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Reception «in general» OFDM-signals in a Doppler channel with memory


V.G. Kartashevsky – Dr. Sc. (Eng.), Professor, Head of Department of Multiservices Networks and Information Security, Volga State University of Telecommunications and Informatics (Samara)
K.A. Belsky – Post-graduate Student, Department of Multiservices Networks and Information Security, Volga State University of Telecommunications and Informatics (Samara)
E.S. Semenov – Ph. D. (Eng.), Associate Professor, Head of Department of Telecommunication Systems, Volgograd State University

The paper considers the solution of the problem during processing orthogonal frequency multiplexing (OFDM) signals using QAM modulation in channels with scattering and Doppler shift based on the methodology of «reception in general». As it was shown in the previous works of the authors, «the reception in general» for processing the multichannel OFDM signal can be realized on the basis of the RGEWDM (reception in general with element-wise decision-making) algorithm, which realizes «reception in general» on channel memory interval, which is determined by the duration of the impulse response. The principal difference of the RGEWDM algorithm is the use of «feedback by decision», which helps compensate the aftereffect for the symbols on which the decision was made on the previous interval is made in the current processing interval.
In contrast to the traditionally used OFDM signal reception scheme, in which the inevitable arising scattering in the channel is controlled by insertion of guard intervals at the beginning and at the end of the OFDM symbol to maintain orthogonality of the plurality of oscillation subcarriers, reception by the RGEWDM algorithm is implemented so that the main task becomes estimation of envelope OFDM-symbol samples by criterion of minimum mean-square error of estimation. This is the modification essence of the RGEWDM algorithm, when the task of making a decision about the transmitted discrete symbols is replaced by the task of forming the estimation of the envelopes of the OFDM symbol. «Feedback by decision» is thus transformed into «feedback by assessment». After forming estimates of the envelope samples, the final decision about the transmitted discrete symbols is made traditionally using the discrete Fourier transform (DFT).
In both the original and the modified version of the RGEWDM algorithm, the accuracy of estimating the impulse response of the channel determines as a result the noise immunity of the type of modulation that is used in the creation the OFDM symbol. Initial estimates of the communication channel characteristics are formed from pilot signals whose information content is known at the receiving site. Earlier it was shown that in a channel with a slowly varying impulse response of 10% impulse response estimation error has little effect on the noise immunity of the RGEWDM algorithm when processing OFDM signals with QAM-16 modulation.
The frequency of the arrangement of the pilot signals in the structure of the group signal depends on the rate of change in communi-cation channel parameters. The Doppler frequency shift, which arises due to the movement of the object, even with a small absolute value due to the phase change, can lead to signal fading, which will lead to a sharp deterioration in the noise immunity of reception.
In the work the statistical simulation of the modified RGEWDM algorithm in the Doppler channel with memory was performed based on Jakes model for impulse channel characteristics. It is shown that in the channel whish was modeled the RGEWDM algorithm preserves high noise immunity of reception.

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