Investigation on optical code division multiplex access (OCDMA)

After successful application of CDMA techniques in wireless communications, optical CDMA has been considered as a candidate for future access networks. We will focus on passive optical networks (PONs) in particular. This work pursues three main goals: First, due to the depth and extension of the topic, offer a comprehensive overview. Several classes of optical codes are reviewed and different technologies for implementing codes are described and compared. Second, provide some guidelines in order to facilitate the design of OCDMA-based PONs. Since OCDMA allows flexible network design, different approaches can be considered. Finally, outline a possible solution, discussing and analyzing key issues as optical coding and encoding/decoding hardware. System performance or network architecture are also studied

The application of SOA for dispersion management of 2D-WH/TS codesin incoherent OCDMA system

In high data rate optical fibre communication networks, dispersion phenomenon plays a pivotal role. It is important to investigate the dispersion effects in a multi-wavelength picosecond optical code division multiple access (OCDMA) system. This research is focused on the analysis of the effects of fibre dispersion on the OCDMA autocorrelation; and how these effects can be resolved in a tuneable way so that the originally recovered OCDMA autocorrelation function at the decoder receiver can be revived without further manual adjustment of fibre (SMF-28) cable lengths.;The environmental effects and the subsequent mitigation process are also investigated further in this research. The chirp in OCDMA is examined experimentally and analytically in an initiative to find the more in-depth understanding of finely tuneable chromatic dispersion (CD) compensation technique in a coarsely compensated link by using semiconductor optical amplifier (SOA). A practical investigation was carried over a partially CD compensated 17 km bidirectional testbed between the University of Strathclyde and the University of Glasgow to perform the fine-tuning of CD adjustment using SOA.;A 19.5 km SMF-28 fibre spool was also used in an environmental chamber to investigate the temperature induced dispersion effects and subsequent mitigation. The tuneable dispersion compensation measures are vital to ensure the high data rate optical communication using an all-optical approach in future data network end-points where the advantages of ultra-high speed optical communication bandwidth are at present disrupted due to opto-electronic conversions commonly known as 'electronic bottlenecks'.In high data rate optical fibre communication networks, dispersion phenomenon plays a pivotal role. It is important to investigate the dispersion effects in a multi-wavelength picosecond optical code division multiple access (OCDMA) system. This research is focused on the analysis of the effects of fibre dispersion on the OCDMA autocorrelation; and how these effects can be resolved in a tuneable way so that the originally recovered OCDMA autocorrelation function at the decoder receiver can be revived without further manual adjustment of fibre (SMF-28) cable lengths.;The environmental effects and the subsequent mitigation process are also investigated further in this research. The chirp in OCDMA is examined experimentally and analytically in an initiative to find the more in-depth understanding of finely tuneable chromatic dispersion (CD) compensation technique in a coarsely compensated link by using semiconductor optical amplifier (SOA). A practical investigation was carried over a partially CD compensated 17 km bidirectional testbed between the University of Strathclyde and the University of Glasgow to perform the fine-tuning of CD adjustment using SOA.;A 19.5 km SMF-28 fibre spool was also used in an environmental chamber to investigate the temperature induced dispersion effects and subsequent mitigation. The tuneable dispersion compensation measures are vital to ensure the high data rate optical communication using an all-optical approach in future data network end-points where the advantages of ultra-high speed optical communication bandwidth are at present disrupted due to opto-electronic conversions commonly known as 'electronic bottlenecks'

Investigation on optical code division multiplex access (OCDMA)

After successful application of CDMA techniques in wireless communications, optical CDMA has been considered as a candidate for future access networks. We will focus on passive optical networks (PONs) in particular. This work pursues three main goals: First, due to the depth and extension of the topic, offer a comprehensive overview. Several classes of optical codes are reviewed and different technologies for implementing codes are described and compared. Second, provide some guidelines in order to facilitate the design of OCDMA-based PONs. Since OCDMA allows flexible network design, different approaches can be considered. Finally, outline a possible solution, discussing and analyzing key issues as optical coding and encoding/decoding hardware. System performance or network architecture are also studied

Investigation of the impact of fibre impairments and SOA-based devices on 2D-WH/TS OCDMA codes

