Jean-Michel Dumas

2-Dimensional optical CDMA system performance with parallel interference cancellation

In this paper, we investigate the feasibility of a 2 Dimensional Optical Code Division Multiple Access system (2DOCDMA), with electronic coding and decoding functions. We present a modified construction method of Multi-Wavelength Optical Orthogonal Codes (MWOOC) which permits high flexibility for the code parameters choice. Our objective is to evaluate the 2D code performance, with a Conventional Correlation Receiver (CCR) and a more complex one named Parallel Interference Cancellation receiver (PIC). For example, for a Bit Error Rate (BER)10-9, 30 simultaneous users and an electrical implementation, we show that contrary to the CCR, the use of a PIC receiver leads to workable 2D code solutions.

Parallel multiple access interference cancellation in optical DS-CDMA systems

This paper investigates the Parallel Interference Cancellation technique in Direct Sequence Optical Code Division Multiple Access (ds-ocdma) system. In the proposed system, the estimated interference is removed from the received signal. We have developped a new approach to obtain the analytical expression of error probability in chip synchronous case, for Optical Orthogonal Codes (ooc). We have shown that under specific conditions between codes parameters and users’ number, the interference can be completely neutralized. Simulation results have validated the theoretical analysis. It is shown that the proposed receiver is effective in reducing significantly the effects of Multiple Access Interference (mai) compared to other interference cancellation systems.RésuméCet article présente l’étude d’un récepteur basé sur l’annulation parallèle d’interférence d’accès multiple dans un système d’accès multiple par répartition en code à séquence directe (ds-ocdma). Le principe consiste à estimer l’interférence due aux utilisateurs non désirés pour la soustraire du signal reçu à l’entrée du récepteur. Nous avons obtenu la formule théorique de la probabilité d’erreur dans le cas synchrone, pour les codes optiques orthogonaux (ooc). Nous avons prouvé que, pour des conditions spécifiques concernant les paramètres des codes et le nombre d’utilisateurs, l’effet de l’interférence peut être complètement éliminé Les résultats de simulation ont validé l’analyse théorique. On montre enfin que le récepteur proposé est plus efficace que d’autres systèmes d’annulation d’interférence pour réduire de manière significative les effets de l’interférence d’accès multiple (mai).

Comparative investigation of gate leakage current in single and double channel InP HEMT

The InP HEMT is a key electron device for monolithic optoelectronic integration. However, the additional gate current due to impact ionization in the low band-gap InGaAs channel is a serious limitation for photoreceiver applications and for high breakdown voltage. In order to overcome this limitation InGaAs/InP double channel HEMTs (DC HEMT) have been fabricated, characterised and compared with InGaAs single channel HEMTs (SC HEMT). As expected the gate leakage current is much lower for the DC HEMT. A physical quasi-2D simulation supports this improved behaviour and shows that the gate leakage current peak is about four orders of magnitude lower for the DC HEMT than for the SC HEMT. This is verified for different gate lengths, even though the drain current for the DC HEMT is two times higher than that of the SC HEMT.

Experimental procedure for the evaluation of GaAs-based HBT's reliability

Abstract This work describes the implementation of an experimental procedure to evaluate the reliability of Heterojunction Bipolar Transistors (HBT) on a GaAs substrate. It is based on the separation of aging test accelerating factors applied on two test vehicles: HBT and Transmission Line Model (TLM) structures associated with emitter, base and collector layers. To identify the physical origin of the degradation mechanism, analysis techniques are used: EDX, SEM and TEM observations for which a new sample preparation method has been worked out. Three different technological fabrication processes, are investigated: AlGaAs/GaAs double-mesa HBT, GaInP/GaAs self-aligned HBT and GaInP/GaAs fully planar HBT. These investigations have revealed two major failure mechanisms: the degradation of SiN–GaAs interface correlated with the increase of emitter-to-base leakage current of HBT submitted to combined bias and temperature stresses; the detachment of Ge/Mo/W emitter ohmic contact related to the base and collector current decrease for high level injection in forward regime. A 2D simulation has lead to modify the interface electrical properties to evaluate the impact of the two degradations on HBT dc characteristics.

