a 120-ghz-band 10-gbps wireless link
TRANSCRIPT
54 September/October 2012
Digital Object Identifier 10.1109/MMM.2012.2205830
1527-3342/12/$31.00©2012IEEE
Date of publication: 13 September 2012
Hiroyuki Takahashi, Toshihiko Kosugi, Akihiko Hirata, and Koichi Murata
Hiroyuki Takahashi ([email protected]) and Akihiko Hirata are with Nippon Telegraph and Telephone Corporation (NTT), Microsystem Integration Laboratories. Toshihiko Kosugi and Koichi Murata are with NTT Photonics Laboratories.
FOCUSED
ISSUE FEATU
RE
Supporting Fast and Clear Video
September/October 2012 55
In communications networks, 10-Gb Ethernet
(10GbE) and gigabit Ethernet passive optical
networks (GE-PON) have been widely used and
10 Gb/s Ethernet PON (10G-EPON) and 100GbE
were established in 2009 and 2010, respectively.
Wireless technologies that can handle optical commu-
nications standards are useful for last-mile wireless
access and setting up temporary connections to restore
a network after a disaster or other disruptions. In the
broadcasting field, high-definition television (HDTV),
which requires a 1.5-Gb/s data rate, has been accepted
in studio and live-relay broadcasts. Moreover, three-
dimensional HD movies (3 Gb/s), 4K digital cinema
(6 Gb/s) and super-high-vision (SHV) (24 Gb/s) [1] have
been developed to catch up with the demand for high-
presence applications. There is a strong need for broad-
band wireless equipment that can transmit uncom-
pressed HD videos in various situations. To support
the data rate of high-speed protocols and HD videos,
there has been a lot of interest in high-speed wireless
technologies using the millimeter-wave (MMW) band
from 30 to 300 GHz, because this band can provide suf-
ficient bandwidth.
The license-free frequency band from 57 to 66 GHz,
the so-called 60-GHz band, is attracting attention
for multigigabit wireless systems suitable for con-
sumer wireless devices. Some wireless standards,
such as Wireless HD, ECMA 387, IEEE802.15.3c, and
IEEE802.15ad (WiGig), have been established toward
commercialization. The 60-GHz band wireless system
is mainly used for indoor applications because the
atmospheric attenuation induced by oxygen absorp-
tion in that band is large. For long-range applications,
the 71–76 GHz, 81–86 GHz, 94-GHz-band and 120-GHz
band are expected for multigigabit or 10-Gb wireless
communications because atmospheric attenuation in
these bands is smaller than that of the 60-GHz band.
Wireless systems using these bands are expected for
field pickup units (FPUs), fixed wireless access (FWA),
and fourth generation (4G) mobile backhaul. There are
studies that explore frequencies of above 200 GHz for
wireless communications. These frequencies are not
yet fully exploited industrially and could lead to the
development of broadband wireless systems using
simple modulation schemes. Devices operating above
200 GHz use state-of-the-art semiconductors and com-
binations of photonics and electronics technologies.
Nippon Telegraph and Telephone Corporation
(NTT) laboratories are developing a 10-Gb/s wire-
less link system using the 120-GHz band to meet
the demands for wireless transmission of 10GbE,
10G-EPON, and uncompressed HD videos over a
distance of several kilometers. The 120-GHz band
is promising for wideband FWA because it provides
sufficient bandwidth and small atmospheric absorp-
tion (about 1 dB/km). A key technology of this link
is a radio-frequency (RF) device that can transmit
a high-power MMW signal modulated at 10 Gb/s
and receive the signal with high sensitivity. We have
developed monolithic microwave-integrated circuits
(MMICs) to make a 10-Gb/s transmitter and receiver
in the 120-GHz band and to extend the wireless link’s
transmission distance. This article covers the MMIC
technologies and system architecture.
Recent Broadband Wireless and Its Device TechnologiesFigure 1 shows bit rates and distances between wire-
less terminals for recently reported MMW wireless
transmissions. This figure covers only experimen-
tal results for wireless transmission using antennas.
