Commit 5726b930 authored by Tran Huy Vu's avatar Tran Huy Vu

Update discussion (expand the XXX)

parent 8a0c6231
......@@ -7,7 +7,7 @@ Our research results, using an admittedly `proof-of-concept' wearable platform,
\textbf{Multi-AP Operation:} Our controlled user studies have been performed using a single WiFi AP. In a practical campus or factory environment, multiple APs are likely to `cover' a specific location (from measurements, the typical number of APs overhead at a location in our campus buildings is 5-6). This opens up additional possibilities. First, the presence of multiple APs can lead to an additive increase in the harvested energy, as a single client device can receive RF energy from multiple transmitters. On the other hand, as suggested by Figure~\ref{fig:edev2beam}, beams on the same channel may also interfere destructively. However, this will also require enhancements to the RF harvester module on the wearable (similar to the design innovations in~\cite{talla2015powering}), to allow the harvester to simultaneously have multiple resonant frequencies, each corresponding to a different AP's operating frequency. The benefits of such added harvested energy vs. additional receiver complexity are unclear and require further investigation. Second, our current WARP-based AP implementation focused only on power packet transmissions. In practice, each AP will have to perform CSMA-based channel access to avoid contentions with the data traffic transmissions. Moreover, if RF charging may be viewed as a secondary benefit of WiFi APs, it is important to adjust the schedule \& duty cycle of power packet transmissions (e.g., by using multiple virtual queues similar to NAPman~\cite{rozner2010}) to ensure that they do not cause unacceptable loss or latency of data packets.
\textbf{Additional \& Improved Energy Harvesting:} One of our key contributions is to demonstrate that, with appropriate beamforming, standards-compliant WiFi transmissions can provide harvestable power levels of O(100$\mu$W) at reasonable distances (2-3m). In contrast, as an exemplary alternative,
ambient light energy (indoors) is projected to provide~\cite{yildiz2007} a harvestable power density of 100$\mu$W/cm$^2$, implying a harvesting capability of roughly around 1200$\mu$W for a typical $12cm^2$ smartwatch surface. It is entirely possible that, depending on the use case, wearables may combine WiFi/RF harvesting with other other ``traditional'' harvestable energy alternatives, such as ambient light and vibrations. The attractiveness of the \name prototype comes from our belief that WiFi is more pervasive and that WiFi-based harvesting can occur continuously (e.g., at night, in a dark room). In addition, our wearable prototype uses a straightforward XXX\am{Vu:say a couple of words about the antenna} antenna for energy harvesting. It is very likely that alternative antenna form factors (e.g., a metallic strip-based ``patch antenna, illustrated in Figure~\ref{fig:patchantenna}) can increase the harvested energy significantly (see~\cite{chen2013})--the development of suitable design forms for such RF-powered wearables remains an open question.
ambient light energy (indoors) is projected to provide~\cite{yildiz2007} a harvestable power density of 100$\mu$W/cm$^2$, implying a harvesting capability of roughly around 1200$\mu$W for a typical $12cm^2$ smartwatch surface. It is entirely possible that, depending on the use case, wearables may combine WiFi/RF harvesting with other other ``traditional'' harvestable energy alternatives, such as ambient light and vibrations. The attractiveness of the \name prototype comes from our belief that WiFi is more pervasive and that WiFi-based harvesting can occur continuously (e.g., at night, in a dark room). In addition, our wearable prototype uses a straightforward whip antenna for energy harvesting. The antenna has a gain of 2.1 dBi, but it's performance is poor in the vertical direction. It is very likely that alternative antenna form factors (e.g., a metallic strip-based ``patch antenna, illustrated in Figure~\ref{fig:patchantenna}) can increase the harvested energy significantly (see~\cite{chen2013})--the development of suitable design forms for such RF-powered wearables remains an open question.
\begin{figure}[!h]
\centering
......
......@@ -70,7 +70,7 @@ Figure~\ref{fig:residual2} shows the results of this experiment. We see that, i
We next study how the residual average power varies with a change in the WARP transmitter's duty cycle (varied between 20-100\%). At present, the WARP transmitter does not implement any CSMA/CA mechanism. Accordingly, a duty cycle of 100\% would imply that the AP was constantly transmitting `power packets', without leaving any opportunity for data packet transmission. Experimenting with smaller duty cycles allows us to mimic the case of a realistic WiFi AP, where such power packet transmissions occur only intermittently, only when the channel is free from other data packet transmissions.
Figure~\ref{fig:dutycycle} plots the residual energy (computed, as before, from the voltage change in the wearable's supercapacitor) as function of the duty cycle. As expected, the average differential power is approximately linear with the duty cycle. More interestingly, even with a duty cycle of 20\%, the differential power is positive ($\sim 50 \mu$W), indicating that the harvested energy is more than sufficient to operate the entire sensing device. From our prior studies~\cite{XXX}, we know that the average WiFi AP utilization in our campus deployment is below 20\%. These results thus strongly suggest that, in a well-engineered WiFi deployment, it may be possible to support the coexistence of useful WiFi RF energy harvesting with regular WiFi communication.
Figure~\ref{fig:dutycycle} plots the residual energy (computed, as before, from the voltage change in the wearable's supercapacitor) as function of the duty cycle. As expected, the average differential power is approximately linear with the duty cycle. More interestingly, even with a duty cycle of 20\%, the differential power is positive ($\sim 50 \mu$W), indicating that the harvested energy is more than sufficient to operate the entire sensing device. From our prior studies~\cite{XXX}\vt{which paper}, we know that the average WiFi AP utilization in our campus deployment is below 20\%. These results thus strongly suggest that, in a well-engineered WiFi deployment, it may be possible to support the coexistence of useful WiFi RF energy harvesting with regular WiFi communication.
\subsection{Effect of Number of Antennas}
\begin{figure}
......@@ -93,26 +93,26 @@ Figures~\ref{fig:multiplexclose},~\ref{fig:multiplexfar} and~\ref{fig:edev2beam}
\begin{figure*}[!tbh]
\centering
\begin{minipage}{.25\textwidth}
\begin{minipage}{.33\textwidth}
\centering
\includegraphics[height=1.6in, width=2.2in]{multiplexclose.pdf}
\includegraphics[height=1.6in, width=2.1in]{multiplexclose.pdf}
\caption{Time-multiplexed (30cm)}
\label{fig:multiplexclose}
\end{minipage}%
\begin{minipage}{.25\textwidth}
\begin{minipage}{.33\textwidth}
\centering
\includegraphics[height=1.6in, width=2.2in]{multiplexseparated.pdf}
\includegraphics[height=1.6in, width=2.1in]{multiplexseparated.pdf}
\caption{Time-multiplexed (1.7m)}
\label{fig:multiplexfar}
\end{minipage}%
\begin{minipage}{.25\textwidth}
\begin{minipage}{.33\textwidth}
\centering
\includegraphics[height=1.6in,width=2.2 in]{2device2beams.pdf}
\includegraphics[height=1.6in,width=2.1 in]{2device2beams.pdf}
\caption{Concurrent Charging}
\label{fig:edev2beam}
\end{minipage}
\end{figure*}
\am{Someone pls. format the captions for Figures 12-14}
%\am{Someone pls. format the captions for Figures 12-14}
% We continue to evaluate our system to support multiple device concurrently. We placed 2 devices on the table with two different spacing of 30cm and 1.7m respectively. We then run the beamforming algorithm with two operational mode: time multiplexing and concurrent beams.
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