Null Transmission Research Articles

Physical Layer Security in Distributed Antenna Systems Using One-Bit Feedback Information

This article concerns the physical layer security (PLS) problem in distributed antenna systems (DASs), where eavesdroppers are equipped with multiple antennas. If both legitimate transmitter (Alice) and receiver (Bob) broadcast wireless signals via a DAS, the channel state information (CSI) between Alice and Bob may be exposed to an eavesdropper (Eve) due to the broadcast nature of wireless signals. Based on the exposed CSI, Eve can use multiple antennas to improve the eavesdropping capability on the confidential message transmitted from Alice. The prior PLS approaches to defend against the eavesdropping attack have two limitations. The first limitation is to assume that the CSI of Bob is available to Alice. The second limitation is to assume that there is no error in the feedback CSIs. In this article, we propose a new PLS scheme of a DAS by using one-bit feedback information. The basic idea of our approach is to generate a null transmission vector from the distributed transmitters of Alice to Bob while using different artificial noise components to protect the exposure problem of CSI between Alice and Bob. The proposed scheme has the following advantages: improved secrecy rate, rapid convergence, and good performance under a time-varying channel. We implemented our scheme and conducted extensive performance comparisons through simulations. Our experimental results show that the proposed scheme improves the secrecy rate with 1.20 b/s/Hz as compared to prior approaches. At last, we present some interesting future works for the PLS problem in a DAS.

Electronic cloaking of confined states in phosphorene junctions

We study the electronic transport of armchair (AC) and zigzag (ZZ) gated phosphorene junctions. We find confined states for both direction-dependent phosphorene junctions. In the case of AC junctions confined states are reflected in the transmission properties as Fabry–Pérot resonances at normal and oblique incidence. In the case of ZZ junctions confined states are invisible at normal incidence, resulting in a null transmission. At oblique incidence Fabry–Pérot resonances are presented in the transmission as in the case of AC junctions. This invisibility or electronic cloaking is related to the highly direction-dependent pseudospin texture of the charge carriers in phosphorene. Electronic cloaking is also manifested as a series of singular peaks in the conductance and as inverted peaks in the Seebeck coefficient. The characteristics of electronic cloaking are also susceptible to the modulation of the phosphorene bandgap and an external magnetic field. So, electronic cloaking in phosphorene junctions in principle could be tested through transport, thermoelectric or magnetotransport measurements.

M-QAM signal transmission at the photonically generated K-band over thermal-induced turbulent FSO links with different turbulence distributions

We present a theoretical and experimental study on the impact of different thermal-induced free-space turbulence distributions on the M-quadrature amplitude modulation (M-QAM) signal transmission in radio frequency K-band over hybrid optical links of standard single mode fiber (SSMF) and free-space optics (FSO). Frequency multiplication using an external intensity modulator biased at the null transmission point has been employed to photonically generate radio signals at a frequency of 25 GHz , included for the frequency bands for fifth-generation (5G) mobile networks. Moreover, extensive simulations have been performed for 10Gb/s with 4-, 16-, and 64-QAM over 5 km of SSMF and 500 m long FSO channels under scenarios with different turbulence levels and distributions. Proof-of-concept experiments have been conducted for 20 MHz with 4- and 64-QAM over 5 km of SSMF and 2 m long FSO channels under turbulence conditions. Both theoretical and experimental systems have been analyzed in terms of error vector magnitude (EVM) performance showing feasible transmission over the hybrid links in the received optical power range. Non-uniform turbulence distributions are shown to have a different impact on M-QAM modulation formats, i.e., turbulence distributions with higher strength in the middle of the FSO link reveal a 1.9 dB penalty when using 64-QAM signals compared to a 1.3 dB penalty using 4-QAM signals, whereas higher penalties have been measured when 4-QAM format is transmitted over turbulence distributions with larger magnitude in the second half of the FSO link. The results have been validated by theoretical predictions and lead to practical consequences on future networks' deployment.

