COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Baltimore; August 25-26, 1999

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1 COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1. (DSL Access) Baltimore; August 5-6, 1999 T1E1./ CONTRIBUTION TITLE: SOURCE: PROJECT: ISDN, HDSL, and HDSL Adaptation and SNR with Short-Term Stationary Crosstalk Telcordia Technologies (formerly Bellcore), pursuant to work supported by Ameritech, BellSouth, SBC, and U S WEST. T1E1., Spectral Compatibility ABSTRACT This contribution presents calculations of the impact of some cases of short-term stationary crosstalk on some guarded systems: ISDN, HDSL, and HDSL. The adaptation of the guarded systems in changing crosstalk environments is considered. Specifically, the short-term stationary transmitters are assumed to be OFF for most of the time while the ISDN, HDSL, and HDSL transceivers are adapting their equalizer taps. Then, the SNR margin is calculated after the short term stationary crosstalk turns ON and the equalizer taps are still adapted to what are now non-optimal values. Relative to continuous crosstalk, the SNR margin is calculated to drop as much as 0 db, assuming pessimistic types of short-term stationary crosstalk. Absolute SNR margins are calculated to be as low as 11 db below the target. The margins increase with increases in the percentage of time that the short-term stationary transmitter is on while the guarded system adapts. NOTICE This contribution has been prepared to assist Accredited Standards Committee T1 Telecommunications. This document is offered to the Committee as a basis for discussion and is not a binding proposal on Telcordia Technologies or any other company. The requirements are subject to change in form and numerical value after more study. Telcordia Technologies specifically reserves the right to add to, amend, or withdraw the statements contained herein. CONTACT: Ken Kerpez; Telcordia, kkerpez@ telcordia.com; Tel: ; Fax:

2 1. Introduction Largely because of the popularity of transporting IP datagrams, a number of DSLs are appearing that transmit bursts of data with quiet in between. These DSLs are ON and transmit a frame or a packet as the need arises, and then are OFF and transmit nothing otherwise. The output of these DSLs is called "shortterm stationary." There are a number of advantages of short-term stationary DSL systems: they can use less power than continuous transmitters, it is possible to multiplex a number of them onto a single line, and the time-averaged power of crosstalk from them is lower than it is from continuous transmitters. Placeholder text is currently in the draft Spectrum Management standard [1] describing conformance criteria for short-term stationary systems. This text is based on work by Paradyne and other vendors [][3], and to some extent it uses work from the author of this contribution []. A potential problem with the new short-term stationary systems is that guarded DSL transmission systems do not anticipate the type of crosstalk that arises from short-term stationary systems. This crosstalk can adversely effect adaptation algorithms [5]. Today's DSL systems use adaptive equalizers that maximize the performance with steady-state noise. Adaptation is performed over a time period on the order of seconds long, longer than the time that short-term stationary crosstalk takes to turn ON and OFF which may be on the order of tens of milliseconds. Current DSL equalizers generally do not adapt fast enough to track changing short-term stationary crosstalk. This contribution presents some simulations where an HDSL, HDSL, or ISDN receiver adapts to a certain type of crosstalk environment, then transceives data in a different crosstalk environment. The receiver's equalizer is adapted to short-term stationary crosstalk that has a PSD equal to it's full power PSD lowered by the average proportion of time that it is ON. This mimics the effect of crosstalk from short-term stationary transmitters rapidly turning on and off. While the crosstalk environments here are entirely plausible and only have one or two disturbers turning on and off, these environments were also chosen to try and find the worst-case impact, so they may be considered pessimistic. Some cases presented here have alarmingly high degradations in SNR relative to continuous crosstalk. However, besides this relative impact, the absolute impact must be assessed, the probability of occurrence of such events must be ascertained, and the adaptation of current guarded systems must be fully understood to make definitive guidelines for the spectrum management of short-term stationary systems.. Common Simulation Parameters The simulated systems here use baseband pulse amplitude modulation (PAM) with minimum mean squared error (MMSE) decision feedback equalizer (DFE) receivers. NEXT is generated with the Unger model. The received signal to noise ratio (SNR) equals the average received signal power divided by the sum of average power of -10 dbm/hz background noise, received NEXT, and residual intersymbol interference at the output of the equalizer. All simulated NEXT disturbers have PSD and total average power that comply with the draft spectrum management standard..1 HDSL Simulation Parameters HDSL is simulated with the transmit spectra and other parameters in the HDSL draft standard. HDSL is modulated with trellis coded 16 level PAM transmitting 155 kbps on a single pair. The simulated HDSL receiver has a DFE with T/-spaced forward-filter taps and 10 baud-spaced feedback taps. MMSE T/-fractional-spaced forward-filter taps were calculated and used for finding the received SNR of HDSL. The required SNR for a 10-7 BER is 7.6 db. SNR margins are presented for uncoded HDSL. The trellis code would add 5 db to the SNR margin, and HDSL should achieve a 5 db SNR 1

