EP2452457A1

EP2452457A1 - Method and device for generating an aggregate signal to reduce the transmission power

Info

Publication number: EP2452457A1
Application number: EP09780364A
Authority: EP
Inventors: Thomas Ahrndt; Matthias Schnitter
Original assignee: Nokia Siemens Networks Oy
Current assignee: Nokia Solutions and Networks Oy
Priority date: 2009-07-09
Filing date: 2009-07-09
Publication date: 2012-05-16
Also published as: WO2011003449A1

Abstract

A method and a device for processing an aggregate signal are provided, said aggregate signal comprising several subcarriers, wherein a first set of subcarriers is assigned to a reduced transmission power; and wherein a second set of subcarriers is utilized for a normal transmission mode.

Description

METHOD AND DEVICE FOR GENERATING AN AGGREGATE SIGNAL TO REDUCE THE TRANSMISSION POWER The invention relates to a method and to a device for processing an aggregate signal.

Current standards for digital subscriber line transmission such as ADSL, HDSL, VDSL, ADSL2/ADSL2+ or VDSL2 follow large- Iy a traditional "leased line philosophy" such that after an initial synchronization procedure a high bit rate connection is running 24 hours a day irrespective of whether there is payload requested by a user or not. In ADSL2/2+ adaptation of transmission power is achieved by a so-called "L2 mode" that allows reducing the power for transmission when the required payload data rate falls below a predetermined threshold. However, this L2 mode needs to run a protocol between an ATU- C, i.e. an ADSL-modem in the central office (CO), and an ATU- R, i.e. a remote modem within the premises of the subscriber. Both, ATU-C and ATU-R negotiate terms, in particular a start and an end, of an L2 mode session. This leads to a certain complexity within both devices ATU-C and ATU-R. Also, swit- ching-on as well as switching-off the L2 mode is delayed due to such negotiation.

The problem to be solved is to overcome the disadvantages as stated before and to provide an efficient solution to reduce transmission power, in particular of an xDSL transmission system.

This problem is solved according to the features of the inde- pendent claims. Further embodiments result from the depending claims . In order to overcome this problem, a method for processing of an aggregate signal is provided, said aggregate signal comprising several subcarriers,

- wherein a first set of subcarriers is assigned to a reduced transmission power;

- wherein a second set of subcarriers is utilized for a normal transmission mode.

Hence, this approach allows reducing a transmit power in a DSL environment without any need to change a bit allocation table. Also, the switching to a reduced power mode can be achieved without any significant delay. The aggregate signal, e.g., a transmit signal, thus has a reduced mean transmit power .

The second set of subcarriers can be used to transmit system immanent information, e.g., via an overhead channel. Also or as an alternative, the second set of subcarriers may be utilized for conveying user traffic (in particular at a low bit rate) .

The approach suggested allows utilizing a mode of reduced transmission power, i.e., the first set of subcarriers may utilize constellation points of low amplitude in a constella- tion diagram, which may not be utilized for conveying user traffic at all or which may be utilized for user traffic at a low data rate. This mode of reduced transmission power can be switched on and off, said switching between modes can be provided without any significant delay.

Hence, the transmission power of an xDSL system is reduced by an appropriate configuration of QAM transmit signals utilized. The content of the data packets on the one hand and the system immanent frame structure and the overhead information on the other hand may determine per each DMT symbol and per each subcarrier the amplitude and phase for the QAM of the trans- mit signal. Hence, the transmit power can be adjusted accordingly in a direct manner.

In an embodiment, the first set of subcarriers is associated with at least one constellation point of a constellation diagram with low amplitude.

Hence, constellation points of the constellation diagram (i.e., I-Q-coordinates in the QAM constellation diagram) can be used that are adjacent to the origin. Such constellation points are of a reduced amplitude (and thus require less transmission energy) compared to constellation points located far-off the origin of the constellation diagram. QAM or any modulation scheme conveying two digital bit streams or two analog message signals by changing (modulating) the amplitudes of two carrier waves, using an amplitude-shift keying (ASK) digital modulation scheme or an amplitude modulation (AM) analog modulation scheme can be utili- zed.

