Modulated Backscatter for Ultra-Low Power Uplinks fromWearable and Implantable Devices

Stewart J. Thomasstewart.t@gmail.com

Jordan S. Besnoffjordan.besnoff@duke.edu

Matthew S. Reynoldsmatt.reynolds@duke.edu

Department of Electrical and Computer EngineeringDuke University

Durham, NC 27708

ABSTRACTWearable and implantable wireless biomedical devices areoften constrained by the limited bandwidth and high powerconsumption of their communication links. The VHF orUHF transceivers (e.g. MICS radios) traditionally used forthis communication function have relatively high power con-sumption, on the order of mW, due to the high bias currentsrequired for the analog sections of the radio. To reduce over-all power consumption, both the data rate and the duty cy-cle of the radio are usually minimized, because the lifetimeof the device is limited by the energy density of availablebattery technologies.

Recent innovations in modulated backscatter techniquesoffer the possibility of a radical reduction in the power costand complexity of the data uplink, while significantly im-proving data rate. This is achieved by a re-partitioning ofthe communication link. Backscatter techniques shift theburden of power cost and complexity from the remote de-vice to a base station. Instead of actively transmitting anRF signal, the remote device uplinks data to the base stationby modulating its reflected field.

We present two ultra-low power biotelemetry systems thatleverage modulated backscatter in both the near-field andfar-field propagation regimes. The first example operates inthe far field and is designed to telemeter multiple channels ofneural/EMG signals from dragonflies in flight. This devicehas a mass of 38 mg, a data rate of 5 Mbit/s, and a rangeof approximately 5 m. The second example operates in thenear field and is designed to be implanted in mice. The sen-sor has a maximum implant depth of 6cm and can transmitat data rates of up to 30 Mbit/s. The power cost of theanimal side of both data links is 4.9 pJ/bit and 16.4 pJ/bitrespectively.

Categories and Subject DescriptorsB.4.4 [Input/Output and Data Communications]: Per-formance Analysis and Design AidsSimulation, Verifica-

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tion; B.4.1 [Input/Output and Data Communications]:Data Communications Devicesreceivers, transmitters

General TermsDesign, Experimentation

Keywordsbackscatter, biotelemetry, low-power communication

1. INTRODUCTIONWearable and implantable biomedical devices are often

severely power limited due to the limited energy density ofavailable battery technologies. Wireless communication usu-ally accounts for a substantial portion of the power budgetof these devices, due to the high currents required by on-chip very-high or ultra-high frequency (VHF / UHF) oscil-lators and frequency synthesizers for Medical Implant Com-munication Service (MICS), Bluetooth, 802.11, and mostother wireless standards. To reduce overall power consump-tion, both the amount of data transferred and the duty cy-cle of the radio are carefully minimized, leading to manypublished schemes for minimizing radio duty cycle in powerconstrained operation.

Because the communication subsystem can account for asignificant fraction of total system power [14], research in thearea of low-power sensor networks and devices often assumesthat power cost to transmit data is significantly greater thanon-chip processing or data storage [9]. This power tradeoffbetween on-chip processing and data uplink has been furtheranalyzed in detail [8].

In this paper we consider an alternative communicationmethod using modulated backscatter, which offers the pos-sibility of continuous, high data rate telemetry uplink atattractively low power consumption at the wearable / im-plantable device. This is achieved by a re-partitioning ofthe data link in terms of both circuit complexity and powerallocation. Circuit complexity is shifted from the wearable/ implantable device to a base station which is responsiblefor generation and transmission of the RF carrier used forcommunication. The wearable / implantable device doesnot generate its own RF carrier, so its on-chip RF oscilla-tor or frequency synthesizer can be eliminated. Instead, theremote device transmits data to the base station by modu-lating its reflected power. This can be realized simply witha single transistor in parallel with an antenna, switching atbaseband frequencies. Modulated backscatter thus allows

1

Data

Forward Link(Power + Data Carrier)

Return Link(Modulated Data)

Figure 1: Conceptual schematic of a backscatterdata uplink. The backscatter modulator can be assimple as a single FET switched by a baseband dig-ital data signal.

data to be uplinked over a high-frequency carrier with noon-chip transistors biased for UHF operation.

Single-chip backscatter modulators were originally devel-oped for passive UHF RFID tags employing binary ASKor PSK at relatively low data rates up to 640 kbps [10].However, recent work by the authors has shown that mod-ulated backscatter can be extended to include modulationschemes of arbitrary complexity, such as M -ary quadratureamplitude modulation (QAM) [17, 21]. This technique hasbeen used to wirelessly telemeter data over a backscatterlink with a data rate of 96 Mbit/s with an energy consump-tion of only 15.5 pJ/bit [20]. This is comparable to 802.11(WiFi) in terms of data rate, but at power levels over 100Xlower than previously reported WiFi systems.