3 margin, so positive margins displayed here are acceptable for HDSL while negative margins are not acceptable. Only Upstream HDSL was simulated here, with transmit signal power of 16.5 dbm.. HDSL Simulation Parameters HDSL simulations use -level PAM (B1Q) at a baud rate of 39 kbaud and bit rate of 78 kbps on each pair in both upstream and downstream directions. The transmit signal power is 13.5 dbm. The signal modulates square pulses which are passed through th order Butterworth with low pass filters (LPFs) with 3 db point at 0 khz at both the transmitter and receiver. The simulated HDSL receiver uses a MMSE DFE that has 16 baud-spaced forward-filter taps and 6 feedback taps. The required SNR of uncoded B1Q at a 10-7 bit error rate (BER) is 1.3 db. The SNR margin equals the computed SNR minus the required SNR..3 ISDN Simulation Parameters ISDN was simulated by modulating 160 kbps with B1Q at a symbol rate of 80 kbaud, and a transmit power of 13.5 dbm. The signal modulates square pulses which are passed through nd order Butterworth LPFs with 3 db point at 80 khz at both the transmitter and receiver. The simulated ISDN receiver uses an MMSE DFE with 10 baud-spaced forward-filter taps and 6 feedback taps. The required SNR of uncoded B1Q at a 10-7 bit error rate (BER) is 1.3 db. The SNR margin equals the computed SNR minus the required SNR. 3. Simulation Methodology with Short-Term Stationary Crosstalkers Simulations here mimic a situation where a guarded service (HDSL, HDSL, or ISDN) adapts its equalizer taps while a short-term stationary crosstalker is OFF or is infrequently transmitting, then the guarded service operates while the short-term stationary crosstalker is ON with full power. For the receiver of the guarded service, MMSE DFE tap values were calculated for a particular crosstalk environments. Then, using the same tap values but with a different crosstalk environment, the SNR at the output of the DFE is calculated. The loop is constant and the intersymbol interference does not change as the crosstalk environment changes. A DFE is comprised of two parts: a feed-forward filter with floating point precision taps that lowers the noise and shapes the pulse, and a feedback filter that subtracts quantized post-cursors. For each loop the received pulse response was calculated, and the correlations of the received noise were calculated for the initial crosstalk environment. From these, the MMSE DFE taps were computed using matrix theory [6]. Then, the crosstalk changes and the SNR is computed for a receiver with the DFE taps that were optimized for the previous crosstalk environment. Since the pulse response and the feed-forward filter taps do not change, there is no change in the amount of uncancelled pre-cursor and post-cursor interference as the crosstalk environment changes. However, there can be significant enhancement in the filtered noise power. For example, if the original crosstalk is at low frequencies, then the equalizer will tend to enhance the high frequency received signal content. Then if the crosstalk moves to high frequencies and the equalizer does not re-adapt, then the equalizer will enhance the high frequency noise. Let the probability that a short term stationary transmitter is transmitting at any given time be denoted as p, with 0 p 1, so that the average percentage of time that a short term stationary transmitter is transmitting is 100 x p %. The long-term time-averaged power of the short-term stationary crosstalk equals its full power times p. It is assumed here that the short-term stationary transmitters turn on and off at a rapid rate relative to the speed at which the guarded services adapt their equalizers. So, the equalizers here are adapted to short-term stationary crosstalk with PSD equal to it's full power PSD times p. In db,

