Adaptive time varying doppler shift compensation algorithm for OFDM-based underwater acoustic communication systems

This paper presents an adaptive approach to address the two main problems associated with the time varying Doppler shift, the first being the acceleration effects on the cyclic-prefix (CP) correlation and the second, the effect of a sudden change in the velocity direction between packets on the entire orthogonal frequency division multiplexing (OFDM) symbols. In addition, this paper considers the residual Doppler shift or carrier frequency offset (CFO) that was estimated iteratively within a range according to a design based on the sub-carrier spacing using pilots, which are basically utilized for the purpose of channel estimation. Furthermore, the proposed algorithm adopts three estimations of the symbol timing offset. These estimations are centroid-based localization over an anticipated CP window, first order expectation and autocorrelation of the received CP with its replica. Subsequently, a penalization algorithm is applied in order to drop the anomalous parameter among them. Therefore, the consequences of the inflection point that accompanies the abrupt change in the velocity are mitigated and a reliable time varying Doppler shift is obtained. This Doppler shift is fine tuned in an iterative manner. Compared with the block-based Doppler compensation approach, the proposed technique works with variable speed during packet duration. In addition, it exploits the available bandwidth more efficiently by utilizing a single preamble of linear-frequency-modulation (LFM) to detect the start of the packet. The proposed receiver was evaluated through simulations and sea trials conducted over 500 m and 1000 m channel ranges. In simulations, a model was designed to imitate the time varying Doppler shift with two scenarios (expansion/compression) in combination with a various multipath delay spread. The simulation results confirm that the proposed system accommodates an acceleration up to ± 1  m/s2 during the packet time.