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ABSTRACT: In this paper, we address the problem of identifying convolutive channels using a Gaussian maximum-likelihood (ML) approach when short training sequences (possibly shorter than the channel impulse-response length) are periodically inserted in the transmitted signal. We consider the case where the channel is quasi-static (i.e., the sampling period is several orders of magnitude smaller than the coherence time of the channel). Several training sequences can thus be used in order to produce the channel estimate. The proposed method can be classified as semiblind and exploits all channel-output samples containing contributions from the training sequences (including those containing contributions from the unknown surrounding data symbols). Experimental results show that the proposed method closely approaches the Cramer-Rao bound and outperforms existing training-based methods (which solely exploit the channel-output samples containing contributions from the training sequences only). Existing semiblind ML methods are tested as well and appear to be outperformed by the proposed method in the considered context. A major advantage of the proposed approach is its computational complexity, which is significantly lower than that of existing semiblind methods.
IEEE Transactions on Wireless Communications 12/2005; · 2.59 Impact Factor
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Vehicular Technology Conference, 2005. VTC-2005-Fall. 2005 IEEE 62nd; 10/2005
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ABSTRACT: We present a block minimum mean-squared error (MMSE) equalizer for orthogonal frequency-division multiplexing (OFDM) systems over time-varying multipath channels. The equalization algorithm exploits the band structure of the frequency-domain channel matrix by means of a band LDL<sup>H</sup> factorization. The complexity of the proposed algorithm is linear in the number of subcarriers and turns out to be smaller with respect to a serial MMSE equalizer characterized by a similar performance.
IEEE Communications Letters 08/2005; · 0.98 Impact Factor
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ABSTRACT: Transmitted reference ultra wideband (TR-UWB) communication systems have gained increasing interest lately, because of their ability to solve the timing and channel estimation problems encountered in conventional UWB systems. Traditionally, an autocorrelation receiver is employed in a TR-UWB system, which is based on correlating the signal with a delayed version of itself. To reduce the noise squaring effect related to this autocorrelation receiver, we propose to split up the correlation interval leading to a number of smaller correlations, which are then linearly combined in some efficient manner. We refer to this receiver as a weighted autocorrelation receiver. We present both optimal and practical design algorithms for this receiver. We also carry out a number of simulations using real UWB channel measurements, and show that the weighted autocorrelation receiver significantly outperforms the conventional one. Moreover, we illustrate that the proposed TR-UWB system can compete favorably with a realistic digital implementation of a conventional UWB system.
Signal Processing Advances in Wireless Communications, 2005 IEEE 6th Workshop on; 07/2005
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ABSTRACT: In this paper, equalization of time-varying channels is discussed, where the channel is modeled by a complex exponential basis expansion model (CE-BEM). We consider a system where on the transmit side a precoder is employed to achieve maximum diversity. Along with an FFT decoder on the receive side, the resulting channel resembles an FIR filter on both matrix and scalar-level. We propose therefore a decision feedback equalizer (DFE), which bears a similar FIR structure as the channel. The equalizer taps can either be computed based on the channel knowledge, or adaptively estimated with the assistance of pilots. Simulations show that the proposed equalizer yields a satisfactory performance at a low hardware cost.
Signal Processing Advances in Wireless Communications, 2005 IEEE 6th Workshop on; 07/2005
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ABSTRACT: Recently, a minimum mean-squared error (MMSE) block linear equalizer based on a band LDL<sup>H</sup> factorization has been proposed for equalization of orthogonal frequency-division multiplexing (OFDM) systems affected by Doppler spread. In this paper, we extend this approach towards two directions. First, we design an MMSE block decision-feedback equalizer (DFE) based on the band LDL<sup>H</sup> factorization. Both performance and complexity are analyzed. Second, we enhance the performance of the linear equalizer by means of receiver windows tailored to the band LDL<sup>H</sup> factorization approach. Simulation results show that the proposed techniques are effective in reducing the error floor caused by the intercarrier interference (ICI).
