FR2767990A1 - Signal atmospheric effect reduction - Google Patents

Abstract

The two polarization received are perpendicular to each other and the second received polarization is used to attenuated undesired transmission responses.

Description

 The present invention relates generally to the attenuation of the effects linked to a telecommunications channel which are present in the received signals and, more particularly, to the use of several polarized signals in the attenuation of these effects.

 In satellite communications systems, it is necessary to transmit signals that go from satellites in orbit around the Earth to the surface of the Earth, and vice versa. These signals travel in the Earth's atmosphere, which includes the ionosphere and the troposphere. Since the characteristics of the different atmospheric layers vary over time and according to other modifications of the transmission medium, such as occultation trees, the signals moving between the satellite and terminals are subject to variable effects of fainting, blocking and distortion.

 Satellites in Low Earth Orbit (LEO) are particularly sensitive to rapid physical variations in the path over which signals are transmitted. LEO satellites move, sometimes very fast, relative to the Earth's surface, which brings the conditions (for example atmospheric conditions) present in the transmission path between a user placed on Earth and the satellite, to also change quickly. These rapid changes in the telecommunications channel commonly cause distortion and fading of the signals.

 To maintain a robust link between users on the Earth's surface and LEO satellites in orbit, it is desirable to mitigate the fading and distortion effects resulting from variations in the telecommunications channel. One known method for mitigating the effects of fading in a telecommunications channel is to work with an increased power level.

 Initially, when designing a satellite telecommunications system, a link budget is created which includes tolerances for the fading of signals due to atmospheric variations. A power level is then fixed such that, if any fading effect occurs, the power remains sufficient to overcome the effect produced. While this leads to a robust connection, excessive power is used when there is little or no fading in the channel. The use of excessive power in cases where this is not necessary is very undesirable, and could be avoided if the effects related to the telecommunications channel could be mitigated without involving a constant increase in the level of power .

 There is therefore a need for an apparatus and a method for rapidly characterizing a telecommunications channel and mitigating the effects that the telecommunications channel imposes on a signal while allowing uninterrupted transmission of user data.

The following description, intended to illustrate the invention, aims to give a better understanding of its characteristics and advantages; it is based on the appended drawings, among which
FIG. 1 shows a satellite telecommunications system according to a preferred embodiment of the invention,
FIG. 2 is a diagram showing a user terminal according to a preferred embodiment of the invention,
Figure 3 is a diagram of a user terminal according to another embodiment of the invention
FIG. 4 is a diagram of a user terminal according to another embodiment of the invention,
FIG. 5 is a diagram of a user terminal according to another embodiment of the invention,
FIG. 6 is a diagram showing a satellite according to a preferred embodiment of the invention, and
FIG. 7 is a flowchart which represents a method making it possible to attenuate the effects linked to a telecommunications channel according to a preferred embodiment of the invention.

 Reference is now made to the drawings, in which identical reference symbols designate corresponding elements, and, to begin with, in FIG. 1. FIG. 1 represents a satellite telecommunications system according to a preferred embodiment of the invention. The satellite communications system 10 includes a user terminal 30, an antenna 35, a satellite 20 and an antenna 25. Also shown in Figure I is a data signal 40 and a pilot signal 50. The satellite 20 can be a satellite moving in any orbit, including a geosynchronous orbit (GEO), a medium terrestrial orbit (MEO), and an LEO orbit. The satellite 20 communicates with the user terminal 30 by transmitting a data signal 40. The user terminal 30 is a terrestrial terminal, which can be fixed, but which is preferably mobile.

 As indicated previously, data signals moving along the telecommunications path undergo variable effects of attenuation and distortion which are due for example to the terrestrial atmosphere. To help attenuate the attenuation and the distortion, the satellite 20 transmits a pilot signal 50 substantially in the same telecommunications channel as the data signal 40. The data signal 40 and the pilot signal 50 are polarized in a substantially orthogonal manner, so they can be received separately under conditions where there is little or no cross-polarization between the two signals.

 Linear polarization of signals in free space is a known method which allows several distinct communications using the same carrier wave. For example, a first signal may be a vertically polarized signal using a carrier, and a second signal may be a horizontally polarized signal using a carrier of the same frequency. As long as the two signals are perfectly orthogonal in their polarizations, they can be completely separated in a receiver. Several polarization methods are contemplated, which include circular polarization and linear polarization.

