Biomedical Engineering Reference
In-Depth Information
compared against a pre-defined threshold at the coordinator end. Since all the
sensor nodes transmit using a pre-defined 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) [
19
] 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. In
addition, an extensive amount of processing is allocated to the sensor nodes, which
leads to an 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 [
21
], a power controlling approach might be well suited for power
stringent WBAN applications of UWB. It also assumes the presence of resyn-
chronization per every data packet, which results in increased overhead. Instead,
synchronization per session is recommended for WBAN applications.
2.9 Multiband MAC for IR-UWB
Multiple access through allocation of a unique frequency band per each sensor-
coordinator data communication link is suggested in [
22
]. A common control
channel, which is assigned with a unique frequency band, is used for sensor
initialization 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, which 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 trans-
mission 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 occu-
pancy of that frequency band. By sensing those UWB pulses within the corre-
sponding time slots of the availability frame, other sensor nodes can determine the
