Article

Method of Estimating Human Orientation Using
Array Antenna
Nobuyuki Shiraki 1,†,∗ , Zhixiong Chen 1,† , Dai Sasakawa 1,† , Naoki Honma 1,† ,Takeshi Nakayama 2,† and Shoichi Iizuka 2,†
1
2

*
†

Graduate School of Arts and Science, Iwate University, Morioka 020-8551, Japan;
yukio.en.r5@dc.tohoku.ac.jp (Z.C.); t5616003@iwate-u.ac.jp (D.S.); honma@iwate-u.ac.jp (N.H.)
Panasonic Corporation, Kadoma 571-8501, Japan; nakayama.takeshi@jp.panasonic.com (T.N.);
iizuka.shoichi@jp.panasonic.com (S.I.)
Correspondence: n718taf@gmail.com; Tel.: +81-19-621-6945
These authors contributed equally to this work.

Received: 24 April 2018; Accepted: 4 June 2018; Published: 7 June 2018

Abstract: This paper presents a method that uses microwaves to estimate human body orientation.
The antennas are arranged to surround the human and observe vital signs such as respiration andheart beat from the microwaves reflected from the human. Since the signal reflected from the front of
the human will fluctuate the most, mainly due to respiration, human body orientation is estimated byfinding the antenna that captures the largest rhythmic fluctuation. In experiments with three subjects,
the median value of angular error of human orientation was 9.01∼23.35◦ .
Keywords: MIMO radar; prominence; human orientation estimation

1. Introduction
Recently, demand for effective and efficient monitoring systems has been rising due to theincreased number of elderly people. As for the monitoring of such people, detecting their orientations
is an important issue because this information is convenient in estimating the status of the observee.
Though monitoring them by camera [1] is the easiest way to know the orientation information of theobservee, this may violate privacy. Infrared-based systems [2] will not violate the privacy, but they are
weak against sunlight and changes in air temperature.
To avoid these problems, the use of microwave sensors is being studied [3–5]. Since this technology
uses radio waves, there is little invasion of privacy. It is also more robust to sunlight and temperature.
WiTrack technology is a conventional method that uses microwaves [6]. In this technique, the distance
range between the human and the antenna is estimated from the time of flight, the time taken for thesignal to be reflected from the living body and captured by the receiver; human position is estimatedusing the time of flight from the multiple antennas. However, there is a disadvantage that a broadband
width more than 1.5 GHz is necessary to obtain sufficient distance resolution.
Multiple-Input Multiple-Output (MIMO) radar offers high localization accuracy even if the
bandwidth is limited [7–10]. Many studies use MIMO radar for estimating body position. MIMO radar,which uses the direction of arrival/departure (DOA/DOD) of the vital sign signal, estimates the
position of the living body from the positional relation of the transceiver. Even though position can bedetected accurately, this technique cannot estimate body orientation.
This paper presents a method of estimating body orientation by using the temporal responses
captured by multiple antennas. The multiple antennas are arranged around the human body so as toobserve the directional characteristics of the signals reflected from the human body. It is expected thatthe amplitude of the signal observed by the antenna in the front of the human is largest as respiration

Electronics 2018, 7, 92; doi:10.3390/electronics7060092

www.mdpi.com/journal/electronics

Electronics 2018, 7, 92

2 of 9movement is most significant at the front of the body. The technique proposed in this paper canestimate the orientation of the subject by observing the subject from various directions, but this is notsuitable for the target positioning. In contrast, the array configuration for positioning cannot estimate
the subject’s orientation because it must have the antenna element spacing around a half wavelengthfor estimating DOD/DOA and this means the all antenna elements are in similar direction from thesubject. To detect the orientation of the subject by using the new antenna arrangement, we propose
a new algorithm, which is explained more detailly in the following part of the paper. This is majordifference from our previous works [8–10].
2. Method of Estimating Body Orientation Using Array Antenna
Figure 1 shows a conceptual diagram of the proposed method. Antennas are placed so as
to surround the human and capture their respective time-variant channel. Using a transmitter andreceiver with Mt and Mr element antennas, respectively, creates Mr × Mt MIMO time-variant channels,
defined as

···
..
.
···



h11 (t)..
H (t) = .
h Mr 1 (t)h1Mt (t)

..

