  Massive antenna arrays form physically large apertures with a beam-focusing
capability, leading to outstanding wireless power transfer (WPT) efficiency
paired with low radiation levels outside the focusing region. However,
leveraging these features requires accurate knowledge of the multipath
propagation channel and overcoming the (Rayleigh) fading channel present in
typical application scenarios. For that, reciprocity-based beamforming is an
optimal solution that estimates the actual channel gains from pilot
transmissions on the uplink. But this solution is unsuitable for passive
backscatter nodes that are not capable of sending any pilots in the initial
access phase. Using measured channel data from an extremely large-scale MIMO
(XL-MIMO) testbed, we compare geometry-based planar wavefront and spherical
wavefront beamformers with a reciprocity-based beamformer, to address this
initial access problem. We also show that we can predict specular multipath
components (SMCs) based only on geometric environment information. We
demonstrate that a transmit power of 1W is sufficient to transfer more than 1mW
of power to a device located at a distance of 12.3m when using a (40x25) array
at 3.8GHz. The geometry-based beamformer exploiting predicted SMCs suffers a
loss of only 2dB compared with perfect channel state information.

Signal Processing