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Deep ultraviolet wavelengths have been proposed for low-probability-of-detection (LPD) communications, particularly for non-line-of-sight (NLOS) links, because of the increased atmospheric absorption at these wavelengths. Motivated by this favorable feature, we develop a modeling framework to quantitatively study the LPD characteristics of ultraviolet communications (UVC). We then demonstrate the application of our modeling framework by considering various friendly and adversarial system configurations and quantifying the proposed LPD metric (the range at which an adversary can detect communications that uses the minimum power needed to meet given communications performance requirements), as well as investigating the sensitivity of the analysis to various scenario parameters. The results demonstrate the potential for this modeling and analysis approach to provide key insights into the design and operation of LPD NLOS UVC systems.

In this paper, we investigate a wireless network where multiple distributed transmitters adjust the phases of their signals so that they can be constructively added at an intended receiver (client). Unlike conventional beamforming with co-located and phase-synchronized antennas, geographically separated transmitters may have phase offsets induced by individual local carrier oscillators, that pose a challenge for coherent distributed beamforming. This is especially true for transmitters that are far apart, when distributed clock synchronization protocols may be more difficult to implement. There may also be a desired spatial repulsion among the positions of the transmitters in order to mitigate mutual coupling effects and extend the coverage region. In this regard, we analyze the performance of distributed beamforming with phase offsets by modeling the spatial distribution of the transmitters as a $\beta$-Ginibre point process that models the repulsive behavior. We consider two transmission strategies: (i) Transmitter selection in which the client chooses the transmitter providing the highest received power at the client, and (ii) Coherent beamforming in which multiple transmitters simultaneously send their signals to the client. From numerical simulations, we examine the impact of the phase offsets on the performance and confirm the accuracy of our analysis. It is shown that even with significant phase offset errors, employing coherent beamforming can be an effective strategy.

A low-power, low-frequency, ad hoc networking paradigm is considered for robust communications among mobile agents in complex non-line-of-sight (NLOS) indoor and urban-type scenarios. Compared with higher frequency, the lower portion of the very high frequency (VHF) band offers improved penetration and reduced multipath in such scenarios. Low VHF is underutilized for mobile ad hoc networking due to the lack of compact low-power systems and efficient miniature antennas. We investigate the proposed approach through experiments in realistic scenarios. In order to carry out the experiments, we leverage a compact, low-power ZigBee radio operating seamlessly in the low-VHF band by introducing a bi-directional frequency converter, which translates ZigBee signals into low-VHF carriers, along with a recently developed highly miniaturized efficient antenna. The experimental low-VHF radio system and a conventional ZigBee operating at 2.4 GHz are both integrated on a compact ground robotic platform for autonomous experimentation and comparison in NLOS indoor and outdoor settings. Measurements quantify the significant advantages of the low-VHF radio system in terms of packet error rate, fading, radio signal strength, and extended spatial coverage, in a number of complex communication environments.

We investigate low-frequency communications with a highly miniature bandwidth-enhanced antenna. To achieve improved bandwidth while maintaining a small form factor, a non-Foster matching technique is applied to a λ/50 low-Vhfantenna. The practical use of existing non-Foster matched antennas has been hindered due to their sensitivity to small system induced variations as well as nearby scatterers. We carry out software defined radio based wireless data transmission experiments with varying data rates to investigate the performance of our proposed antenna for practical applications in indoor environments. Experimental results show the non-Foster matched antenna enables transmitting data with rates three times higher than the same antenna matched with passive elements. We present communication quality measures by transmitting several packets and computing the signal-to-interference-plus-noise-ratio (SINR), the symbol error rate, and cross-correlation between transmitted and received pulses.

Reliable, ad hoc communications among multiple users is a crucial objective in complex tactical scenarios. A major challenge is that synchronization mechanisms (such as GPS) are intermittent, and fixed centralized coordinators (e.g., base stations) are infeasible. Classical approaches to multi-user communications relying on tight time and frequency synchronization and/or power control are impractical in such scenarios. In this paper, we study the effect of coarse time synchrony and phase response variation on a class of code division multiple access (CDMA) strategies known as multi-carrier (MC) CDMA. We study this sensitivity when binary codes achieving the Welch bound in a potentially user-overloaded system are used. The codes are selected to minimize the L 2 and the L ∞ bounds at small or negligible inter-user time offsets and phase-response variation. We show that variations in phase response lead to significant loss of code orthogonality; this motivates employing MC-CDMA systems in scenarios with benign channel responses. Of particular interest is the near-ground lower VHF band, which has improved penetration and reduced multipath relative to microwave, in particular in non-line of sight environments, such as through buildings. This paper aims at studying the reduction in the necessary inter-user time synchrony required of bands having a lower phase variation.

