U.S. patent application number 11/641622 was filed with the patent office on 2007-07-26 for simultaneous two-way transmission of information signals in the same frequency band.
Invention is credited to Bernard Widrow.
Application Number | 20070173195 11/641622 |
Document ID | / |
Family ID | 26898186 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070173195 |
Kind Code |
A1 |
Widrow; Bernard |
July 26, 2007 |
Simultaneous two-way transmission of information signals in the
same frequency band
Abstract
This invention provides designs for communication systems that
use adaptive filters in circuits whose purpose is to enable two-way
transmission of information signals in the same frequency band at
the same time over twisted pair channels, coaxial cable channels,
fiber optic channels, or wireless channels. The methodology allows
two-way DSL transmission over telephone lines, making use of
existing DSL hardware and signal standards, so that the upload
speed is increased by an approximate factor of ten. Applied to
wireless systems with single antennas at the two ends of the
channel, a doubling of the data rate is achieved for a given
bandwidth. Applied to wireless systems with 2-way adaptive antenna
arrays at a central location and a 2-way adaptive antenna array at
each of a plurality of subscriber locations, the data rate for a
given bandwidth is increased by a large factor.
Inventors: |
Widrow; Bernard; (Stanford,
CA) |
Correspondence
Address: |
PERKINS COIE LLP
P.O. BOX 2168
MENLO PARK
CA
94026
US
|
Family ID: |
26898186 |
Appl. No.: |
11/641622 |
Filed: |
December 18, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09852469 |
May 9, 2001 |
7187907 |
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11641622 |
Dec 18, 2006 |
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60202974 |
May 9, 2000 |
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Current U.S.
Class: |
455/3.05 ;
455/15 |
Current CPC
Class: |
H04B 3/36 20130101; H04L
5/14 20130101 |
Class at
Publication: |
455/003.05 ;
455/015 |
International
Class: |
H04H 1/00 20060101
H04H001/00; H04B 7/14 20060101 H04B007/14 |
Claims
1. A two-way communication system, based on adaptive filtering, for
simultaneous transmission and reception of information signals
through a channel of coaxial cable or twisted pair cable
comprising: (a) a first source of data signals, a first data
receiver, and a first two-way terminus device connected by means of
its two-way terminal to the first end of said channel, connected by
means of its input terminal to said first source of data signals,
and connected by means of its output terminal to said first data
receiver; and (b) a second source of data signals, a second data
receiver, and a second two-way terminus device connected by means
of its two-way terminal to the second end of said channel,
connected by means of its input terminal to said second source of
data signals, and connected by means of its output terminal to said
second data receiver, so that signals can be sent from the first
source of data signals to the second receiver and from the second
source of data signals to the first receiver without
interference.
2. A two-way repeater amplifier device, based on adaptive
filtering, comprising: (a) a first cable or channel and a second
cable or channel; (b) a first two-way terminus device and a second
two-way terminus device; (c) a connection connecting the one end of
the first cable or channel to the two-way terminal of said first
two-way terminus device, and a connection connecting one end of the
second cable or channel to the two-way terminal of said second
two-way terminus device; and (d) a crisscross connection between
said first and second two-way terminus devices, said crisscross
connection connecting the output terminal of said first two-way
terminus device to the input terminal of said second two-way
terminus device, and connecting the output terminal of said second
two-way terminus device to the input terminal of said first two-way
terminus device.
3. The two-way repeater amplifier device of claim 2, wherein said
two-way terminus devices connected with a crisscross connection
have digitally-implemented adaptive filters and have at least one
sample time or one unit of delay along the closed-loop path which
includes said crisscross connection, said digitally-implemented
adaptive filters, and two difference amplifiers or signal
subtractors.
4. A two-way communication system, based on adaptive filtering, for
simultaneous transmission and reception of information signals
through a channel of coaxial cable or twisted pair cable requiring
repeater amplification comprising: (a) a first two-way terminus
device having an input terminal connected to input signal source A,
an output terminal for outputting an amplified signal B, and a
two-way terminal connected to the first end of a first coaxial
cable or twisted pair channel; (b) a two-way repeater amplifier
device whose first two-way terminal is connected to the second end
of said first channel and whose second two-way terminal is
connected to the first end of a second coaxial cable or twisted
pair channel; and (c) a second two-way terminus device whose
two-way terminal is connected to the second end of said second
coaxial cable or twisted pair channel, whose input terminal is
connected to input signal source B, and whose output terminal
outputs an amplified signal A.
5. The two-way communication system of claim 4, wherein said
coaxial cable or twisted pair channel incorporates two or more
two-way repeater amplifier devices for two way signal transmission
over long distances.
6. A two-way signal or information transmission system, based on
adaptive filtering and capable of transmission and reception of DSL
(Digital Subscriber Line) signals and simultaneously capable of
providing conventional telephone service over a conventional
twisted-pair telephone line, configured to utilize DSL signal
standards and DSL hardware such as DSLAM (DSL access multiplexer)
and DSL modems, comprising: (a) a telephone central office; (b) a
high-speed download-data stream, a low-speed download-data stream,
a high speed upload-data stream, and a low-speed upload-data
stream, and a two-way internet connection located at said telephone
central office, capable of downloading from the internet said
high-speed download-data stream and said low-speed download-data
stream, and capable of uploading to the internet said high-speed
upload-data stream and said low-speed upload-data stream; (c) a
first DSLAM located at said telephone central office whose input is
connected to said Internet connection to receive said high-speed
download-data stream, and whose output is connected to provide said
low-speed upload-data stream to the Internet connection; (d) a
first DSL modem located at said telephone central office whose
input is connected to said internet connection to receive said
low-speed download-data stream, and whose output is connected to
provide said high-speed upload-data stream to said internet
connection; (e) a first, second, and a third two-way terminus
device, a first signal summer device, a first POTS ("plain old
telephone service") splitter capable of passing through
high-frequency DSL signals while separating out low-frequency
telephone signals, and a telephone exchange switch, all located at
said telephone central office; (f) a connection between the two-way
terminal of said first DSLAM and the two-way terminal of said first
two-way terminus device, and a connection between the two-way
terminal of said first DSL modem and the two-way terminal of said
second two-way terminus device; (g) a connection between the output
terminal of said first terminus device and a first input of said
first summer device, a connection between the output terminal of
said second terminus device and a second input of said first summer
device, a connection between the output of said summer device and
the input terminal of said third two-way terminus device, and a
connection between the output terminal of said third way two-way
terminus device and the input terminals of both said first and
second two-way terminus devices; (h) a connection between the
two-way terminal of said third two-way terminus device and a first
wideband terminal of said first POTS splitter, and a connection
between the narrowband terminal of said first POTS splitter and the
telephone exchange switch; (i) a subscriber location, and said
twisted pair telephone line strung between the telephone central
office and said subscriber location; (j) a connection between said
telephone line and the second wideband terminal of said first POTS
splitter; (k) a computer capable of downloading and uploading
high-speed data streams, a second DSLAM, a second DSL modem, a
fourth, fifth and a sixth two-way terminus device, a second signal
summer, a second POTS splitter, and a standard telephone
instrument, all located at the said subscriber location; (l) a
first high-speed data stream, a first low-speed data stream, a
second high-speed data stream, a second low-speed data stream, a
first computer terminal connected to said computer for outputting
said first high-speed data stream, a connection between said first
computer terminal and the input terminal of said second DSLAM, a
second computer terminal connected to said computer for outputting
said first low-speed data stream, a connection between said second
computer terminal and the input terminal of said second DSL modem,
a third computer terminal connected to said computer for inputting
said second high-speed data stream, a connection between said third
computer terminal and the output terminal of said second DSL modem,
a fourth computer terminal connected to said computer for inputting
said second low-speed data stream, and a connection between said
fourth computer terminal and the output terminal of said second
DSLAM; (m) a connection between the two-way terminal of said second
DSLAM and the two-way terminal of said fourth two-way terminus
device, a connection between the two-way terminal of said second
DSL modem and the two-way terminal of said fifth two-way terminus
device, a connection between the output terminal of said fourth
two-way terminus device, a connection between the output terminal
of said fourth two-way terminus device and the first input of said
second signal summer, a connection between the output terminal of
said fifth two-way terminus device and the second input of said
second signal summer, a connection between the output of said
second summer and the input terminal of said sixth two-way terminus
device, and a connection between the output terminal of said sixth
two-way terminus device and the input terminals of both the said
fourth and fifth two-way terminus devices; and (n) a connection
between the two-way terminal of said sixth two-way terminus device
and a first wideband terminal of said second POTS splitter, a
connection between the second wideband terminal of said second POTS
splitter and said telephone line, and a connection between the
narrowband terminal of said second POTS splitter and said telephone
instrument.
7. A method for two-way transmission and reception of DSL signals
over conventional telephone lines using existing asymmetrical DSL
signal standards and existing DSL hardware so that upload and
download data rates will be equal to conventional download plus
upload rates, comprising the steps of: (a) receiving and
transmitting DSL and telephone signals with a telephone line at the
central office, said telephone line connecting said office to a
subscriber location; (b) separating said telephone signals from the
DSL signals, by means of a POTS splitter, for connection to a
telephone exchange switch; (c) separating said receiving and
transmitting DSL signals into receiving and transmitting data
streams by means of a first two-way terminus device; (d) processing
said receiving and transmitting data streams with bandpass filters
or with two-way terminus devices, applying the received signal to
both a DSLAM and a DSL modem, obtaining and combining
high-frequency and low-frequency transmitted signal components from
the said DSLAM and DSL modem for transmission to the said telephone
line through the first two-way terminus device, inputting said
high-frequency and low-frequency transmitted signal components from
an internet connection to the DSLAM and the DSL modem respectively,
connecting high and low frequency components of said received
signal from the DSL modem and the DSLAM respectively to the
internet connection; (e) separating the DSL signals from the
telephone signals at the subscriber location by means of a POTS
splitter; (f) utilizing the telephone signal by a conventional
telephone instrument; and (g) performing the same operations on the
DSL signal at the subscriber location as was done at the telephone
central office while substituting a computer with data transfer
interfaces in place of the said Internet connection.
