U.S. patent application number 11/682130 was filed with the patent office on 2007-06-28 for digital compensation for optical transmission system.
Invention is credited to Seb J. Savory, James Whiteaway.
Application Number | 20070147850 11/682130 |
Document ID | / |
Family ID | 38193897 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070147850 |
Kind Code |
A1 |
Savory; Seb J. ; et
al. |
June 28, 2007 |
Digital Compensation for Optical Transmission System
Abstract
A receiver for an optical transmission system, has a
polarization diverse and phase diverse coherent optical receiver,
and a digital adaptive equalizer for compensating for distortions
in the optical signal introduced by the optical path. The entire
field of the optical signal is mapped including phase and
polarization information, to enable more complete compensation for
impairments such as chromatic dispersion and PMD. Furthermore, it
can also reduce the problems which have so far held back coherent
optical detection from widespread implementation, such as
polarization alignment and phase tracking. This can be applied to
upgrade existing installed transmission routes to increase capacity
without the expense of replacing the old fiber.
Inventors: |
Savory; Seb J.; (Royston,
GB) ; Whiteaway; James; (Sawbridgeworth, GB) |
Correspondence
Address: |
BARNES & THORNBURG LLP
P.O. BOX 2786
CHICAGO
IL
60690-2786
US
|
Family ID: |
38193897 |
Appl. No.: |
11/682130 |
Filed: |
March 5, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10425809 |
Apr 29, 2003 |
|
|
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11682130 |
Mar 5, 2007 |
|
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Current U.S.
Class: |
398/208 |
Current CPC
Class: |
H04B 10/6166 20130101;
H04B 10/65 20200501; H04B 10/6161 20130101; H04B 10/614 20130101;
H04B 10/613 20130101; H04B 10/61 20130101; H04B 10/6162
20130101 |
Class at
Publication: |
398/208 |
International
Class: |
H04B 10/06 20060101
H04B010/06 |
Claims
1. A receiver for an optical transmission system, having a
polarization diverse and phase diverse coherent optical receiver,
for receiving an optical input signal on an optical path and
generating digital electrical outputs, and a digital adaptive
equalizer coupled to the digital outputs, for compensating for
distortions in the optical signal introduced by the optical
path.
2. The receiver of claim 1, having a bit error detector, the
adaptive equalizer being arranged to minimize a bit error rate.
3. The receiver of claim 1, the optical receiver having four
outputs, in-phase and quadrature on two polarizations.
4. The receiver of claim 3, the optical signal having two or more
information channels modulated orthogonally, the adaptive equalizer
being arranged to maximise an orthogonality of the information
channels.
5. The receiver of claim 4, having a correlator for determining the
orthogonality.
6. The receiver of claim 1, the adaptive equalizer being a
transversal filter.
7. The receiver of claim 6, the transversal filter being adapted by
iteration of a limited set of independent variables including three
defining the orientation and magnitude of PMD, and one each for the
chromatic dispersion, the orientation of polarizing element at the
receiver, and the phase of a local oscillator used for phase
diverse detection in the optical receiver.
8. The receiver of claim 1, the adaptive equalizer being one of a
maximum likelihood sequence estimator and a maximum a posteriori
detector.
9. The receiver of claim 1, the adaptive equalizer being arranged
to have an update rate of at least 1 kHz.
10. The receiver of claim 1, the adaptive equalizer having
fractional spacing.
11. The receiver of claim 1, the optical receiver having a
polarization beam splitter feeding a pair of 90.degree. optical
hybrids, and convertors for converting outputs of the optical
hybrids into digital electrical signals.
12. The receiver of claim 1, the optical input signal being
quadrature amplitude modulated, the adaptive equalizer having one
or more in phase and quadrature outputs for each polarization.
13. The receiver of claim 1, the optical input signal being
modulated by any of phase, amplitude, frequency, and polarization,
the adaptive equalizer being arranged to output demodulated
signals.
