Letters10-Gbit/s Wireless Link Technology Using
the 120-GHz Band
Tadao Nagatsuma† and Akihiko Hirata
Abstract
The appearance of high-speed, large-capacity information-processing devices is driving the need for
ultrahigh-speed wireless communications that can achieve nearly instantaneous transfer of large
amounts of data. This article introduces a 120-GHz, millimeter-wave wireless link capable of 10-Gbit/s
data transmission, which is a full order of magnitude faster than existing wireless communications technologies.

1. Rise in transmission speeds

2. Benefits of 10-Gbit/s wireless links

As with computers, there seems to be no end to theincreases in speed and capacity in information communications. Figure 1 shows how transmission
speeds have increased over time for long-distance
optical backbone transmission, Ethernet LAN, andwireless techniques. It seems as if all three of thesetransmission techniques are converging on the sametarget, that is, transmission speeds from 10 to 100
Gbit/s. This result heralds the coming of an era inwhich wired communications using optical fiber andwireless communications will connect seamlessly atsimilar bit rates.
Figure 2 compares the transmission speeds ofrecent wired and wireless techniques for local areanetwork (LAN) and personal area network (PAN)
environments. Wired LAN/PANs using optical fiber
have already been commercialized in Ethernet form
at a bit rate of 10 Gbit/s. In contrast, the various versions of wireless LANs currently in use feature bit
rates of only 20–50 Mbit/s, much lower than those of
wired LAN/PANs. But the appearance of new techniques such as ultra-wide band (UWB) and millimeter-band wireless 1394 is expected to enable speedsof 1 Gbit/s in the near future.

We examined the need for 10-Gbit/s wireless links,
which far exceed the transmission speeds of existing
wireless techniques. At present, the market for fixedwireless access (FWA) is considered to be the mostpromising. This is because of the current state of corporate in-house LANs where the introduction of optical fiber networks is driving the broadband boom.
Gigabit Ethernet (1 Gbit/s) is already widely used
here, and the introduction of 10-Gigabit Ethernet(10GbE) to corporate in-house LANs can be expected in the coming years. But if such a 10GbE corporate LAN needed to be extended to an adjacent building, for example, it would take considerable time andexpense to lay new optical fiber. If 10-Gbit/s wirelesslinks were used, two separate 10GbE networks could
be connected in a relatively short time at low cost.
10-Gbit/s wireless links are also expected to meetthe demand for temporary broadband networks innatural-disaster recovery efforts, at event halls andvenues, and in remote broadcasts. There is a particular need for circuit capacities of several gigabits per
second for the uncompressed transmission of highresolution moving pictures in high-definition television (HDTV) and digital cinema, but the maximum
transmission speed of millimeter-wave wireless links
typically in the 60-GHz band is only about 1.5 Gbit/s,
which is insufficient for such moving-picture transmission. A 10-Gbit/s wireless link could play anactive role at broadcast stations and conference sites
where there is strong demand for the transmission of† NTT Microsystem Integration Laboratories
Atsugi-shi, 243-0198 Japan
E-mail: ngtm@aecl.ntt.co.jp

58NTT Technical ReviewLetters

Optical fiber communications1TRadiowaves

Optical backbonetransmission

10 G
Bit rate (bit/s)

E-TDM

Wavelength
multiplexing

100 G(120 GHz)
(60 GHz)

1G
Ethernet

Wireless

100 M
10 M
10 BASE-T
1M

E-TDM: electrical time divisionmultiplexing

1980

1985

1990

1995

2000

2005

2010

Year

Fig. 1. Increase in bit rate for various transmission techniques.

Wired LAN/PAN

10 Gbit/s

Ethernet

IEEE 1394 800 Mbit/s 3.2 Gbit/s

Wireless LAN/PAN
Implemented

IEEE 802.11b&g 54 Mbit/s
2.4 GHzIn development

IEEE 802.11a IEEE 802.11n
5 GHz
Wireless 1394
5 GHz

1.6 Gbit/s
60 GHzUltra-wide band (UWB)

1101001000
Transmission speed (Mbit/s)

10,000Fig. 2. Development of high-speed LAN.

large amounts of moving-picture data.
Another application of 10-Gbit/s wireless links is in
the storage area network (SAN) field where large
files and applications are stored on centralized
servers to simplify their management and sharing. In
corporate in-house networks, efforts are being madeto enable individual terminals to instantaneously
download data as needed from these servers. Here,the introduction of 10-Gbit/s wireless links to a SAN
configured by 10GbE would enable each terminal todownload large amounts of data at speeds equivalent
to those of an optical-fiber connection without having
Vol. 2 No. 11 Nov. 2004

to use a cable.
10-Gbit/s wireless links can also be considered for
hotspot-like applications such as downloading movie
files in a movie-rental service. The transmission of
the 4.7 gigabytes of data stored on a DVD, for example, takes nearly seven minutes over Fast Ethernet(100 Mbit/s) but would take only about four seconds
over a 10-Gbit/s wireless link. In the future, the use of
10-Gbit/s wireless links might make scenarios likethe instantaneous exchange of large amounts of data
between two passersby a reality (Fig. 3).

