UC San Diego UC San Diego Electronic Theses and Dissertations Title Downclocking WiFi to Improve Energy Efficiency in Mobile Devices / Permalink https://escholarship.org/uc/item/7c83n5cg Author Lu, Feng Publication Date 2014 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA, SAN DIEGO Downclocking WiFi to Improve Energy Efficiency in Mobile Devices A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Electrical Engineering (Computer Engineering) by Feng Lu Committee in charge: Professor Tara Javidi, Co-Chair Professor Alex C. Snoeren, Co-Chair Professor Geoffrey M. Voelker, Co-Chair Professor Ramesh Rao Professor Stefan Savage 2014 Copyright Feng Lu, 2014 All rights reserved. The Dissertation of Feng Lu is approved and is acceptable in quality and form for publication on microfilm and electronically: Co-Chair Co-Chair Co-Chair University of California, San Diego 2014 iii DEDICATION To my wife who is always there for me. iv EPIGRAPH The best way to predict the future is to invent it. Alan Kay v TABLE OF CONTENTS Signature Page . iii Dedication . ......... iv Epigraph . ........... v Table of Contents . vi List of Figures . ix List of Tables . xii Acknowledgements . xiii Vita................................................. xv Abstract of the Dissertation . xvii Chapter 1 Introduction . 1 1.1 Downclocking . 3 1.2 Contribution . 4 1.3 Organization . 7 Chapter 2 Background and Motivation . 9 2.1 A Communication System Primer . 9 2.2 WiFi Overview . 11 2.2.1 Physical Layer . 12 2.2.2 Rate Selection . 14 2.2.3 Media Access Control . 14 2.3 The Potential of Downclocking . 15 2.4 Downclocked Transmission . 18 2.5 Related Work . 19 2.5.1 Efficient Power Save Modes . 20 2.5.2 Device Duty Cycling . 21 2.5.3 Transmission Power Control . 21 2.5.4 Downclocking . 22 2.6 Summary . 23 Chapter 3 Downclocking 802.11b . 24 3.1 Reception . 24 3.1.1 DSSS Modulation. 25 3.1.2 Compressive Sensing . 27 vi 3.2 Transmission . 29 3.3 Practical Design Considerations . 32 3.3.1 Fixed-Length Integrate-and-Dump . 33 3.3.2 Synchronization . 34 3.4 Interacting with Existing Networks . 35 3.4.1 Rate Selection . 35 3.4.2 Protocol Interactions . 36 3.5 Network Impacts . 37 3.6 Prototype . 38 3.6.1 Implementation . 38 3.6.2 Phase Tracking & Automatic Gain Control . 39 3.6.3 Experimental Evaluation . 40 3.7 Summary . 49 Chapter 4 Downclocking 802.11a/g . 50 4.1 Harnessing Aliasing . 51 4.1.1 Downclocked OFDM Modulation . 51 4.1.2 Decoding Aliasing Induced Modulation . 55 4.2 WiFi Reception . 56 4.2.1 Frame Structure . 56 4.2.2 Reception Pipeline . 57 4.3 Downclocking WiFi . 58 4.3.1 Timing Synchronization . 58 4.3.2 Frequency Compensation . 62 4.3.3 Channel Estimation . 63 4.3.4 Phase Compensation . 64 4.3.5 Data Precoding . 64 4.3.6 Additional Considerations . 67 4.4 Implementation . 69 4.4.1 Sender . 69 4.4.2 Receiver . 70 4.4.3 Network interactions . 72 4.5 Evaluation . 73 4.5.1 Microbenchmark Results . 74 4.5.2 Prototype System Evaluation . 77 4.6 Summary . 86 Chapter 5 Energy Consumption Evaluation . 87 5.1 Methodology . 87 5.1.1 WiFi Power Model . 88 5.1.2 Smartphone App Traces . 89 5.2 SloMo Energy Consumption . 90 5.3 Network Impact . 95 vii 5.4 Alternative Approaches. 96 5.5 Extending Energy Saving with Enfold . 98 5.6 Summary . 101 Chapter 6 Conclusion and Future Work . 103 6.1 Future Directions . ..