CA1201783A - Digital transmission systems - Google Patents
Digital transmission systemsInfo
- Publication number
- CA1201783A CA1201783A CA000407458A CA407458A CA1201783A CA 1201783 A CA1201783 A CA 1201783A CA 000407458 A CA000407458 A CA 000407458A CA 407458 A CA407458 A CA 407458A CA 1201783 A CA1201783 A CA 1201783A
- Authority
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- Canada
- Prior art keywords
- network
- control
- output
- retimer
- regenerator
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Abstract
Abstract of the Disclosure Feedback equalization is employed for digital repeaters of transmission systems having twisted-wire pairs, in order to reduce intersymbol interference while minimizing near-end crosstalk gain. Previously the incoming signal was selec-tively amplified at the higher frequencies in order to negate intersymbol interference, with subsequent near-end crosstalk gain. According to the present invention the intersymbol interference element of an incoming signal is cancelled, prior to amplification in an adder by means of a compensation signal derived from the output signal of a regenerator and a retimer.
An output stage serves to amplify and shape the resultant signal to a form suitable for transmission. The compensation signal comprises the inverted output of a filtering and scaling network.
An output stage serves to amplify and shape the resultant signal to a form suitable for transmission. The compensation signal comprises the inverted output of a filtering and scaling network.
Description
The present invention relates to d.igital information transmission systems including -twisted wire pair transmission lines, and in paxticular to equalization for digital repeaters in the presence of near-end cross-talk.
It is well known that a transmission system involving transmission over symmetric (balanced) wire pairs re~uires repeaters to be placed at intervals, typically less than 2 km (kilome-ters) apart. Such repeaters reduce signal distortion by equalization, and by employing amplification restore the :LO signal to a suitable level for retransmission well above the level of the near-end crosstalk from other systems using the same route.
The background of the invention and the invention it-self are illustrated in the accompanying drawings, in which:
Figure 1 is a block diagram of a sec-tion of a known transmission system having symmetric wire pairs and repeaters described in -the second paragraph of page 1 of this specifi-cation; and Figures 2, 3 and 4 are simplified block diagrams of three diE:Eeremt embodiments oE a digital repeater circuit arrangement in accordance with the principles of the present invent.ion~
~ igure 1 of the accompanying drawing shows a section of such a route in which a repeater 1 transmits high amplitude signals into a symmetric wire pair 3 at point 2. I'hese si~nals are attenuated while travelling down the wire pair 3 to point 4, where they are received by a subse~uent repeater 5. A
repeater 6 transmitting at point 7 into another symmetric wire pair 8, in the opposing direction and towards point 9 and a repeater 10, will induce crosstalk. into -the aforemen-tioned signals near point 4. This crosstalk is near-end cross-talk coupling due to imperfect balance of the cable pairs and is indicated in Fi~ure 1 by a curved arrow between points 7 and 4.
A similar ~oupling occurs between points 2 and 9. Other cross-talk mechanisms may also exist. For example, near-end cross-talk can occur between systems transmitting in the same direc-tion if transmitted and received siynals come within close proximity to each other.
For a digital system the runctions of each repeater are usually to re-amplify, regenerate and retime the symbols being transmitted. Conventionally, the function of re-ampli-fieation involves equalization (inversion) of the linear fil-tering effect of the cable pair such that the subsequent pulse speetrum has a predetermined shape meeting Nyquist's criterion for no intersymbol interference. The intersymbol interference is negated by selectively amplifying the higher frequencies by the magnitude o~ attenuation sufEered by them in transmission.
As the attenuation of the cable pair increases with frequency, the gain of the re-amplification Eunetion eorrespondingly in-exeases with frequency, at least up to a frequency oE approx-~0 i.mately one halE o:E the symbol rate. This increasing of gainwith Erequency ls unfortunate, since it simultaneously ampli-Eies crosstalk, while the eoupling mechanism producing the e.rosstalk deteriorates with inereasing frequency.
