Multi-carrier telecommunication system with power adaptation means

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Multi-carrier telecommunication system with power adaptation means

Alcatel Alsthom CompagnieGenerale d'Electricite C.111540

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MULTI-CARRIER TELECOMMUNICATION SYSTEM WITH POWER

ADAPTATION MEANS

The present invention relates to a telecommunication transmitter for amulti-carrier transmission system, said transmitter including coding means 5 coupled between a transmitter input and a transmitter output and able tomodulate a plurality of carriers with data received at said transmitter input and toderive therefrom symbols which are transmitted towards said transmitter output. A multi-carrier transmission system including such atelecommunication transmitter is already known in the art and is for instance a 10 Digital Subscriber Line [DSL] Transceiver Unit [TU] as defined in internationalstandards such as, for instance, the Asymmetrical Digital Subscriber Line [ADSL]standard. This standard is the "ANSITI.413 - American National Standard forTelecommunications - Network and Customer Installation Interfaces - AsymmetricDigital Subscriber Line (ADSL) Metallic Interface" of the American National 15 Standards Institute [ANSI], New York - 1995. The digital data received at theinput of such a known transmitter is grouped into frames comprising apredetermined number of bits and wherefore the coding means generatescorresponding symbols which are transmitted to a telecommunication lineconnected to the transmitter output. Since the voltage of each symbol may have 20 different peak values, the signal voltage transmitted on the telecommunicationline may vary within a relatively large range. As a consequence, the powerdissipated in the telecommunication transmitter is relatively high because it has tosupport a large value of signal voltage to avoid, or at least minimize, occasionalclipping. 25 An object of the present invention is to provide a telecommunication transmitter of the above known type but whereof the power dissipation isdramatically reduced.

According to the invention, this object is achieved due to the fact thatsaid telecommunication transmitter further includes carrier selection means 30 adapted to apply said carriers to said coding means, and data traffic detectionmeans adapted to detect if idle data are received at said transmitter input and, if - 2 - idle data are detected, to control said carrier selection means to apply apredetermined reduced set of said carriers to said coding means.

In this way, when idle data are received at the transmitter input, thesymbols generated by the coding means are merely derived from a few carriersinstead of from all the carriers as it is the case when effective data are applied tothe transmitter input. The power on the line is thereby dramatically reduced, aswell as the relatively high power dissipated in the coding means and thus in thetelecommunication transmitter.

Since the symbols, which are generated when idle data are receivedat the* transmitter input, contain only a few carriers, the power dissipated in thetransmitter is reduced. In other words, if the number of carriers drops, thedissipated power is reduced accordingly.

Another characteristic feature of the present invention is that saidpredetermined reduced set of carriers comprises only one carrier that is appliedto said coding means.

For frequency synchronization purposes, the only one remainingsymbol transmitted on the line can be chosen to be derived from a so-called"pilot tone". The transmission system remains thus efficient in that it allows a fastrestart owing to the fact that the synchronization on the telecommunication line ismaintained, while the power dissipated is dramatically reduced with respect tothat of the known system.

In a preferred embodiment, the present invention is furthercharacterized in that said telecommunication transmitter further includes dataselection means adapted to apply said data to said coding means, and in thatsaid data traffic detection means is further adapted, if idle data are detected, tocontrol said data selection means to discard the idle data received at saidtransmitter input.

As idle data may, in some circumstances, be dummy data, the lattercould be used by the coding means to modulate the few or single carrier(s) fromwhich symbols could then be derived and transmitted on the telecommunicationline. However, owing to the present invention such dummy data would be -3-

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discarded and the single carrier will remain un-modulated. As a result, thepower dissipation is then minimal.

It should be noted that the switching between symbols derived fromeffective data and symbols derived from idle data, and vice-versa, can be fast 5 whereby the power savings can be improved.

Preferably, said telecommunication transmitter operates according to a predetermined mapping and modulating protocol defined by a digitalsubscriber line [DSL] standard such as the Asymmetrical Digital Subscriber Line[ADSL] standard, and said symbols derived from said data are Discrete 10 Multi-Tone [DMT] symbols.

