Eigen-Based Transceivers for the MIMO Broadcast Channel with Semi-Orthogonal User Selection

This paper studies the sum rate performance of two low complexity eigenmode-based transmission techniques for the MIMO broadcast channel, employing greedy semi-orthogonal user selection (SUS). The first approach, termed ZFDPC-SUS, is based on zero-forcing dirty paper coding; the second approach, termed ZFBF-SUS, is based on zero-forcing beamforming. We first employ new analytical methods to prove that as the number of users K grows large, the ZFDPC-SUS approach can achieve the optimal sum rate scaling of the MIMO broadcast channel. We also prove that the average sum rates of both techniques converge to the average sum capacity of the MIMO broadcast channel for large K. In addition to the asymptotic analysis, we investigate the sum rates achieved by ZFDPC-SUS and ZFBF-SUS for finite K, and show that ZFDPC-SUS has significant performance advantages. Our results also provide key insights into the benefit of multiple receive antennas, and the effect of the SUS algorithm. In particular, we show that whilst multiple receive antennas only improves the asymptotic sum rate scaling via the second-order behavior of the multi-user diversity gain; for finite K, the benefit can be very significant. We also show the interesting result that the semi-orthogonality constraint imposed by SUS, whilst facilitating a very low complexity user selection procedure, asymptotically does not reduce the multi-user diversity gain in either first (log K) or second-order (loglog K) terms.Comment: 35 pages, 3 figures, to appear in IEEE transactions on signal processin

Remote multispectral imaging with PRISMS and XRF analysis of Tang Tomb paintings

PRISMS (Portable Remote Imaging System for Multispectral Scanning) is a multispectral/hyperspectral imaging system designed for flexible in situ imaging of wall paintings at high resolution (tens of microns) over a large range of distances (less than a meter to over ten meters). This paper demonstrates a trial run of the VIS/NIR (400-880nm) component of the instrument for non-invasive imaging of wall paintings in situ. Wall painting panels from excavated Tang dynasty (618-907AD) tombs near Xi’an were examined by PRISMS. Pigment identifications were carried out using the spectral reflectance obtained from multispectral imaging coupled with non-invasive elemental analysis using a portable XRF

Globally Polarized Quark-gluon Plasma in Non-central A+A Collisions

Produced partons have large local relative orbital angular momentum along the direction opposite to the reaction plane in the early stage of non-central heavy-ion collisions. Parton scattering is shown to polarize quarks along the same direction due to spin-orbital coupling. Such global quark polarization will lead to many observable consequences, such as left-right asymmetry of hadron spectra, global transverse polarization of thermal photons, dileptons and hadrons. Hadrons from the decay of polarized resonances will have azimuthal asymmetry similar to the elliptic flow. Global hyperon polarization is predicted within different hadronization scenarios and can be easily tested.Comment: 4 pages in RevTex with 2 postscript figures, an erratum is added to the final published versio

Pricing Interest-Rate Risk for Mortgage REITs

Using tax-qualified mortgage REITs over three periods (1976-79, 1980-82, and 1983-90), this paper investigates the pricing of interest-rate risk for mortgage REITs at equilibrium. A system of nonlinear equations is estimated to determine the monthly interest-rate risk premium over each of the three time intervals. There is evidence to support the hypothesis that interest-rate risk is not diversifiable and hence commands a risk premium.

SU(2)-invariant spin liquids on the triangular lattice with spinful Majorana excitations

We describe a new class of spin liquids with global SU(2) spin rotation symmetry in spin 1/2 systems on the triangular lattice, which have real Majorana fermion excitations carrying spin S = 1. The simplest translationally-invariant mean-field state on the triangular lattice breaks time-reversal symmetry and is stable to fluctuations. It generically possesses gapless excitations along 3 Fermi lines in the Brillouin zone. These intersect at a single point where the excitations scale with a dynamic exponent z = 3. An external magnetic field has no orbital coupling to the SU(2) spin rotation-invariant fermion bilinears that can give rise to a transverse thermal conductivity, thus leading to the absence of a thermal Hall effect. The Zeeman coupling is found to gap out two-thirds of the z = 3 excitations near the intersection point and this leads to a suppression of the low temperature specific heat, the spin susceptibility and the Wilson ratio. We also compute physical properties in the presence of weak disorder and discuss possible connections to recent experiments on organic insulators.Comment: 26 pages, 11 figure

Confronting Synchrotron Shock and Inverse Comptonization Models with GRB Spectral Evolution

The time-resolved spectra of gamma-ray bursts (GRBs) remain in conflict with many proposed models for these events. After proving that most of the bursts in our sample show evidence for spectral "shape-shifting", we discuss what restrictions that BATSE time-resolved burst spectra place on current models. We find that the synchrotron shock model does not allow for the steep low-energy spectral slope observed in many bursts, including GRB 970111. We also determine that saturated Comptonization with only Thomson thinning fails to explain the observed rise and fall of the low-energy spectral slope seen in GRB 970111 and other bursts. This implies that saturated Comptonization models must include some mechanism which can cause the Thomson depth to increase intially in pulses.Comment: (5 pages, 3 figures, to appear in Proceedings of the Fourth Huntsville Symposium on Gamma-Ray Bursts

Probing Neutral Majorana Fermion Edge Modes with Charge Transport

We propose two experiments to probe the Majorana fermion edge states that occur at a junction between a superconductor and a magnet deposited on the surface of a topological insulator. Combining two Majorana fermions into a single Dirac fermion on a magnetic domain wall allows the neutral Majorana fermions to be probed with charge transport. We will discuss a novel interferometer for Majorana fermions, which probes their Z_2 phase. This setup also allows the transmission of neutral Majorana fermions through a point contact to be measured. We introduce a point contact formed by a superconducting junction and show that its transmission can be controlled by the phase difference across the junction. We discuss the feasibility of these experiments using the recently discovered topological insulator Bi_2 Se_3.Comment: 4 page

Non-invasive investigations of a wall painting using optical coherence tomography and hyperspectral imaging

Multispectral and hyperspectral imaging are efficient methods of measuring spectral reflectance at high spatial resolution. This non-invasive technique has been applied to the imaging of paintings over the last 20 years. PRISMS (Portable Remote Imaging System for Multispectral Scanning) was designed specifically for imaging wall paintings. Optical Coherence Tomography (OCT) is a low coherence interferometric technique capable of fast non-invasive imaging of subsurface microstructure. This paper shows the first application of in situ OCT imaging of a wall painting. The combination of PRISMS and OCT gives information on the varnish and paint layer structure, pigment identification, the state of degradation of the paint and varnish layers and informing curators on the painting schemes and techniques

Testing the Invariance of Cooling Rate in Gamma-Ray Burst Pulses

Recent studies have found that the spectral evolution of pulses within gamma-ray bursts (GRBs) is consistent with simple radiative cooling. Perhaps more interesting was a report that some bursts may have a single cooling rate for the multiple pulses that occur within it. We determine the probability that the observed "cooling rate invariance" is purely coincidental by sampling values from the observed distribution of cooling rates. We find a 0.1-26% probability that we would randomly observe a similar degree of invariance based on a variety of pulse selection methods and pulse comparison statistics. This probability is sufficiently high to warrant skepticism of any intrinsic invariance in the cooling rate.Comment: 4 pages, 1 figure, to appear in Proceedings of the Fourth Huntsville Symposium on Gamma-Ray Burst