Orthogonal frequency division multiplexing¡]OFDM¡^systems play an important role in modern wireless communications due to following advantages: bandwidth saving¡Bcombat with frequency selective fading channel and high throughput. The performance of wireless communications is often degraded by fading channel . adaptive modulation and subcarrier allocation are proposed to overcome the degration to meet the quality of servie¡]QoS¡^. Lagrange method and heuristics method, two of the subcarrier allocation technology under multi-user OFDM, can achieve the goal that maximizing bit rate with minimizing transmitted power. However, significantly high complexity of either Lagrange method or heuristics method makes the implementation difficult. Zhang and Letaief proposed a method of making subcarriers detected one by one to reduce the complexity. However, in piratical, an OFDM system accommodates hundred of , or even thousand of subcarriers, so the method can be improved. In this thesis, we propose a subcarrier allocation method. The users that are not satisfied with the QOS requirement are named demander, and the users satisfied with the QOS requirement are named supplier. In the proposed subcarrier allocation method, we evaluate the number of subcarriers that demanders need and remove the subcarriers from supplier to directly compensate demander. Then the system has lower complexity due to less iterations.

This thesis focuses on the design and analysis of an optimal filter that is capable of making one-step-ahead prediction of a bandlimited signal while attenuating unwanted noise. First, the filter optimization based on the least mean-square-error criterion is presented. Then, an exact expression for the achievable minimum mean square error (MMSE) is derived with the aid of the Toeplitz form method and Szego theory. Based on this MMSE expression, the formulae for estimating the optimal filter¡¦s in-band prediction error and out-of-band noise attenuation are derived. Finally, the optimal filter is applied to sigma-delta modulation. It shows that the modulation performance and stability are intimately related to the filter performance and can be accurately estimated by the derived formulae.

Conventionally, the free distance is taken as the principle criterion for computing the error of convolutional and linear block codes. In other words, a larger free distance implies a better correction ability for the error correction codes. Distance spectrum is also an important factor for Maximum likelihood decoding. In this thesis, distance spectrum for different convolutional codes and CPM systems are investigated by us. Experiments results has demonstrate that a better correction ability of a shorter free distance does exist in some cases if they have a better distance spectrum. We also improve the fast algorithm for computing the distance spectrum developed by M. Cedervall and R. Johannesson. Their success is based upon the strategy of a traveling along the coding tree to find the distance spectrum. However, they need a new traveling for ever new distance computation. In contrast, we compute all the distance spectrum just in one travel by taking the advantage of the storing nodes of previous distance computation.

In this study, we have successfully developed an acoustic modulation microscopy that is based on a laser scanning confocal microscopy and operates in the range of a few tens of kilohertz. The induced submicron oscillation is detected through the combination of differential confocal microscopy and lock-in circuit. In this way, the mechanical properties, such as elasticity and stiffness, can be mapped in a two-dimensional way rapidly.

Signal model and observation model are commonly used to describe a dynamic system model in system identification or estimation such as Kalman filtering. The signal model is usually described by a linear dynamical equation driven by generating noise. The observation model is composed of a linear transformed signal and an additive white Gaussian noise. In this thesis, we set the generating noise to be a white binary sequence. This discrete generating noise makes the generating signal to be discrete. In contrast, the conventional generating signal is continuous. Discrete signal is simpler than the continuous signal. However, there still are too many states for this discrete signal. Therefore, defining the states and reducing the number of states are important in our work. In this thesis, we apply the tree structure to define the states. The number of states is reduced by focusing on the most probable working states. Afterwards, we apply two methods to recover the white sequence using the observation data. One is the Viterbi method; the other is Extended Kalman filter. Both methods are based upon the concept of signal states. Finally, we compare the error rates with the signal generated by continues phase modulation method.

The wurtzite GaN has either Ga or N polarity. The direction of polarization, hence it¡¦s associated polarization-induced electric fields (Fp), is determined by the polarity of the sample. In the present work, we prepared both N-face polarity and n-type doping of GaN with thicknesses are 1.1£gm and 70nm. Photoreflectacne (PR) and contactless electroreflectance (CER) were used in combination to study the nature of the surface band bending which was found to be determined by the type of doping for the thick sample and by the polarity for the polarity for the thin sample. This is in agreement with a theoretical calculation by Poisson-Schrödinger solver. Hence, CER can determine the polarity of GaN film as long as the sample is thin enough of the Fp to become dominant in the surface region.

