U.S. patent application number 10/129356 was filed with the patent office on 2004-12-02 for method and system for optimazation of an optical transmission signal which is modulated with a binary data signal.
Invention is credited to Glingener, Christoph, Gottwald, Erich.
Application Number | 20040240030 10/129356 |
Document ID | / |
Family ID | 7928053 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040240030 |
Kind Code |
A1 |
Glingener, Christoph ; et
al. |
December 2, 2004 |
Method and System for Optimazation of an Optical Transmission
Signal which is Modulated with a Binary Data Signal
Abstract
A method and system for optimization of an optical transmissions
signal which is modulated with a binary data signal, wherein a
control device ensures that the operating point and the modulation
signal of a Mach-Zehnder modulator are set optimally, with the
fundamental frequency and/or the first harmonic frequency of the
transmission signal being selected for this purpose, and an optimum
setting is reached when the amplitude of the fundamental frequency
has reached a maximum value and the amplitude of the harmonic
frequency has reached a minimum value.
Inventors: |
Glingener, Christoph;
(Feldkirchen-Westerham, DE) ; Gottwald, Erich;
(Holzkirchen, DE) |
Correspondence
Address: |
BELL, BOYD & LLOYD, LLC
P. O. BOX 1135
CHICAGO
IL
60690-1135
US
|
Family ID: |
7928053 |
Appl. No.: |
10/129356 |
Filed: |
August 6, 2002 |
PCT Filed: |
November 3, 2000 |
PCT NO: |
PCT/DE00/03869 |
Current U.S.
Class: |
359/276 |
Current CPC
Class: |
H04B 10/505 20130101;
G02F 1/0123 20130101; H04B 10/50575 20130101 |
Class at
Publication: |
359/276 |
International
Class: |
G02F 001/01 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 1999 |
DE |
199 53 332.6 |
Claims
1-10. (canceled)
11: A method for amplitude modulation of an optical signal with a
binary data signal, the method comprising the steps of: supplying
both the optical signal and the binary data signal to a modulator,
whose operating point is adjustable, for producing an optical
transmission signal; demodulating the transmission signal or a
measurement signal, which is tapped off the transmission signal;
converting the demodulated signal to a recovered binary data
signal; selecting by filtering out from the recovered data signal
both a fundamental frequency and at least one harmonic frequency;
deriving from the recovered binary data signal a fundamental
frequency control signal and the at least one harmonic frequency
control signal; and controlling the operating point of the
modulator for optimum adjustment such that the fundamental
frequency reaches for a maximum amplitude or power and the at least
one of the harmonic frequencies reaches for a minimum amplitude or
power.
12. (canceled)
13: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 11, the method further
comprising the steps of: converting the data signal to a modulation
signal which is supplied to the modulator; and regulating an
amplitude of the modulation signal at a value at which one of the
fundamental frequency and a control signal formed from the
fundamental frequency and the inverted harmonic frequency reaches a
maximum.
14: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 13, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value.
15: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 13, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value and the harmonic
frequency has reached a minimum value at the same time.
16: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 14, wherein the operating
point and the modulation signal are optimally set alternately.
17: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 15, wherein the operating
point and the modulation signal are optimally set alternately.
18: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 13, wherein the operating
point control signal and the amplitude of the modulation signal are
optimized by sweeping respective frequencies of the operating point
control signal and the modulation signal, and by control being
carried out in each case based on a lock-in principle.
19: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 11, the method further
comprising the steps of: producing the recovered data signal by
demodulation of an optical received signal; deriving control
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end; and
transmitting control signals via the control device and via a
service channel to the modulator.
20: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 11, the method further
comprising the steps of: producing the recovered data signal by
demodulation of an optical received signal; deriving controlled
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end; and
transmitting control signals to a control device arranged at a
receiving end, the control device controlling the modulator via a
service channel.
21: A system for amplitude modulation of an optical signal with a
binary data signal, comprising: a modulator with an adjustable
operating point, the modulator having an output at which a
modulated transmission signal is emitted; a demodulator, in a
control loop, through which one of the transmission signal or a
measurement signal which corresponds to the transmission signal is
supplied in order to obtain a recovered binary data signal from it;
a first filter, in the control loop, which selects a fundamental
frequency of the recovered binary data signal; at least one second
filter, in the control loop, which selects at least one harmonic
frequency of the recovered binary data signal; at least one
measurement transducer, in the control loop, for obtaining control
signals from at least the first and second filters; and a control
device, in the control loop, for controlling the operating point of
the modulator such that the amplitude of the fundamental frequency
reaches for a maximum value; and the amplitude of the at least one
harmonic frequency reaches for a minimum value.