Photonic filterless scheme to generate V-band OFDM vector mm-wave signal without precoding

A novel and relatively simple scheme for the generation of high-quality V-band (50–75 GHz) OFDM vector millimeter-wave (mm-wave) is presented, which is only enabled by a dual-parallel Mach–Zehnder modulator (DP-MZM). In this scheme, precoding technique and optical filter are not needed. At the transmitter, electrical dual single sideband (SSB) signals are generated by a digital signal processing (DSP) module, in which the frequency of modulated signals could be adjusted flexibly. The two push–pull sub-MZMs are biased at quadrature transmission points to generate dual optical single sideband (OSSB) signals. The DP-MZM is biased at null transmission point to suppress the center optical carrier and an asymmetrical OSSB signal can be obtained. Then, a higher frequency OFDM vector mm-wave signal is generated after the beating of the asymmetrical OSSB signal at the photodiode. Theoretical analysis is given out in the paper, and V-band OFDM vector mm-wave at the frequency of 65 GHz is generated in the simulation. The results show that the bit-error ratio (BER) of the transmission system is smaller than forward-error correction (FEC) threshold of 3.8×10−3, when the optical power into photodiode (PD) of 10/20/30 Gbaud cases are not less than −19.13 dB, −17.81 dB, −16.50 dB respectively after 50km single model fiber transmission.

Photonic generation of frequency-doubled microwave waveform based on a PDM-MZM modulator

A novel and simple photonic approach for frequency-doubled microwave waveforms generation based on a polarization division multiplexing Mach–Zehnder modulator (PDM-MZM) is proposed and experimentally demonstrated. The PDM-MZM is driven by a local oscillation (LO) signal through a 90 degree hybrid coupler. One sub-modulator of the PDM-MZM is biased at the peak transmission point and the other sub-modulator is biased at the null transmission point. The optical signals from the two arms are polarization multiplexed at the output of the PDM-MZM and then injected into a photodetector (PD) for photoelectrical conversion. By properly controlling the amplitudes of the LO signal, the triangular and square waveforms with a repetition rate twice the frequency of the LO signal can be generated. This scheme has features of compact, low-cost structure and simple operation. Simultaneously, the frequency of the generated waveform can be tuned over a wide bandwidth since there is no optical or electrical filter involved.

Data-Aided Secure Massive MIMO Transmission Under the Pilot Contamination Attack

In this paper, we study the design of secure communication for time division duplex multi-cell multi-user massive multiple-input multiple-output (MIMO) systems with active eavesdropping. We assume that the eavesdropper actively attacks the uplink pilot transmission and the uplink data transmission before eavesdropping the downlink data transmission of the users. We exploit both the received pilots and the received data signals for uplink channel estimation. We show analytically that when the number of transmit antennas and the length of the data vector both tend to infinity, the signals of the desired user and the eavesdropper lie in different eigenspaces of the received signal matrix at the base station provided that their signal powers are different. This finding reveals that decreasing (instead of increasing) the desired user's signal power might be an effective approach to combat a strong active attack from an eavesdropper. Inspired by this observation, we propose a data-aided secure downlink transmission scheme and derive an asymptotic achievable secrecy sum-rate expression for the proposed design. For the special case of a single-cell single-user system with independent and identically distributed fading, the obtained expression reveals that the secrecy rate scales logarithmically with the number of transmit antennas. This is the same scaling law as for the achievable rate of a single-user massive MIMO system in the absence of eavesdroppers. Numerical results indicate that the proposed scheme achieves significant secrecy rate gains compared to alternative approaches based on matched filter precoding with artificial noise generation and null space transmission.

Anopheles mosquitoes may drive invasion and transmission of Mayaro virus across geographically diverse regions.

The Togavirus (Alphavirus) Mayaro virus (MAYV) was initially described in 1954 from Mayaro County (Trinidad) and has been responsible for outbreaks in South America and the Caribbean. Imported MAYV cases are on the rise, leading to invasion concerns similar to Chikungunya and Zika viruses. Little is known about the range of mosquito species that are competent MAYV vectors. We tested vector competence of 2 MAYV genotypes in laboratory strains of six mosquito species (Aedes aegypti, Anopheles freeborni, An. gambiae, An. quadrimaculatus, An. stephensi, Culex quinquefasciatus). Ae. aegypti and Cx. quinquefasciatus were poor MAYV vectors, and had either poor or null infection and transmission rates at the tested viral challenge titers. In contrast, all Anopheles species were able to transmit MAYV, and 3 of the 4 species transmitted both genotypes. The Anopheles species tested are divergent and native to widely separated geographic regions (Africa, Asia, North America), suggesting that Anopheles may be important in the invasion and spread of MAYV across diverse regions of the world.