In seeking efficient last-mile solutions for high-capacity, optical code division multiple access (OCDMA) emerges as a promising alternative high-speed optical network that can securely support a multitude of simultaneous users without requiring extensive equipment. This multiplexing technique has recently been the subject of comprehensive research, highlighting its potential for facilitating high-bandwidth multi-access networking. When contrasted with techniques such as wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM), OCDMA offers a more effective and equitable split of available fibre bandwidth among the users. This thesis presents my research focused on the incoherent OCDMA under the influence of optical fibre impairments that uses picosecond multiwavelength pulses to form two-dimensional wavelength hopping time-spreading (2D-WH/TS) incoherent OCDMA codes. In particular, self-phase modulation, temperature induced fibre dispersion, chromatic dispersion, as well as the impact of semiconductor optical amplifier SOA devices deployment on 2D-WH/TS OCDMA code integrity were investigated. These aspects were investigated using a 17-km long bidirectional fibre link between Strathclyde and Glasgow University. In particular, I investigated the impact of temporal skewing among OCDMA code carriers and the importance of selecting small range of wavelengths as code carriers where wide range manifest high dependency on wavelength. This wavelength dependency is exploited furthermore to measure the induced temperature dispersion coefficient accurately and economically. I have conducted experiments to characterise the impact of SOA-device on 2D OCDMA code carries which is evaluated under different bias conditions. This evaluation addressed the potential challenges and ramifications of the gain recovery time of SOA and its wavelength dependency with respect to gain ratio and self-phase modulation (SPM). The OCDMA code was built using multiplexers and delay lines to create a 2D OCDMA code to allow studying the impact of deploying a SOA under different conditions on each wavelength. The concept described above is then extended to the investigation of the SOA’s impact on a 2D-WH/TS OCDMA prime code under high bias current/gain conditions. The overall performance of two different 2D-WH/TS OCDMA systems deploying the SOA was also calculated. I have also investigated the possibility of manipulating chirp in 2D-WH/TS incoherent OCDMA to counteract the self-phase modulation-induced red shift by using single mode fibre and lithium crystals. I have investigated the performance of the picosecond code based optical signal when subjected to temperature variations similar to that experience by most buried fibre systems. I have proposed and demonstrated a novel technique, which I examined analytically and experimentally, that utilises a SOA at the transmitter to create a new code with a new wavelength hopping and spreading time sequences to achieve a unique physical improved secure incoherent OCDMA communication method. A novel fully automated tuneable compensation testbed is also proposed of an autonomous dispersion management in a WH/TS incoherent OCDMA system. The system proposed manipulates the chirp of OCDMA code carriers to limit chromatic dispersion detrimental effect on transmission systems.In seeking efficient last-mile solutions for high-capacity, optical code division multiple access (OCDMA) emerges as a promising alternative high-speed optical network that can securely support a multitude of simultaneous users without requiring extensive equipment. This multiplexing technique has recently been the subject of comprehensive research, highlighting its potential for facilitating high-bandwidth multi-access networking. When contrasted with techniques such as wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM), OCDMA offers a more effective and equitable split of available fibre bandwidth among the users. This thesis presents my research focused on the incoherent OCDMA under the influence of optical fibre impairments that uses picosecond multiwavelength pulses to form two-dimensional wavelength hopping time-spreading (2D-WH/TS) incoherent OCDMA codes. In particular, self-phase modulation, temperature induced fibre dispersion, chromatic dispersion, as well as the impact of semiconductor optical amplifier SOA devices deployment on 2D-WH/TS OCDMA code integrity were investigated. These aspects were investigated using a 17-km long bidirectional fibre link between Strathclyde and Glasgow University. In particular, I investigated the impact of temporal skewing among OCDMA code carriers and the importance of selecting small range of wavelengths as code carriers where wide range manifest high dependency on wavelength. This wavelength dependency is exploited furthermore to measure the induced temperature dispersion coefficient accurately and economically. I have conducted experiments to characterise the impact of SOA-device on 2D OCDMA code carries which is evaluated under different bias conditions. This evaluation addressed the potential challenges and ramifications of the gain recovery time of SOA and its wavelength dependency with respect to gain ratio and self-phase modulation (SPM). The OCDMA code was built using multiplexers and delay lines to create a 2D OCDMA code to allow studying the impact of deploying a SOA under different conditions on each wavelength. The concept described above is then extended to the investigation of the SOA’s impact on a 2D-WH/TS OCDMA prime code under high bias current/gain conditions. The overall performance of two different 2D-WH/TS OCDMA systems deploying the SOA was also calculated. I have also investigated the possibility of manipulating chirp in 2D-WH/TS incoherent OCDMA to counteract the self-phase modulation-induced red shift by using single mode fibre and lithium crystals. I have investigated the performance of the picosecond code based optical signal when subjected to temperature variations similar to that experience by most buried fibre systems. I have proposed and demonstrated a novel technique, which I examined analytically and experimentally, that utilises a SOA at the transmitter to create a new code with a new wavelength hopping and spreading time sequences to achieve a unique physical improved secure incoherent OCDMA communication method. A novel fully automated tuneable compensation testbed is also proposed of an autonomous dispersion management in a WH/TS incoherent OCDMA system. The system proposed manipulates the chirp of OCDMA code carriers to limit chromatic dispersion detrimental effect on transmission systems