14. A digital adaptive equalizer for the receiver of claim 1.
15. Software for implementing the equalizer of claim 14.
16. A method of offering a data transmission service over the
receiver of claim 1.
17. A receiver according to claim 1 having six outputs, three on
each polarization state.
18. A receiver according to claim 17 wherein the three outputs on
each polarization have 120.degree. relative phase or polarization
difference.
Description
FIELD OF THE INVENTION
[0001] This invention relates to receivers for optical transmission
systems, to digital adaptive equalizers for such receivers, to
software for such equalizers, and to methods of offering a
transmission service over such apparatus.
BACKGROUND TO THE INVENTION
[0002] Known optical transmission systems can be broadly
categorized as direct detection, or coherent detection systems. In
direct detection systems, at the receiver, the signal power is
measured and therefore any phase and polarization information in
the optical signal is ignored and lost. In coherent detection
systems, the phase and/or polarization information is detected
which enables the use of polarization and/or phase modulation as
well as amplitude modulation, and so much greater information
carrying capacity is possible, than in direct detection systems for
a given optical signal to noise ratio. Direct detection systems
have nevertheless dominated the market for long haul transmission
systems due to their simplicity. In contrast coherent receivers
require careful polarization alignment and phase tracking, which is
difficult and can limit the cost/performance trade off. In typical
systems, the polarization may change at rates up to kHz levels,
while phase variations can be typically up to MHz levels.
[0003] An example of a coherent system intended to be insensitive
to polarization and phase fluctuations is shown in Cheng et al,
Journal Lightwave Technology Vol 7, No 2, 1989. The incoming
optical signal is split into two polarizations (e.g. vertical and
horizontal polarizations using a Wollaston prism). Each of these
two optical signals are then combined with a common optical local
oscillator using an optical 90 degree hybrid to give in-phase and
quadrature waveforms for the two polarization states. On detection
using a photodiode this results in four electrical signals
corresponding to the in-phase and quadrature waveforms for the two
polarizations.
[0004] Both coherent and direct detection systems are also limited
in high capacity systems by distortions introduced by the optical
path, mostly optical fiber. There are many such distortions,
including nonlinearities, four wave mixing, and so on, but the
principal ones are usually chromatic dispersion (CD) and
polarization mode dispersion (PMD). Chromatic dispersion is usually
approximately fixed with respect to time, but may drift over the
life of the fiber, or undergo step changes if the optical path is
altered, for example by a protection switching operation, or
switching in a wavelength routed network. PMD can vary over periods
of minutes, and so needs adaptable control. Many complex solutions
have been tried to compensate for PMD and CD with limited success.
Solutions which correct the distortion in the optical domain
involve expensive optical components.
[0005] An example of an electronic compensator for a conventional
10 Gb/s optical transmission system is described in a press release
of Aug. 12, 2002 by Santel Networks, of Newark, Calif. They claim
that it provides a single solution for mitigating impairments from
PMD and CD, which may otherwise limit the reach of optical systems,
impair quality-of-service levels or prevent deployment of service
on legacy fiber. However, any such compensator will have a limited
performance since in conventional direct detection systems, the
optical field is not fully recovered, for example the phase and
polarization information is lost.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide improved
apparatus and methods. According to a first aspect of the present
invention, there is provided a receiver for an optical transmission
system, having
[0007] a polarization diverse and phase diverse coherent optical
receiver, for receiving an optical input signal on an optical path
and generating digital electrical outputs, and a digital adaptive
equalizer coupled to the digital outputs, for compensating for
distortions in the optical signal introduced by the optical
path.
