U.S. patent application number 10/257424 was filed with the patent office on 2003-09-11 for ofdm apparatus and method.
Invention is credited to Goldenberg, Efraim, Gross, Yoram, Medvedovsky, Lev, Mendlovic, David, Miron, Yehuda, Sariel, Aviram, Shabtay, Gal, Zalevsky, Zeev.
Application Number | 20030169683 10/257424 |
Document ID | / |
Family ID | 27271928 |
Filed Date | 2003-09-11 |
United States Patent
Application |
20030169683 |
Kind Code |
A1 |
Mendlovic, David ; et
al. |
September 11, 2003 |
Ofdm apparatus and method
Abstract
A method for encoding a data symbol vector in an OFDM symbol and
decoding an OFDM symbol to recover a data symbol vector encoded
therein, the method comprising: receiving a vector of values;
generating at least one input spatial light pattern responsive to
the vector; generating for each input spatial light pattern an
output spatial light pattern that is an interference pattern
produced by light from the input spatial light pattern; sensing the
output spatial light pattern at discrete spatial points and
generating signals responsive to the sensed light; and if the
vector represents a data symbol vector, using the signals to encode
the data symbol vector in an OFDM signal and if the vector
represents an OFDM symbol, using the signals to recover a data
symbol vector encoded in the OFDM symbol.
Inventors: |
Mendlovic, David;
(Petach-Tikva, IL) ; Goldenberg, Efraim; (Ashdod,
IL) ; Shabtay, Gal; (Petach-Tikva, IL) ;
Miron, Yehuda; (Tel-Aviv, IL) ; Medvedovsky, Lev;
(Tel-Aviv, IL) ; Gross, Yoram; (Rishon-Lezion,
IL) ; Zalevsky, Zeev; (Rosh-Haayin, IL) ;
Sariel, Aviram; (Ramot-Hashavin, IL) |
Correspondence
Address: |
William H Dippert
Reed Smith
599 Lexington Avenue
29th Floor
New York
NY
10022-7650
US
|
Family ID: |
27271928 |
Appl. No.: |
10/257424 |
Filed: |
February 24, 2003 |
PCT Filed: |
April 10, 2001 |
PCT NO: |
PCT/IL01/00331 |
Current U.S.
Class: |
370/208 |
Current CPC
Class: |
G02B 26/06 20130101;
G06E 3/00 20130101; G01B 9/02098 20130101; G06E 3/001 20130101 |
Class at
Publication: |
370/208 |
International
Class: |
H04J 011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2000 |
IL |
135576 |
Jan 23, 2001 |
IL |
141041 |
Mar 7, 2001 |
IL |
141856 |
Claims
1. A method for encoding a data symbol vector in an OFDM symbol and
decoding an OFDM symbol to recover a data symbol vector encoded
therein, the method comprising: receiving a vector of values;
generating at least one input spatial light pattern responsive to
the vector; generating for each input spatial light pattern an
output spatial light pattern that is an interference pattern
produced by light from the input spatial light pattern; sensing the
output spatial light pattern at discrete spatial points and
generating signals responsive to the sensed light; and if the
vector represents a data symbol vector, using the signals to encode
the data symbol vector in an OFDM signal and if the vector
represents an OFDM symbol, using the signals to recover a data
symbol vector encoded in the OFDM symbol.
2. A method according to claim 1 wherein generating at least one
input spatial light pattern responsive to the vector comprises
partitioning the vector into a plurality of data sub-vectors and
generating at least one input spatial light pattern for each
sub-vector.
3. A method according to claim 1 or claim 2 wherein generating at
least one input spatial light pattern comprises expressing the
vector in terms of binary vectors, where a binary vector has
components equal to one or zero and a vector expressed in terms of
binary vectors is equal to a sum of the binary vectors each of
which is multiplied by a different power of two, and generating at
least one spatial input light pattern for each binary vector.
4. A method according to any of claims 1-3 wherein if the vector
comprises a complex component, generating at least one input
spatial light pattern comprises parsing the vector into a real and
an imaginary vector and generating at least one input spatial light
pattern for the real vector and at least one input spatial light
pattern for the imaginary vector.
5. A method according to any of claims 1-4, wherein processing the
signals to encode or decode the OFDM symbol comprises processing
the signals-to determine a DCT and DST of the vector.
6. A method for generating a DCT and DST of a vector comprising:
partitioning the vector into a plurality of sub-vectors; generating
at least one input spatial light pattern responsive to each
sub-vector; generating for each input spatial light pattern an
output spatial light pattern that is an interference pattern
produced by light from the input spatial light pattern; sensing the
output spatial light pattern at discrete spatial points and
generating signals responsive to the sensed light; and processing
the signals to generate the DCT and DST of the vector.
7. A method according to any of claims 1-6 wherein generating the
at least one input spatial light pattern comprises providing a
plurality of point-like light sources and controlling each light
sources to radiate light at a desired intensity.
8. A method according to claim 7 wherein the light sources are
coplanar in a first plane.
9. A method according to claim 8 wherein generating the output
spatial light pattern comprises generating a virtual mirror image
for each light source wherein the virtual images are reflections of
the light sources across a same mirror plane that is perpendicular
to the first plane.
10. A method according to claim 9 and comprising positioning the
light sources in the first plane so that along a line that lies in
the first plane and is perpendicular to the mirror plane, each
light source has a projection at a different point on the line and
a distance between any two adjacent projection points is the
same.
11. A method according to claim 10 wherein positioning the light
sources comprises positioning the light sources along a straight
line.
12. A method according to claim 11 wherein the straight line is
perpendicular to the mirror plane.
13. A method according to any of claims 10-12 wherein the output
spatial light pattern is a light pattern in a second plane parallel
to the first plane at a distance "z" from the first plane.
14. A method according to claim 13 wherein each of the light
sources provides light having a same characteristic wavelength
".lambda.".
15. A method according to claim 14 wherein the plurality of light
sources comprises N light sources and the n-th light source, where
n is an integer satisfying 0.ltoreq.n.ltoreq.N, is located at a
distance xo+n.DELTA.x from the image plane and the light is sensed
at N points in the second plane.