An object of the present invention is to provide a digital transmission system having redueed in-tersymbol inter-ferenee for an ineoming signal while minimizing near~end crosstalk gain, Another object of the present inven-tion is -to provide a digital repeater having redueed intersymbol interference for an ineoming signal while minimizing near~end erosstalk gain
It is well known that a transmission system involving transmission over symmetric (balanced) wire pairs re~uires repeaters to be placed at intervals, typically less than 2 km (kilome-ters) apart. Such repeaters reduce signal distortion by equalization, and by employing amplification restore the :LO signal to a suitable level for retransmission well above the level of the near-end crosstalk from other systems using the same route.
The background of the invention and the invention it-self are illustrated in the accompanying drawings, in which:
Figure 1 is a block diagram of a sec-tion of a known transmission system having symmetric wire pairs and repeaters described in -the second paragraph of page 1 of this specifi-cation; and Figures 2, 3 and 4 are simplified block diagrams of three diE:Eeremt embodiments oE a digital repeater circuit arrangement in accordance with the principles of the present invent.ion~
~ igure 1 of the accompanying drawing shows a section of such a route in which a repeater 1 transmits high amplitude signals into a symmetric wire pair 3 at point 2. I'hese si~nals are attenuated while travelling down the wire pair 3 to point 4, where they are received by a subse~uent repeater 5. A
repeater 6 transmitting at point 7 into another symmetric wire pair 8, in the opposing direction and towards point 9 and a repeater 10, will induce crosstalk. into -the aforemen-tioned signals near point 4. This crosstalk is near-end cross-talk coupling due to imperfect balance of the cable pairs and is indicated in Fi~ure 1 by a curved arrow between points 7 and 4.
A similar ~oupling occurs between points 2 and 9. Other cross-talk mechanisms may also exist. For example, near-end cross-talk can occur between systems transmitting in the same direc-tion if transmitted and received siynals come within close proximity to each other.
For a digital system the runctions of each repeater are usually to re-amplify, regenerate and retime the symbols being transmitted. Conventionally, the function of re-ampli-fieation involves equalization (inversion) of the linear fil-tering effect of the cable pair such that the subsequent pulse speetrum has a predetermined shape meeting Nyquist's criterion for no intersymbol interference. The intersymbol interference is negated by selectively amplifying the higher frequencies by the magnitude o~ attenuation sufEered by them in transmission.
As the attenuation of the cable pair increases with frequency, the gain of the re-amplification Eunetion eorrespondingly in-exeases with frequency, at least up to a frequency oE approx-~0 i.mately one halE o:E the symbol rate. This increasing of gainwith Erequency ls unfortunate, since it simultaneously ampli-Eies crosstalk, while the eoupling mechanism producing the e.rosstalk deteriorates with inereasing frequency.
An object of the present invention is to provide a digital transmission system having redueed in-tersymbol inter-ferenee for an ineoming signal while minimizing near~end crosstalk gain, Another object of the present inven-tion is -to provide a digital repeater having redueed intersymbol interference for an ineoming signal while minimizing near~end erosstalk gain
-2-According to a first broad aspect of -the present invention r there is provided a digital transmission system comprising: a twisted~wire pair transmission line; and at least one digital repeater coupled to said -transmission line having an arrangement to reduce intersymbol interference in an incoming signal while minimizing near-end crosstalk gain, said arrangement including a pulse regenerator and a retimer net-work having an input and an output, an adder ha~ing two inpu-ts and an output, a feedback network coupled to said output of said regenerator and retimer network to produce a compensation signal, said compensa-tion signal being coupled to one of said two inputs of said adder, an additional network disposed be~
tween said output of said adder and said input of said regen-erator and retimer network to provide a suitable input signal for said regenerator and retimer network, a filter network coupling said incoming signal to the other of said two inputs of said adder, and a control network coupled to said inpu-t of said regenerator and retimer network and at least a selected one of said feedback network, said additional network and sald filter network.
According to a second broad aspect of the invention, there is provided a digital repeater for use with a twisted-pair transmission line to reduce intersymbol interference in an incoming signal while minimizing near-end crosstalk gain comprising: a pulse regenerator and a retimer network having an input and an output, an adder having two inputs and an output~
a feedback network coupled to said output of said regenerator and retimer network to produce a compensation signal, said compensation signal being coupled to one of said two inputs of said adder, an additional network disposed between said output o~ said adder and said input of said regenerator and retimer network to provide a suitable input signal for said regener-ator and retimer network, a fil-ter network coupling said in-coming signal to the other of said two inputs of said adder, and a con~rol ne-twork coupled to said input of said regener-ator and retimer network and at least a selected one of said feedback network, said additional network and said filter network.
