The LPS RF wireless program is actively engaged in several projects to realize wireless networks. Development efforts range from utilizing COTS parts to build ultra small form factor wireless nodes to a collaborative effort with the University of Maryland called the SMARTDUST program to design our own ultra low power wireless nodes on the order of a cm3 using CMOS technology. In addition, we are currently exploring RF and vibration power harvesting, 3D integration, low power RF circuitry, high density battery technologies and compact antenna design.
RF Power Harvesting
The RF subsection is one of the most energy consuming elements of a wireless sensor node. To reduce this energy consumption, we are designing a transceiver using the CMOS process given the defining characteristics of a wireless sensor node:
- • Low data rate
- • Short bursts
- • Low voltage
- • Low battery capacity
- • Long lifetime
- • Low cost
- • 10 –20 m (51 dB loss) range
Typical low power transceivers require on the order of 50 mW of power to operate and usually require at least a 1.8 V power source. The designs under development at the Laboratory for Physical Sciences require only 1 mW of power from a 1 V source and still provide up to -90 dBm sensitivity.
Figure 5 - OOK Receiver Topology
Figure 6 – Receiver chip layout
utilizing IBM 8RF DM process.
To meet the performance goal of 1 mW operation for the receiver circuit, techniques must be explored for lowering power requirements. Despite frequency limitations (Fig 1), the utilization of weak inversion is a promising approach that could significantly reduce power requirements.
Figure 1 - Frequency vs inversion coefficient at selected current gains for IBM .13 um process.
Figure 2 - Gain to supply current ratio versus bias voltage for common source LNA design. Used to determine optimal operating point for LNA
RF POWER HARVESTING
Given a radiated frequency of 915 MHz and a simple quarter wavelength antenna, .5 uW.cm2 equates to approximately 60 uW of received power. Fundamentally, we have 60 uW of power to recharge our battery or power our system from the ambient radiated power of an FM signal. Similar power densities can be detected at higher frequencies (including cellular bands) both domestically and internationally. The complexity lies in the conversion from AC to DC and delivering this power to our system at the correct voltage.
The environment is full of radiated emissions. Even at a kilometer away from an FM radio tower, indoor power densities better than .5 uW/cm2 can be detected on average.
The Laboratory for Physical Sciences is actively researching novel circuit designs that can improve the efficiency of RF energy harvesting systems. Systems utilizing these improvements are currently under development to successfully recharge batteries on the ground from ambient RF energy sources such as the cellular network and broadcast television/radio.
Figure - Layout of RF power harvesting circuits with efficiency improvements
ANTENNA MODELLING CAPABILITY
The RF Wireless Group is currently building a novel antenna design capability. This capability will be used for the design and testing of novel antenna structures, as well, as to determine environmental affects on antenna design and placement on existing communication systems.
Figure - Antenna Gain pattern from ultra thin transceiver using CADFEKO modeling software