U.S. patent application number 14/487675 was filed with the patent office on 2015-01-01 for method and device for adjusting a power allocation of users in a digital subscriber line environment.
The applicant listed for this patent is ADTRAN GMBH. Invention is credited to ANJA KLEIN, MARTIN KUIPERS, DOMINIQUE WUERTZ.
Application Number | 20150003597 14/487675 |
Document ID | / |
Family ID | 43216770 |
Filed Date | 2015-01-01 |
United States Patent
Application |
20150003597 |
Kind Code |
A1 |
KUIPERS; MARTIN ; et
al. |
January 1, 2015 |
METHOD AND DEVICE FOR ADJUSTING A POWER ALLOCATION OF USERS IN A
DIGITAL SUBSCRIBER LINE ENVIRONMENT
Abstract
A method and a device adjust a power allocation of users in a
digital subscriber line environment. An intermediate power
allocation is determined for at least one user initializing with
the digital subscriber line environment based on a new power
allocation determined for the digital subscriber line environment
containing the at least one user. The intermediate power allocation
provides a predefined minimum signal-to-noise ratio margin for the
active users of the digital subscriber line environment.
Furthermore, a communication system can contain such a device.
Inventors: |
KUIPERS; MARTIN;
(Dallgow-Doeberitz, DE) ; KLEIN; ANJA; (Darmstadt,
DE) ; WUERTZ; DOMINIQUE; (Darmstadt, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ADTRAN GMBH |
Berlin-Siemenstadt |
|
DE |
|
|
Family ID: |
43216770 |
Appl. No.: |
14/487675 |
Filed: |
September 16, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13697644 |
Jan 25, 2013 |
8867712 |
|
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PCT/EP2010/056577 |
May 12, 2010 |
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14487675 |
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Current U.S.
Class: |
379/93.08 |
Current CPC
Class: |
H04B 3/32 20130101; H04L
5/00 20130101; H04L 5/0037 20130101; H04B 3/487 20150115; H04L
5/0007 20130101 |
Class at
Publication: |
379/93.08 |
International
Class: |
H04B 3/32 20060101
H04B003/32 |
Claims
1. A method for adjusting a power allocation of users in a digital
subscriber line environment, the method comprising: determining an
intermediate power allocation for at least one user initializing
with the digital subscriber line environment based on a new power
allocation determined for the digital subscriber line environment
containing the at least one user; the intermediate power allocation
providing a predefined minimum signal-to-noise ratio (SNR) margin
for active users of the digital subscriber line environment.
2. The method according to claim 1, which further comprises
determining at least one intermediate power allocation that
converges towards the new power allocation.
3. The method according to claim 1, wherein the digital subscriber
line environment contains at least one dynamic spectrum management
system, which is managed by a spectrum management center.
4. The method according to claim 1, wherein the predefined minimum
SNR margin is provided for the intermediate power allocation or
individually for each of the users for which the intermediate power
allocation is determined, for every subcarrier, a portion of the
users or a portion of the subcarriers.
5. The method according to claim 1, which further comprises
determining the intermediate power allocation utilizing spectral
limitation masks as well as a limited power budget per the at least
one user, the limited power budget being distributed among tones of
a discrete multi-tone modulation scheme.
6. The method according to claim 1, which further comprises
determining the intermediate power allocation such that a distance
metric between an actual power allocation and the new power
allocation is reduced.
7. The method according to claim 6, wherein the distance metric is
reduced meeting at least one of the following constraints: a total
transmit power is limited; a transmit power on each sub-carrier or
tone is limited individually by a power spectrum density mask; and
for the minimum SNR margin, a data rate achieved by each of the
users with an obtained power allocation equals or exceeds a
predetermined target data rate.
8. The method according to claim 6, wherein the distance metric
according to .DELTA.(s(i), s.sub.new) comprises at least one
property selected from the group consisting of: .DELTA. is convex
in s.sub.k.sup.n; .DELTA. is separable in s.sub.k.sup.n; and
.DELTA.(s(i), s.sub.new) has a unique global minimum for
s(i)=s.sub.new.
9. The method according to claim 6, wherein the distance metric
comprises a distance function selected from the group consisting
of: .DELTA. ( s ( i ) , s new ) = n k ( s k n ( i ) s k , new n - 1
) 2 ; ##EQU00019## .DELTA. ( s ( i ) , s new ) = n k .alpha. k n (
s k n ( i ) - s k , new n ) 2 ; and ##EQU00019.2## .DELTA. ( s ( i
) , s new ) = n k .alpha. k n ( s k n5 ( i ) s k , new n - s k ,
new n s k n ( i ) ) 2 ; ##EQU00019.3## wherein s.sub.k.sup.n
denotes a power spectrum density of a transmit signal of a user n;
k denotes a subchannel or tone; s.sub.new is the new power
allocation; and s(i) is the intermediate power allocation at a step
i.
10. The method according to claim 1, wherein the intermediate power
allocation is determined by solving a following optimization
problem: min s k n ( i ) .A-inverted. n .di-elect cons. i , k
.DELTA. ( s ( i ) , s new ) s . t . R n ( .gamma. _ ) s ( i )
.gtoreq. R target n .A-inverted. n k s k n ( i ) .ltoreq. P max n
.A-inverted. n 0 .ltoreq. s k n ( i ) .ltoreq. s k , mask n
.A-inverted. n , k ##EQU00020## wherein n.di-elect cons.G.sub.i.OR
right.N is the user; G.sub.i denotes a group containing the at
least one user initializing with the digital subscriber line
environment; N denotes the users sharing a same binder;
s.sub.k.sup.n denotes a power spectrum density of a transmit signal
of a user n; s.sub.k,mask.sup.n is a PSD mask determined by a band
profile used; k determines a subchannel or tone; s.sub.new is the
new power allocation; s(i) is the intermediate power allocation at
a step i; .gamma. is the predefined minimum SNR margin; R.sup.n is
a data rate of the user n; R.sub.target.sup.n is a target data rate
of user n; and P.sub.max.sup.n is a maximum aggregate transmit
power of the user n.
11. The method according to claim 10, which further comprises
solving the optimization problem via a dual decomposition combined
with a convex relaxation.
