COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Ottawa, Canada; June 7, 1999

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1 COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Ottawa, Canada; June 7, 1999 T1E1.4/ CONTRIBUTION TITLE: SOURCE: PROJECT: Regarding the Spectral Compatibility of 2B1Q SDSL Telcordia Technologies (formerly Bellcore); pursuant to work supported by Metalink. T1E1.4, Spectrum Management ABSTRACT This contribution examines the spectral compatibility of Metalink s symmetric DSL (SDSL) system that is modulated with 2B1Q. The transmitted bit-rates of this SDSL system addressed in this contribution are between 28 kbps and 232 kbps. Because these SDSL systems have at most 14dBm average transmit power, they will generally be spectrally compatible with ISDN and HDSL. Results for SDSL compatibility with T1 lines are reported here. The impact of NEXT from SDSL on full-rate T1.413 ADSL and on G splitterless ADSL (G.lite) is also calculated for the aforementioned SDSL bit rates and is presented in detail here. Results are given for same-binder NEXT. Achievable upstream and downstream ADSL bit rates in the presence of SDSL crosstalk are obtained as a function of loop length. For the 1168, 1552, and 232 kbps SDSLs, the self-next loop limit is determined, and the ADSL bit rates are determined as a function of ADSL loop deployment length with the high-rate SDSLs deployed at the above mentioned self-next loop limits. It is shown that SDSL at 784 kbps or less is spectrally compatible with and depending on it s rate is a member of the very low band symmetric (VLBS), low band symmetric (LBS) or mid-band symmetric (MBS) spectrum management class. Same binder SDSL at all rates addressed here is completely spectrally compatible with repeatered T1 lines. Same-binder SDSL at 784 kbps or less is compatible with T1.413 full-rate ADSL on all CSA range test loops, and same-binder SDSL at 528 kbps or less is compatible with T1.413 full-rate ADSL on all CSA and T1.61 (RRD range) test loops. Higher rate SDSL at 1168, 1552 and 232 kbps, when deployed at their self-next limit is not compatible with T1.413 ADSL on all loops. When deployed on shorter than the self-next limit, these SDSL are compatible with full-rate ADSL. SDSL at all rates considered in this study appear to be mostly compatible with splitterless ADSL on most loops of G G.lite splitterless ADSL. 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 (formerly known as Bellcore), Metalink, or any other company. The requirements are subject to change in form and numerical value after more study. Telcordia and Metalink specifically reserve the right to add to, amend, or withdraw the statements contained herein. CONTACT: Craig Valenti; cvalenti@telcordia.com; Tel: ; Fax:

