides a respective component yij (j=0, ..., Nl) of a vector yi of size N, corresponding to a subcarrier. The time samples yi o - yi^.i are transformed from the time domain to the frequency domain by the DFT transformation circuit, for example by applying a spectral transform of order N to the frequency samples Yi>0 - YijN4. The spectral transform can be for example a fast Fourier transform or a wavelet transform.The frequency samples Yi,0 - YijN4 are grouped by the serializer circuit PS into a binary stream Yi which is successively processed by the demodulation circuit DMD, the deinterleaving module DTL, the decoding module DCD and the descrambling module DSB. The DMD and DTL circuits apply inverse processing to those carried out by the circuits INTL and MOD. The DCD decoder decodes the deinterleaved data, performing, if necessary and if possible, transmission error corrections using the redundancy data. The module . DSB descrambling applies the opposite processing to that performed by the SCB scrambling module.

[0062] For example, the number N of subcarriers can be set to 512, the number of transmission channels in the allocated transmission band can be set to 8, 16 or 32 channels.

[0063] Since the MDM modem only processes digital signals, it can be entirely implemented by a processor of the microprocessor type controlled by a program or implemented by a programmable circuit such as an FPGA ("Field-Programmable Gate Array").

[0064] The SCB scrambling and CD encoding modules may require high computing power. If the programmable circuit has insufficient computing power, the SCB and CD modules may be duplicated to process only part of the data to be transmitted, the other parts of the data to be transmitted being processed in parallel by other duplicated SCB and CD modules.

[0065] Figure 8 shows an embodiment of a digital conversion stage NST and an analog stage AST of the transmission system. The digital stage NST comprises a conversion stage TCNV in transmission and a conversion stage RCNV in reception. The conversion stage TCNV is configured to convert the digital signal x; to be transmitted in baseband at the output of the modem MDM into a translated analog signal of an intermediate frequency IF. For this purpose, the conversion stage TCNV comprises a digital oscillator NO1 connected to a digital mixer of FM frequency, and a digital / analog converter DAC. The FM mixer adds to the frequency of the output samples x; of the modem MDM the intermediate frequency IF generated by the oscillator NO1, and the converter DAC converts the output signal of the mixer of FM frequency, into an analog signal which can then be processed by the analog stage AST.

[0066] Thus, the signals modulated in the different transmission channels are generated into a single digital time signal. This digital signal is then translated from the intermediate frequency IF still in the digital domain, before being converted into the analog domain at the output of the TCNV conversion stage. As a result, all the signals modulated in the different transmission channels at the output of the MDM modem can be transported by a single link to the NST digital stage. This overcomes the implementation constraints posed by traditional analog transmission systems which require, for each transmitted channel, a modem, a digital-to-analog converter, a coaxial cable transporting the signal at a low intermediate frequency to limit losses and an external ODU unit (Figure 1).

[0067] The analog stage AST comprises a transmission chain TXC, a reception chain RXC and a duplexer DX coupling the chains TXC, RXC to an antenna AT. The transmission chain TXC receives the analog signal x(t) at the output of the converter DAC of the conversion stage TCNV. The transmission chain TXC comprises a local oscillator LOI, a frequency mixer FMI, an amplifier PA, and two bandpass filters BPF1, BPF2. The oscillator LOI generates a signal at the intermediate frequency RF which it transmits to the mixer FMI. The analog signal at the output of the stage TCNV is translated to the final frequency RF+IF by the mixer FMI, then filtered by the bandpass filter BPF1 before being amplified by the amplifier PA. The amplified signal at the output of 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 reception chain from interference caused by the transmission signal which could "dazzle" or desensitize the RXC reception chain.

[0068] The RXC reception chain comprises a low-noise LNA amplifier, a frequency mixer FM2, a voltage-controlled local oscillator LO2, and two bandpass filters BPF3, BPF4. The filter BPF3 receives signals from the antenna AT and transmitted by the duplexer DX. The filtered signals at the output of the filter BPF3 are amplified by the LNA amplifier, then translated by an intermediate frequency IF' by the mixer FM2 also receiving a frequency RF' from the oscillator LO2. The translated signals of the intermediate frequency provided by the mixer FM2 are filtered by a bandpass filter BPF4 before being transmitted to the conversion stage RCNV. The filter BPF3 can have a template similar to the template G2, but only allowing the RXB band to pass (centered on the frequency IF'+RF'). The BPF4 filter can have the G1 mask so as to reject signals having frequencies lower than or equal to the intermediate frequency RF'.

