A Near-Threshold, Multi-Node, Wireless Body Area Network Sensor Powered by RF Energy Harvesting

Jiao Cheng1, Lingli Xia1, Chao Ma1, Yong Lian2, Xiaoyuan Xu3, C. Patrick Yue4, Zhiliang Hong5, Patrick Y. Chiang1

1Oregon State University, Corvallis, OR; 2NUS, Singapore; 3CVPL, Singapore;4HKUST, HK; 5Fudan University, Shanghai, China

Abstract - A wirelessly-powered, near-threshold, body area network SoC supporting synchronized multi-node TDMA operation is demonstrated in 65nm CMOS. A global clock source sent from a base-station wirelessly broadcasts at 434.16MHz to all sensor nodes, with where each individual BAN node sensor is phase- locked to the base-station clock using a super-harmonic injection-locked frequency divider. To eliminate the need for a battery E, each near-threshold SoC harvests energy from and phase locks to this broadcasted 434.16MHz waveform, eliminating the need for a battery. A Near-VT MICS-band OOK transmitter sends the synchronized local sensor data back to the base-station in its pre-defined TDMA slot. For an energy-harvested local VDD=0.56V, measurements demonstrate full functionality over 1.4m between the base-station and four worn sensors, including two that are NLOS. The sensitivity of the RF energy harvesting and the wireless clock synchronization are measured at -8dBm and -35dBm, respectively. ECG Lead-II / Lead-III waveforms are experimentally captured, demonstrating the end-to-end system application.

I. INTRODUCTION

The simultaneous acquisition of multiple vital signs from the human body, such as ECG, EEG, EMG, pulse oximetry, activity, heart-rate, and temperature, will be a key differentiating feature for next generation wireless body area network (WBAN) systems. Energy-efficient designs have been previously demonstrated that optimize an individual wireless sensor node for low power operation [1-2]. However, the necessary scheme for operating multiple nodes coherently has been largely overlooked. The challenge is to minimize the network protocol complexity and system power consumption, while providing precise timing synchronization to enable duty-cycled wake-up simultaneously of each node. Finally, battery-free operation is desirable, since the battery is a significant limitation to cost, size, hygiene, and sensor lifetime. In this work, we demonstrate a battery-less, multi-node WBAN system that features: (1) wireless energy harvesting using power broadcast from a base station such as a smart-phone; (2) duty-cycling and TDMA synchronization of multiple nodes by employing injection-locked wireless clock distribution.

II. WBAN ARCHITECTURE

A. Overall ArchitectureFig. 1 illustrates the system architecture of the proposed

WBAN for multiple wearable sensors. The base station (for example, smart phone) broadcasts power and clock which within occupy the 433MHz ISM band to all the nodessensors,

and also receives each node’s allotted TDMA-based wirelessly- transmitted data which that occupy occupies the 402-405MHz MICS band. The core of each sensor node (Fig. 2) is a near-threshold SoC consisting of an RF energy-harvesting front-end, a micro-power bandgap reference generator, a low-dropout (LDO) regulator, a super-harmonic injection locked frequency divider for clock synchronization, a digital TDMA slot generator, and a 402-405MHz MICS-band OOK transmitter. The A biomedical signal acquisition chip

Dummy144MHz

/2

BB CLK SEL

Counter 12b

Start code 12b

End code 12b

DigitalComparator

MICS TxEnable

DigitalComparator

on

off

AnalogComparator

TDMASlot

16MHz

Wakeup

/2/2/8

Clock Synchronization

SHILRO80MHz

EdgeCombiner

Pre Amp

PABB_clk

Data Gen & Sel

En

fREF

MICS TX

VDD (0.56V)

LDO

Bandgap

Rectifier

VoltageReferenceGenerator

VREF1

VREF2

VREF1

VREF2

Energy Harvesting

Match.Net

Match.Net+

Balun

100uF

VBG(1.1V)

433MHz wireless power and clock from

base station

402MHz data to base

station

NODE1

Energy-Harvested VDD = 0.56V

VDD (0.56V)

VDD (0.56V)

ILFD /9

Bio[3] Sensor

VDD (1.1V)ECG

Electrodes

This Work

DIN

Fig. 2. System block diagram for of each sensor node.

Sensor3 (EEG)

Sensor2 (Temp.)

Sensor1 (ECG)

SensorN (EMG)

Smart Phone(base station)

402-405MHzOOK data

Data Path

433MHz 30dBm18dBm

Energy & Clock Path

Fig. 1. Proposed multi-node synchronized body area network powered by RF energy harvesting.

from [3] provides the sensor data input from a captured ECG waveform.

