Configuring the virtual noise parameters of a digital subscriber line

An apparatus comprising a digital subscriber line (DSL) transmitter configured to transmit a transmitter referred virtual noise for a tone in a subscriber line, wherein the transmitter referred virtual noise is based on a time history of a noise condition in the subscriber line. Also disclosed is an apparatus comprising at least one processor configured to implement a method comprising obtaining a transmitter referred virtual noise for a DSL tone using a plurality of noise condition measurements, and determining a DSL bitloading using the transmitter referred virtual noise.

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Digital subscriber line (DSL) technologies can provide relatively large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent twisted-pair phone lines, for example in a same or nearby bundle of lines. Crosstalk limits the performance of some DSL technologies, such as asymmetric DSL 2 (ADSL2) and very high bit rate DSL 2 (VDSL2). For example, significant levels of crosstalk can occur because of the relatively high frequencies used in VDSL2. The crosstalk is highly non-stationary and varies dramatically as lines within the binder are activated and deactivated. Such rapidly varying noise environment causes frequent re-initializations of the lines, loss of service, and low customer satisfaction.

One method for increasing the stability of digital subscriber lines is using virtual noise, a technique that limits the maximum bitloading allowed on each tone. Virtual noise is used to improve DSL stability by ensuring operation of a line at a bitloading level that can be sustained when worse noise conditions are encountered. The performance of a DSL is substantially dependent on the virtual noise configuration for the line. For example, if the virtual noise is set too low, the line becomes unstable. Alternatively, if the virtual noise is set too high, the line's data-rate is unnecessarily reduced. Finding an appropriate configuration for the virtual noise can be difficult due to a variety of practical issues, such as difficulties in line noise data collection, limitations on the number of line profiles in DSL access multiplexers (DSLAMs), finding appropriate balance between line stability and data-rate, and adaptability to changes in the noise environment.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising a DSL transmitter configured to transmit a transmitter referred virtual noise for a tone in a subscriber line, wherein the transmitter referred virtual noise is based on a time history of a noise condition in the subscriber line.

In another embodiment, the disclosure includes an apparatus comprising at least one processor configured to implement a method comprising obtaining a transmitter referred virtual noise for a DSL tone using a plurality of noise condition measurements, and determining a DSL bitloading using the transmitter referred virtual noise.

In yet another embodiment, the disclosure includes a method comprising defining a plurality of virtual noise templates for a plurality of subscriber lines, calculating a transmitter referred virtual noise for a tone in one of the subscriber lines based on a time history of noise conditions in the subscriber line, and selecting from the defined virtual noise templates a virtual noise template that best matches the transmitter referred virtual noise for the subscriber line.

DETAILED DESCRIPTION

Disclosed herein are systems and methods for configuring virtual noise in DSL systems to improve line stability and limit abrupt changes in line data-rate. Specifically, a transmitter referred virtual noise for a tone in the line may be calculated based on a standard operating signal to noise ratio (SNR) margin, a minimum SNR margin required to ensure line stability, and a transmitter referred line noise for a point in time. The virtual noise may be configured based on a history of the line noise condition using a plurality of time samples or measurements of the line noise condition. To protect against the worst case noise condition over a time period, the transmitter referred virtual noise may be calculated based on a maximum transmitter referred line noise over that time. Alternatively, the transmitter referred virtual noise may be calculated based on a mean of the transmitter referred line noise and a standard deviation of the transmitter referred line noise if a better balance between line stability and line data-rate is desired. In another alternative embodiment, the transmitter referred virtual noise may be adjusted to adapt faster to line noise condition based on a last recent time sample of the of the line noise condition. Additionally, a quantity of virtual noise templates may be defined to configure a larger quantity of subscriber lines in the system. The quantity of the virtual noise templates may be limited by the quantity of supported line profiles in the system. Hence, a virtual noise template that best matches the transmitter referred virtual noise in the line may be selected from the set of virtual noise templates.

FIG. 1illustrates one embodiment of a DSL system100. The DSL system100may be a VDSL2 system, an ADSL2 system, an ADSL2 plus (ADSL2+) system, or any other DSL system. The DSL system100may comprise an Exchange102, optionally a Cabinet104coupled to the Exchange102by a cable105, and a plurality of customer premise equipments (CPEs)106, which may be coupled to the Exchange102and/or the Cabinet104via a plurality of subscriber lines108. At least some of the subscriber lines108may be bundled in a binder109. Additionally, the DSL system100may optionally comprise a network management system (NMS)110and a public switched telephone network (PSTN)112, both of which may be coupled to the Exchange102. In other embodiments, the DSL system100may be modified to include splitters, filters, management entities, and various other hardware, software, and functionality.

