U.S. patent application number 11/429686 was filed with the patent office on 2010-01-07 for variable spectral phase encoder/decoder based on decomposition of hadamard codes.
Invention is credited to Ronald C. Menendez.
Application Number | 20100003031 11/429686 |
Document ID | / |
Family ID | 39344807 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100003031 |
Kind Code |
A1 |
Menendez; Ronald C. |
January 7, 2010 |
VARIABLE SPECTRAL PHASE ENCODER/DECODER BASED ON DECOMPOSITION OF
HADAMARD CODES
Abstract
The invention is directed toward a variable spectral phase
encoder. The variable spectral phase encoder includes a plurality
of switches and at least one encoder. The encoder is coupled
between a first switch and second switch among the plurality of
switches. The first switch selectively routes an optical signal to
some combination of fixed encoders such that their collective
product applies one of the Hadamard sequences to the optical
signal.
Inventors: |
Menendez; Ronald C.;
(Chatham, NJ) |
Correspondence
Address: |
TELCORDIA TECHNOLOGIES, INC.
ONE TELCORDIA DRIVE 5G116
PISCATAWAY
NJ
08854-4157
US
|
Family ID: |
39344807 |
Appl. No.: |
11/429686 |
Filed: |
May 8, 2006 |
Current U.S.
Class: |
398/78 |
Current CPC
Class: |
H04J 14/005 20130101;
H04B 2201/70715 20130101; H04J 14/02 20130101; H04J 14/007
20130101 |
Class at
Publication: |
398/78 |
International
Class: |
H04J 14/00 20060101
H04J014/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0001] Funding for research was partially provided by the Defense
Advanced Research Projects Agency under federal contract
MDA972-03-C-0078. The federal government has certain rights in this
invention.
Claims
1-3. (canceled)
4. A variable spectral phase encoder comprising: a plurality of
switches; at least one encoder coupled between a first switch and
second switch of the plurality of switches, the first switch being
operable to selectively route an optical signal to the encoder to
apply a Hadamard sequence to the optical signal; a third switch and
a fourth switch; at least one encoder coupled between the second
switch and the third switch; at least one encoder coupled between
the third switch and the fourth switch; and at least one encoder
coupled after the fourth switch.
5. The spectral phase encoder according to claim 4, wherein each of
the first, second, third, and fourth switch is operable to
selectively route the optical signal through a combination of the
encoders to apply a Hadamard sequence to the optical signal.
6-11. (canceled)
12. A spectral phase decoder comprising: a plurality of switches;
at least one decoder coupled between a first switch and second
switch of the plurality of switches, the first switch being
operable to selectively route an Hadamard encoded optical signal to
the decoder to decode the optical signal; a third switch and a
fourth switch; at least one decoder coupled between the second
switch and the third switch; at least one decoder coupled between
the third switch and the fourth switch; and at least one decoder
coupled after the fourth switch.
13. The spectral phase decoder according to claim 12, wherein each
of the first, second, third, and fourth switch is operable to
selectively route the optical signal through a combination of
decoder to decode the Hadamard encoded optical signal.
14-16. (canceled)
Description
FIELD OF THE INVENTION
[0002] The present invention relates to optical communication and,
more particularly, to a dynamic encoder/decoder suitable for use in
optical code division multiple access (OCDMA) communication
networks.
BACKGROUND OF THE INVENTION
[0003] Various communications schemes have been used to increase
data throughput and to decrease data error rates as well as to
generally improve the performance of communications channels. As an
example, frequency division multiple access ("FDMA") employs
multiple data streams that are assigned to specific channels
disposed at different frequencies of the transmission band.
Alternatively, time division multiple access ("TDMA") uses multiple
data streams that are assigned to different timeslots in a single
frequency of the transmission band. FDMA and TDMA are quite limited
in the number of users and/or the data rates that can be supported
for a given transmission band.
[0004] In many communication architectures, code division multiple
access (CDMA) has supplanted FDMA and TDMA. CDMA is a form of
spread spectrum communications that enables multiple data streams
or channels to share a single transmission band at the same time.
The CDMA format is akin to a cocktail party in which multiple pairs
of people are conversing with one another at the same time in the
same room. Ordinarily, it is very difficult for one party in a
conversation to hear the other party if many conversations occur
simultaneously. For example, if one pair of speakers is excessively
loud, their conversation will drown out the other conversations.