4 the guarded service's equalizers are adapted to short-term stationary crosstalk with PSD equal to the crosstalk's full power transmit PSD lowered by 10log 10 (p) db. These simulations reflect typical DSL behavior. Short-term stationary transmitters typically send packets that are a small number of milliseconds long. According to their respective standards, the training sequence for HDSL is at most 5 seconds long, the training sequence for HDSL is at most 10 seconds long, and the training sequence for ISDN is at most 5 seconds long. Start-up equalizer training of these systems usually takes at least a second or two, a relatively long amount of time compared to the shortterm stationary bursts of milliseconds duration. Of course, the short-term stationary crosstalkers may not be ON for exactly a proportion p of the time that an equalizer adapts, but this effect may be inferred from the results because they are plotted for a wide range of values of p. In practice, the feed-forward filter taps may adapt a little while the short-term stationary transmitter is sending a short burst, but this is not considered here and the tap values do not change at all. If independent, then it is not likely that more than one or two short-term stationary crosstalkers turn ON and OFF at the same time, so only one or two disturbers of each type of crosstalk are simulated here.. Simulations with Short-Term Stationary SM PSD Template NEXT In this section NEXT is generated by a hypothetical transmitter that transmits a PSD that is equal to a Spectrum Management (SM) power spectral density (PSD) template [1]. There are two disturbers, one disturber of each of two types: a continuous crosstalk disturber and a short-term stationary crosstalk disturber. The equalizer of the guarded service (HDSL, HDSL), adapts to full power NEXT from the continuous crosstalker plus low-power NEXT from the short-term stationary crosstalker (with power proportional to it's percentage of ON time). Then, during transmission the SNR of the guarded service is calculated while receiving full power NEXT from both crosstalkers. Uncoded HDSL Upstream SNR Margin (db) CSA loop 6 CSA loop Percent Short-Term Stationary On Time While Adapting Fig. 1. SNR margin of upstream HDSL. Equalizer taps were adapted to one continuous SM class PSD template NEXT disturber plus one short-term stationary (lower average power) SM class 5 downstream (downstream ADSL) PSD template NEXT disturber. SNR was calculated with both disturbers fully ON. 3

5 The results in Fig. 1 are for upstream HDSL, with receiver at a CO. Here there is a single continuous crosstalk disturber transmitting the class SM PSD template, and there is also a single short-term stationary crosstalk disturber transmitting the downstream class 5 SM PSD template (the downstream ADSL PSD). The HDSL equalizer adapts to low-frequency SM class crosstalk by emphasizing the received high frequency signal content, causing a significant SNR loss when the downstream ADSL crosstalk is present. 17 HDSL SNR Margin (db) CSA loop 6 CSA loop Percent Short-Term Stationary On Time While Adapting Fig.. SNR margin of HDSL. Equalizer taps were adapted to one continuous SM class 1 PSD template NEXT disturber plus one short-term stationary (lower average power) SM class 5 downstream (Downstream ADSL) PSD template NEXT disturber. SNR was calculated with both disturbers fully ON. The results in Fig. are for HDSL. Here there is a single continuous crosstalk disturber transmitting the class 1 SM PSD template, and there is also a single short-term stationary crosstalk disturber transmitting the downstream class 5 SM PSD template (the downstream ADSL PSD). The equalizer adapts to lowfrequency SM class 1 crosstalk by emphasizing the received high frequency signal content, causing a significant SNR loss when the downstream ADSL crosstalk is present. Some simulations were run of ISDN with crosstalk equal to SM PSD templates, but the time variance of the short term stationary crosstalk had little effect because all the SM PSD templates have wider bandwidth than ISDN. 5. Simulations with Concocted NEXT Disturbers This section makes the results a little more pessimistic by increasing the number of short-term stationary crosstalk disturbers from one to two, and by concocting crosstalk disturbers with damaging PSDs. Although some of the crosstalkers are concocted and do not represent an actual DSL, they all conform to the spectrum management draft standard and are entirely possible [1]. There are four crosstalkers, with two disturbers of each of two types: two continuous crosstalkers and two short-term stationary crosstalkers. The equalizer of the guarded service (HDSL, HDSL, ISDN), adapts to full power NEXT from the two continuous crosstalkers plus low-power NEXT from the two short-term stationary

