EE359 Wireless Communications

capatow@stanford.edu

Introduction

Technology has progressed to a point in which humans can be connected and able to send or receive data at almost any point in time. This mobility has enabled significant changes in the type of information used as well as provided. For example, a sensor on your lower back could monitor your posture while providing real time feedback to your smartphone device. While on the go, a utility engineer can ensure that her transmission lines are fault free. Using wireless power in combination with wireless communication, solutions have been provided for building automation, energy management, defense, industrial monitoring, data centers, structural monitoring and more. Regardless of the application, sensor networks consisting of one or more nodes can now wirelessly acquire, process and communicate data that can be used to make integral decisions.

Recent advances in ultra-low power chip design techniques, many originally targeting wireless sensor networks, will enable a new generation of body-worn devices for health monitoring[7]. This means that it may now be possible to monitor and share health updates with your family, doctors or emergency responders. The idea is to initially have a monitoring device on the skin that can at the very least relay information to others in a time of critical need. This device must be location independent, reliable and safe for human contact. Furthermore, it will be required that it is extremely low powered and able to communicate to a base station regardless of the users whereabouts, which could be multiple kilometers away. In the future, this discussion may be extended to include subdermal implants and monitoring devices.

Small, low-cost, wireless sensor nodes are integral for ubiquitous sensing. For most applications, remembering to replace a battery can severely impact the usefulness of a device at all. More specifically, in body monitoring devices, a missed call for emergency response could be fatal. In addition, batteries are bulky and constant replacement means more money. Utilizing ambient energy as a power source will both reduce battery replacement costs and support a long period operation in biosensor networks. These have been major pain points in wireless sensing, which is why we will focus our attention to utilizing Wireless Power Transmission for a reliable, life changing technology.

Background and Brief History

A Wireless Power Transmission system can be defined as one “which efficiently transmits electric power from one point to another through the vacuum of space or the Earth’s atmosphere without the use of wires or any other substance” [5]. One can compare this phenomenon to that of wireless Internet. Before 3G, WiFi and other wireless protocols became popular, people were restricted to a desk wired with either DSL or dial-up Internet. Once wireless Internet became popular, however, it became possible to connect to the Internet without the use of cords or wires, no matter where they were. Well of course, as long as they had a charger near by. We can use this analogy to understand the future of body monitoring systems and the necessity for wireless power.

Many think of wireless technology as a modern discovery, but initial developments of wireless energy transfer can be traced back into the 1800s. “William Sturgeon developed the first electromagnet in 1825, which cleared the way for discovery of the basic principle of electromagnetic induction in 1831. Within just a few years, Nicholas Joseph Callan made use of these two scientific developments to successfully demonstrate that the receipt and transmission of electric energy could take place without the presence of any type of wiring to connect the points of origin and termination” [6]. Over the 19th century there was minimal development in Wireless Power Transmission due to the difficulty of overcoming obstacles such as drastic attenuation over distance or interferences. This then limited the practical applications and popularity of this technology.

Basic Principles

Wireless Power Transmission over a distance can be achieved using either inductive coupling or RF Harvesting. Inductive coupling, is a very popular method of transmitting power wirelessly. A classic example of which is found in nearly every U.S. bathroom: an electric toothbrush. Exposure to water is very common in a bathroom setting, which puts electronic devices at a high risk for damage if penetrated. As a result, designers have utilized inductive coupling to wirelessly transmit power to charge the battery. Inductive coupling is very popular in devices that are very close together because they achieve nearly perfect transmission at a distance of around 1mm. Due to the present limitations and issues associated with inductive couplers, they are generally more practical for exceptionally close distances rather than broadcasting power over longer ranges.

Inductive coupling is a specious method when considering wireless power in the instance of body monitoring on the dermal surface over a large distance. Because wireless sensors are built to work with extremely low power, it is at times possible to power them intermittently using super capacitors or solid-state storage devices instead of batteries. Through RF harvesting, the range can be extended significantly because ambient Wireless Power can be absorbed by a tuned antenna and then converted to DC. This energy will be stored until it is used by the sensors or microcontroller. This concept is displayed in Figure 1. In recent studies, researchers have been able to construct a completely ambient RF-powered prototype which measured temperature and light level and wirelessly transmitted these measurements at a distance of over 4 km[3]. Ideally, using a number of spatially distributed sensor notes over the body would increase the amount and reliability of the sensor data[2].

Wirelessly Powered Node Architecture
Figure 1. Ambient RF-powered Sensor Node Architecture [3]

Frequency Consideration

Frequency selection and interference problems play a large part in an efficient and reliable Wireless Power Transmission system with RF harvesting. In general, the frequency range for RF is between 30 kHz to 300 GHz. The problem with the lower range of the RF spectrum is that older technologies heavily occupy this bandwidth. In addition, natural interferences may have operating frequencies in this range. Therefore, most modern RF applications utilize frequencies from 800 MHz up to and including the optical range. Ideally, there is no transmission loss at these frequencies. However, the conditions of a vacuum do not apply to the real world. In the Earth’s atmosphere, especially, there is significant attenuation at higher frequencies according to weather conditions. Therefore, lower frequencies of ambient power will be more efficiently received. Since we are looking to harvest ambient power, it is not necessary to worry about dedicated transmission bands. In fact, old reliable systems that broadcast over a large area can be taken advantage of for power harvesting.

