Simulating the Role of Stellar Rotation in the Spectroscopic Effects of Differential Limb Magnification

Finite-source effects of gravitationally microlensed stars have been well discussed in the literature, but the role that stellar rotation plays has been neglected. A differential magnification map applied to a differentially Doppler-shifted surface alters the profiles of absorption lines, compromising their ordinarily symmetric nature. Herein, we assess the degree to which this finite-source effect of differential limb magnification (DLM), in combination with stellar rotation, alters spectroscopically derived stellar properties. To achieve this, we simulated a grid of high-magnification microlensing events using synthetic spectra. Our analysis shows that rotation of the source generates differences in the measured equivalent widths of absorption lines supplementary to DLM alone, but only of the order of a few percent. Using the wings of H alpha from the same simulated data, we confirmed the result of Johnson et al. (2010) that DLM alters measurements of effective temperature by < 100 K for dwarf stars, while showing rotation to bear no additional effect.Comment: Accepted for publication in PASA, 7 pages, 5 figures, 1 tabl

Variable-Length Coding with Feedback: Finite-Length Codewords and Periodic Decoding

Theoretical analysis has long indicated that feedback improves the error exponent but not the capacity of single-user memoryless channels. Recently Polyanskiy et al. studied the benefit of variable-length feedback with termination (VLFT) codes in the non-asymptotic regime. In that work, achievability is based on an infinite length random code and decoding is attempted at every symbol. The coding rate backoff from capacity due to channel dispersion is greatly reduced with feedback, allowing capacity to be approached with surprisingly small expected latency. This paper is mainly concerned with VLFT codes based on finite-length codes and decoding attempts only at certain specified decoding times. The penalties of using a finite block-length $N$ and a sequence of specified decoding times are studied. This paper shows that properly scaling $N$ with the expected latency can achieve the same performance up to constant terms as with $N = \infty$. The penalty introduced by periodic decoding times is a linear term of the interval between decoding times and hence the performance approaches capacity as the expected latency grows if the interval between decoding times grows sub-linearly with the expected latency.Comment: 8 pages. A shorten version is submitted to ISIT 201

A Rate-Compatible Sphere-Packing Analysis of Feedback Coding with Limited Retransmissions

Recent work by Polyanskiy et al. and Chen et al. has excited new interest in using feedback to approach capacity with low latency. Polyanskiy showed that feedback identifying the first symbol at which decoding is successful allows capacity to be approached with surprisingly low latency. This paper uses Chen's rate-compatible sphere-packing (RCSP) analysis to study what happens when symbols must be transmitted in packets, as with a traditional hybrid ARQ system, and limited to relatively few (six or fewer) incremental transmissions. Numerical optimizations find the series of progressively growing cumulative block lengths that enable RCSP to approach capacity with the minimum possible latency. RCSP analysis shows that five incremental transmissions are sufficient to achieve 92% of capacity with an average block length of fewer than 101 symbols on the AWGN channel with SNR of 2.0 dB. The RCSP analysis provides a decoding error trajectory that specifies the decoding error rate for each cumulative block length. Though RCSP is an idealization, an example tail-biting convolutional code matches the RCSP decoding error trajectory and achieves 91% of capacity with an average block length of 102 symbols on the AWGN channel with SNR of 2.0 dB. We also show how RCSP analysis can be used in cases where packets have deadlines associated with them (leading to an outage probability).Comment: To be published at the 2012 IEEE International Symposium on Information Theory, Cambridge, MA, USA. Updated to incorporate reviewers' comments and add new figure

Increasing Flash Memory Lifetime by Dynamic Voltage Allocation for Constant Mutual Information

The read channel in Flash memory systems degrades over time because the Fowler-Nordheim tunneling used to apply charge to the floating gate eventually compromises the integrity of the cell because of tunnel oxide degradation. While degradation is commonly measured in the number of program/erase cycles experienced by a cell, the degradation is proportional to the number of electrons forced into the floating gate and later released by the erasing process. By managing the amount of charge written to the floating gate to maintain a constant read-channel mutual information, Flash lifetime can be extended. This paper proposes an overall system approach based on information theory to extend the lifetime of a flash memory device. Using the instantaneous storage capacity of a noisy flash memory channel, our approach allocates the read voltage of flash cell dynamically as it wears out gradually over time. A practical estimation of the instantaneous capacity is also proposed based on soft information via multiple reads of the memory cells.Comment: 5 pages. 5 figure

The Impact of Biofuel Mandates and Switchgrass Production on Hay Markets

The Renewable Fuel Standard mandate in the Energy Independence and Security Act of 2007 requires 16 billion gallons out of 36 billion gallons of ethanol be produced from cellulosic feedstocks in 2022, but the mandate was apparently enacted without critical assessments of the agricultural impacts of attempting to achieve energy independence. The feedstock production will likely compete with lands currently used for producing other traditional crops of which hay is likely to be affected the most since it has comparatively lower net returns. Thus ruminant production will consequently be affected greatly. This study uses ordinary least squares (OLS) to estimate and predict Oklahoma hay price which is used as objective value in linear programming (LP) model that determines the profitability options between hay and switchgrass production. The OLS results show that Oklahoma hay price is fairly stable, and hay is shipped across adjoining states. The LP results show that switchgrass production would be more profitable than hay and that switchgrass for biofuel production likely will bid land away from hay if biofuel production becomes fully operational.biofuel mandates, switchgrass production, hay production, hay markets., Agricultural and Food Policy, Environmental Economics and Policy, Production Economics, Research and Development/Tech Change/Emerging Technologies, Resource /Energy Economics and Policy,

Feedback Communication Systems with Limitations on Incremental Redundancy

This paper explores feedback systems using incremental redundancy (IR) with noiseless transmitter confirmation (NTC). For IR-NTC systems based on {\em finite-length} codes (with blocklength $N$) and decoding attempts only at {\em certain specified decoding times}, this paper presents the asymptotic expansion achieved by random coding, provides rate-compatible sphere-packing (RCSP) performance approximations, and presents simulation results of tail-biting convolutional codes. The information-theoretic analysis shows that values of $N$ relatively close to the expected latency yield the same random-coding achievability expansion as with $N = \infty$. However, the penalty introduced in the expansion by limiting decoding times is linear in the interval between decoding times. For binary symmetric channels, the RCSP approximation provides an efficiently-computed approximation of performance that shows excellent agreement with a family of rate-compatible, tail-biting convolutional codes in the short-latency regime. For the additive white Gaussian noise channel, bounded-distance decoding simplifies the computation of the marginal RCSP approximation and produces similar results as analysis based on maximum-likelihood decoding for latencies greater than 200. The efficiency of the marginal RCSP approximation facilitates optimization of the lengths of incremental transmissions when the number of incremental transmissions is constrained to be small or the length of the incremental transmissions is constrained to be uniform after the first transmission. Finally, an RCSP-based decoding error trajectory is introduced that provides target error rates for the design of rate-compatible code families for use in feedback communication systems.Comment: 23 pages, 15 figure