16. A method according to claim 15 wherein the k-th point at which
light is sensed, where k is an integer satisfying
0.ltoreq.k.ltoreq.N, is located at a distance from the image plane
that is equal to k.DELTA..xi., (k+2N).DELTA..xi. or
(2N-k).DELTA..xi. and wherein .lambda.z=4N.DELTA.x.DELTA..xi..
17. A method according to claim any of claims 16 wherein the at
least one input spatial light pattern comprises first and second
input spatial light patterns and the intensity of the n-th light
source in the second input spatial light pattern is substantially
equal o the intensity of the (N-n) light source of the first input
spatial light pattern.
18. An OFDM modem for coding a data symbol vector into an OFDM
symbol comprising: an optical processor having an input port and an
output port that receives the data symbol vector and optically
processes the data symbol vector to generate signals at the optical
processor's output port that are responsive to a DCT and a DST of
the data symbol vector; a signal processor that receives the
signals generated by the optical processor and processes the
signals to determine the DCT and DST of the data symbol vector and
generates the OFDM symbol from the DCT and DST.
19. An OFDM modem for decoding an OFDM symbol and recovering a data
symbol vector encoded therein comprising: a signal processor that
receives a first vector representing the OFDM symbol and generates
a second and a third vector therefrom, wherein the second vector
has components, each of which is proportional to a sum of two
different components of the first vector, and the third vector has
components, each of which is proportional to a difference between
two different components of the first vector; and an optical
processor that receives the second and third vectors and optically
processes the received vectors to generate signals responsive to a
DCT and a DST of the second vector and signals responsive to a DCT
and DST of the third vector; wherein the signal processor receives
the signals generated by the optical processor and processes them
to decode the OFDM symbol and recover the data symbol vector
encoded therein.
20. An OFDM modem according to claim 18 or claim 19 wherein the
optical processor is a shearing processor.
21. An OFDM communication network comprising an OFDM modem
according to any of claims 18-20.
Description
FIELD OF THE INVENTION
[0001] The invention relates to transmitting information using
orthogonal frequency division multiplexing (OFDM) systems and in
particular to OFDM systems in which OFDM symbols are generated
optically.
BACKGROUND OF THE INVENTION
[0002] In OFDM communication systems information transmitted by a
transmitter to a receiver is coded in a plurality of symbols, such
as for example phase shift keying (PSK) symbols or quadrature
amplitude modulation (QAM) symbols, which are simultaneously
transmitted by the transmitter to the receiver via a set of
orthogonal "OFDM" carriers. The OFDM carriers are orthogonal to
each other over a time interval T. An OFDM modem uses each symbol
of the plurality of symbols as a coefficient that multiplies a
different one of the orthogonal carriers and adds the carriers,
each multiplied by its coefficient, to generate a time varying
signal having a period T, which is transmitted to the receiver. The
symbols are hereinafter referred to as "data symbols", the time
varying signal is hereinafter referred to as an "OFDM symbol" and
the period T is referred to as an OFDM symbol period. The OFDM
symbol is transmitted for a duration ST=T+G where G is a guard time
that separates sequential OFDM symbols to reduce inter symbol
interference that might result from multipath delay spread.
[0003] The orthogonal carriers in the OFDM symbol period T are
harmonic carriers exp(i2.pi.nt/T), where t is time, n is an integer
and i is the imaginary i. Let the plurality of data symbols coded
into an OFDM symbol be represented by a vector d(n,N) of N,
generally complex, data symbols d(n), where N is the order of the
vector d(n,N) and n is an index that distinguishes components of
the vector and satisfies the relation 0.ltoreq.n.ltoreq.N-1. A
convention is used herein that vectors are represented by a letter
(or letters) in bold type, which is generally followed by the index
of the vector in lower case and the order of the vector in upper
case in parenthesis.. The letter or letters that designate the
vector, followed by an index in parentheses in regular, i.e.
non-bold, type represents a component of the vector corresponding
to the index.
[0004] If an OFDM symbol formed from data symbols d(n) is
represented by s(t), where t is time, then 1 s ( t ) = n = 0 N - 1
y ( n ) cos 4 nk x z o .
[0005] Assume that time is quantized in units of a sampling period
.DELTA.T so that t=k.DELTA.T and T=N.DELTA.T, then s(t) may be
written as a time ordered set of values, i.e. a vector s(k,N)
having components 2 s ( k ) = n = 0 N - 1 y ( n ) sin 4 nk x z o
.
[0006] From the expression for s(k,N) it is seen that s(k,N), which
is in the time domain, is an inverse discrete Fourier transform
(IDFT) of the vector d(n,N) of data symbols d(n), which are in the
frequency domain. An OFDM modem in the receiver that receives the
time varying OFDM symbol s(k,N) can determine the vector of data
symbols d(n,N), which represents the transmitted information, by
discrete Fourier transforming (DFT) the OFDM symbol from the time
domain to the frequency domain.
[0007] However, the OFDM symbol s(k,N) described above can be, and
often is, a complex symbol and for signal transmission, a real OFDM
symbol that contains the information of the above complex OFDM
symbol is advantageous. In addition it is generally desirable that
the DC component of the OFDM symbol be equal to zero. To assure
that the OFDM symbol has a zero DC component, for any vector of
data symbols d(n,N), if d(0) is not equal to zero a null data
symbol is added to the symbol vector so that d(0) is zero. To
assure that an OFDM symbol corresponding to a data symbol vector
d(n,N) is a real symbol, an Hermitian vector is formed from d(n,N)
and an IDFT of the Hermiitian vector is used to generate an OFDM
symbol containing the information of the data symbol vector
d(n,N).
[0008] If a vector D(n,2N) having components D(n) represents the
Hermitian vector formed from d(n,N), components D(n) are defined as
follows: D(n)=d(n) for 0.ltoreq.n.ltoreq.(N-1), D(N)=0, (or an
arbitrary real number), and D(n)=d(2N-n)* for
(N+1).ltoreq.n.ltoreq.(2N-1). If a vector S(k,2N) having components
S(k) represents the OFDM symbol that is generated by inverse
Fourier transforming D(n,2N), then 3 S ( k ) = n = 0 2 N - 1 D ( n
) exp ( 2 nk / 2 N ) = 2 R n = 0 N - 1 d ( n ) exp ( 2 nk / 2 N )
,
[0009] where R stands for the real part of the last sum in the
above expression. (It is noted that the vector D(n) is not quite
Hermitian because component D(0) does not have a matching complex
conjugate component in the vector D(n,2N). However, for convenience
D(n) is referred to as an Hermitian vector.) The real OFDM symbol
S(k,2N) is used to transmit the data symbol vector d(n,N) to an
intended OFDM receiver. In some OFDM networks S(k,2N) is
transmitted directly to the receiver, while in other OFDM networks
the OFDM symbol is used to modulate a suitable bandpass carrier
which is transmitted to the receiver.
