IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 63, NO. 5, JUNE 2014 2197 Optimal Windowing and Decision Feedback Equalization for Space–Frequency Alamouti-Coded OFDM in Doubly Selective Channels Ahmed Attia Abotabl, Amr El-Keyi, Yahya Mohasseb, and Fan Bai Abstract—Space–frequency block coding with orthogonal frequency-division multiplexing (SFBC-OFDM) suffers from the effect of intercarrier interference (ICI) in doubly selective chan- nels. In this paper, a scheme is proposed in which windowing is applied to the received signal to reduce the effect of ICI to a limited number of neighboring subcarriers. The subcarriers holding the SFBC components of each codeword are separated by a number of subcarriers larger than the ICI range, and hence, they do not interfere with each other. To preserve the structure of the SFBC, the separation between the codeword components is also selected within the coherence bandwidth of the channel. As a result, the diversity gain of the SFBC is preserved. By proper selection of the pilot locations, each OFDM symbol can be divided into subsymbols that can be decoded independently. We show that the proposed windowing technique allows the use of decision feedback equalization to estimate the data transmitted in each subsymbol with low complexity. Simulation results are presented showing the ability of the proposed scheme to significantly improve the performance of SFBC-OFDM and preserve its diversity gain. Index Terms—Alamouti and Doppler shift, orthogonal frequency-division multiplexing (OFDM), SFBC, vehicle-to- vehicle. I. I NTRODUCTION V EHICULAR communication systems are characterized by their challenging channel that is doubly selective in both time and frequency [1]. Due to the presence of many scat- terers at different locations, the received signal is composed of multiple versions of the transmitted signal with different phases and amplitudes arriving at distinct delays, and hence, frequency Manuscript received April 27, 2013; revised September 20, 2013; ac- cepted October 17, 2013. Date of publication November 20, 2013; date of current version June 12, 2014. This work was supported by a grant from General Motors Corporation, Warren, MI, USA. This paper was presented in part at the 2012 IEEE Vehicular Technology Conference, Quebec City, QC, Canada, September 2012. The review of this paper was coordinated by Prof. S.-H. Leung. A. A. Abotabl is with the University of Texas at Dallas, Richardson, TX 75080 USA (e-mail: ahmed.atyia@nileu.edu.eg). A. El-Keyi is with the Wireless Intelligent Networks Center, Nile University, Giza 12677, Egypt (e-mail: aelkeyi@nileuniversity.edu.eg). Y. Mohasseb was with the Wireless Intelligent Networks Center, Nile University, Giza 12677, Egypt. He is now with the Department of Com- munications, The Military Technical College, Cairo 11787, Egypt (e-mail: ymohasseb@nileuniversity.edu.eg). F. Bai is with the Electrical and Control Integration Laboratory, Research and Development and Planning, General Motors Corporation, Warren, MI 48090- 9055 USA (e-mail: fan.bai@gm.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TVT.2013.2291872 selectivity arises. On the other hand, the temporal selectivity arises from the movement of the transmitter, receiver, and/or the scatterers, which causes fast variations in the impulse response of the channel. Several measurement campaigns for vehicular channels have been reported in the literature. An overview of some existing vehicle-to vehicle channel measurement campaigns in a variety of important settings and the channel characteristics such as de- lay and Doppler spreads can be found in [2]. In [3], the authors utilized a channel characterization platform to study the large- scale path-loss models at 5.9 GHz. It is found that the fading statistics change from near Rician to Rayleigh as the vehicle separation increases. Furthermore, Cheng et al. [4] provided analysis of Doppler spread and coherence time and their depen- dence on the vehicular environment. For example, it was found that, for a highway environment, the coherence bandwidth and the coherence time can be on the order of 400 kHz and 0.3 ms, respectively. Orthogonal frequency-division multiplexing (OFDM) [5], [6] has been selected as the modulation scheme in the 802.11p protocol for wireless access in vehicular environments (WAVE) due to its ability to handle frequency-selective channels [7]. In OFDM, the number of subcarriers N is selected such that the frequency-selective channel is decomposed into a set of parallel noninterfering frequency flat channels. Furthermore, in order for OFDM-based systems to operate properly, the channel im- pulse response has to be constant over the OFDM symbol dura- tion. In fact, in time-selective channels that are not quasi-static over the OFDM symbol duration, intercarrier interference (ICI) arises, and the orthogonality between the OFDM subcarriers is lost. Due to the doubly selective characteristics of vehicular channels, several modifications are needed for the 802.11p pro- tocol to be able to increase the throughput and communication reliability. For example, it was shown in [7] that the original time-scaled IEEE 802.11a waveforms being proposed for the IEEE 802.11p WAVE standard for operation in the 20-MHz bandwidth would not be suitable due to inadequacy of the guard interval. On the other hand, if the same packet length is preserved, a 5-MHz packet would take longer transmission time than the coherence bandwidth of the channel. To overcome the temporal selectivity of the channel in OFDM systems, ICI cancellation is required. Complete removal of ICI in OFDM systems requires the inversion of an N × N channel matrix. This might be prohibitive, particularly if the number of subcarriers is large. To combat this problem, Lu et al. 0018-9545 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.