Athermal photonic devices and circuits on a silicon platform

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 153-166).In recent years, silicon based optical interconnects has been pursued as an eective solution that can offer cost, energy, distance and bandwidth density improvements over copper. Monolithic integration of optics and electronics has been enabled by silicon photonic devices that can be fabricated using CMOS technology. However, high levels of device integration result in signicant local and global temperature fluctuations that prove problematic for silicon based photonic devices. In particular, high temperature dependence of Si refractive index (thermo-optic (TO) coefficient) shifts the filter response of resonant devices that limit wavelength resolution in various applications. Active thermal compensation using heaters and thermo-electric coolers are the legacy solution for low density integration. However, the required electrical power, device foot print and number of input/output (I/O) lines limit the integration density. We present a passive approach to an athermal design that involves compensation of positive TO effects from a silicon core by negative TO effects of the polymer cladding. In addition, the design rule involves engineering the waveguide core geometry depending on the resonance wavelength under consideration to ensure desired amount of light in the polymer. We develop exact design requirements for a TO peak stability of 0 pm/K and present prototype performance of 0.5 pm/K. We explore the material design space through initiated chemical vapor deposition (iCVD) of 2 polymer cladding choices. We study the eect of cross-linking on the optical properties of a polymer and establish the superior performance of the co-polymer cladding compared to the homo-polymer. Integration of polymer clad devices in an electronic-photonic architecture requires the possibility of multi-layer stacking capability. We use a low temperature, high density plasma chemical vapor deposition of SiO2/SiNx to hermetically seal the athermal. Further, we employ visible light for post-fabrication trimming of athermal rings by sandwiching a thin photosensitive layer of As2S3 in between amorphous Si core and polymer top cladding. System design of an add-drop filter requires an optimum combination of channel counts performance and power handling capacity for maximum aggregate bandwidth. We establish the superior performance of athermal add-drop filter compared to a standard silicon filter treating bandwidth as the figure-of-merit.by Vivek Raghunathan.Ph.D

Exploiting Wireless Broadcast by Opportunistic Packet Splaying

The IEEE 802.11 random access MAC suffers from expensive channel acquisition costs which are exacerbated by the use of TCP. This floor acquisition mechanism is primarily a way to fashion unicast "links" out of what is fundamentally a broadcast medium. The main thesis of this paper is that such unicast usage of the wireless channel is unnecessary, and that once the floor has been acquired, we can instead "splay" packets to as many potential receivers as possible using the fact that wireless is a broadcast medium. Splay is placed between the IP and MAC layers and reduces the expensive cost of channel acquisition by opportunistically combining packets intended for different receivers. Splay allows for the use of sophisticated coding approaches to augment the IEEE 802.11 stop-and-go ARQ. This also helps prevent IEEE 802.11 from incorrectly backing off exponentially in response to fading losses on intermediate quality links, which are quite common in practice. We are currently implementing Splay as a Linux kernel module.DARPA / N66001-06-C-2021 and N00014-0-1-1-0576AFOSR / F49620-02-1-0217NSF / NSF CNS 05-19535, ANI 02-21357, and CCR-0325716USARO / DAAD19-01010-465DARPA/AFOSR / F49620-02-1-0325Vodafone Graduate FellowshipOpe

