Doppler Shift Estimation In OFDM Communication Systems

OFDM is an efficient data transmission, by reducing and eliminating the impact of ISI to overcome the frequency selective fading channel. It had used in many fields of application. To achieve this overlapping multi-carrier technology, we must consider how to reduce interference between different subcarriers, which is required between the various sub-carrier modulation strictly orthogonal. Because OFDM system has this feature, so it is very sensitive to frequency deviation, and ICI caused by carrier frequency offset and Doppler shift in OFDM systems is greater than be in ordinary single-carrier communication system. Especially as technology advances, carrier frequency becomes larger and larger, and the transceiver relative motion between the more and faster rate, the Doppler effect will make the problem worse, so we must study the Doppler shift issues.In this paper, a technique based on the cycle preamble of OFDM symbols was studied at first. By calculating the auto-correlation function of cycle preamble of OFDM symbol, the maximum value of Doppler frequency excursion is obtained. Then a novel method of Doppler shift estimation for OFDM systems was analyzed. The method is a frequency domain scheme that utilizes pilot subcarriers, which are adopted in most practical OFDM systems. Finally, we use the principle of the Doppler shift estimation algorithm based on the cyclic prefix and method of channel estimation to propose a channel atuo-correlation function algorithm for Doppler shift estimation. We insert the OVSF into OFDM symbol. Through the OVSF we find the channel impulse response, and then use the principle of he algorithm based on cyclic prefix. The Doppler frequency shift information can be gotten. The algorithm is simple structure, can provide high estimation accuracy. It can be easily applied to practical OFDM systems.