Signal Processing Advances in Wireless Communications, 2005 IEEE 6th Workshop on; 07/2005
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ABSTRACT: Coping with time-selective fading channels is challenging but also rewarding, especially with multiantenna systems, where joint space-Doppler diversity and coding gains can be collected to enhance performance of wireless mobile links. These gains have not been quantified, and space-time coded systems maximizing joint space-Doppler benefits have not been designed. Based on a parsimonious basis expansion model for the underlying time-selective (and possibly correlated) channels, we quantify these gains in closed form. Furthermore, we develop space-time-Doppler coded systems that guarantee the maximum possible space-Doppler diversity, along with the largest coding gains within all linearly coded systems. Our three novel designs exploit knowledge of the maximum Doppler spread, and each offers a uniquely desirable tradeoff, including high spectral efficiency, low decoding complexity, and high performance. Our analytical results are confirmed by simulations and reveal the relative of merits of our three designs in comparison with an existing approach.
IEEE Transactions on Signal Processing 07/2005; · 2.63 Impact Factor
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ABSTRACT: Transmit maximum-ratio combining (transmit MRC) is a popular antenna diversity technique that provides both spatial diversity and array gain in downlink multiple-input single-output (MISO) links. These gains, however, critically depend on the availability of the downlink channel state information (CSI). In time-division duplexing systems, channel reciprocity has been commonly put forth to justify the convenient use of the CSI already acquired from the uplink, in the calculation of the transmit-MRC weights. Recent work has questioned this practice, based on the non-reciprocity of multi-antenna RF transceivers, due to significant amplitude and phase mismatches across the antennas. Furthermore, expensive digital calibration solutions have been proposed to enforce the reciprocity of the multi-antenna RF transceivers. Both the impact of multi-antenna amplitude and phase mismatches and the performance of the proposed calibration approaches have only been assessed via simulations. In this contribution, we propose an alternative statistical analysis of the impact of these mismatches on transmit MRC. This analysis allows a faster and more reliable characterization as well as provides insight into the relative importance of these mismatches. Consequently, sufficient matching requirements can be extracted for the multi-antenna RF transceivers, for which simpler and cheaper calibration solutions can be devised.
Acoustics, Speech, and Signal Processing, 2005. Proceedings. (ICASSP '05). IEEE International Conference on; 04/2005 · 4.63 Impact Factor
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G. Leus
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ABSTRACT: In this paper, we discuss a semi-blind channel estimation algorithm for rapidly time-varying channels, relying on a complex exponential basis expansion model (CE-BEM) for the channel. However, whereas the original CE-BEM approach models a rectangularly windowed version of the channel, the proposed CE-BEM approach models a smoothly windowed version of the channel. This allows for a much better fit, and leads to better channel estimates. The obtained semi-blind channel estimates are subsequently used to construct a recently developed CE-BEM serial decision-feedback equalizer for CE-BEM channels. Simulations are carried out to validate the proposed ideas.
Acoustics, Speech, and Signal Processing, 2005. Proceedings. (ICASSP '05). IEEE International Conference on; 04/2005 · 4.63 Impact Factor
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ABSTRACT: We present a low-complexity equalizer for multicarrier code-division multiple-access (MC-CDMA) downlink systems over time-varying (TV) multipath channels with non-negligible Doppler spread. The equalization algorithm, which is based on a block minimum mean-squared error (MMSE) approach, exploits the band structure of the frequency-domain channel matrix by means of a band LDL<sup>H</sup> factorization. The complexity of the proposed block MMSE equalizer is linear in the number of subcarriers, and smaller with respect to a serial MMSE equalizer characterized by a similar performance.