12. The method according to claim 11, which further comprises
solving the optimization problem by decomposing a Lagrangian
.LAMBDA. = .DELTA. ( s ( i ) , s new ) + n .omega. n ( R target n -
R n ( i ) ) + n .lamda. n ( k s k n ( i ) - P max n ) ##EQU00021##
wherein .omega..sup.n is a dual variable corresponding to a data
rate constraint of the user n; and .lamda..sup.n is a dual variable
corresponding to a power constraint, into per-tone Lagrangians
.LAMBDA..sub.k according to .LAMBDA. = k .LAMBDA. k + n .omega. n R
target n - n .lamda. n P max n const . in s ( i ) ##EQU00022## with
##EQU00022.2## .LAMBDA. k = n ( s k n ( i ) s k , new n - 1 ) 2 + n
.lamda. n s k n ( i ) - f s n .omega. n log 2 ( 1 + 1 .gamma.
.GAMMA. g k n , n s k n ( i ) m .noteq. n g k n , m ( i ) + .sigma.
k 2 ) . ##EQU00022.3##
13. The method according to claim 12, wherein a dual problem max
.omega. n , .lamda. n .A-inverted. n .di-elect cons. i min s k n (
i ) .A-inverted. n .di-elect cons. i , k .LAMBDA. s . t . .omega. n
, .lamda. n .gtoreq. 0 .A-inverted. n 0 .ltoreq. s k n ( i )
.ltoreq. s k , mask n .A-inverted. n , k ##EQU00023## of the
optimization problem is solved by solving K independent
sub-problems min s k n ( i ) .A-inverted. n .di-elect cons. i
.LAMBDA. k s . t . 0 .ltoreq. s k n ( i ) .ltoreq. s k , mask n
.A-inverted. n ##EQU00024## per Lagrange multiplier search
step.
14. The method according to claim 10, wherein: at an initial step
i=0, it is determined whether the minimum SNR margin .gamma. with
1.ltoreq. .gamma..ltoreq..gamma..sub.target exists so that the
optimization problem is feasible for s(0); in case where the
minimum SNR margin .gamma. exists, the value is used to determine
the at least one intermediate power allocation; in case no such
minimum SNR margin .gamma..gtoreq.1 exists, a set G.sub.0 is
augmented by at least one additional user whose power allocation is
re-shaped at the time instant t=t.sub.0.
15. The method according to claim 1, which further comprises
determining the intermediate power allocation such that a distance
metric between an actual power allocation and the new power
allocation is minimized.
16. A device, comprising: a processing unit programmed to adjust a
power allocation of users in a digital subscriber line environment
by determining an intermediate power allocation for at least one
user initializing with the digital subscriber line environment
based on a new power allocation determined for the digital
subscriber line environment containing the at least one user; the
intermediate power allocation providing a predefined minimum
signal-to-noise ratio margin for active users of the digital
subscriber line environment.
17. The device according to claim 16, wherein the device is
selected from the group consisting of a modem, a digital subscriber
line access multiplexer, a centralized network component, and a
spectrum management center.
18. A method for adjusting a power allocation of users in a digital
subscriber line environment, the method comprising: upon receiving
a request by a new user to be initialized to the digital subscriber
line environment, determining an intermediate power allocation for
the new user initializing with the digital subscriber line
environment based on a new power allocation determined for the
digital subscriber line environment containing the new user; the
intermediate power allocation providing for a predefined minimum
signal-to-noise ratio margin for active users of the digital
subscriber line environment.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation, under 35 U.S.C.
.sctn.120, of copending patent application Ser. No. 13/697,644,
filed Jan. 25, 2013, which was a .sctn.371 of international
application No. PCT/EP2010/056577, filed May 12, 2010, which
designated the United States; the prior applications are herewith
incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The invention relates to a method and to a device for
adjusting a power allocation of users in a digital subscriber line
environment. In addition, a system comprising at least one such
device is suggested.
[0003] DSL or xDSL, is a family of technologies that provide
digital data transmission in particular over wires of a local
telephone network.
[0004] High speed Internet access is gaining importance and can be
via xDSL services using existing copper lines. Also, other
applications emerge that require broadband transmission services,
e.g., triple play offers comprising subscriber access to Internet,
TV and voice data transmission. A bandwidth consuming application
is the transmission of TV data via xDSL, wherein one HDTV channel
may require a data rate amounting to 12 Mbit/s.
[0005] Therefore, requirements for high speed Internet access are
increasing. Operators are optimizing services that are offered to
their customers. This becomes a difficult task as an increasing
amount of users as well as high data rates leads to higher
crosstalk between subscriber lines in a cable binder. In most
cases, crosstalk noise limits the performance. However, also
crosstalk noise may vary over time: There may be low crosstalk
noise when a significant amount of customers have switched off
their equipment and there may be a considerable amount of crosstalk
noise during business hours when the majority of customers use
their devices.
[0006] FIG. 1 shows a schematic diagram of a cable or binder 101
comprising several lines 102, 103 of a DSL system. The lines 102
and 103 are connected at one side to a DSLAM 104 that could be
deployed at a central office or at a remote terminal and on the
other side the line 102 is connected to a CPE 105 and the line 103
is connected to a CPE 106.
[0007] Crosstalk occurs between the lines 102 and 103 that are
coupled by the binder 101: The crosstalk comprises a near-end
crosstalk (NEXT) 107 and 108 as well as far-end crosstalk (FEXT)
109 and 110.
[0008] Such crosstalk is perceived at a receiver of a victim
(coupled) line as noise and therefore decreases a signal-to-noise
ratio (SNR) at this receiver thereby reducing an attainable data
rate on this line.
[0009] The twisted pair communication channel is frequency
selective, i.e. the direct channel attenuates higher frequencies
more than lower frequencies, but the electromagnetic coupling
between twisted pair lines provides higher crosstalk with
increasing frequency.