A major bottleneck of low frequency communication applications is relatively low-data-rate transmissions due to narrow bandwidth with miniaturized antennas. To increase the bandwidth of small antennas, we propose a non-Foster matched antenna designed for a low-VHF transmitter system. A non-Foster element realized by a single negative impedance converter is applied to a passive-matched miniature dipole at 40 MHz. A rigorous stability analysis based on system sensitivity is performed via circuit simulation and measurement. A power transmission coefficient with a reference antenna in an indoor setting along with impedance bandwidth of the non-Foster matched dipole is measured and compared with those of the passive-matched one. The results show 3dB power bandwidth is improved by a factor of two with active matching and more than 80 % of the bandwidth has a power efficiency advantage, compared to passive matching.

We investigate a time of arrival based localization technique for nodes deployed in complex GPS-challenged environments. The proposed technique exploits spatial and polarization diversity via anchors equipped with polarization diversity antennas operating at low frequencies. Empirically derived effective dielectric constants are used for penetration delay compensation. Our full-wave simulations of a complex scene suggest that sub-meter localization error can be achieved, with low-power transmission at upper HF and lower VHF bands.

Localizing a node in complex propagation environments, such as in obstacle-rich indoor scenes, is a challenging task. Conventional approaches employ a dense and/or highly calibrated infrastructure of sensors assisting in the localization process, which is costly and difficult to achieve in ad-hoc situations. In this paper, we propose a minimalistic design based on a small number of access points. Our method uses the received signal strength (RSS) gradient as a direction of arrival (DoA) estimator. Though non-uniform shadowing and multipath adversely affect RSS, these are much less at lower VHF. We show through high fidelity simulations that our approach has promise as a rapidly-deployable localization system.

Reliable, ad hoc communication among multiple users is a crucial objective in complex tactical scenarios. A major challenge is that synchronization mechanisms (such as GPS) are intermittent and fixed centralized coordinators (e.g., base stations) are infeasible. Classical approaches to multiuser communications relying on tight time synchronization and/or power control are impractical in such scenarios. Furthermore, state-of-the-art systems operating at microwave frequencies suffer from lack of penetration and multipath effects causing significant channel variability and exacerbating the near-far effect. In this paper, we investigate the potential advantages of the favorable channel characteristics of the lower VHF band for reliable multiuser communications in highly cluttered environments among near ground nodes. We focus on a class of code division multiple access (CDMA) strategies known as quasi-synchronous (QS) direct sequence (DS) CDMA and use moderate to high QAM constellations. A key aspect of QS CDMA codes is their robustness to coarse inter-user time synchrony and power mismatch, which greatly relaxes the rate that nodes must re-synchronize with each other. Our study experimentally explores the possibilities of multiuser communications in harsh environments among near-ground nodes. Our measurements involve three canonic indoor-to-outdoor, indoor-to-indoor and through-building channels characterized by obstacles and clutter that represent very difficult propagation environments, especially at microwave frequencies. We also study the spatial and temporal variability of our system and characterize the impact of frequency offset on code orthogonality with simulations and experiments. We show that our system has strong potential for excellent multiuser performance in infrastructure-poor regimes.