[0010] OFDM communication networks are relatively efficient in use
of bandwidth and are substantially immune to inter symbol
interference resulting from multipath time delays. However, as
noted above, they require generating an inverse Fourier transform
of data to be transmitted and generating a Fourier transform of
received data. Performing the Fourier transform and its inverse are
computation intensive and time consuming, even when performed by
DSPs using fast Fourier transform (FFT) algorithms.
[0011] In addition, in order to use FFT algorithms to process OFDM
data, the number 2N of data symbols D(n), and as a result the
number of orthogonal carriers in an OFDM carrier set, should be
equal to 2.sup.m, where m is an integer. As a result, data capacity
of a prior art OFDM communication system is conveniently expanded
only by at least doubling the data carrying capacity of the
network, which requires at least doubling the number of its
orthogonal carriers. It is therefore relatively difficult to expand
data carrying capacity of prior art OFDM systems. In particular,
data carrying capacity of such systems cannot easily be adjusted to
provide an increase in capacity that is less than double the data
capacity of the system.
SUMMARY OF THE INVENTION
[0012] An aspect of some embodiments of the present invention
relates to providing an OFDM communication network that processes
data faster than many conventional prior art OFDM communication
networks.
[0013] An aspect of some embodiments of the present invention,
relates to providing an OFDM communications network that can
relatively easily be expanded to accommodate increases in data
carrying demand.
[0014] An aspect of some embodiments of the present invention
relates to providing an OFDM modem comprising an optical processor
that processes OFDM symbols faster than many conventional prior art
OFDM modems. Preferably, the optical processor comprises a shearing
interferometer, and is similar to an optical processor, hereinafter
referred to as a "shearing processor", based on shearing
interferometry described in Israel Application 135576 entitled
"OFDM", filed on Apr. 10, 2000, Israel Application 141041, entitled
"An Optical Discrete Transform Method and System" filed on Jan. 23,
2001 and PCT Application entitled "Optical Transform Method and
System", filed on even date with the present application, the
disclosures of which are incorporated herein by reference.
[0015] When an OFDM modem, in accordance with an embodiment of the
present invention, receives a data symbol vector d(n,N) to encode
in an OFDM symbol S(k,2N), the modem uses the shearing processor to
generate an IDFT of the Hermitian vector D(n,2N) that corresponds
to d(n,N) and thereby the OFDM symbol S(k,2N) corresponding to
d(n,N). When the OFDM modem receives an OFDM symbol S(k,2N), the
modem uses the shearing processor to generate a DFT of the OFDM
symbol to recover a data symbol vector d(n,N) encoded in the OFDM
symbol.
[0016] Explicitly writing out the real part of the sum in the
expression given above for a component S(k) of S(k,2N), 4 S ( k ) =
2 n = 0 N - 1 [ Rd ( n ) ] cos ( 2 nk / 2 N ) - 2 n = 0 N - 1 [ Id
( n ) ] sin ( 2 nk / 2 N ) ,
[0017] where Rd(n) represents the real part of d(n) and Id(n)
represents the imaginary part of d(n). The first sum in the
expression for S(k) is a discrete cosine transform (DCT) of a
vector Rd(n,N) having components Rd(n), evaluated for index k, and
the second sum is a discrete sine transform (DST) of a vector
Id(n,N) having components Id(n), evaluated for index k. S(k,2N) may
therefore be written S(k,2N)=2DCT[Rd(n,N),k]-2DS- T[Id(n,N),k]. The
modem uses the shearing processor it comprises, in accordance with
an embodiment of the present invention, to generate the DCT and DST
of d(n,N) to provide OFDM symbol S(k,2N).
[0018] The modem receives the data symbol vector d(n,N) as
electronic signals. Assume that the electronic signals represent
the real and imaginary parts of each component d(n) of d(n,N) as
binary numbers having B bits so that 5 d ( n ) = 0 B - 1 ( Rb ( n )
p + iIb ( n ) p ) 2 p ,
[0019] where Rb(n).sub.p is the p-th bit of the binary number
representing the real part of d(n) and Ib(n).sub.p is the p-th bit
of the binary number representing imaginary part of d(n). The data
symbol vector d(n,N) can then be expressed as a sum, 6 d ( n , N )
= 0 ( B - 1 ) ( BRd ( n , N ) p + iBId ( n , N ) p ) 2 p .
[0020] In the expression for d(n,N), BRd(n,N).sub.p is an N
dimensional vector {Rb(N-1).sub.p, Rb(N-2).sub.p, . . .
Rb(0).sub.p} having components that are the p-th bits of the real
parts of the components d(n,N). Similarly, BId(n,N).sub.p is a
vector {Ib(N-1).sub.p, Ib(N-2).sub.p, . . . Ib(0).sub.p} having
components that are the p-th bits of the imaginary parts of the
components of d(n,N). A vector having components that are bits,
i.e. that can assume only a value one or zero, is hereinafter
referred to as a "binary vector". Binary vectors BRd(n,N).sub.p and
BId(n,N).sub.p correspond to "bit planes" discussed in PCT
Publication WO 00/72267, the disclosure of which is incorporated
herein by reference.
[0021] In accordance with an embodiment of the present invention,
for each real and imaginary binary vector BRd(n,N).sub.p and
BId(n,N).sub.p the shearing processor converts electronic signals
representing the binary vector to at least one first spatial light
pattern to represent the vector optically. The shearing processor
generates a second spatial light pattern from each of the at least
one first spatial light pattern, which second spatial light pattern
is a function of the discrete cosine transform (DCT) and/or
discrete sine transform (DST) of the vector d(n,N). The second
spatial light pattern is sensed by a suitable optical sensor
comprised in the shearing processor, which converts the second
spatial light pattern to electronic signals. The electronic signals
from the second spatial light patterns are processed by a suitable
electronic processor to determine the DCT and DST of d(n,N) and
thereby the OFDM symbol S(k,2N) corresponding to d(n).
