Fixed Phase Shifters Research Articles

Dynamic-Subarray With Fixed Phase Shifters for Energy-Efficient Terahertz Hybrid Beamforming Under Partial CSI

Terahertz (THz) communications are regarded as a pillar technology for the 6G systems, by offering multi-ten-GHz bandwidth. To overcome the huge propagation loss, THz ultra-massive MIMO systems with hybrid beamforming are proposed to offer high array gain. Notably, the adjustable phase shifters considered in most existing hybrid beamforming studies are power-hungry and difficult to realize in the THz band. Moreover, due to the ultra-massive antennas, full channel-state-information (CSI) is challenging to obtain. To address these practical concerns, in this paper, an energy-efficient dynamic-subarray with fixed phase shifters (DS-FPS) architecture is proposed for THz hybrid beamforming. To compensate for the spectral efficiency loss caused by the fixed phase of FPS, a switch network is inserted to enable dynamic connections. In addition, by considering the partial CSI, we propose a row-successive-decomposition (RSD) algorithm to design the hybrid beamforming matrices for DS-FPS. A row-by-row (RBR) algorithm is further proposed to reduce the computational complexity. Extensive simulation results show that, the proposed DS-FPS architecture with the RSD and RBR algorithms achieves much higher energy efficiency than the existing architectures. Moreover, the spectral efficiency of the DS-FPS architecture with the proposed algorithms is robust to the CSI error.

W-Band Beam-Tilted H-Plane Horn Array Antenna with Wideband Integrated Waveguide Feed Network Based on MMPTE

A W-band H-plane horn array antenna with tilted radiation beam based on waveguide structure is proposed in this paper. The designed antenna array consists of four H-plane antenna elements and a broadband feed network. The distribution of excitations is determined by the theory of maximum power transmission efficiency (MMPTE). A multiple branch coupler, two T-junctions and three fixed phase shifters are employed to construct the feed network, which can generate the required amplitude and phase in broadband frequency range from 80 GHz to 100 GHz. The computer numerical control (CNC) milling machines technology is employed to machine the feed network and antenna. All measured and simulated results are in good agreement, which verify the feasibility of the theory of MMPTE to generate a radiation beam directed to any angle from -35° to 35° with suitable excitation provided by the proposed feed network in this paper.

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.

Hardware Phase Shift Hybrid Precoding Designs For Multi-User Massive MIMO Systems

Hybrid precoding is a challenging design for massive MIMO systems that involves a combination of analog and digital processing, aiming to maximize the spectral efficiency (SE). Most works on hybrid precoding focus on the single phase shifter (SPS) implementation to adapt the phase from RF chains to antennas. In this paper we propose to develop the double phase shifter (DPS) and the fixed phase shifter (FPS) in both single-path and multi-path configuration. Simulation results certify a significant improvement for both proposed implementations with high hardware efficiency (HE) and high spectral efficiency especially in multi-path environment.

Features of the implementation of fixed phase shifters on coupled microstrip transmission lines with a stub

Fixed phase shifters with a phase-shifting channel based on coupled transmission lines of class II with a stub have a number of advantages in comparison with previously known structures for similar purposes. However, their practical implementation causes difficulties associated with the topological features of the structure, which, in turn, leads to a deterioration in frequency characteristics. In this regard, a numerical analysis of inhomogeneities in a class II structure with a stub and an assessment of their influence on the frequency characteristics of fixed phase shifters is required. In this work, circuitry and electrodynamic modeling of class II structure elements has been carried out, which showed that the greatest parasitic effect on the frequency characteristics of the phase shifter has a nonzero coupling coefficient of single transmission lines and an inhomogeneity localized at the junction of the short-circuited stub and the output arms of the associated transmission lines. A fixed phase shifter with a phase-shifting channel based on a smoothly stepped structure with a stub has been developed. The presented results of the study of fixed phase shifters based on microstrip coupled transmission lines of class II with a stub can be useful to scientific and technical workers engaged in the development of functional devices for the phase shift of microwave signals.

An Efficient Low-Complexity Method to Calculate Hybrid Beamforming Matrices for mmWave Massive MIMO Systems

In the context of a massive MIMO system, the number of RF chains increases prohibitively. To solve the problem, hybrid beamforming (BF) has been proposed. Nevertheless, the amplitude constraint of the analog part in hybrid BF limits achieving the optimal performance given by the optimal digital BF. Some studies have been conducted recently to relax the amplitude constraint by deploying multiple phase shifters and switches (that select phase shifters) in the hybrid beamforming structures. The tremendous number of switches in the structure makes the problem of switch state selection very complex. In this paper, we propose an efficient low-complexity algorithm to obtain the optimal switch states. In the proposed algorithm, we decompose the main problem into sub-ones to reduce the complexity and solve the problem using a finite search space. The results of our study suggest that the optimal solution will be attainable by searching through less than 1% of the search space, if the number of phase levels exceeds 14. Moreover, the proposed algorithm has the potential to perfectly implement linear beamformers, designed for fully digital architectures, in a hybrid structure, while applying the minimum number of RF chains. Finally, the simulation results prove the efficiency of the algorithm and confirm the sufficiency of the analog part of the BF to achieve the optimal digital BF performance just by applying a few number of fixed phase shifters.