8. A method for providing wireless two-way signal or information
transmission between a central antenna array and a plurality of
subscriber antenna arrays, all of said information transmission
taking place simultaneously in a single frequency band, said method
comprising the steps of: (a) connecting a plurality of two-way
subscriber's adaptive beamformers to said central antenna array,
the number of individuals beamformers being equal to the number of
subscribers, connecting sources of input baseband signals to all of
the signals input terminals of said two-way adaptive beamformers,
deriving the respective baseband output signals from the output
terminals of said two-way adaptive beamformers, providing mutually
uncorrelated random pilot signals to be used by the two-way
adaptive beamformers during training; (b) connecting a two-way
adaptive beamformer to each of the distant subscriber antenna
arrays, connecting a source of input baseband signals to the input
terminal of each subscriber's two-way adaptive beamformer, deriving
baseband output signals from the output terminal of each
subscriber's two-way adaptive beamformer, providing random pilot
signals to be used during training times for training the said
subscriber's two-way adaptive beamformers, said pilot signals being
mutually uncorrelated and uncorrelated with the pilot signals used
by all the beamformers connected to the central antenna array; and
(c) providing adaptive canceling filter means for subtracting all
transmitted signals from the radio receiver inputs of the two-way
adaptive beamformers connected to the central antenna array.
9. The method for providing wireless two-way signal or information
transmission of claim 8, wherein the step of connecting a plurality
of two-way adaptive beamformers to said central antenna array
comprises connecting said beamformers to an array of one or more
antenna elements, and wherein the step of connecting a two-way
adaptive beamformer to each of the distant subscriber antenna
arrays comprises connecting said beamformers to an array of one or
more antenna elements.
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 09/852,469, filed on May 9, 2001, which claims priority to
Provisional Application No. 60/202,974 filed May 9, 2000. These
applications are herein incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to the field of
telecommunications, and more particularly to the use of adaptive
filters in circuits which enable two-way transmission of
information signals in the same frequency band at the same time
over twisted pair channels, coaxial cable channels, fiber optic
channels, and wireless channels.
BACKGROUND OF THE INVENTION
[0003] At the present time, transmission of information via the
Internet, whether digital data, digital audio, digital video, or
other forms of data signals, is vital to the world's business.
[0004] These signals are carried by twisted-pair cable, coaxial
cable, filter optic cable, or by wireless radio or satellite
communication links. These channels may be narrow-band or
wide-band. In addition to the Internet, there are many other forms
of electronics communication channels, both analog and digital.
[0005] For many applications, there is need for two-way
simultaneous communication. Currently, this is done by separating
the inbound and outbound signals by placing them in different
frequency bands. In order to conserve bandwidth and increase
channel capacity, this invention provides means for two-way
transmission of information signals in the same channel, in the
same frequency band, at the same time.
[0006] Bi-directional amplification and communication systems. have
been proposed in the prior art. Separating inbound and outbound
signals by placing them in separate frequency bands is commonly
done (see for example U.S. Pat. No. 5,365,368). Separation of
inbound and outbound signals by transmitting them at mutually
exclusive times for radar and television applications is taught by
U.S. Pat. Nos. 5,105,166 and 4,714,959. In microwave radio systems,
separation can be achieved by utilizing horizontal and vertical
polarizations for inbound and outbound radiation (see U.S. Pat. No.
5,481,223). This approach is workable, except for transmission
through multipath where horizontal and vertical polarization
components would become mixed. A different approach is taken by
U.S. Pat. No. 5,119,365, which shows means for cancellation of
transmitted signal components that leak into the received signal
path at the head end of a bi-directional wire or cable
communication line. Further along this direction, U.S. Pat. No.
5,187,803 shows a means for cancellation of transmitted signal
components that leak into the received signal path within a
bi-directional amplifier located in the middle of a two-way wire or
cable communication line. The problem with the prior-art
cancellation methods is that they depend critically on analog
circuits whose component values must be adjusted, tuned, and
balanced to create cancellation. There is no automatic means for
initial tuning or for maintaining balance over time in the presence
of line and component impedance changes, generally due to
temperature changes and aging.
[0007] Limitations of the prior art are overcome by the methods of
this invention. Inbound and outbound signals are separated by means
of cancellation techniques, which are based on adaptive filtering.
Learning and self-adaptive circuits are used in combinations to
make initial tuning for cancellation automatic, and to continually
and automatically maintain the circuit balance necessary for
separation of inbound and outbound signals.
OBJECTS AND SUMMARY OF THE INVENTION
[0008] It is an object of this invention to provide designs for
components of communication systems that allow simultaneous two-way
signal and data transmission over the same transmission channel, in
the same band of frequencies. These components include two-way
terminal devices for the ends of the transmission channel. On each
end, they connect to sources of signal to be transmitted into the
channel, and they separately connect to receivers of signal
arriving from the channel. Also included are two-way repeater
amplifiers that may b e inserted into the channel at various
distances, if required, to compensate for transmission losses.
Included in addition are line tap circuits that allow "T"
connections to the transmission channel. Also included are schemes
for interconnection of three or more transmission lines with
various paths and directions for information flow. For wireless
channels, circuits are provided for full duplex operation in the
same frequency band. Channel capacity with multiple users can be
greatly increased by incorporating adaptive antenna arrays for
transmission and reception in the same frequency band. Fiber-optic
data transmission systems are described for two-way transmission
which include 2-way terminus devices for the ends of the channel
and 2-way repeater amplifiers, as may be required, to compensate
for signal loss. These systems afford multiple wavelength
transmission and they incorporate repeater amplifiers whose data
signal paths are all optical or optical and electronic. These and
other circuits for two-way communication systems are provided. They
all make use of adaptive filters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The foregoing and other objects of the invention will be
more clearly understood from the following detailed description
when read in conjunction with the accompanying drawings,
wherein:
[0010] FIGS. 1A-1B show an adaptive filter of the type used with
the invention, and a functional representation of it.
[0011] FIGS. 2A-2B show a system that can transmit data signals in
two directions at the same time in the same frequency band, and a
functional representation of it.
[0012] FIGS. 3A-3E show a system for two-way transmission of data
signals which includes a 2-way repeater amplifier, experimental
data showing learning, convergence, and low steady-state error for
simultaneous two-way signal transmission, a functional
representation of this system and a functional representation of a
system whose 2-way repeater includes circuits that de-modulate, and
recreate the digital baseband signals, then re-modulate and amplify
them for further transmission.
[0013] FIGS. 4A-4C show several methods for making "T"-connections
that allow two-way communication between three transmission
lines.
[0014] FIGS. 5A-5E show methods for making connections between
three or more transmission lines that provide communication paths
other than simple two-way links between all the transmission
lines.
[0015] FIGS. 6A-6B show two-way radio systems capable of
transmitting and receiving simultaneously with the same antenna in
the same frequency band.
[0016] FIGS. 7A-7B show a 2-way terminus utilizing an adaptive
filter in conjunction with a directional coupler, and a functional
representation of this terminus.
[0017] FIG. 8 shows a two-way signal and data transmission system
with means for supplying DC power over the transmission line for
the 2-way terminus devices and for the 2-way repeater
amplifier.
[0018] FIGS. 9A-9B show DSL (digital subscriber line) systems that
allow high-speed internet service in both directions and in the
same frequency band over a twisted-pair telephone line while at the
same time providing ordinary telephone service over the same
telephone line, with the high-frequency DSL signals separated from
low-frequency telephone signals in one case by means of capacitors
and inductors, or by means of telephone splitter devices.
[0019] FIG. 10 shows a two-way fiber-optic transmission system,
each end of the transmission line having a 2-way terminus that
converts signals from electronic to optical and optical to
electronic, and having a 2-way repeater amplifier in the middle of
the line that also converts signals from optical to electronic and
from electronic to optical.
[0020] FIG. 11 shows a two-way fiber-optic transmission system,
each end of the transmission line having a 2-way terminus that
converts signals from electronic to optical and from optical to
electronic, and having a 2-way repeater amplifier in the middle of
the line whose signal path is all optical and avoids conversion of
signals from electronic to optical and optical to electronic, and
whose adaptive canceling filters have electronically controlled
optical signal paths.
[0021] FIG. 12 shows an asymmetrical DSL (ADSL) hookup, simplified
by omitting the telephone circuits (prior art);
[0022] FIG. 13A shows a symmetrical DSL hookup using 2-way terminus
devices, band pass filters and resistor networks and standard ADSL
hardware and signal formats to couple a telephone line to the
internet at one end and to couple the telephone line to a
subscriber's computer at the other end, allowing both download and
upload speeds to be equal to the ADSL standard download speed plus
the ADSL standard upload speed.
[0023] FIG. 13B shows another symmetrical DSL hookup using three
2-way terminus devices at each end of the telephone line and
standard ADSL hardware and signal formats to couple the subscriber
computer to this line and to couple this line to the internet,
allowing both download and upload speeds to be equal to the ADSL
standard download speed plus the ADSL standard upload speed.
[0024] FIG. 14 shows a receiving adaptive beamformer whose main
beam points in the direction of a coded training signal or pilot
signal, this beamformer capable of forming nulls in the directions
of unwanted interference (prior art).
[0025] FIG. 15 shows a 2-way adaptive beamformer, able to learn to
place nulls in the directions of unwanted signals while receiving,
and having the same directivity pattern while transmitting.
[0026] FIG. 16 shows a central adaptive central adaptive beamformer
capable of simultaneous two-way communication on the same frequency
band with individual subscribers.
[0027] FIG. 17 shows a scheme for canceling many simultaneously
transmitted signals from the inputs of a bank of receivers at the
central adaptive beamformer site.
DESCRIPTION OF PREFERRED EMBODIMENT
[0028] FIGS. 1A and 1B show an adaptive filter of the type used in
the present invention. This filter has an input signal 1, and
output signal 2, and a special input called the "error" 21. The
impulse response of the filter is variable. This impulse response
is controlled by a set of variable coefficients or "weights",
w.sub.1k, 5, w.sub.2, 6, . . . . The values of the weights, in
turn, are controlled by an adaptive algorithm whose purpose is to
find the best combination of weight values so that the mean square
of the error is minimized. The weights are shown as circles, and
the arrows through them represent their variability. In FIG. 1B, a
functional diagram of the adaptive filter is shown, with an input
and an output like a conventional filter, but with the special
error input shown as an arrow through the adaptive filter
indicating the variability of the filter with the purpose of
minimizing the error.