It is obvious for a person skilled in the art that the above multi-carriertransmission technique is not limited to ADSL applications using DMT symbols,but can for instance also be used in High speed Digital Subscriber Line [HDSL]applications, in Very high speed Digital Subscriber Line [VDSL] applications, in 15 Symmetrical Digital Subscriber Line [SDSL] applications as well as in relation withOrthogonal Frequency Division Multiplexing [OFDM] and/or Discrete WaveletMulti Tone [DWMT] applications. The latter are also multi-carrier transmissiontechniques differing from ADSL in that, for instance for DWMT, the Fourier andinverse-Fourier transformers used therein are replaced by filterbanks, wavelets 20 transformers and/or Discrete Cosine Transformers [DCT].

Also another characteristic feature of the present invention is that said coding means is adapted to count the number of symbols transmitted towardssaid transmitter output and to transmit at least one synchronization symbol aftereach group of N symbols, said synchronization symbol being derived from a 25 frame of synchronization signals received at said data selection means, and thatsaid N symbols and said one synchronization symbol form together asuper-frame.

In case of idle data, and as already mentioned, the frequencysynchronization between the transmitter and a receiver at the other end of the 30 telecommunication line is maintained owing to the presence of the pilot tone. By -4- sending one synchronization symbol for every N symbols, the framingsynchronization is also maintained between the transmitter and the receiver.

The invention is further also characterized in that said coding means isadapted to transmit at least one line monitoring super-frame after each group of 5 M super-frames, said line monitoring super-frame including N symbols and atleast one synchronization symbol, in that said M super-frames and said linemonitoring super-frame form together a hyper-frame, and in that said datatraffic detection means is adapted to control said carrier selection means to applyall the carriers of said plurality to said coding means during the transmission of 10 said line monitoring super-frame.

In the digital subscriber line transmission, the receiver measures the quality of the transmission on the line and informs the transmitter about thisquality. The measurement is performed by means of the line monitor symbolstransmitted in the line monitoring super-frame. If the quality of the transmission 15 becomes to low, the receiver may request the transmitter to go back to thepower-up status even if idle data are received at the transmitter input.

Still another characteristic feature of the present invention is that saiddata traffic detection means is adapted to control said data selection means todiscard idle data received at said transmitter input after idle data has been 20 detected for a predetermined period of time.

In a preferred application, said data received at said transmitter inputare Asynchronous Transfer Mode [ATM] cells.

Also according to the present invention, the power dissipated in thetelecommunication transmitter can be even more reduced owing to the 25 characteristic feature that said telecommunication transmitter further includes linedriver means coupled between said coding means and said transmitter outputand adapted to amplify the symbols generated by said coding means prior totransmit them to said transmitter output, said line driver means comprising theparallel connection of a relatively low efficiency line amplifier and a relatively 30 high efficiency line amplifier, that said amplifiers operate in a complementaryway so that only one of said amplifiers is operational at a predetermined timemoment, and that said data traffic detection means is further adapted to control -5-

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the operation of said relatively high efficiency line amplifier if idle data aredetected, and to control the operation of said relatively low efficiency lineamplifier else.

When sending only the pilot tone, the line drive means do not need to5 comprise a dass-A or class-AB amplifier that is a high voltage and highperformant line amplifier. It may then advantageously be replaced by a lowervoltage and thus also less power consuming line amplifier, i.e. an amplifier with a smaller idle power dissipation or a smaller supply voltage.

It is to be noted that this last characteristic feature of the present10 invenfion is preferably, but not necessary, combined with the features mentionedabove. It is thus for instance possible to have a telecommunication transmitterwith line driver means comprising the parallel connection of a low efficiency lineamplifier and a high efficiency line amplifier operating in a complementary way,but without carrier selection means and possibly data selection means as 15 mentioned above.

In a preferred embodiment, if idle data are detected, said one carrierapplied to said relatively high efficiency line amplifier is a sinusoidal wave.

In a variant embodiment, said data traffic detection means is furtheradapted, if idle data are detected, to control the quiescent current of said line 20 driver means in order to reduce the power consumption thereof.

Thereby, instead of changing from line amplifier as above, the powerconsumption may also be reduced by making the amplifier less linear but moreefficient only for a few number of carriers.

It is to be noted that, in the two last cases, the quality of the symbols25 derived, e.g., from the pilot tone and transmitted on the telecommunication lineby the high efficiency line drive means remains sufficient to maintain the frequency synchronization between the transmitter and the receiver.

The present invention is also characterized in that said data trafficdetection means is further adapted to compute the mean traffic of effective data 30 received at said transmitter input over a predetermined period of time, and,accordingly, to control said carrier selection means to apply a predetermined -6-

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reduced set of said carriers to said coding means in function of the computedmean traffic.

