When the spectra of the CAP RADSL upstream and downstream channels are compared with those of DMT, it is evident that there is some overlap between the CAP RADSL upstream and DMT downstream channel. A result of this overlap is that when the systems are deployed in the same cable, they inject NEXT into each other. However, as seen in Figures 13 and 14, this overlap is minimal.

Figure 13. Spectral Compatibility of CAP RADSL with DMT ADSL

Figure 14. Spectral Compatibility of CAP RADSL with DMT ADSL

Figure 15 shows a composite plot of 49 NEXT from each of CAP RADSL upstream, DMT upstream, HDSL, and ISDN signals. Clearly, the level of spectral compatibility of each of these signals with the DMT downstream channel is a function of the amount of overlap with the downstream channel. The upstream DMT crosstalk spectrum is that defined T1.413.1 The CAP RADSL upstream crosstalk spectrum is that of 136–kbps, square-root, raised-cosine spectrum with 15 percent excess bandwidth, and the spectrum assumes to have 50 dB out-of-band attenuation. To quantify the spectral compatibility of DSLs into the DMT downstream, we present the margin computed in the DMT downstream channel in the presence of crosstalk from each of the above DSL disturbers as presented in G. Zimmerman's Spectral Compatibility of CAP/QAM RADSL and DMT ADSL.²

Figure 15. CAP RADSL Upstream, DMT Upstream, HDSL, and ISDN Crosstalk Spectra

Table 1 shows the margin of an FDM–based 6.784–Mbps DMT receiver in the presence of various disturbers on a 9-kft, 26-gauge test loop. The margin of the downstream channel with crosstalk from 20 FDM DMT upstream disturbers and that from 20 CAP RADSL upstream disturbers are both about 5.5 dB. The margin from 20 HDSL disturbers is about 0.5 to 0.6 dB worse (i.e., 4.9 dB). In summary, NEXT from HDSL dominates disturbance into 6.784–Mbps DMT downstream.

DSL Interferer	Downstream 6.784–Mbps FDM DMT Margin (dB)
20 HDSL	4.9
20 FDM DMT upstream	5.4
20 CAP RADSL upstream	5.5

Table 1. Spectral Compatibility into 6.784–Mbps DMT Downstream

Table 2 shows the margin of an FDM–based 1.72–Mbps DMT receiver in the presence of various disturbers on a 13.5-kft, 26-gauge test loop. The margin of the downstream channel with crosstalk from 24 FDM DMT upstream disturbers and that from 24 CAP RADSL upstream disturbers are 7.0 dB and 7.4 dB respectively. The margin from 24 ISDN disturbers is about 3 to 3.4 dB worse (i.e., 4.0 dB). Disturbance from HDSL was not considered because HDSL is not deployed on loops greater than CSA range. In summary, NEXT from ISDN dominates disturbance into 1.72–Mbps DMT downstream.

DSL Interferer	Downstream 1.72–Mbps FDM DMT Margin (dB)
24 ISDN	4.0
224 FDM DMT upstream	7.0
24 CAP RADSL upstrea	7.4

Table 2. Spectral Compatibility into 1.72–Mbps DMT Downstream

In summary, although the CAP RADSL upstream channel has a slightly greater bandwidth than the DMT upstream, its out-of-band energy is less than that of the upstream DMT channel defined in T1.413. With 50 dB out-of-band attenuation of the CAP RADSL upstream spectrum, the spectral compatibility into the DMT downstream channel is the same as that from the DMT upstream channel. In either case, HDSL and ISDN are greater disturbers into the downstream channel than either of the DMT or CAP RADSL upstream channels. So CAP RADSL is spectrally compatible with T1.413 ADSL (see Figure 15).