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

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

Wideband data-independent beamforming for subarrays

The desire to operate large antenna arrays for e.g. RADAR applications over a wider frequency range is currently limited by the hardware, which due to weight, cost and size only permits complex multipliers behind each element. In contrast, wideband processing would have to rely on tap delay lines enabling digital filters for every element.As an intermediate step, in this thesis we consider a design where elements are grouped into subarrays, within which elements are still individually controlled by narrowband complex weights, but where each subarray output is given a tap delay line or finite impulse response digital filter for further wideband processing. Firstly, this thesis explores how a tap delay line attached to every subarray can be designed as a delay-and-sum beamformer. This filter is set to realised a fractional delay design based on a windowed sinc function. At the element level, we show that designing a narrowband beam w.r.t. a centre frequency of wideband operation is suboptimal,and suggest an optimisation technique that can yield sufficiently accurate gain over a frequency band of interest for an arbitrary look direction, which however comes at the cost of reduced aperture efficiency, as well as significantly increased sidelobes. We also suggest an adaptive method to enhance the frequency characteristic of a partial wideband array design, by utilising subarrays pointing in different directions in different frequency bands - resolved by means of a filter bank - to adaptively suppress undesired components in the beam patterns of the subarrays. Finally, the thesis proposes a novel array design approach obtained by rotational tiling of subarrays such that the overall array aperture is densely constructed from the same geometric subarray by rotation and translation only. Since the grating lobes of differently oriented subarrays do not necessarily align, an effective grating lobe attenuation w.r.t. the main beam is achieved. Based on a review of findings from geometry,a number of designs are highlight and transformed into numerical examples, and the theoretically expected grating lobe suppression is compared to uniformly spaced arrays.Supported by a number of models and simulations, the thesis thus suggests various numerical and hardware design techniques, mainly the addition of tap-delay-line per subarray and some added processing overhead, that can help to construct a large partial wideband array close in wideband performance to currently existing hardware.The desire to operate large antenna arrays for e.g. RADAR applications over a wider frequency range is currently limited by the hardware, which due to weight, cost and size only permits complex multipliers behind each element. In contrast, wideband processing would have to rely on tap delay lines enabling digital filters for every element.As an intermediate step, in this thesis we consider a design where elements are grouped into subarrays, within which elements are still individually controlled by narrowband complex weights, but where each subarray output is given a tap delay line or finite impulse response digital filter for further wideband processing. Firstly, this thesis explores how a tap delay line attached to every subarray can be designed as a delay-and-sum beamformer. This filter is set to realised a fractional delay design based on a windowed sinc function. At the element level, we show that designing a narrowband beam w.r.t. a centre frequency of wideband operation is suboptimal,and suggest an optimisation technique that can yield sufficiently accurate gain over a frequency band of interest for an arbitrary look direction, which however comes at the cost of reduced aperture efficiency, as well as significantly increased sidelobes. We also suggest an adaptive method to enhance the frequency characteristic of a partial wideband array design, by utilising subarrays pointing in different directions in different frequency bands - resolved by means of a filter bank - to adaptively suppress undesired components in the beam patterns of the subarrays. Finally, the thesis proposes a novel array design approach obtained by rotational tiling of subarrays such that the overall array aperture is densely constructed from the same geometric subarray by rotation and translation only. Since the grating lobes of differently oriented subarrays do not necessarily align, an effective grating lobe attenuation w.r.t. the main beam is achieved. Based on a review of findings from geometry,a number of designs are highlight and transformed into numerical examples, and the theoretically expected grating lobe suppression is compared to uniformly spaced arrays.Supported by a number of models and simulations, the thesis thus suggests various numerical and hardware design techniques, mainly the addition of tap-delay-line per subarray and some added processing overhead, that can help to construct a large partial wideband array close in wideband performance to currently existing hardware

