Ionospheric Polarization Techniques for Robust NVIS Remote Sensing Platforms

SummaryAlthough many physical layer solutions have appeared for remote sensors and Internet of things during the recent years, none of them is suited to very remote sensors in areas away from any mobile operator coverage. In that case, a solution on the basis of near vertical incidence skywave (NVIS) with reflection in the ionosphere may be very attractive. Using NVIS, no line of sight is needed and the coverage is much bigger than any other system operating in either the very high frequency (VHF) or ultra high frequency (UHF) band. In this paper, we present a new transmission scheme for very remote sensors using the NVIS transmission technique.

The bit error rate (BER) performance for quadrature amplitude modulation (QAM) with different diversity combining scheme is derived for Rayleigh fading channel using Log-likelihood ratio (LLR). Three combining techniques, maximal ratio combiner (MMC), selection combining (SC), and Equal Gain Combiner (EGC) are considered for the BER analysis in this paper. The average BER performance of QAM symbol through a flat fading channel is derived from the individual bits forming the QAM symbol with MRC, SC, and EGC space diversity. The analytical and simulation of BER performance of the QAM symbol through the fading channel with MMC, SC, and EGC combiner is compared. The both analytical and simulation result shows that the probability of bit error decreases with the order of diversity for all the cases of MRC, SC, and EGC. It is also seen from the results that MRC provides better BER performance than the other two combiners (SC and EGC) for the same environment of the fading channel.

The use of near vertical incidence skywave (NVIS) propagation for HF radio communications somewhat beyond the groundwave range has assumed increasing importance years, particularly when no artificial means of radio relay is available. When tracing its history however there has been a tendency to assume that it first made its appearance in military communications during the Vietnam war in the late 1960s. Certainly, in that conflict considerable use was made of NVIS techniques and the basic characteristics of the antenna systems intended to launch it were subjected to exhaustive measurement. However, the genesis of what is now almost universally referred to as NVIS can be traced back at least to the Second World War when both the American and British forces used it-but never referred to it by that acronym.

The benefits of using high frequency (HF) circuits for both long-range beyond line of sight (BLOS) and short-range near vertical incidence skywave (NVIS) military applications are well documented. STANAG 4538 specifies an automated HF system that incorporates robust waveforms and protocols with the aim of providing faster link establishment and higher message throughputs under a variety of channel conditions. Under the auspices of the. NATO BLOS Communication WG, international trials have evaluated the link establishment and data link performance of S4538. Comparisons have been made with existing technologies such as MS-ALE and FS-1052. The testing has shown that S4538 offers a considerable improvement over MS-ALE in terms of increasing communications availability and decreasing LSU times. S4538 was also shown to be more reliable under poor channel conditions, such as those prevalent in the tactical environment. The operation of both point-to-point (PTP) and point-to-multipoint (PTM) protocols have been observed. Furthermore, the benefits of using RTCE information from sounding (or previous successful communications) has been observed.

In high-frequency (HF) frequency band radio waves can propagate over a few thousand kilometers owing to the reflection from ionosphere. In doing so, the ionosphere channel constitute an alternative to satellite communications when smaller data rates (bandwidths) are of interest. The entire range of distances can be covered if an antenna system supports all three modes of propagation: near vertical incidence skywave (NVIS), long range skywave and surface wave. Design of antennas for NVIS mode is the most challenging since they have to operate over the low end of HF frequency band. Due to size constrains, these antennas are electrically small, and thus they are inherently narrowband and have small radiation resistance. In addition, antenna pattern should have maximum at zenith, though there is no stringent requirement for the signal polarization.

The method of characteristic modes is used for the design of vehicular antennas for near vertical incidence skywave (NVIS). The technique allows the treatment of any vehicle-antenna combination as one, computing a unique set of characteristic, orthogonal currents and associated fields that may be optimized. >

We have combined the relatively mature numerical technique of the moment method computer modelling with a powerful, though less well-known, method of antenna synthesis which is based on the procedure of characteristic modes (CM), to both analyse and then to design optimum antenna systems for vehicular NVIS applications. Regardless of the maturity of an analytical technique however, the acid test of its accuracy can only be determined by means of a series of careful measurements of the performance of the candidate antennas when mounted on their appropriate host vehicle. This paper presents both analytical and experimental results for the existing whip antenna, at various angles of inclination, as well as for the vertically orientated loop, which has been proposed as a more appropriate antenna for this application. In addition, a loop configuration with an unconventional feed system is analysed and shown to provide improved performance.

Near vertical incidence skywave (NVIS) is suitable for short distance (ground distance less than 400 kilometres) short-wave communication, and there are errors using the existing calculation methods of transmitter elevation angle in this communication range. This paper presents a synthetic method for predicting the transmitter elevation angle of NVIS links. According to whether the measured data can be obtained in the application area, different models are selected to predict the transmitter elevation angle. The accuracy of this method is verified by theoretical values constructed using three-dimonsional ray-tracing technology. The research results can be used for frequency selection and field strength prediction of short-range skywave communication.

