  In this paper, we propose a novel channel estimation scheme for pulse-shaped
multicarrier systems using smoothness regularization for ultra-reliable
low-latency communication (URLLC). It can be applied to any multicarrier system
with or without linear precoding to estimate challenging doubly-dispersive
channels. A recently proposed modulation scheme using orthogonal precoding is
orthogonal time-frequency and space modulation (OTFS). In OTFS, pilot and data
symbols are placed in delay-Doppler (DD) domain and are jointly precoded to the
time-frequency (TF) domain. On the one hand, such orthogonal precoding
increases the achievable channel estimation accuracy and enables high TF
diversity at the receiver. On the other hand, it introduces leakage effects
which requires extensive leakage suppression when the piloting is jointly
precoded with the data. To avoid this, we propose to precode the data symbols
only, place pilot symbols without precoding into the TF domain, and estimate
the channel coefficients by interpolating smooth functions from the pilot
samples. Furthermore, we present a piloting scheme enabling a smooth control of
the number and position of the pilot symbols. Our numerical results suggest
that the proposed scheme provides accurate channel estimation with reduced
signaling overhead compared to standard estimators using Wiener filtering in
the discrete DD domain.
