Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/14—Two-way operation using the same type of signal, i.e. duplex
  • H04L5/143—Two-way operation using the same type of signal, i.e. duplex for modulated signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/02—Channels characterised by the type of signal
  • H04L5/023—Multiplexing of multicarrier modulation signals

Definitions

• the present invention relates to digital systems for subscriber lines allowing communication at high bit rates and in particular at very high bit rates.
• each direction the information is modulated on a plurality of carriers having distinct frequencies, the modulation and carriers being selected generally according to the method called OFDM (Orthogonal Frequency Division Multiplexing) utilizing orthogonal carriers.
• OFDM Orthogonal Frequency Division Multiplexing
• the carrier frequencies are divided in such a way that some of them are used in the downstream direction, i.e. towards the subscriber, and other frequencies in the upstream direction, i.e. from the subscriber towards the switch.
• a true duplex communication is obtained, in which information is transmitted all the time in both directions.
• This method may be called OFDD (Orthogonal Frequency Division Duplexing).
• every second carrier frequency is used in one direction and the remaining ones, thus also every second frequency, is used for signals propagating in the opposite direction.
• even carriers can be used in the downstream direction and odd carriers in the upstream direction.
• a disadvantage of the this proposed method is that at each end of a communication channel both FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) have to be calculated at the same time what for instance is avoided in the competing method TDD (Time Division Duplexing). This may require processors working at very high clock frequencies at both ends of the communication channel.
• FFT Fast Fourier Transform
• IFFT Inverse Fast Fourier Transform
• DFT Discrete Fourier Transform
• IDFT Inverse Discrete Fourier Transform
• the DMT transmitter only uses carriers having indices that are multiples of four a sequence will be repeated four times, the DFT or IDFT will have a quarter of the length of the original one and the resulting sequence will then have to be repeated four times, etc.
• a telecommunication network is designed in the general way proposed in the cited International patent application WO 97/06619 and the document by Mikael Isaksson et al. of Telia Research AB.
• the network generally comprises at least two nodes, which communicate information bidirectionally with each other.
• Each of the nodes comprises first transforming means which are arranged to transform digital information to be transmitted to the other node from a frequency domain to a time domain and in this transformation the first transforming means uses orthogonal carriers in the time domain, the transformation then producing transformed information.
• Each node further comprises forwarding means connected to the first transforming means, the forwarding means forwarding the transformed information to the other nodes.
• each node receiving means for receiving information from the other node further comprises second transforming means connected to the receiving means.
• the second transforming means is arranged to transform information received from the receiving means from a time domain to a frequency domain.
• the first transforming means and the second transforming means are arranged to transform information according to a discrete fourier transform or an inverse discrete fourier transform.
• the first transforming means in one node uses frequencies of the carriers different from those used by the other node and the first transforming means further determines the discrete fourier transform or the inverse discrete fourier transform for only a fraction of the carriers and then it essentially repeats the transformed values obtained when making the transforming a required or an appropriate number of times.
• the carriers used for communication in one of the directions can include all frequencies within a low frequency band.
• a second transforming means in the other node may determine the inverse discrete fourier transform or the discrete fourier transform respectively by dividing the received information in segments, then essentially adding at least two consecutive segments to each other and finally calculating transformed values from the result of the addition.
• the first transforming means in one node and the second transforming means in the other node use only carriers which have even indices.
• the first transforming means in a node can comprise encoder means arranged to encode information to be transformed to symbols which have been adapted to be transformed according to a discrete fourier transform or an inverse discrete fourier transform using the fraction of the carriers.
• the carriers included in the fraction have preferably indices, which are multiples of powers of 2, i.e. of 2, 4 or 8, etc.
• Fig. 1 is a block diagram of a portion of a network for communicating with a subscriber
• Fig. 2 is a block diagram illustrating the operation of an efficient transforming unit used for modulating information to be transmitted
• Figs. 3a and 3b are schematic pictures illustrating an adding operation for recovering a transmitted short sequence
• Fig. 4 is a diagram illustrating the operation of an efficient transforming unit used for demodulating received information
• Fig. 5a and 5b are diagrams illustrating the assignment of carrier frequencies for asymmetric communication.
• the system depicted as a block diagram in Fig. 1 is a portion of a telecommunication network illustrating the connections from a transport network 1 to sub- scribers 3 using DMT (Discrete Multi Tone) modulation of the transmitted information.
• Bit streams arrive to and come from a line unit 5 from and to the transport network 1 respectively.
• a twisted copper wire-pair 7 extends up to a subscriber unit 9, which can be attached to a telephone set 11 , a computer 13, a telefacsimile device 15 and other electronic input and output devices used by the subscriber 3.
• the bit stream arriving to the line unit 5 from the transport network 1 is encoded in an encoder 17 in which appropriate symbols are formed which are input to an IFFT unit 19 connected to the output terminal of the encoder 17.
• the output terminal of the IFFT unit 19 is connected to a digital-to-analog converter 21 , the output of which is connected through a hybrid circuit 23 to the wire 7.
