Conference Paper

# EXIT Chart Evaluation of a Receiver Structure for Multi-User Multi-Antenna OFDM Systems

Dept. of Electr. & Inf. Technol., Lund Univ., Lund, Sweden

DOI: 10.1109/GLOCOM.2009.5426288 Conference: Global Telecommunications Conference, 2009. GLOBECOM 2009. IEEE Source: IEEE Xplore

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**ABSTRACT:**Joint time-variant channel estimation and multi-user detection are key building-blocks for wireless broadband communication for mobile users at vehicular speed. We propose an iterative receiver for a multi-carrier (MC) code division multiple access (CDMA) system in the uplink. Multi-user detection is implemented through iterative parallel interference cancellation and conditional linear minimum mean square error (MMSE) filtering. MC-CDMA is based on orthogonal frequency division multiplexing (OFDM), thus time-variant channel estimation can be performed for every subcarrier individually. The variation of a subcarrier over the duration of a data block is upper bounded by the maximum Doppler bandwidth which is determined by the maximum velocity of the users. We exploit results from the theory of time-concentrated and bandlimited sequences and apply a Slepian basis expansion for time-variant subcarrier estimation. This approach enables time-variant channel estimation without complete knowledge of the second-order statistics of the fading process. The square bias of the Slepian basis expansion is one order of magnitude smaller compared to the Fourier basis expansion. The square bias of the basis expansion is the determining factor for the performance of the iterative joint channel estimation and data detection. We present an iterative linear MMSE estimation algorithm for the basis expansion coefficients in a multi-user system. The consistent performance of the iterative receiver using the Slepian basis expansion is validated by simulations for a wide range of velocitiesIEEE Transactions on Wireless Communications 07/2006; · 2.76 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Iterative channel estimation can improve the channel-state information (CSI) with respect to noniterative estimation. New iterative channel estimators based on the expectation-maximization (EM) algorithm are proposed in this paper. A first estimator, called the unbiased EM (UEM), is designed to unbias the EM estimates. A second estimator is then put forward, which is based on the expectation-conditional-maximization (ECM) algorithm, and its complexity is lower than that of the EM. An unbiased ECM (UECM) estimator is also proposed. Although the unbiasedness of the UEM and UECM estimators is not rigorously proved, the use of these names is explained in the paper. The new estimators are compared with well-known ones, such as the EM, the decision-directed (DD), and the data-aided (DA) estimators. Simulations are reported for a turbo receiver operating over frequency-selective multiple-input multiple-output channels. It is shown that the UEM channel estimator outperforms the EM, and that the ECM-based estimators are very close to the EM-based onesIEEE Transactions on Communications 02/2007; · 1.98 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**In this paper, we investigate two reduced-complexity iterative soft interference cancellation minimum mean square error (SIC-MMSE) receivers for frequency-selective multiple-input-multiple-output (MIMO) channels. In the first receiver, the extrinsic information is exchanged between the SIC-MMSE equalizer and the channel decoding stages at each iteration. In the second receiver, the extrinsic information obtained from the SIC-MMSE equalizer is fed back to itself only up to a certain number of iterations and then passed to the channel decoder at the end of the last iteration only to reduce the computational complexity. Moreover, to better understand the convergence behavior of the proposed iterative receivers, we study the notion of extrinsic information transfer (EXIT) characteristics. Using simulations, we derive the extrinsic information trajectory on the EXIT chart at various bit-energy-to-noise-spectral-density ratio ( Eb / No ) ranges to predict the number of iterations required to converge and the turbo cliff region. The predicted behavior of the proposed receivers is then confirmed by the bit-error-rate (BER) performance curves.IEEE Transactions on Vehicular Technology 02/2009; · 2.64 Impact Factor

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