Dark signalling and code division multiple access in an optical fibre LAN with a bus topology

This thesis describes an optical fibre network that uses a bus topology and Code Division Multiple Access (CDMA). Various potential configurations are analysed and compared and it is shown that a serious limitation of optical CDMA schemes using incoherent correlators is the effect of optical beating due to the presence of multiple incoherent optical signals at the receiver photodiode. The network proposed and analysed in this thesis avoids beating between multiple optical fields because it only uses a single, shared, optical source. It does this through the SLIM (Single Light-source with In-line Modulation) configuration in which there is a continuously-operating light source at the head-end of a folded bus, and modulators at the nodes to impose signals on the optical field in the form of pulses of darkness which propagate along the otherwise continuously bright bus. Optical CDMA can use optical-fibre delay-line correlators as matched filters, and these may be operated either coherently or incoherently.Coherent operation is significantly more complex than incoherent operation, but incoherent correlators introduce further beating even in a SLIM network. A new design of optical delay-line correlator, the hybrid correlator, is therefore proposed, analysed and demonstrated. It is shown to eliminate beating. A model of a complete network predicts that a SLIMbus using optical CDMA with hybrid correlators can be operated at TeraBaud rates with the number of simultaneous users limited by multiple access interference (MAI), determined only by the combinatorics of the code set

Optical code division multiple access systems in AlGaInAs/InP

The rise of photonic integration makes optical code division multiple access (OCDMA) worth revisiting due to its promising role in future all-optical networks. OCDMA has the potential to exploit the surplus bandwidth of optical fibres and to carry over to the optical domain the benefits seen CDMA radio communication systems, such as the effective sharing of the spectrum for multiple network subscribers, and resistance to jamming and eavesdropping. One of the major requirements for the deployment of OCDMA in networks is integration. This thesis presents a research study of integrated OCDMA systems using the AlGaInAs/InP semiconductor material system. This material is considered due to its useful intrinsic properties such as thermal stability, strong electron confinement, and low threshold, making it suitable for fabricating optoelectronic devices. Two bespoke OCDMA systems are considered for integration: coherent temporal phase coding (TPC), and incoherent wavelength-hopping time-spreading (WHTS) OCDMA systems. TPC systems are excellent for high speed communications due to their static en/decoding enabling features. In this research, a 2×2 asymmetric Mach Zehnder interferometer (AMZI) is used to generate a 2-bit phase code, allowing multiplexing for up to four users. A semiconductor mode-locked ring laser is also embedded in the circuit, and using a synchronous mode-locking method, adequate signal en/decoding is achieved. WHTS systems on the other hand fully exploit the spectral and temporal space available in networks by assigning each user with a unique wavelength-time hop sequence for en/decoding data signals. Here, a mode-locked laser array is used with intracavity distributed Bragg reflectors (DBRs) for spectrally tuning each laser, and a 4:1 multimode interference coupler is used to combine the laser signals into a single channel for amplification, modulation and transmission. The integrated system is fully characterised and synchronisation experiments are performed to show the potential for its use in high speed multi-user networks. Mode-locked lasers play an important role in many OCDMA implementations due to their wide spectrum and discrete temporal properties, which can be easily exploited during data en/decoding. Various mode-locked laser devices have been studied during this research with additional embedded components such as intracavity DBRs and phase controllers for precise tuning of the wavelength and pulse repetition frequency. However, the noisy nature of passively operating mode-locked lasers make them prone to high jitter, which can result in high bit error rates. Synchronisation schemes are thereby explored in order to temporally stabilise the pulse oscillations to make them suitable for use in long haul transmission systems. This includes synchronous and hybrid mode-locking, as well as a passive technique using an optical fibre loop to provide phase feedback, which is shown to promote ultralow RF linewidths in mode-locked lasers

