Effect Of Multiple Interference Research Articles

Low temperature crystallization of diamond-like carbon films to graphene

Plasma surface modification was used to fabricate graphene on the top of insulating diamond-like carbon films. It is shown that by a combination of pulsed argon plasma treatment and thermal annealing at 350̊C it is possible to achieve crystallization of amorphous carbon to graphene. The observed Raman spectra are typical for defected graphene-splitted D- and G-peaks and a broad 2D-peak. Because interpretation of Raman spectra of such complicated system is not easy we have calculated Raman signals of graphene on an amorphous hydrogenated carbon film deposited on a Si substrate. Our simulation results show that multiple reflections and interference effects lead to enhancement of Raman signal of the system. The characteristic for graphene G and 2D bands reach maximal enhancement for thicknesses of the amorphous hydrogenated carbon film of about 75nm and 230nm. We estimate that the interference enhancement of the 2D graphene Raman signal is very weak in contrast to that of the G band signal simulated for the underlying diamond-like carbon films on silicon substrate only. Therefore experimentally measured Raman spectra of the whole graphene/a-C:H/Si system probably will consist of interference enhanced but still weak 2D graphene peak and stronger D and G peaks dominated by G and D Raman bands of the a-C:H. This conclusion is in line with observed experimental Raman spectra. Electrical field effect measurements of the samples show ambipolar dependence, typical for single-layer graphene.

Performance of joint transmit scheme assisted multiple-input multiple-output multi-carrier IDMA system

In this paper, we present the performance of a multiple-input multiple-output multi-carrier interleave division multiple access (MC-IDMA) system assisted by combined vertical Bell Laboratories layered space–time architecture and space–time block code processing over correlated frequency-selective channels for downlink communication. In MC-IDMA system, multi-carrier is realized by orthogonal frequency division multiplexing, which can eliminate the effects of time dispersion. At the mobile station, we employ zero forcing maximum-a-posteriori and minimum mean square error MAP (MMSE/MAP) multi-user detectors (MUD) to mitigate the effects of multi stream interference and iterative chip detection to combat the effects of multiple access interference. The performance of the considered system is compared with double space–time transmit diversity (DSTTD) aided MC-IDMA system. The simulation results show that the considered scheme outperforms DSTTD MC-IDMA and MC-CDMA system with MMSE/MAP MUD in terms of achievable bit error rate.

Optical Interference Substrates for Nanoparticles and Two-Dimensional Materials

Interference substrates are useful in enhancing Raman and luminescence signals and in increasing the optical contrast of nanoparticles and atomically thin layers. Interference substrates played a crucial role in the discovery of the electric field effect on electronic conduction in graphene a few years ago. They are now used for a wide range of two-dimensional materials and recently for the heterostructures of atomically thin films. The same or similar substrates can be employed for the exploration of a large variety of nanoparticles. Although optical interference has been known to occur within the proximity of surfaces for more than one century, optical interference has been only gradually used over the last two decades to enhance the optical response of nanoparticles and layered materials. We review the different forms of interference substrates used over time. While multiple interference effects are frequently put forward to explain the enhancement in interference substrates, we show here that the formation of optical surface standing waves near strongly reflecting surfaces is the main cause of field enhancement. In addition, we demonstrate how a metal layer improves optical field enhancement.

One-User/One-Group Soft-Decision Aided Multiuser Detection for 2D Spread MC DS-CDMAs

We consider the uplink of a multiuser, multiple-input multiple-output (MIMO), frequency-time-domain spread, multi- carrier (MC), direct sequence code division multiple access (DS-CDMA) system. As other CDMA-like systems, the multiple access interference (MAI) effect still exists in such an MC DS-CDMA system. To mitigate the MAI effect, we propose user-based and group-based layered detection schemes. Specifically, to enable a trade-off between the performance and the computational complexity, the schemes only use one user's/group's soft decisions for user-based/ group-based layered detection. The results of simulations demonstrate that the proposed schemes outperform existing approaches, and their computational complexity is modest.

