Coding over space and time for wireless systems

In wireless communications, channel coding is used to combat impairments such as noise or fading. Redundant information is added at the transmitter, to enable reliable detection and decoding of the message at the receiver. With the advent of multiple-antenna techniques, coding for the wireless channel has become an attractive topic of research. Several original schemes have been devised over the past decade that benefit particularly well from the added spatial dimension: clever space-time diversity mappings, coined "space-time coding," increase the reliability of the wireless link, while "spatial multiplexing" and its corresponding demultiplexing and detection algorithms achieve high data rates at unprecedented spectral efficiencies. The combination of channel coding with numerous variations and mixtures of the above poses interesting design challenges. In this article we, admittedly, take a more channel-coding-centric view of a wireless communication link, and outline the current state of the art as well as future trends in coding over space and time.     

The Impact of Stoichiometry on the Photophysical Properties of Ruddlesden–Popper Perovskites

2D Ruddlesden–Popper perovskites are interesting for a variety of applications owing to their tunable optical properties and their excellent ambient stability. As these materials are processable from solution, they hold the promise of procuring flexible and cost‐effective films through large‐scale fabrication techniques. However, such solution‐based deposition techniques often induce large degrees of heterogeneity due to poorly controlled crystallization. The microscopic properties of films of (PEA)₂PbI₄ cast from precursor solutions of different stoichiometry are therefore investigated. The stoichiometry of the precursor solution is found to have a large impact on the crystallinity, morphology, and optical properties of the resulting thin films. Even for films cast from stoichiometric precursors, differences in photoluminescence intensities occur on a subgranular level. The heterogeneity in these films is found to be thermally activated with an activation energy of 0.4 eV for the emergence of local variations in nonradiative recombination rates. The spatial variation in the distribution of trap states is attributed to local fluctuations in the stoichiometry. In line with this, the surface can successfully be passivated by providing an excess of phenylethylammonium iodide (PEAI) to an as‐cast film, enhancing the photoluminescence by as much as 85% without significantly altering the film's morphology.     

A close-to-capacity dirty paper coding scheme

The "writing on dirty paper"-channel model offers an information-theoretic framework for precoding techniques for canceling arbitrary interference known at the transmitter. It indicates that lossless precoding is theoretically possible at any signal-to-noise ratio (SNR), and thus dirty-paper coding may serve as a basic building block in both single-user and multiuser communication systems. We design an end-to-end coding realization of a system materializing a significant portion of the promised gains. We employ multidimensional quantization based on trellis shaping at the transmitter. Coset decoding is implemented at the receiver using "virtual bits." Combined with iterative decoding of capacity-approaching codes we achieve an improvement of 2dB over the best scalar quantization scheme. Code design is done using the EXIT chart technique.     

Magnetophonon oscillations in the transverse and longitudinal magnetoresistance of Hg1−xCdxTe

The transverse and longitudinal magnetoresistance (MR) as well as the longitudinal magneto-thermoelectric coefficient of n-type Hg_1-xCd_xTe (MCT) (0.20 < x < 0.33) have been measured at various temperatures (40 ≤ T≤ 140K) as a function of magnetic field (0 ≤ B≤18 kG). Both the transverse and the longitudinal MR clearly exhibit oscillations which are described in terms of magnetophonon (MP) transitions involving the HgTe-like and the CdTe-like longitudinal optical (LO) phonons of MC.T. The field positions of the transverse MR maxima agree with the calculated MP resonances taking into account nonparabolic bands (k • p model for narrow-gap zinc-blende-type semiconductors) and the polaron effect. Those of the longitudinal MR minima are found to coincide with the oscillation minima in the longitudinal magneto-thermoelectric coefficient. However, these minima are shifted by π/2 to lower fields with respect to the positions of the MP resonances. This phase shift was predicted by Barker1314 for the case of strong Landau level damping but has not been previously observed. In contrast, the MP oscillation minima of the longitudinal MR and the oscillation maxima of the transverse MR of n-type InSb (investigated here for comparison) occur exactly at the fields of the MP resonances. Only the oscillation minima of the longitudinal magneto-thermoelectric coefficient are slightly shifted to the side below the MP resonance fields. With regard to the band parameters and the dominant polar optical mode scattering of charge carriers InSb very much resembles MC.T. InSb, however, is a binary compound whereas MC.T is a solid solution. Thus, the phase shift by π/2 to lower fields observed for the oscillation minima in the longitudinal MR and magneto-thermoelectric coefficient of MC.T may be due to alloy scattering. The temperature coefficients of the MP resonance fields of MC.T are found to be substantially smaller than those reported by Takita et al.¹¹ and McClure et al.¹⁰ The larger temperature coefficents are presumably due to unresolved two-phonon structures of the MP oscillations.     