[0029] Referring now to FIG. 1A, the input is digitized by an
analog-to-digital converter (ADC) 26, and then fed to a tapped
delay line. Unit delays are 10, 11, 12, . . . , and they are
designated by z.sup.-1, which is standard in the field of digital
signal processing. The input signal at the first tap is x.sub.k,
the signal at the second tap is x.sub.k-1, and so forth. The set of
signals at all the taps is represented by the vector X.sub.k. X k =
[ x k x k - 1 x k - n + 1 ] . ##EQU1## These signals are multiplied
by or weighted by the weights w.sub.1k, w.sub.2k, . . . . The
weight vector is represented by W k = [ w 1 .times. k w 2 .times. k
w nk ] . ##EQU2## The number of weights is n. The ADC 26 samples
the input regularly in time, and the time index or sample time
number is k.
[0030] The weighted signals are summed by the summer 15 to provide
a weighted sum signal y.sub.k, 29. The weighted sum y.sub.k can be
written as the inner product of the input signal vector and the
weight vector. That is, y.sub.k =X.sub.k.sup.T W.sub.k. The filter
output signal 2 is obtained from y.sub.k by digital-to-analog
conversion, by DAC 27. The DAC includes an analog low pass filter,
so that output 2 is a continuous signal.
[0031] A desired response signal 3 is generally supplied as a
training signal. Subtracting the filter output signal 2 from the
desired response 3 gives an error signal 21, that is used by the
adaptive algorithm to train or adapt the weights. The error signal
21 is digitalized by the ADC 28 to form the discrete error signal
e.sub.k, 20 for the adaptive algorithm. The mean square of the
error is known to be a quadratic function of the weights. This
function has a global minimum and no local minima. The method of
steepest descent is generally used to iteratively find the global
optimum.
[0032] The most widely used adaptive algorithm in the world is the
LMS algorithm of Widrow and Hoff (see B. Widrow and S. D. Stearns,
"Adaptive Signal Processing", New Jersey: Prentice-Hall, Inc.,
1985, incorporated herein by reference). This algorithm was
invented in 1959 and patented by B. Widrow and M. E. Hoff, Jr.
under U.S. Pat. No. 3,222,654. It is an iterative algorithm, based
on the method of steepest descent and is given by
W.sub.k+1=W.sub.k+2.mu.e.sub.kX.sub.k, where
e.sub.k=d.sub.k-y.sub.k. The parameter .mu. is chosen to control
rate of convergence and stability. This algorithm causes the weight
vector to converge in the mean to a Wiener solution, the best
linear least squares solution W*, given by W*=R.sup.-1 P, where
R=E.left brkt-bot.x.sub.kx.sub.k.sup.T.right brkt-bot. and P=E.left
brkt-bot.d.sub.kx.sub.k.sup.T.right brkt-bot.. The algorithm is
stable as long as 1>.mu.traceR>0. This is the condition for
convergence of the variance of the weight vector. Various proofs of
convergence and formulas for speed of convergence are given in the
literature. Typical convergence time of the adaptive filter is a
number of sample periods equal to ten times the number of weights
n, or about ten times the length of the filter impulse response.
Many algorithms other than LMS exist for adapting the weights and
can be used with the present invention. The literature is
extensive. An excellent summary is given by S. Haykin, "Adaptive
Filter Theory", Third Edition, Prentice-Hall, Englewood Cliffs,
N.J., 1996, incorporated herein by reference. This books describes
the recursive least squares algorithm (RLS) which is often used to
adapt an adaptive filter having a lattice architecture
[0033] The adaptive filter of FIG. 1B has an analog interface in
that it accepts an analog (continuous) input 1, and produces an
analog (continuous) output 2. The adaptive filter of FIG. 1A
converts the analog input to digital form, and converts its digital
output y.sub.k, 29, into analog form. The sampling rate of the
adaptive filter should be the Nyquist rate, or preferably several
times that, for the signals flowing through it. The filter of FIG.
1A could be built to directly accept an analog input however, and
then the ADC's 26, and 28, and DAC 27 could be eliminated. The
tapped delay line could be an analog delay line. An example is a
surface acoustic wave device (SAW). The LMS algorithm can be
implemented in continuous form. A way to do this is shown in B.
Widrow et. al., "Adaptive Antennas Systems", Proceedings of the
IEEE, Vol. 55, No. 12, December, 1967, pp 2143-2159, incorporated
herein by reference. The analog form of the LMS algorithm is
illustrated in FIGS. 7 and 8, page 2149.
[0034] An analog-input analog-output type of adaptive filter is
desirable for inclusion in most of the circuits of the present
invention. If, however, the input to the adaptive filter is already
in digital form, and a digital output is desired, then ADC's 26 and
27 and DAC 27 can be eliminated. The sampling rate of the data
signals flowing through the adaptive filter would need to be
synchronized with the clock rate of the adaptive filter itself,
however.
[0035] Further referring to FIG. 2A, the adaptive filter 36 would
need only a single weight if this filter were completely
implemented in analog form, if the delay unit 37 had no delay, if
the amplifier 39 had a perfectly flat frequency response with no
phase shift, if the resistor R.sub.C, 40 were purely resistive and
if the characteristic impedance of the transmission line 30 were
purely resistive, if the difference amplifier 41 had a perfectly
flat frequency response with no phase shift, and the analog
adaptive weighting coefficient of filter 36 had a flat frequency
response with no phase shift. Since these idealizations are not
perfectly realized in practice, better performance is obtained when
the delay 37 provides a small signal delay, and the adaptive filter
36 had more than a single weight. If this adaptive filter were
implemented as diagrammed in FIG. 1A as a digital realization with
an analog interface to the rest of the circuit, the
analog-to-digital converters 26 and 28, and the digital-to-analog
converter and low-pass filter 27 would need to have clock rates at
the Nyquist rate for the bandwidth of input signal 43, or
preferably several times that. Because of the above mentioned
circuit imperfections and imperfections in the ADC's 26 and 28, and
in the DAC circuit 27, more than one weight will generally be
required by adaptive filter 36 so that it can adjust its frequency
response to balance out and cancel the transmitted signal
components from the signal to be received.
[0036] There are many other forms of adaptive filter that could be
used in place of the adaptive filter shown in FIG. 1A. One of the
most widely used is the adaptive lattice filter adapted by the
recursive least squares (RLS) algorithm. Various lattice forms are
taught in the Widrow and Stearns book and in the Haykin Book. Other
adaptive filtering structures are also taught in these books, such
as adaptive filters with both adaptive feedforward and feedback
filters. Such filters can be used with the present invention.
[0037] Referring now to FIG. 2A, a circuit is shown that can
transmit signals or data in two directions at the same time via the
same communication channel, using the same frequency band for
transmission in both directions. The circuit contains an electric
transmission line, six operational amplifiers, two delay units, two
adaptive filters, and two line termination impedances. The
transmission line 30 could be twisted pair, coaxial cable, or
waveguide. At the left end of the channel, input A, 31 is applied.
After a transmission delay, this signal appears amplified as output
A, 34 at the right end of the channel. Input B, 33 is applied at
the right end of the channel, and it appears amplified as output B,
32 at the left end of the channel. Once the adaptive filters 35 and
36 adapt and converge, and remain converged by allowing their
respective adaptive algorithms to be continually executed, inputs A
and B can be simultaneously transmitted and received without
distortion and without interfering with each other.
[0038] Referring once again to FIG. 2A, the input B, signal 33, is
applied to operational amplifier 38, whose output drives a
small-delay unit .DELTA., 37, whose output drives another
operational amplifier 39 whose output drives the transmission line
30 through an impedance 40 whose value equals the characteristic
impedance R.sub.C of the transmission line. It is clear that input
B is driving the transmission line. Input B will be transmitted
through the transmission line and will be available for reception
at the left end. The amplifier 38 and 39 can have gains that are
greater than or less than unity, so a signal of any desired
amplitude can be transmitted. Depending on signal levels and
impedance levels, some of the amplifiers may be able to be omitted
and replaced with direct wires.
[0039] As shown in FIG. 2A, the operational amplifiers have
essentially infinite input impedances and zero output impedances,
and so the transmission line is properly terminated on its right
end. The same is true for the left end, since the associated
circuit is the same on the left as on the right. There will be no
reflections at the ends of the channel.
[0040] Operational amplifier 41 is connected to the right end of
the transmission line in order to obtain output A, 34. This
connection to the transmission line also allows amplifier 41 to
receive input B, which is troublesome. To solve the problem,
adaptive filter 36 is enlisted to cancel the input B components so
that they do not appear at the right end output mixed with the
output A signal. The adaptive filter is connected so that its input
43 receives the input B signal early in time, before going through
the small time delay .DELTA., 37. The transmission line receives
the input B signal after it goes through delay .DELTA.. This gives
the adaptive filter a slight head start in doing its processing. If
the head start is not necessary, the delay .DELTA., 37, could be
omitted. The output 44 of the adaptive filter is subtracted from
the transmission line signal by operational amplifier 41 in order
to cancel the input B signal from the transmission line signal. The
output 34 of amplifier 41 is used as the error signal for
adaptation of the filter 36. Minimizing the mean square error by
means of the adaptive algorithm minimizes the power of output
signal 34. Output signal 34 contains output A plus the uncancelled
residue of input B. Since input B and input A will be separate
trains of information and will therefore be uncorrelated with each
other, the power of signal 34 will be the sum of the respective
powers. Adapting the weights of the adaptive filter to minimize the
total power of output signal 34 will therefore minimize the residue
of input B contained in output A. So, input B is transmitted
without interfering with the reception of output A. In like manner,
at the other end of the channel, input A is transmitted without
interfering with the reception of output B.
[0041] FIG. 2B shows a functional diagram of the data transmission
system of FIG. 2A. The circuit at the right end of the
communication channel 30 in FIG. 2 A is symbolically represented by
the irregular hexagon 46 in FIG. 2B. This is a "2-way terminus".
