A system for providing underwater communication using orbital angular momentum (OAM) includes a transmitter that processes input data to be transmitted using pre-coding information based on current transmission channel conditions to maximize data rate based on channel conditions. A receiver receives a transmitted multiplexed OAM optical signal and analyzes the received signal for channel state information. The channel state information is used to determine a set of pre-coding values that allow the transmitter to pre-code the input data to maximize the data rate based on current channel conditions. The pre-coding values are mapped to a codebook entry which identifies the pre-coding values. The codebook entry is transmitted from the receiver to the transmitter. The transmitter uses the received codebook entry to identify pre-coding values used to process subsequent input data to be transmitted in order to enhance data rate across the transmit channel.

Free-space communication systems and methods are provided. The systems include a transmitter that combines multiple sets of radio-frequency-modulated optical carrier frequencies for transmission across free space using multiple transmission apertures. Different sets of signals are filtered to form single sideband signals. The different sets of single sideband signals are then combined to form dense wavelength division multiplexed signals. In addition, combined sets of signals of different polarizations can be combined. A receiver can include a single receive aperture.

This application provides a signal transmission method and apparatus. The method includes: obtaining, by a transmitter side, a first signal with N points; performing signal separation on the first signal with N points, to obtain two groups of signals (for example, a second signal with N points and a third signal with N points); determining four signals with N/2 points based on the two groups of signals obtained through separation, and combining the four signals with N/2 points, to obtain a to-be-sent signal with 3N/2 points; and sending the signal with 3N/2 points to a receiver side, to enable the receiver to restore the first signal with N points from the received signal with 3N/2 points.

A system for providing underwater communication using orbital angular momentum (OAM) includes a transmitter that processes input data to be transmitted using pre-coding information based on current transmission channel conditions to maximize data rate based on channel conditions. A receiver receives a transmitted multiplexed OAM optical signal and analyzes the received signal for channel state information. The channel state information is used to determine a set of pre-coding values that allow the transmitter to pre-code the input data to maximize the data rate based on current channel conditions. The pre-coding values are mapped to a codebook entry which identifies the pre-coding values. The codebook entry is transmitted from the receiver to the transmitter. The transmitter uses the received codebook entry to identify pre-coding values used to process subsequent input data to be transmitted in order to enhance data rate across the transmit channel.