This paper is intended to provide physical insight into the phased array design parameters that affect the symmetry of its beam pattern (and frequency response in case of a series-fed array). Qualitative conclusions are derived from the quantitative analysis of the effects of a spare antenna added to a uniform linearly spaced array. Two cases are described in detail depending on the positioning of the spare antenna and on the phase of its radiated signal. Analytical equations are derived to perform the zero forcing of a sidelobe on a single side (or both sides) of the radiation pattern, in case the antenna has a non-zero phase shift (or an additional true-time delay) with respect to that related to the steering direction. This analysis is particularly dictated to simplify the beam synthesis in case of ground-reflection or sensing applications.

The reflection characteristics of electromagnetic (EM) waves can be used to estimate certain environmental properties of the Earth surfaces. In this study, a low-cost and low-complexity dual circularly polarized reception (DCPR) system is used to measure the relative permittivity of land and seawater. This system consists of a custom, phase stable, dual circularly polarized antenna (DCPA) and off-the-shelf receivers for simultaneous reception of direct and reflected global navigation satellite system (GNSS) signals. DCPR offers two distinct improvements in comparison to the dual linear polarization reception. First, the DCPR system enables two low-correlation data streams due to the utilization of GNSS polarization and its orthogonal counterpart. Second, it enables easy observation of the surface scattering characteristics through the behavior of the reflected cross-polarized carrier-to-noise density ratio (C/N0). The right-hand (RHCP) and left-hand circularly polarized (LHCP) reflection coefficients were extracted from the received C/N0 streams using the GNSS-interference pattern technique (GNSS-IPT). The extracted signal peaks were fit with low-order curve functions for a better approximation of the Brewster angles. The measured RHCP and LHCP reflection coefficients enabled the determination of the Brewster angle position, leading to the relative permittivity estimate for the reflecting surfaces. The measured mean relative permittivity for the sea was 84.5 and for the land 13.57. The corresponding standard deviations (stds) were 10.48 and 8.05, and the standard error of mean was 0.23 and 0.17, respectively. The main factors for the uncertainties were the topography of sea and land surfaces, randomly distributed vegetation growth on land, and the receiver quantization process.

This article presents an integrated quadrature balanced power amplifier (PA) operating at a 26-GHz frequency range and techniques to mitigate the frequency-dependent amplitude response of quadrature hybrids used in the balanced amplifier design. The overall structure consists of two stacked pseudo-differential PAs and transformer-based quadrature hybrids designed with 22-nm CMOS FDSOI. Two techniques to compensate frequency-dependent amplitude response of the quadrature hybrid when operating away from the center frequency are proposed. The first one involves a dual input drive and the second one involves asymmetric biasing. With distortion contribution analysis, it is shown that asymmetric biasing compensates quadrature hybrid asymmetry but also produces mutually compensating third-order nonlinearity, resulting in improved linearity. Measurements with continuous wave (CW) and high dynamic range fifth generation (5G) modulated signal demonstrate that the described techniques improve output power that can be reached within the linearity specifications when operating away from the center frequency.

To optimize the linearity of mixers, one needs to recognize the origins and mixing mechanisms of dominant nonlinearities. This article presents a time-varying (TV) nonlinear analysis, where TV polynomial models are used to yield harmonic mixing functions in four simple mixer structures. Local oscillator (LO) waveform engineering has been utilized to tailor the nonlinearity coefficients, which were later fed into a numerical distortion contribution analysis tool to calculate the nonlinearity using a Volterra series-based approach. The spectra of the nonlinear voltages have been drawn, followed by plots breaking the final IM3 distortion down into its contributions. The effect of filtering on distortion has been illustrated in the end.

We propose a new power amplifier (PA) behavioral modeling approach, to characterize and compensate for the signal quality degrading effects induced by a PA with a machine learning (ML) aided piece-wise (PW) modeling approach. Instead of using a single pruned Volterra model, we use multiple small-size pruned Volterra models by classifying the input data into different classes. For that purpose, an ML classifier model is trained by extracting some crucial features from both the input signal statistics and the PA operating point. The simulation results indicate that our approach contributes to an improved performance/complexity trade-off than a single generalized memory polynomial (GMP) model in terms of PA behavior modeling and linearization.

Behavioral amplifier models are heavily used in system-level simulations to understand the fundamental impact of a certain kind of nonlinearity for the designed transmit and receive signals. However, every model requires an accurate set of parameters to gain realistic results, which are not often publicly available, especially for higher frequency bands at millimeter-wave and above that. This applies also to transistor models used in amplifier simulations, which are, in most cases, characterized by measurements only up to around 100 GHz. In this paper, we fit parameters of common memoryless amplifier models, such as Rapp, Ghorbani, and Saleh, against measurement and circuit-level simulation data of a 300 GHz amplifier. The derived model parameters are given such that they can be used by other researchers when studying the impacts of nonlinearity. As an example, the models are utilized to simulate error vector magnitude performance with different input and output signal powers.