Direct RF modulation on semiconductor lasers of wavelength 1310 nm has been widely applied to amplitude modulation vestigial side-band(AM-VSB)lightwave cable television (CATV) systems. The channel capacity is limited mainly by the nonlinear distortion of laser diode, which induces system performance degradation of composite second order (CSO) and composite triple beats (CTB). Though the linearity of laser diode has been improved during the fabrication process, carrier to noise ratio (CNR), CSO, and CTB were still degraded by increasing either RF output power level or the channel loading. Thus, some linearization techniques were proposed to extend the channel capacity. The predistortion approach is the simplest and the lowest cost one among the techniques. This paper describes a distortion compensation method with an electrical ¡§branch¡¨ circuit configuration, which is designed to reduce the nonlinear distortion induced by the laser diode. In this architecture, we utilize the method of impedance non-matching and the reflection of signals. According to the experiment results, we have found that before predistortion compensation, CNR / CSO / CTB are 51.6 dB / 62 dBc / 70 dBc, respectively. After compensation, CNR / CSO / CTB are 51.5 dB / 70 dBc / 71 dBc, respectively. Comparing with and without the distortion compensation, we found that CSO has an improvement with 8 dB, which reveals that the predistorion circuit improves the linearity of laser obviously. This study may give a guideline of predistortion technique and help to design optical transmitter.

The generation and propagation of fluorescence light within biological tissue offers the potential for biomedical diagnostics and analyte sensing. Arising from an exogenous fluorescent dye injected as a contrast agent or immobilized in a polymer implant, the fluorescent decay kinetics can be sensitive to the tissue??s biochemical environment, providing quantitative in vivo information of the confined tissue site. The impact of light propagation and decay kinetics upon the measured signals is important for consideration, simply because tissue scatters light, giving rise to nanosecond photon time-of-flights that are comparable to fluorescence relaxation kinetics. The goal of this study is to develop a time-dependent model describing (i) the generation of fluorescence from dyes exhibiting multi-exponential or more complex kinetics and (ii) its propagation in scattering media. In the preliminary study, fluorescence lifetime spectroscopy is investigated in tissue-like scattering solution. Two fluorescent dyes, 3,3-diethylthiatricarbocyanine iodide (DTTCI) and Indocynanine Green (ICG), which exhibit distinctly different lifetimes and each exhibits single-exponential decay kinetics, were employed. Measurements of phase-modulation as a function of modulation frequency were made at varying concentration ratios of the two dyes to experimentally simulate fluorescence multi-exponential decay kinetics in non-scattering and scattering solutions. The results suggest that frequency-domain measurements of fluorescent decay kinetics along with models of light propagation may be enhanced by scatter in order to probe kinetics more sensitively than in non-scattering solutions. The next study involved fluorescence lifetime sensing in scattering and non-scattering solutions with a pH sensitive dye, Carboxy Seminaphthofluorescein-1 (C-SNAFL-1), which is known to exhibit multi-exponential decay kinetics. The results demonstrate accurate pH sensing in scattering solution via fluorescence kinetics using a simplified propagation model incorporating an average lifetime. Finally, fluorescence lifetime sensing in immobilized systems were investigated. C-SNAFL-1 was immobilized in poly(ethylene glycol) (PEG) microparticles that were immersed in buffered polystyrene solutions. The results demonstrate the ability to perform pH sensing with fluorescence lifetime without the confounding effect of fluorophore loading or the use of 'reference' measurement within multiply scattering systems. In addition, the stability of the immobilized fluorescence sensor and the reliability of fluorescence lifetime measurement verify the prospect of this technology for implantable purposes.

The use of optical feedback for improving response linearity of electrooptic light modulators has been investigated. The modulator is configured as a straight channel waveguide flanked by electrodes in a lithium niobate (LiNbO3) substrate. Light is coupled into the waveguide in both TE and TM polarizations, and a voltage applied across electrodes causes a relative phase shift between two polarization components. An output analyzer converts the phase modulation to intensity modulation. Optical feedback of light in both polarization modes results from reflection of light at the polished edges of the substrate. Channel waveguides supporting a single guided mode for TE and TM polarizations were fabricated in x-cut LiNbO3 substrates using titanium-indiffusion technique. The waveguides and modulators were characterized at a wavelength of 1.55 ??m using a distributed feedback laser. The modulators were driven with a sinusoidal voltage waveform. To minimize harmonics of the modulating frequency in the intensity output, the magnitude of the optical feedback and the substrate temperature were adjusted. The feedback level was altered by applying refractive index-matching liquid to one or both ends of the waveguide at the edges of the crystal. It was found that a high degree of response linearity in the presence of feedback was achievable at certain substrate temperatures. The spurious-free dynamic range (SFDR) relative to the noise floor was measured at different feedback levels and substrate temperatures in an effort to maximize the modulator response linearity. An SFDR of 68.04 dB, limited by third-order nonlinearity, was achieved by applying index-matching fluid to the input end of the substrate. This compares with an SFDR of 64.84 dB limited by second-order nonlinearity when index-matching fluid was applied at both ends of the substrate. By changing the temperature of the same substrate to adjust the phase shifts experienced by TE and TM polarizations, the SFDR with index-matching fluid at the input end increased to 71.83 dB, limited by third-order nonlinearity. In tests at constant modulation depth, an improvement of as much as 9.6 dB in SFDR vs. the theoretical value for an interferometric modulator without feedback was achieved.

Thèse doctorat : Physique : Dijon : 2004. / Titre provenant de l'écran-titre. Bibliogr. p. 209-217, [230] réf.