Algorithms and Circuits for Analog-Digital Hybrid Multibeam Arrays

Fifth generation (5G) and beyond wireless communication systems will rely heavily on larger antenna arrays combined with beamforming to mitigate the high free-space path-loss that prevails in millimeter-wave (mmW) and above frequencies. Sharp beams that can support wide bandwidths are desired both at the transmitter and the receiver to leverage the glut of bandwidth available at these frequency bands. Further, multiple simultaneous sharp beams are imperative for such systems to exploit mmW/sub-THz wireless channels using multiple reflected paths simultaneously. Therefore, multibeam antenna arrays that can support wider bandwidths are a key enabler for 5G and beyond systems. In general, N-beam systems using N-element antenna arrays will involve circuit complexities of the order of N2. This dissertation investigates new analog, digital and hybrid low complexity multibeam beamforming algorithms and circuits for reducing the associated high size, weight, and power (SWaP) complexities in larger multibeam arrays. The research efforts on the digital beamforming aspect propose the use of a new class of discrete Fourier transform (DFT) approximations for multibeam generation to eliminate the need for digital multipliers in the beamforming circuitry. For this, 8-, 16- and 32-beam multiplierless multibeam algorithms have been proposed for uniform linear array applications. A 2.4 GHz 16-element array receiver setup and a 5.8 GHz 32-element array receiver system which use field programmable gate arrays (FPGAs) as digital backend have been built for real-time experimental verification of the digital multiplierless algorithms. The multiplierless algorithms have been experimentally verified by digitally measuring beams. It has been shown that the measured beams from the multiplierless algorithms are in good agreement with the exact counterpart algorithms. Analog realizations of the proposed approximate DFT transforms have also been investigated leading to low-complex, high bandwidth circuits in CMOS. Further, a novel approach for reducing the circuit complexity of analog true-time delay (TTD) N-beam beamforming networks using N-element arrays has been proposed for wideband squint-free operation. A sparse factorization of the N-beam delay Vandermonde beamforming matrix is used to reduce the total amount of TTD elements that are needed for obtaining N number of beams in a wideband array. The method has been verified using measured responses of CMOS all-pass filters (APFs). The wideband squint-free multibeam algorithm is also used to propose a new low-complexity hybrid beamforming architecture targeting future 5G mmW systems. Apart from that, the dissertation also explores multibeam beamforming architectures for uniform circular arrays (UCAs). An algorithm having N log N circuit complexity for simultaneous generation of N-beams in an N-element UCA is explored and verified

Deep Learning based Fast and Accurate Beamforming for Millimeter-Wave Systems

The widespread proliferation of mmW devices has led to a surge of interest in antenna arrays. This interest in arrays is due to their ability to steer beams in desired directions, for the purpose of increasing signal-power and/or decreasing interference levels. To enable beamforming, array coefficients are typically stored in look-up tables (LUTs) for subsequent referencing. While LUTs enable fast sweep times, their limited memory size restricts the number of beams the array can produce. Consequently, a receiver is likely to be offset from the main beam, thus decreasing received power, and resulting in sub-optimal performance. In this letter, we present BeamShaper, a deep neural network (DNN) framework, which enables fast and accurate beamsteering in any desirable 3-D direction. Unlike traditional finite-memory LUTs which support a fixed set of beams, BeamShaper utilizes a trained NN model to generate the array coefficients for arbitrary directions in \textit{real-time}. Our simulations show that BeamShaper outperforms contemporary LUT based solutions in terms of cosine-similarity and central angle in time scales that are slightly higher than LUT based solutions. Additionally, we show that our DNN based approach has the added advantage of being more resilient to the effects of quantization noise generated while using digital phase-shifters.Comment: 7 pages, 5 figures, accepted to Milcom2023. Not published yet(Sep 2023

