Method of and apparatus for determining properties of telecommunication channel elements under load

Description of the invention

The present invention relates to a method and device for characterizing elements of a communication channel, in particular a transponder in a communication satellite during operation.

The characteristics of the communication channel may change during the life of the equipment. Various tests are carried out not only at the beginning but also repeatedly during the service life to check that the telecommunications channel meets the specified technical requirements. Typically, these tests are performed in the absence of normal traffic, that is, without using a communication channel to transmit a communication signal. This situation is explained in more detail below with reference to telecommunications satellites, but without limiting the present invention to this application only.

A telecommunications channel in a telecommunications satellite is defined by the satellite transponder, including several elements such as a receive antenna, an input demultiplexer, a power amplifier, an output multiplexer, and a transmit antenna. Transponder characteristics, such as amplitude response and group delay, are measured not only at the beginning of a spacecraft on Earth and after launch into orbit, but also during its lifetime. The measurements are normally made in the absence of normal movement in the transponder, i.e. without transmitting or retransmitting the useful signal by the transponder.

The necessity to turn off the user signal during the tests is a serious obstacle not only for the user of the transponder, since there is a communication interruption, but also for the satellite operator, since the tests must be performed quickly and efficiently to possibly shorten the interruption. In some cases, it is impossible to interrupt the communication via the communication channel, so that the elements of these channels cannot be checked after putting them into service.

It is known from US Patent No. 4,637,017 to control the transponder input power of a TDMA satellite telecommunications system. In TDMA systems, there is only one carrier frequency at the input of a traveling-wave tube amplifier, so it operates near the saturation point of the traveling-wave tube TWT, with no non-linearity and cross-modulation. In order to measure an input disconnect, the control station transmits a pilot signal CW on the band of amplifiers. During the guard time between the bursts of pulses, the control station measures the level of the undamped pilot at the output of the amplifier. While the ground station transmits an unmodulated carrier during recovery of a carrier or a clock frequency modulated carrier during clock reproduction, the control station measures the level of the pilot suppressed by the nonlinear interplay of pilot and carrier. The pilot attenuation value is related to the input carrier interruption by a relationship previously measured or obtained theoretically. The ratio of carrier to suppressed noise is determined by measuring the carrier level during carrier recovery and by measuring suppressed noise during carrier or clock recovery through a noise filter centered on all transmitted signals or their cross modulation products.

German Patent No. 36 44 175 describes a method for transmitting data and auxiliary information via a satellite for the control of a data channel or data network. The side information is transmitted as a pseudo-noise sequence so that the same frequency can be used for transmitting the side information and data.

There is known from US Patent No. 5,546,421 a self-compensating hybrid circuit with a spread spectrum, which is used in a telecommunication station connected to a bidirectional input-output signal path. Such a communication station is, for example, a simple telephone line terminal. A hybrid circuit is a multiport component that routes an input signal to an adjacent port without affecting other ports, which is referred to as directivity with a quality determined by isolation. Maximum isolation is achieved when the bidirectional signal path has an impedance leading to the bidirectional signal port of the hybrid circuit, matched to the impedance for which the hybrid circuit is designed. This impedance of the bidirectional signal path can change dynamically during operation, which is prevented by stopping the spread spectrum control signal. The control signal covers the bandwidth of the information signal and is reflected by a bidirectional signal path. This signal is phase detected, processed and fed to the hybrid circuit for

Minimizing the amount of the transmitted signal supplied to the receiving plate in closed-circuit operation.

The method according to the invention consists in generating a first pseudo-noise PN (t) signal, modulating with the first pseudo-noise PN (t) pure carrier signal f (t) to generate a pure carrier signal s (t) modulated by PN signal, transmitting clean carrier signal s (t) modulated by the PN signal simultaneously with the payload signal, via the telecommunication channel, at a level below the level of the payload, receive signal s '(t) corresponding to a pure carrier signal s (t) modulated by a PN signal, after passing through the telecommunication channel, receive signal s' (t) and a first pseudo-noise signal PN (t) to generate a reconstructed carrier signal f '(t) and the amplitude response and group delay of the communication channel at the selected discrete frequencies is determined based on a comparison of the clean carrier signal f (t) and the reconstructed carrier signal f' (t).