6 crosstalkers (with power proportional to the percentage of ON time). Then, during transmission the SNR is calculated with the guarded service receiving full power NEXT from all four crosstalkers. Uncoded HDSL Upstream SNR margin (db) CSA loop 6 CSA loop Percent Short-Term Stationary On Time While Adapting Fig. 3. HDSL Upstream, DFE adapted to two continuous disturbers that transmit the SM class 3 PSD template at frequencies below 150 khz and zero power elsewhere plus two short-term stationary (lower average power) SM class 5 downstream (downstream ADSL). The SNR is then calculated with all four disturbers ON full-power. Fig. 3 shows results for upstream HDSL. The two continuous crosstalkers transmit a signal with PSD equal to the SM class 3 PSD template at frequencies up to 150 khz and no power above 150 khz, and the two short-term stationary crosstalkers transmit a signal with PSD equal to the SM class 5 downstream PSD template (downstream ADSL). The case in Fig. 3. is similar to, but more pronounced than, the case in Fig. 1. The fractional-spaced HDSL equalizer appears to be particularly vulnerable to time-varying crosstalk, probably because it is highly optimized. 5

7 HDSL SNR margin (db) CSA loop 6 CSA loop T1E1./ Percent Short-Term Stationary On Time While Adapting Fig.. HDSL, DFE adapted to two continuous disturbers that transmit the SM class PSD template from 0 to 100 khz and zero power elsewhere plus two short-term stationary (lower average power) class 5 downstream (downstream ADSL) disturbers. The SNR is calculated with all four disturbers ON fullpower. The next simulation is of HDSL with results in Fig.. The two continuous crosstalkers transmit a signal with PSD equal to the SM class PSD template at frequencies from 0 to 100 khz and no power elsewhere, the two short-term stationary crosstalkers transmit a signal with PSD equal to the SM class 5 downstream PSD template (downstream ADSL) CSA loop 6 6 CSA loop Percent Short-Term Stationary On Time While Adapting HDSL SNR margin (db) Fig. 5. HDSL, DFE adapted to two continuous disturbers that transmit the SM class PSD template from 5 to 10 khz and zero power elsewhere plus two short-term stationary (lower average power) class upstream (upstream HDSL) disturbers. The SNR is calculated with all four disturbers ON full-power. The next simulation is of HDSL with results in Fig. 5. The two continuous crosstalkers transmit a signal with PSD equal to the SM class PSD template at frequencies from 5 to 10 khz and no power 6

8 elsewhere, the two short-term stationary crosstalkers transmit a signal with PSD equal to the SM class upstream PSD template (upstream HDSL). 1 ISDN SNR Margin (db) T1.601 loop 1 T1.601 loop Percent Short-Term Stationary On Time While Adapting Fig. 6. ISDN, DFE adapted to two continuous disturbers that transmit.5 db above the SM class 1 PSD template at frequencies below 5 khz with zero power elsewhere plus two short-term stationary (lower average power) disturbers that transmit.5 db above the SM class 1 PSD template at frequencies from 30 to 83 khz with zero power elsewhere. The SNR is then calculated with all four disturbers ON fullpower. The final simulation here is of ISDN with results in Fig. 6. It is harder to find short-term stationary crosstalk that damages ISDN adaptation, so the disturber PSDs are more involved here. The two continuous crosstalkers transmit a signal with PSD.5 db above the SM class 1 PSD template at frequencies below 5 khz and no power elsewhere, and the two short-term stationary crosstalkers transmit a signal with PSD.5 db above the SM class 1 PSD template at frequencies from 30 to 83 khz and no power elsewhere. 6. Adjusting the SNR to Account for Silent Intervals of Short-Term Stationary Crosstalk Short-term stationary crosstalkers transmits in bursts, and they should only cause an appreciable bit error rate (BER) while they are transmitting those bursts. This effectively lowers the BER from what it would be if the crosstalkers were always ON, or equivalently, lowers the SNR required to achieve a 10-7 BER. A brief derivation is presented that converts the average percentage of time that a short term stationary crosstalker is ON into the equivalent SNR gain in db. Let the probability that short term stationary crosstalker is ON equal p, assume that there is a significant bit error rate only when the crosstalkers ON, and that the crosstalk is Gaussian. For B1Q there is an average of 1.5 nearest neighbors to any transmitted point, and so the BER = p*1.5*(1 - Q(z)), where Q(z) is the integral of a unit Gaussian density from z to infinity, and z is the normalized received SNR. For continuous crosstalk p = 1, and if the bit-error rate = 10-7 then z = If the crosstalk is only ON for a 7

