GB2039449A

GB2039449A - Telecommunications systems

Info

Publication number: GB2039449A
Application number: GB7901006A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1979-01-11
Filing date: 1979-01-11
Publication date: 1980-08-06
Also published as: FR2446566A1; JPS55115742A

Abstract

Methods of and apparatus for minimising the third order intermodulation noise content of an information signal which acquires this noise during transmission along an information transfer system line (2) between an input (3) and an output (4). The line includes at least one repeater (1). The apparatus employs a group delay equalising network (9) to reconstitute the line input signal from the line output signal, and a mini-system (10), whose intermodulation noise pattern is equivalent to that of the line but with a ratio of intermodulation noise to signal and basic noise larger than that of the line for the same overload point. The output of the mini-system (10) is fed through an attenuator 13 and subtracted from the line output signal to provide a modified line output signal, containing the information, in which the line intermodulation noise has been reduced. <IMAGE>

Description

SPECIFICATION Telecommunications systems This invention relates to the minimisation of third order intermodulation noise particularly in long information transfer systems such as submarine telecommunications systems.

In a long submarine telecommunications system the relative delays between the various signals vary along the route, largely due to the group delay distortion of directional filters of the systems, so that the phases of the intermodulation products from a remote repeater differ from the phases of the intermodulation products from a local repeater. The phase of the route intermodulation compared with locally produced intermodulation is, therefore, a complicated function of frequency, route length etc., so that cancellation of the intermodulation noise is a complex procedure.

According to one aspect of the present invention there is provided a method of minimising the third order intermodulation noise content of an information signal, which noise is acquired by the signal during transmission along an information transfer system line between an input and an output, the line including at least one repeater, which method includes reconstituting the line input signal from the line output signal, which reconstituted line input signal and line output signal include line basic noise and the intermodulation noise, and operating on the line output signal and/or the reconstituted line input signal whereby to produce a modified line output signal containing the information and from which the line intermodulation noise has been minimised if not eliminated.

According to a further aspect of the present invention there is provided apparatus for use in minimising the third order intermodulation noise content of an information signal, which noise is acquired by the signal during transmission along an information transfer system line between an input and an output, the line including at least one repeater, which apparatus comprises means for reconstituting the line input signal from the line output signal, which reconstituted line input signal and the line output signal includes line basic noise and the line intermodulation noise, and means for operating on the line output signal and/or the reconstituted line input signal whereby to produce a modified output signal containing the information and from which the line intermodulation noise has been minimised if not eliminated.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a schematic block circuit diagram of a submarine telecommunications system incorporating an intermodulation noise cancellation arrangement according to a first embodiment of the invention, and Figure 2 shows a schematic block circuit diagram of a submarine telecommunications system incorporating an intermodulation noise cancellation arranaement according to a second embodiment of the invention.

In order to minimise the system intermodulation noise content of an information signal at a system terminal it is first necessary to reconstitute the or a part of the original transmitted signal. This involves the use of a network whose group delay distortion, when added to the line (main system) group delay distortion, is a constant. The output of this network, when arranged in tandem with the line, is the or part of the original transmitted signal plus basic noise and the system intermodulation noise. The reconstituted signal must then be operated upon. Figures 1 and 2 show two different "reconstituting and cancelling" circuit arrangements which are such that the output signal of the circuit arrangement is the original transmitted signal or part thereof plus basic noise but with the system intermodulation noise minimised if not eliminated.

In Figure 1 are shown a plurality of repeaters 1 arranged in a telecommunications (information transfer) system line 2 between two terminals 3 and 4. The direction of transmission is indicated by arrow 5. The signal received at terminal 4 is applied to a wideband modulator 6 and the output thereof divided by means of band splitting filters 7 and 8, and only the upper part of the line spectrum processed to remove the intermodulation noise. The modulator 6 and the band splitting filters 7 and 8 are group distortion delay equalised together. This partial processing is possible since on a heavily preemphasised system, particularly a long one, the intermodulation noise in the upper part of the band tends to come mainly from signal loading in the neighbourhood of the disturbed channel.If only the upper part of fthe spectrum is subject to severe intermodulation noise, and this is usually true, the overall apparatus can be simplified to the form shown in Figure 1 by restricting the band operated upon to, for instance, the upper third of the spectrum by introducing the group delay distortion equalised band splitting filters 7 and 8.