Input B, 33, is transmitted, and output A, 34 is received. The
terminus is equipped to drive transmission line 30 and to receive
signals from it. At the left end of the transmission line is
another 2-way terminus 45, whose input is 31 and whose output is
32. The transmission system of FIG. 2 has been tested and works
well. The terminus units terminate the transmission line with its
characteristic impedance, so no reflections will take place at the
ends of the transmission line. The 2-way terminus units serve as
amplifiers and line drivers and provide ports for input and output
signals. The 2-way terminus device 45 has an input terminal 31, an
output terminal 32, and a 2-way terminal 47.
[0042] If attenuation from wave travel in the transmission line is
excessive, as will be the case with long distance transmission,
amplification along the line will be necessary. With 2-way signal
transmission, this presents a problem since repeater amplifiers are
normally unilateral. What is needed is a 2-way repeater
amplifier.
[0043] A design for such an amplifier is shown in connection with
the data transmission system of FIG. 3A. A 2-way terminus is
connected to transmission line 53, which in turn is connected to a
2-way repeater amplifier, which in turn is connected to
transmission line 54, which is connected to a 2-way terminus. Input
A, 50, is applied at the top of the diagram. It emerges as output
A, 86, at the bottom of the diagram. Likewise, input B, 85, is
applied at the bottom of the diagram and it emerges as output B,
51, at the top of the diagram. In the middle of the diagram is the
2-way repeater amplifier. On close examination, one can see that
the 2-way repeater consists of two 2-way terminus units connected
"back-to-back" by crisscross wires 77 and 78.
[0044] Following input A through the system, it is amplified by its
2-way terminus and then it drives transmission line 53. This signal
propagates through the line and arrives at the 2-way repeater. It
is amplified by operational amplifier 63 and outputted on line 77.
This line provides an input to a small-delay unit .DELTA., 66 whose
output drives operational amplifier 69. The output of this
amplifier drives transmission line 54 through impedance 64 whose
value is R.sub.C, the characteristic impedance of the line. The
signal travels through line 54 to the second 2-way terminus where
it is further amplified and outputted as output A, 86. In like
manner, input B travels through the system in the reverse
direction.
[0045] The adaptive filters 52 and 55 when converged, prevent
outbound signals from interfering with inbound signals in both of
the 2-way terminus units. The same function is served in the 2-way
repeater circuit by adaptive filters 60 and 61, when they are
converged. It should be noted that both ends of transmission lines
53 and 54 are properly terminated to prevent reflections.
[0046] The inputs of adaptive filters 60 and 61 are delayed by unit
delays 70 and 71. These delays are incorporated in the system so
that the closed-loop path starting with delay 70, through adaptive
filter 60, through amplifier 63, through crisscross wire 77,
through delay 71, through adaptive filter 61, through amplifier 62,
through crisscross wire 78, and back to delay 70 has at least one
unit of delay along this closed path. This is required for all
digital closed-loop systems. Accordingly, .DELTA. delays 67 and 66
must each have at least one unit of delay, and with more delay than
that, the adaptive filters will have head starts if this is needed.
The adaptive filters could be all analog or they could be
implemented as in FIG. 1A. They will generally need more than one
weight. They could also be adaptive lattice filters or they could
be adaptive feedforward and feedback filters.
[0047] The system of FIG. 3A has been tested by turning on input A
and input B and starting the adaptive processes. Convergence was
fast and sure, and transmission in both directions without
interference was observed. Test results are shown in FIGS. 3B and
3C. The error in transmission of signal A is obtained by
subtracting input A, 50 from output A, 86. The difference, "signal
A error", is plotted in FIG. 3B as a function of time. Time is
accounted for in terms of number of data samples that have traveled
through the system. At the beginning of time, the error is large,
but it becomes small exponentially as the adaptive filters learn.
After convergence, the residual is very small. FIG. 3C shows the
"signal B error" versus time. The residual error here is also very
small after the adaptive filters converge.
[0048] FIG. 3D is a functional diagram of the system of FIG. 3A.
The transmission lines 53 and 54 could be coaxial cable, twisted
pair, parallel wires, or waveguide. For long transmission lines,
more 2-way repeaters can be inserted to overcome transmission
loss.
[0049] The 2-way repeater amplifier has two terminals. They may be
called 2-way terminals. These terminals connect to the two
transmission lines 53 and 54 in FIG. 3D. They carry signals both
ways.
[0050] When digital data is transmitted over long transmission
lines with many repeater amplifiers, noise can accumulate and cause
bit errors. To avoid such errors, it is common in one-way
transmission systems to design repeaters that receive the
transmitted signal, equalize the line, demodulate and detect the
transmitted signal to recover the baseband bit stream, and then
re-modulate and amplify the signal for further transmission.
Equalization, demodulation, and detection are standard well-known
operations and are generally all done in one circuit or subsystem.
A basic reference on the subject is the book by R. Gitlin, et. al.,
"Data Communications Principles", Plenum Press, New York, 1992.
[0051] The same idea can be applied to two-way transmission
systems. A functional representation of such a system is shown in
FIG. 3E. The equalizer, demodulator, and detector function block,
and the re-modulator function block are located in the correct
places in the system so that these operations do not interfere with
adaptive functions that take place in the 2-way terminus devices
45, 91, 92, and 93. In the 2-way repeater, crisscross wire 95
carries the baseband input B signal, and crisscross wire 96 carries
the baseband input A signal.
[0052] There is often a need to connect three transmission lines
together. What is needed is a "T-connection" for 2-way
transmission. There are many ways to do this, three of which are
shown in FIG. 4. The purpose here is to provide a means of
connection that would allow two-way communication among the three
transmission lines.
[0053] The simplest connection is shown in FIG. 4A. This connection
allows communication in the direction indicated by the arrows. The
three impedances 103, 104, 105 have values equal to R.sub.C, the
characteristic impedance of lines 100, 101, and 102. Accordingly,
all three lines are properly terminated to prevent reflections. The
coupling is passive, and signal losses of 6 dB will be experienced
when transmitting from one transmission line to another.
[0054] In order not to loose signal level and indeed increase
signal level when coupling line 102 to either 100 or 101, an active
coupling of the type shown in FIG. 4B could be used. The coupling
means includes a 2-way terminus 108 and a controlled current source
110. Connecting these to the direct wire between lines 100 and 101
with the two-way terminus connected to line 102 as shown leaves all
three lines properly terminated in the characteristic impedance
R.sub.C, thus there will be no reflections. Lines 100 and 101 will
be able to directly communicate two-ways. Line 102 will be driven
by the 2-way terminus 108 with a signal originating either from
line 100 or line 102. A signal arriving from line 102 appearing at
the output 109 of the 2-way terminus will control the current
source 110 and cause a signal to propagate in both lines 100 and
101, Thus all three lines will be able to communicate two-way with
each other. Each line will be able to transmit to the other two and
will be able to receive whatever is transmitted by the other two.
The arrows indicate the allowed directions of signal
transmission.
[0055] Another way to make this connection is shown in FIG. 4C.
This is a symmetrical active coupling permitting amplification in
each direction. Three 2-way terminus units, 120, 121, and 122 are
utilized. The output signals of each pair of terminus units are
added by summers 135, 136 and 137. The sums drive the inputs of the
terminus units which are connected to transmission lines 100, 101
and 102. This connection allows 2-way communication among the three
transmission lines, as before. The circuit of FIG. 4C is a 2-way
repeater amplifier in the form of a T-junction, enabling the
interconnection of three transmission lines. The same idea can be
used to interconnect four or more transmission lines.
[0056] FIG. 5 shows examples of other ways of connecting three
cables together in order to realize unusual communication paths.
FIG. 5A shows a connection that allows two-way communication
between cables 100 and 101, and two-way communication between
cables 100 and 102, but no communication in either direction
between cables 102 and 101. FIG. 5B shows a connection that allows
two-way communication between cables 100 and 101, but only allows
one-way communication from cable 102 to cable 101, and one-way
communication from cable 100 to 102. FIG. 5C shows a circuit that
affords a set of only one-way communication links from cable 100 to
cable 102, from cable 102 to cable 101, and from cable 101 to cable
100. FIG. 5D shows a circuit that allows two-way communication
between cables 100 and 101, one-way communication from cable 100 to
cable 102, one-way communication from cable 102 to cable 101, and a
return echo path from cable 102 back to cable 102. These are
examples that give some idea of what can be done with
interconnection of three cables, each carrying signals and data in
two directions. These connections can be switched, mechanically or
electronically, to change the communication paths as desired. The
same methods can be used for joining four cables, or more.
[0057] FIG. 5E shows an application of these methods to a simple
communication network. At the head end of the network, there is a
2-way terminus 140 that allows two-way connection to the main line.
Each of the T-connections 142 is configured like the circuit of
FIG. 5A, and connected to a set of 2-way terminus units 144 via
cables 143. The terminus units allow one to connect information
devices 145, such as computers, to the network. The head end is
able to have two-way communication with all the information
devices, according to this configuration, yet the information
devices cannot communicate with each other. Information security
requirements might require information paths like those of FIG. 5A.
Other networks can of course be realized by using the principles
taught by the diagrams of FIG. 5.
[0058] FIG. 6A shows a two-way radio system capable of transmitting
and receiving simultaneously with the same antenna in the same
frequency band. The received signal comes from a distant
transmitter and carries information that is independent of the
baseband signal to be transmitted 150. The received signal drives
the coil 161, which acts as the primary of an RF transformer. This
coil couples with coil 162 to provide an RF input to the radio
receiver 168. Subtracted from this RF input by summer 172 is a
signal 170, which is made equal to an RF interference signal that
comes from the transmitter. The transmitter couples from coil 160
to coil 161 and feeds the antenna. At the same time, coil 160
couples with coil 161, and in this way introduces strong
interference into the radio receiver input. An adaptive filter 166
produces the canceling signal 170 to remove the interference from
the radio receiver input. This is critical because the weak signal
to be received is in the same frequency band as is the strong
interference from the transmitter.
[0059] The baseband information signal 150 to be transmitted is fed
to a modulator 151 whose output is the RF signal to be transmitted.
