# Study of NVIS Channel for USN Protocol Definition in Antarctica
## 2 Sounding System
### Frame Design  
To measure the main channel characteristics, we transmit 150 packets made up of a 600 Hz tone, a 6 th order PN sequence and a 250 symbols modulation of 4 th order phase shift keying (PSK) and Frequency Shift Keying (FSK) as we can see in Figure 4. All the packets have to be carefully designed to be consistent with the Doppler spread and the delay spread of the channel. In particular, each 600 Hz tone has a time duration of 60 ms, the PN sequence a duration of 5 ms and the modulation a duration of 105 ms. All 150 packets in the transmitted frame have a duration of 25.5 s. To estimate the effects of the channel on the modulations, we transmit a different 4 th order modulation in each packet. The two packets have a total duration of 340 ms, which is less than the expected coherence time of the channel [23]. The tests were performed 24 h a day every 10 min between the 10 and 22 of February of 2019.  
Figure 4: Frame design.  
First of all, the 600 Hz tone is used for analyzing and correcting the Doppler shift effect on the received frame. This frequency shift is not mainly produced by the ionosphere, but it is caused by the low stability of the clocks of the Red Pitaya platform. This makes the estimation of the ionosphere Doppler shift effect not possible. The use of a 600 Hz tone for correcting the Doppler shift instead of a DC constant is because with a 600 Hz tone the measure of the Doppler shift frequency can be done with a shorter packet. In this case, the Doppler shift effect will variate the 600 Hz tone between 580 Hz and 620 Hz. For assuring the measure of the Doppler shift, we estimate that the Doppler shift can be in the worst case of 50 Hz. In this case estimating the frequency of 550 Hz tone, the measure could be done with 33 cycles (60 ms). The case of using a DC tone, for measuring 1 Hz of Doppler shift, the measure would consider only a 16 th part of a cycle (60 ms), which is not enough for an accurate result.  
Then, a PN sequence is used for both estimating the channel response in terms of delay spread and Doppler spread and time synchronization. It is a 6 th order PN sequence with a resampling of 8 that has been carefully designed in order not to be affected by the delay spread, the Doppler spread and Doppler shift. The nodes located at Rockie Glacier and Argentina Cove will receive the frame and correlate the received sequence with a local replica, so the output is an estimation of the channel response as a function of time.  
Finally, the data blocks allow us to study the robustness of 4 th order modulations under the NVIS channel. The test has been done with two different modulations: QPSK and 4 FSK. The modulation QPSK shares the same features as the standards STANAG and MIL-STD 188 110C Appendix D in terms of bandwidth and bitrate. In this case, the QPSK has been equalized with a decision feedback equalizer (DFE) with a recursive least squares (RLS) algorithm to mitigate the effects of the intersymbolic interference (ISI). The ISI produced is caused by multiples reflections on the ionosphere and the terrain or by the presence of multiple ionospheric layers. According to previous studies [14], we selected a transmission power of 24 W to analyze the robustness of low power transmissions.