 The data signal 40 includes information in the form of data to be transmitted from the satellite 20 to the user terminal 30, and vice versa. The data contained in the data signal 40 which is received in the user terminal 30 is generally not known to the user terminal 30 before reception. On the contrary, the pilot signal 50 is a signal which is known to the user terminal 30 before it is received. In addition, the pilot signal 50 and the data signal 40 desirably share certain attributes, such as frequency.

 The user terminal 30 receives both the data signal 40 and the pilot signal 50 using the antenna 35. The antenna 35 is an antenna which is able to receive several polarized signals with a view to their treatment with a receptor. The antenna 35 can be made of several hardware antennas, for example two linearly polarized antennas or two circularly polarized antennas, but, according to a preferred embodiment presented by way of example in FIG. 1, the antenna 35 is an antenna unique, for example a group of radiating elements with phase shift.

 In operation, the pilot signal 50 undergoes the same channel effects as the data signal 40. Since the pilot signal 50 and the data signal 40 can share attributes such as frequency, then the channel effects, in particular the fading and distortion, affect the pilot signal 50 and the data signal 40 in much the same way. Since the pilot signal 50 is known to the user terminal 30, the user terminal 30 can effectively measure the characteristics of the telecommunications channel prevailing between the satellite 20 and the user terminal 30. Once the characteristics are known of the channel, it is possible to correct the data signal 40 in the user terminal 30 so as to help mitigate the effects related to the telecommunications channel that are present in the received data signal. This correction can be carried out via conventional equalization methods, such as for example with transversal filters provided with taps having time delays, or any other method which "corrects" the desired signal and minimizes the mean square distortion or any other analog error measurement (e.g. peak distortion) of the input data signal. Alternatively, or in conjunction with the above, it is possible to employ other signal processing methods which could use the information contained in the pilot signal to minimize the measurements downstream performance errors, such as bit or symbol error rates.

 Alternatively, the user terminal 30 can return the channel information that is extracted from the pilot signal 50 to the satellite 20 using the data signal 40. The satellite 20 can then modify or "preform" any other way the data signal 40 before transmission in order to overcome the effects related to the telecommunications channel as they were measured by the pilot signal 50.

 The satellite communications system shown in Figure I can mitigate the effects of the channel almost in real time. When the characteristics of the channel change, the pilot signal 50 experiences changes, and the channel information which is provided by the pilot signal 50 can be used to ensure better reception of the data signal 40.

By thus mitigating the effects linked to the channel almost in real time, it is possible to maintain a robust link at lower power levels than would otherwise have been necessary. The savings in power consumption obtained are an advantage in that the satellite and the user terminal can then operate longer from a power source on the signals.

 Since the data signal 40 and the pilot signal 50 go from the satellite to the surface of the Earth, passing through the ionosphere, they generally undergo the same Faraday rotation, that is to say by saying this more simply, the same polarization variation. Including the pilot signal 50 known to the user terminal 30 allows the data signal 40 to have a polarization value which is measured relative to the polarization value of the pilot signal 50. The fact that a polarization is established with respect to a reference polarization allows the system to operate independently of the Faraday rotation which is caused by the ionosphere.

 The apparatuses and methods according to the invention can be advantageously used in a variety of frequency bands, but since the effects of Faraday rotation become less pronounced at higher frequencies, the invention preferably acts at the band level Ka, or above.

 Of course, the use of a polarized data signal and a polarized pilot signal is not limited to satellite telecommunications systems, but can be broadly applied to various possible transmission systems. The method and apparatus according to the invention are therefore not limited to satellite telecommunications. One of the many other possible uses lies in the field of terrestrial telecommunications networks and, in particular, of networks which carry out communications in direct visibility (LOS) between transmitters and receivers, or of networks which are, moreover, less affected. by reflections resulting from multiple paths. Terrestrial telecommunications networks could take great advantage of the improved quality of communications, power regulation and power savings which are offered by the invention.

 Figure 2 is a diagram showing a user terminal according to a preferred embodiment of the invention. The user terminal 200 comprises an antenna 35, a receiver 210, a circuit 250 for recovering channel information, a circuit 240 for attenuating effects linked to the channel, and a transmitter 280. The receiver 210 receives the signals of the antenna 35. More particularly, the receiver 210 receives the data signal 40 (FIG. 1) and the pilot signal 50 (FIG. 1).