[0010] xDSL systems employing multi-carrier modulation schemes like
Discrete Multi-Tone (DMT) are able to flexibly shape their transmit
power spectrum in order to adapt to frequency-selective
characteristics of the channel. Dynamic Spectrum Management (DSM)
Level 2 is an approach to improve an overall system performance by
centrally shaping transmit spectra (which corresponds to shaping of
power allocations) of interfering lines so that a performance loss
due to crosstalk effects is minimized. With enough (in particular
full) knowledge about the channel characteristics, a Spectrum
Management Center (SMC) is able to compute an optimal power
allocation for each user and reports these allocations to the
individual modems, which utilize the power allocation determined by
the SMC to configure a transmit power level for each tone (of the
DMT modulation scheme).
[0011] FIG. 2 shows a schematic diagram of an optimal downstream
power allocation for a VDSL2 system with two users (with different
loop lengths amounting to 300 m for the first user and to 600 m for
the second user) determined by a SMC using DSM. It is noted that
user in this regard may in particular refer to a CPE or a terminal.
The user may in particular be a DSL modem. Hence, FIG. 2 comprises
a power spectrum density (PSD) mask 201 which can be utilized by
both users, wherein a PSD allocation of the first user (the 300 m
user) is indicated by a graph 202 and a PSD allocation of the
second user (the 600 m user) is indicated by a graph 203.
[0012] The SMC assigns the frequency band above ca. 8 MHz
exclusively to the 300 m user (see graph 202), because the 600 m
user (see graph 203) cannot efficiently transmit data at this range
due to high direct channel attenuation. In a range below 8 MHz, the
SMC instructs the 300 m user (see graph 202) to reduce its transmit
power in order to limit its interference with the 600 m user.
[0013] The power allocation provided by the SMC may provide a
target SNR margin, which protects the users of the DSM system from
(arbitrary or not expected) noise fluctuations, e.g., crosstalk
from legacy systems or other interferers. In this regard, a target
margin of, e.g., 6 dB can be provided to ensure a specified service
quality, i.e. bit-error-ratio (BER) and data rate. Hence, the
actual noise may increase by up to 6 dB relative to the noise level
that has been assumed by the SMC when computing the power
allocations.
[0014] FEXT between copper wires in a binder is the dominant
impairment in current DSL systems, severely limiting achievable
data rates. DSM Level 2 tries to mitigate the capacity loss due to
crosstalk by centrally coordinating the modem's transmit power
allocation, effectively introducing politeness between users.
Existing solutions, however are not able to cope with a scenario
that some optimal joint power allocation computed by an SMC
according to current channel conditions may become invalid at some
point in the future when the channel or DSM system parameters
change, e.g., [0015] (a) when a user joins or leaves the system.
This is likely to happen in an unbundled environment where
customers change service providers, which operate their proprietary
DSM system. [0016] (b) when a user changes a service. If a user
upgrades, e.g., from an ADSL2 service to a VDSL2 service, the
transmit spectrum will change thereby affecting the crosstalk
profile on other users' lines in the binder.
[0017] In any such event, the SMC has to determine a new joint
allocation that corresponds to the new situation. However, it is a
significant problem that transmit spectra of modems that are
already active (in show-time) cannot be reconfigured without
interrupting their service, which in most cases is inacceptable and
should therefore be avoided. Instead, updating the transmit power
profile is thus delayed until a modem enters a (re-)initialization
phase.
[0018] However, updating spectra for a part of the users only leads
to a power allocation comprising a mixture of merely partially
optimized spectra. Such mixture bears the risk that a desired
target BER cannot be guaranteed as long as the final state of the
new power allocation is not reached, i.e. as long as not all modems
have been re-initialized according to the new power allocation.
Until such final state is reached, it is likely to obtain severe
drops of the SNR margin thus seriously affecting an overall line
stability.
BRIEF SUMMARY OF THE INVENTION
[0019] It is accordingly an object of the invention to provide a
method which overcomes the disadvantages of the heretofore-known
devices of this general type and which provides for an efficient
solution to adapt power allocations in a DSM system.
[0020] This problem is solved according to the features of the
independent claims. Further embodiments result from the depending
claims.
[0021] In order to overcome this problem, a method is provided for
adjusting a power allocation of users in a digital subscriber line
environment,
[0022] wherein an intermediate power allocation is determined for
at least one user initializing with the digital subscriber line
environment based on a new power allocation determined for the
digital subscriber line environment comprising this at least one
user;
[0023] wherein the intermediate power allocation provides a
predefined minimum SNR margin for the active users of the digital
subscriber line environment.
[0024] It is noted that the new power allocation corresponds to a
target power allocation value comprising in particular a power
spectrum density distribution over a frequency range. It is also
noted that the power allocation can be also referred to as a (e.g.,
transmit) spectrum.
[0025] It is further noted that user refers to a terminal, a CPE or
a modem. The user can be initialized to become an active user (i.e.
to enter an active mode, also referred to as "show-time").
[0026] Advantageously, said minimum SNR margin can be provided for
all active users of the digital subscriber line environment, i.e.
for all users that are in show-time. Hence, the PSD of the at least
one user that joins the digital subscriber line environment is
adjusted such that it converges towards the new power allocation
and that it does not interfere with already active users in a way
that their SNR drops below said minimum SNR margin. The
intermediate power allocations (of the transition phase towards the
new power allocation) can be determined such that an SNR margin for
all users does not fall below said predefined minimum SNR
margin.
[0027] Via said intermediate power allocation, a transition is
provided that allows the DSL environment to gradually adjust to and
eventually reach the new power allocation without a significant
impairment to existing users.
[0028] Hence, the solution provided allows to gradually update
power allocations in a DSM system and assures that at each point
during such gradual transition, an actual SNR margin for each user
does not fall below a given minimum value.
[0029] Advantageously, only transmit spectra (or power allocations)
of users initializing a new session are modified and a forced
retraining can largely be avoided.
[0030] In an embodiment, at least one intermediate power allocation
is determined that converges towards the new power allocation.
[0031] In particular, with at least one user that is about to
(re-)initialize with the DSL environment, an(other) intermediate
power allocation stage can be determined. Hence, the approach can
be iteratively applied to converge via several intermediate power
allocations toward the new (target) power allocation.
[0032] It is noted that initializing refers to a user that wants to
get connected (or re-connected) to the DSL environment.
[0033] The updating scheme suggested advantageously updates or
adjusts the power allocation if a user (re-)initializes a new
session.
