Radio Frequency Phase Shifter Research Articles

UAV-Assisted Wideband Terahertz Wireless Communications with Time-Delay Phased UPA under Beam Squint

Future Unmanned Aerial Vehicle (UAV)-assisted wireless communication systems are expected to utilize wide bandwidths available at terahertz (THz) frequencies to enhance system throughput. To compensate for the severe path loss in the THz band, it is essential to have a multitude of antennas in the UAV to generate narrow beams for directional transmission. However, narrow beams severely limit its spatial coverage, which greatly affects the efficiency of large-scale access UAV-assisted THz systems. Moreover, the combination of massive antennas and large bandwidth at THz makes the misalignment of the beams caused by beam squint non-negligible and also high energy consumption. UAV-assisted communication technology can effectively increase spatial coverage and provide reliable LoS communication links. In addition, reducing the number of radio frequency (RF) chains while ensuring the number of transmitted data streams and space division multiplexing capability is also an effective way to reduce energy consumption in the UAV communication. In this paper, a single RF chain uniform planar array (UPA) with true-time-delays (TTDs) is equipped on the UAV to achieve two dimensional (2D) beams and split spatial beams to improve transmission efficiency. We analyze the 2D beam squint of the UPA and design a time-delay phased UPA for UAV-assisted THz communication systems. By introducing TTDs between the single RF chain and phase shifters, the beam squint can be controlled flexibly by introducing the delay between each antenna. When TTDs are arranged in both the horizontal and vertical dimensions, the coverage of the beams becomes more complicated compared to uniform linear arrays (ULA). Simulation results show that the proposed time-delay phased UPA can achieve better performance in both single-beam and multi-beam modes for single user and multi-user scenarios compared with conventional phased UPA, respectively. In addition, we propose frequency division beam multiple access (FDBMA) multi access technology, which achieves more efficient multi access by reusing resources from different frequency beam pairs. Finally, the results also show that the enlargement of the beamwidth through the proposed FDBMA strategy can also increase the performance in multi-user scenarios.

Lattice-Based mmWave Hybrid Beamforming

Conventional hybrid precoding and combining based transceivers require a large number of high-resolution radio frequency (RF) phase shifters (PSs), which impose prohibitive hardware costs and power consumption. To address the above issue, both partially connected RF PSs and low-resolution PSs have been proposed. However, the performance limits of these low-cost designs have not been investigated theoretically. Furthermore, there is room for improvement in their spectral efficiency. To fill this knowledge gap, we derive the mean square error performance discrepancy between an optimal precoder/combiner and the hybrid analog-digital precoder/combiner under the constraint of 1-bit PSs relying on lattice theory. Then, by observing that this performance gap can be reduced by deactivating parts of the PSs whilst improving both the spectral and energy efficiency, we develop an adaptive RF PS connection network. To resolve the associated hybrid precoding and combining problems, we appropriately adapt Babai's algorithm from the lattice decoding literature. Our simulation results demonstrate the superiority of the proposed scheme both in terms of its spectral and energy efficiency.

Broadly tunable and low power penalty radio frequency phase shifter using a coupled silicon microcavity.

We present a continuously tunable silicon photonics assisted radio frequency (RF) phase shifter using a coupled microring resonator. Using the coupled cavity, we demonstrate a sub-1 dB power penalty for a RF bandwidth of 34.5 GHz (9-43.5 GHz) and a phase shift of π over the reported frequency range. Rigorous optimization of the cavity design using the coupled-mode theory is carried out to realize ultranarrow resonance peaks with a low-extinction ratio and large phase shift. Thermal tuning of the cavity is used to tune the phase while all-optical tuning is exploited to achieve broadband operation. We present a detailed simulation and experimental study of the proposed configuration. The proposed device configuration exhibits a configurable resonance linewidth and extinction ratio that allows for a broad bandwidth and an extremely low power penalty microwave phase shifter. We believe the demonstration would allow better integration of the on-chip functional elements of integrated microwave photonics.

A Silicon Photonic RF Phase Shifter With Linear Phase Response and Low RF Power Variation

We propose and experimentally demonstrate a microwave photonic phase shifter based on a tunable silicon photonic interleaver. This device consists of a microring resonator, a tunable interleaver, and a $2\,\times \,1$ multimode interferometer (MMI). By thermally tuning the interleaver, a linear phase shift of 188° is realized with a linearity of 0.08 and a radio-frequency (RF) power variation within 1 dB for a 40-GHz signal.