9 fraction p, then the SNR gain relative to continuous crosstalk, in db, is 0*log 10 (5.75/z), and is plotted in Fig. 7. Figures 1-6 should really be normalized by adding the equivalent SNR gain in Fig. 7 to them so that all systems have 10-7 BER. However, the db gain in Fig. 7 is small relative to the db losses in figures 1-5. SNR Gain at 10-7 BER (db) Percent Disturber On Time Fig. 7. Equivalent increase in SNR from the decrease in bit-error rate (BER) due to the fact that the shortterm stationary crosstalk is not always ON. 7. Multiple Short-Term Stationary Crosstalkers A cable with a number of short-term stationary crosstalkers may have time varying crosstalk that displays a variety of behaviors. A wide number of short-term stationary transmitters may be on or off at any given time, and the resulting time series of crosstalk seen in the cable may have complicated statistics and PSDs. The complexity of these statistics may be limited by enforcing certain rules, such as mandating that the short-term stationary systems transmit certain minimum length bursts, or that they are ON for certain percentages of certain size time intervals. The general problem of determining statistics of multiple short-term stationary crosstalkers is beyond the scope of this contribution. Only a brief description of the statistics of a simplified case is given here. Assume that there are independent short-term stationary crosstalk disturbers in a binder, and let the probability that any one disturber is transmitting at any given time equal p. The probability that exactly n of the disturbers are simultaneously transmitting is in Table I, as calculated by the binomial density: Pr( n) = p n n n ( 1 p) Table I. Probability that n out of independent short term stationary transmitters are simultaneously ON. 8

10 p Pr( n = 0) Pr( n = 1) Pr( n = ) Pr( n >= 3) E E E E E E-06.0E Summary A brief summary of results from Sections and 5 is in Table II. For HDSL, 5 db coding gain is added to the uncoded SNR margins presented in figures 1 and 3. The "Maximum adaptation error (db)" is the change in the curve from the left side of figures 1-6 to the right side; from continuous 100% ON to.01% ON (nearly OFF) crosstalk while adapting. Results for figures 1 and have one simple short-term stationary crosstalker, and results for figures 3-6 have two somewhat more complicated short-term stationary crosstalkers. The results should be adjusted for equal BER as discussed in Section 6, and in that case the maximum adaptation error would lower by to 3 db and the minimum SNR margin would increase by to 3 db. Table II. Summary of simulation results in figures 1-5. A coding gain of 5 db was added to HDSL. Scenario Fig. 1 Fig. Fig. 3 Fig. Fig. 5 Fig. 6 Guarded system Upstream HDSL Upstream HDSL HDSL ISDN HDSL HDSL Maximum adaptation error (db) Minimum SNR margin (db) Although the scenarios here were specifically designed to be damaging, they are entirely possible and are by no means pathological. The guarded services here generally adapted to low-frequency NEXT, pushing the equalized signal into high frequencies, and then calculated the SNR with high frequency NEXT. Simulations were also run where the guarded services adapted to high-frequency NEXT, pushing the equalized signal into low frequencies, and then calculated the SNR with low frequency NEXT, but the SNR margins did not decrease as much as those reported here. For developing future specifications, all results in this contribution should be further verified and extended. SNR loss relative to continuous crosstalk as high as 0 db was observed due to the guarded service's equalizer's suboptimal adaptation in time-varying short-term stationary crosstalk. However, if the shortterm stationary transmitters are not synchronized, then only a few disturbers are likely to be turning on and off during adaptation, and the impact is less than that of a full or 9 disturbers. This tends to limit the absolute impact (the minimum SNR margin). For the worst case here (Fig. 5), the minimum SNR margin was -8 db, then assuming that 6 db is required for HDSL, and adjusting for equal BER (+ 3 db), the result is an SNR margin that is 11 db too low. This result could be interpreted to mean that TBD should equal 11 db in the text currently in the draft SM Standard [1], "Equipment to which short-term stationary criteria are applied shall transmit at TBD db below the SM mask." However, since only a few disturbers were considered here instead of a full or 9, the change relative to continuous crosstalk is more appropriate (the change in the SNR margin from the left side of figures 1-6 to the right side). 9