[0034] In another embodiment, the digital subscriber line
environment comprises at least one DSM system, which is managed by
an SMC.
[0035] The SMC may be a centralized component that provides the
adjustment of power allocations. Also the SMC may be realized as at
least one physical entity or it may be combined with an existing
physical entity.
[0036] In a further embodiment, the predefined minimum SNR margin
can be provided for the intermediate power allocation or
individually for each user for which the intermediate power
allocation is determined or for every subcarrier or a portion of
users or subcarriers.
[0037] In a next embodiment, the intermediate power allocation is
determined utilizing spectral limitation masks as well as a limited
power budget per the at least one user, which limited power budget
is in particular distributed among tones of a DMT modulation
scheme.
[0038] It is also an embodiment that the intermediate power
allocation is determined such that a distance metric between an
actual power allocation and the new power allocation is reduced, in
particular minimized.
[0039] Pursuant to another embodiment, the distance metric is
reduced meeting at least one of the following constraints: [0040] a
total transmit power is limited; [0041] a transmit power on each
sub-carrier or tone is limited individually by a power spectrum
density mask; [0042] for the minimum SNR margin, a data rate
achieved by each user with the obtained power allocation equals or
exceeds a predetermined target data rate.
[0043] In a next embodiment, the distance metric according to
.DELTA.(s(i), s.sub.new) comprises at least one of the properties:
[0044] .DELTA. is convex in s.sub.k.sup.n; [0045] .DELTA. is
separable in s.sub.k.sup.n; and [0046] .DELTA.(s(i), s.sub.new) has
a unique global minimum for s(i)=s.sub.new.
[0047] According to an embodiment, the distance metric comprises a
distance function as follows:
.DELTA. ( s ( i ) , s new ) = n k ( s k n ( i ) s k , new n - 1 ) 2
, or ##EQU00001## .DELTA. ( s ( i ) , s new ) = n k .alpha. k n ( s
k n ( i ) - s k , new n ) 2 , or ##EQU00001.2## .DELTA. ( s ( i ) ,
s new ) = n k .alpha. k n ( s k n ( i ) s k , new n - s k , new n s
k n ( i ) ) 2 , ##EQU00001.3##
[0048] wherein [0049] s.sub.k.sup.n denotes a PSD of the transmit
signal of a user n; [0050] k determines a subchannel or tone;
[0051] s.sub.new is the new power allocation; [0052] s(i) is the
intermediate power allocation at a step i.
[0053] It is noted that the term s.sub.k,new.sup.n can be set to a
small positive value. According to another embodiment, the
intermediate power allocation is determined by solving the
following optimization problem:
min s k n ( i ) .A-inverted. n .di-elect cons. i , k .DELTA. ( s (
i ) , s new ) s . t . R n ( .gamma. _ ) s ( i ) .gtoreq. R target n
.A-inverted. n k s k n ( i ) .ltoreq. P max n .A-inverted. n 0
.ltoreq. s k n ( i ) .ltoreq. s k , mask n .A-inverted. n , k
##EQU00002##
[0054] wherein [0055] n.di-elect cons.G.sub.i.OR right.N is a user;
[0056] G.sub.i determines a group comprising the at least one user
initializing with the digital subscriber line environment; [0057] N
determines the users sharing the same binder; [0058] s.sub.k.sup.n
denotes a PSD of the transmit signal of a user n; [0059]
s.sub.k,mask.sup.n is a PSD mask determined by a band profile used;
[0060] k determines a subchannel or tone; [0061] s.sub.new is the
new power allocation; [0062] s(i) is the intermediate power
allocation at a step i; [0063] .gamma. is the predetermined minimum
SNR margin; [0064] R.sup.n is a data rate of the user n; [0065]
R.sub.target.sup.n is a target data rate of user n; [0066]
P.sub.max.sup.n is a maximum aggregate transmit power of the user
n.
[0067] In yet another embodiment, the optimization problem is
solved by a dual decomposition combined with a convex
relaxation.
[0068] According to a next embodiment, the optimization problem is
solved by decomposing a Lagrangian
.LAMBDA. = .DELTA. ( s ( i ) , s new ) + n .omega. n ( R target n -
R n ( i ) ) + n .lamda. n ( k s k n ( i ) - P max n )
##EQU00003##
[0069] wherein [0070] .omega..sup.n is a dual variable
corresponding to the data rate constraint of user n; and [0071]
.lamda..sup.n is a dual variable corresponding to the power
constraint, [0072] into per-tone Lagrangians .LAMBDA..sub.k
according to
[0072] .LAMBDA. = k .LAMBDA. k + n .omega. n R target n - n .lamda.
n P max n const . in s ( i ) ##EQU00004## with ##EQU00004.2##
.LAMBDA. k = n ( s k n ( i ) s k , new n - 1 ) 2 + n .lamda. n s k
n ( i ) - f s n .omega. n log 2 ( 1 + 1 .gamma. _ .GAMMA. g k n , n
s k n ( i ) m .noteq. n g k n , m s k m ( i ) + .sigma. k 2 ) .
##EQU00004.3##
[0073] Pursuant to yet an embodiment, a dual problem
max .omega. n , .lamda. n .A-inverted. n .di-elect cons. i min s k
n ( i ) .A-inverted. n .di-elect cons. i , k .LAMBDA. s . t .
.omega. n , .lamda. n .gtoreq. 0 .A-inverted. n 0 .ltoreq. s k n (
i ) .ltoreq. s k , mask n .A-inverted. n , k ##EQU00005## [0074] of
the optimization problem is solved by solving K independent
sub-problems
[0074] min s k n ( i ) .A-inverted. n .di-elect cons. i .LAMBDA. k
s . t . 0 .ltoreq. s k n ( i ) .ltoreq. s k , mask n .A-inverted. n
##EQU00006## [0075] per Lagrange multiplier search step.
[0076] Hence, this approach renders the overall algorithm
complexity linear in K.
