  Cell-free widely distributed massive multiple-input multiple-output (MIMO)
systems utilize radio units spread out over a large geographical area. The
radio signal of a user equipment (UE) is coherently detected by a subset of
radio units (RUs) in the vicinity of the UE and processed jointly at the
nearest baseband processing unit (BPU). This architecture promises two orders
of magnitude less transmit power, spatial focusing at the UE position for high
reliability, and consistent throughput over the coverage area. All these
properties have been investigated so far from a theoretical point of view. To
the best of our knowledge, this work presents the first empirical radio wave
propagation measurements in the form of time-variant channel transfer functions
for a linear, widely distributed antenna array with 32 single antenna RUs
spread out over a range of 46.5 m. The large aperture allows for valuable
insights into the propagation characteristics of cell-free systems. Three
different co-located and widely distributed RU configurations and their
properties in an urban environment are analyzed in terms of time-variant
delay-spread, Doppler spread, path loss and the correlation of the local
scattering function over space. For the development of 6G cell-free massive
MIMO transceiver algorithms, we analyze properties such as channel hardening,
channel aging as well as the signal to interference and noise ratio (SINR). Our
empirical evidence supports the promising claims for widely distributed
cell-free systems.