The output 165 of the modulator is fed to a delay unit 152 whose
output goes to the final RF power amplifier 154 that drives the
antenna 171 through coils 160 and 161. The modulator output 165 is
further provided as an input signal to the adaptive filter 166. The
delay 152 provides a small time delay that compensates for the
delay through the coupling transformer, coils 160, 161 and 162, and
gives the adaptive filter a small head start in processing time.
The adaptive filter filters the RF output of the modulator and
provides the canceling signal 170. The error signal 167 of the
adaptive filter is actually the input signal to the radio receiver.
The adaptive filter minimizes the mean square of signal 167. The
power of signal 167 is the sum of the powers of two uncorrelated
components, the received RF signal and the uncancelled residue of
the transmitted signal. Minimizing mean square error minimizes the
power of the residue of the transmitter interference.
[0060] The length of the impulse response of the adaptive filter
166 is proportional to the number of its weights. The length of
this impulse response minus the delay time of 152 determines the
time extent over which the adaptive canceller will cancel received
echoes reflecting from structures near the antenna. The adaptive
filter cancels the transmitted signal and its near reflections that
arrive within a time window determined by the length of the impulse
response of filter 166. The system designer can choose the length
of this impulse response. The longer the impulse response however,
the more adaptive weights will be used and the slower will be the
convergence of the adaptive filter. Convergence time is
proportional to the number of weights.
[0061] When the transmitter power is very high and when the
received signal is very faint, the use of a directional coupler or
"hybrid device" 180 of FIG. 6B improves performance. Substituting
the directional coupler for the transformer coils 160, 161, 161 of
FIG. 6A, the transmitter interference at the receiver input could
be reduced by 30-40 dB before the adaptive filter begins to cancel
residue. Transfer of power from the transmitter output stage 154 to
the antenna would not be significantly impaired. Transfer of
received power from antenna to the radio receiver would also not be
significantly impaired. The rest of the circuit of FIG. 6B works
exactly like the rest of the circuit of FIG. 6A. The directional
coupler of FIG. 6B has three ports or sets of terminals. A 2-way
terminal connects to the antenna 171. An input terminal connects to
the RF amplifier 154. An output terminal connects to the summer 185
via line 187. Directions of signal flow are indicated by the
arrows.
[0062] Many other ways exist for coupling transmitters and
receivers to the same antenna. The ideas taught here apply to them
also and are not restricted only to the two coupling means that are
illustrated in FIGS. 6A and 6B.
[0063] Using two transmitter/receiver units of the type shown in
FIG. 6, full duplex operation in the same frequency band is
possible. Since separate frequency bands would not be needed for
transmitting and receiving, two-way communication requires only
half the bandwidth. An application for this is in cellular
telephony. Telephone traffic within a given cell could be doubled.
This could be very significant for all forms of two-way wireless
Internet traffic.
[0064] The circuits of FIG. 6 could also be used in Doppler radar
systems of the pulsed type or the continuous wave (CW) type. All
the circuit components of FIG. 6 can be realized with microwave
devices. The Doppler return echo is of a slightly different
frequency from that of the transmitted RF energy and is
uncorrelated with it.
[0065] In the CW case, the adaptive filter or the combination of
directional coupler and adaptive filter would remove the
interference from the transmitter before it enters the receiver. In
the pulsed radar case, the transmitted pulse would be removed from
the receiver input so that close-in echoes could be detected even
though their arrival takes place before the transmitted pulse
stops.
[0066] The circuits of FIG. 6 use a single antenna for transmitting
and receiving at each end of the wireless channel. Transmitting and
receiving are done simultaneously in the same frequency band so
that the utilization of the band is doubled. This utilization
factor can be increased considerably for many users operating in a
local geographic area who are communicating simultaneously in the
same band by replacing each single antenna with an antenna array
connected to comprise an adaptive beamformer.
[0067] Adaptive beamformers or adaptive antennas of the type
preferred herein were first described in the paper "Adaptive
Antenna Systems," by B. Widrow, P. E. Mantey, L. J. Griffiths, and
B. B. Goode, published in the Proceedings of the IEEE, Vol. 55, No.
12, December 1967, pp. 2143-2159. This and other forms of adaptive
antennas are described in the Widrow and Stearns book, Chapters 13
and 14. Many other books and papers have since been published on
these subjects.
[0068] Referring to FIG. 14, an adaptive beamformer of the
preferred type is diagrammed. The objective of this system is that
the adaptive beamformer 743 receive a signal from source A, 725, in
spite of strong interference in the same band from sources B, 726,
and C, 727. The adaptive beamformer receives signal from an array
of antenna elements 730, 731, and 732. More antenna elements can be
used as required. The antenna elements, which could be dipole
antennas, are connected to supply signals to adaptive filters 733,
734, and 735. The outputs of the adaptive filters are summed by
summer 737 to form the input signal for radio receiver 740. The
baseband output of the adaptive beamformer is the radio output 741.
If signal C, 725, is coded with a known random code, and the same
coded signal 739 is used as the desired summed response of the
adaptive filters, the filters will automatically train themselves
to cause the adaptive array to receive the coded signal 725 from
source A. An antenna array receives a signal by pointing a main
beam at it. In this case, the main beam 720 is formed by the
adaptive process. The interfering signals B, 726, and C, 724, being
independent of and therefore uncorrelated with the coded signal
739, will be rejected by the beamformer. An antenna array rejects a
signal by placing a null in its direction. Thus nulls 722 and 723
form automatically, as a result of the adaptive process, in the
directions of the interferences 726 and 727. In addition to the
main beam 720 and the nulls 722 and 723, the antenna pattern
includes "sidelobes" 721. The shape of the antenna pattern develops
from the adaptive process to reproduce the coded signal (which is
the desired response 739) as best possible in the least squares
sense. This to implies reproduction of the coded signal 725 and
elimination of the interferences 726 and 727. The main beam points
at source A, the signal of interest, and nulls are pointed at
sources B and C, the interferers.
[0069] This works even in the presence of severe multipath. The
goal is seeking the coded signal and rejecting all else by the
action of adaptive filtering. The desired signal will be received
and the undesired signals will be rejected even though the main
beam would not look "beam-like" and the nulls would not look
"null-like", and this would be perfectly fine as long as the goal
is achieved. For sake of discussion, the adaptive antenna methods
and algorithms will be described below in terms of idealized main
beams and nulls.
[0070] The coded signal of source A is often called a "pilot"
signal. It is used to train the adaptive beamformer to reproduce
signal A and to reject all else as well as possible. The pilot
signal could be turned on at predetermined times, known to the
adaptive beamformer, which turns on the coded pilot signal 739 at
the same times for training. No information is transferred during
the training episodes. At other times, no training takes place and
the weights of the adaptive filters remain fixed while data is
conveyed by the wireless link. There are methods for training while
data is transferred, and the pilot signal is not turned on, methods
such as decision directed learning and constant modulus algorithms.
These are well known in the adaptive filtering literature. A good
reference is the book by Simon Haykin, "Adaptive Filter Theory,"
third edition, Prentice Hall, 1996.
[0071] The adaptive beamformer of FIG. 14 is intended for receiving
only. Referring to FIG. 15, a 2-way adaptive beamformer 752 is
shown. It makes use of the principle of reciprocity to transmit and
receive with the same antenna directivity pattern. Thus, nulls that
are learned from receiving also appear in the transmitting pattern
and cause essentially no interference to be transmitted by the
2-way adaptive beamformer toward its sources of interference. The
same antenna pattern exists for both transmitting and receiving
because both transmitting and receiving are using the same
frequency band with the same array of antenna elements with the
same filtering.
[0072] The receiving portion of the system of FIG. 15 is very
similar to that of FIG. 14. The transmitting portion of the system
is introduced here together with the adaptive filter 771 whose
purpose is to subtract the transmitted signal from the radio
receiver input 779. Two adaptive beamformers, A and B, can
communicate with full duplex in spite of the presence of the
interfering signal C, 776, once the two adaptive beamformers have
been trained to receive each other. After having learned to receive
each other, they will then be able to transmit to each other
without further adaptation. It is not even necessary for either one
of them to have knowledge of the location and direction of the
other party, or of the sources of interference. Antenna A would
only need to know the code of the pilot signal transmitted by B,
and B would only need to know the code of the pilot signal
transmitted by A. The different pilot signals should be designed to
be mutually uncorrelated. They will find each other, and eliminate
interference, even in the presence of multipath.
[0073] Referring now to FIG. 15, the antenna elements of beamformer
B, 754, 755, 756, 757, and others, are coupled to adaptive filters
761, 762, and others. Coupling is through transformers such as 758
and 759, but many other coupling networks could be used. It is
important, however, that all coupling networks be identical in
order to have the same antenna directivity pattern for transmitting
and receiving, in accord with the principle of reciprocity. The
outputs of adaptive filters 761, 762, etc. are summed, and the
resulting signal 765 provides an input to the radio receiver 768.
Its output is the received baseband signal 770. This is the
receiving path, from the antenna elements to the radio receiver
output. The transmitting path begins with the transmitted baseband
signal 769, which is applied to the RF modulator 767, whose output
774 is the input to a set of controlled filters, including 760 and
763. Corresponding to each adaptive filter such as 761 and 762 in
the receiving path, there are controlled filters in the
transmitting path (such as 760 and 763). The outputs of the
controlled filters couple to the corresponding coupling networks
(such as 758 and 759) to the corresponding antenna elements (such
as 754 and 757). The controlled filters are identical to the
adaptive filters in having the same architecture and the same
adjustable parameters. The parameters of the controlled filters are
set at every moment to correspond exactly (digital copies) to the
corresponding parameters of the corresponding adaptive filters. The
receiving system learns to form the receiving directivity pattern,
and thereby controls and makes identical the directivity pattern of
the transmitting system. The relative phases and relative
amplitudes, at all frequencies in the band, of the currents driving
the antenna elements when transmitting will be identical to the
relative phases and relative amplitudes, at all frequencies in the
band, of the received voltages from the antenna elements when
receiving. In accord with reciprocity theory, this causes the
directivity pattern to be the same for transmitting and receiving,
at each frequency in the band.
