Multiplex communications – Wide area network – Packet switching
Patent
1983-07-15
1985-09-03
Griffin, Robert L.
Multiplex communications
Wide area network
Packet switching
370 24, 1791702, 1791708, H04B 320
Patent
active
045396752
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a digital transmission system in which digital data information such as PCM is conveyed in either one of two directions over a single transmission path.
In such a system the stations at the end of the transmission path each have a GO and RETURN path, these paths being coupled to the transmission path via a hybrid or its electronic equivalent. The transmission path may be a two-wire twisted-pair which would normally be one of a number of such pairs in a cable. Unfortunately imperfections in the hybrid or its equivalent cause an unwanted signal to find its way from the GO to the RETURN path, and it is an object of this invention to provide for a substantial reduction in or elimination of this unwanted signal.
According to the present invention there is provided a digital transmission station for use in a system in which digital information such as PCM is conveyed in either one of two directions over a single transmission path, wherein the station includes GO and RETURN paths coupled to the transmission path via a hybrid or its equivalent, wherein in the RETURN path the received information is applied to an analog-to-digital converter which produces one output per received symbol which output has a digital format and represents the signal value at a specific sampling instant, wherein said output is applied via a digital filter of response (1-Z.sup.-1) to one input of a subtractor circuit, the information thus applied to said one input of the subtractor circuit including, due to imperfections in the hybrid or its equivalent, an unwanted signal, and wherein a sample and hold circuit receives its input from the GO path and its output is applied via an echo simulator to another input of the subtractor circuit, so that the latter subtracts from the information in the return path a version of the signal in the GO path, whereby the output from the subtractor is a version of a received signal from which the unwanted signal has been substantially removed.
Thus this arrangement involves the use of digital signal processing techniques to achieve full duplex transmission of digital information, in the present case PCM, on a single pair of a multiple twisted-pair cable, in particular 144 kbt/s on exchange to subscriber loops. To simplify the system, the processing operations are performed at the line symbol rate and to minimize both near-end and far-end cross-talk, and also noise sensitivity, the non-linear process of decision feedback equilization (DFE) is used.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which
FIG. 1 is a digital transmission station embodying the invention.
FIG. 2 is a function diagram of an echo canceller arrangement usable to connect the scrambler and the subtractor in FIG. 1.
FIG. 3 is a functional diagram of an adaptive decision feedback equalizer (ADFE) usable in FIG. 1.
FIG. 4 is a functional diagram of an ADFE, similar to that of FIG. 3 but with transient and lock up protection.
FIG. 5 is a graph of channel impulse response, useful in explaining the invention.
FIG. 6 is a functional diagram of an ADFE with means to generate a timing extraction control signal.
FIG. 7 is a diagram explanatory of sample and hold control at a slave end.
FIG. 8 shows sampling phase clock diagrams.
FIG. 9 is a functional diagram of an echo canceller with transferable coefficients.
In a system using the present invention there are two ends, the master end being the one with a master oscillator which controls the PCM transmission rate, the other or slave end being synchronized to the master end by a clock synchronization circuit. The two ends differ because of the clock extraction and sampling time adjustment needs, but the system operates at each end substantially as will be described with reference to FIG. 1, which shows schematically the component parts of one end of the echo cancelling transmission system.
The binary data to be transmitted in the GO (IN) path is initially scrambled by a scrambler 1 to eliminate auto-correlation in the t
REFERENCES:
patent: 4117277 (1978-09-01), van den Elzen et al.
Nielsen, P. Tolstrup, "A Digital Hybrid for Two-Wire Digital Subscriber Loops", 1978 National Telecommunications Conference, vol. 2, Dec. 3-6, 1978.
di Tria, P. et al., "Design and Simulation of a Digital DPSK Modem for 80 Kbit/S Full-Duplex Data Transmission on the Subscriber Loop", 1981, International Conference on Communications, Conference Record, vol. 2.
Monsen, Peter, "Feedback Equalization for Fading Dispersive Channels", IEEE Transactions on Information Theory, Jan. 1971, pp. 56-64.
Greer Timothy K.
Griffin Robert L.
International Standard Electric Corporation
Morris Jeffrey P.
O'Halloran John T.
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