[0022] When the modem receives an OFDM symbol S(k,2N), in
accordance with an embodiment of the present invention, the modem
reverses the process by which the OFDM signal is generated to
recover a data symbol vector d(n) encoded in the OFDM symbol. The
modem converts electronic signals representing the OFDM symbol into
at least one first spatial light pattern to represent the OFDM
symbol optically. The shearing processor generates a second spatial
light pattern for each of the at least one first spatial light
pattern, which second spatial light pattern is sensed by the
optical sensor. Electronic signals generated by the optical sensor
for each of the second spatial patterns are processed to provide
the data symbol vector d(n,N) encoded in the OFDM symbol.
[0023] According to an aspect of some embodiments of the present
invention, a data symbol vector d(n,N) is partitioned into
subvectors prior to processing by the shearing processor.
[0024] A shearing processor in an OFDM modem, in accordance with an
embodiment of the present invention, comprises a plurality of
preferably point-like, non-coherent, light sources that the
processor controls to, generate first spatial light patterns
representing vectors that the processor processes. To represent a
binary vector, such as BRd(n,N).sub.p and BId(n,N).sub.p, each bit
of the binary vector is represented by a different one of the light
sources. For example a bit of the binary vector that has a value 1
may be represented by a turned on light source while a bit that has
a value 0 is represented by a turned off light source.
[0025] In some cases a binary vector BRd(n,N).sub.p or
BId(n,N).sub.p to be processed by the shearing processor so as to
generate a DCT or DST of the vector contains more bits than the
number of the plurality of light sources in the shearing processor.
In such cases, in accordance with an embodiment of the present
invention, the vector is partitioned into binary subvectors. Each
binary subvector is in turn optically processed to generate a DCT
and DST of the binary subvector. The DCT and DST of the binary
subvectors of a vector BRd(n,N).sub.p or BId(n,N).sub.p are used to
determine the DCT and DST of the binary vector BRd(n,N).sub.p or
BId(n,N).sub.p.
[0026] In a prior art process for generating an OFDM symbol from a
data symbol vector d(n,N), generation of a DCT and DST transform of
the vector d(n) are generally the most computationally intense and
time consuming portion of the process. In an "optical OFDM modem",
in accordance with an embodiment of the present invention, the
optical signal processor generates the second spatial light
pattern, which is used to determine the DCT and DST of d(n,N), for
each of the at least one first spatial image substantially
instantaneously. As a result, an optical OFDM modem, in accordance
with an embodiment of the present invention, codes and decodes OFDM
symbols substantially faster than many prior art OFDM modems. An
OFDM communication network comprising optical modems, in accordance
with an embodiment of the present invention, therefore processes
data faster than many prior art OFDM communication networks.
[0027] In addition, an optical signal processor and thereby an OFDM
modem, in accordance with an embodiment of the present invention,
does not generally require that the number of data symbols in a
data symbol vector d(n,N) processed by the optical signal processor
be a multiple of two. As a result, data carrying capacity of an
OFDM communication network, in accordance with an embodiment of the
present invention, does not necessarily have to be doubled each
time an increase in data carrying capacity of the network is
desired. The OFDM system can therefore be relatively easily
expanded to accommodate increases in data carrying capacity.
[0028] There is therefore provided, in accordance with an
embodiment of the present invention a method for encoding a data
symbol vector in an OFDM symbol and decoding an OFDM symbol to
recover a data symbol vector encoded therein, the method
comprising: receiving a vector of values; generating at least one
input spatial light pattern responsive to the vector; generating
for each input spatial light pattern an output spatial light
pattern that is an interference pattern produced by light from the
input spatial light pattern; sensing the output spatial light
pattern at discrete spatial points and generating signals
responsive to the sensed light; and if the vector represents a data
symbol vector, using the signals to encode the data symbol vector
in an OFDM signal and if the vector represents an OFDM symbol,
using the signals to recover a data symbol vector encoded in the
OFDM symbol.
[0029] Optionally, generating at least one input spatial light
pattern responsive to the vector comprises partitioning the vector
into a plurality of data sub-vectors and generating at least one
input spatial light pattern for each sub-vector. Alternatively or
additionally, generating at least one input spatial light pattern
comprises expressing the vector in terms of binary vectors, where a
binary vector has components equal to one or zero and a vector
expressed in terms of binary vectors is equal to a sum of the
binary vectors each of which is multiplied by a different power of
two, and generating at least one spatial input light pattern for
each binary vector.
[0030] In some embodiments of the present invention, if the vector
comprises a complex component, generating at least one input
spatial light pattern comprises parsing the vector into a real and
an imaginary vector and generating at least one input spatial light
pattern for the real vector and at least one input spatial light
pattern for the imaginary vector.
[0031] In some embodiments of the present invention, processing the
signals to encode or decode the OFDM symbol comprises processing
the signals to determine a DCT and DST of the vector.
[0032] There is further provided in accordance with an embodiment
of the present invention a method for generating a DCT and DST of a
vector comprising: partitioning the vector into a plurality of
sub-vectors; generating at least one input spatial light pattern
responsive to each sub-vector; generating for each input spatial
light pattern an output spatial light pattern that is an
interference pattern produced by light from the input spatial light
pattern; sensing the output spatial light pattern at discrete
spatial points and generating signals responsive to the sensed
light; and processing the signals to generate the DCT and DST of
the vector.
[0033] In some embodiments of the present invention, generating the
at least one input spatial light pattern comprises providing a
plurality of point-like light sources and controlling each light
sources to radiate light at a desired intensity. Optionally, the
light sources are coplanar in a first plane.
[0034] Optionally, generating the output spatial light pattern
comprises generating a virtual mirror image for each light source
wherein the virtual images are reflections of the light sources
across a same mirror plane that is perpendicular to the first
plane.
[0035] Optionally, the method comprises positioning the light
sources in the first plane so that along a line that lies in the
first plane and is perpendicular to the mirror plane, each light
source has a projection at a different point on the line and a
distance between any two adjacent projection points is the
same.
[0036] Positioning the light sources optionally comprises
positioning the light sources along a straight line. Optionally the
straight line is perpendicular to the mirror plane.
[0037] In some embodiments of the present invention, the output
spatial light pattern is a light pattern in a second plane parallel
to the first plane at a distance "z" from the first plane.
