WO2002093822A1

WO2002093822A1 - Methods and arrangements relating to a telecommunication system

Info

Publication number: WO2002093822A1
Application number: PCT/SE2001/001110
Authority: WO
Inventors: Albin Johansson; Mattias Hyll
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 2001-05-17
Filing date: 2001-05-17
Publication date: 2002-11-21

Abstract

The present invention relates to a method of selecting at least one 'filler' symbol that is suitable to be transmitted as a Q-mode signal in a XDSL system. In contrast to proposed methods the present method involves: transmitting a number of symbols, recording a power values for each symbol that is transmitted, and selecting, in response to the recorded power value, at least one of said transmitted symbols as a suitable 'filler' symbol. The selection step may more in detail involve comparing of the power value of each symbol with a threshold parameter Tp, and if a symbol has a power value that is lower than the threshold parameter Tp then said symbol is selected as a suitable 'filler' symbol. The present invention further relates to a transmitter unit (80) for a XDSL system.

Description

Patent Application

Applicant: Telefonaktiebolaget LM Ericsson

Methods and arrangements relating to a telecommunication system

Field of the Invention

The present invention relates to a method and an arrangement for selecting a suitable standby signal in a XDSL system. More specifically, the invention relates to a method and transmitter unit for selecting at least one "filler" symbol that is suitable to be transmitted as a Q-mode signal in a XDSL system.

Background of the invention

In recent years telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications includes not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately DC to approximately 3.4 kilohertz.

New, high-speed data communications provided over digital subscriber lines, provide for high speed data transmissions, as is commonly used in communicating over the Internet. As is known, the bandwidth for such transmissions is generally defined by a lower cutoff frequency of approximately 30 kilohertz, and a higher cutoff frequency which varies depending upon the particular technology. Since the POTS and high-speed data signals are defined by isolated frequency bands, both signals may be transmitted over the same two-wire loop.

One well known method of transmitting high-speed data is by dividing data into several interleaved bit streams, and using these bit streams to modulate several carriers, e.g., Discrete Multitone (DMT) and Orthogonal Frequency Division Multiplex (OFDM) modulation and demodulation systems. These types of multi- carrier modems are being used or considered for use in such applications as cellular radio and Digital Subscriber Lines (DSLs) such as High rate Digital Subscriber Lines (HDSLs), Asymmetric Digital Subscriber Lines (ADSLs), etc (more broadly denoted XDSL).

In a Discrete Multitone system, the input bit stream is first serial-to-parallel converted. The parallel output is then grouped into N groups of bits corresponding to the number of bits per symbol. Portions of bits are allocated to each DMT carrier or subchannel. The power transmitted over each subchannel is preferably approximately the same.

FIG. 1 shows an example Discrete Multitone (DMT) communication system in which the present invention may be advantageously employed. Transmitter 10 includes a serial- to-parallel converter 14, a multicarrier modulator 16, and a pretransmit processor 18. Receiver 12 includes a post channel processor 20, a multicarrier demodulator 22, and a parallel-to-serial converter 24. The transmitter and receiver are linked in this example by a digital subscriber line (DSL) or other form of communication channel 26. Serial input data at a rate of b_totai /T bits per second are grouped by converter 14 into blocks of btotai bits for each multicarrier symbol, with a symbol period of T. The btotai bits in each multicarrier symbol are used to modulate N separate carriers in modulator 16 with bibits modulating the i-th carrier.

Generally, the multicarrier modulator 16 uses an Inverse Fast Fourier Transform (IFFT), and the corresponding multicarrier demodulator 22 performs a Fast Fourier Transform (FFT). As depicted in FIG. 2, the carriers or subchannels in a DMT system are spaced 1/T Hz apart across N/T Hz of the frequency band. More detailed discussion of the principles of multicarrier transmission and reception in general is given by J. A. C. Bingham in "Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come", IEEE Communications Magazine, Volume 28, Number 5, pp. 5-14, May 1990.

