Optimized hybrid QPSK/8QAM for CO-OFDM transmissions

In optical communications, a high spectral efficiency can be realized by applying high order modulation formats such as 8QAM, 16QAM and 64QAM. However, depending on the system's requirements (bandwidth, performance and transmission distance), the maximum spectral efficiency may not be achievable with the regular 2m-array QAM formats. In this case, a hybrid modulation format, such as QPSK/8QAM, can provide an effective solution. In this work, we deliver the optimum design for single channel coherent optical orthogonal frequency division multiplexing systems with hybrid QPSK/8QAM modulation format. We also discuss a simple but effective strategy for applying hybrid QAMs for long-haul optical communications without considering sophisticated bit and power loading algorithms developed for wireless communications.

Demonstration of phase-conjugated subcarrier coding for fiber nonlinearity compensation in CO-OFDM transmission

In this paper, we demonstrate through computer simulation and experiment a novel subcarrier coding scheme combined with pre-electrical dispersion compensation (pre-EDC) for fiber nonlinearity mitigation in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. As the frequency spacing in CO-OFDM systems is usually small (tens of MHz), neighbouring subcarriers tend to experience correlated nonlinear distortions after propagation over a fiber link. As a consequence, nonlinearity mitigation can be achieved by encoding and processing neighbouring OFDM subcarriers simultaneously. Herein, we propose to adopt the concept of dual phase conjugated twin wave for CO-OFDM transmission. Simulation and experimental results show that this simple technique combined with 50% pre-EDC can effectively offer up to 1.5 and 0.8 dB performance gains in CO-OFDM systems with BPSK and QPSK modulation formats, respectively.

Quasi-Orthogonal Space-Time Block Coding Using Polynomial Phase Modulation

Recently, polynomial phase modulation (PPM) was shown to be a power- and bandwidth-efficient modulation format. These two characteristics are in high demand nowadays specially in mobile applications, where devices with size, weight, and power (SWaP) constraints are common. In this paper, we propose implementing a full-diversity quasiorthogonal space-time block code (QOSTBC) using polynomial phase signals as modulation format. QOSTBCs along with PPM are used in order to improve the power efficiency of communication systems with four transmit antennas. We obtain the optimal PPM constellations that ensure full diversity and maximize the QOSTBC's minimum coding gain distance. Simulation results show that by using QOSTBCs along with a properly selected PPM constellation, full diversity in flat fading channels and thus low BER at high signal-to-noise ratios (SNR) can be ensured. More importantly, it is also shown that QOSTBCs using PPM achieve a better error performance than those using conventional modulation formats.

Optimum power allocation for multiuser OFDM with arbitrary signal constellations

This paper formulates power allocation policies that maximize the region of mutual informationsachievable in multiuser downlink OFDM channels. Arbitrary partitioning ofthe available tones among users and arbitrary modulation formats, possibly different forevery user, are considered. Two distinct policies are derived, respectively for slow fadingchannels tracked instantaneously by the transmitter and for fast fading channels knownonly statistically thereby. With instantaneous channel tracking, the solution adopts theform of a multiuser mercury/waterfilling procedure that generalizes the single-user mercury/waterfilling introduced in [1, 2]. With only statistical channel information, in contrast,the mercury/waterfilling interpretation is lost. For both policies, a number of limitingregimes are explored and illustrative examples are provided.

La plasticité cérébrale dans le vieillissement normal : effet de l’éducation formelle et de l’entraînement cognitif sur les mesures de potentiels évoqués

