La Salle Campus, Ramon Llull University, 08022 Barcelona, Spain.
Sensors (Basel). 2021 Mar 22;21(6):2210. doi: 10.3390/s21062210.
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 (E/N) 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.
近年来，传感器网络变得越来越流行，现在具有多种选择和功能。尽管如此，远程位置在研究和监测方面仍存在许多困难。高频 (HF) 通信被提出作为卫星通信的替代方案，是一种低成本且易于部署的解决方案。近垂直入射天波 (NVIS) 技术利用电离层作为通信通道，提供大约 250 公里的覆盖范围（取决于使用的频率和电离层条件），无需视线。本文专注于研究电离层及其特征波作为两个独立的通道，以改善任何 NVIS 链路，通过应用特定技术提高其稳健性或减小节点天线的尺寸。我们研究了普通波和超常波及其各自通道的信道探测，分析了延迟扩展和每个波的信道可用性等参数。还测量了硬件的频率不稳定性。此外，研究了两个信号之间的脉冲响应的相关系数。最后，我们应用了极化分集和两种不同的合并技术。这些测量是在调谐到 5.4 MHz 的单频链路中进行的。接收的平均比特能量与噪声功率谱密度 (E/N) 有所提高，实现了比特误码率 (BER)。得到的结果表明，超常模式在一天中的可用性更高（高 15%），但延迟扩展较大（平均值约为 0.3 ms），与普通波相似。此外，使用极化分集可提高 4 dB 左右，从而减少传输误差。