One-user/one-group soft-decision aided layered detection for multiuser MIMO 2D spread multi-carrier DS-CDMA systems

We consider the uplink of a multiuser, multiple input multiple output (MIMO), frequency-time-domain spread, multi-carrier (MC), direct sequence code division multiple access (DS-CDMA) system. In this two-domain (2D) spread system, users are arranged in groups. Each user is assigned a unique time-domain (T-domain) signature code and also shares a frequency-domain (F-domain) signature code with users in the same group. Moreover, each user employs his/her individual T-domain signature code and the shared F-domain signature code to spread multiple symbols, and then sends the spreading signals in parallel from multiple transmit antennas to the antenna array at the base station. However, the multiple access interference (MAI) effect, which results from the non-orthogonality of signature codes and is the main performance limitation, still exists in such an MC DS-CDMA system. To mitigate the MAI effect and improve the performance, we propose user-based and group-based layered detection schemes. Specifically, to enable a trade-off between the performance and the computational complexity, the schemes only use one user's/group's soft decisions for user-based/group-based layered detection. The results of simulations demonstrate that the proposed schemes outperform existing approaches, and their computational complexity is modest.

Collusion resilient spread spectrum watermarking in M-band wavelets using GA-fuzzy hybridization

This paper proposes a collusion resilient optimized spread spectrum (SS) image watermarking scheme using genetic algorithms (GA) and multiband (M-band) wavelets. M-band decomposition of the host image offers advantages of better scale-space tiling and good energy compactness. This bandpass-like decomposition makes watermarking robust against frequency selective fading-like gain (intelligent collusion) attack. On the other hand, GA would determine threshold value of the host coefficients (process gain i.e. the length of spreading code) selection for watermark casting along with the respective embedding strengths compatible to the gain of frequency response. First, a single bit watermark embedding algorithm is developed using independent and identically distributed (i.i.d) Gaussian watermark. This is further modified to design a high payload system for binary watermark image using a set of binary spreading code patterns. Watermark decoding performance is improved by multiple stage detection through cancelation of multiple bit interference (MBI) effect. Fuzzy logic is used to classify decision magnitudes in multiple group combined interference cancelation (MGCIC) used in the intermediate stage(s). Simulation results show convergence of GA and validate relative performance gain achieved in this algorithm compared to the existing works.

Comparative Analysis Of Peak Correlation Characteristics Of Non-Orthogonal Spreading Codes For Wireless Systems

The performance of a CDMA based wireless system is largely dependent on the characteristics of pseudo-random spreading codes. The spreading codes should be carefully chosen to ensure highest possible peak value of auto-correlation function and lower correlation peaks (side-lobes) at non-zero time-shifts. Simultaneously, zero cross-correlation value at all time shifts is required in order to eliminate the effect of multiple access interference at the receiver. But no such code family exists which possess both characteristics simultaneously. That's why an exhaustive effort has been made in this paper to evaluate the peak correlation characteristics of various non-orthogonal spreading codes and suggest a suitable solution.

On the effect of Multiple Access Interference in a Space Time Spreading Time Hopping PPM UWB system

This study investigates the effect of multiple sources using STS across a Time-Hopping-PPM Modulation that is over an UWB Saleh-Valenzuela channel with each source using two transmit antennas. We model and analyze this system, showing that the presence of such MAI results in a significant degradation in BER performance. Simulation results show that for strong signal strength of interfering MAI signals degradation in BER performance is observed for the cases where we have five and 10 distinct MAI sources present when using Walsh-Hadamard thirty two chip spreading sequences. Using simulation, we investigate the use of other sets of orthogonal spreading sequences with improved cross correlation properties and find that for some sequences a significant improvement is seen compared to Walsh-Hadamard sequences when they are similarly misaligned. We show that MAI degrades the BER performance of such systems and that selection of improved cross correlation spreading code sequence sets improves BER.

Femtosecond time-resolved Faraday rotation in thin magnetic films and magnetophotonic crystals

Femtosecond time-resolved Faraday rotation is studied in magnetic garnet films and magnetophotonic crystals. Femtosecond dynamics of Faraday angle governed by multiple reflection interference and Faraday effect non-reciprocity is revealed by using polarization-sensitive ultrafast correlation technique.

The effects of multiple access interference and polarization mode dispersion on the outage probability and blocking probability in optical code paths

ABSTRACTIn this work, we investigate through a formal approach the effects of multiple access interference and polarization mode dispersion on the blocking probability in optical code paths (OCP)s. These paths are determined by the data encoding with non‐coherent codes from code division multiplexing using prime code sequences. We modeled the blocking probability in the OCPs considering the outage probability like a threshold to determine the number of acceptable active OCPs in each wavelength. In this model, new OCPs are granted until the number of active OCPs on the wavelength reaches the blocking threshold; after that, requests are blocked. Results show quantitatively the contribution of multiple access interference effects and polarization mode dispersion constraints on the outage probability and their impact on the blocking probability in the OCPs. Copyright © 2012 John Wiley & Sons, Ltd.

Semi-Blind HEBB-PPIC Multiuser Detection Algorithm

Inter-cell interference greatly impairs the capacity of CDMA system. 4G era is coming with much severer inter-cell inte-rference, and it's important to find a good solution to inter-cell Interference cancellation. In this paper, the semiblind Hebb-PPIC Multi-User Detection(MUD algorithm which applied in the CDMA uplink is proposed. This approach fully utilizes all known users’ information, performs Hebb-PPIC detector to suppress the int-erferers within the cell, and exploits the blind detector to suppress the inter-cell interference. Simulation results demonstrate that the proposed Hebb-PPIC semi-blind algorithm offers substantial perfo-rmance gains over the blind multi-user detection methods, and the Multiple Access Interference and Near-Far effect can be suppressed effectively in a CDMA uplink environment.

Solving the Near-Far Problem in Dynamic Frequency Hopping-Optical Code Division Multiple Access using Power Control

In this study, a distributed power control algorithm is proposed for Dynamic Frequency Hopping Optical-CDMA (DFH-OCDMA) system. In general, the DFH-OCDMA can support higher number of simultaneous users compared to other OCDMA techniques. However, the performance of such system degrades significantly as the received power does lower than its minimum threshold. This may obviously occur in a DFH-OCDMA network with near-far problem which consist of different fiber lengths among the users, that resulting to unequal power attenuation. The power misdistribution among simultaneous active users at the star coupler would degrade the Bit Error Rate (BER) performance for users whose transmitting signals with longer fiber lengths. In order to solve these problems, we propose an adaptive distributed power control technique for DFH-OCDMA to satisfy the target Signal to Noise Ratio (SNR) for all users. Taking into account the noise effects of Multiple Access Interference (MAI), Phase Induced Intensity Noise (PIIN) and shot noise, the system can support 100% of users with power control as compared to 33% without power control when the initial transmitted power was −1dBm with 30 simultaneous users.

Solving the Near-Far Problem in Dynamic Frequency Hopping-Optical Code Division Multiple Access using Power Control

In this study, a distributed power control algorithm is proposed for Dynamic Frequency Hopping Optical-CDMA (DFH-OCDMA) system. In general, the DFH-OCDMA can support higher number of simultaneous users compared to other OCDMA techniques. However, the performance of such system degrades significantly as the received power does lower than its minimum threshold. This may obviously occur in a DFH-OCDMA network with near-far problem which consist of different fiber lengths among the users, that resulting to unequal power attenuation. The power misdistribution among simultaneous active users at the star coupler would degrade the Bit Error Rate (BER) performance for users whose transmitting signals with longer fiber lengths. In order to solve these problems, we propose an adaptive distributed power control technique for DFH-OCDMA to satisfy the target Signal to oise Ratio (S R) for all users. Taking into account the noise effects of Multiple Access Interference (MAI), Phase Induced Intensity oise (PII ) and shot noise, the system can support 100% of users with power control as compared to 33% without power control when the initial transmitted power was -1dBm with 30 simultaneous users.

Effect of multipath fading and multiple access interference on broadband code division multiple access systems

SUMMARYIn this paper, the performance of uncoded uplink transmissions in a broadband code division multiple access (CDMA) system using tapped delay line (TDL) antenna array under multipath conditions is investigated. An expression for broadband CDMA system is derived to calculate the optimal weights of TDL antenna array. Using the derived broadband expression, the performance of TDL antenna array is investigated and compared with that of the already existing narrowband systems. Simulation results show that TDL antenna array can reduce multipath fading and multiple access interference (MAI). It is shown that if the signal‐to‐noise ratio (SNR) level is maintained at 20 dB and two‐ray channel model is used, two antennas with four taps each can support 30 users with a BER of 10−3. If three‐ray channel model is used, the same antenna arrangement is not enough to support 30 users. To support these 30 users with BER of 10−3, the number of taps needs to be increased to eight. Finally, the number of taps needs to be further increased to 16 if four‐ray channel model is used. Not only that, the SNR should also be increased to 25 dB to achieve the same BER. Copyright © 2011 John Wiley & Sons, Ltd.

Development of a new code family based on SAC-OCDMA system with large cardinality for OCDMA network

We have proposed a new Multi-Diagonal (MD) code for Spectral Amplitude – Coding Optical Code Division Multiple Access (SAC-OCDMA). Although this new MD code has many properties, one of the important properties of this code is that the cross correlation is always zero. Simplicity in code construction and flexibility in cross correlation control has made this code a compelling candidate for future OCDMA applications. The Multiple access interference (MAI) effects have been successfully and completely eliminated. Based on the theoretical analysis MD code is shown here to provide a much better performance compared to Modified Quadratic Congruence (MQC) code and Random Diagonal (RD) code. Proof-of-principle simulations of encoding with 5 and 10 users with 622 Mb/s data transmission at a BER of 10 −12 have been successfully demonstrated together with the DIRECT detection scheme.

Subterahertz hypersound attenuation in silica glass studied via picosecond acoustics

We report picosecond acoustic measurements in silica-glass films grown by wet thermal oxidation on a $(111)$ silicon substrate. The longitudinal acoustic phonons are observed over the range from 150 to 300 GHz using an infrared pump and a second harmonic blue probe. The transducer is an aluminum thin film deposited on top. Multiple interference effects are analyzed and fully taken into account. They lead to a signal presenting rapid oscillations as a function of the sample thicknesses. The latter are determined by separate interferometry. Our remarkably precise acoustic attenuation results are found to follow rather well a model combining thermally activated relaxations and anharmonicity. New values for the optical absorption of silicon in the 400-to-500-nm region of the spectrum are obtained as a by-product.

Modeling of reflectometric and ellipsometric spectra from the skin in the terahertz and submillimeter waves region

The human skin is modeled as a stack of homogeneous layers in the terahertz and submillimeter waves regions with some anisotropy due to the helical sweat glands and other elongated entities. A dielectric model for the skin is presented, valid for a wider frequency range (up to the terahertz region) taking into account the dispersive nature of the effective conductivity. Polarized reflectivity and generalized ellipsometric parameters are calculated versus angle and wavelength. Recent studies have claimed that the helical sweat ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the sub-terahertz region. We found that water absorption, dispersion and multiple interference effects play the major role in shaping the spectrum without the need for the assumption of the sweat ducts acting as low-Q helical antennae. High sensitivities to the water content are found particularly in the ellipsometric parameters at large incidence angles. Hence a new methodology is proposed to detect skin cancer using variable angle ellipsometry or polarized reflectometry. The parameter found with the highest sensitivity to water content is cos Δ(pp) with Δ(pp) being the phase of the on-diagonal reflection matrix ratio between p-to-p polarization.

Distributed power control algorithms for asynchronous CDMA systems in frequency-selective fading channels

In Code Division Multiple Access (CDMA) radio environments, the maximum number of supportable users per cell is limited by multipath fading, shadowing, multiple access interference and near-far effects which cause fluctuations of the received power at the base station. In this context, power control and signal detection are essential to provide satisfactory Quality of Service (QoS) and to combat the near-far problem in CDMA systems. In this paper, we raised the uplink power control problem for a generalize asynchronous direct-sequence (DS) CDMA system that explicitly incorporate into the analysis: (1) the propagation delays in the network (generally neglected in the literature), (2) the adverse effect of multipath fading for wireless channels, and (3) the asynchronous transmissions in the uplink channels. This framework is used to propose a distributed power control strategy enhanced with linear multiuser receivers. It is shown that through a proper selection of an error function, the nonlinear coupling among active users is transformed into individual linear loops. A Linear-Quadratic-Gaussian (LQG) power control strategy is derived and compared with other approaches from the literature. Simulation results show that the uplink channel variations do not destroy the stability of these power control structures. However, delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. It is also shown that the use of multiuser detection at the base station can bring significant improvements to the performance of power control.

Does human skin truly behave as an array of helical antennae in the millimeter and terahertz wave ranges?

The sweat ducts of the human perspiration system are helically shaped tubes, filled with a conductive aqueous solution. Recent studies have claimed that these ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the subterahertz region. Using local homogenization theory for the skin embedded with sweat ducts, we found that multiple interference effects from the skin layers play the major role in determining the skin electromagnetic characteristics in the millimeter and terahertz regions without the need for the assumption of the sweat ducts acting as low-Q helical antennae.

Performance analysis of asynchronous time-spreading phase coding and wavelength-time OCDMA systems

We compare the theoretical bit-error-rate (BER) and throughput performance of asynchronous time-spreading (TS) phase coding and two-dimensional (2D) wavelength-time (WT) optical code division multiple access (OCDMA) communication systems. Particular consideration is given, however, to both beat noise (BN) and multiple access interference (MAI) effects. BER performance of the TS OCDMA scheme is much superior to that of the WT scheme in the MAI limited case, while the WT scheme has better tolerance to beat noise. Improvement of BER performance and throughput can be obtained by utilization of optimum threshold detection. The two OCDMA schemes have comparable bit-error probability, and the WT scheme achieves a higher throughput for a heavy load when the optimum threshold is employed.