US 20070173195 A1
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.
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
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
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
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.
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.
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.
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.
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.
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.
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.
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.
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:
Referring now to
The weighted signals are summed by the summer 15 to provide a weighted sum signal yk, 29. The weighted sum yk can be written as the inner product of the input signal vector and the weight vector. That is,
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 ek, 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.
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
The adaptive filter of
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.
Further referring to
There are many other forms of adaptive filter that could be used in place of the adaptive filter shown in
Referring now to
Referring once again to
As shown in
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 Δ, 37. The transmission line receives the input B signal after it goes through delay Δ. This gives the adaptive filter a slight head start in doing its processing. If the head start is not necessary, the delay Δ, 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.
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.
A design for such an amplifier is shown in connection with the data transmission system of
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 Δ, 66 whose output drives operational amplifier 69. The output of this amplifier drives transmission line 54 through impedance 64 whose value is RC, 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.
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.
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, Δ 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
The system of
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
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.
The same idea can be applied to two-way transmission systems. A functional representation of such a system is shown in
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
The simplest connection is shown in
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
Another way to make this connection is shown in
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.
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.
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
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
Using two transmitter/receiver units of the type shown in
The circuits of
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.
The circuits of
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.
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.
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.
The adaptive beamformer of
The receiving portion of the system of
Referring now to
Referring now to
Antenna directivity patterns are drawn for each of the subscriber arrays 802, 803, and 804, in
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.
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.
A directional coupler like the one used in
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
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.
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.
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
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.
The circuits of
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
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
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.
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
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.
Referring now to
Referring now to
How the system of
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.
For example, another approach, illustrated in
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.
Using either the circuit of
Wideband fiber optic systems would also benefit from two-way simultaneous transmission in the same frequency band.
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.
The optical transmission system of
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.
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
If the optical transmission system of
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
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.
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.
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”, 4th 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.
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°) whose magnitude is proportional to the magnitude of the negative voltage.
The Mach-Zehnder device 540 is fed an optical signal that is approximately 90° phase shifted from the optical signal that feeds device 541. The phase shift comes from light traveling through the optical delay 544. A 90° 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° 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.
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
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.
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.
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.
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.
Other adaptive algorithms that can be used for adjusting and optimizing the weights of the 2-way repeater of
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.
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.