Preferably, a pure carrier signal s (t) modulated by a PN signal with a low bandwidth is used, while the course of a pure carrier signal s (t) modulated by a PN signal corresponds to the group delay at the center frequency.

Preferably, the reproduced carrier signal f '(t) is attenuated in accordance with the amplitude response of the communication channel.

Preferably, correlation of the reception signal s '(t) and the first pseudo noise PN (t) is achieved by delaying the first pseudo noise PN (t) and multiplying the delayed first pseudo noise PN (t) by the reception signal s' (t).

Preferably, the pseudo-noise signal PN (t) is delayed until the amplitude of the reconstructed carrier signal f '(t) becomes maximum.

Preferably, the amplitude response and the group delay are determined in the passband of the communication channel relative to the amplitude response and the group delay at the center frequency of the passband.

Preferably, the amplitude and delay at the center frequency are subtracted from the amplitude and delay at any other frequency of the passband.

Preferably, a second PNR pseudo-noise signal (t) is generated, a second PNR pseudo-noise signal (t) is modulated with a reference carrier signal fR (t) to generate a PN-modulated reference carrier signal sR (t), and a PN-modulated reference carrier signal sR (t) is transmitted. with the PN signal simultaneously with the useful signal, through the telecommunications channel, at a level below the level of the useful signal, a reference reception signal sR '(t) corresponding to a PN-modulated reference carrier signal sR (t) is received, after passing through the telecommunication channel, a reference receive signal sR' (t) and a second pseudo-noise PNR signal (t) are correlated to generate a reconstructed signal reference carrier fR '(t) and the amplitude response and group delay of the communication channel are determined at the selected discrete frequencies, also by comparing the reference carrier signal fR (t) and the reconstructed reference carrier signal fR '(t).

Preferably, the corrected amplitude response and the corrected group delay of the communication channel are obtained by subtracting the values ​​obtained by the reference carrier signal fR (t) and the reconstructed reference carrier signal fR '(t) from the values ​​obtained by the clean carrier signal f (t) and the reconstructed carrier signal f '(t).

The inventive apparatus comprises a generator for generating a first pseudo-noise signal PN (t), connected to a first multiplier for modulating with a first pseudo-noise signal PN (t) the clean carrier signal f (t) to generate a clean carrier signal s (t) modulated by a PN signal, fed to it. to a clean carrier signal transmission system s (t) modulated by a PN signal, simultaneously with the payload signal through a telecommunication channel, at a level below the payload level, at this transmission circuit is connected to the input of the second multiplier to the reception signal s' (t) corresponding to the pure carrier signal s (t) modulated by the PN signal, after passing through the communication channel, and the second input of said multiplier is coupled to a first delay circuitry to correlate the reception signal s '(t) and the first pseudo-noise signal PN (t) to generate a reconstructed carrier signal f' (t) and the communication channel amplitude response and delay determination circuit at at the selected discrete frequencies, it compares the clean carrier signal f (t) and the reconstructed carrier signal f '(t).

PL 192 542B1

Preferably, the correlation circuit comprises delay means for delaying the first pseudo noise PN (t) and multipliers for multiplying the delayed first pseudo noise PN (t) by the reception signal s' (t).

Preferably, the apparatus comprises a second generator for generating a second PNR pseudo-noise signal (t), connected to a third multiplier for modulating a second PNR pseudo-noise signal (t) to a reference carrier signal fR (t) to generate a PN-modulated reference carrier signal sR (t) fed to it. to a system for transmitting the reference carrier signal sR (t) modulated by the PN signal, simultaneously with the payload signal, via a telecommunications channel, at a level below the payload level, the transmission circuitry being coupled to an input of a fourth multiplier for receiving a reference reception signal sR '(t) corresponding to a PN-modulated reference carrier signal sR (t) after passing through the communication channel, and the second input of said multiplier is coupled to a second delay circuit to correlate the reference reception signal sR '(t) and the second pseudo noise PNR signal (t) to generate a reconstructed reference carrier signal fR' (t) and the channel amplitude response and group delay determination circuit At the selected discrete frequencies, it compares the reference carrier signal fR (t) and the reconstructed reference carrier signal fR '(t).

