# Study of NVIS Channel for USN Protocol Definition in Antarctica
## 3 NVIS Channel Analysis
### Results Obtained  
In this section are the detailed results obtained in terms of availability of the NVIS channel, height of layers, delay spread, Doppler spread, and bit error rate (BER) for both groundwave and skywave channels. First of all, we evaluate the availability of the two implemented nodes. In Figure 7 we can see the percentage of packets detected and demodulated at 4.3 MHz with the node of Rockie Glacier. It is important to analyze this graphic for knowing the impact of the received frames in other graphics based on the time. As we can see the best availability of the channel is between 12 UTC and 21 UTC when the solar activity is higher. We have to consider that, for that time of year, the sunrise is at 8 UTC and the sunset at 1 UTC in Livingston Island. Furthermore, between 15 UTC and 19 UTC there is a reduction of the received packets. This is produced as we can see in Figure 8 by the presence of two layers which increase the delay spread. Figure 7: Availability of the Livingston Island NVIS channel. The solar activity is highly related to the height of the ionospheric layers during the day for a 4.3 MHz transmission. In Figure 8 we can see between 11 UTC and 23 UTC the emergence of the E layer due to the major solar activity, which coincides with the sunrise at 8 UTC and the sunset at 1 UTC of Livingston Island. As we can see in Figure 8, the height of the F2 layer is around 400 km during the hours of low solar incidence (between 0 UTC and 7 UTC), while it decreases to 275 km during the hours of high solar incidence. During the hours of the emergence of the E layer, the skywave will be reflected at two layers physically separated 250 km approximately, increasing the multipath and directly the BER of the demodulated data due to the ISI. The delay spread will be even higher in the presence of the groundwave. It is also important to denote that during the major solar activity, a layer situated below the E layer, the D layer will appear. In Figure 8, we can see that the F2 layer disappears, due to the attenuation of the PN sequence signal at the D layer [26] making weak the signal reflected at the F2 layer and not being detectable by our system. Once analyzed the channel availability and height of the ionosphere layers for different hours, in Figure 9 we analyze the total delay spread of the NVIS channel received at Rockie Glacier for different hours (see Figure 9). The delay spread is a very important parameter to determine the ISI effect on the transmitted frame. For an NVIS channel the maximum delay spread is 2.75 ms, so if we consider 0.42 ms the time of a symbol transmitted according to the standards STANAG and MIL-STD-188 110, the symbols transmitted will present ISI effect. This maximum delay spread calculated corresponds to a coherence bandwidth of 363.64 Hz. We can also observe that the minimum delay spread is 0.05 ms. In this case, the delay spread is less than the symbol time transmitted so there will not be ISI effect. Even so, the mean delay spread is a bit more than the symbol time, so the ISI effect will be present for most of the transmissions. In this case, the minimum delay spread corresponds to a coherence bandwidth of 20 kHz. In Figure 10, we can analyze the total delay spread of both the NVIS channel and the groundwave channel at different hours for the node located at Argentina Cove. This is a key issue for designing the frame protocol frame since the presence of the groundwave which becomes the shorter path. This new path changes the skywave to a multipath making the delay spread highest than Rockie Glacier. In this case we can observe the maximum delay spread presented during the day, which will define how the frame protocol has to be prepared to avoid ISI for both skywave and groundwave. As we can see the highest delay spread is 5.5 ms which is equal to a coherence bandwidth of 181.8 Hz. If we consider  
Figure 8: Height of F2 and E layer vs. Time.