Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B15/00—Suppression or limitation of noise or interference
  • H04B15/02—Reducing interference from electric apparatus by means located at or near the interfering apparatus
  • H04B15/04—Reducing interference from electric apparatus by means located at or near the interfering apparatus the interference being caused by substantially sinusoidal oscillations, e.g. in a receiver or in a tape-recorder
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
• • 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
  • H04L5/06—Channels characterised by the type of signal the signals being represented by different frequencies
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B2215/00—Reducing interference at the transmission system level
  • H04B2215/064—Reduction of clock or synthesizer reference frequency harmonics
  • H04B2215/068—Reduction of clock or synthesizer reference frequency harmonics by avoiding a reception frequency range

Definitions

• the invention relates generally to implementation of an electric wire ⁇ line transmission connection, and especially to implementation of a transmis ⁇ sion connection with the aid of VDSL (Very high bit rate Digital Subscriber Line) technology.
• VDSL Very high bit rate Digital Subscriber Line
• Optical fiber is a self-evident choice as transmission medium for a trunk network, because trunk connections usually need a high transmission capacity, the transmission distances used are long and existing routes are often found for the cables. Even for subscriber connections (the line between the local exchange and the subscriber) the situation is rapidly changing, because various services implemented with multimedia and demanding a high transmission rate will be everyday services also from the viewpoint of the private consumer.
• ADSL Analog Subscriber Line
• HDSL High bit rate Digital Subscriber Line
• the ADSL transmission connection is asymmetrical in that the transmission rate from network to subscriber is considerably higher than from subscriber to network. This is due to the fact that the ADSL technique is intended mainly for various so-called "on-demand" services.
• the transmission rate from network to sub ⁇ scriber is in an order of 2...6 Mbit/s and from subscriber to network in an order of 16...640 kbit/s (a mere control channel).
• the HDSL transmission technique concerns transmission in a metal wire pair of a digital signal of 2 Mbit/s level.
• HDSL represents a symmetrical technique, that is, the transmission rate is the same in both directions.
• the individual HDSL transceiver system comprises transceivers using echo- cancellation technology, which are interconnected by way of a bi-directional transmission path formed by a wire pair.
• the number of such individual transceiver systems may be one or two or three in parallel, whereby in a case of two or three pairs in parallel the rate to be used in each parallel transmission connection is 2 Mbit/s sub-rate; 784 kbit/s if there are three pairs in parallel and 1168 kbit/s if there are two pairs in parallel.
• radio-frequency interferences be elimi ⁇ nated, for example, with the aid of an adaptive channel equalizer in the receiver.
• an accurate and expensive AD-converter is then needed, because the interferences, which must also be converted into digital form, may be very strong.
• it would be difficult to adjust the equalizer since the interference begins quickly and its frequency changes quickly.
• DMT is a method of implementing an ADSL connec ⁇ tion defined by ANSI (American National Standards Institute), where many such subchannels (carriers) are brought about in the same modulation process which are equally spaced in the frequency domain and have the same width.
• a primary drawback of this solution is that it leads to either a compli ⁇ cated and expensive implementation or to an inefficient use of the frequency range. This is due to the fact that an efficient use of a non-continuous fre- quency range requires narrow subchannels, the number of which is high. An added number of subchannels for its part will make the system more complex. The efficiency of band utilization is also reduced by the circumstance that DMT subchannels will overlap partly in the frequency domain, whereby in order to eliminate interferences also such carriers must be taken out of operation which are outside the bands exposed to interferences. Implementation of multicarrier modulation methods is also complicated (forming and detecting signals are complicated processes).
• DMT-based solution Another drawback of the DMT-based solution is that padding bits (so-called cyclic prefix) must be added to the data to be transmitted for the elimination of distortions caused by the channel to be possible.
• padding bits so-called cyclic prefix
• the purpose of the present invention is to bring about an improve- ment on the drawbacks presented above by providing a solution allowing a very simple implementation of a transmission connection, especially a VDSL transmission connection, having a good performance.
