18 research outputs found

    A Comparative Study of Asynchronous and Synchronous OCDMA Systems

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    Minimizing information asymmetry interference using optimal channel assignment strategy in wireless mesh networks

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    Multi-radio multi-channel wireless mesh networks (MRMC-WMNs) in recent years are considered as the prioritized choice for users due to its low cost and reliability. MRMCWMNs is recently been deployed widely across the world but still these kinds of networks face interference problems among WMN links. One of the well-known interference issue is information asymmetry (IA). In case of information asymmetry interference the source mesh nodes of different mesh links cannot sense each other before transmitting data on the same frequency channel. This non-coordination leads to data collision and packet loss of data flow and hence degrades the network capacity. To maximize the MRMC-WMN capacity and minimize IA interference, various schemes for optimal channel assignment have been proposed already. In this research a novel and near-optimal channel assignment model called Information Asymmetry Minimization (IAM) model is proposed based on integer linear programming. The proposed IAM model optimally assigns orthogonal or non-overlapping channels from IEEE 802.11b technology to various MRMC-WMN links. Through extensive simulations we show that our proposed model gives 28.31% network aggregate network capacity improvement over the existing channel assignment model

    Applications of perfect difference codes in fiber-optics and wireless optical code-division multiplexing/multiple-access systems

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    After establishing itself in the radio domain, Spread spectrum code-division multiplexing/multiple-access (CDMA) has seen a recent upsurge in optical domain as well. Due to its fairness, flexibility, service differentiation and increased inherent security, CDMA is proved to be more suitable for the bursty nature of local area networks than synchronous multiplexing techniques like Frequency/Wavelength Division Multiplexing (F/WDM) and Time Division Multiplexing (TDM). In optical domain, CDMA techniques are commonly known as Optical-CDMA (O-CDMA). All optical CDMA systems are plagued with the problem of multiple-access interference (MAI). Spectral amplitude coding (SAC) is one of the techniques used in the literature to deal with the problem of MAI. The choice of spreading code in any CDMA system is another way to ensure the successful recovery of data at the receiving end by minimizing the effect of MAI and it also dictates the hardware design of the encoder and decoder. This thesis focuses on the efficient design of encoding and decoding hardware. Perfect difference codes (PDC) are chosen as spreading sequences due to their good correlation properties. In most of the literature, evaluation of error probability is based on the assumptions of ideal conditions. Such assumptions ignore major physical impairments such as power splitting losses at the multiplexers of transmitters and receivers, and gain losses at the receivers, which may in practice be an overestimate or underestimate of the actual probability of error. This thesis aims to investigate thoroughly with the consideration of practical impairments the applications of PDCs and other spreading sequences in optical communications systems based on spectral-amplitude coding and utilizing codedivision as multiplexing/multiple-access technique. This work begins with a xix general review of optical CDMA systems. An open-ended practical approach has been used to evaluate the actual error probabilities of OCDM/A systems under study. It has been concluded from results that mismatches in the gains of photodetectors, namely avalanche photodiode (APDs), used at the receiver side and uniformity loss in the optical splitters results in the inaccurate calculation of threshold level used to detect the data and can seriously degrade the system bit error rate (BER) performance. This variation in the threshold level can be compensated by employing techniques which maintain a constant interference level so that the decoding architecture does not have to estimate MAI every time to make a data bit decision or by the use of balanced sequences. In this thesis, as a solution to the above problem, a novel encoding and decoding architecture is presented for perfect difference codes based on common zero code technique which maintains a constant interference level at all instants in CDM system and thus relieves the need of estimating interference. The proposed architecture only uses single multiplexer at the transmitters for all users in the system and a simple correlation based receiver for each user. The proposed configuration not only preserves the ability of MAI in Spectral-Amplitude Coding SAC-OCDM system, but also results in a low cost system with reduced complexity. The results show that by using PDCs in such system, the influence of MAI caused by other users can be reduced, and the number of active users can be increased significantly. Also a family of novel spreading sequences are constructed called Manchestercoded Modified Legendre codes (MCMLCs) suitable for SAC based OCDM systems. MCMLCs are designed to be used for both single-rate and Multirate systems. First the construction of MCMLCs is presented and then the bit error rate performance is analyzed. Finally the proposed encoding/decoding architecture utilizing perfect difference codes is applied in wireless infrared environment and the performance is found to be superior to other codes

