22-01-2013, 12:51 PM
Wireless Integrated Network Sensors: Low Power Systems on a Chip
Systems on a Chip.pdf (Size: 182.18 KB / Downloads: 64)
Abstract
Wireless Integrated Network Sensors (WINS) now
provide a new monitoring and control capability for
transportation, manufacturing, health care,
environmental monitoring, and safety and security.
WINS combine sensing, signal processing, decision
capability, and wireless networking capability in a
compact, low power system. WINS systems combine
microsensor technology with low power sensor interface,
signal processing, and RF communication circuits. The
need for low cost presents engineering challenges for
implementation of these systems in conventional digital
CMOS technology. This paper describes micropower
data converter, digital signal processing systems, and
weak inversion CMOS RF circuits. The digital signal
processing system relies on a continuously operating
spectrum analyzer. Finally, the weak inversion CMOS
RF systems are designed to exploit the properties of
high-Q inductors to enable low power operation. This
paper reviews system architecture and low power
circuits for WINS.
Introduction
Wireless integrated network sensors (WINS)
combine sensing, signal processing, decision capability,
and wireless networking capability in a compact, low
power system. Compact geometry and low cost allows
WINS to be embedded and distributed at a small fraction
of the cost of conventional wireline sensor and actuator
systems. For example, on a global scale, WINS will
permit monitoring of land, water, and air resources for
environmental monitoring. On a national scale,
transportation systems, and borders will be monitored for
efficiency, safety, and security. On a local, wide-area
scale, battlefield situational awareness will provide
personnel health monitoring and enhance security and
efficiency. Also, on a metropolitan scale, new traffic,
security, emergency, and disaster recovery services will
be enabled by WINS. On a local, enterprise scale, WINS
will create a manufacturing information service for cost
and quality control. WINS for biomedicine will connect
patients in the clinic, ambulatory outpatient services, and
medical professionals to sensing, monitoring, and
control.
Wireless Integrated Network Sensor
(WINS) System Architecture
The primary limitation on WINS node cost and
volume arises from power requirements and the need for
battery energy sources. As will be described, low power
sensor interface and signal processing architecture and
circuits enable continuous low power monitoring.
However, wireless communication energy requirements
present additional severe demands. Conventional
wireless networks are supported by complex protocols
that are developed for voice and data transmission for
handhelds and mobile terminals. These networks are
also developed to support communication over long
range (up to 1km or more) with link bit rate over
100kbps.
In contrast to conventional wireless networks, the
WINS network must support large numbers of sensors in
a local area with short range and low average bit rate
communication (less than 1kbps). The network design
must consider the requirement to service dense sensor
distributions with an emphasis on recovering
environment information. The WINS architecture,
therefore, exploits the small separation between WINS
nodes to provide multihop communication.
Multihop communication (see Figure 2) yields large
power and scalability advantages for WINS networks.
First, RF communication path loss has been a primary
limitation for wireless networking, with received power,
PREC, decaying as transmission range, R, as PREC µ R-a
(where a varies from 3 – 5 in typical indoor and outdoor
environments). However, in a dense WINS network,
multihop architectures may permit N communication link
hops between N+1 nodes. In the limit where
communication system power dissipation (receiver and
transceiver power) exceeds that of other systems within
the WINS node, the introduction of N co-linear equal
range hops between any node pair reduces power by a
factor of Na-1 in comparison to a single hop system.
Multihop communication, therefore, provides an
immediate advance in capability for the WINS narrow
bandwidth devices. Clearly, multihop communication
raises system complexity. However, WINS multihop
communication networks permit large power reduction
and the implementation of dense node distribution.
WINS Microsensors
Many important WINS applications require the
detection of signal sources in the presence of
environmental noise. Source signals (seismic, infrared,
acoustic, and others) all decay in amplitude rapidly with
radial distance from the source. To maximize detection
range, sensor sensitivity must be optimized. In addition,
due to the fundamental limits of background noise, a
maximum detection range exists for any sensor. Thus, it
is critical to obtain the greatest sensitivity and to develop
compact sensors that may be widely distributed. Clearly,
microelectromechanical systems (MEMS) technology
provides an ideal path for implementation of these highly
distributed systems. WINS sensor integration relies on
structures that are flip-chip bonded to a low temperature,
co-fired ceramic substrate. This sensor-substrate
“sensorstrate” is then a platform for support of interface,
signal processing, and communication circuits.
Examples of WINS microseismometer and infrared
detector devices are shown in Figure 3.[1]
WINS Summary
New architectures and circuits are required for
wireless integrated network sensors. A series of
interface, signal processing, and communication systems
have been implemented in micropower CMOS circuits.
Chopper input data converters for the WINS
requirements of high stability and micropower have been
demonstrated. Also, a micropower spectrum analyzer
has been developed to enable low power operation of the
entire WINS system. Finally, the lowest reported power
dissipation CMOS RF oscillator and mixer circuits have
been demonstrated. Recently, complete, prototype
WINS networks have been demonstrated in defense.