Le vieillissement normal est souvent associé à des changements cognitifs négatifs, notamment sur les performances cognitives. Cependant, des changements comportementaux et cérébraux positifs ont aussi été observés. Ces modifications indiquent l’existence d’une plasticité cérébrale dans le vieillissement normal. Ainsi, plusieurs facteurs ont été étudiés afin de mieux connaitre les modulateurs de cette plasticité dite positive. La plupart des études évaluant ce phénomène ont utilisé la technique d’imagerie par résonance magnétique alors que la technique des potentiels évoqués a été beaucoup moins utilisée. Cette technique est basée sur les enregistrements de l’activité électrique cérébrale très sensible aux changements anatomiques associés au vieillissement et permet donc d’observer de manière précise les variations du décours temporel des ondes éléctrophysiologiques lors du traitement des informations. Les travaux de cette thèse visent à étudier les modifications de plasticité cérébrale induites par des facteurs protecteurs/préventifs du vieillissement normal et notamment lors de la réalisation de tâches impliquant le contrôle attentionnel, grâce à l’analyse de signaux électroencéphalographiques en potentiels évoqués. Dans un premier temps, une description de l’analyse des données EEG en potentiels évoqués sera fournie, suivie d’une revue de littérature sur le contrôle attentionnel et les facteurs de plasticité dans le vieillissement normal (Chapitre 1). Cette revue de littérature mettra en avant, d’une part la diminution des capacités de contrôle de l’attention dans le vieillissement et d’autre part, les facteurs protecteurs du vieillissement ainsi que la plasticité cérébrale qui leur est associée. Ces facteurs sont connus pour avoir un effet positif sur le déficit lié à l’âge. La première étude de ce projet (Chapitre 2) vise à définir l’effet d’un facteur de réserve cognitive, le niveau d’éducation, sur les composantes des potentiels évoqués chez les personnes âgées. Cette étude mettra en avant une composante des potentiels évoqués, la P200, comme indice de plasticité lorsqu’elle est liée au niveau d’éducation. Cet effet sera observé sur deux tâches expérimentales faisant intervenir des processus de contrôle attentionnel. De plus, une différence d’épaisseur corticale sera observée : les personnes âgées ayant un plus haut niveau d’éducation ont un cortex cingulaire antérieur plus épais. La deuxième étude (Chapitre 3) cherche à déterminer, chez les personnes âgées, les modifications comportementales et en potentiels évoqués induites par trois entraînements cognitifs, entrainements visant l’amélioration de processus attentionnels différents : l’attention focalisée, l’attention divisée, ainsi que la modulation de l’attention. Au niveau comportemental, les entraînements induisent tous une amélioration des performances. Cependant, l’entraînement en modulation de l’attention est le seul à induire une amélioration du contrôle attentionnel. Les résultats éléctrophysiologiques indiquent la N200 comme composante sensible à la plasticité cérébrale à la suite d’entraînements cognitifs. L’entraînement en modulation de l’attention est le seul à induire une modification de cette composante dans toutes les conditions des tests. Les résultats de ces études suggèrent que les facteurs protecteurs du vieillissement permettent des changements positifs observés en potentiels évoqués. En effet, nous mettons en évidence des phénomènes de plasticité cérébrale des personnes âgées qui diffèrent selon leurs origines. L’impact de ces résultats ainsi que les limites et perspectives futures seront présentés en fin de thèse (Chapitre 4).

Implementazione su fpga di un pre-distorsore digitale a banda larga per sistemi a microonde di backhaul

The development of next generation microwave technology for backhauling systems is driven by an increasing capacity demand. In order to provide higher data rates and throughputs over a point-to-point link, a cost-effective performance improvement is enabled by an enhanced energy-efficiency of the transmit power amplification stage, whereas a combination of spectrally efficient modulation formats and wider bandwidths is supported by amplifiers that fulfil strict constraints in terms of linearity. An optimal trade-off between these conflicting requirements can be achieved by resorting to flexible digital signal processing techniques at baseband. In such a scenario, the adaptive digital pre-distortion is a well-known linearization method, that comes up to be a potentially widely-used solution since it can be easily integrated into base stations. Its operation can effectively compensate for the inter-modulation distortion introduced by the power amplifier, keeping up with the frequency-dependent time-varying behaviour of the relative nonlinear characteristic. In particular, the impact of the memory effects become more relevant and their equalisation become more challenging as the input discrete signal feature a wider bandwidth and a faster envelope to pre-distort. This thesis project involves the research, design and simulation a pre-distorter implementation at RTL based on a novel polyphase architecture, which makes it capable of operating over very wideband signals at a sampling rate that complies with the actual available clock speed of current digital devices. The motivation behind this structure is to carry out a feasible pre-distortion for the multi-band spectrally efficient complex signals carrying multiple channels that are going to be transmitted in near future high capacity and reliability microwave backhaul links.

Deflexiones transversales de radiación luminosa en estructuras prismáticas de cristal líquido

A novel structure , based on a wedge shaped configuration, is presented . This structure , previously used in one of his forms,for refraction index measurements is analysed in this paper. The results obtained give the possibility of his use in electro snd magneto-optical modulation and deflection.

Scalable high-order UWB pulse generation employing an FBG-based photonic superstructure

In this letter, we propose and experimentally demonstrate a compact, flexible, and scalable ultrawideband (UWB) generator based on the merge of phase-to-intensity conversion and pulse shaping employing an fiber Bragg Grating-based superstructure. Our approach offers the capacity for generating high-order UWB pulses by means of the combination of various low-order derivatives. Moreover, the scheme permits the implementation of binary and multilevel modulation formats. Experimental measurements of the generated UWB pulses, in both time and frequency domain, are presented revealing efficiency and a proper fit in terms of Federal Communications Commission settled standards.

