||In this thesis, we consider AF cooperative networks with multiple radio frequency energy-harvesting (RFEH) relays, and investigate power allocation and relay selection strategies to prolong the network lifetime since relays are battery-powered with limited energy. Through cooperation among multiple relays, reliability and transmission rate of cooperative networks are enhanced by exploiting spatial diversity. If relays rely on wireline power supplies, implementation cost must increase and mobility is lacked. Therefore, we consider relays apply RFEH technology to harvest energy by transferring ambient RF signal into electrical energy, so that they can be self-sustained. Consequently, the network lifetime can be further prolonged. In this thesis, we investigate relay selection strategies to choose one relay to forward messages while the others perform energy harvesting. Those selection strategies |
aim to minimize the loss of energy dissipation, reducing wasted energy and increase the average harvested energy, in order to prolong the network lifetime. In this thesis, we define network lifetime based on the outage performance of the network. Specifically, if outage probability of network is lower than a threshold, the system is treated as "well-functioned", and the network lifetime is defined as the longest period in which network is well-functioned. From the expression of outage probability, network lifetime is related to the residual energy of relays.Compares with existing relay selection strategies which take Channel State Information(CSI),Residual Energy Information(REI), and/or Harvested Energy Information(HEI) into account, we proposed a novel relay selection scheme based on opportunity cost of each choice. Because the proposed scheme properly exploits CSI, REI, and HEI, the proposed scheme outperforms others with stringent outage requirement. Moreover, since all relay selection strategies require simply local CSI, REI, and HEI, they can be accomplished in a distributed manner through opportunistic carrier sensing without extra overhead.