  We propose a novel joint activity, channel, and data estimation (JACDE)
scheme for cell-free multiple-input multiple-output (MIMO) systems compliant
with fifth-generation (5G) new radio (NR) orthogonal frequency-division
multiplexing (OFDM) signaling. The contribution aims to allow significant
overhead reduction of cell-free MIMO systems by enabling grant-free access,
while maintaining moderate throughput per user. To that end, we extend the
conventional MIMO OFDM protocol so as to incorporate activity detection
capability without resorting to spreading informative data symbols, in contrast
with related work which typically relies on signal spreading. Our method
leverages a Bayesian message passing scheme based on Gaussian approximation,
which jointly performs active user detection (AUD), channel estimation (CE),
and multi-user detection (MUD), incorporating also a well-structured
low-coherent pilot design based on frame theory, which mitigates pilot
contamination, and finally complemented with a detector empowered by bilinear
message passing. The efficacy of the resulting JACDE-based grant-free access
scheme without spreading data sequences is demonstrated by simulation results,
which are shown to significantly outperform the current state-of-the-art and
approach the performance of an idealized (genie-aided) scheme in which user
activity and channel coefficients are perfectly known.