A NOVEL CONSTRUCTION OF VECTOR COMBINATORIAL (VC) CODE FAMILIES AND DETECTION SCHEME FOR SAC OCDMA SYSTEMS

There has been growing interests in using optical code division multiple access (OCDMA) systems for the next generation high-speed optical fiber networks. The advantage of spectral amplitude coding (SAC-OCDMA) over conventional OCDMA systems is that, when using appropriate detection technique, the multiple access interference (MAI) can totally be canceled. The motivation of this research is to develop new code families to enhance the overall performance of optical OCDMA systems. Four aspects are tackled in this research. Firstly, a comprehensive discussion takes place on all important aspects of existing codes from advantages and disadvantages point of view. Two algorithms are proposed to construct several code families namely Vector Combinatorial (VC). Secondly, a new detection technique based on exclusive-OR (XOR) logic is developed and compared to the reported detection techniques. Thirdly, a software simulation for SAC OCDMA system with the VC families using a commercial optical system, Virtual Photonic Instrument, “VPITM TransmissionMaker 7.1” is conducted. Finally, an extensive investigation to study and characterize the VC-OCDMA in local area network (LAN) is conducted. For the performance analysis, the effects of phase-induced intensity noise (PIIN), shot noise, and thermal noise are considered simultaneously. The performances of the system compared to reported systems were characterized by referring to the signal to noise ratio (SNR), the bit error rate (BER) and the effective power (Psr). Numerical results show that, an acceptable BER of 10−9 was achieved by the VC codes with 120 active users while a much better performance can be achieved when the effective received power Psr > -26 dBm. In particular, the BER can be significantly improved when the VC optimal channel spacing width is carefully selected; best performance occurs at a spacing bandwidth between 0.8 and 1 nm. The simulation results indicate that VC code has a superior performance compared to other reported codes for the same transmission quality. It is also found that for a transmitted power at 0 dBm, the BER specified by eye diagrams patterns are 10-14 and 10-5 for VC and Modified Quadratic Congruence (MQC) codes respectively

A NOVEL CONSTRUCTION OF VECTOR COMBINATORIAL (VC) CODE FAMILIES AND DETECTION SCHEME FOR SAC OCDMA SYSTEMS

There has been growing interests in using optical code division multiple access (OCDMA) systems for the next generation high-speed optical fiber networks. The advantage of spectral amplitude coding (SAC-OCDMA) over conventional OCDMA systems is that, when using appropriate detection technique, the multiple access interference (MAI) can totally be canceled. The motivation of this research is to develop new code families to enhance the overall performance of optical OCDMA systems. Four aspects are tackled in this research. Firstly, a comprehensive discussion takes place on all important aspects of existing codes from advantages and disadvantages point of view. Two algorithms are proposed to construct several code families namely Vector Combinatorial (VC). Secondly, a new detection technique based on exclusive-OR (XOR) logic is developed and compared to the reported detection techniques. Thirdly, a software simulation for SAC OCDMA system with the VC families using a commercial optical system, Virtual Photonic Instrument, “VPITM TransmissionMaker 7.1” is conducted. Finally, an extensive investigation to study and characterize the VC-OCDMA in local area network (LAN) is conducted. For the performance analysis, the effects of phase-induced intensity noise (PIIN), shot noise, and thermal noise are considered simultaneously. The performances of the system compared to reported systems were characterized by referring to the signal to noise ratio (SNR), the bit error rate (BER) and the effective power (Psr). Numerical results show that, an acceptable BER of 10−9 was achieved by the VC codes with 120 active users while a much better performance can be achieved when the effective received power Psr > -26 dBm. In particular, the BER can be significantly improved when the VC optimal channel spacing width is carefully selected; best performance occurs at a spacing bandwidth between 0.8 and 1 nm. The simulation results indicate that VC code has a superior performance compared to other reported codes for the same transmission quality. It is also found that for a transmitted power at 0 dBm, the BER specified by eye diagrams patterns are 10-14 and 10-5 for VC and Modified Quadratic Congruence (MQC) codes respectively

Fiber-optic code division multiple access : multi-class optical orthogonal codes, optical power control, and polarization encoding