Moreover, when different pairs of people are speaking in the same
language, the dialogue from one conversation may bleed into other
conversations of the same language, causing miscommunication. In
general, the cumulative background noise from all the other
conversations makes it harder for one party to hear the other party
speaking. It is therefore desirable to find a way for everyone to
communicate at the same time so that the conversation between each
pair, i.e., their "signal", is clear while the "noise" from the
conversations between the other pairs is minimized.
[0005] The CDMA multiplexing approach is well known and is
explained in detail, e.g., in the text "CDMA: Principles of Spread
Spectrum Communication," by Andrew Viterbi, published in 1995 by
Addison-Wesley. Basically, in CDMA, the bandwidth of the data to be
transmitted (user data) is much less than the bandwidth of the
transmission band. Unique "pseudonoise" keys are assigned to each
channel in a CDMA transmission band. The pseudonoise keys are
selected to mimic Gaussian noise (e.g., "white noise") and are also
chosen to be maximal length sequences in order to reduce
interference from other users/channels. One pseudonoise key is used
to modulate the user data for a given channel. This modulation is
equivalent to assigning a different language to each pair of
speakers at a party.
[0006] During modulation, the user data is "spread" across the
bandwidth of the CDMA band. That is, all of the channels are
transmitted at the same time in the same frequency band. This is
equivalent to all of the pairs of partygoers speaking at the same
time. The introduction of noise and interference from other users
during transmission is inevitable (collectively referred to as
"noise"). Due to the nature of the pseudonoise key, the noise is
greatly reduced during demodulation relative to the user's signal
because when a receiver demodulates a selected channel, the data in
that channel is "despread" while the noise is not "despread." Thus,
the data is returned to approximately the size of its original
bandwidth, while the noise remains spread over the much larger
transmission band. The power control for each user can also help to
reduce noise from other users. Power control is equivalent to
lowering the volume of a loud pair of partygoers.
[0007] CDMA has been used commercially in wireless telephone
("cellular") and in other communications systems. Such cellular
systems typically operate at between 800 MHz and 2 GHz, though the
individual frequency bands may only be a few MHz wide. An
attractive feature of cellular CDMA is the absence of any hard
limit to the number of users in a given bandwidth, unlike FDMA and
TDMA. The increased number of users in the transmission band merely
increases the noise to contend with. However, as a practical
matter, there is some threshold at which the "signal-to-noise"
ratio becomes unacceptable. This signal-to-noise threshold places
real constraints in commercial systems on the number of paying
customers and/or data rates that can be supported.
[0008] CDMA has also been used in optical communications networks.
Such optical CDMA (OCDMA) networks generally employ the same
general principles as cellular CDMA. However, unlike cellular CDMA,
optical CDMA signals are delivered over an optical network. As an
example, a plurality of subscriber stations may be interconnected
by a central hub with each subscriber station being connected to
the hub by a respective bidirectional optical fiber link. Each
subscriber station has a transmitter capable of transmitting
optical signals, and each station also has a receiver capable of
receiving transmitted signals from all of the various transmitters
in the network. The optical hub receives optical signals over
optical fiber links from each of the transmitters and transmits
optical signals over optical fiber links to all of the receivers.
An optical pulse is transmitted to a selected one of a plurality of
potential receiving stations by coding the pulse in a manner such
that it is detectable by the selected receiving station but not by
the other receiving stations. Such coding may be accomplished by
dividing each pulse into a plurality of intervals known as "chips".
Each chip may have the logic value "1", as indicated by relatively
large radiation intensity, or may have the logic value "0", as
indicated by a relatively small radiation intensity. The chips
comprising each pulse are coded with a particular pattern of logic
"1"'s and logic "0"'s that is characteristic to the receiving
station or stations that are intended to detect the transmission.
Each receiving station is provided with optical receiving equipment
capable of regenerating an optical pulse when it receives a pattern
of chips coded in accordance with its own unique sequence but
cannot regenerate the pulse if the pulse is coded with a different
sequence or code.
