Query: Linearity and Efficiency in 5G Transmitters

Candidate papers:

abstract: Every new generation of mobile systems should provide higher capacity, serve more users, be more energy efficient, and have lower cost. Radio access hardware constitutes a major bottleneck for reaching these goals, which is a particularly noticeable issue now that 5G communication systems are being developed. A combination of breakthroughs in communication theory as in massive multiple input/multiple output (MIMO) [1] highly integrated semiconductor and packaging technologies, and extended spectrum allocations has enabled a paradigm shift in the way radio hardware will be realized. Today's high power, fewantenna, sectorized systems will soon be replaced with highly integrated active antenna systems having up to hundreds of individually driven low power radios operating with very wideband signals.
author_names: Christian Fager

Thomas Eriksson

Filipe M Barradas

Katharina Hausmair

Telmo Reis Cunha

Jose Carlos Pedro
title: Linearity and Efficiency in 5G Transmitters: New Techniques for Analyzing Efficiency, Linearity, and Linearization in a 5G Active Antenna Transmitter Context
venue: IEEE Microwave Magazine
year: 2019

abstract: Phased array transmit systems use several antennas to combine RF power amplifiers in free space. If all the RF channels are identical, then the transmit spectrum in the far field, including both linear and nonlinear components, would be a scaled version of the output spectrum of each channel. However, random variations in the nonlinear components (AM AM and AM PM conversion) between the channels improves the nonlinearity of the overall array as the number of elements increases. In this paper, measured results are used to show that the adjacent channel power ratio, which is one metric of linearity, improves with the number of elements at a fixed backoff from the 1 dB compression point. Also, for a 100 Mbaud 64QAM signal and a fixed ACPR of 32 dBc, a 256 element phased array can be operated at $P_{1dB} 2\mathbf{dB} while an 8 element phased array would need to be operated at $P_{1dB} 4\mathbf{dB} for the same ACPR level. This work has great implications on the overall efficiency of 5G phased arrays since it implies that large phased arrays can be operated at less back off than small phased arrays (or single antennas with high power amplifiers) Thus, in reality and taking the ACPR as the figure of merit, phased arrays are 2 dB more efficient than what is predicted by standard system simulations which assume the same non linear response for all the phased array channels.
author_names: Bhaskara Rupakula

Abdurrahman H Aljuhani

Gabriel M Rebeiz
title: Linearity and Efficiency Improvements in Phased Array Transmitters with Large Number of Elements and Complex Modulation
venue: 2018 IEEE/MTT S International Microwave Symposium IMS
year: 2018

abstract: Linearity, efficiency, and power consumption are the key figures of merit for modern communication system design [1] as data rates increase toward millimeter wave communication systems like 5G. A common problem is that both high linearity and efficiency are not simultaneously feasible in power amplifier (PA) design. Hence, when a PA is operated near saturation for highest efficiency, undesired harmonic and intermodulation distortions have to be taken into account [2] [3] A transceiver comprises several circuit elements, such as a low noise amplifier, mixer, and PA. In this transceiver chain, the PA usually exhibits the highest power consumption (i.e. it is worthwhile to maximize the efficiency of this circuit component to increase overall system performance) [4]
author_names: Thomas Ackermann

Julian Potschka

Tim Maiwald

Amelie Hagelauer

Georg Fischer

Robert Weigel
title: A Robust Digital Predistortion Algorithm for 5G MIMO: Modeling a MIMO Scenario With Two Nonlinear MIMO Transmitters Including a Cross Coupling Effect
venue: IEEE Microwave Magazine
year: 2020

abstract: Energy efficiency of 5G communication networks and beyond is a major challenge that is yet solved due to high power consumption and low linearity of massive array transmitters. This paper contributes to the existing framework of research in this area that is today mainly dominated by the RF component level and algorithmic solutions by investigating various possible design trade offs for highly integrated active antenna elements as employed in such array transmitters. We demonstrate how to exploit antenna element design to synthesize the optimal loading conditions of a given power amplifier (PA) to, e.g. maximize its power added efficiency (PAE) minimize nonlinear behavior, or find the desired trade off between both. The numerical example with a K band PA integrated antenna element illustrates that moderate nonlinear effects of the PA can be significantly reduced (with up to 10 20 dB in third order intermodulation distortion) by tuning the antenna design, with a relatively small PAE loss 5% The effects of antenna array mutual coupling and corresponding cross talk between PAs when beamsteering are discussed for a small scale linear array of such elements.
author_names: Wan-Chun Liao

