COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Costa Mesa, California, March 8 12, 1999

Transcription

1 COMMITTEE T1 TELECOMMUNICATIONS Working Group T1E1.4 (DSL Access) Costa Mesa, California, March 8 1, 1999 T1E1.4/99-16 CONTRIBUTION TITLE: SOURCE*: PROJECT: Proposal for an Improved Upstream FEXT Model for FEXT-limited xdsl Systems Nortel Networks T1E1.4, Spectrum Management Standards Project ABSTRACT DSL systems providing duplex operation via frequency-division-multiplexing (FDM) or timedivision-multiplexing (TDM (also known as time-compression-multiplexing (TCM)) are impacted by far-end crosstalk (FEXT), in the upstream direction, from disturber systems using the same duplexing scheme on adjacent pairs. The current simplified FEXT model assumes that the disturber transmitters are at the same distance as the transmitter of the disturbed signal. The real situation can be much worse than this. This paper discusses the underestimation of ADSL self- FEXT into the upstream ADSL receiver and proposes a realistic FEXT model, and text, for incorporation into the Spectrum Management standard for the assessment of ADSL upstream capacity in the presence of crosstalk. NOTICE This contribution has been prepared to assist Standards Committee T1 Telecommunications. This document is offered to the Committee as a basis for discussion and is not a binding proposal on Nortel Networks. The proposed requirements are subject to change in form and numerical value after more study. Specifically, the right to add to, or amend, the statements contained herein is reserved by Nortel Networks. * CONTACT: Edward J. Eckert; ejeckert@nortelnetworks.com; Tel: ; Fax: AUTOR: Scott McClennon; scottmcc@nortelnetworks.com; Tel ; Fax :

2 T1E1.4/ Introduction Current models for FEXT cross-talk [1] use the same loop length for the cross talker and the victim channel. This is an overly optimistic model that does not address typical deployment scenarios, where the upstream xtu-r transmitters may be much closer to the CO than the upstream transmitter of the disturbed system. Assessment of the performance of FEXT-limited systems, in particular full-rate (G.99./T1.413) and splitterless ADSL (G.999.) with nonoverlapped spectra, requires more realistic FEXT models to predict performance in the field. This paper proposes models for FEXT crosstalk based on realistic service deployment scenarios and illustrates the impact of FEXT on upstream ADSL capacity. 1.1 FEXT Underestimation The simplified FEXT loss model given in [1] is: ( f ) = 1 f ) k l1 ( f FEXT CANNEL Where: CANNEL1 (f) = the channel transfer function of the disturber at the same distance from the CO as the victim signal k = coupling constant l 1 = coupling path length = length of the test (disturbed system) loop in the current model f = frequency This model miscalculates the FEXT crosstalk whenever the disturber transmitters are at a different distance, l, from the CO than the victim system transmitter (at distance l 1 ). It overestimates the contribution from disturbers that are farther away from the central office. In this case the coupling length is unchanged, however the CANNEL (f) is reduced due to the additional cable loss in the crosstalk path. Of more concern is the case where the disturbers are closer to the central office than the victim signal transmitter. In this case the CANNEL (f) increases as an exponential function of distance due to reduced cable loss, while at the same time the coupling path reduces as a linear function of distance. The FEXT level is then given by the product of these two terms. 1

3 T1E1.4/99-16 The proposed FEXT loss model uses the same underlying equation, except the coupling length and related channel transfer function is no longer that of the test loop: ( f ) = ( f ) k l f FEXT CANNEL Where: CANNEL (f) = the channel transfer function of the disturber at distance l from the CO k = coupling constant l = coupling path length (not necessarily = l 1 ) f = frequency Models for selection of CANNEL (f) and l as a function of l 1 need to be based on outside plant and CO cabling scenarios that better reflect the spread of loop lengths seen in actual and expected real-life deployments. These are the basis of the FEXT model outlined in the proposed text for the Spectrum management standard as follows.