Table 1 shows a comparison of frequency, modulation
scheme, key technology, and antenna in the reports.
Most integrated circuits (ICs) for 60-GHz wire-
less technologies use silicon-based transistors such as
complementary metal–oxide–semiconductor (CMOS)
and silicon germanium (SiGe) bipolar CMOS (BiC-
MOS) because the ICs can be mass produced at low
cost [2]–[7]. Okada et al. reported a 60-GHz direct
conversion transceiver using 65-nm CMOS [3]. The
transceiver supports IEEE802.15.3c full-rate wireless
communication for all modulation schemes and trans-
mits 16-quadrature-amplitude-modulation (16-QAM)
data of 11.1 Gb/s over a distance of 0.17 m with in-
package antennas. Emami et al. designed transceivers
using 65-nm CMOS that support maximum bit rates
for Wireless HD and WiGig [6]. The range between dif-
ferent transceivers is 50 m for 3.8 Gb/s in a line of sight
Figure 1. Recently reported bit rates and transmission distances in experimental demonstrations of MMW wireless transmission.
[20]
[21][21]
[19]
60-GHz Band70–100 GHzOver 100 GHz[16]
[18]
[12][13]
[3]
[3]
[7][6]
[6]
[11][10]
[10][8] [9]
[3][15] [5]
[4]
[17] [2]
100
10
10.01 0.1 1 10 100 1,000 10,000
Link Distance (m)
Bit
Rat
e (G
b/s)
[2][16]NTT
Wireless technologies that can handle optical communications standards are useful for last-mile wireless access.
56 September/October 2012
(LOS) environment. In addition, the transceivers have a
large number of elements for beam steering of antenna
arrays. This function enables finding the new optimal
path between transceivers in non-LOS environments.
Since the frequency bands of 71–76, 81–86, and
94 GHz are promising for a long-range wireless com-
munication, the link systems in these bands use high-
gain antennas and compound semiconductors, such as
gallium arsenide (GaAs) and indium phosphide (InP)
to achieve high output power [8]–[11]. BridgeWave
Communications, Inc. provides a commercial multi-
gigabit wireless links for flexible access and a backhaul
solution [10]. This system yields up to 3 Gb/s through
the use of two wireless terminals and an orthogonal
mode transducer (OMT). Dyadyuk et al. have reported
a multigigabit wireless link, which provides 6-Gb/s
data transmission over a distance of 250 m with a link
margin of over 10 dB [11]. Fabricated modules using
GaAs MMICs achieved spectral efficiency of 2.4 b/s/Hz
at 81–86 GHz.
To handle RF frequencies of over 100 GHz, some
research groups are studying wireless transmitter and
receiver ICs using state-of-the-art semiconductors [12]–
[16]. Laskin et al. reported a double-sideband trans-
ceiver using SiGe BiCMOS in the 140-GHz band [13].
Their 4-Gb/s wireless transmission was conducted
over a distance of 1.15-m using a reflector. Kallfass et al.
reported 220-GHz transmitter and receiver MMICs using
50-nm metamorphic high-electron-mobility transistors
(mHEMTs), which are based on a composite InGaAs/
InGaAs channel [16]. These MMICs packaged into split-
block waveguide modules transmit 25-Gb/s data over
50 cm with an eye diagram quality factor of 3 and trans-
mit 10 Gb/s over a distance of 2 m with a bit error rate
(BER) of 1.6 # 10–9.
Some research groups have adopted photonics tech-
nologies because these technologies provide the wide
bandwidth in signal generation and modulation [17]–[21].
TABLE 1. Comparison of frequency bands, modulations, device technologies and antennas in experimental demonstrations of MMW wireless transmission.