Analysis of the surface base current drift in GaAs HBT's

A field-induced degradation mechanism responsible for the surface base current drift in GaAs HBTs is studied on the basis of accelerated life-tests : two stress conditions are applied to HBTs and associated TLM structures. An EDX analysis performed on the base ohmic contact confirms the migration of metallic species which are assessed to spread into the extrinsic base region of AlGaAs/GaAs HBTs. The self-passivated GaInP/GaAs technology is expected to improve the HBT reliability.

ML Time-Frequency Synchronization for MIMO-OFDM Systems in Unknown Frequency Selective Fading Channels

This paper addresses maximum-likelihood (ME) time-frequency synchronization algorithm joint with channel estimation for MIMO-OFDM systems in frequency selective fading channels. In proposed algorithm, we use two steps to maximize a proposed metric to obtain first frequency offset and then timing. Using these two estimates, the channel is identified. An iterative algorithm is proposed to improve frequency offset estimation. The performance of the proposed synchronization approach, in terms of timing failure probability and mean square error of the estimated frequency offset, was compared with literature. Based on simulation results, the Cramer-Rao lower bound is obtained with a few iterations

2-Dimensional Optical CDMA System Performance with Parallel Interference Cancellation

In this paper, we investigate the feasibility of a 2 Dimensional Optical Code Division Multiple Access system (2DOCDMA), with electronic coding and decoding functions. We present a modified construction method of Multi-Wavelength Optical Orthogonal Codes (MWOOC) which permits high flexibility for the code parameters choice. Our objective is to evaluate the 2D code performance, with a Conventional Correlation Receiver (CCR) and a more complex one named Parallel Interference Cancellation receiver (PIC). For example, for a Bit Error Rate (BER)10-9, 30 simultaneous users and an electrical implementation, we show that contrary to the CCR, the use of a PIC receiver leads to workable 2D code solutions.

Analysis of GaAs HBT failure mechanisms: impact on life test strategy

A field-induced degradation mechanism responsible for the surface current drift in GaAs HBT is identified on the basis of accelerated ageing tests under bias. Degradations of ohmic contact and metallisation are highlighted under high temperature storage. These results bring further evidence of both bias and temperature-induced degradation mechanisms in GaAs HBTs. As a consequence, a specific life test strategy similar to that implemented for FETs has been derived.

2D Optical CDMA Technique Dedicated to Multimedia Transmission

In this paper, we investigate the application of optical code division multiple access (OCDMA) to multimedia transmission (voice, data, video). The various services are differentiated by assigning to each user a specific two-dimensional (2D) code signature. The coding method used is adapted from 2D multi-wavelength optical orthogonal codes (MWOOC). We first generate 2D multi-weight codes to differentiate the quality of service (QoS). Then, by applying a parallel mapping technique, we achieve multi-rate services. The theoretical error probability corresponding to the quality of each service is established in the noiseless case. This permits evaluating the potentiality of such systems in terms of number of communicating users and data rates. This study has been realized considering a conventional correlation receiver (CCR) with and without hard limiting (HL) device.

2-Dimensional code design for an optical CDMA system with a parallel interference cancellation receiver

The objective of this paper is to design a two-Dimensional Optical Code Division Multiple Access system (2D-OCDMA) for application in access networks, with coding and decoding functions performed by electronic devices. We present a new construction method of Multi-Wavelength Optical Orthogonal Codes (MWOOC), which permits a high flexibility in the code parameters choice. This work evaluates in the noiseless case, the MWOOC potentialities for two receiver structures: a Conventional Correlation Receiver (CCR) and a Parallel Interference Cancellation receiver (PIC). We show that with a PIC receiver, it is possible to design two-Dimensional codes that respect the access specifications. Copyright