U.S. patent application number 13/411462 was filed with the patent office on 2013-03-21 for communication through phase-conjugated optical variants.
The applicant listed for this patent is Andrew Roman Chraplyvy, Xiang Liu, Robert William Tkach, Peter J. Winzer. Invention is credited to Andrew Roman Chraplyvy, Xiang Liu, Robert William Tkach, Peter J. Winzer.
Application Number | 20130070786 13/411462 |
Document ID | / |
Family ID | 47879630 |
Filed Date | 2013-03-21 |
United States Patent
Application |
20130070786 |
Kind Code |
A1 |
Liu; Xiang ; et al. |
March 21, 2013 |
Communication Through Phase-Conjugated Optical Variants
Abstract
An optical transport system configured to transmit at least two
phase-conjugated optical variants carrying the same modulated
symbols, with the phase-conjugated optical variants in being
different from one another in one or more of polarization of light,
the time of transmission, spatial localization, optical carrier
wavelength, and subcarrier frequency during transmission. The two
phase-conjugated optical variants can be generated by a single
polarization-diversity transmitter to be orthogonally polarized,
and propagate through an optical transmission link with the same
wavelength and spatial path. The optical variants are detected and
processed at the receiver in a manner that enables coherent
summation of the corresponding electrical signals prior to
constellation de-mapping. The coherent summation tends to cancel
out the deleterious effects of nonlinear distortions imparted on
the individual phase-conjugated optical variants in an optical
fiber transmission link because said nonlinear distortions tend to
be opposite to each other.
Inventors: |
Liu; Xiang; (Marlboro,
NJ) ; Chraplyvy; Andrew Roman; (Matawan, NJ) ;
Tkach; Robert William; (Little Silver, NJ) ; Winzer;
Peter J.; (Aberdeen, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Liu; Xiang
Chraplyvy; Andrew Roman
Tkach; Robert William
Winzer; Peter J. |
Marlboro
Matawan
Little Silver
Aberdeen |
NJ
NJ
NJ
NJ |
US
US
US
US |
|
|
Family ID: |
47879630 |
Appl. No.: |
13/411462 |
Filed: |
March 2, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61535548 |
Sep 16, 2011 |
|
|
|
Current U.S.
Class: |
370/464 ;
398/115 |
Current CPC
Class: |
C02F 1/78 20130101; H04B
10/516 20130101; Y02W 10/37 20150501; H04B 10/00 20130101; H04B
10/61 20130101 |
Class at
Publication: |
370/464 ;
398/115 |
International
Class: |
H04B 10/00 20060101
H04B010/00; H04J 14/00 20060101 H04J014/00 |
Claims
1. An apparatus comprising an optical receiver, the optical
receiver comprising: a front-end circuit configured to convert at
least two phase-conjugated optical variants carrying a same
modulated payload symbol into a corresponding plurality of digital
electrical signals; and a processor configured to: process the
plurality of digital electrical signals to generate a set of
complex values representing the same modulated payload symbol; sum
the complex values of the set to generate a summed complex value;
map the summed complex value onto a constellation; and determine
based on the mapped summed complex value a bit-word represented by
the same modulated payload symbol.
2. The apparatus of claim 1 wherein the at least two
phase-conjugated optical variants differ from one another in one or
more of polarization, time of arrival at the optical receiver,
spatial localization, optical carrier wavelength, and subcarrier
frequency.
3. The apparatus of claim 1 wherein the at least two
phase-conjugated optical variants are complex conjugates in the
time domain.
4. The apparatus of claim 1 wherein the at least two
phase-conjugated optical variants are complex conjugates in the
frequency domain.
5. The apparatus of claim 1 wherein one of the at least two
phase-conjugated optical variants includes an optical version of a
symbol for transmission.
6. The apparatus of claim 1 wherein another of the at least two
phase-conjugated optical variants includes a complex conjugate
version of the optical version of the symbol for transmission with
a constant phase rotation.
7. The apparatus of claim 1 wherein the processor is configured to
undo phase conjugation of the at least two phase-conjugated optical
variants, and generate at least two complex values representing the
symbol intended for transmission.
8. The apparatus of claim 1 wherein the at least two
phase-conjugated optical variants are orthogonally polarized.
9. The apparatus of claim 1 further comprising a
polarization-diversity transmitter for generating at least two
orthogonally-polarized phase-conjugated optical variants.
10. The apparatus of claim 1 wherein the front-end circuit
comprises at least one polarization-diversity optical hybrid and at
least one optical local oscillator.
11. The apparatus of claim 1 wherein the front-end circuit
comprises at least four analog-to-digital convertors (ADCs).
12. The apparatus of claim 1 wherein: the front-end circuit
comprises a wavelength de-multiplexer configured to de-multiplex
the at least two phase-conjugated optical variants.
13. The apparatus of claim 1 wherein: the front-end circuit
comprises an optical coupler configured to spatially de-multiplex
the at least two phase-conjugated optical variants.
14. The apparatus of claim 1 further comprising a medium for
conveying the at least two phase-conjugated optical variants,
wherein the medium is one or more of single-mode fiber,
multi-core-fiber, fiber bundle, and multi-mode fiber.
15. The apparatus of claim 1 wherein the processor configured to
determine the bit-word represented by the same modulated payload
symbol is configured to determine a FEC-based error correction
based on a sequence of mapped constellations for a sequence of same
modulated payload symbols.
16. The apparatus of claim 1, wherein the processor configured to
process the plurality of digital electrical signals to generate the
set of complex values representing the same modulated payload
symbol is configured to perform one or more of time
synchronization, channel estimation, channel compensation,
frequency estimation, frequency compensation, phase estimation, and
phase compensation.
17. The apparatus of claim 16, wherein processing the plurality of
digital electrical signals includes use of pilot symbols.
18. The apparatus of claim 1, further comprising: an optical
transmitter configured to generate a second set of at least two
phase-conjugated optical variants in response to a symbol of an
input payload data stream, the at least two phase-conjugated
optical variants of the second set differing from one another in
one or more of polarization, time of transmission, spatial
localization, optical carrier wavelength, and subcarrier
frequency.
19. A method of optical communication comprising: converting, at an
optical receiver, at least two phase-conjugated optical variants
carrying a same modulated payload symbol into a corresponding
plurality of digital electrical signals; processing the plurality
of digital electrical signals to generate a set of complex values
representing the same modulated payload symbol; summing the complex
values of the set to generate a summed complex value; mapping the
summed complex value onto a constellation; and determining based on
the mapped summed complex value a bit-word represented by the same
modulated payload symbol.
