10.5 Foundation for Determining Acceptable Levels for Spectral Compatibility
| As discussed in the previous chapter on spectral compatibility, near-end crosstalk (NEXT) is a much stronger disturber than far-end crosstalk (FEXT) because the crosstalk coupling between the wire pairs in the cable is much greater. Signals in the cable that have overlapping upstream and downstream frequency components will experience near-end crosstalk with each other. A special case of overlapping frequency spectra occurs with systems that use echo-cancellation for upstream and downstream duplexing . When the upstream and downstream signals use the same spectrum, there is full overlap of the frequency components, and the echo canceler removes the disturbance from the near-end transmitter into the near-end receiver. However, if systems with the same spectrum are deployed in a cable, then the amount of near-end crosstalk injection would be greatest due to the full spectrum overlap. Such near-end crosstalk is termed self near-end crosstalk or simply SNEXT. ISDN, HDSL, and SDSL are systems that have fully overlapping upstream and downstream signal spectra, and their reach is limited by SNEXT; hence, they are SNEXT limited systems. ADSL (per that on the basis systems list) uses separate frequency bands to separate upstream and downstream transmission. Because there are no overlapping frequencies, ADSL has no SNEXT but it does have SFEXT. As we observed in the previous chapter on spectrum compatibility, the crosstalk coupling for FEXT is significantly lower than that for NEXT coupling. Hence, if only ADSL is in the cable, then SFEXT would limit ADSL reach. In fact, if such a situation in the cable could be assured, then ADSL could be deployed at much higher distances than would SNEXT limited systems. However, because the FCC prohibits binder group service segregation , we can expect that the cable will have a mixture of overlapping and nonoverlapping systems such that NEXT from other systems would limit the reach of ADSL to a distance significantly less than the ADSL SFEXT reach. The question now is, What is the maximum level of NEXT acceptable into ADSL? Given that symmetric echo canceled systems are performance-limited by SNEXT and ADSL has best performance in a SFEXT environment, determination of a maximum level of NEXT for spectral compatibility with ADSL is somewhat arbitrary. Specifically, the issue is determining the spectral compatibility of 2B1Q-based SDSL with ADSL. Industry consensus is required when determining this criterion for spectral compatibility. A debate with technical and political dimensions was conducted in T1E1.4 to balance the interests of ADSL and SDSL technologies. Eventually all parties agreed to a compromise that placed some limitations on both ADSL and SDSL. Following is the agreed-upon rationale that defines the acceptable spectral compatibility between SDSL and ADSL. The reach of SDSL is limited by SNEXT. To determine the reach of SDSL, we assume a 50 pair cable and the worst-case crosstalk of 49 SNEXT. In the computation of SDSL reach, 6 dB of SNR margin was included. A value of 6 dB SNR margin is a common practice in the industry. Figure 10.2 shows a plot of the SDSL bit rate versus the distance for the case of 49 SNEXT disturbers and 6 dB of margin. For each bit rate, the distance represents the maximum distance for which the SDSL would be deployed to guarantee 10 -7 bit error rate with 6 dB of margin. Figure 10.2. SDSL reach in the presence of 49 SNEXT. For each of the bit rates in Figure 10.2 we compute the maximum bit rate of ADSL at that same distance with 24 SDSL NEXT disturbers within a 50 pair binder group. The transmit signal power for SDSL is 13.5 dBm for all bit rates. In each case we assume 6 dB of margin and 3 dB of coding gain in the ADSL bit rate computations . The upstream channel PSD is -38 dBm/Hz in the passband region, and the downstream channel PSD is -40 dBm/Hz in its passband region. The resulting ADSL bit rate for each distance corresponds to a maximum level of near-end crosstalk from the 2B1Q SDSL system configured with its bit-rate for the same distance. Table 10.1 lists the ADSL bit rates resulting from injection of twenty-four SDSL NEXT disturbers, where the SDSL spectrum is that of the bit rate corresponding to each distance, and Figure 10.3 shows the corresponding plots. Figure 10.3. ADSL reach in presence of 24 2B1Q SDSL NEXT. Table 10.1. SDSL SNEXT Reach and ADSL Bit Rates
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