Bandgap widening in macroporous silicon photonic crystals by multiperiodic structures

Large bandgaps with low transmission in 3D macroporous silicon photonic crystals have been proved as an interesting technology for the development of optical filters and spectroscopic MIR gas sensors. The aim of this study is the investigation of different bandgap widening methods based on multiperiodic structures for 3D macroporous silicon photonic crystals. To do so, chirped modulations and structures with different periodicity groups have been modelled and theoretically analysed by means of 3D FDTD simulations. They have revealed that by using different decreasing periodicity groups, bandgaps with null transmission and widths as high as 1800 nm, 4 times the original single periodicity photonic crystal bandgap, can be obtained. Furthermore, it has been shown that a resonant cavity with a 20% transmission can be placed in a 1 μm wide bandgap. The results open a way to use this type of structures not only for gas sensing but also for other purposes such as wide stop-band filters, selective filters or broadband mirrors.

Changes in structure and performance during diafiltration of binary protein solutions due to repeated cycles of fouling/alkaline cleaning

The purpose of this work was to evaluate the effect of the temperature (50 and 60°C) of a NaOH cleaning solution during the diafiltration of a binary mixture of bovine serum albumin and β-lactoglobulin, through a 300kDa tubular ceramic membrane along repeated operational cycles. To this aim, final permeate volume, membrane and fouling resistances and individual protein concentration were analyzed. At the end of each individual study, the membranes were characterized by liquid–liquid displacement porosimetry. As a result, 50°C was found to be the most appropriated temperature due to its higher capability to restore the initial membrane resistance and the higher efficiency achieved in terms of protein separation. Both conditions fulfilled without altering the structural properties of the membrane as given by porosimetric analysis. In contrast, a great fouling resistance involving null protein transmission occurred when cleaning was performed at 60°C.

Modes Manipulation Within Subwavelength Metallic Gratings

Surface plasmon polariton (SPP) and cavity modes supported by the plasmonic gratings are often of hybrid nature in ruling light-nanostructure interactions. The coupling of incident light to SPP waves through the excitation of SPP modes usually traps the light at the air-metal interface, while the coupling to standing waves through cavity modes confines the light within the cavities. However, in a specific application, the highly selective coupling of incident light to either SPP or cavity mode is always required. We present here, for a single metal-insulator-metal (MIM) groove, the coupling strength of incident light to a certain mode can be engineered by varying the groove width and depth. To further enhance the coupling selectivity, the periodicity was introduced to the MIM grooves: When the period approaches (deviates from) the wavelength of SPPs, the incident light highly couples to SPP (cavity) mode, due to the constructive (destructive) interference between the propagating SPPs at metal-air interfaces. Two types of metallic gratings, aluminum grating with its period approaching the λSPP and iridium grating with its period far away deviating from the λSPP, were fabricated and measured. The almost null TM transmission through aluminum grating and detection of surface enhanced Raman scattering (SERS) signal from iridium grating solidly guarantee the validity of the proposed method in manipulating the modes, through which the energy flow can be guided to its supposed-to-appear regions.

Molecular mechanisms of action of herbal antifungal alkaloid berberine, in Candida albicans.

Candida albicans causes superficial to systemic infections in immuno-compromised individuals. The concomitant use of fungistatic drugs and the lack of cidal drugs frequently result in strains that could withstand commonly used antifungals, and display multidrug resistance (MDR). In search of novel fungicidals, in this study, we have explored a plant alkaloid berberine (BER) for its antifungal potential. For this, we screened an in-house transcription factor (TF) mutant library of C. albicans strains towards their susceptibility to BER. Our screen of TF mutant strains identified a heat shock factor (HSF1), which has a central role in thermal adaptation, to be most responsive to BER treatment. Interestingly, HSF1 mutant was not only highly susceptible to BER but also displayed collateral susceptibility towards drugs targeting cell wall (CW) and ergosterol biosynthesis. Notably, BER treatment alone could affect the CW integrity as was evident from the growth retardation of MAP kinase and calcineurin pathway null mutant strains and transmission electron microscopy. However, unlike BER, HSF1 effect on CW appeared to be independent of MAP kinase and Calcineurin pathway genes. Additionally, unlike hsf1 null strain, BER treatment of Candida cells resulted in dysfunctional mitochondria, which was evident from its slow growth in non-fermentative carbon source and poor labeling with mitochondrial membrane potential sensitive probe. This phenotype was reinforced with an enhanced ROS levels coinciding with the up-regulated oxidative stress genes in BER-treated cells. Together, our study not only describes the molecular mechanism of BER fungicidal activity but also unravels a new role of evolutionary conserved HSF1, in MDR of Candida.