20. An apparatus comprising an optical transmitter configured to
generate at least two phase-conjugated optical variants in response
to a symbol of an input payload data stream, the at least two
phase-conjugated optical variants differing from one another in one
or more of polarization, time of transmission, spatial
localization, optical carrier wavelength, and subcarrier frequency.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application No. 61/535,548, filed on Sep. 16, 2011, and U.S.
patent application Ser. No. 13/245,160, filed on Sep. 26, 2011,
both entitled "PERFORMANCE ENHANCEMENT THROUGH OPTICAL VARIANTS,"
which are incorporated herein by reference in its entirety.
BACKGROUND
[0002] 1. Field
[0003] The invention(s) relate to optical communication equipment
and, more specifically but not exclusively, to equipment for
managing data transport through a nonlinear and/or noisy optical
channel.
[0004] 2. Description of the Related Art
[0005] This section introduces aspects that may help facilitate a
better understanding of the invention(s). Accordingly, the
statements of this section are to be read in this light and are not
to be understood as admissions about what is in the prior art or
what is not in the prior art.
[0006] Forward error correction (FEC) uses systematically generated
redundant data to reduce the bit-error rate (BER) at the receiver.
The cost of this reduction is a concomitant increase in the
required forward-channel bandwidth, with the latter being dependent
on the overhead of the FEC code. In general, an FEC code with a
larger overhead or lower net data rate is used for a noisier
channel. When the channel conditions change over time, the net data
rate and/or the FEC code can be adaptively changed to maintain an
acceptable BER. However, one problem with FEC coding, as applied to
optical transport systems, is that the coding-gain differences
among various implementable FEC codes usually do not exceed a
certain maximum value, as given by Shannon's information capacity
theory. In addition, the digital signal processing (DSP) complexity
for capacity-approaching FEC codes can be forbiddingly high.
Therefore, for certain optical channels, additional and/or
alternative performance-enhancement techniques may be needed to
overcome these and other pertinent limitations of FEC coding.
SUMMARY
[0007] Improvement in the quality of an optical signal after
transmission may be obtained by performing digital constructive
summation of a set of two or more optical variants. Optical
variants are correlated optical signals which carry the same piece
of payload data, bit-word, or bit sequence but differ from each
other in at least one of their degrees of freedom, e.g., in one or
more of the time of transmission, spatial localization,
polarization of light, optical carrier wavelength and subcarrier
frequency. The constructive summation tends to average out the
deleterious effects of both linear and nonlinear noise/distortions
imparted on the individual optical variants in the optical
transport link because said noise/distortions are incoherent in
nature. The optical variants can be the same as the original
optical signal intended for transmission, or phase-scrambled copies
of original signal.
[0008] Nonlinear distortions imparted on two phase-conjugated
signals can be essentially opposite to each other when the phase
conjugation is removed at the receiver. Therefore, when two
phase-conjugated optical variants carrying the same modulated
payload symbols are coherently summed after removing the phase
conjugation between them, the nonlinear distortions imparted on the
two phase-conjugated optical variants would essentially cancel.
This methodology effectively improves signal quality after
nonlinear fiber transmission, beyond that which can be achieved by
coherently summing two optical variants that are either the
duplicated copies or phase-scrambled copies of a same optical
signal. In one embodiment, the two phase-conjugated optical
variants can differ from one another in one or more of
polarization, time, spatial localization, optical carrier
wavelength, and subcarrier frequency during optical transmission.
Two "phase-conjugated optical variants" refers to two optical
variants that are complex conjugates after removing a constant
phase offset and/or time delay between them. Further, more than two
phase-conjugated optical variants may be utilized in the provided
methodology; in those instances, the third, fourth, etc.
phase-conjugated optical variant is a copy of one of first two
complex conjugates after removing a constant phase offset and/or
time delay from the third, fourth, etc. phase-conjugated optical
variant.
[0009] According to a first embodiment, at least two
phase-conjugated optical variants are orthogonally polarized, and
are generated by a polarization-diversity transmitter and share the
same wavelength and spatial path in an optical fiber transmission
link. A polarization-diversity receiver is used to receive the at
least two orthogonal polarization components and jointly process
them to recover the transmitted optical variants. Then, the phase
conjugation between these two variants is removed, before the
variants are constructively summed to provide a constellation
representation of the original signal.
[0010] According to a second embodiment, at least two
phase-conjugated optical variants for an optical signal intended
for transmission are time delayed with respect to each other by T,
which may be multiple modulation symbol periods, and modulated onto
a polarization component of a Polarization Division Multiplexed
(PDM) signal. At the receiver, the time delay and the phase
conjugation between these two variants are removed, before their
constructive summation to provide a constellation representation of
the original signal.
[0011] According to a third embodiment, at least two
phase-conjugated optical variants are modulated onto different
optical carrier wavelengths, and are wavelength-division
multiplexed for transmission. These wavelengths can travel through
the same spatial path in an optical fiber transmission link. At the
receiver, these optical variants are first wavelength-division
de-multiplexed and jointly processed. Then, the phase conjugation
between these variants is removed, before they are constructively
summed to provide a constellation representation of the original
signal.
[0012] According to a fourth embodiment, at least two
phase-conjugated optical variants are space-division multiplexed
for transmission. These at least two optical variants can travel
through different cores of a multicore fiber link or different
spatial modes of a multi-mode fiber as long as the nonlinear
effects impacting them are approximately the same. At the receiver,
these at least two optical variants are first space-division
de-multiplexed, either optically or digitally, and jointly
processed. Then, the phase conjugation between these at least two
variants is removed, before they are constructively summed to
provide a constellation representation of the original signal.
[0013] As the linear noises impacting each of the optical variants
is uncorrelated, the constructive summation process aforementioned
also effectively increases the optical signal-to-noise (OSNR).
Together with the cancellation of nonlinear distortions, the use of
phase-conjugated optical variants in a constructive summation
process can substantially improve the signal quality in long-haul
optical fiber transmission. In various embodiments, the signal
quality improvement or the reduction in the received bit error
ratio (BER) enabled by the use of optical variants can be
implemented in addition to or instead of that provided by FEC
coding.