[0077] According to another embodiment, [0078] at an initial step
i=0, it is determined whether the minimum SNR margin .gamma. with
1.ltoreq. .gamma..ltoreq..gamma..sub.target exists so that the
optimization problem is feasible for s(0); [0079] wherein in case
such minimum SNR margin .gamma. exists, this value is used to
determine the at least one intermediate power allocation; [0080]
wherein in case no such minimum SNR margin .gamma..gtoreq.1 (0 dB)
exists, the set G.sub.0 is augmented by at least one additional
user whose power allocation is re-shaped at the time instant
t=t.sub.0.
[0081] By enlarging the set of feasible power allocations, a low
intermediate minimum SNR margin .gamma. increases the flexibility
of shaping the power allocations and thus tends to reduce the
number of required intermediate steps i before all users reach the
value of the new (target) power allocation s.sub.new. Hence, a
trade-off decision can be made between a faster convergence and a
reduced protection against fluctuation of noise.
[0082] In the latter case (in case no such minimum SNR margin
.gamma..gtoreq.1 exists), a forced resynchronization of these
augmented users can be conducted.
[0083] The problem stated above is also solved by a device
comprising or being associated with a processing unit that is
arranged such that the method as described herein is executable
thereon.
[0084] It is further noted that said processing unit can comprise
at least one, in particular several means that are arranged to
execute the steps of the method described herein. The means may be
logically or physically separated; in particular several logically
separate means could be combined in at least one physical unit.
[0085] Said processing unit may comprise at least one of the
following: a processor, a microcontroller, a hard-wired circuit, an
ASIC, an FPGA, a logic device.
[0086] Pursuant to an embodiment, the device is a modem, a DSLAM or
a centralized network component, in particular a spectrum
management center.
[0087] The solution provided herein further comprises a computer
program product directly loadable into a memory of a digital
computer, comprising software code portions for performing the
steps of the method as described herein.
[0088] In addition, the problem stated above is solved by a
computer-readable medium, e.g., storage of any kind, having
computer-executable instructions adapted to cause a computer system
to perform the method as described herein.
[0089] Furthermore, the problem stated above is solved by a
communication system comprising at least one device as described
herein.
[0090] Embodiments of the invention are shown and illustrated in
the following figures.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0091] FIG. 1 is a schematic diagram of a cable having several
lines;
[0092] FIG. 2 is a schematic diagram of an optimal downstream power
allocation for a VDSL2 system with two users;
[0093] FIG. 3 shows a schematic diagram of a transition of a
multi-user power allocation after a user 3 has joined the DSM
system at a time instance t=t.sub.0;
[0094] FIG. 4 shows a diagram visualizing a PSD over a frequency,
wherein a PSD mask provides an admissible power allocation range,
which is utilized by three users at an initial power allocation
state s.sub.old;
[0095] FIG. 5 shows based on FIG. 4 a new (target) power allocation
s.sub.new in case an additional user joins the DSM system;
[0096] FIG. 6 shows based on FIG. 4 an intermediate power
allocation s(i) that allows adjusting the DSM system towards the
new power allocation s.sub.new, but avoids a re-configuration of
users that are still active (in show-time) and provides sufficient
SNR margin to allow the DSM system to efficiently maintain its
operation.
DESCRIPTION OF THE INVENTION
[0097] The solution provided herewith suggests determining
intermediate power allocations for users initializing a new session
and to provide a favorable transition phase so that the xDSL system
can eventually reach a new (target) multi-user power allocation
that may be determined by an SMC.
[0098] It is noted that the power allocation is also referred to as
a (transmit) spectrum. A user referred to herein may also be
regarded as terminal, CPE or modem (or vice versa).
[0099] The intermediate power allocations (of the transition phase
towards the new power allocation) can be determined such that an
SNR margin for all users does not fall below a predefined
threshold.
[0100] The updating scheme suggested in particular updates or
adjusts the power allocation if a user (modem) initializes a new
session. Hence, forced retraining of modems can be largely avoided.
The intermediate power allocations can be determined utilizing
spectral limitation masks as well as a limited power budget per
modem, which is distributed among tones of a DMT modulation
scheme.
[0101] Overview: Determining Intermediate Power Allocations
[0102] The solution provided in particular utilizes the following
input: [0103] (a) a new (target, multi-user) power allocation
s.sub.new to which all users should eventually be updated; and
[0104] (b) a minimum SNR margin .gamma..
[0105] At each time instance when at least one user is about to
initialize a new session, a (further) intermediate power allocation
for this at least one user is determined such that a distance
metric between the intermediate (multi-user) power allocation and
the new power allocation s.sub.new is reduced, in particular
minimized and at least one of the following constraints can be met:
[0106] (a) a total transmit power is limited; [0107] (b) a transmit
power on each sub-carrier (DMT tone) is limited individually by a
PSD mask; [0108] (c) for a given minimum SNR margin .gamma. a data
rate achieved by each user with the obtained power allocation
equals or exceeds a predetermined target data rate.
[0109] These constraints lead to an optimization problem (details,
see below with regard to equation (14)), which can be solved in an
efficient manner by a dual decomposition approach combined with a
convex relaxation technique.
[0110] The minimum SNR margin .gamma. may be chosen individually
for every optimization stage, for every subcarrier (or a portion
thereof) and/or individually for each user whose transmit spectrum
is to be (re-)computed.
[0111] FIG. 4 shows a (downstream) multi-user power allocation at
an initial state, also referred to as power allocation s.sub.old
for a DSM system comprising three VDSL users with loop lengths of X
meters, wherein X amounts to 400, 800 and 1000. The respective user
is also referred to as the X m user. Hence, FIG. 4 depicts a PSD
mask 405 and power allocations for the 400 m user 401, the 800 m
user 403 and the 1000 m user 404.
[0112] At a time instance t=t.sub.0, a new user, also referred to
as 600 m user 402 (because of its loop length amounting to 600 m)
joins the DSM system and starts a new session so that the SMC has
to re-calculate the power allocation, i.e. determine the new
optimized allocation s.sub.new for the 4-user system. Such new
power allocation s.sub.new is shown in FIG. 5. It is noted that
both power allocations s.sub.old and s.sub.new provide a target SNR
margin amounting to 6 dB.
[0113] In case the 400 m user 401, the 800 m user 403 and the 1000
m user 404 users are already in show-time at the time instance
t=t.sub.0, they cannot be updated instantly to the new power
allocation s.sub.new without causing an interruption of service.
