1. 1. Limitations

Given today’s FCC 1 W limit on transmitters in the ISM band that Wi-Fi uses, power over Wi-Fi is limited to low-power sensors and devices and cannot, for example, recharge smartphones that require 5 W. Further, the range of our system is determined by the sensitivity of our harvester hardware, which is built with off-the-shelf components. We believe that an ASIC design would be able to improve the harvester’s sensitivity and double Po WiFi’s power-delivery range. Finally, while our current design uses a single antenna, in principle we can use multiple antennas to focus more power toward a sensor and increase the range, but such optimizations are beyond the scope of this paper.

2. UNDERSTANDING WI-FI POWER

To understand the ability of a Wi-Fi router to deliver power, we run experiments with our organization’s Asus RT-AC68U router and a temperature sensor. The router operates on Channel 5 and is set to transmit 23 dBm power on each of its three 4.04 dBi antennas. The temperature sensor is battery free and uses our RF harvester to draw power from Wi-Fi signals. An RF harvester is a device that converts incoming alternating current (AC) radio signals into direct current (DC). A typical RF harvester consists of two stages: a rectifier that converts the incoming radio signal oscillating at 2. 4 GHz into DC voltage, and a DC–DC converter that boosts this voltage to a higher value. Every sensor or microcontroller requires a minimum voltage to run meaningful operations and the DC–DC converter ensures that these requirements are met. The key limitation in harvesting power is that every DC–DC converter has a minimum input voltage threshold below which it cannot operate. We use the DC–DC converter with the lowest threshold of 300 mV. 12

We place the sensor 10ft from the router for 24h and measure the voltage at the rectifier’s output throughout our experiments. We also capture the packet transmissions from the router using a high-frequency oscilloscope connected through a splitter. Over the tested period, the sensor did not reach the 300 mV threshold. Figure 1 plots both the packet transmissions and the rectifier voltage during a period of peak network utilization. It shows that while the sensor can harvest energy during the Wi-Fi packet transmission, there is no input power during the silent slots. The energy leakages during these periods ensure that the voltage does not cross the 300 mV threshold.

3. Po WiFi

Po WiFi combines two elements: ( 1) a Wi-Fi transmission strategy that delivers power on multiple Wi-Fi channels and ( 2) energy-harvesting hardware that can efficiently harvest from multiple Wi-Fi channels simultaneously. See the companion technical report14 for details on the design of the energy-harvesting hardware.

3. 1. Po WiFi router design

Our key insight is that, at any moment, it is unlikely that all Wi-Fi channels will be occupied. Thus, Po WiFi opportunistically injects power packets across multiple Wi-Fi channels with a goal of maximizing cumulative occupancy.

Specifically, it injects 1500-byte UDP broadcast datagrams with a 100us inter-packet delay at the highest 802.11g bit rate of 54 Mbps on the three nonoverlapping 2. 4 GHz Wi-Fi channels ( 1, 6, and 11). A Po WiFi router enqueues these broadcast packets only when the number of frames in the wireless interface’s transmit queue is below a threshold (five frames). If the queue’s depth is at or above this threshold, then there are already enough power and Wi-Fi client packets in the queue to maximize channel occupancy.

Po WiFi must also provide fairness to traffic from nearby networks. Since the Po WiFi router performs carrier sensing and transmits broadcast packets at the highest 802.11g bit rate, its individual frames are as short and unintrusive as possible. Po WiFi thereby provides better-than-equal-share fairness for transmissions from other networks. The rest of this section describes two techniques that further reduce Po WiFi’s effect on neighboring networks.

Rectifier-aware Po WiFi transmissions. When a PoWiFi transmitter knows a harvester’s electrical characteristics, it can tune its transmission strategy to precisely fit the device’s power requirements. For example, suppose we need to read a temperature sensor once per minute. Po WiFi can modulate its occupancy to deliver energy to the harvester so that the sensor reaches its required voltage of 2. 4 V just in time, minimizing the total channel occupancy subject to this goal and thereby minimizing its effect on other networks.

Empirically modeling rectifier voltage. A rectifier converts incoming Wi-Fi transmissions into DC voltage to charge a storage capacitor. Once the voltage on the capacitor reaches the required threshold (Vth = 2. 4 V for the temperature sensor), a reading occurs. Suppose the average power at the harvester after multipath reflections and attenuation is Pin and the channel occupancy of the Po WiFi router packets is C. To a first approximation, the harvester’s behavior can be modeled as a DC voltage source charging a capacitor through a resistor. The difference, however, is that the approximated resistance value depends on the impedance of the harvester’s diodes, which is a function of Pin and C. We can write the voltage as a function of time as where V0 is the initial voltage, t is the time constant, and Vmax is the maximum achievable voltage. Note that both t and Vmax are functions of Pin and the channel occupancy.

Given the nonlinearities of diodes, it is difficult to obtain closed-form solutions for t (Pin, C) and Vmax(Pin, C). We instead connected the harvester through a cabled setup to a Wi-Fi source with variable input power and channel occupancy and measured the output voltage. We fitted the resulting data with the proposed exponential model to estimate how t and Vmax vary with input power and channel occupancy. The key properties of our model fitting are: ( 1) Vmax is inverse-linearly proportional to the input power and channel occupancy; ( 2) the time constant t is exponentially proportional to the input power and/or the channel occupancy; and ( 3) it takes exponentially more time for the same increment in the voltage at a higher voltage value than at a lower one.