GB1522921A - Method of transmitting information for transmission paths distorted in a linear manner and having frequency shifts - Google Patents

Abstract

1522921 A.C. systems; correcting frequency shift LICENTIA PATENT VERWALTUNGS GmbH 28 Feb 1977 [28 Feb 1976] 08419/77 Headings H4L and H4P In a suppressed or reduced carrier single or double sideband data communication system wherein the incoming signal is demodulated in quadrature channels sin and cos, the intermediate frequency signal ZF being mixed with in-phase and quadrature versions of a local oscillator signal at carrier frequency #/2#, the mixer outputs being low pass, match filtered, thence a/d converted, the transmitted signal comprises Barker code test sequences sandwiching data packages, see Fig. 3a (not shown). The demodulated test sequences pass to a processor wherein, in virtue of their character, they can be pulse compressed to form pulses whose distribution and movement between the cos and sin channels enables the detection and correction of frequency errors; including those due to the Doppler effect between the transmitter and receiver; at the receiver local oscillator. Examples of pulse distributions for correct, and incorrect local oscillator frequencies, are displayed in Figs. 3c, 3e; 3b, 3d (not shown). Frequency errors up to 180‹ per test sequence spacing (#t, Fig. 3a) can be corrected without ambiguity. For coarse frequency correction, a pair of close spaced test sequences, Fig. 1 (not shown) precedes the first bit sequence before a data package, enabling coarse correction, Figs. 4a-4d (not shown) before the ongoing fine correction occurring during data transmission.