A "line-card," containing line interface circuitry, is provided at the central office. The line interface circuitry provides the interconnections among XDSL circuitry, POTS or PSTN voice circuitry, off-hook (or tip /ring) detection circuitry, ring generator circuitry, and the local loop. As is known, the line interface circuitry includes a POTS filter that is interposed between the various POTS circuits and the XDSL circuit. This filter protects the POTS circuitry from the high frequency signals of the XDSL transmission circuitry and the XDSL circuit from the high voltage of the POTS circuitry. Fig. 4 is a simplified diagram over the amplifying parts of the "line- card". A modulated multi-carrier signal is converted into analog format in a digital- to-analog converter 30 and the supplied to a line driver amplifier circuit, hereafter referred to as line driver, 32 which drives the line 34 via a coupling transformer. The line driver is the most power consuming part of the transceiver circuit and since the power dissipation is high it produces a lot of heat. Both the power consumption and the generated heat become a severe problem if many "line-cards" are stacked together as could be the case in the local exchange.

It should be appreciated that, for any given local loop, the line driver spends a significant percentage of time in standby operation (i.e., not transmitting). Unfortunately it has been found that if no signal is transmitted during this "standby" time, then several problems arise in a XDSL system:

1) It changes the interaction dependent characteristics for adjacent wire pairs in the cable bundle, whereby the crosstalk change for modems on said adjacent wire pairs. If then several modems in the bundle power-down and up the line driver circuitry, the crosstalk would fluctuate uncontrollably reducing the preformance of the modems, possibly forcing them to retrain, or in the worst case causing them to loose synchronization altogether.

2) The receiver will loose synchronization and needs to train up when the user data resume, thereby causing a degraded connection.

Therefore it has been proposed to send a communication-like signal during standby operation. The signal to be transmitted to achieve these goals is hereafter referred to as the Q-mode signal.

As such a Q-mode signal will consume more power than if no signal is transmitted during standby operation, great effort has to be put in to save power. Therefore, an acceptable Q-mode signal must have a minimal Peak- to-Average (PAR), which is a commonly used measure for output power on the DSL. Reducing the peak of the Q- mode signal while keeping the average power spectral density the same helps reduce the power consumption in the modem line driver that typically consumes a significant portion of the total power consumed in normal operation of the XDSL modems.

The current Q-mode proposals utilize a Q-mode "filler" symbol with low PAR properties in order to save power at the transmitter. Such a filler symbol may be defined by the transmitter and communicated to the receiver during initialization. The most desirable features for Q-mode operation are stationary characteristics of the crosstalk, transparency to the receiver during showtime and the ability to optimize the PAR characteristic of the Q-mode signal.

In XDSL systems the modulation output is approximately a normal distribution. Normal distribution means that the peak-to-average ratio of the output is relatively high. The graph in FIG. 3 shows an example output signal from a DMT or OFDM modulation transmitter having a normal distribution. The tails of the normal distribution curve are quite long and indicate a very low probability of occurrence for symbols of the lowest energies.

Proposed techniques for choosing the Q-mode "filler" symbol with optimal PAR properties, all include advanced optimization algorithms that requires a large amount of processor time and memory space. This is especially true since the characteristics for each wire pair are unique, and each wire pairs therefore need a unique Q-mode signal, each time it is about to enter Q-mode.

Summary of the invention

Obviously an improved method and a system for finding the Q-mode "filler" symbol with optimal power properties is needed. This should be achieved without increasing the need for processor capacity or memory space and within reasonable time periods. The object of the invention is to provide a method that overcomes the drawbacks of the prior art devices. This is achieved by the methods as defined in claim 1 and 9, and the devices as defined in claim 12.

One object of the invention is to provide a method of selecting at least one symbol that is suitable to be transmitted as a Q-mode "filler" signal by transmitting a number of symbols, recording a power value for each symbol that is transmitted, and selecting, in response to the recorded power value, at least one of said transmitted symbols as a suitable "filler" symbol.

Another object of the invention is to select a suitable "filler" signal by comparing the power value of each power value with a threshold value Tp. The threshold value Tp being either a predefined value or dynamically determined from a distribution of power values.

Yet another object of the invention is to provide a method of selecting the symbol with the lowest power value from a number of previously transmitted symbols by searching for the symbol with the lowest power value.

