Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-01-15
2001-11-27
Vo, Don N. (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S242000
Reexamination Certificate
active
06324232
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to processes that reduce the effect of baseline wander and intersymbol interference in signals such as those used in communications.
2. Description of Related Art
A modem or other transmitter using pulse code modulation (PCM) to transmit data generates a signal that includes a series of pulses representing a series of symbols. Each pulse has an amplitude that indicates a value for a corresponding symbol. A receiver determines the symbol values by measuring the amplitudes of the pulses relative to a baseline voltage. However, when the pulses are sent through a channel that blocks or attenuates DC or low frequency signal components, the level of each pulse sags toward zero during the duration of the pulse. This sag affects the level of the next pulse. In particular, the received amplitude of the next pulse is larger if the next pulse is of opposite polarity from the preceding pulse, and the amplitude of the next pulse is smaller if the next pulse is of the same polarity as the preceding pulse. This inter-symbol interference changes the baseline for accurate determination of pulse amplitudes and is commonly referred to as baseline wander. Similar baseline wander occurs in signals using other modulation protocols common to many standard communication signals. Baseline wander can cause a receiver to incorrectly identify symbol values associated with a portion of a received signal.
A transmitter can reduce baseline wander by reducing or avoiding the low frequency components in the transmitted signal. However, avoiding low frequency components reduces the bandwidth available for information transmission. Alternatively, a receiver can compensate for baseline wander using a feedback mechanism similar to the one described in “Quantized Feedback in an Experimental 280Mb/s Digital Repeater for Coaxial Transmission,” by F. D. Waldhauer, IEEE Trans. Communications, Vol. COM-22, No. 1, January 1974, pp 1-5. These feedback mechanisms commonly assume a particular model for the characteristics of the transmission channel and have little or no ability to adjust for actual performance of the channel. Thus, improved systems for handling baseline wander are needed.
SUMMARY
In accordance with the invention, a receiver compensates for baseline wander using a multi-tap filter to predict the amount of baseline shift in each sample of a received signal. The multi-tap filter is adaptive through coefficients that are updated during operation of the receiver. Generally, the coefficients converge to values that minimize differences between values extracted from a compensated signal and allowed symbol values for the signal according to the signal's protocol.
One particular system that compensates for baseline shift includes an adder that adds a predicted baseline shift to an uncorrected sample to generate a corrected sample. A slicer then compares values extracted from the corrected samples to values allowed for the symbols according to the signal protocol of the transmitted signal. The difference between an extracted value and the nearest allowed value indicates an error in the prediction or equivalently the amount of shift the prediction did not correct. A multi-tap finite impulse response (FIR) filter and an integrator predict the baseline shift for the next sample based on preceding predicted baseline shifts and the preceding errors in the predicted shifts. Additionally, for each prediction, filter coefficients of the multi-tap FIR filter can be changed according to the error in the preceding prediction so that the changed coefficients would have provided a better prediction of the baseline shift for the preceding sample. The changed filter coefficients generally provide better fixture predictions. If the channel characteristics are constant, the filter coefficients tend to converge to values that are adapted for the channel. If the channel changes, the filter coefficients adapt to the changes. The number of taps in the multi-tap filter is selected according to the desired correction of intersymbol interference in the received signal and the available computing power for signal conditioning.
REFERENCES:
patent: 5272730 (1993-12-01), Clark
patent: 5406560 (1995-04-01), Kondo et al.
patent: 5561687 (1996-10-01), Turner
patent: 5602602 (1997-02-01), Hulyalkar
patent: 5604769 (1997-02-01), Wang
patent: 5812333 (1998-09-01), Colineau
patent: 5852630 (1998-12-01), Langberg et al.
patent: 0704968A1 (1995-09-01), None
Djen, Wing Shing et al., “Performance Improvement Methods for Dect and Other Non-coherent GMSK Systems,”Proceedings of the Vehicular Technology Society Conference, vol. Conf. 42, pp. 97-100.
Bergmans,Digital Baseband Transmission and Recording, Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 424-437 (1997).
Haykin,Adaptive Filter Theory, Prentice Hall, Englewood Cliffs, NJ, pp. 216-217, 237, 34-36 (1985).
Waldhauer, “Quantized Feedback in an Experimental 280-Mb/s Digital Repeater for Coaxial Transmission”, IEEE Transactions on Communications, vol. COM-22, No. 1, pp. 1-5 (Jan., 1974).
“Baseline Wander and Spectral Shaping”, Hayes Microcomputer Products, Inc., ITU-Telecommunications Standardization Sector, Study Group 16-Question 23, 5 pages (Jun., 1997).
PC-Tel, Inc.
Skjerven Morrill & MacPherson LLP
Tran Khai
Vo Don N.
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