The invention will now be described in greater detail with reference to Figure 2 of the drawings.
Figure 2 shows a basic repeater circuit arrangement including a linear network 11 fo~lowed by an adder 12, a second linear network 13 whose output is connected to a retim~r 14, regenerator 15, and a control network 18~ Regenerator 15 has its output connected to a netwo:rk 16 and an outpu~ driver (out-put stage) 17. Thus, an input signal with crosstalk at 19 is filtered by network 11, and the filtered signal at 20 is applied to adder 12 which sums it with an inverted replica of the sig-nal at 21, which is the output signal (regenerated signal) at 22 of regenerator 15 when filtered and scaled by network 16.
~0 The output s.ignal of adder 12 is applied to network 13 which .is such as to produce a signal at 23 suitable to feed regener-ator 15, and also feed retimer 14 comprising the usual arrange-ment of a linear network followed by a non-linear network, such as full-wave rectifier, and a further linear network of a re~
sonant variety. The output signal at 22 of regenerator 15 is -3a-jl amplified and shaped by output stage 17 to form a signal at Z4 suitable to be transmitted to line and to buffer the effects of different impedances of different pairs from the signal through network 160 Alternatively, network 16 can include its own buffering function. Finally, the ~ \}~
R.J. Catchpole et al 5-2-2-1 (Revision) signal at 23 before regeneration is sensed by the control network 18, typically including a high impedance peak detector followed by a low pass network, to produce a direct -~
current at 25 that can affect the linear characteristics of ---network 11 prior to the point of entry of the signal at 21.
This has the advantage that all properties of most signals in the repeater, such as that of regenerator 15, remain constant. It has the disadvantage, however, that the crosstalk spectrum of the signal at 23 depends on the pair length and cannot be optimally chosen for more than one length, such as the maximum.
The embodiment of Fig. 3 differs from Fig. 2 only in that the network under control is network 13, that is after -....
the point of entry of the signal at 21. This may be useful where the cancellation/reduction techniques are required to be effective on the longest lines, but not where crosstalk amplification is to be reduced also on short lines. -~
In the embodiment of Fig. 4, the network under control is network 16, that is affecting the signal at 21 itself.
This results in the near-end crosstalk spectrum at entry to the regenerator 15 being independent of pair length, but with signal properties at this point that do vary with pair length.
Practical application may thus benefit from an appro-25 priate combination of the embodiments of Figs. 2, 3 and 4. --The networ]c 16 should, prefera~ly, be chosen in such a way that the signal passed to the non-linear circuit in the retimer 14 has the re~uired properties to minimize pattern dependent jitter (e.g. a pulse shape that i5 symmetxic in time or whose spectrum is symmetrically disposed about a frequency equal to half the symbol rate in the freguency --domain). The signal at 21 caused by a symbol must be small -or nil until the corresponding signal at 20 is considerable ---and regeneration is complete. Therefore, the properties --~or low jitter are achieved by ensuring that the signal at 21 a~ter bus time has then the shape that, when added to the ;
signal at 20, produces the required symmetry.
R. J Catchpole et al 5-2-2-1 (Revision) When regeneration occurs incorrectly/ perhaps due to -~
an exceptionally large crosstalk transient, an in~orrect signal ~lows through network 16, causing distortion that --will tend to encourage subsequent errors. This error multiplication effect is dependent on the choice of networks 11, 13 and 16. Preferably, network 16 has a low-pass charac-teristic that can result in a situation where following an errorl typically only one additional error occurs.
In the method of the present invention there is no attempt to introduce re-amplification necessary to cancel the pair attenuation as in the conventional method to meet Ny~uist's above-mentioned criterion. Rather, intersymbol interference is cancelled by means of a compensation signal derived from the output of the regeneration and retiming functions as illustrated, for example, in Fig. 2. For example, a system carrying a 2048 Kbit/s multiplex of 30 telephony channels and e~lploying AMI r HDB3, 4B-3T or a similar line code can with the techniques described above achieve, on the same cable type, greater cable section --c length, a greater number of channels on each system, or a combination of these.
