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