For quick reference, we show the RECAP structure. The channel measurement data was obtained by using a 5 × 5 RECAP array at Alice, with a center feed element (in orange) and 24 other parasitic half-wave dipole antennas spaced λ/4 apart. The 24 parasitic dipoles are reconfigurable elements loaded with electrically tunable capacitances. The active reconfigurable elements (REs) are shown in blue, for different configurations.A random variation in the channel can be generated by changing the capacitive loads at the reconfigurable elements. For the measurements, Bob was equipped with a single dipole antenna.