A prime objective in tactical environments is reliable, ad hoc communication among multiple users robust to lapses in coordination methods such as GPS or centralized base stations. Classical approaches to multi-user communications relying on tight node synchronization and/or power control are infeasible in ad hoc infrastructure-poor scenarios. In this paper, we study multi-user coding strategies with greatly reduced synchronization requirements (compared to conventional systems such as cellular) ideally suited for mobile near-ground nodes in harsh tactical environments. We exploit quasi-synchronous (QS) direct sequence (DS) code division multiple access (CDMA) codes transmitted over multiple carriers (MC). These codes are characterized by a zero cross-correlation zone (ZCZ), a measure of the delay tolerance among transmitters over which perfect code orthogonality still holds at the receiver. Using multiple carriers extends the ZCZ in time, thereby relaxing the time synchronization requirement, but tightening the frequency synchronization requirement. We study this tradeoff in this paper. We also argue that the QS MC-DS-CDMA system will have superior performance at lower VHF because of reduced multipath, reduced frequency offset, and better penetration relative to UHF or microwave systems in dense urban environments [1]–[4]. We develop an experimental testbed based on software-defined radios (SDR) and analyze the proposed approach at the lower VHF band.

This paper investigates the advantages offered by radiation pattern diversity using a new physics-based analysis that takes into account the complex radiation patterns of the transmit(Tx) and receive(Rx) diversity antennas in conjunction with an accurate deterministic, coherent, and polarization preserving propagation model for a complex indoor scenario. Unlike techniques that utilize spatial diversity, radiation pattern diversity offers a unique opportunity to achieve compact diversity antenna systems especially with the advent of enabling antenna miniaturization techniques. In this work, a co-located antenna radiation pattern diversity system is proposed and its performance is analyzed using an accurate physics-based diversity analysis technique. The proposed analysis technique utilizes an efficient deterministic propagation model modified by incorporating the complex gain of the Tx and the complex effective heights of the Rx antennas into the rays launched and received by the antennas in a coherent manner. The proposed system is realized and tested in complex indoor scenarios based on which complex correlation coefficients between various channels and the effective diversity gain are computed which are then utilized as a figure of merit for improved channel reliability. For the scenarios considered, it is shown that the apparent diversity gain is at least 9.4 dB based on measured results.

Direction of arrival (DoA) estimation has wideranging applications and is particularly challenging in complex propagation environments. Virtually all current approaches estimate the DoA based on the phase of the impinging signal on a sensor array. This approach has several challenges: for example, it requires tight synchronization amongst array elements, and the estimated DoA is very sensitive to the specification of the array geometry. We propose a Bayesian DoA estimation procedure based on the received signal strength (RSS) gradient. RSS is readily computed by most sensors, and the RSS gradient points towards the source when the received signal's dominant component is the direct path. Though non-uniform shadowing, small-scale fading, and multipath generally corrupt the accuracy of the RSS gradient as a DoA estimate, these effects are significantly less at lower VHF. We show through simulations and applications to actual datasets that our approach provides a simple and accurate method to estimate DoA and its associated uncertainty, even in cluttered environments.

A cost-effective, low-power, ZigBee-based compact radio system tuned for operation at low VHF band is proposed for mobile communications in complex environments. Existing off-the-shelf radios at this band are bulky, usually operate at high power levels (a few watts), and require relatively large antennas to enable long range communications. To remedy these shortcomings, we develop a small-form-factor low-VHF radio that leverages the ZigBee technology with a frequency conversion technique. A bi-directional frequency converter for the radio is designed to translate ZigBee signals into low-VHF carriers and optimized to minimize power consumption, and simultaneously maximize system gain and sensitivity. This radio also comes with a newly developed miniaturized antenna [1] (≤ 0.02λ 0 in height and lateral dimensions at 40 MHz) having much higher efficiency compared to antennas of similar sizes. This antenna allows the compact radio system to operate at lower power and on small robotic platforms needed for military or rescue missions.

In complex propagation environments such as urban canyons, tunnels, and indoor environments, achieving a reliable communications link is very challenging at conventional microwave frequencies. However, the lower VHF band has significant potential to support low complexity, low power, and reliable one-hop communications, because obstacles are much smaller relative to wavelength at this frequency (Dagefu et. al, Trans. Ant. Prop., vol. 61, no. 6, 2252).