• the twisted wire 7 is in a similar way connected to a hybrid circuit 25.
• the signals from the hybrid circuit 25 are fed to an analog-to-digital converter 27, the output of which is connected to an FFT unit 29 performing calculations, the result of which are input to a decoder 31 connected to the output of the FFT unit 29.
• the output of the decoder 31 is connected to a subscriber interface 33 in which the received and decoded bits are processed according to the unit intended to receive the bitstream, i.e. according to whether it is a e.g. a voice message or a data signal.
• the subscriber interface 33 thus has terminals to which the various subscriber devices 11, 13, 15, etc. are connected.
• a signal fed to the subscriber unit 9 from one of the subscriber devices 11, 13, 15, etc. are processed by the subscriber interface 33 and the resulting bit stream is encoded in an encoder 35.
• the encoded symbols are then processed in an IFFT unit 37 and the transformed data are converted to analog shape in a digital-to-analog converter 39.
• the signal output from the digital-to-analog converter 39 is fed to the hybrid circuit 25 from which the analog signal is transmitted on the twisted wire pair 7.
• the transmitted signal is received by the corresponding hybrid circuit 23 in the line unit 5 and is converted to digital shape by an analog-to-digital converter 41.
• the digital signal is processed by an FFT unit 43 and the resulting symbols are decoded in a decoder 45, which produces a bit stream on a line to the transport network.
• a hybrid circuit 23, 25 thus receives signals from a line or subscriber unit and transmits them on the twisted wire 7, producing a smallest possible deflected signal which in a non-desired way is simultaneously or automatically fed to the output terminal of the hybrid circuit ⁇ o which is connected to the inner portion of the respective line and subscriber units, i.e. to the analog-to-digital converters provided therein.
• the DMT (Discrete Multi Tone) modulation and demodulation used in the subscriber connection illustrated in Fig. 1 generally comprises that first the input symbols arriving serially are converted to a suitable parallel shape by the encoders 17,
• a plurahty of carriers are modulated by the transforming or IFFT units 19, 37.
• the modulated carriers are added to each other in a final step in the IFFT units and the result is fed to the digital-to-analog converters 21 , 23.
• a corresponding process is performed when demodulating the sampled received signal.
• the modulating and demodulating processes are very efficiently implemented by
• IDFT inverse discrete fourier transform
• DFT direct discrete fourier transform
• W n is periodic with a period N and thus x n is periodic with the same period, i.e.
• time domain sequence ⁇ x 0 , Xj, ... * 2N -i ⁇ mav ⁇ e obtained by repeating the sequence ⁇ x 0 , Xi, ... XN-I) being the inverse discrete fourier transform of the symbol vector with the zeroes removed, i.e. the time domain sequence is actually ⁇ X Q , X 1 ; ... x N _
• a received symbol is divided into the relevant number of smaller portions having all the same length, and these portions are added.
• a reduced FFT for a smaller number of indices is then calculated. For instance, for carriers having even indices each received symbol is divided into two equally long parts which are added to each other.
• the adding operation results in that all non-desired carriers are cancelled and that in the receiver noise and interference is reduced and further, for example in the case of only even indices being used, that an FFT for only half the number of indices has to be calculated in the receiver.
• FIG. 4 A simple circuit for making the addition is illustrated in Fig. 4.
• the incoming sample stream passes through a summation node 401 , from one input thereof to the output, to a memory 403 having such a length that it can only accommodate the appropriate divided, shorter length.
• the first part of the incoming block of time domain samples passes through the memory 403 and is guided by a switch 405 back to the summation node 401 , to a second input thereof, and is there summed to the next part of the incoming stream. This is repeated the required number of times and then the switch 405 is brought to another position to feed the added, smoothed stream to an FFT unit 407 calculating the frequency domain values for a reduced number of input values.
• symmetric communication is not required, e.g. for sending video data such as movies to a home, and then the available carrier frequencies can be assigned in an asymmetric way, e.g. so that every fourth or every eighth frequency of the total number of frequencies are used for sending information from the subscriber end. Then the amount of calculations needed in the direction from the subscriber, in the transform units 37 and 43, will be 1/4 and 1/8 respectively of the amount of calculations required in the transform using all carriers what will giving savings of calculations amounting to 3/8 and 7/16 respectively. In the preferred, non-limiting embodiment thus the transform using all carriers may always be calculated in the downstream direction, towards the subscriber.
• every fourth frequency can be used for upstream signalling, as is indicated by the rectangles filled with a cross-hatching including thick hnes.
• the lower frequencies thereof can be transferred to transmission in the downstream direction as indicated by the rectangles filled with a cross-hatching having thin lines.

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• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Computer Networks & Wireless Communication (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
• Compression, Expansion, Code Conversion, And Decoders (AREA)

Applications Claiming Priority (3)

Publications (1)

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ID=20407609

Family Applications (1)

Country Status (5)

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• 1997
  • 1997-06-30 SE SE9702550A patent/SE514736C2/sv not_active IP Right Cessation
• 1998
  • 1998-06-30 EP EP98931186A patent/EP0988732A2/en not_active Withdrawn
  • 1998-06-30 CA CA002294766A patent/CA2294766A1/en not_active Abandoned
  • 1998-06-30 WO PCT/SE1998/001283 patent/WO1999000926A2/en not_active Application Discontinuation
  • 1998-06-30 AU AU81368/98A patent/AU742816B2/en not_active Ceased

Non-Patent Citations (1)

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