.

(1)

h Mr Mt (t)hij represents the channel from the jth transmitter element to the ith receiver element. The absolutechannel value, |hij |, is used for identifying body orientation in the following procedure.

Rx1

Rx‫ܯ‬௥

Tx1

Rx2
Tx2

Tx‫ܯ‬௧

.
.

Tx3
Rx3
Tx4
Tx5

.

Rx4
Rx5
Figure 1. Conceptual diagram.

Figure 2 shows the method of extracting the change of intensity, termed ‘prominence’ [11].
Prominence of a peak means stand out compared to around other peaks. This method consistsof three steps.
( L)

Step 1: When absolute value the channel |hij (t)| has N peaks, the time of the Lth peak is t p . We draw
( L)

a line horizontally from |hij (t p )| until it hits a higher peak to the left and right; let t1 and t2 be
the times at which the line intersects the left and right peaks, respectively. When there is nohigher peak, t1 , t2 are taken to be the ends of the signal.
( L)
( L)
( L)
( L)
Step 2: When the minimum values between t p and t1 and between t p and t2 are hmin1 and hmin2 ,
ij

( L)

ij

hmin1 = min(|hij (t)|)( t1 ≤ t ≤ t p )

( L)

(2)

( L)

( t p < t ≤ t2 )

(3)

ij

hmin2 = min(|hij (t)|)
ij

( L)

Electronics 2018, 7, 92

3 of 9( L)( L)

Step 3: If hstandard is the larger of h1ij and h2ij , the prominence at t = t p is expressed as
ij

( L)

Pij

( L)

( L)

= |hij (t p )| − hstandard

(4)

ij

( L)

Pij is called prominence. The total value of the prominence of the channel s pij fromtransmitting antenna j to receiving antenna i is expressed asNs pij = ∑ Pij

( L)

(5)

L =1

The sum of the prominences of all receiving antennas generated by transmitting antenna j isexpressed as
Mr

Mr

i =1

i =1 L =1

N

s p j = ∑ s pij = ∑ ∑ Pij

( L)

(6)


ሺ௅ሻ

௣ 
‫ ݇ܽ݁݌‬ሺଶሻ‫ ݇ܽ݁݌‬ሺேିଵሻ

‫ ݇ܽ݁݌‬ሺ௅ሻ‫ ݇ܽ݁݌‬ሺଵሻ

⋯

⋯

ଵ

ሺ௅ሻ

௣

‫ ݇ܽ݁݌‬ሺேሻ



ଶ

(a)


‫ ݇ܽ݁݌‬ሺଶሻ‫ ݇ܽ݁݌‬ሺேିଵሻ

‫ ݇ܽ݁݌‬ሺ௅ሻ‫ ݇ܽ݁݌‬ሺଵሻ⋯⋯‫ ݇ܽ݁݌‬ሺேሻ

ሺ௅ሻ

௠௜௡ଶ
ሺ௅ሻ
௠௜௡ଵ
ଵ

ሺ௅ሻ

௣



ଶ

Range1 Range2

(b)


‫ ݇ܽ݁݌‬ሺଶሻ‫ ݇ܽ݁݌‬ሺேିଵሻ

‫ ݇ܽ݁݌‬ሺ௅ሻ‫ ݇ܽ݁݌‬ሺଵሻ⋯⋯‫ ݇ܽ݁݌‬ሺேሻ

ሺ௅ሻ

௠௜௡ଶ
ሺ௅ሻ
௠௜௡ଵ

ሺ௅ሻ
ଵ

ሺ௅ሻ

௣

ଶ



Range1 Range2(c)Figure 2. Calculation method using prominence in three steps: (a) Step 1; (b) Step 2; (c) Step 3.