Development of a compact Local Power Distribution System for the ILC

The Local Power Distribution System (LPDS) of the International Linear Collider (ILC) transmits RF power from a 10 MW klystron to 39 cavities. A variable hybrid and variable phase shifter are adopted in the LPDS to have been adopted in the LPDS to achieve the efficient beam acceleration in the ILC. As it is difficult to construct the LPDS in an accelerator tunnel, the LPDS can be assembled on a cryomodule on the ground. Then, the cryomodule integrated with the LPDS can be installed in the tunnel. Furthermore, production cost can be decreased by utilizing identical variable hybrids and variable phase shifters. In this study, a compact LPDS was developed to satisfy these two requirements. A compact variable hybrid and variable phase shifter were designed for a compact LPDS. The power transmitted to a cavity was adjusted by the variable hybrid to achieve the maximal accelerating field, which exhibited a variation of ±20% depending on cavity performance. The variable phase shifter compensated the phase shift of the variable hybrid. Fixed phase shifters were designed to adjust the phase difference between adjacent cavities for beam acceleration. These RF components were manufactured, and the S parameters and power handling capability of the components were tested. An L-band resonant ring was constructed at the Super-conducting RF Test Facility (STF) at KEK. The power handling capability of these RF components were tested using a power of 5.2 MW with a pulse width of 200 μs and a power of 2.0 MW with a pulse width of 2 ms. The S parameters and power handling capability satisfied the requirements of the compact LPDS. The compact LPDS will be constructed using these RF components and demonstrated at the STF.

Analysis of SVD-Based Hybrid Schemes for Massive MIMO With Phase Noise and Imperfect Channel Estimation

In hybrid analog-digital schemes, proposed to reduce the number of RF chains especially at millimeter waves, the precoding at the transmitter and the combining at the receiver are split into digital and analog parts. We analyze the sensitivity of hybrid schemes to phase noise and channel estimation errors and we compare them to a full-digital approach. The scheme adopted for the analog part employs fixed phase shifters, then the digital part is optimized by a singular-value decomposition. We derive analytical expressions for the interference and the SNR degradation arising from the imperfect decomposition due to phase noise and the channel estimation error, for typical millimter-wave massive MIMO channels. In particular we show that when the channel estimation is made in the beam-space, this hybrid scheme is more robust to the phase noise and to the channel estimation errors than a full-digital approach.

Laser Application and Sparse Imaging Analysis of Diffractive Optical System

In recent years, the diffractive optical systems have developed rapidly. Diffractive devices such as binary optical device and membrane-based lens are equivalent to fixed phase shifters of microwave antennas. Thus, the mature theories and methods of a microwave phased-array antenna could be used for diffractive devices’ performance analysis. Both laser Synthetic Aperture Radar (SAR) and laser communication feature a single color and long wavelength, and they are specifically suitable for non-imaging diffractive optical systems. A signal wave front control realized by a diffraction device reduces the focal length and the weight of a system. Research on laser SAR and laser communication technology has important theoretical significance and application value for diffractive optical system. In this paper, we provide a phased-array interpretation of a diffractive optical system and introduce research that has been conducted on airborne and spaceborne laser SAR with respect to diffractive optical systems. We propose the concept of shipborne 1 m diffraction aperture laser communication and an interferometric positioning system and analyze its performance. The results indicated that, using a 10 m short baseline, this system can reach 400 million km with a corresponding positioning accuracy of 6 km that is suitable for use during deep space probes. We also discuss the sparse-sampling laser-imaging problem using a laser to illuminate the target, transforming the laser image signal into the frequency domain with Fourier lens, using the small-scale detector to perform sparse sampling in the low-frequency domain, and reconstructing the target image using a computer. Some preliminary simulation results are provided.