Ref. Frequency Modulation Technology Antenna
[2] 60-GHz band QPSK/16-QAM 90 nm CMOS Liquid-crystal-polymer planar antenna
[3] 60-GHz band BPSK/QPSK/8-PSK/16-QAM
65 nm CMOS Packaged antenna (2.2 dBi)
[4] 60-GHz band QPSK 90 nm CMOS Horn (25 dBi)
[5] 60-GHz band 16-QAM OFDM 65 nm CMOS HTCC and glass antennas
[6] 60-GHz band 16-QAM OFDM 65 nm CMOS Packaged array antennas
[7] 60-GHz band 16-QAM OFDM SiGe BiCMOS Packaged patch-array antennas
[8] 71–76 GHz/ 81–86 GHz
QPSK — Cassegrain
[9] 71–76 GHz/ 81–86 GHz
BPSK — Cassegrain (51 dBi)
[10] 71–76 GHz/ 81–86 GHz
QPSK — Cassegrain (44 dBi, 51 dBi)
[11] 81–86 GHz 8 PSK GaAs pHEMT Conical lens horn (45 dBi)
[12] 73–93 GHz Impluse radio InP HEMT Horn (23 dBI)
[13] 140-GHz band ASK 130-nm SiGe BICMOS Horn
[14], [15] 120/140-GHz band ASK 65 nm CMOS Horn (25 dBi)
[16] 220-GHz band OOK 50 nm mHEMT Lens and horn
[17] 300-GHz band ASK Photonics-based transmitter Dielectric lens and horn (~25 dBi)
[18] 300-GHz band ASK Photonics-based transmitter Dielectric lens and horn (~25 dBi)
[19] 57.4–64.4 GHz 16-QAM OFDM Photonics-based transmitter Horn (23 dBi)
[20] W band (75–110 GHz) 16-QAM Photonics-based transmitter Horn
[21] W band (75–110 GHz) 16-QAM Photonics-based transmitter Horn (24 dBi)
NTT 120-GHz band ASK 100-nm InP HEMT Cassegrain (49 dBi)
Devices operating above 200 GHz use state-of-the-art semiconductors and combinations of photonics and electronics technologies.
September/October 2012 57
Nagatsuma and Song et al. demonstrated up to
14-Gb/s wireless transmission over a distance of 0.5 m
using an RF of 300 GHz [17], [18]. They integrated a
photonics-based transmitter by using a unitraveling-
carrier photodiode (UTC-PD) [22]. Pang et al. reported
a hybrid optical fiber-wireless link system using the W
band (75–110 GHz) that can transmit 100-Gb/s with an
air distance of 1.2 m [21]. The link also uses a photon-
ics-based 16-QAM modulator and dual-polarization
multiplexing.
As shown Figure 1, reported demonstrations cover
bit rates of up to 100 Gb/s in short-distance transmis-
sions, up to 6 Gb/s for several hundred meters, and
3 Gb/s for several kilometers. However, there is no
demonstration of 10-Gb/s transmission over a distance
of several kilometers. A 10-Gb/s wireless system with
a long transmission distance is suitable for last-mile
access of 10GbE, live-relay transmission for 4K cin-
ema, and multiplexed HD videos. To meet these appli-
cations, NTT laboratories are developing a 10-Gb/s
120-GHz-band wireless link system with the link
distance of several kilometers. An important point in
this development is to extend the link distance while
maintaining the capacity of 10 Gb/s. In the next sec-
tion, we explain the progress in the link distance of the
wireless link.
Technologies of 120-GHz Wireless and the Progress in the Link Distance In order to increase the transmission distance, we need
to increase the output power of the wireless transmit-
ter and decrease the received power necessary for
error-free transmission. However, it has been difficult
to generate high-power radio signals because semicon-
ductor device characteristics deteriorate as the opera-
tion frequency increases.
Figure 2 shows the progress in the transmission
distance of the 120-GHz-band wireless link [23]. The
research of the 120-GHz-band wireless link started
with indoor data transmission using photonics tech-
nologies, because photonics
technologies have broad-
band characteristics and
are suitable for generating
high-frequency signals. The
key device of this system is
a UTC-PD [22]. A UTC-PD
can generate 4.4-dBm out-
put power at 120-GHz-band.
Data transmission over a dis-
tance of 2 m at 1.25 Gb/s was
achieved in 2000 [24], [25]. In
2002, we achieved the world’s
first 10-Gb/s data trans-
mission over a radio wire-
less link, which was made
possible by the development
of a broadband Schottky barrier diode receiver with a
silicon lens antenna [26].