The noise-tee - a lightwave device for microwave noise measurements

An innovative lightwave method is proposed to insert noise in electronic circuits in favor of microwave noise measurements. The proposed noise-tee has attractive additional features compared to the use of 50 Ω noise sources: 1) the inserted noise level and noise bandwidth is continuously variable over a wide dynamic range; 2) the wideband scaling accuracy of this level, relative to a pre-calibrated level, equals the accuracy of simple DC-current measurements; 3) level-induced impedance variations are negligible, compared to the 20% impedance variation of a commonly used microwave noise source; and 4) noise-tees enable the realization of 100% reflective noise sources, in favor of two-port noise-parameter measurements     

N‐Type Organic Thermoelectrics: Improved Power Factor by Tailoring Host–Dopant Miscibility

In this contribution, for the first time, the polarity of fullerene derivatives is tailored to enhance the miscibility between the host and dopant molecules. A fullerene derivative with a hydrophilic triethylene glycol type side chain (PTEG‐1) is used as the host and (4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)phenyl)dimethylamine n ‐DMBI) as the dopant. Thereby, the doping efficiency can be greatly improved to around 18% (<1% for a nonpolar reference sample) with optimized electrical conductivity of 2.05 S cm^?1, which represents the best result for solution‐processed fullerene derivatives. An in‐depth microstructural study indicates that the PTEG‐1 molecules readily form layered structures parallel to the substrate after solution processing. The fullerene cage plane is alternated by the triethylene glycol side chain plane; the n ‐DMBI dopants are mainly incorporated in the side chain plane without disturbing the π–π packing of PTEG‐1. This new microstructure, which is rarely observed for codeposited thin films from solution, formed by PTEG‐1 and n ‐DMBI molecules explains the increased miscibility of the host/dopant system at a nanoscale level and the high electrical conductivity. Finally, a power factor of 16.7 µW m^?1 K^?2 is achieved at 40% dopant concentration. This work introduces a new strategy for improving the conductivity of solution‐processed n‐type organic thermoelectrics.     

Discrimination of polymers by using their characteristic collision energy in tandem mass spectrometry

The characteristic collision energy to obtain 50% fragmentation, expressed as the characteristic collision voltage (CCV), was used as a tool to discriminate different classes of polymers. The CCV value of different polymers was determined in a quadrupole ion trap mass spectrometer. Good linear correlation (0.980 < R(2) < 0.999) between the CCV values and precursor ion mass was found for all polymers studied. The position of the various linear trend lines varied among the various polymers, which allowed their grouping based on the respective CCV values. The collision energy necessary to drive fragmentation was decreasing in the order of polyethers > polymethacrylates > polyesters > polysaccharides. This suggests that polysaccharides fragment most easily (low CCVs), while polyethers require the highest collision energy among the polymers studied. The effect of end group on the CCV was also studied, showing a minor influence in most cases. In addition, the applicability of CCV as discriminator was studied for a mixture of (1) polylactic acid (PLA), (2) poly(tetramethylene glycol) (PTMEG), and (3) PLA-block-PTMEG-block-PLA block copolymer. Differences between the CCV values of four nominally isobaric polymers (of which two were copolymers and two were homopolymers) were observed. These results demonstrate that the insertion of a "weak" link into a polymer chain significantly affects the energy required for fragmentation.     

Ionization Measurements of SuperCDMS SNOLAB 100 mm Diameter Germanium Crystals

Scaling cryogenic Germanium-based dark matter detectors to probe smaller WIMP-nucleon cross-sections poses significant challenges in the forms of increased labor, cold hardware, warm electronics and heat load. The development of larger crystals alleviates these issues. The results of ionization tests with two 100?mm diameter, 33?mm thick cylindrical detector-grade Germanium crystals are presented here. Through these results the potential of using such crystals in the Super Cryogenic Dark Matter Search (SuperCDMS) SNOLAB experiment is demonstrated.     

Present Status of the SuperCDMS program

The expected final reach of the Weakly Interacting Massive Particle (WIMP) search experiment CDMS-II by the end of 2007 is a WIMP-nucleon cross-section sensitivity of 2.1×10⁻⁴⁴ cm². To proceed further in our search, we have proposed the SuperCDMS Phase A project that would deploy 42 1-inch thick Ge detectors, at a site deeper than the location of CDMS II, and reach a desired sensitivity goal of 1.3×10⁻⁴⁵ cm². These cross-sections are of interest and are complementary to Supersymmetry searches at the Large Hadron Collider (LHC) and future linear colliders. P.L. Brink for the SuperCDMS Collaboration.     

On Freedman's Lattice Models for Topological Phases

Freedman proposes a family of Hamiltonians which define quantum loop gas models on any celluated compact surface. We study the simplest nontrivial cases: celluations of the torus. Our numerical data support Freedman's conjecture about the ground states of the Hamiltonians. PACS: 71.10.-w; 71.35.-y