59

Letters

Storage area network (SAN)
Millimeter-wave
wireless linkServer

Fixed-wireless/temporary networks
Natural-disaster recovery

10 GbE
File system
SAN
Inter-building
communications

Digital cinema

10 GbE

Storageequipment

ONU
LAN

Hot spot
Optical fiber

Transmission of uncompressed HDTV signals
Sporting-event relays
Remote medicine

10 GbE

Access point
PDA
File serverMovies,
Music

ONU: optical network unit
PDA: personal digital assistantFig. 3. Application examples of 10-Gbit/s wireless links.
50120-GHz millimeter-wave
wireless link

Transmission speed (Gbit/s)

101FWA (P-P)–1

10

Wireless LAN

P-P: point to point
P-MP: point to multipoint

10–2
FWA (P-MP)
Cellular phone
10–3
1

10
100
Carrier frequency (GHz)

500Fig. 4. Relationship between transmission speed and carrier frequency in various wireless communication systems.

3. Why 120 GHz?
To achieve 10-Gbit/s transmission speeds, wechose to use the 120-GHz electromagnetic-wave
band as a carrier. This band has never before beenused for this purpose. Why use such a high radio frequency above 100 GHz? The answer lies in the relationship between transmission speed and carrier frequency in wireless communications, as shown in Fig.
4. Examining the plots for transmission speed versus
carrier frequency starting with 2-GHz cellular phones
60

and then continuing with the previously described
wireless LAN and 60–70-GHz FWA, we see that carrier frequencies of over 100 GHz would be effective
for achieving bit rates in excess of 10 Gbit/s.
In selecting a particular frequency, we must alsoconsider the problems of radio-wave propagation andradio-wave regulation (frequency allocation). Figure
5 shows attenuation characteristics for radio and optical waves under various atmospheric and rainfall
conditions. In the range from 10 GHz to 1 THz, theatmospheric attenuation of radio waves increases asNTT Technical Review

Letters

Attenuation (dB/km)

1000

120-GHz band
H2O

60-GHz band

100

Fog (0.1 g/m3)

H2O

Storm

O2

Downpour

10O2
1Drizzle
H2O

0.1
0.01
10 GMillimeterwaves

Sub-millimeter
waves

100 G

1TVisible
radiation

Infrared radiation10 T100 T

1000 T

Frequency (Hz)Frequency allocation
Wirelesscommunications
Radio astronomy

116–130 GHz is unused

Ham radio
Car radar
Broadcasting
50

100150200Frequency (GHz)250300Fig. 5. Millimeter-wave propagation characteristics and frequency allocation in Japan.

frequency rises and reaches 10 dB/km at 200 GHz
and above. Absorption bands due to oxygen andwater molecules can also be seen at 60 and 200 GHz
and elsewhere. The 120-GHz band, however, lies in avalley between such absorption bands, so it is a frequency region with relatively little atmospheric attenuation. Millimeter waves, though, are known to suffer from large attenuation during rainfall, and even in
the 120-GHz band, attenuation can reach 50 dB/km
during severe rainfall caused by storms. Such large
attenuation during severe rainfall, however, is not
limited to the 120-GHz band. It is essentially constantfor millimeter waves above 60 GHz.
Given that radio-wave frequencies are a limited
resource, they are regulated in Japan by the Radio
Law to ensure fair and efficient use. Figure 5 also
shows the frequency allocation in Japan in the50–300-GHz range. Radio waves in the 50–90-GHz
range are widely used for wireless communications,radar, and other applications, but above 100 GHz,
they are allocated only to radio astronomy and hamradio. There are also a number of unused regions
including the one from 116 to 130 GHz.
4. Configuration of wireless link
There have been no examples to date of using a millimeter-wave band above 100 GHz for Gbit/s-classhigh-speed wireless communications. One reason forVol. 2 No. 11 Nov. 2004

this is that transistors and other electronic components that can operate at such high frequencies
account for only part of the required circuitry, so it isstill difficult to obtain all the components needed for
wireless transmission of this type. In response to thisproblem, we have been testing the introduction ofphotonics technology for the generation, modulation,and transmission of 120-GHz millimeter-wave signals [1]. This technology, which was developed in thefield of optical-fiber communications, can generallyaccommodate a broader band of signals than electronic technology. It is relatively easy for photonics
technology to process signals having a frequency
above 100 GHz.
Figure 6 shows the configuration of a 120-GHz
wireless link. First, a 125-GHz intensity-modulated
optical signal is generated using an optical millimeter-wave signal generator. Then, this 125-GHz opticalsignal is modulated with a data signal having a maximum bit rate of 10 Gbit/s by an optical modulator,
and the resulting optical signal is amplified and input
to a photonic emitter. A uni-traveling-carrier photodiode (UTC-PD) [2] known for its high speed and highoutput characteristics is used to perform optoelectronic conversion on this optical signal, which is thenamplified by a millimeter-wave amplifier using ahigh electron-mobility transistor (HEMT) [3] andradiated from an antenna [4]. The millimeter-wave
signal that reaches a receiver’s antenna is then ampli61