The receiver 210 receives signals which are substantially orthogonally polarized and produces signals 220 and 230, which are internal to the user terminal 200. The signal 230 corresponds to the received pilot signal. The channel information retrieval circuit 250 receives the signal 230 from the receiver 210, and extracts information relating to the channel. Since the user terminal 200 knows the pilot signal 50 (FIG. 1) which is transmitted by the satellite 20 (FIG. 1), the circuit information retrieval circuit 250 can easily obtain the effects that the telecommunications channel produces on the data signal 40 (Figure 1).

 The circuit 250 for recovering channel information is implemented using known circuits and methods, such as for example a method consisting in producing filter coefficients. The channel information retrieval circuit 250 provides channel information 260 to the channel 240 mitigating circuit effects. In addition to the channel information 260, the circuit 240 for attenuating the effects linked to the channel receives the signal 220 from the receiver 210. The signal 220 represents the data signal 40 (FIG. 1) which is received by the receiver 210 Signal 220 includes the cumulative effects caused by the telecommunications channel. The channel effects attenuation circuit 240 attenuates the effects of the telecommunications channel on the signal 220, and produces a data signal 270.

 The channel-related effects attenuation circuit 240 uses known circuits and methods to attenuate the effects of the channel on the received signal. An example of a channel-related effects attenuation circuit is a filter whose coefficients are produced in the form of channel information 260. When the channel-related effects attenuation circuit 240 is a filter, this filter can be considered as a reverse channel filter, or an equalizer. The attenuation can be carried out via conventional equalization methods, such as for example using filters having time delay taps, or any other method which "corrects" the desired signal and minimizes the mean square distortion or any other measure (e.g. peak distortion) of the input data signal. The apparatus and method according to the invention can advantageously continuously update the coefficients of the filter, which brings the advantage of a continuous "learning sequence" provided by the pilot signal. This method gives the update rate of an adaptive equalizer, with the additional advantage of being more robust as regards the error performance of the channel. On the contrary, it is known that adaptive algorithms "diverge" in presence of significantly significant channel errors. Alternatively, or in conjunction with what has just been said above, other signal processing methods can be employed which could use the information from the pilot signal to minimize measurements of performance errors in downstream, such as bit or symbol error rates.

 The part of the user terminal 200 which has been described so far mitigates the effects linked to the channel only by means of the apparatus and the method implemented in the user terminal. In addition to the user terminal, it is envisaged that the satellite will play a certain role in mitigating the effects associated with the telecommunications channel.

 To this end, the channel information recovery circuit 250 also supplies the transmitter 280 with information 265 on the channel, so that it is retransmitted to the satellite 20 (FIG. 1) via the antenna 35.

 Figure 2 shows the antenna 35 receiving signals to the receiver 240 and transmitting signals from the transmitter 280. While the figures which show a preferred embodiment of the invention by way of example show that only one antenna 35 is used, we also consider the case where several antennas are used.

 The user terminal 200 as shown in FIG. 2 helps to mitigate the effects associated with a telecommunications channel almost in real time.

Even when the communications channel changes rapidly because a satellite is moving rapidly relative to the Earth, the user terminal 200 mitigates the undesirable effects produced by the communications channel.

 FIG. 3 is a diagram showing a user terminal according to another embodiment of the invention. FIG. 3 represents a user terminal 300 which is largely identical to the terminal 200 of FIG. 2, except for the fact that it does not have the circuit for attenuating effects linked to the channel.

In this other embodiment presented in FIG. 3, the entire operation for attenuating the effects linked to the channel is carried out at a location other than the user terminal 300. The circuit 250 for recovering channel information continues to receive the signal 230, which is the version of the pilot signal 50 after reception (FIG. 1), and it continues to supply the channel information 265 to the transmitter 280. The transmitter 280 transmits the channel information 265 to the using antenna 35. The channel information which is transmitted by transmitter 280 is used to modify or "pre-shape" in some other way the data signal to be received by user terminal 300, the modifications or predeformations taking place at the source of the program.

 The receiver 2 10 then receives a data signal 40 (FIG. 1) which has undergone modifications in its source and which has been subjected to the effects of the telecommunications channel. Therefore, the data signal received in the receiver 210 is a modified or pre-deformed data signal which has undergone the effects of the telecommunications channel, so that the effects related to the channel have been attenuated. The data signal 310 is then produced at the output of the receiver 210.

