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.
Highlights
The ionosphere has an essential function for our planet, which is the protection against external radiations
A poor definition of the data frame could imply intersymbol interference (ISI) and our tests, which were designed on the basis of earlier studies and the soundings of the signal-to-noise ratio [27]
5 displays a graphical representation ofto the first type tests, which were designed on the basis of earlier studies and the soundings of the ionoof signal sent (Frame number 1), which was composed by a total of 50 data groups
Summary
The ionosphere has an essential function for our planet, which is the protection against external radiations. As a result of this almost-circular spin, the radio wave has its polarization changed by the ionosphere This leads to the return to the Earth of two different rays (the ordinary and extraordinary rays) with different properties, such as different critical frequencies, phase, amplitude, and arrival time [2]. For the Northern Hemisphere, the ordinary wave has the greater delay and left-hand circular polarization (LHCP), and the extraordinary wave presents the lesser delay and right-hand circular polarization (RHCP) [13] These different properties can be used to improve telecommunication links, as polarization diversity techniques are an option in ionospheric channels. We studied two different techniques: equal-gain combining, a method that sums all the received signals coherently, and selection combining, a technique that selects the strongest signal received (a higher signal-to-noise ratio (SNR)) and ignores the other
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