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.