Letters

Photonic emitter

Millimeter-wave
detector
Schottky
diodeUTC-PD

Opticalamplifier

HEMT
amplifierBaseband
amplifier

10 Gbit/s
Opticalmodulator

HEMT
amplifier

Data
input

Data output

125 GHz
Opticalmillimeter-wave
signal generator

Optical signal
Electrical signal

Fig. 6. Configuration of a 120-GHz millimeter-wave wireless link and a photograph of the transmitter.

fied and subjected to envelope detection using anultrahigh-speed Schottky diode. The figure also
shows a photograph of the millimeter-wave wirelesstransmitter that we developed. A maximum transmission distance of 1 km can be achieved by using a
Gaussian optics lens antenna having high directivity
for both the transmitter and receiver.

[4]suma, and H. Kyuragi, “A 120-GHz Millimeter-wave MMIC Chipset
for Future Broadband Wireless Application,” in Tech. Dig. IMS 2003,
Vol. 1, pp. 129-132, 2003.
A. Hirata, T. Kosugi, T. Shibata, and T. Nagatsuma, “High-Directivity Photonic Emitter for 10-Gbit/s Wireless Link,” International Topical Meeting on Microwave Photonics 2003, pp. 35-38, 2003.

Tadao Nagatsuma

5. Future outlook
On July 8, 2004, we received a license from theKanto Bureau of Telecommunications of the Ministryof Public Management, Home Affairs, Posts andTelecommunications for an experimental station
using the millimeter-wave communication equipment
described above. This is the first license given in
Japan for a radio experimental station in a frequency
region above 100 GHz. In the future, we plan to perform both laboratory and field experiments to evaluate propagation characteristics in various environments with the aim of making a practical working
system. We also plan to take up the challenge ofdeveloping the electromagnetic region from 300 GHz
to 1 THz by using the same photonics approach as
described here.
References
[1]

[2]

[3]

62

A. Hirata, M. Harada, and T. Nagatsuma, “120-GHz Wireless Link
Using Photonic Techniques for Generation, Modulation, and Emission of Millimeter-Wave Signals,” IEEE Journal of Lightwave Technology, Vol. 21, pp. 2145-2153, 2003.
T. Ishibashi, N. Shimizu, S. Kodama, H. Ito, T. Nagatsuma, and T.
Furuta, “Uni-Traveling Carrier Photodiodes,” in Tech. Dig. UltrafastElectron. and Optoelectron., pp. 166-169, 1997.
T. Kosugi, T. Shibata, T. Enoki, M. Muraguchi, A. Hirata, T. Nagat-Group Leader, Senior Research Engineer,
Leader of the Advanced System Devices
Research Group, Smart Devices Laboratory,
NTT Microsystem Integration Laboratories.
He received the B.S., M.S., and Ph.D. degrees
in electronic engineering from Kyushu University, Fukuoka in 1981, 1983, and 1986, respectively. He joined NTT Atsugi Electrical Communications Laboratories in 1986. From 1999 to 2003,
he was a Distinguished Technical Member atNTT Telecommunications Energy Laboratories.
His current research involves ultrahigh-speed
electronics and millimeter-wave photonics, andtheir application to sensors and communications.
He was the recipient of the 1989 young engineers
award presented by the Institute of Electronics,
Information and Communication Engineers(IEICE), the 1992 IEEE Andrew R. Chi best
paper award, the 1997 Okochi Memorial Award,
the 1998 Japan Microwave Prize, the 2000 Minister’s Award of the Science and Technology
Agency, the 2002 Asia Pacific Microwave Conference APMC Prize, and the 2004 YRP Award.
He is a senior member of the IEEE and a memberof the Optical Society of America and IEICE.

Akihiko Hirata
Research Engineer, Advanced System Devices
Research Group, Smart Devices Laboratory,
NTT Microsystem Integration Laboratories.
He received the B.S. and M.S. degrees inchemistry from the University of Tokyo, Tokyoin 1992 and 1994, respectively. Since joining
NTT in 1994, he has worked on development ofLSI interconnection, microelectromechanicalsystems, and the design and integration of millimeter-wave devices. He is currently developing
120-GHz-band wireless links using photonictechnology. He received the 2002 Asia PacificMicrowave Conference APMC Prize and the2004 YRP Award. He is a member of IEEE andIEICE.

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