Jason Merlo

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).

Timing synchronization plays a critical role in many high performance software defined radio applications. However, currently, to achieve the level of synchronization required to perform distributed open loop beamforming, cabled techniques such as precision PPS fanout buffers or White Rabbit must be used to align the system clocks to within a small fraction of a sample. However, we have recently presented techniques to achieve picosecond-level synchronization wirelessly for distributed phased array beamforming accomplished using the RF front-end on Ettus X300 software defined radios. This talk will address the waveform design, time delay estimation and refinement process, and software implementation strategies used to achieve this high level of performance using host-controlled processing in GNU Radio.

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.