Tham khảo tài liệu 'autonomous underwater vehicles part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Short-Range Underwater Acoustic Communication Networks 189 The impact of changes in range can be seen if the vehicles moved from 100 m to 500 m at wind state 0 m s the optimum signal frequency to maintain highest SNR decreases from 38 kHz to ÍV 28 kHz Figure 9 b . Reduction in signal frequency implies a potential reduction in absolute bandwidth and with that a reduction in data rate which needs to be managed. This will be investigated further in the next sub sections. Figure 10 a and b show the optimum signal frequency verses range up to 500 m for the various parameters temperature and depth within the Thorp and Fisher and Simmons Absorption Loss models as well as the wind in the Ambient Noise model. The optimum frequency decreases with increasing range due to the dominating characteristic of the absorption loss. It can be seen in Figure 10 a that as the range increases there is an increasing deviation between the two models and between the parameters within the Fisher and Simmons model. There is approximately a kHz difference between the models themselves at 500 m and up to 6 kHz when temperature increases are included. When wind is included Figure 10 b there is a dramatic change in optimum signal frequency at very short ranges and this difference reduces substantially over the range shown. This is due to the increasing significance of the Absorption Loss term relative to the constant Ambient Noise term as it is not range dependent which reduces the affect of the Noise term and therefore the wind parameter. In both Figure 10 a and b the Fisher and Simmons model provides higher optimum frequencies due to the more accurate inclusion of the relaxation frequencies of boric acid and magnesium sulphate. a Comparison of Absorption Loss Parameters b Comparison with changes in wind from Ambient Noise Characteristics Fig. 10. Optimum Frequency determined from frequency dependent component of narrowband SNR Channel bandwidth Having established that at different .
