2,159 research outputs found
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
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
Hybrid Dy-NFIS & RLS equalization for ZCC code in optical-CDMA over multi-mode optical fiber
For long haul coherent optical fiber communication systems, it is significant to precisely monitor the quality of transmission links and optical signals. The channel capacity beyond Shannon limit of Single-mode optical fiber (SMOF) is achieved with the help of Multi-mode optical fiber (MMOF), where the signal is multiplexed in different spatial modes. To increase single-mode transmission capacity and to avoid a foreseen âcapacity crunchâ, researchers have been motivated to employ MMOF as an alternative. Furthermore, different multiplexing techniques could be applied in MMOF to improve the communication system. One of these techniques is the Optical Code Division Multiple Access (Optical-CDMA), which simplifies and decentralizes network controls to improve spectral efficiency and information security increasing flexibility in bandwidth granularity. This technique also allows synchronous and simultaneous transmission medium to be shared by many users. However, during the propagation of the data over the MMOF based on Optical-CDMA, an inevitable encountered issue is pulse dispersion, nonlinearity and MAI due to mode coupling. Moreover, pulse dispersion, nonlinearity and MAI are significant aspects for the evaluation of the performance of high-speed MMOF communication systems based on Optical-CDMA. This work suggests a hybrid algorithm based on nonlinear algorithm (Dynamic evolving neural fuzzy inference (Dy-NFIS)) and linear algorithm (Recursive least squares (RLS)) equalization for ZCC code in Optical-CDMA over MMOF. Root mean squared error (RMSE), mean squared error (MSE) and Structural Similarity index (SSIM) are used to measure performance results
Neural networks for optical channel equalization in high speed communication systems
La demande future de bande passante pour les donnĂ©es dĂ©passera les capacitĂ©s des systĂšmes de communication optique actuels, qui approchent de leurs limites en raison des limitations de la bande passante Ă©lectrique des composants de lâĂ©metteur. LâinterfĂ©rence intersymbole (ISI) due Ă cette limitation de bande est le principal facteur de dĂ©gradation pour atteindre des dĂ©bits de donnĂ©es Ă©levĂ©s. Dans ce mĂ©moire, nous Ă©tudions plusieurs techniques de rĂ©seaux neuronaux (NN) pour combattre les limites physiques des composants de lâĂ©metteur pilotĂ©s Ă des dĂ©bits de donnĂ©es Ă©levĂ©s et exploitant les formats de modulation avancĂ©s avec une dĂ©tection cohĂ©rente. Notre objectif principal avec les NN comme Ă©galiseurs de canaux ISI est de surmonter les limites des rĂ©cepteurs optimaux conventionnels, en fournissant une complexitĂ© Ă©volutive moindre et une solution quasi optimale. Nous proposons une nouvelle architecture bidirectionnelle profonde de mĂ©moire Ă long terme (BiLSTM), qui est efficace pour attĂ©nuer les graves problĂšmes dâISI causĂ©s par les composants Ă bande limitĂ©e. Pour la premiĂšre fois, nous dĂ©montrons par simulation que notre BiLSTM profonde proposĂ©e atteint le mĂȘme taux dâerreur sur les bits(TEB) quâun estimateur de sĂ©quence Ă maximum de vraisemblance (MLSE) optimal pour la modulation MDPQ. Les NN Ă©tant des modĂšles pilotĂ©s par les donnĂ©es, leurs performances dĂ©pendent fortement de la qualitĂ© des donnĂ©es dâentrĂ©e. Nous dĂ©montrons comment les performances du BiLSTM profond rĂ©alisable se dĂ©gradent avec lâaugmentation de lâordre de modulation. Nous examinons Ă©galement lâimpact de la sĂ©vĂ©ritĂ© de lâISI et de la longueur de la mĂ©moire du canal sur les performances de la BiLSTM profonde. Nous Ă©tudions les performances de divers canaux synthĂ©tiques Ă bande limitĂ©e ainsi quâun canal optique mesurĂ© Ă 100 Gbaud en utilisant un modulateur photonique au silicium (SiP) de 35 GHz. La gravitĂ© ISI de ces canaux est quantifiĂ©e grĂące Ă une nouvelle vue graphique des performances basĂ©e sur les Ă©carts de performance de base entre les solutions optimales linĂ©aires et non linĂ©aires classiques. Aux ordres QAM supĂ©rieurs Ă la QPSK, nous quantifions lâĂ©cart de performance BiLSTM profond par rapport Ă la MLSE optimale Ă mesure que la sĂ©vĂ©ritĂ© ISI augmente. Alors quâelle sâapproche des performances optimales de la MLSE Ă 8QAM et 16QAM avec une pĂ©nalitĂ©, elle est capable de dĂ©passer largement la solution optimale linĂ©aire Ă 32QAM. Plus important encore, lâavantage de lâutilisation de modĂšles dâauto-apprentissage comme les NN est leur capacitĂ© Ă apprendre le canal pendant la formation, alors que la MLSE optimale nĂ©cessite des informations prĂ©cises sur lâĂ©tat du canal.The future demand for the data bandwidth will surpass the capabilities of current optical communication systems, which are approaching their limits due to the electrical bandwidth limitations of the transmitter components. Inter-symbol interference (ISI) due to this band limitation is the major degradation factor to achieve high data rates. In this thesis, we investigate several neural network (NN) techniques to combat the physical limits of the transmitter components driven at high data rates and exploiting the advanced modulation formats with coherent detection. Our main focus with NNs as ISI channel equalizers is to overcome the limitations of conventional optimal receivers, by providing lower scalable complexity and near optimal solution. We propose a novel deep bidirectional long short-term memory (BiLSTM) architecture, that is effective in mitigating severe ISI caused by bandlimited components. For the first time, we demonstrate via simulation that our proposed deep BiLSTM achieves the same bit error rate (BER) performance as an optimal maximum likelihood sequence estimator (MLSE) for QPSK modulation. The NNs being data-driven models, their performance acutely depends on input data quality. We demonstrate how the achievable deep BiLSTM performance degrades with the increase in modulation order. We also examine the impact of ISI severity and channel memory length on deep BiLSTM performance. We investigate the performances of various synthetic band-limited channels along with a measured optical channel at 100 Gbaud using a 35 GHz silicon photonic(SiP) modulator. The ISI severity of these channels is quantified with a new graphical view of performance based on the baseline performance gaps between conventional linear and nonlinear optimal solutions. At QAM orders above QPSK, we quantify deep BiLSTM performance deviation from the optimal MLSE as ISI severity increases. While deep BiLSTM approaches the optimal MLSE performance at 8QAM and 16QAM with a penalty, it is able to greatly surpass the linear optimal solution at 32QAM. More importantly, the advantage of using self learning models like NNs is their ability to learn the channel during the training, while the optimal MLSE requires accurate channel state information
Optical Communication
Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries
- âŠ