# Asynchronous Chirp Slope Keying for Underwater Acoustic Communication
## 6 Conclusions and Future Works  
The proposed demodulator for preamble-free Chirp Slope Keying was implemented and the complete signal chain simulated and tested inside a measurement pool in a laboratory scale experiment for transmission rates of 100 bit/s for single band communications, as well as 200 bit/s for dual-band communication. The available bandwidth and symbol length was kept constant. The achieved BER estimated through the bitwise xor operation was simulated to drop below 0.001, i.e., 0.1% for SNR above \(-6\) dB for a TB of 50 in the single band mode and for SNR above 0 dB for a TB of 25 in dual-band mode. The correct detection of packages and the demodulation was successfully implemented, verified and simulated as well. The PER follows the BER with an SNR offset of approximately 1 dB. The simulated channel contained Rayleigh fading and set the SNR through Additive White Gaussian Noise. A model for fitting the simulation results and parameters were disclosed, and required extensions for a more realistic simulation model discussed. The approach removes the necessity of preambles for multi-band communication that consume the limited available time slots. While the achieved data rates are low compared to narrow band communication schemes, the feasibility in a highly reverberating water tank has been shown, where those schemes will tend to exhibit high error rates if the preamble is not found. Equalization techniques can be employed to achieve better bit and packet error rate for lower signal-to-noise ratio, together with the design of specific RAKE devices to mitigate the multi-path characteristic of the underwater environment. We also strongly believe that investigations on the animals' underwater hearing behaviour have to be done in order to better estimate the impact of artificial noise on the underwater environment. Further investigations can include the issues arising from the multiple access to the medium and in situations, where oxygen bubbles coming from the divers equipment are disturbing the channel. Conceptualization, D.J.S. and A.G.; methodology, D.J.S. ; software, D.J.S. ; validation, D.J.S., A.G., W.X. and G.F.; formal analysis, D.J.S. ; investigation, D.J.S. ; resources, D.J.S. ; data curation, D.J.S. ; writing-original draft preparation, D.J.S. ; writing-review and editing, D.J.S., A.G., W.X., G.F. and S.J.R. ; visualization, D.J.S. ; supervision, F.H., J.W., C.S. and S.J.R. ; project administration, F.H., J.W., C.S. and S.J.R. funding acquisition, F.H., J.W., C.S. and S.J.R. All authors have read and agreed to the published version of the manuscript. Figure 11: Plots of the simulated bit error rate (BER) and packet error rate (PER) for both single band (**a**) and dual-band transmission (**b**). The markers indicate each simulated SNR condition, the lines are manually fitted curves. **Funding:** This work has been supported financially under the KMU innovativ initiative of the German Federal Ministry of Education and Research (BMBF), funding number (FKZ) 01 IS16010A \"ULTA\" and 01 IS18011C \"Smart Diver 4.0\", as well as by the state of Baden-Wurttemberg in the framework of the MERLIN project. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available on request from the corresponding author. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.