In 2003, the development of MMICs for the
120-GHz wireless system was started. We used
0.1-nm-HEMT technology on an InP substrate. The
devices have a current-gain cut-off frequency ( fT)
of 170 GHz and a maximum oscillation frequency
( fmax) of 350 GHz. InP HEMT MMICs feature high-
speed and high-power operation, and we have suc-
ceeded in making low-noise amplifiers (LNAs),
power amplifiers (PAs), and demodulators. A PA
was used to amplify the UTC-PD output power, and
the receiver used receiver MMICs that integrated an
LNA and amplitude shift keying (ASK) demodulator
and achieved high sensitivity. We developed wire-
less equipment using these devices with a high-gain
cassegrain antenna (CA) and achieved an output
power of 0 dBm. The first experimental radio station
license from the Ministry of Internal Affairs and
Communications of Japan was obtained in 2004, and
we conducted the first outdoor transmission experi-
ments over a distance of 200 m [27].
Since 2007, the 120-GHz-band wireless signals
were generated using standard InP HEMT MMIC
technologies. In the transmitter MMIC, a frequency
multiplier, ASK modulator, and amplifiers are inte-
grated in one chip [28], [29]. Most of the receiver cir-
cuit blocks, including LNAs, narrow bandpass filters,
and demodulators have been improved and imple-
mented in a receiver MMIC. The LNA has a noise
Figure 2. Progress in the transmission distance of the 120-GHz-band wireless link.
10
1
10–1
10–2
10–3
2000 2002 2004 2006 2008 2010Year
Tra
nsm
issi
on D
ista
nce
(km
)
[27]
[31]
[32]
[26][25]
In order to increase the transmission distance, we need to increase the output power of the wireless transmitter and decrease the received power necessary for error-free transmission.
58 September/October 2012
figure of 5.6 dB and a small group delay variation of
less than 14 ps [30]. These multifunction MMICs bring
us higher reproducibility compared to the previous
versions of our transceivers by multichip packaging.
We developed transmitter (Tx) and receiver (Rx) mod-
ules that have a Tx or Rx MMIC chip in the same metal
waveguide package. Figure 3 shows photographs of
fabricated MMICs and waveguide modules. Then,
we implemented Tx, Rx, and PA modules in wireless
equipment with a CA. The averaged output power
of the equipment reached 10 dBm. We succeeded in
800-m 10-Gb/s data transmission using this wireless
equipment in 2007 [31].
In 2009, we developed wireless equipment with
an output power of 16 dBm [32]. The increase in the
output power was achieved by the development of
InGaAs/InP composite channel (CC) InP HEMT.
The use of an InGaAs/InP CC increases the break-
down voltage dramatically while maintaining high-
frequency performance. The 0.08-nm-gate CC InP
HEMTs were developed to have a fT of 180 GHz and a
fmax of 580 GHz [33]. The off state breakdown voltage
of the HEMTs is around 10 V, and reliable operation
can be expected below 4.0 V. These values are almost
two times higher than those of conventional lattice-
matched InP HEMTs. We fabricated a PA MMIC
using the CC InP HEMTs. A photograph of the PA
MMIC using CC InP HEMTs is shown in Figure 4.
The PA module was fabricated by integrating the PA
MMICs in a metal package. The P1dB output power
of the PA module is about 19 dBm, and the satura-
tion output power is about 21 dBm at 125 GHz. We
compare the maximum output powers of reported
PAs in Figure 5. At frequencies above 100 GHz, InP
HEMT devices show higher output power than other
devices at the same operation frequency. The PA has
the highest output power in the 120-GHz band. When
we use the PA module for an ASK-modulation wire-
less transmitter, the average power of the transmit-
ter should be 16 dBm for linear operation. Moreover,
we introduced forward error correction (FEC) tech-
nologies to reduce the received power necessary for
error-free transmission. We used Reed Solomon (RS)
(255,239) coding, which has a coding gain of about
6 dB at a BER of 10–12. Using the 16-dBm output power
wireless equipment and these FEC technologies, we
achieved error-free transmission of 10.3125 Gb/s
(11.1 Gb/s with FEC) data and six-channel multi-
plexed uncompressed HD video signals (1.5 Gb/s #
6 channels) from Tokyo Heliport (Koto-ku) to the Fuji
Television coastal studio (Minato-ku) in Tokyo, Japan,
over a distance of 5.8 km in fine weather. This is the
first time that 10-Gb/s data was transmitted by a
radio wireless link over a distance of more than 5 km.
