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
\sqrt{f}
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.