  For frequency-division-duplexing (FDD) systems, channel state information
(CSI) should be fed back from the user terminal to the base station. This
feedback overhead becomes problematic as the number of antennas grows. To
alleviate this issue, we propose a flexible CSI compression method using
variational autoencoder (VAE) with an entropy bottleneck structure, which can
support multi-rate and variable-length operation. Numerical study confirms that
the proposed method outperforms the existing CSI compression techniques in
terms of normalized mean squared error.