B. TDMA ProtocolAs shown in Fig. 3, the WBAN base station initiates

operation by broadcasting a 2-ASK, 434.16MHz waveform in the 433MHz ISM band. During the energy-harvesting phase, when the incoming power received by each sensor exceeds the on-chip rectifier sensitivity (-8dBm), two off-chip surface-mount capacitors (100uF) are charged to 1.1V and 562mV, respectively. The higher supply is used for powering the bandgap reference and the comparator (2uW of total power), while the lower supply powers the rest of the SoC. After the energy-harvesting phase, the base-station transitions into data transmission phase, signified controlled by the base station reducing its 434.16MHz broadcast signal strength by 12dB. This signal amplitude reduction is then detected by the sensor’s analog comparator, generating which then generates a wake-up signal. Local clock synchronization to the base-station clock is achieved by utilizing a divide-by-3 injection-locked frequency divider (ILFD) that produces a 144.72MHz signal from the incoming 434.16MHz base station signal. As a result, the local baseband clocks of all the sensor nodes are phase-locked to the central base station. Once the wake-up signal is detected, a digitally programmable counter within each sensor node begins counting. The nodes interleave transmission based on their pre-programmed TDMA time slot, set by the begin and end codes (Bx and Ex in Fig. 3). The guard band interval between two adjacent TDMA slots can be set either extremely short (one data period) to minimize dead time and power dissipation, or relatively long in order to provide margin for any differences in time-of-flight between physically separated nodes on the body..

C. Merged Rectifier-Limiter A rectifier with a cross-coupled bridge configuration is

adopted here for both low on-resistance and small reverse leakage [4]. Six identical rectifier units are stacked to boost as small as a -8dBm incoming energy up to over 1.2V for

powering the bandgap. The second highest output voltage of the rectifier (~0.75V) is fed into the LDO that supplies the rest of the system-on-chip.

In the proposed multi-node WBAN system, the received signal power at the input of each sensor node’s rectifier may exhibit an extremely large dynamic range, due to distance and channel loss variations between the base-station and the sensors. As a result, a voltage limiter is needed to prevent any over-harvested charge stored on the capacitors from damaging any subsequent circuit block that employs thin-oxide transistors. Fig. 4 illustrates the proposed merged rectifier-limiter circuit along with its simulated and measured characteristics. Since the gate voltages of M1~M5 are set to VBG = 1.1V, their drain nodes can only be charged up to VBG, independent of the transistors’ source voltages. As a result, for all received input powers (Pin) up to 0dBm, the output voltage (VTOP) of the rectifier-limiter is limited to below 2.5V, which is below the tolerance limit of the thick-oxide I/O devices implemented within the rectifier.

-12 -10 -8 -6 -4 -2 01

2

3

4

5w/o limiter (simulated)w/ limiter (simulated)w/ limiter (measured)

Rectifier Unit 1

M1

M2

M3

M4

M5

PIN

VRF +

VRF -

VBG (1.1V)VTOP

VDD (0.56V)

-8dBm

-20dBm

LDO

Bandgap

Matching Network

Input Power (dBm)

V TO

P (V

)

Analog Comp.

20u65n

3pF

3pF

Rectifier Unit 2

Rectifier Unit 3

Rectifier Unit 4

Rectifier Unit 5

Rectifier Unit 6

in

VRF+ VRF-

20u150n

20u150n

10u150n

10u150n

out

in

out

Fig. 4. Merged rectifier-limiter energy-harvesting circuit.

Basestation

Harvesting Synchronized TDMA Transmission

Rectifier Output

Wakeup (Comp.)

Counter

BB Clock

10 2 … B1 …… E1 …

Enable

TX Out

Counter 0 1 2 …… B2 …… E2 …

Enable

TX Out

… X X …

……X X …on off

on off

Slot 1

Slot 2 …

…

…

…

…

…

…

…

…

…

Node1

Node2

…

BB Clock …

Fig. 3. The TDMA handshaking flow chart for multiple nodes.

A1

VBN

fREF (16MHz)

VBN

In

VBP

Out

Subharmonic Injection-Locked Ring Oscillator

Edge Combiner

A1 A2 A3 A4 A5

A1 A2 A3 A4 A5Pre-Amp

Power Amplifier

MatchNet

A2 A3 A4 A5

A1 A2 A3 A4 A5

A1A2 A3 A4 A5

A1A2 A3 A4 A5

Constant Gm Bias

VBP

VBN

VDD = 0.56V

VDD = 0.56V

VDD = 0.56V

3b

VDD = 0.56V

Fig. 5. Near-VT MICS-band OOK transmitter.

