  This paper investigates over-the-air computation (AirComp) in the context of
multiple-access time-varying multipath channels. We focus on a scenario where
devices with high mobility transmit their sensing data to a fusion center (FC)
for averaging. To combat the time-varying channel and Doppler effect, each
device adopts orthogonal time frequency space (OTFS) modulation. After signals
are received by the FC, the aggregated data undergoes demodulation and
estimation within the delay-Doppler domain. We leverage the mean squared error
(MSE) as a metric for the computational error of OTFS-based AirComp. We then
derive the optimal transmit power at each device and signal scaling factor at
FC for minimizing MSE. Notably, the performance of OTFS-based AirComp is not
only affected by the noise but also by the inter-symbol interference and
inter-link interference arising from the multipath channel. To counteract the
interference-induced computational errors, we incorporate zero-padding
(ZP)-assisted OTFS into AirComp and propose algorithms for interference
cancellation. Numerical results underscore the enhanced performance of
ZP-assisted OTFS-based AirComp over naive OTFS-based AirComp.
