Abstract: In a multi-carrier system employing OFDM, for example DMT, an adaptive channel equalizer is normally used, operating in the frequency domain. The sampling clock is controlled so that the time delay between the transmitter and the receiver is effectively eliminated. If the information used to control the sampling clock is received from the equalized data stream, it will introduce an ambiguity between the operation of the channel equalizer and the mechanism used to control the sampling clock. Operation of the equalizer can mask an increasing time difference, between transmitter and receiver, which the sample clock controller should be tracking. The present invention eliminates the ambiguities in the operation of the equalizer and sample clock controller by preventing the equalizer accepting time differences which should be corrected by operation of the sample clock controller.

Abstract: When delivering a broadband service, such as xDSL, without inband POTS, it is necessary to separate the analogue POTS signal and the xDSL signal from each other at both the CO (Central Office) and the CP (Customer's Premises). This can be achieved by using an active POTS splitter. The present invention incorporates test functionality for the line between the CP and the CO, or ONU (Optical Network Unit), in the POTS splitter. This enables two-sided measurements on the line, both during installation and during operation. The measurements are performed at the CO and upon request, or when the test device automatically sends a test message/signal. In this way there is no need for field technicians at the CP side. The POTS splitter can have a unique identity code that is transmitted to the CO each time a test is started, or on receipt of a request from the CO.

Abstract: Zipper is the time-synchronized frequency-division duplex implementation of discrete multi-tome (DMT) modulation. Two communicating Zipper modems transmit DMT symbols simultaneously with a common clock. When all transmitters are time synchronized, the near end cross-talk (NEXT) and near end echoes injected into the received signal are orthogonal to the desired signal. The present invention provides a telecommunications transmission system using zipper and having at least two VDSL systems. Each VDSL system comprises a pair of zipper modems communicating over a cable transmission path. The telecommunications transmission system handles zipper transmission transmitted over the common cable; at least partly mitigates NEXT; and permits transmissions in a first VDSL system which are asynchronous with transmissions in a second VDSL system.

Abstract: In a multi-carrier system employing OFDM, for example DMT, an adaptive channel equalizer is normally used, operating in the frequency domain. The sampling clock is controlled so that the time delay between the transmitter and the receiver is effectively eliminated. If the information used to control the sampling clock is received from the equalized data stream, it will introduce an ambiguity between the operation of the channel equalizer and the mechanism used to control the sampling clock. Operation of the equalizer can mask an increasing time difference, between transmitter and receiver, which the sample clock controller should be tracking. The present invention eliminates the ambiguities in the operation of the equalizer and sample clock controller by preventing the equalizer accepting time differences which should be corrected by operation of the sample clock controller.

Abstract: In a multi-carrier system employing OFDM, for example DMT, an adaptive channel equalizer is normally used, operating in the frequency domain. The sampling clock is controlled so that the time delay between the transmitter and the receiver is effectively eliminated. If the information used to control the sampling clock is received from the equalized data stream, it will introduce an ambiguity between the operation of the channel equalizer and the mechanism used to control the sampling clock. Operation of the equalizer can mask an increasing time difference, between transmitter and receiver, which the sample clock controller should be tracking. The present invention eliminates the ambiguities in the operation of the equalizer and sample clock controller by preventing the equalizer from accepting time differences which should be corrected by operation of the sample clock controller.

Abstract: One problem, frequently encountered with VDSL transmission systems, is that upstream FEXT produced by system users having short wires can be very strong. Users having shorter wires get high bit rates whereas users having longer wires get low bit rates. In extreme cases it may happen that users with wire lengths greater than 1000 meter cannot transmit data upstream. The present invention provides a VDST transmission system with a plurality of modems. The modems are located at varying distances from a central station. There is a target bit rate for each modem. That modems on shorter wires have control means for reducing their transmit power. This reduces the FEXT produced by these modems enabling modems on longer wires to transmit at higher bit rates.

