Low-Complexity Banded Equalizers for OFDM Systems in Doppler Spread Channels
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Low-Complexity Banded Equalizers for OFDM Systems in Doppler Spread Channels Luca Rugini,1 Paolo Banelli,1 and Geert Leus2 1 Department
of Electronic and Information Engineering, University of Perugia, Via G. Duranti, 93-06125 Perugia, Italy of Electrical Engineering, Faculty of Electrical Engineering, Mathematics, and Computer Science, Delft University of Technology, 2628 CD Delft, The Netherlands
2 Department
Received 23 June 2005; Revised 19 January 2006; Accepted 30 April 2006 Recently, several approaches have been proposed for the equalization of orthogonal frequency-division multiplexing (OFDM) signals in challenging high-mobility scenarios. Among them, a minimum mean-squared error (MMSE) block linear equalizer (BLE), based on a band LDL factorization, is particularly attractive for its good tradeoff between performance and complexity. This paper extends this approach towards two directions. First, we boost the BER performance of the BLE by designing a receiver window specially tailored to the band LDL factorization. Second, we design an MMSE block decision-feedback equalizer (BDFE) that can be modified to support receiver windowing. All the proposed banded equalizers share a similar computational complexity, which is linear in the number of subcarriers. Simulation results show that the proposed receiver architectures are effective in reducing the BER performance degradation caused by the intercarrier interference (ICI) generated by time-varying channels. We also consider a basis expansion model (BEM) channel estimation approach, to establish its impact on the BER performance of the proposed banded equalizers. Copyright © 2006 Luca Rugini et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1.
INTRODUCTION
Orthogonal frequency-division multiplexing (OFDM) is a well established modulation scheme, which mainly owes its success to the capability of converting a time-invariant (TI) frequency-selective channel in a set of parallel (orthogonal) frequency-flat channels, thus simplifying equalization [1]. Conversely, a time-variant (TV) channel destroys the orthogonality among OFDM subcarriers, introducing intercarrier interference (ICI) [2, 3], and therefore making the OFDM BER performance particularly sensitive to Doppler-affected channels. Thus, the widespread use of OFDM in several communication standards (e.g., DVB-T, 802.11a, 802.16, etc.) and the increasing request for communication capabilities in high-mobility environments have recently renewed the interest in OFDM equalizers that are able to cope with significant Doppler spreads [4–10]. Among those, a low-complexity MMSE block linear equalizer (BLE) has been recently proposed in [9], which, similarly to other equalizers, exploits the observation that ICI generated by TV channels is mainly induced by adjacent subcarriers [8]. Thus, assuming that the ICI induced by faraway subcarriers can be
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