  Energy-efficient communication is an important issue in wireless sensor
networks (WSNs) consisting of large number of energy-constrained sensor nodes.
Indeed, sensor nodes have different energy budgets assigned to data
transmission at individual nodes. Therefore, without energy-aware transmission
schemes, energy can deplete from sensor nodes with smaller energy budget faster
than from the rest of the sensor nodes in WSNs. This reduces the coverage area
as well as the lifetime of WSNs. Collaborative beamforming (CB) has been
proposed originally to achieve directional gain, however, it also inherently
distributes the corresponding energy consumption over the collaborative sensor
nodes. In fact, CB can be seen as a physical layer solution (versus the media
access control/network layer solution) to balance the lifetimes of individual
sensor nodes and extend the lifetime of the whole WSN. However, the
introduction of energy-aware CB schemes is critical for extending the WSNs
lifetime.
  In this paper, CB with power allocation (CB-PA) is developed to extend the
lifetime of a cluster of collaborative sensor nodes by balancing the individual
sensor node lifetimes. A novel strategy is proposed to utilize the residual
energy information available at each sensor node. It adjusts the energy
consumption rate at each sensor node while achieving the required average
signal-to-noise ratio (SNR) at the destination. It is a semi-distributed
strategy and it maintains average SNR. Different factors affecting the energy
consumption are studied as well. Simulation results show that CB-PA outperforms
CB with Equal Power Allocation (CB-EPA) in terms of extending the lifetime of a
cluster of collaborative nodes.