In this paper, we summarize two ultra-low power wirelessbiotelemetry systems currently being developed. These sys-tems employ modulated backscatter in both the far field(d >> ) and the near field (d

Table 1: Comparison of Selected Low-Power Far-Field Wireless Biotelemetry SystemsAndo Takeuchi Mohseni Yeager Harrison Thomas

[1] [16] [12] [22] [4] [18,19]Year 2002 2004 2005 2010 2011 2012Power Consumption 3.6 mW 10 mW 2.2 mW 16.56 W 1.0 mW 1.23 mW

Signals Transmitted 2 EMG 1 neural 3 neural 1 temperature2 neural2 EMG

10 neural4 EMG

2 auxiliary inputs

Signal Bandwidth 200Hz2kHz 20Hz10kHz 50Hz9.1kHz 50Hz5.2kHz (neural) 10kHz (neural)280Hz (EMG) 700Hz (EMG)

Operating Range 5 m 16 m 0.5 m 3 m 2 m 5 mBattery Life 0.5 h 0.5 h - Battery-free 5 h Battery-free

Total Mass 250 mg100 mg

1,100 mg 250 mg 170 mg 38 mg(w/out battery)

Approximate Size 7.5 x 6.25mm 15 x 8mm 17 x 12mm 9.1 x 7.3mm 6 x 5mm 6.8 x 4.6mmOff Chip Components - 18 3 16 1 3Supply Voltage 1.55 V 3.0 V 3.0 V 1.8 V 1.5 V 1.31 VTelemetry Frequency 8285 MHz 85 MHz 96 MHz 902928 MHz 905 MHz 902928 MHz

Telemetry Modulation analog FM analog FM analog FM digital backscatter digital FSKdigital BPSKbackscatter

Target animal(s) hawkmoth cockroachmarmoset

hawkmothlocust, elec. fish,

dragonflymonkey dragonfly

Using commercial off-the-shelf (COTS) EPC Gen 2 UHF RFID reader

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Figure 3: Block diagram of the far-field system.

16-to-1

analog

MUX

11-bit

ADC

11

10 neural

amplifiers

neural amp

reference

4 EMG

amplifiers

2 DC

amplifiers

data serializer;

Hamming encoder;

frame sequencer;

Manchester

encoder

digital

control

logic

crystal

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clock

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select

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impedance

modulator

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serial data

stream

antenna

RF rectifier;

voltage

multiplier

1.23V

LDO

regulator

off-chip

smoothing

capacitors

chip Vdd

= bond pad

Figure 4: Simplified schematic of the telemetry IC.

data rate of 5.0 Mbps, the entire communication subsys-tem accounts for only 2% (24.6 W) of the total DC systempower consumption (1.23 mW), so continuous telemetry isachieved with minimal power penalty.

The dragonfly chip is capable of a 14X improvement intransmitted data rate when compared to similar deviceswhich use an active radio [4]. The power breakdown, Fig. 5,shows the primary advantage of the backscatter communi-cation modality a majority of power is dedicated to thecircuits used in signal acquisition, so power allocation can

Table 2: Telemetry IC power breakdown (measure-ment vs. simulation)

Neural amplifiers 160 AEMG amplifiers 1 ADC amplifiers 90 AAnalog mux 139 AAmplifier / mux bias generators 80 A11-bit ADC and digital control logic 320 ALDO voltage regulator 60 ACrystal oscillator 70 ABackscatter modulator 15 ASimulated Total IC current 935 AMeasured Total IC current 940 A

Min. unregulated supply voltage 1.31 VSimulated Total IC power 1.22 mWMeasured Total IC power 1.23 mW

Amps + Mux

50%ADC + Logic

34%

LDO Reg.

6%

Osc.

8%

Backscatter

2%

Figure 5: Power dissipation by subystem of the chip.

be focused on the essential task of obtaining high qualitysignals.

3. NEAR FIELD MODULATED BACKSCAT-TER FOR IMPLANTED DEVICES

For implantable devices, near field modulated backscat-ter is a promising approach to high bandwidth communica-tion. In contrast to far field electromagnetic communication(where d >> ) in which energy is propagated by a com-bination of electric and magnetic field components, evanes-cent modes dominate in the near field where d

Table 3: Performance of selected high data rate implantable communication linksInductively Coupled Load Modulation On-Chip RF Oscillator Besnoff

Mollazadeh [13] Mandal [7] Hu [6] Harrison [5] Harrison [4] [2]2008 2008 2005 2007 2011 2012

Operating Frequency 4 MHz 25 MHz 10 MHz 433 MHz 905 MHz 915 MHzRadio Power 1.8 mW (Rx) 100 W (Tx) 0.61 mW (Rx) 1.81 mW 0.27 mW 0.493 mWData Rate 32 kbps 2.8-4.0 Mbps 1.12 Mbps 330 kbps 345.6 kbps 30 MbpsEnergy/bit - - - 5.48 nJ/bit 0.781 nJ/bit 16.4 pJ/bit

implantable devices, particularly in small animals, have animplant depth of less than 10 cm. At UHF carrier frequen-cies (902-928 MHz), the wavelength 33 cm, so evanes-cent modes dominate. In the near field regime, signals canbe coupled via the magnetic component of the evanescentmode. Because animal tissues have low magnetic perme-ability, the magnetic field is relatively unaffected by tissueproximity, and even at high carrier frequencies the losses intissue are manageable.

