Experimental Evaluation of Fourth Generation DSLin Different DSM Scenarios

Diogo Acatauassu∗, Marcio Monteiro†, Francisco Muller∗ and Aldebaro Klautau∗∗Signal Processing Laboratory, Federal University of Para, Brazil

†Dept. of Electrical and Information Technology, Lund University, SwedenEmail: {diogoaca, fmuller, aldebaro}@ufpa.br and marcio.monteiro@eit.lth.se

Abstract—Recently, the industry and the academy started todiscuss the development of the fourth generation DSL systems.For example, within this context, ITU started working in G.fastDSL standard. A trend in this new generation is to increasethe end-user bit rates by extending the frequency rage oftransmission signals and reducing the copper loop length. For thefrequency range being considered for use in G.fast for example,crosstalk is an even worse impairment than it is for current DSLdeployments. Hence, it is sensible to study dynamic spectrummanagement (DSM) techniques to overcome the crosstalk effects.This work presents an experimental evaluation in order to verifythe behavior of fourth generation DSL systems when differentlevels of DSM techniques are used, highlighting the coexistence ofvectored, DSM level 3, and non-vectored sub-systems. The resultswere obtained based on short copper cable measurements andpower spectrum density simulations.

I. INTRODUCTION

Digital subscriber lines (DSL), the broadband data transmis-sion technologies that use the copper cable as channel, are themost used Internet media around the world with more than300 million users [1]. Much of the DSL success is relatedto the cost-benefit for both operators and served consumers.As the transmission channel is the common copper twistedpair telephone cable, there is no need for large investmentsin infrastructure, because the telephone network is largelyconsolidated and active in almost all the world.

Over the years, DSL systems suffered a step-by-step evo-lution, being divided in different generations according tothe increase of its data rates and technology improvements[2]. A lot of elements were evolved in this process. Two ofthem can be highlighted. The first one was the developmentof modern signal processing techniques to avoid crosstalkeffects in DSL transmissions, these are the so-called dynamicspectrum management (DSM) techniques [1], [3], [4], [5], [6].The second one was the reduction of the used copper cablelength, resulting in a consequent reduction in the attenuationimposed on the transmission signals. This fact allowed theincrease of the used bandwidths, and consequently the datarates.

Recently, the International Telecommunication Union (ITU)started a standardization effort under the working name G.fastthat can be positioned as part of the fourth DSL generation[7]. A comparison between this new DSL generation proposaland older DSL generations, in terms of desirable data rates,

TABLE ICOMPARISON OF DSL GENERATIONS.

Generation Standards Desirable Transmission Typical LengthData Bandwidth of CopperRates Cable

1st ISDN 10 Mb/s 1.1 MHz 5 KmADSL

2nd ADSL2 20 Mb/s 2.2 MHz 5 KmADSL2+

3rd VDSL 100 Mb/s Up to 1 KmVDSL2 30 MHz

4th Next approx. 100, 200 50 m to 300 m(4GBB) Standard 1 Gb/s even

(G.fast) 300 MHz

transmission bandwidth and typical length of the used coppercable is shown in Table I [8], [9], [10], [11], [12], [13].

A trend of the next generation DSL systems, also referredas the fourth generation broadband (4GBB) concept [2], [7],is to explore the copper twisted pair cable to the maximum,reducing it to few meters and joining it to hybrid optical fiberaccess architectures, such as fiber to the home (FTTH) [2], [7],[14]. The copper cable length reduction process will allowunused frequencies, far above the 30 MHz VDSL2. Recentworks described that this frequency values can achieve 100MHz [2], [7], [14], even reaching 300 MHz, depending on theused cable quality [7]. Results of capacity simulations showedthat this transmission channels can achieve near 1 Gb/s, andthe use of DSM techniques to avoid crosstalk effects can helpensuring these data rates [2], [14], [15].

This work describes an experimental evaluation in order toverify the behavior of 4GBB systems when different levelsof DSM techniques are used, highlighting the coexistence ofvectored (DSM level 3) and non-vectored sub-systems. Thework shows how DSM level 2 techniques can be importantto improve the achievable rates when DSM 3 is not used.The results were obtained based on power spectrum density(PSD) simulations and short copper cable measurements. Asa secondary contribution, this work illustrates the frequencybehavior, in terms of loop attenuation and crosstalk coupling,of some short copper cables at 4GBB frequencies, obtainedby the performed measurements.

