Telecommunications Industry Association
(TIA)
TR-30.3/00-12-876
TR-30.3 Meeting, Scottsdale, AZ, Dec 5-8, 2000
COMMITTEE CONTRIBUTION
Technical Committee TR-30 Meetings
TITLE:
Need for Revision of PN4254 Annex F
SOURCE:
Paradyne
CONTRIBUTOR:
Ken Ko
Paradyne
PROJECT:
TR-30:
Office Tel:
Office Fax:
E-mail:
(727) 530-2024
(727) 532-5244
kko@paradyne.com
PN4254/PN4255
_______________________________
ABSTRACT
Sections F.1.1 and F.1.2 of the current draft of PN4254 [1] contain equations for Near End and Far End
Crosstalk (NEXT and FEXT) coupling which are inconsistent with current practice for worst case
crosstalk modeling and combinations as addressed within T1E1.4 in the draft Spectrum Management
Standard [2]. Application of the existing equations in PN4254 will result in overly pessimistic test
results for scenarios incorporating multiple disturbers. Proposed corrected text with references to [2] is
provided.
Copyright Statement
The contributor grants a free, irrevocable license to the Telecommunications Industry Association (TIA)
to incorporate text contained in this contribution and any modifications thereof in creation of a TIA
standards publication; to copyright in name any standards publication even though it may include
portions of this contribution, and at TIA’s sole discretion to permit others to reproduce in whole or in
part the resulting TIA Standards publications.
TR-30.3/00-12-876
Introduction and Background
Sections F.1.1 and F.1.2 of the current draft of PN4254 [1] contain equations for Near End and Far End
Crosstalk (NEXT and FEXT) coupling in which the power contributions from each individual disturber
are summed to generate a composite PSD before scaling by f3/2. The equations shown are inconsistent
with current practice for crosstalk modeling as documented in the draft T1E1 Spectrum Management
Standard [2] and will result in overly pessimistic test results for scenarios incorporating multiple
disturbers.
The basic problem stems from the application of a 1% worst case coupling model to multiple disturbers,
and especially to multiple disturbers incorporating different PSDs. The physical representation of the 1%
worst-case model can be viewed as a case in which the disturbers are on the pairs in closest proximity to
the disturbed pair, resulting in the highest coupling coefficients. As the number of disturbers increases,
they can’t all be on the pairs closest to the disturbed pair, and so the cumulative effect of additional
disturbers is lessened with increasing numbers. This effect is modeled for like disturbers by applying a
scalar n0.6 to the NEXT and FEXT equations, where n represents the number of disturbers.
Combination of unlike disturbers (those with differing PSDs) is more complicated. FSAN proposed a
method for combining unlike disturbers [3] which has been adopted by T1E1 and is used in the
Spectrum Management Standard [2]. A good description of the rationale behind the FSAN method is
available in [3] and will not be repeated here. Likewise, a good overall description of cable crosstalk
modeling is provided in section B.4 of [2].
This contribution proposes that the methods used in T1E1 and FSAN for crosstalk modeling be adopted
in PN4254 in place of the current text in sections F.1.1 and F.1.2 (and the placeholder section F.1.3).
Proposed text is provided which is copied nearly verbatim from Annex A in [2] (Only the simplified
model for NEXT coupling is included in the proposed text below. The two-piece Unger model is not
defined for the full range of disturber numbers required for a test implementation.). Alternatively, a
normative reference to [2] could be provided in PN4254.
Proposed Text
F.1.1
Cable crosstalk models
F.1.1.1
Near end crosstalk, NEXT
Crosstalk noise that occurs when a receiver on a disturbed pair is located at the same end of the cable as
the transmitter of a disturbing pair is called Near-End-Crosstalk (NEXT).
F.1.1.1.1
Simplified NEXT model
The simplified NEXT model has loss values of 57 dB, 61.1 dB, and 67.1 dB for 49 disturbers, 10
disturbers, and 1 disturber, respectively, at a frequency of 80 kHz and a linear (log-log) slope of –15 dB
per decade. The simplified NEXT model is expressed by
NEXT f , n  S ( f )  X N  n 0.6  f 3 / 2
Page 2 of 4
TR-30.3/00-12-876
15
where X N  8.536  10 , n  number of disturbers, f  frequency in Hz, and S ( f ) is the power
spectrum of the interfering system.
F.1.1.2
Far end crosstalk, FEXT
Crosstalk noise that occurs when a receiver on a disturbed pair is located at the other end of the cable as
the transmitter of the disturbing pair is called Far-End-Crosstalk (FEXT). The FEXT model is expressed
by
2
FEXT[f , n, l ]  S(f )  H(f )  X F  n 0.6  l  f 2
where H (f ) is the magnitude of the insertion gain transfer function affecting the disturber signal,
X F  7.74  1021 , n  number of disturbers, l  the coupling path length in feet, f  frequency in Hz,
and S(f ) is the power spectrum of the interfering system.
The FEXT model assumes the insertion gain transfer function is computed for the total cable path
located between the interfering transmitter and the victim receiver. On the other hand, the coupling loss
is computed only over the coupling path length l . The coupling path length is the length of cable over
which the victim receiver and far-end disturbing transmitter have a common cable path.
F.1.1.3
FSAN method for combining crosstalk contributions from unlike types of disturbers
Instead of directly adding the crosstalk power terms, each term is first arbitrarily raised to the power
1/0.6 before carrying out the summation. Then, after the summation, the resultant expression is raised to
the power 0.6. This can be expressed as:
N

