EE359 Wireless Communications

capatow@stanford.edu

In the past, people have looked to solar energy as a power source for outdoor nodes. This requires that each system is equipped with large batteries to maintain power when it is not being charged. For practical low power sensing applications, the cost benefit analysis does not support this alternative especially given the size, mobility, and maintenance requirements. This has led researchers to further investigate wireless power transmission (WPT), which has been possible for decades but still remains in its infancy. We are finally at a point in time where our technical capability will be able to push the envelope for what the future holds for wireless power.

Within wireless power transmission, we will focus on the harvesting of ambient RF energy with the specific application of biological sensors. More specifically, the feasibility of a reasonable solution for biosensors that are wirelessly powered regardless of time and location will be discussed. For example, we want a user to move about their daily life with a relatively continuous, connected, battery free body sensor network.

In RF harvesting systems, power is harnessed from the incident electric field within a given frequency band. Ambient sources of wireless power could range anywhere from WiFi to AM/FM radio and beyond. Recent studies in Seattle and Tokyo used TV, cellular, and radio broadcast towers as sources for ambient RF due to their coverage area and uniform spacing[2,3]. These types of signals are designed to be high power and able to cover the whole spectrum of a persons geographical location. Therefore, the preceding sources can be viewed as nearly perpetual and time-invariant[1]. Meaning that the amount of time that the received ambient power is off is much less than the amount of time on. This allows for the power management system to maintain a constant charge without a battery, and hopefully will provide for the foundation of a body sensor network.

Researchers were not able to obtain a proper estimate for the percentage of energy harvested because of the attenuated power due to multipath effects, shielding, fading etc[1]. However, we will discuss their results demonstrating the ability to receive adequate power from about 6-10km away[2,3]. The receiver height, antenna gains, and other elements of the system will be thoroughly examined and used to determine the feasibility for extended use of autonomous wireless body sensor networks.

[1]Nishimoto, Hiroshi, Y. Kawahara, and T. Asami. "Prototype implementation of ambient RF energy harvesting wireless sensor networks." Sensors, 2010 IEEE. IEEE, 2010.

[2]Parks, Aaron N., et al. "A wireless sensing platform utilizing ambient RF energy." Radio and Wireless Symposium (RWS), 2013 IEEE. IEEE, 2013.

[3]Vyas, Rushi, et al. "A battery-less, energy harvesting device for long range scavenging of wireless power from terrestrial TV broadcasts." Microwave Symposium Digest (MTT), 2012 IEEE MTT-S International. IEEE, 2012.