In this paper, we study optimal resource allocation strategies forsimultaneous information and power transfer (SWIPT) focusing on the systemenergy efficiency. We consider two-user multiple access channels in whichenergy harvesting (EH) and information decoding (ID) nodes are spatiallyseparated. We formulate optimization problems that maximize system energyefficiency while taking harvested energy constraints into account. These areconcave-linear fractional problems, and hence Karush-Kuhn-Tucker (KKT)conditions are necessary and sufficient to obtain globally optimal solution.Solving these optimization problems, we provide analytical expressions foroptimal transmit power allocation among the source nodes, and identify thecorresponding energy efficiency. We confirm the theoretical analysis vianumerical results. Furthermore, we also characterize the effect of circuitpower consumption on the system's efficiency as the harvested energy demandvaries.
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