High-resolution Phase Shifters Research Articles

60 GHz 7-Bit Passive Vector-Sum Phase Shifter With an X-Type Attenuator

This letter presents the design of a 60 GHz passive vector-sum phase shifter with 7-bit phase resolution. An analog X-type attenuator with complementary voltage control was used for vector modulation. The proposed attenuator is suitable for high-resolution phase shifter design because it is capable of phase inversion and shows a large attenuation range with low-phase variation. The prototype phase shifter fabricated through the 65 nm CMOS process occupies an area of 0.24 mm2 (excluding the bonding pads). The measurement results show that the proposed phase shifter achieves a phase resolution of 2.815∘ while exhibiting an average insertion loss of 14 dB without DC power consumption. The calculated RMS gain and phase error were below 0.55 dB and 0.83-2.36∘, respectively, in the range of 57-67 GHz.

Machine Learning-Inspired Hybrid Precoding for HAP Massive MIMO Systems with Limited RF Chains

Energy efficiency (EE) is the main target of wireless communication nowadays. In this paper, we investigate hybrid precoding (HP) and massive multiple-input multiple-output (MIMO) systems for a high-altitude platform (HAP). The HAP is an emerging solution operating in the stratosphere at an amplitude of up to 20–40 km to provide communication facilities that can achieve the best features of both terrestrial and satellite systems. The existing hybrid beamforming solution on a HAP requires a large number of high-resolution phase shifters (PSs) to realize analog beamforming and radio frequency (RF) chains associated with each antenna and achieve better performance. This leads to enormous power consumption, high costs, and high hardware complexity. To address such issues, one possible solution that has to be tweaked is to minimize the number of PSs and RFs or reduce their power consumption. This study proposes an HP sub-connected low-resolution bit PSs to address these challenges while lowering overall power consumption and achieving EE. To significantly reduce the RF chain in a massive MIMO system, HP is a suitable solution. This study further examined adaptive cross-entropy (ACE), a machine learning-based optimization that optimizes the achievable sum rate and energy efficiency in the Rician fading channel for HAP massive MIMO systems. ACE randomly generates several candidate solutions according to the probability distribution (PD) of the elements in HP. According to their sum rate, it adaptively weights these candidates’ HP and improves the PD in HP systems by minimizing the cross-entropy. Furthermore, this work suggests energy consumption analysis performance evaluation to unveil the fact that the proposed technique based on a sub-connected low-bit PS architecture can achieve near-optimum EE and sum rates compared with the previously reported methods.

Switch-and-inverter based adaptive subarrays for hybrid mmWave multi-user MIMO systems

ABSTRACT The hybrid precoding design is considered for a millimeter-wave (mmWave) multi-user (MU) multiple-input multiple-output (MIMO) downlink communication system. A small quantity of more energy-efficient switches and inverters (SIs) are being substituted for a large amount of high-resolution phase shifters (PSs) in the analog precoder of the mmWave MIMO system. This SI-based hybrid precoding architecture reduces significantly both the hardware complexity and energy consumption and thus enhances substantially the transmission energy efficiency (EE). On the basis of the achievable sum-rate (ASR) maximization, the SI-based overlapped subarray (OSA) hybrid precoding is exploited. Compared with the traditional SI-based sub-connected (SC) array configurations, the greater the overlapping antenna elements, the better the transmission ASR performance, and the nearer the fully PS-based precoding configuration. Finally, the SI-based hybrid precoding is equipped with the Gram–Schmidt orthogonalization (GSO) based dynamic subarray mapping and antenna selection (AS) mechanism to improve further the EE measure while maintaining excellent ASR performance.

Codebook-Based Hybrid Beamforming Using Combined Phase Shifters of High and Low Resolutions

The hybrid beamforming architecture where a combination of analog and digital precoders is employed has attracted much attention recently for its flexible trade-off between power consumption and performance. A codebook-based beamforming scheme is generally considered as a more practical solution rather than the adaptive beamforming schemes due to the large number of antennas. However, the constant amplitude constraint brought by the phase shifters (PSs) in the analog precoder always makes the codebook design problem a tricky and complicated one. In this work, we consider a novel combined PS (CPS) architecture where one high-resolution PS is connected to each RF chain in parallel with low-resolution PSs to each antenna. The corresponding codebook design aiming for ideal flat beam gains is then proposed for the CPS architecture. Simulation results reveal that the proposed scheme achieves a good trade-off between performance and energy efficiency compared with conventional single PS (SPS) and the recently proposed double PS (DPS) architecture.