A Checkered Network for Implementing Arbitrary Overlapped Feed Networks

Existing overlapped subarray techniques for beamforming arrays aim to realize flat-topped subarray radiation patterns, where pattern synthesis is constrained by the hardware configuration. A generalized framework for designing compressive arrays with unconstrained feed network responses has recently been proposed with promising results, but no hardware implementation has been proposed. A planar checkered network of directional couplers and fixed phase shifters is proposed for implementing arbitrary complex-valued feed network responses, including completely overlapped networks. An algorithm is proposed for realizing a desired response while minimizing the range of coupling ratios and phase shifts required. The technique is validated by a comparison to the standard 4 x 4 Butler matrix implementation, and its versatility illustrated by sharing an eight-element aperture between three independently designed nonscanning arrays. A manufactured microstrip 2 x 4 compressive array feed network achieves the desired aperture illuminations to within 0.6 dB and 5.2° at 3.15 GHz, and to within 1.4 dB and 10.3° across the impedance bandwidth of 4.5%.

A Fixed Miniaturization 90° Phase Shifter for VHF-Band High-Power Applications

This paper presents a novel miniaturization 90° phase shifter using parallel coupled inductors and capacitors. The size reduction is achieved by replacing the quarter-wavelength transmission lines in a conventional fixed 90 degrees phase shifter with the resonator circuit. The resonator circuit, formed by the parallel coupled inductors and capacitor, provides a 90° phase shift at 45 MHz. In this way, the circuit size can be largely reduced without sacrificing the operation bandwidth and power handling. The inductors structure used in the phase shifter carries high power, which consists of two full coupled rectangle metal coils. The proposed miniature fixed phase shifter is then applied to the design of a directional coupler centered at 45 MHz. The measured results show a bandwidth of 40–50 MHz for better than 35 dB return loss with a phase shift of ~ 90°, a deviation within ~ 2°, and power handling of 63 dBm (2 kW). Notably, the proposed fixed phase shifter occupies an area of 58.5 mm $\times30.5$ mm $\times33$ mm, which is only about $0.009\,\,\lambda _{\mathbf {0}}\times 0.0045\,\,\lambda _{\mathbf {0}}\times 0.005\,\,\lambda _{\mathbf {0}}$ at 45 MHz.

A Low Complexity $16\times16$ Butler Matrix Design Using Eight-Port Hybrids

Beamforming networks such as Butler Matrices are important for multibeam array antenna applications. The challenge for Butler Matrix design is that their complexity increases with the number of ports. In this paper, a novel approach of designing a $16\times 16$ Butler Matrix with significant structure simplification is presented. The eight-port hybrids with no crossovers are used to simplify the network. To ensure the network has the same magnitude and phase responses as the standard one, the location and phase shifting value of each fixed phase shifter are derived from the $S$ -matrix of each hybrid. A $16\times 16$ Butler Matrix network operating from 9 GHz–11 GHz is designed to validate this concept. The compensated microstrip 3-dB/90° directional coupler, the phase shifter with a shunt open-and-short stub and the crossover with a resonating patch are used to reduce the transmission loss and enable broadband operation.

A Hardware-Efficient Analog Network Structure for Hybrid Precoding in Millimeter Wave Systems

Hybrid precoding has been recently proposed as a cost-effective transceiver solution for millimeter wave (mm-wave) systems. While the number of radio frequency (RF) chains has been effectively reduced in existing works, a large number of high-precision phase shifters are still needed. Practical phase shifters are with coarsely quantized phases, and their number should be reduced to a minimum due to cost and power consideration. In this paper, we propose a novel hardware-efficient implementation for hybrid precoding, called the fixed phase shifter (FPS) implementation. It only requires a small number of phase shifters with quantized and fixed phases. To enhance the spectral efficiency, a switch network is put forward to provide dynamic connections from phase shifters to antennas, which is adaptive to the channel states. An effective alternating minimization (AltMin) algorithm is developed with closed-form solutions in each iteration to determine the hybrid precoder and the states of switches. Moreover, to further reduce the hardware complexity, a group-connected mapping strategy is proposed to reduce the number of switches. Simulation results show that the FPS fully-connected hybrid precoder achieves higher hardware efficiency with much fewer phase shifters than existing proposals. Furthermore, the group-connected mapping achieves a good balance between spectral efficiency and hardware complexity.

New structure of ultrawideband fixed phase shifters based on stepped coupled transmission lines with stubs

Problems of the synthesis of ultrawideband fixed phase shifters with a new structure of the electric circuit of the phase-shifting channel formed by means of connection of two identical elements consisting of weakly coupled stepped transmission lines with stubs have been solved. The advantage of the proposed structure over the available structures is shown.

Power splitting and switching in a multi-core fiber based on the multimode interference effect.

