  Chirp signals have been exploited extensively in radar and underwater
acoustic (UW-A) communication systems for their robustness to multipath and
superior correlation properties. We present a multiuser multicarrier
chirp-division multiplexing (MU-MCDM) system which orthogonal chirp spread
spectrum are utilized in frequency subcarriers for multiuser applications.
Furthermore, orthogonal chirp transform (OCT) and discrete orthogonal chirp
transform (DOCT) are developed for continuous and discrete implementations of
MCDM systems. In addition, we design a maximum likelihood (ML) receiver capable
of synchronization, channel estimation, and symbol detection. MU-MCDM systems
consider factors, including preamble length, number of subcarriers, portion of
pilots, guard period, and multiuser access scenarios to adapt time-variant UW-A
channels. The bit-error-rate (BER) performance and adaptive transmission are
evaluated in simulation studies. Therefore, we have demonstrated MCDM
modulation systems in simulations that they improve performance of UW-A
communications adaptively and support multiple users. Moreover, MCDM
architectures can be applied in higher order modulations for providing higher
transmission rates in UW-A channels.
