T-duality Twists and Asymmetric Orbifolds

We study some aspects of asymmetric orbifolds of tori, with the orbifold group being some $\mathbb{Z}_N$ subgroup of the T-duality group and, in particular, provide a concrete understanding of certain phase factors that may accompany the T-duality operation on the stringy Hilbert space in toroidal compactification. We discuss how these T-duality twist phase factors are related to the symmetry and locality properties of the closed string vertex operator algebra, and clarify the role that they enact in the modular covariance of the orbifold theory, mainly using asymmetric orbifolds of tori which are root lattices as working examples.Comment: 67 pages. v2: references added and typos correcte

Adaptive beamforming for binary phase shift keying communication systems

system

On multi-user EXIT chart analysis aided turbo-detected MBER beamforming designs

Abstract—This paper studies the mutual information transfer characteristics of a novel iterative soft interference cancellation (SIC) aided beamforming receiver communicating over both additive white Gaussian noise (AWGN) and multipath slow fading channels. Based on the extrinsic information transfer (EXIT) chart technique, we investigate the convergence behavior of an iterative minimum bit error rate (MBER) multiuser detection (MUD) scheme as a function of both the system parameters and channel conditions in comparison to the SIC aided minimum mean square error (SIC-MMSE) MUD. Our simulation results show that the EXIT chart analysis is sufficiently accurate for the MBER MUD. Quantitatively, a two-antenna system was capable of supporting up to K=6 users at Eb/N0=3dB, even when their angular separation was relatively low, potentially below 20?. Index Terms—Minimum bit error rate, beamforming, multiuser detection, soft interference cancellation, iterative processing, EXIT chart

Thin-shell deployable reflectors with collapsible stiffeners: part 1 - approach

Thin-shell deployable reflector structures that are folded elastically in a nearly inextensional mode have been recently realized, exploiting the recent availability of high-modulus, ultrathin composite materials. An inherent and significant limitation of this approach is that these structures remain “floppy” in their deployed configuration. This paper presents a general concept for increasing the deployed stiffness of such structures, through the addition of a collapsible edge stiffener around the rim of a reflector dish. Ananalytical expression of the frequency/stiffness related to the softest deformation mode of a thin-shell reflector structure is presented, both with and without the stiffener. During folding, the stiffener collapses elastically, and this behavior is facilitated by the introduction of suitable discontinuities within the stiffener, or between the dish and the stiffener. A detailed study of a range of different options is presented, and one particular scheme is selected and optimized. For a specific example, a stiffness increase by a factor of 31 and a fundamental frequency increase by a factor of 4 are achieved, with a mass increase of only 16%

AlAsSb avalanche photodiodes with a sub-mV/K temperature coefficient of breakdown voltage

The temperature dependence of dark current and avalanche gain were measured on AlAsSb p-i-n diodes with avalanche region widths of 80 and 230 nm. Measurements at temperatures ranging from 77 to 295 K showed that the dark current decreases rapidly with reducing temperature while avalanche gain exhibits a weak temperature dependence. No measurable band to band tunneling current was observed in the thinner diodes at an electric field of 1.07 MV/cm, corresponding to a bias of 95% of the breakdown voltage. Temperature coefficients of breakdown voltage of 0.95 and 1.47 mV/K were obtained from 80 and 230 nm diodes, respectively. These are significantly lower than a range of semiconductor materials with similar avalanche region widths. Our results demonstrated the potential of using thin AlAsSb avalanche regions to achieve low temperature coefficient of breakdown voltage without suffering from high band to band tunneling current

Ultra Thin Deployable Reflector Antennas

19 - 22 Apr 200