Hence, these 401, 403 and 404 users are gradually updated, i.e. an
update for the user in show-time is delayed until this user
conducts a re-initialization phase for a next session.
[0114] On the other hand, the power allocation for the newly joined
600 m user 402 cannot immediately be configured to the new power
allocation s.sub.new without the risk of causing a severe drop of
the SNR margin to any (or to all) of the 4 users. Therefore, an
intermediate power allocation s(i) is determined as shown in FIG. 6
for this 600 m user 402 to be applied at the time instance
t=t.sub.0, which guarantees that all users operate with an SNR
margin equal or above a given SNR minimum margin (which can be set
to, e.g., 2 dB). Also, the spectrum allocated for the 600 m user
402 for this intermediate power allocation s(i) is selected to
converge towards the new power allocation s.sub.new; the other
users (which are in show-time according to this example) maintain
operation with their previously set configurations. The same
procedure can then iteratively be applied to each user starting a
new session after this time instance t=t.sub.0 until the entire DSM
system reaches and utilizes the new power allocation s.sub.new. In
practice, this may require for some users multiple intermediate
transmit power allocations until the DSM system reaches said new
power allocation s.sub.new.
[0115] System Model for a Static Scenario
[0116] A channel model for a static DSL system comprises a set N of
users sharing the same binder, thus causing mutual FEXT on each
other's lines. By employing DMT transmission with K orthogonal
tones k=1, . . . , K, the interference channel is divided into K
independent subchannels k. Applying a sufficiently small tone
spacing .DELTA.f, the direct channel of user n.di-elect cons.N on
tone k can be described by a single complex coefficient
h.sub.k.sup.n,n. Similarly, a crosstalk channel from a disturber m
to a victim line n on the tone k can be given by a complex scalar
h.sub.k.sup.n,m (m.noteq.n).
[0117] The term s.sub.k.sup.n denotes a PSD of the transmit signal
of a user n and .sigma..sub.k.sup.2 denotes a combined PSD of alien
FEXT and receiver background noise on the tone k.
[0118] Using a Shannon gap approximation, a number of bits
b.sub.k.sup.n(.gamma.) per symbol that a user n can load onto the
tone k with a given SNR margin .gamma..gtoreq.1 amounts to
b k n ( .gamma. ) = log 2 ( 1 + 1 .gamma..GAMMA. g k n , n s k n m
.noteq. n g k n , m s k m + .sigma. k 2 ) , ( 1 ) ##EQU00007##
where [0119] .GAMMA.>1 denotes a so-called gap to capacity,
which is a function of a target BER; [0120]
.theta..sub.k.sup.n,m=|h.sub.k.sup.n,m|.sup.2 are the crosstalk and
direct channel gain coefficients. Furthermore, a total utilized
power P.sup.n and a data rate R.sup.n of the user n, are given
by
[0120] P n = .DELTA. f k s k n and ( 2 ) R n ( .gamma. ) = f s k b
k n ( .gamma. ) , ( 3 ) ##EQU00008##
respectively, where f.sub.s is a symbol rate of the DMT system.
[0121] Update of Multi-User Power Allocation in a Non-Static
Scenario
[0122] In a DSM system, regardless whether operating in
rate-adaptive, margin-adaptive or fixed-margin mode, the optimal
joint power allocation is determined using a spectrum balancing
algorithm which typically accounts for at least three per-user
constraints in the optimization process: [0123] (a) A total power
constraint
[0123] P.sup.n.ltoreq.P.sub.max.sup.n.A-inverted.n, (4) [0124]
where P.sub.max.sup.n is a maximum aggregate transmit power
specified in the respective xDSL standard; [0125] (b) A spectral
mask constraint
[0125]
0.ltoreq.s.sub.k.sup.n.ltoreq.s.sub.k,mask.sup.n.A-inverted.n,k,
(5) [0126] where s.sub.k,mask.sup.n is a PSD mask determined by a
band profile used; and [0127] (c) A rate constraint
[0127] R.sup.n(.gamma.).gtoreq.R.sub.target.sup.n.A-inverted.n, (6)
[0128] where R.sub.target.sup.n is a target data rate of user n
chosen according to a Service Level Agreement and .gamma. amounts
to a value .gamma..sub.target>1 which is a target SNR margin
selected by the provider.
[0129] Next, a non-static scenario is considered in which an
optimized power allocation
s.sub.old={.sub.k,old.sup.n|n.di-elect cons.N;k=1, . . . ,K},
(7)
computed by the SMC becomes invalid at some time instance t=t.sub.0
when a user joins or leaves the DSM system or in case a user
changes the service. In this case, a new power allocation
s.sub.new={s.sub.k,new.sup.n|n.di-elect cons.N;k=1, . . . ,K}
(8)
is required, which is optimized for a time t.gtoreq.t.sub.0, but
cannot be applied for those users that are already in show-time
without interrupting their service.
[0130] FIG. 3 shows a schematic diagram of a transition of a
multi-user power allocation after a user 3 (n*=3) has joined the
DSM system at a time instance t=t.sub.0. The new power allocation
is determined such that the constraints according to equations (4)
and (6) are met for all users n.di-elect cons.N with N={1,2,3}; the
constraints according to equations (4) and (6) are met for the old
power allocation s.sub.old for users n.di-elect cons.{1,2} without
the user n*(user 3) being active prior to the instant of time
t<t.sub.0, i.e. the PSD s.sub.k,old.sup.n*=0.A-inverted.k.
[0131] If all users n.noteq.n* are already in show-time at the time
instant t=t.sub.0, only the transmit PSD s.sub.k.sup.n* of this
newly joined user n* can be updated.
[0132] Time instances or steps i=0, 1, 2, . . . are defined in a
discrete time range corresponding to time instances t=t.sub.i
(t.sub.i<t.sub.i+1) in a continuous time range at which point
any of the users initiates a new session and therefore its transmit
PSD can be re-configured towards the new power allocation
s.sub.new.
