U.S. patent application number 14/027514 was filed with the patent office on 2014-01-09 for pilot symbol aided carrier phase estimation.
This patent application is currently assigned to TYCO ELECTRONICS SUBSEA COMMUNICATIONS LLC. The applicant listed for this patent is Dmitri Foursa, Hongbin Zhang. Invention is credited to Dmitri Foursa, Hongbin Zhang.
Application Number | 20140010532 14/027514 |
Document ID | / |
Family ID | 44902003 |
Filed Date | 2014-01-09 |
United States Patent
Application |
20140010532 |
Kind Code |
A1 |
Zhang; Hongbin ; et
al. |
January 9, 2014 |
PILOT SYMBOL AIDED CARRIER PHASE ESTIMATION
Abstract
Methods and systems for processing an optical signal in a
communication system are disclosed. The disclosed methods yield
benefits for estimation and tracking of carrier phase of received
signals at a digital coherent receiver. Specifically, phase
ambiguity is removed by the insertion of pilot symbols into a
transmitted data stream. Pilot symbols are detected from a received
signal, and carrier phase is estimated for the detected pilot
symbols. If carrier phase track of received data symbols was lost,
a correction is applied to recover the track. Coherent symbol
decoding may be used which has not been possible with prior art
techniques due to the possibility of phase tracking loss.
Inventors: |
Zhang; Hongbin; (Marlboro,
NJ) ; Foursa; Dmitri; (Colts Neck, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zhang; Hongbin
Foursa; Dmitri |
Marlboro
Colts Neck |
NJ
NJ |
US
US |
|
|
Assignee: |
TYCO ELECTRONICS SUBSEA
COMMUNICATIONS LLC
Eatontown
NJ
|
Family ID: |
44902003 |
Appl. No.: |
14/027514 |
Filed: |
September 16, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12775770 |
May 7, 2010 |
8588624 |
|
|
14027514 |
|
|
|
|
Current U.S.
Class: |
398/25 |
Current CPC
Class: |
H04B 10/61 20130101;
H04B 10/0779 20130101; H04B 10/6165 20130101; H04B 2210/074
20130101; H04B 10/616 20130101 |
Class at
Publication: |
398/25 |
International
Class: |
H04B 10/61 20060101
H04B010/61 |
Foreign Application Data
Date |
Code |
Application Number |
May 6, 2011 |
US |
PCT/US2011/035623 |
Claims
1. A method, comprising: receiving an optical signal having a pilot
symbol and multiple data symbols; estimating a carrier phase of
multiple data symbols using an M.sup.th power scheme with joint
polarization carrier phase estimation; and detecting the carrier
phase of at least one of said multiple data symbols adjacent said
pilot symbol.
2. The method of claim 1, further comprising determining a
differential phase between the phase of at least one of said
multiple data symbols adjacent said pilot symbol and a phase of
said pilot symbol.
3. The method of claim 2, further comprising: determining whether
the differential phase is less than a phase tolerance; and decoding
the multiple data symbols based on the determination that the phase
differential is less than the phase tolerance.
4. The method of claim 1, wherein the phase tolerance is .pi./3
radians.
5. The method of claim 1, wherein the pilot symbol is a set of
pilot symbols comprising at least two adjacent pilot symbols.
6. The method of claim 1, wherein the pilot symbol is a set of
pilot symbols comprising a first pilot symbol and a second pilot
symbol, the first pilot symbol corresponding to the first data
symbol in the optical signal and the second pilot signal
corresponding to the last data symbol in the optical signal.
7. The method of claim 1, wherein said data symbols comprise 50
consecutive data symbols.
8. The method of claim 1, wherein M equals 4.
9. The method of claim 1, wherein estimating the carrier phase of
the multiple data symbols comprises estimating the carrier phase of
X and Y polarization tributaries associated with the multiple data
symbols.
10. The method of claim 1, wherein estimating the carrier phase of
the multiple data symbols is based at least in part on a coupling
coefficient, wherein the coupling coefficient has a value between 0
and 1.
