The Bridge - Issue 3, 2021 - 27

Feature
Facilitating Satellite-Airborne-Balloon-Terrestrial
Integration for Dynamic and Infrastructure-less Networks
* A set of ground users that contain user IDs, twodimensional
locations, and QoS requirements.
* A set of fixed gateways that contain the gateway
IDs and two-dimensional locations.
The major challenge is managing the ground users'
and stations' resources. The ground users' data rate
must be maximized considering the (i) bandwidth and
power constraints, (ii) association constraints, and (iii)
HAP- and TB-placement constraints. In other words, the
ground users' throughput depends on several factors
such as the maximum ground users' allowable transmit
power, TBs' and HAPs' available bandwidth, and
station placement. Therefore, control links between the
stations and corresponding users are required to allow
stations to track users' locations under its coverage
area and manage resources. The utility rate of the
system can be characterized by various utility metrics,
the selection of which can be based on required
fairness levels. Some examples are (i) sum rate utility
(maximize the sum rate throughput of all users), (ii)
minimum rate utility (maximize the minimum user
throughput), and (iii) proportionally fair rate utility
(maximize the geometric mean of the data rate).
IV. RESOURCE MANAGEMENT
The key goal is to attain high throughput of ground
users and energy efficiency. Several metrics can
be implemented to achieve this goal, such as (1)
minimizing the total consumed energy while satisfying
a certain user's throughput, or (2) maximizing the
energy efficiency utility. In this context, several highlevel
research questions need to be addressed:
* Resource Optimization: How to optimize the transmit
power allocation of the users and various types of
stations and, given a certain available bandwidth, how
to allocate this bandwidth for the control links (for
management) or serving links (i.e., access and backhauling
links)
* Associations: How to configure (a) access link
associations (the associations between users and
GBSs, TBs, and HAPs) and (b) the back-hauling link
associations (the associations of GBSs and TBs with
HAPs and between HAPs and gateways), as shown in
Figure 2.
* HAPs and TBs Placement: How to find the
best locations for HAPs and TBs considering backhauling
linkquality.
* FSO Alignment: How to optimize the FSO alignment
angles between different FSO transceivers.
The RF and FSO choices will depend on several factors,
such as environmental or weather conditions and the
feasibility of LoS. Note that the use of a hybrid FSO/
RF link will be for back-hauling links, while the RF
band only can be used for the access link due to the
difficulties of tracking the movement of ground users
and maintaining the LoS, as shown in Figure 2.
A. Access Link Optimization
In this work, we propose using multiple stations
(i.e., GBSs, RE TBs, and HAPs) to provide wireless
connectivity to multiple ground users. Because we
involve different types of stations, the access link can
be established based on the form of communications.
(i) For uplink, two possible links can be established:
ground users to GBS or ground users to TBs. (ii) For
downlink, three possible links can be established: HAP
to ground users, GBS to ground users, or TB to ground
users, as shown in the dashed lines in Figure2. In this
case, the mathematical formulation should include an
access link binary variable to indicate that ground users
are associated with certain stations for the specific form
of communications (either downlink or uplink). For
simplicity, we assume that each user can be associated
with one station at most; however, each station can
be associated with multiple users. For station peak
power and user peak power, an optimization problem
can be formulated that maximizes user utility given
the following constraints: (i) back-hauling bandwidth
and rate, (ii) station and user peak powers, (iii) access
link associations, and (iv) user QoS. Therefore, the
following parameters can be optimized to achieve
the best objective function: (i) the transmit power
levels of the user and station transmission power, (ii)
bandwidth allocation to each user, and (iii) access link
associations.
Other factors can play significant roles in determining
the access link associations: first, the back-hauling
available rates, where a user can be associated with a
distant station if it has a good back-hauling rate rather
than being associated with a nearby station with a
low back-hauling rate, and second, the energy stored
in TBs because they are assumed to be RE batterypowered
stations and the amount of stored energy by
each TB at the end of a given time slot is considered
an additional limitation. In this case, each TB should
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The Bridge - Issue 3, 2021

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