Ever since the mid- 1980s when the single-mode fiber-optic media were believed to become the main highways of future telecommunications networks for transporting high-volume high-quality multipurpose information, the need for all-optical multi-access networking became important. An all-optical multi-access network is a collection of multiple nodes where the interconnection among various nodes is via single- or multi-mode fiber optics and for which they perform all their essential signal processing requirements such as switching, add-drop, multiplexing/demultiplexing and amplification in the optical domain. Optical CDMA networking is one possible technique that allows multiple users in local area networks to access the same fiber channel asynchronously with no delay or scheduling. Optical CDMA networks are not without their own problems. Search for codes suitable to the optical domain is one of the important topics addressed in the literature on optical CDMA. Existing codes developed in the late 80's are limited to single class traffic or can support multiclass traffic but with restrictions on code lengths and weights. Also the number of generated codes is severely limited due to orthogonality issues. In this thesis, we pay particular attention to propose new codes that can support multiclass traffic with arbitrary code weights and lengths. Therefore, data sources with varying traffic demands can be accommodated by optical CDMA networks using the proposed codes. We also present a simple generation technique for the proposed multiclass codes and analyze their performance. The number of users supported by the proposed multiclass codes will be limited since it is an extension of existing code designs with such limitation. We then propose the use of polarization dimension in order to double the number of supported users. On the other hand, incoherent optical CDMA systems are considered as positive systems meaning that only unipolar codes can be considered for such systems. Therefore, multiple access interference will be quite high in optical CDMA due to the nature of incoherent power detection. Reducing the effect of the interference on the performance of optical CDMA is an important topic. We propose the use of power control to decrease the effects of interference in optical star networks in which users' fiber lengths and data rates are not equal. We consider the case of optically amplified network with amplifier noise as the main source. We then elaborate by considering the nonlinearity in the photodetection process and propose the use of an iterative algorithm to find the solution of the non-linear optical power control problem. Finally, we propose an optical CDMA system based on polarization encoding. Since the encoding is performed in the spatial domain, therefore, positive and negative levels can be realized. This approach leads to increasing the number of supported users of optical CDMA by the use of known codes, such as Gold and Hadamard codes, with enhanced performance.reviewe

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

Dans cette thèse, la performance du système CDMA à chevauchement optique (OVCDMA) au niveau de la couche de contrôle d'accès au support (MAC) et l'allocation des ressources au niveau de la couche physique (PRY) sont étudiées. Notre but est d'apporter des améliorations pour des applications à débits multiples en répondant aux exigences de délai minimum tout en garantissant la qualité de service (QoS). Nous proposons de combiner les couches PRY et MAC par une nouvelle approche d'optimisation de performance qui consolide l'efficacité potentielle des réseaux optiques. Pour atteindre notre objectif, nous réalisons plusieurs étapes d'analyse. Tout d 'abord, nous suggérons le protocole S-ALOHA/OV-CDMA optique pour sa simplicité de contrôler les transmissions optiques au niveau de la couche liaison. Le débit du réseau, la latence de transmission et la stabilité du protocole sont ensuite évalués. L'évaluation prend en considération les caractéristiques physiques du système OY-CDMA, représentées par la probabilité de paquets bien reçus. Le système classique à traitement variable du gain (YPG) du CDMA, ciblé pour les applications à débits multiples, et le protocole MAC ±round-robin¿ récepteur/émetteur (R31), initialement proposé pour les réseaux par paquets en CDMA optique sont également pris en compte. L'objectif est d ' évaluer comparativement la performance du S-ALOHA/OY-CDMA en termes de l'immunité contre l'interférence d'accès lTIultiple (MAI) et les variations des charges du trafic. Les résultats 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 schéma de répartition optimale des ressources pour allouer des taux de transmission à chevauchement optique et de puissance optique de transmission dans le système OY-CDMA comme des ressources devant être optimalement et équitablement réparties entre les utilisateurs qui sont regroupés dans des classes de différentes qualités de service. La condition d'optimalité est basée sur la maximisation de la capacité par utilisateur de la couche PHY. De ce fait, un choix optimal des ressources physiques est maintenant possible, mais il n'est pas équitable entre les classes. Par conséquent, pour améliorer la performance de la couche liaison tout en éliminant le problème d'absence d'équité, nous proposons comme une approche unifiée un schéma équitable et optimal pour l'allocation des ressources fondé sur la qualité de service pour des multiplexages temporels des réseaux par paquets en CDMA à chevauchement optique. Enfin, nous combinons cette dernière approche avec le protocole MAC dans un problème d'optimisation d'allocation équitable des ressources à contrainte de délai afin de mieux améliorer le débit du réseau et le délai au niveau de la couche liaison avec allocation équitable et optimale des ressources au niveau de la couche PHY