Performance Analysis of Directional Modulation With Finite-Quantized RF Phase Shifters in Analog Beamforming Structure

The radio frequency (RF) phase shifter with finite quantization bits in analog beamforming (AB) structure forms quantization error (QE) and causes a performance loss of received signal to interference plus noise ratio (SINR) at the receiver (called Bob). By using the law of large numbers in probability theory, the closed-form expression of SINR performance loss is derived to be inversely proportional to the square of sinc (or sin(x)/x) function. Here, a phase alignment method is applied in directional modulation transmitter with AB structure. Also, the secrecy rate (SR) expression is derived with QE. From numerical simulation results, we find that the SINR performance loss gradually decreases as the number L of quantization bits increases. This loss is less than 0.3dB when L is larger than or equal to 3. As L exceeds 5, the SINR performance loss at Bob can be approximately trivial. Similarly, SR performance loss gradually reduces as L increases. In particular, the SR performance loss is about 0.1 bits/s/Hz for L=3 at signal-to-noise ratio of 15dB.

Tunable Silicon Photonic RF Phase Shifter With Low RF Power Variation Based on Constructive Interference of an Add-Drop Ring Resonator

We propose and experimentally demonstrate a tunable silicon photonic radio frequency (RF) phase shifter with low power variation based on constructive interference of an add-drop ring resonator. The thermal nonlinear effect of the add-drop ring resonator is used to realize a nearly 2π-phase shift, and the RF power variation is minimized by allowing two lights at the outputs of the add-drop ring resonator to combine at a directional coupler to achieve constructive interference. By thermally tuning the add-drop ring resonator, a nearly 2π-phase shift is realized and the operation is verified by an experiment. Experimental results show that a 0-5.2-rad phase-shift range with an RF power variation within 0.2 dB for a frequency range of a 40-GHz signal is achieved by thermally tuning the ring resonator. A phase shift up to 4.1 rad over a frequency range of 20-40 GHz is also realized.

Design of an optical comb generator based on harmony search algorithm

A method based on harmony search (HS) algorithm for designing an optical comb generator with excellent flatness is proposed. The comb generator with simple structure consists of four phase modulators with a single laser as the input, single radio frequency (RF) bias with different amplitudes, and DC bias for each modulator. No RF phase shifter, filter, or fiber loop is needed. Instead of following a classic mathematical model for finding the values of RF amplitudes and DC voltages for generating an optical comb, HS algorithm is implemented, which is an intelligent algorithm inspired from the improvisation of music players and seeks for the most harmonized answers corresponding to flat optical combs. The simulation results of generating two sets of optical combs by HS algorithm with 43 and 53 lines with flatness of 0.51 and 0.59 dB and then after inclusion of power conversion efficiency in the objective function of HS algorithm, two other sets of combs with 47 and 55 lines, flatness of 1.64 and 1.77 dB, and power conversion efficiency of 28% and 32% are presented. Such a device can be used in wavelength-division multiplexing systems and many other applications.

A Tone-Based AoA Estimation and Multiuser Precoding for Millimeter Wave Massive MIMO

In this paper, we investigate channel estimation and multiuser downlink transmission of a time division duplex massive multiple-input multiple-output (MIMO) system in millimeter wave (mmWave) channels. We propose a tone-based linear search algorithm to facilitate the estimation of angle-of-arrivals (AoAs) of the strongest line-of-sight (SLOS) channel component as well as the scattering components of the users at the base station. Based on the estimated AoAs, we reconstruct the SLOS component and scattering components of the users for downlink transmission. We then derive the achievable rates of maximum-ratio transmission (MRT) and zero-forcing (ZF) precoding based on the SLOS component and the SLOS-plus-scattering components (SLPS), respectively. Taking into account the impact of pilot contamination, our analysis and simulation results show that the SLOS-based MRT can achieve higher data rate than that of the traditional pilot-aided-CSI-based (PAC-based) MRT, under the same mean square errors of channel estimation. As for ZF precoding, the achievable rates of the SLPS-based and the PAC-based are identical. Furthermore, we quantify the achievable rate degradation of the SLOS-based MRT precoding caused by phase quantization errors in the large number of antennas regime. We show that the impact of phase quantization errors on the considered systems cannot be mitigated by increasing the number of antennas and therefore the resolutions of radio frequency phase shifters is critical for the design of efficient mmWave massive MIMO systems.