The output signal a of the filter 8, which is the signal received at terminal 4 plus line basic noise and line intermodulation noise, is then applied to a group delay equalising network 9 which is inverse to the line and whose group delay distortion, when added to the line group delay distortion, is a constant. The output signal b of network 9 is the original signal applied at terminal 3 plus line basic noise and line intermodulation noise. The network 9 therefore reconstitutes the original signal.

The output signal b is then applied to what is in effect a miniature replica of the main system 2 and is known as a mini-system. The mini-system 10 should have an intermodulation noise pattern like the main system 2 but with much higher distortion coefficients so that its ratio of intermodulation noise to signal and basic noise is much larger than the main system for the same overload point. It is here that feed forward amplifiers may be usefully employed since the output of the intermodulation cancelling amplifier can have a very large ratio of intermodulation to signal and basic noise and the phase and amplitude of the intermodulation noise are adjustable. Alternatively a diode/network combination could be used.The main problem here is in the characterisation of the intermodulation production to be a scaled-up model of the line amplifiers in the variation of product levels with signal levels and frequency. It should be noted that a line amplifier, even with its negative feedback reduced, is unsuitable because of its low overload point for a given intermodulation level and because its noise level is too great.

The components of the mini-system 10 are shown as a large number of distortion generators 11, but not necessarily the same number as for the main system, interspersed with group delay networks 12 intended to produce the same total group delay distortion as the real line. The mini-system 10, when in tandem with the main system 2 and the reconstituting network 9, produces intermodulation, like the main system in its phase relationships between the products, but at a much higher level. The output signal c of the mini-system 10 is the received signal plus the line basic noise, the line intermodulation noise, the local (mini-system) basic noise and the local intermodulation noise.

The output signal c is then applied to an attenuator 13 so that its intermodulation noise level is attenuated to that of the main system. The spurious signals and noise outputs of signal care then attenuated to be much less than that of the main system. The output signal dot the attenuator 13 contains local intermodulation noise, which is smaller to that of the main system, but is now relatively free of signal and basic noise and of the intermodulation noise produced by the main system.

The output signal a from the filter 8 is also applied to a constant delay network 14 to produce a signal e which is the same as signal a but delayed by the same (constant) amount as signal d. The output signal dot the attenuator 13 is then subtracted from signal e in subtractor 15 to give a modified line output signal fcontaining the information and which is free of intermodulation noise (since that has been minimised or cancelled out by the subtraction process) and with little enhancement of basic noise or alteration to signal levels since the locally pro duced (in the mini-system) basic noise and signal is much less than that of the line (main system).The subtraction process is adjusted by successive adjust ment of the amplitude and phase of the distortion generators 11 in the mini-system and it is here the feed forward amplifiers previously referred to can be of value. If required the modulator 6 and the filters 7 and 8 can be omitted.

The essential elements of the Figure 1 arrange ment to minimise or cancel intermodulation noise are therefore a network to reconstitute the original signal and a mini-system of much enhanced inter modulation noise to signal and basic noise but with an overload point comfortably higher than that of the main system with which it is in tandem.

Some simplification of the apparatus can be obtained by employing the arrangement of Figure 2 which combines some of the steps employed in the Figure 1 arrangement. In Figure 2 group delay compensators (equalisers) 16 (instead of simulators) and intermodulation cancellors 17 (instead of simulators) are employed in a mini-system 18, which is such as to cancel the main system rather than simulate it as in the Figure 1 arrangement. In principle the arrangement of Figure 2 operates in the same manner as that of Figure 1, the original signal at the appropriate intermodulation generator being reconstituted in steps by the preceding group delay compensating network. The Figure 2 arrangement requires cancellation at each repeater or at each block of n blocks of repeaters instead of at the end as in the Figure 1 arrangement.Each cancellor must reproduce part of the Figure 1 arrangement or must produce inverse distortion. Whereas in Figure 2 all of the apparatus is shown at the receive terminal 19, it can alternatively be at the transmit terminal 20 or distributed between the two. The direction of transmission is indicated by arrow 21.