The text is organized as follows. Section II presents a briefdescription of DSM techniques and describes the coexistence

978-1-4673-0279-1/11/$26.00 c⃝ 2010 IEEE

of vectored and non-vectored systems. Section III illustratesthe simulated scenarios. Section IV shows the adopted systemmodel and describes the simulation assumptions. Section Vdescribes the used channel data, based on measurements ofshort copper cables. Section VI shows the obtained resultsand, finishing, Section VII describes the conclusions of thework.

II. DSM: CONCEPTS AND COEXISTENCE OF

HETEROGENEOUS SYSTEMS

DSM can be defined as a set of techniques for spectrumcoordination. It can minimize or eliminate crosstalk effects inDSL access networks. The applied coordination is similar toan adaptive control of the transmission spectrum of each user’smodem. DSM can be classified in different categories (levels),in agreement with the coordination applied on its algorithms[1], [5].

For the frequency range being considered for use by 4GBBsystems (100, 200, up to 300 MHz), crosstalk is an evenworse impairment than it is for current DSL deployments[7]. Hence, DSM level 3 (vectored) techniques, are highlyrecommended for these systems in order to achieve the bestpossible performance. However, there are situations whichvectoring cannot be applied for all the lines inside a binder(e.g., the binder is shared by different service providers) or it issimply too complex to cancel crosstalk from all the interferinglines, especially for a high number of vectored lines. In thesesituations, DSM level 2 methods for crosstalk mitigation canbe considered. The coexistence of vectored and non-vectoredDSL systems is described in [16], being, however, focused inthe VDSL2 family.

In this coexistence situations the crosstalk of even onenon-vectored DSL line can reduce the vectored DSL linesperformance. This alien crosstalk cannot be avoided by thevectored system, which can mitigate only the crosstalk fromthe vectored ones. The non-vectored DSL lines, however,are affected by the two crosstalk sources, the vectored line’scrosstalk and the self non-vectored line’s crosstalk.

III. SIMULATED SCENARIOS

In order to verify the coexistence effects of vectored andnon-vectored 4GBB systems, two simulation scenarios weredefined. The first scenario was similar to the one proposedby [16] where three DSL users share the same binder. Here,users 1 and 2 were connected to a vectored 4GBB accessnode and user 3 was connected to a non-vectored 4GBB accessnode. The two access nodes were then connected to a spectrummanagement center (SMC) which could apply a DSM level 2algorithm, this case the iterative spectrum balancing (ISB) [4].The objective of this simulation was to observe, at differentcopper loop lengths (50 m, 100 m and 200 m), the datarates obtained for the vectored and non-vectored lines, with orwithout ISB, highlighting the rates degradation of the vectoredlines due to the crosstalk of the non-vectored ones. Figure 1illustrates the first scenario overview.

Spectrum Management

Center

Vectored Gigabit DSL access node

Non-vectored Gigabit DSLaccess node

Copper loop length (50 m, 100 m, 200 m)

FTTH

Service provider

N Users

Fig. 1. First simulated scenario. The SMC can apply ISB in both vectoredand non-vectored lines.

The second simulated scenario evaluated a transmissionenvironment of 24 users in a binder (using 100 m and 200 mcopper loop lengths). This time it was observed the influenceof vectored lines in the binder’s total average rate gains. Itwas done including lines from the 4GBB non-vectored sub-system to the 4GBB vectored sub-system and then observingthe increase of average rate gains. Besides, as in the firstscenario, it was observed the rates degradation resulted fromthe crosstalk leakage of the 4GBB non-vectored lines (withor without DSM 2) to the vectored ones. This time the SMCwas connected only to the non-vectored 4GBB access node,while the vectored 4GBB access node was connected to a staticspectrum management (SSM) that means no DSM 2 was usedin this node, as shown in Figure 2.

Spectrum Management

Center

Vectored Gigabit DSL access node

Non-vectored Gigabit DSLaccess node Copper loop

length (100 m, 200 m)

L Vectored lines L=24-Q

Q Non-vectored linesQ=24-L

N UsersService provider

Static Spectrum

Management

FTTH

Fig. 2. Second simulated scenario. The SMC can apply ISB only in thenon-vectored lines.

IV. SYSTEM MODEL AND SIMULATION ASSUMPTIONS

The system was modeled as copper access binders of Nusers (i.e. 3 lines in the first scenario and 24 lines in the secondscenario) equipped with 4GBB transceivers. Each transceiveremployed discrete multitone modulation (DMT), as mostcurrent DSL standards, and operated over a twisted-pair linewith K independent parallel subchannels (tones). By assumingonly frequency division duplex (FDD) DMT transmission,where upstream and downstream frequency bands are non-overlapping, only the far-end crosstalk (FEXT) was considered[17]. The weak near-end crosstalk (NEXT) influence wasneglected during simulations.

