Theoretical and practical limits of next-generation high-speed digital subscriber loops

For the single-pair HDSL (HDSL2), the concept of having different upstream and downstream power spectral densities is highlighted. The signal-to-noise ratio (SNR) at the input of the central office HDSL terminal unit (HTU-C) can be enhanced by boosting the power output of the remote user HDSL terminal unit (HTU-R). Further improvement in performance can be achieved by reducing the HTU-C NEXT interference. Two techniques are proposed to reach this goal. The first approach is to reduce the HTU-C transmitter power in the downstream direction for shorter reach cables. The second approach is borrowed from the analog frequency-modulation preemphasis/deemphasis frequency tilting in the upstream direction. Combining the three techniques leads to at least 6-dB SNR margin improvement for the full reach carrier serving area loops.