# Study of NVIS Channel for USN Protocol Definition in Antarctica
## 4 Frame Protocol Definition  
As we analyzed in the previous chapter, there are several parameters to consider for the definition of the signal frame according to the channel requirements depending on the distances to communicate. The current HF Frame Protocol Standards MIL-STD 188 110C Appendix D and STANAG are designed for both oblique and NVIS transmissions without considering the possibility of receiving the groundwave. These facts make not at all the most efficient way for short range NVIS transmissions, moreover with groundwave presence. As we have seen, the delay spread is a very important parameter to take into account, with very different values between an NVIS channel and an NVIS channel with groundwave presence. For that reason, it is advisable to design a signal frame different for each situation. The current standards make use of narrowband modulations for the transmission of data. These modulations are especially affected by the delay spread producing ISI on several contiguous transmitted symbols. The correction of this type of channel effect is hardware expensive to achieve low \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & \multicolumn{2}{c}{**Skywave**} & \multicolumn{2}{c}{**Skywave + Groundwave**} \\ \hline  & **Maximum** & **Minimum** & **Maximum** & **Minimum** \\ \hline Availability (12 UTC to 00 UTC) & 84\% & 51\% & 100\% & 100\% \\ Delay Spread & 2.75 ms & 0.05 ms & 5.5 ms & 0.05 ms \\ Doppler Spread & 0.1 Hz & 0.088 Hz & 0.05 Hz & 0.062 Hz \\ Doppler Shift & \(-17.5\) Hz & \(-12.5\) Hz & \(-17.5\) Hz & \(-12.5\) Hz \\  & QPSK & 4 FSK & QPSK & 4 FSK \\ Probability for BER \(<2\times 10^{-4}\) & 84.2\% & 57.2\% & 81.1\% & 53.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measured NVIS channel parameters. Figure 15: NVIS with groundwave CDF of QPSK and 4 FSK. BER values. The equalization of narrowband modulations in time domain needs a long frame header of known symbols that decrease the efficiency of the frame, especially for high delay spread values. To assure the correct transmission, we can make use of different techniques to avoid the different effects produced by the channel. First of all, spread spectrum (SS) techniques can be applied to increase the bandwidth of the signal. The use of these methods can make more secure and increase the robustness in front of interferences and noise. However, in this study, we want to maintain the HF standards signal bandwidth. For that purpose, we propose the use of OFDM modulation to ensure a robust communication without equalization in time domain to avoid ISI. The OFDM modulation is based on dividing a wideband modulation on several narrowband modulations. This makes each narrowband modulation to have a flat fading channel even if the channel is frequency selective, avoiding the use of complex equalizations. In our proposal, we use a simple and low cost Zero-Forcing equalizer instead of the standard decision feedback equalizer (DFE). Moreover, we can find different OFDM variations as single carrier-frequency division multiple access (SC-FDMA) or orthogonal frequency division multiple access (OFDMA). These OFDM variations are designed to assign different OFDM subcarriers to users as multiple access techniques. In addition, these variations have some advantages as a low Peak to Average Power Ratio (PAPR). In this study, the communication is based without differentiation of users and we wanted to base our design in the basis of the OFDM modulation to show the advantages of the cyclic prefix in front of the ISI as an alternative to narrowband modulations used on the HF Frame Protocol Standards. Even so, the modulation to be used will be a single carrier-frequency domain equalization (SC-FDE) as it is proposed at [34] by the addition of a DFT on the symbols to be transmitted and an IDFT after demodulating the symbols at reception. For the frame protocol design, there are two different scenarios, depending on the presence of the groundwave we propose two different cyclic prefix values to cancel the ISI [35].