The NMS110may be a network management infrastructure that processes data exchanged with the Exchange102and may be coupled to one or more broadband networks, such as the Internet. The PSTN112may be a network that generates, processes, and receives voice or other voice-band signals. In an embodiment, the Exchange102may be a server located at a central office and may comprise switches and/or splitters, which may couple the NMS110, the PSTN112, and the subscriber lines108. For instance, the splitter may be a 2:1 coupler that forwards data signals received from the subscriber lines108to the NMS110and the PSTN112, and forwards data signals received from the NMS110and the PSTN112to the subscriber lines108. Further, the splitter may optionally comprise one or more filters to help direct data signals between the NMS110, the PSTN112, and the subscriber line108. Additionally, the Exchange102may comprise at least one DSL transmitter/receiver (transceiver), which may exchange signals between the NMS110, the PSTN112, and the subscriber lines108. The signals may be received and transmitted using the DSL transceiver, such as a modem. In an embodiment, the DSL transceiver may comprise a forward error correction (FEC) codeword generator that generates FEC data, an interleaver that interleaves the transmitted data across a plurality of tones, or both. For instance, the DSL transceiver may use a discrete multi-tone (DMT) line code that allocates a plurality of bits for each sub-carrier or tone in each symbol. The DMT may be adjusted to various channel conditions that may occur at each end of a subscriber line. In an embodiment, the DSL transceiver of the Exchange102may be configured to transmit data at similar or different rates for each subscriber line108.

In an embodiment, the Cabinet104may be located at a distribution center between the central office (CO) and customer premises and may comprise switches and/or splitters, which may couple the Exchange102to the CPEs106. For instance, the Cabinet104may comprise a DSL access multiplexer (DSLAM) that couples the Exchange102to the CPEs106. Additionally, the Cabinet104may comprise a DSL transceiver, which may be used to exchange signals between the Exchange102and the CPEs106. The DSL transceiver may process the received signals or may simply pass the received signals between the CPEs106and the Exchange102. The splitter in the Cabinet104may be a N:1 coupler (where N is an integer) that routes data signals received from the Exchange102to N CPEs106, and routes data signals received from the N CPEs106to the Exchange102. The data signals may be transmitted and received using the DSL transceiver, which may be a modem. Further, the splitter of the Cabinet104may optionally comprise one or more filters to help direct data signals between the Exchange102and the CPEs106via the corresponding subscriber lines108. In an embodiment, the DSL transceiver may be configured to transmit data to the CPEs106at similar or different rates and/or power for each subscriber line108, as described in detail below.

In an embodiment, the CPEs106may be located at the customer premises, where at least some of the CPEs106may be coupled to a telephone114and/or a computer116. The telephone114may be hardware, software, firmware, or combinations thereof that generates, processes, and receives voice or other voice-band signals. The CPE106may comprise a switch and/or a splitter, which may couple the subscriber lines108and the telephone114and the computer116. The CPE106may also comprise a DSL transceiver to exchange data between the CPE106and the Exchange102via the subscriber line108. For instance, the splitter may be a 2:1 coupler that forwards data signals received from the subscriber line108to the telephone114and the DSL transceiver, and forwards data signals received from the telephone114and the DSL transceiver to the subscriber line108. The splitter may optionally comprise one or more filters to help direct data signals to and from the telephone114and the DSL transceiver. The DSL transceiver, e.g. a modem, may transmit and receive signals through the subscriber lines108. For instance, the DSL transceiver may process the received signals to obtain the transmitted data from the Exchange102, and pass the received data to the telephone114, the computer116, or both. The CPEs106may be coupled to the Exchange102directly via the subscriber lines108and/or via the subscriber lines108and the Cabinet104. For example any of the CPEs106may be coupled to a subscriber line108from the Exchange102and/or a subscriber line108from the Cabinet104. The CPEs106may access the NMS110, the PSTN112, and/or other coupled networks via the subscriber lines108deployed by the Exchange102and/or the Cabinet104.