The amount of carriers used by the coding means and their power isthen a function of the data traffic. By using only the number of carriers that are 5 necessary to transmit the effective data, and each with less power, i.e. modulatedwith fess bits of the data, instead of always using all the carriers when the actualdata are reduced, the power consumption of the transmitter is reduced infunction of the data traffic. This switching between high and low capacity couldbe done in a similar fashion as switching between high capacity and idle. 10 · The above and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to thefollowing description of an embodiment taken in conjunction with theaccompanying drawings wherein: FIG. 1 represents a telecommunication transmitter TU according to the 15 invention; FIG. 2 shows, at different time scales, a super-frame SF and ahyper-frame HF of signals used in the transmitter of the FIG. 1; and FIG. 3 represents a possible implementation of the line driver circuitLDC included in the telecommunication transmitter TU of FIG. 1. 20 The telecommunication transmitter TU shown in FIG. 1 is of the type "ADSL Transceiver Unit - Central office side" [ATU-C] used in multi-carriertransmission system as for instance defined in the above mentioned AsymmetricalDigital Subscriber Line [ADSL] standard. Several of such transmitters aremounted in a rack which is subjected to maximum power dissipation 25 requirements. The transmitter TU has an input IN whereat digital data, e.g.Asynchronous Transfer Mode [ATM] cells, are applied, and an output OUT whereso-called "symbols" are provided. These symbols are the result of, amongstother, mapping and modulating operations performed by a data handlingcircuit, generally referred to as DHC, on the data received at the input IN. The 30 transmitter TU further also includes a line driver circuit LDC coupling the datahandling circuit DHC to the output OUT via a terminal LI and amplifying the * - 7 - symbols prior to transmit them on a telecommunication line connected to theoutput OUT. The telecommunication line is a copper twisted pair of wireswhereof the other end is connected to a remote ADSL transceiver (not shown).

It is to be noted that in FIG. 1, the schematic representation of the 5 ADSL transmitter TU has been largely simplified in order to show only theelements which are essential to understand the scope of the invention.

The data received at the input IN is applied to an input DATA of adata selector DS forming part of the data handling circuit DHC. The data selectorDS has a second input SYNC at which synchronization signals are supplied, as 10 will b'e explained later. An output of DS is connected to an input of a mappingand modulating circuit, hereafter merely called coding circuit MMC, of whichanother input is connected to an output of a carrier selector CS. An output of thecoding circuit MMC is connected to a terminal LI further connected to an input ofthe line driver circuit LDC whereof an output is connected to the transmitter 15 output OUT.

Both MMC and CS form part of the data handling circuit DHC.Several carriers Cl, ..., Cp, .., Cn, of which the frequencies are spread over thefrequency spectrum of the Digital Subscriber Line [DSL] system and that may bepre-selected amongst a full set of carriers for their good transmission qualities for 20 the particular line connected to the transmitter TU, are applied to like-namedinput terminals of the carrier selector CS. These carriers are sinusoidal waves andone of them, referred to as Cp, is the so-called "pilot tone" that insures thefrequency synchronization between the present transmitter and a receiverincluded in the ADSL transceiver at the other end of the telecommunication line. 25 The data selector DS and the carrier selector CS are controlled by a data traffic detection circuit DDC via a terminal LX connected to control terminalsDX and CX of DS and CS respectively. The incoming data of the transmitter TU isreceived in DDC via an input thereof that is connected to the transmitter input IN.The data traffic detection circuit DDC is adapted to detect the type of digital data: 30 idle or effective, as well as the mean traffic of this data, received at the input IN. -8-

According to the result of this detection, DDC controls the selectors DS and CS aswill be explained below.

If effective data are received at the transmitter input IN, and thus alsoat the data input DATA of the data selector DS, the latter circuit groups the bits of 5 the data into "frames". These frames are then transferred to the coding circuitMMC that maps them to the carriers Cl -Cn received via the carrier selector CS.MMC further modulates these carriers Cl -Cn in function of the results of themapping thereby generating Discrete Multi-Tone [DMT] symbols that aretransferred to the line driver circuit LDC. It is to be noted that the pilot tone Cp is 10 preferably not modulated to ensure the above mentioned frequencysynchronization. The purpose of the line driver circuit LDC is to amplify thesymbols prior to transmit them on the telecommunication line, its operation willbe described in more detail later.

