Sub-µW Signal Power Doppler Radar Heart Rate Detection

Shuhei Yamada, Mingqi Chen, and Victor Lubecke

Department of Electrical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, 96822

Tel: 1-808-956-9948, E-mail: shuheiy@hawaii.edu

Abstract — Periodic motion, such as that resultingfrom cardiopulmonary activity, can be measured bydirect-conversion microwave Doppler radar. In suchsystems, motion is measured as phase modulation and measurement sensitivity is significantly affected byphase noise and amplitude modulation sidebands in the local oscillator. Presented here are experimentalresults of heart rate measurements under varioussignal output power conditions, with consideration forthe impact of respiration. Measurements were madeusing a custom compact quadrature direct conversionDoppler radar circuit, and results indicate that heartrate can be accurately assessed with transmit powerfor levels as low as 20nW for a subject distance of one meter. This is the lowest power ever reported for ISMband CW doppler radar heart beat detection.

Index Terms — Heart rate monitoring, biomedicalsignal detection, continuous phase modulation,Doppler radar, homodyne detection.

I. INTRODUCTION

Direct-conversion microwave Doppler-radar has

been introduced for non-contact cardiopulmonary

monitoring [1-4], with potential for health

monitoring and personnel detection. Doppler radar

systems generally transmit a continuous wave signal

while receiving and demodulating the signal’s

reflection from a target. According to Doppler

theory, when the target has time-varying movement

with zero net velocity the reflected signal is phase-

modulated in proportion to the position of the target

rather than the velocity. A stationary human body

presents two independent time varying sources of

motion with zero net velocity resulting from

respiration and cardiac activity, and the largest

reflection of incident RF power occurs at the body

surface. According to range correlation theory,

performance limits in Doppler radar sensing depend

on the phase noise and amplitude modulation noise

sidebands of the local oscillator signal.

In this paper, heart rates were measured using a

Doppler radar system with incrementally reduced

transmit power levels in order to assess the lower

limits for system performance. Measurements were

made both for subjects holding their breath and for

subjects breathing normally, with the former used to

assess absolute performance limits for the radar

circuit, and the latter used to asses secondary

impacts resulting from the more complex chest

motion. The design and fabrication of the quadrature

direct-conversion circuit used is also presented.

Results measured with this circuit indicate that

subject heart rates can be accurately monitored for

transmit power levels as low as 20nW, for subjects

at one meter displacement. This is the lowest power

ever reported for ISM band CW doppler radar heart

beat detection.

II. DOPPLER RADAR SYSTEM

The power of the received signal in a Doppler

radar system can be affected by many parameters

including transmit power level, path obstructions,

target composition and cross section, and antenna

patterns. In this work, only transmit power was

varied with all other parameters held constant. A

custom compact quadrature direct-conversion

SignalGenerator

Signal Processing

RFout

DigitizationAmplification,DC block andAnti aliasing filters

2Way 0 degreePower Splitter

2Way 90 degreePower Splitter

2Way 0 degreePower Splitter

Mixer

Mixer

LO

RFin

Iout

Qout

LO

LO

RFin

RFin

PhaseShifter

Variable attenuator

Object

1[m]

Fig. 1. Block diagram of Doppler radar system with

low power measurement approach. A single external

signal source was used to provide the LO and RF

output signals, and an attenuator used to incrementally

decrease the transmitted signal. The optimal output

from the I or Q mixer was processed to assess the heart

beat-to-beat interval for a seated subject at 1m.

Proceedings of Asia-Pacific Microwave Conference 2006

Copyright 2006 IEICE

Doppler radar circuit was used with the subject

seated at one meter from the co-located transmit and

receive antennas, with unobstructed air space

between them. The Doppler radar system and

measurement approach are illustrated in Fig. 1. The

circuit local oscillator (LO) and transmit signals

originated form a stable external signal source, with

LO power maintained at an efficient, though not

optimal, level. Under these conditions it was

anticipated that the system could easily provide

accurate heart rate measurements with a nominal

2mW signal at the transmit antenna feed, and thus

allow an effective means of quantifying the

detrimental effects on radar performance of a

progressive reduction in transmit signal power.

Similar effects could be expected for systems

transmitting higher power signals with signal power

reduced by other factors mentioned above.

The manner in which the mixer output in a direct

conversion Doppler radar is related to the target

motion is known to be acutely dependent on the

nominal target distance [5]. The simplest case of

direct proportionality results for an “optimum”

position that occurs periodically for displacements

of 2/ , with correspondingly distorted conditions

occurring near the “null” positions in between. The

use of a system with both in-phase (I) and quarature

(Q) mixer output channels provides a means for

avoiding null case measurements either through

channel selection or appropriate combination. In this

work channel selection was used, with an effort to

maintain an optimum-case position for the subject at

all times. This approach also helped to restrict all

observed detrimental effects to those related to

transmit power level.

The quadrature direct conversion circuit used was

fabricated as a compact microstrip circuit with

surface mount components on a 3-cm by 4-cm PC

board, with coaxially connected commercial patch

antennas (Antenna Specialists ASPPT2988 2.4 GHz)

and an external Agilent 83640B signal source. The

block diagram of the quadrature direct conversion

circuit is shown in Fig. 1. The two LO signals were

derived from the external signal source. The signals

were divided first by a two-way 0° power splitter

(Mini-Circuits RPS-2-30) to separate the LO and RF

output signals. The latter signal was directed to the

transmit patch antenna, with the former signal

further divided by on-board two-way 90° power

splitters (Mini-Circuits QCN-27) to make quadrature

outputs. The radiated signal reflected by the subject

was received by the receiving patch antenna to

create the RF input signal, which was divided using

a two-way 0° power splitter to feed the two receiver

chains. Each chain had its own mixer (Mini-Circuits

SKY-42) and produced the separate output channel

signals as Iout and Qout at the corresponding SMA

connectors.

