Computer modelling and analysis of the photodegradation effect in a-Si:H p-i-n solar cell

Using a previous model, which was developed to describe the light-induced creation of the defect density in the a-Si:H gap states, we present in this work a numerical modelling of the photodegradation effect in the a-Si:H p-i-n solar cell under continuous illumination. We first considered the simple case of a monochromatic light beam with a wavelength λ between 530-540 nm non uniformly absorbed, then the global standard solar spectrum (AM 1.5) illumination is taken into account. The photodegradation is analysed on the basis of the resulting changes in the free carrier's densities, recombination rate, band structure, electrical potential and field, space charge, and current densities. Changes in the cell's external parameters: the open circuit voltage V_oc, the short circuit current density J_sc, the fill factor FF and the maximum power density P_max are also presented.     

Defects creation in sapphire by swift heavy ions: A fluence depending process

Single crystals of sapphire (α-Al₂O₃) were irradiated at GANIL with 0.7 MeV/amu xenon ions corresponding to an electronic stopping power of 21 keV/nm. Several fluences were applied between 5 × 10¹¹ and 2 × 10¹⁴ ions/cm². Irradiated samples were characterized using optical absorption spectroscopy. This technique exhibited the characteristic bands associated with F and F⁺ centers defects. The F centers density was found to increase with the fluence following two different kinetics: a rapid increase for fluences less than 10¹³ ions/cm² and then, a slow increase for higher fluences. For fluences less than 10¹³ ions/cm², results are in good agreement with those obtained by Canut et al. [B. Canut, A. Benyagoub, G. Marest, A. Meftah, N. Moncoffre, S.M.M. Ramos, F. Studer, P. Thévenard, M. Toulemonde, Phys. Rev. B 51 (1995) 12194]. In the fluences range: 10¹³-10¹⁴ ions/cm², the F centers defects creation process is found to be different from the one evidenced for fluences less than 10¹³ ions/cm².     

Unequal Error Protection Using Partially Regular LDPC Codes

In this paper, we propose a scheme to construct low-density parity-check (LDPC) codes that are suitable for unequal error protection (UEP). We derive density evolution (DE) formulas for the proposed unequal error protecting LDPC ensembles over the binary erasure channel (BEC). Using the DE formulas, we optimize the codes. For the finite-length cases, we compare our codes with some other LDPC codes, the time-sharing method, and a previous work on UEP using LDPC codes. Simulation results indicate the superiority of the proposed design methodology for UEP     

Stability‐ and performance‐robustness tradeoffs: MIMO mixed‐µ vs complex‐µ design

We present a non‐trivial case study designed to highlight some of the practical issues that arise when using mixed‐µ or complex‐µ robust synthesis methodologies. By considering a multi‐input multi‐output three‐cart mass–spring–dashpot (MSD) with uncertain parameters and dynamics, it is demonstrated that optimized performance (disturbance‐rejection) is reduced as the level of uncertainty in one or two real parameters is increased. Comparisons are made (a) in the frequency domain, (b) by RMS values of key signals and (c) in time‐domain simulations. The mixed‐µ controllers designed are shown to yield superior performance as compared with the classical complex‐µ design. The singular value decomposition analysis shows the directionality changes resulting from different uncertainty levels and from the use of different frequency weights. The nominal and marginal stability regions of the closed‐loop system are studied and discussed, illustrating how stability margins can be extended at the cost of reducing performance. Copyright © 2008 John Wiley & Sons, Ltd.     

Detailed numerical simulation of the effect of defects created by electron irradiation on the performance degradation of a p+–n–n+ GaAs solar cell

Solar cells exposed to irradiation undergo severe degradation in their performance due to induced structural defects. To predict this effect, the current–voltage characteristics under AM0 illumination for a constant dose of electron irradiation are numerically calculated. From these characteristics the following solar cell output parameters: the short circuit current density J_sc, the open circuit voltage V_oc, the fill factor FF and the conversion efficiency η are extracted. The irradiation induced defects introduce in the energy gap either recombination centres or traps. The irradiation induced degradation is widely attributed to the first type of defects. A strategy is adopted to check the truthfulness of this by simulating the effect of each single trap level separately on the output parameters of the cell. The simulation results show that only the shallowest deep electron trap is responsible for the degradation of J_sc while V_oc is mostly affected by other electron and hole traps especially the deepest one. This more detailed study is an extension of another work in which the effect of a group instead of individual levels is investigated.     

Fast and robust estimation of the multivariate errors in variables model

In the multivariate errors in variables models, one wishes to retrieve a linear relationship of the form y=β ^t x+α, where both x and y can be multivariate. The variables y and x are not directly measurable, but observed with measurement error. The classical approach to estimate the multivariate errors in variables model is based on an eigenvector analysis of the joint covariance matrix of the observations. In this paper, a projection-pursuit approach is proposed to estimate the unknown parameters. The focus is on projection indices based on half-samples. These lead to robust estimators which can be computed using fast algorithms. Fisher consistency of the procedure is shown, without the need to make distributional assumptions on the x-variables. A simulation study gives insight into the robustness and the efficiency of the procedure.     

Public-key cryptography using paraunitary matrices

In this paper, we propose an algebraic approach for designing multivariate cryptosystems. Our method is based on formulating a general system of multivariate polynomial equations by paraunitary matrices. These matrices are a special family of invertible polynomial matrices that can be completely parameterized and efficiently generated by primitive building blocks. Using the general formulation that involves paraunitary matrices, we design a one-way function that operates over the fields of characteristic two. In order to include a trapdoor, we make some approximations to the paraunitary matrix. The result is a trapdoor one-way function that is efficient to evaluate but hard to invert unless secret information about the trapdoor is known. Using this function, we propose a paraunitary asymmetric cryptosystem (PAC). We present an instance of the PAC and show how it can be efficiently implemented. This instance operates on the finite field GF(256 ). The message block consists of 16 to 32 symbols from GF(256 ), i.e., the block size n is an integer between 16 and 32. The ciphertext block takes its elements from the same field and has at least ten extra symbols. We show that the encryption and decryption can be efficiently performed with complexities O(n/sup 3/) and O(n/sup 2/), respectively, where n is the size of the message block. Comparing complexities of the PAC to those in the hidden-field equation (HFE) family, we show that the PAC is faster in public-key generation and decryption. We study the computational security of the PAC. In addition, we show that the attacks developed for the HFE are not applicable on the PAC.     

Connectivity properties of large-scale sensor networks

In wireless sensor networks, both nodes and links are prone to failures. In this paper we study connectivity properties of large-scale wireless sensor networks and discuss their implicit effect on routing algorithms and network reliability. We assume a network model of n sensors which are distributed randomly over a field based on a given distribution function. The sensors may be unreliable with a probability distribution, which possibly depends on n and the location of sensors. Two active sensor nodes are connected with probability p _e (n) if they are within communication range of each other. We prove a general result relating unreliable sensor networks to reliable networks. We investigate different graph theoretic properties of sensor networks such as k-connectivity and the existence of the giant component. While connectivity (i.e. k = 1) insures that all nodes can communicate with each other, k-connectivity for k > 1 is required for multi-path routing. We analyze the average shortest path of the k paths from a node in the sensing field back to a base station. It is found that the lengths of these multiple paths in a k-connected network are all close to the shortest path. These results are shown through graph theoretical derivations and are also verified through simulations.