  In this work, we propose two methods that utilize data symbols in addition to
pilot symbols for improved channel estimation quality in a multi-user system,
so-called semi-blind channel estimation. To this end, a subspace is estimated
based on all received symbols and utilized to improve the estimation quality of
a Gaussian mixture model-based channel estimator, which solely uses pilot
symbols for channel estimation. Both of the proposed approaches allow for
parallelization. Even the precomputation of estimation filters, which is
beneficial in terms of computational complexity, is enabled by one of the
proposed methods. Numerical simulations for real channel measurement data
available to us show that the proposed methods outperform the studied
state-of-the-art channel estimators.
