Biomedical Engineering Reference
In-Depth Information
order to mitigate the multiple access interference at the PHY layer level. The received
UWB pulse amplitude is compared against a predefined threshold at the coordinator
end. Since all the sensor nodes transmit using a predefined transmit power, the
expected receive power for a particular sensor node can be determined by the
coordinator using UWB ranging techniques. If received pulse amplitude exceeds the
threshold level, it indicates a collision at the coordinator. This concept is used in the
DCC-MAC in order to identify and eliminate erroneous data at the coordinator end.
Rate-compatible punctured convolution (RCPC) [
34
] codes are used in order
to achieve dynamic channel coding. The multiple access to the shared medium is
achieved through TH-codes as in the case of UWB
2
and U-MAC. The TH-codes are
generated locally at the sensor node using a random number generator.
Drawbacks:
This MAC protocol has the same drawbacks as UWB
2
and U-MAC
when it comes to the use of a UWB receiver at the sensor node end. Additionally, it
tries to mitigate the interference at the expense of physical layer complexity. Also,
an extensive amount of processing is allocated to the sensor nodes, which leads into
increased power consumption. It also assumes that the sensor nodes always transmit
at the maximum allowable transmit power. Although this has some advantages when
it comes to interference mitigation and optimizing the throughput [
36
], a power-
controlling approach might be well suited for power-stringent WBAN applications of
UWB. It also assumes the presence of resynchronization per every data packet, which
results in increased overhead. Instead, synchronization per session is recommended
for WBAN applications.
Multiband MAC for IR-UWB
Multiple access through the allocation of a unique frequency band per each sensor-
coordinator data communication link is suggested in [
37
]. A common control
channel, which is assigned with a unique frequency band, is used for sensor ini-
tialization and control message transfer in this MAC protocol. Both control and
data communication bands are allocated with a 500 MHz bandwidth. TH-codes are
used in the common control channel in order to share it with multiple users. The
main advantage of this MAC protocol is that it can be used for concurrent data
transmissions from multiple numbers of sensor nodes, because of the use of dif-
ferent frequency bands. This assists in reducing the probability of collision, hence
increases the throughput and results in low latencies that are ideal properties for high
data rate WBAN applications.
A super frame structure is used for data and control message transfer. A super frame
is divided into 15 sequence frames. Each sequence is used for data transmission in
each band. An
availability frame
is used between two super frames in order to indicate
the availability of a particular band for data transmissions. If a sensor node intends
to continue data transmission in a particular band, it has to send consecutive UWB
pulses in the relevant slot allocated to indicate the occupancy of that frequency band.
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