Thomas Emanuelsson

Rob Maaskant

Artem R Vilenskiy

Thomas Eriksson

Marianna V Ivashina
title: Power Efficiency and Linearity of Highly Integrated Transmitting Array Antennas
venue: 2021 15th European Conference on Antennas and Propagation (EuCAP)
year: 2021

abstract: One of the biggest challenges in modern transmitter (TX) design, when going from the fourth generation (4G) to fifth generation (5G) communications network, is to handle the increased linearity requirements without introducing any compromise in the energy efficiency of the TX line up. In analog systems, high quality for the TX signal can be only achieved when using very linear operation of the (analog) power amplifier (PA) This severely limits the achievable efficiency in practical TX line ups. Alternatively, a nonlinear PA can be used, which is linearized by digital pre distortion (DPD) circuitry. This later approach is commonly used in (4G) macro cell base stations, but it comes at the cost of increased system complexity and high supply power for the advanced DPD unit. When going towards 5G handset, or massive multiple input multiple output (mMIMO) 5G base station units, that facilitate beamforming and higher data rates to their end users. The required RF output power per individual transmitter is rather low (at most only a few watts) However, since many more transmitters are used in 5G applications (e.g. a factor 64 x to 256 x more than in 4G base stations) the use of an advanced DPD units in each individual TX lineup, with their related high power consumption becomes simply impractical. Consequently, to address these changing needs, it is highly desirable to find new circuit level TX solutions, that overcome the traditional linearity efficiency trade off. To achieve this goal, this PhD work is focused on the utilization and tailoring of digital device operation, as facilitated by advanced CMOS technologies, towards the needs of modern wireless applications with their wideband complex modulated TX signals. The circuit techniques developed within this thesis, target an inherently linear amplitude code word (ACW) to TX output signal transfer, as such omitting completely the need for a power hungry advanced DPD unit, or alternatively, rely on a much more simple and consequently less power hungry DPD unit for the most demanding applications (e.g. when handling large modulation bandwidths) The circuit techniques developed in thesis, allow excellent drain and TX line up efficiency, while being compatible with wideband efficiency enhancement techniques like Doherty. The proposed circuit techniques are also able to correct for process, voltage, load and temperature variations of the application.
author_names: Mohsen Hashemi
title: Energy Efficient and Intrinsically Linear Digital Polar Transmitters
year: 2020

abstract: Highly efficient power amplifiers (PAs) and associated linearization techniques have been developed to accommodate the explosive growth in the data transmission rate and application of massive multiple input multiple output (mMIMO) systems. In this paper, energy efficient integrated Doherty PA monolithic microwave integrated circuits (MMICs) and linearization techniques are reviewed for both the sub 6 GHz and millimeter wave (mm Wave) fifth generation (5G) mMIMO systems; different semiconductor processes and architectures are compared and analyzed. Since the 5G protocols have not yet been finalized and PA specifications for mMIMO are still under consideration, it is worth investigating novel design methods to further improve their efficiency and linearity performance. Digital predistortion techniques need to evolve to be adapted in mMIMO systems, and some creative linearity enhancement techniques are needed to simultaneously improve the compensation accuracy and reduce the power consumption.
author_names: Xin Liu

Guansheng Lv

Dehan Wang

Wenhua Chen

Fadhel M Ghannouchi
title: Energy efficient power amplifiers and linearization techniques for massive MIMO transmitters: a review
venue: Frontiers of Information Technology Electronic Engineering
year: 2020

abstract: The modern spectral, and thereby linearity, requirements force 5G phased array transmitter systems to operate at reduced power efficiency, as they can no longer use voluminous filtering. To reduce the linearity requirements of the transmitter, we consider the case of an array consisting of closely spaced radiating elements operating at different frequencies. The coupled tones from one element to another create reverse intermodulation distortion (RIMD) We explain how RIMD is created within a power amplifier (PA) and derive an estimate for the power of the RIMD components. We provide a set of measurements for an X Band GaAs PA and draw a direct comparison between RIMD and IMD. We show that RIMD has a third order behaviour up to very high reverse power levels, opening up the perspective for higher output power operation as well as simpler and lower power predistortion in multi tone array systems such as 5G and radar.
author_names: Anton N Atanasov

