In a large class of sensor network deployments, a small subset of the sensors covering the sensor field is equipped with special communications capability to communicate with operators outside the sensor field. These sensors play the role of gateways for off-field communication in the sense that all communications to- or out of the field is through these nodes, and the other non-gateway nodes are only capable of sensor-to-sensor communication. This design achieves a lower cost by concentrating expensive communication devices in a small subset of nodes. An important problem in designing such gateway-based sensor networks is determining the number of gateway nodes needed, their location in the sensor field, and the automation of the sensor-to-gateway association for off-field communication. These design considerations are addressed in this paper. In determining the number of gateways the tradeoff is between performance and cost. As the number of gateways increases, less traffic load is placed on each gateway and its surrounding nodes, resulting in longer network lifetime and larger off-field aggregate transmission capacity. However, with a larger number of gateways the network may be too costly to deploy as gateway nodes are more expensive than non-gateway sensor nodes. We develop and analyze models that allow us to determine the optimal number of gateways and their location in the sensor field. We also provide initial results with respect to determining the needed number of fusion nodes. While the presence of multiple gateways offers a higher degree of off-field communication reliability, a sensor will need to select one of the gateways at a time for off-field communication. In this paper, we also propose a dynamic sensor-to-gateway association protocol. Based on current energy levels, the distributed protocol dynamically assigns sensors to gateways in such a way that the overall transmission load is balanced among the different gateway regions over the lifetime of the sensor field.
We describe a large sensor field whose mission is to protect coastal
waters by detecting objects like submarines. The system is buoy-based
and distributed over a littoral
area. The opportunities for detection are short and intermittent and
the signal to noise ratio is low. The topology of the field changes
with time due to currents, wind, tides and storms. The field has a
number of gateway nodes that have the capability to transmit
off-field through a satellite, a ship or a plane.
We propose an approach to fusion that includes on-buoy processing,
cooperative processing with nearest neighbors and the potential for
off-field processing. Each stage of processing tries both to minimize
false positive events and to maximize the probability of detection
when an object is present. It also tries to minimize
power used in order to prolong the life of the field.
We analyze the optimal placement of gateway nodes in the field to
minimize power consumption and maximize reliability and probability
of successful off-field transmission. We analyze the duty cycles of
the sensor and gateway nodes to optimize lifetime. We also analyze
the traffic that the field will be expected to handle in order to
support network control and coordination, distributed fusion,
off-field communication (including queries and responses, and
reporting of detection events), forwarding of traffic through
individual sensor nodes toward gateways or for fusion.