Biomedical Engineering Reference
In-Depth Information
Beacon on NB channel
Beacon
Guard
(SF: n+1)
Beacon
Guard
(SF: n)
Initialization
Slots
210 µs
Periodic
Data Slot
(PDS)
Guard
Period
Max. up to
16 data slots
Inactive
Period
(Variable
Length)
Max. up to
7 GTS
GTS 1
GTS 2
max.
100
µs
20µs
max. 20µs
5
µs
35
µs
CAP
CFP (max. 735
µs)
Beacon Interval (1000
µs)
Fig. 3.10
Super frame structure
The total length of the super frame including the beacon is kept at 1 ms. The
transmit slot durations for each sensor node type is decided based on the data rate
requirement, peak transmit power limitation and allowed PPB value range for a
particular sensor type. The duty cycle for all the sensor nodes is kept equal or
below the threshold limit of 18.75 % in order to utilize the maximum power limit
according to the analysis in
Sect. 3.3.1
.
For example, consider the case of a continuous sensor transmitting at 5 Mbps.
The allowable set of PPB values is chosen to be 1 and 2 PPB for this sensor type,
considering the data rate requirement, duty cycle requirement and the need to
facilitate the data transfer for multiple sensor nodes within the super frame. When
a continuous sensor node is transmitting at 2 PPB, which is the maximum
allowable PPB value, it needs to transmit 10,000 pulses every 1 ms in order to
achieve the given data rate of 5 Mbps. At a PRF of 100 MHz this can be done
within a 100 ls time slot, resulting in a duty cycle of 10 %. However, if a con-
tinuous sensor node is transmitting at 1 PPB, it will transfer data in a 50 ls time
slot. Similarly a maximum of 20 ls time slot duration is chosen for periodic sensor
nodes assuming a PPB range of 1-20 PPB. A larger PPB value range is allocated
for periodic sensor nodes in order to compensate for the possible increase in
interference level that might occur during the CAP. Two initialization slots of fixed
20 ls duration are allocated at the beginning of the super frame for sensor ini-
tialization. Initialization requests are sent during these two time slots at a fixed
20 PPB value. This value is assumed to be known to all the sensor nodes. The
synchronization beacons are sent using the narrow band channel in parallel with
the UWB transmission in order to reduce the data rate requirement of the narrow
band feedback (Fig.
3.10
). Narrow band beacon duration of 0.210 ms is used in the
simulations, which contains 4 bits transmitting at 19.2 kbps [
6
]. The CAP for each
super frame starts after a fixed 35 ls beacon guard slot from the end of the beacon
period. A fixed guard period of 5 ls is allocated at the end of each time slot. This
super frame structure is shown in Fig.
3.10
. Note that it is the maximum time
slot duration that determines the maximum limit of time slots that can be
