Page 39 - 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

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