Adaptive-Reference Near- and Far-Echo Cancellation in the Presence of Frequency Offset
Abstract
This report presents the results of a study of the applicability of various echo-cancelling techniques to the problem of improving the transmission of data over voiceband telephone channels. The principal result of the study is a design for a novel echo-canceling configuration which permits the cancellation of a far-echo component containing a frequency offset. The modern design is based on a combined adaptive-reference echo canceller and adaptive-channel equalizer. The adaptive-reference algorithm has the advantage that interference to the echo canceller caused by the far-end signal can be eliminated by subtracting an estimate of the far-end signal based on receiver decisions. This technique is extended to provide a new approach for full-duplex far-echo cancellation in which the far echo can be cancelled in spite of carrier frequency offset. To estimate the frequency offset, the system uses a separate receiver structure for the far echo which provides equalization of the far-echo channel and tracks the frequency offset in the far echo. The feasibility of the echo-canceling algorithms is demonstrated by computer simulation with realistic channel distortions and with 4800 b/s data transmission, at which rate frequency offset in the far echo becomes important. The design requirements for a modem implementing this algorithm are examined briefly.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 22, 1987
- Accession Number
- ADA182212
Entities
People
- Gerald C. O'leary
- Thomas F. Quatieri
Organizations
- Massachusetts Institute of Technology