The Bridge - Issue 3, 2021 - 26

Feature
Facilitating Satellite-Airborne-Balloon-Terrestrial
Integration for Dynamic and Infrastructure-less Networks
aerial relay stations can generate wireless connectivity
in challenging areas. Note that, TBs can be powered by
renewable energy (RE) sources, and therefore, can be
placed in remote areas to help in uplink and downlink
services. This is a plausible proposal since in remote
areas, the TBs would consume less energy by serving
fewer users. Additionally, some sleeping strategy (i.e.,
powering on-off the TBs) can be used to reduce the
overall energy consumption.
B. Hybrid FSO/RF
Compared to the current wireless networks, the
expected next generation wireless demand is
ambitious and may require throughput around 1,000
times higher with round-trip latency around 10 times
lower [16]. The radio frequency (RF) spectrum is
anticipated to be more and more congested for
emerging technologies and applications in future
wireless networks. Therefore, the RF technology
may be insufficient to accommodate the expected
increase in the demand of wireless devices. Focusing
on improving the RF spectrum only in legacy bands
may be not enough, thus it is critical to embrace a
comprehensive technology with high spectral reuse
by supplementing RF technology with other wireless
technologies in directional higher frequency bands [17].
FSO communications is considered as a promising
complementary solution with RF to meet the exploding
demand for wireless networks. FSO transceivers are
amenable to dense integration and provide spatial
reuse and security through highly directional beams.
RF's huge unlicensed spectrum presents a unique
opportunity to overcome the expected future spectrum
scarcity problem. Its potential integration with solidstate
lighting technology also presents an attractive
commercialization possibility [17]. The authors in [18]
proposed a vertical framework consisting of networked
HAPs that supported back-hauling links and access links
of small cell base stations in a multi-tier heterogeneous
network. However, that work was limited to supporting
small cells, did not consider the integration of all types
of GBSs (e.g., macro cell base stations) and satellite
stations with HAPs, and did not discuss providing
global connectivity in remote areas. This paper focuses
on infrastructure-less operation, where the HAPs and
TBs are equipped with directional FSO transceivers to
provide wireless connectivity for the back- hauling links.
HAP-TB integration offers another dimension to legacy
wireless networking by enabling spatial reuse. TBs
can effectively amplify the signals between the HAPs
and ground users using FSO links without causing
any major interference for the rest of the ground
users. If this spatial technique is not used and relaying
is used only on the RF band, then the aggregated
throughput will be limited due to (i) the scarcity of
the RF band and (ii) the possibility of interference
between HAPs, TBs ,and ground users. Therefore,
the potential of using spatial reuse is possible only
if HAPs, TBs, and gateways are equipped with FSO
directional antennas and their positioning and precision
steering of the antennas are feasible. However, the
FSO directional transceivers require not only the FSO
link establishment, but also line-of-sight maintenance
between different transceivers.
Figure 2: All possible uplinks and downlinks scenarios
III. SYSTEM MODEL
We define the following system model as a hybrid
FSO link:
* A set of stations, including a satellite, HAPs, TBs,
and GBSs, where each station contains its station
ID, 3D lo- cation, battery level at any given time (if
applicable, e.g., HAPs and TBs), and back-hauling rate.
All this information is shared with some central units.
Since the TBs are RE stations, the consumed power
during different station states, such as operating and
transmission, is included.
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The Bridge - Issue 3, 2021

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