Modeling Approaches for Active Antenna Transmitters

The rapid growth of data traffic in mobile communications has attracted interest to Multiple-Input-Multiple-Output (MIMO) communication systems at millimeter-wave (mmWave) frequencies. MIMO systems exploit active antenna arrays transmitter configurations to obtain higher energy efficiency and beamforming flexibility. The analysis of transmitters in MIMO systems becomes complex due to the close integration of several antennas and power amplifiers (PAs) and the problems associated with heat dissipation. Therefore, the transmitter analysis requires efficient joint EM, circuit, and thermal simulations of its building blocks, i.e., the antenna array and PAs. Due to small physical spacing at mmWave, bulky isolators cannot be used to eliminate unwanted interactions between PA and antenna array. Therefore, the mismatch and mutual coupling in the antenna array directly affect PA output load and PA and transmitter performance. On the other hand, PAs are the primary source of nonlinearity, power consumption, and heat dissipation in transmitters. Therefore, it is crucial to include joint thermal and electrical behavior of PAs in analyzing active antenna transmitters. In this thesis, efficient techniques for modeling active antenna transmitters are presented. First, we propose a hardware-oriented transmitter model that considers PA load-dependent nonlinearity and the coupling, mismatch, and radiated field of the antenna array. The proposed model is equally accurate for any mismatch level that can happen at the PA output. This model can predict the transmitter radiation pattern and nonlinear signal distortions in the far-field. The model\u27s functionality is verified using a mmWave active subarray antenna module for a beam steering scenario and by performing the over-the-air measurements. The load-pull modeling idea was also applied to investigate the performance of a mmWave spatial power combiner module in the presence of critical coupling effects on combining performance. The second part of the thesis deals with thermal challenges in active antenna transmitters and PAs as the main source of heat dissipation. An efficient electrothermal modeling approach that considers the thermal behavior of PAs, including self-heating and thermal coupling between the IC hot spots, coupled with the electrical behavior of PA, is proposed. The thermal model has been employed to evaluate a PA DUT\u27s static and dynamic temperature-dependent performance in terms of linearity, gain, and efficiency. In summary, the proposed modeling approaches presented in this thesis provide efficient yet powerful tools for joint analysis of complex active antenna transmitters in MIMO systems, including sub-systems\u27 behavior and their interactions

Active Transmitter Antenna Array Modeling for MIMO Applications

The rapid growth of data traffic in mobile communications has attracted interests to the Multiple-Input-Multiple-Output (MIMO) communication systems at millimeter-wave (mmWave) frequencies.\ua0 MIMO systems exploit active transmitter antenna arrays for higher energy efficiency and providing beamforming flexibility. The close integration of multiple PAs and antennas increases the transmitter analysis complexity. Moreover, due to the small antenna element spacing at mm-wave frequencies, isolators are too bulky and cannot be used. Therefore, including the effects of interactions between the antenna array and PAs is a significant aspect in the analysis of MIMO transmitters. For large active arrays, applying joint circuit and EM simulation tools for the analysis is a complicated and time-consuming task. In these occasions, behavioral models are the key to the fast and accurate evaluation of active transmitter antenna arrays.In this thesis, a technique for modeling the active transmitter antenna array performance is presented. The proposed model considers the effect of PAs nonlinearity as well as the coupling and mismatch in the antenna array. With this model, a comprehensive prediction of radiation pattern and signal distortions in the far-field is feasible. The model is experimentally verified by a mmWave active subarray antenna for a beam steering scenario and by performing over-the-air measurements. The measurement results effectively validate the modeling technique for a wide range of steering angles.\ua0\ua0 Furthermore, a linearity analysis is provided to predict transmitter performance in conjunction with beam-dependent digital predistortion (DPD) linearization. The study reveals the model potential in evaluating different DPD approaches as well as predicting the performance of linearized transmitters. The demonstration shows that the variation of nonlinear distortion versus steering angle depends significantly on the array configuration and beam direction.In summary, the proposed model allows for the prediction of the active transmitter antenna array performance in the early design stages with low computational effort. It can provide design guides for developing large-scale active arrays and can be employed for evaluating the DPD and transmitter linearity performance

Methods for Control, Calibration, and Performance Optimization of Phased Array Systems