Impact and modeling of phase noise in mmW beamforming systems

Abstract. Due to the exponential growth of wireless communication, mobile communication applications require more bandwidth available in higher operating frequencies. High centre frequency makes the systems sensitive for phase variations caused by the phase noise (PN) of the imperfect local oscillators (LOs) used in wireless transceivers. Moreover, wide bandwidth also makes the faster phase variations of the phase noise spectra have an impact on the overall system performance by reducing effective signal-to-noise-ratio. These fast variations seen in the high offset frequencies in the phase noise spectra are typically ignored in the communication systems because the traditional system bandwidths are in order of megahertz, or in maximum few gigahertz. In mmW frequencies, i.e., at 30–300 GHz, the transceivers are typically using multiple antenna elements to achieve the required link range by highly directional beams. Often so-called phased arrays are used to implement the multi-antenna transceiver, where the beamforming is mostly performed in the analog domain by digitally controllable mmW phase shifters. For generating multiple beams from the same transceivers, more than one phased array is typically used in the same platform. The phased arrays often share a single LO, for multiple antenna elements. A typical LO generation architecture is containing a base clock, phased-locked loop (PLL), and some frequency multipliers to achieve the target mmW operating frequency. In multi-array systems, the LO signal can be divided into phased arrays in multiple domains, i.e., the arrays can have an independent clock, and a shared clock, but independent PLLs, shared PLL, or even the final mmW LO can be shared. In different architectures, the phase noise has different behavior, and it can have an impact for example on the beamforming accuracy. This thesis focuses on the effects of phase noise on milimeter-wave (mmW) beamforming systems to study different LO routing architectures. We mainly focus on LO architecture with multiple phased arrays that intend to make a common beamformer and their impact on overall system-level phase noise performance. The specific focus is given to the behavior of the wideband phase noise. The phase noise is modeled by using baseband equivalent models where a gaussian phase noise source is filtered by a filter modeling the equivalent phase noise spectra. The parameterization of the model is based on commercial LO phase noise spectra. The behavior is studied in different LO schemes in single-beam and multi-beam scenarios by using simple examples. The simulations are mostly carried out by using continuous-wave signals, but also the single-carrier modulated QAM waveform is demonstrated. The simulations are performed in MATLAB

Digital phase tightening for improved spatial resolution in millimeter-wave imaging systems

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.Includes bibliographical references (leaves 68-69).Imaging systems using millimeter-wave frequencies allow for the possibilities of vehicular radar and concealed weapons detection. By using silicon technology, the integration of millimeter-wave circuits can reach new levels that were previously impossible. This thesis discusses the challenge and design of a mm-wave imaging system using a technique called digital phase tightening for improved spatial resolution. Digital phase tightening uses feedback and oversampling to accurately measure the amplitude and phase of an incoming signal. Furthermore, it can be implemented using only a delay-lock loop, an analog-to-digital converter, and a counter. A proof of concept system utilizing a 2.4GHz delay-lock loop with supporting circuitry is designed in 90nm CMOS. Test results demonstrate a proof of concept system with a measured DLL resolution of 41.7ps that consumes 36mW of power. The goal of the system is to reduce the jitter of phase measurements to the order of femto-seconds. In the proto system, the quantization error is larger than the Gaussian noise; therefore, significant improvements in the accuracy of the phase measurements were not observed.by Ke Lu.S.M

Simulation analysis of algorithms for interference management in 5G cellular networks using spatial spectrum sharing

In this thesis we completely overhaul past techniques to the new millimeter wave frequencies used in 5G and the aim is to study algorithm, protocols and architectures enablers to allow spatial spectrum sharing between different networks at these frequencies. With the use of specific modules of the network simulator ns-3, studies of simulations has been made in order to analyse performance of several sharing procedure with the goal of increase performance in a 5G mobile networkope

Design and Realization of Fully-digital Microwave and Mm-wave Multi-beam Arrays with FPGA/RF-SOC Signal Processing

