Gunjan Verma

Short-Range Low-VHF Channel Characterization in Cluttered Environments

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.

Wireless physical layer authentication via fingerprint embedding

Authentication is a fundamental requirement for secure communications. In this article, we describe a general framework for fingerprint embedding at the physical layer in order to provide message authentication that is secure and bandwidth-efficient. Rather than depending on channel or device characteristics that are outside of our control, deliberate fingerprint embedding for message authentication enables control over performance trade-offs by design. Furthermore, low-power fingerprint designs enhance security by making the authentication tags less accessible to adversaries. We define metrics for communications and authentication performance, and discuss the trade-offs in system design. Results from our wireless software-defined radio experiments validate the theory and demonstrate the low complexity, practicality, and enhanced security of the approach.

Physical Layer Authentication via Fingerprint Embedding Using Software-Defined Radios

The use of fingerprint embedding at the physical layer enables a receiver to authenticate a transmitter by detecting a low-power authentication tag superimposed upon the message waveform; a theoretical framework for such fingerprinting has been outlined. We carry out single-carrier single-antenna software defined radio (SDR) experiments with a wireless communications link over which we transmit and receive packets with the embedded fingerprinting. We analyze these experimental results and find they match well with theoretical predictions. This paper demonstrates that the method of superimposed fingerprints can deliver high probability of authentication without additional bandwidth and with minimal impact on bit-error rate in SDR systems.

ITA Sensor Fabric

The diverse sensor types and networking technologies commonly used in fielded sensro networks provide a unique set of challenges [1] in the areas of sensor identification, interoperability, and sensor data consumability. The ITA Senor Fabric is a middleware infrastructure - developed as part of the International Technology Alliance (ITA)[2] in Network and Information Science - that addresses these challenges by providing unified access to, and management of, sensor networks. The Fabric spans the network from command and control, through forward operating bases, and out to mobile forces and fielded sensors, maximizing the availability and utility of intelligence information to users.

The effect of zealotry in the naming game model of opinion dynamics

An individual in the process of forming or updating an opinion on any given subject is heavily influenced by the opinions expressed by others in his/her social network. However, not all individuals are flexible or open-minded; some are zealots, proselytizing an opinion in hopes of convincing others, while themselves completely immune to influence. Zealotry is a common phenomenon surrounding opinions involving significant personal investment, such as political or religious affiliation. In this work, we study the effect of zealotry in the context of binary opinion dynamics (denoted as opinions A and B, such as for/against increasing taxes). We derive several key results as a function of the zealot fractions backing each of the two opinions, including the number and nature of steady state solutions. We present analytically-grounded answers to questions such as: Does the opinion with larger zealot backing always win a majority of the non-zealot population, or are there conditions under which the opinion with smaller zealot backing can win? We extend our analysis with simulations to finite population and networks with finite connectivity.

Direction of arrival estimation with the received signal strength gradient 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 signals 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.

Zealotry promotes coexistence in the rock-paper-scissors model of cyclic dominance.

Cyclic dominance models, such as the classic rock-paper-scissors (RPS) game, have found real-world applications in biology, ecology, and sociology. A key quantity of interest in such models is the coexistence time, i.e., the time until at least one population type goes extinct. Much recent research has considered conditions that lengthen coexistence times in an RPS model. A general finding is that coexistence is promoted by localized spatial interactions (low mobility), while extinction is fostered by global interactions (high mobility). That is, there exists a mobility threshold which separates a regime of long coexistence from a regime of rapid collapse of coexistence. The key finding of our paper is that if zealots (i.e., nodes able to defeat others while themselves being immune to defeat) of even a single type exist, then system coexistence time can be significantly prolonged, even in the presence of global interactions. This work thus highlights a crucial determinant of system survival time in cyclic dominance models.

Implications of time/frequency synchronization tradeoff of quasi-synchronous multi-carrier DS-CDMA for robust communications at lower VHF

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.

Full-wave analysis of time of arrival based localization with polarization diversity

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.

Localization via the received signal strength gradient at lower VHF

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.