IEEE Circuits and Systems Magazine - Q4 2022 - 10

scales with the number of antenna elements N, thus,
the beam-training overhead becomes a bottleneck for
low-latency communications. Sub-array based PAAs
with NRFRF chains can reduce the overhead by probing
NRF beams simultaneously. However, since NRF < N,
they need to rotate the NRF beams in consecutive training
symbols to cover all angular directions. Besides
a lower beamforming gain for each beam, sub-array
based PAAs also come at the cost of a higher power
consumption due to the presence of NRF radio frequency
(RF) chains. SOTA IA approaches based on
5G requirements are limited to narrowband pilots and
relatively small antenna arrays, so the latency of PAAbased
IA is acceptable. However, the future evolution
of mmW-nets will be IA latency limited and it would
require the use of wideband pilots and new radio
architectures that can probe a large number of AoAs/
AoDs in parallel as illustrated in Figure 7(b).
To address this fundamental issue in mmW PAAs,
introduce a fast beam training
this article will
method leveraging TTD arrays. Instead of suppressing
the beam squint effect evident in mmW arrays, we
will exploit this effect to our advantage by introducing
intentionally large time delays in each antenna
branch to realize frequency-dependent probing
beams which can be exploited to accelerate the
channel probing capability. These frequency-dependent
beams will be fully controlled by adjusting the
delay introduced in TTD circuits [61].
In addition,
the number of simultaneously probed frequencydependent
beams in TTD arrays can be rather large,
even when there is only one RF chain. These features
give an advantage to TTD arrays over conventional
sub-arrays based PAAs for fast and power-efficient
beam-training.
In summary, this article presents reconfigurable
delay compensating circuits with large delay range and
bandwidth, and finer delay resolution to realize wideband
mmWSSP. Section III summarizes the TTD SSP
architectures highlighting the challenges associated
(a)
(b)
Figure 7. Beam training comparison of (a) Ps based array with (b) ttd array.
10
IEEE cIrcuIts and systEms magazInE
Fourth quartEr 2022

IEEE Circuits and Systems Magazine - Q4 2022

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