This calculation is performed for all transmitting antennas. As expected, the signal from thetransmitter facing the front of the subject tends to be more strongly influenced by respiration thanthose of the other transmitters. Therefore, the sum of the prominences of the transmitter facing theElectronics 2018, 7, 92

4 of 9front of the body is the largest, and has the largest variation. The values of the transmitters on eitherside of the transmitter facing the front of the body exhibit the next largest variation. The orientationof the body is estimated by subjecting the top three transmitters (ranked in decreasing order of total
prominence) to Lagrangian interpolation to find the peak orientation. However, when the transmitterwith the second largest total prominence is not next to the transmitter with the largest prominence,
it is assumed that it cannot be estimated and this data is not used.
3. Experiment3.1. Calibration
Before carrying out the experiment, we performed calibration of the system, where the sensitivity
and transmission power (amplitude) are conditioned. To do this, we connected the transmitter andreceiver directly using cables.
3.2. Measurement Conditions
Figure 3 shows the antenna arrangement used in this experiment and the antenna elements.
Figure 4 shows a photo of a typical measurement run and Table 1 shows the experimental conditions.
Eight transmitter and eight receivers were used, both in a circular array. The frequency is 2.47 GHz,and the antenna height is 80 cm from the floor, the observation time is set to 25 seconds. The sampling
frequency of MIMO channel is set to 100 Hz, which is much higher than the maximum frequencyof the vital-sign, i.e., 3.3 Hz. The antennas were arranged at 45◦ intervals around the subject.
In order to measure vertical reflection wave, the distance between transmitter and receiver is 1 λ.
Three subjects sat on a chair centered on the array facing 16 directions at 22.5◦ intervals. In addition,( L)

the instant prominence values, Pij , less than 0.1 were neglected to alleviate the noise effect. The powertransmitted by the antenna is 10 dBm.
Our experiment was conducted considering the influence on the living body. The Ministry ofInternal Affairs and Communications of Japan mandates radio radiation protection guidelines forhuman exposure to electromagnetic fields. From the guideline, it is supposed that there is no problem
when the distance between antenna and living body is more than 20 cm and the power is less than
20 dBm. Since the power used in our experiment was 10 dBm, our experiment complied with theguidelines. Also, from expression of attenuation constant of lossy dielectrics, penetration depth ofskin and muscle are less than 2 cm. Since the penetration depth is short, the microwave does notpass through the human body. Furthermore, 51.71% of transmit power are reflected by skin from thecalculation result of Fresnel’s reflection coefficient, and very limited amount of the power propagates
into the body.

Rx1

Rx8

Tx1

Rx2
Tx2

Tx8

Tx3

Tx7

Tx4

Tx6

Rx7

Transmitterelement(antenna1)

1ߣ

Rx3
Tx5

Receiverelement(antenna2)

Rx6

Rx4
Rx5
(a)

(b)

Figure 3. Used antennas and antenna arrangement. (a) Tx/Rx configuration; (b) The antennas used in
the experiments.

Electronics 2018, 7, 92

5 of 9subject

antennaFigure 4. Photo of a measurement run.
Table 1. Measurement conditions.
Antenna Element

8-Element Circular Array Antenna

Distance between Tx and Rx
Height of the Rx
Distance between the antenna and the subject
Frequency
Sampling frequency
Channel measurement time
Human orientation
Transmission power