[0069] The RCNV conversion stage in reception receives the analog signals y(t) translated to the intermediate frequency, provided by the filter BPF4. The RCNV conversion stage is configured to convert the analog signal y(t) into baseband and generate digital signals y; capable of being processed by the MDM modem. For this purpose, the RCNV conversion stage comprises an analog / digital converter ADC connected to a digital frequency mixer FM3 also receiving the intermediate frequency IF' from a digital oscillator NO2 with digital control. The ADC converter converts the signal y(t) into digital samples. The frequency mixer FM3 lowers the frequency of the samples, from the intermediate frequency to the baseband. The samples y; in baseband are transmitted to the reception part MRX of the MDM modem.

[0070] The BPF1 filter makes it possible to obtain a single useful image of the signal at the output of the FMI mixer. Figure 9 represents the spectrum of the output signal of the mixer FMI, as well as the template of the BPF1 filter. The output signal of the FMI mixer has three components, namely two components centered respectively on the RF-IF and RF+IF frequencies and modulated by the analog signal from the TCNV stage, and one component at the RF frequency. In the example in Figure 9, the BPF1 filter eliminates the image of the modulated signal centered on the RF-IF frequency and the component at the RF frequency.

[0071] According to one embodiment, the intermediate frequency IF is set to a value greater than 1 GHz. For example, for an emission band located around 18 GHz and having a typical width of 1 GHz, the frequency IF is set to at least 2 GHz. It is then possible to use only a BPF2 filter, the BPF1 filter then becoming accessory, to reject the complete emission frequency band.

[0072] By setting the intermediate frequency IF to a relatively high value, greater than 1 GHz, it is possible to implement a relatively inexpensive filter (BPF1). Indeed, the choice of a relatively high intermediate frequency IF makes it possible to reduce the selectivity of the filter BPF1 compared to the filter BF2 (figure 1). Thus, the filter BPF1 can have a selectivity of 20 dB at the RF frequency. The filter BPF1 can therefore be produced using a technology that is much less expensive than that of cavity filters, such as microstrip technology. Furthermore, since the rejection requirement is lower, it is possible to place the filter BPF1 before the amplifier PA, whereas it is necessary in systems of the type in Figure 1 to place the filter BF2 after the amplifier AL.

[0073] Figure 9 also shows 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.

[0074] For example, the intermediate IF and final RF+IF frequencies are respectively equal to 1.5 GHz and 11 GHz, and the width of the TXB transmission bandwidth is 480 MHz, between 10.76 and 11.24 GHz. The RF frequency is therefore set at 9.5 GHz. The BPF1 filter can therefore be configured to reject frequencies below 10 GHz to achieve a rejection ratio at the RF frequency of 9.5 GHz of 20 dB.

[0075] As the frequency of the analog output signal of the TCNV conversion stage is relatively high (around the IF frequency), higher than the frequencies of signals transmitted by cable of several meters, it is desirable to limit the length of the link between the output of the TCNV conversion stage and the analog stage AST. The digital conversion stage NST can then be placed in the same external unit EXU with the analog stage AST. On the other hand, the signals to be transmitted between the MDM modem and the NST conversion stage are digital signals and therefore less sensitive to noise. These digital signals can therefore be transmitted over a relatively longer distance than the signals passing between the NST conversion stage and the AST analog stage. Thus, the connection between the MDM modem and the TCNV conversion stage can be made by coaxial cable or optical fiber.

[0076] Figure 10 shows in more detail the transmission channels C1-C8 and C1'-C8' located in the frequency bands TXB, RXB allocated respectively to transmission and reception, as well as the template G2 of the filter BPF2. The filter BPF2 is configured to have a wideband template, allowing the entire frequency band TXB allocated to transmission to pass, and having sufficient RJ rejection in the frequency band allocated to reception RXB. In particular, the RJ rejection of the filter BPF2 in the band RXB allocated to reception is significantly lower (at least 6 dB) than the amplitude of the minimum signal received by the reception chain RXC reduced by the signal-to-noise ratio of the chosen modulation. The properties required for the filter BPF2 can be obtained by a cavity filter.However, it should be noted that the selectivity required for the BPF2 filter is much less than that required for the BF1, BF2 filters (f Igure 3) which must pass only one transmission channel and reject adjacent channels.

