Patrick Vandamme

Decision Feedback Equalization of Dispersive Radio Channels

The performance of adaptive decision feedback equalization applied to high bit rate digital radio systems in the presence of multipath propagation is analytically investigated. Minimum phase (MP) and nonminimum phase (NMP) type fades as well as the transition periods between these two states are considered. Insight is given into the IF recovered timing epoch and its consequences on the decision feedback equalizer (DFE) behavior, especially during transition periods. Required conditions on the DFE structure for ensuring its maximum efficiency are derived and a modified updating algorithm is presented. Finally, dynamic simulation results are reported and compared to theoretical results. They show that the proposed structure is capable of coping with most propagation conditions.

Baseband Equalization and Carrier Recovery in Digital Radio Systems

The influence of adaptive baseband equalization on decision-feedback carrier recovery performance is investigated. First, it is shown that the delay introduced by the equalizer in the carrier recovery loop does not have basic consequences on system performance. Second, the interaction between the two adaptive circuits is studied for four different equalizer adaptation algorithms. It is found that algorithms based on the zero-forcing criterion are unstable unless they are used with an appropriate constraint. With gradient-type algorithms based on the minimum mean square error criterion, the interaction leads to an improved carrier acquisition performance at the expense of a larger phase jitter. It is therefore concluded that even with these latter algorithms it is generally worthwhile to avoid the interaction by forcing the imaginary part of the equalizer reference tap to zero. The theoretical analysis (carried out with a single complex tap equalizer) is supported by experimental results and computer simulation.

On the synthesis of digital transmit filters

A simple method for calculating the coefficients of a digital transversal filter when cascaded with analog filters is proposed. The optimization criterion takes into account both the spectral characteristics of the signal and the residual amount of intersymbol interference at the decision device input. In contrast with standard methods no a priori constraints are applied to the filter in the time or in the frequency domain. >

All-digital implementation of the carrier recovery loop in digital radio systems

The authors address the issue of complete digitization of the carrier recovery loop in digital microwave radio systems. First, they present digital equivalents of conventional carrier recovery loops employing either a passive filter or an active filter with a high gain. Next, they address the issue of extending the acquisition range of these loops, which is a critical problem in digital radio receiver design. For carrier recovery loops with the digital equivalent of an active loop filter, the loop acquisition range was investigated using two previously proposed PFDs (phase and frequency detectors). With the best of these two detectors, it was shown that the acquisition range can be very close to 1/8 T, which constitutes a fundamental limit for decision-feedback carrier recovery loops with symbol-rate phase error measurements.<<ETX>>

Performance of equalization techniques in a radio interference environment

The performance of equalization techniques for digital radio systems in the presence of sinusoidal and modulated interference signals is investigated. Linear and nonlinear equalizers, as well as synchronous and T/2-spaced structures, are considered. It is shown that synchronous decision-feedback equalizers are powerful countermeasure devices for radio channels affected by both selective fading and sinusoidal interferers, and that both T/2-spaced linear and nonlinear equalizers can provide a significant improvement of the adjacent channel interference margin, whether the channel is selective or not, especially in the case of multicarrier interleaved frequency arrangements. >

Les dispositifs de lutte contre les effets des évanouissements sélectifs dans les faisceaux hertziens numériques

AnalyseL’article présente une synthèse des dispositifs utilisés dans les faisceaux hertziens numériques pour combattre l’effet des évanouissements sélectifs en période de propagation par trajets multiples. Leurs principes sont analysés et leurs performances comparées à partir des différents résultats déduits de mesures expérimentales conduites à travers le monde. On se limite à la transmission sur un canal isolé: les dispositifs utilisant la diversité de fréquence ou tendant à minimiser l’influence de canaux perturbateurs ne sont donc pas évoqués. Les perspectives d’évolution dans les années futures sont également abordées.AbstractAn overview of the countermeasure techniques used in digital radio systems for combatting the effects of selective fading due to multipath propagation is presented. Basic principles are analyzed. Their performances, based upon experimental measurements carried out throughout the world are compared. Frequency diversity techniques and channel interference cancellation techniques specific to a multichannel environment are not investigated. Future trends in the field are finally discussed.

