# Study of NVIS Channel for USN Protocol Definition in Antarctica
## 4 Frame Protocol Definition
\begin{table}
\begin{tabular}{c c} \hline \hline
**Bandwidth** & **3 kHz** \\ \hline Useful symbol length & T\({}_{\text{S}}\) = 9.33 ms \\ Prefix cyclic length & T\({}_{\text{CP}}\) = 3 ms \\ Number of subcarriers & N\({}_{\text{SC}}\) = 28 \\ Number of symbols OFDM & N\({}_{\text{SOFDM}}\) = 7 \\ Number of pilot OFDM symbols & N\({}_{\text{SP}}\) = 1 \\ Number of data OFDM symbols & N\({}_{\text{SD}}\) = 6 \\ Number of subcarriers DC NULL & N\({}_{\text{DC}}\) = 1 \\ Time duration of OFDM packet & N\({}_{\text{PT}}\) = 86.31 ms \\ Bits in packet & Bits = 324 bits \\ Modulation & QPSK \\ Equalization & Zero forcing \\ Bitrate of signal frame & 2.139 Kbps \\ \hline \hline \end{tabular}
\end{table} 
Table 2: OFDM configuration for NVIS transmission. Figure 16: OFDM matrix configuration for NVIS transmission. comprehension of the designed data block, in Figure 17, we can observe the OFDM matrix in a time and frequency domain. Once the OFDM data block of the new frame protocol is designed, it is important to take into account some preamble to avoid effects as Doppler shift and time synchronization. The preamble time required in the HF standards frames is about 34.33 ms. For a better comparison, we will take the same preamble time. First of all, the measured critical Doppler shift of Figure 13 is about \(-17.5\) Hz. To avoid critical problems of frequency offset on the reception of the frame protocol and with the OFDM subcarriers, we will consider \(-20\) Hz and we will introduce a 600 Hz tone at the preamble of all data blocks to measure the Doppler shift as we mentioned at Section 2.2. Taking 2.5 Hz of margin, in the worst case, we will receive a 580 Hz tone which can be correctly measured with 17 cycles being a total time of 29.31 ms. The estimation of the frequency offset can be corrected with  
Figure 17: OFDM matrix configuration for NVIS with groundwave transmission. \begin{table}
\begin{tabular}{c c} \hline \hline
**Bandwidth** & **3 kHz** \\ \hline Useful symbol length & T\({}_{\text{S}}\) = 12.15 ms \\ Prefix cyclic length & T\({}_{\text{CP}}\) = 5.75 ms \\ Number of subcarriers & N\({}_{\text{SC}}\) = 36 \\ Number of symbols OFDM & N\({}_{\text{SOFDM}}\) = 5 \\ Number of pilot OFDM symbols & N\({}_{\text{SP}}\) = 1 \\ Number of data OFDM symbols & N\({}_{\text{SD}}\) = 4 \\ Number of subcarriers DC NULL & N\({}_{\text{DC}}\) = 1 \\ Time duration of OFDM packet & N\({}_{\text{PT}}\) = 89.5 ms \\ Bits in packet & Bits = 280 bits \\ Equalization & Zero forcing \\ Modulation & QPSK \\ Bitrate of signal frame & 1.810 Kbps \\ \hline \hline \end{tabular}
\end{table} 
Table 3: OFDM configuration for NVIS with groundwave transmission. an error of 0.01 Hz, being enough to assure the correct reception and demodulation of the OFDM subcarriers. Furthermore, to assure the time synchronization of the system with the data blocks received, the preamble will include 5.12 ms of a 6 th order PN sequence with a resampling of 8 in order to not be affected by the delay spread, the Doppler spread and Doppler shift. Furthermore, every OFDM symbol takes 12.33 ms and 17.9 ms, with 1233 samples and 1790 samples respectively, which is enough to avoid time synchronization errors. Finally, it is important to take into account for the amplifier used to transmit the frame, the PAPR produced at the OFDM modulation. To avoid the PAPR, we can apply different techniques, such as clipping, coding, partial transmit sequence (PTS), and selected mapping (SLM). We have chosen the clipping technique, with a controlled input back-off (IBO) as it is the less complex method. The IBO is defined as the difference between the squared clipped power and the signal mean power without generating a non-linear distortion as we can see at [35]. In Figure 18, we can observe the designed frame protocol for USN NVIS systems. The OFDM data block can be adapted depending on the communication scenario.