2026/07/27 by Ruhul Amin Khalil
#eess.SP
Acoustic backscatter enables low-power Internet of Underwater Things (IoUT) nodes, but localizing passive/semi-passive tags is difficult under weak returns and surface/seabed multipath. This letter proposes a clock-free common-transmitter time-difference-of-arrival (TDoA) framework, in which one anchor interrogates the tag and synchronized receivers measure the backscattered arrivals. Receiver-side differencing cancels the unknown interrogation time, forward anchor-to-tag delay, and tag switching delay. A weighted nonlinear least-squares estimator combines signal-to-noise ratio (SNR), timing variance, and root-mean-square (RMS) delay spread. Simulations against received signal strength indicator (RSSI)-only, unweighted, SNR-weighted, and robust TDoA baselines show that at 20 dB SNR, the root-mean-square error (RMSE) decreases from 4.71 m to 4.13 m, and the 5 m success probability improves from 0.793 to 0.835.