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Reception of OFDM signals with convolutional coding in channels with memory and fast fading

DOI 10.18127/j00338486-201906(7)-09

Keywords:

V.G. Kartashevsky – Dr.Sc.(Eng.), Professor, Head of Department «Information Security»,  Volga State University of Telecommunications and Informatics (Samara)
E-mail: kartashevskiy-vg@psuti.ru
A.A. Filimonov – Engineer, Department «Information Security», Volga State University of Telecommunications and Informatics (Samara)
E-mail: alfhero@gmail.com
A.N. Bespalov – Post-graduate Student, Volga State University of Telecommunications and Informatics (Samara)
E-mail: andreybespalov@mail.ru
K.V. Kotkov – Director for Development, JSC «SIB RS» (Samara)
E-mail: kkv@siprs.ru


The paper discusses the problem of coherent reception of signals with orthogonal frequency multiplexing (OFDM) in a channel with memory using convolutional coding. For a comparative analysis of the effectiveness of the «reception in the whole» with element-wise decision-making (RWED) with a convolutional code, two different channels are modeled in the OFDM system – a channel with constant parameters and a fast fading channel. The analysis is carried out with different modulations - QPSK modulation and QAM modulation. The use of the algorithm of RWED makes it possible to abandon the use of prefixes and postfixes in the structure of the OFDM symbol, which leads to an increase in throughput.
The described method of processing the received signal, based on the use of the algorithm of the RWED, has an acceptable computational complexity and is easily implemented by digital signal processors.
The article deals with the problem of processing orthogonal frequency multiplexing (OFDM) signals in QAM and QPSK modulation using convolutional coding in channels with scattering and fast fading. When using the traditional structure of an OFDM symbol, to protect against the dispersion in time of the energy of the transmitted signal, guard gaps – prefixes and postfixes – are used. This allows to maintain the orthogonality of the subcarrier oscillations on the extended processing interval, but leads to a loss of system capacity. Losses can be up to 20%. For processing at the place of reception of the OFDM-symbols stream, an algorithm of «reception in the whole» with element-wise decision-making (RWED) is proposed, which it implements the demodulation task through the evaluation procedure of the complex samples of the envelope of the OFDM symbol by the criterion of the minimum mean square error. The demodulation is carried out by forming the DFT of the sequence of estimates of the samples of the envelope of the OFDM symbol. The use of the algorithm for a RWED does not require the formation of prefixes and postfixes in the transmission, which eliminates bandwidth losses due to their presence in the signal structure.
It is shown that under the assumption of high accuracy in measuring the impulse response of a communication channel, the use of a simple convolutional code (when decoding with the Viterbi algorithm) leads to a significant energy gain in processing by the RWED algorithm both in the channel with constant parameters and in the channel with fast fading.
The paper shows that with fast fading and high transmission speed it is preferable to use QPSK modulation.
The simulation of the operation of the RWED algorithm with coding in a channel with fast fading showed its high noise immunity in a wide range of changes in the signal-to-noise ratio at the receiving point.

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June 24, 2020
May 29, 2020

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