It is also possible to provide a single bit stream or a single analog message signal. In this case, the amplitude of a single carrier wave could be changed (modulated) accordingly.

In another embodiment, the constellation points for the first set of subcarriers are changed, in particular rotated or according to a given hopping pattern. This approach efficiently allows for reducing (or compensating) crosstalk.

In a further embodiment, a constellation point is transmitted for a predetermined number of consecutive symbols.

This efficiently enables a receiver to determine an interference for such several symbols. Hence, such interference may be subtracted from received signals. Said symbols may in particular be DMT symbols.

In a next embodiment, the first set of subcarriers is associ- ated with signals of a given amplitude and/or phase pattern.

Hence, a fixed assignment can be provided for said first set of subcarriers to signals that are mapped to a certain amplitude and/or a certain phase pattern.

It is also an embodiment that the first set of subcarriers is associated with signals that vary in amplitude and/or phase.

Pursuant to another embodiment, the aggregate signal is a signal in the frequency domain and in particular each subcar- rier of the aggregate signal is modulated by an amplitude modulation, by a quadrature amplitude modulation and/or by Amplitude Shift Keying, in particular in combination with a phase modulation.

It is noted that a single carrier signal may also be provided and/or processed instead of a multi-carrier signal.

According to an embodiment, said aggregate signal is proces- sed on at least one of the following components:

- a network element;

- a central office device;

- a DSLAM;

- an access point;

- an access multiplexer;

- a customer premises equipment;

- a modem.

According to another embodiment, a network element informs another network element about the first set of subcarriers and the second set of subcarriers. One network element may be realized as a CPE. The other network element may be located in or associated with the CO or DSLAM. Hence, CPE and CO device each may know which subcarriers may be modified in terms of coding and a protocol running between these network elements only has to convey information regarding the subcarriers that are actually modified. Therefore, informing the respective other network element (e.g., CPE or CO) about any modification is much faster compared to an existing L2 mode protocol.

In yet another embodiment, unused subcarriers are at least temporarily assigned to the first set of subcarriers.

According to a next embodiment, the second set of subcarriers utilize lower frequencies than the first set of subcarriers.

Pursuant to yet an embodiment, after a predetermined period of time and/or by external trigger the assignment of subcarriers associated with the first set and/or the second set is changed.

Hence, advantageously, subcarriers associated with the first set of subcarriers can be mapped to the second set of subcarriers in case additional user traffic needs to be conveyed or the energy saving mode is no longer possible or required. This process also works the other way round, i.e., subcarriers may also be mapped to the first set of subcarriers in case, e.g., less user traffic is requested.

The problem stated above is also solved by a device comprising a and/or being associated with a processor unit and/or a hard-wired circuit and/or a logic device that is arranged such that the method as described herein is executable thereon . According to an embodiment, the device is a communication device, in particular a or being associated with a network element, a central office device or a customer premises equipment .

The problem stated supra is further solved by a communication system comprising the device as described herein.

Embodiments of the invention are shown and illustrated in the following figures:

Fig.l shows constellation diagrams according to ITU-T

G.992.1 (06/99); Fig.2 shows a block diagram comprising a central office device (CO) and a customer premises equipment (CPE) that are connected with one another, wherein an aggregate signal is conveyed from the CO to the CPE (or vice versa) .

This approach suggests reducing a transmit power in a DSL environment in a faster manner without any need for changing a bit allocation table. It is an objective to generate an aggregate transmit signal with a mean transmit power (or the mean amplitude) being as low as possible.

Since the discrete multi-tone modulation (DMT) is used, the aggregate transmit signal in the frequency domain comprises a sum of many subcarriers with each subcarrier being modulated using QAM.