[0009] Alternatively, the optical network utilizes CDMA that is
based on optical frequency domain coding and decoding of
ultra-short optical pulses. Each of the transmitters includes an
optical source for generating the ultra-short optical pulses. The
pulses comprise N Fourier components whose phases are coherently
related to one another. The frequency intervals around each of the
N Fourier components are generally referred to as frequency bins. A
"signature" is impressed upon the optical pulses by independently
phase shifting the individual Fourier components comprising a given
pulse in accordance with a particular code whereby the Fourier
components comprising the pulse are each phase shifted a different
amount in accordance with the particular code. The encoded pulse is
then broadcast to all of or a plurality of the receiving systems in
the network. Each receiving system is identified by a unique
signature template and detects only the pulses provided with a
signature that matches the particular receiving system's
template.
[0010] The availability of variable spectral phase
encoders/decoders or dynamic encoders/decoders (i.e., one capable
of changing its coding state under user control) in OCDMA networks
makes possible a variety of code-based network configurations and
user-to-user connectivity configurations. For spectral-phase
encoding, the number of possible orthogonal codes is equal to the
number of frequency bins. Previous methods of producing an encoder
capable of generating all N codes may have operated by: (1)
physically switching in an entirely new phase mask (a relatively
slow process), (2) incorporating a variable phase mask based on
either mechanical adjustments of phase bins (via
Micro-Electro-Mechanical devices (MEMs) or other mechanical means)
or by means of liquid crystal phase modulators, (3) thermally
rearranging the frequencies of integrated ring resonators to create
new codes, or (4) using a bank of N fixed coders and two 1:N
optical switches (before and after the bank of coders).
[0011] Options 2 through 4 may be reconfigured more rapidly than
the physical mask replacement approach. Options 1 through 3 can
function with a single coder unit but at best are expected to
operate on millisecond time scales and typically require that all N
elements of the phase mask are adjustable. While option 4 is in
many ways the most straightforward and, being switch based, could
be fast, the fact that it would require N fixed coders means that
it will likely scale poorly with increasing N. As such, there is a
need for a dynamic encoder that may be rapidly reconfigured and
scalable as N increases.
SUMMARY OF THE INVENTION
[0012] In an aspect of the invention a spectral phase encoder is
provided. The encoder includes a plurality of switches and at least
one Walsh encoder coupled between a first switch and second switch
of the plurality of switches, the first switch being operable to
selectively route an optical signal to the Walsh encoder to apply a
Hadamard sequence to the optical signal.
[0013] In another aspect of the invention, each switch among the
plurality of switches is a 2.times.2 crossbar switch.
[0014] In yet another aspect of the invention, the spectral phase
encoder further includes a third switch and a fourth switch.
[0015] In yet another aspect of the invention, the spectral phase
encoder further includes at least one Walsh encoder coupled between
the second switch and the third switch, at least one Walsh encoder
coupled between the third switch and the fourth switch, and at
least one Walsh encoder coupled after the fourth switch.
[0016] In yet another aspect of the invention, each of the first,
second, third, and fourth switch is operable to selectively route
the optical signal through a combination of the Walsh encoders to
apply a Hadamard sequence to the optical signal.
[0017] In yet another aspect of the invention, the first switch,
second switch, third switch, and fourth switch are 2.times.2
crossbar switches.
[0018] In yet another aspect of the invention, the spectral phase
encoder further includes an additional 2.times.2 crossbar
switch.
[0019] In yet another aspect of the invention, the spectral phase
encoder further includes a passive coupler.
[0020] In an aspect of the invention a spectral phase encoder is
provided. The encoder includes a plurality of switches and at least
one Walsh encoder coupled between a first switch and second switch
of the plurality of switches, the first switch being operable to
selectively route an Hadamard encoded optical signal to the Walsh
decoder to decode the optical signal.
[0021] In another aspect of the invention, each switch among the
plurality of switches is a 2.times.2 crossbar switch.
[0022] In yet another aspect of the invention, the spectral phase
decoder further includes a third switch and a fourth switch.
[0023] In yet another aspect of the invention, the spectral phase
decoder further includes at least one Walsh decoder coupled between
the second switch and the third switch, at least one Walsh decoder
coupled between the third switch and the fourth switch, and at
least one Walsh decoder coupled after the fourth switch.
