QPSK-Modulation Modem Invariant to the Rotation of the Signal Constellation Plane

Abstract

In order to increase the efficiency of dedicated frequency channels, i.e. to increase the specific data transfer rate, one needs to use multipositional quadrature phase shift keying (QPSK, aka 4‑PSK). The problems with QPSK signal demodulation is a rotation of the signal constellation plane by an angle multiple of 90° and a slow response of the carrier oscillation recovery scheme. The study considers existing methods for eliminating the phase ambiguity of the recovered carrier frequency in typical QPSK modems, and identifies the shortcoming of a low-speed response oscillation recovery circuit. The authors propose a QPSK demodulator circuit with a fast adjustment of the reference oscillator, which is due to the fact that no loop filter is used in the feedback and that a digital calculator of the required phase shift is used. An algorithm for the frame synchronization restoration with the simultaneous elimination of the phase ambiguity multiple of 90° was also developed using synthesized binary sequences with an ideal non-periodic autocorrelation function (NACF) at even shifts that do not have the rotary symmetry property. The phase ambiguity elimination algorithm proposed in the article can be used as an alternative to standard modems with differential coding.