[0074] Referring now to FIG. 16, a wireless communication system
that provides 2-way communication between a central array 801, and
a set of subscriber arrays 802, 803, and 804, is shown. The central
array is mounted on a local tower 800. The subscriber arrays only
communicate with the central array and do not communicate with each
other directly. Thus there are three 2-way communication paths
shown in FIG. 16, and if each path had its own frequency band, six
frequency bands would be required. However, if each subscriber
array were equipped with 2-way adaptive beamformers, and if the
central array were equipped with three separate 2-way adaptive
beamformers, only a single frequency band would be required to
achieve the same objective.
[0075] Antenna directivity patterns are drawn for each of the
subscriber arrays 802, 803, and 804, in FIG. 16. The main lobes all
point toward the central array 801. For array 802, nulls appear in
the directions of arrays 803 and 804. Its main lobe points at the
central array 801 because, during training, the pilot signal for
adaptive beamformer (ABF) 810 is identical to the pilot signal
transmitted by adaptive beamformer 807 connected to array 801. The
nulls form the directions of arrays 803 and 804 because these
arrays transmit coded signals during the training period that are
uncorrelated with the pilot signal of adaptive beamformer 810. For
like reasons, nulls form in the directivity patterns of arrays 803
and 804 in the direction of the other subscriber arrays, and main
lobes form in the direction of the central array. Separate adaptive
beamformers are connected to central array 801 to provide
communications with each of the subscriber arrays. Adaptive
beamformer 807 communicates two-ways with adaptive beamformer 810,
and so forth. The directivity pattern 815 shown in FIG. 16 is that
of adaptive beamformer 806, resulting from communication with array
803. Two other directivity patterns, for adaptive beamformers 805
and 807, are not shown in the figure. The nulls in one of the three
directivity patterns of the central array allow communication
between this array and the array of the selected subscriber,
without radiating the transmitted signal toward the other
subscribers and interfering with them. Three separate directivity
patterns are needed for the central array to independently
communicate with the three subscribers. The transmitted signals of
each of the subscribers are not received by the other subscribers
because of the nulls in their directivity patterns. Thus, the
subscribers do not interfere with each other. The adaptive
beamformers connected to the central array communicate with their
respective selected subscriber adaptive beamformers also without
interference.
[0076] There is one question that remains, about the three
transmitters in the three beamformers 805, 806, and 807, all
transmitting through the same central array 801 while their
respective receivers are simultaneously receiving. Each receiver
must have each of the transmitted signals removed from its input.
FIG. 17 shows how this would be done for the case of two 2-way
adaptive beamformers operating with the same antenna array 840.
[0077] Referring to FIG. 17, radio receiver 828 of adaptive
beamformer 825 has signals from its own RF modulator 827 removed
from its input 828 by the action of adaptive filter 831, and from
the RF modulator 829 by the action of adaptive filter 832. Both of
these adaptive filters adapt to minimize the mean square of the
common error signal 837, which is the input of radio receiver 828.
By minimizing this error, the transmitted signals leaking back into
the radio receiver are removed. The same result is obtained within
the 2-way adaptive beamformer 826, and would be obtained within any
other adaptive beamformer connected to antenna array 840. The same
method would be used to cancel the transmitted signals from the
radio receiver inputs. This would need to be done with the 2-way
adaptive beamformers 805, 806, and 807, shown in FIG. 16.
[0078] FIG. 16 shows a central array communicating with a cluster
of three subscribers. One frequency band would be used for this
instead of six bands. The utilization factor for the band is six
fold. If a cluster of 10 subscribers were communicating with a
central array, the utilization factor would be 20, and so
forth.
[0079] This 2-way adaptive beamformer scheme could be used with
signals having a variety of modulation types, such as TDMA, CDMA,
etc. Whatever efficiency that they achieve in the spectrum usage,
the rate of data transmission would be multiplied by the above
utilization factor.
[0080] A directional coupler like the one used in FIG. 6B could be
included in the 2-way terminus circuit of FIG. 2A, and this could
enhance the performance of this circuit and that of the 2-way
repeater circuit of FIG. 3 A that is based on it. FIG. 7A shows a
2-way terminus device using a directional coupler 201 together with
an adaptive filter 204. The 2-way terminal of the directional
coupler connects to transmission line 200. Its input terminal
connects to impedance 202. Its output terminal connects to
impedance 203. The adaptive filter will typically need more than a
single weight. The impedance level of the directional coupler is
chosen to be equal to the characteristic impedance of transmission
line 200. Accordingly, the transmission line 200 connected to the
directional coupler will be properly terminated with its
characteristic impedance R.sub.C. A symbolic diagram of the 2-way
terminus 210 incorporating a directional coupler (or hybrid) with
an adaptive filter is shown in FIG. 7 B having the designation "DC"
on the terminus 210.
[0081] When using 2-way terminus devices and 2-way repeater
amplifiers in connection with coaxial cable networks and with other
copper transmission circuits such as telephone lines, it is often
convenient to power these devices with DC current carried by the
cable and telephone transmission lines themselves. A way of doing
this is shown in FIG. 8. A DC power supply 230 is located near
2-way terminus 90 and provides its power. The same supply is
connected to the transmission line 53 through the inductor L, 232,
providing a DC component of voltage on line 53. The 2-way terminus
90 is also connected to line 53 with capacitor C, 231. The
capacitor C is an open circuit at DC, and no DC current flows into
the 2-way terminus through it. At the high frequencies of signal
input A, 50, capacitor C acts like a short circuit and provides a
direct connection to cable 53. At these frequencies, inductor L,
232, acts like an open circuit and does not bypass the signal to
ground. Further along line 53, at the location of the first 2-way
repeater amplifier, the line is connected to a DC regulator 235
through inductor L, 236. The regulator supplies power to the 2-way
terminus devices 91 and 92. The DC voltage on line 53 is received
by the regulator since the inductor L, 236, is essentially a short
circuit at zero frequency. At high frequency, it acts like an open
circuit and does not bypass the high frequency signals to ground.
The capacitor C, 237 blocks the DC voltage of line 53 from entering
the 2-way terminus 91. The inductor L, 239 serves as a DC bridge
across the 2-way repeater, applying a DC voltage to transmission
line 54. Capacitor C, 240, blocks this DC voltage from the input of
the 2-way terminus 92. At signal frequencies, the capacitor acts
like a short circuit, making a direct high-frequency connection to
line 54. If there were a second 2-way repeater further along the
line, it would be wired in the same way as the first 2-way
repeater, and so forth. At the end of the line, 2-way terminus 93
is wired with inductor L, 251, connected to pass DC and block
signal frequencies, and capacitor C, 250 is connected to block DC
and pass signal frequencies as was done at the other end of the
line. A DC regulator 252 receives the DC voltage from line 54, and
supplies power to the 2-way terminus 93.
[0082] The DC power supply 320 could alternatively be connected
through an inductor L to any point on the line to supply DC power
to all of active devices in the system, all along the line. The
same principles would be used to insure DC continuity along the
line, proper DC supply voltages to the active devices, and solid
high-frequency connections between the active devices and the
transmission lines.
[0083] A wideband Internet service over conventional twisted-pair
telephone lines known as DSL (digital subscriber line) could
benefit from the two-way communication methods of the present
invention. Conventional DSL uses different frequency bands for the
two directions. An increased bandwidth would result from
transmitting two ways in the same band of frequencies at the same
time. FIGS. 9A and 9B show means for doing this.
[0084] Conventional DSL uses the telephone line for two different
purposes. At low frequencies, up to about 3 kHz, the line is used
for "dial-up" telephone service in the usual way. At higher
frequencies, the line is used for constant high-speed Internet
connection. The circuits of FIGS. 9A and 9B do all of this except
that they effectively double the transmission rate of the Internet
connection.
[0085] In FIG. 9 A, the telephone line 310 is connected to the
telephone exchange switch 300 through inductor 303. The inductor
passes the low frequency telephone signal to line 310. Further
along line 310, there may be a 2-way repeater to compensate for
line signal loss. The low frequency telephone signal bypasses the
repeater through inductor 311 and connects to the line 320. The
conventional touch-tone or dial telephone 335 connects to line 320
through inductor 334. Thus at the low frequencies of the telephone
signal, telephone 335 is connected to the telephone exchange switch
300 and operates in a normal way. Dial tone and the ringing signal
are carried over this path in the usual way.
[0086] The high-frequency Internet signals are superposed on lines
310 and 320 through capacitors 304, 312, 317 and 320. These
capacitors are open circuits at the telephone signal frequencies,
but act as short circuits at the digital Internet signal
frequencies. The internet connection 301 couples in both directions
through DSL modulator 305 and through DSL equalizer, demodulator,
and detector circuit 306 to the telephone line 310 through the
2-way terminus 302 and through capacitor 304. The internet signal
couples through the 2-way repeater to line 320. Inductor 311 is an
open-circuit to the internet signal. The internet signal couples in
both directions through DSL modulator 307 and through DSL
equalizer, demodulator, and detector circuit 308 to computer 333
through capacitor 330 and the 2-way terminus 332. The inductor 334
keeps the internet signal away from telephone 335. The computer 333
is in constant two-way communication with the Internet, without
experiencing any interference from the telephone operation.
Likewise, the telephone can be used normally while the computer is
logged into the Internet.
[0087] In FIG. 9B, the coupling capacitors and inductors that are
used in FIG. 9A to separate the high-frequency DSL signals from the
low-frequency telephone signals are replaced by frequency-sensitive
directional couplers used in conventional DSL systems called POTS
splitters. They split the "plain old telephone signal" (POTS) from
the DSL signal. FIG. 9B shows how these directional couplers can be
connected to provide two-way internet and telephone service over
the same telephone line.
[0088] The circuits of FIGS. 9A and 9B perform equivalently. The
2-way repeater amplifier can be omitted for short distances, of the
order of one, to two miles. For longer distances, one or more
repeaters should be used. With 2-way repeater amplifiers inserted
as required, DSL signals can be transmitted over a twisted pair
telephone line over very long distances. This cannot be done with
conventional DSL.
