Communications of the ACM

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.