Cooperative Distributed MIMO Channels in Wireless Sensor Networks

The large number of network nodes and the energy constraints make Wireless Sensor Networks (WSN) one of the most important application fields for Cooperative Diversity. Node cooperation increases the spatial diversity of wireless channels and, thus, reduces the transmitted power. In this paper, we propose a multi-hop WSN with nodes grouped in cooperative clusters that exploits transmit and receive cooperation among cluster nodes. Multi-hop transmission is carried out by concatenating single cluster-to-cluster hops, where every cluster-to-cluster link is defined as a cooperative distributed multiple-input-multiple-output (MIMO) channel. Transmit diversity is exploited through a time-division, decoder-and-forward, relaying scheme based upon two time slots: the Intracluster Slot, used for data sharing within the cluster, and the Intercluster Slot, used for transmission between clusters. At the receiver side, a distributed reception protocol is devised based upon a Selection Diversity algorithm. The proposed multi-hop cooperative WSN is optimally designed for minimum end-to-end outage probability by deriving the optimum time and power allocated on the intracluster and intercluster slots of every single hop, given a per-link energy constraint. A simplified suboptimum resource allocation is also proposed, which performs close to the optimal policy. Results show that the proposed scheme achieves diversity equal to the equivalent MIMO system and significantly reduces energy consumption with respect to. the non-cooperative channel     

X-ray based tools for the investigation of buried interfaces in organic electronic devices

X-ray reflectivity combined with grazing incidence diffraction is a valuable tool for investigating organic multilayer structures that can be used in devices. We focus on a bilayer stack consisting of two materials (poly-(3-hexylthiophene)) (P3HT) and poly-(4-styrenesulfonic acid) (PSSA) spin cast from orthogonal solvents (water in the case of PSSA and chloroform or toluene for P3HT). X-ray reflectivity is used to determine the thickness of all layers as well as the roughness of the organic–organic hetero-interface and the P3HT surface. The surface roughness is found to be consistent with the results of atomic force microscopy measurements. For the roughness of P3HT/PSSA interface, we observe a strong dependence on the solvent used for P3HT deposition. The solvent also strongly impacts the texturing of the P3HT crystallites as revealed by grazing incidence diffraction. When applying the various PSSA/P3HT multilayers in organic thin-film transistors, we find an excellent correlation between the determined interface morphology, structure and the device performance.     

Identification of the source of an interferer by comparison with known carriers using a single satellite

We describe a method for identifying the source of a satellite interferer using a single satellite. The technique relies on the fact that the strength of a carrier signal measured at the downlink station varies with time due to a number of factors, and we use a quantum‐inspired algorithm to compute a “signature” for a signal, which captures part of the pattern of variation that is a characteristic of the uplink antenna. We define a distance measure to numerically quantify the degree of similarity between two signatures, and by computing the distances between the signature for an interfering carrier and the signatures of the known carriers being relayed by the same satellite at the same time, we can identify the antenna that the interferer originated from, if a known carrier is being relayed from it. As a proof of concept, we evaluate the performance of the technique using a simple statistical model applied to measured carrier data.     

Deployment of RSS-Based Indoor Positioning Systems

Location estimation based on Received Signal Strength (RSS) is the prevalent method in indoor positioning. For such positioning systems, a massive collection of training samples is needed for their calibration. The accuracy of these methods is directly related to the placement of the reference points and the radio map used to compute the device location. Traditionally, deploying the reference points and building the radio map require human intervention and are extremely time-consuming. In this paper we present an approach to reduce the manual calibration efforts needed to deploy an RSS-based localization system, both when using only one RF technology or when using a combination of RF technologies. It is an automatic approach both to build a radio map in a given workspace by means of a signal propagation model, and to assess the system calibration that best fits the required accuracy by using a multi-objective genetic algorithm.     

Cooperative energy management in distributed wireless real-time systems

This work is based on the observation that existing energy management techniques for mobile devices, such as Dynamic Voltage Scaling (DVS), are non-cooperative in the sense that they reduce the energy consumption of a single device, disregarding potential consequences for other constraints (e.g., end-to-end deadlines) and/or other devices (e.g., energy consumption on neighboring devices). This paper argues that energy management in distributed wireless real-time systems has to be end-to-end in nature, requiring a coordinated approach among communicating devices. A cooperative distributed energy management technique (Co-DVS) is proposed that (1) adapts and maintains end-to-end latencies within specified timeliness requirements (deadlines) and (2) enhances energy savings at the devices with the highest pay-off factors that represent the relative benefits or significance of conserving energy at a device. The proposed technique employs a feedback-based approach to dynamically distribute end-to-end slack among the devices based on their pay-off factors.     

