IEEE Circuits and Systems Magazine - Q4 2022 - 7

based arrays in next-generation network infrastructure supporting
3d beam training in planar arrays, low latency massive multiple
access, and emerging wireless communications standards.
I. Introduction
W
IRELESS networks have fueled socioeconomic
growth worldwide and are expected
to further advance to enable new applications
such as autonomous vehicles, virtual and augmented
reality, and smart cities. Due to shortage of sub-6 GHz
spectrum, millimeter-wave (mmW) frequencies play an
important role in the fifth generation (5G) communication
networks. Recent research and development of
5G mmW networks has revealed that the propagation
loss in the mmW band [1] needs to be compensated by
antenna array gain [2] and densification of base stations
with cell radius as small as a hundred meters [3], [4].
To make radio chipsets power and cost- efficient, state
of the art (SOTA) 5G mmW transceivers are designed
with phased antenna array (PAA) based subarray architecture
[5], [6]. As a consequence, signal processing
techniques [7] and network protocols [8] for 5GmmW
networks are designed under constraints of PAA
architectures.
Future generations of mmW networks will operate in
the upper mmW frequency band where more than 10GHz
bandwidth can be used to meet
the ever-increasing
demands [9], [10]. Their realization will demand addressing
a completely new set of challenges including wider
bandwidths, larger antenna array size, and higher cell
density at the physical infrastructure level as highlighted
in Fig. 1. These new system requirements demand fundamental
rethinking of radio architectures, signal processing
and networking protocols. Major breakthroughs are
thus required in radio front-end architectures to enable
wideband mmW networks, as most commonly adopted
PAA based radios face many challenges in meeting the
demanding requirements for different SSP functions.
A large portion of PAA implementations approximate
the inter-element delay between the received signals
with a phaseshift and hence the spatial processing is
performed based on the phase difference between the
received signals. This approximation simplifies the physical
integrated circuit implementation, as compensating
a phase shift is much simpler than a time delay. However,
this simplification and approximation comes at the cost
of limited operating fractional bandwidth of the receiver
and will be analyzed further in Section II.
This article brings forward advances in true-timedelay
(TTD) arrays that have the potential to significantly
impact SSP for emerging wireless communication standards.
We use TTD-based PAA [11]-[26] to first establish
a contrast with the current phase shift only PAAs [27][46]
that will be used for both data communications and
direction finding. The application of TTD-based PAAs for
different SSP functions will have ramifications for future
mmW network infrastructure and emerging wireless
standards. We next develop a TTDbased PAA exploiting
the so called beam squint phenomenon to achieve precision
direction finding overcoming fundamental latency
bottlenecks in earlier beam training methods.
II. Exploiting Delay Compensation in
Spatial Signal Processors
PAA operating over a very wide frequency band exhibits
frequency-dependent beam pattern in an uncontrollable
manner and it often degrades beamforming gain and
directionality of the beam. This phenomenon is referred
as spatial wideband effect [47] or beam squint [48]-[50],
that becomes more significant in large antenna arrays.
This section will describe recent advances in SSP
algorithms and architectures leveraging delay compensating
circuits to overcome fundamental limits in analog
PAAs. Interested readers are referred to [51] for a
similar analysis for hybrid TTD PAAs. We will describe
application of TTD arrays to realize different SSP modes
when the array is used for data communications or for
direction finding. In the former SSP mode, the TTD array
will alleviate beam squint effects while in the latter
(A)
Compact
Co-located
Massive
MIMO Arrays
on Roof
(B)
Extremely
Large
Aperture
Array
on Walls
Figure 1. Emerging large multi-antenna based beamforming
infrastructure array systems feeding to a baseband ssP unit.
Chung-Ching Lin, Erfan Ghaderi*, Mohammad Ali Mokri, Jayce Jeron Gaddis*, Chase Puglisi*, Soumen Mohapatra, Qiuyan Xu, Sreeni Poolakkal,
Deukhyoun Heo, and Subhanshu Gupta are with the School of Electrical Engineering and Computer Science, Washington State University, Pullman,
WA 99164 USA (e-mail: chung-ching.lin@wsu.edu). (*-work performed at Washington State University)
Veljko Boljanovic, Han Yan, Aditya Wadaskar, and Danijela Cabric are with the Department of Electrical and Computer Engineering, University of
California at Los Angeles, Los Angeles, CA 90095 USA (e-mail: vboljanovic@ucla.edu).
Fourth quartEr 2022
IEEE cIrcuIts and systEms magazInE
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IEEE Circuits and Systems Magazine - Q4 2022

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