Abstract: The hybrid circuit can be used to substantially reduce near echo signals. The circuit includes a balanced 2-wire to 4-wire hybrid for interconnecting a two wire receive path and a two wire transmit path to a two wire transmission line. The two wire receive path connects the balanced hybrid to an A/D converter and the two wire transmit path connects a D/A converter to the balanced hybrid. The two wire receive path contains a filter, dimensioned to remove signals transmitted from the D/A converter. The invention is particularly adapted for use with FDD and OFDD.

Abstract: A VDSL-modem which is divided into an analog part which is placed in an optical node and a digital part which is placed in a local station. The analog part of the VDSL-modem includes an A/D-converter and D/A-converter, a filter, an amplifier, a hybrid/balun, an adaptive noise attenuator, an optical interface, and possibly an echo canceller. The digital part of the modem includes an FFT/IFFT-processor, a synchronizer, an equalizer, an interleaving unit, an error correction unit, a protocol manager, and an optical interface. The VDSL-modem simplifies synchronization of the modem and reduces power consumption in the optical node. A multiplexor function in the optical node in addition is simpler because it need not manage a protocol.

Abstract: A multi-carrier transmission system has a plurality of transceivers, each having a receiver and a transmitter. Data is transmitted between the transceivers by modulating the data onto a multiplicity of carrier waves in the form of multi-bit symbols, wherein each carrier wave constitutes a channel, and wherein the number of bits per symbol (the bit loading), varies between channels and, within a channel, with time, so that each channel has associated therewith a bit loading parameter.

Abstract: With OFDM systems the frequency domain data is the Fourier transform of the received time domain OFDM frames. The time domain frames must be sampled, at the receiver, in synchronism with the transmitter, so that each received frame contains data from only a single transmitted frame. It is vital for this synchronism to be maintained in order to maintain the orthogonality of the frames. A typical multi-carrier system, of the OFDM type, which uses a cyclic prefix permits orthogonality to be maintained when there is a small deviation from exact frame synchronisation. Because the signalling interval includes both an entire frame and the cyclic prefix, which is a repetition of part of the frame, a frame sampled within the signalling interval will contain data from only one frame. Since the signalling interval is greater than the frame period, this gives some leeway in frame alignment. In a multi-carrier system of the OFDM type, an adaptive channel equalizer, operating in the frequency domain, is often used.

Abstract: A RFI canceller, for use in a subscriber line system using multi-carrier modulation, measures a RFI disturbance in carriers falling within a band of frequencies causing the RFI. The RFI ingress into carriers outside the band of frequencies is estimated, and an error correcting signal derived from the estimation of the RFI ingress is subtracted from a received signal. The subscriber line system may be a VDSL system and the multi-carrier modulation may be DMT. The RFI canceller includes a demodulator for demodulating an incoming data stream to provide a first parallel data stream. A parallel to serial convertor converts the first parallel data stream to a first serial data stream. A digital to analogue convertor converts the first serial data stream to a first analogue signal. An analogue RFI canceller circuit combines the analogue signal with an analogue error correcting signal, to produce a second analogue signal.

Abstract: An arrangement for synchronization of nodes in VDSL-systems, or more exactly, synchronization of optical VDSL-nodes which share a common part of a cable in the access network between subscribers and a local station. A time synchronization is provided towards an external system, for instance GPS, which gives a time reference by which the different nodes can be synchronized. The synchronization reduces the near end cross-talk between the VDSL-systems in the different nodes. Preferably, each respective node includes a receiver for a synchronization signal and an internal oscillator with high stability to deliver a stable clock signal.

Abstract: A mechanism for achieving frame synchronization in the frequency domain. In order to synchronize a receiver with a transmitter, on signal acquisition, the interval in which orthogonality exists is determined. Once this has been achieved, an argument function is calculated from the received frame. This argument function can then be used to improve the synchronization. This system is particularly suitable for use in ADSL and VDSL modems which can be used to give broad band access over copper networks. It is also relevant to broad band transmission in mobile and semi-mobile systems for transmission over radio channels.

