**Microwave Power Transmission Based on Retroreflective Beamforming**

Xin Wang and Mingyu Lu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62855

#### **Abstract**

Microwave power transmission has the potential to supply wireless power to portable/ mobile electronic devices over long distances (on the order of meters or even kilome‐ ters) efficiently. Nevertheless, several technical challenges remain to be resolved in order to accomplish practical microwave power transmission systems, including (i) minimiz‐ ing power loss due to microwave propagation, (ii) preventing humans and other electrical systems from exposure to excessive microwave radiation, and (iii) reconfiguring wireless power transmission in reaction to environmental changes (such as physical movements of portable devices) in real time. In this chapter, a microwave power transmission scheme based on retro-reflective beamforming is proposed to address the above challenges. In the retro-reflective beamforming, wireless power transmission is guided by pilot signals. To be specific, one or more than one mobile device(s) broadcast pilot signals to their surroundings, and based on analyzing the pilot signals, a wireless power transmitter delivers focused power beam(s) onto the mobile device(s). Preliminary numerical and experimental results are presented to demonstrate the feasibility of the proposed retroreflective beamforming scheme.

**Keywords:** microwave power transmission, antenna array, retro-reflective beamform‐ ing, pilot signals, microwave power focusing

#### **1. Introduction**

Microwave power transmission has been actively pursued for more than 50 years as one of the possible technologies to deliver electrical power wirelessly [1–3]. The microwave power transmission technique employs propagating electromagnetic waves in the microwave frequency range as the carrier of wireless power. Compared with techniques based on induc‐

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tive coupling [4], microwave powertransmission has the potentialto reach longer distances (on the order of meters or even kilometers). Meanwhile, microwave power transmission enjoys several advantages relative to optical power transmission [5]. First, microwave has better penetration compatibility than optical waves. Second, the conversion efficiency between DC power and microwave poweris usually higherthan that betweenDC power and optical power. Third, microwave beams can be steered straightforwardly through the phase control, where‐ as beam steering without resorting to mechanical motion is much more difficult in the optical regime. In 1960s, Brown [6] successfully demonstrated supplying microwave power from a ground station to a helicopter, which is probably the first impactful demonstration of micro‐ wave power transmission in the history. Since 1970s, a range of research efforts are conducted onusingmicrowavebeamtodeliverpowerfromsatellites totheearth,albeittodateits feasibility is still under evaluation [7]. A case study from 1997 to 2004 is reported in the studies of Lan Sun Luk [8] and Celeste et al. [9] to construct a point-to-point wireless electricity transmission to a small isolated village called Grand-Bassin in France. An antenna array was developed by the University of Colorado in 2008 to harvest 100 mW power from a transmitter 1 meter away [10]. In 2009, the feasibility of using a car-borne power broadcaster to power sensors installed over a bridge is studied in the work of Mascarenas et al. [11]. Today, many researchers are investi‐ gating taking advantage of microwave power transmission to eliminate/relieve the battery life bottleneckofmobile/portable electronicdevices [12].Despiteofits longresearchhistory, several technical challenges remaintoberesolvedbeforemicrowavepowertransmissioncanbeapplied in practice, as elaborated below.

#### (i) Efficiency improvement

In microwave power transmission, power loss is mainly attributed to two factors: microwaveto-DC conversion and microwave propagation. With the development of novel rectennas, the microwave-to-DC conversion loss has been reduced to less than 20% [13, 14]. In order to improve the microwave propagation efficiency, beamforming (i.e., focusing electromagnetic fields in space) is the only effective means. Beamforming can be achieved straightforwardly using highly directive antennas when both the wireless power transmitter and wireless power receiver are stationary [8]; however, it is challenging when multiple mobile/portable devices residing in a large region need wireless power simultaneously. Traditional phased-array beamforming [15] does not constitute an ideal solution, since it fails when the line-of-sight path between the phased array and the target receiver is blocked by obstacles.

#### (ii) Safety assurance

Excessive microwave radiation may produce hazardous effects to human bodies as well as electrical devices. Thus, while wireless power is transmitted to target receivers, it is vital to ensure that humans and other electrical systems in the surroundings are not under intensive microwave illumination. As a matter of fact, a range of regulations have been established by various agencies for microwave radiation to safeguard human safety and electromagnetic compatibility [16–18].

