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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.

In this paper, we study the problem of designing resource allocation algorithms for communication systems equipped with multiple different communication hardwares. We consider a point-to-multipoint scenario, in which communication occurs between an access point and a series of agents. Both the agents and the access point are equipped with multiple hardware (e.g., VHF, UHF, and mmWave antennas). Under this multi-mode setting, we aim to jointly optimize the power allocation along with the mode selection such that the rate requirements of the agents are satisfied. We pose this resource allocation problem under a stochastic optimization framework, and show that the problem exhibits null duality gap. To obtain the optimal solution to the problem, we resort to a primal-dual algorithm, which can be run in an online fashion. Numerical results show that agents employing the proposed algorithm are capable of adapting their transmission mode in complex environments.

Challenging Army-relevant communications and networking environments require novel methods and systems with sufficient robustness and resilience to enable operation in a covert fashion, especially in the presence of sophisticated adversaries. Recent studies suggest that regions of electromagnetic spectrum, such as deep ultraviolet (UV) spectrum, that are not being utilized in existing systems can provide unique advantages, particularly for more-covert short- range operations. However, the UV channel is not well understood, and techniques for the effective and efficient use of this channel for networking, such as spatial multiplexing, are still being investigated. In this paper, we report on experiments that validate previously hypothesized channel modeling behaviors, where we isolate and accurately measure the line-of-sight (LOS), single-scattering, and multiple-scattering components of measured returns and compare the measurements with theory. These experimental measurements exhibit good agreement with theoretical predictions, and demonstrate, in particular, predicted variations from ideal behavior in the multiple-scattering case. The validated behaviors play a key role in a previously proposed UV networking optimization framework. As such, we conclude this paper with additional development and examples of that framework in order to demonstrate the utility of the validated channel modeling.

In this paper, we focus on the problem of distributed coherent beamforming in wireless networks where multiple distributed transmitters adjust the phases of their signals to form a directional and targeted communication link to a client receiver. The quality-of-service (QoS) and security are key aspects of robust and covert wireless networks. Although the security can be enhanced by exploiting information about the locations of adversaries, such information may not be available in practical networks since the adversaries are often passive. Therefore, we propose transmission strategies which divide transmitters into two groups where one group forms a beam towards the client and the other group broadcasts interference in order to obfuscate adversaries. As the interference may degrade the QoS of the client, the latter group steers a null to the client to alleviate the interference at the client. The proposed scheme can be performed in a fully distributed manner with only two bits of feedback information from the client and without any knowledge about the locations of potential adversaries.

An electrically small dipole antenna that can be tuned from 35 MHz to 90 MHz is reported. The proposed design is low profile and lightweight, and is integrated on the landing gear of a compact UAV. The antenna operates as an inductively loaded, folded dipole to efficiently utilize the available space. The antenna is tuned with both RF switches and varactor diodes. The RF switches short out different sections of the inductive coil enabling discrete frequency tuning. The varactor diodes in parallel with the coil allow for continuous frequency adjustments. Full wave simulation results show that the antenna can be tuned from 35 to 90 MHz. The instantaneous bandwidth and gain at 42 MHz are 1% and -8 dB, respectively. The gain and bandwidth increase when the antenna is tuned to the higher end of the frequency band of interest.

This paper presents a design strategy for covert platform-based antennas that operate on electrically-small platforms such as compact unmanned ground vehicles (UGVs). Specifically, we report the design of platform-based antennas operating at the lower end of the very high frequency (VHF) band for a small UGV having electrical dimensions of 0.13λ × 0.09λ × 0.05λ at 40 MHz. To do this without increasing the size or visual signature of the platform, the bottom of the UGV is modified in a nondestructive fashion. Specifically, a quarter-wavelength slot antenna is created in the bottom of the vehicle by placing a false bottom in the undercarriage area. The proposed antenna generates a vertically-polarized, monopole-like radiation pattern as verified through simulations and experiments.

Tactical military ad-hoc networks usually rely on easily and quickly deployable infrastructure. In such systems reliable communication may be compromised due to imperfect synchronization. QS-MC-DS-CDMA is a solution that allows loose synchrony by employing, so called, LS-codes that are characterized by a zero-correlation zone (ZCZ) that enables preserving code orthogonality in the presence of delay within that zone. Another highly desirable characteristic of tactical networks is low probability of detection (LPD). CDMA inherently favors LPD due to its property to spread the signal in the frequency domain thus making it look like noise. However, manmade signals have periodic characteristics that render them detectable using advanced techniques. Our goal is to reduce the patterns in such signals by employing random dithering in time, i.e. by introducing small random delays that do not affect orthogonality as long as they remain within the ZCZ. In addition to the ZCZ size, the amount of dithering that can be performed is also limited by the time uncertainty/synchronization requirement of the system. The periodic characteristic reduction is quantified using the Degree of Cyclostationarity (DCS) and simulation results are presented. We advocate the implementation of such a system in the lower VHF band due to its favorable channel characteristics and and especially its reduced multipath and frequency offset sensitivity. The latter is the key to our strategy towards increasing LPD protection via time dithering under time uncertainty.