An Energy-Efficient Hybrid Precoding Design in mmWave Massive MIMO Systems

The hybrid precoding (HP) technique has been widely considered as a promising approach for millimeter wave communication systems. In general, the existing HP structure with a complicated high-resolution phase shifter network can achieve near-optimal spectral efficiency, however, it involves high energy consumption. The HP architecture with an energy-efficient switch network can significantly reduce the energy consumption. To achieve maximum energy efficiency, this paper focuses on the HP architecture with switch network and considers a novel adaptive analog network HP structure for such mmWave MIMO systems, which can provide potential array gains. Moreover, a multiuser adaptive coordinate update algorithm is proposed for the HP design problem of this new structure. Simulation results verify that our proposed design can achieve better energy efficiency than other recently proposed HP schemes when the number of users is small.

Energy-Efficient Power Allocation with Hybrid Beamforming for Millimetre-Wave 5G Massive MIMO System

Massive multiple-input and multiple-output systems combined with hybrid beamforming technique is a key approach to achieve high data rate and extended cell coverage in the fifth generation (5G) cellular networks . Specifically, the precoding for analog/digital hybrid beamforming technique is absolutely essential for millimeter-wave (mm-wave) frequency based transmission for 5G, simply because the technique reduces hardware complexity and energy consumption for high-frequency design. In order to prove the aforementioned advantages, the present paper will investigate the energy efficiency (EE) and optimization of low complexity hybrid precoding (EELCHP) algorithm to enable the reduction of radio frequency (RF) chains within the base station (BS). The proposed solution will also incorporate high-resolution phase shifters (HRPS) specifically for downlink multi-user mm-wave system. The intelligence within the EELCHP algorithm for hybrid beamforming is based on the fully complex zero-forcing (ZF) precoding mechanism that allowed the maximization of energy efficiency by reducing the RF chains, without affecting the potentially increasing number of antennas. In addition, maximum EE is possible by eliminating the nonlinear concave fractional issue based on the Lagrange duality method. During the simulation process of the proposed method, we further realized that hardware complexity and power consumption can be reduced by allowing the RF chains to select only one beam per switch within the beamforming solution. Furthermore, with the implementation of EELCHP algorithm, more energy is saved due to low complexity system by reducing the required RF chain.

Low-Complexity Hybrid Precoding for Multi-User MmWave Systems With Low-Resolution Phase Shifters

Large-scale antennas systems have been attracted significant attention to provide considerable array gains for millimeter wave (mmWave) communications systems. However, huge hardware complexity and power consumptions make the conventional precoding architectures impractical to realize. Recently, hybrid precoding structures for mmWave communications systems are widely investigated to save the cost and achieve satisfactory performance. Unfortunately, most existing precoder designs lie in the assumption of infinite or high-resolution phase shifters (PSs), which are also difficult to realize in real-world. Motivated by this fact, a low complexity hybrid precoder and combiner design for multi-user systems with two-bit resolution PSs is proposed in this paper, which provides an appropriate trade-off between energy efficiency and system sum-rate. Specifically, the candidate sets for analog beamforming are constructed by introducing the auxiliary variables, in which the number of elements is the same as that of antennas. Accordingly, the computational complexity of searching optimal beamformer is significantly reduced over proposed codebooks. Then, to avoid high complexity singular value decomposition (SVD) of channel matrix, we propose a rank-1 approximate decomposition algorithm based-on principal component analysis technique. At last, the user scheduling algorithm is presented to further improve the performance in multiuser-scenario. Simulation results verified the effectiveness of the proposed algorithms, the performance of which is close to the optimal unquantized precoder.

