Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Patent
1997-07-24
1999-09-28
Pham, Chi H.
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
370203, 375261, H04L 2302, H04L 2728
Patent
active
059599677
DESCRIPTION:
BRIEF SUMMARY
This invention relates to telecommunications systems and in particular to an arrangement and method for transmitting signals over a subscriber loop.
BACKGROUND OF THE INVENTION
Telecommunication networks were originally developed to provide voice communications between subscriber terminals. With the experience of such networks from the traditional telephone or POTS service into the provision of a variety of digital series, there is now a need to provide improved subscriber access to the network. Traditionally subscribers are coupled to the network with a twisted pair wire loop commonly referred to us the subscriber loop. The costs of replacing these loops with more efficient or higher bandwidth connections, e.g. a coaxial connection, is prohibitive and various techniques have therefore been proposed for providing effective digital across over the subscriber loop. A recently introduced transmission technique is the orthogonal frequency division multiplex (OFDM) protocol. This technique comprises a multi-carrier modulation scheme which achieves a very low co-channel interference over dispersive channels by the use of a cyclic prefix on each data symbol. In contrast to conventional frequency division multiplex (FDM) techniques, no complex time division equalisation is required. A description of this technique is given by J Bingham in IEEE Communications Magazine 28(4) pp 5-14 --April 1990 and by A Peled and A Ruiz in `International Conference on Acoustics, Speech and Signal Processing` April 1980, Denver, pp 964 to 967. Further descriptions of OFDM techniques are given in specification No EP-A2-0656705 and specification No U.S. Pat. No. 4,884,139.
Orthogonal frequency division multiplex is ideally suited to digital transmission applications, and has been implemented using standard fast Fourier transform (FFT) techniques. However, the application of this technique to telecommunications subscriber loop technology, where the transmission path comprises a twisted copper pair, has been restricted by its susceptibility to high level tone interference. In such circumstances, the rejection offered by conventional FFT processing has been found insufficient to prevent significant data corruption.
The object of the invention is to minimise or to overcome this disadvantage.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided an orthogonal frequency division multiplex (OFDM) transmission system, including a transmitter having means for generating from an input bit stream sets of N time division samples in respective channels corresponding to an input data sequence comprising a series of bits, N being an integer, and a receiver coupled to the transmitter via a transmission path, the receiver incorporating a time variant digital filter and Fourier transform means for recovering the data stream from the output of said filter, wherein the time variant digital filter comprises a set of wavelet filters one for each transmission channel and whose tap weights are determined so as to provide specific interference rejection, and wherein the filter has an impulse response which is short in comparison with the Fourier transform order.
According to a further aspect of the invention there is provided a method of transmitting a digital data sequence comprising a series of bits over a transmission path, the method including generating sets of time division samples in respective channels corresponding to the input data sequence, transmitting said samples over the transmission path to a receiver incorporating a time variant digital filter having a plurality of elements one for each said channel, and determining a Fourier transform of the outputs of the filter elements whereby to recover the data stream, wherein the time variant digital filter comprises a set of wavelet filters one for each transmission channel and whose tap weights are determined so as to provide specific interference rejection.
The arrangement provides a means of digital transmission, e.g. over a telephone subscriber twisted pair loop
REFERENCES:
patent: 5483529 (1996-01-01), Baggen et al.
patent: 5818813 (1998-10-01), Saito et al.
Humphrey Leslie Derek
Tate Christopher
Northern Telecom Limited
Pham Chi H.
Qureshi Afsar M.
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