2-Dimensional optical CDMA system performance with parallel interference cancellation

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Abstract

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

Introduction

The OCDMA (Optical Code Division Multiple Access) inspired from radio frequency communications, is nowadays studied for application in optical networks, such as: Local Area Networks (LANs) [1] and optical wireless communications [2].

The multiple access method is based on spread spectrum technique and consists in allocating to each user a specific and distinct code. Different coding methods have already been proposed. The oldest solutions deal with spreading codes in one dimension (1D): temporal [3], [4] or spectral [5], [6].

Since coherent optical systems are costly and difficult to implement, most of the studies concern the non-coherent optical systems. In incoherent system, codes are unipolar and cannot be strictly orthogonal. Thus, one of the main limitations is linked to Multiple Access Interference (MAI). To manage the system performance regarding MAI, the 1D-code lengths have to be large enough. But, in this case, it is difficult to realize an OCDMA system with simple and cheap optical and/or optoelectronic devices.

To overcome this problem, new coding schemes have been proposed, based on the both 1D coding methods (temporal and spectral) simultaneously. These approaches are called two dimensions (2D) coding methods. A 2D code can be viewed as a matrix of dimension (m × n), with m and n related, respectively, to spectral and temporal spreading. Most of the 2D matrix constructions are issued from the 1D code families such as: prime/prime [7], OOC (Optical Orthogonal Code)/prime [8], prime/EQC (Extended Quadratic Congruence) [9], OOC/OOC [10], [11].

We focus in this paper on 2D codes based on the OOC/OOC spreading codes called Multi-Wavelength Optical Orthogonal Code (MWOOC) [10], [11], and we develop a new efficient method that permits choosing the code parameters with high flexibility.

The main objective of this work is to evaluate the performance of these codes for high-speed optical links, with a Parallel Interference Cancellation (PIC) receiver, which is a suitable method to mitigate MAI [12]. We particularly investigate the 2D-OCDMA for an optical link with an electrical implementation of coding and decoding functions.

In the first part, the 2D-OCDMA system and the coding method are described. Then, we evaluate the codes performance when a conventional correlation method is used at the receiver end. We investigate in the last part, a PIC receiver. We show that the use of a PIC receiver with 2D codes improves the system performance, and permits respecting the high-speed optical link specifications.

Section snippets

The 2D-OCDMA system

We consider an asynchronous incoherent OCDMA system. Each user employs an On/Off Keying (OOK) modulation to transmit independent and equiprobable binary data upon an optical channel.

Before transmission, data are coded by multiplication with a 2D code matrix. 2D codes are defined by: (L × F,  W, ha, hc) where L is the number of wavelength, F is the temporal code length (the bit period is subdivided in F intervals called chips), W is the weight corresponding to the number of chips set to one, ha and hc

The conventional correlation receiver (CCR)

The CCR (Fig. 3) has the knowledge of the desired user code matrix. At the reception end, each wavelength is separated. The electrical signal corresponding to the optical signal rk, F(t) at wavelength λk is multiplied by the sequence dk,Fj of the desired user #j; then, the resulting signal is integrated over the bit duration. The values obtained for each wavelength are summed. We get the decision variable value of the desired user, which is compared to the threshold level S of the decision

Parallel interference cancellation (PIC) receiver

To improve the performances and so to reduce the constraints on the temporal code length (F) and the number of wavelength (L), we use a PIC receiver [12]. The aim of such receiver is to estimate the interference term due to all interfering users and to remove it from the received signal.

The PIC receiver first detects the Nu  1 interfering users with the CCR defined in the previous part with a threshold level S = ST. Each receiver provides the estimation bˆip of the non-desired user #p data. Next,

Conclusion

We have presented a 2D-OCDMA transmission scheme and a new construction method to obtain MWOOC whose parameters have high flexibility. The new 2D code construction method permits generating code matrices with a number of wavelength Lmin = W and a short temporal length. A CCR and a PIC receiver have been applied to Optical CDMA system with 2D electric coding. From numerical calculation and simulation, we have validated the theoretical analysis. The performance study has shown that the CCR is not

Acknowledgment

This work was supported in part by France Telecom “Recherche et Développement”.

Mikaël Morelle received his Diplôme d’Ingénieur degree from the Institut d’Ingénierie Informatique de Limoges (3IL), University of Limoges, France in 2005. He continued his studies as a doctorate student (Ph.D.) in the research group of the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL). His research work deals with the study of coding techniques applied to optical communications.

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Mikaël Morelle received his Diplôme d’Ingénieur degree from the Institut d’Ingénierie Informatique de Limoges (3IL), University of Limoges, France in 2005. He continued his studies as a doctorate student (Ph.D.) in the research group of the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL). His research work deals with the study of coding techniques applied to optical communications.

Claire Goursaud received her Diplôme d’Ingénieur degree from the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL), University of Limoges, France in 2003. She received the Ph.D. degree in Microwave and Optical Telecommunications in 2006. She is currently in the research group of ENSIL. Her research work deals with the study of the digital signal processing techniques applied to Optical Code Division Multiple Access.

Anne Julien-Vergonjanne received her Ph.D. in Microwave and Optical Communications from the University of Limoges in 1987. She joined the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL), University of Limoges, as Assistant Professor of Electronics and Telecommunications in 1997. Her current research interests deal with digital signal processing for optical communications and optical CDMA.

Christelle Aupetit-Berthelemot received her Diplôme d’Ingénieur degree from the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL), University of Limoges, France in 1995. She received her Ph.D. degree from University of Limoges and Optical Communications from the University of Limoges in 1998. She is now an assistant-professor in Electronics and Telecommunications at ENSIL, University of Limoges. Her current research interests include optoelectronic devices, fiber-optic communication systems and Optical CDMA systems.

Jean-Pierre Cances graduated in Electrical Engineering from Ecole Nationale Supérieure des Télécommunications (ENST) Bretagne in 1990. He received his Ph.D. degree from ENST Paris in satellite communications engineering in 1993. He is now an assistant professor at the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL), University of Limoges. His current research interests include satellite communication systems, CDMA multiuser detection, multicarrier modulation and synchronization algorithms.

Jean-Michel Dumas received his Diplôme d’Ingénieur degree from the Institut National des Sciences Appliquées of Toulouse, University of Toulouse, in 1973 and the Doctorat ès-Sciences Physiques degree from the University of Limoges in 1985. He was a member of the technical staff of France Télécom/Centre National d’Etudes des Télécommunications (FT.CNET) in Lannion till 1994. Then, he joined the Ecole Nationale Supérieure d’Ingénieurs de Limoges (ENSIL), University of Limoges, as Professor of Electronics and Telecommunications. At ENSIL, he leads a research group involved in the insertion of high-speed devices into systems and the simulation of digital communication systems.

Philippe Guignard received his engineer degree from the Ecole Nationale Supérieure des Télécommunications de Bretagne, Brest, France, in 1983. He joined the Centre National d’Etudes des Télécommunications (CNET), Lannion, France, in September 1983, where he has been working on the introduction of optical fibre in LANs and in the access networks. In 1988, he received his PhD degree from the Université de Limoges (France). He has been mainly involved in mid and long-term studies on the use of WDM technologies and of optical ultrashort pulses in access networks. He is currently working in France Telecom Research and Development (FTR&D), on access and home networks.

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