Acoustics, Speech, and Signal Processing, 2005. Proceedings. (ICASSP '05). IEEE International Conference on; 04/2005 · 4.63 Impact Factor
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ABSTRACT: We propose a time-varying (TV) finite impulse response (FIR) equalizer for doubly selective (time- and frequency-selective) channels. We use a basis expansion model (BEM) to approximate the doubly selective channel and to design the TV FIR equalizer. This allows us to turn a complicated equalization problem into an equivalent simpler equalization problem, containing only the BEM coefficients of both the doubly selective channel and the TV FIR equalizer. The minimum mean-square error (MMSE) as well as the zero-forcing (ZF) solutions are considered. Comparisons with the block linear equalizer (BLE) are made. The TV FIR equalization we propose here unifies and extends many previously proposed serial equalization approaches. In contrast to the BLE, the proposed TV FIR equalizer allows a flexible tradeoff between complexity and performance. Moreover, through computer simulations, we show that the performance of the proposed MMSE TV FIR equalizer comes close to the performance of the ZF and MMSE BLE, at a point where the design as well as the implementation complexity are much lower.
IEEE Transactions on Wireless Communications 02/2005; · 2.59 Impact Factor
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ABSTRACT: Ultra wideband (UWB) wireless communication systems are based on the transmission of a stream of narrow pulses (each shorter than a ns). In such a system it is of great importance to estimate the beginning of the data packet of interest in order to subsequently estimate the data symbols. In this paper we present a combined blind synchronization and detection scheme based on the transmit-reference ultra wideband (TR-UWB) transceiver model taking into account a channel with a long impulse response. The proposed algorithm processes a block of received data samples, takes advantage of a shift invariance structure in the frequency domain, and applies a MUSIC-like search to estimate the delay of the data packet.
Signals, Systems and Computers, 2004. Conference Record of the Thirty-Eighth Asilomar Conference on; 12/2004
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ABSTRACT: Transmitted reference (TR) systems have recently been proposed for ultra wideband (UWB) communications. They considerably simplify synchronization and channel estimation, which are known to be difficult problems in UWB communications. We extend existing receivers for TR-UWB systems by replacing the correlation operation by a linear combination of specific parts of the correlation and weighting the parts that have a small noise contribution more than parts that have a large noise contribution. This turns out to improve the performance considerably.
Signal Processing Advances in Wireless Communications, 2004 IEEE 5th Workshop on; 08/2004
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ABSTRACT: The joint linear precoder and decoder minimum mean squared error (MMSE) design represents a low complexity yet powerful solution for spatial multiplexing MIMO systems. Its performance can be further boosted through optimally selecting the number of spatial streams to be used according to the available channel state information (CSI), so-called spatial-mode selection. The performance of both the latter MMSE design and the related spatial-mode selection criteria, however, critically depends on the availability of timely CSI at both transmitter and receiver. In practice, the latter assumption can be severely challenged, due to channel time variations that lead to imperfect CSI at the transmitter. State-of-the-art designs mistakenly use this imperfect CSI to design the linear precoder and rely on the receiver to reduce the induced degradation. We have alternatively proposed a robust Bayesian joint linear precoder and decoder solution that takes into account the uncertainty on the true channel, given the channel mean feedback at the transmitter. In this paper, we further improve the performance of our aforementioned robust design using a new spatial-mode selection criterion based on channel mean feedback. We also illustrate, via Monte-Carlo analysis, the robustness of the resulting improved design to channel time variations, which outperforms the state-of-the-art approach.
Signal Processing Advances in Wireless Communications, 2004 IEEE 5th Workshop on; 08/2004
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ABSTRACT: Recently, serial linear equalizers (SLEs) and serial decision feedback equalizers (SDFEs) have been proposed to mitigate the doubly-selective channel effects. To design the SLE/SDFE and to model the doubly-selective channel, a so-called finite impulse response basis expansion model (FIR-BEM) is used. Initially, the FIR-BEM coefficients of the SLE/SDFE were designed based on the exact knowledge of the FIR-BEM coefficients of the doubly-selective channel. In practice, we can use a direct SLE/SDFE design procedure, which avoids an intermediate channel estimation step. In this paper, we describe this idea for the SLE and focus on direct semi-blind design of the FIR- BEM coefficients of the SLE. Simulation results demonstrate the validity of the proposed approach.