[0014] In an embodiment, an apparatus includes optical receiver
comprises a front-end circuit and a processor. The front-end
circuit is configured to convert at least two phase-conjugated
optical variants carrying a same modulated payload symbol into a
corresponding plurality of digital electrical signals. The
processor is configured to process the plurality of digital
electrical signals to generate a set of complex values representing
the same modulated payload symbol, sum the complex values of the
set to generate a summed complex value, map the summed complex
value onto a constellation, and determine based on the mapped
summed complex value a bit-word represented by the same modulated
payload symbol.
[0015] In another embodiment, the at least two phase-conjugated
optical variants differ from one another in one or more of
polarization, time of arrival at the optical receiver, spatial
localization, optical carrier wavelength, and subcarrier
frequency.
[0016] In another embodiment, the at least two phase-conjugated
optical variants are complex conjugates in the time domain. In
another embodiment, the at least two phase-conjugated optical
variants are complex conjugates in the frequency domain.
[0017] One of the at least two phase-conjugated optical variants
may include an optical version of a symbol for transmission.
Another of the at least two phase-conjugated optical variants may
include a complex conjugate version of the optical version of the
symbol for transmission with a constant phase rotation.
[0018] In one embodiment, the processor is configured to undo phase
conjugation and undo phase rotation of the at least two
phase-conjugated optical variants, and generate a complex value
representing the symbol intended for transmission.
[0019] The at least two phase-conjugated optical variants may be
orthogonally polarized. In another embodiment, the apparatus may
include a polarization-diversity transmitter for generating at
least two orthogonally-polarized phase-conjugated optical
variants.
[0020] In one embodiment, the front-end circuit includes at least
one polarization-diversity optical hybrid and at least one optical
local oscillator. In another embodiment, the front-end circuit
includes at least four analog-to-digital convertors (ADCs).
[0021] In one embodiment, the front-end circuit includes a
wavelength de-multiplexer configured to de-multiplex the at least
two phase-conjugated optical variants. In yet another embodiment,
the front-end circuit includes an optical coupler configured to
spatially de-multiplex the at least two phase-conjugated optical
variants.
[0022] In one embodiment, the apparatus may also include a medium
for conveying the at least two phase-conjugated optical variants,
wherein the medium is one or more of single-mode fiber,
multi-core-fiber, fiber bundle, and multi-mode fiber.
[0023] In one embodiment, the processor may determine the bit-word
represented by the same modulated payload symbol by determining a
FEC-based error correction based on a sequence of mapped
constellations for a sequence of same modulated payload symbols. In
another embodiment, processing the plurality of digital electrical
signals to generate the set of complex values representing the same
modulated payload symbol may include performing one or more of time
synchronization, channel estimation, channel compensation,
frequency estimation, frequency compensation, phase estimation, and
phase compensation. This processing of the digital electrical
signals may include the use of pilot symbols.
[0024] In one embodiment, the apparatus of claim 1 may also include
an optical transmitter configured to generate a second set of at
least two phase-conjugated optical variants in response to a symbol
of an input payload data stream, the at least two optical variants
of the second set differing from one another in one or more of
polarization, time of transmission, spatial localization, optical
carrier wavelength, and subcarrier frequency.
[0025] An example method of optical communication includes
converting, at an optical receiver, at least two phase-conjugated
optical variants carrying a same modulated payload symbol into a
corresponding plurality of digital electrical signals; processing
the plurality of digital electrical signals to generate a set of
complex values representing the same modulated payload symbol;
summing the complex values of the set to generate a summed complex
value; mapping the summed complex value onto a constellation; and
determining based on the mapped summed complex value a bit-word
represented by the same modulated payload symbol.
[0026] According to one embodiment, an apparatus includes an
optical transmitter configured to generate at least two
phase-conjugated optical variants in response to a symbol of an
input payload data stream, the at least two optical variants
differing from one another in one or more of polarization, time of
transmission, spatial localization, optical carrier wavelength, and
subcarrier frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Other aspects, features, and benefits of various embodiments
of the invention will become more fully apparent, by way of
example, from the following detailed description and the
accompanying drawings, in which:
[0028] FIG. 1 shows a block diagram of an optical transport system
according to one embodiment of the invention;
[0029] FIG. 2 shows a flowchart of a signal-processing method that
can be implemented in the receiver of the optical transport system
shown in FIG. 1 according to one embodiment of the invention;
[0030] FIG. 3 shows a flowchart of a signal-processing method that
can be implemented in the receiver of the optical transport system
shown in FIG. 1 according to one embodiment of the invention;
[0031] FIG. 4 shows a block diagram of an optical transport system
according to another embodiment of the invention;
[0032] FIG. 5 shows a block diagram of an optical transport system
according to yet another embodiment of the invention; and
[0033] FIG. 6 shows a flowchart of a signal-processing method that
can be implemented in the receiver of the optical transport system
shown in FIG. 4 and FIG. 5 according to one embodiment of the
invention;
DETAILED DESCRIPTION
[0034] An optical transport link is typically configured to support
multiple degrees of freedom, such as time, space, carrier frequency
(wavelength), and polarization. Each of these degrees of freedom
can be used for optical-signal multiplexing. Multiplexing
techniques corresponding to these four different individual degrees
of freedom are referred to in the literature as time-division
multiplexing, space-division multiplexing, wavelength-division
multiplexing, and polarization-division multiplexing.
[0035] In addition to or instead of using the various degrees of
freedom supported by an optical transport link for multiplexed
transmission of independent optical signals, various embodiments of
the invention employ these degrees of freedom for the transmission
of correlated optical signals, referred to as optical variants. In
a representative embodiment, two optical variants are two optical
signals that carry the same piece of payload data, bit-word, or bit
sequence, but differ from each other in the way they carry the
payload data: these two optical variants are complex
conjugates.
[0036] Assuming that the E-field of an optical signal intended for
transmission is E, the E-field of one of the two optical variants
can be E, and the other can be E*, where "*" denotes complex
conjugate.