[0008] The use of such digital adaptive equalizing can be more
efficient or more cost effective than optical domain methods of
trying to compensate for impairments such as chromatic dispersion
and PMD. Furthermore, it can also reduce the problems which have so
far held back coherent optical detection from widespread
implementation, such as polarization alignment and phase tracking
associated with the local oscillator in the receiver. This could be
commercially significant if it enables the upgrade of existing
installed transmission routes to increase capacity without the huge
expense of replacing the old, relatively poor quality fiber. The
optical receiver need not necessarily generate digital outputs
directly, it can be arranged in stages, separating the
polarizations and phases in the optical domain, followed by
conversion into the electrical domain, followed by analog to
digital conversion. The term diverse is defined as meaning the
output of the optical receiver retains all of the phase and
polarization information in the input optical signal.
[0009] An additional feature of some embodiments is a bit error
detector, the adaptive equalizer being arranged to minimize the bit
error rate. This is one way of controlling the adaptation of the
equalizer.
[0010] An additional feature of some embodiments is the optical
receiver having four outputs, composed of the in-phase and
quadrature waveforms on two polarizations. This is an optimal way
of providing a complete mapping of the optical field of the optical
input signal into the electrical domain. This retains information
which is lost by receivers using direct detection of the optical
signal.
[0011] An additional feature of some embodiments is the optical
signal having two or more information channels modulated
orthogonally, the adaptive equalizer being arranged to maximise the
orthogonality of the information channels.
[0012] An additional feature of some embodiments is a correlator
for determining the orthogonality.
[0013] An additional feature of some embodiments is the adaptive
equalizer being a transversal filter. This is one of several
possibilities. Transversal filters offer linear equalization and
are well suited to compensating for chromatic dispersion, but are
less suitable for compensating for non-linear transmission
distortions. The tap weights on the transversal filters can be
obtained by passing training data through the system, but there are
other possibilities such as direct calculation if the system
transmission characteristics are sufficiently understood.
[0014] An additional feature of some embodiments is the transversal
filter being adapted by iteration of a limited set of independent
variables including three defining the orientation and magnitude of
PMD, and one each for the chromatic dispersion, the orientation of
the polarizing element at the receiver, and the phase of a local
oscillator used for phase diverse detection in the optical
receiver. Minimising the number of independent variables, to be
fewer than the number of taps in the transversal filters, can
increase the chances of successful and sufficiently rapid
convergence on a true global minimum.
[0015] An additional feature of some embodiments is the adaptive
equalizer being a maximum likelihood sequence estimator (MLSE) or a
maximum a posteriori (MAP) detector. These alternatives to the
transversal filter have the advantage of being non-linear and so
can take into account non additive effects more readily. The MLSE
and MAP equalizers require training, as do transversal filters, in
order to set up the algorithms required for the signal
processing.
[0016] An additional feature of some embodiments is the adaptive
equalizer being arranged to have an update rate of at least 1 kHz
which is suitable for adaptation to PMD.
[0017] An additional feature of some embodiments is the adaptive
equalizer having fractional spacing corresponding to sampling at an
integer multiple of the bit rate. This can improve performance, but
at the cost of requiring higher speed electronics. An additional
feature of some embodiments is the optical receiver having a
polarization beam splitter feeding a pair of 90.degree. optical
hybrids, and convertors for converting outputs of the optical
hybrids into digital electrical signals.
[0018] An additional feature of some embodiments is the optical
input signal being quadrature amplitude modulated, the adaptive
equalizer having one or more in phase and quadrature outputs for
each polarization.
[0019] An additional feature of some embodiments is the optical
input signal being modulated by any of phase, amplitude, frequency,
and polarization, the adaptive equalizer being arranged to output
demodulated signals. It is efficient to have the equalizer output
such signals rather than intermediate signals which need further
processing.
[0020] Another aspect provides a digital adaptive equalizer for the
receiver. Another aspect provides software for implementing the
equalizer. This acknowledges that software can be a valuable,
separately tradable commodity. It is intended to encompass
software, which runs on or controls "dumb" or standard hardware, to
carry out the desired functions, (and therefore the software
essentially defines the functions of the equalizer, and can
therefore be termed an equalizer, even before it is combined with
its standard hardware). For similar reasons, it is also intended to
encompass software which "describes" or defines the configuration
of hardware, such as HDL (hardware description language) software,
as is used for designing silicon chips, or for configuring
universal programmable chips, to carry out desired functions.
