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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical communications, and, more
particularly, to an optical communications system for multiple users or
transmitters.
2. Background Information
A major area of development in light or optical communications concerns
multiple access communications, i.e., a mechanism by which multiple users
may share the same communications channel. This has been done successfully
in optical communications with frequency division (FDMA) and time division
multiplexing techniques (TDMA). Optical TDMA, however, requires
synchronization for the transmitters to avoid mutual interference. Optical
FDMA, on the other hand, may severely restrict the number of transmitters
because of limitations on the availability and range of tunable high
powered light sources, such as lasers. A need thus exists for a multiple
access optical communications system that does not require transmitter
synchronization and is not limited by division of the bandwidth.
SUMMARY OF THE INVENTION
One object of the invention is to provide an optical communications system
that permits several transmitters to communicate with the same receiver or
receiving aperture.
Another object is to provide an optical communications system that is
relatively simple to implement in comparison with optical communications
systems employing FDMA.
Still another object is to provide an optical communications system in
which a large number of transmitters may operate aperiodically and with
small duty cycles.
One more object is to provide an optical communications system capable of
being used with multiple receivers and multiple transmitters, such as with
local area fiber optics networks or for identify friend or foe (IFF)
signaling.
Yet another object is to provide an optical communications system with a
channel capacity exceeding that currently available.
In accordance with the invention, an optical communications system includes
apparatus for interferometric signaling comprising a substantially
coherent light source and a plurality of adjacent, substantially coplanar
optical modulators for encoding the light incident upon the modulators. A
method for coding light signals in accordance with the invention comprises
the steps of propagating a plurality of light signals from a first
location so as to form at least one predetermined interference pattern at
a second location remote from the first location and demodulating the
light signals received at the second location.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter regarded as the invention is particularly pointed out
and distinctly claimed in the concluding portion of the specification. The
invention, however, both as to organization and method of operation,
together with further objects and advantages thereof, may best be
understood by reference to the following detailed description when read
with the accompanying drawings in which:
FIG. 1A is a schematic diagram of an embodiment of a device for
interferometric signaling for incorporation in an optical communications
system in accordance with the invention.
FIG. 1B is a schematic diagram of an alternative embodiment of a device for
interferometric signaling for incorporation in an optical communications
system in accordance with the invention.
FIG. 2 illustrates a typical geometric arrangement for interferometric
signaling between a transmitter and a receiver.
FIG. 3 illustrates the autocorrelation of the interference intensity
pattern for a particular interferometric signaling mask in an embodiment
of the invention.
FIG. 4 illustrates the autocorrelation and crosscorrelations for eight
particular interferometric signaling masks in an alternative embodiment of
the invention.
FIG. 5 is a schematic diagram of an embodiment of a device for
interferometrically decoding a transmitted light signal for incorporation
in an optical communications system in accordance with the invention.
FIG. 6 is a schematic diagram of yet another alternative embodiment of a
device for interferometric signaling for incorporation in an optical
communications system in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
An optical communications system in accordance with the present invention
provides a space-time signaling or coding technique particularly suited
for private, multiple access optical communications. In radio frequency
code division multiple access (CDMA) the same channel or bandwidth may
support simultaneous transmission by many users. The users are "sorted
out" or discriminated by different signaling waveforms that have low cross
correlations, as described in Spread Spectrum Communications, Vol iii, M.
K. Simon, J. K. Omura, R. A. Scholtz, and B. K. Levitt, Computer Science
Press, 1985, Satellite communications, R. M. Garliardi, Lifetime Learning
Publications, 1984, and Coherent Spread Spectrum Systems, J. K. Holmes,
John Wiley & Sons, 1982. In optical communications, by contrast, short
wavelengths of light, such as light in the range from infrared to
ultraviolet, may provide the basis for interferometric signaling by
spatially encoding the light. An interferometric signal encoder, for
example, may include a substantially coherent light source, such as a
laser, electrical or electronic circuitry to perform the interferometric
signal modulation, and optical elements. Likewise, an interferometric
signal decoder may include components for performing incoherent optical
and signal processing to interferometrically decode the signal.