1λ
80 cm
50 cm
2.47125 GHz
100 Hz25 s
16 directions (22.5◦ intervals)
10 dBmFigure 5 shows the S-parameter of the antenna using in this experiment. |S11 || and |S22 | mean
reflection of transmitter and receiver, respectively. |S12 | and |S21 | mean mutual coupling of transmitterand receiver antennas. From this figure, mutual coupling and reflection of the used antenna at theoperating frequency are about −22 dB and −14 dB, respectively. Normally, the antenna spacing larger
than a wavelength is sufficient for obtaining the mutual coupling lower than −15 dB. In fact, the mutual
coupling in our antenna was |S21 | = −22 dB, which is sufficient for observing the reflected signal for
our testbed. Also, Fraunhofer distance is calculated by (2D2 )/λ = 0.25 m (D is the antenna aperture,
which is 0.125 m), which is the boundary of near and far fields. Therefore, it can be understood that
this measurement is performed at the far field range because the distance between the antenna and thesubject was set to 0.5 m.
0ܵଵଵ , ܵଶଶS-paramater [dB]

-10

-14 [dB]-20-22 [dB]ܵଵଶ , ܵଶଵ

-30
-40
-50
-60
2

2.2

2.4 2.47 2.6Frequency [GHz]2.8

3

Figure 5. S-parameter.

Figures 6 and 7 show vertical- and horizontal-polarization radiation pattern, respectively.
From Figure 6, the main lobes of both antennas direct forward (0 degree), whereas their back lobe at
180 degree has low gain. In contrast, Figure 7 shows the gain of both antennas is quite low because
this is a cross-polarization components.

6 of 9Antenna gain [dB]

Antenna gain [dB]

Electronics 2018, 7, 92

90
120

10
0
-10

60
30

-20
-30

0

210

120

60

0
-10

30

-20
-30

0

210

330
240

90
10

330
240

300

300

270

270

Receiver 1

Receiver 2

(a)

(b)

90
120

10
0
-10
-20
-30

60
30

0

210

330
240

300
270

Antenna gain [dB]

Antenna gain [dB]

Figure 6. vertical-polarization radiation pattern; (a) antenna 1; (b) antenna 2.
90
10
0
-10

120

60
30

-20
-30

0

210

330
240

300
270

Receiver 1

Receiver 2

(a)

(b)

Figure 7. horizontal-polarization radiation pattern; (a) antenna 1; (b) antenna 2.

3.3. Measured Results
Figure 8 shows the absolute value transmitted from each transmitter and received all receiver.
In these figures, the target is directed to TRx1. In terms of the diagonal components of the channel
matrix, it can be seen that the largest channel fluctuation is observed by TRx1, i.e., both of thetransmitter and receiver antennas are at the front side of the living body, whereas only the small
fluctuations are observed by other antennas. This behavior is due to a respiratory movement.
Furthermore, the non-diagonal components of the channel matrix have weak intensity, but stillseem to contain the vibration due to the respiration.
Figure 9 show the amplitude transmitted from each transmitter and received all receiver.
In these figures, the target faces the direction to TRx1. From these figures, it can be seen that the
combination of the antennas transmitted and received from the front of the living body has the largest
amplitude and regular cycle. The largest amplitude and regular cycle are assumed to be affected bybreathing fluctuation.

