Performance Bounds for Bi-Directional Coded Cooperation Protocols

In coded bi-directional cooperation, two nodes wish to exchange messages over a shared half-duplex channel with the help of a relay. In this paper, we derive performance bounds for this problem for each of three protocols. The first protocol is a two phase protocol were both users simultaneously transmit during the first phase and the relay alone transmits during the second. In this protocol, our bounds are tight and a multiple-access channel transmission from the two users to the relay followed by a coded broadcast-type transmission from the relay to the users achieves all points in the two-phase capacity region. The second protocol considers sequential transmissions from the two users followed by a transmission from the relay while the third protocol is a hybrid of the first two protocols and has four phases. In the latter two protocols the inner and outer bounds are not identical, and differ in a manner similar to the inner and outer bounds of Cover's relay channel. Numerical evaluation shows that at least in some cases of interest our bounds do not differ significantly. Finally, in the Gaussian case with path loss, we derive achievable rates and compare the relative merits of each protocol in various regimes. This case is of interest in cellular systems. Surprisingly, we find that in some cases, the achievable rate region of the four phase protocol sometimes contains points that are outside the outer bounds of the other protocols.Comment: 15 page

Nanoscale electrochemical patterning reveals the active sites for catechol oxidation at graphite surfaces

Graphite-based electrodes (graphite, graphene, and nanotubes) are used widely in electrochemistry, and there is a long-standing view that graphite step edges are needed to catalyze many reactions, with the basal surface considered to be inert. In the present work, this model was tested directly for the first time using scanning electrochemical cell microscopy reactive patterning and shown to be incorrect. For the electro-oxidation of dopamine as a model process, the reaction rate was measured at high spatial resolution across a surface of highly oriented pyrolytic graphite. Oxidation products left behind in a pattern defined by the scanned electrochemical cell served as surface-site markers, allowing the electrochemical activity to be correlated directly with the graphite structure on the nanoscale. This process produced tens of thousands of electrochemical measurements at different locations across the basal surface, unambiguously revealing it to be highly electrochemically active, with step edges providing no enhanced activity. This new model of graphite electrodes has significant implications for the design of carbon-based biosensors, and the results are additionally important for understanding electrochemical processes on related sp2-hybridized materials such as pristine graphene and nanotubes

Disorder versus two transport lifetimes in a strongly correlated electron liquid

We report on angle-dependent measurements of the sheet resistances and Hall coefficients of electron liquids in SmTiO3/SrTiO3/SmTiO3 quantum well structures, which were grown by molecular beam epitaxy on (001) DyScO3. We compare their transport properties with those of similar structures grown on LSAT [(La0.3Sr0.7)(Al0.65Ta0.35)O3]. On DyScO3, planar defects normal to the quantum wells lead to a strong in-plane anisotropy in the transport properties. This allows for quantifying the role of defects in transport. In particular, we investigate differences in the longitudinal and Hall scattering rates, which is a non-Fermi liquid phenomenon known as lifetime separation. The residuals in both the longitudinal resistance and Hall angle were found to depend on the relative orientations of the transport direction to the planar defects. The Hall angle exhibited a robust T2 temperature dependence along all directions, whereas no simple power law could describe the temperature dependence of the longitudinal resistances. Remarkably, the degree of the carrier lifetime separation, as manifested in the distinctly different temperature dependences and diverging residuals near a critical quantum well thickness, was completely insensitive to disorder. The results allow for a clear distinction between disorder-induced contributions to the transport and intrinsic, non-Fermi liquid phenomena, which includes the lifetime separation.Comment: In press, Sci. Re

Residential Segregation in General Equilibrium

This paper studies the causes and consequences of racial segregation using a new general equilibrium model that treats neighborhood compositions as endogenous. The model is estimated using unusually detailed restricted Census microdata covering the entire San Francisco Bay Area, and in combination with a rich array of econometric estimates, serves as a powerful tool for carrying out counterfactual simulations that shed light on the causes and consequences of segregation. In terms of causes, and contrasting with prior research, our GE simulations indicate that equalizing income and education across race would be unlikely to result in significant reductions in racial segregation, as minority households would sort into newly formed minority neighborhoods. Indeed, among Asian and Hispanic households, segregation increases. In terms of consequences, this paper provides the first evidence that sorting on the basis of race gives rise to significant reductions in the consumption of local public goods by minority households and upper-income minority households in particular. These consumption effects are likely to have important intergenerational implications.Segregation, General Equilibrium, Endogenous Sorting, Urban Housing Market, Locational Equilibrium, Counterfactual Simulation, Discrete Choice

What Drives Racial Segregation? New Evidence Using Census Microdata

This paper sheds new light on the forces that drive residential segregation on the basis of race, assessing the extent to which across-race differences in other household characteristics can explain a significant portion of observed racial segregation. The central contribution of the analysis is to provide a transparent new measurement framework for understanding segregation patterns. This framework allows researchers to characterize patterns of segregation, to decompose them in meaningful ways, and to carry out partial equilibrium counterfactuals that illuminate the contributions of a variety of non-race characteristics in driving segregation. We illustrate our approach using restricted micro-Census data from the San Francisco Bay Area that provide a rich joint distribution of household and neighborhood characteristics not previously available to the research community. In contrast to findings in the prior literature, our analysis indicates that individual household characteristics can explain a considerable fraction of segregation by race, explaining almost 95% of segregation for Hispanic, over 50% for Asian, and 30% for White and Black households.Residential Segregation, Racial Segregation, Sorting, Housing Markets

An Equilibrium Model of Sorting in an Urban Housing Market: The Causes and Consequences of Residential Segregation

This paper presents a new equilibrium framework for analyzing economic and policy questions related to the sorting of households within a large metropolitan area. We estimate the model using restricted-access Census data that precisely characterize residential and employment locations for households the San Francisco Bay Area, yielding accurate measures of preferences for a wide variety of housing and neighborhood attributes across different types of household. We use these estimates to explore the causes and consequences of racial segregation in general equilibrium. Our results indicate that, given the preference structure of households in the Bay Area, the elimination of racial differences in income and wealth would significantly increase the residential segregation of each major racial group, as the equalization of income leads, for example, to the formation of new wealthy, segregated Black and Hispanic neighborhoods. We also provide evidence that sorting on the basis of race itself (whether driven by preferences or discrimination) leads to large reductions in the consumption of housing, public safety, and school quality by Black and Hispanic households.Segregation, Sorting, Housing Markets, Locational Equilibrium, Residential Choice, Discrete Choice