Page 416 - Kaleidoscope Academic Conference Proceedings 2024

2024 ITU Kaleidoscope Academic Conference

coverage by the UAV swarm is A T , the number of UAVs user is at distance 0.5R c from the center of the cell, and the
required in the swarm are given as UAV is at an altitude h, the path loss experienced by the user
for a signal transmitted by the UAV can be written as
A T
SS = Ceil( ) (9)
2
π(mh + nh + o) 2 A
PL ( at r = 0.5R COV ) =
In the previous sub-section it was observed that the 1 + a exp −b arctan h − a
0.5R COV
communication power model suggests that communication (15)
power is constant with the UAV altitude. Also, for the altitude 2 2
+10 log h + (0.5R COV ) + B
range of 0 to 5000 m, the slope of the hovering power
(16)
function is so small that the increase in power with altitude
is very small. Thus, the hovering power is also assumed to The signal-to-noise-ratio (SNR) at the ground-based receiver
be constant. can thus be written as:
SNR(dB) = P T − PL − P N (17)
The total power consumed by a single UAV is denoted
by P 0 and is constant with h. Thus, the expression for the Similarly, the expression for bitrate capacity can be written as:
total power consumption as
C = B log (1 + SNR) (18)
2
A T
P = P 0 Ceil( ) (10)
2
π(mh + nh + o) 2 The transmission delay for the packet of length L bits in the
down link direction is:
D. Latency Model
L
From (6), coverage radius has been approximated, and stated T D = (19)
B log (1 + SNR)
2
as in (8). Number of users inside the UAV cell depends upon
the radius of the cell. Assuming user density to be ρ users per The propagation delay for the packet to travel from UAV at
square meters, the expression for the number of users inside elevation h, to the ground user at distance 0.5R c is expressed
the UAV macro cell is given as: as:
p 2 2
h + (0.5R c )
A T
N = ρ π Ceil( ) (11) T P = (20)
2
π(mh + nh + o) 2 c
8
It was earlier assumed that the total bandwidth available where, c is the speed of light (3x10 ).
with a UAV (B 0 ) is evenly distributed to all the users inside
the cell. Hence the channel bandwidth of a single user inside From the above expressions of propagation and transmission
the UAV macro cell is given as delay, one can deduce that both the delays are the function of
altitude, thus the overall latency is also a function of altitude,
B 0
B = (12) and expressed as:
N
Here, N denotes the number of users in the cell. Similarly, the T = T P + T D (21)
total available transmit power (in dB) is also evenly distributed
p
2
over the users, and can be expressed as h + (0.5R COV ) 2 L
T = + (22)
P T (Total) c B log (1 + SNR)
2
P T = 10 log (13)
N III. PROBLEM FORMULATION
where P T is the transmit power per user and P T (Total) is This paper formulates the optimization problem for UAV
the total available transmit power. altitute optimization, along with determination of optimal
power levels.
Similarly the noise power at the receiver can be written
as: To determine the optimal height of the flying BS, service
threshold is defined in terms of PL max . If the path loss at
P N = B k T (14)
any instance exceeds PL max , it is termed as link failure.
Here, B is the channel bandwidth of a single user in the cell, This service threshold acts as an indicator to the coverage
k is the Boltzmann constant with value 1.38X10 23 and T is radius on ground, R c , for the UAV, which is expressed as
the temperature (300 kelvin).
R c = r|(Γ = PL m ax) (23)
It was observed that the average path loss performance Substituing η as LOS and NLOS in (2), the subsequent
for the uniformly distributed users inside the cell area is expressions for PL LOS and PL NLOS can be deduced. Con-
equivalent to the path loss performance for the user at sidering (2) and (3),
0.5R c (See Appendix I). Therefore, for simplicity, network
(24)
performance for the user at 0.5R COV is evaluated. Now, if a Γ = P(LOS) ∗ PL LOS + P(NLOS) ∗ PL NLOS

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