Experimental Bidirectional Subscriber Loop Transmission System

This paper describes a complete hardware implementation of an experimental two-wire digital subscriber loop transmission system using balanced pairs of wires. It is based on the time division burst mode transmission scheme (ping-pong), and provides two information channels, 64 kbits/s and 8 kbits/s, and a signaling channel both ways. The burst cycle is 2 ms and line transmission rate is 192 kbits/s. The salient feature of the system is that it is equipped with a powerful bridged tap equalizer which has the capability to compensate a large fraction of existing two-wire subscriber loops in Japan. It is realized by a fifth-order recursive filter using analog tapped delay lines, and a modified zero forcing algorithm is adopted for the adaptation of the tap coefficients. The experimental results showed that the combination of the AGC and the bridged tap equalizer enables the digital transmission up to 4 km on the 0.4 mm φ subscriber line with three bridged taps each of 400 m length.