Prioritization of Patient Vital Signs Transmission Using Wireless Body Area Networks - E-Healthcare Systems and Wireless Communications - page 268

(Haykin, 2005). In our case, spectrum sensing is
achieved by probing and sensing the quality of
the 16 channels 1 . The measured channel quality
serves as a metric to characterize channel activi-
ties and variations such as fading. A channel with
the best quality, i.e., the channel experiencing the
least interference or fading, is chosen for packet
forwarding. Moreover, as the channel quality is
observed at the receiver, the channel is selected by
the receiver in our MAC. By using the measured
LQI information on multiple channels, our MAC
captures channel variations, maximizes channel
utilization, reduces interference, and allows si-
multaneous transmissions.
The spectrum (channel) sensing protocol to
probe and track the LQI values of all the chan-
nels is designed as follows. A dedicated Common
Control CHannel (CCCH) is used for all the
nodes to exchange control information. During
initialization, all nodes switch to the same data
channels according to a predefined or dynamically
agreed schedule. On each data channel, a node
sends probes and then listens. This can be done by
competing with probes. The winner sends a probe
over each channel while the losers back off and
retry until their probes are sent. Upon reception
of these probes, the receivers build their LQI RX
tables (Table 3), which records the LQI values
of the channels. LQI RX is also updated upon
the reception of a packet on one channel. This
LQI RX table is broadcast periodically on CCCH
for the transmitters to build up their LQI TX table
(similar to LQI RX table), which will be used by
the transmitter for its egress (outgoing) channel
information. LQI TX is used by the transmitter for
routing, as discussed in Sect. 4.3.
Our MQ-MAC is shown in Figure 2(a). After
LQI RX is built, nodes can forward data to each
other with MQ-MAC. At the start of a slot, when
node i has a data packet for j , it first sends out a
Request-To-Send (RTS) packet on CCCH with
the un-preferred channels and l , the data packet's
length. Here, l is used by the receiver and i 's
neighbors to estimate the time length that the

channel will be used. Upon receiving the RTS, j
selects the best available channel as,

* RX

CH = arg max { LQI }, (1)

ij CH ICH j

CH

∈
∉Λ

( )
( , )

i

i j

where LQI i RX is the set of ingress LQIs from i
in table LQI RX , ICH (j) is the idle channel set
(estimated by overhearing RTS/CTS messages)
in j is neighborhood, and L(i, j) is the set of the
unavailable channels just received from i and the
adjacent channels of those active channels near
j . Channels in L(i, j) are not selected in such a
way as to avoid interference at the transmitter and
from adjacent channel (Jonsrud, 2006). Clear-To-
Send (CTS) is then broadcast with CH ij * and l on
CCCH. If no channel can be selected, CTS is
broadcast with channel number 0, telling i to back
off.
After receiving CTS, i sends back an AC-
Knowledgement (ACK) message with CH ij * if
this channel is not reserved or active in the neigh-
borhood. On hearing this message, i i's neighbors
will mark CH ij * as reserved. Otherwise, i sends
back a new RTS to ask j to re-select another best
channel. This negotiation is repeated until the
transmitter i and receiver j settle on a common
available channel.
Finally, i tunes its channel to CH ij * and starts
the transmission of the data packet to j . An ACK
message will be sent back to i for acknowledge-
ment. If all these steps are successful, i.e., all
messages are received, i and j will tune their

Table 3. LQI RX at Receiver

Transmitter DCH11 DCH12 … DCH26

( ) ( ) ( 16 )

LQI i 1 LQI i 1 LQI i 1

i 1 …

( ) ( ) ( 16 )

LQI i 2 LQI i 2 LQI i 2

i 2 …

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