Optimal Transmission Policy In Wireless Sensor Network With Energy Harvesting

In conventional wireless communications, transmissions are limited by energy constraints toprolong operating time for battery-powered devices. An alternative technique that has been appliedto address the problem of finite node lifetime is the use of energy harvesting. Energy harvestingcommunication systems have been widely used in many wireless networking applications as theybring improved lifetime and ease of deployment. A significant feature of these systems is harvestingenergy in data transmission.Firstly, the transmission completion time minimization problem in an energy harvestingtwo-user broadcast channel is studied, and the harvested energy is stored in infinite sized battery.The transmitter has fixed number of packets for each receiver. We first analyze the structuralproperties of the optimal transmission policy via the dual problem of maximizing the departureregion by a fixed time, and prove that the optimal total transmit power has the same structure as inthe single-user channel. We also prove that there exists a cut-off power for the stronger user. If theoptimal total transmit power is lower than this cut-off level, all power is allocated to the strongeruser, and otherwise, all power above this level is allocated to the weaker user. Based on thestructure of the optimal power allocation policy, we developed an iterative algorithm to obtaincut-off power, and obtain the globally optimal off-line transmission policy.Secondly, finite sized battery is considered in two-user broadcast channel, unlike the infinitesized battery, energy may overflow without being utilized for data transmission. Hence, no energyoverflow constraint is needed. By analyzing the structural properties of the optimal transmissionpolicy via the dual problem of maximizing the departure region by a fixed time, and prove that theoptimal total transmit power has the same structure as in the single-user power allocation, which isfound by directional water filling. Due to the finite battery capacity, the energy transfer is limitedby battery capacity. We also prove that there exists a cut-off power for the stronger user. Unlike theinfinite battery capacity, the optimal total power allocation is not monotonically increasing withtime. Based on the structure of the optimal power allocation policy, we developed an iterativealgorithm to obtain the globally optimal off-line transmission policy. Simulation show that themaximum departure region is smaller compared with infinite battery capacity.