[0089] DC power is generally supplied to the telephone instrument
by the telephone exchange switch. This power can be used to supply
all of the 2-way terminus units from the telephone line by
combining the techniques of FIG. 8 with those of FIGS. 9A and 9B,
as would be done by one skilled in the art.
[0090] The range of DSL can be extended by using 2-way repeater
amplifiers, as many as would be required to compensate for line
loss and noise. This overcomes a limitation of conventional DSL
since DSL cannot serve over very great distances from the telephone
exchange switch. If many repeaters are to be used along the line,
the systems of FIGS. 9A and 9B could be used to insure very low bit
error rates in transmission. The modulator circuit and the
equalizer, demodulator, detector circuit would be standard types
used with DSL signal format. Doubling the bandwidth of DSL
transmission could be very advantageous, particularly when
transmitting two-way video during a teleconference.
[0091] Present day DSL systems use standardized signal formats and
are implemented with chip-sets and circuits that are designed to
work with these standards. It is possible to utilize the technology
of the present invention together with the existing DSL circuits
and hardware and thereby enhance the overall system performance,
without requiring a complete redesign of DSL technology.
[0092] Asymmetrical DSL or ADSL is the prevalent form of DSL at the
present time. Approximately 90% of the channel bandwidth is
dedicated to "download" transmission from the internet to the
subscriber's computer and 10% of the channel bandwidth is used to
"upload" from the subscriber's computer to the internet. Low
frequencies low bandwidths are generally used for downloading,
while high frequencies and wider bandwidths are used for uploading.
A block diagram illustrating the existing art of ADSL is shown in
FIG. 12. The circuits that provide telephone service are not shown
in this simplified diagram.
[0093] The objective is to increase the data rate for uploading and
make it the same as for downloading. This could be done by sharing
the channel bandwidth equally for transmission in both directions.
But this would almost halve the download speed, a most undesirable
effect. The goal is to allow full-speed data transmission in both
directions simultaneously, using a single telephone line. This can
be done using standard signal formats and standard electronic
circuits, with the addition of 2-way terminus devices and certain
other circuit components. In operation, fast 2-way data
transmission would make applications such as 2-way video over the
internet more practical.
[0094] Referring now to FIG. 12, at the telephone central office
650, an internet connection is made to a subscriber's telephone
line 652 through a DSLAM 653, where DSLAM is an acronym for DSL
access multiplexer. The DSLAM provides a 2-way connection to the
internet. A high-speed download connection 655 is shown, as well as
a low-speed upload connection 656. The low-speed connection has
about one tenth the data rate of the high-speed connection. The
high-speed connection therefore has about ten times the bandwidth
of the low-speed connection, and the frequency band for the
high-speed connection is separate and higher than the frequency
band for the low-speed connection. These signals are coupled to the
telephone line and travel in different frequency bands in the two
directions. On the other end of the phone line at the subscriber
location 651, a computer 658 transmits and receives simultaneously
through the phone line by coupling to it with the DSL modem 657.
The DSLAM and the DSL modem perform similar functions except that
the DSLAM is configured to transmit at high speed and receive at
low speed, while the DSL modem is configured to receive at high
speed and to transmit at low speed.
[0095] Referring now to FIG. 13A, a symmetrical DSL system is shown
that provides high-speed connection in both directions between the
internet and a subscriber's computer. The telephone line 682
provides a 2-way connection between the telephone central office
680 and the subscriber location 681. Signals of both the DSL high
band and the DSL low band travel simultaneously in both directions
over channel 682. With ADSL on the other hand the low-band signals
travel in one direction (from the computer to the internet), while
the high-band signals travel in the other direction (from the
internet to the computer).
[0096] How the system of FIG. 13A functions can be explained as
follows. We begin at the central office 680. Both a DSLAM 684 and a
DSL modem 685 are connected to the internet, downloading through
high-bandwidth connection 686 and low-bandwidth connection 689,
while uploading through high-bandwidth connection 687 and
low-bandwidth connection 688. The total download speed and the
total upload speed are thus the sum of the download and upload
speeds of conventional ADSL. The DSLAM and the DSL modem connect to
a network coupling and line terminating resistors, and band pass
filters 695, 696, 697, and 698. The bandpass filters pass either
the high-bandwidth signals or the low-bandwidth signals, as
indicated. High-bandwidth and low-bandwidth download signals are
summed by operational amplifier 699 and applied to 2-way terminus
700 by the connection 675. High-bandwidth and low-bandwidth upload
signals arrive from the 2-way terminus 700, and connect via line
676 to low-bandwidth and high-bandwidth band pass filters 696 and
697, and thereby through a network of resistors to the DSLAM and
the DSL modem. The band pass filters and resistors provide proper
line termination impedances for the DSLAM and the DSL modem, and
provide coupling between the DSLAM and DSL modem and the 2-way
terminator 700. They prevent uploading signals on line 616 from
leaking into the downloading path 675. The 2-way terminus couples
the uploading signal path 676 and the downloading signal path 675
to the telephone line.
[0097] The circuits at the subscriber location 681 work in the same
way as at the telephone central office 680, only in reverse order.
It should be clear to one skilled in the art that variations in
these circuits would be possible, yet the same system functions as
described above could be realized.
[0098] For example, another approach, illustrated in FIG. 13B,
eliminates the band pass filters and the resistor networks, and
substitutes 2-way terminus devices in their place. The phone line
682 carries independent low plus high bandwidth signals in both
directions between the internet connection 683 and the computer
713. At the telephone central office 680, the phone line connects
to the 2-way terminus 700 which separates the received signal 676
from the transmitted signal 675. The received signal is inputted to
2-way terminus devices 714 and 715. They output this to DSLAM 684
and DSL modem 685. The DSLAM is designed to respond only to the low
frequency, low bandwidth line signal, and it outputs a demodulated
low bandwidth bit stream 688 to the internet connection 683. The
DSL modem in turn is designed to respond only to the high
frequency, high bandwidth line signal, and it outputs a demodulated
high bandwidth bit stream 687 to the internet connection 683. Thus,
the internet connection receives both bitstreams, the high
bandwidth and the low bandwidth. These bitstreams correspond
respectively to the high and low bandwidth bitstreams, 710 and 711,
transmitted from the computer 713 at the subscriber's location 681.
This computer's total upload bitstream is assumed to be broken into
bitstreams 710 and 711.
[0099] At the telephone central office 680, the total bitstream
downloaded from the internet is assumed to be broken in to two
bitstreams, the high bandwidth one 686 and the low bandwidth one
689. Bitstream 686 is inputted to the DSLAM and it transmits a
modulated high bandwidth wave to 2-way terminus 714, which in turn,
outputs this same signal to the summer 699. Bitstream 689 is
inputted to the DSL modem and it transmits a modulated low
bandwidth wave to 2-way terminus device 715, which in turn, outputs
this same signal to summer 699. The output of this summer is a
low-plus-high bandwidth signal that is inputted to 2-way terminus
700. This terminus connects to the phone line and imparts a
low-plus-high bandwidth wave in the direction toward the
subscriber. The circuits at the subscriber location work in the
same way as at the telephone central office, only in reverse
order.
[0100] Using either the circuit of FIG. 13A or that of FIG. 13B,
one obtains more than full speed data transmission between the
internet and a subscriber's computer in both directions at the same
time. Use is made of standard signal formats and existing hardware;
plus 2-way terminus devices and other circuit components. Although
the explanation of these circuits has been given in the context of
DSL communication, the same circuits work for cable modem systems
as well. Symmetrical high-speed 2-way communication over cable
networks is obtained with these methods.
[0101] Wideband fiber optic systems would also benefit from two-way
simultaneous transmission in the same frequency band. FIG. 10 shows
a fiber optic transmission system having 2-way terminus units at
its ends, and if needed to compensate for transmission loss, a
2-way repeater in between.
[0102] For very long transmission lines, more 2-way repeaters would
be installed along the line. For short transmission lines,
typically shorter than 50 km, repeaters would not be needed. The
line would simply be a fiber cable with 2-way terminus units at its
ends.
[0103] The optical transmission system of FIG. 10 works in a manner
similar to that of the transmission system of FIGS. 3A and 3B, but
it differs in detail because it makes use of various
opto-electronic components. At the top of FIG. 10, a 2-way terminus
is shown whose purpose is to provide a means for injection of the
baseband input A signal 350 into the transmission system and to
receive from it the baseband output B signal 360. Input A drives
amplifier 351 that drives the small delay .DELTA., 352, that in
turn feeds an input signal to a drive circuit 354. The drive
circuit modulates input signal A and amplifies the modulated signal
to power optical source 355. This source could be a laser or
light-emitting diode (LED). The light from the optical source
drives a fiber cable 361 that connects to coupler 359. This injects
a light signal representative of input A into the transmission
fiber 370. In the other direction, signals coming from fiber 370 go
to the optical detector 358 via the coupler 359 and a length of
fiber cable 362 to detector 358. The detector is a photo diode. It
connects to receiver 357 to provide an electrical signal to drive
amplifier 356, whose output is output B, 360. The receiver
amplifies and de-modulates to recreate the baseband output signal
B.
[0104] When optical source 355 couples to the fiber cable 370 by
means of coupler 359, a small amount of its light energy leaks
through the coupler into fiber line 362 and from there to the
optical detector 358. This is unfortunate, and it necessitates
adaptive canceling of the leakage component at the output of
receiver 357. This is done by adaptive filter 353 whose input comes
from input A and whose output is subtracted from the receiver
output by the difference amplifier 356. The error signal for the
adaptive filter is output B, 360.
[0105] Two-way light signals are carried by fiber cable 370. If a
2-way repeater is used, optical signals are brought to it by fiber
cable 370. At the repeater, coupler 371 delivers signal A via fiber
372 to an optical detector, and receives signal B from an optical
source via fiber 373. The 2-way repeater consists of a pair of
2-way terminus units connected "back-to-back" by electrical means.
This repeater is a symmetric device that connects to fiber cable
400 in the same way that it connects to fiber cable 370. As shown
in FIG. 10, fiber cable 400 at its end connects to another 2-way
terminus. This terminus couples an optical signal to fiber cable
400 that is representative of input B, 401, and extracts from fiber
cable 400 a light signal that, when demodulated, yields output A,
402. This second 2-way terminus is identical in structure to the
first 2-way terminus and works in the same way. The functioning of
the electrical parts, the adaptive filters, the delay units, the
differencing amplifiers, and the crisscross connections in the
2-way repeater is the same as for the system shown in FIG. 3A.
