  Current trends in communication system design precipitate a change in the
operating regime from the traditional far-field to the radiating near-field
(Fresnel) region. We investigate the optimal transmit antenna placement for a
multiple-input single-output (MISO) wireless power transfer (WPT) system
designed for a three-dimensional cuboid room under line-of-sight (LoS)
conditions in the Fresnel region. We formulate an optimisation problem for
maximising the received power at the worst possible receiver location by
considering the spherical nature of the electromagnetic (EM) wavefronts in the
Fresnel region while assuming perfect knowledge of the channel at the
transmitter. For the case of two transmit antennas, we derive a closed-form
expression for the optimal positioning of the antennas which is purely
determined by the geometry of the environment. If the room contains locations
where the far-field approximation holds, the proposed positioning is shown to
reduce to the far-field solution. The analytical solution is validated through
simulation. Furthermore, the maximum received power at the locations yielding
the worst performance is quantified and the power gain over the optimal
far-field solution is presented. For the considered cuboid environment, we show
that a distributed antenna system is optimal in the Fresnel region, whereas a
co-located antenna architecture is ideal for the far-field.
