Biomedical Engineering Reference
In-Depth Information
Drawbacks:
This MAC protocol possesses all the drawbacks in the IEEE802.15.4a
standard MAC protocol when it comes to WBAN applications. The TDMA-based
multiple-access mechanism relies on precise synchronization in timing. However, a
synchronization mechanism has not been proposed for the MAC protocol.
Transmit-Only MAC
Most of the MAC protocols discussed above have limitations when it comes to UWB-
based WBAN applications. Authors have not paid attention towards the practical
constraints that occur in hardware design. Although IR-UWB transmitters consume
low power, IR-UWB receiver needs to detect pulses with low power level. This leads
to a complex and high power-consuming receiver architecture. For example, the
CMOS IR-UWB transmitter discussed in [
39
] has a power consumption of 2 mW,
whereas the IR-UWB receivers consumes up to 32 mW of power [
40
]. Addition of
an IR-UWB receiver in the sensor node will increase its power consumption as well
as the design complexity. Implantable or wearable sensor nodes are battery powered.
Hence, power consumption in sensor nodes is a critical factor that determines the
efficiency of a MAC protocol. The transmit-only MAC protocol suggested in [
17
]
and [
21
] enables the use of a transmit-only hardware design at the sensor node end.
The suggested transmit-only MAC protocol is of asynchronous nature; hence, it
faces several challenges when it comes to collision avoidance and synchronization
at the receiver end. It has been designed with following characteristics in order to
overcome the challenges:
•
Gateway nodes are designed in a way that it is possible to self-synchronize using
the received UWB pulses
•
Data packets are transmitted at a much higher data rate than the required data rate,
so it is possible to get an optimum sleep time for the sensor nodes while it waits
for the next set of data
•
Each sensor transmits at a pre-allocated unique transmission slot in order to
minimize the occurrence of collisions
•
A unique pulse rate is assigned for each WBAN in the same region
•
Sensor nodes transmit without prior knowledge of the channel condition
•
There is no feedback in the network
The frame structure for this WBAN system is shown in Fig.
9
.
When a sensor node is first connected to the network, synchronizing frame struc-
ture is used in order to assist the self-synchronization at the gateway node end. A
guard interval follows immediately after the initial synchronization process to al-
low the receiver to prepare for the reception of information in the PHR. The PHR
contains information on the chirp rate, symbol rate and the timing of the next trans-
mission window. After establishing initial communication with the gateway node,
the data frame will be used in the successive transmissions. The data frame has a
short preamble, which helps the receiver achieve fine synchronization followed by a
Search WWH ::
Custom Search