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:
The present invention simplifies known data scramblers by making use of the synchronisation frames, normally used for measuring channel characteristics, as a source of pseudo-random data which can be combined with incoming user data. The present invention has particular application to multi-carrier transmission systems which employ DMT, or OFDM. Many of these transmission systems send known data, usually referred to as synchronisation frames, to measure channel characteristics such as signal to noise ratio. The known data contained in a synchronisation frame is selected to have a suitable statistical distribution, e.g. pseudo-random. In the data scrambler of the present invention, user data bits are combined with the known synchronisation frame data using an exclusive-OR function. This results in a statistically and computationally efficient scrambling of the user data.
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 present invention relates to a receiver for use in an OFDM type transmission system, in which data is transmitted in frames. Each frame has a cyclic prefix which is a repetition of part of the frame. Control means are provided and the control means controls a sampling oscilator. The control means include estimation means for estimating timing deviations of the sampling clock. The estimation means operates entirely on frequency domain input data.
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:
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. The present invention provides a mechanism for achieving frame synchronisation, in the frequency domain, by utilising this fact. The first step in synchronising a receiver with a transmitter, on signal acquisition, is to determine the interval in which orthogonality exists. Once this has been achieved an argument function is calculated from the received frame. This argument function can then be used to improve the synchronisation. The present invention is particularly suitable for use in ADSL and VDSL modems which can be used to give broadband access over copper networks. The invention is also of relevance to broadband transmission in mobile and semi-mobile systems for transmission over the radio channels.
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.