  Extending network lifetime of battery-operated devices is a key design issue
that allows uninterrupted information exchange among distributive nodes in
wireless sensor networks. Collaborative beamforming (CB) and cooperative
transmission (CT) have recently emerged as new communication techniques that
enable and leverage effective resource sharing among collaborative/cooperative
nodes. In this paper, we seek to maximize the lifetime of sensor networks by
using the new idea that closely located nodes can use CB/CT to reduce the load
or even avoid packet forwarding requests to nodes that have critical battery
life. First, we study the effectiveness of CB/CT to improve the signal strength
at a faraway destination using energy in nearby nodes. Then, a 2D disk case is
analyzed to assess the resulting performance improvement. For general networks,
if information-generation rates are fixed, the new routing problem is
formulated as a linear programming problem; otherwise, the cost for routing is
dynamically adjusted according to the amount of energy remaining and the
effectiveness of CB/CT. From the analysis and simulation results, it is seen
that the proposed schemes can improve the lifetime by about 90% in the 2D disk
network and by about 10% in the general networks, compared to existing schemes.
