Page 39 - 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






























          Fig. 4 – Reverse waterfall plot for M‑QAM links, M = 16, 64, over distances from 0 to 20 km with various SNRs. SNR values of 0, 10, 20, 30, 40, and 50 dB
          are designated by markers 5 k, 5, 3 6  1 respectively, and in inite SNR is denoted by a thick, solid line with no marker. Atmospheric conditions  ,  ,  , and
          are the same as in Fig. 2.


         While  equalizers  are  certainly  theoretically  capable  of   Accordingly, we have not included equalization routines
          compensating dispersion, whether they will be physically   in our simulations for concern they would produce results
          realizable for terahertz frequencies (and,  if so,  when) is   that are not necessarily realistic.  Furthermore, we wish
          still yet to be determined.  There are still questions that   to limit the scope of this paper to a characterization and
         remain to be answered before we can con idently assert   description of the GVD induced by the atmosphere,  and
         which  equalizer  architectures  will  be  most  suited  to   its interaction with modulation type.  If and when disper‑
         operation  in  the  terahertz  bands.  In  4G  architectures   sion compensating technology is implemented in future
                                                               terahertz communication systems, the judicious selection
         utilizing  orthogonal  frequency  division  multiplexing,
                                                               of modulation type will reduce the performance require‑
         equalizers  operate  on  channels  at  most  20  MHz  wide,
                                                               ments placed on such technology by utilizing modulation
         and  this  is   the   dominant   wireless   technology.
                                                               schemes  naturally  resistant  to  dispersion‑induced  bit
         However,  in  the terahertz  bands,  the  signal  bandwidth
                                                               errors.  This  could  be  especially  important  for  relaxing
         may be up to 100 GHz,  potentially  over  three  orders  of
                                                               the signal processing burden in terahertz transceivers.
         magnitude  larger!  Even  the   iber  optic  equalizers
         referenced  previously  typically  have  bandwidths  less
                                                               4.   CONCLUSION
         than  100  GHz  [30].  This  high  bandwidth  signi icantly
         complicates   ilter  design.  If  terahertz  sub‑bands  are   In this work, we leveraged highly accurate models of the
         kept  only  a  few  tens  of  kilohertz wide in order to avoid   atmosphere  to  predict  the  effects  of  uncompensated  at‑
                                                               mospheric GVD on the bit error rate of high‑capacity ter‑
         this  problem,  then  the  number  of sub‑bands  (and  thus
         equalizers)  scales  up  by  potentially  four  orders  of   ahertz links using various orders of M‑QAM. A signi icant
                                                                inding was that, due to GVD, unintuitive situations arise
         magnitude.  Further  complications  include  10  to  100
                                                               in  which  higher‑order  modulations  offer  superior  error
         times  greater  Doppler  shifts,  noise  bandwidths  two  to
                                                               rate performance than lower‑order modulations.  This is
         three orders of magnitude larger, and dispersion pro iles
                                                               contrary to what would be expected in a traditional wire‑
         that change with weather, not to mention the is‑ sues of
                                                               less link with a lower bandwidth, in which the selection
         receiver linearity, phase noise, and dynamic range which
                                                               of modulation type is dominated by the SNR alone.  It is
         are already challenges for 3G and 4G hardware [31]. While
                                                               anticipated that this will need to be taken into account by
         none of this changes the fact that dispersion is theoretically
                                                               both future link designers and adaptive modulation algo‑
         reversible,  it  does  raise  the  question:  are  current
                                                               rithms attempting to select the ideal modulation scheme
         equalization  algorithms  and  the  digital  hardware  on
                                                               for present channel conditions. A related  inding was that,
         which  they  are  implemented  capable  of  performing  the   in  uncompensated  links,  there  are  some  modulation  or‑
         task?  Current research is presently being undertaken to   ders  that  should  not  be  used  (or  are  at  least  never  the
         investigate these issues [32],  and bottlenecks related to   best  choice).    ically,  high‑order  modulations,  such
         sampling rate and signal processing limitations have been   as  256‑QAM  (and  above)  suffer  from  stringent  require‑
         identi ied [33,  34].  Presently,  it seems premature to as‑   ments on both SNR and maximum allowable symbol shift
         sume  that  the  equalization  and  signal  processing  tech‑   due to dispersion, which when combined lead to subopti‑
         nologies  we  currently  have  will  carry  over  to  terahertz   mal  performance  for  all  or  nearly  all  combinations  of
         channels without signi icant modi ication and innovation.  links distance and SNR.


                                             © International Telecommunication Union, 2021                    27
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