IEEE Circuits and Systems Magazine - Q2 2019 - 34

Abstract
millimeter wave (mmW) communications is viewed as the key
enabler of 5G cellular networks due to vast spectrum availability
that could boost peak rate and capacity. Due to increased propagation loss in mmW band, transceivers with massive antenna array are required to meet a link budget, but their power consumption and cost become limiting factors for commercial systems.
radio designs based on hybrid digital and analog array architectures and the usage of radio frequency (rF) signal processing via phase shifters have emerged as potential solutions to
improve radio energy efficiency and deliver performances close
to the conventional digital antenna arrays. in this paper, we provide an overview of the state-of-the-art mmW massive antenna
array designs and comparison among three array architectures,
namely digital array, partially-connected hybrid array (subarray), and fully-connected hybrid array. the comparison of performance, power, and area for these three architectures is performed for three representative 5G downlink use cases, which
cover a range of pre-beamforming signal-to-noise-ratios (sNr)
and multiplexing regimes. this is the first study to comprehensively model and quantitatively analyze all design aspects and
criteria including: 1) optimal linear precoder, 2) impact of quantization error in digital-to-analog converter (DAc) and phase shifters, 3) rF signal distribution network, 4) power and area estimation based on state-of-the-art mmW circuits including baseband
digital precoding, digital signal distribution network, high-speed
DAcs, oscillators, mixers, phase shifters, rF signal distribution
network, and power amplifiers. our simulation results show that
the fully-digital array architecture is the most power and area
efficient compared against optimized designs for sub-array and
hybrid array architectures. our analysis shows that digital array
architecture benefits greatly from multi-user multiplexing. the
analysis also reveals that sub-array architecture performance
is limited by reduced beamforming gain due to array partitioning, while the system bottleneck of the fully-connected hybrid
architecture is the excessively complicated and power hungry
rF signal distribution network.

I. Introduction
illimeter-wave (mmW) communications is a promising technology for the future fifth-generation
(5G) cellular network [1], [2]. In the US, the Federal Communications Commission (FCC) has voted to
adopt a new Upper Microwave Flexible Use service in the
licensed bands, namely 28 GHz (27.5-28.35 GHz band),
37 GHz (37-38.6 GHz band), 39 GHz (38.6-40 GHz) with a
total 3.85 GHz bandwidth [3]. The abundant spectrum facilitates key performance indicators (KPI) of 5G, including 10Gbps peak rate, 1000 times higher traffic throughput than the current cellular system [4]. As shown in
theory and measurements, mmW signals suffer higher
free-space transmission loss [5], and is vulnerable to
blockage [6]. As a consequence, radios require beamforming (BF) with large antenna arrays at both base
station (BS) and user equipment (UE) to combat severe

m

propagation loss [7]. This makes reliable communication range short and as a consequence, mmW BSs will be
deployed in an ultra-dense manner with inter-site distance in the order of hundreds of meters [8], [9]. Due to
these facts, performance, energy, and cost efficiency in
the future mmW base station (BS) radios become more
important than ever before.
Implementation and deployment of transceiver arrays
in sub-6 GHz have shown great success. In the 4G Long
Term Evolution Advanced (LTE-A) system, BS supports
up to 8 antennas [10] and arrays with even larger size are
being actively prototyped [11] and will be soon available
in the LTE-A PRO (the pre-5G standard). Those systems
exclusively have digital array architecture based on a
dedicated radio-frequency transceiver chain, with data
converter and up/down-conversion, per each antenna,
and rely on digital baseband for array processing. Many
implementation challenges arise in scaling up array size
[12] by an order of magnitude or more required for mmW
bands. System designers are also concerned about the
high cost and power consumption in digital array architecture with massive number of RF-chains and ultrawide processing bandwidth [13].
Recently, an emerging concept of hybrid array has been
proposed. A hybrid array uses two stage array processing. The analog beamforming implemented with variable phase shifters (PS) provides beamforming gain and
the digital beamforming in the baseband provides flexibility for multiplexing multiple user streams [14], [15].
As a result, hybrid arrays support an RF transceiver
count which is smaller than the array size. Such an architecture intends to reduce the power and cost penalty
due to numerous tranceivers. Based on the connectivity between RF-chain and antenna, there are two major
variations, fully-connected hybrid array and partially
connected hybrid array. Although both architectures
were used for radar application [16] and were introduced for telecommunication application as early as a
decade ago [17], they have recently gained much attention for mmW radios. Signal processing techniques,
including channel estimation and beamforming, using
hybrid architecture have been comprehensively studied [18]. Proposals for using hybrid architectures
in mmW 5G have been considered in standardization
organizations [19].
A handful comparative analyses exists for different
mmW array architectures, with an emphasis on the
signal process algorithms [19]-[22]. Authors in [23] discussed circuits design challenges in implementing

Han Yan and Danijela Cabric are with the Electrical Engineering Department, University of California, Los Angeles, Los Angeles, CA 90095
(e-mail: yhaddint@ucla.edu; danijela@ee.ucla.edu), Sridhar Ramesh, Timothy Gallagher, and Curtis Ling are with Maxlinear, Inc, Carlsbad, CA 92008.
(e-mail: sramesh@maxlinear.com; tgallagher@maxlinear.com; cling@maxlinear.com).

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