a 120-ghz-band 10-gbps wireless link

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.


(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.


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.


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)


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.


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.


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.


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


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

To LIMsand EDs

Waveguide TypeVariable ATT.

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–46 –44 –42 –40 –38 –36

Bit

Err

or R

ate

Received Power (dBm)

(a)

(b)


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“10-Gbit/s BPSK modulator and demodulator for a 120-GHz-band

wireless link,” IEEE Trans. Microwave Theory Tech., vol. 59, no. 5, pp.

1361–1368, 2011.


“10-Gbit/s Quadrature phase-shift-keying modulator and demod-

ulator for 120-GHz-band wireless links,” IEEE Trans. Microwave Theory Tech., vol. 58, no. 12, pp. 4072–4078, 2010.

a 120-ghz-band 10-gbps wireless link

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