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]. “[G]overnments restrict the transfer of intense RF signals because they can block other types of radio transmissions; thus electro-magnetic fields are considered as a shared property. On the other hand, radio and TV broadcast signals are designed to cover the entire range of human activities; thus usually transmitted using intense RF signals. RF energy harvesting would be attractive if the harvested energy is sufficient for powering small devices such as widely distributed sensor nodes.”[2]. Although the broadcast power of the stations is on the order of 10-100 kilowatts of effective radiated power, because of the non ideal environment and the distance between the base station and receiver the received power will be in milliwatts to microwatts. In 2010, Nishimoto et al published a Prototype Implementation of Ambient RF Harvesting Wireless Sensor Networks where they exploit the use of UHF and VHF TV broadcasts in Japan to power the nodes. After continuously monitoring the received ambient energy for 7 days, they were able to demonstrate that RF energy could always be harvested. UHF and VHF Readings

Figure 2. Measured UHF and VHF TV and Radio Broadcasts measured in downtown Tokyo 6.5km from Tokyo TV Tower [2]

Rectenna

The rectenna is also a very important variable in the harvesting of RF for sensing applications. A RF receiving antenna is a form of tuned circuit that has its own capacitance and inductance. Therefore, they also have a resonant frequency, or frequency at which the reactive elements cancel out and a purely resistive load is left. The antenna will operate around this resonant point over a limited bandwidth, where even small deviation will cause operational efficiency to drop off quite steeply. Antennas require the use of frequency for sending/receiving information. When receiving a quickly fluctuating signal, the antenna absorbs the electromagnetic energy and converts it into electrical pulses.

The Nishimoto research in Japan tuned a rectenna to several broadcast stations to increase the amount of received power. They found that lower input power yielded lower efficiencies in the rectifier circuit. “RF signals ranging from 15 MHz to 800 MHz can be rectified at an efficiency higher than 50%. When the rectifier circuit is equipped with an antenna, the frequency range will be chosen.”[2]. As a result, it is clear that the rectifier circuit must be optimized in the future and will play a large role in the feasibility of this technology for body nodes. Using Friis equation, one can estimate the captured power of a polarization matched linear antenna to be 10-100’s of microwatts as previously mentioned[4].

In the far field range of the TV broadcasting antenna, the useful power harvested from the incident electric field is a function of the antenna aperture area, and therefore antenna gain. It is also dependent on its conjugate load matching with the RF to the DC charge pump, polarization mismatch and, as mentioned previously, the frequency of the incident wave. “Since TV broadcasts are linear in polarization, linear antennas such as dipoles or monopoles or its arrays offer lower losses due to polarization mismatch and therefore present a larger Antenna aperture area to the incident wireless electric fields.”[4].

Advancements in rectenna design include various array schemes. The most widely used scheme is the rectenna array with full DC power combination. However, significant efficiency improvement is achievable by considering RF summation architectures, seen in Figure 3 [1]. Novel methods have been proposed, but such schemes need the development and implementation of tracking strategies also on the receiver side. Further work to gather incident power at the receiver and make the conversion of RF to DC more efficient will lay the foundation for successful WPT systems.

RF Summation Architecture
Figure 3. RF Summation Architecture [1]

Advancements in Research

Although the Nishimoto researchers were unable to measure the amount of energy harvested from the broadcast signals due to the multipath effect, reflection, shielding objects, etc, they were able to obtain RF at 6.6km from the broadcast station over a 7 day period, seen in Figure 4. In that time, they demonstrated a very consistent energy profile that, in combination with a power management system, could provide constant measurements. They tested the UHF harvesting prototype at a distance of 500m from the broadcast tower. The experimental prototype network consisted of an access point node (AP), an RF energy harvesting powered end node (RFN), and a battery powered end node. The RFN would sense and transmit 3 bytes of data to the access point every 5 seconds. Transmission to the AP is quite costly in terms of power, so the RFN must wait for the capacitor to charge to a suitable level for more transmission.

RF Measurements from Tokyo TV Tower
Figure 4. Energy Profile for RF Harvesting in Tokyo [2]

Multiple ways to optimize RFN power use have been created and expanded upon by Nishimoto and others that have followed. Nishimoto et al offer the Adaptive Duty Cycle Determination Method to enable low overhead power management. The group had to determine the balance of spending too much power on energy management and the potential of the node suffering from energy shortage if it is not checked enough. Therefore, the group would take two voltage measurements and use them to determine the time needed to sufficiently charge the capacitor.