Abstract: A circuit, for digitizing an analogue signal includes an analogue to digital converter, a clip processor adapted to estimate a value for clipped digital signal samples, and a buffer adapted to dynamically store a plurality of digitized samples produced by the analogue to digital converter. The clip processor is adapted to read digitized samples from the buffer and replace clipped digitized samples with the estimated values, thereby mitigating the effects of clipping in an output of the circuit.

Abstract: A transmission system using nQAM. A control channel is superimposed on a nQAM data channel. It is desirable to separate control channel data from user data without using a higher level protocol layer. In multi-carrier systems with variable bit-loading it is particularly desirable that the control channel data be separable from user data when the value of n. that is the bit-loading, or constellation identifier are unknown. Binary data is transmitted using nQAM modulation. The binary data includes both user data and control data. This means that a control channel is superimposed on a user data channel within the same nQAM constellation. Separation of the control data is facilitated by arranging control data points to have either a unique phase or unique amplitude values. These unique values are not employed for user data points.

Abstract: A multi-carrier transmission system using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol. Such systems place high demands on the synchronization of the receiver with the transmitter. The maximum permitted deviation from exact synchronization is usually a small fraction of a sampling interval. A reserve carrier, the pilot carrier, which is given a fixed phase, is usually used as the reference to achieve this high accuracy. The receiver sampling clock oscillator is phase locked to the pilot carrier. It is therefore necessary to estimate the phase of the pilot carrier. Using a bandpass filter to recover the pilot carrier, regardless of the frame structure of the DMT signal, does not eliminate the influence of neighboring carriers on the pilot carrier.

Abstract: A multi-carrier transmission system. A received multi-carrier signal is sampled and digitized before FFT processing. Multi-carrier signals consist of many narrow band carrier waves which convey data via a wide band channel. The amplitude distribution of the sum of all the individual carriers is Gaussian. This means that high amplitudes occur with low probability. At the present time, analog to digital converters have a limited dynamic range. It is therefore normal practice to provide a compromise between cutting and quantization noise. The adverse affects of a course quantization have to be balanced between signal distortions caused by the loss of the low probability high amplitude signals.

Abstract: A multi-carrier transmission system such as a DMT system. Channel information is transmitted between two transceivers using a plurality of sub-carriers modulated with symbols, each of which represents a multiplicity of bits. Each of the transceivers includes a receiver and a transmitter in which a fixed maximum value was determined for the number of bits for each symbol and in which the system is adapted to determine the bit capacity per symbol of each of the plurality of sub-carriers and to increase the number of bits represented by a symbol transmitted over those sub-carriers having a capacity less than a fixed maximum value to the maximum value by the addition of channel coding bits. The fixed maximum value for the number of bits for each symbol may be determined on the basis of the bit capacity of that one of the sub-carriers having the highest theoretical bit capacity may be at least as large as the theoretical bit capacity.

Abstract: In certain types ol transmission system using nQ,1M it may be desirable to use a control applications in muIticarrier snrstems with variable bitloading it is particularly desirible that the control channel data be separable from user data when the value of n. i.e the bitloading. or constellation identifier are unknown. Binary data is transmitted using nQAM modulation. The binary data comprises both user data and cointro,l clata. This means that a control channel is superimposed on a user data channel within the same nQAM constellation. Separation of said control data is lacilitated by arranging control data points to have either unique phase, or unique amplitude, values. These unique values are not employed for user data points.

Abstract: A multi-carrier transmission system such as a DMT based VDSL system. The system uses orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol. This estimate includes digital receiver and transmitter units with the receiver unit including a symbol detection unit. This system is adapted to determine a parameter for each single carrier with the parameter being indicative of a deviation of a received signal from a corresponding constellation point. It compares the parameter with an upper lower limit and if the parameter is outside the limits, changes the constellation used to modulate the carrier to a neighboring constellation. The symbol detection unit may be used to determine a parameter such as the ratio d2/&sgr;2, where d is the shortest distance between neighboring constellations, &sgr; is a standard deviation and &sgr;2 is the variance of the deviations of the input and output signal values of the symbol detection unit.