Subarray-Based Multibeam Antenna Frontend for Millimeter-Wave Hybrid Beamforming

With the paradigm shift from sub-6 GHz to millimeter-wave (mm-Wave) for wireless communications, beamforming becomes essential for mm-Wave access points to mitigate losses. Due to the small wavelength, a compact circuit could accommodate a large number of antenna elements. This favors the principle of beamforming to achieve high array-gain and spatial resolution through a large-scale N × M array. For such antenna frontends, full-digital beamforming circuitry requires N × M RF chains, which is unfeasible and energy inefficient. Likewise, a higher-order mm-Wave analog beamforming network is highly lossy to generate N × M beams. Hybrid beamforming addresses this dilemma by partitioning the beamforming between the analog and digital domains appropriately. For this purpose, the antenna frontend needs to be segmented into subarrays, such that the subarray-based analog beamspace patterns are digitally processed rather than processing element patterns individually. Thus, hybrid beamforming requires a suitable subarray-based N × M multibeam antenna frontend. In this thesis, a study of the subarray antennas is presented for hybrid beamforming operation. A simplified model is considered in which the analog beam-switching is performed in the azimuth plane (H-plane) and the digital beamspace beamforming in the elevation plane (V-plane). This is to reduce the number of RF chains as well as to achieve fine-tuned digital beam-steering in V-plane along with predefined analog switched-beams in H-plane. In this research work, the frequency band of 28 – 32 GHz is considered for prototyping purposes. For practical use at mm-Wave, the microstrip line technology is augmented with the perfect magnetic conductor (PMC) packaging. The fixed-beam and switched-beam subarrays with an order of n × m = 1 × 4, 2 × 2, and 4 × 4 are investigated. A dual-polarized aperture-coupled magneto-electric dipole antenna is designed as a single element with 20% bandwidth, ports' isolation better than 35 dB, cross-polarization less than -25 dB, and gain of 8 dBi. Using this element, a fixed-beam 4 × 4 dual-polarized subarray is designed that maintains a bandwidth of 16.7% at 30 GHz with a maximum gain of 19.3 dBi and symmetrical radiation patterns. The fixed-beam limitation of the 2n × 2m subarray leads to building the efficient switched-beam subarray antennas for hybrid beamforming. For this purpose, a 2 × 2 dual-polarized analog beamforming network is designed for 28-32 GHz. Two identical PMC packaged microstrip line networks, one for each polarization, are designed on a single substrate surface. However, to be processed for beamspace digital beamforming, this topology exhibits physical layout and array factor problems. Thus, further designs are investigated to meet the hybrid beamforming frontend requirements. To this end, as switched-beam subarrays for hybrid beamforming, two PMC packaged 4 × 4 Butler matrices are presented with a longitudinal layout and a folded layout for the end-fire and broadside radiation characteristics, respectively. The former design achieves a 5 GHz (28-33 GHz) bandwidth with return loss and isolation, both better than 15 dB. At 30 GHz, the insertion loss is 0.8 ± 0.3 dB, and antenna-ports' phase distributions are ±45° and ±135°. E-plane-flared horn antennas terminate the Butler matrix antenna-ports as a linear array. The double-ridge gap waveguide horn antenna is designed to reduce the scan loss within a subarray environment. The H-plane fan-beam switching covers ±42° with a maximum gain of 11.7 and 11.2 dBi for the inner (1R) and outer (2R) radiation beams. The latter novel topology of the folded Butler matrix is laid out for a compact tiled planar antenna frontend to accommodate a beamforming network beneath the antenna array's physical footprints. As compared to the conventional longitudinal layout, the size is reduced by more than 50 %. The PCB aperture-coupled antenna elements are integrated within the PMC packaged environment for a broadside radiation characteristic. The folded Butler matrix and antenna element are designed for a bandwidth of 4 GHz (28-32 GHz). The single antenna element's directivity is 5.22 dBi; whereas, for a 1 × 4 switched-beam subarray antenna, the directivities are 11.1 dBi and 10.6 dBi for 1R and 2R beams, respectively. Using Butler matrices-based 1 × 4 switched-beam subarrays, two types of multibeam antenna frontends with order N × M = 4 × 4 are constructed. Post-processing for the digital beamforming is applied over the subarray-based analog beamspaces. The first hybrid beamforming model maintains a scan range of ± 42o in the H-plane and ± 28o in the V-plane with BV × BH = 3 × 4 = 12 beams. Similarly, the second model maintains a scan range of ± 38o in the H-plane and ± 40o in the V-plane with BV × BH = 4 × 4 = 16 beams. As compared to full-analog two-dimensional (2-D) beamforming, these models are capable of fine-tuned beam-steering in the V-plane because the complex beamforming coefficients are not fixed but calculated digitally. Furthermore, compared to full-digital 2-D beamforming, it reduces the number of active RF chains from N × M to N

Volumetric Phased Arrays for Satellite Communications

The high amount of scientific and communications data produced by low earth orbiting satellites necessitates economical methods of communication with these satellites. A volumetric phased array for demonstrating horizon-to-horizon electronic tracking of the NASA satellite EO-1 was developed and demonstrated. As a part of this research, methods of optimizing the elemental antenna as well as the antenna on-board the satellite were investigated. Using these optimized antennas removes the variations in received signal strength that are due to the angularly dependent propagation loss exhibited by the communications link. An exhaustive study using genetic algorithms characterized two antenna architectures, and included optimizations for radiation pattern, bandwidth, impedance, and polarization. Eleven antennas were constructed and their measured characteristics were compared to those of the simulated antennas. Additional studies were conducted regarding the optimization of aperiodic arrays. A pattern-space representation of volumetric arrays was developed and used with a novel tracking algorithm for these arrays. This algorithm allows high-resolution direction finding using a small number of antennas while mitigating aliasing ambiguities. Finally, a method of efficiently applying multiple beam synthesis using the Fast Fourier Transform to aperiodic arrays was developed. This algorithm enables the operation of phased arrays combining the benefits of aperiodic element position with the efficiency of FFT multiple beam synthesis. Results of this research are presented along with the characteristics of the volumetric array used to track EO-1. Experimental data and the interpretations of that data are presented, and possible areas of future research are discussed.Ph.D.Committee Chair: Steffes, Paul; Committee Member: Durgin, Gregory; Committee Member: Peterson, Andrew; Committee Member: Roper, Robert; Committee Member: Williams, Dougla