Phased array radar systems have proven advantageous in a variety of research applications, offering faster volume scans and unparalleled time-resolution as compared to traditional parabolic dish antenna systems that rely solely on mechanical systems for controlling the direction of radiation. As such, research has accelerated the development of practical phased array systems to realize their full vision. In particular, next generation phased array systems aim to provide additional advantages in the form of re-configurable beam patterns, adaptive digital beamforming, multiple-input multiple-output (MIMO) radar modes, and other software-defined technologies. However, to fully realize a paradigm shift in phased array technology, especially as the ratio of array to sub-array size becomes greater, this requires a corresponding increase in novel digital backend architectures to fully achieve this vision. Therefore, new methods for control, calibration, and performance optimization are required to enable next-generation phased array systems to reach their potential. In this thesis, a variety of practical engineering challenges related to phased array system design are discussed, with system-level implications and relevant theory included where necessary. For instance, for the first time, as explained in this thesis, a GPS disciplined, time-interleaved measurement technique that leveraged real-time control of a beamformer was developed to enable accurate post-processing correction of the phase drift that results from clocking differences between noncoherent physically separated bistatic nodes. In addition, laboratory efficacy of digital predistortion using the memory-polynomial model has been confirmed for the purpose of maximizing an element's usable power while minimizing spectral spreading and achieving desirable output linearity during operation, and a novel method for training predistortion models comprised of a combined software-defined and physical mechanism for measuring transmitter front-end distortion for elements within a digital-at-every element array has been proposed and verified in the lab

A 37-40 GHz Dual-Polarized 16-Element Phased-Array Antenna with Near-Field Probes

With the development of fifth-generation (5G) communication networks, in order to meet the growing demand for high-speed and low-latency wireless communication services, channel capacity has become the main driving force for choosing millimeter wave (mm-wave) over over-crowded sub-6 GHz frequency bands. Recently, beamforming phased array attracts significant research efforts as it is a promising solution and unique in its ability to overcome the high path-loss at high frequency, provide fast beam steering and deliver better user-ends experience. However, to alleviate the issues that associated with beamforming phased array, such as imbalance between array elements and non-linearity caused by power-amplifiers (PAs) in beamforming channels, far-field (FF) based array calibration and digital pre-distortion (DPD) need to be performed, which is not practical in real world scenario. This thesis presents a low-cost 16-element dual-polarized mm-wave antenna-on-printed circuit board (PCB) transmitter RF beamforming array with embedded near-field probes (NFPs) at 37-40 GHz. The elements are orthogonal, proximity-coupled feed dual-polarized patch antenna with a spacing of 0.5λ within 2x2 subarray and 0.6λ between 2x2 subarray at 38.5 GHz, resulting in maximum 17.7 dB gain with a scan angle of +/-50◦, +/-20◦ in azimuth and +/-20◦, +/-50◦ in elevation for vertical polarization and horizontal polarization, respectively. Without affecting phased array performance, the NFPs achieve flat and comparable coupling magnitude and group delay to the closet RF chain for both polarizations, across operating frequency range. This ensures the quality of received output signal from phased array to implement array calibration and DPD. The configuration of embedded NFPs maintains the scalability of phased array and eliminate the needs of impractical FF reference probe for array calibration and DPD

Nonlinear Equalization and Digital Pre-Distortion Techniques for Future Radar and Communications Digital Array Systems

Modern radar (military, automotive, weather, etc.) and communication systems seek to leverage the spatio-spectral efficiency of phased arrays. Specifically, there is an increasingly large demand for fully-digital arrays, with each antenna element having its own transmitter and receiver. Further, in order to makes these systems realizable, low-cost, low-complexity solutions are required, often sacrificing the system's linearity. Lower linearity paired with the inherent lack of RF spacial filtering can make these highly digital systems vulnerable to high-power interferering signals-- potentially introducing spectral regrowth and/or gain compression, distorting the signal-of-interest. Digital linearization solutions such as Digital Pre-Distiortion (DPD) and Nonlinear Equalization (NLEQ) have been shown to effectively mitigate nonlinearities for transmitters and receivers, respectively. Further, DPD and NLEQ seek to extend the effective dynamic range of digital arrays, helping the systems reach their designed dynamic range improvement of $10\log_{10}(N)$~dB, where $N$ is the number of transmitters/receivers. However, the performance of these solutions is ultimately determined by training model and waveform. Further, the nonlinear characteristics of a system can change with temperature, frequency, power, time, etc., requiring a robust calibration technique to maintain a high-level of nonlinear mitigation. This dissertation reviews the different types of nonlinear models and the current NLEQ and DPD algorithms for digital array systems. Further, a generalized calibration waveform for both NLEQ and DPD is proposed, allowing a system to maximize its dynamic range over power and frequency. Additionally, an \textit{in-situ} calibration method, leveraging the inherent mutual coupling in an array, is proposed as a solution to maintaining a high level of performance in a fielded digital array system over the system's lifetime. The combination of the proposed training waveform and \textit{in-situ} calibration technique prove to be very effective at adaptively creating a generalized solution to extending the dynamic range of future low-cost digital array systems