D. Transmitter ArchitectureThe near-threshold MICS-band transmitter operating with a

harvested supply of VDD=0.56V, is shown in Fig. 5. To enhance the transmitter global efficiency (defined as the ratio of the transmitter output power divided by the entire transmitter power consumption), a sub-harmonic injection-locked ring oscillator (SHILRO), and edge combiner are employed to generate the 402-MHz carrier [5]. Compared with traditional phase-locked loops, this SHILRO structure has the advantage of fast start-up time, which facilitates the precise duty cycling requirements for the multi-node TDMA operation. The 16.08-MHz local reference clock derived from the 434.16-MHz RF input is injected into the 80.4-MHz, 5-stage SHILRO, eliminating the need for an off-chip crystal oscillator for each sensor. An inverter-based pre-amplifier is added between the edge combiner and the class-C power amplifier to ensure sufficient driving capability. Programmable output power for the power amplifier is achieved by a 3-bit current DAC, which tunes the current flowing through the bottom resistor, modifying the bias voltage.

III. EXPERIMENTAL RESULTS

Fig. 6 shows the lab setup and the measured waveforms for four sensor nodes operating simultaneously. A signal generator (Agilent 8643A), used as the base station, transmits a 2-ASK 434.16MHz signal with a +30dBm output power to four sensor nodes placed on a user standing 1.4m away. The measured sensitivity of the rectifier and the ILFD are -8dBm and -35dBm, respectively, using a quarter-wavelength antenna for the base station and 1.8inch 433MHz antennas for the sensors. Sensor Node3 and Node4 are placed on the back side of the user to demonstrate full functionality for non-line-of-sight operation.

As shown in Fig. 7, using two 100uF surface-mount capacitors, the energy-harvested supply voltages (1.1V and 0.56V) can hold remain stable for 5.55ms before exhibiting a 10mV drop at the LDO output, when the MICS transmitter is sending data at a 1Mbps data rate with -16dBm output power. When the minimum rectifiable input power at -8Bm is received, 5.8ms is required to charge up the 100uF capacitors by 10mV. Hence, for a 25% duty-cycle duration between harvesting and transmission modes for a network of four nodes, the overall effective data rate per sensor is over 180kbps. Furthermore, the proposed periodic harvesting scheme allows the trade-off between the storage capacitance size and the duty-cycle ratio between harvesting and transmitting, expanding the range of applications to other cost and size constrained scenarios.

The phase noise of the 402MHz carrier shows only minor degradation as the 434.16MHz received power decreases (Fig.

Fig. 6. Measured time-domain waveforms for TDMA transmission of four sensor nodes from the front/back of person.

Energy-Harvested VDD

TX Output

Wakeup

10mV

5.55ms -8 -6 -4 -2 010-3

10-2

10-1

100

101

PIN (dBm)

Tim

e (s

) for

Cha

rgin

g 10

0uF

LDO

Loa

d

From 0 to 0.56V

From 0.55V to 0.56V

Fig. 7. Measurement results of the energy-harvesting front-end.

(a) (b)

Fig. 8. a) Measured TX output spectrum and b) TX & ILFD phase noise.

0 10 20 30 40 500

5

10

15

20

25

Output Power (uW)

Glo

bal E

ffici

ency

(%)

Fig. 9. Measured TX global efficiency vs. output power.

8), insuring robust radio operation even as the surrounding environment and wireless channel conditions alter. The carrier-to-spur ratio at the transmitter output is a measured 31.2dBc for a 16.08MHz spacing. The measured global transmitter efficiency is over 16% when the output power is 25uW, as shown in Fig. 9.

Fig. 10 shows a die photo of the 1mm x 1mm body-area network prototype, fabricated in a 65nm CMOS technology.

Fig. 11 shows the ECG waveforms of both Lead-II and Lead-III of the subject under test, when the BAN chip is connected with a biomedical sensor chip interface [3] that is powered by the bandgap on from our chip. The RF data transmitted by the MICS-band TX is sampled by an oscilloscope (Tektronix TDS7404) and reconstructed in MATLAB.

The performance summary and comparison with previous body area network prototypes are summarized in TABLE-I and TABLE-II, respectively.

IV. CONCLUSION

This work proposed a wirelessly-powered, body area network SoC supporting synchronized multi-node operation. Wireless clock synchronization based on an injection-locked frequency divider enables low-overhead TDMA duty-cycled

transmission for multiple nodes. RF energy harvesting further eliminates the requirement of the battery. Both techniques help reduce the sensor node’s size, weight and cost, and enable the future possibility for disposable wearable sensors.

