Iterative Pilot-Layer Aided Channel Estimation with Emphasis on Interleave-Division Multiple Access Systems
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Iterative Pilot-Layer Aided Channel Estimation with Emphasis on Interleave-Division Multiple Access Systems Hendrik Schoeneich and Peter Adam Hoeher Information and Coding Theory Lab, Faculty of Engineering, University of Kiel, Kaiserstrasse 2, 24143 Kiel, Germany Received 1 June 2005; Revised 22 May 2006; Accepted 4 June 2006 Channel estimation schemes suitable for interleave-division multiple access (IDMA) systems are presented. Training and data are superimposed. Training-based and semiblind linear channel estimators are derived and their performance is discussed and compared. Monte Carlo simulation results are presented showing that the derived channel estimators in conjunction with a superimposed pilot sequence and chip-by-chip processing are able to track fast-fading frequency-selective channels. As opposed to conventional channel estimation techniques, the BER performance even improves with increasing Doppler spread for typical system parameters. An error performance close to the case of perfect channel knowledge can be achieved with high power efficiency. Copyright © 2006 H. Schoeneich and P. A. Hoeher. 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.
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INTRODUCTION
Spread-spectrum multiple access is a popular technique allowing several users to share the same bandwidth at the same time. Spread spectrum is often equated with direct-sequence code-division multiple access (DS-CDMA), where data detection is based on orthogonal or near-orthogonal spreading sequences. In [1–3], a spread-spectrum technique without the need for spreading sequences has been proposed. In this technique, data separation is based on chip-level interleavers. Therefore we refer to it as interleave-division multiple access (IDMA) [3, 4]. Processing is done on a chip-level basis. No orthogonal design is necessary. According to the results in [5, 6], the power and bandwidth efficiency of DS-CDMA can theoretically be maximized when devoting the entire bandwidth expansion (spreading) to FEC coding and removing the spreading sequences. IDMA fulfills this requirement and still allows for user separation. In conjunction with an optimized power allocation scheme, IDMA is able to reach the channel capacity—even when binary antipodal signaling is applied [7]. Like DSCDMA, IDMA is well suited to make use of the diversity that is introduced by frequency-selective fading, as will be shown by the subsequent numerical results. IDMA is currently discussed as a candidate for upcoming 4G systems [8– 11]. In this paper, channel estimation schemes for IDMA are proposed. Parts of this paper are published in [12]. Robust
channel estimation is especially important for spread-spectrum systems with iterative receiver structures, where channel estimation is performed before despreading, as in this case the signal-to-noise ratio is typically very low due to lowrate encoding. This is especially true for
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