Compressed sensing based cyclic feature spectrum sensing for cognitive radios

Spectrum sensing is currently one of the most challenging design problems in cognitive radio. A robust spectrum sensing technique is important in allowing implementation of a practical dynamic spectrum access in noisy and interference uncertain environments. In addition, it is desired to minimize the sensing time, while meeting the stringent cognitive radio application requirements. To cope with this challenge, cyclic spectrum sensing techniques have been proposed. However, such techniques require very high sampling rates in the wideband regime and thus are costly in hardware implementation and power consumption. In this thesis the concept of compressed sensing is applied to circumvent this problem by utilizing the sparsity of the two-dimensional cyclic spectrum. Compressive sampling is used to reduce the sampling rate and a recovery method is developed for re- constructing the sparse cyclic spectrum from the compressed samples. The reconstruction solution used, exploits the sparsity structure in the two-dimensional cyclic spectrum do-main which is different from conventional compressed sensing techniques for vector-form sparse signals. The entire wideband cyclic spectrum is reconstructed from sub-Nyquist-rate samples for simultaneous detection of multiple signal sources. After the cyclic spectrum recovery two methods are proposed to make spectral occupancy decisions from the recovered cyclic spectrum: a band-by-band multi-cycle detector which works for all modulation schemes, and a fast and simple thresholding method that works for Binary Phase Shift Keying (BPSK) signals only. In addition a method for recovering the power spectrum of stationary signals is developed as a special case. Simulation results demonstrate that the proposed spectrum sensing algorithms can significantly reduce sampling rate without sacrifcing performance. The robustness of the algorithms to the noise uncertainty of the wireless channel is also shown.

Compressed sensing approach to ultra-wideband receiver design

One of the scarcest resources in the wireless communication system is the limited frequency spectrum. Many wireless communication systems are hindered by the bandwidth limitation and are not able to provide high speed communication. However, Ultra-wideband (UWB) communication promises a high speed communication because of its very wide bandwidth of 7.5GHz (3.1GHz-10.6GHz). The unprecedented bandwidth promises many advantages for the 21st century wireless communication system. However, UWB has many hardware challenges, such as a very high speed sampling rate requirement for analog to digital conversion, channel estimation, and implementation challenges. In this thesis, a new method is proposed using compressed sensing (CS), a mathematical concept of sub-Nyquist rate sampling, to reduce the hardware complexity of the system. The method takes advantage of the unique signal structure of the UWB symbol. Also, a new digital implementation method for CS based UWB is proposed. Lastly, a comparative study is done of the CS-UWB hardware implementation methods. Simulation results show that the application of compressed sensing using the proposed method significantly reduces the number of hardware complexity compared to the conventional method of using compressed sensing based UWB receiver.

Electrochemical characterization of self-assembled Ferrocene-terminated alkanethiol monolayers on low-index gold single crystal electrodes

We present a voltammetric and in situ STM study of 11-ferrocenyl-1-undecanethiol (FcC11) assembled on low-index single crystal and polycrystalline gold electrodes. The influence of electrode surface structure as well as of structure defects in the self-assembled FcC11 monolayers on the electrochemical response during the oxidation and reduction of the terminal ferrocene group is explored. The nature of the redox peaks is discussed in detail. We identified the coexistence of disordered FcC11 regions with 2D patches of “locally ordered” FcC11 species. We demonstrate that close-packed domains are preferentially formed at atomically flat terraces. Increasing the defect density of the substrate surface leads to a decreasing amount of locally ordered FcC11 molecules.