[0038] Optionally, each of the light sources provides light having
a same characteristic wavelength ".lambda.".
[0039] Optionally, the plurality of light sources comprises N light
sources and the n-th light source, where n is an integer satisfying
0.ltoreq.n.ltoreq.N, is located at a distance xo+n.DELTA.x from the
image plane and the light is sensed at N points in the second
plane.
[0040] Optionally, the k-th point at which light is sensed, where k
is an integer satisfying 0.ltoreq.k.ltoreq.N, is located at a
distance from the image plane that is equal to k.DELTA..xi.,
(k+2N).DELTA..xi. or (2N-k).DELTA..xi. and wherein
.lambda.z=4N.DELTA.x.DELTA..xi..
[0041] The at least one input spatial light pattern, optionally,
comprises first and second input spatial light patterns and the
intensity of the n-th light source in the second input spatial
light pattern is substantially equal to the intensity of the (N-n)
light source of the first input spatial light pattern.
[0042] There is further provided, in accordance with an embodiment
if the present, invention an OFDM modem for coding a data symbol
vector into an OFDM symbol comprising: an optical processor having
an input port and an output port that receives the data symbol
vector and optically processes the data symbol vector to generate
signals at the optical processor's output port that are responsive
to a DCT and a DST of the data symbol vector; a signal processor
that receives the signals generated by the optical processor and
processes the signals to determine the DCT and DST of the data
symbol vector and generates the OFDM symbol from the DCT and
DST.
[0043] There is further provided, in accordance with an embodiment
of the present invention, an OFDM modem for decoding an OFDM symbol
and recovering a data symbol vector encoded therein comprising: a
signal processor that receives a first vector representing the OFDM
symbol and generates a second and a third vector therefrom, wherein
the second vector has components, each of which is proportional to
a sum of two different components of the first vector, and the
third vector has components, each of which is proportional to a
difference between two different components of the first vector;
and an optical processor that receives the second and third vectors
and optically processes the received vectors to generate signals
responsive to a DCT and a DST of the second vector and signals
responsive to a DCT and DST of the third vector; wherein the signal
processor receives the signals generated by the optical processor
and processes them to decode the OFDM symbol and recover the data
symbol vector encoded therein. Additionally or alternatively, the
optical processor is a shearing processor.
[0044] There is further provided, an OFDM communication network
comprising an OFDM modem in accordance with an embodiment of the
present invention.
BRIEF DESCRIPTION OF FIGURES
[0045] Non-limiting examples of embodiments of the present
invention are described below with reference to figures attached
hereto. In the figures, identical structures, elements, or parts
that appear in more than one figure are generally labeled with the
same numeral in all the figures in which they appear. Dimensions of
components and features shown in the figures are generally chosen
for convenience and clarity of presentation and are not necessarily
shown to scale. The figures are listed below.
[0046] FIG. 1 schematically shows an OFDM communication network in
which an OFDM symbol is being transmitted from a first OFDM modem
to a second OFDM modem, in accordance with an embodiment of the
present invention; and
[0047] FIG. 2 schematically shows a shearing processor suitable for
use in an OFDM modem in accordance with an embodiment of the
present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0048] FIG. 1 schematically shows an OFDM communication network 20
in which a first modem 22 generates an OFDM symbol that it
transmits to a second OFDM modem 24, in accordance with an
embodiment of the present invention. Modem 22 comprises a processor
26 that processes electronic signals, an optical processor 28 and
optionally an analogue front end (AFE) 30. Optionally, optical
processor 28 is a shearing processor similar to a shearing
processor described in Israel Applications 135576 and 141041. Modem
24 is optionally similar to modem 22 and comprises a processor 32,
an optical processor 34 and optionally an analogue front end (AFE)
36.
[0049] Modem 22 receives an N dimensional data symbol vector
d(n,N), schematically represented by a block arrow 23, in the form
of electronic signals generated by a data source (not shown). Data
symbol vector d(n,N) is routed to processor 26, which optionally,
generates at least one binary vector, generally a real binary
vector BRd(n,N).sub.p and an imaginary binary vector BId(n,N).sub.p
and, as might be required, suitable subvectors thereof, to
represent the data symbol vector.
[0050] Processor 26 transmits each binary vector, schematically
represented by a block arrow labeled BV, in the form of electronic
signals to shearing processor 28. Shearing processor 28 converts
the electronic signals into a preferably discrete spatial light
pattern and optically processes the light pattern to generate
electronic "transform" signals that are functions of a DCT and/or
DST transform of binary vector BV. The transform signals,
represented by a block arrow labeled TBV are input to processor 26.
Processor 26 processes transform signals TBV from vectors BV to
generate an OFDM signal expressed as a vector S(k,2N) of order 2N,
represented by a block arrow labeled S(k,2N), that encodes data
symbol vector d(n,N). S(k,2N) is input to analogue front end 30
which uses S(k,2N) to modulate a suitable carrier and generate a
signal XTR encoded with S(k,2N) that is transmitted to OFDM modem
24 over a suitable communication channel, represented by dashed
line 40 of communication network 20.
[0051] OFDM modem 24 receives a copy of transmitted signal XTR
after transmission via channel 40 as a received signal RCV.
Analogue front end 36 of OFDM modem 24 recovers a copy of S(k,2N)
from received signal RCV, using methods known in the art, which
copy is input to processor 32. Processor 32 generates two real
vectors of order N from S(k,2N). A first vector, RS(k,N) having
components RS(k)=[S(2N-k)+S(k)]/2 and a second vector IS(k,N)
having components IS(k)=[S(2N-k)-S(k)]/2. Processor 32 optionally
decomposes RS(k,N) into binary vectors BRS(k,N).sub.p, where 7 RS (
k , N ) = 0 ( B - 1 ) BRS ( n , N ) p 2 p
[0052] and IS(k,N) into binary vectors BIS(k,N).sub.p where 8 IS (
k , N ) = 0 ( B - 1 ) BIS ( n , N ) p 2 p .
[0053] Processor 32 inputs each binary vector BRS(k,N).sub.p and
BIS(k,N).sub.p generically represented by a block arrow BV in modem
24 into shearing processor 34. Shearing processor 32 optically
processes the vector and transmits transform signals TBV that are
functions of the DCT and DST of the binary vector to processor 32.