22: A system for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 21, further comprising an
adjustable amplifier for converting the data signal to a modulation
signal, wherein the modulation signal is regulated at a maximum
value at which the amplitude of the fundamental frequency is at a
maximum.
23: A system for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 21, further comprising an
adjustable amplifier for converting the data signal to a modulation
signal, wherein the modulation signal is regulated at a maximum
value at which the amplitude of the fundamental frequency is at a
maximum and the amplitude of the at least one harmonic frequency is
at a minimum.
24. (canceled)
25: A method for amplitude modulation of an optical signal with a
binary data signal, the method comprising the steps of: supplying
both the optical signal and the binary data signal to a modulator
for producing a transmission signal whose operating point is
adjustable; demodulating one of the transmission signal and a
measurement signal, which is tapped off the transmission signal;
converting the demodulated signal to a recovered data signal;
filtering out at least one harmonic frequency of the recovered data
signal; converting the data signal to a modulation signal which is
supplied to the modulator; regulating an amplitude of the
modulation signal at a value at which one of the fundamental
frequency and a control signal formed from the fundamental
frequency and the inverted harmonic frequency reaches a maximum;
using a signal at the at least one harmonic frequency, which has
been filtered out, to derive a control signal which controls an
operating point of the modulator such that the at least one
harmonic frequency reaches one of a minimum amplitude and a minimum
power; and varying one of the operating point and an amplitude of
the modulation signal on a trial basis until the fundamental
frequency has reached a maximum value.
26: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 25, the method further
comprising the steps of: selecting both a fundamental frequency and
the at least one harmonic frequency from the recovered data signal;
and using control signals derived from the fundamental frequency
and the at least one harmonic frequency for optimum adjustment of
the operating point.
27: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 25, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value and the harmonic
frequency has reached a minimum value at the same time.
28: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 25, wherein the operating
point and the modulation signal are optimally set alternately.
29: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 27, wherein the operating
point and the modulation signal are optimally set alternately.
30: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 25, wherein the operating
point control signal and the amplitude of the modulation signal are
optimized by sweeping respective frequencies of the operating point
control signal and the modulation signal, and by control being
carried out in each case based on a lock-in principle.
31: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 25, the method further
comprising the steps of: producing the recovered data signal by
demodulation of an optical received signal; deriving control
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end; and
transmitting control signals via the control device and via a
service channel to the modulator.
32: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 25, the method further
comprising the steps of: producing the recovered data signal by
demodulation of an optical received signal; deriving controlled
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end; and
transmitting control signals to a control device arranged at a
receiving end, the control device controlling the modulator via a
service channel.
33: A method for amplitude modulation of an optical signal with a
binary data signal, the method comprising the steps of: supplying
both the optical signal and the binary data signal to a modulator
for producing a transmission signal whose operating point is
adjustable; demodulating an optical received signal; converting the
demodulated signal to a recovered data signal; deriving control
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end;
transmitting control signals via the control device and via a
service channel to the modulator; filtering out at least one
harmonic frequency of the recovered data signal; and using a signal
at the at least one harmonic frequency, which has been filtered
out, to derive a control signal which controls an operating point
of the modulator such that the at least one harmonic frequency
reaches one of a minimum amplitude and a minimum power.
34: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 33, the method further
comprising the steps of: selecting both a fundamental frequency and
the at least one harmonic frequency from the recovered data signal;
and using control signals derived from the fundamental frequency
and the at least one harmonic frequency for optimum adjustment of
the operating point.
35: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 33, the method further
comprising the steps of: converting the data signal to a modulation
signal which is supplied to the modulator; and regulating an
amplitude of the modulation signal at a value at which one of the
fundamental frequency and a control signal formed from the
fundamental frequency and the inverted harmonic frequency reaches a
maximum.
36: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 35, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value.
37: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 35, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value and the harmonic
frequency has reached a minimum value at the same time.
38: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 36, wherein the operating
point and the modulation signal are optimally set alternately.
39: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 37, wherein the operating
point and the modulation signal are optimally set alternately.
40: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 35, wherein the operating
point control signal and the amplitude of the modulation signal are
optimized by sweeping respective frequencies of the operating point
control signal and the modulation signal, and by control being
carried out in each case based on a lock-in principle.
