  The purpose of the study is to investigate potential benefits of using
Alamouti-like orthogonal space-time-frequency block codes (STFBC) in
distributed multiple-input multiple-output (D-MIMO) systems to increase the
diversity at the UE side when instantaneous channel state information (CSI) is
not available at radio units (RUs). Most of the existing transmission
techniques require instantaneous CSI to form precoders which can only be
realized together with accurate and up-to-date channel knowledge. STFBC can
increase the diversity at UE side without estimating the downlink channel.
Under challenging channel conditions, the network can switch to a robust mode
where a certain data rate is maintained for users even without knowing the
channel coefficients by means of STFBC. In this study, it will be mainly
focused on clustering of RUs and user equipment, where each cluster adopts a
possibly different orthogonal code, so that overall spectral efficiency is
optimized. Potential performance gains over known techniques that can be used
when the channel is not known will be shown and performance gaps to
sophisticated precoders making use of channel estimates will be identified.