Contrast sensitivity for complex and random gratings

This thesis studied the effect of (i) the number of grating components and (ii) parameter randomisation on root-mean-square (r.m.s.) contrast sensitivity and spatial integration. The effectiveness of spatial integration without external spatial noise depended on the number of equally spaced orientation components in the sum of gratings. The critical area marking the saturation of spatial integration was found to decrease when the number of components increased from 1 to 5-6 but increased again at 8-16 components. The critical area behaved similarly as a function of the number of grating components when stimuli consisted of 3, 6 or 16 components with different orientations and/or phases embedded in spatial noise. Spatial integration seemed to depend on the global Fourier structure of the stimulus. Spatial integration was similar for sums of two vertical cosine or sine gratings with various Michelson contrasts in noise. The critical area for a grating sum was found to be a sum of logarithmic critical areas for the component gratings weighted by their relative Michelson contrasts. The human visual system was modelled as a simple image processor where the visual stimuli is first low-pass filtered by the optical modulation transfer function of the human eye and secondly high-pass filtered, up to the spatial cut-off frequency determined by the lowest neural sampling density, by the neural modulation transfer function of the visual pathways. The internal noise is then added before signal interpretation occurs in the brain. The detection is mediated by a local spatially windowed matched filter. The model was extended to include complex stimuli and its applicability to the data was found to be successful. The shape of spatial integration function was similar for non-randomised and randomised simple and complex gratings. However, orientation and/or phase randomised reduced r.m.s contrast sensitivity by a factor of 2. The effect of parameter randomisation on spatial integration was modelled under the assumption that human observers change the observer strategy from cross-correlation (i.e., a matched filter) to auto-correlation detection when uncertainty is introduced to the task. The model described the data accurately.

Digital back-propagation for spectrally efficient WDM 112 Gbit/s PM m-ary QAM transmission

We report the performance of coherently-detected nine-channel WDM transmission over high dispersion fibers, using polarization multiplexed m-ary quadrature amplitude modulation (m = 4, 16, 64, 256) at 112 Gbit/s. Compensation of fiber nonlinearities via digital back-propagation enables up to 10 dB improvement in maximum transmittable power and similar to 8 dB Q(eff) improvement which translates to a nine-fold enhancement in transmission reach for PM-256QAM, where the largest improvements are associated with higher-order modulation formats. We further demonstrate that even under strong nonlinear distortion the transmission reach only reduces by a factor of similar to 2.5 for a 2 unit increase in capacity (log(2)m) when full band DBP is employed, in proportion to the required back-to-back OSNR.

Optimal packing for cascaded regenerative transmission based on phase sensitive amplifiers

We investigate the transmission performance of advanced modulation formats in nonlinear regenerative channels based on cascaded phase sensitive amplifiers. We identify the impact of amplitude and phase noise dynamics along the transmission line and show that after a cascade of regenerators, densely packed single ring PSK constellations outperform multi-ring constellations. The results of this study will greatly simplify the design of future nonlinear regenerative channels for ultra-high capacity transmission

Serial dark soliton for 100-Gb/s applications

We analyze the performance through numerical simulations of a new modulation format: serial dark soliton (SDS) for wide-area 100-Gb/s applications. We compare the performance of the SDS with conventional dark soliton, amplitude-modulation phase-shift keying (also known as duobinary), nonreturn-to-zero, and return-to-zero modulation formats, when subjected to typical wide-area-network impairments. We show that the SDS has a strong chromatic dispersion and polarization-mode-dispersion tolerance, while maintaining a compact spectrum suitable for strong filtering requirement in ultradense wavelength-division-multiplexing applications. The SDS can be generated using commercially available components for 40-Gb/s applications and is cost efficient when compared with other 100-Gb/s electrical-time-division-multiplexing systems.

Shannon capacity of nonlinear communication channels

The exponentially increasing demand on operational data rate has been met with technological advances in telecommunication systems such as advanced multilevel and multidimensional modulation formats, fast signal processing, and research into new different media for signal transmission. Since the current communication channels are essentially nonlinear, estimation of the Shannon capacity for modern nonlinear communication channels is required. This PhD research project has targeted the study of the capacity limits of different nonlinear communication channels with a view to enable a significant enhancement in the data rate of the currently deployed fiber networks. In the current study, a theoretical framework for calculating the Shannon capacity of nonlinear regenerative channels has been developed and illustrated on the example of the proposed here regenerative Fourier transform (RFT). Moreover, the maximum gain in Shannon capacity due to regeneration (that is, the Shannon capacity of a system with ideal regenerators – the upper bound on capacity for all regenerative schemes) is calculated analytically. Thus, we derived a regenerative limit to which the capacity of any regenerative system can be compared, as analogue of the seminal linear Shannon limit. A general optimization scheme (regenerative mapping) has been introduced and demonstrated on systems with different regenerative elements: phase sensitive amplifiers and the proposed here multilevel regenerative schemes: the regenerative Fourier transform and the coupled nonlinear loop mirror.

Kerr nonlinear switching in a hybrid silicasilicon microspherical resonator

A hybrid silicon-core, silica-clad microspherical resonator has been fabricated from the semiconductor core fiber platform. Linear and nonlinear characterization of the resonator properties have shown it to exhibit advantageous properties associated with both materials, with the low loss cladding supporting high quality (Q) factor whispering gallery modes which can be tuned through the nonlinear response of the crystalline core. By exploiting the large wavelength shift associated with the Kerr nonlinearity, we have demonstrated all-optical modulation of a weak probe on the timescale of the femtosecond pump pulse. This novel geometry offers a route to ultra-low loss, high-Q silica-based resonators with enhanced functionality.

Efficient packing for phase regenerative channels

We compare the performance of advanced modulation formats in cascaded phase regenerative systems and demonstrate the importance of constellation optimization to the transfer function characteristics of the regenerator. © 2013 IEEE.