[0037] Here, introduced is a more general term "phase-conjugated
optical variants", which refers to two optical variants that are
complex conjugates after removing a constant phase offset and/or
time delay between them. By complex conjugates is meant a pair of
complex numbers, both having the same real part, but with imaginary
parts of equal magnitude and opposite signs. For example,
E.sub.1(t) and E.sub.2(t) are phase-conjugated optical variants of
E(t) when the following conditions are satisfied
E.sub.1(t-t.sub.1)=exp(j.phi..sub.1)E(t),
E.sub.2(t-t.sub.2)=exp(j.phi..sub.2)E(t)*, (1)
where j denotes the imaginary unit, t denotes time, t.sub.1 and
t.sub.1 are time offsets, and .phi..sub.1 and .phi..sub.2 are phase
offsets. From the above equations, we have
E.sub.1(t-t.sub.1)=exp[j(.phi..sub.1+.phi..sub.2)]E.sub.2(t-t.sub.2)*,
(2)
i.e., E.sub.1(t) and E.sub.2(t) are complex conjugates after
removing a constant phase offset of (.phi..sub.1+.phi..sub.2) and a
time delay of (t.sub.1-t.sub.2). When there are more than two
phase-conjugated optical variants, the additional phase-conjugated
optical variants take the form:
E.sub.n(t-t.sub.n)=exp(j.phi..sub.n)E(t), or
E.sub.n(t-t.sub.n)=exp(j.phi..sub.n)E(t)*, (3)
where n is 3, 4, . . . .
[0038] These two phase-conjugated optical variants are transmitted
over an optical transmission link in different dimensions, e.g., in
one or more of the time of transmission, spatial localization,
polarization of light, optical carrier wavelength, and subcarrier
frequency. For example, a first transmission of an optical symbol
using a first (e.g., X) polarization and a second transmission of
that same optical symbol using a second (e.g., Y) polarization
represent two different optical variants of the bit-word that the
optical symbol encodes. As a second example, a first transmission
of an optical symbol at time t1 and a second transmission of that
same optical symbol at time t2>t1 represent two different
optical variants of the bit-word that the optical symbol encodes.
As a third example, a first transmission of an optical symbol using
carrier wavelength .lamda..sub.1 and a second (e.g., concurrent)
transmission of that optical symbol using carrier wavelength
.lamda..sub.2 similarly represent two different optical variants of
the bit-word that the optical symbol encodes. As a fourth example,
a first transmission of an optical symbol via a first propagation
path of a multipath fiber or fiber-optic cable (e.g., via a first
core of a multi-core fiber or a first guided mode of a multi-mode
fiber) and a second transmission of that optical symbol via a
second propagation path of that multipath fiber or fiber-optic
cable (e.g., via a second core of the multi-core fiber or a second
guided mode of the multi-mode fiber) represent two different
optical variants of the bit-word that the optical symbol
encodes.
[0039] Note that, in each of these examples, the two corresponding
optical variants are described as differing from one another in the
parameters of just one degree of freedom. However, optical variants
may differ from one another in the parameters of two or more
degrees of freedom, such as: (i) polarization and time; (ii) time
and space; (iii) time and wavelength; (iv) space and wavelength;
(v) space and polarization; (vi) wavelength and polarization; (vii)
time, space, and wavelength; (viii) time, space, and polarization;
(ix) time, wavelength, and polarization; (x) space, wavelength, and
polarization; or (xi) time, space, wavelength, and
polarization.
[0040] The concept of optical variants also applies to (i) optical
symbol sequences that carry multiple bit-words and (ii) optical
signals that carry the same bit-word using different optical
symbols. Further, more than two phase-conjugated optical variants
may be transmitted/received over an optical path according to the
principles of the invention. Assuming that the E-field of an
optical signal intended for transmission is E, the E-field of the
third, fourth, etc. optical variant can be either E or E*, where
"*" denotes complex conjugate. Other pertinent features of "optical
variants" will become more fully apparent, by way of example, from
the following more detailed description that is given below in
reference to FIGS. 1-6.
[0041] Various embodiments rely on an inventive concept, according
to which the receiver adds, in a phase-coherent manner, the
electrical signals corresponding to at least two phase-conjugated
optical variants of the same symbol stream prior to de-modulation
and de-coding. Each pair of phase-conjugated variants are conveyed
from the transmitter to the receiver on orthogonal transmission
paths or dimensions, but experience similar nonlinear effects,
which in effect impart opposite nonlinear distortions on these
variants when the phase conjugation between the pair is removed.
Accordingly, while the number of phase-conjugated optical variants
utilized in any one embodiment may be even or odd, the use of a
larger number of phase-conjugated optical variants is preferable to
minimize nonlinear effects when the number is odd.
[0042] FIG. 1 shows a block diagram of an optical transport system
100 according to one embodiment of the invention. System 100 has an
optical transmitter 110 that is configured to transmit optical
variants that differ from each other in polarization or time, or
both. System 100 also has an optical receiver 190 that is
configured to process the received optical variants to recover the
corresponding original data in a manner that reduces the BER
compared to the BER attainable without the use of optical variants.
Transmitter 110 and receiver 190 are connected to one another via
an optical transport link 140.
[0043] Transmitter 110 receives an input stream 102 of payload data
and applies it to a digital signal processor (DSP) 112. Processor
112 processes input stream 102 to generate digital signals
114.sub.1-114.sub.4. In each signaling interval (time slot),
signals 114.sub.1 and 114.sub.2 carry digital values that represent
the in-phase (I) component and quadrature (Q) component,
respectively, of a corresponding constellation symbol intended for
transmission using X-polarized light. Signals 114.sub.3 and
114.sub.3 similarly carry digital values that represent the I and Q
components, respectively, of the corresponding constellation symbol
intended for transmission using Y-polarized light.
[0044] An electrical-to-optical (E/O) converter (also sometimes
referred to as a front end) 116 of transmitter 110 transforms
digital signals 114.sub.1-114.sub.4 into a modulated optical output
signal 130. More specifically, digital-to-analog converters (DACs)
118.sub.1 and 118.sub.2 transform digital signals 114.sub.1 and
114.sub.2 into an analog form to generate drive signals I.sub.X and
Q.sub.X, respectively. Drive signals I.sub.X and Q.sub.X are then
used, in a conventional manner, to drive an I-Q modulator
124.sub.X. Based on drive signals I.sub.X and Q.sub.X, I-Q
modulator 124.sub.X modulates an X-polarized beam 122.sub.X of
light supplied by a laser source 120.sub.X, thereby generating a
modulated optical signal 126.sub.X.
[0045] DACs 118.sub.3 and 118.sub.4 similarly transform digital
signals 114.sub.3 and 114.sub.4 into an analog form to generate
drive signals I.sub.Y and Q.sub.Y, respectively. Based on drive
signals I.sub.Y and Q.sub.Y, an I-Q modulator 124.sub.Y modulates a
Y-polarized beam 122.sub.Y of light supplied by a laser source
120.sub.Y, thereby generating a modulated optical signal 126.sub.Y.
