HIGH BIT RATE DIGITAL SUBSCRIBER LINE ECHO CANCELLATION

During the 1980's, the digital subscriber line (DSL) was developed for ISDN basic rate access, applying adaptive equalization and echo cancellation techniques to the subscriber loop for the first time. Adaptive filtering techniques are particularly beneficial in loop applications because of the wide range of different channel characteristics encountered due to different cable makeups. At the time the DSL was developed, several research organizations proposed extension of adaptive filtering techniques to higher rates. Over the last several years, there has been much study of the feasibility of delivering 1.544 Mb/s over subscriber loops conforming to carrier serving area (CSA) guidelines. There is a general consensus that such a system is practical within the next several years if two loops are used, each carrying full duplex data at 800 Kb/s. An adaptive echo canceller is therefore needed to separate the two directions of transmission. This raises many technical questions regarding the feasibility of echo cancellation at these high rates. This paper explores issues of complexity, precision, and dynamic range and practical implementation issues such as nonlinearities in HDSL echo cancellers. The approach taken is to study these issues analytically using computer simulations, and then to verify simulation results with laboratory prototyping. One of the objectives of the study was to determine whether HDSL echo cancellation will be practical and cost effective within the next several years, consistent with the desire to develop and deploy a "repeaterless T1" capability by 1993. Thus, attention is given to practical design issues.