Ferrocene-terminated alkanethiol self-assembled monolayers: An electrochemical and in situ surface-enhanced infra-red absorption spectroscopy study

The effect of anions on the redox behavior and structure of 11-ferrocenyl-1-undecanethiol (FcC11) monolayers (SAM) on Au(1 1 1) single crystal and Au(1 1 1-25 nm) thin film electrodes was investigated in 0.1 M solutions of HPF6, HClO4, HBF4, HNO3, and H2SO4 by cyclic voltammetry (CV) and in situ surface-enhanced infrared reflection-absorption spectroscopy (SEIRAS). We demonstrate that the FcC11 redox peaks shift toward positive potentials and broaden with increasing hydrophilicity of the anions. In situ surface-enhanced IR-spectroscopy (SEIRAS) provided direct access for the incorporation of anions into the oxidized adlayer. The coadsorption of anions is accompanied by the penetration of water molecules. The latter effect is particularly pronounced in aqueous HNO3 and H2SO4 electrolytes. The adlayer permeability increases with increasing hydrophilicity of the anions. We also found that even the neutral (reduced) FcC11 SAM is permeable for water molecules. Based on the property of interfacial water to reorient upon charge inversion, we propose a spectroscopic approach for estimating the potential of zero total charge of the FcC11-modified Au(1 1 1) electrodes in aqueous electrolytes.

Search for pair-produced third-generation squarks decaying via charm quarks or in compressed supersymmetric scenarios in pp collisions at √s=8 TeV with the ATLAS detector

Results of a search for supersymmetry via direct production of third-generation squarks are reported, using 20.3  fb −1 of proton-proton collision data at √s =8  TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojetlike and c -tagged event selections are carried out to optimize the sensitivity for direct top squark-pair production in the decay channel to a charm quark and the lightest neutralino (t 1 →c+χ ˜ 0 1 ) across the top squark–neutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the m ˜t 1, m ˜ X0 1 ) parameter space. A top squark of mass up to about 240 GeV is excluded at 95% confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojetlike analysis are also interpreted in terms of compressed scenarios for top squark-pair production in the decay channel t ˜ 1 →b+ff ′ +χ ˜ 0 1 and sbottom pair production with b ˜ 1 →b+χ ˜ 0 1 , leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.

Gas adsorption and desorption effects on cylinders and their importance for long-term gas records

It is well known that gases adsorb on many surfaces, in particular metal surfaces. There are two main forms responsible for these effects (i) physisorption and (ii) chemisorption. Physisorption is associated with lower binding energies in the order of 1–10 kJ mol−¹, compared to chemisorption which ranges from 100 to 1000 kJ mol−¹. Furthermore, chemisorption only forms monolayers, contrasting physisorption that can form multilayer adsorption. The reverse process is called desorption and follows similar mathematical laws; however, it can be influenced by hysteresis effects. In the present experiment, we investigated the adsorption/desorption phenomena on three steel and three aluminium cylinders containing compressed air in our laboratory and under controlled conditions in a climate chamber, respectively. Our observations from completely decanting one steel and two aluminium cylinders are in agreement with the pressure dependence of physisorption for CO₂, CH₄, and H₂O. The CO₂ results for both cylinder types are in excellent agreement with the pressure dependence of a monolayer adsorption model. However, mole fraction changes due to adsorption on aluminium (< 0.05 and 0 ppm for CO₂ and H₂O) were significantly lower than on steel (< 0.41 ppm and about < 2.5 ppm, respectively). The CO₂ amount adsorbed (5.8 × 1019 CO₂ molecules) corresponds to about the fivefold monolayer adsorption, indicating that the effective surface exposed for adsorption is significantly larger than the geometric surface area. Adsorption/desorption effects were minimal for CH₄ and for CO but require further attention since they were only studied on one aluminium cylinder with a very low mole fraction. In the climate chamber, the cylinders were exposed to temperatures between −10 and +50 °C to determine the corresponding temperature coefficients of adsorption. Again, we found distinctly different values for CO₂, ranging from 0.0014 to 0.0184 ppm °C−¹ for steel cylinders and −0.0002 to −0.0003 ppm °C−¹ for aluminium cylinders. The reversed temperature dependence for aluminium cylinders points to significantly lower desorption energies than for steel cylinders and due to the small values, they might at least partly be influenced by temperature, permeation from/to sealing materials, and gas-consumption-induced pressure changes. Temperature coefficients for CH₄, CO, and H₂O adsorption were, within their error bands, insignificant. These results do indicate the need for careful selection and usage of gas cylinders for high-precision calibration purposes such as requested in trace gas applications.

