Abstract:
The invention relates to a multi-carrier transmission system, particularly a DMT system, in which data is transmitted between two transceivers using a plurality of carriers, the frequency bandwidth of the system being divided between said plurality of carriers, the transmission system being adapted for operation in a heterogeneous network including a number of subscriber equipments having different channel characteristics and coexisting on the same cable, the length of cable for each subscriber terminal varying in dependence on their respective locations. In accordance with the invention the transmission system includes allocation means for allocating the traffic of subscriber equipments having a shorter length of cable to tones starting from a higher frequency band of said system bandwidth.
Abstract:
The invention provides a multi-carrier transmission system, for example, a DMT system, in which channel information is transmitted between two transceivers using a plurality of sub-carriers, characterised in that each sub-carrier, or symbol, has a parameter associated therewith, and in that said transceivers are adapted to transmit said channel information as a sequence of n groups, in which each of said n groups contains information concerning the number of adjacent sub-carriers which have the same value as said parameter, together with the actual value of the parameter. The parameter which may have a plurality of discrete values, may be a bit-loading value, or a QAM constellation identifier. 00000
Abstract:
Modern multi-carrier transmission systems, using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, place high demands on the synchronisation of the receiver with the transmitter. The maximum permitted deviation from exact synchronisation is usually a small fraction of a sampling interval. A reserved 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 neighbouring carriers on the pilot carrier.
Abstract:
In multi-carrier transmission systems using, for example, DMT, it is known to recover a receiver sampling clock from a reserved carrier, a pilot carrier, having a fixed phase. A sampling clock oscillator in a receiver is then phase-locked to the pilot carrier. Multi-carrier receivers, such as DMT receivers, are normally equipped with a FFT processor. A complex number representing the pilot carrier is then available from the FFT processor output. If a FFT processor is not available, a one-frequency DFT processor can be provided to produce a complex estimate of the pilot carrier. In a DMT system, frame synchronisation is handled separately from sampling clock synchronisation, although the two processes are intimately related and frame synchronisation must be acquired before sampling clock synchronisation.
Abstract:
In a two-way multi-carrier transmission system, such as a DMT system, in which there can be dynamic changes in the transmission parameters, some means must be provided for maintaining synchronisation between transmitter and receiver when the transmission parameters change. The first stage of such a process requires that changes of parameter be notified by one transceiver to the other, involved in an active communication process, over a slow transmission channel, the control channel. Subsequently the synchronisation of the transceivers is adjusted simultaneously, i.e. from a predetermined DMT symbol. Such adjustements in time synchronisation must be achieved with a minimum of overhead. 00000
Abstract:
The invention provides a multi-carrier transmission system, for example, a DMT based VDSL system, using orthogonal carriers with high order QAM constellations for the transmission of multiple bits per carrier and symbol, said system including digital receiver and transmitter units, said receiver unit including a symbol detection unit, characterised in that said system is adapted to determining a parameter for each single carrier, said parameter being indicative of a deviation of a received signal from a corresponding constellation point; comparing the parameter with an upper and lower limit; and, if the parameter is outside said limits, changing the constellation used to modulate the carrier to a neighbouring constellation. The symbol detection unit may be used to determine the parameter which may be a ratio d?2/σ2¿, where d is the shortest distance between neighbouring constellations, σ is a standard deviation, and σ2 is the variance of the deviations of the input and output signal values of said symbol detection unit.
Abstract:
The invention provides a multi-carrier transmission system, for example, a DMT system, in which 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 said transceivers including a receiver and a transmitter, in which a fixed maximum value is 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 said 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 the fixed maximum value, to said 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 and may be at least as large as said theoretical bit capacity.
Abstract:
When a multi-carrier signal is received, it is sampled and digitised before FFT processing, (or wavelet transform 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, analogue to digital convertors have a limited dynamic range. It is, therefore, normal practice to provide a compromise between cutting and quantisation noise. This means that the adverse effects of a coarse quantisation have to be balanced against signal distortions caused by loss of the low probability high amplitude signals. 00000
Abstract:
Modern multi-carrier techniques using orthogonal carriers with high order QAM constellations for the transmission of a plurality of bits per carrier and symbol, use some method for the determination of an inverse channel model to be used in an equalisation process. In the frequency domain, this is normally based on an estimate of the channel attenuation and phase for each carrier. The present invention provides a simplified method of equalisation that uses data at the symbol detector input and output to adaptively estimate an inverse channel model.
Abstract:
In certain types of transmission system using nQAM it may be desirable to use a control channel superimposed on a nQAM data channel. In such circumstances it is very desirable to be able to separate control channel data from user data without using a higher level protocol layer. For applications 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, i.e. the bit-loading, or constellation identifier are unknown. Binary data is transmitted using nQAM modulation. The binary data comprises 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 said control data is facilitated by arranging control data points to have either unique phase, or unique amplitude, values. These unique values are not employed for user data points.