[0133] In addition,
s(i)={s.sub.k.sup.n(i)|n.di-elect cons.N;k=1, . . . ,K} (9)
denotes a power allocation used by the system during an
intermediate time interval
.theta..sub.i=t.sub.i.ltoreq.t<t.sub.i+1. Hence, if a user n is
not re-initialized at an instance i, no convergence towards the new
power allocation s.sub.new is reached, i.e.
s.sub.k.sup.n(i)=s.sub.k.sup.n(i-1).A-inverted.k.
[0134] In order to achieve an intermediate power allocation that
converges toward the new power allocation s.sub.new, the user n*'s
PSD s.sub.k.sup.n*(0) could be initialized at an instance i=0 to
correspond to the new optimal allocation s.sub.k,new.sup.n*, while
the other users n.noteq.n* maintain transmission with the
previously determined (then optimal) spectra, i.e.
s k n ( 0 ) = { s k , new n n = n * s k , old n n .noteq. n *
.A-inverted. k = 1 , , K . ( 10 ) ##EQU00009##
[0135] At a next instance i=1, a user 2 is re-initialized and its
transmit PSD s.sub.k.sup.2(1) could be set to s.sub.k,new.sup.2,
wherein the other users maintain their spectra (as they are still
in show-time), i.e.
s k n ( 1 ) = { s k , new n n = 3 s k n ( 0 ) n .noteq. 3
.A-inverted. k = 1 , , K . ( 11 ) ##EQU00010##
[0136] If each user has been re-initialized, e.g., at least one
time, the DSM system is fully updated and has reached its new power
allocation s.sub.new.
[0137] On the other hand, during the transition phase described,
another event could invalidate the previously determined new power
allocation s.sub.new. In this case, a revised a new optimal power
allocation s.sub.new may be determined and the power allocation
s.sub.old is set to the current power allocation.
[0138] During each interval .theta..sub.i, an actual SNR margin
.gamma..sup.n(i) of the user n resulting from a given multi-user
power allocation s(i) can be obtained by solving the equation
R.sup.n(.gamma..sup.n(i))|.sub.s(i)-R.sub.target.sup.n=0, (12)
[0139] However, it cannot be guaranteed that any of the
intermediate allocations s(i), which are a mixture of old and new
optimized power spectra, are feasible, i.e. yield a solution
.gamma..sup.n(i).gtoreq.1 for equation (12).
[0140] Proposal for New Updating Scheme
[0141] An approach is suggested that enables seamless transition
from the old power allocation s.sub.old to the new power allocation
s.sub.new in the DSM system. Hence, intermediate power allocations
s(i) are determined such that at all times the actual SNR margin
.gamma..sup.n(i) is guaranteed not to fall below a specified
minimum value .gamma.. This can in particular be achieved by
shaping the intermediate spectra s(i) at each instance i towards
(in particular as similar as possible) the new (target) power
allocation s.sub.new, while accounting for per-user power and
target rate constraints. Such similarity between the intermediate
power allocation s(i) and the new (target) power allocation
s.sub.new can be determined based on a distance function
.DELTA. ( s ( i ) , s new ) = n k ( s k n ( i ) s k , new n - 1 ) 2
, or ( 13 a ) .DELTA. ( s ( i ) , s new ) = n k .alpha. k n ( s k n
( i ) - s k , new n ) 2 , or ( 13 b ) .DELTA. ( s ( i ) , s new ) =
n k .alpha. k n ( s k n ( i ) s k , new n - s k , new n s k n ( i )
) 2 , ( 13 c ) ##EQU00011##
which reaches 0 for s(i)=s.sub.new.
[0142] It is noted that a distance metric according to
.DELTA.(s(i), s.sub.new) may comprise at least one of the
properties: [0143] .DELTA. is convex in s.sub.k.sup.n; [0144]
.DELTA. is separable in s.sub.k.sup.n; and [0145] .DELTA.(s(i),
s.sub.new) has a unique global minimum for s(i)=s.sub.new.
[0146] In order to avoid division by zero, the term
s.sub.k,new.sup.n can be lower-bounded to some sufficiently small
positive value s.sub.min. For example, a value of -130 dBm/Hz could
be useful for DSL applications.
[0147] At an instance i, users n.di-elect cons.G.sub.i.OR right.N
are about to resynchronize. Based on a predetermined minimum SNR
margin .gamma., the intermediate power allocation s(i) is obtained
by solving the following optimization problem
min s k n ( i ) .A-inverted. n .di-elect cons. i , k .DELTA. ( s (
i ) , s new ) s . t . R n ( .gamma. _ ) s ( i ) .gtoreq. R target n
.A-inverted. n k s k n ( i ) .ltoreq. P max n .A-inverted. n 0
.ltoreq. s k n ( i ) .ltoreq. s k , mask n .A-inverted. n , k ( 14
) ##EQU00012##
Wherein the spectra for users nG.sub.i are maintained unchanged
according to
s k n ( i ) = { s k , old n i = 0 s k n ( i - 1 ) i > 0
.A-inverted. n i ; k = 1 , , K . ( 15 ) ##EQU00013##
[0148] An efficient solution of the problem according to equation
(14) will be shown and explained below.
[0149] A convergence analysis of the proposed scheme could be
summarized as follows: Basically, a sequence of optimized power
allocations {.DELTA.(s(i), s.sub.new)} is monotonously decreasing,
i.e.
.DELTA.(s(i),s.sub.new).ltoreq..DELTA.(s(i-1),s.sub.new). (16)
[0150] In practical scenarios, DSL sessions are of limited (finite)
duration and for every instance i with s.noteq.s.sub.new, there
will always be a succeeding instance j>i such that
.DELTA.(s(j),s.sub.new)<.DELTA.(s(i),s.sub.new), (17)
which implies convergence of the system to finally reach the new
power allocation s.sub.new within a finite number of (time)
steps.
[0151] An existence of a feasible intermediate power allocation
s(i) can be shown by the following induction: If a feasible
solution for the intermediate power allocation s(i) exists, this
solution will also be feasible for a succeeding intermediate power
allocation s(i+1). The remaining issue is to find an initial power
allocation s(0) that also is feasible.
[0152] As discussed above, there is no guarantee that a service
with pre-defined target rates and pre-defined target BER can be
maintained for all users once the newly joined user becomes active.
Thus, at the initial step i=0, it has to be determined whether a
minimum SNR margin .gamma. with 1.ltoreq.
