  In this paper, we study optimal resource allocation strategies for
simultaneous information and power transfer (SWIPT) focusing on the system
energy efficiency. We consider two-user multiple access channels in which
energy harvesting (EH) and information decoding (ID) nodes are spatially
separated. We formulate optimization problems that maximize system energy
efficiency while taking harvested energy constraints into account. These are
concave-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 for
optimal transmit power allocation among the source nodes, and identify the
corresponding energy efficiency. We confirm the theoretical analysis via
numerical results. Furthermore, we also characterize the effect of circuit
power consumption on the system's efficiency as the harvested energy demand
varies.