11 The draft SM standard currently requires that "Equipment to which short-term stationary conformance criteria are applied shall transmit in the ON condition for a cumulative total of 10 milliseconds minimum in any second period," i.e., the percentage short-term stationary ON time must be at least 0.5%. Results in Figs. 1-5 here show that this requirement is not likely to yield any benefits to HDSL, HDSL, or ISDN. The curves flatten out at from 1% to 0.1%. The percentage short-term stationary ON time would need to be about 30% or more to ensure that there is no appreciable SNR degradation, or about 5% or more to allow only about half the worse-case SNR degradation. The original 0.5% requirement was derived assuming optimal detection of short-term stationary crosstalk [3], but HDSL, HDSL, and ISDN do not perform this optimal detection. It is recommended here to lower the PSD template and PSD mask power for short-term stationary systems as a function of the percentage of short-term stationary on time. For the two worst case results, in figures 3 and 5, the db loss in margin was averaged for a given percentage of short-term stationary on time while adapting. Then the equivalent increase in SNR from the decrease BER due to the fact that the short-term stationary crosstalk is not always ON, in Fig. 7, was subtracted from this average and this is the final recommended number of db to lower PSD templates and PSD masks for short-term stationary crosstalk. 9. Proposal In the draft spectrum management standard, page 1, Section 6..1, replace the first sentence of the second paragraph: "Equipment to which short-term stationary criteria are applied shall transmit at TBD db below the SM mask" with: Equipment to which short-term stationary criteria are applied shall conform to a spectrum management class if it meets all the criteria defined in Sections 5 and 6 for that class except with a PSD template and a PSD mask that is lowered by a fixed number of db at every frequency, with that number given in Table 1. That is, the PSD mask and PSD template used for testing conformance of a short-term stationary system with this standard is the same as that defined in Section 5 for continuous systems except that PSD mask and PSD template are lowered by the number of db given in Table 1 at all frequencies. This number of db is a function of the minimum percentage of time that the short-term stationary transmitters are on and transmitting their full power in any second time period. Table 1. The number of db that are subtracted from spectrum management PSD templates and PSD masks for testing conformance of short-term stationary systems. Minimum percentage of time transmitting at full power in any second time period db that short-term stationary PSD mask and PSD template are below those in Section 5 30% % 3.0 3% 7.0 1% % 1.0 < 0.3% 16.0 REFERENCES 10

12 [1] "Draft Proposed American National Standard Network to Customer Installation Interfaces Spectrum Management for Loop Transmission Systems," T1 LB 785 (T1E1./99-00R), July [] R. Brown, P. Stanley, M. Sorbara, K. Ko, and J. Carlo, "Proposed Text for Conformance Criteria Applied to Short-Term Stationary DSL Transmitters," T1E1./99-63, June 7-11, [3] K. Ko, "Modeling and Estimation of Short-Term Stationary Crosstalk From Multiple Disturbers," T1E1./99-6, June 7-11, [] K. J. Kerpez, "Stationarity and Time-Domain Specifications for Spectrum Management," T1E1./99-103, March 11, [5] J. Stiscia and D. Johnson "Selected Measurements of Non-Stationary and Stationary Crosstalk Effects Upon FDM ADSL," T1E1./99-00, February 1, [6] K. Sistanizadeh and K. J. Kerpez, "A Comparison of Passband and Baseband Transmission Schemes for HDSL," IEEE Journal on Selected Areas in Communications, special issue on High-Speed Digital Subscriber Lines (HDSL), Vol. 9, pp , August