[0077] Similarly, the intermediate frequency IF' used in the RXC reception chain can also be set to a value greater than 1 GHz. Here again, the choice of a relatively high intermediate frequency IF' makes it possible to produce the BPF4 filter with relatively low selectivity, for example 20 dB at the RF' frequency. The BPF4 filter can therefore be produced using a technology that is much less expensive than that of cavity filters, such as microstrip technology.

[0078] The transmission system formed by the MDM modem and the EXU external unit allows multi-channel transmissions to be carried out in the microwave bands by configuring the different transmission channels and the subcarrier channels by software. It is therefore possible to dynamically modify the configuration of the different channels and subcarriers, the hardware architecture of the transmission system depending 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.

[0079] On the contrary, traditional systems require, by combining the carriers in analog and final frequency, the use of expensive cavity filters which must be manufactured according to the frequency configuration of the different transmission channels.

[0080] Thus, the configuration of the transmission system formed of the MDM and EXU stages makes it possible to significantly reduce manufacturing costs, and to very significantly reduce the manufacturing and assembly time of a multi-channel system. Indeed, the system can be manufactured and stored, and the configuration of the carriers which is done only by software, can be done following the installation of the system.

[0081] The transmission system formed by the MDM and EXU stages also has the advantage of using the entire transmission power of the PA amplification stage to transmit the signals in the different transmission channels. In contrast, a large part of the power generated in traditional systems is dissipated in cavity filters and circulators. It can thus be envisaged to achieve much greater transmission distances than with traditional systems.

[0082] Furthermore, since it is not necessary to have to use cavity filters covering a single transmission channel, it becomes possible to use higher frequency bands, in particular in the bands at 15 GHz, 18 GHz, 23 GHz and beyond, knowing that it is possible to produce cavity filters having the properties required for the BPF1 and BPF2 filters at these frequencies.

[0083] It may be desirable to carry out a transmission in several non-contiguous frequency bands. Thus, Figure 11 represents a transmission chain DTX1 of a multi-channel transmission system, according to another embodiment. The transmission chain DTX1 comprises a modem stage MDST and a conversion stage TCV1 replacing the conversion stage TCNV. The modem stage MDST comprises several modems MDM1, MDM2, ... MDMn, which may be identical to the MDM modem described with reference to Figures 4 and 7. The conversion stage TCV1 comprises digitally controlled frequency converters UFC, UFC1, UFC2, ... UFCn and a digital / analog converter DAC. Each frequency converter UFC, UFC 1-UFCn comprises a digital frequency mixer connected to a digitally controlled digital oscillator. Each UFCl-UFCn frequency converter is connected to the output of one of the MDM 1-MDMn modems.

[0084] Each MDM 1-MDMn modem of the MDST modem stage receives data to be transported Dx; in binary form, and provides baseband samples in several adjacent channels, to a respective UFCl-UFCn converter. Each MDM 1-MDMn modem generates samples in a respective set of channels. Each UFCl-UFCn converter increases the frequency of the output samples of the MDM 1-MDMn modem to a respective first intermediate frequency centered in a respective frequency band, the frequency bands thus assigned to the output samples of the modems being spaced from each other, so that, after translation to the RF frequency, the output signals of each of the MDM 1-MDMn modems are located in the frequency band assigned to the modem. The frequency bands thus assigned respectively to the modems MDMl-MDMn can be non-contiguous, i.e. separated by frequency bands not to be used. The output samples of the UFCl-UFCn converters are added by an adder ADD into a single sample stream which is processed by the UFC converter to raise the frequency of the output samples of the UFCl-UFCn converters to the intermediate frequency. The DAC converter converts the output signal of the UFC converter into an analog signal which can then be processed by the analog stage AST.

