ALGORITHM FOR ESTIMATING AND CORRECTING MULTIPATH CHANNEL PARAMETERS FOR DEMODULATION OFDM SIGNALS

Abstract

OFDM signals form the basis of the radio interface of 4G mobile networks. Among the most well-known systems using OFDM signals, the LTE / LTE-Advanced, IEEE 802.11, IEEE 802.16 family, as well as DVB-T, DVB-T2 digital television standards, should be noted. Due to the high spectral efficiency and noise immunity in channels with multipath propagation of radio waves, OFDM signals are also used in other systems under development. Radio systems with OFDM signals have two significant drawbacks: the high peak factor (PAPR), which reduces the efficiency of the radio transmitter, and the high sensitivity of the demodulator to frequency synchronization errors. Because of them, in the LTE system in the Up lines (from a subscriber - mobile station – MS - to the base station - BS), the SC-OFDM transmission method is used instead of the classical OFDM, which reduces the spectral efficiency. High-speed user movement in an environment with many diffusers characteristic of dense urban development leads to frequency dispersion, when a Doppler spectrum occurs near each subcarrier of the OFDM signal. In addition, in any radio communication system, there are random fluctuations in the phase of the radio signal caused by the instability of the generators of the radio receiver and radio transmitter. As a result, the orthogonality of the subcarriers is violated and mutual interference between them (ICI - inter-carrier interference) occurs. This can significantly impair the noise immunity of the transmission system. Therefore, an integral part of the demodulator processing the OFDM signal is the channel parameter estimation and correction unit (adaptive equalizer). There are two categories of equalizers that determine the parameters of the channel – in the frequency domain and in the time domain. Since the OFDM signal demodulators use the fast Fourier transform at the subcarrier separation stage, as well as the pilot subcarriers being contained in the signal structure, it is advisable to consider equalizers that calculate the channel parameters in the frequency domain for efficient hardware implementation and good performance. Such an equalizer joins in with a demodulator after the stage of fast Fourier transformation. Traditionally for OFDM signals, an adaptive equalizer is built based on approximation of frequency description of channel by averaging of results obtained on the pilot subcarrier. Frequency dependence of total phase change of multibeam signal is not considered in such methods. Accordingly, good results can be expected at the dense location of the pilot subcarrier that reduces frequency efficiency in turn. An offered method allows considering a frequency-dependent phase change and interference losses, arising due to addition of copies of the signal from the different ways of distribution. It is special topically in systems with a small number of the pilot subcarriers.