Adaptive Transmission Power for Optimal Energy Reliable Multi-hop Wireless Communication

Abstract — We define a transmission power adaptation-based routing technique that finds optimal paths for minimum energy reliable data transfer in multi-hop wireless networks. This optimal choice of the transmission power depends on the link distance between the two nodes and the channel characteristics. Typical energy efficient routing techniques use a transmission power such that the received signal power at the destination minimally exceeds a desired threshold signal strength level. In this paper we argue that such a choice of the transmission power does not always lead to optimal energy routes, since it does not consider differences in the receiver noise levels. We first analyze the optimal transmission power choices for both the ideal case from an information-theoretic perspective, and for realistic modulation schemes. Subsequently we define our technique for transmission power adaptation that can be used in existin

Optimisation of superconducting thin film growth for next generation superconducting detector applications

There is a growing demand for superconducting detectors with single photon sensitivity from near- to far infrared wavelengths. Emerging application areas include imaging, remote sensing, astronomy and free space communications. Two superconducting device technologies, superconducting nanowire single-photon detectors (SSPDs/SNSPDs) and microwave kinetic inductance detectors (MKIDs) have the potential to outperform off-the-self semiconductor technologies and offer scalability to large arrays. Fabrication of high efficiency superconducting detectors strongly depends on the quality of superconducting thin films. The original work presented in this thesis has explored the growth and optimization of several superconducting thin film materials for next generation superconducting detectors. Films have been grown in an ultra-high vacuum sputter deposition system and an atomic layer deposition system. Since its initial demonstration, NbN and NbTiN have been predominantly used as the base material for SNSPDs. In this work, we have explored the optimization of both the materials with an emphasis on NbTiN. NbTiN is optimized by heating the substrates to 800 ̊C achieving a Tc of 10.4 K for a film thickness of 5.5 nm on silicon substrate. Due to their crystalline nature superconducting properties of NbN or NbTiN thin films are strongly correlated with the lattice parameters of substrate properties. This causes a restriction on the substrate choice and integration of SNSPD devices with complex circuits. Amorphous superconducting materials can be promising alternatives for this purpose. We have explored growth and optimization of amorphous MoSi and MoGe thin films. Both the materials are co-sputtered to tune the composition. For 5 nm thick MoSi film on silicon substrate we obtain Tc of 5.5 K. For MKID fabrication, TiN can be an useful base material due to its high sheet resistance and widely tuneable superconducting properties. TiN thin films have been sputtered on heated (500 ̊C) silicon substrates with a Tc of 3.9 K for a 90 nm thick film. The dielectric constants of the thin films as a function of wavelength (270-2200 nm) have been determined via variable angle spectroscopic ellipsometry (VASE). Atomic structure and stoichiometry of the films have been characterized in high resolution transmission electron microscopy (HRTEM). This study enables us to precisely control film properties and thus tailor superconducting films to the requirements of specific photon-counting applications

MRPC: Maximizing network lifetime for reliable routing in wireless environments

We propose MRPC, a new power-aware routing algorithm for energy-efficient routing that increases the operational lifetime of multi-hop wireless networks. In contrast to conventional power-aware algorithms, MRPC identifies the capacity of a node not just by its residual battery energy,but also bythe expected energy spent in reliably forwarding a packet over a specific link. Such a formulation better captures scenarios where link transmission costs also depend on physical distances between nodes and the link error rates. Using a max-min formulation, MRPC selects the path that has the largest packet capacity at the `critical' node (the one with the smallest residual packet transmission capacity). We also present CMRPC, a conditional variant of MRPC that switches from minimum energy routing to MRPC only when the packet forwarding capacity of nodes falls below a threshold. Simulationbased studies have been used to quantify the performance gains of our algorithms. I

Minimum Energy Paths for Reliable Communication in Multi-Hop Wireless Networks

Current algorithms for minimum-energy routing in wireless networks typically select minimum-cost multi-hop paths. In scenarios where the transmission power is fixed, each link has the same cost and the minimum-hop path is selected. In situations where the transmission power can be varied with the distance of the link, the link cost is higher for longer hops; the energy-aware routing algorithms select a path with a large number of small-distance hops. In this paper, we argue that such a formulation based solely on the energy spent in a single transmission is misleading --- the proper metric should include the total energy (including that expended for any retransmissions necessary) spent in reliably delivering the packet to its final destination

