Biomedical Engineering Reference
In-Depth Information
6.3 Cross Layer MAC Protocol Implementation at Sensor
Node
A sensor node synchronises to the super frame structure used in the MAC protocol
by listening to the beacon packets transmitted on the narrowband channel. A super
frame structure consists of two initialization time slots at the beginning of the
super frame. Position of these initialization time slots is pre-known to all the
sensor nodes. This is determined using a time delay measured with reference to the
end of beacon reception period. At sensor initialization, a sensor node transmits an
initialization request message using the packet structure shown in Fig.
6.1
c and
starts a time-out operation in order to determine whether the sensor node gets an
initialization respond from the coordinator node within a given amount of time. A
unique pre-programmed sensor address is used in the initialization request packet.
Sensor node then waits until the reception of initialization acknowledgement
packet that contains transmit time slot and PPB value assigned for that sensor
node. If a time-out occurs without receiving the initialization acknowledgement
packet, sensor node then retransmits an initialization request message during the
initialization slots of the following super frame. The maximum number of ini-
tialization attempts is limited to 10. After initialization, sensor node transmits data
in the pre-allocated time slot and listens to any BER correction control messages
on the narrowband receiver. Implementation of the BER correction procedure in
discussed in the following sub section. Figure
6.2
depicts the overall operation of
sensor node control program that is implemented in the micro-controller module.
6.4 Cross Layer MAC Protocol Implementation
at the Coordinator Node
The coordinator node is responsible for organizing and controlling the data
communication of sensor nodes. Main operations carried out by the control pro-
gram of coordinator node are listed below;
1. IR-UWB pulse synchronization
2. Data bit detection and packet synchronization
3. Dynamic BER detection and feedback control.
6.4.1 Pulse Synchronization for IR-UWB WBAN
Communication
IR-UWB pulse detection is done by an ADC module and a Field Programmable
Gate Array (FPGA) that are integral parts of the coordinator node. The ADC
module samples the broadened base band pulse stream that is generated by
IR-UWB front-end. Pulse detection program is implemented in the FPGA module.