4 T1E1.4/ Suggested text for Spectrum Management standard Original text underlined B.4. Far end crosstalk, FEXT Far-end crosstalk (FEXT) is defined as the effect of crosstalk due to adjacent transmitters. In other words, FEXT is due to crosstalk from adjacent transmitters at the transmitter end that couples to the receiver of another system. See Figure B.9. FEXT loss is similar but not equal to the combination of NEXT and the subscriber loop channel losses over the coupling length. FEXT was also considered during T1 transmission engineering efforts but was classified as a minor factor compared with NEXT. The effect of FEXT for DSL and DSL is very small and, hence, has been omitted in test procedures. The effect of ADSL system self-fext can not be simply ignored. At high frequencies, and for upstream transmitter disturbers on short loops, ADSL self-fext noise power can exceed that of DSL NEXT and white background noise combined. A simplified FEXT model has been most frequently used by ANSI T1E1.4 and is expressed by 49 = f ) FEXT channel ( klf Where channel (f ) is the channel transfer function, k = ( n 49) 0. 6, n = number of disturbers, l = the loop length in feet, and f = frequency in z. Experimental results for a 5-pair binder group of a 4-AWG PIC cable, support this model. In Figure B.10, those results are shown fitted to the model. The difference between the fitted results and the ANSI model can be explained as the difference between a 50-pair binder group of -AWG PIC cable (ANSI model) and a 5-pair binder group of 4-AWG PIC cable The simplified FEXT model assumes the channel transfer function and length of the coupling path match those of the disturbed system or more simply that the disturber system FEXT sources (transmitters) are co-located with the transmitter of the disturbed system. In the upstream direction, this underestimates the FEXT where the disturbers are closer to the central office than the victim signal transmitter. 3

5 T1E1.4/ Central Office FEXT FEXT1 Victim Signal Transm itter ADSL Disturbers1 Tx ADSL Disturbers Tx Figure x FEXT versus Distance from the Central Office A more realistic FEXT loss model uses same underlying equation, except the coupling length and related channel transfer function is no longer that of the victim system test loop: ( f ) = ( f ) k l f FEXT CANNEL Where: CANNEL (f) = the channel transfer function of the disturber at distance l from the CO over 6AWG PIC l = coupling path length (not necessarily = length of victim system loop (l 1 )). Models for FEXT noise at upstream receivers need to be based on outside plant and CO cabling scenarios that better reflect the spread of loop lengths seen in actual and expected real-life deployments. We consider three cases here as the basis for a new FEXT model: - Spread of loop lengths in a distribution cable binder, downstream of the JWI ((Junction Wire Interface). - Spread of loop lengths seen in adjacent binders in a feeder cable - Spread of loop lengths seen in the cabling between the ATU-C equipment bay and the MDF B.4..1 Loop lengths in a distribution cable At the JWI, the feeder cable binder groups required to serve the distribution area are peeled off. A separate distribution cable is then run with binder groups peeling off as required. This places a tight limit on the geographic spread within a distribution cable binder group. The distribution cable is also typically only 50% occupied to allow for changes in service use. 4

6 T1E1.4/99-16 The binder groups of the feeder cable are filled up at the JWI on a first come first served basis, without regard to the binder groups in the distribution cable. Thus, an apartment building near the JWI could be occupied at one date filling up all but one pair of the feeder binder group, the last pair could be filled with a home from the far side of the distribution area. This situation could also occur as vacated feeder pairs get re-assigned. The feeder cable binder groups tend to be fully occupied, and within a group the sources can be distributed anywhere within a distribution area. Thus, the most severe upstream FEXT will occur within the feeder cable. Feeder Cable, Distribution Cable Central Office Distribution binder group JWI Distribution Cable Distribution Area Drop Cable Furthest Subscriber Figure y Example showing subscriber spread occurring on feeder cable binder groups due to size of distribution area The 1983 Bell Loop Survey [] gives the following data on distribution cable length: Min. Length:3 ft Max. Length: 3,17 ft Mean Length: 1,888 ft If the maximum length result is ignored as an anomaly, a reasonable maximum length can be derived from the mean length. Assuming a rectangular distribution area, the maximum length would be 3776 ft = * mean length. A length of 4.5 kft is selected to provide a reasonable margin to allow for increased length due to irregular distribution area shapes. 5