The need for unrestricted, high-quality, and high-speed communications in planned sixth generation (6G) wireless systems drives the development and research towards the sub-terahertz (sub-THz) bands which so far have been relatively unused for wireless communications applications. Additionally, the sub-THz bands have gained an increasing interest as a potential spectral region at which to go even beyond the well-known Shannon limits. This review paper provides a technological overview on some of the key hardware aspects of sub-THz wireless communications (at 100–300 GHz), namely antennas, reconfigurable intelligent surfaces (RISs), and reconfigurable antenna systems based on state-of-the-art technologies reported in recent literature. Different technologies of antennas and RISs are compared to understand their possibilities and limitations, and to identify the most promising technological approaches to transform 6G from a vision into a commercially viable solution. The paper also presents the authors’ interpretations of possible hardware and design trends that can shape the future research directions.

This letter presents analysis of frozen sea surface properties using low cost and low complexity terrestrial Global Navigational Satellite System (GNSS) receivers. Monitoring sea ice thickness and the mean sea level (MSL) of the frozen sea are performed using the interference frequency obtained by the GNSS interference pattern (IP) technique. The height variations between the GNSS antenna and the sea surface evaluated using the IP of the direct and reflected carrier-to-noise density ratio (C/N0) is used to find the corresponding MSL. The GNSS-R derived MSL for open sea conditions agreed well with the mareograph data with root-mean-squared error (RMSE) of 2.72 cm with R-squared value of 0.9644. For frozen sea, a notable difference was observed between the measured MSL and ground truth MSL values. This difference was caused by the combined thickness of snow and ice above the frozen sea surface, also known as the total freeboard. Assuming the conditions for hydrostatic equilibrium is satisfied, total freeboard was converted to ice thickness. The ice thickness values agreed well with the published ice charts by Finnish Meteorological Institute. The main uncertainty in the extracted ice-thickness was due to the thick snow accumulation and unknown snow properties.

Stacked MOS power amplifiers (PA) are commonly used in SOI nodes but also have the potential to be realized in bulk CMOS nodes. In this paper they are analyzed in millimeter wave regimes. The study focuses on the key limiting factors and in particular the optimum number of transistors from which the key performance parameters such as maximum possible operating frequency, output power, and efficiency are achieved. Based on the analysis, design trade-offs of stacked MOS PAs are presented. The frequency dependency of the optimum load presented to each stack is analyzed to express the overall performance of the mentioned PA topologies as a new optimization method. Additionally, it is shown how the optimal load variations translate into amplitude-to-amplitude/phase (AM-AM/PM) conversion distortions. The validity of the analysis is examined against simulations. The simulations are performed based on 8M1P CMOS 28nm technology and electromagnetic simulations in ADS Momentum.

This paper describes the design and post-layout simulations of a 2/3/4- modulus frequency divider circuit, accompanied with an accumulator that controls the division count. The circuit is capable of operating as an integer or as a fractional divider. Key topic of this paper is the merging of div-2/3 and div-3/4 circuits into a single compact circuit that solves an issue of a forbidden state in fractional-division operation. The circuit is designed with 28-nm CMOS technology and the post-layout simulations indicate an operating input frequency range of 0.3 - 5.4 GHz with 13-bit fractional frequency resolution between division ratios of 2-4. The divider occupies only 40 pm × 30 pm while consuming 2.0 mW at 5.4 GHz input frequency.

Multiantenna systems can increase the spatial reuse of the wireless spectrum by nulling the interfering directions. However, the efficiency of the spatial interference suppression is highly sensitive to errors in the progressive phase applied to the antenna elements. We examine the impact of one particular source of error resulting from the quantization of the analog beamforming weights. In particular, we focus on vector-modulator based RF beamforming system. To improve the quantization error, we propose a sliding reference approach for using a floating phase reference minimizing the quantization error of the RF beamformer. Monte-Carlo simulations are performed to analyze the performance of the zeroforcing with quantized coefficients. With small arrays, the simulation results showed almost 10 dB of improvement in the average null depth compared to the conventional round-off method.

