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354 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 57, NO. 2, FEBRUARY 2022
TABLE I
C
OMPARISON WITH RECENT PHASED ARRAYS AND HYBRID BEAMFORMERS
also enables STAR operation along with MIMO, which will
be demonstrated in our future publications.
E. Concurrent 28- and 37-GHz Band Operations
Concurrent 28- and 37-GHz band performance is character-
ized next in transmit and receive modes for a single element.
In the concurrent receive mode, two independently modulated
data streams from two external up-converters (one at the
28-GHz band and the other at the 37-GHz band) are power
combined and applied to one antenna element [see Fig. 25(a)].
One received stream is set to operate in the 28-GHz band and
the other at the 37-GHz band to simultaneously down-convert
both the incoming streams. The baseband received outputs
from both streams are simultaneously collected through an
oscilloscope. As shown in Fig. 25(b), for 1.5-Gb/s (64-QAM)
data rate in each carrier independently, the RX EVM is
−27.9 and −26.3 dB, respectively (peak received power is
set as iP1dB of RX). Similarly, in the concurrent TX mode,
two on-chip up-conversion paths of two streams operate: one
in the 28-GHz band and the other in the 37-GHz band. Two
upconverted data streams are combined on-chip at the input of
the PA and are transmitted through one PA. The PA output is
downconverted using two external down-converters operating
one in each band [see Fig. 25(c)]. As shown in Fig. 25(d),
for 1.5-Gb/s (64-QAM) data rate in each carrier, the TX
EVM is −27.7 and −27.1 dB, respectively (peak TX power
is set as oP1dB of TX). The above measurements demonstrate
concurrent dual-band operation for a single element. Although
the prototype can support concurrent carrier aggregation from
multiple elements, the demonstration of multi-antenna carrier-
aggregation requires designing a dual-band/wideband antenna
array (a field of ongoing research [31]–[33]), which can be
considered as future work.
VI. C
ONCLUSION
This work makes two major contributions. First, a new
two-layer hybrid MIMO architecture is introduced, which
comprises multiple low-RF-complexity single-stream (PC) or
multi-stream (FC) tiles in the first spatial-processing layer,
followed by an FC analog/digital baseband second layer. This
architecture mitigates the complexity versus spectral-efficiency
tradeoffs between conventional single-layer PC and (our previ-
ously proposed) FC architectures and enables upward scaling
of the number of MIMO streams. Second, an ultra-compact
bidirectional reconfigurable dual-band two-layer beamforming
circuit architecture is introduced, which includes various inno-
vative circuit design techniques. While achieving state-of-the-
art transceiver performance (see Table I), the eight-element
four-stream dual-band prototype in this work demonstrates, for
the first time, multi-user (up to four) OTA MIMO functionality
using low-complexity highly energy-efficient FC RF-tiles with
spectral-performance and flexibility approaching that of digital
beamformers.
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