  Precoding techniques are key to dealing with multiuser interference in the
downlink of cell-free (CF) multiple-input multiple-output systems. However,
these techniques rely on accurate estimates of the channel state information at
the transmitter (CSIT), which is not possible to obtain in practical systems.
As a result, precoders cannot handle interference as expected and the residual
interference substantially degrades the performance of the system. To address
this problem, CF systems require precoders that are robust to CSIT
imperfections. In this paper, we propose novel robust precoding techniques to
mitigate the effects of residual multiuser interference. To this end, we
include a loading term that minimizes the effects of the imperfect CSIT in the
optimization objective. We further derive robust precoders that employ clusters
of users and access points to reduce the computational cost and the signaling
load. Numerical experiments show that the proposed robust minimum mean-square
error (MMSE) precoding techniques outperform the conventional MMSE precoder for
various accuracy levels of CSIT estimates.