This paper describes the design and measurement of an open-loop fractional frequency divider implementation. The fractional divider consists of a multi-modulus integer frequency divider (MMD), a sigma-delta modulator (SDM) and a pipelined phase interpolator. The fractional frequency division is achieved with the MMD and the 13-bit SDM toggling the integer division ratio. The resulting signal is then processed by the phase interpolator which significantly reduces the spurs by 22 dB and generates spectrally clean signal with correct output frequency. The prototype is implemented in 28-nm CMOS technology and it operates within input frequency range of 1.9 GHz - 3.5 GHz with fractional division ratio in between 2-3. As an example of the operation, with a setting of an arbitrary division ratio of 2.3164 and input frequency of 2.4 GHz, the output sets correctly to 1.0361 GHz with RMS jitter of 2.1 ps.

This paper concerns with the nonlinearity analysis and compensation of active phase shifters based on vector modulator topologies which utilize Gilbert cell type multipliers. In this respect, both frequency dependence and expansion behavior of such topologies are analyzed. Based on translinear circuits theory, log/anti-log nonlinear equations are transformed to algebraic linear equations and/or multiplication equations, which facilitates solution for such nonlinear equations. To validate the analyses, a 5-bit active phase shifter was designed and simulated in 0.13um HBT technology node. 3D electromagnetic simulation results verify the consistency of the analysis and proposed compensation technique.

At sub-THz frequencies, the local oscillation for transceivers is often generated by an analog frequency multiplier tree. This paper studies the origins and minimization of spurious tones in a local oscillator multiplier consisting of a chain of frequency doublers. It is found that the nearest spurious components depend on the leakage of the input signal in the first multiplier, which can be minimized by active bias tuning, filtering in the first stage or better balance in the driving differential signal. To verify the nonlinearity modeling, 0.13-μm SiGe BiCMOS technology has been utilized.

The 5G system enables flexible and high data rate communication between nearby vehicles, infrastructure nodes, or pedestrians to enable smart traffic use-cases. However, fully autonomous driving vehicles supporting real-time, high-quality, and high data rate sensor data sharing will require data rates beyond 5G, which a future 6G system will support. Mobile devices (UE) and base stations (BTS) supporting the 5G and the future 6G are implemented as multi-mode devices, where multiple radios supporting different systems are integrated into one. The highest 5G data rates are enabled with 5G millimeter wave (mmW) radios, but those face interoperability challenges of harmonic transmissions (TXs) of Long Term Evolution (LTE) and Wi-Fi radios. The 6G radios will suffer co-channel interference from the fundamental TXs of legacy cellular systems. An analog baseband signal of 6G radio will be wider than the frequency allocations of current wireless systems. Similarly, the fundamental TXs of LTE and Wi-Fi may introduce co-channel interference to the analog intermediate frequency (IF) interface of 5G mmW. A minimum 111 dB isolation requirement is calculated for the co-channel interference from the LTE antenna to the 5G mmW analog IF interface. The measured isolation in the 5G mmW proof-of-concept radio was 67 dB, significantly lower than the requirement. The 5G mmW OTA link measurements show that a narrow interference can block the 5G mmW operation if the IF signal has co-channel interference distorting the 5G synchronization signals. The 5G mmW analog interface measurement verifies that the future 6G multi-mode UEs and BTSs will face severe radio interoperability challenges with 5G, LTE, and Wi-Fi transmissions.

The performance of multi-user millimeter-wave (mmW) systems is limited by relatively high sidelobe levels (SLLs) of antenna arrays. Per-antenna amplitude control can be used to adjust the amplitudes to reduce the SLL, but the reduction is often achieved at the cost of reduced transmitted power. Large two-dimensional (2D) antenna panels used in mmW phased arrays, however, allow the 2D antenna configuration to be reconfigured to reduce the SLL. In this paper, we present a simplified approach for sidelobe reduction by stacking multiple uniform linear arrays of different size to reduce the sidelobes across the horizontal plane. The approach is based on the relation between the number of antenna elements and the directions of null and sidelobe maxima. The sidelobe reduction is demonstrated by both simulations and measurements. The measurements are carried out in an anechoic chamber at 28 GHz center frequency using a 100 MHz wide modulated 5GNR waveform.

Phased arrays used in millimeter-wave systems challenge the concept of power amplifier (PA) linearization by digital predistortion (DPD). This is due to the shared digital path and inaccuracies in analog beamforming and other component variations. However, the group behavior of multiple parallel nonlinear branches can be expected to be more predictable due to averaging effect compared to a single branch behavior. In this paper, we use a power adaptive nonlinear model to mimic the average behavior of a single PA and utilize the probability distribution of the input power of each individual PA to approximate the expected nonlinear behavior of the array over-the-air. The approximated array response is used for the DPD training. The simulation results indicate that the proposed approach provides good linearization performance for large arrays that have varying amplitude and phase weights.