In this paper, we propose a novel 1 to N optical power splitter and a 1 to N optical switch for a multi-core fiber (MCF) with N circularly aligned cores. The splitter and the switch are based on the multimode interference (MMI) effect inside a ring core fiber. The MMI effect will convert one image into N output images in the ring and therefore, the ring shape MMI coupler can act as a 1 to N power splitter. These images will have different phases. If two ring shape MMI couplers are used and a tunable phase shifter array and a fixed phase shifter array are placed between them, by properly setting the phases of the N images in the middle of the MMI couplers, the images will converge to one output port of the 2nd MMI coupler. The output port number can be changed by tuning the phase shifters. In this way, the input signal at one of the cores of the MCF can be switched to the other core, and a 1 to N switch can be realized. In the analysis, it is found that only one control parameter is required for the phase adjustment of the tunable phase shifter array in order to achieve the switching between the cores.

Ultrawideband fixed phase shifters based on coupled transmission lines with stubs

The synthesis problem is solved for fixed phase shifters with a new structure of the phase-shifting channel, which is formed by cascading several two-ports. Each two-port is section of distributed–coupled transmission lines with a short-circuited stub. An advantage of the proposed structure over existing analogues is shown.

Single-Sideband Time-Modulated Phased Array

A new in-phase/quadrature (I/Q) complex modulation technique to realize single-sideband time-modulated phase-only weighting on an array antenna is proposed. Based on modulating rectangular pulses and trapezoidal pulses, the theories of generating a scanning beam at the single positive sideband are presented. It was found that using trapezoidal pulse modulation can obtain excellent performance in power efficiency and undesired sideband level than using rectangular pulse modulation. Unlike conventional time-modulated antenna array where each array element is connected to a RF switch, in the proposed scheme, each element is connected to an I/Q channel modulator which is composed of two Wilkinson power dividers, two RF switches, two $0/\pi $ phase shifters in RF channels, and one $\pi/2 $ fixed-phase shifter in the control circuit. By properly controlling both the phase shifters and the switches in time domain, the single-sideband modulation can be realized with the uniform amplitude and variable phase in frequency domain. The I/Q modulator acts as a phase shifter and can be used to form a phased array. In this paper, the main efforts have been paid to find the pulse sequences to suppress the spurious bands. To validate the proposed technique, simulated results of an eight-element single-sideband time-modulated linear array obtained from the array factor and full-wave analysis are reported. In the case of using rectangular pulses, the power efficiency of the scanning beam is 91.47% and the peak level of the highest undesired harmonic is $-13.98\;\mathrm{dB} $ . In the case of using trapezoidal pulses, the best values can reach 99.79% and $-27.98\;\mathrm{dB} $ .

OAM states generation/detection based on the multimode interference effect in a ring core fiber.

In this paper, we propose to generate/detect orbital angular momentum (OAM) states based on the multimode interference (MMI) effect in a piece of ring core fiber. A comprehensive theory for the MMI process inside ring core fibers is presented. The MMI process inside ring core fibers will convert one input image into multiple equally spaced duplicated output images. After phase adjustment by a fixed phase shifter array, these output images will stimulate OAM states in the ring core fiber or in free space. Henceforth, a novel OAM multiplexer/de-multiplexer (MUX/DEMUX) can be realized by a piece of ring core fiber and a fixed phase shifters array.

Switched‐beam antenna array with broadside beam fed by modified butler matrix for radar receiver application

ABSTRACTA novel approach to the design of switched‐beam antennas with broadside beam has been presented. The presented antenna utilizes a modified Butler matrix as a feeding network. The proposed feeding network consists of only two 3‐dB/0/180 directional couplers, a 3‐dB/90° directional coupler and a 180° phase shifter. Moreover, an innovative arrangement of the network has been proposed, which allows for avoiding any crossings in the network, and allows for achieving fully planar realization of a complete antenna. Measurement results of the designed three‐beam antenna are shown, and also the application of the presented concept in a simple Doppler radar receiver is proposed, in which fixed phase shifters are eliminated. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:732–735, 2014

Miniaturization Design for 8 × 8 Butler Matrix Based on Back-to-Back Bilayer Microstrip

A low-cost, compact 8 × 8 Butler matrix based on a novel bilayer microstrip configuration is presented and implemented for 4.3 GHz telecommunication application. A back-to-back placed bilayer microstrip structure has been proposed to avoid using crossover. To expand operational bandwidth of the Butler matrix, a three-branch line directional coupler has been employed as 3 dB/90° bridge, and a kind of improved two-order Schiffman phase shifter has been adopted as fixed phase shifter. For application of indoor wireless communication, a compact broadband 8 × 8 Butler matrix has been designed and fabricated. The measured results show that the return loss of the matrix is lower than −10 dB, the isolation is better than 17 dB, the power distribution error is less than ±2.0 dB, the phase error is less than ±15°, and the relative bandwidth is more than 23%.