9.2 Crosstalk in the Loop Plant

Crosstalk generally refers to the interference that enters a communication channel, such as twisted wire pairs, through some coupling path . The diagram in Figure 9.2 shows two examples of crosstalk generated in a multi-pair cable. On the left-hand side of the figure, signal source V _j ( t ) transmits a signal at full power on twisted wire pair j . This signal, when propagating through the loop, generates two types of crosstalk into the other wire pairs in the cable. The crosstalk that appears on the left-hand side, x _n ( t ) in wire pair i , is called near-end crosstalk (NEXT) because it is at the same end of the cable as the crosstalking signal source. The crosstalk that appears on the right-hand side, x _f ( t ) in wire pair i , is called far-end crosstalk (FEXT) because the crosstalk appears on the end of the loop opposite to the reference signal source. In the loop plant, NEXT is generally far more damaging than FEXT because NEXT has a higher coupling coefficient and, unlike far-end crosstalk, near-end crosstalk directly disturbs the received signal transmitted from the far end after it has experienced the propagation loss from traversing the distance from the far end down the disturbed wire pair.

Figure 9.2. NEXT and FEXT in a multipair cable.

graphics/09fig02.gif

In a multipair cable, relative to the desired receive signal on one twisted wire pair, all the other wire pairs are sources of crosstalk. For DSL systems, the reference cable size for evaluating performance in the presence of crosstalk is a 50-pair cable [2]. So by reviewing the example shown in Figure 9.1, we see that relative to the received signal on wire pair i , the other 49 wire pairs are sources of crosstalk (both near-end and far-end).

In the United States, 25-pair binder groups are most common; however the T1.417 spectrum management standard employs the 50-pair binder model for conservatism. The difference between a 50-pair and a 25 pair model is approximately 1.8 dB. This is shown in Figure 9.3.

Figure 9.3. Plot of NEXT coupling versus frequency per number of disturbers.

graphics/09fig03.jpg

9.2.1 Near-End Crosstalk Model

As described in references [2, 3, 5 and 6], for the reference 50-pair cable, the near-end crosstalk coupling of signals into other wire pairs within the cable is modeled as

where is the coupling coefficient for 49 NEXT disturbers, N is the number of disturbers in the cable, and f is the frequency in Hz. Note that the maximum number of disturbers in a 50-pair cable is 49. A signal source that outputs a signal with power spectral density PSD _Signal ( f ) will inject a level of NEXT into a near-end receiver that is

So as illustrated in Figure 9.2, if there are N signals in the cable with the same power spectral density PSD _Signal ( f ), the PSD of the NEXT at the input to the near-end receiver on wire pair i is PSD _NEXT ( f ).

Note from the above expressions that the crosstalk coupling is very low at the lower frequencies and the coupling increases at 15 dB per decade with increasing frequency. For example, at 80 kHz, the coupling loss is 57 dB for 49 disturbers. The loss (in dB) for 49 disturbers at other frequencies may be computed using the following formula:

where L ₄₉ is the near end crosstalk coupling loss in dB and f is the frequency in kHz.

9.2.2 Far-End Crosstalk Model

Correspondingly, in the same 50-pair cable, the far-end crosstalk coupling of signals into other wire pairs is modeled as

where H _Channel ( f ) is the channel transfer function, k =8 x 10 ^- ²⁰ is the coupling coefficient for 49 FEXT disturbers, N is the number of disturbers, d is the coupling path distance, and f is the frequency in Hz.

Note that the coupling is small at low frequencies and large at higher frequencies. The coupling slope increases at 20 dB/decade with increasing frequency.

Top