7 T1E1.4/99-16 ( f ) = ( f k l f FEXT _ DCABLE CANNEL ) Where: l = coupling path length (kft) = max(100, l ) B.4.. Loop lengths seen in adjacent binders in a feeder cable Inter-binder disturbers can occur anywhere along the feeder cable. Thus, the model should assume they are at the most damaging distance (at roughly1.kft from the CO for upstream ADSL frequencies). ( f ) = k _ ( f k l FEXT _ ADJ adj binder CANNEL ) Where: l = coupling path length (kft) = 100 and k adj_binder = additional adjacent binder coupling loss (10dB) factor = 0.1 With the additional adjacent binder coupling loss of 10 db, this FEXT scenario is less severe than that described by FEXT_DCABLE for short victim loops; however, for long loops this inter-binder crosstalk will dominate. The crossover point between the two models occurs around l 1 = 13.5kft for the case where the disturber transmitters transmit with the same PSD level. Jointly, these two outside plant scenarios define the distributed FEXT model: DISTR _ FEXT ( f ) = FEXT _ DCABLE FEXT _ ADJ ( f ) ( f ) for l for l kft; > 13.5kft; As noted above, this model applies to the case where the disturber transmitters all transmit at the same level (same transmit PSD), independent of their target rate and loop length/losses. An xdsl system that reduces its upstream transmitter on short loops, where it has excess margin for a given capacity, will generate less FEXT; the dominant FEXT scenario in this case also needs to consider the upstream transmit power as a function of victim loop length, l 1. 6

8 T1E1.4/99-16 l 1 < 5.7kft Victim System US Receiver l = 1.kft Victim System US Transmitter Disturber System US Transmitters Victim System US Receiver l = l 1-4.5kft 5.7kft < l 1 < 13.5kft Victim System US Transmitter Disturber System US Transmitters Victim System US Receiver l = 1.kft 13.5kft < l 1 Victim System US Transmitter Adjacent Binder Coupling Loss Disturber System US Transmitters Figure z Topology of Distributed FEXT Model B.4..3 Loop lengths seen in cabling between ATU-C equipment and CO MDF Pairs in cabling between the the ATU-C equipment and the MDF can be from any number of separate binders in the outside plant, serving subscribers at any distance from the CO. This cabling can run 100s of feet in length, enough to provide significant FEXT coupling between pairs. In the worst case, there will be inter-binder FEXT from upstream ADSL transmitters at the 7

9 T1E1.4/99-16 most damaging distance from the ATU-C equipment into ATU-C receivers serving subscribers on long loops and the FEXT model is given by: ( f ) = ( f k l f WC _ FEXT CANNEL ) Where: l = coupling path length (kft) = 100 This is referred to as the worst case FEXT model as all disturbers are placed at the worst case location and all are assumed to be in the same binder as the disturbed system. For the purpose of assessing spectral compatibility as per Analytical method of determining spectral compatibility (method B), the distributed FEXT for upstream ADSL FEXT should be used. Where an operator faces a deployment scenario as described above, the worst case FEXT model can be applied to assess the impact of FEXT coupling over the cabling between the ATU- C equipment and the main distribution frame (MDF). [End of proposed text] 8

10 T1E1.4/ Impact of revised FEXT models on upstream capacity 3.1 Upstream capacity vs. victim system loop length Figures 1-4 show the 1% worst case upstream capacity (line rate) vs. loop length (6AWGequivalent) with ADSL disturbers using the distributed and worst case FEXT models above. Both (parallel telephone set) on-hook and off-hook (with 5dB power reduction for filter-less operation) US capacity are shown. Each figure also includes (see solid lines) capacity for the same numbers of disturbers where the disturber upstream transmitters are co-located with the disturbed system transmitter (as per the current simplified FEXT model). Frequency domain simulations have been run to assess the U/S capacity with the distributed and worst case FEXT models above. No impairments due to imperfect equalization or timing recovery have been taken into account and the following system parameters have been used: Used tones: from #7 to #6 Margin: 4 db Coding gain: 3 db Max. bits/carrier: 15 Background noise = -140 dbm/z Figure 1 Upstream Capacity with Distributed FEXT Model No UPC 9

