  The challenge for next-generation underwater optical wireless communication
systems is to develop optical transceivers that can operate with low power
consumption by maximizing the transmission capacity according to the
transmission distance between transmitters and receivers. This study proposes
an underwater wireless optical communication (UWOC) system using an optical
transceiver with an optimum transmission rate for the deep sea with near-pure
water properties. As a method for actualizing an optical transceiver with an
optimum transmission rate in a UWOC system, time-domain hybrid pulse amplitude
modulation (PAM) (TDHP) using a transmission rate and distance-adaptive
intensity modulation/direct detection optical transceiver is considered. In the
TDHP method, variable transmission capacity is actualized while changing the
generation ratio of two intensity-modulated signals with different noise
immunities in the time domain. Three different color laser diodes (LDs), red,
blue, and green are used in an underwater channel transmission transceiver that
comprises the LD and a photodiode. The maximum transmission distance while
changing the incidence of PAM 2 and PAM 4 signals that calibrate the TDHP in a
pure transmission line and how the maximum transmission distance changes when
the optical transmitter/receiver spatial optical system is altered from the
optimum conditions are clarified based on numerical calculation and simulation.
To the best knowledge of the authors, there is no other research on data-rate
and distance adaptive UWOC system that applies the TDHP signal with power
optimization between two modulation formats.