A Hardware-Efficient Hybrid Beamforming Solution for mmWave MIMO Systems

In mmWave communication systems, existing hybrid beamforming solutions generally require a large number of high-resolution phase shifters (PSs) to realize analog beamformers, which still suffer from high hardware complexity and power consumption. Targeting at this problem, this article introduces a novel hardware-efficient hybrid precoding/combining architecture, which only employs a limited number of simple phase over-samplers (POSs) and a switch (SW) network to achieve maximum hardware efficiency while maintaining satisfactory spectral efficiency performance. The POS can be realized by a simple circuit and simultaneously outputs several parallel signals with different phases. With the aid of a simple switch network, the analog precoder/ combiner is implemented by feeding the signals with appropriate phases to antenna arrays or RF chains. We analyze the design challenges of this POS-SW-based hybrid beamforming architecture and present potential solutions to the fundamental issues, especially the precoder/combiner design and the channel estimation strategy. Simulation results demonstrate that this hardware-efficient structure can achieve comparable spectral efficiency but much higher energy efficiency than that of traditional structures.

Hybrid Precoder and Combiner Design With Low-Resolution Phase Shifters in mmWave MIMO Systems

Millimeter wave (mmWave) communications have been considered as a key technology for next generation cellular systems and Wi-Fi networks because of its advances in providing orders-of-magnitude wider bandwidth than current wireless networks. Economical and energy efficient analog/digial hybrid precoding and combining transceivers have been often proposed for mmWave massive multiple-input multiple-output (MIMO) systems to overcome the severe propagation loss of mmWave channels. One major shortcoming of existing solutions lies in the assumption of infinite or high-resolution phase shifters (PSs) to realize the analog beamformers. However, low-resolution PSs are typically adopted in practice to reduce the hardware cost and power consumption. Motivated by this fact, in this paper, we investigate the practical design of hybrid precoders and combiners with low-resolution PSs in mmWave MIMO systems. In particular, we propose an iterative algorithm which successively designs the low-resolution analog precoder and combiner pair for each data stream, aiming at conditionally maximizing the spectral efficiency. Then, the digital precoder and combiner are computed based on the obtained effective baseband channel to further enhance the spectral efficiency. In an effort to achieve an even more hardware-efficient large antenna array, we also investigate the design of hybrid beamformers with one-bit resolution (binary) PSs, and present a novel binary analog precoder and combiner optimization algorithm with quadratic complexity in the number of antennas. The proposed low-resolution hybrid beamforming design is further extended to multiuser MIMO communication systems. Simulation results demonstrate the performance advantages of the proposed algorithms compared to existing low-resolution hybrid beamforming designs, particularly for the one-bit resolution PS scenario.

Millimeter Wave Analog Beamforming With Low Resolution Phase Shifters for Multiuser Uplink

We consider millimeter wave multiuser uplink system with low resolution phase shifters, where users transmit simultaneously to base station. Transmit and receive beamforming through large antenna arrays is used to compensate severe path loss of millimeter waves. We first propose a joint precoder and detector design based on the low-complexity local search algorithm that iteratively finds the preferred transmit and receive beamforming vectors, which maximizes the sum-rate of the multiuser uplink system. Although the joint design achieves similar sum-rate to a fully digital system, the computation complexity to determine good beamforming vectors is high. To reduce complexity, we then propose nonjoint designs of precoder and detector. For the precoder design, the transmit beamforming vectors are chosen to maximize either the signal to noise ratio or the signal to interference plus noise ratio of each user. For the detector design, the receiver beamforming vectors are selected using either an approximate ML detector or a successive cancellation detector. Through simulations, we show that our designs with low resolution phase shifters outperform the traditional methods using steering vectors as beamforming vectors with high resolution phase shifters.