11 T1E1.4/99-16 Figure Upstream Capacity with Worst Case FEXT Model No UPC Figure 3 Off-hook Upstream Capacity with Distributed FEXT Model - No UPC 10

12 T1E1.4/99-16 Figure 4 Off-hook Upstream Capacity with Worst Case FEXT Model - No UPC 3. G.99. system performance test cases with proposed FEXT models System performance against the required and extended reach North American test cases (Annex D of the G.99. draft) with Annex A (FDM) G.99. disturbers with the proposed distributed and worst case FEXT models are shown in Table 1 below. These figures are also based on frequency domain simulations (no impairments due to imperfect equalization or timing recovery) with system parameters as per section 3.1 above, with the exception that we restrict the maximum number of bits/carrier to 8. The upstream test case capacity targets are sufficiently modest in that use of the distributed FEXT model has little impact on the ability to achieve them. Use of the worst case FEXT model with this reference system, however, leaves little margin to achieve the target rate set in extended reach test case #14 and results in an inability to meet the target rate in test case #1. (Note also that using all upstream carriers (7-31) improves capacity by only ~10kbps in these two cases). 11

13 T1E1.4/99-16 Case # Loop Noise Required Upstream net data rate T1.601 #7 49 FDM G.lite 4 T1.601 #13 49 FDM G.lite 8 T FDM #8 G.lite 10 * T FDM #1 G.lite 11 * T FDM # G.lite 1 * T FDM #5 G.lite 14 * T FDM #9 G.lite Upstream net data rate Distributed FEXT Upstream net data rate Worst Case FEXT 4kbps 518kbps 346kbps 4kbps 494kbps 34kbps 96kbps 518kbps 36kbps 96kbps 370kbps 178kbps 96kbps 354kbps 18kbps 56kbps 466kbps 80kbps 56kbps 39kbps 44kbps Table 1. Required and *Extended Reach Test Cases 3.3 G.99.1 system performance test cases with proposed FEXT models Similarly, system performance against the North American test cases for G.99.1 full-rate ADSL with the proposed distributed and worst case FEXT models, has also been investigated (system parameters as above, but with a maximum of 15 bits/carrier). In this case, even with the worst case FEXT model, the achievable net upstream capacity never fell below ~870kbps, well above the modest 4kbps category I (and even the 640kbps category II) requirements. 1

14 T1E1.4/ Recommendations It is recommended that the distributed and worst case FEXT models be included in the Spectrum Management standard for a evaluation of DSL systems claiming strict (non-overlapping) spectral compatibility with FDM ADSL systems. The recommended text is included in section of this contribution. To avoid the immediate impact of forcing vendors and test houses to re-work test suites for the proposed FEXT models, the existing simplified FEXT model may continue to be used for equipment compliance testing against the G.99. recommendation. The new model should, however, be used for performance benchmarking (as under consideration in TIA TR30.3 project PN454 - Telephone network transmission model for evaluating xdsl systems ) and for spectral compatibility assessments. The impact of upstream FEXT between FDM ADSL systems also highlights an opportunity to for overall capacity gains via strategies such as short loop upstream power in future versions of G.99.1 and G.99. ADSL [3]. 5. References: [1] T1E1.4/98-007R, John Bingham, Frank Van der Putten (editors), Standards Project for Interfaces Relating to Carrier to Customer Connection of Symmetrical Digital Subscriber Line (ADSL) Equipment. [] Characterization of Subscriber Loops for Voice and ISDN Services, Bell Communications Research Inc., Science and Technology Series ST-TSY [3] T1E1.4/99-107, S. McClennon, Proposal for Short Loop Remote Power Cutback in FEXTlimited xdsl Systems, Costa Mesa California, March 8 1,