11. The method of claim 9, wherein the coupling coefficient is
0.7.
12. An optical receiver comprising: a plurality of photodetectors
each configured to generate an electrical signal proportional to a
received optical signal, said optical signal having a plurality of
channels each including a pilot symbol and a plurality of data
symbols; an analog to digital converter configured to convert each
of said electrical signals to digital signals; and a digital signal
processor (DSP) communicating with said analog to digital
converter, said DSP comprising a carrier phase estimation module
configured to: detect a carrier phase of one of the plurality of
data symbol that is adjacent said pilot symbol; determine a
differential phase between the one of the plurality of data symbols
adjacent said pilot symbol and said pilot symbol; and estimate the
phase of the data symbols using an M.sup.th power scheme with joint
polarization carrier phase estimation.
13. The optical receiver of claim 12 further comprising: a local
oscillator circuit; and a plurality of hybrid interferometers
communicating with said local oscillator circuit, each of said
hybrid interferometers configured to receive the plurality of
optical signal channels and extract signals representing a phase
and amplitude between the received optical channels and a signal
received from said local oscillator and supply said phase and
amplitude signals to a corresponding one of said plurality of
photodetectors.
14. The optical receiver of claim 13 further comprising a
polarization beam splitter configured to supply the plurality of
optical signal channels to each of said plurality of hybrid
interferometers.
Description
FIELD OF THE INVENTION
[0001] Embodiments of the present disclosure relate to the field of
optical communication systems. More particularly, the present
disclosure relates to the addition of pilot symbols in modulated
optical signals to aid in joint polarization phase estimation in
coherent optical receivers.
DISCUSSION OF RELATED ART
[0002] In optical communication systems, various modulation schemes
are used to transmit optical signals using dense wavelength
division multiplexing (DWDM) where a plurality of optical channels
each at a particular wavelength propagate over fiber optic cables.
However, various non-linearities including, for example, self-phase
modulation (SPM) and cross-phase modulation (CPM) may induce
nonlinear phase shifts which effect signal reception and decoding
of these transmitted signals. After propagation over long
distances, the impact of these non-linearities create processing
and decoding difficulties at the receiver which may compromise the
integrity of the transmitted information.
[0003] Coherent decoding detects not only an optical signal's
amplitude but phase and polarization as well. In other words, it
detects whole characteristics of the received signal across the
spectrum. This allows the receiver to compensate for the linear
channel transfer function. Moreover, quadrature phase Shift Keying
(QPSK) and polarization division multiplexing (PDM) schemes can be
implemented and thus increase detection capability and spectral
efficiency.
[0004] In coherent phase modulated optical communication systems,
the carrier phase of the transmitter must be estimated and tracked
at the receiver in order to decode the transmitted information
signals. However, an inherent problem associated with coherent
receivers in phase modulated optical communication systems is that
of phase ambiguity associated with phase detection at the receiver.
This is due to the presence of data modulation in the phase
detection. When the phase error reaches .pi./4 for QPSK or .pi./2
for Binary Phase Shift Keying (BPSK), a "cycle slip" can occur in
which symbols may be erroneously interpreted as lying in an
adjacent quadrant. For systems utilizing these types of modulation
techniques, this results in erroneous interpretation of the symbols
following the cycle slip. Therefore, it is important to
differentially pre-code data to avoid error propagation. However,
differential pre-coding suffers from about 1 dB Q penalty at low
optical signal to noise ratios because a single symbol error
becomes a pair of consecutive symbol errors. Accordingly, it is an
object of the present disclosure to overcome these problems and
provide better system performance at a lower optical signal to
noise ratio in optical communication systems.