[0021] Another aspect provides a method of offering a data
transmission service over the network. The advantages of the
invention can enable improvements to be made in the system or
network performance such as being more reliable or more flexible,
having a greater capacity, or being more cost effective.
Consequently data transmission services over the network can be
enhanced, and the value of such services can increase. Such
increased value over the life of the system, could prove far
greater than the sales value of the equipment.
[0022] Any of the features can be combined with any of the aspects
of the invention as would be apparent to those skilled in the art.
Other advantages will be apparent to those skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] To show by way of example how the invention can be
implemented, embodiments will now be described with reference to
the figures in which:
[0024] FIG. 1 shows a transmission system having a receiver
according to an embodiment of the invention,
[0025] FIG. 2 shows a BPSK receiver
[0026] FIG. 3 shows a QPSK receiver, and
[0027] FIG. 4 shows a receiver using MLSEs.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] Adaptive equalization is a technique known for compensating
for channel distorting effects in transmission systems, using
digital filters, e.g. transversal filters, maximum likelihood
sequence estimators, or maximum a posteriori detectors. What is
notable is the application of this with a coherent optical detector
to compensate for optical distortions. Embodiments of the invention
use coherent detection of an incoming optical signal to map the
optical field into four electrical signals. A subsequent digital
adaptive equalizer can use these signals, containing phase and
polarization information, for compensation of effects such as
polarization mode dispersion (PMD) and chromatic dispersion which
otherwise cause an increase in the bit error rate.
[0029] FIG. 1 shows an optical transmission system using an
embodiment of the invention. A transmitter 20 is shown for
modulating user data onto an optical signal. This is fed along an
optical path including a fiber 90, typically many kilometers or
hundreds of kilometers long. The signal may be on a single
wavelength and be wavelength division multiplexed with many other
signals. There may be many components in the optical path,
including optical amplifiers, optical compensators, wavelength
multiplexers and demultiplexers and so on. These will all introduce
the distortions mentioned above. At the receiver 50, a polarization
and phase diverse optical receiver 30 takes the optical signal and
produces a digital output signal or signals in the electrical
domain ready for digital processing. Being a polarization and phase
diverse receiver, its outputs have not lost the phase and
polarization information inherent in the optical signal. There are
various ways to implement such an optical receiver. A digital
adaptive equalizer 40 makes use of this phase and polarization
information to compensate for the optical distortions. The
equalizer can also complete the demodulation of the original user
data channels. In principle the user data can be modulated by any
of phase, amplitude, frequency, and polarization. In practice, the
more common modulation types include IM/DD (Intensity
Modulation/Direct Detection), BPSK (Binary Phase Shift Keying),
QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude
Modulation), FSK (Frequency Shift Keying), multilevel FSK,
Polarization Shift Keying, and combinations of these. The optical
receiver can be arranged to effectively optically resolve the
waveform into components with orthogonal phase and polarization,
but resolution into the components corresponding to the transmitted
channels, and/or demodulation of further levels of coding in the
phase/polarization/amplitude/frequency space are better carried out
after or simultaneously with the equalization to compensate for the
optical distortions. Otherwise the loss of orthogonality from these
distortions will cause cross talk between the channels, resulting
in increased bit error rates. Equalization can be implemented by
for example linear equalizers, decision-feedback equalizers, and
maximum-likelihood sequence-estimation (MLSE) equalizers. A linear
equalizer tries to undo the linear distortions of the channel by
filtering the received signal. A decision-feedback equalizer
exploits previous symbol detections to cancel out the intersymbol
interference from echoes of these previous symbols. Finally, an
MLSE equalizer hypothesizes various transmitted symbol sequences
and, with a model of the dispersive channel, determines which
hypothesis best fits the received data. These equalization
techniques are well known to those skilled in the art, and can be
found in standard textbooks such as J. G. Proakis, Digital
Communications, 2nd ed., New York: McGraw-Hill, 1989.
