IEEE - Aerospace and Electronic Systems - December 2022 - 6

Joint Multi-UAV Deployments for Air-Ground Integrated Networks
Static multi-UAV BS deployment: The static UAV functions
as a fixed aerial BS and serves the users within its communication
coverage. The coverage area of each UAV can
be seen as a microcellular, where the users can exchange
information via the corresponding UAV BS. The distances
between the UAVs should be long enough, so that the communications
ofan UAV will not be disturbed by the adjacent
UAVs. In addition, the UAVs should be deployed according
to the distribution of the users to ensure the UAV BSs can
cover as many users as possible and provide high-quality
communication services to users. Therefore, the key to the
deployment of the static UAV BSs is to optimize the threedimension
position of the UAVs under the constraints of
energy and coverage ofeach UAV as well as the communication
performance required by each user.
Mobile multi-UAV BS deployment: The mobile UAV
serves the users as a dynamic aerial BS, which can improve
the service quality by flying close to the users who need information
transmissions. As the position of the UAV changes
constantly, the air-to-ground channel is time varying. To
improve the communication performance, the flight trajectory
of the UAV should be optimized according to the channel
conditions. Since the mobile UAV BSs have no fixed serving
users and each user can access any UAV, the user scheduling
needs to be optimized to improve the service quality and
decrease the interuser interference. Therefore, joint optimization
ofUAV trajectory and user scheduling should be considered
in the deployment ofmobile UAVBSs.
UAV RELAY
When the ground communication link is blocked by the
shadow fading, due to the aerial LoS link, the UAV as a
cooperative relay can forward the source information to the
destination. Compared with the ground relay, the UAV relay
can flexibly choose the cooperation location with the best
channel states, thus improving the cooperative communication
performance. There are two deployment schemes for
the multi-UAV relays as follows.
UAV multihop relay: When the distance between the
source user and the target user is relatively far, multiple
UAVs are connected in turn to form a multihop cooperative
communication link. The UAV can relay the source
information by AF mode or decode-and-forward (DF)
mode. The distance between UAVs should be selected
properly. If the distance is too large, the relaying performance
of the UAV will be reduced; while the number of
cooperative UAVs will be increased if the distance is
very small. In addition, the air-to-air channel accords
with the LoS loss, whereas the air-to-ground channel is
affected by the scattering and fading near the ground
users. Thus, the optimal deployment should consider
different channel types and reduce the number of cooperative
UAVs by optimizing the cooperative locations
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of the UAVs with the best cooperative communication
performance.
UAV formation relay: When the transmit power of a
UAV is low, multiple UAVs can formate to form a multirelay
system to forward the source information. Since the
UAV formation and the ground terminals form a virtual
multi-input multioutput (MIMO) system, the relaying performance
of the UAV formation will be significantly
enhanced [19]. The MIMO capacity of the cooperative
link depends on the placement of the UAV formation,
which is directly affected by both the source-to-UAV and
the UAV-to-destination links. Therefore, the deployment
of the UAV formation should be optimized to maximize
the cooperative throughput. Furthermore, the transmit
beamforming and OSTBC can be applied to the UAV formation
to further improve the cooperative communication
performance.
STATIC MULTI-UAV BS DEPLOYMENT
When the ground BS is damaged, the UAVBS with stronger
environmental adaptability can provide reliable high-speed
communication services to the ground users through proper
operation and deployment. If a quantity of static users need
to be served, multiple static UAV BSs can be deployed. To
achieve better communication performance under the circumstances
of limited energy and resources, the optimal
three-dimensional deployment locations of static UAVs
should be determined according to the distribution ofground
users. The static UAVBS deployment should get the following
information: 1) the number ofdeployedUAVBSs; 2) the
deployment locations ofUAV BSs; 3) the expected service
levels ofUAVBSs.
According to the maximum number of users that
accessing each UAV and the total number of ground
users, we can estimate the number of deployed UAV
BSs. There are two methods to estimate the number of
UAVs. One is to get the coverage radius by setting the
optimal height of UAV and determine the number of
UAV BSs according to the total coverage area and the
maximum coverage radius of UAV. The other is to
estimate the number of UAV BSs according to the
maximum capacity of UAV BS and the total service
capacity of ground users [20]. Assuming that the maximum
capacity of the UAV BS is C and the user rate is
r, the maximum number of accessing users is q ¼ C
Therefore, the initial number of deployed UAV BSs is
r .
N ¼ NT
q ,where NT is the total number of ground
users. By using the K-means method, the NT ground
users are divided into N clusters, each of which is
managed by one UAV BS, as shown in the deployment
of Figure 2. Assume the horizontal location coordinate
of each UAV is ðv; mÞ, the location coordinate of each
clusteruseris ðxk;ykÞ, and the maximum coverage
IEEE A&E SYSTEMS MAGAZINE
DECEMBER 2022
IEEE - Aerospace and Electronic Systems - December 2022

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