Secure Wireless Communication Using Coherent Distributed Transmission and Spatial Signal Decomposition

We present a new approach to secure wireless communications using coherent distributed transmission of signals that are spatially decomposed between a two-element distributed antenna array. High-accuracy distributed coordination of microwave wireless systems supports the ability to transmit different parts of a signal from separate transmitters such that they combine coherently at a designated destination. In this paper we explore this concept using a two-element coherent distributed phased array where each of the two transmitters sends a separate component of a communication signal where each symbol is decomposed into a sum of two pseudo-random signal vectors, the coherent summation of which yields the intended symbol. By directing the transmission to an intended receiver using distributed beamforming, the summation of the two vector components is largely confined to a spatial region at the destination receiver. We implement the technique in a 50 wavelength array operating at 3 GHz. We evaluate the symbol error ratio. (SER) in two-dimensional space through simulation and measurement, showing the approach yields a spatially confined secure region where the information is recoverable(i.e., the received signal has low SER), and outside of which the information is unrecoverable (high SER). The proposed system is also compared against a traditional beamforming system where each node sends the same data. We validate experimentally that our approach achieves a low SER of 0.0082 at broadside and a SER above 0.25 at all other locations compared to a traditional beamforming approach that achieves a SER of 0 at all locations measured.

Three-Dimensional Millimeter-Wave Imaging Using Active Incoherent Fourier Processing and Pulse Compression

We present a novel three-dimensional (3D) imaging approach that combines two-dimensional spatial Fourier-domain imaging techniques with traditional radar pulse compression to recover both cross-range and down-range scene information. The imaging system employs four transmitters, three of which emit spatially and temporally incoherent noise signals, while the fourth transmits a known linear frequency modulated (LFM) pulsed signal. The spatial incoherence of the noise signals enables sampling of the 2D spatial Fourier spectrum of the scene from which two-dimensional cross-range (azimuth and elevation) images can be formed via interferometric processing. Simultaneously, the LFM signal enables high-resolution downrange imaging through matched filtering. The received signals consist of a superposition of the noise sources and the known pulse allowing for joint recovery of all three dimensions. We describe the system architecture and waveform design, and demonstrate the imaging technique using both simulations with a linear array and experimental data from a 38 GHz active incoherent millimeter-wave imaging system with 23-element randomized array. Results show the reconstruction of targets in three dimensions.

Collaborative Beamforming for Communication Applications Using a Two-Element Fully-Wireless Open-Loop Coherent Distributed Array

In this work we demonstrate a proof of concept of a fully-wireless two-node open-loop coherent distributed communication system and evaluate its performance by transmitting QPSK , 64-, and 256-QAM constellations at a symbol rate of 2 MBd over a 58 m link in an urban environment. The system is implemented in a distributed manner with on-node processing using software-defined radios (SDRs) and wireless internode communication to share coordination information and does not rely on external time or frequency references such as the global navigation satellite system (GNSS). In each experiment ~100 messages were transmitted and a mean coherent gain of 0.936 was achieved across all measurements with a mean symbol error ratio of below $1.4\times 10^{-4}$ achieved up to 64-QAM, demonstrating a reliable bandwidth of up to 12 Mbps.

A Compact Dynamic Omnidirectional Antenna

We propose a novel omnidirectional antenna design incorporating directional modulation for secure narrow planar information transmission. The proposed antenna features a compact size and stable omnidirectional radiation performance by employing two tightly spaced, printed meander line monopole antennas, acting as a single radiating element. To achieve a narrow information secure region, the proposed antenna is fed by differential power excitation of two ports with real-time dynamic switching. This leads to phase pattern modulation only along the electrical polarization, resulting in directionally confined information recoverable region in the E-plane, while maintaining highly constant or static omnidirectional H-plane pattern, inducing a $360^\circ$ information recoverable region. The dynamic antenna is designed and fabricated on a single layer of Rogers RO4350B which provides a miniaturized planar size of $0.36 \times 0.5 , \lambda_0^2$ at 2.7 GHz and easy integration. To validate the wireless communication performance, the fabricated antenna is directly fed with a 10 dB power ratio by a radio frequency (RF) switching system and evaluated for 16-QAM and 256-QAM transmission in a high signal-to-noise ratio (SNR) environment. Experimental results demonstrate that for 16-QAM transmission, a narrow E-plane information beam (IB) of approximately $34^\circ$ and omnidirectional H-plane IB are obtained, and a narrower E-plane IB is achieved around $15^\circ$ for 256-QAM. These results confirm that the proposed antenna offers a simple yet effective approach to enhance planar physical information security with a compact dynamic antenna system.