SUMMARY OF THE INVENTION
[0005] Exemplary embodiments of the present disclosure are directed
to a method for estimating the carrier phase of a modulated optical
signal. In an exemplary method, an optical signal is received
having data symbols and pilot symbols which have predefined data
modulation. The carrier phase of the received pilot symbols are
detected. The carrier phase of the data symbols is estimated by an
M.sup.th power scheme. A differential phase between the pilot
symbol and the adjacent data symbol is determined. The differential
phase is compared to a defined phase tolerance to determine if the
cycle slip or phase error of the data symbols occurred before the
pilot symbols. If cycle slip occurred, then the carrier phase of
the data symbols are estimated by linear interpolation of the phase
between a pair of pilot symbols.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a simplified block diagram of an optical
communication system in accordance with the present disclosure.
[0007] FIG. 1A is a simplified block diagram of an optical receiver
of the communication system of FIG. 1 in accordance with the
present disclosure.
[0008] FIG. 2 is a depiction of a pilot symbol insertion scheme in
accordance with the present disclosure.
[0009] FIG. 3 is a depiction of correction of phase tracking in
accordance with the present disclosure.
[0010] FIG. 4 is a flow diagram illustrating a process in
accordance with the present disclosure.
[0011] FIG. 5 is a graph showing performance of carrier phase
estimation in accordance with the present disclosure.
[0012] FIGS. 6A, 6B are graphs showing the loss of tracking without
pilot symbols and pilot symbol aided joint polarization estimation
in accordance with the present disclosure.
DETAILED DESCRIPTION
[0013] The present invention will now be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention,
however, may be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. In the drawings, like
numbers refer to like elements throughout.
[0014] Presently disclosed embodiments mitigate the impact of
nonlinear phase noise through improved carrier phase estimation
with increased tracking capability. Phase ambiguity associated with
carrier phase estimation may be removed by inserting one or more
pilot symbols, which are reference symbols, inserted at known
positions in a modulated optical signal. The one or more pilot
symbols represent known data information and are inserted
periodically among data symbols in the modulated signal. The
carrier phase of the one or more pilot symbols is estimated by
determining the differential phase between the one or more pilot
symbols and the carrier phase of an adjacent or previous data
symbol. By utilizing pilot symbols in the modulated signal,
coherent PSK without precoding may be employed which provides
better system performance at a lower optical signal to noise ratio
than previously realized.
[0015] FIG. 1 generally illustrates a simplified optical
communication system 10 including an exemplary transmit terminal
11, receive terminal 12 and an optical transmission medium 15
disposed therebetween. The optical transmission medium may be a
fiber optic cable having a plurality of fiber pairs configured to
propagate communication signals between terminals 11 and 12.
Terminal 11 includes a plurality of transmitters 20 which supplies
a modulated optical signal 25 having a particular one of a
plurality of wavelengths to multiplexer 30. Multiplexer 30 combines
the modulated optical channels from the transmitters 20 and
combines them in a dense wavelength division multiplexed (DWDM)
signal for propagation over fiber optic cable 15. It should be
understood that the exemplary transmitter 20 may also be included
in terminal 12 for bidirectional transmission. Terminal 12 includes
a demultiplexer 40 used to separate the received DWDM optical
signal into individual wavelengths or channels 50. Once separated,
each channel is supplied to a respective receiver 60 and processed
to provide a demodulated optical data signal. It should be
understood that the exemplary receiver 60 may also be included in
terminal end 11 for bidirectional transmission.
[0016] As described earlier, in order to transmit and receive these
optical signals long distances, various modulation techniques are
employed to provide a detectable optical signal at receiver 60.
These modulation techniques include, for example, QPSK (quadrature
phase-shift keying); DP-QPSK (dual polarization quadrature
phase-shift keying), etc., in which data is identified by phases of
an optical carrier. However, various known non-linear effects
impact the integrity of the optical signal received by receiver 60
after propagation over cable 15. For example, the light sources
used to provide the optical signal at transmitters 20 are typically
external cavity lasers capable of providing linewidths in the range
of 100 KHz. However, after propagation over cable 15 the received
linewidth of the light may have broadened to tens of MHz due to
non-linear effects such as four-wave mixing, cross-phase modulation
(CPM), self-phase modulation (SPM), etc. Because of these nonlinear
effects, the phase of the optical signal received at receiver 60
may have rotated such that the decoded symbols do not match those
which were generated by transmitter 20. This causes symbol or data
errors.
