Abstract-In this paper, pilot-aided in-phase and quadraturephase (IQ) imbalance compensation for orthogonal frequency division multiplexing (OFDM) systems operating over doubly selective channels is addressed. Based on a reformulated system model and the least squares (LS) criterion, a joint IQ imbalance and channel estimation method is developed, and the corresponding compensation scheme is also proposed. Moreover, to further enhance the compensation performance, an iterative compensation algorithm is derived via an expectation-maximization (EM) algorithm. Simulation results show that the iterative compensation algorithm initialized by the proposed LS compensation scheme converges in a few iterations and its performance after convergence is close to the ideal case with perfect IQ imbalance and channel state information.Index Terms-Channel estimation, doubly selective channel (DSC), in-phase and quadrature-phase (IQ) imbalance, intercarrier interference (ICI), orthogonal frequency division multiplexing (OFDM).
Abstract-Data detection for OFDM systems over unknown doubly selective channels (DSCs) and carrier frequency offset (CFO) is investigated. A semiblind iterative detection algorithm is developed based on the expectation-maximization (EM) algorithm. It iteratively estimates the CFO, channel and recovers the unknown data using only limited number of pilot subcarriers in one OFDM symbol. In addition, efficient initial CFO and channel estimates are also derived based on approximated maximum likelihood (ML) and minimum mean square error (MMSE) criteria respectively. Simulation results show that the proposed data detection algorithm converges in a few iterations and moreover, its performance is close to the ideal case with perfect CFO and channel state information.Index Terms-Carrier frequency offset (CFO), orthogonal frequency division multiplexing (OFDM), doubly selective channel (DSC), data detection, expectation-maximization (EM).
In this paper, an orthogonal frequency division multiplexing (OFDM) system operating in a fast fading environment modeled by a doubly selective channel (DSC) is considered. The paper first reformulates a commonly adopted system model using the generalized complex exponential basis expansion technique. The resulting model enables the IQ imbalance and DSC to be estimated in the time domain with a small number of scattered pilots within an OFDM symbol. A joint estimation and compensation scheme is then proposed which compensates all the inter-carrier interference terms. Simulation results show that the proposed compensation method achieves better symbol error rate performance than previous proposed methods.
In this paper, a joint channel estimation and data detection algorithm is proposed for OFDM systems under doubly selective channels (DSCs). After representing the DSC using Karhunen-Loève basis expansion model (K-L BEM), the proposed algorithm is developed based on the expectationmaximization (EM) algorithm. Basically, it is an iterative algorithm including two steps at each iteration. In the first step, the unknown coefficients in K-L BEM are first integrated out to obtain a function which only depends on data, and meanwhile, a maximum a posteriori (MAP) channel estimator is obtained. In the second step, data are directly detected by a novel approach based on the function obtained in the first step. Moreover, a Bayesian Cramer-Rao Lower Bound (BCRB) which is valid for any channel estimator is also derived to evaluate the performance of the proposed channel estimator. The effectiveness of the proposed algorithm is finally corroborated by simulation results. I. INTRODUCTIONWith high spectral efficiency and robustness against frequency selective fading, orthogonal frequency division multiplexing (OFDM) has been widely recognized as an efficient transmission technique for wireless communications and has been considered as a strong candidate for next generation mobile systems. Generally, in mobile systems high speed movement would cause Doppler spread and result in a multi-path time-varying channel, i.e., doubly selective channel (DSC). Due to the time variation of the channel, the number of channel parameters in one OFDM symbol significantly increases, which makes the channel estimation a challenge. Meanwhile, this doubly-selectivity in channel will destroy the orthogonality among subcarriers and induces intercarrier interference (ICI) in OFDM systems, which also complicates the data detection.So far, channel estimation and data detection for OFDM systems over DSCs are usually tackled separately. In the literature [1]-[6], a number of pilot-aided channel estimation methods have been proposed by adopting various kinds of basis expansion models (BEMs). With the channel estimate, a minimum mean square error (MMSE) data detection method and an improved version with low-complexity have been proposed in [5] [6] respectively. In recent years, joint channel estimation and data detection methods have been developed for single carrier systems in [7] [8]. It is shown that those joint schemes can generally provide better performance than the separate approaches, since the data are also exploited for channel estimation. However, their extension to OFDM systems is by no means straightforward.In this paper, a joint channel estimation and data detection algorithm is proposed for OFDM systems over DSCs. K-L
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