In an embodiment, the subscriber lines108may be telecommunications paths between the Exchange102and the CPE106and/or between the Cabinet104and the CPEs106, and may comprise one or more twisted-pairs of copper cable. Typically, crosstalk interference may occur between the tones or signals transported through the subscriber lines108. The crosstalk interference may be related to the power, frequency, and travel distance of the transmitted signals and may limit the communications performance in the network. For instance, when the transmitted signals have higher power spectral densities (PSDs), e.g. over a range of frequencies, the crosstalk between the adjacent subscriber lines108(e.g. in a binder) may increase and hence the signal data-rates may decrease. The crosstalk may be highly non-stationary and may vary substantially during activation, use, and/or deactivation of a subscriber line108, which may cause rapidly varying noise conditions and reduce line stability.

To guarantee sufficient line stability during rapidly varying noise conditions, for example due to non-stationary crosstalk, the transmissions in the subscriber lines108may be configured using virtual noise. For instance, the CO exchange102or the CPE104may receive a DSL signal that comprises a tone i and a transmitter referred virtual noise TXREFVN(i) associated with the tone i. The transmitter referred virtual noise may be used to determine the SNR of the tone i, SNR(i):

SNR⁡(i)=TXPSD⁡(i)·H⁡(i)H⁡(i)·TXREFVN⁡(i),
where TXPSD(i) is a transmit spectrum power density of the tone i and H(i) is a channel gain on the tone i.

In an embodiment, the transmitter referred virtual noise may be configured to ensure line stability and may be calculated based on a noise level in the receiver, RXNOISE(i):

TXREFVN⁡(i)=RXNOISE⁡(i)·SNRMminH⁡(i)·SNRM,
where SNRM is a standard operating SNR margin, e.g. equal to about six decibel (dB), and SNRMminis a minimum margin required to ensure line stability, which may be as low as about zero dB. The transmit power spectrum density (PSD) may be calculated based on a medley reference PSD, MREFPSD(i) and a gain adjustment parameter gi:
TXPSD(i)=MREFPSD(i)·|gi|2.
The SNR of the tone i may be obtained by substituting for TXREFVN(i) in the equations above:

SNR⁡(i)=TXPSD⁡(i)·H⁡(i)·SNRMRXNOISE⁡(i)·SNRMmin.
The obtained SNR may then be used to determine the line bitloading for the tone i, bi:

bi=log2⁡(1+1Γ⁢SNR⁡(i)SNRM)=log2⁡(1+1Γ⁢TXPSD⁡(i)·H⁡(i)RXNOISE⁡(i)·SNRMmin),
where Γ is a SNR gap to capacity parameter, which may be typically set to about 9.75 dB minus a coding gain. Thus, line stability and operation may be maintained by adjusting the bitloading level in the line according to the line noise conditions. When the line noise is equal to the noise level in the receiver RXNOISE(i), the SNR margin SNRM may be equal to about the minimum margin SNRMmin, which may ensure line stability. The minimum margin SNRMminmay be set to about zero dB or to a higher tolerated value, e.g. at about three dB. The minimum margin SNRMminmay be substantially smaller than the SNR margin SNRM, which may reduce the virtual noise and increase the data-rate.

The RXNOISE(i) may be determined using a reported quite line noise QLN(i), which may not be updated during showtime (e.g. line activation time), and therefore give limited information about the noise condition during a modem's operation time. Further, QLN(i) may be measured when a near-end transmitter (e.g. at a cabinet) and a far-end transmitter (e.g. at a CO exchange) may be silent or inactive. During showtime, the near-end and far-end transmitters become active, and the powers of the transmitted signals may substantially increase, which may change the noise spectrum in the lines. For this reason, the reported quite line noise QLN(i) may provide a poor indication of the actual noise during operation time. In an embodiment, to overcome this problem, the noise level in the receiver RXNOISE(i) may be determined based on a reported SNR, reportedSNR(i), for a particular point in time:

RXNOISE⁡(i)=TXPSD⁡(i)·H⁡(i)reportedSNR⁡(i).
Hence, the transmitter referred virtual noise TXREFVN(i) may be calculated:

To improve line stability, the virtual noise may be configured by addressing a plurality of practical issues related to line operations. For instance, a history of the line's noise condition may be needed to accurately configure the virtual noise in the line. In an embodiment, the virtual noise may be configured accurately using the history of the noise condition based on a representative sample or measurement of line noise conditions. For instance, the virtual noise for each tone may be calculated based on a plurality of samples or measurements of TXPSD(i) and reportedSNR(i) that may be obtained over a time period, e.g. in a periodic manner. The virtual noise may be calculated to protect against the worst case noise condition over that time using a maximum TXPSD(i) to reportedSNR(i) ratio over that time:

TXREFVN⁡(i)=SNRMminSNRM⁢maxt⁢{TXNOISEt⁡(i)},(1)
where max{ } indicates a function for selecting a maximum sample from a set,

TXNOISEt⁡(i)=TXPSDt⁡(i)reportedSNRt⁡(i)
is a transmitter referred line noise, and TXPSDt(i) and reportedSNRt(i) are samples obtained at a time t. Using the maximum transmitter referred line noise over time

(maxt⁢{TXNOISEt⁡(i)})⁢⁢to⁢⁢calculate⁢⁢TXREFVN⁡(i)
may ensure relatively high line stability but may also substantially reduce the line data-rate, which may be unnecessary.

