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  • Ph.D. Dissertation
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  • Frequency-Multiplexed Array Digitization for MIMO Receivers: 4-Antennas/ADC at 28 GHz on Xilinx ZCU-1285 RF SoC
    Communications at mm-wave frequencies and above rely heavily on beamforming antenna arrays. Typically, hundreds, if not thousands, of independent antenna channels are used to achieve high SNR for throughput and increased capacity. Using a dedicated ADC per antenna receiver is preferable but it’s not practical for very large arrays due to unreasonable cost and complexity. Frequency division multiplexing (FDM) is a well-known technique for combining multiple signals into a single wideband channel. In a first of its kind measurements, this paper explores FDM for combining multiple antenna outputs at IF into a single wideband signal that can be sampled and digitized using a high-speed wideband ADC. The sampled signals are sub-band filtered and digitally down-converted to obtain individual antenna channels. A prototype receiver was realized with a uniform linear array consisting of 4 elements with 250 MHz bandwidth per channel at 28 GHz carrier frequency. Each of the receiver chains were frequency-multiplexed at an intermediate frequency of 1 GHz to avoid the requirement for multiple, precise local oscillators (LOs). Combined narrowband receiver outputs were sampled using a single ADC with digital front-end operating on a Xilinx ZCU-1285 RF SoC FPGA to synthesize 4 digital beams. The approach allows M -fold increase in spatial degrees of freedom per ADC, for temporal oversampling by a factor of M. FULL PAPER
  • Spacetime Frequency-Multiplexed Digital-RF Array Receivers with Reduced ADC Count
    Wireless systems operating at mm-wave frequencies require dense antenna arrays to achieve directional gain for overcoming high path loss. Digital mm-wave arrays retain spatial degrees of freedom, but require a dedicated analog to data converter (ADC) per spatial channel, leading to undesirably high receiver complexity, large ADC count, and power consumption. This paper exploits directional sparsity to reduce the number of receivers and ADCs with minimal loss in performance. A multidimensional (MD) linear transformation using transmission lines and a K:1 combiner is used to reduce the number of ADCs by a factor K. Simulations verify that the proposed method can lead to better than 50% ADC complexity reductions (for K≥2) for linear arrays and more than 75% ADC complexity reduction (for K≥4) for rectangular arrays when sparsity conditions are met. Unlike in analog-digital hybrid beamforming, where a phased-array combines K channels to a single ADC, the proposed method does not lead to loss of spatial degrees of freedom. FULL PAPER
  • Massive-MIMO and Digital mm-Wave Arrays on RF-SoCs using FDM for M-Fold Increase in Antennas per ADC/DAC
    Communication systems of the future will require hundreds of independent spatial channels achieved through dense antenna arrays connected to digital signal processing software defined radios. The cost and complexity of data converters are a significant concern with systems having hundreds of antennas. This paper explores frequency division multiplexing as an approach for augmenting the baseband signals of multiple antenna channels such that a single ADC can sample a multitude of antennas in an array. The approach is equally applicable to both massive MIMO and mm-wave digital wireless arrays. An example design based on Xilinx RF SoC for combining 4 antenna channels at 28 GHz into a single ADC is provided. FULL PAPER
  • Sampling H- & V-Polarized Antennas using a Single ADC for Digital Antenna Arrays by Exploiting Multi-Dimensional Signal Processing RF Circuits
    Digital array receivers increasingly require both H and V polarization of the incident RF waves while supporting full-band operation. A wideband ADC is required for each polarization at every location of the array, leading to 2N ADCs for N locations. The paper proposes exploiting multidimensional sparsity in the spatio-temporal frequency domain to reduce the number of ADCs from 2N to N, while supporting two polarizations and wideband RF-digital operation. By using spatio-temporal sparsity with multi-dimensional linear transforms, it is proposed to combine the H and V array signals without interference, such that, the combined array signal can be sampled using just N ADCs. This allows a modern RFSoC or array receiver with N wideband ADC inputs to process N spatial locations in an array where each location contains a cross-polarized element measuring both H and V components, doubling the information carrying capacity of the N-ADC system provided that the elements limit the field-of-view to about 60°. FULL PAPER
  • Improving ADC Figures-of-Merit in Wideband Antenna Array Receivers using Multidimensional Space-time Delta-Sigma Multiport Circuits
    To add a new question go to app settings and press "Manage Questions" button.This paper describes recent work on the topic of multi-dimensional (MD) spatio-temporal noise and distortion shaping for radio-frequency (RF) antenna arrays with applications in wireless communications, phased-array radar sensing, microwave/mm-wave imaging, and radio astronomy instrumentation. The MD spectral properties of propagating plane-waves that arise from electromagnetics are combined with MD circuits and signal processing theory based on passive resistively-terminated 2-D filters. The result, for the first time in the literature, is MD multi-port extensions of the analog and digital electronics found within wireless transceivers, including RF amplifiers, mixers, and data converters. In particular, a multi-port extension of conventional analog-to-digital converters (ADCs) is proposed in which the distortion that is generated from non-linear operations (such as quantization) is spectrally shaped in multiple spatiotemporal dimensions such that the region of support (ROS) of the desired RF signals corresponding to desired planar waves is ideally mutually exclusive with that of these undesired components. Because of spectral shaping, both distortion and noise are non-overlapping with the signal of interest, and can be filtered out after sampling by using a MD digital filter. This theoretical advance in MD noise and distortion shaping across both space and time domains ensures that both electronic noise and non-linear distortion arising from coarse low-complexity quantization do not appreciably reduce the signal to noise and distortion ratio (SNDR) of antenna array receivers. The proposed method is an extension of Δ - Σ modulation that is commonly used in conventional single-input single-output ADCs, but does not require either temporal or spatial over-sampling. Moreover, to the best of our knowledge this paper is the first to combine 2-D analog filters derived from resistively-terminated classical passive low-pass filter prototypes with active analog feedback control in multiple dimensions (space, time) to realize 2-D analog-digital mixed-signal electronics for RF array processing applications. FULL PAPER
