An ICI Canceling Underwater OFDM Communication System with 2-Step Modified Delay and Doppler Profiler

In 2023, the first prototype of an Inter-Carrier-Interference (ICI) Canceling Underwater Orthogonal Frequency Division Multiplexing (OFDM) Communication system was proposed using modified Delay and Doppler Profiler (mDDP). However, the Bit Error Rate (BER) reduction was limited. As a result of continuous research, we found that the two-stage configuration of mDDP can dramatically improve BER reduction performance. This paper presents the results of pool experiments of OFDM communications in forward and reverse two-wave Doppler-shift environments. To mitigate the ICI effect, a modified Delay and Doppler Profiler (mDDP), which estimates not only attenuation, relative delay and Doppler shift but also sampling clock shift of each multi-path component, is utilized. After the 1st stage mDDP, the effect of ICI has been reduced from the Channel Transfer Function (CTF) and the more accurate CTF is processed by the second-stage mDDP, resulting in highly accurate channel parameter estimation and improved ICI cancellation in the final multi-tap equalizer. To verify this effect, two transmit transducers and one receive transducer were used in the pool experiment to create a two-wave inverse Doppler-shifted multipath environment. By increasing the number of equalizer taps, the BER reduction performance of 64QAM modulation was improved by approximately one order of magnitude for 2step mDDP compared to 1step mDDP.