Balancing line stability vs. data-rate is another practical issue that may be addressed when configuring the virtual noise. In an embodiment, a tradeoff may be achieved between line stability and data-rate based on statistics of the line noise measurements, for instance using a mean of the transmitter referred line noise, TXNOISEmean(i), and a standard deviation of the transmitter referred line noise TXNOISEstd(i):

TXREFVN⁡(i)=SNRMminSNRM⁢(TXNOISEmean⁡(i)+v·TXNOISEstd⁡(i)).(2)
The mean of the transmitter referred line noise TXNOISEmean(i), and the standard deviation of the transmitter referred line noise TXNOISEstd(i) may be calculated:

TXNOISEmean⁡(i)=1T⁢∑t⁢TXNOISEt⁡(i)⁢⁢andTXNOISEstd⁡(i)=1T⁢∑t⁢(TXNOISEt⁡(i)-TXNOISEmean⁡(i))2,
where T is the total quantity of time samples obtained. In equation (2), v is a multiple of standard deviations away from the mean of the virtual noise. The value of v may be selected to determine a confidence interval or amount of certainty that the line noise is below the virtual noise. For example, the value of v may be equal to about two, which may correspond to an amount of certainty at about 97 percent.

Another practical issue of virtual noise configuration is the memory requirement. Using the history of the noise condition to configure the virtual noise may require collecting and storing the samples of noise conditions over a period of time, which may require substantial memory allocation, e.g. by the NMS. The memory requirement may further increase as the quantity of lines in the network increases. In an embodiment, the virtual noise configured based on the line noise condition may be adjusted based on a most recent or current virtual noise setting. For instance, the virtual noise TXREFVNt(i) may be configured based on an update rule that depends on the last calculated virtual noise TXREFVNt-1(i), such as

The factor α in the equation above may be changed to allow different trade-offs between line stability and data-rate. The factor α may be determined based on a half-life λ for virtual noise adaptation to line noise condition. The half-life λ may be a time required for the virtual noise value to decay or decrease to its half value if the line noise is equal to bout zero. For instance, if

TXNOISEt⁡(i)={1,t=00,t>0⁢⁢and⁢⁢SNRMmin=SNRM,then⁢⁢TXREVFVNt⁡(i)=0.5⁢⁢when⁢⁢t=λmeasurementPeriod,
where measurementPeriod is a period between line noise measurements. To achieve a half-life equal to about one, the factor α may be set:

α=2-measurementPeriodλ.
For example, if the period between line noise measurements is equal to about 15 minutes, e.g. the measurements of reportedSNR(i) and TXPSD(i) are updated every about 15 minutes, then a half-life of about six hours may be achieved by setting

Another consideration is the adaptability of the virtual noise configuration, where changes to the noise environment may be monitored over time and the virtual noise may be adjusted accordingly. Adaptability may be improved when a relatively small time window is used to monitor the changing noise environment. Using the last time sample of noise condition to configure the virtual noise, as shown above, may allow faster tracking of changes in line conditions and faster response. Additionally, the last sample of noise condition in time may be based on preceding samples of noise condition, and thus may comprise information about the line noise condition over a time window substantially larger than last monitored time window. As such, the virtual noise may be configured to adapt to the line noise condition over time and may be calculated accurately based on sufficient line history of the noise condition.

FIG. 2illustrates an embodiment of a virtual noise configuration200, which may be adaptive to changing line conditions over a period of time. The virtual noise configuration200is represented by a curve210that comprises a plurality of points. The points correspond to the calculated virtual noise values over a plurality of hours using the update rule in equation (3). Accordingly, the virtual noise value at a time t (TXREFVNt(i)) in the curve210is calculated using the last calculated virtual noise value at time t−1 (TXREFVNt-1(i)) and the obtained transmitter referred line noise at time t (TNOISEt(i)). The curve220comprises a plurality of points that correspond to the obtained TXNOISEt(i) values over the same time as the curve210.

