Biomedical Engineering Reference
In-Depth Information
TDMA based multiple access mechanism relies on precise synchronization in
timing. However, a synchronization mechanism has not been proposed for the
MAC protocol.
2.11 Transmit-only MAC
Most of the MAC protocols discussed above have limitations when it comes to
UWB based WBAN applications. Many MAC protocol designs have not paid their
attention towards the practical constraints that occur in hardware design. Even
though 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 [
24
] has a power consumption of 2 mW while the IR-UWB receivers
consumes up to 32 mW of power [
25
]. 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 con-
sumption in sensor nodes is a critical factor that determines the efficiency of a
MAC protocol. The transmit-only MAC protocol suggested in [
5
,
26
] 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;
• Data packets are transmitted at a much higher data rate than the required data
rate so that it is possible to get an optimum sleep time for sensor nodes while it
waits for the next set of data.
• Each sensor transmits at a pre allocated unique transmission slot in order to
minimise 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.
2.8
.
When a sensor node is first connected to the network, synchronizing frame
structure is used in order to assist the self-synchronization at the gateway node. A
guard interval follows immediately after the initial synchronization process to
allow the receiver to prepare for the reception of information in the Physical
Header (PHR). The PHR contains information on the chirp rate, symbol rate, and
the timing of the next transmission window. After establishing initial communi-
cation with the gateway node, the data frame will then be used in the successive
transmissions. The data frame has a short preamble, which helps the receiver
achieve fine synchronization followed by a guard interval to prepare the receiver