A ray tracing simulator for urban and indoor environments is introduced. The simulator uses NVIDIA graphics processing unit (GPU) accelerated CUDA parallel computing platform and programming mode and the OptiX Ray Tracing Engine. As a use case, channel characteristics for Global Navigation Satellite System (GNSS) satellites are simulated and compared with measurements in an urban area. The speedup achieved by parallel processing allows computation of multiple relevant reflections characteristic of a satellite channel.

This paper proposes how to define the optimum number of stacked transistors in a multi-stacked CMOS power amplifier (PA) topology, based on several physical as well as circuit design aspects. Starting with a systematic concept, the analysis then goes through the relevance of transistor transconductance, aspect ratio, parasitics, operating frequency, and the number of transistor stages in a pentagonal trade-off concept. While this is done based on theoretical circuit analysis, the results, then, are evaluated using simulations based on 45nm CMOS technology.

Practical radio frequency beamforming can suffer from high sidelobe levels which cause inter-beam interference (IBI) in multibeam transceivers. IBI can be reduced by shaping the amplitude and phase excitation over the individual antenna elements. However, such methods do not exploit the available power efficiently in practical arrays, where each antenna is driven with a dedicated power amplifier. In this paper, we show a simplified approach for hybrid beam synthesis in subarray-based beamforming architecture and propose a two-stage beamforming method for cancelling the IBI. The proposed technique cancels the interference between the subarrays while it maximizes the effective isotropic radiated powers in the desired directions. Simulation show typically over 40 dB IBI rejection for randomly spread beams and more than 4 dB improvement for radiated power compared to amplitude tapering.

We study the performance of a prototype of a millimeter-wave transceiver with radio frequency (RF) beamforming capabilities. More specifically, the focus is on the architecture and software implementation of a smart-RF that allows self-beam-alignment. Simulation studies shows the expected performance in terms of probability of misalignment and coverage using different beam search strategies as well as realistic antenna beam patterns and phase-shifters quantization constraints.

This paper shows how digital predistortion of a phased array can benefit from the parametric variations over parallel power amplifiers (PAs). Different antenna configurations are simulated by varying the PA input drive levels by the Monte-Carlo method. The error vector magnitude (EVM) at the steering angle and total radiated adjacent channel power ratio (TRACPR) are used as performance metrics. The simulation results indicate that array predistortion can benefit from the variations between the PAs to improve the EVM significantly. However, at the same time, the TRACPR performance is reduced. This gives a new trade-off to balance between in-band and out-off-band distortion in the fifth generation beamforming systems.

5G system design is a complex process due to a great variety of applications and their diverse requirements. This article describes our experiences in developing a centimeter waves mobile broadband concept satisfying future capacity requirements. The first step in the process was the radio channel measurement campaign and statistical modeling. Then the link level design was performed tightly together with the radio frequency (RF) implementation requirements to allow as large scalability of the air interface as possible. We started the concept development at 10 GHz frequency band and during the project World Radiocommunication Conference 2015 selected somewhat higher frequencies as new candidates for 5G. Thus, the main learning was to gain insight of interdependencies of different phenomena and find feasible combinations of techniques and parameter combinations that might actually work in practice, not only in theory.

In-band full-duplex sets challenging requirements for wireless communication radios, in particular their capability to prevent receiver sensitivity degradation due to self-interference (transmit signals leaking into its own receiver). Previously published self-interference rejection designs require bulky components and/or antenna structures. This paper addresses this form-factor issue. First, compact radio transceiver feasibility bottlenecks are identified analytically, and tradeoff equations in function of link budget parameters are presented. These derivations indicate that the main bottlenecks can be resolved by increasing the isolation in analog/RF. Therefore, two design ideas are proposed, which provide attractive analog/RF-isolation and allow integration in compact radios. The first design proposal targets compact radio devices, such as small-cell base stations and tablet computers, and combines a dual-port polarized antenna with a self-tunable cancellation circuit. The second design proposal targets even more compact radio devices such as smartphones and sensor network nodes. This design builds on a tunable electrical balance isolator/duplexer in combination with a single-port miniature antenna. The electrical balance circuit can be implemented for scaled CMOS technology, facilitating low cost and dense integration.

This paper proposes a novel symmetric super- regenerative transceiver architecture for pulsed short-range communication. The proposed architecture is targeted for wireless applications where ultra low power consumption and data-rate in tens of megabits per second are needed over distances in the range of tens of centimeters. The transmitter and receiver are both based on super-regenerative principle and both functions in the transceiver utilize one mutual super-regenerative oscillator. Additionally, the transceiver architecture allows fast synchronization of transceivers thanks to the delayed reflection phenomenon which makes it possible to detect correct timing simultaneously at both ends of the system. Measurement results demonstrate the feasibility of the proposed principle.