CSEM Scientific and Technical Report 2008

Riselon – Using IEEE 802.15.4 for Real-time Visual People Tracking
L. von Allmen, S. Pangaud, A. Hutter
In this article, we present the realization of a real-time people tracking system focusing on the implementation of a suitable communication network,
proper sensors pre-processing and fusion of the data harvested by the sensors.
This project targeted the elaboration of a heterogeneous
sensor network for the purpose of real time person detection
and tracking system within home and building areas. It
combines:
• 2D high dynamic range vision sensors for people motion
detection under any illumination conditions
• 3D camera for object (people, groups) height detection
to enhance the robustness of the detection and tracking as
well as rejecting false triggering occasioned by pets and
shadows artefact.
The sensors are wirelessly connected in a star network
configuration to a supervisor node.
Figure 1: Network architecture
The area to be covered by the sensing network can be divided
into clusters (see Figure 1). Every cluster has its own
supervisor collecting the pre-processed data from the sensors
via a wireless link. Pre-processing of the data means here the
detection of moving parts in the Field-Of-View (FOV), the
discrimination of people from pets, and reporting of
trajectories. This reduction of the raw data to relevant
information enables the use of low-power and low-cost
communication means.
The wireless link has to support three kinds of communication:
• Time-stamp broadcasts from supervisor to sensor nodes.
• Regular reporting from sensors: data is collected by the
supervisor node on a periodic basis at a rate of 123 ms.
The system can deal with occasional loss of regular
reports if they occur sporadically.
• On-request reporting from sensors: to furnish additional
information such as compressed captures (pictures) of the
FOV, statistical analysis results, general conditions
(illumination) ... This traffic does not require deterministic
time of delivery.
The challenging part in communications is the regular
reporting, where each node may be transmitting up to
86 bytes every 123 ms and with up to six sensors in a cluster.
This results in an aggregated throughput of 33.6 kb/s on the
supervisor.
The network configuration typically calls for the use of
IEE802.15.4. This standard defines an operation mode to
build a network in a star configuration with a coordinator
transmitting beacon signals in order to synchronize the
network. The coordinator periodically emits beacons. Beacon
signals can have a payload. The payload will carry the timestamp
from the supervisor, which at the same time is also the
network coordinator. The standard defines a super-frame
structure as illustrated in Figure 2.
Figure 2: IEEE 802.15.4 super-frame structure
The super-frame starts with the transmission of the beacon to
all network nodes. Then – as shown in Figure 2 – after the
beacon follows the active period, where communication
between nodes and coordinator takes place. This active
period can be organised in two periods: the contention access
period (CAP) and the contention-free access period (CFP).
In CFP up to six slots could be reserved to guarantee
communication between a sensor and the supervisor. The
CFP accommodates the regular traffic. The CAP provides
space for the on-request reporting traffic.
In the current system there is no inactive period in the superframe.
This is imposed by the requirements of the traffic of the
application.
Figure 3: The cameras and the monitored area
The communication system has been implemented on a board
containing a micro-controller and a radio chip dedicated to
802.15.4. The Medium Access Control (MAC), part of the
standard, is the CSEM implementation of 802.15.4. This MAC
version is one of the very few implementations available that
provides support for the guaranteed traffic.The concept was
validated and a prototype installed covering the reception area
of the CSEM building (Figure 3). This prototype network is
now in operation and serving as a show case.
9