The loading problem of bits allocation per tone k and foruser n can be expressed as follows [17]

𝑏𝑘𝑛 = log2

(1 +

1

Γ𝛾𝑛

∣∣ℎ𝑘𝑛,𝑛

∣∣2 𝑝𝑘𝑛∑𝑛∕=𝑚

∣∣ℎ𝑘𝑛,𝑚

∣∣2 𝑝𝑘𝑚 + 𝜎𝑘𝑛

), (1)

where

∙ ∣ℎ𝑘𝑛,𝑛∣2 is the the square-magnitude of the direct channel

transfer function gain for user 𝑛 at tone 𝑘;∙ ∣ℎ𝑘

𝑛,𝑚∣2 denotes the square-magnitude of the far-endcrosstalk channel transfer function from transmitter 𝑚 toreceiver 𝑛 at tone 𝑘;

∙ 𝑝𝑘𝑚 denotes the power transmitted by user 𝑚 at tone 𝑘;∙ 𝜎𝑘

𝑛 represents the background noise power on tone 𝑘 atreceiver 𝑛;

∙ Γ is the signal-to-noise (SNR) ratio gap, which is afunction of the desired bit error rate;

∙ 𝛾𝑛 is the noise margin of user 𝑛.

The total used power 𝑃𝑛 and the total data rate 𝑅𝑛 of eachuser n can be calculated as

𝑃𝑛 =

𝐾∑𝑘=1

𝑝𝑘𝑛 𝑅𝑛 = 𝑓𝑠

𝐾∑𝑘=1

𝑏𝑘𝑛, (2)

where 𝑓𝑠 is the DMT symbol rate [17].When DSM 2 was used, the ISB algorithm was set to solve

the optimization problem, which considering the DMT system,is described as [4]

max𝑝1,...𝑝𝑛

𝑅1 𝑠.𝑡. 𝑅𝑛 ≥ 𝑅𝑡𝑎𝑟𝑔𝑒𝑡𝑛 , ∀𝑛 > 1, (3)

𝑠.𝑡

𝐾∑𝑘=1

𝑝𝑘𝑛 ≤ 𝑃𝑚𝑎𝑥𝑛 , ∀𝑛,

where 𝑅𝑡𝑎𝑟𝑔𝑒𝑡𝑛 denotes the target data rate of user n, 𝑝𝑛 =

[𝑝1𝑛, ..., 𝑝𝐾𝑛 ] is the power spectrum density vector of user n on

all K tones and 𝑃𝑚𝑎𝑥𝑛 is the maximum available power of user

n.The transmission PDS masks were defined according to

the ones proposed by [14]. This masks are derived from theingress and egress limits described in the CISPR 22 norm.It follows the standard VDSL2 up to 30 MHz, which is -60 dBm/Hz. For frequencies above 30 MHz, the PSD maskdecays from -60 dBm/Hz to -80 dBm/Hz at 100 MHz. Figure3 shows the used PSD, divided in upstream and downstreambands. Note that the spectrum was divided in a symmetricalway between upstream and downstream transmission, but withhigher priority for downstream, as this one uses, for most of itstones, lower (less attenuated) frequencies. The tone spacing,Δf, was set to 25 kHz.

Table II shows the main simulation assumptions used duringthe experiments (which were imported to our PSD based DSLsimulator). It is also important to notice the simplified modelused for vectoring. It assumes that crosstalk is completelycanceled among vectored lines, while the crosstalk from andto non-vectored lines is still present and unmodified.

0 10 20 30 40 50 60 70 80 90 100−100

−90

−80

−70

−60

−50

−40

Frequency (MHz)

PS

D M

ask

(dB

m/H

z)

DownstreamUpstream

Fig. 3. PSD mask used by the 4th generation DSL systems adopted in thiswork.

TABLE IISIMULATION ASSUMPTIONS.

Parameter Value

Background noise AWGN, -140 dBm/HzTone spacing Δf 25 kHz

(4096 FFT for a 100 MHz bandwidth)DMT symbol rate 21.562 kHz

DS max power 14.5 dBmUS max power 12.2 dBm

Target SNR margin 6 dBCoding gain 3.5 dBSNR gap Γ 9.8 dB

Max number of bits per tone 15

V. CHANNEL MEASUREMENTS DATA

In order to obtain results closer to the actual deployments,it was used in this work real channel measurements datathat includes 50 m, 100 m and 200 m copper cables. Asthe signal attenuation reduces if the channel length reduces,these copper twisted pair cables are appropriate to exploring4GBB frequencies, this case up to 100 MHz [14]. As thesystem model defined in Section IV applied FDD, the NEXTinfluence was neglected, and thus, NEXT measurements werenot performed.