2 1. INTRODUCTION This contribution examines the spectral compatibility of symmetric digital subscriber line (SDSL) systems that are modulated with 2B1Q (baseband 4-level pulse amplitude modulation). These systems transmit the same bit rate in upstream and downstream directions, full-duplex with echo-cancelers. The transmitted line bit-rates of SDSL for this contribution are 28, 272, 4, 528, 784, 1168, 1552, and 232 kbps. Zimmerman [1] studied spectral compatibility of 2B1Q transmission systems running at bit-rates of 26 kbps and 384 kbps, and concluded that these systems are spectrally compatible with other copper transmission systems out to CSA range. Kerpez [2] subsequently reinforced those results by showing that variable bit-rate 2B1Q systems at rates up to 1 Mbps are spectrally compatible with all other systems out to CSA range. Results for a particular implementation [3] that scales the LPF 3-dB point in proportion to the baud-rate and has better compatibility have also been reported [4]. Recently, there have been a number of other studies [5][6][7] on a different multi-rate 2B1Q SDSL implementation. This contribution reports on new spectral compatibility calculations for this specific implementation of SDSL. Symmetric systems such as ISDN and HDSL are generally limited by self-next. Because the SDSL systems have 13.5 dbm transmit power they are nearly certain to be spectrally compatible with ISDN and HDSL, because ISDN and HDSL are self-next limited. Compatibility of the SDSLs with T1 lines are specifically calculated and shown in this contribution. This contribution focuses on SDSL compatibility with ADSL. Calculations are performed to determine the deployment ranges of multi-rate SDSL that are spectrally compatible with full-rate T1.413 ADSL [8] and splitterless ADSL [9] (G.992.2, formerly called G.lite.). Achievable upstream and downstream ADSL bit rates in the presence of SDSL crosstalk is obtained as a function of loop length. For the 1168, 1552, and 232 kbps SDSLs, the self-next loop limit is determined, and the ADSL bit rates are determined as a function of ADSL loop deployment length with the high-rate SDSLs deployed at the above mentioned self-next loop limits. In Section 2 it is shown that SDSL at 784 kbps or less is spectrally compatible with and depending on it s rate is a member of the very low band symmetric (VLBS), low band symmetric (LBS) or mid-band symmetric (MBS) spectrum management class. In Section 3 self-next limits for high rate SDSL determined through computer simulation are presented. In Section 4, it is shown that same binder SDSL at all rates addressed here is completely spectrally compatible with repeatered T1 lines. In Section 5, it is shown through computer simulation that same-binder SDSL at 784 kbps or less is compatible with T1.413 full-rate ADSL on all CSA range test loops, and same-binder SDSL at 4 kbps or less is compatible with T1.413 full-rate ADSL on all T1.61 (RRD range) test loops. In Section 5, we see also that higher rate SDSL at 1168, 1552 and 232 kbps, when deployed at their self-next limit is not compatible with T1.413 ADSL on all loops. When deployed on shorter than the self-next limit, these SDSL are compatible with full-rate ADSL. In Section 6, it is shown that SDSL at all rates considered in this study appear to be mostly compatible with splitterless ADSL on most loops of G G.lite splitterless ADSL. 2

3 2. SDSL System Under Study This SDSL uses the same modulation as ISDN basic access [1] and HDSL [11]: 2B1Q, which is 4-level baseband pulse amplitude modulation. SDSL is bi-directional and echo-canceled, with the same bit rate in each direction. SDSL runs at a variable clock rate and can transmit 2B1Q at any bit-rate up to about 232 kbps [3]. The baud (or symbol) rate is one-half of the bit-rate. There is no error coding and no trellis coding. The measured transmit PSD of Metalink s SDSL running at a number of bit rates are shown in Figures 1-6. The total average transmitted signal powers of this SDSL system were measured as between 13.5 and 14dBm. As indicated, 28 kbps and 272 kbps SDSL are members of the Very Low Band Symmetric spectrum management class and are therefore spectrally compatible. 4 kbps and 528 kbps SDSL are members of the Low Band Symmetric spectrum management class and are spectrally compatible. 784 kbps SDSL is a member of the Mid Band Symmetric spectrum management class and is spectrally compatible kbps SDSL PSD (dbm/hz) Measured VLB Tmp Frequency (Hz) Figure 1: Transmit PSD of 2B1Q SDSL at 28 kbps 3

4 kbps SDSL T1E1.4/ PSD (dbm/hz) Measured VLB Tmp Frequency (Hz) Figure 2: Transmit PSD of 2B1Q SDSL at 272 kbps PSD (dbm/hz) kbps SDSL Measured LB Tmp Frequency (Hz) Figure 3: Transmit PSD of 2B1Q SDSL at 4 kbps 4

5 kbps SDSL PSD (dbm/hz) Measured LB Tmp MB Tmp Frequency (Hz) Figure 4: Transmit PSD of 2B1Q SDSL at 528 kbps kbps SDSL PSD (dbm/hz) Measured MidBand Tmp Frequency (Hz) Figure 5: Transmit PSD of 2B1Q SDSL at 784 kbps 5

