Efficient QPSK signal demodulation in channels with unfavorable conditions of radio reception
Keywords:QPSK signal demodulation, adverse radio reception conditions, TDMA, feedforward, feedback, ISI, frequency distortion
The main task of the receiver in digital communication systems is to provide a minimum bit error when transmitted data is received. Therefore, the quality of digital communication largely depends on the quality of demodulation of signals in general. In addition, in channels with unfavorable conditions of radio receiving, which are characterized by rapid change in signal parameters due to time and frequency Doppler spread, as well as unknown characteristics of the main transmission channel, there are situations where the most modems don’t allow for high-quality receiving of the signal. Thus, increasing the efficiency of demodulation was and remains a relevant task for modern technology of radio receiving and signal processing. The article is devoted to the problem of signal reception with phase manipulation in channels with unfavorable transmission conditions, since in most modern digital communication systems, phase manipulation is the basis of the physical level of transmission. The existing solutions are analyzed and the method of hybrid signal demodulation is considered, which combines the algorithms of feedforward estimation of signal parameters with subsequent synchronization of parameters in feedback schemes. The initial estimates of the carrier frequency offset, phase, and time delay of the signal are calculated according to the known unique insertion and the obtained estimates are used when initializing the feedback circuits for instant entering into tracking mode. A comparative modeling of known traditional methods of demodulation and method of hybrid demodulation is done on an example of a signal with quadrature phase-shift keying (QPSK). Consequently, the obtained results confirm that in the conditions of rapid change of signal parameters the hybrid algorithm provides the minimum value of bit error, depending on SNR, in comparison with traditional methods. The considered synchronization approach can also be used for signals with other types of phase manipulation. Moreover, the proposed hybrid combination of feedback and feedforward algorithms allowed the use of feedback algorithms for signals with packet data transmission.
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