A Two-Dimensional Chirp-MFCSK Modulation Method for Underwater LoRa System

In the design of low-power long-range (LoRa) modulation systems in underwater Internet of Things (IoT) applications, multipath and noise in underwater acoustic (UWA) channels need to be considered. Commonly used LoRa modulation technologies, such as chirp-MFSK and chirp-MCSK, perform data modulation in a single dimension, and cannot provide reliable anti-multipath and anti-noise performance at the same time. This article proposes a two-dimensional modulation method chirp-MFCSK for the LoRa system. This method applies the <inline-formula> <tex-math notation="LaTeX">$M_{2}$ </tex-math></inline-formula>-ary chirp-MCSK subsystem in the <inline-formula> <tex-math notation="LaTeX">$M_{1}$ </tex-math></inline-formula> subbands of the FSK dimension. In specific UWA channels, the chirp-MFCSK modulation obtains a larger processing gain under the premise of suppressing the influence of multipath effects by flexibly adjusting the orders of the two dimensions. Simulation results and field experimental data show that in different UWA channels, the bit error rate performance of the optimal-ordered chirp-MFCSK modulation is several orders of magnitude better than that of the single-dimensional modulation methods.