The embodiments described above include automatic compensation for different lengths of the cable~section, and involvè automatic control of a network or networks 25 such that the signal entering the regeneration function --....
remains unchanged in its important properties, such as amplitude, and ensures freedom from intersymbol inter~
ference.
Typically, the linear network 11 in Fig. 2 is formed by an equalizer, comprising one or more resonant brldged T sections, followed by an automatic line build out circuit. ^
....
This last circuit can include filter sections in which each of several resistance elements is created by the slope ---resistance of a diode through which the control current 25 is passed. If the signals at 20 and 21 can be from current sources the adder 12 can be simply a common impedence ~2~
through which those currents are passed. ~he linear network 13 can be an amplifier, and its yain may be arranged to vary with frequency ln order to fulfill part of the e~ualization and pulse shaping functions. The retimer 14, regenerator 15 and output stage 17 can be as described in Chapter 14 and earlier chapters of "Digital Transmission Systems", Revised 2nd Edition by P. Bylanski and D.G.W. Ingram, Peter Peregrinus, 1980, ISBN 0906048427. The filtering and scaling network 16 may be a simple low pass network comprising a series resis-tance followed by a shunt capacitance, together with any neces-sary a.c. (alternating current) coupling. The control ne-twork 18 may he a peak detector formed by a half wave rectifier fol-lowed by a large capacitance with large shunt resistance whose potential difference is used to dri~e a current source.
tween said output of said adder and said input of said regen-erator and retimer network to provide a suitable input signal for said regenerator and retimer network, a filter network coupling said incoming signal to the other of said two inputs of said adder, and a control network coupled to said inpu-t of said regenerator and retimer network and at least a selected one of said feedback network, said additional network and sald filter network.
According to a second broad aspect of the invention, there is provided a digital repeater for use with a twisted-pair transmission line to reduce intersymbol interference in an incoming signal while minimizing near-end crosstalk gain comprising: a pulse regenerator and a retimer network having an input and an output, an adder having two inputs and an output~
a feedback network coupled to said output of said regenerator and retimer network to produce a compensation signal, said compensation signal being coupled to one of said two inputs of said adder, an additional network disposed between said output o~ said adder and said input of said regenerator and retimer network to provide a suitable input signal for said regener-ator and retimer network, a fil-ter network coupling said in-coming signal to the other of said two inputs of said adder, and a con~rol ne-twork coupled to said input of said regener-ator and retimer network and at least a selected one of said feedback network, said additional network and said filter network.
The invention will now be described in greater detail with reference to Figure 2 of the drawings.
Figure 2 shows a basic repeater circuit arrangement including a linear network 11 fo~lowed by an adder 12, a second linear network 13 whose output is connected to a retim~r 14, regenerator 15, and a control network 18~ Regenerator 15 has its output connected to a netwo:rk 16 and an outpu~ driver (out-put stage) 17. Thus, an input signal with crosstalk at 19 is filtered by network 11, and the filtered signal at 20 is applied to adder 12 which sums it with an inverted replica of the sig-nal at 21, which is the output signal (regenerated signal) at 22 of regenerator 15 when filtered and scaled by network 16.
~0 The output s.ignal of adder 12 is applied to network 13 which .is such as to produce a signal at 23 suitable to feed regener-ator 15, and also feed retimer 14 comprising the usual arrange-ment of a linear network followed by a non-linear network, such as full-wave rectifier, and a further linear network of a re~
sonant variety. The output signal at 22 of regenerator 15 is -3a-jl amplified and shaped by output stage 17 to form a signal at Z4 suitable to be transmitted to line and to buffer the effects of different impedances of different pairs from the signal through network 160 Alternatively, network 16 can include its own buffering function. Finally, the ~ \}~
R.J. Catchpole et al 5-2-2-1 (Revision) signal at 23 before regeneration is sensed by the control network 18, typically including a high impedance peak detector followed by a low pass network, to produce a direct -~
current at 25 that can affect the linear characteristics of ---network 11 prior to the point of entry of the signal at 21.
