Mohammad-Reza Nezhad-Ahmadi

Bio: Mohammad-Reza Nezhad-Ahmadi is an academic researcher from University of Waterloo. The author has contributed to research in topics: Antenna (radio) & Phased array. The author has an hindex of 11, co-authored 44 publications receiving 562 citations.

Optimized Microstrip Antenna Arrays for Emerging Millimeter-Wave Wireless Applications

Abstract: Two compact planar antennas operating in the unlicensed 60 GHz frequency band are presented based on the physical layer specifications of IEEE 802.15.3c and ECMA 387 standards for different classes of wireless applications. Each antenna is an array of 2 × 2 microstrip antennas covering at least two channels of the 60 GHz spectrum. The first antenna is optimized to achieve the highest gain, while the second antenna is optimized to give the largest beamwidth. The maximum measured radiation gain of the first antenna is 13.2 dBi. The measured beamwidth and gain of the second antenna are 76° and 10.3 dBi, respectively. The areas of these two antenna are only 0.25 and 0.16 cm2. The variation of radiation gain of each antenna over the frequency range of 57-65 GHz is less than 1 dB.

High-Efficiency On-Chip Dielectric Resonator Antenna for mm-Wave Transceivers

Abstract: A high radiation efficiency on-chip antenna is presented in a low-resistivity silicon technology. The proposed antenna configuration consists of a high-permittivity rectangular dielectric resonator excited by an H-slot antenna implemented in a silicon integrated circuit process. Using the Wheeler method an efficiency of 48% has been measured for the integrated antenna at 35 GHz. The maximum size of this low profile antenna (h = 0.5 mm) is close to λ0/5 (considering the dielectric resonator), and its radiation gain is around 1 dBi at 35 GHz. Moreover, the bandwidth of this antenna is 4.15 GHz (12%). Simulations and measurements show that by removing the passivation layer on top of the H-slot aperture the radiation efficiency increases by 10%.

CMOS Phased Array Transceiver Technology for 60 GHz Wireless Applications

Abstract: Based on the indoor radio-wave propagation analysis, and the fundamental limits of CMOS technology it is shown that phased array technology is the ultimate solution for the radio and physical layer of the millimeter wave multi-Gb/s wireless networks. A low-cost, single-receiver array architecture with RF phase-shifting is proposed and design, analysis and measurements of its key components are presented. A high-gain, two-stage, low noise amplifier in 90 nm-CMOS technology with more than 20 dB gain over the 60 GHz spectrum is designed. Furthermore, a broadband analog phase shifter with a linear phase and low insertion loss variation is designed, and its measured characteristics are presented. Moreover, two novel beamforming techniques for millimeter wave phased array receivers are developed in this paper. The performance of these methods for line-of-sight and multipath signal propagation conditions is studied. It is shown that one of the proposed beamforming methods has an excess gain of up to 14 dB when the line of sight link is obstructed by a human.

Millimeter-Wave Reflective-Type Phase Shifter in CMOS Technology

Abstract: The design and measurement of a compact, wide-band reflective-type phase shifter in 90 nm CMOS technology in V-band frequency is presented. This phase shifter has a fractional bandwidth of 26% and an average insertion loss of 6 dB over all phase states. The chip area is only 0.08 mm 2. Measurement results show that the developed phase shifter provides 90° continuous phase shift over the frequency range of 50-65 GHz. The measured return loss is greater than 12 dB at 50 GHz. The output power is linear up to at least 4 dBm input power.

A Modular Architecture for Wide Scan Angle Phased Array Antenna for K/Ka Mobile SATCOM

Abstract: In this paper a low cost/complexity modular architecture for electronically scanned active phased array antenna (A-PAA) for high throughput K/Ka-band mobile satellite communication (SATCOM) is presented. With this approach, A-PAAs of any size, shape, and configuration can be built of intelligent building blocks (active sub-array modules) to minimize the design complexity and fabrication cost. The design and fabrication aspects of K/Ka A-PAA with wide scanning angle range is presented. The measured results of the modular array show that the antenna main beam can be steered to ± 70o off the boresight at 20/30 GHz. Furthermore, the antenna is able to achieve EVM ~ 7.60 % during the constellation test for 16 QAM and 1.2 Gb/s.

Channel Estimation and Hybrid Precoding for Millimeter Wave Cellular Systems

Abstract: Millimeter wave (mmWave) cellular systems will enable gigabit-per-second data rates thanks to the large bandwidth available at mmWave frequencies. To realize sufficient link margin, mmWave systems will employ directional beamforming with large antenna arrays at both the transmitter and receiver. Due to the high cost and power consumption of gigasample mixed-signal devices, mmWave precoding will likely be divided among the analog and digital domains. The large number of antennas and the presence of analog beamforming requires the development of mmWave-specific channel estimation and precoding algorithms. This paper develops an adaptive algorithm to estimate the mmWave channel parameters that exploits the poor scattering nature of the channel. To enable the efficient operation of this algorithm, a novel hierarchical multi-resolution codebook is designed to construct training beamforming vectors with different beamwidths. For single-path channels, an upper bound on the estimation error probability using the proposed algorithm is derived, and some insights into the efficient allocation of the training power among the adaptive stages of the algorithm are obtained. The adaptive channel estimation algorithm is then extended to the multi-path case relying on the sparse nature of the channel. Using the estimated channel, this paper proposes a new hybrid analog/digital precoding algorithm that overcomes the hardware constraints on the analog-only beamforming, and approaches the performance of digital solutions. Simulation results show that the proposed low-complexity channel estimation algorithm achieves comparable precoding gains compared to exhaustive channel training algorithms. The results illustrate that the proposed channel estimation and precoding algorithms can approach the coverage probability achieved by perfect channel knowledge even in the presence of interference.

