A Channel-Aware Adaptive Modem for Underwater Acoustic Communications

Abstract

Acoustic underwater channels are very challenging, because of limited bandwidth, long propagation delays, extended multipath, severe attenuation, rapid time variation and large Doppler shifts. A plethora of underwater communication techniques have been developed for dealing with such a complexity, mostly tailoring specific applications scenarios which can not be considered as one-size-fits-all solutions. Indeed, the design of environment-specific solutions is especially critical for modulations with high spectral efficiency, which are very sensitive to channel characteristics. In this paper, we design and implement a software-defined modem able to dynamically estimate the acoustic channel conditions, tune the parameters of a OFDM modulator as a function of the environment, or switch to a more robust JANUS/FSK modulator in case of harsh propagation conditions. The temporal variability of the channel behavior is summarized in terms of maximum delay spread and Doppler spread. We present a very efficient solution for deriving these parameters and discuss the limit conditions under which the OFDM modulator can work. In such scenarios, we also calibrate the prefix length and the number of sub-carriers for limiting the inter-symbol interference and signal distortions due to the Doppler effect. We validate our estimation and adaptation techniques by using both a custom-made simulator for time-varying underwater channels and the well-known Watermark simulator, as well as real in field experiments. Our results show that, for many practical cases, a dynamic adjustment of the prefix length and number of sub-carriers may enable the utilization of OFDM modulations in underwater communications, while in harsher environments JANUS can be used as a fall-back modulation.

Deep reinforcement learning-based adaptive modulation for OFDM underwater acoustic communication system

“…The first scheme adjusted only the modulation level and evenly distributed power among subcarriers; the second scheme adaptively adjusted the modulation level and power and then gave the effectiveness of UWA link adaptive modulation results through real-time marine experiments for the first time. Mangione [7] and others designed and implemented a software-defined modem, which can dynamically estimate the acoustic channel conditions, adjust the parameters of the OFDM modulator according to the environment, or switch to a more robust JANUS/FSK modulator under harsh propagation conditions.…”

“…, where the modulation mode a i adopted for the ith time slot is selected from the given constellation M system. (7) (3) Immediate reward function: each time slot obtains the timely reward function R t = {r 1 , r 2 , r 3 , . .…”

Review of Underwater Sensing Technologies and Applications

“…Commonly used underwater modulation methods include frequency-shift keying (FSK), phase-shift keying (PSK), and orthogonal frequency-division multiplexing (OFDM). Compared with other modulation methods, OFDM has advantages due to its resilience against frequency selective channels with long delay spreads [ 61 , 62 , 63 , 64 ]. In addition, although underwater acoustic communication has been relatively mature, it also faces some problems.…”

Adaptive Underwater Acoustic OFDM Modulation Method Based on Improved Deep Reinforcement Learning

“…To combat the complexity of underwater acoustic channel, researchers usually establish underwater acoustic communication system according to the worst environment to ensure low bit error rate before obtaining relatively accurate channel information. [3] .…”