23-05-2012, 03:59 PM
MAC Protocols for WBANs
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Introduction
A Wireless Body Area Network (WBAN) allows the integration of intelligent, miniaturized, lowpower,
invasive/non-invasive sensor nodes that monitor body functions and the surrounding
environment. Each intelligent node has enough capability to process and forward information to a base
station for diagnosis and prescription. A WBAN provides long term health monitoring of patients
under natural physiological states without constraining their normal activities. It is used to develop a
smart and affordable health care system and can be a part of diagnostic procedure, maintenance of a
chronic condition, supervised recovery from a surgical procedure, and can handle emergency events.
WBAN MAC Requirements
The most important attribute of a good MAC protocol for a WBAN is energy efficiency. In some
applications, the device should support a battery life of months or years without intervention, while
others may require a battery life of only tens of hours due to the nature of the applications. For
example, cardiac defibrillators and pacemakers should have a lifetime of more than 5 years, while
swallowable camera pills have a lifetime of 12 hours [6]. Power-efficient and flexible duty cycling
techniques are required to minimize the idle listening, overhearing, packet collisions and control
packet overhead problems. Furthermore, low duty cycle nodes should not receive frequent
synchronization and control information (beacon frames) if they have no data to send or receive.
Heartbeat Driven MAC Protocol (H-MAC)
A Heartbeat Driven MAC protocol (H-MAC) is a TDMA-based protocol originally proposed for a
star topology WBAN. The energy efficiency is improved by exploiting heartbeat rhythm information
in order to synchronize the nodes. The nodes do not need to receive periodic information to perform
synchronization. The heartbeat rhythm can be extracted from the sensory data and hence all the
rhythms represented by peak sequences are naturally synchronized. The H-MAC protocol assigns
dedicated time slots to each node to guarantee collision-free transmission. In addition, this protocol is
supported by an active synchronization recovery scheme where two resynchronization schemes
are implemented.
PB-TDMA Protocol
The performance of a Preamble-based TDMA (PB-TDMA, see Section 4.3) for a WBAN has been
analyzed in [17]. The authors used NS-2 for extensive simulations where the wireless physical
parameters were considered according to low-power Nordic nRF2401 transceiver and the simulation
area was 3 × 3 meters. For the performance comparison, many other protocols such as S-MAC and
IEEE 802.15.4 were also simulated in the same environment. Simulation results showed that
PB-TDMA protocol outperformed S-MAC and IEEE 802.15.4 protocol in terms of energy efficiency.
The results are valid for normal traffic only and do not consider the behaviour of emergency and ondemand
traffic.
Power-efficient Mechanisms for WBANs
Power-efficient mechanisms play an important role in the performance of a good MAC protocol.
These mechanisms are categorized into Low-power Listening (LPL), Scheduled Contention, and
TDMA mechanisms. The following sections briefly explain each mechanism with examples.