The adaptability of the virtual noise to the transmitter referred line noise over the time period may be observed by comparing the patterns of the curve210and the curve220. As the values of the curve220that represent line noise conditions increase over time, the values of the curve210may also increase at about the same rate. For example the increasing portions of the curve210and220may have about the same slope, e.g. between about zero hour and about 2.5 hours and between about 13 hours and about 15 hours. The matching rate between the increasing portions in the two curves may indicate fast virtual noise response to increasing line noise.

Further, as the values of the curve220decrease over time, the values of the curve210may decrease at a slower rate (smaller slope), e.g. between about 2.5 hours and about five hours and between about 15 hours and about 17 hours, in comparison to its increasing rate. The rate of decrease or decay in the curve210may be determined by the factor α in equation (3), which may be equal to about 0.9715 and correspond to a measurement period equal to about 15 minutes and a half-life decay equal to about six hours. The slower rate of the curve210may indicate a slow virtual noise response to decreasing line noise in comparison to the case of increasing line noise. A slower virtual noise response to decreasing line noise may be desired to account for sudden noise rising and/or relatively strong noise fluctuations, e.g. between about 7.5 hours and 13 hours and between about 16 hours and about 18 hours. The relatively slow virtual noise response to such sharp and sudden changes in line conditions may prevent substantial and abrupt changes in line data-rate and hence improve line stability.

Another practical issue for virtual noise configuration may be the limited quantity of line profiles that may be supported by the DSL system. For instance, a DSLAM (e.g. in a CO exchange or cabinet) may support a limited quantity of line profiles, which may be less than the quantity of subscriber lines in the system. Each line profile may be associated with a virtual noise configuration, which may be stored as part of the line profile. Thus, it may not be possible to use a distinct virtual noise configuration for each subscriber line. In an embodiment, a plurality of virtual noise templates may be established and used to service the subscriber lines, as described below.

FIG. 3illustrates one embodiment of a downstream transmission scenario300in a DSL system. The DSL system may comprise a combination transmitter302comprising a plurality of transmitters310, which may be coupled to a plurality of receivers320. For instance, the combination transmitter302may be a modem or a DSLAM in a CO exchange or a cabinet, where the transmitters310may be located. The receivers320may be located at separate locations in the system, for example at a plurality of CPEs. The transmitters310and the receivers320may be configured to transmit and receive, respectively, DSL signals or tones via a plurality of subscriber lines, e.g. similar to the components of the DSL system100. Some of the lines between the transmitters310and receivers320may suffer from crosstalk. The crosstalk may be non-stationary and may vary substantially and hence cause rapidly varying noise conditions and reduce line stability.

Ideally, line stability may be ensured by configuring the virtual noise for each of the subscriber lines individually. However, configuring the virtual noise for each line may require allocating a separate profile for each line, which may not be possible due to the limited quantity of available profiles for the transmitters310(e.g. in the DSLAM). In an embodiment, to improve line stability in the subscriber lines between the transmitters310and the receivers320, a set of virtual noise templates may be defined. The quantity of virtual noise templates may be equal to about the quantity of available profiles and may be configured based on a plurality of noise conditions that may be encountered. The virtual noise templates (and associated profiles) may be assigned to the lines that have actual noise conditions that most closely match the noise conditions of the virtual noise templates. Thus, the noise conditions that may be encountered in the lines may be addressed or accounted for using fewer virtual noise templates than the quantity of lines in the system. Consequently, the quantity of line profiles that may be needed for virtual noise configuration may be substantially reduced.

For instance, the transmitters310may comprise at least one victim transmitter (TX)311that may communicate with a victim receiver (RX)321in the receivers320in a downstream direction, e.g. from transmitter to receiver. The transmitters310may also comprise about N disturber TXs312(e.g. disturber TX1, disturber TX2. . . disturber TX N) that may communicate downstream with N corresponding disturber RXs322(e.g. disturber RX1, disturber RX2. . . disturber RX N) via N individual lines, where N is an integer. The signals in a victim line from the victim TX311to the victim RX321may be disturbed due to crosstalk interference by transmissions from at least one of the disturber TXs312, e.g. disturber TX N. Thus, the victim line may comprise a worst case downstream crosstalk channel from the victim TX311and one of the disturber TXs312, e.g. disturber TX N.