.gamma..ltoreq..gamma..sub.target exists so that equation (14) with
G.sub.0={n*} is feasible for s(0). If such a minimum SNR margin
.gamma. is found, this value can be used to determine all
intermediate power allocations.
[0153] By enlarging the set of feasible power allocations, a low
intermediate margin .gamma. increases the flexibility of shaping
the spectra and thus tends to reduce the number of required
intermediate steps i, before all users can be set to the new
(target) power allocation s.sub.new. Hence, a trade-off decision
can be made between a faster convergence and a reduced protection
against fluctuation of noise.
[0154] If, however, no feasible .gamma..gtoreq.1 exists, the set
G.sub.0 can be augmented by one or more additional users whose
spectra are to be re-shaped at the time instant t=t.sub.0. In this
case, a forced resynchronization of these users may be
required.
[0155] Low-Complexity Solution
[0156] The objective to minimize the term .DELTA.(s(i), s.sub.new)
is convex in s.sub.k.sup.n(i) and separable with regard to the
tones k while the target rate constraint R.sup.n(
.gamma.)|.sub.s(i).gtoreq.R.sub.target.sup.n leads to a non-convex
set of feasible solutions, making it difficult to find a solution
that is guaranteed to be globally optimal.
[0157] It is thus suggested to decompose a Lagrangian
.LAMBDA. = .DELTA. ( s ( i ) , s new ) + n .omega. n ( R target n -
R n ( i ) ) + n .lamda. n ( k s k n ( i ) - P max n ) ( 18 )
##EQU00014##
wherein [0158] .theta..sup.n is a dual variable corresponding to
the data rate constraint of user n; and [0159] .lamda..sup.n is a
dual variable corresponding to the power constraint, into per-tone
Lagrangians .LAMBDA..sub.k according to
[0159] .LAMBDA. = k .LAMBDA. k + n .omega. n R target n - n .lamda.
n P max n const . in s ( i ) with ( 19 ) .LAMBDA. k = n ( s k n ( i
) s k , new n - 1 ) 2 + n .lamda. n s k n ( i ) - f s n .omega. n
log 2 ( 1 + 1 .gamma. _ .GAMMA. g k n , n s k n ( i ) m .noteq. n g
k n , m s k m ( i ) + .sigma. k 2 ) . ( 20 ) ##EQU00015##
[0160] This allows solving the dual problem
max .omega. n , .lamda. n .A-inverted. n .di-elect cons. i min s k
n ( i ) .A-inverted. n .di-elect cons. i , k .LAMBDA. s . t .
.omega. n , .lamda. n .gtoreq. 0 .A-inverted. n 0 .ltoreq. s k n (
i ) .ltoreq. s k , mask n .A-inverted. n , k ( 21 )
##EQU00016##
of the problem according to equation (14) by solving K independent
sub-problems
min s k n ( i ) .A-inverted. n .di-elect cons. i .LAMBDA. k s . t .
0 .ltoreq. s k n ( i ) .ltoreq. s k , mask n .A-inverted. n ( 22 )
##EQU00017##
per Lagrange multiplier search step, thus rendering the overall
algorithm complexity linear in K.
[0161] As .LAMBDA..sub.k is non-convex, minimization may however
still require an exhaustive search with exponential complexity in
the number of users N. For the rate-adaptive spectrum management
problem, [P. Tsiaflakis, J. Vangorp, M. Moonen and J. Verlinden:
Convex Relaxation Based Low-Complexity Optimal Spectrum Balancing
for Multi-User DSL. In Acoustics, Speech and Signal Processing,
2007, ICASSP 2007. IEEE International Conference, volume 3, pages
II-349 to III-352, April 2007] suggests an efficient algorithm
based on convex relaxation by noting that the Lagrangian can be
rewritten as a difference of convex (d.c.) functions. Rewriting
.LAMBDA..sub.k as
.LAMBDA. k = n ( s k n ( i ) s k , new n - 1 ) 2 + n .lamda. n s k
n ( i ) - f s n .omega. n log 2 ( m .noteq. n g k n , m s k m ( i )
+ .sigma. k 2 + g k n , n s k n ( i ) .gamma. .GAMMA. ) A + f s n
.omega. n log 2 ( m .noteq. n g k n , m s k m ( i ) .sigma. k 2 ) B
( 23 ) ##EQU00018##
where part A is a convex and part B is a concave portion. Hence,
the problem according to equation (14) exposes a d.c. structure and
can thus be solved using the approach as described in [P.
Tsiaflakis, J. Vangorp, M. Moonen and J. Verlinden: Convex
Relaxation Based Low-Complexity Optimal Spectrum Balancing for
Multi-User DSL. In Acoustics, Speech and Signal Processing, 2007,
ICASSP 2007. IEEE International Conference, volume 3, pages II-349
to III-352, April 2007].
[0162] The solution for the per-tone sub-problem pursuant to
equation (22) can be approximated by iteratively solving a sequence
of relaxed convex minimization problems, wherein the solution of
one iteration is used as an approximation point for finding a
convex relaxation of .LAMBDA..sub.k in the next iteration. An
adaption of the low-complexity algorithm to the optimization
problem according to equation (14) can be realized accordingly.
[0163] Further Advantages:
[0164] The approach presented guarantees a minimum SNR margin for
each user during each (intermediate) stage of an iterative
optimization of the power allocation towards a target values
s.sub.new. Hence, by ensuring such minimum SNR margin, the service
stability can be significantly improved as the DSM system can be
well protected against fluctuations of noise that is not managed by
the DSM system (i.e. the SMC). In addition, forced re-configuration
or re-training of users that are already in show-time and hence
service interruptions can be largely avoided.
LIST OF ACRONYMS
[0165] BER Bit Error Rate [0166] CPE Customer Premises Equipment
(DSL modem) [0167] d.c. difference of convex [0168] DMT Discrete
Multi-Tone [0169] DSL Digital Subscriber Line [0170] DSLAM DSL
Access Multiplexer [0171] DSM Dynamic Spectrum Management [0172]
PSD Power Spectrum Density [0173] SMC Spectrum Management Center
[0174] SNR Signal-to-Noise Ratio
* * * * *