Xtalk  f , n   n i

i 1





N

  Xtalk f , n i 1/ 0.6 


 i 1

0.6
,
where Xtalk is either NEXT or FEXT, n is the total number of crosstalk disturbers, N is the number of
n
types of unlike disturbers and j is the number of each type of disturber. Example uses of this equation
are given in the following subsections.
F.1.1.3.1
Example application of two NEXT terms
Take the case of two sources of NEXT at a given receiver. In this case there are n1 disturber systems of
spectrum S1 (f ) and n2 disturber systems of spectrum S 2 (f ) .
The combined NEXT is expressed as:


3
0.6
NEXT f , n    S1 (f ) X N f 2 n1

F.1.1.3.2

1
0.6

 S (f ) X
2
N
f
3
2
n2
0.6

1
0.6



0.6
Example application of three FEXT terms
Take the case of three sources of FEXT at a given receiver. In this case there are n1 disturber systems of
spectrum S1 (f ) at range l 1 , a further n 2 disturber systems of spectrum S 2 (f ) at range l 2 and yet another
n3
disturber systems of spectrum S3 (f ) at range l 3 .
Page 3 of 4
TR-30.3/00-12-876
The expected crosstalk is built in exactly the same way as before, taking the base model for each source,
raising it to power 1/0.6, adding these expressions, and raising the sum to power 0.6:
1
1


  S1 (f )] H 1 (f ) 2 X F f 2 l 1 n10.6  0.6   S 2 (f ) H 2 (f ) 2 X F f 2 l 2 n 2 0.6  0.6






FEXT [f , n, l ]  

1
2
0 .6 
0.6

  S 3 (f ) H 3 (f ) X F f 2 l 3 n 3 





F.1.1.4
0 .6
Total noise power spectral density
The NEXT term and the FEXT term are computed to arrive at separate NEXT and FEXT disturbance
power spectra. These power spectra should then be summed with the background noise to determine the
total disturbance seen by the victim receiver. The total noise seen by the victim receiver is given by:
N(f )  NEXT f , n  FEXT[f , n, l ]  1014 mW/Hz
References
[1]
North American Network Access Transmission Model for Evaluating xDSL Modem Performance,
TIA TR-30.3/00-12-801R5, December 2000.
[2]
Editor, T1E1.4, Draft Proposed American National Standard, Spectrum Management for Loop
Transmission Systems, T1E1.4/2000-002R6, November 28 2000.
[3]
FSAN VDSL working group, A new analytical method for NEXT and FEXT noise calculation,
T1E1.4/98-189, June 1 1998.
Page 4 of 4