    Two-dimensional permutation vectors’ (PV) code for optical code division multiple access systems

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    In this paper, we present a new algorithm to generate two-dimensional (2D) permutation vectors’ (PV) code for incoherent optical code division multiple access (OCDMA) system to suppress multiple access interference (MAI) and system complexity. The proposed code design approach is based on wavelength-hopping time-spreading (WHTS) technique for code generation. All possible combinations of PV code sets were attained by employing all permutations of the vectors with repetition of each vector weight (W) times. Further, 2D-PV code set was constructed by combining two code sequences of the 1D-PV code. The transmitter-receiver architecture of 2D-PV code-based WHTS OCDMA system is presented. Results indicated that the 2D-PV code provides increased cardinality by eliminating phase-induced intensity noise (PIIN) effects and multiple user data can be transmitted with minimum likelihood of interference. Simulation results validated the proposed system for an agreeable bit error rate (BER) of 10−9

    High Dimensional Modulation and MIMO Techniques for Access Networks

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    Exploration of advanced modulation formats and multiplexing techniques for next generation optical access networks are of interest as promising solutions for delivering multiple services to end-users. This thesis addresses this from two different angles: high dimensionality carrierless amplitudephase (CAP) and multiple-input multiple-output (MIMO) radio-over-fiber (RoF) systems. High dimensionality CAP modulation has been investigated in optical fiber systems. In this project we conducted the first experimental demonstration of 3 and 4 dimensional CAP with bit rates up to 10 Gb/s. These results indicate the potentiality of supporting multiple users with converged services. At the same time, orthogonal division multiple access (ODMA) systems for multiple possible dimensions of CAP modulation has been demonstrated for user and service allocation in wavelength division multiplexing (WDM) optical access network. 2 x 2 MIMO RoF employing orthogonal frequency division multiplexing (OFDM) with 5.6 GHz RoF signaling over all-vertical cavity surface emitting lasers (VCSEL) WDM passive optical networks (PONs). We have employed polarization division multiplexing (PDM) to further increase the capacity per wavelength of the femto-cell network. Bit rate up to 1.59 Gbps with fiber-wireless transmission over 1 m air distance is demonstrated. The results presented in this thesis demonstrate the feasibility of high dimensionality CAP in increasing the number of dimensions and their potentially to be utilized for multiple service allocation to different users. MIMO multiplexing techniques with OFDM provides the scalability in increasing spectral effciency and bit rates for RoF systems. High dimensional CAP and MIMO multiplexing techniques are two promising solutions for supporting wired and hybrid wired-wireless access networks

    Overlapped CDMA system in optical packet networks : resource allocation and performance evalutation

