Design of a system for cm-range wireless communication Simone Gambini Jan M. Rabaey Elad Alon Electrical Engineering and Computer Sciences University of California at Berkeley Technical Report No. UCB/EECS-2009-184 http://www.eecs.berkeley.edu/Pubs/TechRpts/2009/EECS-2009-184.html December 18, 2009 Copyright © 2009, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. Design of a system for cm-range wireless communications by Simone Gambini A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Electrical Engineeing and Computer Sciences in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor Jan Rabey, Chair Professor Elad Alon Professor Paul K. Wright Fall 2009 The dissertation of Simone Gambini is approved. Chair Date Date Date University of California, Berkeley Fall 2009 Design of a system for cm-range wireless communications Copyright c 2009 by Simone Gambini Abstract Design of a system for cm-range wireless communications by Simone Gambini Doctor of Philosophy in Electrical Engineeing and Computer Sciences University of California, Berkeley Professor Jan Rabey, Chair The continuous growth in the number of mobile phone subscribers, which exceeded 3 billions by 2007 , and of the number of wireless devices and systems, led to visions of a near future in which wireless technology is so ubiquitous that 1000 Radios per person will exist. In this context, ad-hoc area networks between several consumer electronic devices located within a meter of each other will be necessary in order to reduce traffic towards the base station and reduce power consumption and interference generation. As existing air interfaces still both radiate and dissipate an order of magnitude higher power than what this future scenario requires, a new, low-power radio technology must be developed. In this thesis, we develop a low-power transceiver with range of a few centimeters targeted to mobile-mobile data ex- change. The same transceiver could also be employed in some implanted applications, as well as in distributed industrial control environments. The design of the air-interface for this cm-range communication system at the propagation, system and circuit levels. First, we describe an optimization methodology that enables the designer to choose, for any given antenna design, which carrier frequency results in the maximum receiver Signal-To-Noise-Ratio (SNR). We then show how by using impulse-radio signaling, the chosen high-SNR channel can be leveraged to simplify the radio receiver archi- tecture to the simplest possible RF receiver-consisting only of an RF rectifier. To mitigate the known issues of sensitivity to interference for these rectifier-based receivers, a technique to improve selectivity that uses only baseband processing and requires no RF prefilter is introduced. These techniques are demostrated by a transceiver test chip, implemented in a 65nm CMOS process . The transceiver dissipates 250µW in receive mode, and 25µW in transmit mode when operating at 1Mbps , and it integrates a timing-recovery loop that achieves jitter lower than 2nS while consuming 45µW .This figures correspond to 1 an energy per bit of 300pJ, which compares favorably with current state-of-the art. Professor Jan Rabey Dissertation Committee Chair 2 To my mother Silvia, my brother Francesco and my father Diego. If I have seen a little further it is by standing on the shoulders of Giants I.Newton i Contents List of Figures vi List of Tables xii Acknowledgements xiii 1 1000 Radios per Person And the Case for Ad-Hoc Networking 1 1.1 Short range wireless systems today . .4 1.1.1 Commercially available, low-power short range bi-directional commu- nication devices . .4 1.1.2 Bi-directional,narrowband short range radios resulting from academic papers . .5 1.1.3 Impulse Ultra-wideband radios resulting from academic papers . .6 1.1.4 Other short-range, low energy communication systems . .7 1.2 Thesis structure . .8 2 Link Margin Modeling for cm-range radios 9 2.1 Improving path loss modeling . 10 2.1.1 Electromagnetic wave propagation at short range . 10 2.1.2 Magnetic Coupling . 15 2.1.3 Model Verification . 16 2.1.4 Comparison with power transfer . 23 2.1.5 Chosen design point and channel measurements with cm-scale antennas 24 2.1.6 Channel impulse response . 24 2.2 Antenna Design for a 3-10GHz wideband system . 25 2.2.1 Slotted Bowtie Antenna Design . 27 ii 2.2.2 Monopole antenna design . 30 2.3 Conclusions . 