Pulse or digital communications – Synchronizers – Network synchronizing more than two stations
Patent
1996-12-09
1998-12-22
Chin, Stephen
Pulse or digital communications
Synchronizers
Network synchronizing more than two stations
375356, 375364, 375362, 455502, 455503, H04L 700
Patent
active
058526412
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Digital communications systems usually require a synchronization of the carrier oscillators in the transmitter and receiver. Two types of reception are: coherent reception: the carrier oscillators in the transmitter and receiver should be as phase-synchronized as possible; and incoherent reception: the carrier oscillators in the transmitter and receiver do not need to be phase-synchronized, but the phase difference between the two carrier oscillators should not vary too greatly with time, that is, the two carrier oscillators should be as frequency-synchronized as possible.
The present invention relates to the second case, that is, the synchronization of carrier frequencies between the transmitter oscillator and the receiver oscillator. It must be pointed out, however, that the present invention can also be of great significance for coherent transmission methods; the carrier phase synchronization is often performed in two steps: first, carrier frequency synchronization is performed in an attempt to keep the carrier phase difference between the transmitter oscillator and receiver oscillator as constant as possible so that the carrier phase difference can eventually be eliminated in a second step.
In many communications systems, particularly in mobile radio systems, carrier frequency synchronization is supported by special transmission base stations radiate a sinusoidal oscillation (hereinafter called "sinusoidal tone") that is constant with respect to frequency; this tone is electromagnetically radiated either continuously or intermittently. The mobile stations use this sinusoidal tone in receiver-side carrier frequency synchronization. The mobile stations are additionally required to derive their transmitter-side carrier frequency from the synchronized reception frequency. This automatically assures the carrier frequency synchronization of the receivers in the base stations.
This method has two notable disadvantages. terrestrial radio channels are exposed to occurrences of so-called fading. The more narrow-banded the transmitted signal, the more detrimental the effects of these occurrences. Because a sinusoidal tone has an infinitely small bandwidth, such signals are impacted most severely by fading. frequencies, because otherwise they could mutually cancel each other out through destructive interference. Contrary to this requirement, however, is the desire for the sinusoidal tones of all base stations to have the same frequency so that certain diversity effects result for the mobile stations in carrier frequency synchronization.
SUMMARY OF THE INVENTION
The present invention is intended to provide a solution to the aforementioned contradiction so the occurrences of fading do not cause perceptible disturbances and lead to breakdowns or interruptions in synchronization in the mobile stations.
The method of the invention has the advantages that the same synchronization signal can be radiated in the base stations, resulting in a unified arrangement and equipment of the same, and that the synchronization of the mobile stations is assured because cancellations of the transmitted synchronization signals due to fading are not perceptibly disturbing. Unified circuits designed for identical frequencies can also be used in the mobile stations. These advantages prove useful when the following two conditions are met: affected less by fading than pure sinusoidal tones. each other out directly, but, at the same time, they should be closely-enough matched in their essential features that all possible synchronization signals can be evaluated with the same receiver circuit; this means that a plurality of synchronization signals transmitted from different base stations and superimposed at the receiving site can be evaluated simultaneously.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an arrangement for generating a sinusoidal tone group according to the invention.
FIG. 2 shows an arrangement for determining the carrier frequency of a synchronization signal according to the invention
REFERENCES:
patent: 4719619 (1988-01-01), Crookshanks
patent: 5444697 (1995-08-01), Leung et al.
patent: 5521937 (1996-05-01), Kondo et al.
Binham, Multicarrier Modulation for Data Transmission, IEEE Communication Magazine, 5-14, May 1990.
Couch, Digital and Analog Communication Systems, Macmillan Publishing Company, 405, 1990.
Chin Stephen
Liu Shuwang
Robert & Bosch GmbH
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