Energy-Efficient Hybrid Precoding Design for Millimeter-Wave Massive MIMO Systems Via Coordinate Update Algorithms

This paper considers an energy-efficient, hybrid digital, and analog precoding design problem in millimeter-wave massive multiple-input multiple-output systems, in which the analog precoder is realized with a small number of energy-efficient switches and inverters rather than with a large number of high-resolution phase shifters. However, finding the optimal weights of such energy-efficient hybrid precoding requires solving a challenging combinatorial problem. A known solver, namely “adaptive cross-entropy (ACE),” can provide a reasonable solution but its computational complexity is still high. Thus, a simple yet effective coordinate update algorithm (CUA) is proposed in this paper to reduce the computational complexity while maintaining and even improving the achievable rate performance. In addition, we propose a new hybrid precoding architecture that introduces the antenna selection mechanism into the conventional energy-efficient-based hybrid precoding architecture to further reduce energy consumption. Although, the resulting design problem of this new architecture becomes more complex in that the traditional ACE-based solver cannot be applied directly, the proposed CUA-based solver can still handle this problem. The simulation results demonstrate that the proposed CUA-based solver provides the same or even an improved achievable rate performance than the ACE-based solver at a lower complexity. Furthermore, the energy consumption of the proposed hybrid precoding architecture is lower than that of the conventional energy-efficient hybrid precoding architecture.

Analog Beamforming With Low Resolution Phase Shifters

In this letter, we consider analog beamforming using low resolution phase shifters for millimeter waves communications. We propose a hierarchical codebook design, where the beamforming vectors in the codebook are grouped into multiple levels and the preferred beamforming vector at each level is constructed to approximate an amplitude beamforming gain mask by using a low complexity local search algorithm. We show, by simulations, that the proposed codebook using low resolution phase shifters outperforms the existing schemes using high resolution phase shifters.

A High Resolution Digital Phase Shifter using Distributed MEMS Transmission Lines-A New Approach

Distributed Micro Electromechanical systems Transmission Line (DMTL) is used for designing digital phase shifters, by using an array of shunt switches. In the present paper, a new approach for designing DMTL for small phase steps is proposed. In the present work, an additional single switch section is introduced, having half the phase shift as the other switch sections in the DMTL. The addition of the new switch section does not alter the characteristic impedance or total phase shift of the DMTL phase shifter. A high resolution phase shifter is designed using the proposed model and the simulation results are presented. The proposed method can be further extended for achieving smaller phase steps.

A 120-GHz Electrical Interferometer for Contactless Permittivity Measurements With Direct Digital Read-Out

This work describes an electrical interferometer for contactless permittivity measurements working at 120 GHz. It was fabricated in a 130 nm SiGe process featuring an ft and fmax of 240 and 330 GHz. The on-chip system contains a 120 GHz VCO with a tuning range of 7 GHz featuring a divide-by-64 circuit to enable external PLL operation. The subsequent buffer provides 7 dBm of output power at 120 GHz. Additionally, the IC contains high-precision and high-resolution phase shifters based on a slow-wave transmission line approach with digital control for direct readout ability. A 120 GHz LNA with 17 dB gain and a power detector to provide DC output signals were realized on chip. It enables sample emulation capability by phase shift inducement in the measurement as well as a reference transmission line. In terms of phase detection, the system shows a sensitivity of 907.36 MHz/°.

Dense Delta-Sigma Phased Arrays

The high cost of high-resolution phase shifters required to maintain precise control over the array beam pattern in traditional phased arrays preclude their use in a variety of emerging millimeter-wave applications. We develop a phased array architecture that obviates the need for such precise phase shifters, based on the use of sub-half-wavelength array element spacing and novel spatial domain delta-sigma processing. We characterize the performance of this architecture in terms of the array signal-to-quantization-noise ratio (SQNR) and the array power transfer efficiency, and demonstrate a tradeoff between these two metrics. As an illustrative design, we show that when constrained to two-bit phase shifters, a four-fold increase in the array density can provide a roughly 6 dB improvement in SQNR over standard design techniques, with an average efficiency loss of less than 1.5 dB with respect to a perfectly tuned ideal array. In our analysis, we account for the effects of mutual coupling, and describe a simple, practical impedance matching network for this architecture. The resulting framework allows a system designer with a given set of circuit, device, and antenna fabrication and integration technologies to choose from a spectrum of tradeoffs between array density and RF component complexity.