Biomedical Engineering Reference
In-Depth Information
transmitters have similar performance because they have the same bandwidth and
equal transmission power that is in compliance with the FCC transmission regulation.
There is a slight difference between the pulse shapes of amplifier- and VCO-based
sensor nodes. The pulse shape of the amplifier-based UWB pulse always contains two
spikes as illustrated in Fig.
15
due to the rise and fall time of the narrow rectangular
pulse. Because the VCO-based method uses a low pass filtered version of the narrow
pulse train for up-conversion, the shape of the pulse is more consistent. The power
consumption of the amplifier-based design is 10 nJ per data bit while the VCO-based
sensor node operates at slightly low power consumption of 8 nJ per data bit.
UWB Sensor Node Integrated with a Narrowband Receiver
Although the transmit-only based design of the UWB sensor nodes avoids the com-
plexities introduced by the UWB receiver, it lacks the coordination required for
upscaling of the network architecture. It cannot reconfigure itself according to the
varying data rate requirements of the network. In an IR-UWB network, it is possible
to vary the data rate by changing the PRF of UWB pulses. In the previous designs,
it was not possible to change the PRF of the UWB pulse generator without manual
intervention to the circuit.
A low-power NB receiver circuit can be incorporated into the sensor node to
enhance the adaptability of the system. An example of a UWB system transmitter
incorporated with a 433-MHz ISM band-based NB feedback is illustrated in Fig.
16
.
The typical current consumption of a UWB receiver is around 16 mA, but NB
receivers can operate at a current as low as 1.65 mA [
38
] in a low-current mode.
Another advantage of NB receiver is the simplicity of circuit design. Using a feedback
path will also reduce the computational complexity at the sensor node end. In a
transmit-only system, the position of the receiver has to be fixed and it does not
allow for reconfiguration for changing channel conditions. By using an NB receiver,
the gateway node can use the NB feedback to reconfigure the system to adapt to the
changing channel conditions without the user intervention. It is also important to
note that the feedback path does not require high data rate because it will be mostly
used for sending acknowledgment and control messages. A simple NB receiver does
not consume much of additional design space. It also consumes very small power
because it is only powered on when required (sleep mode operation). Compared
to the transmit-only method it gives a considerable advantage when scheduling the
network and avoiding collisions. It also helps to optimize the power consumption of
the sensor node, introducing additional power savings.
As a further improvement to the UWB-based sensor nodes, we have chosen to in-
clude an NB receiver operating in the 433-MHz ISM band. We have chosen RX5500
ISM band receiver chip by RFM [
38
] due to its low operating power. It can operate
using both on-off keying (OOK) and amplitude shift keying (ASK) modulation
methods. It operates in low-current mode around a power consumption of 5 mW even
under the continuous operation. The power overhead introduced by this addition is
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