University of California Los Angeles MIMO Enhancements for Air-to-Ground Wireless Communications A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Electrical Engineering by Jesse Thomas Chen 2014 c Copyright by Jesse Thomas Chen 2014 Abstract of the Dissertation MIMO Enhancements for Air-to-Ground Wireless Communications by Jesse Thomas Chen Doctor of Philosophy in Electrical Engineering University of California, Los Angeles, 2014 Professor Babak Daneshrad, Chair In order to introduce the benefits of Multiple-Input/Multiple-Output (MIMO) wireless so- lutions into the airborne environment for maximal effect, the airborne channel must be fully understood. While there have been theoretical models proposed for the airborne channel, there has been very little work toward providing a practical channel model which has been validated by actual an airborne platform. This work presents a characterization of practical performance gains of a MIMO sys- tem over a conventional SISO, in a mobile air-to-ground environment. Field measurements were collected with an airborne 4x4 MIMO-OFDM channel-sounding platform at altitudes, speeds and flight patterns approximating medium-endurance vehicles flying over various ter- rain. Ground stations placed in multiple locations (different scattering scenarios) measured channel responses in addition to actual throughput statistics. Our studies indicate that sig- nificant throughput and range gains are achievable with MIMO. We also show that depending on application requirements, these MIMO-enabled gains can be converted into considerable power savings. We also present a study of the effects of introducing MIMO-enabled signaling techniques (such as eigen beamforming and spatial multiplexing) on the total link-capacity of a system of uncoordinated, air-to-ground link-pairs deployed to a single area of operations. Captured channel measurements from the earlier real-world airborne study were inserted into our multi- ii link simulation environment. Trials were run under several representative aerial-deployment scenarios, revealing significant gains in link capacity. Finally, we consider the potential throughput enhancement delivered by full-duplex sig- naling and its limitations due to desensitization of receiver hardware by self-generated in- terference (SI). Existing SI cancellation solutions are prohibitive for long-range/airborne applications due to power handling limitations. They are also not easily scalable for an arbitrary number of MIMO antennas in arbitrary positions. A host-agnostic, high-power, adaptive SI canceler design is proposed and a hardware prototype is presented. Performance enhancement with an off-the-shelf host radio was demonstrated in the presence of varying SI signal profiles. iii The dissertation of Jesse Thomas Chen is approved. Kung Yao William Kaiser Mario Gerla Babak Daneshrad, Committee Chair University of California, Los Angeles 2014 iv To my dad. v Table of Contents 1 Introduction :::::::::::::::::::::::::::::::::::::: 1 1.1 Motivation . .1 1.2 Scope of the Dissertation . .2 2 The Airborne Wireless MIMO Channel :::::::::::::::::::: 4 2.1 Overview . .4 2.1.1 Background . .4 2.1.2 Prior Art . .9 2.1.3 Objectives . 10 2.2 Measurement Platform . 10 2.2.1 Packet Structure . 12 2.2.2 Post Processing Methodology . 13 2.3 Field Test Overview . 16 2.4 Analysis of Channel Measurements . 21 2.4.1 Eigenmode Analysis . 21 2.4.2 Theoretical MIMO Capacity Gain . 21 2.4.3 Eigen Beamforming Analysis . 24 2.4.4 MIMO Power Savings Analysis . 26 2.5 Analysis of Actual Data Performance . 28 2.5.1 MIMO Throughput Gain . 28 2.5.2 Spatial Stream Analysis . 30 2.5.3 MIMO Range Extension . 31 2.6 Discussion and Conclusions . 33 vi 3 Airborne MIMO Concurrent Link Capacity :::::::::::::::::: 35 3.1 Overview . 35 3.1.1 Background . 36 3.1.2 Prior Art . 38 3.1.3 Objectives . 39 3.2 Mode Selection Algorithm . 40 3.2.1 Transmit Bandwidth and Power Constraints . 40 3.2.2 Resource Mode Selection Engine . 41 3.2.3 Mode Selection Engine in Action . 42 3.3 Simulation Engine . 44 3.3.1 Framework Overview . 44 3.3.2 Air-to-Ground Channel Assumptions . 45 3.3.3 Post-Processing SNR Calculation . 46 3.3.4 Network Topology Generation . 47 3.3.5 Practical Considerations . 53 3.4 MIMO Concurrent Link Performance . 