For every N = 68 DMT symbols transmitted on the line, at least one 15 synchronization symbol is sent. To this end, at the occurrence of the 69th symbol,the data selector DS selects its second input SYNC to get the synchronizationsignals instead of the data from its first input DATA. As for the data, thesynchronization signals are also grouped into frames by the data selector DS.The synchronization symbol derived from such a frame is used for performing 20 framing synchronization between the transmitter TU and the receiver at the otherend of the line. A set of N=68 DMT symbols (numbered from 0 to 67) and onesynchronization symbol SS (numbered 68) form together a so-called"super-frame" SF as shown in FIG. 2.

Furthermore, after having generated M, e.g. M=256, of the above 25 super-frames, the coding circuit MMC generates at least one so-called"line-monitoring super-frame" MSF. This special super-frame MSF contains linemonitoring information used by the receiver to measure the quality of thetransmission on the line. A set of M super-frames and one or more linemonitoring super-frame(s) MSF form together a so-called "hyper-frame". An 30 example of such a hyper frame HF with M=256 super-frames SF (numberedfrom 0 to 255) and one line monitoring super-frame MSF (numbered 0) is shown * -9- in FIG. 2 at α time period t' that is larger than the time period t showing thesuper-frame SF in the same figure. A line monitoring super-frame MSF includesN=68 line monitoring symbols· and the synchronization symbol SS. Each linemonitoring symbol being generated by applying all the carriers Cl to Cn to thecoding circuit MMC.

The operation of the data handling circuit DHC and more particularlythat of the coding circuit MMC will not be described in more detail here since allthe details of the functioning of these circuits may be found in the abovementioned Asymmetrical Digital Subscriber Line [ADSL] standard.

The power dissipated in the transmitter TU if effective data aretransmitted is of about 3 Watt for 100 milliWatt effectively transmitted on thetelecommunication line. This low efficiency is due to several ADSL requirementssuch as the signal quality requirement. Hereafter will be explained how thispower dissipation will be reduced in function of the traffic on the line.

If idle data are received at the transmitter input IN, it is detected bythe data traffic detection circuit DDC which then controls, via the terminal CX, thecarrier selector CS to allow only a few of the carriers Cl -Cn to be transferred tothe coding circuit MMC. By generating symbols that are derived from a reducednumber of carriers, the power dissipated in the transmitter TU is dramaticallyreduced. If the set of selected carriers is reduced to one, the pilot tone Cp ispreferably chosen, with respect to the other carriers Cl-Cn, as the singleremaining carrier because it allows to maintain the frequency synchronizationbetween the transmitter and the receiver.

In some cases, such as for instance for the transmission of ATM cells,the idle data are in fact dummy data that could be grouped in frames by thedata selector DS and then be transferred to the coding circuit MMC. To avoid thegeneration of symbols derived from such dummy data, the data traffic detectioncircuit DDC also controls, via the terminal DX, the data selector DS to discard anyidle data received at the input DATA. As a result, no data are then applied to thecoding circuit MMC. - 10-

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10 15 20 25 30

In other words, if idle data are received in the transmitter TU anddetected by the data traffic detection circuit DDC, the latter controls the carrierselector CS and the data selector DS in such a way that the coding circuit MMCgenerates for instance un-modulated symbols, generally called "pilot symbols",instead of the above discrete multi-tone DMT symbols. The frequencysynchronization is ensured by the choice of the pilot tone Cp forming always partof the selected carriers, whilst the frame synchronization is ensured by thesynchronization symbols SS that are generated as usual, i.e. as with effectivedata. The line monitoring super-frame MSF is composed of idle or effectivesymbols but in any case with all the carriers Cl to Cn selected and applied to thecoding circuit MMC.

Another parameter that influences the power dissipation is theso-called "crest factor". The crest factor is the ratio of the peak voltage over theRoot Mean Square [RMS] voltage on the telecommunication line and is thus afunction of the voltage of the symbol transmitted on the line. The crest factor ofthe line signal in a multi-carrier transmission system is rather unfavorable. Inpractice, the power dissipated in the transmitter is much higher than the poweractually transmitted on the line. It is known that the lowest crest factor is obtainedby a sinusoidal wave wherefore it is then equal to the root of 2. Since all thecarriers, including the pilot tone, are pure sinusoidal waves, the pilot symbolsgenerated if idle data are detected have the lowest crest factor and the powerdissipated is then minimal.

The power dissipated in the transmitter TU when only pilot symbolsare transmitted reduces from the above 3 Watt to about 1 Watt.