The used 50 m cable was a 16 pair 0.4 mm, 26 Americanwire gauge (AWG). This loop measurements resulted in 256channel transfer functions: 16 direct channel transfer functionsand 240 far-end crosstalk (FEXT) transfer functions. The100 m and 200 m cables were 24 pair 0.5 mm, 24 AWG.Their measurements resulted in 576 channel transfer functions:24 direct channel transfer functions and 552 FEXT transferfunctions, totalizing 1408 different channel transfer functions.

Figures 4, 5 and 6 show some recorded data from the 50 m,100 m and 200 m copper cable measurements in terms ofdirect channel transfer functions (insertion loss) and far-endcrosstalk (FEXT) transfer functions. It can be observed thedirect channel attenuation increase due to the copper cablelength increase. For example, at 100 MHz using the measured50 m cable the magnitude was near -15 dB while at the same100 MHz using the measured 200 m cable the magnitude wasnear -40 dB.

The obtained channel data were loaded in our PSD basedDSL simulator and then the simulations were performed.

0 1 2 3 4 5 6 7 8 9 10

x 107

−100

−80

−60

−40

−20

0

20

Frequency (Hz)

Mag

nit

ud

e (d

B)

Direct and FEXT transfer functions of the measured 50m copper cable

Fig. 4. Direct transfer functions (upper curves) and FEXT transfer functions(lower curves) of the measured 50 m copper cable.

0 1 2 3 4 5 6 7 8 9 10

x 107

−100

−80

−60

−40

−20

0

20

Frequency (Hz)

Mag

nit

ud

e (d

B)

Direct and FEXT transfer functions of the measured 100m copper cable

Fig. 5. Direct transfer functions (upper curves) and FEXT transfer functions(lower curves) of the measured 100 m copper cable.

0 1 2 3 4 5 6 7 8 9 10

x 107

−100

−80

−60

−40

−20

0

20

Frequency (Hz)

Mag

nit

ud

e (d

B)

Direct and FEXT transfer functions of the measured 200m copper cable

Fig. 6. Direct transfer functions (upper curves) and FEXT transfer functions(lower curves) of the measured 200 m copper cable.

VI. RESULTS

First of all it is important to describe that the rate regions[5], [6] for upstream and downstream were not evaluated whenDSM level 2 was applied. Instead, only one point in the rateregion was obtained. It is important to notice that the rateregion obtained by a DSM 2 algorithm is rather varied, andsometimes, only one point, may not reach the best possiblesolution for spectrum balancing.

In the first simulated scenario it was observed the trans-mission rate gains in a simple situation of mixed vectored andnon-vectored 4GBB DSL systems, as was illustrated in Figure1. There was an SMC which could be used to apply the ISBalgorithm in both vectored and non-vectored lines. The ISB

algorithm was set to maximize the gains of the vectored linesreducing the power spectral density of the non-vectored ones,and so reducing its rates, such as performed in [16]. Figure7 illustrates the obtained results, where the data rates are theaggregated downstream + upstream .

50 100 200200

400

600

800

1000

1200

1400

Loop length (m)

Ave

rag

e ra

te (

Mb

/s)

First simulated scenario

All lines − DSM2All lines − SSMVectored lines − DSM2Vectored lines − SSMNon−vectored lines − DSM2Non−vectored lines − SSM

Fig. 7. Data rates of vectored and non-vectored lines with and without ISB.

It is clear that the power optimization in the non-vectoredlines can reduce the crosstalk impact in the vectored ones, thatis, the data rate of the vectored lines degrades significantly ifno attempt is done to control the transmitted PSD of the non-vectored ones. For example, using the measured 100 m cable,the average gains of the vectored lines when the non-vectoredones applied DSM 2 were 855 Mb/s, much better than the 614Mb/s achieved when the non-vectored lines did not appliedDSM 2. The results showed that if no attempt is done forreducing the non-vectored crosstalk effects the whole binderperformance falls down. It can be illustrated by the obtainedaverage gains of all lines inside the 50 m loop, which was 1Gb/s when DSM 2 was applied and 850 Mb/s when DSM 2was not applied, an increase of almost 18%.

