High bit rate asymmetric digital communications over telephone loops

Abstract

Asymmetric digital subscriber lines (ADSL) transmit high bit rate data in the forward direction to the subscriber, and lower bit rate data in the reverse direction to the central office, both on a single copper telephone loop. This paper provides a comprehensive description of the performance of ADSL's that use currently available technologies with optimized parameters. Passband ADSL's encounter no self-near-end crosstalk (NEXT) since the forward and reverse channels are separated in the frequency domain. The absence of NEXT allows reliable transmission at high bit rates on long loops. The passband ADSL is disturbed by background noise, self-far-end crosstalk (FEXT), and NEXT from other digital transmission systems that share its spectrum, such as Basic Access DSL, HDSL, and T1 lines. This paper determines the performance of DS1 rate passband ADSL's in the presence of each of these impairments. Although baseband ADSL's are disturbed by NEXT from the reverse channel, they are suitable for data rates at 3 Mb/s or above since they experience less high frequency loss than passband ADSL's. The range of reliable baseband ADSL transmission is determined for forward data rates between 1.5 and 9 Mb/s, and reverse data rates up to 1.5 Mb/s. An analysis and comparison of different codes that could be applied to an ADSL is also presented. The performance of forward error correction codes and trellis codes is found for ADSL's in the presence of crosstalk and impulse noise. >