In a preferred embodiment, the transmitter TU informs the receiverthat pilot symbols will be sent in order to avoid unnecessary decoding operationsat the receiver side. On the other hand, in order to avoid excessive swapsbetween the transmission of DMT symbols and pilot symbols, i.e. swaps betweena power-up and a power-down status respectively, the data traffic detectioncircuit DDC measures the time during which idle data are received at thetransmitter input IN. DDC then controls the carrier selector CS and the data 4 - 11 - selector DS of the data selection handling DHC so that pilot symbols are onlygenerated after a predetermined time of idle data being received. In a variant ofthis implementation, the data traffic detection circuit DDC may count apredetermined number of frames of idle data received. The behavior of DDC is 5 then the same as when it measures time.

As already mentioned, the data handling circuit DHC has an output terminal LI coupled to a line driver circuit LDC that is itself connected to thetransmitter output OUT. LDC amplifies the symbols generated by DHC prior totransmit them on the telecommunication line. To this end, the line driver circuit 10 LDC generally includes a high voltage line amplifier that is generally a dass-ABor a class-A amplifier. A drawback of this amplifier is that it has a low efficiencyand consumes a lot of power, However, in case of transmission of idle data, i.e.of pilot symbols, such a high voltage amplifier is not necessary. The low efficiencyline amplifier may then advantageously be replaced by a lower voltage, higher 15 performant and thereby less power consuming line amplifier. Such animplementation is shown in FIG. 3, where a low efficiency line amplifier LL iscoupled in parallel with a higher efficiency line amplifier LH between theterminals LI and OUT. The operation of the line driver circuit LDC is such thatonly one of the amplifiers LL or LH operates at a predetermined time moment. 20 They are therefore controlled by the data traffic detection circuit DDC via itsoutput terminal LX. In more detail, if effective data are transmitted, DDC controlsthe low performant but high voltage line amplifier LL to operate, while it preventsthe symbols to be transmitted through the amplifier LH. On the contrary, if idledata, i.e. pilot symbols, are transmitted, DDC prevents the amplifier LL to 25 operate, while it controls the higher performant and lower voltage line amplifierLH to operate.

In a variant (not shown) of this embodiment, the line driver circuit LDConly comprises one, low efficiency, line amplifier and, if idle data aretransmitted, the data traffic detection circuit DDC controls the quiescent current 30 [lq] thereof. Although the line amplifier is thereby made less linear, its powerconsumption is dramatically reduced and becomes thus more efficient when onlya few number of carriers are used. - 12 -

In the two last cases, a gain of about 1 Watt may be obtained on thepower consumption. However, the quality of the signals transmitted on thetelecommunication line via the high efficiency line amplifier has to remainsufficient to maintain the synchronization between the transmitter and the 5 receiver. To control this quality, each transmitted hyper-frame HF includes, asalready mentioned, a line monitor super-frame MSF that is used by the receiverto measure the quality of the transmission. The receiver informs the transmitterabout the results of the measurements and, when the quality is too low, thereceiver requests the transmitter to go back to the power-up status, even if only 10 idle data are received at the transmitter input.

Because the power dissipated in the transmitter TU is a function of the number of carriers used and of the kind of symbols transmitted on thetelecommunication line, the data traffic detection circuit DDC is further designedto measure the mean traffic ofthe data received at the input IN. According to the 15 result of this measurement performed for a predetermined time period, the datatraffic detection circuit DDC controls, via the terminal CX, the carrier selector CSto allow a reduced set of carriers Cl -Cp to be transferred to the coding circuitMMC. The number of carriers selected by CS is a function of the mean trafficdetected. By using only the number of carriers that are necessary to transmit the 20 data instead of using all the available carriers, as when effective data arereceived, the power consumption of the transmitter is reduced.

It is to be noted that the different embodiments and variants of thepresent transmitter as described above may be used separately or incombination. For instance, it is possible to have a telecommunication transmitter 25 whereof the power consumption is reduced owing to the use of a line drivercircuit LDC comprising the parallel connection of a low efficiency line amplifier LLand a high efficiency line amplifier LH operating in a complementary way, butwithout a carrier selector CS and possibly a data selector DS.

While the principles of the invention have been described above in 30 connection with specific apparatus, it is to be clearly understood that this - 13-

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description is made only by way of example and not as a limitation on the scopeof the invention, as defined in the appended claims.