[0030] FIG. 2 shows a BPSK receiver. This embodiment can be used in
the system of FIG. 1 or in other systems. The optical receiver 30
includes a polarizing device 120 for receiving the optical signal
from fiber 90. This is for separating the incoming signal into
orthogonal polarizations, e.g. horizontal and vertical. It can be
implemented by a polarization beam splitter, or other components
which have two or more outputs with different polarization
transmission coefficients. The separate polarizations from a
polarization beam splitter are fed to a pair of 90.degree. optical
hybrids 100 for mixing with a local oscillator laser source. The
outputs are the two mixing products with orthogonal phases between
the optical signal and the local oscillator. The generation of the
two mixing products has to be performed in the optical domain to
preserve the phase information. However the local oscillator can be
closely locked in phase to the incoming signal for homodyne
detection, or be separated from the local oscillator by several
times the signal bandwidth in the frequency domain for heterodyne
detection. The latter option then requires further mixing in the
electrical domain to down convert to base band. There is also an
intermediate option known as intradyne detection where the incoming
signal and local oscillator are spaced apart in the frequency
domain by a fraction of the signal bandwidth, which again requires
mixing in the electrical domain.
[0031] It is not essential that there are four outputs of the
optical receiver, nor that the outputs are orthogonal. Other ways
of retaining the phase and polarization information are
conceivable. For example six outputs can be produced, with
120.degree. relative phase or polarization difference rather than
four outputs with 90.degree..
[0032] The outputs of the optical hybrids are fed to convertors 110
if necessary, to convert into the electrical domain and from analog
electrical signals into digital electrical signals. The digital
signals are fed to the digital adaptive equalizer 40. This includes
equalizers which could for example be transversal filters 80, 82,
84, 86, for each of the four orthogonal signals. Transversal
filters are one way of implementing a digital adaptive equalizer in
the form of an FIR (Finite Impulse Response) filter with variable
coefficients time-spaced by an amount equal to the signal sample
interval which is typically the symbol time or an integral
sub-multiple thereof. The tap weights for such filters can be
determined and adapted by calculation from training sequences, and
updated by iteration for example, to minimize the bit error rate. A
tap adaptation algorithm 55 can be implemented in various ways. It
can use iteration techniques, and/or use pre calculation of taps
based on known or predicted values of CD for example. The
equalizers do not have to be FIR transversal filters, but could
also be maximum likelihood sequence estimators, maximum a
posteriori detectors, IIR (Infinite Impulse Response) transversal
filters, decision feedback equalizers or a combination thereof.
[0033] The outputs of the transversal filters are summed by adders
88, 92 and 94, to produce a signal representing the user data.
Optionally this may include FEC (forward error correction) codes to
enable the bit error rate to be reduced from that of the raw data A
FEC element 75 uses these codes to detect bit errors and carry out
the corrections. This element can therefore output an indication of
a bit error rate (BER) which can be used to adapt the adaptive
equalizer. This is achieved by using the BER as an input to the tap
adaptation algorithm.
[0034] One practical issue is the update time for the adaptive
filters. Given that the rate of polarization evolution in a fiber
is of the order of kHz, the refresh rate should be greater than
this to enable such changes to be tracked. This is a realisable
update rate for the adaptive filter in a transmission system
operating at channel data rates of gigabits per second. The update
rate is usually limited by the computational resource available,
the complexity of the adaptation algorithm, the number of taps to
be calculated, the range of dispersion to be compensated, and so
on. The choice of the number of taps will determine the amount of
chromatic dispersion that can be compensated. When chromatic
dispersion is being compensated the filter is only modifying the
phase response and not the amplitude response. This means that the
filter has an all pass response and the noise magnification is the
sum of the squared tap weights which is unity. Hence the noise
level is not affected by a filter that compensates only for
chromatic dispersion. In the more general case, if a non-linear
filter function is implemented, which would not be possible using a
transversal filter, then the noise might be amplified by the
equalizer.