A polarization beam combiner 128 combines modulated optical signals
126.sub.X and 126.sub.Y to generate optical output signal 130.
[0046] In a representative configuration, processor 112 generates
digital signals 114.sub.1-114.sub.4 so that, for each bit-word to
be transmitted to receiver 190, optical output signal 130 contains
at least two phase-conjugated optical variants carrying that
bit-word. Conceptually, this set of phase-conjugated optical
variants can be viewed as comprising one or more overlapping and/or
non-overlapping subsets. For example, there might be a subset
consisting of two or more phase-conjugated optical variants, in
which the phase-conjugated optical variants have the same
polarization, but different temporal positions in signal 130.
Alternatively or in addition, there might be another subset
consisting of two phase-conjugated optical variants, in which the
phase-conjugated optical variants have the same temporal position
(the same time slot) in signal 130, but different polarizations.
Furthermore, there might be yet another subset consisting of
phase-conjugated optical variants, in which the phase-conjugated
optical variants have different temporal positions in signal 130
and different polarizations.
[0047] In one embodiment, two phase-conjugated optical variants are
carried by orthogonal polarization components. In this case,
signals 114.sub.1, 114.sub.2, 114.sub.3, and 114.sub.3 can be
arranged to meet the following conditions
I.sub.x(t)=real(E(t)), Q.sub.x(=imag(E(t)),
I.sub.y(t)=real(E(t-.tau.), Q.sub.y=-imag(E(t-.tau.)), (4)
where E is the E-field of the original signal intended for
transmission, and t is a time delay that can be zero or multiple
modulation symbol periods.
[0048] In another embodiment, two phase-conjugated optical variants
are carried by one polarization component but at different time
intervals. In this case, signals 114.sub.1, 114.sub.2, 114.sub.3,
and 114.sub.3 can be arranged to meet the following conditions
[0049] (1) For t=nT, nT+1, nT+2, . . . , (n+1)T-1,
[0049] I.sub.x(t)=real(E(t)), Q.sub.x(t)=imag(E(t)),
I.sub.y(t)=real(E(t+T)), Q.sub.y(t)=imag(E(t+T)), (5) [0050] (2)
For t=(n+1)T, (n+1)T+1, (n+1)T+2, . . . , (n+2)T-1,
[0050] I.sub.x(t)=real(E(t-T)), Q.sub.x(t)=-imag(E(t-T)),
I.sub.y(t)=real(E(t)), Q.sub.y(t)=-imag(E(t)),
where n is an integer, and T is a time interval which can be, for
example, many modulation symbol periods.
[0051] The processor 112 may also add pilot symbols and/or
pilot-symbol sequences to each of signals 114.sub.1, 114.sub.2,
114.sub.3, and 114.sub.3. One purpose of the added pilot symbols
and/or pilot-symbol sequences is to form an optical frame having a
well-defined structure. This structure can be used at receiver 190
to distinguish the optical symbols corresponding to the payload
data from the pilot symbols/sequences, and to ensure the phase
alignment between the optical variants. The pilot symbols/sequences
can then be used to perform one or more of (i) time
synchronization, (ii) channel estimation and compensation, (ii)
frequency estimation and compensation, and (iv) phase estimation
and compensation. An enabling description of possible frame
structures and suitable pilot symbols/sequences can be found, e.g.,
in commonly owned U.S. patent application Ser. No. 12/964,929
(filed on Dec. 10, 2010), which is incorporated herein by reference
in its entirety.
[0052] System 100 has an optical add-drop multiplexer (OADM)
configured to add signal 130, as known in the art, to other optical
signals that are being transported via optical transport link 140.
Link 140 is illustratively shown as being an amplified link having
a plurality of optical amplifiers 144 configured to amplify the
optical signals that are being transported through the link, e.g.,
to counteract signal attenuation. Note that an optical link that
does not have optical amplifiers can alternatively be used as well.
After propagating the intended length of link 140, signal 130 is
dropped from the link via another optical add-drop multiplexer,
OADM 146, and directed to receiver 190 for processing. Note that
the optical signal applied to receiver 190 by OADM 146 is labeled
130', which signifies the fact that, while in transit between
transmitter 110 and receiver 190, signal 130 may accumulate noise
and other signal distortions due to various linear effects and
nonlinear effects in the optical fiber. One type of a fiber
nonlinear effect is intra-channel four-wave mixing (IFWM), which is
a function of the phases and amplitudes of the corresponding
optical symbols.
[0053] Receiver 190 has a front-end circuit 172 comprising an
optical-to-electrical (O/E) converter 160, four analog-to-digital
converters (ADCs) 166.sub.1-166.sub.4, and an optical local
oscillator (OLO) 156. O/E converter 160 has (i) two input ports
labeled S and R and (ii) four output ports labeled 1 through 4.
Input port S receives optical signal 130'. Input port R receives an
optical reference signal 158 generated by optical local oscillator
156. Reference signal 158 has substantially the same
optical-carrier frequency (wavelength) as signal 130'. Reference
signal 158 can be generated, e.g., using a tunable laser controlled
by a wavelength-control loop (not explicitly shown in FIG. 1) that
forces an output wavelength of the tunable laser to closely track
the carrier wavelength of signal 130'.
[0054] O/E converter 160 operates to mix input signal 130' and
reference signal 158 to generate eight mixed optical signals (not
explicitly shown in FIG. 1). O/E converter 160 then converts the
eight mixed optical signals into four electrical signals
162.sub.1-162.sub.4 that are indicative of complex values
corresponding to the two orthogonal-polarization components of
signal 130'. For example, electrical signals 162.sub.1 and
162.sub.2 may be an analog in-phase signal and an analog
quadrature-phase signal, respectively, corresponding to the
X-polarization component of signal 130'. Electrical signals
162.sub.3 and 162.sub.4 may similarly be an analog in-phase signal
and an analog quadrature-phase signal, respectively, corresponding
to the Y-polarization component of signal 130'.
[0055] In one embodiment, O/E converter 160 is a
polarization-diverse 90-degree optical hybrid (PDOH) with four
balanced photo-detectors coupled to its eight output ports. Various
suitable PDOHs are commercially available, e.g., from Optoplex
Corporation of Fremont, Calif., and CeLight, Inc., of Silver
Spring, Md. Additional information on various O/E converters that
can be used to implement O/E converter 160 in various embodiments
of system 100 are disclosed, e.g., in U.S. Patent Application
Publication Nos. 2010/0158521 and 2011/0038631, and International
Patent Application No. PCT/US09/37746 (filed on Mar. 20, 2009), all
of which are incorporated herein by reference in their
entirety.
