Jason Merlo

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.

Antenna arrays have become an essential piece of modern life enabling applications from high bandwidth mobile communications to radar and earth remote sensing by providing high directivity and electronically controllable beam patterns. Traditionally, these arrays have been assembled using antenna elements with fixed locations, coupled via fixed radio frequency (RF) transmission lines to a central processing location. In recent years, demands for more resilient versatile multifunction antenna arrays have spurred interest in wirelessly coordinated coherent distributed antenna arrays (CDAs) wherein individual elements or groups of elements (i.e., subarrays), referred to as nodes, can be moved freely relative to one another with the RF signals being generated and digitized on node and processed in a partially- or fully-decentralized manner. This provides CDAs with several distinct advantages over traditional fixed arrays: due to the mobility of the elements, the element locations can be reconfigured based on the desired beam pattern for a given operating mode and frequency; nodes may join and leave the array in an ad-hoc manner allowing array deployments over time and graceful degradation upon node failure; and because each node provides its own power source, the overall array gain scales as N2 as opposed to N for conventional arrays, enabling high bandwidth communication and high sensitivity sensing with lower system power handling requirements on any single node. While these advantages are significant, the challenge of wirelessly coordinating the electrical states—time, frequency, and phase—of each element to a sub-wavelength level remains an area of ongoing research. Generally, these arrays can be implemented in a “closed-loop” configuration where the array elements receive feedback from a cooperative receiver node in a communications-type application, or in an “open-loop” configuration where the nodes directly coordinate their states amongst themselves, enabling communication with conventional, noncooperative communications terminals as well as radar and remote sensing operations where the destination is typically passive.In this work, I present a system for high-accuracy coordination of the RF electrical states for CDA nodes consisting of commercial off-the-shelf (COTS) software-defined radios (SDRs). I will discuss a computationally-efficient high-accuracy time and phase synchronization technique which utilizes spectrally-sparse waveforms to minimize spectrum utilization while simultaneously maximizing the lower-bound on accuracy and a multi-stage refinement technique to achieve synchronization on the order of picoseconds. Furthermore, I will discuss a new high-accuracy frequency syntonization technique based on the high-accuracy synchronization method which estimates and compensates for the platform oscillator frequency offsets using temporally-sparse timing estimates. This technique provides advantages over previous techniques in that it is tolerant to relative motion, multipath, and non-line-of-sight (NLoS); it is necessarily both spectrally- and temporally-sparse which directly provides high-accuracy; and it is fully-digital and can be directly implemented on existing COTS SDR hardware without requiring voltage tunable oscillators. Finally, I will demonstrate several applications of CDAs ranging from collaborative beamforming which operates similar to conventional phased arrays to long-baseline interferometry which utilizes the spatial sparsity of the antenna elements to perform direct measurements of a scene in the spatial frequency domain.

In this work we experimentally demonstrate a wirelessly coordinated three-element coherent distributed phased array performing beamforming and beam steering to a target in the far-field over 17 m at a carrier frequency of 1.05 GHz. We build on our previous work utilizing a high accuracy two-way time transfer (TWTT) technique for inter-node time synchronization and ranging and an analog continuous two-tone frequency transfer technique to perform syntonization by moving to a fully wireless architecture, without the need for external reference signals such as global navigation satellite system (GNSS)-derived pulse-per-second (PPS), including a distributed computing system architecture, and the use of fully wireless communication links between nodes. Finally, we perform a far-field beamforming experiment with beam steering to a receiver 17 m away at a carrier frequency of 1.05 GHz and demonstrate a beamforming coherent gain of 0.95 (9.32 dB) with a beamforming inter-element timing accuracy of below 60 ps.

In this work we present a fully wireless coherent distributed interferometric radar for sensing the radial and angular velocities of dynamic targets. We build upon prior work on wireless coordination in both time and frequency by utilizing a high accuracy two-tone frequency transfer method for frequency syntonization and global navigation satellite system-based pulseper-second timing alignment. The interferometric radar is based on software-defined radios and uses a continuous-wave 3.3 GHz signal to measure the Doppler and interferometric frequency shifts of signals scattered from moving targets. The two nodes in the array are separated by 91 cm (10λ). Coordination between the nodes was implemented wirelessly at 4.3 GHz, and received radar signals were transferred for joint processing at one node at 5.8 GHz. We demonstrate the performance of the distributed interferometric radar by measuring the instantaneous radial and angular velocities of a pedestrian carrying a corner reflector tangentially past the array.