In the Figure 2 arrangement a wide band modulator and band splitting filters can be inserted as in the Figure 1 arrangement if required. The signal a' from then blocks of repeaters will be the same as the signal a in Figure 1, that is the received signal plus the line basic noise and the line intermodulation noise. The signal b' will be the same as at the start of block 1' in the system, that is without the intermodulation noise of block 1'. The signal c' will be the same as that at the start of block 2', that is without the inter-modulation noise of blocks 1' and 2'. The signal d' will be the same as that at the start of block n, that is without the intermodulation noise of all of blocks 1' ton.The requirements of each distortion generator of the mini-system of Figure 2 are the same as for the whole mini-system of Figure 1, that is the overload point must be at least as great as the main system but with lower basic noise, and with intermodulation noise which minimises or cancels the intermodulation noise of the appropriate part of the main system. The distortion generators of Figure 2 are therefore of the opposite sign to those of Figure 1.

However, the Figure 1 arrangement is presently preferred since group delay simulators are simpler and more accurate than group delay equalisers so that the mini-system of Figure 1 could be more accurately made than that of Figure 2. It is also easier to produce an intermodulation simulator than an intermodulation cancellor.

The group delay equalising network 9 of Figure 1 can be designed as an entity to suit the telecommunications system as laid, from data measured on the whole system, and adjusted to give zero group delay distortion when in tandem with the system.

The mini-system networks can be adjusted prior to installation and when in tandem with the group delay equalising network 9 to give zero overall delay.

The constant delay network 14 can also be adjusted prior to installation to compensate for the delay of the equalising network 9 and the mini-system 10.

This means that only the intermodulation production controls have to be adjusted after installation of the system. Group delay measurements on long systems after installation are difficult because of the large quick ripples produced in the group delay response from long term echoes of otherwise minor importance. It is necessary to use smoothing prog rammes applied to the measurements to produce accuracy and although on swept frequency measurement it is possible to minimise the confusion from those ripples, it is highly desirable to keep adjustments of group delay after installation to a minimum.

It is not necessary to perform the reconstituting and simulation operations at the line frequencies, for example 27.196 to 44.30 MHz as in a conventional type of wide band system, indeed it is advantageous to reduce the frequency of operation of the equalising network and mini-system etc. to as low as possible a frequency, while still excluding second order intermodulation. It is, for example, possible to use a 6 - 12 MHz (approximately) band to perform most of the signal manipulations, and this is also suitable for intermodulation minimising orcancell- ing operations. If used, the modulator 6 may operate at 50.30 MHz in connection with the line frequencies quoted above.

Claims

1. A method of minimising the third order intermodulation noise content of an information signal, which noise is acquired by the signal during transmission along an information transfer system line between an input and an output, the line including at least one repeater, which method includes reconstituting the line input signal from the line output signal, which reconstituted line input signal and line output signal include line basic noise and the intermodulation noise, and operating on the line output signal and/or the reconstituted line input signal whereby to produce a modified line output signal containing the information and from which the line intermodulation noise has been minimised if not eliminated.

2. A method as claimed in claim 1, wherein the reconstituted line input signal is obtained by applying the line output signal to a group delay equalising network which is inverse to the line and whose group delay distortion when added to the line group delay distortion is a constant.

3. A method as claimed in claim 2, wherein the reconstituted line input signal is applied to a minisystem having an intermodulation noise pattern equivalent to that of the line but with higher distortion coefficients such that the mini-system ratio of intermodulation noise to signal and basic noise is larger than that of the line for the same overload point, the output signal of the mini-system being the line output signal plus the line basic noise, the line intermodulation noise, the mini-system basic noise and the mini-system intermodulation noise.

4. A method as claimed in claim 3, wherein the mini-system output signal is applied to an attenuator whereby to attenuate the overall intermodulation noise to the level of the line intermodulation noise, the attenuator output signal being equivalent to the line intermodulation noise, wherein the line output signal is also applied to a constant delay network whereby to produce a signal equivalent to the line output signal but delayed by the same amount as the output signal from the attenuator has been delayed by the group delay equalising network, the minisystem and the attenuator, and wherein the output signal from the attenuator is subtracted from delayed line output signal to produce the modified line output signal.