6 PSD (dbm/hz) , 1552 and 232 kbps SDSL 1168 kbps 1552 kbps 232 kbps T1E1.4/ Frequency (Hz) Figure 6: Measure PSD of 2B1Q SDSL at 1168, 1552 and 232 kbps Spectral compatibility calculations here have 1 or 24-disturber NEXT from SDSL. The Unger 1% NEXT [11] model is also used, where only 1% of all loops would have worse NEXT than the model. Every system receives -14 dbm/hz white background noise in addition to the crosstalk. All systems should achieve a 1-7 bit error rate with 6dB margin. Loops are simulated by computation of their ABCD transmission matrices. 3. SDSL Self-NEXT Limits The received SNR of 1168, 1552 and 232 kbps SDSL were computed with 1 and 24 self-next disturbers. The simulated 2B1Q SDSL receiver uses a DFE with optimal minimum mean squared error (MMSE) tap values that has 12 baud-spaced forward-filter taps and 64 feedback taps. The required SNR of 2B1Q at a 1-7 bit error rate (BER) is 21.3 db. The SNR margin equals the computed SNR minus the required SNR. A 6dB margin is desired. Calculated margins for different 26-AWG loop lengths are shown in Figure 7 and Figure 8. The self-next limits with 6dB margin are summarized in Table I. Table I: SDSL self-next 26-AWG loop limits SDSL kbps 1 Disturbers 24 Disturbers ft 85 ft ft 75 ft ft 6 ft 6

7 SNR Margin (db) kbps 1552 kbps 232 kbps 6 db T1E1.4/ AWG Loop Length (Ft) Figure 7: Received SNR margin of SDSL with 1 same-binder self-next SNR Margin (db) kbps 1552 kbps 232 kbps 6 db AWG Loop Length (Ft) Figure 8: Received SNR margin of SDSL with 24 same-binder self-next 7

8 4. SDSL Spectral Compatibility with Repeatered T1 T1 is Mbps data modulated with alternate mark inversion (AMI) at a symbol rate of Mbaud. T1 is designed for transmission over first and last repeatered sections that have no more than 22.5 db attenuation at 772 khz [12], with no bridged taps. This corresponds to 3285 feet of 26 AWG PIC cable. Mid-span sections are designed to have no more than 32dB attenuation at 772 khz. This corresponds to 4672 feet of 26 AWG PIC cable. Compatibility calculations were performed using the methodology of contribution T1E1.4/ [13]. Compatibility with T1 was tested for 24 SDSL NEXT disturbers in the same binder group for both 22.5dB and 32dB spans. Defining C(f) as the 1% Unger two-piece model and Gain(f) as given in [13], the conditions End Section (22.5dB) : 1.544MHz PSDSDSL C( f ) Gain( f ) df 36. 5dBm Mid Span Section (32dB) : 1.544MHz PSDSDSL C( f ) Gain( f ) df 49. 5dBm are tested for all SDSL bit rates. The above integration calculations result in Table II: Table II: Results of T1 compatibility integral SDSL Bit- 22.5dB Span 32dB Span Thus, SDSL and T1 in the same binder group have complete compatibility with each other. Note that = 7.96 kft is well beyond the self-next limit of 6 kft that 232 kbps SDSL could be deployed. 5. SDSL Spectral Compatibility with T1.413 Full-Rate ADSL Spectral compatibility results are calculated for same-binder NEXT with the standard Unger 1% NEXT model. The maximum achievable bit-rate of T1.413 full-rate DMT ADSL in the presence of NEXT from 8

