[Show abstract][Hide abstract] ABSTRACT: This paper presents a new frequency allocation scheme for ad hoc networks. Unlike other proposals, the scheme allocates frequencies to groups of nodes, for their intra-group communication needs, in order to guarantee a high level of availability of the needed frequency resources. This scheme uses a distributed decision process with interference measurements and a simple collaboration protocol between groups. Performance figures concerning the optimality of the obtained solution and the convergence time are given. The paper also presents a work that has been done to simulate this allocation scheme in a realistic environment with a mobility scenario on a 3D terrain model. Finally some ideas for future work are proposed.
[Show abstract][Hide abstract] ABSTRACT: We investigate a sub-optimal iterative receiver for joint equalization and decoding called Turbo-equalizer. We view the evolution of the error variance of the transmitted symbols through the iterative processing, obtaining convergence analysis.
[Show abstract][Hide abstract] ABSTRACT: In this paper we propose to apply multi-user detection techniques to the GSM system allowing either more transmitters per cell or higher data rates for point to point communications. In this former context, a optimal maximum likelihood channel estimator is derived. To increase the robustness of the transmission, iterative receivers are proposed including channel decoding and re-estimation of the propagation channel.
[Show abstract][Hide abstract] ABSTRACT: In this paper we propose to apply to the GSM system multi-user detection (MUD) techniques generally used in CDMA networks. The constraint of using the GSM wave form makes it impractical to use the optimum receiver due to its high complexity. Thus, in this paper, we derive the multi-user sub-optimum DFE-MMSE adapted to the GSM wave form. It relies on a linear approximation of the GSM modulation leading to a staggered modulation and thus requires a specific, (and multi-user in our case) equalizer to achieve good performances (Tu 1993). Not taking into account the fact that the modulation is staggered (as in Tripathi et al. 2000) leads to poor performances. The symbol detection is made possible by means of multiple antennas at the receiver. This technique is actually being tested on real data for underground transmissions in an IST funded project called ESCORT.
[Show abstract][Hide abstract] ABSTRACT: We propose in this paper to use multiuser detection techniques on the 2G GSM system allowing either higher data rates for point to point communications or multiuser transmissions thus increasing the uplink capacity. Multiuser detection techniques are applicable in such non spread spectrum systems if the number of reception antennas is equal to or larger than the number of transmitters. This system is actually tested in subway propagation conditions for an IST funded project named ESCORT.
[Show abstract][Hide abstract] ABSTRACT: When channel coding is used on a CDMA transmission, the most appropriate approach for reception is joint Multiuser detection and decoding. This joint problem can be successfully processed by the Turbo-techniques. Therefore, we consider a low-complexity iterative receiver for the CDMA uplink transmission, called Turbo-CDMA, and we propose to further improve it. First, we derive a new DFE to be used at the first iteration. Then, we propose to fully use the decoded information in a new structure called Full Turbo-CDMA. Finally, we derive a new demodulation order that is well suited to the Turbo-CDMA receiver. This improved receiver, called Full Turbo-CDMA, aims to eliminate MUI and ISI almost completely in the presence of frequency selective channels. Improved performance is illustrated by simulations.
No preview · Article · Sep 2001 · Journal of Communications and Networks
[Show abstract][Hide abstract] ABSTRACT: We investigate a sub-optimal iterative receiver for joint equalization and decoding called Turbo-equalizer. We view the evolution of the error variance of the transmitted symbols through the iterative processing, obtaining convergence analysis. This allows us to predict the asymptotic performance (when the Turbo-equalizer has converged) but also the trigger point observed in its performance. 1. INTRODUCTION In high rate communication, where the transmitted signal is subject to intersymbol interference (ISI), we may use equalization to reduce the effect of ISI and channel coding to correct remaining errors. A conventional equalizer does not make use of the redundancy introduced by the channel coding. Equalization and decoding are disjoint which is not optimal in the sense of the minimization of the error probability. Since optimal joint equalization and decoding is an NP-complete problem, we consider a relevant tradeoff between complexity and performance: the Turboequalizer, first prop...
[Show abstract][Hide abstract] ABSTRACT: We investigate a sub-optimal iterative receiver for joint
equalization and decoding called the turbo-equalizer. We view the
evolution of the error variance of the transmitted symbols through the
iterative processing, obtaining the convergence analysis
[Show abstract][Hide abstract] ABSTRACT: Nous analysons ici une solution itérative et sous-optimale au problème de l'égalisation et du décodage conjoints, solution appelée Turbo-égaliseur. Nous observons l'évolution de la variance de l'erreur d'estimation des symboles (transmis sur le canal) au cours du processus itératif, ce qui constitue l'analyse de convergence du Turbo-égaliseur. Ceci nous permet de prédire les performances asymptotiques (i.e. lorsque le Turbo-égaliseur a convergé) mais aussi le point de déclenchement du phénomène Turbo.
No preview · Article · Jan 2001 · Acoustics, Speech, and Signal Processing, 1988. ICASSP-88., 1988 International Conference on
[Show abstract][Hide abstract] ABSTRACT: In order to achieve performant but low-complexity multiuser
detection for coded signals, we propose to use a full turbo-CDMA
structure. Noticing that a good order of demodulation is very effective
in improving performance, we propose a new criterion for the order of
demodulation in an asynchronous and frequency selective CDMA context. It
takes into account both intersymbol and multiuser interference
[Show abstract][Hide abstract] ABSTRACT: Sub-optimal joint equalization and decoding is performed by
iterated equalization and decoding. This processing is named
turbo-equalization with reference to the turbo-decoding of serially
concatenated codes. We propose to optimize the equalizer structure (an
interference canceler) thanks to the training sequence available to
estimate the channel impulse response. The interference canceler,
optimized at each iteration, permits one to reduce the number of
iterations needed to achieve a given performance. The gain is all the
more important that the channel is hard to equalize
[Show abstract][Hide abstract] ABSTRACT: A low-complexity and iterative receiver is proposed for the CDMA
uplink, consisting of a low-complexity equalizer and single-user
decoders. Iterations are used to feed the equalizer with more reliable
data, which improves performance. This receiver called turbo-CDMA is
aimed to eliminate MUI almost completely in the presence of multipath
and to be robust to the near-far effect. Simulation results show that
the performance approaches single-user performance at moderate and also
low signal-to-noise ratios, even for a severe distortion channel
[Show abstract][Hide abstract] ABSTRACT: This paper deals with turbo-equalization as a joint equalization
and decoding algorithm. The performance analysis shows that there is a
trigger point in this iterative process, followed by a breakdown effect.
