  As a new candidate waveform for the next generation wireless communications,
orthogonal chirp division multiplexing (OCDM) has attracted growing attention
for its ability to achieve full diversity in uncoded transmission, and its
robustness to narrow-band interference or impulsive noise. Under high mobility
channels with multiple lags and multiple Doppler-shifts (MLMD), the signal
suffers doubly selective (DS) fadings in time and frequency domain, and data
symbols modulated on orthogonal chirps are interfered by each other. To address
the problem of symbol detection of OCDM over MLMD channel, under the assumption
that path attenuation factors, delays, and Doppler shifts of the channel are
available, we first derive the closed-form channel matrix in Fresnel domain,
and then propose a low-complexity method to approximate it as a sparse matrix.
Based on the approximated Fresnel-domain channel, we propose a message passing
(MP) based detector to estimate the transmit symbols iteratively. Finally,
under two MLMD channels (an underspread channel for terrestrial vehicular
communication, and an overspread channel for narrow-band underwater acoustic
communications), Monte Carlo simulation results and analysis are provided to
validate its advantages as a promising detector for OCDM.