A pre-breeding screening program for transgenic boars based on fluorescence in situ hybridization assay.

For efficient transgenic herd expansion, only the transgenic animals that possess the ability to transmit transgene into next generation are considered for breeding. However, for transgenic pig, practically lacking a pre-breeding screening program, time, labor and money is always wasted to maintain non-transgenic pigs, low or null transgenic transmission pigs and the related fruitless gestations. Developing a pre-breeding screening program would make the transgenic herd expansion more economical and efficient. In this technical report, we proposed a three-step pre-breeding screening program for transgenic boars simply through combining the fluorescence in situ hybridization (FISH) assay with the common pre-breeding screening workflow. In the first step of screening, combined with general transgenic phenotype analysis, FISH is used to identify transgenic boars. In the second step of screening, combined with conventional semen test, FISH is used to detect transgenic sperm, thus to identify the individuals producing high quality semen and transgenic sperm. In the third step of screening, FISH is used to assess the in vitro fertilization embryos, thus finally to identify the individuals with the ability to produce transgenic embryos. By this three-step screening, the non-transgenic boars and boars with no ability to produce transgenic sperm or transgenic embryos would be eliminated; therefore only those boars could produce transgenic offspring are maintained and used for breeding and herd expansion. It is the first time a systematic pre-breeding screening program is proposed for transgenic pigs. This program might also be applied in other transgenic large animals, and provide an economical and efficient strategy for herd expansion.

Performance analysis of automatic feedback control for multiple bias voltages in optical quadrature phase-shift keying modulator

We study the performance of automatic bias control for three bias voltages in optical quadrature phase-shift keying modulator. The interaction between multiple bias controls for two Mach-Zehnder modulators’ (MZMs’) bias voltages and a Mach-Zehnder interferometer’s (MZI’s) bias voltage is analyzed theoretically. The condition for achieving global extremum of the monitored optical power with two MZMs’ bias voltages at the null transmission points is also studied by numerical simulations. The results show that the bias voltages of two MZMs can be controlled regardless of the MZI’s bias voltage and can avoid the risk of trapping in local extremum if the driver signal amplitude operates in the proper range.

Photoacoustic Monitoring of the Macroscopic Orientational Order in Disperse Red 1 Azo-Dye-Based Dissolutions

The polarized pulsed-laser photoacoustic (PLPA) methodology was implemented to characterize the macroscopic molecular orientational order as a function of the applied electric field in an azo-dye-based dissolution. It is shown in this work that photoacoustic signals are highly sensitive to the material anisotropy when PLPA analyses are performed. In particular, the macroscopic molecular symmetries and structural organization of rod-like azo-dyes (Disperse Red 1: DR1) prepared in heavily loaded toluene dissolutions, have been studied. Experiments evidence unambiguous information concerning the molecular organization detected via simultaneous optical transmission and PLPA measurements. Indeed, in the electrically poled stage, it is shown that the Fourier, correlation, and root-mean-square analyses performed on the PLPA signals, as functions of the incident linear polarization of the laser source (orthogonal states $$P_\mathrm{in}$$ and $$S_\mathrm{in}$$ , respectively), reveal the main optical axis, symmetry, and structure of the aligned compounds in the liquid phase. An important advantage of the PLPA method is that it can be implemented with poor or null optical transmission anisotropic materials.

Surface-Surface Interaction in Smectic Liquid Crystal Films

Null transmission ellipsometry was employed to study the field induced transition of the surface arrangements in freestanding films of smectic liquid crystals. The interlayer interaction between the two surfaces obtained from the threshold voltage for the transition is found to be antiferroelectric and is quasilong ranged. The possible microscopic origins of the measured interaction and its relevance to the interlayer interaction in antiferroelectric liquid crystal materials are discussed.

