COHERENT DETECTION OF TWO-DIMENSIONAL DIGITAL MODULATION SIGNALS

Abstract

Two-dimensional digital modulations require coherent detection to eliminate phase uncertainty. The main problem is that it is necessary to exactly match the frequencies and initial phases of the carrier and reference oscillations in the two processing channels and the phase difference in the subchannels at exactly π / 2. This article proposes a joint algorithm for clock recovery and phase adjustment of the reference oscillator of a two-dimensional digital modulation demodulator. If there is a delay in the beginning of the pulse, at the moment of transition from one channel symbol to another there will be a phase jump equal to the phase difference of the transmitted channel symbols. To determine the transition from one channel symbol to another, it is necessary to clearly define the moment of the jump. To do this, it is necessary to form a signal for the difference in phase values across the sampling interval. At the time of a phase jump, a single, substantially non-zero jump will be observed. The moment of the jump will determine the beginning of the channel symbol. The error of the start pulse adjustment will not exceed the sampling interval. Algorithm adjustment of the reference generator. The proposed algorithm is intended to use for determining the phase φ0 of the intermediate results of the clock recovery system. Sampling of values is performed within certain limits of the channel symbol and averaging is performed. Then it is necessary to introduce a correction of the phase value taking into account the clock adjustment. The proposed algorithm eliminates the mutual effects on the results of the estimates of the delay and phase adjustment. That is, the phase increment due to the delay in the channel does not affect the result of evaluating the phase increment due to the mismatch between the phases of the carrier and reference oscillations, despite the fact that the adjustment is made by phase error.