41: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 33, the method further
comprising the steps of: producing the recovered data signal by
demodulation of an optical received signal; deriving controlled
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end; and
transmitting control signals to a control device arranged at a
receiving end, the control device controlling the modulator via a
service channel.
42: A method for amplitude modulation of an optical signal with a
binary data signal, the method comprising the steps of: supplying
both the optical signal and the binary data signal to a modulator
for producing a transmission signal whose operating point is
adjustable; demodulating an optical received signal; converting the
demodulated signal to a recovered data signal; deriving controlled
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end;
transmitting control signals to a control device arranged at a
receiving end, the control device controlling the modulator via a
service channel; filtering out at least one harmonic frequency of
the recovered data signal; and using a signal at the at least one
harmonic frequency, which has been filtered out, to derive a
control signal which controls an operating point of the modulator
such that the at least one harmonic frequency reaches one of a
minimum amplitude and a minimum power
43: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 42, the method further
comprising the steps of: selecting both a fundamental frequency and
the at least one harmonic frequency from the recovered data signal;
and using control signals derived from the fundamental frequency
and the at least one harmonic frequency for optimum adjustment of
the operating point.
44: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 42, the method further
comprising the steps of: converting the data signal to a modulation
signal which is supplied to the modulator; and regulating an
amplitude of the modulation signal at a value at which one of the
fundamental frequency and a control signal formed from the
fundamental frequency and the inverted harmonic frequency reaches a
maximum.
45: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 44, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value.
46: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 44, the method further
comprising the step of varying one of the operating point and an
amplitude of the modulation signal on a trial basis until the
fundamental frequency has reached a maximum value and the harmonic
frequency has reached a minimum value at the same time.
47: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 45, wherein the operating
point and the modulation signal are optimally set alternately.
48: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 46, wherein the operating
point and the modulation signal are optimally set alternately.
49: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 44, wherein the operating
point control signal and the amplitude of the modulation signal are
optimized by sweeping respective frequencies of the operating point
control signal and the modulation signal, and by control being
carried out in each case based on a lock-in principle.
50: A method for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 42, the method further
comprising the steps of: producing the recovered data signal by
demodulation of an optical received signal; deriving control
signals from the recovered data signal; supplying the control
signals to a control device arranged at a transmission end; and
transmitting control signals via the control device and via a
service channel to the modulator.
51. A system for amplitude modulation of an optical signal with a
binary data signal, comprising: a modulator with an adjustable
operating point, the modulator having an output at which a
modulated transmission signal is emitted; a demodulator, in a
control loop, through which one of the transmission signal or a
measurement signal which corresponds to the transmission signal is
supplied in order to obtain a recovered binary data signal from it;
a first filter, in the control loop, which selects a fundamental
frequency of the recovered binary data signal; at least one second
filter, in the control loop, which selects at least one harmonic
frequency of the recovered binary data signal; at least one
measurement transducer, in the control loop, for obtaining control
signals from at least the first and second filters, wherein the
measurement transducers are power measurement devices; and a
control device, in the control loop for controlling the operating
point of the modulator such that the amplitude of the fundamental
frequency reaches for a maximum value; and the amplitude of the at
least one harmonic frequency reaches for a minimum value.
52. A system for amplitude modulation of an optical signal with a
binary data signal as claimed in claim 51, further comprising an
adjustable amplifier for converting the data signal to a modulation
signal, wherein the modulation signal is regulated at a maximum
value at which the amplitude of the fundamental frequency is at a
maximum.
Description
BACKGROUND OF THE INVENTION
[0001] Mach-Zehnder modulators (MZI Mach-Zehnder interferometers)
are used in optical transmission systems whose data rates are 10
Gbit/s or more since, at these high data rates and with the
present-day state of the art, neither direct modulation of a laser
nor modulation using electroabsorption modulators is expedient.
Generally, in addition to the modulation signal, MZI modulators
require a bias voltage for setting the operating point in order to
achieve a balanced output signal and, hence, a balanced eye shape
for the received signal. Any deviation of the operating point from
this value leads to distortion in the optical transmission signal
and, hence, to greater error rates and/or to a reduced range. Like
all interferometer arrangements, which react extremely sensitively
to a very small optical path length change, the operating point
also varies with the environmental conditions, in most available
modulators.