A crosstalk model may be used to define the worst case downstream crosstalk channel, such as based on an American National Standard T1.424-2004 by the Alliance for Telecommunications Industry Solutions (ATIS) or based on a European Telecommunications Standard Institute (ETSI) Standard TS 101 270-1, which are incorporated herein by reference as if reproduced in their entirety. For instance, a 1% worst case downstream crosstalk channel may be defined:

Hxtalk,1⁢%⁢⁢worst⁢⁢case⁡(i)=∑n=1N⁢H⁡(i)·Kxf2·f⁡(i)2·Lcoupling⁢⁢n,
where H(i) is a direct channel gain of the victim line from one of the disturber TXs512, Kxf2is equal to about −45 dB, f(i) is a frequency of tone i in Megahertz (MHz), and Lcoupling nis a coupling length from the disturber TX N into the victim line. Using this model, the 1% worst case crosstalk channel (or noise) at the victim RX321may be calculated:

Each or a plurality of virtual noise templates (VNTEMPLATE) may be based on the 1% worst case crosstalk model and the factor β, which may determine how severe the crosstalk may be in comparison to the worst case model. For example, there may be M virtual noise templates that may be supported by the DSLAM, e.g. VNTEMPLATE1(i), VNTEMPLATE2(i) . . . VTEMPLATEM(i), where M may be an integer less than N. The virtual noise templates may be configured based on the severity of crosstalk, e.g. on increasing severity of crosstalk, obtained:

FIG. 4illustrates an embodiment of a plurality of virtual noise templates400, which may be configured based on the equations above for a plurality of crosstalk or noise levels. The virtual noise templates400are represented by a plurality of curves410that comprise a plurality of points. The points correspond to virtual noise values in decibels per milliwatt per Hertz (dBm/Hz) vs. a range of frequencies in MHz (from about zero MHz to about 30 MHz). The curves410are shown for five virtual noise templates, which may be configured for the first five highest crosstalk levels (e.g. Template1, Template2. . . Template5). As shown in the equations above, the virtual noise templates are obtained using a substantially similar equation with the exception of a varying multiplying factor that may be varied according to the crosstalk or noise level, e.g. at about 2−Mfor the M-th template. Accordingly, the curves410have a substantially similar pattern or profile across the range of frequencies but vary in magnitude (virtual noise value). The first virtual noise template (Template1) may comprise the highest virtual noise values and may be used to configure any of the lines in the case of the highest crosstalk severity. Generally, any of the lines may be configured using any of the virtual noise templates that best matches the crosstalk severity in the line.

FIG. 5illustrates one embodiment of an upstream transmission scenario500in a DSL system, for instance similar to the DSL system ofFIG. 3. As shown inFIG. 5, the DSL system may comprise a combination transmitter502comprising a plurality of transmitters510, which may be coupled to a plurality of receivers520. The combination transmitter502may be a modem or a DSLAM in a CO exchange or a cabinet, where the transmitters510may be located. The receivers520may be located at a separate location in the system, for example at a plurality of CPEs. The transmitters510and the receivers520may be configured to transmit and receive, respectively, DSL signals or tones via a plurality of subscriber lines. The lines may suffer from non-stationary and substantially varying crosstalk levels, which may reduce data-rate stability in the lines. To improve line stability in the lines, a plurality of virtual noise templates may be defined, e.g. for a plurality of crosstalk levels in the lines. The quantity of virtual noise templates may be limited to about the quantity of line profiles supported by the combination transmitter502(e.g. in a DSLAM) and may be less than the quantity of lines in the DSL system.

A victim receiver521in the receivers520may communicate with a victim TX511in the transmitters510in an upstream direction, e.g. from receiver to transmitter. A plurality of N disturber RXs522(e.g. disturber RX1, disturber RX2. . . disturber RX N) may also communicate with a plurality of N corresponding disturber TXs512(e.g. disturber TX1, disturber TX2. . . disturber TX N) in the transmitters510via N individual lines, where N is an integer. The signals in a victim line from the victim RX521to the victim TX511may be disturbed from crosstalk interference by transmissions from at least one of the disturber RXs522, e.g. disturber RX N. Thus, the victim line may comprise a worst case upstream crosstalk channel from the victim RX521and at least one of the disturber RXs522, e.g. disturber TX N.