[0056] Each of electrical signals 162.sub.1-162.sub.4 generated by
O/E converter 160 is converted into digital form in a corresponding
one of ADCs 166.sub.1-166.sub.4. Optionally, each of electrical
signals 162.sub.1-162.sub.4 may be amplified in a corresponding
amplifier (not explicitly shown) prior to the resulting signal
being converted into digital form. Digital signals
168.sub.1-168.sub.4 produced by ADCs 166.sub.1-166.sub.4 are
processed by a digital signal processor (DSP) 170, e.g., as further
described below in reference to FIG. 3, to recover the data of the
original input stream 102 applied to transmitter 110.
[0057] FIG. 2 shows a flowchart of a signal-processing method 200
that can be employed by processor 170 (FIG. 1) to recover data
stream 102 from digital signals 168.sub.1-168.sub.4 according to
one embodiment of the invention where phase-conjugated optical
variants are carried on two orthogonal polarization states of a
same wavelength channel.
[0058] At step 201 of method 200, digital signals
168.sub.1-168.sub.4 are processed to construct two received optical
fields corresponding to two orthogonal polarization components,
E.sub.x(t) and E.sub.y(t). This processing may include one or more
of (i) time and frequency synchronization, (ii) channel estimation
and compensation, and (iii) phase estimation and compensation.
[0059] In a representative implementation, the time-synchronization
procedure of step 202 relies on certain properties of pilot-symbol
sequences to determine the start of each optical frame. The known
structure of the optical frame can then be used to identify time
slots that have digital samples and/or digital-signal portions
corresponding to the optical symbols carrying the payload data. The
frequency-synchronization procedure of step 202 performs electronic
estimation and compensation of a mismatch between the
carrier-frequency of input signal 130' and the frequency of
reference signal 158 (see FIG. 1). After the frequency offset is
determined, frequency-mismatch can be compensated, e.g., by
applying to each digital sample a phase shift equal to the
frequency offset multiplied by 2.pi. and the time elapsed between
the start of the frame and the temporal position of the digital
sample.
[0060] The channel-estimation/compensation procedure of step 203
performs electronic estimation and compensation of the phase and
amplitude distortions imposed by optical transport link 140, due to
effects such as chromatic dispersion and polarization-mode
dispersion. The channel estimation relies on digital samples
corresponding to pilot symbols to determine the channel-response
function, H, of optical transport link 140. The inverse
channel-response function H.sup.-1 is then applied to the digital
samples corresponding to payload data to perform channel
compensation.
[0061] At step 204, phase estimation and phase compensation are
performed, e.g., through the assistance of pilot symbols to correct
or compensate for slowly changing phase shifts between input signal
130' and reference signal 158 (FIG. 1). Various methods that can be
used for this purpose are disclosed, e.g., in U.S. Patent
Application Publication Nos. 2008/0152361 and 2008/0075472 and U.S.
Pat. No. 7,688,918, all of which are incorporated herein by
reference in their entirety. In this manner the plurality of
digital electrical signals are processed to generate a set of
complex values representing a modulated payload symbol. At step
205, the recovered E-fields of phase-conjugated optical variants
are further processed to remove the phase conjugation between them,
followed by coherent summation. The coherent summation is mapped
onto a constellation and a bit-word represented by the modulated
payload symbol is determined based on the mapped summation. For the
transmitter embodiment described by Eq. (4), step 205 is configured
to obtain the original optical signal intended for transmission as
follows
E(t)=E.sub.x(t)+E.sub.y(t+.tau.)* (6)
[0062] At step 206, the recovered original optical signal field
intended for transmission, E(t), is renormalized, demodulated, and
FEC decoded to obtain payload data 102. Both hard-decision (HD) and
soft-decision (SD) FEC codes can be used.
[0063] FIG. 3 shows a flowchart of a signal-processing method 300
that can be employed by processor 170 (FIG. 1) to recover data
stream 102 from digital signals 168.sub.1-168.sub.4 according to
another embodiment of the invention where phase-conjugated optical
variants are carried at different time intervals of a same
wavelength channel.
[0064] Steps 301-304 are the same as steps 201-204 in this
embodiment. For the transmitter embodiment described by Eq. (5),
step 305 is configured to obtain the original optical signal
intended for transmission as follows
E(t)=E.sub.x(t)+E.sub.x(t+T)*, for t=nT, . . . (n+1)T-1
E(t)=E.sub.y(t-T)+E.sub.y(t)*, for t=(n+1)T, . . . (n+2)T-1 (7)
That is, the two sets of phase-conjugated optical variants that are
delayed by T samples are in the x-polarization are summed, after
the phase conjugation between them is removed, to determine the
complex values representing the optical field of an optical version
of a symbol sequence intended for transmission. A similar summation
is done for the two sets of phase-conjugated optical variants in
the y-polarization.
[0065] At step 206, the recovered original optical signal field,
E(t), is renormalized, demodulated, and FEC decoded to obtain
payload data 102.
[0066] FIG. 4 shows a block diagram of an optical transport system
400 according to another embodiment of the invention. System 400
has an optical transmitter 410 that is configured to transmit
phase-conjugated optical variants that differ from each other in
one or more of time, space, polarization, carrier wavelength, and
subcarrier frequency in orthogonal frequency-division multiplexed
(OFDM) systems. System 400 also has an optical receiver 490 that is
configured to process the received optical variants to recover the
corresponding original data in a manner that reduces the BER
compared to the BER attainable without the use of optical variants.
Transmitter 410 and receiver 490 are connected to one another via
an optical transport link 440.
[0067] Transmitter 410 has a front-end circuit 416 having L
electrical-to-optical (E/O) converters 116.sub.1-116.sub.1, (also
see FIG. 1), each configured to use a different respective carrier
wavelength selected from a specified set of wavelengths
.lamda..sub.1-.lamda..sub.L. Transmitter 410 further has a
wavelength multiplexer (MUX) 420 configured to combine optical
output signals 418.sub.1-418.sub.L generated by E/O converters
116.sub.1-116.sub.L, respectively, and apply a resulting WDM signal
430 to an OADM 436 for adding it to the signals that are being
transported through link 440.