Design and analysis of a proof-of-concept checkered-network compressive array

Compressive arrays have recently been proposed as a new technique for reducing the number of controls in a beamforming system for a given array aperture, promising improved performance over existing thinned-array and subarray techniques. Checkered networks, feed networks consisting of interconnected couplers and fixed phase shifters, have been suggested for realizing the required overlapped subarrays. Although a checkered network has previously been implemented in microstrip, an integrated compressive array, comprising both the antenna elements and the feed network, is required to demonstrate the practical feasibility of such systems. Results for the first successfully manufactured checkered-network compressive array with integrated antenna elements are presented, thereby showing that compressive arrays are promising for use in a variety of real-world beamforming applications. Analysis of the results shows that steered-beam squint is a greater issue than previously assumed, and design guidelines are presented for minimizing the risk of excessive steered-beam squint in manufactured compressive arrays.This work is based on a chapter in H.E.A. Laue, “Design of Compressive Antenna Arrays,” Ph.D. dissertation, Univ. Pretoria, Pretoria, South Africa, 2020. (http://hdl.handle.net/2263/73316)http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=8hj2023Electrical, Electronic and Computer Engineerin

Practical investigation of Butler matrix application for beamforming with circular antenna arrays

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Non-Radiative Calibration of Active Antenna Arrays

Antenna arrays offer significant benefits for modern wireless communication systems but they remain difficult and expensive to produce. One of the impediments of utilising them is to maintain knowledge of the precise amplitude and phase relationships between the elements of the array, which are sensitive to errors particularly when each element of the array is connected to its own transceiver. These errors arise from multiple sources such as manufacturing errors, mutual coupling between the elements, thermal effects, component aging and element location errors. The calibration problem of antenna arrays is primarily the identification of the amplitude and phase mismatch, and then using this information for correction. This thesis will present a novel measurement-based calibration approach, which uses a fixed structure allowing each element of the array to be measured. The measurement structure is based around multiple sensors, which are interleaved with the elements of the array to provide a scalable structure that provides multiple measurement paths to almost all of the elements of the array. This structure is utilised by comparison based calibration algorithms, so that each element of the array can be calibrated while mitigating the impact of the additional measurement hardware on the calibration accuracy. The calibration was proven in the investigation of the experimental test-bed, which represented a typical telecommunications basestation. Calibration accuracies of ±0.5dB and 5o were achieved for all but one amplitude outlier of 0.55dB. The performance is only limited by the quality of the coupler design. This calibration approach has also been demonstrated for wideband signal calibration

Design of antenna array and data streaming platform for low-cost smart antenna systems

The wide range of wireless infrastructures such as cellular base stations, wireless hotspots, roadside infrastructures, and wireless mobile infrastructures have been increasing rapidly over the past decades. In the transportation sector, wireless technology refreshes require constantly introducing newer wireless standards into the existing wireless infrastructure. Different wireless standards are expected to co-exist, and the air space congestion worsens if the wireless devices are operating in different wireless standards, where collision avoidance and transmission time synchronisation become complex and almost impossible. Huge challenges are expected such as operation constraints, cross-system interference, and air space congestion. Future proof and scalable smart wireless infrastructures are crucial to harmonise the un-coordinated wireless infrastructures and improve the performance, reliability, and availably of the wireless networks. This thesis presents the detailed design of a novel pre-configurable smart antenna system and its sub-system including antenna element, antenna array, and radio frequency (RF) frontend. Three types of 90° beamforming antenna array (with low, middle and high gain) were designed, simulated, and experimentally evaluated. The RF frontend module or transmit and receive (T/R) module was designed and fabricated. The performance of the T/R module was characterised and calibrated using the recursive calibration method, and drastic sidelobe level (SLL) improvement was achieved using the amplitude distribution technique. Finally, the antenna arrays and T/R modules are integrated into the pre-configurable smart antenna system, the beam steering performance is experimentally evaluated and presented in this thesis. With the combination of practical know-how and theoretical estimation, the thesis highlights how the modern smart antenna techniques that support most cutting-edge wireless technology can be adopted into the existing infrastructure with minimum distraction to the existing systems. This is in line with the global Smart City initiative, where a huge number of Internet of Things (IoT) devices being wired, or wireless are expected to work harmoniously in the same premises. The concept of the pre-configurable smart antenna system presented in this thesis is set to deliver a future-proof and highly scalable and sustainable infrastructure in the transportation market