Andrew G. Klein

MODERN AND LAST LOCAL GLACIAL MAXIMUM SNOWLINES IN THE CENTRAL ANDES OF PERU, BOLIVIA, AND NORTHERN CHILE

Late Pleistocene snowlines in the central Andes were 500–1200 m lower than at present. Radiocarbon dates imply that the late-Pleistocene glacial maximum in the region occurred prior to 20 14C ka, but lack of maximum limiting ages adds considerable uncertainty to the exact timing. Snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the central Andes respond differently to temperature and precipitation changes. In the eastern cordillera, the snowline is near the level of the annual 0°C isotherm and melting occurs throughout the year. Here snowlines are sensitive to temperature changes, but relatively insensitive to accumulation changes. In the western cordillera, the snowline rises 1000 higher owing to increasing aridity, and the snowline exhibits a much stronger sensitivity to accumulation changes. The consistent 1200 m snowline depression along the eastern cordilleras of the central Andes can be modeled by a mean annual cooling of 5–9°C. This is inconsistent with the <2°C cooling in tropical sea-surface temperatures suggested by CLIMAP reconstructions. The 800–1000 m snowline depression in the western cordillera cannot be accounted for solely by cooling, but also requires an increase in precipitation.

Spectral mixture analysis of Landsat thematic mapper images applied to the detection of the transient snowline on tropical Andean glaciers

Abstract The tropical glaciers in the central Andes are sensitive indicators of climatic variability. They are an important water resource, but are presently in a state of rapid retreat. Spectral mixture analysis using Landsat Thematic Mapper (TM) images was used to identify the ablation and accumulation zones and the transient snowline at two tropical sites: Zongo Glacier in the Cordillera Real, Bolivia, and the Quelccaya Ice Cap in Peru. Delineation of the accumulation and ablation zones is relatively insensitive to the endmembers selected to represent each zone. Endmembers selected from Zongo Glacier were successfully used to delineate accumulation and ablation zones on the Quelccaya Ice Cap. Spectral mixture analysis was found to be superior to a single band image or the TM4/TM5 ratio in discriminating the accumulation and ablation zones on these small tropical glaciers. The altitude of the transient snowline identified on Zongo Glacier at the end of the ablation season is consistent with the altitude of the equilibrium line determined from mass balance studies. This suggests that in the tropics, as in mid-latitudes, the highest altitude reached by the transient snowline during a hydrological year can be used as a proxy for the altitude of the equilibrium line.

Natural cooperation in wireless networks

This tutorial provides an introduction to game theoretic analysis of selfish behavior in wireless ad hoc networks with a focus on the packet forwarding and relaying scenarios.

RAKE reception for UWB communication systems with intersymbol interference

Recently, ultra wideband (UWB) technology has been proposed for use in wireless personal area networks (WPAN). Under the conditions where such transceivers are expected to operate, intersymbol interference (ISI) will become a significant performance limitation, and improvements to conventional RAKE reception will be necessary. We propose a modified RAKE receiver that finds an optimal balance between the goal of gathering multipath signal energy avoiding ISI, and suppressing narrowband interference. For fixed RAKE finger delays, we develop a closed-form expression for the minimum mean squared error (MMSE) combining weights that account for ISI. We then examine the optimal choice of RAKE finger delays, and show that significant performance gains can be achieved, particularly in an undermodeled situation when there are more channel paths than RAKE fingers. Several numerical examples are presented which compare our proposed scheme to a conventional RAKE with maximal ratio combining (MRC).

Practical quantize-and-forward schemes for the frequency division relay channel

We consider relay channels in which the source-destination and relay-destination signals are assumed to be orthogonal and thus have to be recombined at the destination. Assuming memoryless signals at the destination and relay, we propose a low-complexity quantize-and-forward (QF) relaying scheme, which exploits the knowledge of the SNRs of the source-relay and relay-destination channels. Both in static and quasistatic channels, the quantization noise introduced by the relay is shown to be significant in certain scenarios. We therefore propose a maximum likelihood (ML) combiner at the destination, which is shown to compensate for these degradations and to provide significant performance gains. The proposed association, which comprises the QF protocol and ML detector, can be seen, in particular, as a solution for implementing a simple relaying protocol in a digital relay in contrast with the amplify-and-forward protocol which is an analog solution.