ACKNOWLEDGEMENTS

This work was funded by grants from the Center for the Design of Digital-Analog Integrated Circuits (NSF-CDADIC), NSF-0901883, and the Catalyst Foundation. The authors thank Yajie Qin for help with the chip fabrication.

REFERENCE

[1] F. Zhang, Y. Zhang, J. Silver, Y. Shakhsheef, M. Nagaraju, A. Klinefelter, J. Pandey, J. Boley, E. Carlson, A. Shrivastava, B. Otis, B. Calhoun, “A Batteryless 19uW MICS/ISM-Band Energy Harvesting Body Area Sensor Node SoC,” ISSCC Dig. Tech. Papers, pp. 298-299, Feb. 2012.

[2] M. Vidojkovic, X. Huang, P. Harpe, S. Rampu, C. Zhou, L. Huang, K. Imamura, B. Busze, F. Bouwens, M. Konijnenburg, J. Santana, A. Breeschoten, J. Huisken, G. Dolmans, H. de Groot, “A 2.4GHz ULP OOK Single-Chip Transceiver for Healthcare Applications,” ISSCC Dig. Tech. Papers, pp. 458-459, Feb, 2011.

[3] X. Zou, X. Xu, L. Yao, Y. Lian, "A 1-V 450-nW Fully Integrated Programmable Biomedical Sensor Interface Chip", IEEE J. Solid-State Circuits, vol. 44, no. 4, pp. 1067-1077, Apr. 2009.

[4] K. Kotani, A. Sasaki, T. Ito, "High-Efficiency Differential-Drive CMOS Rectifier for UHF RFIDs", IEEE J. Solid-State Circuits, vol. 44, no. 11, pp. 3011-3018, Nov. 2009.

[5] J. Pandey, B. Otis, “A 90uW MICS/ISM Band Transmitter with 22% Global Efficiency,” IEEE Radio Frequency Integrated Circuits, pp. 285-288, 2010.

0 0.5 1 1.5 20

0.5

1

1.5

2

Time (s)

ECG

Lea

d-II

(mV)

0 0.5 1 1.5 20

0.5

1

1.5

2

Time (s)

ECG

Lea

d-III

(mV)

Original

Reconstruction

Original

Reconstruction

Fig. 11. Measured ECG Lead-II/Lead-III signals.

Fig. 10. Die photo.

TABLE II: MULTI-NODE BAN PERFORMANCE COMPARISON

TechnologySupply

ChannelPower Source

RangeMultiple Access

TX Energy/bit

[6]0.18um

0.9V

eTextilesRemote Battery

1mTDMA/CSMA

0.7-18pJ

[7]0.18um

1V

BCCN/A

N/AFDMA

0.20nJ

This work65nm0.56V

MICS/ISMRF Energy Harv.

1.4mTDMA

0.15nJTX Data Rate 10Mbps 1k-10Mbps 250k-2Mbps

Frequency Band 10MHz (Clock) 40M - 120MHz 402MHz/433MHz

TABLE I: PERFORMANCE SUMMARYTechnology

Die SizeHarvested VDD for TDMA Slot, Clock Sync. and TX

Frequency Band (Harvesting)Frequency Band (TX Transmission)

MICS TX OOK Data RateTransmission Time Before 10mV Drop in Harvested VDD

Number of Synchronized Nodes (Measured)Sensitivity (Harvesting)

Sensitivity (Clock Synchronization)Max Experimental Distance for a Fully Operational Sensor

BandgapLDO

Digital TDMA SlotClock Synchronization RX

MICS TX

Power Break Down

65nm-CMOS1mm x 1mm

0.56V433MHz

402-405MHz

250kbs-2Mbps5.55ms

4-8dBm

-35dBm1.4m

< 1uW< 1uW< 1uW8uW

150uW (When Pout = -16dBm)

MICS TX Output Power (Pout) -27dBm ~ -13dBm

MICS TX Global Efficiency 16.7% (When Pout = -16dBm)

[6] P. Mercier, A. Chandrakasan, “A 110μW 10Mb/s eTextiles Transceiver for Body Area Networks with Remote Battery Power,” ISSCC Dig. Tech. Papers, pp. 496-497, Feb, 2010.

[7] J. Bae, K. Song, H. Lee, H. Cho, L. Yan, H. Yoo, “A 0.24nJ/b Wireless Body-Area-Network Transceiver with Scalable Double-FSK Modulation,” ISSCC Dig. Tech. Papers, pp. 34-35, Feb, 2011.