V.G. Kartashevsky – Dr.Sc.(Eng.), Professor, Head of Department «Information Security»,  Volga State University of Telecommunications and Informatics (Samara)

E-mail: kartash@psati.ru

E.S. Semenov – Ph.D.(Eng.), Associate Professor, Head of Department «Telecommunication Systems»,  Volgograd State University

E-mail: essemenov@mail.ru

The algorithm of reception of signals of orthogonal frequency multiplexing with QPSK modulation (OFDM-QPSK) in constant channel with memory and additive white gaussian noise is considered. The use of the algorithm «reception in the whole» with element-wise decision-making allows you to abandon the use of prefixes and postfixes in the structure of the OFDM symbol, which leads to an increase in bandwidth. It is shown that when using QPSK modulation at the modulator output, envelope samples are a four-level signal, which at the receiving point can be demodulated by a receive suboptimal algorithm. A method for analyzing the probability of error when an OFDM symbol is received in a memory channel is proposed. It is shown, that the discrete Fourier transform used in the OFDM modulator helps to reduce the bit error probability as compared with the case when the binary code sequence is transmitted directly using QPSK modulation.

The article discusses the analysis of the probability of bit error when using the OFDM-QPSK system in a channel with memory with constant parameters and additive «white» Gaussian noise. At the output of an OFDM-QPSK modulator, the OFDM symbol envelope is a fourlevel sequence of samples that is transmitted to the communication channel using phase modulation signals (QPSK). For processing at the point of reception of the stream of OFDM symbols, an algorithm is proposed for «general reception» with element-by-element decision making (PCRD), which solves the problem of distinguishing four-level signals in a memory channel. At the same time, the operation of the PCRP algorithm does not require the formation of a group signal structure with traditional inserts (prefix and postfix), which are used for non-optimal algorithms for compensating signals of intersymbol interference that inevitably occur in the memory channel. It is known that these inserts reduce the bandwidth of a system with OFDM technology by an average of 20-25%.

The article discusses the method for estimating the noise immunity of the PCRD algorithm during demodulation of signals from fourlevel discrete sequences. This is due to the fact that at the output of the OFDM-QPSK modulator in each quadrature channel, the samples of the envelope of the OFDM symbol can be modeled with a code base of four.

The analysis of the noise immunity of the reception of a four-level signal in the channel with memory for the PCRD algorithm using the Boole inequality is carried out. The boundaries of the error probability are constructed when an OFDM symbol is received. It is shown that the discrete Fourier transform used in the OFDM modulator helps reduce the probability of a code bit error at the modulator input as compared to the case when the binary code sequence is transmitted directly using QPSK modulation.

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