Still another object of the invention is to provide a transmitter unit that comprises a power analyzer that is connected to the digital parts of the transmitter unit and that the power analyzer provides a selection signal indicating a symbol with suitable power value.

One advantage afforded by the invention is that selection of suitable "filler" signal is performed without significantly increasing the needed processor capacity of the modem, nor requiring any extensive memory spaces.

Another advantage afforded by the invention is that the methods can be continuously running in the system an hence react quickly upon changes in the environment. Alternatively the search for the suitable "filler" signal is done periodically or on demand. Yet another advantage is the ability of choosing a threshold value 7p that corresponds to the characteristics of the line driver circuit and/ or the statistical behavior of the transmitted symbols.

Other objects, advantages and novel features of the invention will become apperent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.

Brief description of the figures

The features and advantages of the present invention outlined above are described more fully below in the detailed description in conjunction with the drawings where like reference numerals refer to like elements throughout.

Fig. 1 is a function block diagram showing an example discrete multi-tone (DMT) system in which the present invention may be employed;

Fig. 2 is a graph illustrating the principle of a multi-carrier system; FIG. 3 is a simplified illustration of a line driver in a digital subscriber line environment;

Fig. 3 is a graph showing a probability density function for obtaining a certain PAR value;

Fig. 4 is a simplified diagram of the amplifying part of the modem;

Fig. 5 is a flowchart of a first exemplary embodiment of the invention;

Fig. 6 is a flowchart of a second exemplary embodiment of the invention;

Fig. 7 is a simplified diagram of an amplifying part capable of operating on two different voltages;

Fig. 8 is a simplified diagram of the transmitter unit including a power analyzer; Detailed description of the invention

Embodiments of the invention will now be described with reference to the figures.

A commonly used measure for output power on the DSL is the Peak-to-Average Ratio, PAR. In Fig. 3 the PAR probability density function per symbol is depicted as a function of PAR. The shown distribution is typical for a full transmission, i.e. both a training phase and the transmission of the payload of data, often referred to as Showtime. The purpose of the training phase is to adjust parameters, controlling e.g. modulation and channel coding in the transmitter and receiver to optimise the transmission. The training phase is also used for synchronizing the transmitter and the receiver. During the training phase, the transmitted signal is called Medley. This signal is chosen to have a random pattern. Also during Showtime, when the payload is transmitted, a random pattern is plausible. The PAR values resulting from such signals will produce a Gaussian-like distribution as seen in Fig. 3.

The PAR value is a measure of transmitted power defined according to:

Where x is a vector containing the measured samples. Another commonly used measure of transmitted power is the crest factor, CF, defined as the peak value divided by the Root Mean Square, RMS-value. Other measures of power are possible to define. In the following PAR will be used as an example of a power measure, but the invention should not be considered limited to using PAR.

The distribution of amplitudes during both Medley and Showtime forces the line driver to work at or close to, full power when transmitting symbols having a high PAR value. As discussed above, "filler" symbols giving low PAR value should be transmitted during Q-mode to enable low power consumption in the line driver and yet maintain the communication. Suitable "filler" symbols are found to the left of the indicated threshold parameter T_p in the distribution shown in Fig. 3. Due to the complexity of DMT transmission technology, as previously discussed, it is not a trivial task to a priori find symbols of low PAR. Appropriate symbols will be dependent on the conditions of the subchannels, chosen modulation etc, and will be different for different wires as well as vary over time.

Fig. 5 shows a flowchart of the method according to the invention. In a first recording and analysing step 500 the Peak- to Average Ratio, PAR, of the transmitted signal is analysed per symbol. The analysis is typically made during normal transmission, i.e. Medley and Showtime. If a sufficient number of transmitted symbols were analysed and stored, a distribution as in Fig. 3 would result. However, according to the invention it is not necessary to store all PAR- values for all transmitted symbols. In step 510 the PAR of a transmitted symbol is compared with the threshold value Tp. If the PAR is equal to or below T_v the symbol, SYM is stored in a memory in step 520. In step 530 the normal transmission has ended and the transmitter goes to the idle state, Q-mode. It starts to transmit the SYM symbol as the "filler" symbol and hence save power at the transmitter.

