  Minimizing the cost of deploying and operating a Wireless Sensor Network
(WSN) involves deciding how to partition a budget between competing expenses
such as node hardware, energy, and labor. Most commercial network operators
account for interest rates in their budgeting exercises, providing a financial
incentive to defer some costs until a later time. In this paper, we propose a
net present cost (NPC) model for WSN capital and operating expenses that
accounts for interest rates. Our model optimizes the number, size, and spacing
between expenditures in order to minimize the NPC required for the network to
achieve a desired operational lifetime. In general this optimization problem is
non-convex, but if the spacing between expenditures is linearly proportional to
the size of the expenditures, and the number of maintenance cycles is known in
advance, the problem becomes convex and can be solved to global optimality. If
non-deferrable recurring costs are low, then evenly spacing the expenditures
can provide near-optimal results. With the provided models and methods, network
operators can now derive a payment schedule to minimize NPC while accounting
for various operational parameters. The numerical examples show substantial
cost benefits under practical assumptions.