#### (iii) Real-time reconfigurability

tive coupling [4], microwave powertransmission has the potentialto reach longer distances (on the order of meters or even kilometers). Meanwhile, microwave power transmission enjoys several advantages relative to optical power transmission [5]. First, microwave has better penetration compatibility than optical waves. Second, the conversion efficiency between DC power and microwave poweris usually higherthan that betweenDC power and optical power. Third, microwave beams can be steered straightforwardly through the phase control, where‐ as beam steering without resorting to mechanical motion is much more difficult in the optical regime. In 1960s, Brown [6] successfully demonstrated supplying microwave power from a ground station to a helicopter, which is probably the first impactful demonstration of micro‐ wave power transmission in the history. Since 1970s, a range of research efforts are conducted onusingmicrowavebeamtodeliverpowerfromsatellites totheearth,albeittodateits feasibility is still under evaluation [7]. A case study from 1997 to 2004 is reported in the studies of Lan Sun Luk [8] and Celeste et al. [9] to construct a point-to-point wireless electricity transmission to a small isolated village called Grand-Bassin in France. An antenna array was developed by the University of Colorado in 2008 to harvest 100 mW power from a transmitter 1 meter away [10]. In 2009, the feasibility of using a car-borne power broadcaster to power sensors installed over a bridge is studied in the work of Mascarenas et al. [11]. Today, many researchers are investi‐ gating taking advantage of microwave power transmission to eliminate/relieve the battery life bottleneckofmobile/portable electronicdevices [12].Despiteofits longresearchhistory, several technical challenges remaintoberesolvedbeforemicrowavepowertransmissioncanbeapplied

In microwave power transmission, power loss is mainly attributed to two factors: microwaveto-DC conversion and microwave propagation. With the development of novel rectennas, the microwave-to-DC conversion loss has been reduced to less than 20% [13, 14]. In order to improve the microwave propagation efficiency, beamforming (i.e., focusing electromagnetic fields in space) is the only effective means. Beamforming can be achieved straightforwardly using highly directive antennas when both the wireless power transmitter and wireless power receiver are stationary [8]; however, it is challenging when multiple mobile/portable devices residing in a large region need wireless power simultaneously. Traditional phased-array beamforming [15] does not constitute an ideal solution, since it fails when the line-of-sight path

Excessive microwave radiation may produce hazardous effects to human bodies as well as electrical devices. Thus, while wireless power is transmitted to target receivers, it is vital to ensure that humans and other electrical systems in the surroundings are not under intensive microwave illumination. As a matter of fact, a range of regulations have been established by various agencies for microwave radiation to safeguard human safety and electromagnetic

between the phased array and the target receiver is blocked by obstacles.

in practice, as elaborated below.

90 Wireless Power Transfer - Fundamentals and Technologies

(i) Efficiency improvement

(ii) Safety assurance

compatibility [16–18].

A practical microwave power transmission system must be capable of reconfiguring itself in reaction to the environmental changes (such as physical movements of portable devices) in real time, to maintain high efficiency and safety performance.

In order to address the above challenges, a retro-reflective beamforming scheme is proposed in this chapter. In the proposed scheme, one or more than one wireless power receiver(s) broadcast *pilot signals*, a wireless power transmitter receives the pilot signals, and based on analyzing the pilot signals, the wireless power transmitter constructs focused microwave beam(s) to deliver wireless power to the receiver(s). In other words, before transmitting wireless power, the wireless power transmitter plays the role of radar: it tracks the locations of mobile/portable receivers through analyzing pilot signals broadcasted by the receivers. Based on the outcome of radar tracking, the wireless power transmitter constructs spatially dedicated channels to deliver wireless power to the receivers, which minimizes the power loss associated with microwave propagation. As a radar, the wireless power transmitter is able to identify obstacles along the line-of-sight path toward the wireless power receivers such that it could avoid directly illuminating power beams onto humans or other objects. The proposed scheme is highly reconfigurable because wireless power transmission is guided by pilot signals. Specifically, wireless power is always focused onto the locations from which pilot signals stem. As a result, as long as the target receivers broadcast pilot signals periodically, wireless power beams would follow their motions dynamically. A range of numerical and experimental studies have been conducted to verify the feasibility of the proposed retroreflective beamforming scheme [19–21]. The numerical and experimental results demonstrate that the proposed retro-reflective beamforming scheme is able to track mobile receivers' locations and focus wireless power onto the receivers' locations in real time.