Performance of IJF-OQPSK and Partial Response (PR) IJF-OQPSK Modems in a Nonlinearly Amplified and Adjacent-Channel Interference Satellite Environment

The performance of intersymbol-interference and jitterfree offset-QPSK (IJF-OQPSK) and partial-response (PR) IJFOQPSK modems in a nonlinearly amplified multichannel environment is studied and compared to that of conventional QPSK and OQPSK modems. Power efficiency is maximized by operating the high-power amplifier (HPA) in full saturation. The effect of adjacent-channel interference and flat fade on the performance of illustrative nonlinear satellite systems is investigated. Simulation and measurement results are reported. It is shown that both IJF-OQPSK and PR-IJF-OQPSK modems exhibit a significant performance improvement compared to QPSK and OQPSK modems, in illustrative SCPC and on-board regenerative satellite systems, they provide an improvement of more than 4 dB compared to QPSK.

Conception et réalisation d’un égaliseur auto-adaptatif pour lutter contre les évanouissements sélectifs sur les liaisons hertziennes numériques

AnalyseLes évanouissements sélectifs dus aux phénomènes de trajets multiples affectent les faisceaux hertziens numériques à grande capacité. L’égalisation en bande de base, technique appliquée d’abord aux systèmes de transmission de données sur ligne téléphonique, est un moyen de combattre ces évanouissements sélectifs. L’article donne un modèle très simplifié de ceuxci, présente les différentes structures d’égaliseurs envisageables et donne des résultats expérimentaux obtenus en modulation MAQ16 à 140 Mbit/s en présence d’évanouissements simulés avec un égaliseur construit au cnet à Lannion.AbstractSelective fading drastically affects the high capacity digital radiolinks. Baseband equalization, which has been applied for several years to data transmission over telephone lines, appears to be very efficient for compensating for these fadings. The paper presents a very simplified fade model, the different equalizer structures that could be used and gives experimental results achieved with an equalizer breadboard built in CNETlaboratories in Lannion. The modulation sheme is 16 QAM,the bit rate 140 Mbit/s, and the fades are simulated, using a tworay model.

A robust and fast-locking all-digital carrier recovery loop for QAM signals

The authors present the design and the implementation of an all-digital carrier recovery loop for quadrature amplitude modulation (QAM) signals. Original features concerning the programmability of the loop, its fast-locking properties in acquisition and restart procedures as well as its wide acquisition range are discussed. The emphasis is on its performance in terms of robustness versus noise and intersymbol interference and protection against potential frequency false-locks. Experimental results are reported for a 16-QAM system.<<ETX>>

Etude comparative des techniques d’égalisation synchrone et fractionnée pour transmissions numériques par voie hertzienne

AnalyseCet article présente une étude comparative des techniques d’égalisation synchrone et fractionnée, appliquées aux systèmes de transmission numérique par voie hertzienne. Les performances de systèmes utilisant les modulations MAQ-64 et MAQ-256 et transmettant un débit de 140 Mbit/s sont évaluées en termes de temps de coupure, pour les deux techniques, et pour des égaliseurs linéaires et non linéaires récursifs. Les résultats sont présentés sous la forme de facteurs d’amélioration de la marge nette en fonction de la marge brute. L’utilisation de l’égalisation fractionnée fait apparaître, par rapport à l’égalisation synchrone, un facteur d’amélioration de l’ordre de 2 à 3 dB dans le cas d’égaliseurs linéaires, facteur qui redevient négligeable avec les égaliseurs non linéaires.AbstractThe paper presents a comparative study of synchronous and fractionally-spaced equalization techniques, in a multipath fading environment. The outage performance of 64-QAM and 256-qam 140 Mbit/s digital radio systems equipped with linear and decision feedback equalizers is evaluated. Results are given in terms of net fade margin improvement versus flat fade margin. They show that the 2 to 3 dB gain obtained for linear equalizers vanishes with non-linear equalizers.