Page 38 - 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. 3 – Reverse waterfall plot for M‑QAM links, M = 4, 16, 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. The fact that the 40 dB, 50 dB, and in inite SNR curves are almost indistinguishable because beyond 40 dB, dispersion is the
dominant source of errors, rather than noise.

(according to point ‘c’ on Fig. 3). However, increasing the this is how future terahertz links should operate;
modulation order to 64 offers improvement according to indeed, variable bandwidth channels would be both an
Fig. 4, reducing the error rate to 4.7 × 10 −5 ( point ‘b’ on the engineering and a regulatory challenge, and probably are
plot). In this case, the improvement again relies on the not appropriate for most circumstances. Rather, we
increased spectral ef iciency of the higher order chose to present our data in this manner because
modulation scheme, with the stipulation that the SNR be allowing bandwidth to vary with modulation order
40 dB or greater. This exacting constraint on SNR arises makes the counter‑intuitive behavior of the spectrally‑
from the small spacing between symbol decision ef icient, low bandwidth links (that is, 256‑QAM) the
boundaries for the higher‑order link, made even stricter most clear and explicit. This does not change our
by the fact that dispersion, though reduced, has still simulation results; it is just a data‑ presentation choice.
shifted some of the received symbols closer to the Varying the bandwidth of the link was the best way to
decision boundaries. show that decreasing the bandwidth does not necessarily
decrease the error rate, and that modulation type
becomes an important factor due to atmospheric GVD.
Thirdly, notice from Fig. 2 that, for the 60 Gb/s case pre‑
sented here, there are some modulation schemes that, in
general, constitute poor choices for a link without dis‑
Finally, all discussion up until this point has been focused
persion compensation. Namely, a 256‑QAM scheme has
on communication links in which dispersion is
a worse error rate at all distances and SNRs than ei‑
uncompen‑ sated. Though GVD has not historically
ther 16‑QAM or 64‑QAM (with the exception of a slight
been a concern for wireless communication systems
and insigni icant region around 14 km in the noise‑free
(owing to the com‑ paratively narrow bandwidth of
case, where it has performance marginally better than
legacy microwave com‑ munication links), it has been
the 16‑QAM scheme). While BPSK and 4‑QAM do under‑
extensively investigated in iber optics, where
perform 256‑QAM over long distances with higher SNR,
dispersion‑compensating technology is relatively
there are also two other modulation types (16‑QAM and
mature. Additionally, there are other forms of temporal
64‑QAM) that outperform 256‑QAM in nearly all situa‑
dispersion that have been identi ied, studied, and
tions, so while 256‑QAM is an improvement over some
compensated in existing wireless links, which often
modulation schemes, it is never the best choice (and this
arise from multipath propagation. Although atmospheric
holds true for all higher SNRs as well).
GVD is a new phenomena for wireless links, dispersion
in general is not. This may lead some to think that since
At this point in the discussion, there are a few assump‑ dispersion can and has been compensated by both pho‑
tions that need to be addressed. One point of concern tonic [27] and electronic [28, 29] means, then these tech‑
may be that in most applications, the bandwidth of a nologies would be readily adapted for use in a terahertz
wireless link is ixed and the data rate varies with wireless communication system. Speci ically, it might be
modulation order, while in the results we present, the assumed digital signal processing ilters, also known as
band‑width varies with modulation order while data equalizers, will be able to compensate GVD and thus ren‑
rate is held constant. However, we are not proposing der the problem of GVD irrelevant.
26 © International Telecommunication Union, 2021