Ultraviolet (UV) communications has been extensively studied as a promising technology to overcome challenging communication environments that may include, e.g., lack of line-of-sight and radio-frequency denial. In particular, atmospheric scattering at UV wavelengths can provide for a unique non-line-of-sight optical communication channel, and much research has focused on modeling this channel. However, the computational complexity of the highest-fidelity models, which are based on Monte Carlo simulation, has limited their usefulness. Here, we propose a low-complexity method for model evaluation that is based on precomputation and interpolation. We illustrate this approach in the context of the optimization of networks of UV communications systems. The results demonstrate the utility of our proposed method, show the importance of applying rigorous channel modeling when configuring UV communication systems, and suggest that our approach may be of great use beyond the particular application studied here.

A novel, highly miniaturized, lightweight antenna operating at the low-VHF band is presented. Earlier studies on an extremely short HF monopole antenna consisting of two in-phase vertical elements face an inevitable issue regarding an unbalanced coaxial cable feed due to the very small ground plane . To resolve this problem, as well as to achieve higher bandwidth, we introduce an electrically small folded dipole version of the same antenna having a fully balanced structure. The overall dimension and total mass of the proposed antenna are $10 \times 10 \times 15 ~\hbox{cm}^3$ . ( $0.013{\lambda _0} \times 0.013{\lambda _0} \times 0.02{\lambda _0}$ at 40 MHz) and 98 g, respectively. The gain and pattern of the fabricated antenna are measured in an elevated range that is in nearly free-space conditions. Measurements are shown to be in good agreement with the design predictions from simulation.

Supporting networks with a large number of nodes in infrastructure-poor and complex propagation environments is an important challenge for military and civilian applications. A major problem when using classical approaches, such as code division multiple access (CDMA), is maintaining inter-link coordination while mitigating multi-user interference (MUI). By contrast, loosely synchronous (LS) codes have perfect code orthogonality within a window of inter-link delays at a cost of the number of available spreading codes. Since sporadic communications naturally involves a low probability of transmission, we investigate the potential for LS code reuse to effectively support more users. We study this problem by simulating inter-user channels using a high-fidelity physics-based model. We focus our study of the channel on the low-VHF band, which has improved penetration and channel coherence in complex environments. We perform initial characterization of different levels of code reuse, synchronization and coordination. Of particular interest is a purely random (uncoordinated) spreading code assignment. The results illustrate good performance of a scalable medium access scheme.

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.

The lower VHF band has potential for low-power, short-range communications, as well as for geolocation applications, in both indoor and urban environments. Most prior work at low VHF focuses on longer range path loss modeling, often with one node elevated. In this paper, we study indoor/outdoor near-ground scenarios through experiments and electromagnetic wave propagation simulations. These include the effects of indoor penetration through walls and obstacles, as well as indoor/outdoor cases, for both line of sight (LoS) and nonLoS (NLoS), at ranges up to 200 m. Mounting our receiver (Rx) on a robotic platform enabled the collection of thousands of measurements over an extended indoor/outdoor test area. We measure the channel transfer function, employing bandpass waveform sampling, with pulse and tone probe signals. Based on statistical tests, we show that the measured channels have a nearly ideal scalar attenuation and delay transfer function, with minimal phase distortion, and little to no evidence of multipath propagation. Compared with higher VHF and above, the measured short-range VHF channels do not exhibit small-scale fading, which simplifies communications Rx signal processing, and enables phase-based geolocation techniques.

The focus of this paper is to characterize near-ground wave propagation in the lower very high frequency (VHF) band and to assess advantages that this frequency band offers for reliable short-range low-data rate communications and geolocation applications in highly cluttered environments as compared to conventional systems in the microwave range. With the advent of palm-sized miniaturized VHF antennas, interest in low-power and low-frequency communication links is increasing because (1) channel complexity is far less in this frequency band compared to higher frequencies and (2) significant signal penetration through/over obstacles is possible at this frequency. In this paper, we quantify the excess path loss and small-scale fading at the lower VHF and the 2.4 GHz bands based on short-range measurements in various environments. We consider indoor-to-indoor, outdoor-to-indoor, and non-line-of-sight outdoor measurements and compare the results with measurements at higher frequencies which are used in conventional systems (i.e., 2.4 GHz). Propagation measurements at the lower VHF band are carried out by using an electrically small antenna to assess the possibility of achieving a miniaturized, mobile system for near-ground communication. For each measurement scenario considered, path loss and small-scale fading are characterized after calibrating the differences in the systems used for measurements at different frequencies, including variations in antenna performance.

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.

A high-resolution subsurface imaging technique based on distributed near-ground sensor networks that utilize ultrawideband waveforms in the very high frequency (VHF) range is presented. An accurate scattering model for a target buried in realistic subsurface environment, modeled as a vertically stratified medium, is presented first. Then an inversion technique that uses ultrawideband near-field focusing is described. The signal penetration depth as a function of frequency and various subsurface parameters is calculated based on the developed forward model. The imaging resolution as it relates to the accuracy of background retrieval is also analyzed. A semi-empirical soil dielectric model that is originally developed for the ultra high frequency band is modified and validated at the VHF range with measured results available in the literature. For a given soil textural composition and frequency, the model predicts the real and imaginary parts of the dielectric constant as a function of soil moisture content. This soil dielectric model is utilized to make the inversion more efficient. To address the challenge associated with the design of compact and ultrawideband VHF antennas, a scheme utilizing multiple antennas and reduced number of frequency points is proposed. The sensor arrangement both in terms of spatial distribution and polarization of each antenna as it relates to the lateral resolution, as well as minimizing the direct coupling between the Tx and Rx antennas, is analyzed. The proposed subsurface imaging approach is validated based on numerical techniques and a laboratory scale model measurement results.