Single Input Single Output Digital Pre-Distortion at Millimeter Wave Frequencies for Phased Arrays

The limiting fact that is impeding the increase in data rate in the current generation of wireless communication is the limited available spectrum in the sub-6 GHz bands. This has motivated the shift to higher frequencies such as millimeter waves (mm-wave) and terahertz frequencies where modulation bandwidth of several hundreds of MHz can be utilized to increase the communication link capacity. The deployment of high data rate mm-wave base stations will highly depend on the maximum achievable equivalent isotropic radiated power (EIRP) and on the ability to generate reliable and error free wideband signals. High EIRP and high efficiency operation can be achieved by using active phased arrays operated deep into the power amplifiers (PAs) nonlinear region. In this work, a low power and low complexity compensation schemes to mitigate the impairments exhibited by phase arrays driven with wideband signals and high efficient nonlinear PAs at mm-wave frequencies are proposed. Digital pre-distortion (DPD) techniques can provide an attractive solution to linearize high efficiency and high EIRP nonlinear phased arrays at mm-wave frequencies. However, the viable deployment of DPD solutions call for the reduction in the power consumption of the transmitter observation receiver (TOR) feedback path required to train the DPD function. To that end, a low power DPD scheme for linearizing mm-wave hybrid beamforming antenna systems is presented. The proposed DPD scheme exploits the modularity of hybrid beamforming systems. During the training phase, the constituent sub-arrays, are categorized, into (i) the main sub-array that exhibits non-linear distortion and is to be linearized, and (ii) the auxiliary sub-arrays that operate in the backoff region to avoid nonlinearity. To produce the error signal necessary to train the DPD function (and compensate for the distortions exhibited by the main sub-array), the signals transmitted by the main and auxiliary sub-arrays are combined. This error signal is captured using a TOR with low dynamic range and is digitized using a low-bit resolution analog-to-digital converter (ADC). Proof-of-concept validation experiments are conducted by applying the proposed DPD system to linearize an off-the-shelf hybrid-beamforming array comprised of four 64-element sub-arrays, operating at 28 GHz and driven with up to 800 MHz orthogonal frequency-division multiplexing (OFDM) modulated signals. Using the proposed DPD scheme, a TOR with a 4-bit ADC was sufficient to improve the adjacent channel power ratio (ACPR) by 10 dB and the error vector magnitude (EVM) improved from 5.8% to 1.6%. These results are similar to those obtained using a TOR with 16-bit ADCs. Reducing the complexity of the DPD scheme for phased arrays is also of primordial importance to the successful deployment of DPD solutions. For instance, the DPD function needs to be desensitize to the load modulation effects exhibited by large antenna systems and be able to linearize phased arrays at different steering angles. To address the challenges associated with the load modulation for phased arrays, we propose a generalized SISO DPD scheme as solution to minimize the EVM variation at different steering angles. The measurement results of the proposed scheme, using a 400 MHz OFDM signal with subcarriers modulated using 256 QAM and on a commercial 64-elements beamforming array, was able to maintain the EVM below 2% across the full steering range. This solution, however, failed to maintain the ACPR below -45 dBc. The effect of tapering on the load modulation and the array nonlinearity is also analysed. The measurement results using different tapers are used to validate the theory and the simulation results. Using tapering, the ACPR and EVM variation before and after DPD were minimized versus steering angles. For instance, using taper setting 2, the ACPR and EVM are maintained below -46 dBc and 1% from -38° to 45° and below -42.3 dBc and 1.8% from -45° to 45° respectively. Better results are measured when tapering is used in conjunction with the proposed generalized DPD scheme. In that case, the ACPR is improved from -35.5 to at worst -46.4 dBc and at best -50 dBc and the EVM is improved from at worst 4.5% to at worst 1.2% and at best 0.85%. The EVM is also maintained below 0.95% from -39° to 45°

Modeling and Compensation of Nonlinear Distortion in Multi-Antenna RF Transmitters

