ICI Estimation of MIMO OFDM Sytems

Orthogonal Frequency Division Multiplexing (OFDM) is a multi– channel modulation technique that makes use of Frequency division multiplexing (FDM) being modulated by a low bit rate digital stream. The main reason of using OFDM is because here the symbol detection is easy and also to increase the robustness against frequency selective fading and narrow band interference. In scenarios with time-varying channels such as intelligent traffic systems or high speed trains, the orthogonality between subcarriers in orthogonal frequency division multiplexing (OFDM) is destroyed leading to inter-carrier interference (ICI). In the literature, ICI equalization algorithms have been disscused; however, they assume perfect channel knowledge at sample level. Unfortunately, existing channel estimation algorithms do not provide accurate channel estimates at high Doppler spreads, prohibiting data transmission with high spectral efficiency. In this paper, we studied an algorithm for ICI estimation that can be applied to OFDM systems with an arbitrary pilot structure. ICI estimator models the channel variation by means of a basis expansion model (BEM). Introduction Orthogonal Frequency Division Multiplexing (OFDM) has become the selected transmission technique for several recent wireless standards, such as the IEEE standard for local and metropolitan area networks (better known as WiMAX) , or the 3GPP UTRA Long Term Evolution (LTE) . In OFDM, the transmission bandwidth is divided into multiple narrowband subcarriers. By the addition of a proper cyclic prefix (CP), these subcarriers become fully orthogonal and experience frequency flat fading conditions in time-invariant channels . This allows for simple equalization of the signal at the receiver, while keeping a high spectral efficiency due to the use of orthogonal overlapping subcarriers. In OFDM systems with frequency re-use, however, the signal transmitted from other cells may create cochannel interference which, if not correctly treated, can induce a severe degradation of the receiver performance, especially at the cell edge. Orthogonal frequency division multiplexing (OFDM) is used in most current and upcoming mobile communication systems. Such systems perform well when the channel is not varying during the duration of one OFDM symbol. However, mobile scenarios in which the channel is varying rapidly are becoming more and more important for intelligent traffic systems or high speed trains. If the channel is not constant during the transmission of one OFDM symbol, inter-carrier interference (ICI) occurs and the performance of the system is degraded. Therefore, there is a need to introduce receivers that combat ICI.In [1], the ICI for single input single output (SISO) and multiple input multiple output (MIMO) transmissions is analyzed. Pilot symbols located at adjacent subcarriers in order to estimate the ICI. This approach is in contradiction to the common agreement that scattered pilot symbols are optimal [2, 3]. Nevertheless, such an approach would be suitable for ICI estimation in the case of a SISO system. In the case of a MIMO system, such a pilot symbol pattern results in a huge overhead. In [4–6] ICI estimation and mitigation assume that the channel is varying linearly in the time domain. We can use polynomials for channel estimation in MIMO. However, their estimator works only with a limited order of the polynomials. Numerous different equalization algorithms are proposed in [8–10], in which the authors assume perfect channel knowledge for each signal sample. However, this information is not available at the receiver and algorithms cannot estimate the time-variant channel impulse response at sample level precisely enough at high Doppler spreads. The rest of this paper is organized as follows. Section II presents the system model. The channal estimation is presented in Sect. III. In section IV ICI estimation is disscussed . Section V concludes this paper.