FIG. 1A depicts an interferometric encoder 10 for incorporation in an
optical communications system in accordance with the invention.
Substantially coherent light from an emitter or source, such as a laser
20, which may incorporate conventional temporal signaling, such as pulse
position modulation, or a more sophisticated temporal modulation, such as
CDMA, as described in A Semiclassical Analysis of Optical Code Division
Multiple Access, D. Brady and S. Verdu, IEEE Transactions on
Communications, Vol. 39, No. 1, January 1991, pp. 85-93, may be
respectively expanded and collimated using a beam diverger 30 and a beam
former 40. The collimated light beam from beam former 40 is incident upon
an array of lenses 50 that provide a plurality of beams, such as arranged
in a circular format about a center 52, as illustrated in FIG. 1A. It will
be appreciated that the format of the beams is not limited to a circular
arrangement. As illustrated, the array of lenses 50 comprises a ring of
equiangularly spaced circular lenses mounted on a disk 51 that act as
light apertures, each of which provides beam focusing at a respective
optical modulator 60. A plurality of adjacent, substantially coplanar,
optical modulators 60, liquid crystal (LC) cells in this instance, are
oriented at an angle, such as 90.degree., with respect to the path of the
emitted light and are arranged in an array on a disk 61. Using focussed
beams with the LC cells or pixels has the advantage of avoiding severe
edge-diffraction effects. Each optical modulator may alternatively
comprise a film, such as photographic film, for modulating the incident
light.
FIG. 1A illustrates a 1:1 correspondence between the LC cells and the
number of beams provided by the array of lenses 50 although the invention
is not limited to this 1:1 correspondence. In this particular embodiment,
the center of each lens is spaced a predetermined distance from the center
52 of the array of lenses 50, and each LC cell introduces a phase shift
onto that portion of the laser beam passing through the respective window
60, such as with parallel-rub nematic liquid crystal cells. This
embodiment of the invention employs phase modulation with two phase
shifts, 0 and .pi. radians. Alternative embodiments of the invention may
also introduce magnitude modulation separately or in conjunction with
phase modulation. After phase modulation by the LC cells, the diverging
optical beams may be recollimated, such as by the array of lenses 70 on a
disk 71. The collimated beams arranged in an equi-angular circular format
emerging from array of lenses 70 have the appearance of spatially encoded
mutually coherent point sources that radiate simultaneously into free
space, such as in the direction of a ground-based or spaced-based optical
receiver. It will now be appreciated that so long as at least one of the
optical modulators alters light incident upon it by modifying either the
phase of the incident light, the amplitude of the light, or both, an
interferometrically encoded light signal may be provided.
FIG. 1B illustrates an alternative embodiment of an interferometric encoder
for incorporation in an optical communications system in accordance with
the invention. As illustrated in FIG. 1B, instead of array of lenses 50,
array of optical modulators 60, and array of lenses 70 illustrated in FIG.
1A, this embodiment includes an array of optical modulators 80, such as LC
cells, ganged in a rectangular format. Likewise, such an array may be used
with focusing and recollimating lens arrays and may be used for amplitude
as well as phase modulation.
At a remote location which may be in the far-field of the optical point
sources, at least one interference pattern is formed by propagating a
plurality of light signals from a first location to the remote location,
much like the interference fringes of a Young's two-slit interferometer,
as described in Optics, E. Hecht, Edison-Wesley, 2nd Edition, 1988. In
this particular embodiment the interference pattern obtained at the
detector plane or light gathering aperture of a remote :receiver is
determined by the phase-shifts, i.e., the code settings for the different
phase modulated beams, the spatial positions of the beams at the
transmitter, and the free-space beam travel distances. By simply using
spatial amplitude or phase-based coding or both together, the received
light signal is spatially coded via far-field interference effects.
Because the far-field interference pattern is known for a particular
spatial code, a receiver may have the capability to crosscorrelate the
received spatial interference intensity pattern with previously stored
spatial intensity patterns to achieve user discrimination and to decode
the signal. Further discrimination may be achieved if the signal is also
temporally coded, as previously described.
For an optical communications system in accordance with the present
invention, the intensity I(x,y,L) of the light at a point (x,y,L), on a
plane parallel to the plane of an array of optical modulators, such as
array of lenses 70 illustrated in FIG. 1A, will be proportional to the
product EE*, where E is the electric vector E=E(x,y,L) and E* is its
complex conjugate. A typical geometry is illustrated in FIG. 2. Assuming
the lenses resemble spatial point sources, such as delta functions, in the
far-field region, and that L is substantially larger than R,x, and y, R
being the radius of the array of lenses about a central point 72, then
E(x,y,L), may be approximated as
##EQU1##
where
the {.alpha..sub.p } are the complex values associated with optical
modulations due to the p-th LC cell/point source, with .alpha..sub.p
=r.sub.p .multidot.e.sup.-jw p,r.sub.p being the amplitude modulation and
w.sub.p being the phase modulation (for example, for a zero radian phase
shift, .alpha..sub.p =+1, while for a x radian phase shift, .alpha..sub.p
=e.sup.j.pi. =-1.)
w is the angular frequency of the light source,
N is the number of lenses in the array,
t is time, and
.phi..sub.p (x,y,L) is the phase shift, in radians, acquired from optical
propagation flow from the p-th lens to the point (x,y,L).
The expression for .phi..sub.p (x,y,L) is given as
##EQU2##
where c is the speed of light. Using the well known Fresnel approximation,
the square root is approximated by
##EQU3##
where the fixed phase shift at the receiver e.sup.jkL, with k=w/c, is
omitted. In this particular embodiment, the {.alpha..sub.p } are either +1
or -1 corresponding to the 0 or .pi. radians point source phase shifts,
respectively, and
##EQU4##
Thus, I(x,y,L) is proportional to
##EQU5##
In the context of the present invention, a particular selection for
{.alpha..sub.p } refers to an interferometric signaling mask or spatial
code. The expression immediately above thus provides the Fraunhofer
diffraction pattern of the spatially coded aperture at the receiver based
on coding provided at the transmitter.
FIG. 3 displays a scaled autocorrelation of the interference intensity
pattern at the receiver provided by the signaling mask +1-1-1+1+1-1-1+1
over an approximately 0.6 m.times.0.6 m area for an embodiment of the
present invention in which R=1 cm, L=2 km,
.omega..apprxeq.3.multidot.10.sup.15 Hz, such as from a helium-neon laser
source, and N=8. As provided above, +1 indicates a phase shift of 0
radians and -1 indicates a phase shift of .pi. radians. The {.alpha..sub.p
} for the eight signaling masks were arbitrarily chosen to be the 8 rows
of the Walsh-Hadamard matrix of order eight. The Walsh-Hadamard matrix of
order 2.sup.n, H.sub.2n, is defined by the recursive Cartesian product
H.sub.2n =H.sub.2 .sym.H.sub.2n -1 where H.sub.2 =
##EQU6##
This autocorrelation is given by the integral
R(.DELTA.x,.DELTA.y)=I(x,y,L)I(x+.DELTA.x,y+.DELTA.y)dxdy
The units on the .DELTA.x, and .DELTA.y axes are, in this particular
embodiment, 10 cm.
FIG. 4 displays the cross and autocorrelations of the eight interference
intensity patterns when properly aligned. The autocorrelations are the
peaks on the diagonal and the crosscorrelations are the off-diagonal
values, given by the integral
I.sub.i (x,y,L)I.sub.j (x,y,L)dxdy
where i and j designate the interferometric signaling or "mask"
designations.
An optical communications system employing interferometric signaling may be
used with many conventional optical signaling techniques, such as pulse
position modulation and pulse duration modulation. Furthermore,
interferometric signaling has several useful features. First, the
intensity functions, {I.sub.i (x,y,L)}, transmitted via different codes or
signaling masks will effectively add as the light sources are incoherent
with respect to each other when integration is performed over any
reasonable signaling period. Second, the interferometric decoder at the
receiver need not be designed for coherent light, e.g., it may sin:ply be
a distributed set of light sensitive sensors. This permits inexpensive
extended receiving aperture construction, such as might be required for
ground-to-satellite applications. Third, because the interferometric
decoder correlates the intensity function present at the receiving
aperture with stored templates or intensity patterns, interferometric
signaling may provide protection against optical interference.
In the embodiment of the invention illustrated in FIG. 1A, detection and
demodulation of the information being sent by one or more transmitters may
proceed as follows. Optical or light sensitive detectors at the receiver
spatially sample the interference intensity pattern across the aperture
typically at a rate that appreciably exceeds the maximum signaling rate.
Apparatus for such detection may comprise a plurality of adjacent,
substantially coplanar, optical detectors oriented at a predetermined
angle with respect to the path of the light, such as ninety degrees, so
that different portions of the light are incident upon different optical
detectors. The detectors integrate the photocurrents or detector outputs
during a sampling period. At the end of each sampling period, the
accumulated charges may be measured, quantized, and provided to a signal
processor, such as an electronic or optical signal processor for
performing a matched filter operation. At the end of each sampling period,
the processor correlates the sampled intensity pattern against each
interferometric signaling pattern or mask. Finally, the processor
indicates a particular pattern as being present if the pattern's
crosscorrelation to the sampled intensity pattern exceeds a specified
threshold, and not present otherwise. By this method, apparatus for
interferometric decoding may sort out multiple transmissions to a single
aperture. Interferometric signaling exhibits graceful degradation, i.e.
the effective noise and hence the probability of symbol error increases
gradually with the number of simultaneous users. Likewise, sophisticated
algorithms are possible; apparatus for interferometric decoding may, for
example, first estimate the number of users during a sampling period and
then perform a maximum likelihood estimation of the interferometric
signaling patterns present based on the number of users.
As previously described, apparatus for interferometric decoding for
incorporation in an optical communications system in accordance with the
present invention may operate satisfactorily with incoherent light, as
well as coherent light, permitting use of a relatively inexpensive
receiver for large extent apertures, such as might be desirable for a
satellite. FIG. 5 illustrates such an embodiment. Each of the elements 100
of array 101 is a half-sphere comprised of a transparent material with an
appropriate refractive index such that the half-sphere focuses
substantially all of the incoming light onto a photosensitive sensor (not
shown) positioned at the center of the bottom of the half-sphere. The
half-spheres may be densely packed so that the receiving aperture provides
efficiency. For optical communications over substantial distances, such as
from ground-to-satellite, such apparatus may be of spatial extent greater
than the signaling pattern so that the pattern may be tracked as the beam
wanders or as it is initially acquired. This feature allows highly
accurate satellite tracking and positioning. Furthermore, interferometric
signaling provides a number of possibilities for accomplishing initial
acquisition. For example, a signaling mask may be used for initial spatial
synchronization and the liquid crystal devices or other optical sensors
may correlate this particular mask during synchronization. Thus, the
decoder may search for this particular pattern and when found, the
receiver may either lock onto it and track it, or work with the ground
station of the transmitter through a feedback loop to center it and keep
it centered on the receiving aperture. Initial synchronization may be
performed with an unmodulated laser beam.
In an alternative embodiment of the invention, an optical communications
system employing interferometric signaling, such as interferometric
encoder 110 illustrated in FIG. 6, may be adapted for use with fiber-optic
based local area networks. In this particular embodiment, the free-space
Fraunhofer diffraction pattern, as previously described, is produced by
including a spherical lens 140. The lens produces the Fourier transform of
the light incident upon it and thus has a similar effect on the
transmitted light as propagation through free space. As shown in FIG. 6,
in this particular embodiment the desired interferometric signaling or
spatial code mask is imposed by an LC optical modulator 160, and the
resulting spatial Fourier transform pattern is then transferred to a
remote distribution center via a multi-fiber cable 170. Here, each optical
fiber 150 in multi-fiber cable 170 acts as a point sampler. At the remote
end of multi-fiber cable 170, the received light distribution is
crosscorrelated with various known spatial masks corresponding to a
different code for each user. Depending upon the particular spatial code
correlations, different messages are recovered and routed to the
appropriate users. Likewise, for this particular embodiment, a group of
users might share the same spatial mask and achieve intra-mask isolation
by temporal coding with modulation sequences exhibiting low temporal
crosscorrelation. For example, codes with large Hamming distances offer
maximum errors and erasures correction. Due to practical implementation
issues, interferometric signaling using fiber-optics is most economic when
the number of fibers in the multi-fiber cable is limited to a small
number, although the invention is not so limited in scope. The small
number of fibers does not, however, drastically limit the number of
possible users.
