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.

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.

Decentralized Picosecond Synchronization for Distributed Wireless Systems

We demonstrate a wireless, decentralized time-alignment method for distributed antenna arrays and distributed wireless networks that achieves picosecond-level synchronization. Distributed antenna arrays consist of spatially separated antennas that coordinate their functionality at the wavelength level to achieve coherent operations such as distributed beamforming. Accurate time alignment (synchronization) of the local clocks on each node in the array is necessary to support accurate time-delay beamforming of modulated signals. In this work we combine a consensus averaging algorithm and a high-accuracy wireless two-way time transfer method to achieve decentralized time alignment, correcting for the time-varying bias of the clocks in a method that has no central node. Internode time transfer is based on a spectrally-sparse, two-tone signal achieving near-optimal time delay accuracy. We experimentally demonstrate the approach in a wireless four-node software-defined radio system, with various network connectivity graphs. We show that within 20 iterations all the nodes achieve convergence within a bias of less than 12 ps and a standard deviation of less than 3 ps. The performance is evaluated versus the bandwidth of the two-tone waveform, which impacts the synchronization error, and versus the signal-to-noise ratio.

Design of a Single-Element Dynamic Antenna for Secure Wireless Applications

We introduce a new technique for secure wireless applications using a single dynamic antenna. The dynamic antenna supports a constantly changing current distribution that generates a radiation pattern that is static in a desired direction and dynamic elsewhere, thereby imparting additional modulation on the signal and obscuring information transmitted or received outside of the secure spatial region. Dynamic currents are supported by a single feed that is switched between separate ports on a single antenna, generating two different radiation patterns. We introduce the theoretical concept by exploring an ideal complex dynamic radiation pattern that remains static in a narrow desired direction and is dynamic elsewhere. The impact on the transmission of information is analyzed, showing that the secure region narrows as the modulation order increases, and design constraints on the spatial width of the secure region as a function of modulation format are determined. We design and analyze a 2.3 GHz two-state dynamic dipole antenna and experimentally demonstrate secure wireless transmission. We show the ability to steer the secure region experimentally, and to maintain high throughput in the secure region while obscuring the information elsewhere. Our approach introduces a novel single-element technique for secure wireless applications that can be used independently from the rest of the wireless system, essentially operating as a $``$black box$"$ for an additional layer of security.

Wireless Picosecond Time Synchronization for Distributed Antenna Arrays

Distributed antenna arrays have been proposed for many applications ranging from space-based observatories to automated vehicles. Achieving good performance in distributed antenna systems requires stringent synchronization at the wavelength and information level to ensure that the transmitted signals arrive coherently at the target, or that scattered and received signals can be appropriately processed via distributed algorithms. In this paper we address the challenge of high precision time synchronization to align the operations of elements in a distributed antenna array and to overcome time-varying bias between platforms due to oscillator drift. We use a spectrally sparse two-tone waveform, which obtains approximately optimal time estimation accuracy, in a two-way time transfer process. We also describe a technique for determining the true time delay using the ambiguous two-tone matched filter output, and we compare the time synchronization precision of the two-tone waveform with the more common linear frequency modulation (LFM) waveform. We experimentally demonstrate wireless time synchronization using a single pulse 40$\,$MHz two-tone waveform over a 90$\,$cm 5.8$\,$GHz wireless link in a laboratory setting, obtaining a timing precision of 2.26$\,$ps.

Distortion Mitigation in Millimeter-Wave Interferometric Radar Angular Velocity Estimation Using Signal Response Decomposition

A new method of distortion mitigation for multitarget interferometric angular velocity estimation in millimeter-wave radar is presented. In general, when multiple targets are present, the response of a correlation interferometer is corrupted by intermodulation distortion, making it difficult to estimate individual target angular velocities. We present a distortion mitigation method that works by decomposing the responses at each antenna element into the responses from the individual targets. Data association is performed to match individual target responses at each antenna such that cross-correlation is performed only between associated targets. Thus, the intermodulation distortion (cross-terms) from correlating unlike targets are eliminated, and the result is a frequency response whose individual frequencies are proportional to the angular velocities of the targets. We demonstrate the approach with a custom 40 GHz interferometric radar, a high-accuracy motion capture system which provides ground-truth position measurements, and two robotic platforms. The multitarget experiments consist of three scenarios, designed to represent easy, medium, and difficult cases for the distortion mitigation technique. We show that the reduction in distortion yields angular velocity estimation errors in the three cases of less than $0.008$ rad/s, $0.020$ rad/s, and $0.033$ rad/s for the easy, medium, and hard cases, respectively.