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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.

This paper presents a novel antenna architecture to achieve an extremely small form factor for HF band applications. The approach is based on manipulating the topology of a short monopole antenna without utilizing a high index material. A new architecture incorporating two radiating elements is configured, which allows significant gain enhancement. It is shown that such architecture can render a miniaturized HF antenna on air substrate having lateral and height dimensions as small as 0.0115λ0 × 0.0115λ0 × 0.0038λ0 (150 mm× mm × 50 mm for operation at 22.9 MHz). It is found that the measured gain of such architecture can be as high as - 18.1 dBi, which is 16.7 dB higher than a reference inverted-F antenna realized on a high index material (eR = 10.2) having exactly the same dimensions. The proposed antenna architecture is composed of two in-phase radiating vertical elements connected to two inductors between which a capacitive top load is connected to achieve the desired resonant condition. The two vertical elements act effectively as a monopole having increased height. It is also shown that the gain of the antenna can be increased monotonically by increasing the quality factor (Q) of the phase shifter. High Q air-core inductors that can be accommodated in electrically small monopole antenna are designed and incorporated in the phase shifter to achieve a gain value of - 17.9 dBi. Details about the proposed design approach, simulation, and measurement results are discussed.

A sub-wavelength source tracking system utilizing highly miniaturized antennas in the HF range for applications in GPS-denied environments including indoor and urban scenarios is proposed. A technique that combines a high resolution direction finding and radio triangulation utilizing a compact transmit (Tx) and receive (Rx) antenna system is pursued. Numerical models are used to investigate wave propagation and scattering in complex indoor scenarios as a function of frequency. We choose HF band to minimize attenuation through walls and multipath in indoor environments. In order to achieve a compact system, a low-profile and highly miniaturized antenna ( $\lambda /300$ height and $\lambda /100$ lateral dimensions at 20 MHz) having omnidirectional, vertically polarized field is designed. At such low frequencies, accurate measurement of the phase difference between the signals at the Rx antennas having very small baseline is challenging. To address this issue, a biomimetic circuit that mimics the hearing mechanism of a fly (Ormia Ochracea) is utilized. With this circuit, very small phase differences are amplified to measurable values. The numerical simulations are used to analyze direction of arrival retrieval and source localization in highly cluttered environments. A compact system prototype is also realized and source tracking in complex indoor scenarios is successfully demonstrated.

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.

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.

An efficient semi-analytic model for near-ground wave propagation in indoor scenarios is presented. For transceivers deployed in indoor environments on or near the ground, since RF wave propagation is dominated by Norton surface waves, these higher order waves and their interactions with building walls and other indoor obstacles have to be captured for accurate field calculations. Existing ray tracing routines which are commonly used for indoor field prediction, are inadequate for evaluating signal coverage of transceiver nodes very close to the ground (less than a wavelength above ground) since such routines neglect higher order surface waves. In addition, geometrical optics alone is inadequate to treat finite-size and possible irregular-shaped obstacles at low radio frequencies (VHF and lower UHF). Our approach for calculation of near-ground wave propagation and scattering is based on a hybrid physical optics and asymptotic expansion of dyadic Green's function for a half-space dielectric medium. Equivalence principle in conjunction with physical optics approximation is utilized to handle scattered field from building walls which are the dominant scatterers in indoor settings. Simulation results for various indoor propagation scenarios based on the new approach is validated by using both measurement results and full-wave numerical solvers.

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.

The concept of a subsurface imaging technique based on Unattended Ground Sensor Networks operating in the VHF range using ultra-wideband waveforms was recently proposed in [1]. In this approach a forward model for realistic subsurface environment based on the Dyadic Green's function for a stratified medium and an inversion technique using an ultra-wideband near-field focusing were presented. Simulation results showed that very good lateral and depth resolution could be achieved. Before carrying out an experiment to test the proposed technique, three vital aspects of the work which are imposed by practical limitations are investigated and presented in this paper. First, the sensitivity analysis to assess the signal penetration depth in realistic subsurface environments for various frequencies is performed. Analysis of the frequency requirements of the inversion as they relate to depth resolution is also analyzed. A semi-analytic soil dielectric model originally devised for microwave frequencies is extended to VHF and validated using measurement results available in literature.

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.

A semi-analytic model for near-earth wave propagation in the presence of building walls or similar obstacles is devised. The method is based on physical optics type approximations used in conjunction with the volume equivalence principle. Norton surface waves that are typically neglected in more common indoor propagation techniques such as ray tracing are included in this method as they prove to be vital for the accurate prediction of fields especially for sources close to ground. Simulation results have shown that, this method gives much more accurate results than ray tracing and proves to be computationally better than a numerical method and hence could be used to analyze field propagation in indoor structures that are large in terms of wavelength.

A realistic forward model based on a near-ground distributed sensor grid for a target buried under soil is devised. The soil medium is modeled as a planar stratified medium with a complex dielectric constant profile. The Dyadic Green's function for multilayer dielectric is used to compute the electric field at the target. A high resolution inversion algorithm based on phase-conjugation approach to detect deeply submerged targets is also presented. The sensor network is setup to enhance the lateral resolution by forming a synthetic aperture. The depth resolution is improved by using bandwidth. So as to improve the efficiency of the search algorithm a soil dielectric model to predict the real and imaginary parts of the dielectric constant from the volumetric soil moisture for a given soil textural composition is extended to the frequency of interest (VHF) resulting in less number of variables.