Biomedical Engineering Reference
In-Depth Information
have to be considered when determining the transmission slot duration and the
duty cycle for a certain sensor node that requires a specific data rate. However,
dynamically changing the FBW transmit power involves the use of a variable gain
amplifier at the sensor node end in a practical scenario, while dynamically
changing the PPB value can be done by varying the bit duration of the bits
generated using a microcontroller of a sensor node. The latter is simpler to
implement and less power consuming than the former. Hence, FBW transmit value
is kept constant within the limits specified in
Sect. 3.3.1
for the sensor nodes
considered in the simulations. The PPB value is dynamically changed in order to
optimize the power consumption of the sensor node.
In the WBAN system described in this chapter, a parent node dynamically
assigns the number of PPB for a child node in order to obtain a specified optimum
BER value at the receiver. A BER threshold of 10
-4
is used in the simulation
scenario for all sensor nodes, because a good throughput can be obtained with this
value while keeping the power consumption at the sensor nodes low. During the
data transmission, the sensor nodes insert a certain number of bytes containing a
known bit patterns equally spread among the data bits. For the simulations, six
bytes are chosen for continuous sensors while three bytes are chosen for periodic
sensors. A parent node dynamically determines the BER for a particular sensor
node using the known bit pattern, and compares it against the 10
-4
threshold. Note
that this calculated BER value includes the bit errors due to MPI, MAI as well as the
noise characteristics. Sensor nodes start data transmission with the highest allow-
able PPB value for that particular sensor type during an allocated transmission slot
for that sensor node (time slot allocation is discussed in a following section).
Coordinator node requests a particular sensor node to dynamically reduce the PPB
value in 1 PPB steps until a BER value of 10
-4
or the closest possible value which
is higher than the 10
-4
threshold is achieved using the minimum PPB value pos-
sible. This procedure ensures that the sensor nodes transmit data using the mini-
mum achievable PPB while maintaining a close to 10
-4
. The sensor nodes resend
the same data packet until an agreement is made to use a certain PPB value for
future data transmissions. In the case of degradation in the BER during data
transmission using the pre agreed PPB, parent node repeats the above process to
increase the PPB value to improve the BER. If a certain sensor node receives a
message to increase the previously assigned PPB value, it resends the previously
sent data packet modulated according to the new assignment. Note that changing
the PPB value is achieved in sensor nodes by varying the duration of the bit, not the
PRF. Hence, the receiver node does not have to change the sampling frequency for
each sensor node. It keeps track of the number of PPB assigned to each sensor node.
This can be achieved in the MAC layer. As a result, a single receiver node can be
utilised for all the sensor nodes. The dynamic BER compensation procedure
described above is particularly useful in short range UWB communications systems
where a strong LOS path is present, such as the WBAN system described in this
chapter, to achieve reliable communication during dynamic channel conditions.
