Smartphone-based data collection from wireless sensor networks in an urban environment

• According to the GPS readings falling in each cell, hot cells are found to be close enough to cold cells for cold cell nodes to transmit data in multi-hop manner to a visiting smartphone user traveling in a hot cell.
• We grouped nearby frozen cells and call it an island. Since islands occur adjacent to city roads, multi-hop data collection is feasible from island nodes by smartphone users traveling on city roads.
• If island nodes run a WSN data collection protocol, continuous data collection is a better choice than on-demand data collection regarding to data delivery ratio and latency.
• For data collection by smartphones, to achieve energy-efficiency at the smartphone side, smartphone radio can only be turned on when its user is mobile.
• For conserving energy at islands, island nodes can run various duty cycling algorithms based on smartphone user visits.

Using smartphones as mobile basestations and leveraging human mobility is a promising approach for urban data collection from Wireless Sensor Networks (WSNs). In this paper, we evaluate the feasibility of this approach applying analyses on a city-wide mobility dataset. Our spatial analysis shows that popular locations cluster close to each other and sensor nodes located in rarely visited locations can transmit their data in a few hops to smartphones visiting these popular locations. Our energy-efficiency analysis indicates the feasibility of employing energy-conserving approaches on both smartphones and WSN nodes based on mobility behavior of smartphone users. We evaluated and compared on-demand and continuous data collection protocols on several WSN islands with different size and connectivity regarding to data collection efficiency. Our simulation results show that continuous data collection protocols surpass on-demand data collection protocols in terms of data delivery ratio and latency. We found that data collection protocols run more efficiently in many-connected small islands compared to fewer connected large islands.