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    Dans cette thĂšse, la performance du systĂšme CDMA Ă  chevauchement optique (OVCDMA) au niveau de la couche de contrĂŽle d'accĂšs au support (MAC) et l'allocation des ressources au niveau de la couche physique (PRY) sont Ă©tudiĂ©es. Notre but est d'apporter des amĂ©liorations pour des applications Ă  dĂ©bits multiples en rĂ©pondant aux exigences de dĂ©lai minimum tout en garantissant la qualitĂ© de service (QoS). Nous proposons de combiner les couches PRY et MAC par une nouvelle approche d'optimisation de performance qui consolide l'efficacitĂ© potentielle des rĂ©seaux optiques. Pour atteindre notre objectif, nous rĂ©alisons plusieurs Ă©tapes d'analyse. Tout d 'abord, nous suggĂ©rons le protocole S-ALOHA/OV-CDMA optique pour sa simplicitĂ© de contrĂŽler les transmissions optiques au niveau de la couche liaison. Le dĂ©bit du rĂ©seau, la latence de transmission et la stabilitĂ© du protocole sont ensuite Ă©valuĂ©s. L'Ă©valuation prend en considĂ©ration les caractĂ©ristiques physiques du systĂšme OY-CDMA, reprĂ©sentĂ©es par la probabilitĂ© de paquets bien reçus. Le systĂšme classique Ă  traitement variable du gain (YPG) du CDMA, ciblĂ© pour les applications Ă  dĂ©bits multiples, et le protocole MAC ±round-robinÂż rĂ©cepteur/Ă©metteur (R31), initialement proposĂ© pour les rĂ©seaux par paquets en CDMA optique sont Ă©galement pris en compte. L'objectif est d ' Ă©valuer comparativement la performance du S-ALOHA/OY-CDMA en termes de l'immunitĂ© contre l'interfĂ©rence d'accĂšs lTIultiple (MAI) et les variations des charges du trafic. Les rĂ©sultats montrent que les performances peuvent varier en ce qui concerne le choix du taux de transmission et la puissance de transmission optique au niveau de la couche PRY. Ainsi, nous proposons un schĂ©ma de rĂ©partition optimale des ressources pour allouer des taux de transmission Ă  chevauchement optique et de puissance optique de transmission dans le systĂšme OY-CDMA comme des ressources devant ĂȘtre optimalement et Ă©quitablement rĂ©parties entre les utilisateurs qui sont regroupĂ©s dans des classes de diffĂ©rentes qualitĂ©s de service. La condition d'optimalitĂ© est basĂ©e sur la maximisation de la capacitĂ© par utilisateur de la couche PHY. De ce fait, un choix optimal des ressources physiques est maintenant possible, mais il n'est pas Ă©quitable entre les classes. Par consĂ©quent, pour amĂ©liorer la performance de la couche liaison tout en Ă©liminant le problĂšme d'absence d'Ă©quitĂ©, nous proposons comme une approche unifiĂ©e un schĂ©ma Ă©quitable et optimal pour l'allocation des ressources fondĂ© sur la qualitĂ© de service pour des multiplexages temporels des rĂ©seaux par paquets en CDMA Ă  chevauchement optique. Enfin, nous combinons cette derniĂšre approche avec le protocole MAC dans un problĂšme d'optimisation d'allocation Ă©quitable des ressources Ă  contrainte de dĂ©lai afin de mieux amĂ©liorer le dĂ©bit du rĂ©seau et le dĂ©lai au niveau de la couche liaison avec allocation Ă©quitable et optimale des ressources au niveau de la couche PHY

    Performance improvement of SS-WDM passive optical networks using semiconductor optical amplifiers: Modeling and experiment