Actually after a given point (the trigger one), the BER decreases
steeply as a function of the decoding step p. We compare the performance
of the turbo-equalizer with that of the optimal joint receiver and show
that it can match the bound over the optimal disjoint receiver
[Show abstract][Hide abstract] ABSTRACT: - On étudie la turbo-égalisation en tant qu'algorithme d'égalisation et de décodage conjoints. L'analyse des performances montre que ce traitement itératif présente un point de déclenchement, i.e. qu'il existe une valeur seuil du rapport signal à bruit à partir de laquelle les performances s'améliorent quand le nombre d'itérations augmente, produisant un effet d'avalanche. Nous montrons ici l'influence de différents facteurs sur ce point de déclenchement et cet effet d'avalanche, notamment les performances de l'égaliseur de la première itération et la capacité de l'entrelaceur à bien décorréler le bruit des données.
[Show abstract][Hide abstract] ABSTRACT: In digital communications, conventional linear and decision
feedback equalizers are optimal for stationary signals. However,
modulated signals are polycyclostationary (PCS): their correlation
functions are time-polyperiodic. Cyclic equalization structures
optimized by the MMSE (minimum mean square error) criterion are
proposed. The proposed structure is based on a multidimensional input.
The latter is built from the received signal possibly shifted by its
cyclic frequencies. We show by simulation that the proposed cyclic
structures give better performance in terms of binary error rate than
the conventional equalizers. The paper is organized as follows. First,
we review some background topics on cyclostationarity and optimum
filtering of PCS signals. Then we calculate the expressions of the
forward and backward filters for the cyclic DFE and deduce the
expression of the linear cyclic equalizer. Finally, simulation results
of conventional and cyclic equalizers are given for BPSK and MSK signals
jammed by other modulated signals. The new cyclic structures are
particularly suitable for FDMA networks disturbed by adjacent channel
interference (ACI). They can lead to a significant improvement in the
number of communication channels within a given bandwidth. Besides,
cyclic structures can also be used for transmission links disturbed by
jammers that are modulated signals with the same or different baud rates
[Show abstract][Hide abstract] ABSTRACT: This paper presents a new 9600 bps high data rate HF modem. This
modem uses a coherent parallel modulation thanks to the addition in the
waveform of known pilot tones, the channel as well as the the noise on
each tone can be estimated during demodulation. The coding scheme uses
the turbo-code principle. This new coherent parallel modem offers an
interesting and extremely competitive solution for a high data rate HF
modem in a frequency selective fading channel
[Show abstract][Hide abstract] ABSTRACT: For radiocommunication channels with propagation multipaths, the
presence of intersymbol interference (ISI) at reception must be
mitigated by an equalizer. Furthermore, when the link is perturbed by
cochannel interference (CCI), the use of an adaptive array at reception
may be very useful. In such contexts, it has been shown that the maximum
likelihood sequence estimation (MLSE) multichannel receiver is composed
of a spatio-temporal matched filter (STMF) followed by a symbol rate
sampler and the MLSE-based modified Viterbi algorithm for a mono channel
receiver. However, this optimal receiver has not been implemented in a
practical context. The purpose of this paper is to fill the gap
previously mentioned by introducing a new multichannel receiver, which
is an implementation of the optimal one in temporally white noise (TWN).
The performance of this new receiver is then analyzed in the context of
a GSM link with ISI and CCI
[Show abstract][Hide abstract] ABSTRACT: Pour des systèmes de radiocommunications numériques opérant sur un canal sélectif en fréquence et brouillé, la performance du récepteur peut être améliorée par l'utilisation conjointe des techniques de diversité d'antennes et d'égalisation pour combattre à la fois les évanouissements sélectifs en temps et en fréquence et les effets des brouilleurs. Des égaliseurs à diversité d'antennes d'un tel canal sont développés dans les références [1,2]. Ils sont composés du filtre adapté multidimensionnel, ou filtre adapté spatio-temporel (FAST), suivi d'un échantillonneur au rythme symbole et d'un égaliseur pour récepteurs à une seule antenne. Dans le présent papier, la définition du FAST est établie et une interprétation de ce filtre est donnée.
[Show abstract][Hide abstract] ABSTRACT: For digital radiocommunications systems operating on a jammed frequency-selective fading channel, the receiver performance can be improved by using the joint antenna diversity and equalization techniques to combat both time- and frequency-selective fades and jammers effects. The optimum, in the sense of minimum mean-squared error (MMSE), structure of the linear equalizer (LE) and the decision feedback equalizer (DFE) for coherent receiver antenna diversity has been derived for an unjammed environment. The structure of the MMSE antenna diversity equalizer is determined for a jammed multipath channel. We show that is composed of the multidimensional matched filter (MMF) followed by a symbol-rate sampler and the MMSE LE or DFE equalizer for single antenna receivers.
Preview · Article · Jan 1995 · Proceedings - ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing