Modeling and simulation of long reach high speed inter-satellite link (ISL)

A hybrid OFDM–IsOWC system incorporating diverse modulation schemes like optical double side band (ODSB), optical tandem side band (OTSB) and optical single side band (OSSB) is carried out to optimize a long reach high speed inter-satellite link (ISL) with acceptable SNR and BER. The proposed system is also reported under the influence of antenna efficiency and pointing error at varied values of power levels. Simulations point toward that the proposed hybrid OFDM–OSSB–IsOWC transmission system promises significantly enhanced ISL link compared to conventional IsOWC systems.     

Analysis of inter-satellite free-space optical link performance considering different system parameters

The welcome and adaptation of optical wireless technology by the modern era has brought forward the concept of an inter-satellite free-space optical communication system. In the present work, I study the combined effect of selection of different operating wavelengths and detector types along with the pointing errors at the transmitter and receiver side on the performance of an inter-satellite free-space optical link. The link performance has been optimized by measuring and analyzing the bit error rate and quality-factor of received signal under different scenarios. Performance of the inter-satellite link has also been investigated considering different modulation formats and data rates for LEO and MEO distances.     

2.50 Gbit/s optical wireless communication system using PPM modulation schemes in HAP-to-satellite links

HAP-to-satellite, one of the important applications of FSO/OWC technology, which will be deployed in space in the near future. In this paper, we have presented analysis of 2.50 Gbit/s OWC system using PPM modulation schemes and a distance of 2500 km was achieved at BER 10⁻⁶ in HAP to satellite links.     

Enhanced performance analysis of inter-satellite optical-wireless communication (IsOWC) system

In this paper, we have reported the improved performance by usage of a square root module. By simulation in OPTISYSTEM™ a distance of 5000 km with 1.25 Gbps was achieved with the same performance representing an enhancement of 48% when compared to the traditional detection.     

Enhanced performance analysis of 10 Gbit/s optical OFDM-RoF transmission links

In this paper, we have presented analysis of 10 Gbit/s optical OFDM-RoF transmissions links with distance of 50 km and reported the improved performance by usage of a square root module (SQRT).     

2.50 Gbit/s optical CDMA over FSO communication system

Optical CDMA over FSO communication system is very effective to provide high data rate transmission with very low bit error rate and low amount of multiple access interference. In this paper, we have presented optical CDMA over FSO communication system to the range of 8000 m. The simulative results reveal that the transmission distance is limited mainly by the multi-access interference (MAI) which arises when there are number of users in the system because of the fact that one user data becomes noisy for all other users in the channel.     

Experimental determination and comparison of rain attenuation in free space optic link operating at 532 nm and 655 nm wavelength

The demand for high data rate, security and reliable communication is driving the development of free space optic communication (FSO) technology. The atmospheric effects such as scintillation, absorption and scattering severely affect the availability and range of the FSO system. The atmospheric rain absorbs and scatters the laser beam energy resulting in attenuation of the propagating signal. Initial development of FSO technology primarily used wavelength from infrared spectrum. In the recent years, the interest in visible light carrier for FSO applications is consistently increasing. In this paper, the effect of rain over two optical wavelengths from the visible spectrum i.e. 532 nm and 655 nm has been experimentally evaluated and results for the specific rain attenuation at 532 nm and 655 nm wavelengths have been compared.     

基于神经网络的航天光学遥感器在轨信噪比测试方法

提出一种基于神经网络的航天光学遥感器在轨信噪比的的测试方法。通过模拟得到了大量的包含有不同信噪比等级的遥感图像,并将其作为网络训练和测试的样本。通过对遥感图像进行分析,找到了分别与景物结构和噪声有关的特征向量,并将其作为神经网络的输入。在对大量样本图片进行训练后,可完成对由遥感器传输下来的任意一幅地面景物图像进行信噪比的测试,从而避免了传统方法对特定地面景物目标在成像测量中的诸多弊端,平均测量误差约为10%。     

Optimization of CW-OSL parameters for improved dose detection threshold in Al2O3:C

Continuous wave optically stimulated luminescence (CW-OSL) is relatively a simple technique that offers good signal to noise ratio (SNR) and involves simple instrumentation. This study reports the influence and optimization of CW-OSL parameters on minimum detectable dose (MDD) using α-Al₂O₃:C phosphor. It is found that at a given stimulation intensity MDD in CW-OSL mode depends on signal integration time. At lower integration times MDD is inferior. It exhibits an improvement for intermediate values, shows a plateau region and deteriorates as integration time increases further. MDD is found to be ∼127 μGy at 4 mW/cm² stimulation intensity for integration time of 0.1 s, which improves to ∼10.5 μGy for 60 s. At stimulation intensity of 72 mW/cm², MDD is 37 μGy for integration time of 60 s and improves significantly to 7 μGy for 1 s.     

Spectral efficiency maximization for IRS-assisted wireless communication in cognitive radio networks

In this paper, we consider the Intelligent reflecting surface (IRS)-assisted cognitive radio (CR) network, in which the IRS is applied to improve the spectral efficiency of secondary users. In specific, the advantages of applying IRS is double: it can not only improve the transmission rate of the secondary users, but also reduce the interference from the secondary users to the primary users, which allow the secondary users to increase the transmission power correspondingly. The original problem is formulated by simultaneously consider the maximum power limitation of the secondary users and the Quality of Service (QoS) requirement of the primary users, which expressed by the interference temperature (IT). The formulated problem is non-convex and difficult to solve directly. We apply the alternating optimization (AO) algorithm to split the original problem into two sub-problems. For the first sub-problem, we transform it into a convex problem and for the second sub-problem, we use the successive convex approximation method to obtain the corresponding results. The simulation experiments show that the performance of our proposed scheme is better than existing schemes.     

Core-level shifts of the c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) surfaces

We have studied In-stabilized c(8 × 2)-reconstructed InAs(1 0 0) and InSb(1 0 0) semiconductor surfaces, which play a key role in growing improved III–V interfaces for electronics devices, by core-level photoelectron spectroscopy and first-principles calculations. The calculated surface core-level shifts (SCLSs) for the ζ and ζa models, which have been previously established to describe the atomic structures of the III–V(1 0 0)c(8 × 2) surfaces, yield hitherto not reported interpretation for the As 3d, In 4d, and Sb 4d core-level spectra of the III–V(1 0 0)c(8 × 2) surfaces, concerning the number and origins of SCLSs. The fitting analysis of the measured spectra with the calculated ζ and ζa SCLS values shows that the InSb spectra are reproduced by the ζ SCLSs better than by the ζa SCLSs. Interestingly, the ζa fits agree better with the InAs spectra than the ζ fits do, indicating that the ζa model describes the InAs surface better than the InSb surface. These results are in agreement with previous X-ray diffraction data. Furthermore, an introduction of the complete-screening model, which includes both the initial and final state effects, does not improve the fitting of the InSb spectra, proposing the suitability of the initial-state model for the SCLSs of the III–V(1 0 0)c(8 × 2) surfaces. The found SCLSs are discussed with the ab initio on-site charges.     

Morphology and optical properties of p-type porous GaAs(1 0 0) layers made by electrochemical etching

Porous GaAs layers were formed by electrochemical etching of p-type GaAs(1 0 0) substrates in HF solution. A surface characterization has been performed on p-type GaAs samples using X-ray photoelectron spectroscopy (XPS) technique in order to get information about the chemical composition, particularly on the surface contamination. According to the XPS spectra, the oxide layer on as-received porous GaAs substrates contains As₂O₃, As₂O₅ and Ga₂O₃. Large amount of oxygen is present at the surface before the surface cleaning.Compared to untreated GaAs surface, room temperature photoluminescence (PL) investigations of the porous layers reveal the presence of two PL bands: a PL peak at ∼871 nm and a “visible” PL peak at ∼650-680 nm. Both peak wavelengths and intensities varied from sample to sample depending on the treatment that the samples have undergone. The short PL wavelength at 650-680 nm of the porous layers is attributed to quantum confinement effects in GaAs nano-crystallites. The surface morphology of porous GaAs has been studied using atomic force microscopy (AFM). Nano-sized crystallites were observed on the porous GaAs surface. An estimation of the mean size of the GaAs nano-crystals obtained from effective mass theory and based on PL data was close to the lowest value obtained from the AFM results.     

Self-organized amorphous material in silicon (0 0 1) by focused ion beam (FIB) system

A method using a focused ion beam (FIB) to prepare a silicon amorphous material is presented. The method involves the redeposition of sputtered material generated during the interaction of the Ga⁺ ion beam with a silicon substrate material. The shape and dimensions of this amorphous material are self-organized and reproducible. The stability of this amorphous material under electron irradiation was investigated in the transmission electron microscopy (TEM). Electron irradiation can induce recrystallization of the amorphous material, resulting in the lateral and vertical growth, starting at an amorphous-crystalline interface, of polysilicon containing defects.     