[0068] Each of E/O converters 116.sub.1-116.sub.L generates its
respective optical output signal 418 based on a corresponding set
414 of digital signals supplied by a DSP 412. Each signal set 414
has four digital signals that are analogous to digital signals
114.sub.1-114.sub.4 (FIG. 1). Signal sets 414.sub.1-414.sub.L are
generated by DSP 412 based on an input data stream 402. When each
of E/O converters 116.sub.1-116.sub.L generates two
phase-conjugated optical variants, the total number of
phase-conjugated optical variants is then 2 L.
[0069] After propagating through link 440, signal 430 is dropped
from the link (as signal 430') via another optical add-drop
multiplexer, OADM 446, and directed to receiver 490 for processing.
Receiver 490 has a front-end circuit 472 comprising a wavelength
de-multiplexer (DEMUX) 450 and L front-end circuits
172.sub.1-172.sub.L (also see FIG. 1). Wavelength de-multiplexer
(DEMUX) 450 is configured to de-multiplex signal 430' into its
constituent WDM components 452.sub.1-452.sub.L, each having a
corresponding one of carrier wavelengths X.sub.1-X.sub.L. Each of
front-end circuits 172.sub.1-172.sub.L then processes the
corresponding one of signals 452.sub.1-452.sub.L, as described
above in reference to FIG. 1, to generate a corresponding one of
sets 468.sub.1-468.sub.L of digital signals, with each set
consisting of four digital signals analogous to digital signals
168.sub.1-168.sub.4, respectively (see FIG. 1).
[0070] Signal sets 468.sub.1-468.sub.L generated by front-end
circuit 472 are processed by a DSP 470 to recover the data of
original input stream 402 applied to transmitter 410.
[0071] FIG. 5 shows a block diagram of an optical transport system
500 according to yet another embodiment of the invention. System
500 has an optical transmitter 510 that can be configured to
transmit optical variants, including phase-conjugated optical
variants, that differ from each other in one or more of time,
polarization, and space (as represented by a plurality of different
propagation paths). System 500 also has an optical receiver 590
that is configured to process the received optical variants to
recover the corresponding original data in a manner that reduces
the BER compared to the BER attainable without the use of optical
variants. Transmitter 510 and receiver 590 are connected to one
another via an optical transport link comprising a multi-core fiber
540, different cores of which provide the plurality of propagation
paths.
[0072] Transmitter 510 has an electrical-to-optical (E/O) converter
516 that is analogous to E/O converter 116 (FIG. 1). Transmitter
510 further has an optical splitter 520 and an optical coupler 526.
Optical splitter 520 is configured to split an optical output
signal 518 generated by E/O converter 516 into J (attenuated)
signal copies 522.sub.1-522.sub.J, where J is the number of cores
in multi-core fiber 540. Optical coupler 526 is configured to
couple each of signals 522.sub.1-522.sub.J into a corresponding
core of multi-core fiber 540.
[0073] E/O converter 516 is configured to generate optical output
signal 518 based on a set 514 of four digital signals supplied by a
DSP 512. The four signals of set 514 may be analogous to digital
signals 114.sub.1-114.sub.4, respectively (see FIG. 1). Signal set
514 is generated by DSP 512 based on an input data stream 502.
[0074] In one configuration, the processing implemented in DSP 512
is generally analogous to method 200 (FIG. 2). Note, however, that
optical splitter 520 and optical coupler 526 operate to increase
the number of optical variants per bit-word by a factor of J. Thus,
if signal 518 has n.sub.1 optical variants per bit-word, then an
output signal 530 generated in this configuration by transmitter
510 contains n.sub.2 (=J.times.n.sub.1) optical variants per
bit-word.
[0075] After propagating through multi-core fiber 540, signal 530
is applied (as signal 530') to receiver 590 for processing.
Receiver 590 has an optical coupler 546 and a front-end circuit 572
comprising J front-end circuits 172.sub.1-172.sub.J (also see FIG.
1). Optical coupler 546 is configured to direct light from each
core of multi-core fiber 540 to a corresponding one of front-end
circuits 172.sub.1-172.sub.J. Each of front-end circuits
172.sub.1-172.sub.J then processes the signal received from optical
coupler 546, as described above in reference to FIG. 1, to generate
a corresponding one of sets 568.sub.1-568.sub.J, each having four
digital signals analogous to digital signals 168.sub.1-168.sub.4,
respectively (see FIG. 1). In one embodiment, front-end circuits
172.sub.1-172.sub.J in receiver 590 share a single common OLO 156
(see FIG. 1).
[0076] Signal sets 568.sub.1-568.sub.J generated by front-end
circuit 572 are processed by a DSP 570 to recover the data of
original input stream 502 applied to transmitter 510.
[0077] FIG. 6 shows a flowchart of a signal-processing method 600
that can be employed by processor 470 (FIG. 4) or 570 (FIG. 5) to
recover data stream 102 from digital signals 468.sub.1-468.sub.L or
568.sub.1-568.sub.4 according to another embodiment of the
invention where phase-conjugated optical variants are carried by
wavelength channels or by different spatial paths.
[0078] Steps 601-604 are similar as steps 201-204, but process
E-fields received by at least two front ends. For the transmitter
embodiment described by Eq. (4), step 605 is configured to obtain
the original optical signal intended for transmission as
follows
E(t)=E.sub.1x(t)+E.sub.1y(t+.tau.)*+E.sub.2x(t)+E.sub.2y(t+.tau.)*,
(8)
where E.sub.1x(t) and E.sub.1y(t) are the recovered E-fields for
front end 172.sub.1, and E.sub.2x(t) and E.sub.2y(t) are the
recovered E-fields for front end 172.sub.2. First, two sets of
phase-conjugated optical variants that are delayed by .tau.
samples, orthogonally polarized, and carried by a first optical
channel, are summed, after the phase conjugation between them is
removed, to obtain a first set of summed values. Then another two
sets of phase-conjugated optical variants that are delayed by .tau.
samples, orthogonally polarized, and carried by a second channel,
are summed, after the phase conjugation between them is removed, to
obtain a second set of summed values. Finally, the two sets of
summed values are added to determine the complex values
representing the optical field of an optical version of a symbol
sequence intended for transmission.
[0079] At step 606, the recovered original optical signal field,
E(t), is renormalized, demodulated, and FEC decoded to obtain
payload data 102.
[0080] While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense.
[0081] Although phase-conjugated optical variants have been defined
in the time domain through Eqs. (1) and (2), phase conjugation can
also be realized in the frequency domain. As an example, two OFDM
symbols can be phase-conjugated optical variants when the modulated
subcarriers of the second OFDM symbol are complex conjugates of
those of the first OFDM symbol. In effect, frequency-domain phase
conjugation can be seen as time-domain phase conjugation plus time
reversal.
