  We experimentally investigate the feasibility of evaluating multiple-input
multiple-output (MIMO) radio equipment under adjustable Rician fading channel
conditions in a programmable-metasurface-stirred (PM-stirred) reverberation
chamber (RC). Whereas within the "smart radio environment" paradigm PMs offer
partial control over the channels to the wireless system, in our use case the
PM emulates the uncontrollable fading. We implement a desired Rician K-factor
by sweeping a suitably sized subset of all meta-atoms through random
configurations. We discover in our setup an upper bound on the accessible
K-factors for which the statistics of the channel coefficient distributions
closely follow the sought-after Rician distribution. We also discover a lower
bound on the accessible K-factors in our setup: there are unstirred paths that
never encounter the PM, and paths that encounter the PM are not fully stirred
because the average of the meta-atoms' accessible polarizability values is not
zero (i.e., the meta-atoms have a non-zero "structural" cross-section). We
corroborate these findings with experiments in an anechoic chamber,
physics-compliant PhysFad simulations with Lorentzian vs "ideal" meta-atoms,
and theoretical analysis. Our work clarifies the scope of applicability of
PM-stirred RCs for MIMO Rician channel emulation, as well as electromagnetic
compatibility test.
