CSEM Scientific and Technical Report 2008

Using IEEE 802.15.4 for Athlete Condition Monitoring
L. von Allmen, B. Perrin, J.-M. Koller
CSEM designed and implemented a body-worn electronic system for acquisition, wireless transmission and storage of accelerometer data. The
system has been deployed to monitoring athlete energy consumption
IEEE 802.15.4 is a standard for Low Rate Wireless Personal
Area Networks which defines the physical layer and the
medium access control sublayer. Every PAN (personal area
network) has a PAN coordinator that decides on a number of
basic parameters such as the communication channel and the
operating mode. Two operating modes are defined, a
beaconless mode and a beacon enabled mode. In the first
mode, nodes use a pure CSMA protocol as medium access
control. In the second, special nodes called coordinators emit
at periodic intervals a special packet called "beacon".
This second mode is particularly well suited for systems in a
star configuration where data have to be transferred from a
number of nodes to a sink in direct visibility. The node at the
sink is the coordinator. The standard also defines the rules for
association of new nodes in the network.
The energy saving capability is offered by this standard in the
star configuration, the synchronization provided by the beacon
mode, and the support for management of adding devices to
the network where key factors to select 802.15.4 in the
development of a body-worn electronic system for acquisition
and storage of accelerometer data.
The system is composed of (Figure 1):
• 2 accelerometer sensor units that can be worn on the
body, e.g. one on the knee and the other one on the upper
arm.
• A data logger unit that can be worn on the body, e.g. on a
belt
Figure 1: Data logger and accelerometer
The application requires a lifetime for data acquisition of at
least 24 hours. The sampling rate of the accelerometers is
25 Hz, at a resolution of 8 bits.
The system has to be worn on the body, so the components
should be as small and light as possible. The acceleration
sensors consists of three stacked boards (2.4 GHz radio
module, sensor and battery) and measures 31x21x16 mm3 .
The dimensions of the data logger housing are
16x38.5x57 mm3 . The battery of sensor nodes is dimensioned
to operate for at least 24 hours. On the logger unit storage of
acquired data up to two weeks is possible.
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The microcrontroller has to perform the signal acquisition, the
data processing and the communications management. In
order to provide a solution based on single microcontroller
architecture, CSEM has developed its own highly optimized
implementation of IEEE 802.15.4 MAC (Medium Access
Control). This implementation keeps the energy consumption
at the lowest level allowed by the hardware components.
The required autonomy is achieved by using a beacon interval
of 0.5 s. The duration of the active period after the beacon, for
the transmission of data, is 60 ms, as shown in Figure 2.
Figure 2: Current consumption
The measured current consumption is summarized as:
• Maximum: 22.3 mA
• Minimum: 1.1 mA
• Average: 3.76 mA
With the Lithium-polymer battery used, the device is able to
send data for more than 35 hours.
Four systems have been used for an activity monitoring field
study that was organized and supervised by the federal
administration for sport (BASPO) in Macolin. During this field
study, which ran from July through August 2008, the
acceleration data of 16 persons have been reliably recorded
for a duration of two weeks each. The acquired data serve to
determine the energy consumed by the person.