Biomedical Engineering Reference
In-Depth Information
are allocated to the contending sensor nodes by a central coordinator, which controls
the shared access to the wireless medium.
The IEEE802.15.6 standard supports three communication modes [
20
]:
1. Beacon mode with super frame boundaries:
The super frame structure is divided by beacons transmitted in the downlink by
the coordinator in this communication mode. Several medium access mechanisms
are supported for sensor nodes communicating using this mode: exclusive access,
managed access, random access and contention access. Exclusive access and
managed access periods in the super frame are used to provide guaranteed data
transfer for sensor nodes with high priority while other two methods provide data
transfer for less-priority sensor nodes.
2. Non-beacon mode with super frame boundaries:
This communication mode does not use a downlink beacon in order to indicate the
super frame boundaries to the sensor nodes. Instead it uses the scheduling of data
communication through techniques such as polling. The coordinator schedules
the data transmission of each individual sensor node through polling such that the
data communication from the sensor nodes falls within a super frame structure.
This communication mode falls within the managed access.
3. Non-beacon mode without super frame boundaries:
In this communication mode a pre-defined super frame structure is not used. The
data communication occurs through polling or posted allocation where a certain
amount of time slots are allocated by the coordinator node, which can be accessed
by any sensor node waiting for data transmission.
The access to the shared medium is provided using various mechanisms [
20
]:
1. Random access using slotted ALOHA and carrier sense multiple access with
collision avoidance (CSMA/CA)
2. Improvised and unscheduled access mechanism, where the coordinator nodes
send polling and posting commands without pre-reservation or pre-scheduling in
a random manner
3. Scheduled access using polling
The UWB physical layer (PHY) specifications in the IEEE802.15.6 standard is used
to provide high data rate and low power consuming data transfer using UWB signals.
The UWB spectrum in the range of 3.1-10 GHz is divided into 11 channels with a
channel bandwidth of 499.2 MHz for each channel. The PHY specifications support
both IR-UWB and frequency modulation-UWB (FM-UWB). This chapter will only
discuss the specifications for IR-UWB, as it is better suited for WBAN applications
because of the possibility of implementing low complexity hardware for IR-UWB
transmitters.
The IEEE802.15.6 standard supports three different modulation schemes for IR-
UWB: on-off keying (OOK), differential binary phase shift keying (DBPSK) and
differential quadrature phase shift keying (DQPSK). The physical layer protocol data
unit (PPDU) for the IR-UWB-based data communication is shown in Fig.
2
.
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