$\mathcal{M}$	Set of UAVs
$\mathcal{U}$	Set of users
$d_{b,u}$	3D distance between the HAP b and user u
$d_{b,m}$	3D distance between the HAP b and UAV m
$d_{m,u}$	3D distance between UAV m and user u
$(x_b,y_b)$	Horizontal location of the HAP b
$(x_m,y_m)$	Horizontal location of the UAV m
$x_{m-init}$	Initial values of $x_m$ of the UAVs
$y_{m-init}$	Initial values of $y_m$ of the UAVs
$(x_u,y_u)$	Horizontal location of the user u
$h_b$	Height of HAP b
$h_m$	Height of UAV m
$h_u$	Height of a user u
$r_{b,u}$	Horizontal distance between the HAP b and a user u
$r_{b,m}$	Horizontal distance between the HAP b and UAV m
$r_{m,u}$	Horizontal distance between UAV m and user u
$G_{b,u}^{LoS}$	LoS-channel gain between the HAP b and user u
$G_{b,m}^{LoS}$	LoS-channel gain between the HAP b and UAV m
$G_{m,u}^{NLoS}$	NLoS-channel gain between UAV m and user u
$G_T$	Transmitter antenna gain
$G_R$	Receiver antenna gain
$P_T$	Transmit power of the HAP
c	Speed of light
$f_c$	Carrier frequency
$h_{g,u}^{LoS}$	Small-scale fading factor of the LoS link between user u and the HAP
$h_{g,u}^{NLoS}$	Small-scale fading factor of the NLoS link between user u and UAV m
${\gamma }_{b,u}$	Signal to noise ratio(SNR) at the location of user u from HAP b
$N_0$	Gaussian noise power spectrum density
$a_u$	Bandwidth ratio allocation of user u
B	Available bandwidth $\forall u$
$C_{b,u}$	Achievable data rate at user u served by the HAP b
$C_{m,u}^{\prime }$	Achievable data rate at user u served by the HAP through UAV m
${\beta }_0$	NLoS link reference path loss
$\alpha$	NLoS link path loss factor
$N_m$	Number of users associated with UAV m
$N_{max}$	Maximum capacity of UAV $\forall m$
$h_{min}$	Minimum height of UAV $\forall m$
$h_{max}$	Maximum height of UAV $\forall m$
A	Users–UAVs association vector
$a_{m,u}$	A binary variable equal to 1 if user u is assigned to UAV m, otherwise 0
$q_b$	Location of the HAP b
$q_m$	Location of UAV m
$q_u$	Location of user u