Garrett J. Schneider

Time-reversal (TR) signal processing is a promising approach for mitigating multipath propagation effects in wideband wireless systems by leveraging channel reciprocity to pre-compensate for propagation-induced symbol delays, phase misalignment, and interference at the transmitter. This enables temporal compression at the receiver, resulting in the constructive combination of multipath components, reduced inter-symbol interference (ISI), and improved spatial focusing, while shifting the signal processing burden away from the receiver. However, the practical implementation of TR in distributed multi-antenna systems is fundamentally limited by the need for tight phase coherence and frequency synchronization across transmitters, which is difficult to achieve with conventional RF architectures that rely on independent oscillators. In this work, we present an RF-photonic distribution architecture that enables distributed coherent multi-transmitter TR operation by driving up to 10 transmit antennas from a single optical source, thereby ensuring phase coherence and frequency synchronization across all paths. Using experimentally measured channel states over the 6G carrier frequency of 20 GHz and an instantaneous bandwidth of 600 MHz, we demonstrate time-reversal transmission with strong temporal compression and spatial focusing. These results highlight the critical role of coherence in distributed TR systems and establish RF-photonic architectures as a scalable solution for multi-antenna time-reversal communication in emerging wireless networks.

This article presents the development and demonstration of a photonically actuated ultrawideband (UWB), 2-D transmit array using a Fourier optical lens as a beamformer. The transmitted beam direction is selected using a beam-space optical fiber array fed through an optical switch matrix. A data encoding scheme was developed, in which custom intermediate frequency (IF) signal/data can be modulated on phase-locked tunable optical paired sources (TOPSs). A 2-D RF front-end, consisting of a tapered slot array (TSA), medium power amplifier (MPA), and needle-packaged photodetector (PD) arrays, was also developed. Experimental measurement of the far-field pattern of the developed system was conducted to demonstrate 2-D wide angular beam steering over an ultrawide frequency band from 10 to 20 GHz.

This letter presents low-loss monolithic Barium Titanium Oxide (BaTiO3), hereafter referred to as BTO, waveguides fabricated on a Silicon-on-insulator (SOI) platform, using a novel measurement approach based on optical back-scattering reflectometry (OBR). Utilizing this method, a low average propagation loss of 0.7 dB/cm was achieved for a straight waveguide which, to our knowledge, is the lowest propagation loss reported to date based on long BTO ridge-waveguides. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) highlight the smooth sidewalls responsible for lowering the waveguide loss. Design simulations and fabrication processes are presented, followed by optical characterization.

As the demand for data capacity in wireless networks and mobile communications continues to grow, they are moving toward higher carrier frequencies and wider modulation bandwidths. Unfortunately, electronic device performance degrades in association with increased frequency and modulation bandwidths, which inhibits the application of conventional microwave architectures, particularly in the millimeter wave and terahertz regimes. Alternatively, microwave photonic systems address these challenges by offering device and system performance with exceptionally higher operational bandwidths. The challenge, however, is the ability to monolithically integrate both electronic and photonic devices into functional components that provide ultra-wideband performance up into the millimeter wave and terahertz regions. In particular, such integration remains a major technical challenge due to the high dielectric permittivity of commonly used material platforms for photonic integrated circuits, such as silicon, indium phosphide, and lithium niobate. In this paper, we present a photonic receiver consisting of a broadband antenna and a low-drive-voltage modulator monolithically integrated on thin-film lithium niobate with a quartz handle. A free-space data link is demonstrated, achieving data rates up to 2.7 Gbps using quadrature amplitude modulation, with error vector magnitude as low as 3%. This work demonstrates the potential of thin-film lithium niobate for high-frequency, monolithically integrated radiofrequency and photonic devices to enable ultra-wideband millimeter wave-to-terahertz communication systems.

We present a method for the recovery of two-dimensional angle-of-arrival information by beamspace processing using only a linear array of elements in the beam-forming processor. The method exploits the presence of grating lobes in beam patterns formed when using phased arrays with regularly spaced antenna elements. Periodic distribution of grating lobes in the reciprocal space allows, for some arrays, ‘unrolling’ a two-dimensional beamspace pattern into a linear distribution of beams, which is amenable to recovery using a linear array of elements in the beamspace processor. We develop mathematical underpinning of the approach and analyze several explicit examples of such array-beamspace (AB) mapping.

This paper presents a class of phased-array systems that function as video-rate imagers in the millimeter-wave (mmW) and sub-THz bands. While the systems presented operate in the Ka-band (35 GHz) and W-band (77 and 86 GHz), the approach is scalable to the THz regime. Their operation is based on the upconversion of incident mmW and sub-THz signals to the optical domain using high-speed electro-optic modulators (EOMs) that are connected to each antenna element in a phased-array antenna. The output optical fiber from each EOM is relayed to a fiber bundle, or optical fiber array, from which the upconverted mmW/sub-THz signals are launched into free space. Because the upconversion preserves both the temporal and spatial coherence, through a spatial phase-control loop (SPCL), the launched sideband signals re-form the beamspace of the incident mmW or sub-THz signals, but in the optical domain. At this point, a lens performs a 2D spatial Fourier transform, to produce a real-time image of the mmW or sub-THz signals from the environment on a short-wave infrared (SWIR) camera, which renders the scene at video rates. The fundamental operating principles of these systems are presented, along with the historical progress in their development, and experimental demonstrations.