9 SDSL systems was calculated. The DMT tones are separated by khz, and the received SNR of each tone was calculated. The maximum bit-rate that each tone can carry with a 6dB SNR margin was found and then summed across all tones to get the total achievable T1.413 bit rate. The average transmit power of downstream ADSL is -4 dbm/hz, and the average transmit power of upstream ADSL is -38 dbm/hz, within the passband. T1.413 ADSL is assumed to have trellis coding gain of 3dB and 2dB ripple, and is FDD with non-overlapping upstream and downstream spectra. Downstream T1.413 ADSL is assumed to transmit from 16 khz to 114 khz, and upstream T1.413 ADSL transmits from 26 khz to 138 khz. The pilot tones carry no data. A maximum of 12 bits per Hz can be transmitted by any tone in the T1.413 simulations here, allowing a maximum constellation size of 496 points. ADSL bit rates are rounded down to the nearest integer multiple of 32 kbps. Cyclic prefix redundancy (6.66%) and a minimal 32 kbps EOC redundancy was removed before presenting the bit rates here. In Section 11 of T1.413 [8], test conditions are given with an objective of downstream ADSL supporting 6144 kbps on CSA test loops, and 1696 kbps on RRD range test loops (T1.61 loops 7, 9, and 13). Upstream ADSL should support 64 kbps on CSA test loops, and 16 kbps on RRD range T1.61 loops 7, 9, and 13. The limit of RRD loops equates to about 15.5 kft of 26-AWG cable. However, the ADSL target rates apply only to loops less than 13.5 kft of 26-AWG (T1.61 loop #7). 5.1 Equal Level SDSL NEXT: 28, 272, 4, 528 and 784 kbps In this scenario the ADSL unit and SDSL disturber units are collocated at all loop lengths considered. The results of the calculations as presented in Figures 9 12 indicate that the target rates for full-rate ADSL are met in the presence of 24 SDSL disturbers operating at 28, 272, 4, 528 and 784 kbps. Note that 784 kbps SDSL will not be deployed beyond 9 kft. Also 528 kbps SDSL will be self-next limited to less than 13 kft of 26-AWG. Upstream T1.413 ADSL Bit Rate kbps 272 kbps 4 kbps 528 kbps 784 kbps Figure 9: Achievable upstream T1.413 ADSL bit rates with 1 SDSL same-binder NEXT 9

10 Upstream T1.413 ADSL Bit Rate kbps 272 kbps 4 kbps 528 kbps 784 kbps Figure 1: Achievable upstream T1.413 ADSL bit rates with 24 same-binder SDSL NEXT Downstream T1.413 ADSL Bit kbps 272 kbps 4 kbps 528 kbps 784 kbps Figure 11: Achievable downstream T1.413 ADSL bit rates with 1 SDSL same-binder NEXT 1

11 Downstream T1.413 ADSL Bit kbps 272 kbps 4 kbps 528 kbps 784 kbps Figure 12: Achievable downstream T1.413 ADSL bit rates with 24 SDSL same-binder NEXT 5.2 Unequal Level SDSL NEXT: 1168, 1552 and 232 kbps In this scenario the remote ADSL unit and SDSL disturber units are collocated at loop lengths up to the SDSL self-next limit (1 disturber crosstalk limit is slightly longer than 24 disturber limit). Past this point the ADSL and SDSL systems are no longer collocated. The SDSL remotes are fixed at their self- NEXT loop limit. NEXT from these SDSL systems is attenuated by the difference in loop length between deployed ADSL and SDSL systems. The results of the calculations are presented in Figures Upstream T1.413 ADSL Bit Rate XT(1168) 24 XT(1168) 24-SDSL Limit Figure 13: Achievable upstream T1.413 ADSL bit rates with same-binder 1168 kbps SDSL NEXT 11

12 Downstream T1.413 ADSL Bit XT(1168) 24 XT(1168) 24-SDSL Limit Figure 14: Achievable downstream T1.413 ADSL bit rates with same-binder 1168 kbps SDSL NEXT Upstream T1.413 ADSL Bit Rate XT(1552) 24 XT(1552) 24-SDSL Limit Figure 15: Achievable upstream T1.413 ADSL bit rates with same-binder 1552 kbps SDSL NEXT 12

13 Downstream T1.413 ADSL Bit XT(1552) 24 XT(1552) 24-SDSL Limit Figure 16: Achievable downstream T1.413 ADSL bit rates with same-binder 1552 kbps SDSL NEXT Upstream T1.413 ADSL Bit Rate XT(232) 24 XT(232) 24-SDSL Limit Figure 17: Achievable upstream T1.413 ADSL bit rates with same-binder 232 kbps SDSL NEXT 13

