# R&D of an Innovative OFDM Communication Payload for Small-Size AUV
## 3 Results  
In this section, HIP simulation is performed to evaluate the performance of the proposed UWA OFDM payload onboard small-size AUV platform. The frame structure of transmitted packet is identical to Figure 1, the parameters of which are provided in Table 1. Note that the (2,1,7) convolutional coding with an interleaving depth of 7 is adopted to further improve the communication performance, corresponding to the original communication rate and effective communication rate shown in Table 1.  
While the preamble sequence containing ten linear frequency modulation (LFM) signals is inserted before the entire signal frame for preliminary synchronization and Doppler estimation, one LFM signal is located at the beginning of the OFDM data frame for fine timing synchronization to facilitate the demodulation process of OFDM symbols. The setup parameters of LFM signal are consistent with the OFDM data frame. An artificial UWA channel [26] is generated for HIP simulation, which consists of direct path, surface reflected path and surface-bottom-surface reflected path. The corresponding dominant coefficients are set as the magnitude of 0.76, 0.53 and 0.37, respectively. The other coefficients are set to zero to generate the channel impulse response as displayed in Figure 3. The total multipath delay spread is 10 ms, which is consistent with the scale of some field experimental scenes. The signal-to-noise rate (SNR) of 0 dB to 20 dB is generated by mixing the additive white Gaussian noise with the signal at an interval of 5 dB. Furthermore, three typical \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Items** & **Value** & **Items** & **Value** \\ \hline Modulation mode & DQPSK & OFDM symbol duration (ms) & 42.67 \\ Center frequency (Hz) & 25,000 & Length of guard interval (ms) & 10.67 \\ Bandwidth (Hz) & 8000 & Length of LFM signal (ms) & 17.71 \\ Sampling rate (Hz) & 96,000 & FFT points & 4096 \\ Number of sub-carriers & 340 & Number of OFDM symbol & 40 \\ Interval of sub-carrier (Hz) & 23.44 & Original communication rate (kbps) & 6.38 \\ Length of OFDM data frame (ms) & 2133.33 & Effective communication rate(kbps) & 3.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of transmitted packet. Figure 3: The channel impulse response of simulation channel. working scenarios of small-size AUV platform including unpowered drifting with seawater, low-speed motion and high-speed motion are employed to verify the Doppler compensation performance of the proposed UWA OFDM payload, corresponding to three Doppler scales of 10 Hz, 25 Hz and 40 Hz introduced by artificial resampling. The classic time-frequency differential OFDM receiver with the block chart as shown in Figure 2 is adopted for acquisition, Doppler estimation, Doppler compensation and demodulation of the received packets. The results of Doppler estimation and demodulation BER as shown in Figure 4 are used as the fundamental basis to evaluate the feasibility and effectiveness of the designed UWA OFDM payload. The BER is defined as the number of the received error bits divided by the total number of received bits. The Doppler estimation results under different SNR conditions are provided in Figure 4a, from which we can notice that the three scales Doppler curves are independent with respect to different SNRs. While the Doppler scales introduced by artificial resampling are 10 Hz, 25 Hz and 40 Hz, the evaluated Doppler values are 9.49 Hz, 24.41 Hz and 39.33 Hz due to the limitations of estimation accuracy corresponding the Doppler estimation biases of 5.1%, 2.36% and 1.68%, which indicates that the Doppler estimation approach possesses definite robustness. As revealed in Figure 4b, BER curves of the three Doppler scales tend to decrease with the increase in SNR. Under low SNR conditions, the BER curves are close for the three Doppler scales. When the SNR is greater than 10 dB, the BER results with three Doppler scales are reduced to the order of \(10^{-3}\), which demonstrates the feasibility and effectiveness of the designed UWA OFDM payload.