Exploiting spatial correlation at the link layer for event-driven sensor networks

Wireless sensor networks (WSNs) can generally be classified into two categories: timedriven and event-driven. In an event-driven WSN, sensors report their readings only when they detect events. In such behavior, sensors in the event area may suffer from higher contention. In this paper, we solve this problem by jointly considering two subissues. One is exploiting the spatial correlation of data reported by sensors in the event area and the other is designing a specific MAC protocol. We propose a novel hybrid TDMA/CSMA protocol with the following interesting features that differentiate itself from traditional TDMA-based protocols: (1) the TDMA part is based on very loose time synchronization and is triggered by the appearance of events. (2) the slot assignment strategy associated with the TDMA part takes the spatial correlation of sensor data into consideration and thus allows less strict slot allocation than conventional TDMA schemes. Interestingly, by intentionally allowing one-hop neighbors to share the same time slot, the number of slots required is significantly reduced. (3) by intentionally enlarging the slot size, packets will be separated sufficiently in distance to avoid interference after leaving the event area. In addition, we also discuss how to combine our protocol with the LPL (Low Power Listening) technique to achieve energy efficiency. Simulation results show the efficiency of the proposed protocol.