An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems

Abstract: Communication at millimeter wave (mmWave) frequencies is defining a new era of wireless communication. The mmWave band offers higher bandwidth communication channels versus those presently used in commercial wireless systems. The applications of mmWave are immense: wireless local and personal area networks in the unlicensed band, 5G cellular systems, not to mention vehicular area networks, ad hoc networks, and wearables. Signal processing is critical for enabling the next generation of mmWave communication. Due to the use of large antenna arrays at the transmitter and receiver, combined with radio frequency and mixed signal power constraints, new multiple-input multiple-output (MIMO) communication signal processing techniques are needed. Because of the wide bandwidths, low complexity transceiver algorithms become important. There are opportunities to exploit techniques like compressed sensing for channel estimation and beamforming. This article provides an overview of signal processing challenges in mmWave wireless systems, with an emphasis on those faced by using MIMO communication at higher carrier frequencies.

Limited Feedback Hybrid Precoding for Multi-User Millimeter Wave Systems

Abstract: Antenna arrays will be an important ingredient in millimeter-wave (mmWave) cellular systems. A natural application of antenna arrays is simultaneous transmission to multiple users. Unfortunately, the hardware constraints in mmWave systems make it difficult to apply conventional lower frequency multiuser MIMO precoding techniques at mmWave. This paper develops low-complexity hybrid analog/digital precoding for downlink multiuser mmWave systems. Hybrid precoding involves a combination of analog and digital processing that is inspired by the power consumption of complete radio frequency and mixed signal hardware. The proposed algorithm configures hybrid precoders at the transmitter and analog combiners at multiple receivers with a small training and feedback overhead. The performance of the proposed algorithm is analyzed in the large dimensional regime and in single-path channels. When the analog and digital precoding vectors are selected from quantized codebooks, the rate loss due to the joint quantization is characterized, and insights are given into the performance of hybrid precoding compared with analog-only beamforming solutions. Analytical and simulation results show that the proposed techniques offer higher sum rates compared with analog-only beamforming solutions, and approach the performance of the unconstrained digital beamforming with relatively small codebooks.

State of the Art in 60-GHz Integrated Circuits and Systems for Wireless Communications

Abstract: This tutorial presents an overview of the technological advances in millimeter-wave (mm-wave) circuit components, antennas, and propagation that will soon allow 60-GHz transceivers to provide multigigabit per second (multi-Gb/s) wireless communication data transfers in the consumer marketplace. Our goal is to help engineers understand the convergence of communications, circuits, and antennas, as the emerging world of subterahertz and terahertz wireless communications will require understanding at the intersections of these areas. This paper covers trends and recent accomplishments in a wide range of circuits and systems topics that must be understood to create massively broadband wireless communication systems of the future. In this paper, we present some evolving applications of massively broadband wireless communications, and use tables and graphs to show research progress from the literature on various radio system components, including on-chip and in-package antennas, radio-frequency (RF) power amplifiers (PAs), low-noise amplifiers (LNAs), voltage-controlled oscillators (VCOs), mixers, and analog-to-digital converters (ADCs). We focus primarily on silicon-based technologies, as these provide the best means of implementing very low-cost, highly integrated 60-GHz mm-wave circuits. In addition, the paper illuminates characterization techniques that are required to competently design and fabricate mm-wave devices in silicon, and illustrates effects of the 60-GHz RF propagation channel for both in-building and outdoor use. The paper concludes with an overview of the standardization and commercialization efforts for 60-GHz multi-Gb/s devices, and presents a novel way to compare the data rate versus power efficiency for future broadband devices.

Limited Feedback Hybrid Precoding for Multi-User Millimeter Wave Systems

Abstract: Antenna arrays will be an important ingredient in millimeter wave (mmWave) cellular systems. A natural application of antenna arrays is simultaneous transmission to multiple users. Unfortunately, the hardware constraints in mmWave systems make it difficult to apply conventional lower frequency multiuser MIMO precoding techniques at mmWave. This paper develops low complexity hybrid analog/digital precoding for downlink multiuser mmWave systems. Hybrid precoding involves a combination of analog and digital processing that is inspired by the power consumption of complete radio frequency and mixed signal hardware. The proposed algorithm configures hybrid precoders at the transmitter and analog combiners at multiple receivers with a small training and feedback overhead. The performance of the proposed algorithm is analyzed in the large dimensional regime and in single path channels. When the analog and digital precoding vectors are selected from quantized codebooks, the rate loss due to the joint quantization is characterized and insights are given into the performance of hybrid beamforming compared with analog-only beamforming solutions. Analytical and simulation results show that the proposed techniques offer higher sum rates compared with analog-only beamforming solutions, and approach the performance of the unconstrained digital beamforming with relatively small codebooks.