[0035] Tap Adaptation Algorithm
[0036] There are many possible algorithms known for adapting taps
to minimize an error signal fed back from the output of the filter.
In principle, the algorithm can be based on the iteration of the
tap weights using a general mathematical minimization routine which
does not take any account of the physical reality of the situation.
Alternatively it can be a direct non iterative calculation derived
from, for example, measurements of actual CD or PMD. One example is
the compensation of pure chromatic dispersion, which introduces a
quadratic phase variation with frequency corresponding to a linear
variation in group delay with frequency. The quadratic phase
response in the frequency domain can be transformed into an
equivalent temporal response using an FFT (Fast Fourier Transform).
The temporal response is then windowed and used to set the tap
weights so as to obtain the desired finite impulse response.
[0037] There is a linear relationship between the no. of taps
required, for a given OSNR (Optical Signal to Noise Ratio) penalty,
and the chromatic dispersion for both iterated and calculated tap
weights. In one example using iterated tap weights in a BPSK
system, 29 taps gave 18,000 ps/nm of dispersion compensation (1000
km NDSF) for a 1 dB OSNR penalty at 10.sup.-3 BER. In another
arrangement using tap weights calculated from the chromatic
dispersion using an FFT (Fast Fourier Transform), 65 taps gave
34,800 ps/nm of dispersion (2000 km NDSF) for a 1 dB OSNR penalty
at 10.sup.-3 BER.
[0038] Where an iterative process is used, each of the taps is
usually regarded as an independent variable in the algorithm. If
there are too many taps, the algorithm may be too slow, or may not
converge on a true global minimum. An alternative is to determine
the taps indirectly. The algorithm determines, or is fed with other
independent variables and calculates the taps from the values of
those independent variables. The number of these independent
variables can be minimized by, for example, selecting the following
as independent variables: [0039] 1, 2: angles used to describe the
orientation of one fiber PMD principal state on the Poincare
sphere, [0040] 3: magnitude of PMD. [0041] 4: chromatic dispersion.
[0042] 5: orientation of the polarization beam splitter (PBS)
relative to fiber. [0043] 6: phase of local oscillator LO.
[0044] Fractionally Spaced Equalizer (FSE)
[0045] Improved performance may be obtained using fractionally
spaced equalizers (FSE) consisting of say an adaptive FIR filter
with taps time-spaced by an amount equal to the digital sampling
interval which would be an integral sub-multiple of the symbol
time. An example is shown in U.S. Pat. No. 6,240,134. As it is a
well known and well documented technique, it need not be described
here in more detail. Performance of a fractionally spaced equalizer
with a sufficient number of taps is less dependent on phase
characteristics of the transmission channel and on how well the
phase is 10 recovered by the optical receiver.
[0046] FIG. 3 Shows a QPSK Receiver
[0047] The reference numerals used in this figure correspond to
those used above where appropriate. In this case, instead of four
transversal filters there are eight, labeled TF 1 to TF8. There are
two channels of user data, modulated on orthogonal
polarizations.
[0048] In general orthogonal elliptical states are transmitted and
in the presence of first order PMD they rotate around the Poincare
sphere, while maintaining their orthogonality, as the waveforms
propagate along the fibre. In addition chromatic dispersion applies
equally to both polarization states. At the receiver the objective
is to separate out the orthogonal elliptical states, by taking the
outputs from the arbitrarily orientated polarization beam splitter
and summing them with an appropriate phase difference so as to
select one of the orthogonal states. The other elliptical state is
selected using a different phase difference so as to address the
orthogonal elliptical polarization. Superimposed on this is the
need to compensate for chromatic dispersion.