  • Multiport ADCs for Microwave Focal Plane Array Dish Receivers
    This paper proposes an architecture that reduces the complexity of traditional N-bit ADCs used in focal plane array (FPA) dish receivers by replacing them with multiport ADCs. The proposed ADC architecture uses a multi-dimensional (MD) noise-shaping method based on a Δ-Σ architecture for wideband RF signals that are received on the focal region of a parabolic dish/lens antenna. In the M-port noise shaping technique, the N-bit quantizers of conventional ADCs are replaced by 1-bit quantizers followed by a spatial feedback system based on a Δ-Σ architecture with spatial oversampling, which shapes the quantization noise out of the region of support (ROS) of the electromagnetic (EM) waves received from the dish. The paper discusses the case of a prime-axis pencil beam in detail for the simplified case of a linear FPA. Simulations for 2.1-5.1 GHz wideband dish signals show 16-element FPAs with oversampling ×l, ×2, and ×4 shows ADC effective number of bits (ENoB) improvements of 2.5 bits, 3.2 bits and 4.2 bits, respectively. Extensions to off-axis pencil-beams and rectangular FPAs will be considered in future work. Potential applications exist across microwave and mm-wave bands, for radio astronomy, radar, and wireless communications. FULL PAPER
  • Fast Radix-32 Approximate DFTs for 1024-Beam Digital RF Beamforming
    The discrete Fourier transform (DFT) is widely employed for multi-beam digital beamforming. The DFT can be efficiently implemented through the use of fast Fourier transform (FFT) algorithms, thus reducing chip area, power consumption, processing time, and consumption of other hardware resources. This paper proposes three new hybrid DFT 1024-point DFT approximations and their respective fast algorithms. These approximate DFT (ADFT) algorithms have significantly reduced circuit complexity and power consumption compared to traditional FFT approaches while trading off a subtle loss in computational precision which is acceptable for digital beamforming applications in RF antenna implementations. ADFT algorithms have not been introduced for beamforming beyond N = 32, but this paper anticipates the need for massively large adaptive arrays for future 5G and 6G systems. Digital CMOS circuit designs for the ADFTs show the resulting improvements in both circuit complexity and power consumption metrics. Simulation results show similar or lower critical path delay with up to 48.5% lower chip area compared to a standard Cooley-Tukey FFT. The time-area and dynamic power metrics are reduced up to 66.0%. The 1024-point ADFT beamformers produce signal-to-noise ratio (SNR) gains between 29.2-30.1 dB, which is a loss of ≤ 0.9 dB SNR gain compared to exact 1024-point DFT beamformers (worst case) realizable at using an FFT. FULL PAPER
  • Multi-Beam Radio Frequency (RF) Aperture Arrays Using Multiplierless Approximate Fast Fourier Transform (FFT)
    Fast Fourier transforms FFTs are fast algorithms for the computation of the discrete Fourier transform DFT with low computational complexity. FFT owes its popularity to the fact that the parent algorithm the DFT is of critical importance in a wide range of applications, such as wireless communications, data networks, sensor networks, cognitive radio, radar and beamforming, imaging, filtering, correlation and radio-astronomy. In this report approximate transforms that closely follow the DFT have been studied and found. The approximate-DFT a-DFT transforms are derived to have acceptable performance in terms of achieving spatial multi-beams. FULL REPORT
  • A Direct-Conversion Digital Beamforming Array Receiver with 800 MHz Channel Bandwidth at 28 GHz using Xilinx RF SoC
    This paper discusses early results associated with a fully-digital direct-conversion array receiver at 28 GHz. The proposed receiver makes use of commercial off-the-shelf (COTS) electronics, including the receiver chain. The design consists of a custom 28 GHz patch antenna sub-array providing gain in the elevation plane, with azimuthal plane beamforming provided by real-time digital signal processing (DSP) algorithms running on a Xilinx Radio Frequency System on Chip (RF SoC). The proposed array receiver employs element-wise fully-digital array processing that supports ADC sample rates up to 2 GS/second and up to 1 GHz of operating bandwidth per antenna. The RF mixed-signal data conversion circuits and DSP algorithms operate on a single-chip RFSoC solution installed on the Xilinx ZCU1275 prototyping platform. FULL PAPER
  • Design of 28 GHz 64-QAM Digital Receiver
    This paper presents a MIMO+beamformer architecture for millimeter-wave (mmW) 5G wireless applications. A 64-QAM digital receiver operating at 28 GHz is designed for eventual applications in orthogonal frequency division multiplexing (OFDM) 5G links. The receiver design employs a 32-element linear array, with 850 MHz bandwidth sufficient to support 512-point OFDM modulation. The proposed RF chain is aimed at establishing a wireless communication link with a bit error probability better than 10 -5 . A design procedure is adopted to provide the required signal to noise ratio (SNR) at the input of the digital demodulator. A link budget and noise analysis has been conducted using AWR Microwave Office to evaluate the system's performance under circuit non-idealities and RF impairments, such as noise, distortion, and mismatch. The 64-QAM constellation is simulated for non-ideal conditions using manufacturer test-data and circuit parameters that are embedded into the AWR Microwave Office model. This paper presents the simulation of a single receiver. Physical implementation, measurement, verification, and extension to OFDM based 32-element beamforming arrays is reserved for future work. FULL PAPER
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