19 research outputs found
Advanced optical modulation and fast reconfigurable en/decoding techniques for OCDMA application
With the explosive growth of bandwidth requirement in optical fiber communication
networks, optical code division multiple access (OCDMA) has witnessed tremendous
achievements as one of the promising technologies for optical access networks over the
past decades. In an OCDMA system, optical code processing is one of the key
techniques. Rapid optical code reconfiguration can improve flexibility and security of
the OCDMA system. This thesis focuses on advanced optical modulations and
en/decoding techniques for applications in fast reconfigurable OCDMA systems and
secure optical communications.
A novel time domain spectral phase encoding (SPE) scheme which can rapidly
reconfigure the optical code and is compatible with conventional spectral domain phase
en/decoding by using a pair of dispersive devices and a high speed phase modulator is
proposed. Based on this scheme, a novel advanced modulation technique that can
simultaneously generate both the optical code and the differential-phase-shift-keying
(DPSK) data using a single phase modulator is experimentally demonstrated. A
symmetric time domain spectral phase encoding and decoding (SPE/SPD) scheme using
a similar setup for both the transmitter and receiver is further proposed, based on which
a bit-by-bit optical code scrambling and DPSK data modulation technique for secure
optical communications has been successfully demonstrated. By combining optical
encoding and optical steganography, a novel approach for secure transmission of time
domain spectral phase encoded on-off-keying (OOK)/DPSK-OCDMA signal over
public wavelength-division multiplexing (WDM) network has also been proposed and
demonstrated.
To enable high speed operation of the time domain SPE/SPD scheme and enhance the
system security, a rapid programmable, code-length variable bit-by-bit optical code
shifting technique is proposed. Based on this technique, security improvements for
OOK/DPSK OCDMA systems at data rates of 10Gb/s and 40Gb/s using reconfigurable
optical codes of up to 1024-chip have been achieved.
Finally, a novel tunable two-dimensional coherent optical en/decoder which can
simultaneously perform wavelength hopping and spectral phase encoding based on
coupled micro-ring resonator is proposed and theoretically investigated. The techniques
included in this thesis could be potentially used for future fast reconfigurable and secure
optical code based communication systems
Applications of perfect difference codes in fiber-optics and wireless optical code-division multiplexing/multiple-access systems
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
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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
Passive optical network (PON) monitoring using optical coding technology
Les rĂ©seaux optiques passifs (PON) semblent ĂȘtre la technologie gagnante et ultime du futur pour les "fibres jusqu'au domicile" ayant une haute capacitĂ©. L'Ă©coute de contrĂŽle de ce genre de systĂšme est nĂ©cessaire pour s'assurer un niveau de qualitĂ© de service prĂ©dĂ©terminĂ© pour chaque client. En outre, l'Ă©coute de contrĂŽle rĂ©duit considĂ©rablement les dĂ©penses en capital et de fonctionnement (CAPEX et OPEX), tant pour le fournisseur du rĂ©seau que les clients. Alors que la capacitĂ© des PON est croissante, les gestionnaires de rĂ©seau ne disposent pas encore d'une technologie efficace et appropriĂ©e pour l'Ă©coute de contrĂŽle des rĂ©seaux de capacitĂ© aussi Ă©levĂ©e. Une variĂ©tĂ© de solutions a Ă©tĂ© proposĂ©e. Toutes ces derniĂšres solutions ne sont pas pratiques Ă cause de leur faible capacitĂ© (nombre de clients), d'une faible Ă©volutivitĂ©, d'une grande complexitĂ© et des dĂ©fis technologiques. Plus important encore, la technologie souhaitable pour l'Ă©coute de contrĂŽle devrait ĂȘtre rentable car le marchĂ© des PON est trĂšs sensible aux coĂ»ts. Dans cette thĂšse, nous considĂ©rons l'application de la technologie du codage optique passif (OC) comme une solution prometteuse pour l'Ă©coute de contrĂŽle centralisĂ©e d'un rĂ©seau optique ramifiĂ© tels que les rĂ©seaux PON. Dans la premiĂšre Ă©tape, nous dĂ©veloppons une expression pour le signal dĂ©tectĂ© par l'Ă©coute de contrĂŽle et Ă©tudions ses