As sixth-generation (6G) communication systems manifest at carrier frequencies well into the millimeter-wave (mmW) spectrum, the ability of conventional digital beamforming techniques to handle the beam-bandwidth product is increasingly stressed. Microwave photonic beamforming has been presented as a solution to this problem by up-converting a sampled RF field distribution to an optical carrier for analog beam-space processing, but to date has relied upon fiber arrays with the same dimensionality as the RF array, i.e. a two-dimensional RF array requires a two-dimensional fiber array and a three-dimensional optical processor to perform the Fourier transform required for two-dimensional beamforming. To address this problem, we present an approach to photonic mmW beamforming wherein two-dimensional phase information is preserved through a one-dimensional Fourier transform leveraging grating lobes in the array response. This approach carries several benefits, primarily as an enabler for leveraging photonic integrated circuits for RF-photonic beamforming, carrying with it a footprint reduction of more than ten thousand times. Furthermore, beamforming efficiency is increased for sources near the limits of the RF field-of-view; improvements to throughput power in such cases are as much as double. Theory, simulations, and experimental results in the form of images and videos are presented to validate the approach for a nineteen-element hexagonally-distributed phased array.

Current trends in wireless communications require a Base Transceiver Station (BTS) to support an ever-increasing number of antennas, wider bandwidths, multiple frequency bands, and many simultaneous beams. This is a tall order when considering all-electronic implementations, especially when operating at high carrier frequencies. We describe here Fourier-optics based opto-electronic architectures that include analog and fully connected transmit (TX) and receive (RX) implementations that support simultaneous multi-band, multi-beam phased arrays where optical up- and down-conversion are used to generate IF/RF and RF/IF, respectively, signals for simultaneous multi-user and multi-beam transmission and reception over a wide range of frequencies. The proposed lens-based architectures have the ability to transmit and receive multiple distinct beams simultaneously even when only analog beamforming is performed, thus greatly simplifying the tasks of cell search and initial access without having to resort to complex matrix-based beamforming networks. Additionally, lens-based TX and RX beamforming is performed in the analog optical domain with zero power consumption and negligible latency bounded only by the time the light takes to travel through the lens. Photonic signal processing also reduces the number of RF components required at the remote radio unit (RRU) and the number of costly and power-hungry high-performance analog-to-digital/digital-to-analog converters (ADCs/DACs) required. The optical subsystems are ultra-wideband and frequency agnostic as only the front-end components (antennas, amplifiers) are RF frequency/band specific. Therefore, a single photonic system design may be operated at any band and, furthermore, existing installations may be modified/upgraded to operate in different bands with minimal component replacements needed. Finally, the presented architectures provide near unlimited beam-bandwidth product (BBP) with minimal power requirements and no external cooling. Theoretical analysis and experimental confirmation of the proposed architecture will both be reported, including a receiver with a nominal BBP of 36 GHz with a prototype system that consumed less than 300 W, thus yielding a power efficiency of beam formation of 8 W/GHz, which is more than a 6× improvement over the current state of the art.

In this paper, we propose an architecture wherein the radio-frequency (RF) field at the antenna array is up-converted to an optical carrier and passed through an array of optical fibers, after which an analog Fourier transform is taken in a free-space optical processor. Through the use of multiple temporal dispersion projections, implemented through varied-length optical fiber segments, the locations of RF sources in three-dimensional space spanning angle-of-arrival (AoA) and instantaneous frequency may be determined on a millisecond time scale using commercial computing hardware after detection by a charge-coupled device (CCD) camera. We present a mathematical formulation of the problem, followed by simulated and experimental results showing three-dimensional spatial-spectral localization through the solution to a system of equations brought forth through the use of Fourier optics to process the RF field.

As mobile beam-bandwidth-product requirements accelerate, millimeter-wave (mmW) bands have been opened to telecommunications networks to enable wider channel bandwidths, while Massive Multiple-Input Multiple-Output (mMIMO) technology has been implemented to concurrently address multiple devices at the same frequency from a single base station. Such space-division multiplexing can be combined with spectral multiplexing to enable a very large number of concurrent users, but currently is implemented through computationally intensive digital beamforming networks. We show that a radio-frequency (RF) photonic receiver system, previously shown to be capable of sorting signals into respective spatial-spectral ‘bins’ is further capable, through an injection-locked tunable optical local oscillator (TOLO), of recovering the data upon each signal in the RF scene. The TOLO is combined in free-space with an up-converted optical sideband and the combined optical field impinges upon an array of photodetectors, each corresponding to separate points in k-space, defined by unique combinations of angle-of-arrival (AoA) and carrier frequency. Using this free-space LO insertion, we demonstrate simultaneous recovery of multiple spatially co-located data streams with resilience to interference.