Figure 4. Photograph of a PA MMIC chip using CC InP HEMTs.
The use of an InGaAs/InP CC increases the breakdown voltage dramatically while maintaining high-frequency performance.
Figure 3. Photographs of (a) a 120-GHz-band transmitter/ receiver MMICs and (b) RF modules.
AMP AMP
AMPLNA
DE
T
MOD DoublerBP
F
BPF
Transmitter MMIC
TransmitterModule
First AmplifierModule
Receiver Module
Receiver MMIC
(a)
(b)
September/October 2012 59
HD Video Signal Transmission Trials One of the promising applications of the state-of-
the-art 120-GHz-band wireless link is the uncom-
pressed transmission of TV broadcast contents for
live relay. To investigate whether the 120-GHz-band
wireless link could actually be used for these appli-
cations, we conducted various trials of HD video
wireless transmission.
For this purpose, we developed a compact
120-GHz-band wireless link. There is a strong
demand to reduce as much as possible the time from
arrival at a site to being broadcast ready. As such,
the FPU used to transmit broadcast contents must be
quite simple in structure, easy to assemble quickly,
and easy to operate. Figure 6 shows a photograph of
the 120-GHz-band wireless transmitter and speci-
fications of the link. The transmitter has a simple
architecture, consisting of three components: the
head, which generates the radio signal; the controller,
which supplies power and the data signal and control
signals to the head; and the antenna. The antenna is
attached by a bayonet mechanism, which is a sim-
ple fastening mechanism to connect a small F-band
waveguide (2 mm × 1 mm).
As such, we conducted a trial of the 120-GHz-band
system to transmit raw footage for on-site TV broad-
casting at the Beijing Olympics [34]. The 120-GHz radio
signal was used to transmit an uncompressed HD
video signal shot at the Beijing Media Center (BMC)
to the International Broadcast Center (IBC). The BMC
is a specially built relay studio facing the Olympic
park with an unobstructed view, and many Olympic
updates were reported from there. The receiver was
installed on an RF tower on the roof of the IBC and
the demodulated signal from the receiver was then
transmitted to one of the TV booths in the IBC. Not one
error was observed in the 120-GHz channel, and HD
image transmission was very stable during rain and at
temperatures of over 40 °C.
For further investigation, the 120-GHz-band wire-
less link was used for an SHV transmission trial.
SHV is a digital video format, and it has a resolution
of about 16 times the number of pixels of existing
HDTV. The data rate of an uncompressed SHV signal
based on the dual green method is 24 Gb/s; therefore,
three 120-GHz-band wireless link sets arranged in
parallel are necessary to transmit an uncompressed
SHV signal. The 120-GHz-band wireless link uses
a high-gain antenna, and high-frequency MMW
signals travel straight. Therefore, the interference
between wireless links using the same frequency
is small, even when two sets of wireless equipment
are arranged close to each other. Moreover, we can
Figure 5. Maximum output power of MMW PAs made with semiconductor MMICs.
0
10
20
30
40
50 100 150 200 250 300 350
InPGaAsGaN Si
Out
put P
ower
(dB
m)
Frequency (GHz)
NTTCC InP HEMT
Figure 6. The compact type 120-GHz-band wireless link: (a) a photograph and (b) specifications.
Center Frequency
Occupied Band
Output Power
Modulation
RF Front-End NF
Rx Sensitivity
Data Rate
Antenna
Antenna Gain
125 GHz
116.5–133.5 GHz
16 dBm
ASK
6 dB
–38 dBm for BER of 10–10
1 Mb/s–11.1 Gb/s
Cassegrain (CA), Horn
CA: 37, 49, 50, 51 dBiHorn: 23.3 dBi
(a)
(b)
One of the promising applications of the state-of-the-art 120-GHz-band wireless link is the uncompressed transmission of TV broadcast contents for live relay.