14 Downstream T1.413 ADSL Bit XT(232) 24 XT(232) 24-SDSL Limit T1E1.4/ Figure 18: Achievable downstream T1.413 ADSL bit rates with same-binder 232 kbps SDSL NEXT Results are summarized in Table III and Table IV. Table III: Maximum loop range that ADSL can transmit at least 1696 kbps downstream and 16 kbps upstream on longer than CSA loops, or 6144 kbps downstream and 64 kbps upstream on shorter than CSA loops, while high bit rate SDSL is deployed on shorter than self-next limited range, generating 24 disturber NEXT into T1.413 full-rate ADSL Max RRD Loop Max CSA Loop SDSL 13 kft 7.5 kft kft 6.5 kft kft 5 kft 232 Table IV: Achievable rates for T1.413 ADSL when deployed on 9kft CSA loop and 13.5kft RRD loop while 24 disturber high bit rate SDSL are deployed at their self-next limited range SDSL CSA #6 Test Loop T1.61 #7 Test Loop T1.413 Up T1.413 Down T1.413 Up T1.413 Down The results indicate that T1.413 ADSL target rates are generally not met when subject to 24 disturber NEXT from high bit rate SDSL systems operating at 1168, 1552 and 232 kbps deployed at their self- NEXT limits. The whole subject of ADSL target rates as specified in T1.413 is under review due to the fact that the rates are based in part on an optimistic HDSL NEXT model. A recent contribution [14] proposes downstream ADSL target rates of 4.5 Mbps for CSA loops and 1 Mbps for RRD loops (T1.61 #7, T1.61 #9 and T1.61#13) that are based on more realistic SDSL models. While 1168 and 1552 kbps SDSL would meet these modified target rates, SDSL at 232 kbps does not. The point is that 24 of these 14

15 high rate SDSL systems probably cannot be deployed out to their self-next limits. They must be moved in about 1 kft closer to the CO. 6. SDSL Spectral Compatibility with Splitterless ADSL, G G.lite The Universal ADSL Working Group (UAWG), universal ADSL (UADSL) splitterless ADSL is standardized with the ITU G specification [9]. G G.lite uses discrete-multitone modulation (DMT) and is essentially the same as T1.413 DMT ADSL, except that G G.lite only uses half as many downstream tones. G G.lite is assumed to have trellis coding gain of 4dB and 2dB ripple, and is FDD with non-overlapping upstream and downstream spectra. Downstream G G.lite is simulated with a lowest passband transmit frequency of 17 khz and a highest passband transmit frequency of 552 khz. The average passband transmit power is -4 dbm/hz. The downstream pilot tone, #64, carries no data. A maximum of 12 bits per Hz can be transmitted by any tone in the G G.lite simulations here, allowing a maximum constellation size of 496 points. Cyclic prefix redundancy and a minimal 32 kbps EOC redundancy was removed before presenting the bit rates here. Upstream G G.lite is essentially the same as upstream T1.413, except that upstream G G.lite has no pilot tone and it may transmit reduced power. The performances of upstream G G.lite is nearly identical to that of upstream T1.413 ADSL and so results for upstream G G.lite were not plotted separately (instead see Figures 9, 1, 13, 15 and 17). 6.1 Equal Level SDSL NEXT: 28, 272, 4, 528 and 784 kbps In this scenario the ADSL unit and SDSL disturber units are collocated at all loop lengths considered. Downstream G G.lite Bit kbps 272 kbps 4 kbps 528 kbps 784 kbps Figure 19: Achievable downstream splitterless ADSL G G.lite bit rates with 1 same-binder SDSL NEXT 15

16 Downstream G G.lite Bit kbps 272 kbps 4 kbps 528 kbps 784 kbps T1E1.4/ Figure 2: Achievable downstream splitterless ADSL G G.lite bit rates with 24 same-binder SDSL NEXT 6.2 Unequal Level SDSL NEXT: 1168, 1552 and 232 kbps In this scenario the remote G.lite unit and SDSL disturber units are collocated at loop lengths up to the SDSL self-next limit (1 disturber crosstalk limit is slightly longer than 24 disturber limit). Past this point the G.lite and SDSL systems are no longer collocated. The SDSL remotes are fixed at their self- NEXT loop limit. NEXT from these SDSL systems is attenuated by the difference in loop length between deployed G.lite and SDSL systems. The results of the calculations are presented in Figures Downstream G G.lite Bit XT(1168) 24 XT(1168) 24-SDSL Limit Figure 21: Achievable downstream G.lite bit rates with same-binder 1168 kbps SDSL NEXT 16