[0017] FIG. 1A is a simplified block diagram of receiver 60 shown
in FIG. 1 in accordance with the present disclosure. The
transmitted DWDM channels 50 are received and supplied to a
polarization beam splitter (PBS) 61. The polarization split signal
is provided to a pair of 90.degree. optical hybrid interferometers
62A and 62B which extracts phase and amplitude between the received
signal and local oscillator 63. A plurality of photodetectors
denoted generally as 64 generate respective electrical signals
proportional to the received optical signals from the hybrid
interferometers 62A and 62B. A plurality of Analog to Digital
(A/D)converters 65 connected to respective photodetectors 64
receive the electrical signals and supply corresponding digital
signals to digital signal processor (DSP) 70. The DSP 70 may
include various modules/circuits including dispersion compensation
circuit 71, clock recovery circuit 72, synchronized data resampling
module 73, polarization tracking and polarization mode dispersion
compensation module 74, carrier phase estimation module 75 and
decision module 76. Receiver 60 is configured to decode the
received modulated data symbols.
[0018] Carrier phase estimation associated with decoding QPSK
modulated signals is difficult since the modulated data is present
in the received optical signal. In order to decode the received
signal, the modulated data must be removed from the signal to
determine the carrier phase and thus, synchronize the demodulation
of the data symbols based on the phase of the carrier. However, an
inherent problem associated with typical coherent QPSK systems is
that of phase ambiguity at the receiver. This phase ambiguity is
due to the general inability of the carrier recovery circuit in an
associated receiver to distinguish the reference phase from the
other phase(s) of the received carrier.
[0019] In order to remove the phase ambiguity of the carrier, pilot
symbols having known data information are inserted periodically
within the modulated optical signal in accordance with the present
disclosure. FIG. 2 illustrates an exemplary transmitted sequence of
data 100 having symbols 110N and a period length in a modulated
optical signal 25 generated by transmitter 20 in an exemplary
optical communication system 10. A first set of one or more pilot
symbols 110-0, 110-1 is inserted within a data sequence 100 to be
modulated on a carrier wave. A second set of one or more pilot
symbols 110-50, 110-51 are inserted into the data sequence 100
after data symbols 110-2 . . . 110-49. The data symbols 110-2 . . .
110-49 may have, for example, data information values 1, -1, i, or
-i.
[0020] Each of the sets of pilot symbols may comprise a pair of
adjacent pilot symbols. For example, the first set of pilot symbols
may include a first pilot symbol 110-0 and an adjacent pilot symbol
110-1. The second set of pilot symbols may include a first pilot
symbol 110-50 and an adjacent pilot symbol 110-51. Thus, the fifty
symbol period (i.e. N=50) from symbol 110-1 thru 110-50 is defined
by data symbols 110-2 . . . 110-49 disposed between a first pilot
symbol 110-1 and a second pilot symbol 110-50. Of course,
alternative numbers of pilot symbols and sequencing with
corresponding data symbols may also be employed. Data symbol 110-2
is adjacent pilot symbol 110-1 and data symbol 110-49 is adjacent
pilot symbol 110-50 to form a first and second set of pilot
symbols, respectively. The resultant stream 100 is transmitted to a
receiver 60 via optical cable 15. The above example of the
insertion of two pilot symbols (110-1, 110-50) per fifty symbol
period, increases the symbol rate of the transmitted signal by 4%
which causes approximately a 0.17 dB optical signal to noise ratio
(OSNR) penalty. However, insertion of the pilot symbols
advantageously mitigates phase tracking errors and enables the use
of coherent PSK without the need of symbol precoding, and overall
system performance improves by approximately 1 dB at low OSNR.
[0021] The modulated optical signal 25 is received by receiver 60
where each of the data symbols (e.g. 110-2 . . . 110-49) and pilot
symbols (110-0, 110-1, 110-50, 110-51) have an associated phase
consistent with the modulation format such as, for example, QPSK
supplied by transmitter 20. Receiver 60 detects the received pilot
symbols and estimates the carrier phase of these pilot symbols
using the following equation:
a.