[0049] To remove these distortions, the 4 outputs from the optical
hybrids 110 are coupled via convertors (not illustrated) to each
feed digital inputs to two of the transversal filters. The four
outputs are a horizontal polarization in phase signal H/I, a
horizontal polarization quadrature phase signal H/Q, a vertical
polarization in phase signal, V/I, and a vertical polarization
quadrature phase signal, V/Q.
[0050] Calculation of tap weights for filters TF1-4 can be carried
out using the same algorithm as set out above in relation to FIG.
2. The orthogonality of QPSK channels allows calculation of tap
weights for filters TF5-8, as follows: TF5=-TF2, TF6=+TF1,
TF7=-TF4, TF8=+TF3. Adders 302, 304, 306, 308, 310 and 312 are
provided to sum the outputs of the transversal filters to reproduce
two orthogonal compensated output channels, an in phase channel and
a quadrature channel. The in phase channel is produced by summing
outputs from filters TF1-TF4, fed by all four outputs of the
optical receiver. The quadrature channel is produced by summing the
outputs of filters TF5-TF8, again fed by all four outputs of the
optical receiver. If large numbers of taps are used, it may be
impractical to iterate all the taps in the 8 transversal filters to
obtain an exact equalization of a QPSK signal for chromatic
dispersion and PMD, because the multi-dimensional space being
searched can exhibit local minima that can lead to solutions for
the tap weights that do not correspond to the global minimum. In
other words there may be problems with convergence onto the global
optimum in the equalization space. Nevertheless if the iteration is
carried out over the 6 independent variables mentioned above, from
which all the tap weights can be calculated, irrespective of the
magnitude of the two dispersions, then at least an approximate
solution can be obtained. The option remains to perform a more
detailed iteration of the tap weights using the approximate
solution as a starting point.
[0051] The feedback to the adaptation algorithm can for example be
either BER or a measure of orthogonality of the channels, or both.
The orthogonality of the channels can be determined by a correlator
330. This can be implemented using conventional digital
techniques.
[0052] FIG. 4 shows a BPSK receiver using an MLSE equalizer to
implement the adaptive equalization. Corresponding reference
numerals to those of FIG. 2 have been used as appropriate. In place
of the transversal filters, MLSE functions 480,482, 484, 486 are
provided for processing the digital signals. In an MLSE equalizer,
all possible transmitted symbol sequences up to a certain length
are considered. For each hypothetical sequence, the received signal
samples are predicted using a model of the distortions in the
channel. The difference between the predicted received signal
samples and the actual received signal samples, referred to as the
prediction error, gives an indication of how good a particular
hypothesis is. The squared magnitude of the prediction error is
used as a metric to evaluate a particular hypothesis. This metric
is accumulated for different hypotheses, to give probability
density functions (PDF), stored in pdf store 400, for use in
determining which hypothesis is best. This process can be realized
in various ways, and the Viterbi algorithm, which is a form of
dynamic programming is an efficient example.
[0053] The PDFs can be adapted by a training process, using
feedback from sources such as BER and correlation, (not shown in
the figure) or more effectively, by using known training input
sequences. The PDFs therefore effectively store an indirect
representation or model of the distortions being compensated.
[0054] Concluding Remarks
[0055] As has been described above, a receiver for an optical
transmission system, has a polarization diverse and phase diverse
coherent optical receiver, and a digital adaptive equalizer for
compensating for distortions in the optical signal introduced by
the optical path. The entire field of the optical signal is mapped
including phase and polarization information, to enable more
complete compensation for impairments such as chromatic dispersion
and PMD. Furthermore, it can also reduce the problems which have so
far held back coherent optical detection from widespread
implementation, such as polarization alignment and phase tracking
associated with the local oscillator in the receiver. This
technique can be applied to upgrade existing installed transmission
routes to increase capacity without the expense of replacing the
old fiber.
[0056] Other variations will be apparent to those skilled in the
art, having corresponding advantages to those set out above, within
the scope of the claims.
* * * * *