Based on the ATIS or ETSI standards described above, a 1% worst case upstream crosstalk channel may be defined:

Hxtalk,1⁢%⁢⁢worst⁢⁢case⁡(i)=∑n⁢H⁡(i)disturber⁢⁢n·Kxf2·f⁡(i)2·Lcoupling⁢⁢n,
where Hdisturber n(i) is a direct channel gain of the victim line from one of the disturber RXs522. In the case of upstream transmissions, the receivers520may communicate with the transmitters510using upstream power back-off (UPBO), which is a static spectrum management technique that is used to reduce crosstalk in the lines. As such, the upstream transmit PSD of the n-th disturber RX522, UPBOMASKn(i), may be obtained:

UPBOMASKn⁡(i)=UPBOPSD⁡(i)Hdisturber⁢⁢n⁡(i),
where UPBOPSD(i) denotes the UPBO reference PSD, e.g. based on an International Telecommunication Union (ITU) Standard G.997.1, which is incorporated herein by reference as if reproduced by its entirety. The transmitted PSD or the UPBO reference PSD may be used to obtain the 1% worst case upstream crosstalk channel (or noise) at the victim TX511:

TXNOISE1⁢%⁢⁢worst⁢⁢case=RXNOISE1⁢%⁢⁢worst⁢⁢case⁡(i)H⁡(i)=1H⁡(i)⁢Kxf2·f⁡(i)2·β·UPBOPSD⁡(i).
The term H(i) in the equation above may have different values depending on the length of the victim line or loop. A plurality of virtual noise templates may be defined for a plurality of loop lengths between the transmitters510and receivers520, e.g. at about 200 meters, about 300 meters . . . 1200 meters. The loop lengths may be denoted as L1, L2. . . LV, which correspond to V line or loop lengths, where V is an integer. For each loop length, there may be M virtual noise templates. For example, for the v-th loop length, a plurality of M virtual noise templates may be calculated based on the crosstalk severity in the line:

In some embodiments, the combined transmitter502may be located in a cabinet and the transmitters510may communicate with the receivers520using downstream power back-off (DPBO) to reduce crosstalk in the lines. As such, the downstream transmit PSD of the n-th disturber TX512, DPBOMASKn(i), may be set based on an E-side electric length of the line, DPBOESEL, e.g. according to the ITU standard G.997.1. The downstream transmitted PSD may be used to obtain the transmitter referred line noise TXNOISE1% worst casecorresponding to the 1% worst case crosstalk channel:
TXNOISE1% worst caseKxf2·f(i)2·β·DPBOPSD(i).

A plurality of virtual noise templates may be defined for a plurality of E-side electric lengths corresponding to the different lines, e.g. for about V E-side electric lengths denoted as DPBOESEL1, DPBOESEL2. . . DPBOESELV, where V is an integer. For each E-side electric length, there may be M virtual noise templates. For example, for the v-th E-side electric length, the M virtual noise templates may be calculated based on the crosstalk severity in the line, e.g. in increasing severity of crosstalk:

FIG. 6illustrates an embodiment of a virtual noise configuration method600, which may be used to improve line stability in a subscriber line. The virtual noise may be configured by selecting one of a plurality of defined virtual noise templates for the line. The virtual noise template may be selected according to a transmitter referred virtual noise that may be calculated for a tone in the line. The method600may begin at block605, where a plurality of virtual noise templates may be defined for a plurality of subscriber lines, such as for the downstream transmission scenario300or the upstream transmission scenario500.

At block610, the transmitter referred virtual noise for a line tone may be calculated based on a history of the line noise measurements. For example, the transmitter referred virtual noise TXREFVN(i) may be calculated using equation (1) based on the standard operating SNR margin SNRM, the minimum margin required to ensure line stability SNRMmin, and the maximum transmitter referred line noise over time

maxt⁢{TXNOISEt⁡(i)}.
Alternatively, TXREFVN(i) may be calculated using equation (2) based on SNRM, SNRMmin, a mean of the transmitter referred line noise TXNOISEmean(i), and a standard deviation of the transmitter referred line noise TXNOISEstd(i). In an alternative embodiment, TXREFVN(i) may be calculated using equation (3) for adaptive tracking of line noise condition. In other embodiments, TXREFVN(i) may be calculated using other methods that may be based on the history of the line noise measurements.

