Browse Prior Art Database

Baud Period Synchronization for the Slicer in a Digitally Implemented Demodulator of a PSK Modem

IP.com Disclosure Number: IPCOM000036989D
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Davis, GT: AUTHOR [+2]

Abstract

A technique is described whereby the baud period of a slicer, used in a digitally implemented demodulator of a Phase Shift Keyed (PSK) modem, is synchronized by peak tracking the x and y product. The technique allows the decoding process to be independent of frequency drifts occurring on the receive signal, so as to provide error-free decoding. Processing time is reduced since calculation of the demodulator algorithm is required to be performed only at the baud rate. Programmable resynchronization provides a significant reduction in the total number of computations required.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 54% of the total text.

Page 1 of 3

Baud Period Synchronization for the Slicer in a Digitally Implemented Demodulator of a PSK Modem

A technique is described whereby the baud period of a slicer, used in a digitally implemented demodulator of a Phase Shift Keyed (PSK) modem, is synchronized by peak tracking the x and y product. The technique allows the decoding process to be independent of frequency drifts occurring on the receive signal, so as to provide error-free decoding. Processing time is reduced since calculation of the demodulator algorithm is required to be performed only at the baud rate. Programmable resynchronization provides a significant reduction in the total number of computations required.

Although there are several methods employed in the demodulation of PSK- modulated signals, after demodulation occurs, it is essential that error-free decoding of the demodulation signals be sliced at the center of the baud period. Due to tolerance variations of the incoming carrier frequency, the center of the baud period is not exactly periodic over a specified time duration. The technique described herein allows the slicing point to track the frequency drifts of the carrier. This method is easily programmable for slicing and decoding by the use of a high speed digital signal processor.

The x and y components of the differential phase shift at the outputs of a differential PSK demodulator may be expressed as follows: x = u * (u-v)/2 + v * (u+v)/2

y = u * (u+v)/2 + v * (u-v)/2 where u and v are the real and imaginary components of the incoming signal. Simulations showed that the absolute value of the product of x and y

x * y was a signal with peaks occurring at the baud rate. The amplitudes of the peaks varied, but occurred at the center of the baud period, representing the desired slicing point. The power spectrum of the signal was dependent on the contents of the modulated signal. The simulations also showed that a scrambled mark generated a signal having a peak every baud period, whole the signal had no peaks if there were no phase shifts in the received carrier.

In order to keep track of the center of the baud period, the following algorithm was developed.

If n(j) is the ith sample of the abs...