Simultaneous two-way (full-duplex) data transmission in the same bandwidth normally requires separate channels, and in the telephone network is realized by using a four-wire circuit. This paper reports on investigations directed toward full-duplex operation on a two-wire circuit using a hybrid coupler and a passband automatic transversal filter at each end of the circuit to separate the two signals. Full-duplex binary transmission at a rate of 2000 Bd in each direction has been experimentally demonstrated on a limited number of twowire circuits. Photographs of eye patterns are provided to illustrate the substantial improvement over operation with hybrids alone. Limitations on performance due to unstable distant echoes are described.
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Koll et al. (1973) studied this question.