Adaptive Echo Cancellation/AGC Structures for Two-Wire, Full-Duplex Data Transmission

Adaptive Echo Cancellation/AGC Structures for Two-Wire, Full-Duplex Data Transmission By D. D. FALCONER and K. H. MUELLER (Manuscript received March 2, 1979) Three different receiver arrangements are studied, all of which incorporate provision for joint adaptive echo cancellation and gain adjustment to provide two-wire full-duplex data communication. In each case, the canceler consists of a data-driven transversal filter, but the architectures differ in the way the gain adjustment is provided. For all architectures, we investigate the properties of a joint adaptive LMS algorithm based on the receiver's decisions on far-end data symbols and present appropriate computer simulations. We show that an arrangement where the gain control adjusts the reference level after the decision detector output performs significantly better than AGC schemes attempting to adjust the level of the analog signal. I. INTRODUCTION High-speed full-duplex data communication on a single channel is of immense practical interest. Data transmission via the DDD telephone network and the possibility of future digital subscriber lines are two of the most challenging applications. Techniques for achieving this goal fall in essentially three categories: Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), and echo cancellation. Only echo cancellation allows full-bandwidth continuous use of the channel in each direction. This scheme therefore offers the highest potential bit rates. The transmitter and receiver are jointly coupled to a two-wire line via a hybrid.