An advantage of the invention is to provide a method and a device for characterizing the elements of a communication channel, in particular a satellite transponder, without interrupting the traffic of the communication channel. The operating signal need not be turned off while measurements are being taken. This significantly reduces the downtime required for the maintenance and control of the telecommunications channel and therefore increases the availability of services.

An advantage of the invention is also that it is possible to measure the characteristics of the elements of the telecommunication channel in real conditions. Moreover, in the method according to the invention, the power spectral density of the measurement signal is significantly lower than the power spectral density of the useful signal, so that it is possible to characterize the behavior of the communication channel under the most realistic conditions.

An additional advantage is that, in the case of a satellite communication channel, the Earth-to-satellite / Satellite-to-Earth transition frequency is measured without interrupting the payload and simultaneously with other measurements.

The subject of the invention is shown in the drawing, in which Fig. 1 shows a diagram of a telecommunications satellite transponder, Fig. 2 - in the form of a diagram, a first embodiment of the device according to the invention, Figs. 3a and 3b - graphs representing measurement results and Fig. 4 - in diagrammatic form a second embodiment of the device according to the invention.

Figure 1 shows transponder elements in a communication satellite as an example of a communication channel.

The telecommunication satellite transponder comprises a receiving antenna 1 for receiving an earth-satellite signal transmitted from a earth station (not shown). The output signal from the receiving antenna 1 is fed to the input demultiplexer 3 after a frequency conversion in the frequency converter 2. The input demultiplexer 3 includes a few first filters 4-1 to 4-n for separating the individual signals in the signal from the antenna. Typically one filter is used for each signal to separate it from other signals received by the receiving antenna 1. The n number of output signals from the input demultiplexer 3 are fed to a corresponding number of 5-1 to 5-n high power amplifiers that use a lamp traveling wave TWT to amplify the output signals of the input demultiplexer 3. Each of the 5-1 to 5-n high power amplifiers operates normally in a saturated state, so many signals would produce products of mutual modulation and signal distortion. The output signals of these amplifiers are passed through the second filters 6-1 to 6-n, which are part of the output multiplexer 7 combining the n outputs of the amplifier. The output of the output multiplexer 7 is fed to transmitting antenna 8 for transmission towards a specific area on Earth.

Since the filters used in the input demultiplexer 3 and the output multiplexer 7 have a strong influence on the performance of the transponder, the method according to the invention is explained below in a preferred example with reference to the measurement of two specific characteristics, namely the amplitude response and group delay of these channel elements of the communication transponder .

Figure 2 shows a first embodiment of the device according to the invention. At the ground station, a pseudo noise PN (t) signal is generated by a pseudo-noise signal generator 9, for example a feedback shift register or storage device in which a sequence of pseudo noise signal values ​​is stored. The pseudo-noise signal PN (t) has a very clear autocorrelation function with a zero delay. This makes it possible to determine the delay between the locally generated pseudo-noise PN (t) signal and the received signal which is delayed by the propagation time. A clean carrier signal f (t) having a variable frequency is modulated by a pseudo-noise signal PN (t) by a first multiplier to produce a PN modulated clean carrier signal s (t) = PN (t) xf (t). The transmission rate of the pseudo noise PN (t) signal, which determines the bandwidth of this signal, is chosen such that the signal bandwidth s (t) is small compared to the expected telecommunications channel delay peaks. Typically, the pseudo-noise signal rate is chosen to be less than 5 Mchip / s.

The PN modulated clean carrier signal s (t) is fed to the boost converter 11 and through the high power amplifier 12 to the antenna 13 which transmits the PN modulated clean carrier signal s (t) to the tested telecommunications satellite transponder. From the point of view of the user transmitting the service signal to the satellite, the transponder remains usable during the test and the service signal may be continuously fed to it.

According to the invention, the PN-modulated clean carrier signal level s (t) is sufficiently below the payload level, e.g. by about 15 to 25 dB or more, such that the payload signal does not noticeably deteriorate. For this reason, a pure carrier signal s (t) modulated by a PN signal can be transmitted during the operation of the communication channel, i.e. simultaneously with the payload signal transmitted to the satellite transponder from the same or a different earth station.

