Digital echo cancellation for baseband data transmission

In analogy with analog speech transmission, simultaneous two-way ("full-duplex") transmission of data signals over two-wire circuits can, in principle, be achieved by the application of hybrid couplers. In practice the resulting imperfect isolation between transmitter and receiver at the same end of a connection is disastrous for the detection of the data from the other end and has to be nullified in some way. A solution to this problem can be found in echo cancellation by means of an adaptive digital filter. In this paper an integrated circuit realized in 4Φ-dynamic NMOS technology is presented which has been designed especially for echo cancellation in baseband modems. In the design a number of aspects have extensively been accounted for. These are the choice of adaptation algorithm, the interaction between the digital filter and its analog environment, interpolation, finite precision implementation, and the natural occurrence of an effect akin to dithering. These subjects receive full attention in this paper. The realized IC can be used for the economical upgrading of existing baseband modems to full-duplex service and can pave the way to the use of similar circuits in the local area of the future digital telephone.