Biomedical Engineering Reference
In-Depth Information
Correct detection of preamble assures that the initial threshold value is suffi-
cient for bit detection and that the clock synchronization occurs at the start of
packet structure. Incorrect detection of preamble can occur due to two reasons;
1. High Bit Error Rate (BER) in the UWB channel.
2. Initial threshold level sitting below the noise level of received signal.
The program at the receiver is designed to differentiate between these two
scenarios in the following manner. If the initial threshold level is below the noise
level of the received signal, the preamble detection procedure records all P
1
pulses
during a preamble detection period. In this case, threshold level is increased by
1 mV step and clock synchronization is reattempted during the relevant time slot
of the next super frame. Otherwise, it is considered that an incorrect preamble
detection occurs due to a high BER. BER compensation procedure is discussed
under the following sub section.
A successful preamble detection is followed by the bit detection. The coordi-
nator node has prior knowledge of the parameters, such as the PPB value and the
threshold level, assigned for each sensor node transmitting at a given time slot of
the super frame. These properties are used to decode the bits that are transmitted
by the sensor nodes. Decoded data packets are stored in a memory card attached to
the FPGA board, and sent on request to a computer terminal using a serial
communication.
6.4.3 BER Performance Control Using and the Feedback
in the Network for Reliable Data Communication
The coordinator node detects the BER for all the sensor nodes that transmit data in
real time using the preamble code and a known 10 bit data pattern of
'1011001110' spread among data bits within a transmit packet. Coordinator node
attempts to keep the BER threshold at 10
-4
or the closest possible value using the
detected for a particular sensor node in either the preamble code or known bit
pattern embedded in data section of the packet, data detection program starts
counting the number of bits received for that particular sensor node from there
onwards. If a second erroneous bit is detected before 10,000 bit counts, the bit
counter is reset and feedback message is sent via the narrowband feedback path
requesting for an increase in the PPB value for that particular sensor node. In case
where an erroneous bit is not found, bit counter is reset after reaching the 10000 bit
counts. This BER compensation procedure is further discussed under
Chap. 3
.
Overall operation of the data detection procedure for a particular sensor node is
shown in Fig.
6.7
.
Rest of the MAC protocol operates in the same way as described in
Chap. 3
.It