Processor 32 uses transform signals TBV responsive to binary
vectors BRS(k,N).sub.p generated for RS(k,N) to determine the real
parts of components d(n) of data symbol vector d(n,N). Processor 32
uses transform signals TBV responsive to binary vectors
BIS(kN).sub.p generated for IS(k,N) to determine the imaginary
parts of components d(n) of data symbol vector d(n,N). The real and
imaginary parts of d(n,N) determined by modem 24 are combined in an
output signal 38 provided by the modem, which encodes a copy of
d(n,N).
[0054] FIG. 2 schematically shows a shearing processor 50 suitable
for use in an OFDM modem, in accordance with an embodiment of the
present invention.
[0055] Shearing processor 50 is similar to a shearing processor
described in Israel applications 135576 and 141041. Shearing
processor 50 optionally comprises a planar mirror 52, an optionally
linear array 54 of N substantially evenly spaced light sources
LS(n), 0.ltoreq.n.ltoreq.(N-1), such as LEDs or VCSLs, and an
optionally linear array 56 of N evenly spaced light detectors
LD(k), 0.ltoreq.k.ltoreq.N. To prevent clutter, only some of light
sources LS(n) and light detectors LD(k) are labeled. Optionally,
linear array 54 of light sources LS(n) and linear array 56 of
detectors LD(k) are perpendicular to the plane of mirror 52.
[0056] For convenience, a coordinate system having its origin in
the plane of mirror 52 and x-axis passing through light sources
LS(n) is used to locate features of shearing processor 50.
Optionally, a straight line 58 that passes through light detectors
LD(k) passes through the z-axis of the coordinate system. Let
z.sub.o represent the intersection point of line 58 with the
z-axis.
[0057] Let .DELTA.x represent spacing between light sources LS(n)
and let x.sub.o represent the x coordinate of LS(0), so that x(n),
the x coordinate of LS(n) can be written x(n)=x.sub.o+n.DELTA.x.
Mirror 52 generates an array 60 of virtual light sources LS'(n)
that are mirror images of light sources LS(n). Whereas light from
two different light sources LS(n) is not coherent, light emitted by
a light source LS(N) is coherent with and substantially 1800 out of
phase with light "virtually" emitted by its mirror image light
source LS'(n). The 1800 phase difference is generated when light
from light source LS(n) is reflected from mirror 52.
[0058] Light from light source array 54 interferes with light from
its mirror image light source array 60 to generate an interference
pattern (not shown) in a plane 62 that is parallel to the xy plane
and passes through z.sub.o, in which plane light detectors LD(k)
are located.
[0059] Intensity of light in the interference pattern comprises a
DC bias intensity plus an intensity that varies with position in
z.sub.o plane 62. Assume that each light source LS(n) has an extent
along the x axis "a" and radiates light with an intensity y(n). A
vector y(n,N) of light intensities {y(0), y(1), . . . y(N-1)} at
which light sources LS(n) radiate light is used. to define a
spatial light pattern (i.e. a "first" spatial light pattern
generated by shearing processor 50) of light sources LS(n).
[0060] Let x coordinates of points in plane 62 be represented by
.xi. and let Y(.xi.) represent light intensity of the interference
pattern (a "second" spatial light pattern generated from a first
spatial light pattern by shearing processor 50) in the plane. If
.lambda. represents the wavelength of light emitted by light
sources LS(n) and if (2a.xi./.lambda.z.sub.o)<<1 then 9 Y ( )
= K + n = 0 N - 1 y ( n ) cos 4 ( x o + n x ) z o
[0061] where K is the DC bias intensity and the sum is the variable
intensity. (K depends only on the sum 10 n = 0 N - 1 y ( n )
[0062] and to a first approximation, intensity of the interference
pattern at a point in plane 62 is independent of the y component of
the point for magnitudes of the y component substantially equal to
values of .xi. for which (2a.xi./.lambda.z.sub.o)<<1.)
[0063] Expanding the cosine term in the above expression for
Y(.xi.) gives, 11 Y ( ) = K + [ cos 4 x O z O ] n = 0 N - 1 y ( n )
cos 4 n x z O - [ sin 4 x O z O ] n = 0 N - 1 y ( n ) sin 4 n x z O
.
[0064] Let the even spacing between light detectors LD(k) be
represented by .DELTA..xi. and assume that LD(0) is located at
(x=0, (i.e. .xi.=0), y=0, z=z.sub.o). The .xi. coordinate .xi.(k)
(i.e. the x coordinates in plane 62) of light detector LD(k) can
therefore be written .xi.(k)=k.DELTA..xi.. Replacing .xi. in the
above expression for Y(.xi.) with k.DELTA..xi. and defining
q=x.sub.o/.DELTA.x, intesity of light sensed by light detector
LD(k) can be written, 12 Y ( k ) = K + [ cos 4 kq x z O ] n = 0 N -
1 y ( n ) cos 4 nk x z O - [ sin 4 kq x z O ] n = 0 N - 1 y ( n )
sin 4 nk x z O .
[0065] The bias intensity K can be substantially removed from
signals generated by detectors LD(k) responsive to sensed light
intensity Y(k) using methods of processing the signals and/or
hardware known in the art. For convenience it is therefore assumed
hereinafter that K=0.
[0066] If spacing .DELTA.x between light sources LS(n) and spacing
.DELTA..xi. between light detectors LD(k) satisfy a relation, a
"matching condition", .lambda.z.sub.o=4N.DELTA.x.DELTA..xi. then,
13 Y ( k ) = [ cos 2 kq 2 N ] n = 0 N - 1 y ( n ) cos 2 nk 2 N - [
sin 2 kq 2 N ] n = 0 N - 1 y ( n ) sin 2 nk 2 N = [ cos 2 kq 2 N ]
DCT [ y ( n , N ) , k ] - [ sin 2 kq 2 N ] DST [ y ( n , N ) , k ]
.
[0067] Matching conditions for optical systems that provide
discrete transforms and effects of light source size (e.g. extent
"a" of a light source LS(n) along the x axis) light detector size
on matching conditions are discussed in PCT Publication WO
00/72105, the disclosure of which is incorporated herein by
reference.