Mark S Oude Alink

Frank E van Vliet
title: Reverse Intermodulation in Multi Tone Array Transmitters
venue: 2020 IEEE BiCMOS and Compound Semiconductor Integrated Circuits and Technology Symposium (BCICTS)
year: 2020

abstract: THE growing demand of modern communication network with high data rate including 5G, point to point backhaul systems requires energy efficiency and wideband TXs for beamforming. In the analog intensive TX architectures, DACs are essential to convert the digital baseband to analog signal. However, the power consumption of DACs with gigabits data rate is watt level, which decrease the system efficiency significantly especially for portable device. Meanwhile, to support high data rate wireless transmission with high order QAM modulations, high linearity, large dynamic range are required for TXs. Digital intensive TXs eliminate DACs, and provide solutions to break trade off between PA efficiency and linearity [1] A 4 element digital modulated polar phased array TX based on phase modulation phase shifting is proposed [2] with state of the art phase error, gain error, and system efficiency. However, the delay mismatch between the amplitude and phase path affects the modulation bandwidth, which is even more critical for millimeter wave TX. Power DAC based digital quadrature TX is a promising candidate for the wideband modulation at millimeter wave bands comparing to the polar/outphasing counterparts. However, with the increasing power DAC resolution, the parasitics of the complex interconnections of power DAC unit cells cause efficiency degradation due to the load impedance mismatch of the unit cells. To address above issues, we propose a prototype of 2x10 bit digital quadrature TX [3] with notched matching and modeswitch topology for beamforming transmission to improve the TX efficiency, bandwidth, and data rate. To improve the scanning resolution and gain error of phased array TX, a current limited digital phase shifter for digital assisted phasedarray TX is proposed [4] The design mechanism with high resolution, low gain error, and wideband operation is analyzed, and a 3 7GHz prototype phase shifter is then demonstrated. Based on the mechanism [4] a 90 98GHz digital assisted phased array TX with high phase resolution is reported [5] The gain variation is reduced with an integrated digital assisted variable gain PA. In this report, we descript our recent research progress on a digital quadrature TX [3] and a digital assisted phasedarray TX [5] for high resolution beamforming transmission. Fabricated in 28nm/40nm CMOS technology, the proposed
author_names: Huizhen Jenny Qian

Xun Luo
title: Mixed Signal Architectures of Wideband Transmitters for High Resolution Beamforming Transmission
year: 2019

abstract: The 5G wireless revolution presents some dramatic challenges to the design of handsets and communication infrastructures, as 5G targets higher than 10 Gbps download speed using millimeter wave (mm Wave) spectrum with multiple input multiple output (MIMO) antennas, connecting densely deployed wireless devices for Internet of Everything (IoE) and very small latency time for ultrareliable machine type communication, etc. The broadband modulation bandwidth for 5G RF transmitters (i.e. maximum possibly even above 1 GHz) demands high power efficiency and stringent linearity from its power amplifier (PA) Additionally, the phased array MIMO antennas with numerous RF front ends (RFFEs) will require unprecedented high integration level with low cost, making the design of 5G PA one of the most challenging tasks. As the centimeter wave (cm Wave) 5G systems will probably be deployed on the market earlier than their mm Wave counterparts, we will review in this paper the latest development on 15 GHz and 28 GHz 5G cm Wave PAs extensively, while also covering some key mm Wave PAs in the literature. Our review will focus on the available options of device technologies, novel circuit and system architectures, and efficiency enhancement techniques at power back off for 5G PA design.
author_names: Donald Yu-Chun Lie

Jill C Mayeda

Yan Li

Jerry Lopez
title: A Review of 5G Power Amplifier Design at cm Wave and mm Wave Frequencies
venue: Wirel. Commun. Mob. Comput.
year: 2018

abstract: There is a growing interest in exploring antenna electronic co designs that leverage antennas (including their low loss metal structures, design versatility, multi feed driving capabilities) as a new design paradigm to radically advance mm wave front end performance. In particular, enhancing the peak/back off transmitting efficiency is essential for future mm wave wireless communications, such as 5G, which will predominantly use complex modulations with large peak to average power ratio (PAPR) and extra power back off (PBO) for linearity and reliability. Consequently, mm wave transmitters often operate at deep PBO (9 to 12dB) with poor average efficiency. Although mm wave Doherty/Outphasing PAs have been widely explored, there is limited success in high order active load modulation for deep PBO efficiency enhancement with high PA linearity, mainly due to the complexity and prohibitive loss of on chip passive networks. On the other hand, most existing antenna electronic radiators only follow basic Class AB/B efficiency roll off with rapidly degraded PBO efficiency [1,2] Some use active load modulation on a single multi feed antenna, whose simple structures support only 6dB PBO efficiency enhancement and limited PBO extension [3]
author_names: Huy Thong Nguyen

Sensen Li

Hua Wang
title: 4.6 A mm Wave 3 Way Linear Doherty Radiator with Multi Antenna Coupling and On Antenna Current Scaling Series Combiner for Deep Power Back Off Efficiency Enhancement
venue: 2019 IEEE International Solid State Circuits Conference (ISSCC)
year: 2019