statistiques. Nous trouvons une nouvelle expression explicite pour le rapport signal utile/signal brouillĂ© (SIR) comme outil de mesure mĂ©trique de performance. Nous considĂ©rons cinq distributions PON gĂ©ographiques diffĂ©rentes et Ă©tudions leurs effets sur l'SIR pour l'Ă©coute de contrĂŽle d'OC. Dans la prochaine Ă©tape, nous gĂ©nĂ©ralisons notre modĂšle mathĂ©matique et ses expressions pour le contrĂŽle des signaux dĂ©tectĂ©s par un dĂ©tecteur quadratique et des paramĂštres rĂ©alistes. Nous Ă©valuons ensuite les performances thĂ©oriques de la technologie basĂ©e sur l'Ă©coute de contrĂŽle selon le rapport signal/bruit (SNR), le rapport signal/bruit plus coefficient d'interfĂ©rence (SNIR), et la probabilitĂ© de fausse alarme. Nous Ă©laborons l'effet de la puissance d'impulsion transmise, la taille du rĂ©seau et la cohĂ©rence de la source lumineuse sur le rendement des codes unidimensionnels (ID) et bidimensionnels (2D) de l'Ă©coute de contrĂŽle d'OC. Une conception optimale est Ă©galement abordĂ©e. Enfin, nous appliquons les tests de Neyman-Pearson pour le rĂ©cepteur de notre systĂšme d'Ă©coute de contrĂŽle et enquĂȘtons sur la façon dont le codage et la taille du rĂ©seau affectent les dĂ©penses de fonctionnement (OPEX) de notre systĂšme d'Ă©coute de contrĂŽle. MalgrĂ© le fait que les codes ID et 2D fournissent des performances acceptables, elles exigent des encodeurs avec un nombre Ă©levĂ© de composants optiques : ils sont encombrants, causent des pertes, et ils sont coĂ»teux. Par consĂ©quent, nous proposons un nouveau schĂ©ma de codage simple et plus appropriĂ© pour notre application de l'Ă©coute de contrĂŽle que nous appelons le codage pĂ©riodique. Par simulation, nous Ă©valuons l'efficacitĂ© de l'Ă©coute de contrĂŽle en terme de SNR pour un PON employant cette technologie. Ce systĂšme de codage est utilisĂ© dans notre vĂ©rification expĂ©rimentale de l'Ă©coute de contrĂŽle d'OC. Nous Ă©tudions expĂ©rimentalement et par simulation, l'Ă©coute de contrĂŽle d'un PON utilisant la technologie de codage pĂ©riodique. Nous discutons des problĂšmes de conception pour le codage pĂ©riodique et les critĂšres de dĂ©tection optimale. Nous dĂ©veloppons Ă©galement un algorithme sĂ©quentiel pour le maximum de vraisemblance avec une complexitĂ© rĂ©duite. Nous menons des expĂ©riences pour valider notre algorithme de dĂ©tection Ă l'aide de quatre encodeurs pĂ©riodiques que nous avons conçus et fabriquĂ©s. Nous menons Ă©galement des simulations de Monte-Carlo pour des distributions gĂ©ographiques de PON rĂ©alistes, avec des clients situĂ©s au hasard. Nous Ă©tudions l'effet de la zone de couverture et la taille du rĂ©seau (nombre d'abonnĂ©s) sur l'efficacitĂ© de calcul de notre algorithme. Nous offrons une borne sur la probabilitĂ© pour un rĂ©seau donnĂ© d'entraĂźner l'algorithme vers un temps exorbitant de surveillance du rĂ©seau, c'est Ă dire le dĂ©lai d'attente de probabilitĂ©. Enfin, nous soulignons l'importance du moyennage pour remĂ©dier aux restrictions budgĂ©taires en puissance/perte dans notre systĂšme de surveillance afin de supporter de plus grandes tailles de rĂ©seaux et plus grandes portĂ©es de fibres. Ensuite, nous mettrons Ă niveau notre dispositif expĂ©rimental pour dĂ©montrer un m PON avec 16 clients. Nous utilisons un laser Ă modulation d'exploitation directement Ă 1 GHz pour gĂ©nĂ©rer les impulsions sonde. Les donnĂ©es mesurĂ©es par le dispositif expĂ©rimental est exploitĂ© par l'algorithme de MLSE Ă dĂ©tecter et Ă localiser les clients. Trois dĂ©ploiements PON diffĂ©rents sont rĂ©alisĂ©s. Nous dĂ©montrons une surveillance plus rigoureuse pour les rĂ©seaux ayant une rĂ©partition gĂ©ographique Ă plusieurs niveaux. Nous Ă©tudions aussi le budget de la perte de notre dispositif de soutien plus Ă©levĂ©s de capacitĂ©s du rĂ©seau. Enfin, nous Ă©tudions le budget total admissible de la perte d'exploitation du systĂšme de surveillance dans la bande de frĂ©quences Ă 1650 nm en fonction des spĂ©cifications de l'Ă©metteur/rĂ©cepteur. En particulier, la limite totale de la perte de budget est reprĂ©sentĂ©e en fonction du gain de l'amplicateure de transimpĂ©dance (TIA) et le rĂ©solution de la conversion analogique-numĂ©rique (ADC). Par ailleurs, nous enquĂȘtons sur le compromis entre la distance portĂ©e et la capacitĂ© (taille de fractionnement au niveau du noeud distant) dans notre systĂšme de suivi