17 Downstream G G.lite Bit XT(1552) 24 XT(1552) 24-SDSL Limit Figure 22: Achievable downstream G.lite bit rates with same-binder 1552 kbps SDSL NEXT Downstream G G.lite Bit XT(232) 24 XT(232) 24-SDSL Limit Figure 23: Achievable downstream G.lite bit rates with same-binder 232 kbps SDSL NEXT 17

18 Table V: Maximum bit-rates that G G.lite can transmit over test loop T1.61 #7 in the presence of 24 same binder SDSL disturbers G.lite Loop G G.lite Downstream SDSL T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) T1.61 #7 (13.5 kft) In Table V, the maximum G.lite bit rate is given for the case of collocation of G.lite and SDSL remote transceivers for SDSL bit rates of 28, 272, 4, 528 and 784 kbps. The high rate SDSL (1168, 1552 and 232 kbps) are constrained to the self-next limited loops. An ITU agreement outlined in G.test [15] gives target bit-rates for G G.lite with separate tests for different types of disturbers. However, target bit-rates have not been specified for SDSL disturbers. 7. Conclusions SDSL at 784 kbps or less is spectrally compatible and depending on it s rate is a member of the very low band symmetric (VLBS), low band symmetric (LBS) or mid-band symmetric (MBS) spectrum management class. Same binder SDSL at all rates addressed here is completely spectrally compatible with repeatered T1 lines. Same-binder SDSL at 784 kbps or less is compatible with T1.413 full-rate ADSL on all CSA range test loops, and same-binder SDSL at 528 kbps or less is compatible with T1.413 full-rate ADSL on all CSA and T1.61 (RRD range) test loops. Higher rate SDSL at 1168, 1552 and 232 kbps, when deployed at their self-next limit is not compatible with T1.413 ADSL on all loops. When deployed on shorter than the self-next limit, these SDSL are compatible with full-rate ADSL. SDSL at all rates considered in this study appear to be mostly compatible with splitterless ADSL on most loops of G G.lite splitterless ADSL. REFERENCES [1] G. Zimmerman, Spectral Compatibility of 26, 384, and 768 kbps 2B1Q DSLs, T1E1.4/96-226, July [2] K. J. Kerpez, Variable Bit-Rate 2B1Q Spectral Compatibility within CSA Range, T1E1.4/98-29, March 4,

19 [3] M. Steenstra, J. Yang, and M. Rushing, "Specifications for Sub-Rate HDSL," T1E1.4/98-343, November 3, [4] K. J. Kerpez, The Spectral Compatibility of SDSL with T1.413 ADSL and G G.lite, T1E1.4/99-31, February 2, [5] Avi Kliger, Spectral Compatibility of Multi Rate SDSL Systems, T1E1.4/99-48, February 1, [6] Qi Wang, The Spectral Compatibility of High Rate 2B1Q SDSL with T1.413 ADSL and G G.lite, T1E1.4/99-97, March 8, [7] Qi Wang, The Spectral Compatibility of 1168, 1152 and 232 kbps 2B1Q SDSL, T1E1.4/99-182, April 2, [8] "Network and Customer Installation Interfaces, Asymmetric Digital Subscriber Line (ADSL) Metallic Interface," T (issue 2), American National Standards Institute, Inc. [9] "ITU-T Draft G.992.2, Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers," ITU- T, Draft Recommendation G.992.2, February 4, [1] American National Standard for Telecommunications-Integrated Services Digital Network (ISDN) -Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT-Layer 1 Specification, ANSI T , American National Standards Institute, Inc. [11] Generic Requirements for High-Bit-Rate Digital Subscriber Lines, Bellcore TA-NWT-121, Issue 1, October [12] American National Standard for Telecommunications - Network-to-Customer Installation - DS1 Metallic Interface Specification, ANSI T , American National Standards Institute, Inc. [13] Gary Tennyson, A Revised Model for T1 Repeatered Lines, T1E1.4/99-192, April 2, [14] Jack Yang and Mark Steenstra, Proposal for Reducing ADSL Target Data Rate, T1E , April 2, [15] R. Hamdi, "Proposed Performance Requirements for G.lite System," ITU-T Temporary Document AB-52R3, Q4/SG15, August