IEEE Solid-States Circuits Magazine - Fall 2020 - 26

the signal frequency changes from
150 to 200 GHz. This is in contrast to
the increase from 11.31 to 11.38 when
the signal frequency rises from 700 to
750 GHz. The corresponding dispersion verifies the trend where the dispersion drops from 7.04 ps/ ^km $ nmh
at 140 GHz to 0.69 ps/ ^km $ nmh at
800 GHz. Therefore, a higher operating frequency is preferred for a wider
signal bandwidth with less dispersion. However, another consideration
is the mode conversion loss. At higher
frequencies, multiple signal propagation modes can be supported, leading
to a potential mode conversion loss.
Therefore, to avoid mode conversion,
it is preferred to transmit in the singlemode frequency range, i.e., lower
than the cutoff frequency of a higher
mode, which is approximately 500 GHz
for this waveguide [40].
Figure 6(b) compares SNR degradation versus channel length due to
channel loss (assuming 10 dB/m) and
dispersion [assuming 1 ps/ ^km $ nmh]
for 10- and 30-GHz signal bandwidths.
The SNR due to the loss drops linearly with distance, and, due to channel dispersion, it declines gradually
versus distance L by 20 log ^L h , based
on (4). The intercept point resulting
from the attenuation and dispersion
constraint is roughly a few meters, corresponding to the EI reach limit. At longer distances, the SNR is constrained

Detour From the Dispersion
Constraint: Multiband Structure
Due to its strong bandwidth dependence, the dispersion effect can be

-G

60
6

SNR (dB)

-G

40
Dispersion

10
Epsilon

mitigated through channelization by
transmitting data through different
logical channels while m
- aintaining
large aggregate data rates. Each channel bandwidth requirement is reduced
and thus able to reach a longer distance. There are some forefront re---
search activities in this field, as shown
in Figure 7. The first full-duplex,
plastic waveguide-based link was
demonstrated in 2011 [57]. We have
developed an orthomode DWG to
simultaneously support two orthogonal channels through the same physical link, with an overall data rate of
16.9 Gb/s [39], [41]. A micromachined
orthomode transducer (OMT) is created across a polymer microwave
fiber to support 5.5- and 6-Gb/s twodirection data rates at 120 GHz across
8 m [111]. A high-density polyethylene
(HDPE)-based plastic waveguide is fed
by dipole/slot-coupling structures
to provide polarization-orthogonal
channels, aiming to double the bandwidth density at the E band [58].
However, in addition to the benefit of multichannel transmission to
boost bandwidth density, the cost
and challenges are also apparent. Having more channels on the
same physical link amplifies channel crosstalk, degrades the SNR, and
increases the BER, or it demands a
higher signal power and more signal processing to combat crosstalk

by the channel loss, i.e., signal attenuation. At shorter distances, the SNR is
limited by the dispersion, therefore
restricting the supportable bandwidth.
Besides, a larger bandwidth leads to a
much greater SNR degradation due to
dispersion rather than attenuation.
In addition to waveguide and material dispersion, other components
and design nonidealities contribute
to signal dispersion. For example,
active device bandwidth limitation
also has frequency-dependent features, thus causing dispersion. The
good news is that semiconductor
development has advanced so much
that active device bandwidth capabilities are not the limiting factors in
electronics systems. The more crucial
causes of dispersion come from offchip interfaces, where active components meet passive channels and the
resulting mismatches and reflection.
Field research advances are still not
sufficient, which impedes progress
with the interconnect bandwidth
density. Therefore, to ultimately support ultrawide bandwidth density
interconnects, the ultrawideband
interface design is also a bottleneck
to address.

Channel Attenuation
Channel Dispersion

wid

10-GHz Bandwidth
30-GHz Bandwidth

-20
-40

4
100

200

300

400 500 600
Frequency (GHz)
(a)

700

0
800

-60

4
6
8
Communication Distance (m)
(b)

FIGURE 6: (a) Simulated dielectric constant and dispersion versus signal frequency. (b) SNR degradation versus communication distance for
different signal bandwidths, due to two key factors: signal attenuation and dispersion.

FA L L 2 0 2 0

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IEEE Solid-States Circuits Magazine - Fall 2020

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