Multi-antenna systems are utilized as a way to increase spectral efficiency in wireless communications. In a transmitter, the use of several parallel transmit paths and antennas increases system complexity and cost. Cost-efficient solutions, which employ active antenna arrays and avoid expensive isolators, are therefore preferred. However, such solutions are vulnerable to crosstalk due to mutual coupling between the antennas, and impedance mismatches between amplifiers and antennas. Combined with the nonlinear behavior of the power amplifiers, these effects cause nonlinear distortion, which deteriorates the quality of the transmitted signals and can prevent the transmitter from meeting standard requirements and fulfilling spectrum regulations. Analysis, assessment and, if necessary, compensation of nonlinear distortion are therefore essential for the design of multi-antenna transmitters.In this thesis, a technique for modeling and predicting nonlinear distortion in multi-antenna transmitters is presented. With this technique, the output of every individual transmit path, as well as the radiated far-field of the transmitter can be predicted with low computational effort. The technique connects models of the individually characterized transmitter components. It can be used to investigate and compare the effects of different power amplifier and antenna array designs at early design stages without complicated and expensive measurements.Furthermore, a digital predistortion technique for compensating nonlinear distortion in multi-antenna transmitters is presented. Digital predistortion is commonly used in transmitters to compensate for undesired nonlinear hardware effects. The proposed solution combines a linear function block with dual-input predistorters. The complexity is reduced compared to existing techniques, which require highly complex multivariate predistorter functions. Finally, a technique for identifying multi-antenna transmitter models and predistorters from over-the-air measurements using only a small set of observation receivers is presented. Conventional techniques require a dedicated observation receiver in every transmitter path, or one or more observation receivers that are shared by several paths in a time-interleaved manner. With the proposed technique, each receiver is used to observe several transmitter paths simultaneously. Compared to conventional techniques, hardware cost and complexity can be reduced with this approach. In summary, the signal processing techniques presented in this thesis enable a simplified, low-cost design process of multi-antenna transmitters. The proposed algorithms allow for feasible, low-complexity implementations of both digital and analog hardware even for systems with many antennas, thereby facilitating the development of future generations of wireless communication systems

Experimental Investigation of the Power Amplifiers’ Nonlinearity using a 3.5 GHz 2×2 RF Front-end Prototype

Next generation communication systems will demand extremely high system capacity. Ap-proaches such as complex digital modulation schemes, widening of the instantaneous band-width and massive multi-input multi-output (MIMO) architectures will need to be employed to realize such high capacity systems. However, these approaches impose stringent requirements on the radio hardware. For instance, in conventional wireless transmitters, isolators/circulators are typically used to immunize the radio hardware and its performance from negative effects of the antenna load variation. However, in massive MIMO transmitters, while the antenna active impedance varies significantly, isolators cannot be used due to their unacceptable overhead in terms of cost and space. Thus, within these transmitters the power amplifiers’ (PA) performance in the aspects of linearity, output power and efficiency are significantly impacted by the load modulation introduced by the finite isolation between the antenna elements. To date, studies in the literature have mainly relied on emulating the load modulation in massive MIMO transmit-ters and have used generic PAs rather than those specifically designed for massive MIMO transmission. This work begins by designing a two-by-two RF front-end for a massive MIMO transmitter, comprised of antenna and PA arrays suitable for use in a base station. The antenna array is formed of multilayered patch antenna elements that achieved an enhanced isolation and ex-tended fractional bandwidth of 19 dB and 14%, respectively. The PA array was built using gal-lium nitride transistors and carefully operated in Class J mode. Under continuous wave meas-urements, the PA array element demonstrated high peak-power efficiency of between 54%-66% over the frequency band ranging from 3.2GHz to 3.8GHz. It also showed excellent linear-izability when driven with modulated signals with 200 MHz instantaneous bandwidth. When both the antenna and PA arrays are connected, they form a front-end that was used to study the effects of the antenna load modulation using realistic modulated signals. This study undertook a large set of measurement configurations specifically devised to investigate the effects of cou-pling due to the PA substrate and finite isolation between the antenna elements, as well as the extent of the nonlinearity of the PA elements. Furthermore, a single-input single-output (SISO) digital pre-distortion (DPD) scheme was applied to attempt to linearize the overall response of the PA array. This study revealed that the coupling attributed to the PA substrate had a minor impact on the array’s performance. Furthermore, it highlighted the necessity of jointly designing for both the PA element linearity and the antenna isolation level, so that SISO DPD can be used and MIMO DPD is avoided