The near field UHF modulated backscatter modality isdistinct from near field inductive communication techniquesat low carrier frequencies such as NFC. These systems, wheref < 30 MHz, are sometimes called load modulation. Loadmodulation systems are based on transformer coupling be-tween the readers primary coil and the secondary coil in theimplant. Thus, communication is not achieved through scat-tered fields but rather via a switched load at the transpon-der antenna inducing a voltage change at the external sys-tem [3]. Given the low carrier frequencies of inductive loadmodulation systems, bandwidth is severely limited as shownin Table 3.

To gain insight into the behavior of backscatter based im-plants at UHF frequencies, the two-way backscatter chan-nel has been modeled using a proxy system shown in Fig-ure 6 prior to any testing on a live animal. The proxy sys-tem consists of a polyethylene tank containing 6 liters of0.91% mass / volume saline. The path loss between thesegmented loop antenna and the small loop transponder an-tenna has been characterized using a network analyzer bothin the saline tank and in free space. Not surprisingly, thesmall loop transponder experiences higher path loss in salinethan free space, as saline has a much higher relative per-mittivity (r 80) than air, and a larger loss tangent atthe tested frequencies, and is thus a higher loss dielectricfluid [15].

A prototype implantable backscatter modulator, poweredby a CR2032 coin cell, was fabricated as shown in Figure 7.It uses a 10 mm diameter magnetic loop antenna that isconnected to a low-power backscatter modulator that canbe set to a rate of 7.5 Mbps, 15 Mbps, or 30 Mbps. Inan actual implant, the circuitry can be reduced to a singlechip on a thin substrate, while the bulky connectors shownwould not be required. The external base station transceiverfor this near field system consists of a specialized segmentedloop near field antenna, which creates an approximately uni-form magnetic field in close proximity to lossy, conductivedielectrics such as the saline proxy or actual biological tis-sue. The measured backscatter signal strength at 30 Mbpsis shown in Figure 8.

Near field UHF backscatter naturally lends itself to lowpower implementation, as shown in Table 4. The main com-munication element is a CMOS RF switch that connects oneof two load impedances to the loop antenna. The power con-

Figure 6: Saline tank proxy system used for UHFnear field tests.

Figure 7: Near field modulated backscatter test cir-cuit.

2 3 4 5 695

90

85

80

75

70

65

60

Implant Depth (cm)

Backscatter

Magnitude (

dB

)

875 MHz

900 MHz

925 MHz

Figure 8: Measured 30 MHz backscatter returnpower referenced to base station carrier power.

4

Table 4: Power consumption and energy performanceBackscatter Frequency Supply Voltage (V) Current (A) Power (mW) Energy/bit (pJ/bit)

7.5 MHz 3.01 67.8 0.204 27.215 MHz 3.01 135 0.406 27.030 MHz 3.01 164 0.493 16.4

sumed by the switch increases with backscatter rate, whilethe energy per backscattered bit is as low as 16.4 pJ/bit.

Near field UHF modulated backscatter provides for a low-power, high data rate uplink from implantable biomedicaldevices. This technology is especially applicable to multi-electrode telemetry devices for acquiring data from multipleneurons, where the capability of transmitting large amountsof raw digitized data without compression to an externalbase station is essential. This transfers the burden of com-putation from the implant to the external system, allowingfor reduction of power dissipation on the implant which is incontact with highly temperature sensitive neural tissue. Theproposed system provides penetration depths up to 6 cm ina medium that is expected to be more lossy than actual bi-ological tissue, with communication rates up to 30 Mbps.The proposed system is competitive in terms of power con-sumption with those in Table 3, while far outperformingcompetitive systems in maximum data rate and energy perbit.

4. CONCLUSIONSBecause of their origins in low data rate UHF RFID tags,

backscatter communication links have traditionally been con-sidered as a low-bandwidth communication channel. How-ever, the two example systems in this paper show that high-bandwidth communication using modulated backscatter isboth practical and very attractive from a power efficiency(energy per transmitted bit) perspective. By eliminating al-most all on-chip RF circuitry, the backscatter uplink offerssignificant savings in power cost and implantable/wearabledevice complexity. These benefits are also disruptive to theexisting paradigm for low-power communication links, whereit is often assumed that wireless data transmission has ahigher power cost than on-chip computation. Future de-vices that employ modulated backscatter will offer higheruplink data rates, at much lower power cost, than similardevices using traditional wireless communication. We havealso shown that high-bandwidth modulated backscatter canbe applied to both implantable and wearable devices, giventhe near field and far field RF propagation domains.

AcknowledgmentsThis material is based in part upon work supported by theNational Science Foundation under Grant Numbers CBET-0931924 and CMMI-0800858. Additional support has beenprovided by the Howard Hughes Medical Institute.

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