• the invention is based, firstly, on the insight that although use of a multicarrier modulator (DMT) is defined as a transmission method, for exam ⁇ ple, in the ADSL standard and it would thus be an obvious alternative also for the implementation of a similar but higher speed connection (a VDSL connec ⁇ tion), it is not the most efficient or simplest way of implementing a transmission connection when the available frequency band is not continuous, but certain narrow frequency bands are not used.
• DMT multicarrier modulator
• VDSL connec ⁇ tion VDSL connec ⁇ tion
• the invention is also based on the insight that radio- frequency interferences caused by amateur radio stations in particular are especially strong, because amateur radio stations are usually located close to subscriber lines of telephone networks.
• a sub-band thus means a transmission band which borders on an international amateur radio band at least at one of its edges.
• the idea is thus with the aid of amateur radio bands to divide the fre- quency bands reserved for the connection into sub-bands to which the data is modulated.
• Each carrier is generated by its own modulator.
• the channels are separated from one another and they may be handled independently of each other.
• the band reserved by each channel may thus be adjusted independently with the aid of carrier frequency and symbol rate.
• Figure 1 illustrates a transmission system utilizing a VDSL connection
• Figure 2 illustrates a frequency division for use in the VDSL connection
• Figure 3 llustrates a transmission principle in accordance with the invention
• Figure 4 shows a division of sub-bands between transmission directions in accordance with an advantageous embodiment of the invention
• Figure 5 shows a basic form of apparatus for implementing a VDSL connec ⁇ tion according to the invention
• Figure 6 shows an advantageous embodiment of the apparatus shown in Figure 5.
• FIG. 1 illustrates the structure of a system utilizing a VDSL connection.
• the system architecture complies with the so-called FTTC (fiber to the curb/cabinet) structure.
• the cabinet or curb 11 receives data over a high-speed optical fiber connection indicated by reference number 12.
• Existing metallic lines copper pairs also pass through the same cabinet from exchange to subscriber. These copper pairs are indicated by reference number 13.
• the optical network unit (ONU) located in the cabinet the high-speed data are combined onto the subscriber line so that the subscriber may still use old narrow-band POTS/ISDN services, but besides these a high-speed full-duplex data connection is available to him/her.
• the VDSL connection proper is formed between the ONU and the network terminal NT.
• the network terminal is typically located on the premises of the end user (subscriber) and is connected to the subscriber's terminal equipment, for example, to an ordinary analog telephone or an ISDN telephone (reference number 15) or to a terminal equipment (TE) utilizing broadband services, such as, for example, a microcomputer (reference number 16).
• the network terminal provides the end user with UNI (User to Network Interface) interfaces in accordance with the standard. This interface is indicated by reference mark INT1.
• the broadband VDSL interface provided by the optical network unit is indicated by reference mark INT2. Since the present invention relates only to a VDSL connection to be implemented between the ONU and the network terminal NT, the system shown in Figure 1 will not be described more closely in this context. The VDSL system is described in greater detail, for example, in the ETSI report DTR/TM- 03068, where the reader may find a description in greater detail, should he desire one.
• the frequency division is made in accordance with Figure 2 so that on low frequencies space is left for existing POTS or ISDN services (frequency band A).
• the VDSL channels are transferred on frequency band B, the lower limiting frequency of which is typically 300...600 kHz and upper limiting frequency preferbaly about 18 MHz, as can be found out hereinafter.
• the division of sub-bands between different transmission directions of the VDSL connection will be described more closely hereinafter.
• the bit stream transmitted to the VDSL connection is divided onto several carriers which are located on the frequency band so that they are placed on sub-bands limited by international amateur radio bands.
• the transmitter has several parallel modulators, wherein a suitable known modulation method, for example, QAM modulation is used.
• a suitable known modulation method for example, QAM modulation is used.
• QAM modulation is used in each modulator an individual frequency is used so that the corresponding carrier is placed on the desired sub-band.
• Figure 3 illustrates the transmission principle according to the pres ⁇ ent invention by showing the power spectral density (PSD) of a signal received from a VDSL connection when six different QAM-modulated carriers C1...C6 (sub-bands 1...6 mentioned above) have been used, so that the two lowest ones have been reserved for upstream transmission and the remaining ones for downstream transmission.
• the amateur radio bands are presented by orthogons provided with reference marks AB1...AB6.
• the values in the figure correspond with computer calculations done for a connection distance of 400 m.
• the power density of the VDSL connection was -60 dBm/Hz and its Additive White Gaussian Noise (AWGN) was on a level of -115 dBm/Hz.
• the heights of orthogons AB1...AB6 correspond with the calculatory magnitude of the interference when the transmission power is assumed to be 0 dBm and the band width is assumed to be 4 kHz.
• the transmission rates of the VDSL connection mentioned above can be implemented by using the six lowest sub-bands (sub-bands 1-6). It is preferable to use the lowest sub-band (No. 1) in the downstream direction, because these low frequencies are used in this direction also in the ADSL system. In this way near end crosstalk (NEXT) is avoided, even if both ADSL connections and VDSL connections in accordance with the invention are in the same cable.
• NTT near end crosstalk
• An examination of the subscriber line with Shannon's law indi ⁇ cates that the above-mentioned transmission rates and the targeted connec ⁇ tion distances mentioned in the above-mentioned ETSI report can be best achieved when choosing transmission directions for the sub-bands in accor ⁇ dance with the following table.
• the division described above is advantageous also in that it may be used both for symmetrical and asymmetrical connections, whereby there may be connections of each type in the same cable, without crosstalk from one connection to another.
• This advantageous frequency division is also illustrated in Figure 4 wherein sub-bands of the upstream direction are hatched.
• the VDSL connection may also be implemented so that the upstream direction is given to sub-bands of the lower end of the frequency band (band B, Figure 2), while the down ⁇ stream direction is given to sub-bands of the upper end.
• the 3.5...3.8 MHz amateur radio band is a good split frequency for an upstream/downstream channel division.
• asymmetrical operation is considered important, it can be noted in a similar manner that the 1.810...2.000 MHz amateur radio band constitutes a suitable split frequency. Placing the crossover frequency at the 3.5...3.8 MHz fre ⁇ quency band allows implementation of a symmetrical bit rate of 26 Mbit/s and a bit rate of 52 Mbit/s in the downstream direction.
• Figure 5 shows a basic form of apparatus according to the invention.
• the figure shows both ends of a VDSL connection seen in one transmission direction.
• the high-speed bit stream DATAJN (the rate of which is, for example, 52 or 26 Mbit/s, according to the above presentation) to be trans- mitted to the VDSL transmission connection is fed to symbol-forming block 41 , which forms symbols of the bits and divides the symbols onto different carriers by feeding them to modulator block 42, which comprises a number of n parallel QAM modulators A1...An operating at different carrier frequencies, according to Figure 3.
• Block 41 feeds the symbol which it has formed directly to the input of its corresponding modulator. Since the signal-to-noise ratio varies with different carriers, a different number of bits per symbol may be used with different modulators. The better signal-to-noise ratio a carrier has the more bits (denser constellation diagram) can be used.
• the QAM modulators modulate the signals to different carrier fre ⁇ quencies, which are located in some of the above-mentioned sub-bands 1...9, in each case preferably essentially in the middle of the frequency band in question.
• the QAM modulation method is at the present time the most generally used modulation method, for example, in cable modems, it will not be described more closely in this context. Should the reader so desire, he may find a description of QAM in greater detail, for example, in the publication William Webb, Lajos Hanzo: Modern Quadrature Amplitude Modulation, Pentech Press, London, IEEE Press, New York, ISBN 0-7803-1098-5 (reference 1).
• Output signals from the modulators are fed to summer 43, in which the signals are summed digitally.