A photograph of the fabricated quadrature direct

conversion circuit is shown in Fig 2. The RF and LO

signals were routed via 50 microstrip lines and

the design frequency was 2.4 GHz. An FR4

substrate is used for the PC board with a dielectric

constant of 4.5, a substrate thickness of 0.787 mm, a

conductor thickness of 35µm, a metal conductivity

of 5.5e7, and a delta loss tangent of 0.018.

Fig. 2. Photograph of the custom compact

quadrature direct conversion Doppler radar

circuit. The circuit was connected to an

external signal source, antennas, and

power supply for measurements.

III. MEASUREMENT OF LOW POWER

OPERATION

The measurement setup for the low power

measurements is shown in Fig 1. The external signal

source was set to 11 dBm in order to get a 0 dBm

LO-input power for the mixer. While the ideal LO

power for the mixer is 7 dBm, a 0dBm signal was

used for this measurement to maintain circuit

simplicity. The conversion loss of the mixer is

5.4dB at a 7dBm LO power, and 7.26dB at a 0 dBm

LO power, as shown in Fig. 3. This implies that the

measured system limits might still be improved.

Two variable attenuators were used to

incrementally reduce the RF output power. Each

attenuator provided 0 to 30 dB attenuation in 1 dB

steps. In order to compensate for the phase change

introduced by each different attenuator settings, a

compensating phase shifter was introduced after the

signal generator. With this, the phase change

introduced with each increment of the attenuator

could be completely compensated. Another phase

shifter was also used to further optimize the phase to

0

0.4

0.8

1.2

1.6

2

0 5 10 15 20

Time[sec]

Inte

rval

[sec

]

Reference Radar

Fig. 6. Measured result at 0.2 µW, with

respiration.

0

0.4

0.8

1.2

1.6

2

0 5 10 15 20

Time[sec]

Inte

rval

[sec

]

Reference Radar

`

Fig. 4. Measured result at 2mW, without

respiration.

0

0.4

0.8

1.2

1.6

2

0 5 10 15 20

Time[sec]

Inte

rval

[sec

]

Reference Radar

Fig. 7. Measured result at 20 nW, without

respiration. Degradation in accuracy can be

observed.

0

0.4

0.8

1.2

1.6

2

0 5 10 15 20

Time[sec]

Inte

rval

[sec

]

Reference Radar

Fig. 5. Measured result at 0.2 µW, without

respiration.

0

10

20

30

40

50

-18 -15 -12 -9 -6 -3 0 3 6

LO Power[dBm]

Co

nve

rsio

n L

oss

[dB

]

Fig. 3. Measured LO power vs conversion

loss for the Mixer (Mini-Circuits SKY-42).

The mixer was supplied with a 0 dBm LO

avoid null points. The baseband output signals were

amplified and filtered with SR560 LNAs and then

digitized with a Tektronix 3014 digital oscilloscope.

A wired finger pressure pulse sensor was used as a

reference for comparison with the heart rate data

obtained with the Doppler radar. A peak detector

and digital band pass filter implemented in

MATLAB were used for signal processing for

signals under both breath-hold and normal

conditions.

When the subject breathes during a

measurement, there is some phase variation and

resulting frequency distortion, as described in [5].

To avoid this impacting rate detection accuracy, the

subject did not breathe during the initial

measurements. The result for the system operating at

2 mW is shown in Fig. 4. The plot indicates an

average beat-to-beat interval of about one second,

with expected agreement between finger-pulse

reference and radar results.

The result for 0.2 µW is shown in Fig. 5. At

this point the system still functions well, with no

discernable degradation. A second plot at this power

level is also shown (Fig. 6), also indicating close

agreement with reference measurements in spite of

the additional phase modulation effects introduced

by respiration related chest motion.

The result for 20 nW is shown in Fig. 7.

Here appreciable degradation can be observed in the

rate tracking for the Doppler radar signal. Still, the

rate is tracked with reasonable accuracy and may be

useful for many applications.

IV. CONCLUSION

Low signal power results for Doppler motion

sensor based heart rating monitoring has been

presented. A custom compact quadrature direct-

conversion Doppler radar has been described along

with its use in assessing low power monitoring

limits. Heart rates for subjects holding their breath

and subjects breathing normally have been

successfully tracked for signal power levels as low

as 20 nW. This is the lowest power ever reported for

ISM band CW doppler radar heart beat detection.

These results can be extrapolated to larger power

systems where obstructions and antenna gain

similarly impact the signal power available for heart

motion detection.

ACKNOWLEDGEMNTS

This research was sponsored by the NSF

under contract ECS0428975 and the U.S. army

under contract W15P7T-05-C-B202. The authors

would also like to thank Byung-Kwon Park, Dung

Nguyen, and Olga Boric-Libecke for their assistance.

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[1] V.M. Lubecke, O. Boric-Lubecke, G. Awater, P.-W.

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[2] A. Droitcour, V. M. Lubecke, J. Lin, and O. Boric-

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