In the second simulated scenario, illustrated in Figure 2, itwas observed the average rate gains by including, in steps of2 pairs, lines from the non-vectored sub-system (representedby the non-vectored 4GBB access node) to the vectored sub-system (represented by the vectored 4GBB access node), usingthe 100 m and 200 m measured cables. This time the SMCwas connected only to the non-vectored lines. Figure 8 showsthe average data rate gains obtained (upstream + downstream)using the 100 m cable.

As expected, the average data rate in the binder goes up asthe number of vectored lines increases and the non-vectoredlines decreases. The result shows that the ISB algorithmachieved better gains as the number of crosstalk channels usedfor the algorithm’s calculations reduces. It was observed amaximum average gain of 30 Mb/s when ISB was used in thenon-vectored lines.

Figure 9 shows the results for the 200 m cable, where itwas observed a maximum average gain of 50 Mb/s when ISBwas used in the non-vectored lines.

Moreover, it was observed the impact of the non-vectoredlines to the average rate gains of the vectored ones, which isshown in Figures 10 and 11. It is clear again that the crosstalkleakage of the non-vectored lines can affect the vectored ones,

2 4 6 8 10 12 14 16 18 20 22250

300

350

400

450

500

550

600

650

Number of vectored lines (total of 24 lines inside the binder)

Ave

rag

e ra

te (

Mb

/s)

Second simulated scenario − Gain of all lines − 100m cable

Non−vectored DSM2−ISBNon−vectored SSM

Fig. 8. Average rates including lines from the non-vectored sub-system tothe vectored sub-system using the measured 100 m cable.

2 4 6 8 10 12 14 16 18 20 22200

250

300

350

400

450

500

550

Number of vectored lines (total of 24 lines inside the binder)

Ave

rag

e ra

te (

Mb

/s)

Second simulated scenario − Gain of all lines − 200m cable

Non−vectored SSMNon−vectored DSM2 − ISB

Fig. 9. Average rates including lines from the non-vectored sub-system tothe vectored sub-system using the measured 200 m cable.

reducing the average gains in almost 75 Mb/s for the 100 mcable worts case and almost 100 Mb/s for the 200 m cableworst case. The results show the importance of spectrumbalancing in the non-vectored lines in order to ensure the full-gains of the vectored ones.

2 4 6 8 10 12 14 16 18 20 22250

300

350

400

450

500

550

600

650

Number of vectored lines (24 lines inside the binder)

Ave

rag

e ra

te (

Mb

/s)

Second simulated scenario − Gain of vectored lines − 100m cable

Non−vectored lines − DSM2−ISBNon−vectored lines − SSM

Fig. 10. Average rates of vectored lines including lines from the non-vectoredsub-system to the vectored sub-system using the measured 100 m cable.

VII. CONCLUSION

During the last years, broadband technologies over copperincreased their end-user bit rates due to a step-by-step evo-lution. The next generation DSL systems, also called 4GBBsystems, wishes to improve the service and eventually achievedata rates near 1 Gb/s. As the direct channel attenuation is

2 4 6 8 10 12 14 16 18 20 22200

250

300

350

400

450

500

550

Number of vectored lines (total of 24 lines inside the binder)

Ave

rag

e ra

te (

Mb

/s)

Second simulated scenario − Gain of vectored lines − 200m cable

Non−vectored DSM2−ISBNon−vectored SSM

Fig. 11. Average rates of vectored lines including lines from the non-vectoredsub-system to the vectored sub-system using the measured 200 m cable.

higher if the copper loop is long, the use of short cables willbe inevitable to ensure improved rates. The copper cable re-duction process will allow the use of unexplored transmissionfrequencies, much higher than the current 30 MHz. Allied tothis, the use of DSM techniques to avoid crosstalk effects canbe a powerful tool to ensure this objective.

This work presented experimental evaluation results, whereit was observed the behavior of 4GBB systems in differ-ent DSM scenarios, highlighting the coexistence of vectoredand non-vectored systems. In this coexistence situations thecrosstalk of even one non-vectored DSL line can reduce theperformance of the vectored DSL ones. The results showedthat if no attempt is done for reducing the non-vectoredcrosstalk effects, the whole binder performance falls down.In these cases, the use of DSM level 2 techniques can beconsidered to improve the achievable rates. The obtainedresults were based on real copper cable measurements andPSD based simulations.

ACKNOWLEDGEMENT

This work was supported by the Innovation Center, EricssonTelecomunicacoes S.A., Brazil, and CNPq.

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