This has the advantage that all properties of most signals in the repeater, such as that of regenerator 15, remain constant. It has the disadvantage, however, that the crosstalk spectrum of the signal at 23 depends on the pair length and cannot be optimally chosen for more than one length, such as the maximum.
The embodiment of Fig. 3 differs from Fig. 2 only in that the network under control is network 13, that is after -....
the point of entry of the signal at 21. This may be useful where the cancellation/reduction techniques are required to be effective on the longest lines, but not where crosstalk amplification is to be reduced also on short lines. -~
In the embodiment of Fig. 4, the network under control is network 16, that is affecting the signal at 21 itself.
This results in the near-end crosstalk spectrum at entry to the regenerator 15 being independent of pair length, but with signal properties at this point that do vary with pair length.
Practical application may thus benefit from an appro-25 priate combination of the embodiments of Figs. 2, 3 and 4. --The networ]c 16 should, prefera~ly, be chosen in such a way that the signal passed to the non-linear circuit in the retimer 14 has the re~uired properties to minimize pattern dependent jitter (e.g. a pulse shape that i5 symmetxic in time or whose spectrum is symmetrically disposed about a frequency equal to half the symbol rate in the freguency --domain). The signal at 21 caused by a symbol must be small -or nil until the corresponding signal at 20 is considerable ---and regeneration is complete. Therefore, the properties --~or low jitter are achieved by ensuring that the signal at 21 a~ter bus time has then the shape that, when added to the ;
signal at 20, produces the required symmetry.
R. J Catchpole et al 5-2-2-1 (Revision) When regeneration occurs incorrectly/ perhaps due to -~
an exceptionally large crosstalk transient, an in~orrect signal ~lows through network 16, causing distortion that --will tend to encourage subsequent errors. This error multiplication effect is dependent on the choice of networks 11, 13 and 16. Preferably, network 16 has a low-pass charac-teristic that can result in a situation where following an errorl typically only one additional error occurs.
In the method of the present invention there is no attempt to introduce re-amplification necessary to cancel the pair attenuation as in the conventional method to meet Ny~uist's above-mentioned criterion. Rather, intersymbol interference is cancelled by means of a compensation signal derived from the output of the regeneration and retiming functions as illustrated, for example, in Fig. 2. For example, a system carrying a 2048 Kbit/s multiplex of 30 telephony channels and e~lploying AMI r HDB3, 4B-3T or a similar line code can with the techniques described above achieve, on the same cable type, greater cable section --c length, a greater number of channels on each system, or a combination of these.
The embodiments described above include automatic compensation for different lengths of the cable~section, and involvè automatic control of a network or networks 25 such that the signal entering the regeneration function --....
remains unchanged in its important properties, such as amplitude, and ensures freedom from intersymbol inter~
ference.
Typically, the linear network 11 in Fig. 2 is formed by an equalizer, comprising one or more resonant brldged T sections, followed by an automatic line build out circuit. ^
....
This last circuit can include filter sections in which each of several resistance elements is created by the slope ---resistance of a diode through which the control current 25 is passed. If the signals at 20 and 21 can be from current sources the adder 12 can be simply a common impedence ~2~
through which those currents are passed. ~he linear network 13 can be an amplifier, and its yain may be arranged to vary with frequency ln order to fulfill part of the e~ualization and pulse shaping functions. The retimer 14, regenerator 15 and output stage 17 can be as described in Chapter 14 and earlier chapters of "Digital Transmission Systems", Revised 2nd Edition by P. Bylanski and D.G.W. Ingram, Peter Peregrinus, 1980, ISBN 0906048427. The filtering and scaling network 16 may be a simple low pass network comprising a series resis-tance followed by a shunt capacitance, together with any neces-sary a.c. (alternating current) coupling. The control ne-twork 18 may he a peak detector formed by a half wave rectifier fol-lowed by a large capacitance with large shunt resistance whose potential difference is used to dri~e a current source.