[0024] In yet another aspect of the invention, each of the first,
second, third, and fourth switch is operable to selectively route
the optical signal through a combination of Walsh decoder to decode
the Hadamard encoded optical signal.
[0025] In yet another aspect of the invention, the first switch,
second switch, third switch, and fourth switch are 2.times.2
crossbar switches.
[0026] In yet another aspect of the invention, the spectral phase
decoder further includes an additional 2.times.2 crossbar
switch.
[0027] In yet another aspect of the invention, the spectral phase
decoder further includes a passive coupler.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] A more complete appreciation of the subject matter of the
present invention and the various advantages thereof can be
realized by reference to the following detailed description in
which reference is made to the accompanying drawings wherein like
reference numbers or characters refer to similar elements.
[0029] FIG. 1 illustratively depicts a system in accordance with an
aspect of the present invention;
[0030] FIG. 2 illustratively depicts a spectral phase
encoder-switch cascade in accordance with an aspect of the present
invention;
[0031] FIG. 3 illustratively depicts a one realization of a Walsh
encoder in an aspect of the invention;
[0032] FIGS. 4A through 4D illustratively depict reflective phase
masks in accordance with an aspect of the invention; and
[0033] FIG. 5 illustratively depicts a Walsh decoder in an aspect
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] FIG. 1 illustratively depicts a system 100 in accordance
with an aspect of the present invention. The system comprises a
laser source 110 that generates a sequence of optical pulses 115
that are fed to a data modulator 120. The data modulator 122 also
receives a data stream 122 that is used to modulate the sequence of
optical pulses 115. The modulation data preferably comprises a
digital data stream generated by a subscriber or user station 124.
In a preferred embodiment, the data modulator 122 comprises an
ON/OFF keyed data modulator wherein a "1" symbol or bit in the
digital data stream corresponds to the presence of an optical pulse
and a "0" symbol or bit corresponds to the absence of an optical
pulse. In this way, each pulse represents a bit of information. For
example, a modulated stream 125 is shown where the digital data
stream comprises a "1010" data sequence. As shown, each time slot
with the bit "1" will result in the presence of an optical pulse
(125.sub.1 and 125.sub.3), whereas each time slot with a "0" bit
indicates the absence of an optical pulse (125.sub.2 and
125.sub.4), which are shown as dashed lines to indicate their
absence.
[0035] The modulated data stream 125 is then fed to a spectral
phase encoder 132. As is discussed in further detail below, the
spectral phase encoder 132 applies a phase code associated with a
user to each optical pulse in the data stream to produce an encoded
data stream 135. The phase code operates to provide a "lock" so
that only a corresponding phase decoder with the appropriate "key"
or phase conjugate of the phase code of the spectral phase encoder
may unlock the encoded data stream. Typically, a spectral phase
encoder is associated with a particular user and therefore allows
only another user with the appropriate key to decode or receive
information from the particular user. The information appears as
noise to users that do not have the appropriate key.
[0036] The encoded data stream 135 may then be transported over a
network 140, such as Wavelength Division Multiplex (WDM) network
for example, to a spectral phase decoder 144 that, preferably,
applies the phase conjugate of the phase code of the spectral phase
encoder 132, as discussed above. The spectral phase decoder 144
provides a decoded data stream 149 to an optical time gate 150. The
spectral phase decoder works in a manner similar to that of the
spectral phase encoder as will be described below. The optical time
gate 154 operates to reduce multiple access interference by
temporally extracting only a desired user channel from among the
decoded stream. The optical time gate 154 produces a user data
stream 159, which is fed to a data demodulator 164. Where ON/OFF
keying was employed at the transmitting end, the data demodulator
164 comprises an amplitude detector that reproduces the digital
data stream 124.
[0037] In accordance with an aspect of the present invention, the
laser source 110, data modulator 122 and spectral phase encoder 132
may comprise a transmitting station 170 associated with a user. The
spectral phase decoder 144, optical time gate 154 and demodulator
164 may preferably comprise a receiving station 180 associated with
a user.
