Channel estimation technique assisted by postfixed PN sequences with zero padding for wireless OFDM communications

It is well-known that in order to enhance system performance, channel estimation in coherent OFDM systems is necessary and crucial. In practice, lots of coherent OFDM systems transmit pilot symbols over some of the subcarriers for the purpose of estimating channel attenuation, and also insert cyclic prefix to avoid inter-carrier and inter-symbol interference. In this paper, a pseudo noise (PN) sequence combined with zero padding is inserted in the postfix of each OFDM symbol to estimate the channel impulse response. The major advantage of the proposed channel estimation techniques is the ability of estimating and, then, tracking the channel variations. These proposed channel estimation algorithms include least-square technique with decision-directed data-assistance (DDLS), least-square (LS) estimation and maximum-likelihood (ML) scheme with different observation windows. The proposed techniques are thus very appropriately used in presence of high mobility with large Doppler spreads. Some comparative simulations are given to illustrate the superior performance of the proposed channel estimation techniques.