Optical properties of refractory metal based thin films

There is a growing interest in refractory metal thin films for a range of emerging nanophotonic applications including high temperature plasmonic structures and infrared superconducting single photon detectors. We present a detailed comparison of optical properties for key representative materials in this class (NbN, NbTiN, TiN and MoSi) with texture varying from crystalline to amorphous. NbN, NbTiN and MoSi have been grown in an ultra-high vacuum sputter deposition system. Two different techniques (sputtering and atomic layer deposition) have been employed to deposit TiN. We have carried out variable angle ellipsometric measurements of optical properties from ultraviolet to mid infrared wavelengths. We compare with high resolution transmission electron microscopy analysis of microstructure. Sputter deposited TiN and MoSi have shown the highest optical absorption in the infrared wavelengths relative to NbN, NbTiN or ALD deposited TiN. We have also modelled the performance of a semi-infinite metal air interface as a plasmonic structure with the above mentioned refractory metal based thin films as the plasmonic components. This study has implications in the design of next generation superconducting nanowire single photon detector or plasmonic nanostructure based devices

Design and characterisation of titanium nitride sub-arrays of kinetic inductance detectors for passive terahertz imaging

We report on the investigation of titanium nitride (TiN) thin films deposited via atomic layer deposition (ALD) for microwave kinetic inductance detectors (MKID). Using our in-house ALD process, we have grown a sequence of TiN thin films (thickness 15, 30, 60 nm). The films have been characterised in terms of superconducting transition temperature Tc , sheet resistance Rs and microstructure. We have fabricated test resonator structures and characterised them at a temperature of 300 mK. At 350 GHz, we report an optical noise equivalent power NEPopt≈2.3×10−15 W/√Hz , which is promising for passive terahertz imaging applications

Minimum Energy Reliable Paths using Unreliable Wireless Links

We address the problem of energy-efficient reliable wireless communication in the presence of unreliable or lossy wireless link layers in multi-hop wireless networks. Prior work [1] has provided an optimal energy efficient solution to this problem for the case where link layers implement perfect reliability. However, a more common scenario — a link layer that is not perfectly reliable, was left as an open problem. In this paper we first present two centralized algorithms, BAMER and GAMER, that optimally solve the minimum energy reliable communication problem in presence of unreliable links. Subsequently we present a distributed algorithm, DAMER, that approximates the performance of the centralized algorithm and leads to significant performance improvement over existing singlepath or multi-path based techniques. Categories and Subject Descriptor

Nano-optical photoresponse mapping of superconducting nanowires with enhanced near infrared absorption

Superconducting nanowire single-photon detectors (SNSPDs) play an important role in emerging optical quantum technologies. We report on advanced nanometric characterization of a high efficiency near infrared SNSPD design based on a low roughness Tantalum pentoxide (Ta2O5)/ silicon dioxide (SiO2) distributed Bragg reflector (DBR) cavity structure. We have performed high resolution transmission electron microscopy (TEM) analysis to verify the smoothness of the DBR. Optical reflectance measurements show excellent correspondence with DBR simulations. We have carried out precision nano-optical photoresponse mapping studies at 940 nm wavelength at T = 3.5 K, indicating excellent large area device uniformity (peak efficiency 55 % at 100 Hz dark count rate [DCR]) with a full width half maximum (FWHM) timing jitter of 60 ps. With manual fibre coupling with single mode fibre, we achieve a system detection efficiency (SDE) of 57.5% at 940 nm wavelength (100 Hz DCR) at T = 2.3 K and a low polarization dependence of 1.20 ± 0.03. For coupling with multimode fibre, we achieve SDE of 90% at 940 nm (200 Hz DCR) at T= 2.3 K. These SNSPD devices are promising candidates for use in quantum dot photoluminescence studies and optical quantum technology applications