Fig.l is taken from ITU-T G.992.1 (06/99), figure 7-17, de- picting exemplary constellation diagrams 101 and 102.

Regarding an example with one subcarrier only, a transmit signal could be generated that utilizes one of the values 0, 5, 10 or 15 in the frequency domain according to the constellation diagram 102.

A majority of subcarriers may be modulated such that the mean transmit power is decreased. This can be achieved, e.g., by providing a fixed assignment of certain subcarriers to signals which are forced to certain amplitude and phase patterns. Also, the transmit signal can slightly vary in amplitude and/or phase.

The rest of the subcarriers can then be used to transmit system immanent information contained in an overhead channel as well as low bit rate user traffic. Such an approach reduces the mean amplitude of the transmit signal. Hence, the effective transmit power is reduced.

A chosen constellation can be rotated from one DMT symbol to a subsequent DMT symbol (e.g., with respect to the origin of the constellation diagram) in order to compensate and/or reduce crosstalk. Alternatively (or in addition), special hopping patterns can be defined for choosing constellations.

It is also an alternative to transmit the same constellation for a number of consecutive symbols. The receiver may then calculate an interference by computing a mean interference over such several symbols, e.g., DMT symbols or ASK symbols. Based on such interference calculation, the receiver may subtract an interference value from a received signal.

A protocol could be defined between a central office device (CO) and a customer premises equipment (CPE) , which may inform the receiver about subcarriers comprising user data and subcarriers comprising dummy data. Such an interval can, e.g., be defined as follows: Starting with a subcarrier a and a symbol b, ending at a subcarrier c and a symbol d. Alternatively to such protocol, the subcarriers to be modulated by fixed patterns could be defined in advance (e.g., standardized or negotiated during a start-up phase) . In this case, the protocol may only inform the remote end about a starting symbol indicating a starting point for fixed patterns being used. Accordingly, an end symbol can be provided for switching off said fixed patterns.

The approach provided herein can be used, e.g., for VDSL2 and ADSL2/2+ systems to reduce the transmit power on a physical layer .

Hence, a protocol may be provided known to the transmitter as well as to the receiver. This bears the advantage that less computing power is required, because favorable QAM constellations (amplitudes and matching phases) can be predefined and can be independent from the other (user) data. These constellations can be assigned to the unused subcarriers. Advantageously, the transmit power level is reduced on a physical layer of a network element.

It is also an advantage, that the solution suggested may be implemented at least partially as software. In particular the approach may be provided as a full software solution.

Fig.2 shows a block diagram comprising a central office device (CO) 201 and a customer premises equipment (CPE) 202 that are connected with one another. An aggregate signal is con- veyed from the CO 201 to the CPE 202 (or vice versa) comprising a first set of subcarriers 203 and a second set of sub- carriers 204. The first set of subcarriers 203 utilize a reduced transmission power, i.e. constellations with reduced amplitude that are closer to the origin of a constellation diagram. In contrast, the second set of subcarriers is used for normal operation purposes and may utilize constellations with high amplitude. The number of subcarriers assigned to the first set and to the second set may be adjusted according to, e.g., a requirement of the actual traffic to be conveyed. Further Advantages:

The transmit power of the DSL system can be reduced.

Furthermore, the approach is applicable for the CO and/or for the CPE.

No hardware changes are required.

Also, pursuant to the approach suggested, a faster reaction to data rate variations is possible compared to the current L2 mode solution of ADSL2/2+.

The existing L2 mode requires a protocol between the CPE and the CO to negotiate a starting point and an end of the L2 mode as well as a second bit allocation table to be used during L2 mode. Such protocol deteriorates the switching time of the L2 mode. Also, such protocol adds complexity to the transceivers. The reason for this particular protocol is the change of the bit allocation table.

