Research Article Energy Efficient Doppler Assisted Channel Estimation for Highly Mobile OFDM Systems Mateen Ashraf, Adnan Shahid, Woon-Young Yeo, and Kyung-Geun Lee Department of Information and Communication Engineering, Sejong University, Seoul 143-747, Republic of Korea Correspondence should be addressed to Kyung-Geun Lee; kglee@sejong.ac.kr Received 14 May 2015; Revised 29 July 2015; Accepted 9 August 2015 Academic Editor: Olivier Berder Copyright © 2015 Mateen Ashraf et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Te complexity of channel estimation algorithms is of critical importance in high mobility orthogonal frequency division multiplexing (OFDM) systems. To reduce the complexity of the algorithm, the existing channel estimation algorithms take advantage of the correlations between the channel coefcients. However, they do not take into account the variations of the correlations with the delay tap. In this paper, we consider the change in correlation between channel coefcients for diferent delay taps. By considering these variations, the number of computations needed for channel estimation can be decreased. Tis results in a reduction in the circuit energy. Te simulation results show that the proposed scheme consumes less total energy per bit for transmission distances less than about 170 m when the mobile speed is 324 Kmph. 1. Introduction Recently, OFDM has been adopted in many standards due to its improved performance in the presence of frequency selec- tive channels. Channel state information (CSI) is required at the receiver to perform coherent detection. Pilot symbols are used to assist the channel estimation at the receiver. Te fast variations in the channel state caused by the high speed of the transmitter and/or receiver demand that the channel estimation should be performed instantly. Tis suggests a reduction in the complexity of the channel estimation algorithm for highly mobile users. Diferent channel estimation methods are proposed in the literature that use sof decoded symbols and interpolation between the time domain channel coefcients to assist the channel estimation. A low complexity iterative channel esti- mation technique is proposed in [1]. In this scheme, channel estimation is done by improving a priori information of the decoded data and the preamble by dynamically weighting the statistics according to reliability. However, the Doppler information is not taken into account by this method. Te correlation between channel coefcients depends on the Doppler frequency and therefore this information can be exploited for channel estimation. Accordingly, Aboutorab et al. [2] have proposed an iterative Doppler assisted channel estimation for OFDM systems. A similar idea was used by Aboutorab et al. in [3] for MIMO OFDM systems. In these works, the interpolation coefcients depend on the Doppler information. For estimation of every channel coefcient, only two reference channel coefcients are used. Further improve- ment is done by using iterative estimates of data symbols as additional pilots. However, the variation of the correlation coefcients for diferent delay taps [4] is not considered in these studies. Moreover, the iterative method, although it improves estimates, can also lead to error propagation. In [5, 6] Doppler information is used to form the basis for basis expansion model (BEM) of the wireless channel and performance is enhanced by considering interference. Linear approximation of the variations of channel coefcients is used in [7]; however the performance of the channel estimation becomes poor for high mobility systems. In [8], the channel is assumed to be static over  OFDM blocks and channel estimation is performed on the basis of information acquired over the transmission of  OFDM blocks. However, as  Hindawi Publishing Corporation International Journal of Distributed Sensor Networks Volume 2015, Article ID 892151, 10 pages http://dx.doi.org/10.1155/2015/892151