In this work, we experimentally demonstrate, for the first time, a fully wireless coherent distributed antenna array (CDA) performing distributed transmit and receive beamforming for a down-range sensing application at microwave frequencies without the need for external frequency and time references such as global navigation satellite systems (GNSSs). We build on previous methods utilizing a continuous-wave (CW) two- tone frequency transfer system and a pulsed two-tone time synchronization system to align the distributed platforms in both time and frequency. Coherent transmit and receive beamforming was performed utilizing 100 MHz linear frequency modulation (LFM) waveforms for sensing. Two experiments were performed: one imaging a static scene and one imaging a moving pedestrian holding a corner reflector. The beamforming gain is quantified in the static measurements yielding a median beamforming gain of 2.12 dB and a maximum gain of 2.86 dB (96.5% coherent beamforming gain).

Picosecond-level timing synchronization with wireless frequency transfer is presented in a non-line-of-sight (NLoS) environment using two software-defined radios (SDRs). Using a spectrally sparse pulsed two-tone waveform, high accuracy timing synchronization is achieved. Frequency synchronization is achieved via a continuous 10 MHz two- tone waveform transmitted between nodes which is demodulated by a self-mixing circuit to recover the 10 MHz reference signal used to discipline the local oscillator on the receiving SDR. Total time synchronization, beamforming, and phase accuracies of 8.84 ps, 23.17 ps, and 10\,^∘, respectively, were achieved enabling modulation bandwidths of up to 4.3 Gb/s at a carrier frequency of up to 1.33 GHz.

We present a two node wideband wireless distributed antenna array operating from 3.1–3.3 GHz on software-defined radios (SDRs). To synthesize the total bandwidth of 300 MHz, a 100 MHz bandwidth signal was used over multiple frequency steps, with the carrier frequency re-tuned between each pulse. The nodes were wirelessly synchronized using a high-accuracy two-way time transfer technique which was also used to determine the inter-node range for beamforming. A total beamforming bias \pm standard deviation of 62.46 \pm 45.74 ps and 0.27 \pm 2.28\,^∘at the target beamforming angle at a range of 35.5 m was achieved.

Wirelessly coordinated distributed arrays will have a significant impact in next generation communications and sensing systems in coming years. However, to achieve high modulation bandwidths and carrier frequencies, timing and frequency transfer must be synchronized to small fractions of the modulation and carrier periods respectively. In this work, we demonstrate coordination and beamforming of a wireless distributed antenna array at 1 GHz in a cluttered outdoor environment achieving a coherent gain of 95.4% and beamforming accuracy of 27.6 ps over a 41 m channel.

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 40MHz two-tone waveform over a 90cm 5.8GHz wireless link in a laboratory setting, obtaining a timing precision of 2.26ps.

Due to the rapid increase in 76 GHz automotive spectrum use in recent years, wireless interference is becoming a legitimate area of concern. However, the recent rise in interest of automated vehicles (AVs) has also spurred new growth and adoption of low frequency vehicle-to-everything (V2X) communications in and around the 5.8 GHz unlicensed bands, opening the possibility for communications spectrum reuse in the form of joint radar-communications (JRC). In this work, we present a low frequency 5.9 GHz side-looking polarimetric synthetic aperture radar (SAR) for automotive use, utilizing a ranging waveform in a common low frequency V2X communications band. A synthetic aperture technique is employed to address the angular resolution concerns commonly associated with radars at lower frequencies. Three side-looking fully polarimetric SAR images in various urban scenes are presented and discussed to highlight the unique opportunities for landmark inference afforded through measurement of co- and cross-polarized scattering.

In this work, a method for directly measuring target velocity in three dimensions using a dual axis correlation interferometric radar is presented. Recent advances have shown that the measurement of a target’s angular velocity is possible by correlating the signals measured at spatially diverse aperture locations. By utilizing multiple orthogonal baselines and using conventional Doppler velocity methods to obtain radial velocity, a full three-dimensional velocity vector can be obtained using only three receive antennas and a single transmitter, without the need for tracking. A 41.8 GHz dual axis interferometric radar with a 7.26λ antenna baseline is presented along with measurements of a target moving parallel to the plane of the radar array, and of a target moving with components of both radial and tangential velocity. These experiments achieved total velocity root-mean-square errors of 41.01 mm/s (10.5%) for a target moving along a plane parallel to the array, and 45.07 mm/s (13.5%) for a target moving with components of radial and tangential motion relative to the array; estimated trajectory angle RMSEs of 10.42\,^∘ and 5.11\,^∘ were achieved for each experiment respectively.

The design of a portable frequency-modulated continuous-wave (FMCW) radar system for detecting and localizing drones is discussed in this report. The system utilized the 5.8 GHz industrial, scientific and medical band and occupied a 100 MHz bandwidth. The system is able to localize drones at a range of 10 m or more. The theory behind FMCW radar localization is explained, and an algorithm for detecting the drone response is presented. The design considerations of a radar board and transmit and receive patch antenna arrays are discussed. We include experimental measurements of tracking a drone.