In contrast thereto, the approach presented does not require for complex changes to be conducted at the CPE and the CO. Only a variation of the coding on at least one subcarrier needs to be conveyed to the respective other network element. The coding as such is known to the CPE and CO according to the protocol implementation.

Hence, CPE and CO each knows which subcarriers may be modified in terms of coding and the protocol only has to convey information regarding the subcarriers that are actually modified. Therefore, informing the respective other network element (e.g., CPE or CO) about any modification planned is much faster compared to the existing L2 mode protocol. This approach can be applied in combination with any DSL utilizing QAM, i.e. ADSL2/ADSL2+, VDSL, VDSL2. Also, any other communication method utilizing a modulation technique thereby mapping user information to the carrier signal amplitude and/or to the carrier signal phase can be utilized, e.g., ASK and/or amplitude modulation.

List of Abbreviations:

ADSL Asymmetric Digital Subscriber Line

ASK Amplitude Shift Keying

ATU ADSL Transceiver Unit

ATU-C ATU at the central office end (i.e., network operator)

ATU-R ATU at the remote terminal end (i.e., CP)

ATU-x Any one of ATU-C or ATU-R

BER Bit Error Ratio

CO Central office

CP Customer Premises

CPE Customer Premises Equipment

DMT Discrete Multi-Tone modulation

DSL Digital Subscriber Line

DSLAM Digital Subscriber Line Access Multiplexer

IP Internet Protocol

IPTV IP Television

OFDM Orthogonal Frequency Division Multiplexing

QAM Quadrature Amplitude Modulation

VDSL Very high-speed Digital Subscriber Line

xDSL any DSL (e.g., ADSL, ADSL2, ADSL2+, SHDSL, VDSL,

VDSL2)

Claims

Claims :

1. A method for processing of an aggregate signal, said aggregate signal comprising several subcarriers,

2. The method according to claim 1, wherein the first set of subcarriers is associated with at least one symbol of a constellation with low amplitude.

3. The method according to claim 2, wherein the constellation points for the first set of subcarriers are

changed, in particular rotated and/or according to a given hopping pattern.

4. The method according to any of claims 2 or 3, wherein a constellation point is transmitted for a predetermined number of consecutive symbols.

5. The method according to any of the preceding claims, wherein the first set of subcarriers is associated with signals of a given amplitude and/or phase patterns.

6. The method according to any of the preceding claims, wherein the first set of subcarriers is associated with signals that vary in amplitude and/or phase.

7. The method according to any of the preceding claims, wherein the aggregate signal is a signal in the frequency domain and in particular each subcarrier of the aggregate signal is modulated by an amplitude modulation, by a quadrature amplitude modulation and/or by Amplitude Shift Keying, in particular in combination with a phase modulation.

8. The method according to any of the preceding claims, wherein said aggregate signal is processed on at least one of the following components: - a network element;

- a central office device;

- a DSLAM;

- an access point;

- an access multiplexer;

- a customer premises equipment;

- a modem.

9. The method according to any of the preceding claims, wherein a network element informs another network ele- ment about the first set of subcarriers and the second set of subcarriers.

10. The method according to any of the preceding claims, wherein the first set of subcarriers and the second set of subcarriers are determined during a startup phase of a network element.

11. The method according to any of the preceding claims, wherein unused subcarriers are at least temporarily assigned to the first set of subcarriers.

12. The method according to any of the preceding claims, wherein the second set of subcarriers utilize lower frequencies than the first set of subcarriers.

13. The method according to any of the preceding claims, wherein after a predetermined period of time and/or by external trigger the assignment of subcarriers associ- ated with the first set and/or the second set is

changed.

14. A device comprising a and/or being associated with a

processor unit and/or a hard-wired circuit and/or a logic device that is arranged such that the method ac- cording to any of the preceding claims is executable thereon .

15. The device according to claim 14, wherein said device is a communication device, in particular a or being associ- ated with a network element, a central office device or a customer premises equipment.