[0002] An MZI modulator is described, for example, in the
"Designer's Guide to External Modulation", UTP, 1289 Blue Hills
Avenue, Bloomfield, Conn. pages 4-6. When the modulator is being
fully driven, any change in the operating point leads to
overdriving, as a result of which, after initially rising, the
optical power falls once again, despite the control signal
increasing, during the transition from blocking after switching on
from "0" to "1." In the event of overdriving, a transmitted "1 bit"
peaks in the region of the rising 1 flank, resulting in a
relatively high frequency structure in which some of the spectral
power is contained in higher frequencies in particular; for
example, at twice the fundamental frequency of the data signal, or
at the data rate.
[0003] Patent specification U.S. Pat. No. 5,710,653 discloses a
transmission system having a module for external modulation of a
signal, in which harmonic frequencies of the modulated signal are
suppressed. A first method uses two Mach-Zender interferometers,
arranged in parallel, as modulators, to whose inputs the signal to
be modulated is supplied with different amplitudes (80% and 20%),
and whose two modulated output signals are combined such that a
harmonic frequency from the second output signal is isolated from
the fundamental frequency, is inverted and then amplified such that
the harmonic frequencies in the resultant modulated signal
compensate for one another by addition between the first modulated
output signal and the processed isolated harmonic frequency of the
second modulated output signal. In the second method, only one
modulator is used in order to suppress second-order and third-order
harmonic frequencies in the modulated signal. In this case, a
distortion network is required to suppress the third-order harmonic
frequency in addition to controlling the operating point of the
modulator. These two methods suppress third-order harmonic
frequencies. An additional control loop is provided for setting the
operating point of the modulator or modulators in phase quadrature
in order to eliminate the second-order harmonic frequency.
[0004] Patent Specification U.S. Pat. No. 5,629,792 discloses a
further arrangement and method for modulation of a signal, with
closed-loop control of the operating point of the modulator. The
closed-loop control controls the operating point of the modulator
by measuring the power level of the fundamental frequency of the
modulated signal emitted by the modulator. This closed-loop process
does not overcome, nor does it minimize, the influences of
interference harmonic frequencies in the modulated signal.
[0005] An object of the present invention is, therefore, to specify
a method which provides as simple a solution approach as possible
for suppression of harmonic frequencies. Another aim is to specify
a suitable system for doing the same.
[0006] Additional features and advantages of the present invention
are described in, and will be apparent from, the following Detailed
Description of the Invention and the Figures.
SUMMARY OF THE INVENTION
[0007] This object is achieved by using the signal at the harmonic
frequency which, in each case, has been filtered out to derive a
control signal which controls the operating point of the modulator,
such that the at least one harmonic frequency reaches a minimum
amplitude or power.
[0008] The advantage of this solution is that it requires
considerably less complexity than the prior art for suppressing
harmonic frequencies.
[0009] The method makes use of an effect, which occurs when the
modulator is overdriven, for regulating the operating point. It is
particularly advantageous in this case to combine operating point
control with control of the modulation signal. All major parameters
are kept constant by the control process.
[0010] It is also advantageous for the control criteria to be
obtained at the receiving end, and to be transmitted via a service
channel. This also makes it possible to partially compensate for
distortion caused by the transmission path.
[0011] Additional features and advantages of the present invention
are described in, and will be apparent from, the following Detailed
Description of the Invention and the Figures.
BRIEF DESCRIPTION OF THE FIGURES
[0012] FIG. 1 shows an outline circuit diagram of a system
according to the present invention.
[0013] FIG. 2 shows a waveform of the associated control
voltage.
[0014] FIG. 3 shows a variant of the system according to the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The system illustrated in FIG. 1 contains a Mach-Zehnder
modulator (MZI) 4, to which an optical signal OS is supplied from a
laser 3. The modulator 4 is modulated with a data signal DS, which
is supplied from a data source, as a modulation signal U.sub.DAT,
via a controllable amplifier 2. The modulated optical transmission
signal OSM is transmitted. A small portion of the signal is
supplied via an optical splitter 5 to an optoelectrical transducer
6 with a downstream amplifier 7, and is demodulated. The electrical
data signal DS1 recovered in this way essentially contains the
modulation signal U.sub.DAT or data signal DS. The data signal DS1
is supplied via an amplifier 7 to two filters, the bandpass filters
8 and 10. The first bandpass filter 8 filters the fundamental
frequency GW out of the data signal DS1; that is to say, its pass
frequency is at half the bit rate. A low-pass-filtered 01 bit
sequence essentially results in a sinusoidal voltage at a frequency
corresponding to half the data rate (a derived data signal also may
be used instead of the NRZ data signal). The output voltage from
the first bandpass filter 8 is supplied directly or via a
measurement transducer 9, such as a rectifier or a power
measurement device, as a control signal U.sub.R1 to a control
device 12. In the exemplary embodiment, a second bandpass filter 10
is provided and tuned to a harmonic frequency OW, preferably the
first. Its output voltage is also supplied directly or via a second
measurement transducer 11 as a further control signal U.sub.R2 to
the control device 12. Additional bandpass filters also may be
provided for filtering out further harmonic frequencies, and their
output voltages can be combined. The control device produces, via a
regulator 17, a control signal U.sub.BIAS, which governs the
operating point of the modulator 4.