Design of adaptive dispersion compensation and demodulation system for 40 Gbps RZ-DQPSK optical fiber communication receivers

An adaptive terminal dispersion compensation and demodulation scheme based on dual-feedback strategy for 40 Gbps return-to-zero differential quaternary phase shift keying (RZ-DQPSK) fiber communication receivers is proposed to achieve high reliability receiving performances. In the receiving system introduced in this paper we utilize both the balanced receiver feedback and framer error rate to achieve accurate RZ-DQPSK optical signal demodulation and adaptive dispersion compensation. Compared to traditional RZ-DQPSK receivers, the proposed design combines the chromatic dispersion (CD) compensation and Mach-Zehnder interferometer (MZI) adjusting and automatically estimates the dispersion compensation parameters without human intervention. And another improvement is introducing the framer error rate as feedback quantity to ensure the accuracy of CD and MZI adjusting. Finally, we present the performance of the designed receiver by showing the simulations and experiment results in actual telecommunication engineering projects.     

Structural analysis of the c(4 × 2) reconstruction in Si(0 0 1) and Ge(0 0 1) surfaces by low-energy electron diffraction

The c(4 × 2) structures in (0 0 1) surfaces of Si and Ge have been studied by low-energy electron diffraction (LEED). Using a proper cleaning method for the Si surface, we were able to observe clear c(4 × 2) LEED patterns up to incident energy of ∼400 eV as well as the Ge surface. Extensive experimental intensity-voltage curves allowed us to optimize the asymmetric dimer model up to the eighth layer (including the dimer layer) in depth in the dynamical LEED calculation. Optimized structural parameters are almost the same for the Si and Ge except for the height of the buckled-up atom of the asymmetric dimer. For the Ge surface, the structural parameters are in excellent agreement with those obtained by a previous theoretical calculation. The tilt angle and bond length of the dimer are 18 ± 1 (19 ± 1)° and 2.4 ± 0.1 (2.5 ± 0.1) Å for the Si(0 0 1) (Ge(0 0 1)), respectively.     

Optical millimeter-wave generation and transmission system for 1.25 Gbit/s downstream link using a gain switched laser

In this paper, we propose a novel and cost effective system for optical millimeter-wave (mm-wave) generation and transmission of downstream data based on a gain switched laser (GSL). The GSL produces an optical comb spectrum that can be appropriately filtered to generate two optical sidebands spaced by more than 4 times the repetition rate of the GSL. These sidebands are modulated by baseband data and then transmitted via optical fiber to the remote antenna unit (RAU). At the RAU, the two sidebands are heterodyned using a photodetector to generate the electrical modulated mm-wave signal, before demodulation using self mixing. We demonstrate the distribution of 1.25 Gbit/s data OOK modulated onto a 60 GHz carrier, similar to that used in the IEEE 802.15.3c draft standard, over fiber lengths up to 62 km.     

Analysis of multitone microwave signals on optical fiber system using external laser modulation

In this paper, we Analyze multitone microwave signals on optical fiber with different tone signals and utilizing various possibilities of regeneration and amplification by different means. We compare out the performance analysis of Erbium doped fiber amplifiers (EDFAs) and Semi-conductor Optical Amplifiers (SOA). One tone's frequency varies from 1 to 20 GHz through parametric runs and other has fixed frequency of 5 GHz and third harmonic tones may be monitored.     

Impact analysis on performance optimization of the hybrid amplifier (RA + EDFA)

The configuration of the hybrid amplifier (Raman Amplifier: RA + Erbium-Doped Fiber Amplifiers: EDFA) has been introduced and the restriction conditions of its optimum design have also been analyzed and discussed, and then the ASE noise and Rayleigh noise in Raman Amplifier (RA) as well as their influences on the Signal Noise Ratio (SNR) of the receiver have been analyzed in depth. Furthermore, the influences, which result from these noises, on the performances of the optimum design for the hybrid amplifier (RA + EDFA) have thoroughly been researched. Finally, some significant conclusions to play an important role in guiding the optimum design of the hybrid amplifier have been obtained by the above analyses and researches.     

Analysis of modulation format in the 40 Gbit/s optical communication system

Three different modulation formats including nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed return-to-zero (CS-RZ) and the differential phase keying (DPSK) modulation format of each code are introduced in the article. A method of their modulated signal generation with computer is described, and a comparison of their spectra and waveforms is made. The 40 Gbps signal transmitted in 200 km G.652 fiber by way of single channel with erbium-doped-fiber-amplifier (EDFA) is simulated for these three formats. The ability of anti-dispersion and anti-PMD is analyzed. It is shown that RZ and CS-RZ formats are more tolerant than NRZ format in the same conditions.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.