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 optionsand capabilities. Notwithstanding this, remote locations present many difficulties for their studyand monitoring. High-frequency (HF) communications are presented as an alternative to satellitecommunications, 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 beingused and the ionospheric conditions) without a line of sight using the ionosphere as a communicationchannel. This paper centers on the study of the ionosphere and its characteristic waves as twoindependent channels in order to improve any NVIS link, increasing its robustness or decreasingthe size of the node antennas through the appliance of specific techniques. We studied the channelsounding of both the ordinary and extraordinary waves and their respective channels, analyzingparameters such as the delay spread and the channel’s availability for each wave. The frequencyinstability of the hardware used was also measured. Furthermore, the correlation coefficient of theimpulse response between both signals was studied. Finally, we applied polarization diversity andtwo different combining techniques. These measurements were performed on a single frequencylink, 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 showedthat 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