An alternative embodiment of the invention does not utilise a threshold value T_p for comparison. Instead, during a selected time period or a selected number of symbols, the transmitted symbols are recorded and analysed to find the symbol with the smallest PAR. Shown in Fig. 6 is a flowchart of this embodiment. As above, the first step 600 is to record and analyse the PAR of the transmitted symbols. If the PAR of a symbol is lower than the previously lowest PAR, PAR_min (step 610) a new PARmin as well as the symbol giving it will be stored in step 620 according to: PARmin=PAR and SYM_mi__=SYM. In the next step 630 a check is made if the selected time period has expired or the selected number of transmitted symbols has been reached. If not the process goes back to analysing the transmitted symbols, step 600, otherwise the transmitter is ready for the Q-mode in step 640.

The threshold value T_p can be chosen with different criteria. Laboratory test, simulations and/ or tests of real systems can be used to determine an appropriate value for T_v. Important criteria for choosing T_p can be based on statistical considerations, e.g. by studying the PAR pdf of FIG. 3. Of importance is the observation that symbols with very low PAR have a low probability of occurrence. If T_p is chosen too low, the search for a corresponding symbol would take unacceptable long time, or even fail completely. On the other hand, T_p should be chosen such that the high PAR values are clearly avoided. The statistical analysis can be automated by methods known by the skilled in the art. Chosing T_p can also be made dynamically and without prior knowledge of the exact distribution of PAR by for example defining T_p to be a power value which is lower than the power value of highest probability of occurrence, and which has a probability of occurrence of e.g. 10%.

Another important criterion in choosing T_p is the characteristics of the line driver circuit. The line driver circuit might have a power consumption, that for transmission of symbols having a PAR-values under a certain PAR-value, becomes effectively constant. In that case a T_p corresponding to the lowest power consumption level of the line driver circuit should be chosen.

Known in the art are line driver circuits with more than one of supply voltage. Typically two supply voltages are used, e.g. as described in US Patent 6,028,486, giving the line driver circuits two possible power levels, see Fig. 7. Further reductions in power consumption can be obtained if the present invention is combined with such line driver circuits. The threshold value T_p should preferably be chosen to correspond to the lower power level/lower supply voltage of the line driver circuit, enabling the line driver circuit to go into its low power state during Q-mode. To chose an even lower value of T_p would not significantly further reduce the power consumption, since the low power level is already obtained. Choosing an unnecessary low T_p would only increase the time needed in step 500 to 510 to find a suitable symbol to use for the Q-mode. More advanced implementations of a power reducing mechanism in the line driver are of course possible. However also the power consumption from a circuit with a more continuos supply voltage will level out at some level and the T_p should be chosen accordingly. To conclude, the threshold value T_p should be considering the line driver circuits used and the probability of finding a symbol with corresponding PAR within an acceptable time period.

The above-described methods of finding a symbol with low PAR have the advantage of needing very little processor capacity as well as little memory. Therefore, they can always be kept running in the transmitter and thus, the modem will always be ready to go into Q-mode. Alternatively the process of finding a low PAR symbol can be initiated periodically or on demand e.g. if other parts of the modem logic have detected changes in the wires that require changes in e.g. the modulation. The process of measuring PAR, or other measures of transmitted power is preferably performed in the analogue parts of the line driver circuit, preferably close to the input of the line driver. Shown in FIG. 8 is a transmitter unit 80 comprising a power analyser 86 placed in between the D/A-converter 84 and the Line driver 88. The power analyser 86 is connected to the digital parts 82 of the circuit by a selection connection 90, through which selection signals such as the PAR-values to corresponding symbols or other suitable signals may be sent. Alternatively, a comparator may be comprised in the power analyser 86, sending a selection signal to the digital parts 82 if the voltage peak values of a symbol are below a pre-set power level. The digital part 82 buffers the outputted symbols and responds to the selection signal from the comparator by storing the corresponding symbol as suitable for the "filler" signal.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. Method of selecting at least one "filler" symbol that is suitable to be transmitted as a Q-mode signal in a XDSL system, characterized by the steps:

transmitting a number of symbols,

recording a power value for each symbol that is transmitted, and

selecting, in response to the recorded power value, at least one of said transmitted symbols as a suitable "filler" symbol.

