Maximizing Lifetime per Unit Cost in Wireless Sensor Networks Yunxia Chen Department of Electrical and Computer Engineering University of California, Davis, 95616

Outline Wireless sensor network. Network model. Lifetime per unit cost. Number of sensors and sensor placement. Numerical results. Conclusion. Mar. 18, 2005 EEC 273 Computer Networks

Wireless Sensor Networks Sensors: low-cost, low-power, energy-constrained, limited computation and communication capability. Gateways: powerful. Applications: Transportation monitoring Temperature monitoring …… Gateway nodes Sensor nodes Mar. 18, 2005 EEC 273 Computer Networks

Basic Operation Sensors: Gateways: Monitor certain phenomenon. Report to the gateway nodes. Event-driven: triggered by the event of interest. Demand-driven: triggered by the request from the gateway nodes. Gateways: Collect and process the data from sensors. Ensure end-user can access the data. Mar. 18, 2005 EEC 273 Computer Networks

Sensor Deployment Random deployment Deterministic deployment Battlefield or disaster areas. Generally, more sensors are used to ensure the performance. Deterministic deployment Friendly or accessible environment. Optimal sensor deployment schemes which maximize the lifetime of the network or the coverage of the network. Mar. 18, 2005 EEC 273 Computer Networks

Network Model An event-driven linear wireless sensor network. Each sensor monitors the region between itself and its right neighbor. Generates and sends a packet to its left neighbor when an event occurs. Packets are replayed one after another to the gateway. The event of interest is a Poisson random process. Mar. 18, 2005 EEC 273 Computer Networks

Two Questions How many sensors should we use? How should we place these sensors? Mar. 18, 2005 EEC 273 Computer Networks

Definitions : maximum coverage area of the network. : maximum sensing region of each sensor. : mean arrival rate of the event. : distance to the gateway node . : initial energy of each sensor. : energy required to transmit one packet over 1m. The energy required to transmit one packet over m distance is where is the path loss exponent. : energy required to keep sensors alive. Mar. 18, 2005 EEC 273 Computer Networks

Network Lifetime Sensor lifetime: the amount of time until the sensor runs out of energy. Network lifetime: the amount of time until the first sensor in the network runs out of energy. Given the number of sensors, what is the maximum network lifetime? Mar. 18, 2005 EEC 273 Computer Networks

Motivation Different schemes are developed to maximize the network lifetime with N sensors. Network lifetime can be increased by dividing the sensors into several small groups and enabling one group each time. 4N -> 4T 2N + 2N -> 6T How many sensors should we enable each time? # of sensors Max. Lifetime N T 2N 3T 4N 4T Mar. 18, 2005 EEC 273 Computer Networks

Lifetime per Unit Cost Definition: network lifetime divided by the number of sensors. Characterizes the rate at which the network lifetime increases as the number of sensors increases. Optimal number of sensors in each group = the number of sensors that maximizes the lifetime per unit cost. Mar. 18, 2005 EEC 273 Computer Networks

Greedy Deployment Scheme Intuitively, the network lifetime is maximized when all the sensors run out of energy at the same time. Greedy sensor placement scheme depends on the number of sensors. Maximizing network lifetime = Minimizing the transmission energy [Cheng et. al. 2004]. Maximizing lifetime per unit cost = Minimizing total energy consumption. . Mar. 18, 2005 EEC 273 Computer Networks

Average Energy Consumption The average energy consumption of each sensor per unit time depends on the sensor placement of the network. Mar. 18, 2005 EEC 273 Computer Networks

Problem Formulation Given the coverage area , what is the number of sensors and the corresponding deployment scheme that maximizes the lifetime per unit cost? A multivariate non-linear optimization problem: Mar. 18, 2005 EEC 273 Computer Networks

Numerical Results unit per packet over distance 1 m. All the energy quantities are normalized by . units. . Mar. 18, 2005 EEC 273 Computer Networks

Sensor Placement Mar. 18, 2005 EEC 273 Computer Networks

Lifetime per Unit Cost Mar. 18, 2005 EEC 273 Computer Networks

Optimal Number of Sensors Mar. 18, 2005 EEC 273 Computer Networks

Conclusion We observed that network lifetime can be increased by dividing sensors into small groups and enabling one group each time. We proposed a new performance metric, the lifetime per unit cost. We studied the number of sensors and the sensor deployment scheme that maximizes the lifetime per unit cost. Enable small number of sensors when the mean arrival rate of the event is low or the sensing energy consumption is small. Mar. 18, 2005 EEC 273 Computer Networks

Thanks! Mar. 18, 2005 EEC 273 Computer Networks