Quadrature Amplitude Modulation for Differential Space-Time Block Codes

Differential Space-Time Block Codes (DSTBC) do not require any radio channel measurement and channel state information neither on the transmitter nor at the receiver side. Therefore, they are an attractive alternative to coherent Multiple-Input Multiple-output (MIMO) systems. The classical technical proposal for differential techniques is based on M-ary phase shift keying (M-PSK) modulation schemes for DSTBC (PSK-DSTBC). One advantage of this scheme is the constant envelop of the transmit signal, but it is well known that higher-order PSK is less efficient due to the small distance between adjacent points in the constellation diagram. Therefore, in this paper an alternative modulation technique for DSTBC is discussed, which is based on quadrature amplitude modulation (QAM-DSTBC). The signal envelope of the transmit signal is not any more constant in this case. Therefore, the technical challenge of integrating QAM into the DSTBC system design is to control the transmit power in order to avoid an increase or a decrease in transmit power to some extreme values. The mechanism, which is used in this paper to control the transmit power is based on an extension of the original QAM constellation diagram. The additional points, which are integrated into the constellation diagram are used in this case for the mapping procedure to select one of the modulation symbols with high or alternatively low signal power. This means for each single bit pattern there are almost always two modulation symbols available in the mapping procedure one with low and one with high signal power, which gives the basis for a signal power control algorithm. The resulting bit-error-rate (BER) performance of QAM-DSTBC is compared finally to the performance figures of the original proposal of PSK-DSTBC.

Merocyanine/C60 Planar Heterojunction Solar Cells: Effect of Dye Orientation on Exciton Dissociation and Solar Cell Performance

In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C₆₀ organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post‐annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric‐field‐dependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as‐deposited cells. Fourier‐transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules’ orientations on the charge‐transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials.     

Recent Advances in Birefringence Studies at THz Frequencies

In the last few years, various studies on terahertz (THz) birefringence have been presented, based on polarization-sensitive THz time-domain spectroscopy. The field of THz birefringence is a wide one covering several aspects, such as different materials exhibiting THz birefringence, polarization-sensitive THz technology, as well as numerous methods for extracting the birefringence properties of a sample from the THz data. Therefore, this paper aims at reviewing recent results on THz birefringence measurements presented in literature, addressing the above aspects and giving an overview of the topic. Moreover, it will focus on the investigation of birefringent fibrous samples, where specific results will be discussed in detail. Altogether, this paper should give a comprehensive view on the recent achievements in the measurements of THz birefringence.     

Preparation of Ring-Shaped Thermoelectric Legs from PbTe Powders for Tubular Thermoelectric Modules

Waste heat recovery—for example, in automotive applications—is a major field for thermoelectric research and future application. Commercially available thermoelectric modules are based on planar structures, whereas tubular modules may have advantages for integration and performance in the field of automotive waste heat recovery. One major drawback of tubular generator designs is the necessity for ring-shaped legs made from thermoelectric material. Cutting these geometries from sintered tablets leads to considerable loss of thermoelectric material and therefore high cost. Direct sintering of ring-shaped legs or tubes of thermoelectric material is a solution to this problem. However, sintering such rings with high homogeneity and density faces some difficulties related to the mechanical properties of typical thermoelectric materials such as lead telluride (PbTe)—particularly brittleness and high coefficient of thermal expansion. This work shows a process for production of thermoelectric rings made of p- and n-doped PbTe. Long tubes of PbTe have been sintered in a current-assisted sintering process with specially designed sintering molds, coated with a diffusion barrier, and finally cut into ring-shaped slices. To demonstrate the technology, a tubular thermoelectric module has been assembled using these PbTe rings.     

Effect of Bismuth Nanotubes on the Thermoelectric Properties of BiSb Alloy Nanocomposites

Bismuth nanotubes have been synthesized and successfully included in Bi_1?x Sb _x nanoalloys to form composite structures. The nanotubes were synthesized by transformation of a β-BiI precursor with n-BuLi solution leading to tubular bismuth structures. The Bi_1?x Sb _x nanoalloys were produced by ball milling. Three series of composite structures were synthesized by including different fractions (0 wt.%, 3 wt.%, 5 wt.%) of nanotubes in nanoalloys of different composition x. Investigation of thermoelectric and structural properties revealed a decrease of the thermal conductivity of up to 40% for the composites in comparison with alloys without nanotube inclusions. This effect can be attributed to enhanced phonon scattering. Seebeck coefficients and electrical conductivities were both slightly enhanced in the composite series with 3 wt.% nanotube inclusions, leading to enhancement of $$ ZT \ \left(ZT=\frac {(S^2 \sigma)}{\kappa}\,{ {T}}\right) $$ throughout the series compared with the nanoalloy series without nanotube inclusions.