Electronics 2018, 7, 92

7 of 91
15

73|௜௝  |28|ଵଵ (t) |105|଻ଵ (t) ||ସଵ (t) ||ଷଵ (t) ||ଶଵ (t) ||଼ଵ (t) |

4

6

0
0
|ହଵ (t) |

5

20

40

60

Observation time䠄s䠅

80|଺ଵ (t) |

(a)
1

1

|ଶଶ (t) |

8

2

8

6

3

|଼ଶ (t) |74|ଷଶ (t) |

2

5

40

60

0
0|଻ଷ (t) | 20

580

Observation time䠄s䠅|ସସ (t) |6|ଶସ (t) ||ଵସ (t) | |ହସ (t) | |ଷସ (t) |

7

4

3

2

4

6
5

0
0

60

80

4

6
|଺ସ (t) |2040|଼ସ (t) ||ହହ (t) |660| (t) |80଻ସ

5|௜௝  |

3

8

2

8

8

|௜௝  |

7

40

(c)
1

10

2

8

|଺ଷ (t) |Observation time䠄s䠅(b)1|ଵଷ (t) ||ଶଷ (t) |

4

6
0
0
20
|ହଶ (t) ||଼ଷ (t) | |ହଷ (t) ||ସଷ (t) |

4
2

4

6

3

|ଵଶ (t) ||଺ଶ (t) ||௜௝  |7|௜௝  |6|଻ଶ (t) | |ସଶ (t) ||ଷଷ (t) |

8

2

8

|ସହ (t) |4
2 |ଷହ (t) | |଻ହ (t) | |ଶହ (t) |0020|ଵହ (t) |40|଺ହ (t) |60

Observation time䠄s䠅

Observation time䠄s䠅

80|଼ହ (t) |

(e)

(d)

Figure 8. The absolute value transmitted from each transmitter and received all receiver. (a) transmitted
from Tx1; (b) transmitted from Tx2; (c) transmitted from Tx3; (d) transmitted from Tx4; (e) transmitted
from Tx5.

1.6|ସସ (t) ||ଵଵ (t) ||௜௝ (t) |1.4
1.2|଺଺ (t) |

1
0.8
0.6
0.4

|ଶଶ (t) |݄ଷଷ|ହହ (t) |020|଼଼ (t) ||଻଻ (t) |

40

60

80

Observation time䠄s䠅Figure 9. Absolute value of a channel.

Figure 9 shows an example of the absolute value of a channel when the subject faced the Tx1 andRx1 direction. From the figure, we can see that channel |h11 (t)| yielded by the antenna directly facing
the subject was greatly altered by the biological activity.
Figure 10 shows the Cumulative Distribution Function (CDF) of the orientation estimation error
for each subject. From the figure, median values of estimation error of subjects A, B, C were 9.01◦ ,22.35◦ , and 20.33◦ , respectively.

Electronics 2018, 7, 92

8 of 9W/personA100

CDF [%]

80

W/personB60
W/personC

40
20
0

0

22.5 45 67.5 90 112.5 135 157.5 180
Angle Error [deg]Figure 10. CDF of the orientation estimation error.

Figure 11 shows the CDF of the angular estimation error of three subjects when the only diagonalcomponents of MIMO channel are used for detecting the subjects’ orientation. It is found that theestimation accuracies at 50% value degraded by 5.86 ∼ 53.74◦ compared to the results shown in
Figure 10. This means the MIMO scheme, i.e., use of the non-diagonal components can enhance theaccuracy of the orientation detection.

w/ person A
100

CDF [%]

80
60

w/ person C

40
20

w/ person B

0022.5 45 67.5 90 112.5 135 157.5 180

Angle error [deg.]
Figure 11. CDF of the orientation estimation error(only diagonal components).

4. Conclusions
In this paper, we proposed a method of estimating human body orientation from changes in theintensity of microwave signals. Experiments confirmed that the antenna directly front of the subject
experienced the largest channel fluctuation. The median values of the angular estimation error withthree subjects ranged from 9.01◦ to 23.35◦ , and this means the proposed method can well estimate theorientation of the human body without any prior knowledge.
Also, the wavelength in this frequency is about 12 cm, and the movement of the subject’s body issmaller than this value since the subject is seated. Nevertheless, the positional error is quite important
issue for this system, and this will be investigated in our future study.
Author Contributions: N.S., Z.C., D.S. and N.H. designed the study; N.S., Z.C., T.N. and S.I. performed theexperiments; N.S. and Z.C analyzed the data; N.S. wrote the paper; D.S. and N.H. supervised the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.

Electronics 2018, 7, 92

9 of 9Abbreviations
The following abbreviations are used in this manuscript:
MIMO Multiple-Input Multiple-Output
CDF
Cumulative Distribution Function
DOA
Direction Of Arrival
DOD
Direction Of Departure

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