[0085] Figures 12A, 12B, 12C respectively represent examples of spectra of output signals of the UFCl-UFCn converters, with n = 3, of the output signal of the UFC converter, and of the output signal of the FMI converter of the AST stage, the output signal of the UFC converter being processed by the analog stage AST. The frequency scales (on the abscissa) used respectively in Figures 12A, 12B, 12C are different. In Figure 12A, the spectra S1, S2, S3 of the signals generated respectively by the converters UFC1, UFC2 and UFC3 are superimposed, and respectively comprise a frequency band B1 centered on the intermediate frequency IF1 used by the converter UFC1, a frequency band B2 centered on the intermediate frequency IF2 used by the converter UFC2, and a frequency band B3 centered on the intermediate frequency IF3 used by the converter UFC3.The frequency bands Bl, B2, B3 can have different widths and be subdivided into different numbers of channels.

[0086] In 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. In Figure 12C which represents the spectrum of the output signal of the FMI frequency converter, the frequency bands B1, B2, B3 are centered respectively on the frequencies IF1+IF+RF, IF2+IF+RF and IF3+IF+RF.

[0087] It can be observed that the converter UFC1 can be omitted, the output signal of the modem MDM1, in baseband, then being directly supplied to the converter UFC. According to another embodiment illustrated by FIG. 13 representing a conversion stage TCV2, the converter UFC is omitted. Thus, the conversion stage TCV2 differs from the conversion stage TCV1 in that it comprises frequency converters UFC1', UFC2', UFCn' connected to the adder ADD, and in that the adder ADD is connected directly to the converter DAC. The converters UFC1'-UFCn' are each connected to a respective modem MDM1-MDMn and each provide a digital signal directly in the final output frequency band of the conversion stage TCV1. In this case, Figure 12A shows the spectra of the signals before being combined by the ADI adder, the frequency translation step illustrated by Figure 12B is omitted, and the central frequencies of the bands Bl, B2, B3 in figure 12C are respectively equal to IF1+RF, IF2+RF and IF3+RF.

[0088] The frequency of the analog output signal of the conversion stage TCNV, TCV1, TCV2 is relatively high. The conversion stage TCNV, TCV1, TCV2 can be placed in an external unit with the analog stage AST to be placed as close as possible to the antenna AT. However, if the number of modems in the modulation stage MDST is high, the number of links with the conversion stage is also high. It may therefore be desirable to limit this number of links. Thus, Figure 14 represents the transmission chain DTX2 of a multi-channel transmission system, according to another embodiment. The transmission chain of Figure 14 comprises a modulation stage MDS1 and a conversion stage TCV3. The MDS1 modulation stage 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 MDMl-MDMn modems are multiplexed.The TCV3 conversion stage differs from the TCV1 stage in that it includes a DES deserialization circuit connected to the ENS serialization circuit and configured to extract from the SS serial signal the different signals from the MDM1 modems and transmit them to the UFCl-UFCn converters, the signal from the MDMi modem being transmitted to the UFCi converter, i = 1, ..., n. Thus, the MDS1 modulation stage can be placed in an easily accessible room and the TCV3 stage can be placed with the AST analog stage as close as possible to the AT antenna, the MDS1 and TCV2 stages being connected to each other by a simple SS serial link.

[0089] According to one embodiment, the serial link SS can be established by means of an optical fiber. Furthermore, the conversion stage TCV3 may not include a UFCI converter or only include the UFCl'-UFCn' converters.

[0090] 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 other of the TCV1, TCV2, TCV3 converters.

[0091] It will be clear to those skilled in the art that the present invention is susceptible to various variant embodiments and various applications. In particular, the invention is not limited to the provision of filtering treatments per subcarrier channel, in transmission, following the inverse spectral transformation, and in reception before the spectral transformation. Indeed, it may be provided that the modem inserts cyclic prefixes in transmission and removes these cyclic prefixes in reception.

[0092] It is also not necessary for the connection between the MDM modem or the set of modems MDST, MDS1, on the one hand, and the converter TCV1, TCV2, TCV3, on the other hand, to be made by an optical fiber. Indeed, the use of such a connection makes it possible to avoid drops in the quality of the transmitted signals, particularly when these equipment is far from each other. If these equipments are installed close to each other, the loss of quality of the transmitted signals is insignificant.