[0106] If the optical transmission system of FIG. 10 is carrying
digital data signals, the receiver circuits (such as receiver 357)
would contain demodulators, amplifiers and digital regenerator
circuits. By regenerating the original baseband digital signal,
signal-to-noise ratio is greatly improved. Having a low bit error
rate is most important for transmission systems having many
repeater units.
[0107] It is possible to construct a 2-way repeater having an
all-optical information signal path, so that the optical signal
would not need to be converted to electronic form, and then
converted back to optical form. Amplification is done with laser
amplifiers. An optical 2-way repeater is diagrammed in FIG. 11 that
avoids optical-to-electronic conversion along the signal path.
[0108] In FIG. 11, the 2-way repeater is connected on the left to
fiber cable 370, and on the right to fiber cable 400. As with the
2-way electronic repeater shown in FIG. 10, signal A arrives via
fiber cable 370 and departs via fiber cable 400, while signal B
arrives via fiber cable 400 and departs via fiber cable 370. On the
left, signal A couples to fiber 506 by means of the coupler 501.
Signal A is amplified by laser amplifier 504, whose output signal
travels on fiber 510 to laser amplifier 581. The output of laser
amplifier 581 travels on fiber 509, through coupler 503, then onto
fiber cable 400. In like manner, signal B arrives from fiber cable
400, couples through coupler 503 to fiber 508, drives laser
amplifier 505, whose output travels on fiber 511 to laser amplifier
580, whose output couples to fiber cable 370 via fiber 507 and
coupler 501. This describes the information signal paths of signals
A and B. The laser amplifiers are designed to be unilateral in
order to block unwanted reverse light paths.
[0109] Because of leakage in couplers 501 and 503, adaptive optical
circuits are employed to provide cancellation of this leakage.
Control of the adaptive circuits is exercised by microprocessors
554 and 579 based on correlation information related to the
leakage.
[0110] Under operating conditions, laser amplifier 580 amplifies
signal B and sends its output to fiber cable 370 by way of fiber
507 and coupler 501. Not all of the light couples to cable 370,
however. With about a 30 dB reduction in amplitude, some of the
light leaks through coupler 501 to fiber 506. Light signals from
fibers 562 and 563 via couplers 520 and 521 respectively are added
to the input of laser amplifier 504 to cancel the signal B leakage.
The goal for laser amplifier 504 is to amplify signal A alone.
[0111] The leakage canceling circuits receive optical signal B from
fiber 511 via couplers 522 and 523. The light signals travel on
fibers 560 and 561 through couplers (splitters) to variable weight
devices 540, 541, 542 and 543. The variable weight devices can be
Mach-Zehnder interferometer modulators. These devices are well
known in the fiber-optic communication literature. An excellent
description is given by J. C. Palais, "Fiber Optic Communications",
4.sup.th Edition, Prentice-Hall, Inc., Upper saddle River, N.J.,
1984 (see page 97). Variable optical weight or gains are obtained
by supplying the Mach-Zehnder interferometers with DC control
signals 550, 551, 552, and 553 from digital-to-analog converters
(DAC) connected to microprocessor 554.
[0112] A Mach-Zehnder device can be biased so that its optical gain
is zero. Adding a positive voltage to this bias will cause the gain
to have a positive value approximately proportional to the positive
voltage applied. Adding a negative voltage to this bias will cause
the gain to have a negative value (the phase of the light output is
shifted by 180.degree.) whose magnitude is proportional to the
magnitude of the negative voltage.
[0113] The Mach-Zehnder device 540 is fed an optical signal that is
approximately 90.degree. phase shifted from the optical signal that
feeds device 541. The phase shift comes from light traveling
through the optical delay 544. A 90.degree. phase shift is best,
but is not critical. It mainly needs to be different from a zero
degree shift. The phase shift could also be an odd multiple of
90.degree. or an approximation of this. The optical signals from
weights 540 and 541 are summed by a coupler whose output drives
line 562. By adjusting weights 540 and 541 under computer control,
the light signal carried by fiber 562 can be made to have the
correct magnitude and phase to cancel the leakage of signal B
through coupler 501.
[0114] This works well if the transmission system were operating
with light having a single wavelength. If the system were carrying
light having two channels, i.e. two wavelengths, then the
additional pair of weights 542 and 543 would be needed to cancel
the leakage. For each additional optical wavelength used by the
system, an additional pair of weights, two additional degrees of
freedom, would be needed. It should be noted that the light path
through fibers 561 and 563 and the weights 542 and 543 is made to
be of different length than the corresponding light path through
fibers 560 and 562 and the weights 540 and 541. If the path lengths
were identical, only two degrees of freedom would be available for
leakage canceling rather than four degrees of freedom. Two degrees
of freedom are needed for leakage cancellation per wavelength being
transmitted. Each independent variable weight provides one degree
of freedom. As shown, the system of FIG. 11 works with optical
signals having one or two wavelengths.
[0115] The microprocessor 554 has the job of controlling weights
540, 541, 542 and 543. The weights are adjusted to minimize a
crosscorrelation signal 579, inputted to the computer through its
analog-to-digital converter (ADC). The crosscorrelation signal 579
is the product of two baseband signals multiplied by multiplier 576
and averaged by a low-pass filter consisting of resistor 577 and
capacitor 578. The baseband signals were amplified and detected by
receiver 573 and 574. The inputs to the receivers cane from optical
detector 572 and 574.
[0116] The optical inputs to the optical detectors come from laser
amplifier 504 via fiber 510, coupler 530, and fiber 571, and from
laser amplifier 505 via fiber 511, coupler 532, the optical delay
(a loop of fiber) 534, and fiber 570. If laser amplifier 504
carries only signal A, and laser amplifier 505 carries only signal
B, and since signal A and signal B are independent of each other,
the crosscorrelation signal 579 would be zero. If there were
leakage at coupler 501, then laser amplifier 504 would be
amplifying signal B components along with signal A and the
crosscorrelation signal would be non-zero. The only way to make the
crosscorrelation zero would be to adjust the weights to cancel the
leakage. Note that the length of the optical delay 534 should be
chosen so that the optical delay time from coupler 532 to optical
detector 575 would balance the optical delay time from coupler 532
through fiber 511, laser amplifier 580, fiber 507, coupler 501,
fiber 506, laser amplifier 504, fiber 510, coupler 530, fiber 571,
to optical detector 572. The delay balancing is not critical. The
timing needs only to be optimized from the point of view of time
alignment of the baseband signals, not to within a fraction of the
time period of the optical carrier frequency. With this time
alignment, the magnitude of the crosscorrelation signal is a
quadratic function of the weight values. A unique optimal choice
for the weight values exists that corresponds to perfect
cancellation of the leakage of signal B originating at coupler
501.
[0117] The weights of the adaptive optical circuits, once
converged, would only need to change slowly over time to keep up
with effects of temperature changes and aging of components. The
microprocessor can therefore be slow and inexpensive.
[0118] Several adaptive algorithms could be implemented by
microprocessor 554 to adjust the weights. One algorithm based on a
relaxation method would begin by slowly slewing one of the weights,
say 540, in a given direction while sensing the magnitude of the
crosscorrelation function. If this magnitude goes down, the slewing
should continue until the magnitude begins to get larger, then
stop. If the magnitude got larger at the outset, reverse the
slewing direction and go until a minimum of the magnitude is
reached. Then go to the next weight, say 541, and adjust it to
minimize the magnitude of the crosscorrelation function. Adjust the
next weight, and then the next one, and so on, each adjustment done
by slewing to minimize the magnitude of the crosscorrelation
function. When all of the weights have been adjusted, repeat the
process by starting with the first weight again, and so forth. The
process is repeated indefinitely in order to achieve convergence
and, in steady state, to maintain balance and proper adjustment of
the canceling circuit in the face of temperature changes and
component aging. On the other side of the 2-way repeater,
microprocessor 590 determines in like manner the weights that
cancel the leakage of signal A originating at coupler 503.
[0119] Other adaptive algorithms that can be used for adjusting and
optimizing the weights of the 2-way repeater of FIG. 11 are based
on genetic algorithms. A textbook describing genetic algorithms is
J. R. Koza, et. al., "Genetic Programming III", Morgan Kaufmann
Publishers, San Francisco, Calif., 1999. One form of genetic
operation called "mutation" would have microprocessor 554 make
random changes in weights 540, 541, 542 and 543 and check the
autocorrelation signal 579. If the autocorrelation increases,
remove the changes and restore the weight values. Then make another
set of random changes to the weights. If the autocorrelation
diminishes, keep the changes, otherwise remove them. Then make
another set of random changes. And so forth. The process should be
repeated indefinitely, always trying to improve performance. The
same process would be implemented by microprocessor 590 to control
the rest of the weights.
[0120] Another form of genetic operation of genetic algorithms
called "crossover" could be used to adapt the weights of the 2-way
repeater. All of the weights would be represented as binary numbers
that would be concatenated into a large binary number. Some of the
bits chosen at random would be complemented, and from this and the
original binary number, two "parent" binary vectors are created. By
mating the parents many times, many offspring are created some of
whose bits come from one parent and some from the other. For each
of the offspring, the weights of the 2-way repeater are set and the
autocorrelation is observed. A pair of offspring is selected having
the smallest autocorrelation. They then breed the next generation,
and so on. The objective is to continually improve performance by
selecting weights values that minimize the autocorrelations.
Genetic algorithms generally converge more slowly than the
relaxation algorithm, but they are easier to implement. Many other
algorithms can be also be used to adapt the weights to minimize the
autocorrelations. The microprocessors that control the weights need
not be fast ones. They only need to be able to keep up with slow
changes in the optical paths and optical components due to
temperature changes and aging.
[0121] The above description is based on preferred embodiments of
the present invention; however, it will be apparent that
modifications and variations thereof could be effected by one with
skill in the art without departing from the spirit or scope of the
invention, which is to be determined by the following claims.
* * * * *