In 2012, Vyas et al were able to verify the previously made power estimates by using a radiation meter in downtown Tokyo at a distance of 6.5km from the Tokyo TV Tower. They found that there was indeed significant wireless activity in the VHF and UHF bands from 480-580MHz. Rather than using many off the shelf components, this group improved upon previous research by addressing the variables that we previously discussed including the “design of an optimized antenna, an RF-to-DC charge pump circuit, a proper power matching between them to minimize reflection losses and a low leakage power management circuitry to keep the embedded end device from draining the charge tank super capacitor till it has reaches a high turn on voltage”[4].

According to Antenna Theory and as discussed in Microwave Circuits, it is known that S-parameters are used to describe the I/O relationship between terminals. This research provided the S11 parameter which is the most commonly used parameter for antennas because it shows how much power is reflected from the antenna. Where S11= 0 dB, all of the power is reflected from the antenna and nothing is radiated. The measured S11 of the antenna in Vyas et al “shows a reflection loss of lower than -15dB from 467MHz to well beyond 580 MHz, ensuring that 97% of the transduced wirelesses TV signal power makes it through to the matched RF to DC converter circuit” as seen in Figure 5. From the same distance they were able to show that harvesting circuit can charge the 100uF charge tank to 2.9V in 3 minutes.

S11 Measurements
Figure 5. Measured Dipole Antenna Return Loss [4]

Research presented in 2013 focused on two main goals: to improve operating sensitivity and minimize operation energy. This system spends most of its time in the charging state. Once suitable charge is reached, it will perform and process one measurement to then transmit. Therefore, the duty cycle is determined by the amount of charge available. Using a discrete full wave rectifier (with Schottky barrier diodes) in conjunction with a low-voltage DC-DC integrated charge pump, the harvester placed 10.4 km from the TV broadcasting station operating at 539 MHz in Seattle was able “to cold start and operate with a cycling period of 3 seconds”[3]. With a fully functional node utilizing a five-stage discrete Dickson RF charge pump implemented with Schottky barrier diodes, sensing and communication was achieved at a distance of 4.2 km at a rate of 1 Hz. Additionally, the most sensitive harvester allowed for operation 200m from a cellular base transceiver operating at 738 MHz. The minimal RF input power required for sensor node operation was -18 dBm or 15.8 uW, which can be used as the threshold for future comparison. Of course, the operating power necessary for the node could also be improved by firmware in addition to the variables described.

Viability of Wirelessly Powered Body Sensor Networks

Wearable and implantable human monitoring devices are inevitable. There are over 100 different solutions currently being researched. The largest obstacle at this point for such devices to be wirelessly powered over a large distance is the size of the transceiving antenna. Therefore, there must be high power sources in the low gigahertz range that would be able to cover the whole spectrum of human movement. Although it is currently viewed as a long shot, current research at Google Labs may be brewing an indirect solution. In the mission of Project Loon to provide internet to the world with over 100,000 balloons providing coverage, this obstacle has potential for resolution. The balloons would provide essentially line of site transmission at a significantly higher power than is currently available in a consumer home. The sensor could then relay information to your mobile device that could call an ambulance, alert your family members and more. Another supplementary approach could include transmitters in a place of residence to ensure quick charge times and further reliability.

Although this paper has only discussed RF Harvesting over a relatively large distance, there are substantial efforts currently being made using WPT for medical purposes at a short distance. In the Poon research group at Stanford, similar possibilities have been turned into reality. From wirelessly powered implantable heart monitors to wireless locomotive devices the move through the blood stream, we have yet to reach the tip of the iceberg.

Finally, many applications for Wireless Power Transmission are a viable option today and should not be neglected. For example, the systems discussed in this paper could certainly be used in earthquake monitoring or smart traffic routing. The possibilities are endless in a research area that has the potential to monitor nearly every facet of life and natural occurrences. For now, we can dream and work towards a brilliant future for Wireless Power Transmission may someday, finally, set us free from the wires that snake around the feet of our desks. Just maybe, the chains of that charger will be broken; an amusing momento of the past.

References

[1]Massa, Andrea, et al. "Array Designs for Long-Distance Wireless Power Transmission: State-of-the-Art and Innovative Solutions." (2013): 1-18.

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

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

[4]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.

[5] William C. Brown. The History of Wireless Power Transmission, Solar Energy Vol. 56, No. 1, Great Britain, Elsevier Science Ltd, 1996, pp 3-21.

[6]What is Wireless Energy Transfer?, wiseGeek [Online]. Available: http://www.wisegeek.com/what-is-wireless-energy-transfer.htm

[7]Zhou, Xun, Rui Zhang, and Chin Keong Ho. "Wireless information and power transfer: Architecture design and rate-energy tradeoff." Global Communications Conference (GLOBECOM), 2012 IEEE. IEEE, 2012.