PURSUING AUTOMATED CLASSIFICATION OF HISTORIC PHOTOGRAPHIC PAPERS FROM RAKING LIGHT IMAGES

Abstract Surface texture is a critical feature in the manufacture, marketing, and use of photographic paper. Raking light reveals texture through a stark rendering of highlights and shadows. Though close-up raking light images effectively document surface features of photographic paper, the sheer number and diversity of textures used for historic papers prohibits efficient visual classification. This work provides evidence that automatic, computer-based classification of texture documented with raking light is feasible by demonstrating an encouraging degree of success sorting a set of 120 images made from samples of historic silver gelatin paper. Using this dataset, four university teams applied different image-processing strategies for automatic feature extraction and degree of similarity quantification. All four approaches successfully detected strong affinities and outliers built into the dataset. The creation and deployment of the algorithms was carried out by the teams without prior knowledge of the distributions of similarities and outliers. These results indicate that automatic classification of silver gelatin photographic paper based on close-up texture images is feasible and should be pursued. To encourage the development of other classification schemes, the 120-sample “training” dataset used in this work is available to other academic researchers at http://www.PaperTextureID.org.

Combining Decoded-and-Forwarded Signals in Gaussian Cooperative Channels

The goal of this paper is to investigate the different ways of combining signals that have been decoded-and-forwarded by a bunch of relays. We more deeply look at the case where the relays are in bad reception conditions and the cooperation powers are sufficiently high. In this situation using a conventional MRC severely degrades the receiver performance especially when the number or relays increases. On the other hand the MMSE- and ML-based combiners can almost always extract from their partners some performance improvements

Gaussian Broadcast Channels With Cooperating Receivers: The Single Common Message Case

The main purpose of this paper is to evaluate the benefits of receive cooperation in a broadcast situation where two receivers want to decode the same message. Additionally the cooperation channel is assumed to be orthogonal to the downlink channels. In the case where the cooperation channel is unidirectional, the channel capacity is determined and the performance loss induced by orthogonalizing the cooperation channel is evaluated. In the bidirectional and orthogonal case lower and upper bounds for the capacity are provided. More specifically two decoding schemes are compared. Coding scheme 1: the two receivers use estimate-and-forward. Coding scheme 2: each receiver uses, in a complementary manner, estimate-and-forward a certain fraction of the time and decode-and-forward for the rest of the time. For realistic levels of the cooperation powers the performance of the second scheme is shown to be close to the considered upper bound

Precise timestamp-free network synchronization

This paper describes an approach to master/slave network synchronization based on bidirectional message exchanges without the use of timestamps. Rather than the usual approach of exchanging digital timestamps through a dedicated synchronization protocol, an approach is described in which synchronization information is conveyed implicitly at the physical layer through the timing of the master node¿s responses to the slave nodes. This approach can reduce overhead and allow the embedding of synchronization functions in existing network traffic. A timestamp-free synchronization protocol is described and its performance is quantified in the presence of delay estimation error and stochastic local oscillator dynamics. A filtering framework is also developed to allow each slave node to accurately infer and correct local clock drifts from multiple noisy clock offset estimates. Based on fundamental delay estimation bounds for narrowband signals, numerical results show that synchronization among the slave nodes can be achieved quickly and that the resulting steady-state accuracy can be sufficient to support distributed transmission techniques requiring carrier phase alignment, e.g. distributed beamforming.

Relay selection in cooperative networks with frequency selective fading

In this article, we consider the diversity-multiplexing tradeoff (DMT) of relay-assisted communication through correlated frequency selective fading channels. Recent results for relays in flat fading channels demonstrate a performance and implementation advantage in using relay selection as opposed to more complicated distributed space-time coding schemes. Motivated by these results, we explore the use of relay selection for the case when all channels have intersymbol interference. In particular, we focus on the performance of relaying strategies when multiple decode-and-forward relays share a single channel orthogonal to the source. We derive the DMT for several relaying strategies: best relay selection, random relay selection, and the case when all decoding relays participate. The best relay selection method selects the relay in the decoding set with the largest sum-squared relay-to-destination channel coefficients. This scheme can achieve the optimal DMT of the system under consideration and generally dominates the other two relaying strategies which do not always exploit the spatial diversity offered by the relays. Different from flat fading, we found special cases when the three relaying strategies have the same DMT. We further present a transceiver design which is proven to asymptotically achieve the optimal DMT. Monte Carlo simulations are presented to corroborate the theoretical analysis and to provide a detailed performance comparison of the three relaying strategies in channels encountered in practice.