Biomedical Engineering Reference
In-Depth Information
further discussed under a following section. Secondly, each node is assigned with a
unique address (see
Sect. 3.3.5
). In Topology 1, router nodes gather data from
sensor nodes using the super frame structure described in
Sect. 3.3.4
. Due to the
smaller number of sensor nodes that communicate with each router node, a larger
inactive period is present in the super frame. This inactive period is utilized by the
router nodes to map the data received from the sensor nodes into the super frame
slots of the communication link between router nodes and coordinator node. The
packet acknowledgement delay can be significantly reduced in this manner.
During data reception, the attenuated signal power from other simultaneous
transmissions and simultaneous multipath receptions are added to the noise floor of
the receiver (detected by the BER calculation at the parent node). It was observed
from the simulations that the interference between narrowband signals and UWB
signals are negligible. This is due to the large separation between the operational
frequencies of those two types of signals.
During the simulation, sensor nodes are initiated in a random manner. Data
rates used for sensor nodes are taken from Table
3.2
. The data packets for con-
tinuous sensor nodes consist of nine bytes of physical overhead and 617 bytes of
fragmented data payload, which is sent within one transmission slot of the super
frame. Periodic sensors generate data packets with 6 bytes of overhead and 52 bits
of data payload. For a fair assessment of both topologies, same number of sensor
nodes is simulated in each case. Considering a realistic hospital scenario, five
sensor nodes are attached to each patient out of which, one is a continuous
implantable sensor node, one is a periodic implantable sensor node and others are
periodic wearable sensor nodes. A maximum of seven patients are assumed to
enter the room during the simulation time.
3
This number is decided based on the
limitation of the GTS in Topology 2. More sensor nodes can be assigned in
Topology 1, since the data can be buffered at the router before sending to the
coordinator node.
For demonstrating the advantage of using a narrow band receiver in place of a
UWB receiver in a sensor node, packet delay and energy consumption are ana-
lyzed for both cases. When a UWB receiver is used, the beacons are sent within
the 35 ls guard period of the super frame structure. A 200 ls transmitter inactive
period is added at the end of the super frame to allow for turnaround time and
reception time. The parent nodes send feedback messages during this period. A
100 MHz PRF and 20 PPB is used for the receiver. It should be noted that the
effective data rate is reduce due to the addition of the transmitter inactive time slot
in this case. In the case of the sensor node with the narrow band receiver, the
receiver is kept on during the whole sensor active period (throughout the period
where the sensor is attempting to complete a transmission) in order to receive
simultaneous downlink messages.
3
This assumes that a hospital room accommodates up to seven patients for monitoring.
