as the original router, which we disconnect. We place our
router within a few feet of the original router, with the exact
location determined by user preferences. In all six deployments, we set our router to provide Internet connectivity on
channel 1 and to transmit power packets on channels 1, 6,
and 11 using the algorithm in Section 3. 1.
We log the router’s channel occupancy on each of the
three Wi-Fi channels at a resolution of 60s. Figure 9 plots
the occupancy values for each Wi-Fi channel over the 24-h
deployment duration. We also plot the cumulative occu-
pancy across the channels. The figures show that:
• We see significant variation in per-channel occupancy
across homes. This is because our router uses carrier
sense to enforce fairness with other Wi-Fi networks. It
scales back its transmissions on high occupancy channel
but when the load on neighboring networks is low, the
router occupies a larger fraction of the wireless channel.
• The cumulative occupancy is high over time in all our
home deployments. Specifically, the mean cumulative
occupancies for the six homes are in the 78–127% range.
Although some of these occupancies are much greater
than 100%, once can reduce the rate of the power traffic
based on the cumulative occupancy to ensure that it is
below 100%. Po WiFi does not currently implement this
feature.
• The users in homes 1–4 did not perceive any noticeable
difference in their user experience. The user in home 5,
however, noted a significant improvement in PLTs and
better streaming experience. This was primarily because
home 5 originally was using a cheap low-grade router
with worse specifications. A user in home 6 noted a
slight deterioration in You Tube viewing experience for a
30-min duration. Our analysis showed that our router
operate up to 17ft from the router, with an image capture
every 35 min. On the other hand, the battery-recharging cam-
era has an extended range of 23ft with an image capture
every 34. 5 min in an energy-neutral manner. Both the sensors
have a similar image capture rate up to 15 ft from the router.
We also note that Figure 8b limits the range to 23 ft to focus
on the smaller values. Our experiments, however, show that
the battery-recharging camera can operate up to 26. 5 ft with
an image capture every 2. 6 h.
6. HOME DEPLOYMENT STUDY
In Section 4. 2, we showed that the channel occupancy
of PoWiFi can be optimized for different sensor applications and minimize impact on neighboring Wi-Fi devices.
However, PoWiFi’s ability to efficiently deliver power
depends on the traffic patterns of other Wi-Fi networks in
the vicinity as well as the router’s own client traffic, both of
which can be unpredictable. So, we deploy our system in six
homes in a metropolitan area and measure Po WiFi’s ability
to continuously achieve high channel occupancies.
Table 1 summarizes the number of users, devices, and
other 2. 4 GHz routers nearby in each of our deployments. We
replace the router in each home with a PoWiFi router, and
the occupants use it for normal Internet access for 24 h. Our
router uses the same SSID and authentication information
Table 1. Summary of our home deployment
Home# 123456
Users
21
32
13
Devices
61
64
26
Neighboring APs
17
4
10
15
24
16
Figure 9. Po WiFi channel occupancies in home deployments. We see significant variation in per-channel occupancy values across homes.
This is because Po WiFi uses carrier sense that reduces its occupancy when the neighboring networks are loaded. The cumulative occupancy,
however, is high across time in all home deployments. (a) Home 1, (b) Home 2, (c) Home 3, (d) Home 4, (e) Home 5, and (f) Home 6.
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