[0068] Let Y'(k) represent intensity of light sensed by light
detector LD(k) when the spatial light intensity pattern of light
sources LS(n) is "reversed". In the reversed spatial light pattern
light source LS(n) emits light with intensity y(N-1-n) instead of
with intensity y(n) and the spatial pattern of light intensity of
light sources LS(n) is described by a vector of light intensity
y'(n,N) having components y'(n)=y(N-1-n). Then, 14 Y ' ( k ) = [
cos 2 k ( N - 1 + q ) 2 N ] n = 0 N - 1 y ( n ) cos 2 nk 2 N + [
sin 2 k ( N - 1 + q ) 2 N ] n = 0 N - 1 y ( n ) sin 2 nk 2 N = [
cos 2 k ( N - 1 + q ) 2 N ] DCT [ y ( n , N ) , k ] + [ sin 2 k ( N
- 1 + q ) 2 N ] DST [ y ( n , N ) , k ] .
[0069] From the above discussion it is seen that light intensities
Y(k) and Y'(k) sensed by light detector LD(k) are linear functions
of the discrete cosine and sine transforms DCT[y(n,N),k] and
DST[y(n,N),k]. As a result, DCT[y(n,N),k] and DST[y(n,N),k] are
linear functions of Y(k) and Y'(k) that can be determined by a
suitable processor using signals generated by detectors LD(n)
responsive to Y(k) and Y'(k).
[0070] Therefore, for any general N dimensional vector x(n,N) that
can be represented by a light intensity vector y(n,N), in
accordance with an embodiment of the present invention, the
discrete cosine and sine transforms of the vector can be determined
using shearing processor 50. Light sources LS(n) are first
controlled to emit light with intensities in accordance with a
vector y(n,N) and signals generated by each light detector LD(k)
responsive to intensity of light that it senses are recorded. Light
sources LS(n) are then controlled to emit light with intensities
determined in accordance with a reversed intensity vector y'(n,N),
and signals generated by each light detector LD(k) responsive to
intensity of light that it senses are again recorded. The recorded
signals are processed by a suitable processor to determine
DCT[y(n,N),k] and DST[y(n,N),k] and thereby DCT[x(n,N),k] and
DST[x(n,N),k] .
[0071] In terms of Y(k) and Y'(k),
DCT[x(n,N),k]=DCT[y(n,N),k]=.alpha.(k,q- )Y(k)+.beta.(k,q)Y'(k) and
DST[x(n,N),k]=DST[y(n,N),k]=.gamma.(k,q)Y(k)+.d- elta.(k,q)Y'(k)
where .alpha.(k,q) .beta.(k,q), .gamma.(k,q) and .delta.(k,q) are
coefficients that are dependent only on the parameter q and index
k. The coefficients are, of course, independent of y(n,N) and
values for the coefficients may therefore be calculated once and
stored in the processor for use for determining a DCT and DST for
any intensity vector y(n,N). The possibility of storing
coefficients .alpha.(k,q), .beta.(k,q), .gamma.(k,q) and
.delta.(k,q) reduces processing time required to determine DCTs and
DSTs for vectors optically processed by shearing processor 50.
[0072] In particular, shearing processor 50 can be used in an OFDM
modem, in accordance with an embodiment of the present invention,
to determine the DCT and DST respectively of the real and imaginary
parts, Rd(n,N) and Id(n,N), of a data symbol vector d(n,N) so as to
encode the data symbol vector in an OFDM symbol S(k,2N). For modems
that represent a data symbol vector d(n,N) using binary vectors
BRd(n,N).sub.p and BId(n,N).sub.p, in accordance with an embodiment
of the present invention, each binary vector is converted into
corresponding intensity vectors y(n,N) and y'(n,N). (To represent a
binary vector by an intensity vector y(n,N) each bit is represented
by intensity y(n) of a different corresponding light source LS(n).
Optionally, light sources LS(n) representing bits having a value
one are turned on to radiate light at a substantially same
predetermined intensity, while light sources LS(n) representing
bits having a value zero are turned off.) Vectors y(n,N) and
y'(n,N) are optically processed by shearing interferometer 50 to
determine the DCT or DST of the binary vector and the DCT and DST
of all the binary vectors are used by a suitable processor to
determine the DCT and DST of d(n,N).
[0073] It is noted that OFDM symbol S(k,2N) corresponding to d(n,N)
is determined for 0.ltoreq.k.ltoreq.(2N-1) and that therefore
DCT[Rd(n,N),k] and DST[Id(n,N),k] must similarly be determined for
0.ltoreq.k.ltoreq.(2N-1). However, S(k,2N) has a symmetry property
with respect to k=N such that for 0.ltoreq.k.ltoreq.(2N-1)
[0074] S(k,2N)=2{DCT[Rd(n,N),k]-DST[Id(n,N),k]} and
S(2N-k,2N)=2{DCT[Rd(n,N),k]+DST[Id(n,N),k]}.
[0075] As a result, to determine S(k,2N) for
0.ltoreq.k.ltoreq.(2N-1) it is sufficient to determine
DCT[Rd(n,N),k] and 2DST[Id(n,N),k] for 0.ltoreq.k.ltoreq.(N-1).
[0076] It is further noted that if q (i.e. x.sub.o/.DELTA.x) is an
integer and Q is any positive integer (which may be different from
N) and the matching condition .lambda.z.sub.o=4Q.DELTA.x.DELTA..xi.
is satisfied, then Y(k) is periodic with period 2Q and
Y(k)=Y(k+2Q). In addition Y(k) has a symmetry property that
Y(k)=Y(2Q-k). As a result, of the periodicity and symmetry of Y(k),
light detectors LD(k) can be located at convenient positions in
plane 62 other than positions .xi.=n.DELTA..xi. for which
0.ltoreq.k.ltoreq.(N-1) to determine values for Y(k). If q is not
an integer but a rational number, such that q=r/s where r and s are
integers, then Y(k) has a period equal to sQ and a symmetry
property Y(k)=Y(2sQ-k).
[0077] The above description tacitly assumes that a vector
represented by intensity vector y(n,N) is a positive vector, i.e.
all the components of the vector are either positive or zero.
Representing a vector by a spatial light intensity vector y(n,N) is
relatively straightforward for a vector having components that are
either positive or zero, or components that are either negative or
zero. A positive vector and its negative, i e. a vector that is
equal to the positive vector multiplied by minus one, are
represented by a same intensity vector y(n,N) in shearing processor
50. Shearing processor 50 does not differentiate between a positive
vector and its negative and provides a same DCT and DST for both
vectors. If a vector represented by y(n,N) is a negative vector,
the DCT and DST provided by processing signals from shearing
processor 50 is multiplied by minus one to provide the DCT and DST
of the vector.
