Remote Mobile Patient Monitoring using Diffuse Optical

Transmissions

Pascal Toumieux, Stéphanie Sahuguède and Anne Julien-Vergonjanne

XLIM UMR 7252, University of Limoges, Limoges, France

Keywords: Wireless Optical Communication, Mobile Communications, Infrared Transmission, Remote Monitoring.

Abstract: In the context of remote patient monitoring, we propose in this paper a radio free solution based system to

transmit health related data. We investigate wireless optical technology based on diffuse infrared links,

which presents the advantage to be secure regarding electromagnetic interference, low-cost and easy to

deploy. Two different monitoring scenarios are considered, both composed of an emitter worn by a mobile

patient and receivers fixed in the environment. The first scenario is the continuous remote monitoring of

heart rate and temperature and the second one involves accelerometer data. To evaluate the wireless mobile

link reliability for the two scenarios, we have developed custom made systems using commercially available

components. The experimental performance is established in terms of packet loss to evaluate the potentiality

of wireless optical technology.

1 INTRODUCTION

Nowadays, there is a rapid growth in medical

devices that incorporate wireless Radio-Frequency

(RF) technology. By eliminating the cables

restricting patient mobility during stay in hospital,

wireless devices contribute to improve patient health

outcomes. However, because of an increasingly

crowded RF environment, the risk of

electromagnetic interferences with other medical

devices has to be considered (Periyasam, 2013). In

addition, the question of prospective health effects

of RF signals in particular long exposure impact is

still open (Benson, 2013). Thus, the deployment of

RF devices can be risky and thus limited in sensible

environments such as hospitals or healthcare

organizations, where high data security and high

immunity against interference with other existing RF

and electronic devices are required.

To overcome this limitation, Wireless Optical

Communication (WOC) based systems can be used

as alternative or complement to RF ones for wireless

monitoring in hospitals (Torkestani, 2012). This

technology has the potential of reducing the amount

of RF effects in patient vicinity because WOC

wavelength range ensures that there is no

interference with existing RF and electronic

equipment. In addition, WOC systems have many

advantages over RF ones such as being free of

license, compact, low cost and having a great level

of security because light cannot pass through walls.

Communicate by using optical beams is one of

the oldest solutions. Since (Gfeller, 1979) WOC in

infrared range have been standardized and with the

development and maturity of optoelectronic devices,

this technology has experiencing a renewed interest.

Indeed, for many years, systems based on infrared or

visible wavelength range have been studied for

indoor and outdoor applications (Carruthers, 1997,

Elgala, 2011; Arnon, 2012; Borah, 2012;

Ghassemloy, 2013).

Two main link configurations are commonly

investigated: Line of Sight (LOS) scheme and

diffuse one. In LOS configuration, the transmitter is

directly pointed towards the receiver. This scheme is

easy to implement and offers high performance, but

it is highly sensitive to misalignments and blocking

effects. Diffuse schemes are more robust because

they exploit multiple reflections of optical beams

over the surface environment to establish non-

directed links between emitter and receiver. The

potentialities of using WOC for healthcare

monitoring have been already investigated

(Torkestani, 2011; Torkestani, 2012; Noonpakdee,

2013). In our previous works the theoretical

performance has been established by determining

553

Toumieux P., Sahuguede S. and Julien-Vergonjanne A..

Remote Mobile Patient Monitoring using Diffuse Optical Transmissions.

DOI: 10.5220/0005275505530557

In Proceedings of the International Conference on Health Informatics (HEALTHINF-2015), pages 553-557

ISBN: 978-989-758-068-0

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

link outage probability for both LOS and diffuse

systems considering emitter mobility.

In this paper, we investigate the feasibility of

practical WOC based system using diffuse optical

transmissions to remotely monitor health data. We

have developed a custom made system dedicated to

a realistic monitoring scenario in a medical unit of

Limoges University Hospital Center (CHU)

concerning post-stroke patients. The objective is to

remotely transmit physiological and physical activity

data during the first weeks of rehabilitation and re-

training effort, which is a critical phase for patients.

During this phase, the patients move in a life

experimental set up of Physical Medicine and

Rehabilitation unit of Limoges hospital. They are

placed in a real situation to assess their capacity to

exercise in different situations of everyday life such

as getting out of bed or cooking for example. It is

therefore extremely important to monitor the patient

throughout this phase and to take into account

patient mobility.