31 3 Low Power RF System Design for a cm-range Transceiver 34 3.1 Radio Dynamic Range-Power Tradeoff . 34 3.1.1 Overhead power components . 36 3.1.2 Summary . 38 3.2 Effect of duty-cycling on power dissipation . 39 3.2.1 Overhead Costs in duty-cycled systems . 41 3.2.2 Duty cycling optimization . 41 3.2.3 Inherent Asymmetry of duty-cycled systems . 44 3.2.4 Power consumption of VCO and baseband: a more critical look . 44 3.3 Radio Architecture Design . 45 3.3.1 Technological Constraints . 46 3.3.2 Transmitter section and link budget . 46 3.3.3 Receiver section . 47 3.4 Conclusions . 51 4 Baseband and Timer Subsystem Design 53 4.1 Baseband architectures for UWB systems . 53 4.2 Improving Interferer Performance in Self-Mixing Radios . 55 4.3 Synchronizer and modulation scheme design . 66 4.4 Summary . 76 5 Circuit Design 78 5.1 Introduction . 78 5.2 Transmitter Circuits . 79 5.2.1 Architecture selection . 79 5.2.2 Oscillator . 81 5.2.3 Divider . 83 5.2.4 PSK Modulator and Driver . 85 5.3 Receiver Circuits . 88 5.3.1 Matching network/duplexer . 88 iii 5.3.2 Reconfigurable mixer/diode . 90 5.3.3 Baseband Filter . 97 5.3.4 Comparators . 102 5.3.5 Layout and summary . 107 5.3.6 Bias Circuits and power gating strategy . 107 5.4 Timing Circuits . 110 5.4.1 Baseband oscillator . 112 5.4.2 Baseband divider and comparator . 113 5.4.3 Pulse detection and retiming logic . 114 5.4.4 Controller . 115 5.4.5 Block Layout and summary . 116 5.5 System Layout and summary . 116 6 Measurement Results 121 6.1 FIB Issue . 121 6.2 Transmitter measurements . 122 6.3 Receiver Measurements . 126 6.3.1 Sensitivity Measurements . 126 6.3.2 Duty-cycled Measurements . 128 6.3.3 Interference Rejection Measurements . 128 6.3.4 Wireless Tests . 131 6.3.5 Receiver Performance Summary . 135 6.3.6 Comparison with prior art . 135 6.4 Timer Measurements . 137 7 Conclusions 142 7.1 Research Directions . 143 7.1.1 Power-efficient TCM for wireless sensor networks . 143 7.1.2 Circuit Implementation . 143 A Matching Network Impedance Calculations 144 A.1 Receiver Side Impedance . 144 A.2 Transmit Side Impedance . 146 iv B Statistical Jitter Transfer Analysis 147 v List of Figures 1.1 Pushpin nodes, a first implementation of the paintable computing concept [66]. Approximate node size is 3:24cm2 .....................4 2.1 Schematic of electrical dipoles and frame of reference . 10 2.2 Equivalent circuit for monopoles in the near-field operating range . 13 2.3 Measured, calculated and simulated return loss for wire monopoles . 17 2.4 Calculated and measured path loss as a function of frequency . 17 2.5 Measured and simulated path loss as a function of frequency . 18 2.6 Measured path loss (including matching network) for the TX-RX Antenna pair at different separations . 18 2.7 Experimental setup in the BWRC Lab . 19 2.8 Calculated(top) and measured(bottom) loss as a function of frequency for different antennas separations and quality factors . 20 2.9 Effect of matching network quality factor on voltage gain . 21 2.10 Measured and simulated path loss as a function of range for different frequencies 21 2.11 Antenna impedance quality factor as a function of frequency . 22 2.12 Measured Channel S21 as a function of TX-RX separation . 25 2.13 Received energy as a function of tap number . 26 2.14 Channel Impulse Response Sampled at 2GS/s for good channel: I-component (top) and Q component (bottom) . 26 2.15 Annotated Bowtie Antenna Schematic . 28 2.16 Bowtie Antenna Simulations: Effect of varying the expansion angle (top); effect of varying the inductive stub length(bottom) . 29 2.17 Bowtie Antenna Measured Reflection versus HFSS Simulations . 30 2.18 Monopole antennas annotated schematic:full monopole(top) and loaded hol- low monopole (bottom) . 32 vi 2.19 Measured results from monopole antennas (Full monopole (top) and loaded hollow monopole (bottom) . 33 3.1 Optimal Radio Range as a function of total radio power budget . 35 3.2 Maximum Radio Range as a function of carrier frequency . 36 3.3 Power dissipation versus total baseband gain . 37 3.4 Transceiver duty cycling concept . 39 3.5 Analog Synchronizer . 42 3.6 Power Dissipation of TX+RX as a function of duty-cycling . 42 3.7 Minimum Power Dissipation as a function of receiver turn-on-time Tm .... 43 3.8 Desensitization due to reciprocal mixing . 45 3.9 Comparison between a receiver comprising of LNA+AM detection (left), and lo power oscillator+passive mixer(right) .