Monitoring molecular orientational order in NLO push–pull based polymeric films via photoacoustic measurements

The pulsed-laser photoacoustic-technique (PLPA) was implemented to characterize molecular orientational order and anisotropy in push–pull poled polymeric films as function of temperature and laser polarization. Traditionally, photoacoustic signals are considered to be directly proportional to the linear optical absorption in amorphous media. In this work, however, it is shown that photoacoustic signals can also be highly sensitive to the material anisotropy when convenient polarization dependent photoacoustic analyses are performed. Thus, variation of the molecular orientation in organic films, comprising rod-like polar chromophores, can be unambiguously monitored via rms-analyses performed on the amplitude of the generated opto-acoustical PLPA-signals as function of the incident laser polarization. This result can be useful for the characterization of organic-based nonlinear optical (NLO) poled films and, in general, in studies of anisotropic materials. In fact, in this work we were able to accurately determine the molecular order parameter ( ϕ) of a NLO-active spin-coated polymeric film containing optically active push–pull chromophores. These molecules, previously oriented via an electrical-poling procedure, are capable to exhibit strong second harmonic generation (SHG) effects. The PLPA-measurements were systematically compared to the linear UV–vis optical absorbance spectra while heating the poled film sample in order to monitor the thermally induced molecular disorder, so that the order parameter may be photo-acoustically evaluated via the PLPA-signals generated from the poled to the unpoled film phase. These PLPA-experiments were performed taking into account the UV–vis reference spectra for calibration and comparison purposes in the evaluation of the order parameter. A significant advantage of the PLPA-technique over commonly used optical spectral methodologies is its convenient applicability in samples exhibiting poor or null optical transmission.

Surface-induced reduction of twisting power in liquid-crystal films

Null transmission ellipsometry was employed to study the temperature evolution of the helical structure of the smectic-C(α)* phase. Free-standing films with thickness ranging from 31 to more than 400 layers were prepared and studied. The experimental results show a reduced twisting power in thin films. A simple model was constructed to explain the results. Surface effects were found to be the key reason for this phenomenon. Our findings are consistent with the theoretical studies of helically ordered magnetic films.

Effect of enantiomeric excess on the phase behavior of antiferroelectric liquid crystals

Null transmission ellipsometry and resonant x-ray diffraction are employed to study the effect of enantiomeric excess (EE) on the phase behavior of antiferroelectric liquid crystal 10OTBBB1M7. Phase sequence, layer spacing, and pitch of the helical structures of the smectic-C(α)* and smectic-C* phases are studied as a function of temperature and EE. Upon reducing EE, a liquid-gas-type critical point of the smectic-C(α)* to smectic-C* transition is observed, as well as the disappearance of the smectic-C(d4)* and the smectic-C(d3)* phases. Results are analyzed in a mean-field model.

Surface and Bulk Uniaxial to Biaxial Smectic-ATransition in a Bent Core Liquid Crystal

A null transmission ellipsometer was employed to study the uniaxial to biaxial smectic-A phase transition of a bent core liquid crystal material. Free-standing films of thicknesses ranging from 5 molecular layers to more than 300 were prepared and studied. Critical exponents for both the surface and interior biaxiality were obtained. The results were discussed in the general framework of phase transition in lower dimensions.

Plasmon-Induced Nearly Null Transmission of Light Through Gratings in Very Thin Metal Films

Based on numerical simulations, we show that a very thin metal (Ag) film, otherwise transmissive partially, becomes opaque for transverse magnetic-polarized light in a certain spectrum band when perforated with grating-like slits. Positions of the nearly null transmission band are dependent on the various structure dimensions, particularly on the ridge width for gratings with relatively narrow slit width. Our analyses show that the nearly null transmission is related to resonant excitation of anti-symmetric bound surface plasmon waves at the ridges of the thin metal film gratings and further resulted from destructive interference of waves evolved from the fields at the ridges and slits that are in opposite phase. It is also found that for 2D gratings, the nearly null transmission band appears only for disk array-type gratings and not for the hole array-type gratings. This structure may be applied in novel photonic devices to enhance their performances and functionalities.