[0038] The spectral phase encoder may utilize a set of Hadamard
codes, which are orthogonal and binary, by breaking each Hadamard
code into a multiplicative series of codes. An example of a
multiplicative series of codes that span the set of Hadamard code
is the set of Walsh codes. Note that the Walsh codes form a proper
subset of the Hadamard codes. It should be noted that many (but not
all) other sub-groups of size log.sub.2(N) selected from the
Hadamard codes also form a multiplicative basis that can span the
complete set of Hadamard codes. Any of these spanning groups can be
used as the basis for the variable Hadamard coder and we focus here
on the Walsh subset for specificity.
[0039] Hadamard codes can achieve relatively high spectral
efficiency with minimal multi-user interference (MUI). This coding
scheme offers orthogonally in the sense that MUI is zero at the
time that the decoded signal is maximum. The number of orthogonal
codes is equal to the number of frequency bins; hence, relatively
high spectral efficiency is possible. Binary Hadamard codes are
converted to phase codes by assigning to +1's and -1's phase shifts
of 0 and .pi., respectively. To encode data, which contains a
spread of frequencies, as opposed to the unmodulated pulse stream,
which contains only the initial comb of frequencies produced by a
mode locked laser (MLL), it is preferable to define frequency bins
around the center frequencies. Encoding data then consists of
applying the phase shift associated with a frequency to the entire
bin. The output of the phase encoder is then a signal obtained by
summing the phase-shifted frequency components of the modulated
signal, or equivalently, by convolving the modulated optical signal
at the input of the phase encoder with the inverse Fourier
transform of the phase code. Breaking down the Hadamard code into a
sequence of Walsh codes allows for the development of an
encoder/decoder that can be implemented in relatively small time
scales as discussed below.
[0040] In an aspect of the present invention the spectral phase
encoder is implemented as a dynamic coder that is desirably
reconfigurable at the microsecond to nanosecond time scales. In
addition, the number of adjustable elements required to span a code
space of N codes grows as log.sub.2(N) rather than as N. The
adjustable elements in such coder are not based on .lamda.-scale
adjustments of the phase mask, but instead function as optical
2.times.2 switches arranged in a cascade with fixed Walsh coders.
FIG. 2 shows an example of such a variable encoder. As shown in
FIG. 2, encoder 300 includes a cascade of switches 301, 302, 303,
and 304 and Walsh coders 311, 312, 313, and 314. Each switch is a
2.times.2 optical crossbar switch that routes or directs a signal
into one or more of the Walsh coders to encode a signal with a
particular Hadamard code. Switch 301 can be set so that an incoming
signal can bypass Walsh coder 311 or pass through Walsh coder 311.
Switch 302 can be set so that an incoming signal can bypass Walsh
coder 312 or pass through Walsh coder 312. Switch 303 can be set so
that an incoming signal can bypass Walsh coder 313 or pass through
Walsh coder 313. Switch 304 can be set so that an incoming signal
can bypass Walsh coder 314 or pass through Walsh coder 314. As
such, switches 301, 302, 303, and 304 are used to direct the signal
to either bypass their respective Walsh coders or to pass through
their respective Walsh coders thereby applying a Hadamard code to
the signal.
[0041] Although optical switching is usually fast enough to operate
on the time scale of bits or perhaps packets and with a significant
reduction in the number of adjustable elements, the encoder 300 if
FIG. 2 could simplify the task of using bit-by-bit code state
changes to perform data modulation (code-shift keying).
[0042] The spectral phase encoder 300 generally works under the
principle that Hadamard codes of order N, H.sub.n, where
n.epsilon.1 . . . N, can be decomposed into products of a smaller
basis set of Walsh codes W.sub.m of length N where the maximum
number of Walsh codes required to reconstruct any of these N
Hadamard code is log.sub.2(N). Walsh codes exist for only certain
values of m, specifically for m=2.sup.p-1 where p .epsilon.1 . . .
log.sub.2(N) and m=0.
[0043] The Walsh codes, like the Hadamard codes, are of length N
and all the elements are either +1 or -1. In general, the n.sup.th
code W.sub.n of length N is characterized by alternating blocks of
+1's and -1's where the length of the blocks is given by n. Thus
for order N=16, the Walsh codes are as discussed below.
[0044] W.sub.8 consisting of eight +1's followed by eight -1's (+ +
+ + + + + + - - - - - - - -).
[0045] W.sub.4 consisting of two sets of four +1's and four -1's.
(+ + + + - - - - + + + + - - - -).