.PHI..sub.n=.PHI..sub.n+1=.phi..sub.n-1+arg[.SIGMA..sub.k=0.sup.1r.sub.n-
+k exp(-i.phi..sub.n-1)] (1)
where .PHI.n represents the carrier phase of a first pilot symbol,
.PHI..sub.n+1 represents the carrier phase of an adjacent pilot
symbol, .phi..sub.n-1 represents the carrier phase of a data symbol
adjacent the first pilot symbol and r.sub.n represents the received
data symbol. For example, .PHI.n may represent the phase of pilot
symbol 110-50, .PHI.n+1 represents the carrier phase of pilot
symbol 110-51 (i.e. n and n+1, respectively) and .phi..sub.n-1
represents the carrier phase of data symbol 110-49 all of which are
shown in FIG. 2.
[0022] The last term of Equation (1) represents the differential
phase between the pilot symbol and the previous (immediately
preceding) data symbol. In the above example, this corresponds to
determining the differential phase between the carrier phase of
pilot symbol 110-50 and the carrier phase of data symbol 110-49
since the data symbol 110-49 immediately precedes pilot symbol
110-50. This differential phase is compared to a defined phase
tolerance and if the differential phase exceeds this tolerance,
then this indicates that the phase tracking of the signal at
receiver 60 was lost. This phase tolerance is relatively large such
as, for example, greater than n/3 radians. In the above example, if
the phase differential is greater than n/3 radians, then the phase
tracking of the carrier phase between the pilot symbols 110-50 and
110-1 was lost. Of course, the phase tolerance of n/3 radians is
used herein as an example and other tolerances may be applicable
based on the modulation scheme, length of transmission, etc.
[0023] Once the determination is made that the differential phase
falls outside the phase tolerance and phase tracking has been lost
between the pilot symbols, this loss of phase tracking may be
corrected via linear interpolation since phase is assumed to trend
linearly. The following Equation (2) is used for this linear
interpolation-based correction and assumes that the period has a
length of 50 symbols as described with reference to FIG. 2:
a . .phi. n - j = .phi. n - j + 1 + .PHI. n - 50 - .PHI. n 49 , j =
1 , 2 , 48 ( 2 ) ##EQU00001##
where .PHI.(n) and .PHI.(n-50) are the estimated carrier phase of
the pilot symbols 110-50 and 110-1 and again .phi. is the phase of
an associated data symbol. Of course, the period length may be
modified based on the particular application.
[0024] FIG. 3 is a notional depiction (not drawn to scale) of the
phase tracking correction via linear interpolation utilizing
Equation (2) above. The phase 200 is illustrated between a first
pilot symbol corresponding, for example, to pilot symbol 110-1 at
P1 and a second pilot symbol corresponding, for example, to pilot
symbol 110-50 at P2. The differential phase (denoted graphically at
210) is determined between the pilot symbol at P2 and the phase of
the data symbol preceding the pilot symbol at P2. If the
differential phase is not greater than .pi./3 radians than phase
tracking of the data symbols between the pilot symbols proceeds.
Again, if the differential phase 210 is large enough, for example,
greater than .pi./3 radians, then a correction is applied to the
carrier phase of the data symbols based on linear interpolation as
shown by dashed line 220, to yield a corrected phase track 230.
[0025] The carrier phase of the data symbols (.phi..sub.n) between
the pilot symbols (110-1 and 110-50) is estimated. This may be done
using an M.sup.th power scheme with or without joint polarization
carrier phase estimation. The equation for M.sup.th power scheme
where M=4 (i.e. 4.sup.th power) is as follows:
a . .phi. n = .phi. n - 1 + .mu. 4 arg [ k = 0 N r n + k 4 exp ( -
4 .phi. n - 1 ) ] ( 3 ) ##EQU00002##
where .mu. is a Kalman filter coefficient. It is important to note
that the drawbacks associated with M.sup.th power scheme (i.e.
phase estimation error) referenced above is associated with
removing data modulation from the signal without the use of pilot
symbols to estimate the carrier phase. The use of the M.sup.th
power scheme of Equation (3) is with respect to the carrier phase
of the data symbols between the pairs of pilot symbols.