Communications, 2004 IEEE International Conference on; 07/2004
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ABSTRACT: We propose a per-tone frequency-domain equalization approach for OFDM over doubly-selective channels. We consider the most general case, where the doubly-selective channel delay spread is larger than the cyclic prefix (CP), which results into inter-block interference (IBI). IBI in conjunction with the Doppler effect destroys the orthogonality between subcarriers and hence, results into severe intercarrier interference (ICI). In this paper, we propose a novel per-tone frequency-domain equalizer (PTFEQ) that is obtained through transferring a time-varying time-domain equalizer (TV-TEQ) to the frequency-domain. The purpose of the TV-TEQ is to restore orthogonality between subcarriers and eliminate ICI. We use the mean-square error criterion to design the PTFEQ. An efficient implementation of the proposed PTFEQ is also discussed. Finally, we show some simulation results of the proposed equalization technique.
Communications, 2004 IEEE International Conference on; 07/2004
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ABSTRACT: We construct unitary noncoherent space-time constellations, which can be considered as a concatenation of a training block with an orthogonal design. With a simple construction, our constellations are easy to design, enjoy full antenna diversity, allow for a simplified maximum-likelihood (ML) detector, and achieve error performance comparable to existing designs that rely on computer search. To exploit the constellation structures and improve coding gains, we further pursue a trellis-coded modulation (TCM) approach. Based on the sequence pairwise error analysis, we identify two simple parameters to quantify the asymptotic error performance, which enables us to compare among different TCM schemes or uncoded alternatives.
IEEE Transactions on Information Theory 07/2004; · 3.01 Impact Factor
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ABSTRACT: A new approach has recently been proposed to describe doubly selective channels (i.e., time- and frequency-selective channels) with a limited number of parameters; it is referred to as the basis expansion model (BEM). In the BEM, the true channel coefficients are approximated with a high accuracy using a limited number of complex exponentials. We propose a new method in order to identify the BEM coefficients of the transmission channel. We consider a transmission scheme where several short training sequences (i.e. their length is comparable to the channel order) are inserted in the stream of data symbols. We propose an iterative method that exploits all the received symbols that contain contributions from the training sequences and blindly filters out the contribution of the unknown surrounding data symbols. The proposed method has a low computational complexity and outperforms existing methods proposed in a similar context.
Acoustics, Speech, and Signal Processing, 2004. Proceedings. (ICASSP '04). IEEE International Conference on; 06/2004 · 4.63 Impact Factor
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ABSTRACT: We discuss time-domain equalization of OFDM over doubly-selective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results in inter-block interference (IBI). IBI, in conjunction with the Doppler effect, destroys the orthogonality between subcarriers and, hence, results in intercarrier interference (ICI). The time-domain equalizer (TEQ) is assumed to be a time-varying finite impulse response (TV FIR). The purpose of the TEQ is to convert the doubly-selective channel into a purely frequency-selective channel whose delay spread fits within the CP. In other words, the purpose of the TEQ is to restore orthogonality between subcarriers in the OFDM system.
Acoustics, Speech, and Signal Processing, 2004. Proceedings. (ICASSP '04). IEEE International Conference on; 06/2004 · 4.63 Impact Factor
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ABSTRACT: In a previous paper, we proposed a bitrate maximizing (BM) design criterion for the time-domain equalizer (TEQ) in a discrete multitone receiver. This BM-TEQ and the closely related BM per-group equalizers (PGEQ) get close to the performance of the so-called per-tone equalization (PTEQ). In this paper, we show that the BM-TEQ criterion, despite its nonlinear nature, is well suited for a recursive Levenberg-Marquardt (RLM) based design. This adaptive BM-TEQ also allows us to track slow variations of the transmission channel and the noise. This RLM-based design uses the same second-order statistics (SOS) as the earlier presented recursive least-squares (RLS) based adaptive PTEQ and opens up a complete range of adaptive BM equalizers: from the computationally efficient RLS-based PTEQ with largest memory cost, over the RLM-based BM-PGEQ with intermediate memory cost, towards an RLM-based BM-TEQ with considerably smaller memory cost, but larger equalizer updating complexity.
Acoustics, Speech, and Signal Processing, 2004. Proceedings. (ICASSP '04). IEEE International Conference on; 06/2004 · 4.63 Impact Factor