On the Number of RF Chains and Phase Shifters, and Scheduling Design With Hybrid Analog–Digital Beamforming

This paper considers hybrid beamforming (HB) for downlink multiuser massive multiple input multiple output (MIMO) systems with frequency selective channels. For this system, first we determine the required number of radio frequency (RF) chains and phase shifters (PSs) such that the proposed HB achieves the same performance as that of the digital beamforming (DB) which utilizes $N$ (number of transmitter antennas) RF chains. We show that the performance of the DB can be achieved with our HB just by utilizing $r_t$ RF chains and $2r_t(N-r_t + 1)$ PSs, where $r_t \leq N$ is the rank of the combined digital precoder matrices of all sub-carriers. Second, we provide a simple and novel approach to reduce the number of PSs with only a negligible performance degradation. Numerical results reveal that only $20-40$ PSs per RF chain are sufficient for practically relevant parameter settings. Finally, for the scenario where the deployed number of RF chains $(N_a)$ is less than $r_t$, we propose a simple user scheduling algorithm to select the best set of users in each sub-carrier. Simulation results validate theoretical expressions, and demonstrate the superiority of the proposed HB design over the existing HB designs in both flat fading and frequency selective channels.

A compact frequency tunable radio frequency phase shifter with patterned Py enabled transmission line

A well designed frequency tunable phase shifter using patterned Py with different thickness has been demonstrated. Phase shifter is implemented with a slow wave coplanar wave guide (CPW) transmission line, where the signal line has alternate short narrow and wide sections. Py is patterned on the top of narrow section for high inductance density, and inter-digital capacitor is implemented in wide section for high capacitance density. Compared with phase shifter using regular CPW, the dimension of the developed phase shifter has been reduced from 14.86 mm to 4.70 mm at 2 GHz. Phase shifter based on 100 nm and 200 nm thick patterned Py with the same dimensions (14 μm × 10 μm) are implemented and investigated comprehensively. FMR frequency of 3.2 GHz and 3.6 GHz without any external magnetic field has been achieved for 100 nm and 200 nm thick Py film, respectively. Thicker Py has increased inductance density from 1067.2 nH/m to 1193.2 nH/m while the center frequency of the phase shifter has been shifted to 1.80 GHz. Frequency tunability of the phase shifter has been also demonstrated with DC current. The phase shifter can provide 90° phase shift continuously from 2 GHz to 1.80 GHz with DC current from 0 mA to 150 mA. The design concept has great potential in design arbitrary tunable RF components such as filters and couplers.

Theory study on a photonic-assisted radio frequency phase shifter with direct current voltage control

A photonic-assisted radio frequency phase shifter with direct current voltage control is proposed using a polymer-based integrated Mach—Zehnder modulator. A closed-form expression of radio frequency (RF) signal power and phase is given. Theoretical calculation reveals that by carefully setting the bias voltages, RF signal power variation lower than 1-dB and phase accuracy less than 3° can be achieved and are not degraded by perturbation of modulation index once the bias voltage drift is kept within −3% ∼ 3%.

Active phase drift cancellation for optic-fiber frequency transfer using a photonic radio-frequency phase shifter.

We propose an active photonic phase drift cancellation scheme for frequency transfer over optical fiber based on a linear photonic RF phase shifter. The photonic RF phase shifter consists of a dual parallel Mach-Zehnder modulator and optical filter with the assistance of the local microwave signal. The phase drift induced by fiber transmission can be compensated by simply tuning the bias voltage of the modulator. The principle of the phase cancellation scheme based on the photonic phase shifter is demonstrated and validated experimentally by transferring a 0.5GHz reference signal over a 20km single-mode fiber with a root mean square jitter of less than 0.5ps.