[0026] Alternatively, the carrier phase of the data symbols between
the pairs of pilot symbols may be estimated by using M.sup.th power
scheme with joint polarization carrier phase estimation as
described in ECOC '2008, paper No. 4, D.2, M. Kuschnerov, et al.,
Brussels, September 2008, the contents of which are incorporated
herein by reference. The phase of the X and Y polarization
tributaries associated with the modulated data symbols is
calculated separately using the following equations:
.phi. n ( X ) = .phi. n - 1 ( X ) + .mu. 4 arg [ k = 0 N r n + k 4
( X ) - 4.phi. n - 1 ( X ) + C k = 0 N r n + k 4 ( Y ) - 4.phi. n -
1 ( Y ) ] and i . .phi. n ( Y ) = ( 4 .phi. n - 1 ) ( Y ) + .mu. 4
arg [ k = 0 N r n + k 4 ( Y ) - 4.phi. n - 1 ( Y ) + C k = 0 N r n
+ k 4 ( X ) - 4.phi. n - 1 ( X ) ] ##EQU00003##
where C is the coupling coefficient between X-polarization and
Y-polarization tributaries. Joint polarization phase estimation
improves the performance of carrier phase estimation by effectively
increasing the averaging window size. The optimal coupling
coefficient C.di-elect cons.[0,1] may vary depending on nonlinear
XPM effect which leads to reduced correlation between the carrier
phase of QPSK polarization tributaries. However, the use of pilot
symbols significantly helps to tolerate a non-optimized C value.
This is because the phase of the pilot symbols can be estimated
with high accuracy, but without ambiguity. The value of the
coupling coefficient can be selected by default (e.g., 0.7) and the
accumulated phase error of data symbols due to non-optimal C values
are corrected periodically with the pilot symbols. In this manner
the carrier phase of the data symbols between the pilot symbols in
a received symbol sequence may be estimated and coherent decoding
using QPSK may be utilized to realize better system
performance.
[0027] FIG. 4 is a flow chart graphically illustrating the
preceding method of pilot symbol aided carrier phase estimation
400. In particular, at step 401 pilot symbols having known data
information are inserted periodically within a sequence of data
symbols of a modulated optical signal at an optical transmitter.
The data symbols and the pilot symbols each have an associated
phase consistent with a predefined data modulation for optical
signal transmission. The phase of each of the pilot symbols is
detected at step 402 at a receiver. The carrier phase of the data
symbols between the pilot symbols is estimated. This is done using
M.sup.th power scheme with joint polarization carrier phase
estimation at step 403. The phase of the data symbol (e.g. data
symbol 110-49 of FIG. 2) immediately preceding or adjacent the
pilot symbol (e.g. pilot symbol 110-50 of FIG. 2) is detected at
step 404. The differential phase between the carrier phase of the
pilot symbol and the carrier phase of the data symbol which
immediately precedes the pilot symbol is determined at step 405.
This differential phase is compared to a phase tolerance (e.g.
.pi./3 radians) at step 406 to determine if cycle slip or phase
error of the data symbols occurred before the pilot symbol (e.g.
pilot symbol 110-50).
[0028] If the differential phase is not greater than the phase
tolerance, the process proceeds to step 408 where no loss of phase
tracking was detected and the data is decoded. If the differential
phase is greater than the phase tolerance, then the process
proceeds to step 407 where correction is applied to phase tracking
of the data symbols between the pilot symbols using linear
interpolation. The process proceeds to step 408 where there is no
loss of phase tracking and the data may be decoded.