[0046] W.sub.2 consisting of four sets of two +1's and two -1's. (+
+ - - + + - - + + - - + + - -).
[0047] W.sub.1 consisting of eight sets of +1's and -1's. (+ - + -
+ - + - + - + - + - + -).
[0048] W.sub.0 consisting of all 1's (+ + + + + + + + + + + + + + +
+).
[0049] In general, the i.sup.th element of Hadamard code n of order
N as the following product of the i.sup.th elements of the Walsh
codes, also of order N can be written as:
H n , i = W 0 , i j = 1 log 2 ( N ) ( W 2 j - 1 , i ) b j ,
##EQU00001##
where b.sub.j is j.sup.th digit of the binary representation of
(n-1). As a specific example, any of the 16 codes available in
H.sub.16 can be expressed as a product of W.sub.1, W.sub.2,
W.sub.4, and W.sub.8 (which may correspond to Walsh coders 314,
313, 312, and 311 respectively). The 16 Hadamard codes of order 16
can be represented as:
TABLE-US-00001 H.sub.1 = W.sub.0 H.sub.2 = W.sub.1 H.sub.3 =
W.sub.2 H.sub.4 = W.sub.2*W.sub.1 H.sub.5 = W.sub.4 H.sub.6 =
W.sub.4*W.sub.1 H.sub.7 = W.sub.4*W.sub.2 H.sub.8 =
W.sub.4*W.sub.2*W.sub.1 H.sub.9 = W.sub.8 H.sub.10 =
W.sub.8*W.sub.1 H.sub.11 = W.sub.8*W.sub.2 H.sub.12 =
W.sub.8*W.sub.2*W.sub.1 H.sub.13 = W.sub.8*W.sub.4 H.sub.14 =
W.sub.8*W.sub.4*W.sub.1 H.sub.15 = W.sub.8*W.sub.4*W.sub.2 H.sub.16
= W.sub.8*W.sub.4*W.sub.2*W.sub.1
H.sub.1=W.sub.0 H.sub.2=W.sub.1 H.sub.3=W.sub.2
H.sub.4=W.sub.2*W.sub.1
[0050] H.sub.5=W.sub.4 H.sub.6=W.sub.4*W.sub.1
H.sub.7=W.sub.4*W.sub.2 H.sub.8=W.sub.4*W.sub.2*W.sub.1
H.sub.9=W.sub.8 H.sub.10=W.sub.8*W.sub.1 H.sub.11=W.sub.8*W.sub.2
H.sub.12=W.sub.8*W.sub.2*W.sub.1 H.sub.13=W.sub.8*W.sub.4
H.sub.14=W.sub.8*W.sub.4*W.sub.1 H.sub.15=W.sub.8*W.sub.4*W.sub.2
H.sub.16=W.sub.8*W.sub.4*W.sub.2*W.sub.1
[0051] The sum of the Walsh indices equals one less than the
corresponding Hadamard code index. Thus the settings for the
2.times.2 switches (i.e., cross or bar state) to set the coder to
Hadamard code n are obtained by converting (n-1) to a binary
number. For example, as shown in FIG. 2, if n=14, (n-1)=13=(1 1 0
1).sub.base 2 and the switches 301 and 302 would be set such that
the signal passes through Walsh coders 311 and 312. Switch 303
would be set so that the signal bypasses Walsh coder 313 and switch
304 would be set so that signal passes through Walsh coder 314.
[0052] FIG. 3 shows an example of a Walsh coder 400 in accordance
with an aspect of the invention. As shown in FIG. 3, an optical
signal enters a collimated lens 401. The optical signal passes
through space 403 toward a gradient reflective coating 404. Space
403 may be an air gap or it may be filled with a glass substrate.
Gradient reflective coating 404 may be a metallic coating such as
silver or aluminum which is highly-reflective/low-transmissive near
the collimating lens (where the light is bright) and tapers off to
low-reflectance/high-transmittance further from the lens. The
intention is that each bounce should transmit an equal amount of
light. The coating allows a portion of the optical signal to pass
through and reflects the other portion through space 403 toward a
100% reflective material. The portion(s) of the optical signal that
pass through coating 404 are directed toward a Fourier lens 405.