• the digital sum signal is supplied further to line adapter unit 44 typically comprising in sequence a DA converter, a filter to remove harmonic components occurring in the digital signal, a line driver circuit to raise the output level of the signal to a correct level, a hybrid to separate the transmission and reception branches from each other, a line transformer and a (passive) filter (POTS/ISDN-splitter) to separate POTS/ISDN signals and VDSL signals from each other.
• the filter output is connected to a channel (wire pair).
• line adapter unit 45 typically comprising the above-mentioned (passive) filter (POTS/ISDN- splitter), a hybrid to separate the transmission and reception branches from each other, an adjustable amplifier stage, a filter stage and an AD converter.
• the filter stage is preferably provided with so-called notch filters operating in amateur radio bands. Since the summer and line adapter units can be imple ⁇ mented in accordance with conventional techniques, their structure will not be described further in this context.
• Symbol words received from demodulator outputs are supplied to symbol identification circuit 47, which forms the original bit stream DATAJDUT of the baseband symbol words. Should various delays of the carriers cause problems in the recon ⁇ struction of the original signal, the delays may be measured individually for each carrier at the initial stage of the connection and this information may be utilized in the reception to ensure a correct reception order for the symbols.
• the apparatus presented in the foregoing can be modified further, for example, in accordance with the advantageous embodiments presented hereinafter.
• Trellis-coding is an advantageous choice also for the reason that it is known to be a very useful coding method, especially in transmission along telephone lines.
• Figure 6 shows such an embodiment, wherein symbol-forming unit 51 also performs trellis-coding. Since trellis-coding is known per se, it will not be described further in this context. When desired, a description in closer detail can be found, for example, in reference 1 mentioned above. Since trellis- coding is performed at the transmission end, such decoding as, for example, Viterbi-decoding, is used at the reception end, as this is the usual decoding method in connection with trellis-coding. Thus, symbol identification unit 52 is provided with a Viterbi-decoder in this embodiment.
• the Viterbi-algorithm is described both in reference 1 and also thoroughly, for example, in the publica ⁇ tion Benedetto, Biglieri, Castellani: Digital Transmission Theory, Prentice-Hall Inc., ISBN 0-13-214313-5025.
• each modulator output is provided in accordance with Figure 6 with an adjustable amplifier AMP1...AMPn, allowing the transmitter's control part 48 individually to adjust the amplification of each carrier.
• the receiver informs the transmitter of the signal-to-noise ratios which it has measured, and based on these values the transmitter's control part adjusts the amplification of amplifiers AMP1...AMPn so that the bit error ratios are equal for each carrier.

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• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• Computer Networks & Wireless Communication (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
• Communication Cables (AREA)
• Communication Control (AREA)
• Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
• Telephonic Communication Services (AREA)
• Transmitters (AREA)

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• 1996
  • 1996-06-07 FI FI962362A patent/FI962362A0/fi not_active Application Discontinuation
• 1997
  • 1997-05-09 HU HU0001983A patent/HUP0001983A3/hu unknown
  • 1997-05-09 WO PCT/FI1997/000279 patent/WO1997048206A1/en not_active Ceased
  • 1997-05-09 BR BR9709664-4A patent/BR9709664A/pt unknown
  • 1997-05-09 EP EP97920764A patent/EP0906676A1/en not_active Withdrawn
  • 1997-05-09 AU AU27023/97A patent/AU724605B2/en not_active Ceased
  • 1997-05-09 NZ NZ333199A patent/NZ333199A/xx unknown
  • 1997-05-09 PL PL97330924A patent/PL182887B1/pl unknown
  • 1997-05-09 CZ CZ19983993A patent/CZ288913B6/cs not_active IP Right Cessation
  • 1997-05-09 IL IL12736297A patent/IL127362A0/xx unknown
  • 1997-05-09 CA CA002257799A patent/CA2257799A1/en not_active Abandoned
  • 1997-05-09 JP JP10501233A patent/JP2000512450A/ja active Pending

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