Design of universal multicoset sampling patterns for compressed sensing of multiband sparse signals

Many problems in digital communications involve wideband radio signals. As the most recent example, the impressive advances in Cognitive Radio systems make even more necessary the development of sampling schemes for wideband radio signals with spectral holes. This is equivalent to considering a sparse multiband signal in the framework of Compressive Sampling theory. Starting from previous results on multicoset sampling and recent advances in compressive sampling, we analyze the matrix involved in the corresponding reconstruction equation and define a new method for the design of universal multicoset codes, that is, codes guaranteeing perfect reconstruction of the sparse multiband signal.

Estimation of Symmetric Channels for Discrete Cosine Transform Type-I Multicarrier Systems: A Compressed Sensing Approach

The problem of channel estimation for multicarrier communications is addressed. We focus on systems employing the Discrete Cosine Transform Type-I (DCT1) even at both the transmitter and the receiver, presenting an algorithm which achieves an accurate estimation of symmetric channel filters using only a small number of training symbols. The solution is obtained by using either matrix inversion or compressed sensing algorithms. We provide the theoretical results which guarantee the validity of the proposed technique for the DCT1. Numerical simulations illustrate the good behaviour of the proposed algorithm.

Self-assembly of fibronectin into fibrillar networks underneath dipalmitoyl phosphatidylcholine monolayers: Role of lipid matrix and tensile forces

The cell-mediated assembly of fibronectin (Fn) into fibrillar matrices is a complex multistep process that is incompletely understood because of the chemical complexity of the extracellular matrix and a lack of experimental control over molecular interactions and dynamic events. We have identified conditions under which Fn assembles into extended fibrillar networks after adsorption to a dipalmitoyl phosphatidylcholine (DPPC) monolayer in contact with physiological buffer. We propose a sequential model for the Fn assembly pathway, which involves the orientation of Fn underneath the lipid monolayer by insertion into the liquid expanded (LE) phase of DPPC. Attractive interactions between these surface-anchored proteins and the liquid condensed (LC) domains leads to Fn enrichment at domain edges. Spontaneous self-assembly into fibrillar networks, however, occurs only after expansion of the DPPC monolayer from the LC phase though the LC/LE phase coexistence. Upon monolayer expansion, the domain boundaries move apart while attractive interactions among Fn molecules and between Fn and domain edges produce a tensile force on the proteins that initiates fibril assembly. The resulting fibrils have been characterized in situ by using fluorescence and light-scattering microscopy. We have found striking similarities between fibrils produced under DPPC monolayers and those found on cellular surfaces, including their assembly pathways.

Proton long-range migration along protein monolayers and its consequences on membrane coupling

It has been shown with lipid layers and more recently with purple membranes that protons have slow surface-to-bulk transfer. This results in long-range proton lateral conduction along membranes. We report here that such lateral transfer can take place along a pure protein film. It is strongly controlled by the packing. Subtle reorganizations of the protein–protein contact can be biological switches between interfacial and delocalized proton pathways between sources and sinks.

Equilibration rates in lipid monolayers

A theoretical analysis is given for the rate of change of domain sizes in lipid monolayers at the air–water interface. The calculation is applicable to liquid domains formed from binary mixtures of lipids that form two coexisting liquid phases. Under conditions where the two lipid molecules have approximately equal areas, the equilibration rate does not involve macroscopic hydrodynamic flow in the subphase but rather depends on the diffusion coefficient of the lipid molecules. The calculation shows that the equilibration rate in binary mixtures of cholesterol and phosphatidylcholine is remarkably slow, the radius of a typical 20-μm diameter domain changing by as little as a part in a million per second. Under these circumstances, equilibration times of the order of days or weeks are expected. Even with such long times, the final state reached by the monolayer will in general be a state of metastable equilibrium, rather than true equilibrium.