5. A method as claimed in any one of the preceding claims wherein the line output signal is applied to a modulator and band splitting filters prior to reconstituting the line input signal, and wherein only the part of the line input signal associated with the upper part of the line spectrum is reconstituted and has the intermodulation noise cancelled therefrom.

6. A method as claimed in claim 1, wherein the line input signal is reconstituted and the line intermodulation noise cancelled therefrom in stages, there being a separate stage associated with each repeater or a block of repeaters, wherein for each stage the method includes applying the line output signal, or the output signal from the preceding stage, to an associated group delay equalising network whereby to reconstitute it to the value of the input signal to a particular repeater or block of repeaters, and applying the reconstituted signal to an intermodulation cancellor whereby to cancel the line intermodulation noise associated with the particular repeater or block of repeaters therefrom.

7. A method as claimed in claim 6, wherein the reconstituting and minimising stages are all performed at the line output.

8. A method as claimed in claim 6, wherein the reconstituting and minimising stages are all performed at the line input.

9. A method as claimed in claim 6, wherein the reconstituting and minimising stages are performed during transmission of the information signal along line and between the input and the output of the line.

10. Apparatus for use in minimising the third order intermodulation noise content of an information signal, which noise is acquired by the signal during transmission along an information transfer system line between an input and an output, the line including at least one repeater, which apparatus comprises means for reconstituting the line input signal from the line output signal, which reconstituted line input signal and the line output signal include line basic noise and the line intermodulation noise, and means for operating on the line output signal and/or the reconstituted line input signal whereby to produce a modified output signal containing the information and from which the line intermodulation noise has been minimised if not eliminated.

11. Apparatus as claimed in claim 10, wherein the signal reconstituting means comprises a group delay equalising network which is inverse to the line and whose group delay distortion when added to the line group delay distortion is a constant, which network is connectible to the line output.

12. Apparatus as claimed in claim 11, wherein the operating means comprise a mini-system, an attenuator, a constant delay network and a subtractor, wherein the mini-system is connected to the group delay equalising network and has an intermo dulation noise pattern equivalent to that of the line but with higher distortion coefficients such that the mini-system ratio of intermodulation noise to signal and basic noise is larger than that of the line for the same overload point, wherein the attenuator is connected to the mini-system and is such as to attenuate the output signal therefrom to a signal equivalent to the line modulation noise, wherein the attenuator is connected to one input of the subtractor, wherein a constant delay network is connectible in parallel with the group delay equalising network, the mini-system and the attenuator between the receive terminal and the other input of the subtractor, the constant delay network serving to delay the line output signal by the same amount as the group delay equalising network, the mini-system and the attenuator, the output signal of the subtractor being equivalent to the modified line output signal.

13. Apparatus as claimed in claim 10, wherein associated with each repeater or block of repeaters is a stage comprising a group delay equaliser network and an intermodulation cancellor, each stage serving to reconstitute the signal applied thereto the value of signal applied to a particular repeater or block of repeaters and cancel the line intermodulation noise associated with the particular repeater or block of repeaters therefrom.

14. Apparatus as claimed in claim 13, wherein the stages are arranged in tandem and connectible to the output of the line.

15. Apparatus as claimed in claim 13, wherein the stages are arranged in tandem and connectible to the input of the line.

16. Apparatus as claimed in claim 13, wherein the stages are connectible in the line and distributable between input and output thereof.

17. A method of minimising the third order intermodulation noise content of an information signal substantially as herein described with reference to and as illustrated in Figure 2 of the accompanying drawing.

18. A method of producing a third order intermodulation noise free information signal substantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.

19. Apparatus for obtaining a third order intermodulation noise-free information signal substantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.

20. Apparatus for minimising third order intermodulation noise from an information signal substantially as herein described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings.

21. An information transfer system comprising a line including at least one repeater and apparatus as claimed in any one of claims 10to 16 or 18 for minimising third order intermodulation noise from an information signal transmitted along the line.