Inter-Carrier Interference Estimation in MIMO OFDM Systems with Arbitrary Pilot Structure Michal ˇ Simko ∗ , Christian Mehlf¨ uhrer ∗ , Thomas Zemen † and Markus Rupp ∗ ∗ Institute of Telecommunications, Technische Universit¨ at Wien, Vienna, Austria † FTW Forschungszentrum Telekommunikation Wien, Vienna, Austria Contact: msimko@nt.tuwien.ac.at Web: http://www.nt.tuwien.ac.at/ltesimulator Abstract—In scenarios with time-varying channels such as in- telligent traffic systems or high speed trains, the orthogonality be- tween subcarriers in orthogonal frequency division multiplexing (OFDM) is destroyed leading to inter-carrier interference (ICI). In the literature, ICI equalization algorithms have been proposed; 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 propose an algorithm for ICI estimation that can be applied to OFDM systems with an arbitrary pilot structure. Thus, our algorithm can be applied to any already standardized OFDM system. Our ICI estimator models the channel variation by means of a basis expansion model (BEM). The performance of the estimator and of the subsequent equalization is evaluated in an UMTS long term evolution (LTE) link level simulator. In a Rayleigh fading scenario, the proposed algorithm allows a velocity increase of 150 km/h without throughput degradation. The gain in terms of the post-equalization signal to interference and noise ratio (SINR) is about 3.7 dB at a user speed of 300 km/h. Index Terms—LTE, ICI, Channel Estimation, Fast Fading, OFDM, MIMO. I. I NTRODUCTION 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 traf fic 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. Related Work: ICI estimation and equalization have been addressed in previous work, for example in [1–11]. In [1], the ICI for single input single output (SISO) and multiple input multiple output (MIMO) transmissions is analyzed. The authors propose to employ 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. Hijazi and Ros propose to use polynomials for channel estimation in [7]. 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 proposed so far [4–7] cannot estimate the time-variant channel impulse response at sample level precisely enough at high Doppler spreads. Scientific Contribution: 1) Building on [7] we model the channel variation in the time domain by means of a general set of orthogonal basis functions to optimize the square-bias variance tradeoff [12] for time-variant channel estimation. 2) In Section IV, we address the problem of inverting an ill conditioned matrix in [7] by introducing an additional regularization step, such that the basis function set is orthogonal on the given pilot grid. 3) Our algorithm can be utilized as a simple extension to any existing channel estimator. Therefore, our method does not require any special pilot structure and can be applied in existing standardized systems such as UMTS LTE. 4) In Section V, we evaluate the performance of linear, polynomial and discrete prolate spheroidal basis functions [12] for ICI estimation in a fully standard compliant LTE link level simulator [13]. II. SYSTEM MODEL Long term evolution (LTE) is the current standard of the cellular communication 3rd Generation Partnership Project (3GPP). It supports technologies such as different MIMO schemes, adaptive coding and modulation (ACM) and hybrid automated repeat request (H-ARQ) that allow to transmit data with high spectral efficiency. LTE supports bandwidth from 1.4 MHz up to 20MHz, corresponding to a number of data subcarriers ranging from 72 to 1200. The subcarrier spacing is fixed to 15 kHz. Depending on the cyclic prefix length, being either extended or normal, each LTE subrame consists of 12 or 14 OFDM symbols, respectively. The duration of an LTE subframe is 1 ms. The structure of the pilot symbols is described in [14]. This pilot symbol pattern allows to estimate a MIMO channel as independent SISO channels, neglecting spatial correlation. Copyright 2011 IEEE. Published in the proceedings of VTC-Spring 2011, Hungary, 2011