Biomedical Engineering Reference
In-Depth Information
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 sched-
ules 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 timeslots 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 [
4
]:
1. Random access using slotted ALOHA and CSMA/CA.
2. Improvised and unscheduled access mechanism, where the coordinator node
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 IEEE 802.15.6 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 eleven
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 section 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 IEEE 802.15.6 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.
2.1
.
The Synchronisation Header (SHR) provides a preamble bit pattern (Kasami
sequence with a length of 63) which is essential part in the narrow pulse based
UWB data transmission. The PHY Header (PHR) provides 24 data fields which are
used to indicate communication parameters such as data rate, MAC frame body