Real-Time High-Accuracy Digital Wireless Time, Frequency, and Phase Calibration For Coherent Distributed Antenna Arrays

This work presents a fully-digital high-accuracy real-time calibration procedure for frequency and time alignment of open-loop wirelessly coordinated coherent distributed antenna array (CDA) modems, enabling RF phase coherence of spatially separated commercial off-the-shelf (COTS) software-defined radios (SDRs) without any cables or external references such as global navigation satellite system (GNSS). Building on previous work using high-accuracy spectrally-sparse time of arrival (ToA) waveforms and a multi-step ToA refinement process, a high-accuracy two-way time transfer (TWTT)-based time-frequency coordination approach is demonstrated. By using a high-accuracy time estimation approach, frequency estimates can be derived over long observation intervals leading to a high-accuracy frequency estimate, without the requirement for long pulse durations as is required for direct spectral frequency estimation techniques, minimizing coordination overhead. Furthermore, due to the two-way nature of the high-accuracy TWTT approach, the time and frequency estimates are Doppler and multi-path tolerant, so long as the channel is reciprocal over the synchronization epoch. This technique is experimentally verified by demonstrating wireless distributed array coordination using COTS SDRs in a lab environment in static and dynamic scenarios and with significant multipath scatterers. Time, frequency, and phase stability were measured over coaxial cables to an oscilloscope and achieved time and phase coordination precision of ~60-70 ps, with median coherent gains above 99% using optimized parameters, and a beamforming frequency RMSE of 3.73 ppb in a dynamic scenario. Finally, experiments are conducted to compare the performance of this technique with previous work works using an analog continuous-wave two-tone (CWTT) frequency reference technique in both static and dynamic settings as a benchmark.

A Compact Narrowband Antenna Design for RF Fingerprinting Applications

Radio frequency (RF) fingerprinting is widely used for supporting physical layer security in various wireless applications. In this paper, we present the design and implementation of a small antenna with low-cost fabrication that can be directly integrated with nonlinear passive devices, forming a passive RF tag providing unique nonlinear signatures for RF fingerprinting. We first propose a miniaturized meander line dipole, achieved by two folded arms on two sides of the substrate. This leads to antenna with a simple feeding structure and compact size, making it ideal for planar integration. Two antennas on Rogers 4350B and ultra-thin flexible Panasonic Felios are fabricated, achieving small size at $0.21 \times 0.06 \times 0.004 \lambda_0^3$ and $0.14 \times 0.1 \times 0.0008 \lambda_0^3$ with realized gain of 1.87 dBi and 1.46 dBi. The passive tag consists of the proposed antenna structure and an integrated RF diode, and is further developed on both substrates, aiming to generate inter-modulation products (IMP) due to the nonlinearity of the diode, which can be used for device identification through classification algorithms. We investigate the nonlinearity of the designed tags for transmission at 15 dBm using two-tone signals. All tags produce a significant increased power at IMP frequencies at a range of 0.4 m. The tags on Rogers substrate provide around 23 dB IMP power increase and tags on flexible substrate embedded in lossy material provide around 16 dB power increase. These findings confirm that the proposed solution offers a simple passive tag design to support unique nonlinear signatures for RF fingerprinting applications in a simple, low-cost device.

