IEEE Circuits and Systems Magazine - Q2 2019 - 36

■ Number of RF-chains: M
■ Number of simultaneous streams: U (U # M ).
■ Number of bits in digital-to -analog convert-

er (DAC): B
■ Number of bits in phase shifter: Q. This only ap-

plies to hybrid arrays.
In the rest of the paper, we use DA, SA and FH when referring to digital array architecture, sub-array and fullyconnected hybrid array architecture, respectively. Mathematical symbols with subscript indicate parameters
associated with the specific architecture, e.g., N DA represents number of antennas in digital array. The main differences among three array architecture are:
■ Digital Array: As shown in Figure 1(a), N DA antennas in DA are connected to M DA RF-chains, i.e.,
N DA = M DA . The beamformer precoding occurs in
the baseband (BB) digital signal processor (DSP).
■ Sub-Array: SA consists of multiple phased arrays.
As shown in Figure 1(b), N SA antennas are partitioned into M SA group, each of which has one
dedicated RF-chain, K SA phase shifters (PS), variable gain amplifiers/attenuators (VGA), and power
amplifiers (PAs). The array size, group number,
and number of elements in a group follows relationship N SA = M SA K SA . Using phase shifters, each
group can transmit a beam towards specific direction and SA is capable of transmitting/multiplexing up to M SA simultaneous beams. When the required number of beams U SA is smaller than M SA,
multiple array groups can form a virtual group.
The increased array size for that specific beam
provides better beamforming performance, e.g.,
higher gain and narrower beam-width. DSP facilitates precoding multiple beams in the baseband.
■ Fully-Connected Hybrid Array: This architecture is
also known as overlapped sub-array [16], multibeam active phased array [39], and high definition
active antenna system [40]. Similar to SA, the FH
architecture uses phase shifters for analog beamforming and DSP for digital beamforming. However, FH has different connecting structures between RF-chains and phase shifters. As shown in
Figure 1(c), each of M FH RF chains connects with
all N FH antennas via N FH phase shifters. Combiner
networks are used to add M FH RF signals before
passing through the PAs. As a consequence, a total of M FH N FH phase shifters are required in this
architecture. FH is capable of transmitting up to
M FH simultaneous streams.
Recent integrated circuits (IC) implementations of
all three architectures are summarized in Table I. Apart
from array in 28 GHz band, Table. I includes implementation in 60 GHz band for mmW indoor access, mmW back36

ieee circuits AND sYstems mAGAziNe

haul and radar, because they share the same array architectures. Directly comparing array architectures from the
table is difficult, because they use different silicon technology, and not all circuits components, e.g., local oscillator (LO) and associated up/down-conversion circuits, low
noise amplifier (LNA), and PA, are integrated. It is worth
noting that SA and FH architectures in Table. I implement
phase shifters in the RF domain. A comprehensive survey
of phase shifter implementations is covered in [41], including phase shifters in analog baseband, LO, and RF domain.
Moreover, system level prototyping of 28 GHz arrays together with field test can be found in [19], [42].
There are other architectures that have been recently proposed, e.g., switch based antenna array [43] and
lens antenna array [44]. Due to the lack of implementation details available in the literature, we do not include
quantitative analysis of them in this work.
B. Comparison Metrics Under 5G Use Cases
5G is characterized by a wide variety of use cases having different environments, communication distances,
and performance requirements. Performance, in turn
depends on connectivity density (defined as number
of simultaneous connections for one wireless service
operator in an given area), peak rate, and network traffic throughout. It is our vision that the mmW BS should
be capable of using the same radio front-end arrays to
handle various use cases and meet their demands.
We choose three representative use cases [45]: Dense
Urban Mobile Broadband (MBB), 50+Mbps Everywhere,
and Self-Backhauling. They cover different MIMO processing schemes of transmitter array.
■ Dense Urban MBB: In dense urban area, large number of UEs require high-speed connections for
applications like streaming, high-definition videos, and downloading files. According to 5G
KPI requirement [45], the connection density is
expected to be 150,000 connections per square
kilometer, while the traffic throughput is up to
3.75 Tbps/km2 in such scenario. A typical 5G mmW
BS deployment setting has inter-site distance
(ISD) of 200 m and each BS has 3 radio sectors
[46]. With 850 MHz spectrum at 28 GHz band, the
required SE in this use case is up to 58.8 bps/Hz.
Such a scenario often involves line-of-sight (LOS)
environment and relatively good SNR is expected
for each UEs so that SE greatly benefit from high
multiplexing. We anticipate that at least 8 simultaneous streams are required1.
1
Till the time of writing, there is no specification for multiplexing in 5G
mmW system. However, 8 streams are commonly used as assumption
in the literature [47], [48]. Meanwhile, the next generation of 60 GHz
indoor wireless system also targets to use 8 spatial streams [49].

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