In such context, our contribution in this paper

concerns the experimental evaluation of the wireless

optical technology to transmit health data between

an emitter worn by a mobile patient and receivers

fixed in the environment. We investigate two

different monitoring scenarios, the first one is the

continuous remote monitoring of heart rate and

temperature and the second one involves

accelerometer data.

2 SYSTEM DESCRIPTION

The experiments are performed in a test room of

dimension (6.6m x 6.7m x 3m) as shown in figure 1.

Black areas in the figure correspond to elements of

the furniture. Scratched area corresponds to a

window placed over the entire length of the wall.

In this environment, we consider a patient

equipped with sensors connected to a wearable

hardware unit attached on the body and

communicating by using an optical source which is

an eye safe Infrared (IR) Light Emission Diode

(LED).

Data are sent using Intensity Modulation and

Direct Detection (IM/DD). Four receivers composed

of photo-detectors (named A to D on figure 1) are

fixed on the ceiling and oriented towards the floor.

They are connected to a distant computer

allowing remote access to the data. In such

configuration, because of patient mobility and

source position on patient body, the communication

link is mainly a Non – LOS one, established by

exploiting diffuse reflections over the environment.

A receiver collects beams from all the reflective

surfaces, and thus the total received power is the

sum of all reflective room elements contributions.

Figure 1: Indoor test environment.

2.1 Transmitter

The optical source is a high power infrared diode

(TSAL5100) emitting around 940nm, having a half-

power angle of 10° and generating a maximal optical

intensity of 130 mW/sr.

In the first scenario, we have designed a jacket

worn by the patient equipped with two sensors: a

chest belt for cardiac frequency and a thermistor as

temperature sensor, located under the arm. The LED

is inserted in a plastic part and located on the jacket

shoulder by snaps (figure 2). A cable connection is

realized between the LED and a control unit. In this

configuration, the optical source is mainly oriented

towards the ceiling but it may also point to the walls

and obstacles in the environment according to body

movements.

For the second scenario, the wearable device is

composed of tri-axial accelerometer (MMA7631LC)

and attached on the patient arm (figure 3). The

optical source is mainly oriented towards the ceiling

as in scenario 1, but the orientation can be also

varying according to arm movements.

In both scenarios, the control unit of the device

is composed of a microcontroller based on

Atmega328, battery pack and electronic LED driver.

The signal is emitted with an On-Off Keying (OOK)

modulation, at a baud rate of 4.8 kbps and with a

subcarrier at 38 kbps.

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Figure 2: Wearable device for scenario 1 (cardiac

frequency and temperature).

Figure 3: Wearable device for scenario 2 (accelerometer

data).

2.2 Receiver

The receivers consist of infrared modules composed

of photo-detectors and preamplifiers (TSOP34338)

with FOV of 45° and a minimal irradiance of

100W/m². They are located in the corner of the

false ceiling tiles with a plastic part to ensure that

they are pointing towards the floor (see figure 4).

Moreover, they are arranged uniformly on the

ceiling as presented in figure 1. They are all

switch transmitting the received data to a remote

computer connected to a standard RJ45. The

computer analyses the received data and fills a table

containing the sensor information so that it can be

exported at the end of the measurement, and then

plots the measured parameters on a scrolling graph

as data arrive.

Figure 4: Optical receiver module.

2.3 Data Packets

The two types of health data are transmitted,

following a common scheme represented in figure 5.

In both cases, we consider a frame delimiter, a

patient identifier (4 digits: P

), a verification

code obtained by summation of information data (9

digits: V

) and a measure

increment (3 digits: I

) to identify the information

once when it is received from different receivers.

The verification data and the redundant transmission

of patient identifier are two different ways to check

the integrity of the data.

Figure 5: Packet description.

Depending on the scenario, data packets are not

transmitted at the same period. Let’s denote T

this

time duration. In addition, the packet duration T

depends also on the scenario as presented in the

following. According to the value of T

/ T

ratio, the

maximal forward current through the photodiode is

not the same. In each scenario, we adjust the

emission circuit so that we maximize the current and

thus the resulting emitted power.

For the cardiac frequency plus temperature

transmission, data are transmitted at the rhythm of

heart rate, so T

is of the order of the second. The

information data are carried over N

=2 bytes and

the verification code is obtained by summation of

temperature, cardiac frequency, patient identifier and

measure increment. This scheme leads to T

≈ 10 ms

so that T

/ T

is of the order of 1%. In this case the

forward current is fixed to 325mA. Considering the

temporal occupancy, the average emitted power is

less than 0.5 mW.