60 September/October 2012
decrease the interference using cross-polarized
MMW waves. Nippon Housou Kyoukai (NHK)
reported 1.3-km-long error-free transmission of
SHV signals by using three 120-GHz-band wireless
links in parallel [35]. When the middle link was set
to H-polarization and the other two links to V-polar-
ization and FEC technologies were introduced,
error-free transmission was achieved even when two
of the three links were right next to each other and
the other link was set 8 m from them.
InP HEMT MMICs for 120-GHz Wireless Link
QPSK ModulationAs explained above, 120-GHz-band wireless links
have been developed to achieve wideband opera-
tion over 10 Gb/s in long-distance data commu-
nication. Not only ASK but also binary phase shift
keying (BPSK) transceiver MMICs have been already
reported for a 120-GHz-band 10-Gb/s wireless link
[36] to improve the link margin. One other impor-
tant specification for wireless systems is spectral
efficiency. The 120-GHz wireless link employs an
ASK modulation scheme, which is the simplest
architecture but has poor spectral efficiency due to
binary modulation. quadrature phase shift keying
(QPSK) is a promising modulation scheme that has
double the spectral efficiency of ASK. It lets us use
the 120-GHz-band 10-Gb/s wireless link with less
occupied bandwidth. Though QPSK modulator and
demodulator MMICs are more susceptible to phase
error than ASK, the accuracy of the circuit design
seems to be high enough to integrate them with
other circuit blocks.
MMIC ArchitectureTwo system requirements for a 120-GHz-band QPSK
wireless link are an ability to handle 10-Gb/s data
and a transmission performance that ensures a BER
of less than 10–10 at a very low received power close to
the theoretical limit. Another important requirement
is a simple system architecture. The ASK with direct
modulation and demodulation is a simple architecture
and has a high affinity with 10GbE and the other
high-speed data formats. We were therefore able to
design the ASK modulator or demodulator into a
MMIC with other circuits on one chip and integrate a
very simple wireless system. This integration creates
big cost advantages in such a broadband wireless sys-
tem. For QPSK, we first selected the architecture of
the modulator and demodulator MMICs. One way
to simplify the MMICs is to employ a direct modu-
lation and demodulation scheme, because it doesn’t
have intermediate frequency (IF) circuits. However,
it requires accurate design of MMICs in the MMW
region. In addition, the demodulator MMIC employs
differentially coherent detection, which doesn’t need
carrier recovery circuits. Theoretically, differentially
coherent detection has lower sensitivity than coher-
ent detection, but the degradation is small for our
wireless link as described below.
Figure 7. Photographs of QPSK modulator and demodulator MMICs. (a) QPSK modulator MMIC and (b) QPSK demodulator MMIC.
LO
Doubler
AmplifierBB
Amplifier
PhaseShifter
IQMixers
Delay Line
Distribution Amplifiers
GC AmplifierCouplers
(a) (b)
RF RF
I
I
Q
Q
RSSI
The ASK with direct modulation and demodulation is a simple architecture and has a high affinity with 10GbE and the other high-speed data formats.
September/October 2012 61
The theoretical BERs for PSK with coherent detec-
tion and differentially coherent detection are given as
follows:
,exp
erfc NE
NE
21
21
Coherent
Differentially coherent
b
b
0
0-
c
c
m
m
(1)
where Eb and N0 are bit energy and noise power
spectral density, and erfc(x) is a complementary error
function. As shown in (1), the difference in BER per-
formance between coherent and differentially coher-
ent detection is small in the high Eb/N0 region. The
required Eb/N0 for coherent detection at a BER of 10–10
is only about 0.5 dB smaller than that for differentially
coherent detection. That means that the sensitivity
degradation with differentially coherent detection is
not a big penalty at our target BER of 10–10.