Real-Time Scheduling in Wireless Networks

We study a canonical real-time scheduling problem for time-slotted collocated wireless networks serving users with diverse real-time requirements and wireless loss patterns arising from fading. We study two traffic patterns: periodic arrivals with deadline equal to period, and renewal arrivals, in which a new packet from a user arrives when the current one leaves. Wireless channel conditions are modelled as Bernoulli losses. For periodic arrivals, we prove that the optimal policy that minimizes the expected number of deadline misses has a strong property: it only switches between users on arrivals, successful completions or deadline expiry. When users have similar periods, this optimal policy is a linear switching curve characterized by a single number. Our result explicitly captures the trade-off between two competing aspects of the problem: the real-time tendency to schedule users in earliest-deadline-first (EDF) order, and the wireless tendency to exploit multi-user diversity by scheduling users with good channel conditions first. When users have similar channels, a common occurrence, we establish the optimality of EDF. For renewal arrivals, the optimal policy continues to have a switching structure, although not necessarily characterized by a single parameter. It schedules the user most likely to complete when it also has the earlier deadline. When the "better" user has a later deadline, it is scheduled till a worse user's deadline gets "close'", and then the worse user is scheduled till expiry. Our results for periodic arrivals are strong and significant. They reduce the search space for optimal wireless real-time scheduling policies by an exponential order of magnitude. They establish optimality of "virtual-deadline-first" policies, where each user's deadline is modified to take channel quality into account. Policies in this class are easy to implement in a distributed manner.NSF / CCR-0325716 and CNS 05-19535DARPA/AFOSR / F49620-02-1-0325DARPA / N00014-0-1-1-0576 and N66001-06-C-2021Oakridge-Battelle / 239 DOE BATT 4000044522AFOSR / F49620-02-1-0217Ope

Cross-Layer Exploitation of MAC Layer Diversity in Wireless Networks

The conventional function of the medium access control (MAC) layer in wireless networks is interference management. We show how the MAC can also be used to mitigate the effect of fading. We begin by providing experimental data to demonstrate that multipath fading effects are seen at the MAC layer. These effects appear at timescales on the same order of the IEEE 802.11 protocol and therefore, interact negatively with the RTS-CTS-DATA-ACK handshake. We identify two types of MAC diversities to jointly combat fading and interference, called multi-receiver diversity and multi-channel diversity, respectively, through canonical scenarios. In order to harness these MAC layer diversities, we propose a simple dynamic-binding multi-channel MAC (DB-MCMAC) protocol that is backward compatible with IEEE 802.11. DB-MCMAC exploits MAC diversities by opportunistically acquiring the floor for the best receiver on each channel, and dynamically binding data transmissions after the floor has been acquired. We employ a simple continuous time Markov chain model to analyze the expected performance of the DB-MCMAC protocol. We have carried out a comprehensive performance evaluation of DB-MCMAC using ns-2. Simulation results show that DB-MCMAC can successfully harness multi-receiver and multi-channel fading and interference diversities to provide considerable improvements over a baseline multi-channel MAC in several situations.DARPA/AFOSR, AFOSR, USARO, NSF, and DARPA / F49620-02-1-0325, F49620-02-1-0217, DAAD19-01010-465,Vodafone Graduate FellowshipOpe

A counterexample in congestion control of wireless networks

One of the triumphs of wireline network research of the last decade has been the casting of the Internet congestion control problem within an optimization framework based on utility functions. Such an approach provides a sound understanding of the underlying stability and fairness issues, as well as a post-facto justification of TCP-like additiveincrease multiplicative-decrease (AIMD) algorithms. This paper provides a counter-example showing that the same result cannot be extended to wireless networks, at least not in a straightforward manner. The fundamental difference is that wireless networks are of a broadcast nature. There is no strict notion of a “link,” since transmissions from nearby nodes interfere with each other. Using a simple model of interference in wireless networks, a counter-example of a wireless network is presented in which the congestion control mechanism has an unstable equilibrium point at the desired fair solution. Further, ns-2 simulations of this counter-example manifest an oscillatory behavior. Surprisingly, this oscillatory behavior appears to be fairly typical in wireless networks, with most randomly chosen network examples manifesting it. This loss of stability suggests a possible need for the re-design of wireless TCP and wireless queue management to explicitly account for the wireless nature of the effects of interference

Physical Layer Aware Algorithms and Protocols for Wireless Networks

197 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.Our candidate stack uses traffic adaptive multipath routing to guide packets probabilistically around network bottlenecks, and automatically create interference-aware routes. Recognizing that theoretical arguments require a cross layer re-design of TCP taking wireless interference into account, our work suggests the use of in-network congestion control to automatically adapt flow rates inside the network.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD