IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
3403
Digital Beamforming-Based Massive MIMO
Transceiver for 5G Millimeter-Wave
Communications
Binqi Yang , Zhiqiang Yu , Member, IEEE, Ji Lan, Ruoqiao Zhang ,
Jianyi Zhou, Member, IEEE, and Wei Hong , Fellow, IEEE
Abstract— A 64-channel massive multiple-input multipleoutput (MIMO) transceiver with a fully digital beamforming (DBF) architecture for fifth-generation millimeter-wave
communications is presented in this paper. The DBF-based
massive MIMO transceiver is operated at 28-GHz band with
a 500-MHz signal bandwidth and the time division duplex mode.
The antenna elements are arranged as a 2-D array, which has
16 columns (horizontal direction) and 4 rows (vertical direction)
for a better beamforming resolution in the horizontal plane.
To achieve half-wavelength element spacing in the horizontal
direction, a new sectorial transceiver array design with a bent
substrate-integrated waveguide is proposed. The measured results
show that an excellent RF performance is achieved. The system
performance is tested with the over-the-air technique to verify the
feasibility of the proposed DBF-based massive MIMO transceiver
for high data rate millimeter-wave communications. Using the
beam-tracking technique and two streams of QAM-64 signals,
the proposed millimeter-wave MIMO transceiver can achieve
a steady 5.3-Gb/s throughput for a single user in fast mobile
environments. In the multiple-user MIMO scenario, by delivering
20 noncoherent data streams to eight four-channel user terminals,
it achieves a downlink peak data rate of 50.73 Gb/s with the
spectral efficiency of 101.5 b/s/Hz.
Index Terms— Digital beamforming (DBF), fifth-generation
(5G) communication, massive multiple-input multipleoutput (MIMO), millimeter-wave, multibeam system, transceiver.
I. I NTRODUCTION
I
N THE past few years, there has been growing interest
in the utilization of millimeter-wave bands for fifthgeneration (5G) cellular communication systems [1]–[4].
Compared with current communication systems below 6 GHz,
the millimeter-waveband can offer a wide available spectral
resource which can be used to support a wide signal bandwidth
for the demands of high data throughput in 5G communication
systems. Despite the great potential of millimeter-wave cellular
communications, there are many key technical challenges need
Manuscript received December 6, 2017; revised March 8, 2018; accepted
April 7, 2018. Date of publication May 3, 2018; date of current version July 2,
2018. This work was supported by the National Natural Science Foundation
of China under Grant 61401093 and Grant 61627801. (Corresponding author:
Binqi Yang.)
The authors are with the State Key Laboratory of Millimeter Waves,
School of Information Science and Engineering, Southeast University,
Nanjing 211189, China (e-mail: bqyoung@live.cn; zqyu@seu.edu.cn;
230169036@seu.edu.cn; zrqjoel@163.com; jyzhou@seu.edu.cn; weihong@
seu.edu.cn).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMTT.2018.2829702
to be addressed and the feasibility of millimeter-wave cellular
requires careful verification [2]–[5].
The electromagnetic wave at millimeter-wave frequency
suffers from a high attenuation caused by free space pathloss as well as shadowing [4], [5]. Fortunately, the shorter
wavelength of a millimeter-wave signal enables a greater
antenna gain by using an antenna array with a large number of
antenna elements. It is known that the existing millimeter-wave
point-to-point communication system with a large antenna
array can achieve multigigabit data rates at a line-of-sight
distance of a few kilometers. However, the fixed narrow
beam provides limited geographical coverage so that it cannot
support mobile communication environment well. Due to this
fact, some advanced multibeam or beam-steerable antenna
array techniques are recently adopted to enable 5G millimeterwave cellular communication, such as the passive multibeam
antenna in [6]–[8], the lens-based beam-switching antenna
system in [9] and [10], and the active phased array in [3]
and [11]. Generally, the active beamforming system can provide a higher transmitted power and a better beamforming
flexibility compared with the passive multibeam antenna array.
Combined with MIMO techniques, the performance of the
active beamforming system can be further improved. With
advanced beamforming precoding, the MIMO communication system can generate multiple beams to deliver multiple data steams for supporting single-user (SU) MIMO and
multiuser (MU) MIMO communications [12]. The MIMO
beamforming techniques can offer a high antenna array gain,
anti-interference, a better signal coverage, and a high spectral
efficiency for 5G millimeter-wave cellular communication.
Several potential active beamforming architectures have
been proposed and analyzed in the literature, including analog
beamforming architecture, hybrid beamforming architecture,
and fully digital beamforming (DBF) architecture [11]–[15].
However, immense challenges remain in the practical hardware implementation of millimeter-wave MIMO beamforming
system. The main hardware constrains arise from cost, power
consumption, and size of the transceiver components as well
as wide signal bandwidth, circuit technologies, interconnection
techniques, signal processing techniques, and so on. With the
existing advanced millimeter-wave integrated circuits technology and packaging technology, some integrated analog phased
array solutions for 5G millimeter-wave communication have
been proposed at integrated circuit level [16]–[18]. However,
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
the linear output power of each integrated millimeter-wave
transceiver element is still very limited. Some system level
solutions with millimeter-wave hybrid beamforming architecture based on analog phased subarray have been reported in [3]
and [11]. Until now, the hardware design of millimeter-wave
massive MIMO transceiver with fully DBF architecture has
never been reported. In general, the fully DBF architecture
can offer the greatest flexibility and performance. However,
the fully DBF architecture is more complicated than others in terms of hardware implementation that each antenna
element requires an independent transceiver chain. Although
the millimeter-wave antenna element has small shape, each
millimeter-wave transceiver circuit may occupy a relatively
large size and will lead to the difficulty in integrating many
transceiver channels in limited space. The millimeter-wave
transceiver circuits can be more complicated than the transceiver circuits for sub-6-GHz systems, because it contains
not only the millimeter-wave front-end but also the wideband intermediate frequency (IF) transceiver. To implement
millimeter-wave cellular communication based on the fully
DBF architecture, there are many challenges need to be
addressed.
The substrate-integrated waveguide (SIW) technique
provides a low-loss, low-cost, compact, flexible, and
mass-producible solution for the implementation of highperformance millimeter-wave transceiver front-end [19], [20].
Based on standard printed circuit board processes, some key
passive components in millimeter-wave transceiver front-end,
such as filters, multiplexers, and transmission structures, can
be fabricated and integrated with the millimeter-wave active
circuits in the planar form on the same substrate [21], [22].
Significant efforts have been devoted to the research and
development of the SIW technique as promising solutions
for millimeter-wave circuits and systems with extraordinary
performance.
In this paper, a millimeter-wave MIMO transceiver with
fully DBF architecture is proposed for 5G millimeter-wave
communications at system level. The proposed transceiver,
which integrates 64 complete millimeter-wave transceiver
channels with a high linear output power, is used for verifying
the hardware design and ultimate system performance of
DBF-based millimeter-wave massive MIMO communication
system. The developed DBF-based millimeter-wave MIMO
transceiver is operated at 28-GHz band with a signal bandwidth of 500 MHz and the time division duplex (TDD)
mode. The SIW technique is applied to implement the filter
components and interconnection components in the millimeterwave transceiver front-end array (mmWFEA). A good RF
performance is achieved by the proposed transceiver circuits.
Collaborated with Shanghai Bell Labs, the system performance
of the DBF-based MIMO transceiver was tested and verified
at system level. The 64-channel DBF-based MIMO system
achieved peak data rates of 50.73 Gb/s with the spectral
efficiency of 101.5 b/s/Hz by transmitting 20 data streams to
eight four-channel user terminals (UEs).
The rest of this paper is organized as follows. Section II
describes the DBF-based millimeter-wave MIMO transceiver
architecture and key parameters; Section III will introduce
TABLE I
BASIC L INK B UDGET PARAMETERS
the element circuit and transceiver subchannel design; and
Section IV gives the achieved results of the hardware performance and the experimental results of the whole
DBF-based millimeter-wave MIMO system. Finally, conclusions are drawn in Section V.
II. DBF-BASED M ILLIMETER -WAVE M ASSIVE MIMO
T RANSCEIVER A RCHITECTURE AND K EY PARAMETERS
In order to verify the feasibility and system performance
of DBF-based millimeter-wave MIMO transceiver for high
data rate wireless communications, a 64-channel millimeterwave MIMO transceiver is developed. The basic system
requirements are given as follows: the operation band is
28-GHz frequency band with 500-MHz signal bandwidth;
the maximum linear transmit power should be higher than
30 dBm; the cell coverage range should be greater than 200 m,
and the SU data rate should achieve 2.5 Gb/s at the 200-m
cell edge; the peak data rate for multiple users should meet
20 Gb/s; and the highest modulation scheme is QAM-64.
To meet the required system performance and coverage, some
practical design specifications of millimeter-wave transceiver
need to be satisfied to ensure adequate signal-to-interferenceplus-noise ratio (SINR). The total SINR at the receive side is
mainly contributed by the receive signal-to-noise ratio (SNR)
of wireless link, local oscillator (LO) phase noise, modulation
quality, and the MU interference
−1
−1
−1 −1
(1)
SINRtot = SNR−1
Link + SNRLO + SNRMod + SNRMU
where SNRLink is the ratio of a receive signal to a thermal noise, SNRLO is the relative interference caused by
LO phase noise, SNRMod is the relative interference caused
by modulation imperfection, and SNRMU is the interference
caused by MU-MIMO interferences. The basic link budget
parameters are listed in Table I. Considering the 28-GHz band
with 500-MHz signal bandwidth, the receiver sensitivity is
around −82 dBm under a typical receiver noise figure (NF)
of 5 dB. From the Friis transmission formula, the estimated
path loss at the 200-m cell edge is around 107.4 dB. The
required SINR is at least 22 dB for QAM-64 signals. Thus,
the basic requirements of millimeter-wave transceiver are:
1) the array gain is at least 17.4 dB to meet the required
SNR at the cell edge and the antenna element has extra gain to
provide adequate SNR margin; 2) each transceiver element can
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
Fig. 1.
3405
Simplified block diagram of the 64-channel millimeter-wave MIMO fully DBF transceiver.
Fig. 2. Measured results of commercial 6-bit phase shifter at 28 GHz.
(a) Relative phase and phase error for 64 phase states. (b) Gain fluctuation
for 64 phase states.
provide at least 12-dBm linear power to meet the 30-dBm total
transmit power; 3) the modulation precision and the LO phase
noise should be good enough to meet the requirements of
QAM-64 signal or even higher order modulation schemes;
4) to achieve the peak data rate, the transceiver need to be able
to support eight QAM-64 streams or twelve QAM-16 streams;
and 5) high beamforming precision is required to eliminate the
MU interference.
The simplified block diagram of the DBF-based 64-channel
millimeter-wave MIMO transceiver is shown in Fig. 1. The
64-channel DBF-based MIMO transceiver is divided into two
parts, the mmWFEA and the IF-baseband subsystem. Besides,
the millimeter-wave transceiver also contains an RF LO module (housed in the mmWFEA), an IF LO module (housed in
IF-baseband subsystem), and the power supply module which
are not shown in Fig. 1.
A. Advantages of DBF Architecture and Major Constraints
Compared with other architectures, the fully DBF architecture has the highest precoding freedom, flexible multibeam
ability, fast beam steering speed, and high beamforming
precision. Three major advantages of DBF-based array are
as follows. First, as shown in Fig. 2, the millimeter-wave
analog phased array has a lower phase-shift resolution and
a higher gain fluctuation. By contrast, very high magnitude
and phase resolution can be achieved by digital precoding.
Second, the DBF-based system has a higher capacity. The
DBF-based array can be used to superpose multiple beams for
several data streams. Usually, each analog phased subarray can
only severe one data stream. The maximum number of spatial
multiplexing streams in the DBF-based system is greater than
phased array-based system. Third, for multicarrier signals,
such as orthogonal-frequency-division-multiplexing (OFDM)
signals, the fully DBF architecture can realize independent
beamforming precoding at each subcarrier or resource block to
obtain extraordinary performance at a wide signal bandwidth.
The wideband wireless channel is a frequency-selective channel and the signals in different parts of the band have different
propagation characteristics. The phase shifter can only apply
the same phase to all subcarriers, while the DBF architecture
can assign independent magnitudes and phases to different
subcarriers of the band.
The fully DBF architecture requires many complete transceiver chains. In addition to the technical obstacles, the hardware cost and power consumption are viewed as the major
constrains [5]. The hybrid beamforming architecture is considered as an alternative solution for a good tradeoff between system performance and hardware cost. The simplified diagram
of the typical hybrid beamforming-based millimeter-wave
MIMO transceiver is shown in Fig. 3. A comparison of cost
and power consumption between 64-channel DBF transceiver
and 64-element hybrid beamforming transceiver (grouped into
8 subarray) is given in Table II. The main source of power
consumption is the mmWFEA and the baseband processing
system, which are around 25.7% and 66.1%, respectively.
The total power consumption of the mmWFEA is around
278.8 W at 50% transmit duty cycle. Each millimeter-wave
transmit channel has a power consumption of 5.23 W, of which
the power amplifier (PA) occupies 70%. The total poweradded efficiency of each millimeter-wave transmit channel is
around 7.6% at peak power. The total power consumption
of DBF-based architecture is more than twice that of the
hybrid beamforming architecture. It can be seen that the
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
TABLE II
C OMPARISON OF C OST AND P OWER C ONSUMPTION BETWEEN F ULLY
DBF A RCHITECTURE AND H YBRID A RCHITECTURE
Fig. 3. Simplified block diagram of the typical 64-element millimeter-wave
MIMO transceiver with hybrid beamforming architecture.
cost and power consumption of the millimeter-wave transceiver circuit for the fully DBF architecture and the hybrid
beamforming architecture are almost the same. The major
gap of cost and power consumption is contributed by the
baseband processing system which includes analog-to-digital
converter (ADC), digital-to-analog converter (DAC), and fieldprogrammable gate array (FPGA). However, this bottleneck
is being overcome gradually. Some development trends show
that the cost and power consumption of baseband processing
system can be reduced in the future. First, the complementary
metal–oxide–semiconductor integration circuits are denser and
cheaper and have higher performance and lower power in
the past decade [23]. Some revolutionary technology changes
and advancements, such as FinFET transistors and the 10-nm
process technologies, have continued to promise Moore’s law
for improved density, higher performance, lower power, and
lower cost. Second, the application-specific integrated circuits (ASICs) can provide lower cost and power consumption
than FPGAs [24]. Thus, the cost and power consumption can
be further reduced by future 5G baseband processing ICs, such
as the Qualcomm Snapdragon X50 5G modem chip. Third,
some advanced System-on-Chip (SoC) integration techniques
enable a lower cost and power consumption for baseband
processing [25]. The Xilinx’s UltraScale+ RFSoCs have
integrated multiple high-speed RF data converters and softdecision forward error correction into an all programmable
SoC architecture [26]. By eliminating the high-speed data
transceiver circuits between converters to FPGAs, the cost and
power consumption of baseband process system are expected
to be reduced greatly. From above, the constraints of the DBF
transceiver are likely to change in the future.
B. Proposed DBF-Based Millimeter-Wave MIMO Transceiver
As shown in Fig. 1, the 64-element antenna array of the
DBF transceiver is arranged as a rectangular planar array
with 16 horizontal elements by four vertical elements. Each
antenna element is connected to a separated RF chain. The
DBF precoding is performed at the baseband stage, which
enables controlling both phase and magnitude accurately.
Given that most of UEs and scatters are spread horizontally,
a higher system performance may be obtained by using more
antenna elements in the horizontal direction to achieve a
better beamforming resolution in the horizontal plane. The
DBF architecture has flexible ability for superposing multiple
spatial multiplexing data streams simultaneously. For each data
stream, a narrow beam can be formed by digital precoding
that multiplies a complex coefficient matrix with the baseband
symbols. Multiple narrow beams with different directions
can be formed for multiple noncoherent data streams by
using different precoding weighting matrices. Assume that the
N-element (N = 64) MIMO system is severing K users by S
independent data streams. The kth user uses Sk RF chains to
receive its Sk data streams. The total precoding matrix for the
S data streams is F = [F1 , . . . , F K ], where Fk ∈ C N×Sk and
each column vector of Fk represent a precoding coefficient
matrix for one data stream. The transmitted symbol of all
data streams can be written in a vector d = [d1T , . . . , dTK ]T ,
where dk ∈ C Sk×1 and (·)T denotes transpose. The kth user
decodes its Sk data streams by using a receive decoder matrix
Wk ∈ C Sk×Sk as follows:
K
H
H
H
H
d̂k = Wk (Hk Fd + nk ) = Wk Hk
Fi di + nk
(2)
i=1
C N×Sk
is the MIMO channel between the base
where Hk ∈
station (BS) and the kth user, nk is the additive Gaussian noise
at the kth user, and (·) H denotes the conjugate transpose. The
number of spatial multiplexing streams S is determined by the
rank of MIMO channel matrix. The decoding of entire system
can be given as
K
H
H
H
H
d̂ = W (H Fd + n) = W
Fi di + n
(3)
H
i=1
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
where the block diagonal matrix W = diag[W1 , . . . , W K ] ∈
C S×S is the overall decoder matrix, H = [H1 , . . . , Hk ] ∈
C N×S is the MIMO channel matrix, and n = [n1T , . . . , nkT ]T
is the additive Gaussian noise at all users.
For DBF-based MIMO transceiver, the MU interference
mitigation can be accomplished by using a zero-forcing (ZF)
linear beamforming approach at the BS side. This approach is
based on the generalized inverse matrix of the MIMO channel
matrix H H
√
F = αH(H H H)−1
(4)
where α is power constraint factor. Since the maximum power
of each transceiver element is limited, the row vectors of
precoding matrix F are constrained to satisfy max {F(i) F(i)H ,
for all i ∈ [1, . . . , N]} ≤ PT ,max , where (·)(i) denotes the
i th row vector and PT ,max is the maximum power of each
transceiver element. There is no receive decoding matrix
required at UEs, and the MIMO channel can be decoupled into
a set of independent parallel channels with Gaussian noise
√
d̂ = αd + n.
(5)
It clearly shows the existence of the solutions for the
DBF-based MIMO system. On the other hand, in the case
of each UE equipped with multiple transceiver channels,
the ZF linear beamforming approach at the BS side is suboptimal. The block diagonalization ZF approach can be used
to explore extra beamforming gain at multiple-antenna UEs.
The basic idea is completely removing the MU interference
at the BS side while leaving each user to mitigate the interference among its own data streams with receiver decoding. The MU interference elimination at all users indicates
that
HiH F j = θ, for all i = j.
(6)
As described in [27], the solutions of the precoding
matrix F can be obtained by singular value decomposition
approach. The receiver decoding matrix Wk is performed at
each UE to decouple its own data streams and form receiver
beamforming gain.
In the ideal case, the relationship between maximum effective isotropic radiated power for each stream and the stream
number is as follows:
EIRP[dBm] = Ptot + G A + G Ant − 10 log S
(7)
where Ptot is the total output power of the transceiver, G A is
the array gain, G Ant is the gain of antenna element, and S is
the number of data streams. For each data stream, the column
vector of precoding matrix Fk can be rewritten into a 2-D
complex coefficient weighting matrix A ∈ C4×16 . The array
factor of an Nx × M y 2-D uniform planar array can be written
as [28]
AF(φ, θ ) =
N
y −1
x −1 M
anm e− j 2π sin φ(n cos θd x+m sin θd y)/ λ0
(8)
n=0 m=0
where φ is the azimuth angle, θ is the elevation angle, anm
is complex coefficient at each element, λ0 is the wavelength
in vacuum, and (dx , d y ) represent the horizontal and vertical
3407
Fig. 4. Pattern in spectral domain of the spatial discrete excitation source
sequence.
element spacing. For Nx -element uniform linear array at
horizontal direction, the horizontal far-field radiation pattern
is
P(φ) = E(φ)
N
x −1
an e− j 2πnd sin φ / λ0
(9)
n=0
where E(φ) is the horizontal pattern of element, d is the
element spacing, and an is the complex excitation at the
nth element. Let ξ denote sin φ ∈ [−1, 1], and the array
factor in the horizontal plane can be viewed as a discrete
Fourier transform (DFT) from spatial discrete excitation source
sequence {an } to spectral domain
AF(ξ ) =
N
x −1
an e− j 2πnξ d / λ0 .
(10)
n=0
As shown in Fig. 4, the image spectrum spacing of this DFT
is λ0 /d. If the antenna element spacing d is too large,
additional main beams (grating lobes) will exist due to
the image spectrum in beam space. In the millimeter-wave
front-end array, the horizontal element spacing is around
half-wavelength to avoid forming high grating lobes at the
horizontal plane. The vertical element spacing is around four
wavelength due to the occupied size of the millimeter-wave
transceiver front-end circuit. Thus, vertical beamforming precoding may introduce beam splitting unless the data streams
are noncoherent in the vertical direction. However, the effects
of beam splitting in the wireless communication scenario are
still not clear. It seems not to affect the existence of the
solutions of the DBF-based MIMO system for MU-MIMO
communications.
The hardware architecture of the proposed DBF-based
millimeter-wave MIMO transceiver is given as follows.
As shown in Fig. 1, the SIW is used to connect the antenna
array and the millimeter-wave transceiver front-end. Compared
with the expensive millimeter-wave cables and connecters,
using the SIW has the advantages of low cost, compact size,
and easy integration. The 64-channel millimeter-wave transceiver front-end is grouped into 16 linear subarray daughter
boards which are vertically mounted on one metal base board.
At the top side, each daughter board has four independent
transceiver chains. Each transceiver chain contains PA, subharmonic mixer, low noise amplifier (LNA), RF switch, image
rejection filters, and other related RF components. A 1-to-4
Wilkinson power splitter for RF LO is also on this side. At the
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
Fig. 5. Top view of the sectorial structure of transceiver front-end array with
bent SIW feeding network.
Fig. 6. (a) Architecture and (b) photograph of one baseband daughter board.
bottom side, each daughter board has a control unit and fourchannel power management circuit which can provide proper
power-up sequence and voltage for the transceivers at the top
side. Because the wavelength in millimeter-wave frequency
is very short, there exists a great difficulty in making the
total thickness of the four-channel daughter board to meet the
half-wavelength spacing. One of the practical constraints is
that the size of the RF connectors for the IF signals and the
LO signal is larger than half-wavelength of the millimeterwave carrier frequency. Another key constraint is the heat
dissipation. Due to the low efficiency of the millimeterwave PA, each four-channel front-end daughter board requires
high power consumption for generating required linear power.
Therefore, the front-end daughter board requires a sufficient
area for heat dissipation. To solve above-mentioned problems,
a sectorial transceiver array design is proposed as shown in
Fig. 5. The 16 front-end daughter boards are arranged on a 90°
sector structure. The SIW is bent to ensure that the antenna
elements are uniform and linear. As a compact alternative
solution of the millimeter-wave coaxial cables, the SIW based
on soft substrate can be slightly bent while keeping a good
transmission performance.
The millimeter-wave transceiver front-end is connected with
the IF-baseband subsystem by coaxial cables. The IF signal
has a carrier frequency of 2.75 GHz, while the control signals
are usually lower than dozens of MHz. For brevity and
reliability, the coaxial cables are multiplexed by the IF signals
and the control signals. Each IF-baseband subsystem board
contains a four-channel IF transceiver daughter board and
two baseband daughter boards. The IF transmitter uses direct
conversion architecture. The transmitted baseband signal is
directly upconverted to the 2.75-GHz IF carrier frequency
by a wideband quadrature modulator. The bandpass direct
sampling architecture and digital downconverter (DDC) technique are used in the IF receiver to eliminate the demand
of wideband demodulator and obtain a high demodulation
precision. As shown in Fig. 6, each baseband daughter board
consists of one Xilinx’s Kintex-7 FPGA, two dual-channel
12-bit ADCs, and one quad-channel 16-bit DACs. Each baseband board also has two QSFP + optical interfaces for
common public radio interface. Besides, the multiboard synchronization is very important in the MIMO communication
system. The pulse per second (1PPS) signal from the GPS
module is used as synchronous triggering for the multiboard
synchronization processing. All the clocks on board are locked
and synchronized by external 10-MHz reference clock. The
transmit baseband sampling clock rate is 552.96 MHz and the
DAC sampling clock rate is 2211.84 MHz with four times
interpolation. The ADC sampling clock rate is 2211.84 MHz
and the effective receive baseband sampling rate after DDC is
552.96 MHz which is 20 times of the 30.72-MHz clock rate
of the LTE system.
The key parameters and specifications for the developed
DBF-based millimeter-wave MIMO transceiver are listed
in Table III. The OFDM is employed in the baseband system.
The OFDM symbol consists of 2048 subcarriers with the subcarrier spacing of 270 kHz. The subcarrier spacing should be
much smaller than the coherent bandwidth of the millimeterwave wireless channel but large enough to cover system
frequency offset and Doppler shift. Only 1824 subcarriers are
used due to the guard band requirement.
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
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TABLE III
K EY S YSTEM PARAMETERS OF THE 64-C HANNEL DBF-BASED
M ILLIMETER -WAVE MIMO T RANSCEIVER S YSTEM
Fig. 7. (a) Block diagram and (b) layout of one transceiver front-end element.
III. D ESIGN OF T RANSCEIVER E LEMENTAL C OMPONENTS
The RF analog transceiver is certainly a critical part of
the DBF-based millimeter-wave MIMO transceiver system.
The system performance is highly determined by the RF
performance of the RF analog transceiver. Compared with lowband system, the millimeter-wave transceiver is more sensitive
to the RF impairments, such as phase noise, nonlinearity of PA,
I/Q imbalance, channel flatness, NF, and so on. This section
describes the circuit design of the elemental components in
the RF transceiver system.
A. Millimeter-Wave Transceiver Front-End Design
The block diagram of the millimeter-wave transceiver frontend element is shown in Fig. 7(a). The transceiver front-end
element contains five microwave amplifiers (LO amplifier, IF
amplifier, PA, PA driver, and LNA), three SIW bandpass filters,
one PIN switch, and two passive subharmonic mixers. The
mixers are used to upconvert the IF signal to RF or downconvert the RF signal to IF. Since the passive subharmonic
mixers require high LO power, one LO amplifier and one
power splitter are used to provide coherent LO signals. With
the subharmonic mixer technique, the design difficulty and
complexity of the RF LO module are greatly reduced by
using only half RF LO frequency which is 12.6 GHz in
this system. Generally, the output power compression point
of the millimeter-wave mixer is low, and a drive amplifier
with a high gain is required to drive the PA to output high
power. On the contrary, the receiver chain uses only one LNA
before mixer to avoid compressing the receiving dynamic
range. Three SIW bandpass filters are used to achieve high
image sideband rejection and LO-leakage rejection. The total
Fig. 8. Response and normalized coupling matrix of the four-order SIW
Chebyshev filter.
transmit gain (Tx gain) is around 29 dB and the transmit
output 1-dB power compression point is around 25 dBm. Each
transmitter element can offer 15 dBm of linear average power
with 10-dB power back-off. In addition to the p-i-n switch,
the power supply for the amplifiers at transmit and receive
paths is also switched rapidly in the TDD mode. One major
reason is that the isolation of the millimeter-wave switch is
poor, and the transmit link must be shut down at the receiving
time slot to avoid forming high transmit noise floor at the
receive port. Another well-known benefit is that the total power
consumption can be greatly reduced.
The layout of one millimeter-wave transceiver front-end
element is also shown in Fig. 7(b). The SIW components
are integrated with the active transceiver circuits on the same
substrate. The three bandpass SIW filters are four-resonator
Chebyshev response filters with a bandwidth of 2.4 GHz.
The simulated/measured results and the N + 2 normalized
coupling matrix of the SIW filters are given in Fig. 8. The
measured results show that the SIW filter has 27-dB rejection at 25.2 GHz and 55-dB rejection at 22.45 GHz. Thus,
the transmitted link has 54 dB of LO-leakage rejection and
110 dB of image sideband rejection. The SIW with broadband
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3410
Fig. 9.
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
Geometry of broadband CPWG-to-SIW transition.
Fig. 11.
transition.
Simulated and measured results of the SIW with broadband
Fig. 10.
Simulated results of the SIW interconnect with different bent
angles (Rs = 30 mm).
transition [29], [30] after the TDD switch is served as a
low-loss interconnection component between millimeter-wave
transceiver front-end and the antenna element. The equivalent
dielectric-filled rectangular waveguide width of the SIW can
be approximated by [31]
(11)
Weff = Wg − dv2 (0.95 p)
where Wg is the center spacing between the two rows of
metallic via holes, dv is the diameter of the via holes, and p is
the center spacing of the adjacent via holes. In order to obtain
an optimal operation bandwidth and tolerance, the operation
frequency should be set at the center of the cutoff frequency
of dominant mode TE10 and the cutoff frequency of the first
high mode TE20 . Thus, the optimal width Wg can be given by
√ Wgopt = 3c 4 f εr + dv2 (0.95 p)
(12)
where εr is the relative dielectric constant, c is the light
speed in vacuum, and f is the operation frequency. Rogers
RT/Duroid 5880 substrate with a relative dielectric constant
of 2.2 and a thickness of 0.254 mm is used for our design.
The geometry of the broadband CPWG-to-SIW transition is
shown in Fig. 9, and the related dimensions for 28-GHz
band are: Wg = 5.7 mm, dv = 0.4 mm, p = 0.6 mm,
Rt = 2.5 mm, St = 0.4 mm, θ t = 26°, and W50 = 0.72 mm.
As demonstrated in Section II, the SIW based on soft substrate
is bent for interconnection between transceiver front-end and
antenna array. Fig. 10 shows the simulation results of the
SIW interconnections with different bent angles θ s, where the
bent radius Rs for our design is 30 mm. The SIW based on
soft substrate can be slightly bent while keeping almost the
same good transmission performance. The simulated and measured results of the SIW interconnection with the broadband
Fig. 12. Photograph of four-channel millimeter-wave transceiver front-end
daughter board (top view).
Fig. 13.
Shielding design of millimeter-wave planar circuits with SIW
components.
CPWG-to-SIW transitions are shown in Fig. 11. The measured
insert loss is around 1.3 dB and the reflection coefficient is less
than −16 dB when the frequency ranges from 26 to 32 GHz.
Fig. 12 shows the photograph of the fabricated four-channel
millimeter-wave transceiver front-end daughter board. A good
shielding structure is implemented to minimize interferences
and improve channel isolation. In this design, the total gain
of the driver amplifier and the PA is around 45 dB. So the
isolation should be more than 60 dB (with 15-dB margin) for
a good RF performance and circuit stability. The shielding
design of the millimeter-wave transceiver circuit with SIW
components is shown in Fig. 13. Benefiting from the complete
shielding property of the SIW structure, the metal shielding
structure can be directly pressed on the surface of the SIW
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
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Fig. 15. Simulated and measured reflection coefficient and peak gain of the
antenna element.
Fig. 14. (a) Geometry of printed Yagi–Uda antenna element. (b) 3-D far-field
radiation pattern of the antenna element.
components to achieve a good isolation. The reached isolation
is more than 75 dB. The four transceiver elements share
the same LO by using an in-phase 1-to-4 Wilkinson power
splitter. At the bottom side is a control unit and four-channel
power management circuit which can provide a proper powerup sequence and voltage for the transceiver. The power-up
sequence is indispensable for transceiver circuit protection,
because most of the millimeter-wave devices typically use the
depletion-mode pseudomorphic high-electron-mobility transistor technology. To avoid breakdown of the millimeter-wave
devices, the negative gate voltage should be biased before
powering up the positive drain voltage.
B. Antenna Element Design
The radiation antenna element adopted in this transceiver
is a printed planar Yagi–Uda antenna with an integrated
microstrip balun. This endfire antenna element is promising
for millimeter-wave communication due to its high gain, easy
fabrication, and low cost [32], [33]. The compact size also
makes it very suitable for millimeter-wave array applications.
Fig. 14(a) shows the geometry of the designed antenna element
for 28-GHz band, and the related design dimensions used for
this antenna are: W1 = 0.8 mm, W2 = 0.7 mm, d1 = 0.85 mm,
dr = 3.2 mm, dt = 1.8 mm, L r = 5.85 mm, L 1 = 2.38 mm,
L d = 3 mm, Wt = 6 mm, and L t = 6 mm. The total size
of the antenna is approximately 1.2λ0 × 1.8λ0 . Fig. 14(b)
shows the simulated 3-D far-field radiation pattern. It can be
seen that the antenna element has low a backlobe and a wide
beamwidth in the H -plane.
The measured reflection coefficient and antenna gain are
shown in Fig. 15. The measured reflection coefficient is
less than −14 dB at 28-GHz frequency band with 3-GHz
bandwidth. It can meet the practical engineering requirement
of 5G millimeter-wave communication systems. The measured
impedance bandwidth is more than 5 GHz. The measured
antenna element gain is around 7 dBi, which is in good
agreement with the simulated results. The radiation pattern
for H -plane and E-plane at 28 GHz is shown in Fig. 16.
The measured half-power beamwidth of H -plane and E-plane
Fig. 16.
Simulated and measured radiation patterns of the antenna element.
is 134° and 57°, respectively. Therefore, the designed DBFbased array has a wide horizontal continuous beam scanning
angle of ± 67°. Fig. 17 illustrates the horizontal radiation
beam patterns generated by the horizontal 16 antenna elements
with the DBF precoding of the 16 × 16 DFT matrix. It can be
observed that the array gain fluctuation is less than 3 dB when
the main beam is steered from −60° to 60°. With the digital
precoding, null areas of radiation pattern can be formed. From
these results, more than 30-dB interference rejection can be
obtained at the null areas. Thus, this antenna array can be
used for supporting multiple QAM-64 streams. In practical
applications, the beamforming precoding vector is determined
by the MIMO channel. The beamforming precoding is selected
from the null space of the channel vectors of other user.
C. IF Transceiver Design
The IF-baseband subsystem consists of 16 IF transceiver
daughter boards. Each IF transceiver daughter board has four
IF transceiver elements. The simplified block diagram of one
IF transceiver element is shown in Fig. 18. At the IF transmitter side, the direct conversion architecture is employed.
The analog baseband signals are converted to IF frequency
by a wideband quadrature modulator. In order to obtain a
good modulation precision, the reconstruction filter after DAC
is removed. As a result, the image signals of DAC will be
transferred to the IF carrier frequency and then cause high
out-band spurious signals. This problem is overcome by the
bandpass filter after modulation. A high-selectivity bandpass
filter is used after the quadrature modulation to reject these
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
Fig. 20.
Fig. 17. Beam patterns of 16 antenna elements in horizontal plane with the
DBF precoding of 16 × 16 DFT matrix.
Fig. 18.
Simplified block diagram of one IF transceiver element.
Block diagram of the LO subsystem.
of the four-channel IF transceiver. Given that the high gain of
the IF receive chain, a good shielding design is required to
avoid self-oscillation of the IF chain and reject the transmit
IF LO leakage. The four IF transceivers share the same IF
LO source by using an in-phase 1-to-4 IF Wilkinson power
splitter at the back side of the IF daughter board.
D. Design of Coherent LO Subsystem
The LO performance is critical in the millimeter-wave
mobile communication system. One major contribution of the
degradation of the modulation precision of the millimeter-wave
transceiver system is the phase noise of LO. The influence of
the phase noise on the error vector magnitude (EVM) can be
expressed as [34]
EVMPN =
Fig. 19.
Photograph of one four-channel IF transceiver daughter board
(top view).
transmitted spurious as well as the harmonic signals introduced
by broadband modulator. The transmitter carrier frequency
leakage is eliminated by applying dc offset on the baseband
differential amplifiers. At the IF receiver side, the received
signals are amplified and filtered. In order to obtain a good
demodulation precision for high-order modulation signals,
the demodulation is replaced by the DDC technique after
ADC sampling. The autogain control (AGC) loop of the IF
receiver is closed by the digital baseband to provide proper
signal scale at the ADC input. Each digital attenuator in
the IF transceiver has 31.5-dB attenuation range in 0.5-dB
step precision. Therefore, the IF transceiver has a 31.5-dB
autopower control (APC) range at the transmitter side and a
63-dB AGC range at the receiver side. The maximum gain of
the IF receiver chain is 55 dB. Fig. 19 shows the photograph
∞
−∞
1/2
SPN ( f )d f
= (PNInt )1/2
(13)
where SPN ( f ) is the phase noise spectrum density and PNInt
is the integral phase noise.
Fig. 20 shows the block diagram of the LO subsystem. For
DBF-based millimeter-wave MIMO transceiver, the multiple
transceiver chains need to share the same LO source to ensure
that the initial physical phase differences between multiple
transceivers are certain. Thus, the LO system generates 16 RF
LO signals and 16 IF LO signals by using a 1-to-16 in-phase
Wilkinson power splitter. The phase noise in the loop bandwidth dominated by the phase-locked loop (PLL) can be
approximated by
PNPLL [dBc Hz] = PNnorm + 10 log NPLL + 10 log f out (14)
where PNnorm is the normalized in-band phase noise floor of
the PLL, NPLL is the frequency division ratio, and fout is
the output LO frequency. The normalized phase noise floor
PNnorm is a design constant, which is used for describing the
phase noise specification of a specific PLL device. The major
noise is contributed by the phase detector and the charge pump
of PLL. In this design, the normalized phase noise floor is
−226 dBc/Hz. In order to obtain good phase noise, the frequency division ratio NPLL should be reduced by using a high
phase detection frequency. As shown in Fig. 20, in the LO subsystem, a 100-MHz oven-controlled crystal oscillator (OCXO)
locked by a 10-MHz reference clock from GPS is used as the
reference clock for the RF LO and the IF LO. Compared with
using 10-MHz reference clock directly, the phase noise of the
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
Fig. 21.
3413
Measured phase noise of the RF LO.
Fig. 22. Photograph of the 64-channel DBF-based millimeter-wave massive
MIMO transceiver system.
RF PLL can be reduced 10 dB by using 100-MHz OCXO
as a phase detection clock. Besides, the loop bandwidth of
the PLL should be chosen carefully. The optimal phase noise
performance can be obtained by choosing the loop bandwidth
to be the offset frequency where the VCO free-running phase
noise is equal to the phase noise of the PLL.
The measured phase noise performance of the RF LO
is shown in Fig. 21. In the mobile communication system,
the close-in phase noise below a few hundred Hz has a little
effect on system performance, because the slowly varying
phase can be eliminated by the pilot signals. The measured
integral phase noise is −46.9 dBc. Considering that the LO
frequency is doubled by the subharmonic mixer, the phase
noise would increase 6 dB. Thus, the actual integral phase
noise of the RF LO is −40.9 dBc which is still very good
for high-order modulation signals, such as QAM-64 and
QAM-256.
IV. E XPERIMENTAL R ESULTS
Fig. 22 shows the photograph of the proposed 64-channel
DBF-based millimeter-wave MIMO transceiver. The measurements of the proposed DBF-based millimeter-wave MIMO
transceiver are composed of two parts: the first part is the key
Fig. 23. Measured RF performance. (a) Tx gain response, Rx gain response,
and NF. (b) Rx gain and NF versus AGC state. (c) Transmit spurious rejection.
(d) Transmit power linearity. (e) Modulation accuracy.
RF performance of the transceiver and the second part is the
over-the-air (OTA) performance tests of the whole DBF-based
millimeter-wave MIMO transceiver.
A. Transceiver RF Performance Tests
The system transmission performance is highly dependent
on the RF transceiver performance. The key transceiver performance parameters include: gain response flatness, power
linearity, spurious signal rejection, modulation accuracy, NF,
phase noise, and so on. The measured results of the LO phase
noise has been given in Fig. 21. And the other transceiver
performance measured results are given as follows.
The gain response of the transmitter chain and receiver chain
is shown in Fig. 23(a). The measured Tx gain is around 29 dB
(at APC atten. = 0 dB) and the Tx gain flatness is 1.05 dB
over 500-MHz channel bandwidth. The measured receive gain
(Rx gain) is around 27 dB (at AGC atten. = 55 dB) and the
Rx gain flatness is 1.1 dB over 500-MHz channel bandwidth.
As shown in Fig. 23(a), the measured NF of the receive chain
is around 5.3 dB at maximum gain and is 9.5 dB at minimum
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3414
Fig. 24.
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
Spectrum of received 500-MHz OFDM signal.
gain. Fig. 23(b) shows the variations of Rx gain and NF at
different AGC states. It can be observed that the NF increases
rapidly when the Rx gain is decreased from 40 to 20 dB.
The measured results of transmit spurious signal rejection is
shown in Fig. 23(c). The image sideband rejection, two times
LO leakage rejection, and other unwanted transmit spurious
rejections are more than 60 dBc. The transmit power linearity
is characterized by the adjacent channel power ratio (ACPR).
As shown in Fig. 23(d), the measured ACPR is around
−46 dBc when a QAM-64 signal is applied and the practical
transmit power is around 14.5 dBm. Thus, each transceiver
channel can provide 15-dBm linear transmit power for high
peak-to-average ratio signals, such as OFDM signals. The
measured results of modulation accuracy of wideband signal
at 14.5-dBm output power are shown in Fig. 23(e). The
EVM and the signal constellation are used to characterize the
signal quality. The measured EVM of single-carrier QAM-64
signal is 1.1%. The magnitude error and phase error
are 0.736% and 1.116°, respectively. The measured gain
imbalance and quadrature error are −0.002 dB and
−45.676 m-degree, respectively. It can be seen that the measured QAM-64 constellation has a high quality. The measured
results show that an excellent modulation accuracy is achieved
by the millimeter-wave transceiver.
Fig. 25. (a) Beam-tracking test scenario. (b) Peak data throughput rate test
with the 64-channel BS and eight four-channel UEs.
B. Beam-Tracking and Peak Throughput Performance Tests
The State Key Laboratory of Millimeter-Waves of Southeast
University and Shanghai Bell Labs have collaborated to
demonstrate and verify the system performance of the
64-channel DBF-based millimeter-wave MIMO system prototype. The complete system performance tests of the DBFbased 5G millimeter-wave MIMO transceiver were carried out
at the Research and Innovation Center of Shanghai Bell Labs,
China. The main purposes of the transceiver system performance tests are to verify the feasibility of the millimeter-wave
cellular communication for effective geographical coverage
and the ultimate data throughput performance of the proposed 64-channel DBF-based millimeter-wave massive MIMO
transceiver.
The OTA performance tests use a 500-MHz signal bandwidth at a 28-GHz band. The modulation scheme adopted
is OFDM QAM-64 modulation. Fig. 24 shows the spectrum
of the received baseband signal which contains 1824 useful
Fig. 26.
Field trial environment.
subcarriers between +/− 246.24 MHz at 552.96-MHz sampling rate. In the ideal case, each data stream can achieve a
maximal data rate of 2.95488 Gb/s. The test scenarios of OTA
performance are shown in Fig. 25, which consists of the SU
beam-tracking test and MU-MIMO peak data rate test. The
field trial environment is shown in Fig. 26.
As mentioned in Section I, a fixed narrow beam with a
large antenna array can overcome the high propagation loss
at millimeter-wave bands, but the geographical coverage is
so limited that cannot support the mobility of the cellular
communication scenario. In order to obtain a good geographical coverage, a real-time beam should be rapidly steered
to provide a good signal quality for the mobile users. The
beam-tracking test scenario is shown in Fig. 25(a). Using
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
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TABLE IV
C OMPARISON OF M ILLIMETER -WAVE T RANSCEIVER E LEMENT
scheme and 500-MHz signal bandwidth, up to 20 noncoherent
data streams are generated and transmitted to eight UEs at the
same time. The measured constellation of receive signals and
peak data rate of MU-MIMO test are shown in Fig. 28. The
downlink peak data rate reaches 50.73 Gb/s and the spectral
efficiency reaches to 101.5 b/s/Hz.
Fig. 27. OTA system performance tests. (a) Receive signal constellations of
beam-tracking test. (b) Data rates of beam-tracking test.
the 64-channel DBF-based millimeter-wave MIMO BS system
and one UE, the beam-tracking tests for SU are carried out.
The UE has four-element antenna array and each antenna
element is associated with an independent transceiver chain.
Thus, each UE can support up to four streams for SU-MIMO.
The BS generates two data streams simultaneously to the
fast moving UE. Beam-tracking is performed to form optimal
beam such that the wireless channel response between BS
and UE is good. The received signal constellations of the
two data streams are shown in Fig. 27(a). To compare the
performance, the performance test with fixed narrow beam is
also performed. As shown in Fig. 27 (a), the received signal
quality is pretty good compared with the fixed-beam situation.
Fig. 27(b) shows the real-time throughout of the beam-tracking
tests. The throughput of the beam-tracking test keeps at around
5.3 Gb/s. It can be seen that the fixed narrow beam cannot
support the mobile environment well. In fixed narrow beam
situation, rapid fading of the signal is caused by the movement
of the UE. On the contrary, the beam-tracking technique can
provide stable and reliable channel at the mobile environment.
The maximal cell coverage radius for the SU-MIMO is more
than 300 m. In fact, a much longer distance is possible when
using single stream and beamforming at the UE with multiple
antennas.
The data throughput of massive MIMO systems can be
significantly increased by transmitting multiple independent
data streams to multiple UEs. As shown in Fig. 25(b), the testbed was built with the 64-channel MIMO transceiver and
eight stationary four-channel UEs. Using QAM-64 modulation
C. Discussion
An excellent performance is provided by the proposed
64-channel DBF-based millimeter-wave massive MIMO transceiver. Table IV compares the millimeter-wave transceiver
element with the existing Ka-band millimeter-wave transceivers. The proposed transceiver has a high linear output
power and front-end gain. Compared with other millimeterwave transceivers, its modulation quality and image rejection
level are very good. Besides, the proposed millimeter-wave
transceiver element has high gain control range. Table V
compares this paper with the state-of-the-art 5G MIMO systems. Compared with phased array-based hybrid beamforming systems and beam-switching antenna-based multiple-beam
systems, the DBF-based MIMO system has very high capacity
and spectral efficiency. It can be seen that the peak throughput
and the simultaneous access number of users of DBF-based
architecture are many times of other architecture. In this
regard, the DBF-based MIMO systems are very suitable for the
communication scenarios of high capacity and access number
of users. The MIMO capacity can be written as [38]
ρ H ρ H H =B
log2 1 + λ2i
S
S
S
C = B log2 det I S +
i=1
(15)
where B is the signal bandwidth, I S is identify matrix,
ρ is the average SNR of channel, and λ2i is the eigenvalue
of matrix H H H. Ideally, the upper bound of capacity at
20 streams is 88.36 Gb/s. The ideal total throughput of
20 QAM-64 streams is 59.1 Gb/s. The reached peak data rate
50.73 Gb/s is lower than the upper bound of capacity and
is very close to the theoretical value of 20 QAM-64 steams.
Compared with the DBF-based MIMO system in sub-6-GHz
band in [37], the DBF-based millimeter-wave MIMO
system has comparable spectral efficiency. In SU scenario,
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3416
Fig. 28.
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 66, NO. 7, JULY 2018
Measured constellations and peak data rate of MU-MIMO test.
TABLE V
C OMPARISON OF S TATE - OF - THE -A RT 5G MIMO C OMMUNICATION S YSTEMS
the achieved spectral efficiency is better than other beamforming architecture. Compared with phased array-based hybrid
beamforming system in [3] and [11], the proposed DBF system
has a higher spectral efficiency even if the channel coding is
introduced. The main reasons for the performance improvement are high modulation scheme and precoding capability at
finer level. For wideband signal bandwidth, the channel is a
frequency-selective channel that has different magnitudes and
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YANG et al.: DBF-BASED MASSIVE MIMO TRANSCEIVER FOR 5G MILLIMETER-WAVE COMMUNICATIONS
phases in different parts of the band. The DBF approaches
can assign independent magnitudes and phases to different
subcarriers of the band, while the phase shifter can only
apply the same phase to all subcarriers. Thus, an optimal
SNR at each subcarrier can be achieved and a higher spectral
efficiency can be obtained.
This paper has verified the feasibility to build a high performance DBF-based millimeter-wave massive MIMO system for
high data throughput rate millimeter-wave communications.
The DBF-based system exhibits high ability in spectral utilization and multiple user access. At the moment, there exists a
gap of cost and power consumption between DBF architecture
and hybrid beamforming architecture. This paper also points
out that reducing the cost of digital baseband processing part is
an essential direction for expanding the DBF-based millimeterwave MIMO system in the future. Given that the performance,
flexibility, and cost of DBF-based system, in the practical
network deployment, the BS network can be made of the DBFbased access points and the hybrid beamforming-based access
points in accordance with certain proportion. In addition,
the DBF-based millimeter-wave MIMO systems can be used
for the scenarios of high capacity and high simultaneous access
number.
V. C ONCLUSION
In this paper, a 64-channel DBF-based massive MIMO
transceiver for 5G millimeter-wave communications is proposed and implemented. The DBF-based millimeter-wave
massive MIMO transceiver is operated at 28-GHz band with
a 500-MHz signal bandwidth and the TDD mode. The
64-element antenna array configuration is 16 horizontal elements by four vertical elements for a better beamforming resolution in the horizontal plane. The measured results show that
great RF performances have been achieved by the proposed
millimeter-wave transceiver. The OTA system performance
tests are also carried out. In addition, the test results show
that, with beam-tracking and two data streams, the DBFbased millimeter-wave MIMO system can achieve steady
5.3-Gb/s throughput for an SU in fast mobile environment.
Using eight four-channel UEs and 20 data streams, the multiple user peak data rate has reached 50.73 Gb/s, and the spectral
efficiency reaches to 101.5 b/s/Hz. The test results verify the
feasibility of the proposed 64-channel DBF-based millimeterwave massive MIMO transceiver for significantly improving
the overall throughput for 5G cellular communication systems.
This paper also shows that the DBF-based millimeter-wave
MIMO system has high ability in spectral utilization and MU
access. The system hardware complexity, cost, and power consumption are still the main constraints for implementing DBFbased millimeter-wave MIMO transceiver. Reducing the cost
of digital baseband processing part is the essential direction
for expanding the DBF-based millimeter-wave MIMO systems
in the future. These constraints are gradually broken by
advanced integration techniques and new semiconductor manufacture techniques. In addition, the DBF-based millimeterwave MIMO transceiver architecture is a promising choice
for future 5G cellular communications.
3417
ACKNOWLEDGMENT
The authors would like to thank the researchers at the
Research and Innovation Center of Shanghai Bell Labs
who have made valuable contributions to baseband signal processing and system performance test for this DBFbased millimeter-wave massive MIMO system. The authors
would also like to thank Dr. W. Huang, Dr. F. Huang, and
Dr. W. Yang for their work.
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Binqi Yang received the B.S. and M.S. degrees in
information engineering from Southeast University,
Nanjing, China, in 2013 and 2015, respectively,
where he is currently pursuing the Ph.D. degree in
electromagnetic field and microwave technology.
In 2013, he joined the State Key Laboratory of
Millimeter Waves, Southeast University, where he
has been involved in microwave and millimeterwave circuits and transceiver systems. His current
research interests include planar filters, millimeterwave antennas, microwave and millimeter-wave circuits and transceiver systems, beam-forming networks, and phased arrays for
mobile communications.
Zhiqiang Yu (M’13) received the B.S. degree from
the Nanjing University of Science and Technology,
Nanjing, China, in 2002, and the Ph.D. degree from
Southeast University, Nanjing, in 2013.
From 2002 to 2007, he was a Research Staff in
airborne radar transmitter with the Nanjing Institute
of Electronics, China Electronics Technology Group
Corporation, Nanjing. He is currently a Lecturer
with the School of Information Science and Engineering, Southeast University. His current research
interests include microwave and millimeter-wave
circuit and system design.
Ji Lan received the B.S. and M.S. degrees in electrical engineering from Southeast University, Nanjing,
China, in 2013 and 2015, respectively, where he is
currently pursuing the Ph.D. degree in electromagnetic field and microwave technology.
His current research interests include microwave
circuits, transceiver systems, and antennas in mobile
communication.
Ruoqiao Zhang received the B.S. degree in electrical engineering from Southeast University, Nanjing,
China, in 2014, where he is currently pursuing the
Ph.D. degree in electromagnetic field and microwave
technology.
His current research interests include tunable
circuit components, active antenna array systems,
microwave and millimeter-wave transceiver systems, beamforming networks, and phased arrays for
mobile communication.
Jianyi Zhou (M’05) received the B.S.E.E.,
M.S.E.E., and Ph.D. degrees from Southeast University, Nanjing, China, in 1993, 1996, and 2001,
respectively.
In 1996, he joined the faculty of the Department
of Radio Engineering, Southeast University, as an
Assistant Professor, and became a Lecturer in 1998,
an Associate Professor in 2001, and a Professor
in 2005. His current research interests include RF
circuits and systems in mobile communications.
Wei Hong (M’92–SM’07–F’12) received the B.S.
degree in radio engineering from the University
of Information Engineering, Zhengzhou, China,
in 1982, and the M.S. and Ph.D. degrees in radio
engineering from Southeast University, Nanjing,
China, in 1985 and 1988, respectively.
Since 1988, he has been with the State Key
Laboratory of Millimeter Waves, Southeast University, where he has been serving as the Director
since 2003. He is currently a Professor and the
Dean of the School of Information Science and
Engineering, Southeast University. In 1993, 1995, 1996, 1997, and 1998,
he was a short-term Visiting Scholar with the University of California at
Berkeley, Berkeley, CA, USA, and the University of California at Santa Cruz,
Santa Cruz, CA, USA. He has authored or co-authored over 200 technical
publications and authored 2 books. His current research interests include
numerical methods for electromagnetic problems, millimeter-wave theory
and technology, antennas, electromagnetic scattering, and RF technology for
mobile communications.
Dr. Hong was a three-time recipient of the First-Class Science and Technology Progress Prizes by the Ministry of Education of China and the
Jiangsu Province Government, China. He was also the recipient of awards
from the Foundations for China Distinguished Young Investigators and the
Innovation Group by the National Science Foundation of China. He serves
as the Vice President of the Microwave Society and the Antenna Society of
the Chinese Institute of Electronics, the Chair of the IEEE MTT/AP/EMC
Joint Nanjing Chapter, and an AdCom member of the IEEE Microwave
Theory and Techniques Society. He served as an Associate Editor for the
IEEE T RANSACTIONS ON M ICROWAVE T HEORY AND T ECHNIQUES from
2007 to 2010.
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