MMICsFigure 7 shows photographs of modern QPSK modula-
tor and demodulator MMICs fabricated with 0.1-nm-
gate InP HEMTs [37]. We succeeded in fabricating a
one-chip QPSK modulator and a one-chip demodu-
lator. The chip size of each MMIC is 2 mm # 2 mm.
The modulator and demodulator consume 850 and
650 mW, respectively.
For the modulator MMIC, we chose a simple archi-
tecture consisting of 90º and 180º hybrid couplers and
switches and combiners as shown Figure 8. The total
fundamental loss for the hybrid couplers combiners
is 9 dB. To compensate for that, we designed a gain-
control (GC) amplifier as an on-off switch. An input
local oscillator (LO) signal of 64 GHz is multiplied to
the carrier frequency of 128 GHz by a double circuit.
The carrier is amplified by an amplifier and input
to a direct modulator. In the modulator, the 90º and
180º hybrid couplers divide the carrier to four signals,
which are quadrature phases. The GC amplifier acts as
an on-off switch according to the data signals. When
the level of the data signal is high, an RF signal fed
into the GC amplifier is amplified by 10 dB; when the
level is low, the RF signal is attenuated by over 20 dB,
resulting in a 30 dB on-off ratio. The Wilkinson com-
biner combines the output signals of the GC amplifi-
ers. When in phase, quadrature phase (I, Q) is (1, 1), the
GC amplifiers at the I channel amplify the 0º signal
and the GC amplifiers at the Q channel amplify the 90º
signal. The phase of the combined RF signals therefore
becomes 45º. The equivalent circuit of the GC ampli-
fier is shown in Figure 9. The GC amplifier has three
stages. To avoid impedance mismatch between the
rat-race circuit and the input port of the amplifier, the
first stage doesn’t have the switching function. In the
second and third stages, the gain is changed according
to the level of the input data.
Figure 10 shows a block diagram of a QPSK demod-
ulator MMIC with differentially coherent detection.
The received signal is split into two. One part is
delayed by the duration of the 5-Gb/s data symbol.
The other part goes through a variable phase shifter.
After that, each signal is split again, and the four sig-
nals are fed into gate mixers. The main issues in mak-
ing the MMIC are the design of the one-symbol delay
circuit and control of the phase relationship between
the two split signals. First, we designed the delay
line for the one-symbol delay circuit. A delay line
made of a transmission line provides accurate delay
time, but it has the drawback of being very long. The
I Ch.5 Gb/s
Q Ch.5 Gb/s
Doubler
LO64 GHz
0/90°
0/180°
0/180°
180°
90°
270°
0°GC Amp.
WilkinsonCombiner
RF128 GHz
GC Amp.
GC Amp.
GC Amp.
Figure 8. Block diagram of the 120-GHz-band QPSK modulator.
A 120-GHz-band 10-Gb/s wireless link using an InP-HEMT-based MMIC is suitable for last-mile access of 10GbE, live-relay transmission for 4K cinema, and multiplexed HD videos.
62 September/October 2012
required length for 200 ps is about 25 mm at 128 GHz
if a delay line consists of only a coplanar waveguide
(CPW) with w/s = 15 nm/15 nm on InP substrate. To
reduce the length, we made the delay line by alter-
nating metal-insulator-metal (MIM) shunt capacitors
and CPWs. The length of the designed delay line is
10 mm, and the insertion loss is 18 dB in simulation.
Next, we designed a variable phase shifter to adjust
the phase relation between the received and delayed
signals prior to mixing. The variable phase shifter
consists of CPWs and cold-FETs, which are HEMTs.
Figure 11 shows the equivalent circuit of the vari-
able phase shifter. This circuit can adjust the elec-
trical length continuously by changing the values
of parasitic capacitances of the HEMTs. Thus, we
can tune the phase of the output signal by means of
applied voltage. The designed tuning range of this
circuit is over 180° at 125 GHz, which makes it pos-
sible to respond to any phase error caused by process-
voltage-temperature (PVT) variations.