 FIG. 4 is a diagram showing a user terminal according to another embodiment of the invention. The user terminal 400 includes an antenna 35, a receiver 410, and a processing device, or processor, 440.

As in the embodiments described above, the receiver 410 receives the data signal 40 and the pilot signal 50 (FIG. 1), these signals being polarized in a substantially orthogonal manner. The receiver 410 then supplies signals 420 and 430 to the processor 440. The signal 420 and the signal 430 correspond respectively to the data signal 40 (FIG. 1) and to the pilot signal 50 (FIG. 1).

The signals 420 and 430 can be analog signals, but they are preferably digital signals.

 The processor 440 receives the signal 420, which represents the transmitted data signal, and the signal 430, which represents the transmitted pilot signal. The processor 440 then mitigates the effects of the telecommunications channel on the data stream by using the channel information which has been extracted from the signal 430. The processor 440 may be a digital hardware device specially designed for the intended purpose, or else which is preferable, a programmable digital signal processor. The processor 440 desirably implements a filter which has an inverse function to that of the telecommunications channel. This reverse channel filter is what is commonly called an equalizer. Processor 440 produces data 450, in which the effects of the telecommunications channel have been mitigated by using the reverse channel filter.

 FIG. 5 is a diagram showing a user terminal according to another embodiment of the invention. The user terminal 500 as shown in FIG. 5 comprises an antenna 35, a receiver 410, a processor 540 and a transmitter 310. The user terminal 500 is for the most part analogous to the user terminal 400 ( FIG. 4), with the exception that the processor 540 of the user terminal 500 supplies channel information 260 to the transmitter 3 10 with a view to their retransmission intended for the satellite 20 (FIG. 1). The processor 540 of the user terminal 500 produces a data signal 550 using one of several algorithms. An algorithm is presented by way of example in FIG. 4, where the data 550 is produced by implementing a reverse channel filter inside the digital signal processor 540.

 Another algorithm simply lets the signal 420 pass, which becomes the data signal 550. Another form of this other algorithm provides the satellite 20 (FIG. 1) with channel information 260 via the transmitter 310. The satellite 20 then modifies the data signal, or in some other way applies a preform to it before transmission. Therefore, the data signal received in the receiver 410 is a modified or preformed data signal which has been subjected to the effects of the telecommunication channel, so that the effects related to the channel are attenuated.

 The fact of equalizing the telecommunications channel almost in real time while allowing the continuous transmission of user data is an advantage provided by the method and the apparatus according to the invention. In addition to being equalized almost in real time, the data signals 420 and 430 can be saved by the processor 540, for further processing. The signal 420 can be filtered during the complementary processing so as to attenuate the effects of the telecommunications channel in deferred time.

 Figure 6 is a diagram showing a satellite according to a preferred embodiment of the invention. The satellite 20 comprises an antenna 25, a receiver 610, coding blocks 630 and 640, modulation blocks 650 and 660, a preforming block 670, and a transmitter 680. The satellite 20 transmits two distinct signals. The data signal 40 (Figure 1) is the transmitted version of the data signal 45 as shown in Figure 6. Likewise, the pilot signal 50 (Figure 1) corresponds to the pilot signal 55 as shown in Figure 6.

 The pilot signal 55 is produced inside the satellite 20 by starting up known data. These data are known to the intended recipient and are coded in the coding block 640 and modulated in the modulation block 660. The result obtained is the pilot signal 55, which will be transmitted on the telecommunications channel and received by the intended recipient, which knows the data, the code used, and the type of modulation. The data signal 45 comprises the data carrying the information and is produced when user data is coded in the coding block 630, modulated in the modulation block 650 and pre-deformed as desired in the pre-deformation block 670.

 Receiver 610 receives data signals which include channel information. The channel information received in the receiver 610 may include raw characteristic information of the channel, for example information concerning the amplitude or the phase which describes the user data signal as it is received in the user terminal. . Alternatively, and more preferably, the channel information received in the receiver 610 includes processed channel information which has been transmitted by a user terminal. This processed channel information 620 contains information that controls the encoding and modulation of the data signal and the pilot signal, and also includes preform information regarding the data signal.

