Biomedical Engineering Reference
In-Depth Information
(b)
(c)
(a)
Sensor
Co-ordinator
Sensor
Co-ordinator
Sensor
Co-ordinator
UWB-Tx
UWB-Rx
UWB-Tx
UWB-Rx
UWB-Tx
UWB-Tx
UWB-Tx
NB-Rx
NB-Tx
UWB-Rx
UWB-Rx
Fig. 3.1 Communication techniques for UWB based WBAN a sensor nodes using UWB for
both transmitter and receiver b transmit-only method c sensor nodes using UWB to transmit data
and narrowband to receive control messages
IR-UWB receivers are generally complex in design and consume large amount of
power compared to IR-UWB transmitters. This poses a challenge to use IR-UWB
technology in low-power WBAN devices. It is possible to investigate alternative
approaches that lead to incorporating advantages provided by IR-UWB transmit-
ters, while avoiding the disadvantages of using IR-UWB receivers in wearable and
implantable hardware platforms [
1
,
2
].
Figure
3.1
shows three communication protocols that can be used for an IR-
UWB based WBAN. Figure
3.1
a shows a standard UWB system that requires the
use of a UWB transmitter and a receiver in the sensor node. Figure
3.1
b is based
on the UWB transmit-only technique that does not use a receiver in the sensor
nodes [
3
,
4
]. In this system, each individual sensor transmits periodically without
prior knowledge of other users and the channel condition. This MAC protocol
addresses the problem of high power consumption that arises due to the use of
UWB receiver at the WBAN sensor node. However, it suffers from low scalability
of the network, degraded performance under multiple user environments and
requirement of using individual receivers for individual sensor nodes.
A very efficient technique for a UWB-WBAN network is to use a narrow band
receive link instead of using UWB receiver in the sensor node side (Fig.
3.1
c)
[
1
,
5
]. With the introduction of the narrowband feedback system, it is possible to
achieve a more dynamic power reduction schemes that involve cross layer design.
The typical current consumption of a UWB receiver is around 16 mA [
2
],
1
but
narrowband receivers can operate at a current as low as 3.1 mA [
6
]; hence this can
reduce the power consumption of the WBAN sensor node significantly. Using a
feedback path will also reduce the computational complexity at the sensor node
end. In a transmit-only system, the position of the receiver is fixed and it does not
allow for reconfiguration for changing channel conditions. By using a narrow band
receiver, the gateway node can use the narrowband feedback to reconfigure the
system to adapt to changing channel conditions without any user intervention. It is
also important to note that the feedback path does not require high data rate since it
will be mostly used for sending acknowledgement and control messages. A simple
narrowband receiver does not consume much of additional design space. It also
1
UWB receivers consume more power than narrow-band receivers as the transmission of UWB
signal has low power levels as well as the higher operation frequency.