Distributed Beamforming Using Decentralized Time Synchronization in a Six-Element Array

We demonstrate a distributed beamforming and beamsteering from a six-node distributed phased array using fully wireless coordination with decentralized time synchronization. In wireless applications such as distributed beamforming, high-accuracy time synchronization across the array is crucial for high coherent gain. The decentralized time synchronization method employed is based on the average consensus algorithm and the two-way time transfer method presented in our previous work, which achieved picosecond time synchronization with a cabled frequency reference. The system presented in this paper utilizes a centralized wireless frequency transfer method to achieve wireless frequency syntonization in a fully wireless coordination and a distributed computing system architecture. We experimentally evaluate system performance through beamforming and beamsteering to a receiver 16.3 m away from the six-node non-uniformly distributed antenna array, achieving an average coherent gain of 98% of the ideal gain at a carrier frequency of 1.05 GHz. The average time synchronization accuracy achieved was less than 36 ps.

Additive Frequency Diverse Active Incoherent Millimeter-Wave Imaging

We present an approach for improving spatial frequency sampling in active incoherent millimeter-wave (AIM) imaging systems using frequency diversity. AIM imaging relies on active transmission of spatio-temporally incoherent signals to illuminate a scene, from which interferometric Fourier-domain imaging can be implemented using a sparse receiving antenna array. One of the benefits of Fourier domain imaging is the sparsity of the receiving array, which can form images with equivalent resolution to traditional filled beamsteering arrays, but with a small fraction of the elements. The hardware reduction afforded by the sparse array often leads to an undersampled Fourier space, where even though image formation is possible, the image reconstruction may be degraded when viewing complex objects. To address this challenge without requiring additional receiver channels, we explore the use of frequency diversity in the illuminating and receiving systems. Fourier domain spatial frequency samples are determined by the electrical spacing and rotation of the receiving elements, thus by changing the frequency the sampled spatial frequencies also change. We implement an additive technique where the spatial frequency samples are summed prior to Fourier transform image formation. Importantly, because the system is active, a consistent signal-to-noise ratio is maintained across all frequencies, which may not be possible in traditional passive Fourier-domain imagers.

A Dipole Antenna with a Dynamic Balun for Wireless Security via Amplitude Based Directional Modulation

We present a new approach to secure wireless operations using a simple dipole antenna with a dynamic unbalanced feeding structure. By rapidly switching between two states, a dynamic radiation pattern is generated, resulting in directional modulation. The current distribution on the arms of the dipole antenna are made asymmetric by the balun, which changes the relative phase between the feed currents. By rapidly switching between two mirrored states, the relative phase shift changes sign, causing the current distribution to manifest asymmetrically on the arms of the dipole antenna. The resultant far field radiation pattern is therefore asymmetric in both states, but mirrored between the two states. Rapid switching between the two states results in a far-field pattern that is dynamic in amplitude at all angles except for a narrow region of space, which is referred to as the information beam. The dynamic radiation pattern causes additional modulation on any transmitted or received signals, thereby obscuring the information at all angles outside the information beam. The proposed directional modulation technique is separate from both the antenna and the rest of the wireless system, and can thus be implemented in a black box form in wireless communications or sensing systems. We demonstrate the concept in a 1.86 GHz printed dipole antenna, demonstrating the transmission of 256-QAM signals.

Decentralized Localization of Distributed Antenna Array Elements Using an Evolutionary Algorithm

Distributed phased arrays have recently garnered interest in applications such as satellite communications and high-resolution remote sensing. High-performance coherent distributed operations such as distributed beamforming are dependent on the ability to synchronize the spatio-electrical states of the elements in the array to the order of the operational wavelength, so that coherent signal summation can be achieved at any arbitrary target destination. In this paper, we address the fundamental challenge of precise distributed array element localization to enable coherent operation, even in complex environments where the array may not be capable of directly estimating all nodal link distances. We employ a two-way time transfer technique to synchronize the nodes of the array and perform internode ranging. We implement the classical multidimensional scaling algorithm to recover a decentralized array geometry from a set of range estimates. We also establish the incomplete set of range estimates as a multivariable non-convex optimization problem, and define the differential evolution algorithm which searches the solution space to complete the set of ranges. We experimentally demonstrate wireless localization using a spectrally-sparse pulsed two-tone waveform with 40 MHz tone separation in a laboratory environment, achieving a mean localization error vector magnitude of 0.82 mm in an environment with an average link SNR of 34 dB, theoretically supporting distributed beamforming operation up to 24.3 GHz.