 If a user terminal receiving the polarized signals as transmitted by the transmitter 680 determines that a different code or a different type of modulation is desirable, channel information is sent to satellite 20 and is received by the receiver 610, which information modifies the type of code or the type of modulation.

 In addition, a user terminal may instruct the satellite 20 to pre-transform the data signal to help mitigate the effects of the telecommunications channel on the transmitted data signal.

 Embodiments have been described with respect to user terminals, which are based on Earth, and satellites, which are based in space. It will be clearly understood that the circuits, described above, of user terminals may be located in a satellite and that the circuits, described above, of satellites may be located in a user terminal.

 FIG. 7 is a flowchart which represents a method making it possible to attenuate the effects linked to a telecommunications channel according to a preferred embodiment of the invention. The method 700 begins when a polarized pilot signal is transmitted in step 710. The polarized pilot signal comprises a type of coding and a type of modulation which are both known to the intended recipient.

 Then, in step 720, a polarized data signal is transmitted, where the data signal is polarized substantially perpendicular to the pilot signal. The data signal also has a type of coding and a type of modulation, however, the types of coding and modulation of the data signal are not necessarily the same as those of the pilot signal. The polarized pilot signal obtained in step 710 and the polarized data signal obtained in step 720 can be transmitted from different transmitters, however, they are preferably transmitted by the same transmitter.

 In step 730, the polarized pilot signal and the polarized data signal are received in a receiver. The receiver has the ability to receive perpendicularly polarized signals and process them separately while minimizing cross polarization.

 The pilot signal received in the receiver includes known data, a known coding type and a known modulation type, so that the receiver can extract the channel information from the pilot signal. This channel information is extracted from the pilot signal in step 740. The channel information is preferably in the form of filter coefficients which can be used in a reverse filter which mitigates the effects that the communications channel has had on the data received.

 After the channel information has been extracted from the pilot signal, the channel information is used to attenuate the effects of the channel on the polarized data signal, in step 750. When the channel information is in the form filter coefficients, step 750 includes filtering the received data signal. After step 750, the received data becomes data in which the effects of the telecommunications channel have been mitigated.

Of course, those skilled in the art will be able to imagine, from the devices and methods whose description has just been given by way of
merely illustrative and in no way limitative, various variants and modifications do not
outside the scope of the invention.

Claims (10)

 1. A method for attenuating the effects linked to a telecommunications channel, said method being characterized by the following operations  send, from a transmitter, a first polarized signal,  transmit, from said transmitter, a second polarized signal,  receive in a receiver said first polarized signal and said second polarized signal,  extract from said second polarized signal channel information, and  employing said channel information to mitigate the effects of the channel on said first polarized signal.  2. Method according to claim 1, characterized in that said second polarized signal is polarized substantially perpendicular to said first polarized signal.  3. Method according to claim 2, characterized in that said first polarized signal contains coded data which are not known to said receiver, and said second polarized signal contains coded data which are known to said receiver.  4. Method according to claim 2, characterized in that said extraction operation comprises the operation of determining reverse channel filter coefficients.  5. Device characterized by  a receiver (210) for receiving a first polarized signal (40) and a second polarized signal (50), said first polarized signal and said second polarized signal having been transmitted on a channel  a circuit (240) for attenuating the effects linked to the channel, this circuit being coupled to said receiver, where the circuit for attenuating the effects linked to the channel attenuates the effects of the channel on said first polarized signal (40), and  a channel information recovery circuit (250), this circuit being coupled between said receiver and said channel-related effects attenuation circuit, where said channel information recovery circuit extracts, from said second polarized signal (50) , channel information (260) and provides said channel information to said channel-related effects attenuation circuit.  6. Apparatus according to claim 5, characterized in that said circuit (240) for attenuating the effects linked to the channel comprises a filter.  7. Apparatus according to claim 5, characterized in that said receiver (210) receives said first polarized signal (40) in a form substantially perpendicular to said second polarized signal (50).  8. Apparatus according to claim 5, characterized in that said channel information (260) includes filter coefficients.  9. Apparatus according to claim 5, characterized in that said receiver (210) receives said first polarized signal (40) and said second polarized signal (50) in the form of linearly polarized signals.  10. Apparatus according to claim 5, characterized in that said receiver (210) receives said first polarized signal (40) and said second polarized signal (50) in the form of circularly polarized signals.