Next at block620, a virtual noise template that best matches the transmitter referred virtual noise may be selected for the line. For example, the lowest power virtual noise template VNTEMPLATEv,m(i) that captures TXREFVN(i), e.g. that has larger power than TXREFVN(i), may be selected. Selecting the lowest power virtual noise template VNTEMPLATEv,m(i) that captures TXREFVN(i) may be expressed mathematically:

v*,m*=arg⁢minv,m⁢∑i⁢VTEMPLATEv,m⁡(i)such that TXREFVN(i)≦VTEMPLATEv,m(i), ∀i,
where min{ } indicates a function for selecting a maximum sample from a set, v indicates the line and m indicates the virtual noise template. Alternatively, VNTEMPLATEv,m(i) that has the least of squares fit with TXREFVN(i) may be selected, which may be expressed mathematically:

v*,m*=arg⁢⁢minv,m⁢{∑i⁢TXREFVN⁡(i)-VTEMPLATEv,m⁡(i)2}.
In an alternative embodiment, the virtual noise template that may achieve the highest data-rate and capture TXREFVN(i) may be selected, such as

v*,m*=arg⁢⁢minv,m⁢{∑i⁢log2⁡(1+1Γ⁢TXPSD⁡(i)VTEMPLATEv,m⁡(i))}such that TXREFVN(i)≦VTEMPLATEv,m(i), ∀i,
where Γ is the SNR gap to capacity and may be equal to about 9.75 dB plus SNRM minus a coding gain. For example, SNRM may be equal to about six dB, the coding gain may be equal to about four dB, and Γ may be equal to about 11.75 dB. In other embodiments, the virtual noise template may be selected using other methods that may depend on the transmitter referred virtual noise. The method may end after block620.

In an embodiment, the virtual noise templates may be stored using a set of breakpoints. The breakpoints may correspond to PSD values or levels for a plurality of frequencies, which may define the PSD curves or profiles of the virtual noise templates. The PSD profiles of the virtual noise templates (or virtual noise PSDs) may be calculated, e.g. for the range of frequencies, by interpolating the PSD values at each of the breakpoints. Thus, the virtual noise templates may be converted into PSD profiles for each tone i in the line before comparing the virtual noise PSDs with TXREFVN(i) to find a best match. Alternatively, TXREFVN(i) may be converted into a set of breakpoints corresponding to PSD levels and may then be compared to the breakpoints corresponding to the PSD levels of the virtual noise templates.

FIG. 7illustrates an embodiment of a best matched virtual noise template700, which may be selected for a transmitter referred virtual noise in a subscriber line. The best matched virtual noise template700may be obtained from a plurality of virtual noise templates, which may be obtained as described above. The virtual noise templates may be represented by a plurality of curves710that comprise a plurality of points. The points correspond to virtual noise values in dBm/Hz vs. a range of frequencies in MHz (from about zero MHz to about 30 MHz). The curves710may be the PSD curves of the virtual noise templates. The curves710include a curve712that is matched to a curve714. The curve712represents a virtual noise template that best matches the transmitter referred virtual noise in the subscriber line, which is represented by the curve714. The best matched virtual noise template may capture the TXREFVN(i), e.g. may be the smallest PSD curve that is higher than the PSD curve of the transmitter referred virtual noise in a subscriber line. In an alternative embodiment, a virtual noise template that is different from the virtual noise template corresponding to the curve712may be selected as the best match for the transmitter referred virtual noise in the subscriber line corresponding to the curve714. For instance, the best matched virtual noise template may have the best least squares fit with TXREFVN(i) or may achieve the highest data-rate and capture TXREFVN(i). As such, another curve from the curves710that is different from the curve712may be matched with the curve714.

The components described above may be operated in conjunction with any general-purpose network component, such as a computer or network component with sufficient processing power, memory resources, and network throughput capability to handle the necessary workload placed upon it.FIG. 8illustrates a typical, general-purpose network component800suitable for implementing one or more embodiments of the components disclosed herein. The network component800may include a processor802(which may be referred to as a central processor unit or CPU) that is in communication with any memory devices including secondary storage804, read only memory (ROM)806, random access memory (RAM)808, input/output (I/O) devices810, and network connectivity devices812, or combinations thereof. The processor802may be implemented as one or more CPU chips, or may be part of one or more application specific integrated circuits (ASICs).

The secondary storage804is typically comprised of one or more disk drives or tape drives and is used for non-volatile storage of data and as an over-flow data storage device if RAM808is not large enough to hold all working data. Secondary storage804may be used to store programs that are loaded into RAM808when such programs are selected for execution. The ROM806is used to store instructions and perhaps data that are read during program execution. ROM806is a non-volatile memory device that typically has a small memory capacity relative to the larger memory capacity of secondary storage804. The RAM808is used to store volatile data and perhaps to store instructions. Access to both ROM806and RAM808is typically faster than to secondary storage804.