ESTIMATION THE EFFECT OF CARRIER FREQUENCY OFFSET ON THE NOISE IMMUNITY OF TELECOMMUNICATIONS WITH OFDM

Abstract

OFDM signal transmission is widely used in wireless communication systems Wi-Fi, WIMAX, LTE, digital television systems DVB-T, DVB-T2, and others. Advantages of OFDM: resistance to channel delays, equalization in the frequency domain, reduced sensitivity to frequency-sampling fading, and efficient implementation. Disadvantages of OFDM: loss of spectral efficiency due to higher side lobes; strict requirements for synchronization. Orthogonal frequency division multiplexing (OFDM) provides high-speed data transmission by dividing wideband modulated carrier signals into many modulated narrowband subcarriers. To avoid intersymbol interference (ISI), individual subcarriers must be orthogonal to each other. The OFDM modulator works with a high-speed serial stream of symbols consisting of many parallel low-speed streams. Each orthogonal low-speed stream is transmitted over a relatively uniform channel with minimal MFIs and can be easily aligned. The purpose of the work is the study of synchronization in OFDM communication systems, the assessment of the effect of frequency shift on the bit error rate in the MATLAB environment. Carrier Frequency Offset (CFO) is of great importance to the performance of the decoder in an OFDM system because the phase distortion of symbols increases from symbol to symbol. Two receiver uncertainties with respect to an OFDM symbol are considered. Uncertainty in the arrival time of the OFDM symbol. Uncertainty of the carrier frequency, which arises due to the difference of local dynamism in the transmitter and receiver and leads to a shift in the frequency domain. The effect of the frequency shift is to shift the frequency characteristics of the channel, which leads to a loss of orthogonality between the subcarriers. For the effective operation of the OFDM system, a frequency accuracy of 1-2% of the distance between the carriers is required. Maximum likelihood estimation is used for samples with minimal variance and large size. The dependence of the dispersion of subcarriers on the frequency shift at the receiver input was investigated. From the obtained dependence, it can be concluded that the dispersion of OFDM subcarriers varies according to the Gaussian law. The length of the cyclic prefix is chosen based on the maximum possible frequency shift, at the boundaries of which the dispersion of the subcarriers is zero.