A tunable photonic RF phase shifter using single semiconductor optical amplifier

ABSTRACTA continuously tunable linear radio frequency phase shifter based on polarization interference in single semiconductor optical amplifier is presented, and the theoretical fundamentals of the design are explained. A prototype of the phase shifter with 10‐GHz bandwidth and far beyond 360° tuning range is experimentally demonstrated. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:340–343, 2014

Linear and stable photonic radio frequency phase shifter based on a dual-parallel Mach–Zehnder modulator using a two-drive scheme

We theoretically and experimentally demonstrate a linear and stable photonic RF phase shifter based on a dual-parallel Mach-Zehnder modulator (DPMZM) using a two-drive scheme. To avoid the effect of the residual optical carrier and overcome the lowest frequency limit from the optical filter, a local microwave signal and a signal up-converted from the under-phase-shifted RF signal are applied to the two RF inputs of the DPMZM, respectively. A phase-shifted RF signal is generated by beating the two first-order upper sidebands located in the passband of the optical filter. A continuous and linear phase shift of more than 360° and power variation of less than ±0.15 dB at 1GHz are achieved by simply tuning the bias voltage of the modulator. A phase tuning bandwidth of more than 17MHz and phase drift of less than 0.5° within 2000s are also observed.

A novel tunable optoelectronic oscillator based on a photonic RF phase shifter

A novel tunable optoelectronic oscillator (OEO) based on a photonic radio frequency (RF) phase shifter is proposed and analyzed, which consists of a dual-drive Mach-Zehnder modulator (DMZM), a 90° hybrid coupler and a tunable microwave amplifier (TMA). This tunable OEO has a simple configuration, and the frequency of the oscillating signal can be tuned by adjusting the amplification factor of the tunable amplifier. The simulation results show that the maximum frequency shift from the center oscillation frequency is 1.48 MHz when the amplification factor of the TMA is set at 10.

Tunable 360° Photonic Radio-Frequency Phase Shifter Based on Polarization Modulation and All-Optical Differentiation

A continuously tunable photonic radio-frequency (RF) phase shifter based on polarization modulation and all-optical differentiation using a polarization modulator (PolM) and an optical frequency discriminator is proposed, theoretically analyzed and experimentally demonstrated. In the proposed phase shifter, two light waves at different wavelengths are injected into a PolM which is driven by an RF signal to be phase shifted. For one wavelength, the state of polarization (SOP) of the light wave is aligned with one principal axis of the PolM, thus the PolM operates jointly with an optical frequency discriminator as an optical differentiator to produce a quadrature (Q) signal. For the other wavelength, the SOP of the light wave is oriented with 45° or 135° with the principal axis of the PolM, thus the PolM operates jointly with a polarizer as an intensity modulator to produce an in-phase (I) signal. By detecting the I-Q optical signals at a photodetector (PD), a phase-shifted RF signal is generated. The value of the phase shift is determined by the amplitudes of the I-Q signals, which can be continuously tuned by controlling the powers of the light waves. An experiment is performed. A phase shifter with a full tunable range of 360° at an RF frequency tunable from 3 to 10 GHz is demonstrated.

Tunable 360° photonic radio frequency phase shifter based on optical quadrature double-sideband modulation and differential detection

We propose a novel structure of a photonic RF phase shifter based on the vector-sum principle. The optical signal with quadrature double-sideband modulation passes through a dual-output Mach-Zehnder interferometer (MZI), and the two outputs are differentially detected. Two phase-quadrature RF terms are generated, and their amplitudes can be controlled in a triangularly complementary way by changing the phase of the MZI. A full tuning range of 0°-360° at 14 GHz is demonstrated experimentally accompanied by nearly constant RF amplitude. The validity of using our scheme in all-optical RF phase modulation is also verified.

SOA-based fiber ring laser use in a photonic radio-frequency phase shifter

This investigation demonstrates the feasibility of a semiconductor optical amplifier (SOA)—based fiber ring laser use in a photonic radio-frequency (RF) phase shifter. Experimental results indicate that a fiber ring laser can successfully replace an external pump laser in a photonic RF phase shifter. The RF phase shifter can be turned by adjusting the power of the fiber ring laser.

A novel photonic RF phase shifter based on polarization controller

A novel radio frequency (RF) photonic phase shifter based on polarization controller is presented and the theoretical fundamentals of the design are explained.This phase shifter provides broad operational bandwidth and a full 360° phase shift tuning range with a commercial polarization controller. A prototype of the RF photonic phase shifter with a frequency of 26.75 GHz and 360° tuning range is experimentally demonstrated. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2598–2600, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25533

Linear photonic radio frequency phase shifter using a differential-group-delay element and an optical phase modulator

We demonstrate a stable and linear photonic radio frequency (RF) phase shifter based on a differential-group-delay element and the polarization sensitive effect of an optical phase modulator. The phase shift can be tuned continuously over 360 degrees for RF signals over 40GHz with an electrical control voltage from -7.5 to +7.5V.