QPSK Modules and BER PerformanceFigure 12 shows QPSK modulator and demodulator
modules using the QPSK MMICs described in the
previous section. Thanks to the one-chip integration
of the modulator and demodulator MMICs, we can
obtain compact QSPK modules: The size is only
20 mm × 8 mm × 25 mm, and the weight is 35 g. The
package has three coaxial ports and a WR-8 wave-
guide for the interface of the RF signal in the 120-GHz
band. Rectangular waveguide to CPW transitions
were needed to transfer the RF energy from the CPW
to the WR-8 waveguide and vice versa. To make the
transitions, a coupler fabricated on a quartz substrate
was employed. The modulator module has a quadru-
pler MMIC besides the modu-
lator MMIC. The quadrature
MMIC multiplies the LO fre-
quency of 16–64 GHz and
provides it to the modula-
tor MMIC. This enabled us
to decrease the required LO
frequency and use a commer-
cially available phase-locked
oscillator for the LO.
Figure 13 shows a photo-
graph of the measurement
system and measured BER
characteristics for the I and Q
channels. The modulator and
the demodulator were con-
nected through a waveguide
variable attenuator. We put
an LNA [27] in front of the
demodulator module to mea-
sure the minimum received
power. The MMIC in the LNA
module was the same as the
one in the wireless link using
ASK, and the noise figure
and gain were 5.6 and 19.8 dB,
respectively [30]. In addition,
we put limiting amplifiers
(LIMs) for the baseband sig-
nals after the demodulator
module to ensure that the
In the future, we hope to implement QPSK modules in the 120-GHz-band wireless link equipment.
Figure 11. Equivalent circuit of the variable phase shifter.
In
Out
MatchingNetworks
MatchingNetworks
Vphase
RF128 GHz
VariablePhase Shifter
200-psDelay Line
DistributionAmplifier
GateMixer
90°
I Ch.5 Gb/s
Q Ch.5 Gb/s
RSSI
Figure 10. Block diagram of QPSK demodulator MMIC.
Figure 9. Equivalent circuit of the GC amplifier.
Data
In Out
Drain
September/October 2012 63
error detectors (EDs) received sufficient power. Dif-
ferentially coherent detection of 10-Gb/s QPSK needs
5-Gb/s differentially encoded data for each I and Q
channel. Encoded PRBS 27-1 data was generated and
input into the I and Q ports of the modulator from
pulse pattern generators (PPGs). The BERs of the I
channel and Q channel were smaller than 10–10 at
–38.5-dBm input power for the LNA. In the current
link, the transmitter and receiver modules using
ASK exhibited a BER of 10–10 at the received power of
–38 dBm in a back-to-back test [31]. If we simply com-
pare the values, using the same antennas and a PA as
in the current link, we can achieve a 10-Gb/s QPSK
wireless link with a transmission distance of 2 km.
Conclusion A 120-GHz-band 10-Gb/s wireless link using an InP-
HEMT-based MMIC was introduced. This link is
suitable for last-mile access of 10GbE, live-relay trans-
mission for 4K cinema, and multiplexed HD videos.
The transmitter and receiver MMICs were developed
to extend the link distance while maintaining the
capacity of 10 Gb/s. The 120-GHz wireless link using
the MMICs successfully demonstrated wireless trans-
mission of 10GbE over the link distance of over 5 km.
We also designed QPSK modulator and demodulator
MMICs to improve the spectral efficiency of the wire-
less link. Fabricated QPSK MMICs and modules per-
formed 10-Gb/s transmission with the BER of 10–10 at
the received power of –38.5 dBm. In the future, we hope
to implement QPSK modules in the 120-GHz-band
wireless link equipment. We would also like to advance
the QPSK modulator and demodulator MMICs and
modules to handle bit rates of up to 20 Gb/s.
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I Ch. I Ch.Q Ch.
Q Ch.
LO in(16 GHz)
dcPins
WR-8Waveguide
RSSI
DemodulatorModulator
Figure 13. Photograph of measurement system and BER characteristics.
I Ch: 5 Gb/sQ Ch: 5 Gb/s
Modulator Demodulator
LNAModule
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–46 –44 –42 –40 –38 –36
Bit
Err
or R
ate
Received Power (dBm)
(a)
(b)
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