For the accelerometer data transmission, there

are N

= 6 bytes containing accelerometer values

RemoteMobilePatientMonitoringusingDiffuseOpticalTransmissions

555

over the three dimensions (x, y, z) from the 10-bits

analog to digital converter. The verification code is

in this case obtained by summation of the first five

MSB of accelerometer data over the three axes,

patient identifier and measure increment. This

scheme leads to T

≈ 20 ms. In addition, data are

transmitted each 0,1s so that T

/ T

is around 20%.

The forward current in this case is fixed to 150 mA,

and the average emitted power is less than 4 mW.

3 EXPERIMENTAL RESULTS

To evaluate the wireless optical technology

performance, we have measured packet loss during

mobile transmission within the room presented in

figure 1, for the two scenarios previously described.

We have performed several measurements with

different persons equipped with the two kinds of

devices and different conditions of lighting from sun

and neon lights at room ceiling. Moreover, during

the test, persons moved regularly throughout the

environment.

A packet can be lost for two reasons: the signal is

not received or it is bad received. The signal is not

received if its amplitude is too low to be detected by

the photo-detector, for example when an obstacle

blocks the optical beam. A packet is bad received

when it contains an error so that the receiver rejects

it. An error is detected as soon as the redundant

patient identifier or the verification code is wrong.

Performance is presented in terms of packet loss

rate determined by the number of lost packets over

the total number of packets sent.

We have evaluated the transmission performance

for one active receiver, two and four. The obtained

results are reported on table I, averaged from 10

measurements for each scenario. The measurements

were carried out over a period of 30 min in order to

detect at least 10 packet losses which are necessary

to have reliable paquet loss rate information. For one

active receiver, measures have been performed for

each receiver A to D. For the case with two receivers

we have considered the couples A-C and B-D

corresponding to room diagonals.

As expected, we can see that the performance

improves when the number of receivers increases.

When there are only one or two receivers,

scenario 2 leads significantly to more packet losses

than scenario 1. This can be explained by the fact

that due to the higher ratio T

/ T

in scenario 2, the

forward current in the photodiode is lower than in

scenario 1, which means a lower peak optical

emitted power. In addition, the emitter position is

not the same on the patient body so that the body can

be more blocking in scenario 2.

Table 1: Packet Loss Rate.

Number of

receivers

Scenario 1 Scenario 2

4 1,3.10

-3

1,3.10

-3

2 4.10

-3

1,5.10

-2

1 4.10

-2

0,11

When the number of receivers increases, we first

remark for scenario 2 that the performance is

significantly enhanced. Indeed, the packet loss rate

has been improved by one decade by comparing the

results obtained from 1 to 2 receivers and also from

2 to 4. On the other hand, we can note that for

scenario 1 this improvement is not so important: the

packet loss rate from 2 to 4 receivers is only divided

by 3. In addition it can be noticed that with 4

receivers, performance between both scenarios tends

towards a similar value. These observations can be

explained by the fact that, in scenario 1, as the peak

optical emitted power is larger, the optimal room

coverage is achieved with fewer receivers than in

scenario 2.

Finally, our experimental results show that it is

important to take into account the data packet

parameters in order to deploy the right number of

receivers allowing a given quality of service in terms

of packet loss while optimizing the emitted power.

Besides, we can say that the value obtained with

4 receivers, which is 10

-3

, is a typical one in wireless

transmission system and permits concluding that the

wireless optical transmission reliability is ensured.

4 CONCLUSIONS

In this paper, we have presented the experimental

performance of an alternative solution to radio

frequencies for healthcare mobile monitoring that is

wireless optical technology. We have used as a

criterion the packet loss of the transmitted data for

two health-related monitoring scenarios having

different constraints.

For both scenarios, we have investigated the

wireless data transmission between a mobile emitter

worn by the patient and receivers fixed in the

environment, by exploiting optical diffuse

reflections over the room surfaces.

The measurement results obtained with two

prototypes made of commercially available and low

cost components have shown that the optical

HEALTHINF2015-InternationalConferenceonHealthInformatics

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wireless technology constitutes a reliable solution

for health monitoring. In the best coverage

configuration, a packet loss of 10

-3

has been

obtained for both scenarios.

Future work will consist in conducting this

experimentation in the life experimental unit of

Limoges hospital and to evaluate the reliability in

real context of use, especially considering several

monitored patients. In addition, one important point

is to enhance the system by reducing the

consumption and to work on its miniaturization.

ACKNOWLEDGEMENTS

This work is part of SAPHIRALE project supported

by the Limousin regional council (France) and the

AutonomLab living Lab association. (www.limousin

livinglab.fr)

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