[0082] Although system 500 (FIG. 5) has been described in reference
to multi-core fiber 540, it can be adapted for use with a
multi-mode fiber, wherein different guided modes of the multi-mode
fiber provide the spatial degrees of freedom for the generation and
transmission of optical variants. Representative optical couplers
that can be used in conjunction with the multi-mode fiber in such a
system are disclosed, e.g., in U.S. Patent Application Publication
Nos. 2010/0329670 and 2010/0329671 and U.S. patent application Ser.
Nos. 12/986,468, filed on Jan. 7, 2011, and 12/827,284, filed on
Jun. 30, 2010, all of which are incorporated herein by reference in
their entirety.
[0083] In one embodiment, different cores of multi-core fiber 540
can be configured to concurrently transmit optical variants
corresponding to different bit-words. It may beneficial, however,
to configure multi-core fiber 540 so that, at any time, at least
two of its cores transmit optical variants corresponding to the
same bit-word.
[0084] Furthermore, system 500 can be modified in a relatively
straightforward manner to use optical variants that differ from
each other in one or more of time, polarization, carrier
wavelength, and space. In one embodiment, such a modification can
be accomplished, e.g., by (i) replacing E/O converter 516 by
front-end circuit 416, (ii) replacing each of front-end circuits
172.sub.1-172.sub.J by a corresponding instance of front-end
circuit 472, and (iii) appropriately reconfiguring DSPs 512 and 570
(see FIGS. 4 and 5).
[0085] In various alternative embodiments of methods 200, 300, and
600, the order of certain processing steps may be changed to differ
from the order indicated in FIGS. 2 3, and 6, respectively.
[0086] Various modifications of the described embodiments, as well
as other embodiments of the invention, which are apparent to
persons skilled in the art to which the invention pertains are
deemed to lie within the principle and scope of the invention as
expressed in the following claims.
[0087] Unless explicitly stated otherwise, each numerical value and
range should be interpreted as being approximate as if the word
"about" or "approximately" preceded the value of the value or
range.
[0088] It will be further understood that various changes in the
details, materials, and arrangements of the parts which have been
described and illustrated in order to explain the nature of this
invention may be made by those skilled in the art without departing
from the scope of the invention as expressed in the following
claims.
[0089] The use of figure numbers and/or figure reference labels in
the claims is intended to identify one or more possible embodiments
of the claimed subject matter in order to facilitate the
interpretation of the claims. Such use is not to be construed as
necessarily limiting the scope of those claims to the embodiments
shown in the corresponding figures.
[0090] Although the elements in the following method claims, if
any, are recited in a particular sequence with corresponding
labeling, unless the claim recitations otherwise imply a particular
sequence for implementing some or all of those elements, those
elements are not necessarily intended to be limited to being
implemented in that particular sequence.
[0091] Reference herein to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment can be included in at
least one embodiment of the invention. The appearances of the
phrase "in one embodiment" in various places in the specification
are not necessarily all referring to the same embodiment, nor are
separate or alternative embodiments necessarily mutually exclusive
of other embodiments. The same applies to the term
"implementation."
[0092] Also for purposes of this description, the terms "couple,"
"coupling," "coupled," "connect," "connecting," or "connected"
refer to any manner known in the art or later developed in which
energy is allowed to be transferred between two or more elements,
and the interposition of one or more additional elements is
contemplated, although not required. Conversely, the terms
"directly coupled," "directly connected," etc., imply the absence
of such additional elements.
[0093] The present inventions may be embodied in other specific
apparatus and/or methods. The described embodiments are to be
considered in all respects as only illustrative and not
restrictive. In particular, the scope of the invention is indicated
by the appended claims rather than by the description and figures
herein. All changes that come within the meaning and range of
equivalency of the claims are to be embraced within their
scope.
[0094] A person of ordinary skill in the art would readily
recognize that steps of various above-described methods can be
performed by programmed computers. Herein, some embodiments are
intended to cover program storage devices, e.g., digital data
storage media, which are machine or computer readable and encode
machine-executable or computer-executable programs of instructions
where said instructions perform some or all of the steps of methods
described herein. The program storage devices may be, e.g., digital
memories, magnetic storage media such as a magnetic disks or tapes,
hard drives, or optically readable digital data storage media. The
embodiments are also intended to cover computers programmed to
perform said steps of methods described herein.
[0095] The description and drawings merely illustrate the
principles of the invention. It will thus be appreciated that those
of ordinary skill in the art will be able to devise various
arrangements that, although not explicitly described or shown
herein, embody the principles of the invention and are included
within its spirit and scope. Furthermore, all examples recited
herein are principally intended expressly to be only for
pedagogical purposes to aid the reader in understanding the
principles of the invention and the concepts contributed by the
inventor(s) to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions. Moreover, all statements herein reciting principles,
aspects, and embodiments of the invention, as well as specific
examples thereof, are intended to encompass equivalents
thereof.
[0096] The functions of the various elements shown in the figures,
including any functional blocks labeled as "processors," may be
provided through the use of dedicated hardware as well as hardware
capable of executing software in association with appropriate
software. When provided by a processor, the functions may be
provided by a single dedicated processor, by a single shared
processor, or by a plurality of individual processors, some of
which may be shared. Moreover, explicit use of the term "processor"
or "controller" should not be construed to refer exclusively to
hardware capable of executing software, and may implicitly include,
without limitation, digital signal processor (DSP) hardware,
network processor, application specific integrated circuit (ASIC),
field programmable gate array (FPGA), read only memory (ROM) for
storing software, random access memory (RAM), and non volatile
storage. Other hardware, conventional and/or custom, may also be
included. Similarly, any switches shown in the figures are
conceptual only. Their function may be carried out through the
operation of program logic, through dedicated logic, through the
interaction of program control and dedicated logic, or even
manually, the particular technique being selectable by the
implementer as more specifically understood from the context.
[0097] It should be appreciated by those of ordinary skill in the
art that any block diagrams herein represent conceptual views of
illustrative circuitry embodying the principles of the invention.
Similarly, it will be appreciated that any flowcharts, flow
diagrams, state transition diagrams, pseudo code, and the like
represent various processes which may be substantially represented
in computer readable medium and so executed by a computer or
processor, whether or not such computer or processor is explicitly
shown.
* * * * *