Separate excitation of the characteristic waves in the ionosphere results in two orthogonal propagation channels on the same frequency, which may be used in diversity and multiple-input, multiple-output (MIMO) systems. In this article, a method to measure the isolation between these paths is proposed and demonstrated in a near vertical incidence skywave (NVIS) experiment at a frequency of 7 MHz over a 105-km distance. Characteristic wave isolation exceeding 25 dB is measured during Happy Hour: the interval when the propagation path just opens or closes and only the extraordinary wave propagates.

This paper introduces a compact spider-shaped monopole antenna tailored for tactical HF radio communication using Near Vertical Incidence Skywave (NVIS) propagation. The antenna structure is designed to maximize high-angle radiation, enabling communication across obstructed or mountainous terrain. Featuring a top-loaded radial configuration and choke balun for impedance matching, the prototype antenna achieved a significantly improved VSWR of 1.19. Field deployment confirmed reliable HF communication at a distance of 346 kilometers, highlighting the design’s practical relevance for mobile military operations.

Accurate noise estimation in the HF band is a defining aspect in the evaluation of SNR for communication links. The reference document that specifies how different types of electromagnetic noise can be taken into account by radio communication system planners is the ITU-R Recommendation P.372.Meanwhile a series of studies have demonstrated that Gaussian noise in the HF band is not always valid because this band noise is strongly affected by atmospheric emission sources such as lightning impulse. This leads to the idea that a distribution suited to express the noise variance is the Bi-Kappa distribution. In addition, over the past decade, SDR devices and real-time spectrum analysers, which can be used to evaluate this noise, have registered explosive growth. This article indicates technical solutions and procedures used to estimate the noise level in the HF subband which can be used for Near Vertical Incidence Skywave (NVIS) communications and proposes a measurement system based on a SDR solution which meets these requirements.

An enhanced transmission is presented in a multiple‐input‐multiple‐output (MIMO) dense‐wavelength division multiplexed (DWDM) free‐space‐optical (FSO) communication link using diversity coding techniques under the effect of turbulent weather phenomenon. The findings show good performance with an (8 channels × 2.5 Gbps data rate/channel) 20 Gbps 1500 m transmission distance. The bit‐error‐rate (BER), outage probability (OP), and signal‐to‐noise ratio (SNR) of the diversity combining techniques using maximum‐ratio combining (MRC), selection combining (SC), and equal‐gain combining (EGC) technique are evaluated in this work. The obtained results illustrate that Alamouti, space‐time coding (STC), space‐time block coding (STBC), space‐time trellis code (STTC), orthogonal STBC (O‐STBC), and quasi‐orthogonal STBC ( QO‐STBC ) on the minimum mean‐square‐error, and MRC are worth implementing on the DWDM‐FSO wireless communication systems. The mitigation of atmospheric turbulence is achieved using MIMO diversity combining techniques coding. The simulation results for diversity coding techniques using QO‐STBC/STTC and SC/MRC in the MIMO‐DWDM FSO communication system can improve BER performance, OP, and SNR. The MRC exhibits the lowest OP and BER when compared with the SC and EGC. The numerical results demonstrate that the FSO communication link using DWDM QO‐STBC/STTC improves the power penalty at both BER values under varying atmospheric turbulence conditions for ST, MT, and WT, in comparison to FSO systems without DWDM QO‐STBC/STTC diversity coding techniques.

Bit-error rate investigation of satellite-to-ground downlink optical communication employing spatial diversity and modulation techniques

Inductive power transmission depends on many parameters such as the distance and the alignment between the coils, the working frequency, the current excitation, the geometry and the property of the coils, etc. Exchanging power distance between coils in wireless power transmission using inductive link is generally low, it is shorter than the diameter of the sending coils. Consequently, increasing coil to coil distance causes a harm decrease of the transmitted power and the efficiency of the system. A major technical challenge affecting the use of this kind of power transmission is to find a way of sending energy to the target devices in an efficient and reliable manner taking into account this influencing effect. In this paper, we explored potential solutions to overcome this challenge by adopting a Multiple Input Single Output (MISO) coil system able to decrease the magnetic flux leakage and orientate the magnetic field to the receiving coil by powering the neighbor coils of the active ones in opposite direction. This technique can be implemented in many industrial applications such as supplying wireless sensors installed into a conveyor. Same parameters according the model of the coupled coils on SISO and MISO configurations are developed and simulations by finite element method are done. The investigation shows that the MISO orientated system is capable to transfer 42 mW over a 50 mm distance and reaching 30% efficiency.

In this article, the performance of M-ary phase shift keying (PSK) modulation schemes over mobile-to-mobile (M2M) fading channels with equal gain combining (EGC) in cooperative networks is analyzed. Narrowband M2M fading channels under line-of-sight (LOS) propagation conditions are modeled as double Rice processes. A dual-hop amplify-and-forward relay network, where LOS components can exist in the transmission links between the source mobile station and the destination mobile station via K mobile relays, is studied. Here, the average bit error probability (BEP) is utilized to evaluate the performance of Mary PSK modulation schemes. A simple analytical approximation for the average BEP of M-ary PSK modulations over relay-based M2M fading channels with EGC is derived. The accuracy of the approximation is assessed by simulations. The presented results show that in a dual-hop relay system with EGC, the presence of LOS components in the relay links improves the systems' performance. In addition, a significant enhancement in the diversity gain can be achieved as the number of diversity branches K increases.