  Linear chirp-based underwater acoustic communication has been widely used due
to its reliability and long-range transmission capability. However, unlike the
counterpart chirp technology in wireless -- LoRa, its throughput is severely
limited by the number of modulated chirps in a symbol. The fundamental
challenge lies in the underwater multi-path channel, where the delayed copied
of one symbol may cause inter-symbol and intra-symbol interfere. In this paper,
we present UWLoRa+, a system that realizes the same chirp modulation as LoRa
with higher data rate, and enhances LoRa's design to address the multi-path
challenge via the following designs: a) we replace the linear chirp used by
LoRa with the non-linear chirp to reduce the signal interference range and the
collision probability; b) we design an algorithm that first demodulates each
path and then combines the demodulation results of detected paths; and c) we
replace the Hamming codes used by LoRa with the non-binary LDPC codes to
mitigate the impact of the inevitable collision.Experiment results show that
the new designs improve the bit error rate (BER) by 3x, and the packet error
rate (PER) significantly, compared with the LoRa's naive design. Compared with
an state-of-the-art system for decoding underwater LoRa chirp signal, UWLoRa+
improves the throughput by up to 50 times.
