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Channel Estimation for OFDM System with Two Training Symbols Aided and Polynomial Fi - kadesh s b - 10-04-2017 Channel Estimation for OFDM System with Two Training Symbols Aided and Polynomial Fitting [attachment=529] INTRODUCTION ORTHOGONAL frequency division multiplexing (OFDM) is a high spectral efficiency transmission scheme. It divides a wideband signal into many orthogonal subcarriers and induces a symbol period that is much longer than the bit duration. However, a long symbol period is sensitive to the time-varying channel in wireless communications. The channel response might change during an OFDM symbol period in a high-mobility environment. Therefore, orthogonality among subcarriers is destroyed, and inter-carrier interference (ICI) occurs [1]-[2]. CHANNEL ESTIMATION WITH TWO TRAINING SYMBOLS AND POLYNOMIAL FITTING As mentioned above, H can be transformed from the timedomain matrix g. Because we do not exactly know the number of paths, the worst case that the channel delay spread becomes equal to the length of CP should be considered. If we estimate the time domain matrix g rather than H, then the number of unknowns can be reduced to NL. It has been shown that when the channel response does not vary too fast, such as when the OFDM symbol period is 8% of the channel coherence time or less [11], the channel response varies almost linearly within one OFDM symbol. If the channel response of each path is modeled by a linear function, then only two unknowns for each path need to be solved. PERFORMANCE VERIFICATION the performance of the presented channel estimation method under the COST207 bad urban area (BU) multipath fading channel [9]. A second-order polynomial is used in the simulation. Two normalized Doppler frequencies under various data symbols are tested. The number of subcarriers is 512, and the CP length is 64. The training symbols are pseudo-random noise (PN) sequences with quadrature phase shift keying (QPSK) modulation, which is also employed for all data symbols in the simulations. CONCLUSION A channel estimation method for OFDM systems in fast time-varying channels has been presented. The presented method uses the structure of two training symbols in combination with polynomial fitting. Thus, it can provide enough channel estimation accuracy for even a normalized Doppler frequency as large as 0.1. The major advantage of using the two training symbols is that no ICI can disturb the estimation. The disadvantages of the presented channel method include: (1) increased computational complexity, and (2) requirement of a long buffer with +2 symbols. Furthermore, as the variation cycle of each path response is inversely proportional to the normalized Doppler frequency, the two training symbols and the data symbols are better located within one-half of the variation cycle. |