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ZERO Order Adaptive Equalizer

IP.com Disclosure Number: IPCOM000061700D
Original Publication Date: 1986-Sep-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 1 page(s) / 13K

Publishing Venue

IBM

Related People

Blokkum, DR: AUTHOR [+2]

Abstract

A device is described which adapts certain data modulator-demodulator characteristics to the characteristics of the telephone line in use, and dynamically varies modem characteristics during call, to track telephone line changes. Adaptive equalization of modem signals is a well-known art. Normally, adaptive equalizers are configured as a finite-impulse-response (FIR) filter with a multiplicity of taps. The coefficient of each tap is continually updated based on output level. FIR adaptive equalizers are powerful, but require substantial processing. A typical FIR equalizer for a DPSK modem requires at least 30 taps, and requires about three math operations per sample time per tap. Thus the computational requirement is 180 math operations per sample time.

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ZERO Order Adaptive Equalizer

A device is described which adapts certain data modulator-demodulator characteristics to the characteristics of the telephone line in use, and dynamically varies modem characteristics during call, to track telephone line changes. Adaptive equalization of modem signals is a well-known art. Normally, adaptive equalizers are configured as a finite-impulse-response (FIR) filter with a multiplicity of taps. The coefficient of each tap is continually updated based on output level. FIR adaptive equalizers are powerful, but require substantial processing. A typical FIR equalizer for a DPSK modem requires at least 30 taps, and requires about three math operations per sample time per tap. Thus the computational requirement is 180 math operations per sample time. A much simpler equalizer, suitable for low and medium speed modems, which requires only 0.5 math operation per sample time, is described. The correction obtained is less than with more sophisticated equalizers but is significant. Every telephone line is unique in its transmission characteristics. The primary characteristic for which adaptive equalizers are intended is called envelope delay distortion.

Envelope delay distorts the received (demodulated) data in a number of different ways. One of the serious degradations is that the bit level is shifted. Certain kinds of envelope delay, particularly for DPSK or PSK modems, cause the demodulated data bits to be too positive or too negative in level. If the level is too positive, then ONE bits will be detected properly, but ZERO bits will appear so positive that they will sometimes be detected as ONE bits. If, conversely, the level is too negative, then the ONE bits are subject to mis-detection. This problem is exacerbated by telephone line noise. Envelope delay can shift the level slig...