# Study of NVIS Channel for USN Protocol Definition in Antarctica
## 4 Frame Protocol Definition
In the case of an NVIS without groundwave communication, first of all, we took into account the maximum delay spread measured in Figure 9. This is the key parameter while designing the OFDM to avoid the ISI effects produced by multipath. For this configuration, we set the cyclic prefix to 2.75 ms leaving a margin of 0.25 ms to ensure the correct use of it. The duration of the useful symbol length is a key issue because it determines the number of subcarriers that will be allocated in a 3 kHz band. For a low power transmission, we cannot have too many multiples subcarriers because the energy per bit to the spectral noise (E\({}_{\text{b}}\)/N\({}_{0}\)) received of a single subcarrier will be very low. Moreover, the number of subcarriers has to be consistent with the number of equalization pilots to be positioned in the most efficient way. On the other hand, the minimum number of subcarriers is set to achieve the bitrate requirements. It is also important to obtain an OFDM symbol time nearly multiple to the time data block of the standards at 3 kHz of bandwidth. Bearing all that in mind, we set the useful symbol length to 9.33 ms to configure the OFDM with 28 subcarriers, 7 OFDM symbols and a total data block time of 86.31 ms. To allow a better comparison, the number of OFDM symbols has been set to obtain the most similar data block time with the HF data transmission frames. In addition, the minimum measured coherence time of the channel is 10 s, as we can see both frame structures accomplish the coherence time. According to the accomplishment of the coherence time measured in all the data block, only one OFDM symbol will be set as pilots to estimate the frequency variations. In the other side, the bandwidth of each subcarrier is about 107 Hz to assure the accomplishment of the 363.64 Hz coherence bandwidth of the measured channel and a total of 27 subcarriers will be used as data subcarriers. Finally, as shown in Figure 14, we tested the robustness between the QPSK and the 4 FSK, and the results showed the QPSK as the best modulation for the data subcarriers of the OFDM. Also, we can contrast our results [14], where the QPSK shows to be a robust option for low power transmissions. Taking into account the DC null and the equalization pilots, the useful bitrate of the OFDM configuration is about 1.741 Kpps. In Table 2 is summarized the OFDM configuration for an NVIS communication without groundwave. For a better comprehension of the designed data block, in Figure 16, we can observe the OFDM matrix in a time and frequency domain. In the case of an NVIS with groundwave communication, as the previous configuration, we took into account the maximum delay spread measured in Figure 10 to avoid the ISI effects produced by the multipath. First of all, we configured the cyclic prefix to 5.75 ms leaving a margin of 0.25 ms to ensure the correct use of it. To maintain a similar block data time with the HF Frame Protocol Standards, we set the useful symbol length to 12.15 ms to configure the OFDM with 36 subcarriers, 5 OFDM symbols and a total data block time of 89.5 ms. According to the accomplishment of the coherence time measured in all the data blocks, only one OFDM symbol will be set as pilot to estimate the frequency variations. To be consistent with the coherence bandwidth measured of 181 Hz, the bandwidth of each subcarrier is set to 82.3 Hz increasing the number of subcarriers to 36. Finally, as we measured at Figure 15, the QPSK modulation is more robust in front of the 4 FSK for this scenario. As the previous OFDM configuration, we modulate the data subcarriers with a QPSK due to its robustness for low power transmissions. Taking into account the DC null and the pilots to equalize, the useful bitrate of the OFDM configuration is about 1.130 Kbps. The bitrate for this configuration has decreased because of the increase of the cyclic prefix, the useful symbol length and the number of pilots. In Table 3, the configuration is summarized to avoid the delay spread in an NVIS with groundwave communication. For a better