[0029] The foregoing carrier phase estimation method was tested
using orthogonally launched independent QPSK modulated signals with
pilot symbols and decoded at a receiver. The receiver utilized
carrier frequency estimation (CFE) method without the aid of pilot
symbols and was compared to the carrier phase estimation (CPE)
method with pilot symbols. In order to compare these two methods
along with ideal performance, the bit error rate (BER) was plotted
in FIG. 5. It is important to note that since the pilot symbols
were added in the transmitted signal in accordance with the present
disclosure, coherent QPSK could be used to decode the symbol
information rather than using conventional M.sup.th power CPE which
could only employ differential decoding (DC-QPSK) to decode the
symbols. The decoding results illustrate a 1 dB Q-factor
improvement at low SNR and 0.6 dB improvement at high SNR. As is
known to one of ordinary skill in the art, the term "Q-factor"
suggests the minimum SNR required to obtain a specific BER for a
given received signal.
[0030] FIGS. 6A and 6B graph the phase tracking results without the
use of pilot symbols and the phase tracking results using the pilot
symbols respectively. In particular, FIG. 6A shows plot 600 of the
estimated carrier phase assuming the data phase is known and plot
601 is the estimated carrier phase obtained by the conventional
4.sup.th power estimation method (i.e., not using pilot symbol
aided joint polarization estimation as in a presently disclosed
embodiment). As can be seen from the divergence of plots 600 and
601, track loss was not corrected. In contrast, FIG. 6B shows plot
602 of the estimated carrier phase assuming the data phase is known
and plot 603 of the estimated carrier phase obtained using pilot
symbol aided joint polarization CPE together with coherent QPSK.
The pilot symbol are indicated at points 605A and 605B with
tracking of the data symbols therebetween. As indicated by plots
602 and 603, phase tracking is advantageously maintained, thereby
enabling the use of coherent PSK for symbol decoding.
[0031] With pilot symbol aided joint polarization CPE, phase
tracking is guaranteed because there is no phase ambiguity.
Although loss of tracking may still occur between pilot symbols,
such track loss is advantageously detected and corrected which is
an improvement over the prior art.
[0032] Embodiments of the present disclosure may be implemented at
a transmitter and receiver of an optical communication system. A
processor may be used to effectuate operations associated with a
communication system, as is known to one of ordinary skill in the
art. A processor as used herein is a device for executing stored
machine-readable instructions for performing tasks and may comprise
any one or combination of hardware, software, and firmware. A
processor may also comprise memory storing machine-readable
instructions executable for performing tasks. A processor acts upon
information by manipulating, analyzing, modifying, converting, or
transmitting information for use by an executable procedure or an
information device, and/or by routing the information to an output
device. A processor may use or comprise the capabilities of, e.g.,
a controller or microprocessor. A processor may be electrically
coupled with any other processor, enabling interaction and/or
communication therebetween. A processor comprising executable
instructions may be electrically coupled by being within stored
executable instructions enabling interaction and/or communication
with executable instructions comprising another processor. A user
interface processor or generator is a known element comprising
electronic circuitry or software, or a combination of both, for
generating display images or portions thereof.
[0033] An executable application comprises code or machine readable
instructions for conditioning the processor to implement
predetermined functions, such as those of an operating system, a
context data acquisition system, or other information processing
system, e.g., in response to user command or input. An executable
procedure is a segment of code or machine readable instruction,
sub-routine, or other distinct section of code or portion of an
executable application for performing one or more particular
processes. These processes may include receiving input data and/or
parameters, performing operations on received input data and/or
performing functions in response to received input parameters, and
providing resulting output data and/or parameters.
[0034] The present disclosure is not to be limited in scope by the
specific embodiments described herein. Indeed, other various
embodiments of and modifications to the present disclosure, in
addition to those described herein, will be apparent to those of
ordinary skill in the art from the foregoing description and
accompanying drawings. Thus, such other embodiments and
modifications are intended to fall within the scope of the present
disclosure. Further, although the present disclosure has been
described herein in the context of a particular implementation in a
particular environment for a particular purpose, those of ordinary
skill in the art will recognize that its usefulness is not limited
thereto and that the present disclosure may be beneficially
implemented in any number of environments for any number of
purposes.
* * * * *