Luc Vandendorpe

Multitone spread spectrum multiple access communications system in a multipath Rician fading channel

The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated. >

Soft decode and forward improves cooperative communications

Mobile receivers can benefit from an increased spatial diversity through cooperation. Two relaying techniques are typically used: either amplify and forward (AF) or decode and forward (DF). This contribution presents a new technique, which combines the main advantages of both AF and DF. This technique amounts to a DF scheme where all operations are performed in a soft-input soft-output fashion. In particular, an encoding technique which uses soft values to incorporate reliability information is proposed. The resulting soft-DF relay can be seen as an analog signal-to-noise ratio enhancer. Simulations show that this soft-DF scheme outperforms both AF and DF

Turbo synchronization: an EM algorithm interpretation

This paper is devoted to turbo synchronization, that is to say the use of soft information to estimate parameters like carrier phase, frequency offset or timing within a turbo receiver. It is shown how maximum-likelihood estimation of those synchronization parameters can be implemented by means of the iterative expectation-maximization (EM) algorithm [A.P. Dempster, et al., 1977]. Then we show that the EM algorithm iterations can be combined with those of a turbo receiver. This leads to a general theoretical framework for turbo synchronization. The soft decision-directed ad-hoc algorithm proposed in V. Lottici and M. Luise, [2002] for carrier phase recovery turns out to be a particular instance of this implementation. The proposed mathematical framework is illustrated by simulations reported for the particular case of carrier phase estimation combined with iterative demodulation and decoding [S. ten Brink, et al., 1998].

MIMO Relay Design for Multipoint-to-Multipoint Communications With Imperfect Channel State Information

The problem of designing multiple-input-multiple-output (MIMO) relay for multipoint to multipoint communication in wireless networks has been dealt with by considering the fact that only the imperfect channel state information (CSI) is available at the MIMO relay. In particular, assuming that the second-order terms of the uncertainties of the source-relay and relay-destination channels are negligible, we design an amplify-and-forward (AF) MIMO relay that provides robustness against channel uncertainties. In our proposed robust method, the objective is to design the MIMO relay in which the worst-case relay transmit power is minimized by keeping the worst-case signal-to-interference-and-noise ratio (SINR) for all destinations above a certain threshold value. This paper shows that the aforementioned problem is nonconvex but it can be relaxed to a convex problem consisting of second-order cone (SOC) and semidefinite cone constraints using the semidefinite relaxation technique. The optimal solution of the relaxed problem is utilized to generate the best approximate solution of the original nonconvex problem using the well-known randomization technique. Computer simulations verify the robustness of the proposed MIMO relay when compared to the nonrobust MIMO relay.

Turbo-equalization for multilevel modulation: an efficient low-complexity scheme

This paper addresses the problem of iterative equalization and decoding (turbo-equalization) when bit-interleaved multilevel modulations are considered. The goal is to propose an efficient low-complexity solution in this context, where optimal turbo-equalization is totally untractable. Therefore, we generalize reduced-complexity soft-in/soft-out (SISO) equalizers based on the minimum mean square error (MMSE) criterion (originally proposed in a BPSK context) to this particular scheme. Performances are illustrated through simulations. We show the influence of the mapping on the signal-to-noise ratio (SNR) threshold and on asymptotic performances, and link the obtained results to iterative demodulation and decoding on an additive white Gaussian noise (AWGN) channel.

The M2VTS Multimodal Face Database (Release 1.00)

The primary goal of the M2VTS project is to address the issue of secured access to buildings or multi-media services by the use of automatic person verification based on multimodal strategies (secured access based on speech, face images and other information). This paper presents an overview of the multimodal face database recorded at UCL premises for the purpose of research applications inside the M2VTS project. This database offers synchronized video and speech data as well as image sequences allowing to access multiple views of a face. This material should permit the design and the testing of identification strategies based on speech andro labial analysis, frontal and/or profile face analysis as well as 3-D analysis thanks to the multiple views. The M2VTS Database is available to any non-commercial user on request to the European Language Resource Agency.

Code-Aided Turbo Synchronization

The introduction of turbo and low-density parity-check (LDPC) codes with iterative decoding that almost attain Shannon capacity challenges the synchronization subsystems of a data modem. Fast and accurate signal synchronization has to be performed at a much lower value of signal-to-noise ratio (SNR) than in previous less efficiently coded systems. The solution to this issue is developing specific synchronization techniques that take advantage of the presence of the channel code and of the iterative nature of decoding: the so-called turbo-synchronization algorithms. The aim of this paper within this special issue devoted to the turbo principle is twofold: on the one hand, it shows how the many turbo-synchronization algorithms that have already appeared in the literature can be cast into a simple and rigorous theoretical framework. On the other hand, it shows the application of such techniques in a few simple cases, and evaluates improvement that can be obtained from them, especially in the low-SNR regime.

Power Allocation for OFDM Transmission with DF Relaying

We consider OFDM (orthogonal frequency division multiplexing) transmission helped by a relay. Symbols sent by the source may or may not be retransmitted by a relay during a second time slot. The relay is supposed to operate in Decode-and-Forward (DF) mode. For each carrier the destination implements maximum ratio combining. Assuming perfect CSI (channel state information) knowledge the paper investigates the power allocation problem for rate maximization of the scheme. Both cases of a sum power constraint, and of individual power constraints at the source and at the relay are tackled. The theoretical analysis is illustrated by numerical results for both types of constraints.

MMSE-based fractional turbo receiver for space-time BICM over frequency-selective MIMO fading channels

Space-time bit-interleaved coded multilevel modulation over frequency-selective multiple-input multiple-output (MIMO) fading channels is considered. The receiver is turbo based. A soft-input soft-output (SISO) space-time fractionally spaced equalizer is designed for the minimum mean square error (MMSE) criterion. Simulation results show the ability of the receiver to account for intersymbol interference, co-antenna interference, and its capability to exploit transmit, multipath, and receive diversity. These results are also compared to a bit-error-rate lower bound.

Face verification competition on the XM2VTS database

In the year 2000 a competition was organised to collect face verification results on an identical, publicly available data set using a standard evaluation protocol. The database used was the Xm2vts database along with the Lausanne protocol [14]. Four different institutions submitted results on the database which were subsequently published in [13]. Three years later, a second contest using the same dataset and protocol was organised as part of AVBPA 2003. This time round seven seperate institutions submitted results to the competition. This paper presents the results of the competition and shows that verification results on this protocol have increased in performance by a factor of 3.