Аннотация:
In order to increase the efficiency of dedicated
frequency channels, i.e. to increase the specific data transfer rate,
multipositional quadrature phase shift keying (QPSK, aka 4-PSK)
should be used. 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 the 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.