There has been a constant increase in data-traffic and device-connections in mobile wireless communications, which led the fifth generation (5G) implementations to exploit mm-wave bands at 24/28 GHz. The next-generation wireless access point (6G and beyond) will need to adopt large-scale transceiver arrays with a combination of multi-input-multi-output (MIMO) theory and fully digital multi-beam beamforming. The resulting high gain array factors will overcome the high path losses at mm-wave bands, and the simultaneous multi-beams will exploit the multi-directional channels due to multi-path effects and improve the signal-to-noise ratio. Such access points will be based on electronic systems which heavily depend on the integration of RF electronics with digital signal processing performed in Field programmable gate arrays (FPGA)/ RF-system-on-chip (SoC). This dissertation is directed towards the investigation and realization of fully-digital phased arrays that can produce wideband simultaneous multi-beams with FPGA or RF-SoC digital back-ends. The first proposed approach is a spatial bandpass (SBP) IIR filter-based beamformer, and is based on the concepts of space-time network resonance. A 2.4 GHz, 16-element array receiver, has been built for real-time experimental verification of this approach. The second and third approaches are respectively based on Discrete Fourier Transform (DFT) theory, and a lens plus focal planar array theory. Lens based approach is essentially an analog model of DFT. These two approaches are verified for a 28 GHz 800 MHz mm-wave implementation with RF-SoC as the digital back-end. It has been shown that for all proposed multibeam beamformer implementations, the measured beams are well aligned with those of the simulated. The proposed approaches differ in terms of their architectures, hardware complexity and costs, which will be discussed as this dissertation opens up. This dissertation also presents an application of multi-beam approaches for RF directional sensing applications to explore white spaces within the spatio-temporal spectral regions. A real-time directional sensing system is proposed to capture the white spaces within the 2.4 GHz Wi-Fi band. Further, this dissertation investigates the effect of electro-magnetic (EM) mutual coupling in antenna arrays on the real-time performance of fully-digital transceivers. Different algorithms are proposed to uncouple the mutual coupling in digital domain. The first one is based on finding the MC transfer function from the measured S-parameters of the antenna array and employing it in a Frost FIR filter in the beamforming backend. The second proposed method uses fast algorithms to realize the inverse of mutual coupling matrix via tridiagonal Toeplitz matrices having sparse factors. A 5.8 GHz 32-element array and 1-7 GHz 7-element tightly coupled dipole array (TCDA) have been employed to demonstrate the proof-of-concept of these algorithms

High Performance InP Photonic Integrated Circuits and Devices for Free Space Communications and Sensing

Communication needs have grown tremendously over the past few decades and will continue to increase in the future. In order to address these needs, 5G mobile communication systems are moving towards higher carrier frequencies in the millimeter wave (mmW) regime (30 – 300 GHz). Unlike traditional microwave frequencies, which have a relatively isotropic radiation pattern, the highly directional free space propagation characteristics of mmWs requires beemsteering and tracking between transmitters and receivers. One technology that is promising for future mobile communication systems is optical beam forming networks (OBFN). This technology uses photonic components to provide wide bandwidth and eliminate beam squint associated with RF methods to drive phased array antennas. The optical signals from the OBFN are down-converted using high speed photodiodes, which require high bandwidth, efficiency and RF output power. Here we present results on waveguide uni-traveling-carrier photodiodes integrated with mode converters for efficient coupling to a silicon nitride OBFN photonic integrated circuit (PIC). We demonstrate greater than 67 GHz bandwidth and extract efficiency limitations due to the space charge effect of the high carrier density under large optical input power.In addition to communication, highly directional beams can be used for free space sensors including LIDAR. While various frequency ranges provide benefits for specific applications, by increasing the frequency from the mmW regime to the near infrared (~193 THz), beam size can be further reduced to provide high resolution imaging and sensing. We present an indium phosphide transceiver PIC that incorporates a tunable laser, frequency discriminator, and receiver that can be used for frequency modulated continuous wave (FMCW) LIDAR when integrated with an optical phased array for 2D beamsteering. The transceiver provides wavelength tuning over 40 nm, a method for stabilizing the lasing frequency and imparting frequency modulation, and a balanced receiver for coherent detection. The components of the PIC will be discussed along with experimental verification of the functionality of this transceiver