The frequency of the clean carrier signal f (t) varies such that it ranges from the lowest to the highest frequency of the passband of the filters in the satellite transformer or any other element of the general communication channel under investigation. The pure carrier signal s (t) modulated by the PN signal has a narrow bandwidth due to the pseudo-noise signal PN (t), such that the amplitude response and group delay of the communication channel are determined at selected discrete frequencies as will be described below.

The antenna 13 also serves to receive the signal retransmitted by the satellite transponder, in other words the signal that has traveled through the communication channel. The output of the antenna 13 is passed through a downconverter 14 to obtain a receive signal s' (t), which is fed to a second multiplier 15 also receiving the same but delayed pseudo-noise PN (t) signal. The delay is produced by delayer 16 which is set such that the output of the second multiplier 15 becomes maximum. Hence, the receive signal s '(t) is multiplied, in other words dependent on the exact same pseudo-noise signal PN (t) that was used to generate a pure carrier signal s (t) modulated by PN, and the reconstructed carrier signal f' (t) is obtained ( t), which is only delayed and attenuated compared to the clean carrier signal f (t). Thus, it is easy to determine the amplitude response which corresponds to the attenuation of the reproduced carrier signal f '(t) and the group delay which corresponds to the delay of the reproduced carrier signal f' (t) for the satellite transponder as an example of a general communication channel. The runtime of a narrowband signal at its center frequency corresponds to the group delay of the filters if the phase can be approximated linearly across the signal bandwidth. Accordingly, the PN signal transmission rate is determined.

In the case of telecommunications satellites, it is sufficient to determine the amplitude response and group delay in the passband of the transponder only relative to the amplitude response and group delay at the center frequency of the passband. Thus, it suffices to delay the pseudo-noise signal PN (t) such that the amplitude of the reproduced carrier signal f '(t) becomes maximum and subtract the amplitude and delay at the center frequency from the amplitude and delay at any other frequency in the passband.

Fig. 3a shows typical results of amplitude response measurements and Fig. 3b shows typical group delay measurements obtained by the method of the invention.

In the case of a satellite communication channel, i.e. a transponder, the distance to the satellite changes during measurements due to the movements of the satellite. During measurements, also due to atmospheric phenomena, the attenuation of losses in the path between the ground station and the satellite changes. Since in the above embodiment the amplitude response and the group delay are determined by subtracting the amplitude response and the group delay at the center frequency from

EN 192 542B1 corresponding values ​​at other discrete frequencies, an error may arise due to the aforementioned satellite movements and atmospheric phenomena or other influences.

In Fig. 4, the reference signal sR (t) is used to compensate for the measurement error. Figure 4 is an extension of the solution of Figure 2. The reference signal sR (t) is generated by a third multiplier 18, which receives the second pseudo noise PNR signal (t) independent of the first pseudo noise signal PN (t) and is generated by the second pseudo noise signal generator 17 and receives the reference carrier signal fR (t) which is located on a fixed frequency within a passband of the same transponder or in a passband of a different transponder on the same satellite having a different center frequency. Similar to the above embodiment, the PN-modulated reference carrier signal sR (t) is transmitted to the satellite and the reference receiving signal sR '(t) is multiplied by a second pseudo-noise PNR (T) signal to obtain a reconstructed reference signal fR' (t). The embodiment of Fig. 4 has a fourth multiplier 20 for multiplying the signal from the multiplier 15 by the signal sR '(t) from the downconvertor 14, and has a second delayer 19. The measurement signal has a variable frequency in the passband of the transponder and the frequency of the reference carrier signal fR (t) remains constant. Thus, by subtracting the value of the reference signal from the values ​​of the measurement signals at the appropriate time, the correct amplitude response and group delay is obtained.

The measurement of the group delay is in another embodiment measuring the phase of the reproduced carrier of the PN modulated signal at a particular frequency very close to the first frequency, which allows the group delay at the frequency located in the middle between the two measurement frequencies to be approximated by calculating the phase difference and dividing by the frequency difference.

Pseudo-noise signals have been discussed above because they can be generated relatively easily. However, real noise signals are also used in the method and apparatus of the invention.