23-08-2012, 03:28 PM
A Comprehensive Survey of Wireless Body Area Networks
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Abstract
Recent advances in microelectronics and
integrated circuits, system-on-chip design, wireless
communication and intelligent low-power sensors have
allowed the realization of a Wireless Body Area Network
(WBAN). A WBAN is a collection of low-power,
miniaturized, invasive/non-invasive lightweight wireless
sensor nodes that monitor the human body functions
and the surrounding environment. In addition, it
supports a number of innovative and interesting applications
such as ubiquitous healthcare, entertainment,
interactive gaming, and military applications. In this paper,
the fundamental mechanisms of WBAN including
architecture and topology.
Introduction
Current healthcare systems are facing new challenges
due to the rate of growth of the elderly population
(persons 65 years old and over) and limited financial
resources. According to the US Bureau of the Census,
the number of old people (65–84 years old) is predicted
to double from 35 million to 70 million by 2025 [1].
This trend shows that the world elderly population will
double from 375 million in 1990 to 761 million in 2025.
Furthermore, overall healthcare expenditure in the US
was $1.8 trillion in 2004, and this number is projected
to be triple by 2020, or 20% of the US Gross Domestic
Product (GDP) (http://www.who.int/) [2]. The impending
health crisis attracts researchers, industrialists, and
economists toward optimal and quick health solutions.
The non-intrusive and ambulatory health monitoring of
patient’s vital signs with real time updates of medical
records via the internet provides economical solutions
to the challenges that health care systems face. The
remote monitoring of body status and the surrounding
environment is therefore becoming more important
for sporting activities, members of emergency, military
and health care services.
WBAN architecture
A WBAN consists of in-body and on-body nodes that
continuously monitor a patient’s vital information for
diagnosis and prescription. Some on-body nodes can
be used for multimedia and gaming applications. These
nodes can have different topologies such as star, tree,
and mesh topologies. However, the most common is
a star topology where the nodes are connected to a
central coordinator in star manner. Depending on the
application, several nodes are sometimes combined to
process and transfer data to a central coordinator.
PHY layer communication
There are several ways to communicate with a human
body implant, including methods that use electromagnetic
coupling and Radio Frequency (RF) communication.
Both are wireless and their use depends on the
applications. Comprehensive details about the implant
communication are presented in [5]. In this section, we
briefly discuss electromagnetic coupling, in-body RF
communication, antenna design, and the propagation
pattern in or around a human body. This section is
concluded with useful remarks.
Electromagnetic coupling
Electromagnetic coupling means that the transponder
and the antenna are coupled by the magnetic flux
through coils, much like a transformer. Different applications
still use electromagnetic coupling to provide
a communication link to implanted devices, with an
external coil held very close to the patient that couples
to a coil implanted just below the skin surface.
The implant is powered by the coupled magnetic field
and requires no battery for communication. Data is
transferred from the implanted device by altering the
impedance of the implanted loop that is detected by
the external coil and electronics. This type of communication
is commonly used to identify animals that have
been injected with an electronic tag. Electromagnetic
induction is commonly used when continuous.
Antenna design
According to [5], an in-body antenna needs to be tuneable
using an intelligent transceiver and routine. This
enables the antenna coupling circuit to be optimized
and to obtain the best signal strength. Often size constraints
dictate the choice of a non-resonant antenna.
A non-resonant antenna has lower gain and therefore
is less sensitive on the receiving side and radiates low
power generated by the transmitter. This makes design
of the antenna coupling circuit even more important.
Signal propagation
The propagation pattern of the antenna is required to
predict the performance of an implant. Measurements
can be made using a body phantom and immersing
a battery test implant into it. The authors of [15]
conducted several experiments to analyze the performance
of an implant inside a human body/phantom.
The phantom was filled with a liquid that mimicked
the electrical properties of the human body tissues.
The distance from the body phantom to the basestation
was 3 m. Further details can be found in [15]
where the authors made useful measurements over
a set distance with all combinations of implant and
test antenna polarisations, i.e., Vertical-Vertical (V-V),
Horizontal-Vertical (H-V), Vertical-Horizontal (V-H),
and Horizontal-Horizontal (H-H) polarisations. Typical
results are shown in Fig. 6 where the Effective
Radiated Power (ERP) is calculated from the received
signal power and the antenna characteristics. It can be
seen that there is a significant difference in signal levels
with polarisation combinations and depth.
Conclusions
In this paper, we studied the fundamental mechanisms
of WBAN at PHY, MAC, and Network layers. Each
section was concluded separately with useful remarks.
Starting from the system architecture, for the PHY
layer, we reviewed different methodologies of wireless
communication to/from an implant including RF
communication, in-body antennas, and propagation
patterns. For the MAC layer, we discussed several
low-power mechanisms in the context of WBAN and
concluded that the TDMA mechanism is suitable and
more appropriate for WBAN. We further discussed
our proposed low-power MAC protocol for WBAN
followed by important suggestions.