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    Les sources incohĂ©rentes sont proposĂ©es comme alternatives aux lasers stabilisĂ©s en longueur d'onde pour rĂ©duire le coĂ»t des rĂ©seaux optiques passifs utilisant le multiplexage par longueur d'onde dĂ©coupĂ©e dans le spectre (SS-WDM PONs). À cause de leur nature incohĂ©rente, ces sources gĂ©nĂšrent au rĂ©cepteur un large bruit d'intensitĂ©. Ce bruit limite l'efficacitĂ© spectrale et/ou le taux binaire pouvant ĂȘtre achevĂ©. Cette thĂšse Ă©tudie l'utilisation des amplificateurs optique Ă  semi-conducteur SOAs pour nettoyer le bruit d'intensitĂ©. De plus, lors de cette thĂšse, nous explorons les outils numĂ©riques et expĂ©rimentaux qui nous permettent d'analyser les performances des SOAs dans le cadre de systĂšmes de communication multi-canaux, incluant le SS-WDM. Nous prĂ©sentons des modĂšles mathĂ©matiques pour le bruit d'intensitĂ©, ce bruit Ă©tant celui qui limite les performances des systĂšmes de communication utilisant des sources incohĂ©rentes. Nous discutons les dynamiques complexes des SOAs et prĂ©sentons les Ă©quations qui gouvernent l'Ă©volution des porteurs de charges dans ces amplificateurs. Nous identifions et soulignons l'effet des paramĂštres les plus importants, qui affectent le processus ainsi que la dynamique de nettoyage du bruit d'intensitĂ©. Nous passons en revue, les diffĂ©rentes techniques de nettoyage de bruit avec les SOAs, qui ont dĂ©montrĂ© les meilleurs rĂ©sultats connus. De plus, nous effectuons une revue de littĂ©rature poussĂ©e pour ce qui a attrait au problĂšme de post-filtrage lorsque le SOA est placĂ© au transmetteur, avant la modulation. Notre premiĂšre contribution pendant ce travail de recherche est de dĂ©montrer, en utilisant l'intermodulation de gain d'un SOA au rĂ©cepteur pour convertir le signal incohĂ©rent en laser cohĂ©rent, une amĂ©lioration significative du taux d'erreur binaire BER. Cette mĂ©thode est spectralement efficace, d'autant plus qu'elle ne souffre point la limitation occasionnĂ©e par le post-filtrage au rĂ©cepteur. En contre partie elle nĂ©cessite un ample budget de puissance qui doit assurer une saturation suffisante de l'amplificateur au rĂ©cepteur. Une source laser est aussi nĂ©cessaire au rĂ©cepteur. Ceci est un inconvĂ©nient, mĂȘme si une telle source n'ait pas besoin d'une quelconque stabilisation. Pour contourner le problĂšme causĂ© par le post-filtrage quand le SOA est au transmetteur, nous proposons un nouveau rĂ©cepteur pour les systĂšmes de communication WDM, qui permet de mieux garder le nettoyage de bruit, et ce malgrĂ© le filtrage optique au rĂ©cepteur. La nouvelle mĂ©thode consiste en un dĂ©tecteur balancĂ© utilisĂ© au rĂ©cepteur: d'un bord, tous les canaux sont dĂ©tectĂ©s sans distinction. De l'autre, le signal dĂ©sirĂ© est bloquĂ© pendant que tous les autres canaux sont dĂ©tectĂ©s. Avec cette mĂ©thode, il est facile de saturer l'amplificateur pour une meilleure suppression de bruit tout en Ă©vitant en grande partie la dĂ©gradation causĂ© par le post-filtrage. Nous avons expĂ©rimentalement dĂ©montrĂ© un systĂšme WDM dense de 8 x 10 Gbps avec une source incohĂ©rente et un SOA en saturation. Une autre contribution originale de ce travail est le dĂ©veloppement d'un outil de simulation pour les SOAs qui est numĂ©riquement plus efficace et lĂ©ger que les modĂšles connus Ă  ce jour. Nous avons donc dĂ©veloppĂ© un modĂšle thĂ©orique simple, pouvant ĂȘtre implĂ©mentĂ© par diagramme block, dans le but de simuler les performances des hens de communications WDM. Notre modĂšle dĂ©montre une bonne concordance avec les rĂ©sultats expĂ©rimentaux ainsi qu'une diminution de temps de calcul de l'ordre de 20 fois. Finalement, lors de la derniĂšre partie de ces travaux, nous nous sommes occupĂ©s de mesurer, de façon prĂ©cise, le temps de recouvrement du gain dans un SOA. Le temps de recouvrement des porteurs dans un SOA est un des paramĂštres les plus importants qui sont Ă  l'origine du phĂ©nomĂšne de nettoyage de bruit et qui rĂ©gissent le comportement ainsi que les dynamiques de l'amplificateur. Nous avons Ă©tudiĂ© en particulier, la dĂ©pendance de ce temps de recouvrement r de la longueur d'onde. Pour le SOA utilisĂ© lors de notre Ă©tude expĂ©rimentale, nous avons dĂ©montrĂ© que r dĂ©pendait de la longueur d'onde de façon similaire au spectre de gain. Ces mesures ont Ă©tĂ© possibles grĂące au dĂ©veloppement d'un nouveau dispositif de mesure pompe/sonde, qui permettait de mesurer le recouvrement du gain pour une pompe et une sonde Ă  la mĂȘme longueur d'onde et ayant le mĂȘme Ă©tat de polarisation

    Enabling Technology in Optical Fiber Communications: From Device, System to Networking

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    This book explores the enabling technology in optical fiber communications. It focuses on the state-of-the-art advances from fundamental theories, devices, and subsystems to networking applications as well as future perspectives of optical fiber communications. The topics cover include integrated photonics, fiber optics, fiber and free-space optical communications, and optical networking
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