## 5 Conclusions and Perspectives  
In this paper, a mobile MSS UWA communication scheme is proposed. The quasi orthogonality of a pseudo-random sequence and its thumb-tack peak on the Doppler axis is utilized. The received signal is matched symbol by symbol with the reference signals, which have different frequency shifts and spreading code numbers, to find the maximum correlation value. Then we complete the decoding and Doppler estimation of the current symbol. The conventional Doppler compensation is not used in receiving and processing. By utilizing only the large time-bandwidth product of the spreading signals, the symbol-level synchronization is simply carried out after the Doppler estimation. The sea trial results show a satisfying decoding effect. More importantly, on the premise of ensuring the robustness of communication, we optimize the receiving process. A large Doppler search step and some heuristic information are used to reduce the correlation operations. On the premise of ensuring the real-time implementation of hardware, this method obtains good robustness in a variable speed motion scenario.  
Many researchers are using CAF for channel information estimation, and two-dimensional search for Doppler and time delay in a certain range. In fact, since the Doppler sensitive waveform on time delay and Doppler axis has a relatively sharp peak, it is not necessary to traverse all correlation results in the search range. Using some heuristic conditions to narrow the search range expands the application scope of this method. If the threshold is reasonably set, it is easier to find the optimal solution accurately in a shorter time. Although the search domain proposed in this paper is different from the conventional CAF method, the idea is actually universal.  
In some studies, for better Doppler compensation, more effective methods are used to improve the accuracy of Doppler estimation. The pursuit of extreme Doppler estimation accuracy is not necessarily the only solution in mobile UWA communication. Considering the hardware overhead of underwater equipment research and development, this simple and stable system design needs to be paid attention to.  
Conceptualization, G.Y. and F.Z.; methodology, G.Y. and Y.Z.; software, G.Y.; validation, G.Y., Y.Z., Y.L. ([PERSON]) and Y.L. ([PERSON]); formal analysis, G.Y. and F.Z.; investigation, G.Q.; resources, G.Q. and Y.H.; data curation, Y.L. ([PERSON]) and Y.L. ([PERSON]); writing--original draft preparation, G.Y.; writing--review and editing, G.Y., F.Z. and Y.Z.; visualization, G.Y.; supervision, F.Z.; project administration, F.Z.; funding acquisition, F.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number U1806201; supported by Science and Technology on Underwater Information and Control Laboratory, grant number 6142218200410; funded by the State Administration of Defense Science, Technology and Industry, grant number JCKYS2019604 SSJ006; funded by the Ministry of Science and Technology of the People's Republic of China, grant number 2018 YFC0308500.  
Not applicable.  
The authors are grateful to [PERSON] for his technical support. The authors would like to thank the editors and the anonymous reviewers.  
The authors declare no conflict of interest.