energy-aware context capture

A prevalent problem in battery-powered sensing is minimizing power consumption in order to extend the life of the sensor. While the smartphones used in urban sensing have the benefit of a live user able to recharge the device, the inconvenience of frequent charging must be minimized if collecting data is to remain practical for participants. One approach is to dynamically adapt the sampling rate as the energy level of the battery changes. For example, when the battery life drops below a certain threshold, Campaignr can reduce the sampling frequency to conserve power. This may result in less usable data if a minimum frequency is required to observe a particular event. However, this approach will allow for an extended sampling duration, which may provide data that could otherwise not have been captured.  The trade-off between duration and frequency of sampling should be considered on a per-campaign basis when configuring this feature.

Another area of significant power consumption is radio use.  In order to more efficiently make use of the radios, I propose to have Campaignr detect and use the lowest-power-per-bit connection that is available.  This will require investigation into power consumption of the different radios on the phone.  Power consumption may also increase when the connectivity of the network is poor.  This can result in the retransmission of much of the data, thereby draining energy that could otherwise be used for successful transmissions when the signal strength is better.  Campainr should also detect when the transmission quality is poor or signal strength is very weak.  When this is the case, Campaignr will cease network activity until conditions improve.

An approach that would alleviate all power consumption due to network radio usage is to batch the upload process when the phone is plugged into a power source.  While this will extend the battery life considerably, it is an inconvenience that could deter many users from participation.  Campaignr will take a hybrid approach to this solution in order to minimize in convenience, while still extending battery life.  Different upload strategies can be invoked as a function of battery life.  For example, if the battery is running low, the upload frequency can be reduced significantly in order to allow for a longer sensing duration.  On the other hand, if Campaignr senses the phone is plugged into a power source, it should attempt to upload at the maximum rate possible, taking advantage of the time when the device functionality is not constrained by battery life.