54 3.4.1 Scenario I: Free Roam Topology . 54 3.4.2 Scenario II: Sensor Hotspot Topology . 58 3.4.3 Scenario III: Perimeter Patrol Topology . 59 3.4.4 Scenario IV: Outpost Deployment Topology . 60 3.4.5 Scenario V: Maximum Coverage Topology . 61 3.4.6 Scenario VI: Convoy Topology . 62 3.4.7 MIMO Concurrent Link Performance Summary . 62 3.5 Effectiveness of Individual Features . 63 3.5.1 MIMO Spatial Multiplexing Effectiveness . 63 vii 3.5.2 MIMO RX Eigen Beamnulling Effectiveness . 65 3.5.3 MIMO TX Eigen Beamforming Effectiveness . 66 3.5.4 Power Control Effectiveness . 67 3.5.5 Link Adaptation, Spectral Segmentation, Variable Bandwidth . 68 3.6 Discussion and Conclusions . 69 4 Enabling Airborne Full-Duplex Communications ::::::::::::::: 71 4.1 Overview . 71 4.2 Background . 73 4.2.1 Prior Art . 73 4.2.2 Objectives . 75 4.3 Two-Stage Adaptive Self-Interference Cancellation . 76 4.3.1 General Architecture Trade-offs . 78 4.4 Analog RF Cancellation Design Considerations . 80 4.4.1 Gain Resolution, Phase Precision and Power Handling . 80 4.4.2 Group Delay . 81 4.5 Baseband Cancellation Design Considerations . 81 4.5.1 Digital Adaptive Filter . 81 4.5.2 Non-Linearity . 82 4.5.3 Phase Noise . 85 4.5.4 I/Q Imbalance and DC Offset Cancellation . 87 4.6 Hardware System Implementation . 88 4.6.1 Analog Subsystem . 89 4.6.2 Digital Subsystem . 92 4.7 Hardware Performance . 94 viii 4.7.1 Experimental Setup . 94 4.7.2 Radio System Performance . 95 4.7.3 Narrowband, Wideband and Tracking Performance . 95 4.8 Discussion and Conclusions . 98 5 Conclusions and Future Work ::::::::::::::::::::::::::: 100 5.1 Conclusions . 100 5.2 Future Work . 102 References ::::::::::::::::::::::::::::::::::::::::: 105 ix List of Figures 1.1 DoD spending on UAS development [Dod05] . .1 2.1 Effect of multipath on wireless transmission [Gol05] . .5 2.2 Variation in signal power due to multipath fading [Gol05] . .5 2.3 Channel delay spread (τdelay) and coherence bandwidth (Bcoherence) [Gol05] .6 2.4 OFDM subcarriers in a frequency selective channel [Tsa07] . .6 2.5 General MIMO system diagram [Gol05] . .7 2.6 A 2x2 MIMO channel . .8 2.7 A 2x2 MIMO channel after SVD . .9 2.8 StreamCaster 3500 high-level hardware architecture . 11 2.9 MIMO channel-sounding packet structure . 12 2.10 MIMO channel-sounding symbol structure . 12 2.11 MIMO/SISO data packet structure . 13 2.12 Sounding waveform capture mechanism . 14 2.13 MIMO channel-sounding waveform post-processing flow . 14 2.14 Google EarthTMformat flight logs . 15 2.15 Flightpath for airborne field tests . 16 2.16 Cessna-172S aircraft used for the airborne channel measurement campaign, antenna positions circled in white . 17 2.17 Airborne antenna (left) and radiation pattern (right) . 17 2.18 Ground Station 1, Rooftop Unit 1: patch antennas (spread) . 18 2.19 Ground Station 1, Rooftop Unit 2: omnidirectional (left); and Unit 3: patch antenna (right) . 19 2.20 Ground Station 2, UCLA: omnidirectional antennas . 19 x 2.21 Ground Station 3, Long Beach Airport: omnidirectional antennas . 20 2.22 Ground station omnidirectional antenna (left) and radiation pattern (right) . 20 2.23 Ground station directional antenna (left) and radiation pattern (right) . 20 2.24 Normalized eigenmode values vs. distance, rooftop patch antenna location . 22 2.25 Normalized eigenmode values vs. distance, UCLA omnidirectional antenna location . 22 2.26 Total data transfer capacity vs. time, rooftop patch antenna location . 24 2.27 Channel capacity vs. distance, rooftop patch antenna location . 24 2.28 Beamforming capacity gain, rooftop spread patch antenna location . 25 2.29 Beamforming capacity gain, airport omni. antenna location . 25 2.30 MIMO gain over SISO vs. MPRF, rooftop patch antenna location . 27 2.31 MIMO gain over SISO vs. MPRF, airport omnidirectional antenna location . 27 2.32 MIMO/SISO total data transfered (top) and instantaneous throughput (bot- tom) vs. time, rooftop patch antenna location . 29 2.33 Observed data throughput vs. distance, rooftop omnidirectional antenna lo- cation . 29 2.34 Spatial streams vs. location (blue, red, yellow, green are 1, 2, 3, 4 spatial streams, respectively) . 31 2.35 Flightpath for MIMO range extension experiment . 32 2.36 MIMO range extension (throughput vs. distance); Rooftop patch antenna location . 32.