[0078] However, for a "mixed" vector having both positive and
negative valued components the situation is less straightforward.
To process a mixed vector, the vector can be partitioned into
positive and negative vectors that are separately processed by
shearing processor 50. The results of processing by shearing
processor 50 are then added to provide a desired DCT or DST of the
mixed vector. Alternatively, a known vector can be added to the
mixed vector to generate a positive vector for which a DCT and DST
is provided by shearing processor 50. A DCT or DST, as appropriate,
of the known vector is then subtracted from the DCT and/or DST
provided by shearing processor 50 to provide the DCT and/or DST of
the mixed vector. Methods of representing negative numbers and
mixed vectors by spatial light patterns for processing by optical
processors are discussed in PCT Publication WO 00/72267, the
disclosure of which is incorporated herein by reference.
[0079] In some cases, a vector to be processed by a shearing
processor similar to shearing processor 50, in accordance with an
embodiment of the present invention, might have a number of
components larger than the number of the plurality of light sources
LS(n) comprised in the shearing processor. For example, in many
OFDM communication networks a data symbol vector typically
comprises 256 components and a shearing processor comprised in an
OFDM modem in accordance with an embodiment of the present
invention, might comprise 32 light sources LS(n).
[0080] For such cases, in accordance with an embodiment of the
present invention, the vector to be processed is partitioned into a
plurality of subvectors, each having a number of components equal
to the number of light sources in the shearing processor. (For
simplicity and convenience of presentation it is assumed that the
number of components in the vector is an integer multiple of the
number of light sources in the shearing processor. If the number of
components in the vector is not an integer multiple of the number
of light sources, at least one of the subvectors is "padded" with
suitable "filler" components having, for example, value zero or
other suitable "dummy" value.) Each subvector is then processed by
the shearing processor to determine the DCT and DST of the
subvector. The DCTs and DSTs of the subvectors are then used to
provide the DCT and DST of the vector.
[0081] For example, assume that a vector x(n,N) having components
x(n) is to be processed, in accordance with an embodiment of the
present invention, by a shearing processor having M light sources,
where N>M. Let the components x(n) be arrayed in an (L.times.M)
matrix for which N=LM, having row index l and column index m as
follows: 15 ( x ( 0 ) x ( L ) x ( m L ) x ( ( M - 1 ) L ) x ( 1 ) x
( L + 1 ) x ( m L + 1 ) x ( ( M - 1 ) L + 1 ) x ( l ) x ( L + l ) x
( m L + l ) x ( ( M - 1 ) L + l ) x ( L - 1 ) x ( 2 L - 1 ) x ( m L
+ L - 1 ) x ( M L - 1 ) ) .
[0082] In accordance with an embodiment of the present invention,
each row l of M elements in the matrix is a subvector of vector
x(n,N), which subvector is processed by the shearing processor to
determine the DCT and DST of x(n,N).
[0083] Let x.sub.l(m,M) represent the l-th subvector (i.e the l-th
row of elements in the matrix shown above),
(0.ltoreq.l.ltoreq.(L-1), of x(n,N). x.sub.l(m,M) has components
x.sub.l(m)=x(mL+l), 0.ltoreq.m.ltoreq.(M-1). It can be shown that,
16 DCT [ x ( n , N ) , k ] = l = 0 L - 1 DCT [ x l ( m , M ) , k '
] cos 2 kl 2 N + l = 0 L - 1 DST [ x l ( m , M ) , k ' ] sin 2 kl 2
N
[0084] and 17 DST [ x ( n , N ) , k ] = l = 0 L - 1 DCT [ x l ( m ,
M ) , k ' ] sin 2 kl 2 N + l = 0 L - 1 DST [ x l ( m , M ) , k ' ]
cos 2 kl 2 N ,
[0085] where k'=k(mod 2M).
[0086] If Y(l,k) and Y'(l,k), are intensities of an interference
pattern in plane 62 measured for the l-th subvector of vector
x(n,N) then DCT and DST of x(n,N) can be written, 18 DCT [ x ( n ,
N ) , k ] = l = 0 L - 1 ( k , l ) Y ( l , k ' ) + l = 0 L - 1 ( k ,
l ) Y ' ( l , k ' ) and DST [ x ( n , N ) , k ] = l = 0 L - 1 ( k ,
l ) Y ( l , k ' ) + l = 0 L - 1 ( k , l ) Y ' ( l , k ' ) .
[0087] The number of arithmetical operations required to post
process Y(l,k) and Y'(l,k) to determine the DCT and DST of the
vector x(n,N), in accordance with an embodiment of the present
invention, if the vector is partitioned into is L subvectors is
2N(4L-1). If the DCT and DST are determined using an FFT algorithm,
the number of arithmetical operations is 2Nlog.sub.2N. Therefore if
(4L-1)<log.sub.2N, less arithmetical operations are generally
required to determine the DCT and DST using a shearing processor
and a suitable electronic "post processor" that processes output of
the shearing processor than in using a DSP programmed with an FFT
algorithm. As a result, determining the DCT and DST of the vector
is generally faster using the shearing processor. Furthermore, it
is noted that post processing can be performed using hardware, in
which case, execution time for performing the DCT and DST with a
shearing processor, in accordance with an embodiment of the present
invention, is substantially equal to the processing time of the
shearing processor.
[0088] In the description and claims of the present application,
each of the verbs, "comprise" "include" and "have", and conjugates
thereof, are used to indicate that the object or objects of the
verb are not necessarily a complete listing of members, components,
elements or parts of the subject or subjects of the verb.
[0089] The present invention has been described using detailed
descriptions of embodiments thereof that are provided by way of
example and are not intended to limit the scope of the invention.
The described embodiments comprise different features, not all of
which are required in all embodiments of the invention. Some
embodiments of the present invention utilize only some of the
features or possible combinations of the features. Variations of
embodiments of the present invention that are described and
embodiments of the present invention comprising different
combinations of features noted in the described embodiments will
occur to persons of the art. The scope of the invention is limited
only by the following claims.
* * * * *