The remote-decoder may take any one of a number of possible embodiments.
For example, such a decoder may comprise a plurality of charge-coupled
devices (CCD). Alternatively, the device may comprise a 2-dimensional high
speed photodiode array when high speed processing is desirable. A computer
or signal processor, such as a digital signal processor, may then compute
the crosscorrelations using the stored spatial codes or masks to determine
which users are active based on the received intensity pattern.
Appropriate control signals then (re)generate and route the recovered data
messages to the appropriate recipients.
As described for the first embodiment, in the embodiment of FIG. 6 the
optical modulators may be used in a 1:1 imaging mode with the multi-fiber
cable. The optical modulators may comprise a twisted nematic liquid
crystal cell array sandwiched between cross polarizers to provide the
desired amplitude modulation required for the fiber-based coding
technique. As will be appreciated by those skilled in the art, for a
fiber-optics network as previously described phase modulation of the light
may not be employed to spatially encode the signals.
An optical communications system in accordance with the present invention
accomplishes signaling in three dimensions per signaling interval: two
spatial dimensions, and a time dimension; in contrast with one dimension,
i.e., time, for a conventional communications systems. Thus, to achieve
multidimensional signaling in a conventional communications system,
multiple signaling is used. For a communications system in accordance with
the invention, if the dimension of the signaling waveform is D, then the
number of available dimensions per signaling interval is D.sup.3. Thus, as
will be appreciated by one skilled in the art, a large minimum Hamming
distance is easier to obtain in higher dimensional signaling. For example,
as is well-known, an extended Reed-Solomon code is a code with an alphabet
size equal to the block length. Thus, in another embodiment of the present
invention, the array may include a number of optical modulators arranged
to correspond with the block length of the Reed-Solomon code. Then each
symbol is further encoded by a convolutional code, resulting in a spatial
block code and a temporal convolutional code. Where d.sub.rs is the
minimum distance of the Reed-Solomon code and d.sub.con is the minimum
free distance of the convolutional code, the minimum distance of the
combined concatenated code is underbounded by the product d.sub.rs
d.sub.con. It is thus valuable to chose good concatenated codes, one code
in time and the other in space. For maximum errors and erasures
correction, however, the number of array elements may be much larger than
the code length of the block code to provide for user discrimination codes
with a large Hamming distance.
Finally, another embodiment of the present invention may prove valuable for
identify friend or foe (IFF) signaling in many possible situations, such
as single transmitter/single receiver, multiple transmitter/single
receiver, single transmitter/multiple receivers, and multiple
transmitters/multiple receivers. Moreover, the transmitter(s)/receiver(s)
may be ground based or airborne/spaced based. Thus, in one particular
embodiment the coding or signaling technique disclosed herein may be used
to perform IFF, such as by using laser-based signaling in a single
transmitter/multiple receiver environment with an airborne/space based
transmitter looking down on multiple ground based receivers. A spatially
and temporally coded laser beam emerging from an airborne platform may
mechanically or electronically scan a given region on the ground, such as
with an optical phased array. Thus, ground based receivers may continually
crosscorrelate any impinging optical radiation with known codes. If a code
match occurs, an electromagnetic beacon may be transmitted, making
identification.
The invention has been described herein in accordance with certain
preferred embodiments thereof, many modifications, substitutes, changes
and equivalents will now occur to those skilled in the art. For example, a
number of possible coding techniques may be employed by an optical
communications system in accordance with the present invention. It is
intended to cover all such modifications and changes as are within the
true spirit and scope of the invention by means of the appended claims.
* * * * *
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