IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 57, NO. 5, SEPTEMBER 2008 2969 A Novel Simulation Model for Coded OFDM in Doppler Scenarios Mario Poggioni, Student Member, IEEE, Luca Rugini, Member, IEEE, and Paolo Banelli, Member, IEEE Abstract—This paper proposes a novel simulation model to characterize the bit-error rate (BER) performance of coded or- thogonal frequency-division multiplexing (OFDM) systems that are affected by the Doppler spread. The proposed equivalent frequency-domain OFDM model (EFDOM) avoids the exact gen- eration of the time-varying channel by introducing several param- eters that summarize the statistical properties of the channel and of the intercarrier interference (ICI) that is generated by the time variation of the channel. Simulation results are used to prove that the proposed model can be used to accurately predict the BER of coded OFDM systems in Rayleigh and Rice doubly selective channels. An attractive feature of the proposed model is the significant reduction of the simulation time with respect to the exact model. We show by simulation that the simulation efficiency increases for channels with many multipath components, whereas it is independent of the size of the fast Fourier transform (FFT). Index Terms—Bit-error rate (BER) performance, coded orthog- onal frequency-division multiplexing (OFDM), Doppler spread, intercarrier interference (ICI), simulation, time-varying fading channels. I. I NTRODUCTION O RTHOGONAL frequency-division multiplexing (OFDM) is a well-established technique for high-rate communi- cations in frequency-selective fading channels due to its easy per-subcarrier equalization in the frequency domain [1]. Con- sequently, OFDM is widely used in many popular wireless standards, such as IEEE 802.16e, IEEE 802.11a, digital video broadcasting—terrestrial (DVB-T) and handheld (DVB-H), digital audio broadcasting (DAB), and terrestrial digital mul- timedia broadcasting (T-DMB) [2]–[5]. However, in high- mobility environments, the time variation (i.e., the Doppler spread) of mobile radio channels destroys the orthogonality of the OFDM subcarriers, leading to the so-called intercarrier interference (ICI) [6], [7]. If advanced time-varying equaliza- tion techniques are not used, the ICI can significantly degrade the performance of OFDM systems introducing bit-error rate (BER) floors that channel coding can only try to reduce [6]. Consequently, a statistical characterization of the ICI is neces- sary to analytically assess the BER performance. Several previous works [6]–[8] have shown that, for uncoded OFDM systems, BER performance can be obtained by model- Manuscript received October 27, 2006; revised September 10, 2007 and October 12, 2007. The review of this paper was coordinated by Prof. H.-C. Wu. The authors are with the Department of Electronic and Information Engi- neering, University of Perugia, 06125 Perugia, Italy (e-mail: paolo.banelli@ diei.unipg.it). 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.2007.913178 ing the ICI as an additive white Gaussian noise (AWGN), whose average power can be derived in a closed form [6], [7], [9]. It was shown in [10] that the jointly Gaussian approximation of the ICI is good for phase-shift keying OFDM, whereas for non- constant envelope constellations, such as quadrature amplitude modulation (QAM), the probability density function (pdf) of the ICI is a Gaussian mixture, i.e., a weighted sum of Gaussian functions. However, a more appropriate figure of merit of a communication system is the coded BER performance, and, consequently, we want to model the ICI in coded OFDM (COFDM) systems. We will show that a simple extension of the AWGN-like ICI model from which it is derived [8] is not adequate for assessing the BER performance of COFDM systems. Specifically, we will show that the channel power- delay profile, which does not affect the BER performance of uncoded systems [8], conversely, can greatly impact the coded BER performance. The broader scope of this paper is to assess the BER of COFDM systems that use simple per-subcarrier equalization to combat the adverse effect of doubly selective channels. To this end, we introduce an equivalent frequency-domain OFDM model (EFDOM), which is capable of predicting with good accuracy the BER performance without replicating the entire OFDM transmitter–receiver chain. The main idea of the EFDOM is to replace the exact generation of the ICI by a computer-generated “BER-equivalent” ICI to speed up the simulation time while maintaining the same BER produced by the exact ICI realization. This equivalent ICI is obtained by a moment-matching technique that tries to keep only a few relevant moments of the true ICI, such as its power and its cross-correlation with the useful channel. A specific merit of our model is its capability to highly reduce the simulation time with respect to the simulation of the exact OFDM model. We will show that the saving in simulation time mainly depends on the number of channel paths and is almost independent of the size N of the fast Fourier transform (FFT). This new model could be used, for instance, to compare the BER of different OFDM-based standards, such as DVB-T/H, DAB, and T-DMB. However, the comparison among different standards, although important, would require a significant space, and it is partially addressed in [11]. The rest of this paper is organized as follows. Section II briefly describes the COFDM system model in time-varying multipath channels. We will refer to this model as the exact model. In Section III, we introduce our simplified model, i.e., the EFDOM, by explaining all the constraints that we impose in the ICI generation. The accuracy of the proposed model is validated in Section IV, which illustrates the BER comparison 0018-9545/$25.00 © 2008 IEEE