[0016] As already mentioned, the first bandpass filter BP1 filters
out the fundamental frequency GW. Deviations from the operating
point or overdriving caused by an excessively large modulation
signal lead to a reduction in the amplitude of the fundamental
frequency (sinusoidal signal), since the harmonics which then occur
result in the fundamental frequency spectral component decreasing.
A corresponding situation applies to the control signal U.sub.R1
obtained from the sinusoidal signal. An opposite situation applies
to the wave form for the harmonic frequencies. Their amplitudes
and, thus, the amplitudes of the control signals U.sub.R2, . . .
increase when overdriving occurs.
[0017] The diagram illustrated in FIG. 2 shows the relationship
between the power levels P of the fundamental frequency GW and of
the first harmonic frequency OW, and the operating point of the
modulator. The power (amplitude) of the fundamental frequency has a
clear optimum in the region of the ideal operating point. Here, the
amplitude of the harmonic frequency (of the harmonic frequencies)
is at a minimum which is more strongly dependent on the operating
point and which is particularly highly suitable for optimization of
the operating point. It is particularly advantageous to combine the
two control signals U.sub.R1 and U.sub.R2 for operating point
control, such as by addition, with one of these signals being
inverted, since this leads to the control characteristic having a
steeper profile. FIG. 2 shows the main values for the operating
point control signal (bias voltage). The optimum occurs at about 4.
8 V.
[0018] A control device can at the same time be used to convert the
data signal DS emitted from the data source to an optimum
modulation signal U.sub.DAT by using a further control signal
U.sub.MOD to control the amplifier 2, this optimum modulation
signal U.sub.DAT being that which leads to a transmission signal
with the maximum amplitude and the minimum harmonic content
(maximum modulation level). It is sufficient to use the fundamental
frequency for control purposes in order to maximize the modulation
signal U.sub.DAT.
[0019] During a control process, the control device can be used to
produce changes in the control signals U.sub.BIAS and U.sub.MOD in
both directions on a trial basis in order to reach the respective
optimum setting. The operating point U.sub.BIAS (control
signal/bias voltage) and the modulation signal U.sub.DAT may, for
example, be adjusted alternately. The frequency of the operating
point and/or the amplitude of the modulation signal likewise may be
swept (periodic variation by a small amount via frequency-sweep
voltages U.sub.W1, U.sub.W2) in order to determine the mathematical
sign of any control error, with the control process being carried
out based on the lock-in principle.
[0020] It is, of course, also possible to control the amplitude of
the laser signal OS and, hence, the amplitude of the transmission
signal OSM.
[0021] FIG. 3 shows a variant in which the control signals
U.sub.R1, U.sub.R2 are derived from the received signal OEM at the
end of a transmission path (at the receiving end). The control
process also can, in this way, take account of the line
characteristics. The demodulation of the received signal OEM is
carried out in a receiving device 20. The demodulated data signal
DS1 is once again evaluated via filters 8, 10 and is converted by
the measurement transducers 9, 11 to control signals U.sub.R1,
U.sub.R2 which, after inversion of one control signal via an
inverting amplifier 13, are combined in an adder 16 to form a
resultant control signal U.sub.R.
[0022] The control device 12 may be arranged at the receiving end
or at the transmitting end. In this exemplary embodiment, the
combined control signal U.sub.R is transmitted via a service
channel 22 to the control device 12 arranged at the transmitting
end, in order to optimize the operating point and/or the amplitude
of the modulation signal.
[0023] Although the present invention has been described with
reference to specific embodiments, those of skill in the art will
recognize that changes may be made thereto without departing from
the spirit and scope of the present invention as set forth in the
hereafter appended claims.
* * * * *