Page 33 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

M‑ARY QUADRATURE AMPLITUDE MODULATION ORDER OPTIMIZATION FOR TERAHERTZ
WIRELESS COMMUNICATIONS OVER DISPERSIVE CHANNELS

1
1
Karl Strecker , Sabit Ekin , and John O’Hara 1
1 Department of Electrical and Computer Engineering, Oklahoma State University, Stillwater, OK, 74078 USA
NOTE: Corresponding author: John O’Hara, oharaj@okstate.edu

Abstract – Highly accurate atmospheric models, based on molecular resonance information contained within the HITRAN
database, were used to simulate the propagation of high capacity single‑carrier quadrature amplitude modulated signals
through the atmosphere for various modulation orders. For high‑bandwidth signals such as those considered in this work,
group velocity dispersion caused by atmospheric gases distorts the modulated waveform, which may produce bit errors. This
leads to stricter Signal‑To‑Noise Ratio requirements for error‑free operation, and this effect is more pronounced in high‑order
modulation schemes. At the same time, high‑order modulation schemes are more spectrally ef icient, which reduces the band‑
width required to maintain a given data rate, and thus reduces the total group velocity dispersion in the link, resulting in
less distortion and better performance. Our work with M‑ary quadrature amplitude modulated signals shows that optimal
selection of modulation order can minimize these con licting effects, resulting in decreased error rate, and reducing the perfor‑
mance requirements placed on any equalizers, other dispersion‑compensating technologies, or signal processing hardware.
Keywords – Atmospheric modeling, bit error rate, chromatic dispersion, millimeter wave communications, quadrature
amplitude modulation, terahertz communications

1. INTRODUCTION For example, in 2010, Hirata et al. demonstrated a wire‑
less link operating at 120 GHz, using Binary Phase Shift
Wireless data rates have risen dramatically over the last
Keying (BPSK) that achieved an error‑free data rate of 10
decade, and are projected to continue to do so over the
Gb/s over 5 km [7]. In 2013, Takahashi et al. also
decade to come [1, 2]. This growth has been fueled by de‑
demonstrated a 10‑Gb/s, error free link at 120 GHz, using
mand, created by consumer expectations as well as new
Quadrature Phase Shift Keying (QPSK, or 4‑QAM), over a
technologies such as virtual reality, high‑de inition video
distance of 170 m [8]. However, their calculations in‑
streaming, and (most signi icantly) the Internet of Things
dicated the link could conceivably span up to 2 km. The
(IoT) [3, 4]. This growth has been enabled by the deve-
same year, another wireless link was demonstrated, this
lopment of devices capable of operating at progressively
time at 140 GHz, using 16‑QAM to achieve 10 Gb/s over
higher frequencies and bandwidths. Wireless systems −6
1.5 km, with an error rate of 10 [ 9].
operating at several gigahertz are commercially
available off‑the‑shelf, and networks operating at several
In 2017, another communication link centered at 94 GHz,
tens of gigahertz (millimeter wave) are just on the
using 8‑QAM, achieved a data rate of 54 Gb/s, with an
verge of becoming so. The inevitable next step is −3
error rate of 3.8 × 10 , over 2.5 km [10]. Also in 2017,
systems operating at sub‑millimeter wavelengths, that
Kallfass et al. presented a review of their experimental
is, hundreds of gigahertz [5]. This is frequently
work with point‑to‑point millimeter wave links which in‑
recognized as the beginning of the terahertz
cluded an E‑band link (between 60 and 90 GHz, carrier
communication bands. These bands have been slow in
frequency not speci ied) and a 240 GHz link [11]. The
development for many years, in part due to the challenge
E‑band link used QPSK, 8‑QAM, and 16‑QAMs, and achieved
of atmospheric absorption and in part due to the
data rates in the range of 4 Gb/s up to 21 Gb/s, over
technological dif iculties arising from the fact that few
ranges between 4.1 km and 36.7 km, under various
devices are naturally active in these frequencies. −3
weather con‑ ditions with error rates below 4.8 × 10 .
The 240 GHz link used QPSK modulation, and achieved
However, the so‑called “terahertz gap” is beginning to
−5
64 Gb/s over 0.85 km, with an error rate of 7.9 × 10 .
close [6]. Recent progress in terahertz devices has re‑
Many different link con igurations were investigated in
sulted in hardware not only capable of producing and
the review, and the reader is referred to the work of
processing these high‑speed signals, but also powerful
Kalfass et al. for more detailed information [11].
enough to overcome the atmospheric attenuation, which
is much more severe than at microwave frequencies. Over
the last decade, several prototype terahertz communica‑ Finally, Wu et al. also demonstrated a long‑distance wire‑
tion systems have been demonstrated, operating in the less communication system at 140 GHz in 2017, which
hundreds of gigahertz, achieving communications over spanned 21 km and used 16‑QAM to achieve 5 Gb/s with
multi‑kilometer distances. effectively error‑free operation (a bit‑error rate below
© International Telecommunication Union, 2021 21