Efficient Bidirectional DFE for Doubly Selective Wireless Channels
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Efficient Bidirectional DFE for Doubly Selective Wireless Channels Stefano Tomasin Department of Information Engineering, University of Padova, Via Gradenigo 6/B, 35131 Padova, Italy Received 1 June 2005; Revised 3 November 2005; Accepted 7 November 2005 The bidirectional decision feedback equalizer (DFE) performs two equalizations, one on the received signal and one on its timereversed version. In this paper, we apply the bidirectional DFE to wireless transmissions on rapidly time-varying dispersive channels and we propose an efficient implementation obtained by implementing the feedforward filter in the frequency domain. The feedback filter is adapted to the channel variations within one block and we propose a simplified design of the feedback filter coefficients based on a polynomial model of channel variations. Simulations performed on time-varying channels show that the proposed structure significantly outperforms existing architectures. Copyright © 2006 Hindawi Publishing Corporation. All rights reserved.
1.
INTRODUCTION
Recent developments in wireless communications have seen the extension to mobile applications of existing standards originally conceived for static receivers [1–4]. The push to provide access to wideband services on mobile terminals has posed a number of issues in the design of transmit and receive physical layers. In particular, the channel on which communication takes place is affected by both frequency selectivity and time selectivity, thus yielding a time-varying intersymbol interference (ISI) on the received signal. In order to compensate for the distortion introduced by doubly selective channels, equalization is required. Among various equalization techniques, we mention the block linear equalizer (BLE) [5, 6] which has a high complexity, since it requires the inversion and multiplication of huge matrices. The time-invariant BLE has been considered in [5]. An extension to a time-varying finite impulse response (FIR) equalization has been considered in [6], where the doubly selective channel is described using a basis expansion model. For time-invariant dispersive channels, it is well known that nonlinear equalization outperforms linear equalization, due to its ability of reducing ISI by means of interference cancelation through past detected symbols [7]. In particular, decision feedback equalization (DFE) is able to significantly lower the bit error rate below linear equalization. This comes in general at a relatively high cost in terms of computational complexity, which can be reduced by implementing the filters in the frequency domain (FD) with the use of efficient discrete Fourier transforms (DFTs) [8–10]. The
FD implementation requires a particular transmission format that forces the circularity of the convolution between the channel and the transmitted signal. On the other hand, DFE is prone to error propagation, since errors in symbol detection are propagated to next detections through the feedback part. This phenomenon can be partially alleviated with the use of a bidirectional DFE, w
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