Fourier lens 405 converts the incoming signal into the frequency
domain and directs the converted signal onto a reflective phase
mask 406 as shown in FIG. 3. Reflective phase mask 406 corresponds
to the respective Walsh code.
[0053] Reflective phase mask is different for each Walsh coder. For
instance, as shown in FIG. 4A, for Walsh coder W.sub.8, the phase
mask would be set at a height .lamda./2 for 8 bits and then changed
back to the base level. As shown in FIG. 4B, for Walsh coder
W.sub.4, the phase mask would be set at height .lamda./2 for 4 bits
and then changed back to the base level for 4 bits. This pattern is
then repeated one more time. As shown in FIG. 4C, for Walsh coder
W.sub.2, the phase mask would be set at height .lamda./2 for 2 bits
and then changed back to the base level for 2 bits. This pattern is
then repeated three more times. As shown in FIG. 4D, for Walsh
coder W.sub.1, the phase mask would alternate in between height
.lamda./2 and the base height for a total length of 16 bits.
[0054] Physically, any given Walsh function can be implemented by
phase coders of the type contemplated for standard Hadamard codes
and the successive products of the Walsh codes correspond to
passing through the corresponding Walsh coders in cascade (in any
order). Although FIG. 2 shows Walsh coders W8, W4, W2, and W1 (311,
312, 313, and 314 respectively) in that particular order, that
order may be changed to any combination such as W8, W1, W4, and W2
for example.
[0055] All of the N codes of Hadamard N can be reproduced in a
cascaded structure of log.sub.2(N) fixed Walsh coders interspersed
with log.sub.2(N) 2.times.2 optical crossbar switches as shown in
FIG. 2 for the case N=16. The switches route the signal through the
following stage in the Walsh cascade or cause the signal to bypass
that stage. Each of the 16 possible states of the 4 switches
corresponds to one of the 16 Hadamard codes. For example, if all
four switches are in the "bar" state, the input signal bypasses all
four Walsh coders 311, 312, 313, and 314, emerges unchanged, and
the cascade is equivalent to H.sub.1. If all four switches are in
the "cross state" the signal passes through Walsh coder 311,
bypasses Walsh coder 312, passes through Walsh coder 313, and
bypasses Walsh coder 314. The net effect is equivalent to
W.sub.8*W.sub.2=H.sub.11.
[0056] At the rightmost edge of the cascade, element 305 may be a
passive coupler (which introduces an additional 3 dB loss) or an
additional crossbar switch which may reduce the signal loss by
connecting the output port to the active branch of the previous
state; in short, the state of the rightmost switch (element 305)
would match the state of the switch 304.
[0057] If this final crossbar switch is included, the configuration
above could serve as a dual-code coder by using both of the inputs
on the left and both of the outputs on the right. Consider a
situation where all of the crossbar switches are set such that the
uppermost bypass branch is selected. In this case, signals entering
the upper input port emerge from the upper output port after having
code H.sub.1=W.sub.0 applied to them (unchanged). For that same
configuration of the cascade, signals entering the lower input port
will be guided through all four Walsh coders and emerge at the
lower output port after having code
H.sub.16=W.sub.8*W.sub.4*W.sub.2*W.sub.1 applied to them (note,
this signal could also counter-propagate through the cascade). In
general, when the cascade is set to code Hadamard code m on the
upper branch, it is simultaneously set to code Hadamard code
(N-m+1) on the lower branch. This implies that Hadamard codes m and
(N-m+1) are complementary codes in the sense that their binary
representations in Walsh projection are bit-wise inverted. Consider
a scenario in which a user is assigned two such complementary codes
A and B to use to represent 1 or 0 (code-shift keying/modulation).
Using a rapidly tunable variable coder, this user launches a
continuous sequence of A or B coded pulses to convey his data. (The
use of two codes provides greater signal obscurity.) At the
receiver end, a variable decoder cascade, for example, as shown in
FIG. 5, would be set to the A/B complement. If the received signal
is split and launched into both input ports, the cascade will
operate such that the A pulses are decoded at one output port and
the B pulses are decoded at the other output port in a natural
setup for differential detection of a code-shift-keyed
transmission.