Claims (22)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A digital transmission system comprising: a twisted-wire pair transmission line; and at least one digital repeater coupled to said transmission line having an arrangement to reduce intersymbol interference in an incoming signal while minimizing near-end crosstalk gain, said arrangement including a pulse regenerator and a retimer network having an input and an output, an adder having two inputs and an output, a feed-back network coupled to said output of said regenerator and retimer network to produce a compensation signal, said com-pensation signal being coupled to one of said two inputs of said adder, an additional network disposed between said out-put of said adder and said input of said regenerator and retimer network to provide a suitable input signal for said regenerator and retimer network, a filter network coupling said incoming signal to the other of said two inputs of said adder, and a control network coupled to said input of said regenera-tor and retimer network and at least a selected one of said feedback network, said additional network and said filter net-work.
2. A system according to claim 1 wherein said repeater further includes an output network coupled to said output of said regenerator and retimer network to form a signal suitable for transmission on said transmission line and to buffer said feedback network from the effects of the impedance of said transmission line.
3. A digital repeater for use with a twisted-pair trans-mission line to reduce intersymbol interference in an incoming signal while minimizing near-end crosstalk gain comprising: a pulse regenerator and a retimer network having an input and an output, an adder having two inputs and an output, a feed-back network coupled to said output of said regenerator and retimer network to produce a compensation signal, said compen-sation signal being coupled to one of said two inputs of said adder, an additional network disposed between said output of said adder and said input of said regenerator and retimer net-work to provide a suitable input signal for said regenerator and retimer network, a filter network coupling said incoming signal to the other of said two inputs of said adder, and a control network coupled to said input of said regenerator and retimer network and at least a selected one of said feedback network, said additional network and said filter network.
4. A repeater according to claim 3, further including an output network coupled to said output of said regenerator and retimer network to form a signal suitable for transmission on said transmission line and to buffer said feedback network from the effects of impedance of said transmission line.
5. A system according to claim 1, wherein a control signal output of said control network controls said filter network.
6. A system according to claim 1, wherein a control signal output of said control network controls said feedback network.
7. A system according to claim 1, wherein a control signal output of said control network controls said filter network and said feedback network.
8. A system according to claim 1, wherein a control signal output of said control network controls said additional network.
9. A system according to claim 1, wherein a control signal output of said control network controls said filter network and said additional network.
10. A system according to claim 1, wherein a control signal output of said control network controls said feedback network and said additional network.
11. A system according to claim 1, wherein a control signal output of said control network controls said feedback network, said filter network and said additional network.
12. A repeater according to claim 3, wherein said output network prevents impedance variations of said transmission lines from affecting said compensation signal.
13. A repeater according to claim 3, wherein said feed-back network prevents impedance variations of said transmis-sion lines from affecting said compensation signal.
14. A repeater according to claim 3, wherein said control network has a low-pass characteristic to limit error multipli-cation.
15. A repeater according to claim 3, wherein said control network has a high frequency characteristic to minimize gener-ation of pattern dependent jitter in said retimer of said re-generator and retimer network.
16. A repeater according to claim 3, wherein a control signal output of said control network controls said filter network.
17. A repeater according to claim 3, wherein a control signal output of said control network controls said feedback network.
18. A repeater according to claim 3, wherein a control signal output of said control network controls said filter network and said feedback network.
19. A repeater according to claim 3, wherein a control signal output of said control network controls said additional network.
20. A repeater according to claim 3, wherein a control signal output of said control network controls said filter network and said additional network.
21. A repeater according to claim 3, wherein a control signal output of said control network controls said feedback network and said additional network.
22. A repeater according to claim 3, wherein a control signal output of said control network controls said feedback network, said filter network and said additional network.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000407458A CA1201783A (en) | 1982-07-16 | 1982-07-16 | Digital transmission systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000407458A CA1201783A (en) | 1982-07-16 | 1982-07-16 | Digital transmission systems |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1201783A true CA1201783A (en) | 1986-03-11 |
Family
ID=4123230
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000407458A Expired CA1201783A (en) | 1982-07-16 | 1982-07-16 | Digital transmission systems |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1201783A (en) |
-
1982
- 1982-07-16 CA CA000407458A patent/CA1201783A/en not_active Expired
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