[0093] In the example of Figures 8 to 10, the final frequency or the 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 (or RF'-IF') frequency, in particular according to the transmission and reception frequency bands targeted. Thus, the final frequency can be chosen so that none of the RF-IF, RF and RF+IF images of the frequency translation (FMI) is in the reception band. In the case where the reception band is located at higher frequencies than the transmission band TXB, it is therefore advantageous to choose the RF+IF frequency as the final frequency of the transmission band. In the case where the reception band is located at lower frequencies than the transmission band TXB, it is advantageous to choose the RF-IF frequency as the final frequency of the transmission band.In the latter case, the filters BPF1, BPF2 previously described will have to be adapted to reject the RF frequency and a frequency band around the RF+IF frequency. This arrangement is also applicable to the embodiments of figures 11 to 14 implementing intermediate frequencies RF, IF, IF1, IF2, IF3.

Claims

Claims

1. A method for transmitting multi-channel data, comprising the steps of: generating from a data stream to be transmitted (Dx;), a plurality of frequency digital sample streams (Xij), each digital sample stream modulating a respective subcarrier defining a respective subcarrier channel (CPj), grouping the subcarrier channels (CPj) into transmission channels (C1-C8) having a width of at least 3.5 MHz, assigning nuisance sample streams to the subcarriers of unused transmission channels (C2), converting the frequency digital sample streams into time digital symbol streams (xij), by an inverse spectral transform (IDFT), without inserting cyclic prefixes between the symbols, filtering each time digital symbol stream (xij) by a respective digital filter (PFj), combining the filtered time digital symbol streams into a composite time digital symbol stream (x;), frequency-translating the composite stream of time-domain digital symbols, using a digital oscillator (NO1) at an intermediate frequency greater than 1 GHz, converting the translated composite stream of symbols into an analog signal (x(t)), frequency-translating the analog signal to a final transmission frequency (RF+IF) greater than 2 GHz, using an analog oscillator (LOI) at a translation frequency (RF), filtering the analog signal to attenuate the translation frequency, and transmitting the translated analog signal through an antenna (AT).;

2. A method according to claim 1, wherein the composite stream of time-domain digital symbols (x;) is transmitted over an optical link before being translated to the intermediate frequency (IF).

3. A method according to claim 1, comprising the steps of: generating in parallel several other composite streams of time-dependent digital symbols, and translating each of the other composite streams of time-domain digital symbols to a respective intermediate frequency (IF1, IF2, IF3) using a respective digital oscillator (UFC1-UFCn), each of the translated composite streams of time-domain digital symbols being transmitted in analog form in a respective frequency band (B1, B2, B3).

4. Method according to claim 1, in which the data stream (Dxi) to be transmitted undergoes a scrambling processing (SCB) so that bits of the stream have a random distribution, a coding processing (COD) to introduce redundancy or error correction data into the stream, an interleaving processing (INTL) forming a stream of data blocks, and a modulation processing (MOD) to transform the stream of data blocks into a stream of frequency samples (X;) in the form of complex numbers.

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

6. Transmission device configured to implement the method according to one of claims 1 to 5.

7. Device according to claim 6, comprising: a modem (MDM) for generating the composite stream of time-domain digital symbols (x;), a converter (TCNV) for translating the composite stream of time-domain digital symbols to the intermediate frequency (IF) and converting the composite stream of samples translated to the intermediate frequency into analog signals (x(t)), and an analog stage (AST) connected to an antenna (AT) for generating the translated analog signals.

8. Device according to claim 7, in which the modem (MDM) is connected to the converter by an optical link.

9. Device according to one of claims 7 and 8, in which the modem belongs to a group of a plurality of modems (MDMl-MDMn) connected to a respective input of the converter (TCV1, TCV2).

10. 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 / parallel conversion circuit (DES) connected to the parallel / serial conversion circuit by a fiber optic link (SS) and comprising an output per modem connected to the input of a respective frequency conversion circuit (UFCl-UFCn) for translating the composite stream of time-dependent digital symbols (x;) provided by a respective modem, to a respective intermediate frequency (IF1, IF2, IF3).

11. Device according to one of claims 7 to 10, in which each modem (MDM, MDMl-MDMn) is implemented by a processor controlled by a program or a programmable circuit.