[0058] The different paths through the spectral phase encoder 300
may, for different Hadamard codes, cause the signals to incur
different losses and different levels of bin edge filtering
effects. Although not shown, encoder 300 may include mediation
measures such as in-line amplifiers at each stage, lumped losses in
the bypass legs or a single gain-clamped amplifier to compensate
for the loss variations.
[0059] For full generality log.sub.2(N) Walsh coders are needed,
but for some subsets of the Hadamard codes, the cascade depth might
be reduced. For example, if the variable coder need only provide
access to odd-numbered Hadamard codes, the W.sub.1 stage of the
cascade could be eliminated. If the variable coder need only
provide access to even-numbered Hadamard codes, the W.sub.1 stage
of the cascade need not be switched. If the variable coder need
only provide access to Hadamard codes for n<9, the W.sub.8 stage
of the cascade could be eliminated.
[0060] Finally, this approach is not limited to the standard
real-valued (+1, -1) codes. There exists at least one variety of
complex generalizations of the Hadamard code that can be
implemented via augmenting the cascade. These generalize Hadamard
codes G.sub.N retain the desired orthogonality property of Hadamard
codes and are obtained by pre-multiplying and/or post-multiplying
the Hadamard matrix by monomial matrices with complex elements
according to G.sub.N=M1.sub.N*H.sub.N*M2.sub.N. If the monomial
matrices M1.sub.N and M2.sub.N are diagonal, they can each be
physically realized by a fixed complex phase coder (i.e., with a
complex phase mask). If these coders precede the cascade and follow
the cascade (in appropriate sequence), the N different G.sub.N
codes can be accessed by the same log.sub.2(N) switching elements
as above.
[0061] Although the above description describes how a spectral
phase encoder 300 works, the same principles apply for a spectral
phase decoder 500 illustrated in FIG. 5. An incoming encoded data
stream 135 from network 140 is applied to the spectral phase
decoder 500. The spectral phase decoder 500 or 144 applies the
phase conjugate of the coders applied in the spectral phase encoder
300 or 132. For the special case of Walsh-Hadamard codes, where
every phase element is either +1 or -1, each code is its own
conjugate. As such, the spectral phase encoder may also be used as
a spectral phase decoder when using Walsh-Hadamard codes. Using any
combination of Walsh coders, the spectral phase decoder 500 may
reproduce the phase conjugate of all of the N Hadamard codes in
order to decode the encoded data stream.
[0062] FIG. 5 shows an example of such a decoder. As shown in FIG.
5, decoder 500 includes a cascade of switches 501, 502, 503, and
504 and coders 511, 512, 513, and 514. Coders 511, 512, 513, and
514 of decoder 500 are phase conjugates of coders 311, 312, 313,
and 314 in encoder 300. Each switch is a 2.times.2 optical crossbar
switch that routes or directs a signal into one or more of the
coders to encode a signal with a particular Hadamard code. Switch
501 can be set so that an incoming signal can bypass coder 511 or
pass through coder 511. Switch 502 can be set so that an incoming
signal can bypass coder 512 or pass through coder 512. Switch 503
can be set so that an incoming signal can bypass coder 513 or pass
through coder 513. Switch 504 can be set so that an incoming signal
can bypass coder 514 or pass through coder 514. As such, switches
501, 502, 503, and 504 are used to direct the signal to either
bypass their respective coders or to pass through their respective
coders thereby decoding the signal. At the rightmost edge of the
cascade, element 505 may be a passive coupler (which introduces an
additional 3 dB loss) or an additional crossbar switch which may
reduce the signal loss by connecting the output port to the active
branch of the previous state; in short, the state of the rightmost
switch (element 505) would match the state of the switch 504.
[0063] In a single-stage variable encoder, as the order N of the
Hadamard matrix increases, the number of phase elements which must
be controlled grows linearly with N while the number of switch
elements which must be controlled in the variable cascade grows as
log.sub.2(N).
[0064] Second, optical switch technology promises to be faster than
either MEMS-based, liquid-crystal-based, or thermal phase
adjustments and could make this approach suitable for situations in
which the code must be changed on a rapid timescale.
[0065] Although the invention herein has been described with
reference to particular embodiments, it is to be understood that
these embodiments are merely illustrative of the principles and
applications of the present invention. It is therefore to be
understood that numerous modifications may be made to the
illustrative embodiments and that other arrangements may be devised
without departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *