Analysis of the Ordinary and Extraordinary Ionospheric Modes for NVIS Digital Communications Channels

Abstract

Sensor networks have become more popular in recent years, now featuring plenty of options and capabilities. Notwithstanding this, remote locations present many difficulties for their study and monitoring. High-frequency (HF) communications are presented as an alternative to satellite communications, being a low-cost and easy-to-deploy solution. Near vertical incidence skywave (NVIS) technology provides a coverage of approximately 250 km (depending on the frequency being used and the ionospheric conditions) without a line of sight using the ionosphere as a communication channel. This paper centers on the study of the ionosphere and its characteristic waves as two independent channels in order to improve any NVIS link, increasing its robustness or decreasing the size of the node antennas through the appliance of specific techniques. We studied the channel sounding of both the ordinary and extraordinary waves and their respective channels, analyzing parameters such as the delay spread and the channel’s availability for each wave. The frequency instability of the hardware used was also measured. Furthermore, the correlation coefficient of the impulse response between both signals was studied. Finally, we applied polarization diversity and two different combining techniques. These measurements were performed on a single frequency link, tuned to 5.4 MHz. An improvement on the mean bit energy-to-noise power spectral density (Eb/N0) was received and the bit error rate (BER) was achieved. The results obtained showed that the extraordinary mode had a higher availability throughout the day (15% more availability), but a delayed spread (approximately 0.3 ms mean value), similar to those of the ordinary wave. Furthermore, an improvement of up to 4 dB was achieved with the usage of polarization diversity, thus reducing transmission errors.

A Heterogeneous Layer-Based Trustworthiness Model for Long Backhaul NVIS Challenging Networks and an IoT Telemetry Service for Antarctica

“…However, we can find three main issues that can provoke this architecture to become a challenging network [ 14 ]: Due to the fact of Antarctica’s extreme weather and environmental conditions, both sensors and gateways could experience temporary or persistent malfunctioning; The irregular elevations of the Antarctic terrain might create situations where sensors do not have a LoS path through the gateway. This fact degrades the performance of LoRa communications considerably [ 23 ]; Depending on the ionosphere’s state and the solar activity, NVIS links may become unavailable temporally or intermittently [ 4 , 6 , 7 , 11 ]. …”

“…Depending on the ionosphere’s state and the solar activity, NVIS links may become unavailable temporally or intermittently [ 4 , 6 , 7 , 11 ].…”

“…Moreover, given that the ionosphere characteristics vary considerably throughout a day, we also modeled the probability of a packet being correctly delivered through an NVIS link hour by hour, following the results in [ 11 ]. These results showed that the NVIS links are unlikely to be available from 17:00 until 6:00.…”

“… NVIS link availability depending on solar activity and the ionosphere’s state [ 11 ]. The graph’s legend is defined as follows: OR refers to the performance of the ordinary wave, XOR refers to the performance of the extraordinary wave received.…”

“…Researchers from our university have previously participated in research campaigns on Livingston Island, studying and verifying the NVIS communication system’s viability [ 8 , 9 , 10 ]. A new campaign is planned to be carried out in the Antarctic field, with the goal to test the new improvements of the NVIS link [ 11 ] and deploy an IoT network for three different use cases: a telemetry service for light data (e.g., penguin tracking [ 12 ], a telemetry service for fat data (e.g., lichen observation [ 13 ]), and a distributed computing service to map the ionosphere along Antarctica).…”