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A COMPARATOR FOR DIGITAL SIGNALS

IP.com Disclosure Number: IPCOM000005615D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2001-Oct-19
Document File: 2 page(s) / 164K

Publishing Venue

Motorola

Related People

Paul Bocci: AUTHOR

Abstract

Two-way radio systems covering large geographic areas must often utilize multiple receiver sites in order to provide adequate receive signal quality for in-bound transmissions, This is particularly true of systems employ- ing portable or other low-powered transmitters. In voice radio systems, a comparator is used to compare the various received signals and to select or vote the best signal for use. This is currently done by measuring the amount of high frequency noise in the received signal and selecting that signal with the least amount of noise.

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m MOIOROLA Technical Developments Volume 6 October 1986

A COMPARATOR FOR DIGITAL SIGNALS

by Paul Bocci

   Two-way radio systems covering large geographic areas must often utilize multiple receiver sites in order to provide adequate receive signal quality for in-bound transmissions, This is particularly true of systems employ- ing portable or other low-powered transmitters. In voice radio systems, a comparator is used to compare the various received signals and to select or vote the best signal for use. This is currently done by measuring the amount of high frequency noise in the received signal and selecting that signal with the least amount of noise.

   Digital signals sent over these radio systems have been voted in a similar manner, either by measuring the high frequency noise (for low data rate signals) or by selecting some parameter of the digital signal which varied with the received signal strength and selecting on this basis. These systems are typically highly depen- dent on the type and data rate of the signal and on the information being sent. Advanced coding techniques, as well as encryption of the data being sent (as in digital voice systems), have rendered the actual signal to be extremely random in nature and difficult to compare without at least partially decoding or decrypting the signal. In addition, these digital signals can be very sensitive to interruptions ordiscontinuities in the bit stream. Voting techniques which simply switch from one bit stream to another may disrupt the digial message being sent, rendering it useless. What is often done is to take a single signal quality measurement at the beginning of a transmission, make the selection and remain on that one signal for the duration of the transmission, ignor- ing any deterioration in the signal. This technique is obviously of limited use, especially for mobile transmitters in a fading environment.

   The performance and versatility of land mobile radio systems employing digital signalling for either data or for digitized voice could be improved if the comparator could be made more responsive to digital signals. The comparator must first be insensitive to the type of data transmitted, that is, it should not require any knowledge of the information being transmitted but only of the basic digital signalling rate. It should also be able to vote between the various signals while data is being transmitted to compensate for signal fading without disrupting the data. To accomplish this, the multiple incoming signals must be time aligned. This can be done by inserting variable length delays in the signal paths and cross correlating the signals. The variable length delays are adjusted to maximize correlation. Once the signals are time aligned, switching may occur from one delayed signal to another at any bit boundary with no disruption in the output data. Time alignment, therefore solves one basic problem in current voting systems as regards digital signalling, that of di...