The lower VHF band shows potential for reliable communications in low power, short range scenarios among near-ground nodes in both indoor and urban environments. Such scenarios are of great interest, for example, in military and search-and-rescue settings. Most prior work at low VHF focuses on modeling path loss at long range. In this paper, we study indoor/outdoor near-ground scenarios through experiments focusing on both line-of-sight (LoS) and non-LoS (NLoS), at ranges up to 200 meters. By transmitting tones and pulses from various locations in a realistic environment, we acquire channel data via a mobile data collection platform which gathers data at hundreds of different locations. We show that the measured channels have a nearly ideal scalar attenuation and delay transfer function, with minimal phase distortion, and little evidence of multipath propagation. We further confirm the absence of small scale fading by measuring bit error rate (BER) versus received signal-to-noise ratio (SNR) for QPSK transmission in an indoor setting. Using only timing and carrier estimation at the receiver, the resulting BER curves coincide with theoretical additive white Gaussian noise channel BER predictions.

Summary form only given. An aspect of wireless communication that is of paramount interest is reliable connectivity unhampered by signal fading caused by scatterers such as walls, buildings and other obstacles. For power limited ad hoc networks in complex environments, various phenomenon including multipath, diffraction from sharp corners contribute to the fading and distortion of electromagnetic waves that severely limit the coverage and reliability. Because of the difference in path length among the various signal components and in the absence of a direct signal, the received electric field will have uneven spatial distribution and significant fluctuations. This phenomenon is called fast fading and results in intermittent signal drop-offs causing the communication channel to be unreliable. A viable approach to mitigate fast fading is the use of antenna diversity systems. Antenna Diversity Systems, when used in multipath environments such as indoor and urban environments, enable improvement in signal-to-noise-ratio (SNR) without increasing the transmit power. A thorough and accurate analysis of a diversity system requires three main components: the Tx and Rx antenna radiation patterns (phase and amplitude), the multipath coherent propagation model and calculation of figures of merit for the performance of the diversity system including complex and envelop correlation coefficients and diversity gain. Existing simulation based diversity analysis techniques essentially model the multipath channel using a stochastic model such as Rayleigh and Rician distributions. While these probabilistic models provide generalized approximations to the indoor channel, such models do not accurately capture all the propagation mechanisms such as angle of arrival and polarization, and hence diversity analysis techniques based on such models inherently lack the information needed to assess the true performance of a given diversity system. In this work, we discuss a new diversity system analysis approach that takes into account the complex radiation pattern of the Tx and Rx diversity antennas and make use of an accurate deterministic, coherent, and polarization preserving propagation model for a complex indoor scenario. The new physics-based pattern diversity analysis approach will be introduced. The propagation modeling with specific focus on near-ground antennas will also be discussed. Measurement results in indoor scenarios utilizing a compact, co-located radiation pattern diversity antenna system will be utilized to assess the performance of the diversity system via complex correlation coefficients and diversity gain.

Efficient and versatile wave propagation models for indoor and urban environments can find applications in many areas including wireless channel characterization, environmental sensing and radar applications such as fire and earthquake rescue missions. There are various indoor field prediction techniques available in the literature such as ray tracing, hybrid techniques and empirical models that are based on measurement results [1–2]. Models based on numerical solvers such as MoM, FEM and FDTD are usually not preferred due to high computational cost. Ray tracing techniques, being a high frequency approximation, are not valid for lower frequency (lower UHF and VHF) applications. In addition, existing ray-tracing routines, which are most commonly used for indoor field prediction, are inadequate for evaluating the signal coverage for near-ground networks since the scattered wave from the ground is only approximated by Geometrical Optics (GO) which neglects the more dominant higher order Norton surface waves. Therefore, for near-ground transceivers deployed in indoor and urban environments these higher order waves and their interactions with building walls and other indoor obstacles should be taken into account for accurate field calculations. A semi-analytic propagation model for indoor scenarios that fully takes into account the Norton waves by making use of the Dyadic Green's function for a half-space dielectric medium was recently proposed in [3]. A physical optics type approximation was utilized to handle scattered field from building walls which are the dominant scatterers in indoor settings. The derivation and validation of the technique using a numerical solution for the 2D case were presented. In this work, the method is extended to a more realistic indoor propagation scenarios where multipe scatterers exist. In the first section, the semi-analytic technique for 3D multi-wall scenarios is described followed by measurement results used to validate the new method for 3D single wall-building geometry. To further validate the accuracy of the proposed model, field coverage comparison between the semi-analytic method and a full-wave solver (Semcad X) for various cases is also presented.