1
Advances in terahertz communications accelerated by photonics
Tadao Nagatsuma
1*
, Guillaume Ducournau
2
, and Cyril C. Renaud
3
1
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan
2
Institut d’Electronique de Microélectronique et de Nanotechnologie (IEMN), UMR
CNRS 8520, Université de Lille 1, 59652, Villeneuve d’Ascq CEDEX, France
3
Department of Electronic and Electrical Engineering, University College London,
London, WC1E 7JE, U.K.
*e-mail: nagatuma@ee.es.osaka-u.ac.jp
Almost 15 years have passed, since the initial demonstrations of terahertz
(THz) wireless communications were made using both impulse and
continuous waves. THz technologies have recently gained greater interest
and expectations to meet an ever-increasing demand for the speed of
wireless communications. This article reviews a latest trend of THz
communications research focusing on how photonics technologies have
played a key role in the development of first-age THz communication
systems and how they compare with other competitive technologies such as
THz transceivers enabled by electronic devices as well as free-space light-
wave communications.
2
Trends in wireless communications
It is known that data traffic is increasing exponentially with Internet Protocol (IP)
traffic expected to reach over 130 Exabytes per month by 2018
1
. The fastest growing part
of that increase is on wireless channels, as mobile users increasingly make use of online
services. Such an increase in the network capacity requires much higher wireless
transmission rates in numerous connection links between each base station, between a base
station and an end-user device, between each end-user device, etc. The prospective data
rate for wireless communications in the market place will be 100 Gbit/s within 10 years
2
.
Historically, since the first microwave wireless link developed by G. Marconi in early 20
th
century, carrier frequencies used for wireless communications have been increasing
3, 4
to
meet bandwidth requirements, up to the recent development of wider spectral bands at
millimetre-wave (MMW) frequencies, such as 60 GHz, and 70 GHz~95 GHz
5
. However,
the total allocated bandwidth is less than 7 GHz~9 GHz which will ultimately limit the
total throughput of the channel to an insufficient level for the increasing demand.
It is obvious that the use of even higher carrier frequency in the THz range (0.1
THz~10 THz) is required when the bandwidth is at a minimum several tens of GHz. The
initial demonstrations of THz wireless communications were conducted using both impulse
and continuous waves, which were generated from photoconductors and photodiodes
excited by pulse lasers and intensity-modulated lasers, respectively
6-8
. The latter
continuous-wave wireless link, which employs a 120-GHz band, is the first commercial
THz communication system with an allocated bandwidth of 18 GHz (116 GHz~134 GHz),
which offers 10 Gbit/s with an On-Off-Keying (OOK) modulation and 20 Gbit/s with a
Quadrature Phase Shift Keying (QPSK) modulation at a transmission distance actually
demonstrated of over 5 km
9, 10
. Now, lots of worldwide research groups have developed
3
communication links at frequencies over 100 GHz. In particular, above 275 GHz, there is a
possibility to employ extremely large bandwidth of over 50 GHz for radio
communications, since these frequency bands have not yet been allocated to active services
in the world.
General consideration and expectations of THz waves for
communications
From Shannon formula
11
, the information capacity, C, or the data rate is associated
to the bandwidth, W, and the signal-to-noise ratio, S/N, as is given by C [bit/s] = W log
2
(1
+ S/N). High data-rate THz wireless systems could be possible due to the large available
bandwidth, W, even though the signal power, S, generally tends to decrease with the carrier
frequency. However, one of the big obstacles in the use of THz waves in wireless
communications is the atmospheric attenuation
12
as shown in Fig. 1a. Transmission
distance is limited by the attenuation, and the appropriate carrier frequency or frequency
band should be determined by applications; 100 GHz~150 GHz for long distance (1 km~10
km), < 350 GHz for medium distance (100 m~1 km), < 500 GHz for in-door (10~100 m).
Above 600 GHz, there are two windows also for in-door communications; 625 GHz~725
GHz, and 780 GHz~910 GHz. When the frequency exceeds 1 THz, the radio wave
undergoes a significant absorption by water vapour and oxygen molecules in the
atmosphere, and is attenuated by less than one tenth at only 1-m propagation distance,
which is still useful for near-field communications (NFCs; <0.1 m). In addition, one cannot
ignore attenuation from rainfall
13
. This attenuation is mostly independent on frequency in
the range above 100 GHz, and the attenuation is about 10 dB/km in the case of heavy rain
condition (25 mm/h), and should be considered for outdoor applications.
4
A free-space path loss (FSPL)
14
, L
B
, which is given by L
B
= (4pdf/c)
2
with link
distance, d, carrier frequency, f, and the velocity of light c, is physically inevitable. In the
first THz communication window (200 GHz~320 GHz) and for up to km-range systems,
the link budget is very close to the FSPL, and is not really degraded by the atmospheric
contribution (Fig. 1b). For 1 km (usual backhaul size in cellular networks), THz system
will have to deal with 140-dB total losses at a carrier frequency of 300 GHz. High gain
antenna structures have to be considered to compensate this fundamental limitation.
Indeed, since the antenna gain, G
A
, is given by G
A
= 4pA
h
(f/c)
2
with antenna area, A, and
antenna efficiency,
h
, the free-space path loss can be compensated by the gain of both
transmitter and receiver antennas; total antenna gain in the link can easily be made more
than 100 dBi at 300 GHz, though such antennas are highly directive. Even if technology
will increase in output-power capability, isotropic THz links may not be practically
feasible. Beam steering or beam forming with phased array antennas would be useful as
has already been introduced in 60-GHz wireless technologies
15, 16
.
Depending on the above link distance criteria, promising applications of THz
communications include front- and back-hauling of base stations (BSs) in femto cells,
wireless local area networks in smart offices, wireless personal area networks in smart
homes, near-field communications (NFCs) such as kiosk downloading, wireless
connections in data centers, device-to-device communications (D2D), etc. (Fig. 2a). In
these applications, another key aspect is the power consumption, directly related to
transmitter and receiver architectures, and strongly impacting the real THz link scenario
(mobile user or fixed point to point). For an outdoor link, spectral efficiency has to be
taken into account, both for point-to-point and earth-to-space links
17
in order to limit
interference with established systems
18
. For indoor links, where frequencies can be re-used
5
over several rooms for example, simple amplitude coding/duobinary can be used not only
to drastically simplify the receiver architectures using primitive direct detection but also to
limit the receiver energy requirements.
One of the frequently raised concerns for THz communications is the comparison to
free-space optics (FSO) communications using infrared light (IR) waves. As we described
in the beginning of the section, it would be logical to multiply the carrier frequency by
several order of magnitudes and use optics as the carrier
19-21
. This would definitely offer
more bandwidth while using the same base technology as photonics-based THz for the
modulation of signals. However, the FSO faces lower tolerance in alignment, and requires
stronger beam-steering control systems, though the development of MIMO-based system is
promising
22
. For outdoor applications, the optics suffers more than 2 orders of magnitude
higher losses in foggy conditions than THz waves
23
, which again would limit the use of the
FSO systems
24
. Ultimately, cost, size, performance and usability would determine which
becomes more adopted in the marketplace.
Photonics-based approaches to realizing THz communications
From the various THz technologies continuously developed, system-level efforts have led
to the use of both photonics and/or electronics-based technologies. First key advances have
been realized using III/V semiconductor technologies at a “lab” integration level, where
each device or component separately developed is optimized to show the highest
performance. Among this scheme, the highest data rates have been reached using photonic
devices as transmitters, combined with cutting-edge III/V THz electronic devices as
receivers.