2. Method according to claim 1, characterized in that the selection step comprises:

comparing the power value of each symbol with a predefined threshold parameter T_p, and

if a symbol has a power value that is lower than the threshold parameter T_p, then selecting said symbol as a suitable "filler" symbol.

3. Method according to claim 1, characterized in that the threshold parameter T_p is defined a priori by transmitting a large number of symbols during various conditions and selecting a T_p value, which is lower than the power value of highest probability of occurrence, and which has a probability of occurrence in the interval of 0.5% to 30%

4. Method according to claim 1, characterized in that the selection step comprises:

comparing the power value of each symbol with a threshold parameter T_p defined as a power value, which is lower than the power value of highest probability of occurrence, and which has a probability of occurrence in the interval of 0.5% to 30%, and if a symbol has a power value that is lower than the threshold parameter T_p, then selecting said symbol as a suitable "filler" symbol.

5. Method according to claim 1, characterized in that the selection step comprises:

finding the symbol with the lowest power value, and

selecting said symbol as a suitable "filler" symbol.

6. Method according to any of the claims 1 to 5, characterized in that the step of transmitting is comprised in a normal training phase transmission.

7. Method according to any of the claims 1 to 5, characterized in that the step of transmitting is comprised in a normal Showtime transmission.

8. Method according to any of the claims 1 to 7, characterized in that the steps of the method are repeated during a communication session, such that the "filler" symbol continuously is updated.

9. Method of selecting at least one "filler" symbol that is suitable to be transmitted as a Q-mode signal in a XDSL system, ustilizing a transmitter unit (80) for a XDSL system comprising a line driver (88), a D/A-converter (84) and digital parts (82) connected in series characterized by the steps:

providing a power analyzer (86) in between the line driver (88) and the D/A- converter (84),

providing a selection connection (90) between said power analyzer (86) and the digital parts (82) of the circuit,

transmitting, with the transmitter unit (80), a number of symbols,

recording, with the power analyzer (86), a power value for each symbol that is transmitted, and through which may be sent.

sending, for each symbol that is transmitted, a selection signal from the power analyzer (86) to the digital parts (82) of the circuit

selecting, in the digital parts (82) of the circuit, in response to the selection signal, at least one of said transmitted symbols as a suitable "filler" symbol.

10. Method according to claim 9, characterized in that the step of sending selection signals comprise sending digital selection signals corresponding to the transmission power for each symbol that is transmitted.

11. Method according to claim 9, characterized by the steps:

providing a comparator comprised in the power analyzer (86), and

providing, in the digital part (82), a buffer for the transmitted symbols,

whereby the step of sending selection signals comprise sending a selection signal to the digital parts (82) if the peak voltage of a symbol are below a pre-set level, and the step of selecting a suitable "filler" symbol comprise selecting the symbol corresponding to the selection signal from the comparator.

12. Transmitter unit (80) for a XDSL system comprising a line driver (88), a D/A- converter (84) and digital parts (82) connected in series characterized in that a power analyzer (86) that is arranged in between the line driver (88) and the D/A- converter (84), said power analyzer (88) is further connected to the digital parts (84) of the circuit by a selection connection (90), through which selection signals may be sent.

13. Transmitter unit (80) according to claim 12 characterized in that said power analyzer (86) is arranged to provide digital selection signals corresponding to the transmission power for each symbol that is transmitted.

14. Transmitter unit according to claim 12 characterized in that

a comparator is comprised in the power analyzer (86), sending a selection signal to the digital parts (82) if the peak voltage of a symbol are below a pre-set level, and

that the digital part (82) is arranged to buffer the transmitted symbols and to respond to the selection signal from the comparator by selecting the corresponding symbol as a suitable "filler" symbol.

15. XDSL modem characterized in that it comprises at least one transmitter unit according to any of the claims 12 to 14.

16. Exchange for a XDSL system characterized in that it comprises at least one transmitter unit according to any of the claims 12 to 14.

17. XDSL system characterized in that it comprises at least one transmitter unit according to any of the claims 12 to 14.