Wireless Network Information Flow: A Deterministic Approach

In a wireless network with a single source and a single destination and an arbitrary number of relay nodes, what is the maximum rate of information flow achievable? We make progress on this long standing problem through a two-step approach. First we propose a deterministic channel model which captures the key wireless properties of signal strength, broadcast and superposition. We obtain an exact characterization of the capacity of a network with nodes connected by such deterministic channels. This result is a natural generalization of the celebrated max-flow min-cut theorem for wired networks. Second, we use the insights obtained from the deterministic analysis to design a new quantize-map-and-forward scheme for Gaussian networks. In this scheme, each relay quantizes the received signal at the noise level and maps it to a random Gaussian codeword for forwarding, and the final destination decodes the source's message based on the received signal. We show that, in contrast to existing schemes, this scheme can achieve the cut-set upper bound to within a gap which is independent of the channel parameters. In the case of the relay channel with a single relay as well as the two-relay Gaussian diamond network, the gap is 1 bit/s/Hz. Moreover, the scheme is universal in the sense that the relays need no knowledge of the values of the channel parameters to (approximately) achieve the rate supportable by the network. We also present extensions of the results to multicast networks, half-duplex networks and ergodic networks.Comment: To appear in IEEE transactions on Information Theory, Vol 57, No 4, April 201

Approximate Capacity of Gaussian Relay Networks

We present an achievable rate for general Gaussian relay networks. We show that the achievable rate is within a constant number of bits from the information-theoretic cut-set upper bound on the capacity of these networks. This constant depends on the topology of the network, but not the values of the channel gains. Therefore, we uniformly characterize the capacity of Gaussian relay networks within a constant number of bits, for all channel parameters.Comment: This paper is submited to 2008 IEEE International Symposium on Information Theory (ISIT 2008) -In the revised format the approximation gap (\kappa) is sharpene

Activation Volume for Arsenic Diffusion in Germanium

We have measured the effect of pressure on As diffusion in Ge. Diffusion anneals on ion-implanted samples were carried out in a high-temperature diamond anvil cell using fluid argon as a clean, hydrostatic pressure medium. At 575 °C over the pressure range 0.1–4 GPa, pressure slightly enhances the diffusivity, characterized by an activation volume of –1.7 ± 1.4 cm3/mole or –0.12 ± 0.10 times the atomic volume. The results call into question the prevailing view that diffusion of groups III, IV, and V elements are mediated entirely by vacancies. If diffusion of As is mediated entirely by vacancies then either the vacancy formation volume must be unexpectedly low or the energy of vacancy migration must be unexpectedly high.Engineering and Applied Science

Cytoskeletal and integrin-mediated mechanobiology of the alveolar epithelium

Ventilator-associated lung injury (VALI) describes the process by which mechanical ventilation of the lungs causes systemic injury. Cyclic Mechanical Strain (CMS) of alveolar epithelial cells induces cytokine release, suggesting that mechanotransduction of force into injurious biological signaling occurs in the alveolar epithelium. The work in this thesis investigated the role of integrin and cytoskeletal signalling in CMS-induced cytokine release in these cells. Specifically, the roles of RhoA-Rho associated protein kinase (ROCK) signaling and the integrin-associated protein, CD98hc, were assessed. Inhibition of ROCK and non-muscle myosin II (NMII), and knockdown of CD98hc abrogated CMS-induced IL-8 release. Preliminary data obtained using FRET probes indicated that CD98hc knockdown reduced baseline RhoA activity. To investigate these pathways upon CMS-induced injury in intact human lung, and to generate translational data, we established a model of CMS using precision-cut lung slices (PCLS). CMS induced cytokine release in both murine and human PCLS and there was also an indication that cells spatially correlating with ATII proliferated in response to CMS. These data suggest a key role for the RhoA-ROCK-NMII axis in the mechanotransduction of CMS in the alveolar epithelium. CD98hc may modulate baseline RhoA expression and CMS-induced injurious signalling by affecting baseline cellular pre-stress. PCLS respond to CMS thereby enabling mechanotransduction studies in human tissue with near physiological conditions. This will potentially facilitate future translational research in VALI.Open Acces

Effect of Pressure on Boron Diffusion in Silicon

We are studying the effect of pressure on boron diffusion in silicon in order to better understand the nature of the point defects responsible for diffusion. Si homoepitaxial layers delta-doped with boron were grown using molecular beam epitaxy. Diffusion anneals were performed in a high temperature diamond anvil cell using fluid argon as a pressure medium. Diffusivities were deduced from B concentration-depth profiles measured with using secondary ion mass spectrometry. Preliminary results indicate that pressure enhances B diffusion in Si at 850 ˚C, characterized by an average activation volume of -0.125±0.02 times the atomic volume, and thus appear consistent with an interstitial-based diffusion mechanism. Results are compared with previous hydrostatic-pressure studies, with results in biaxially strained films, and with atomistic calculations of activation volumes for self diffusion.Engineering and Applied Science