Page 156 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 2

m = 0,1,2,…,M-1 j = 1,2,3,…,L L number of 1D array of
y laser receivers
laser RXs at an i th
0 1 2 M-2 M-1 steering position
r m = 0,1,…,M-1 Laser RXs at each 1D
∆x laser array
LxM Size of 2D mesh of
laser RXs from a UAV
i = -N
i = -N,-N+1,…,N Steering azimuth
L 1D arrays
j = 1,2,3,…,L
positions
r The radial distance from
the beam’s central axis
∆x Distance between
consecutive RX elements
∆y Distance between two
i = N-1
∆y ∆P 1D arrays
∆P Distance between two
Steering positions
j = 1,2,3,…,L
consecutive azimuth
i = -N, -N+1, …, 0, 1, 2, …, N
steering positions
i = N-1
j = 1,2,3,…,L
x
Fig. 3 – Top view of the RXs on the ground for laser beams from a UAV (e.g. UAV1 in Fig. 1). Laser mesh steering is used to steer the transmitted laser
beams to different RXs positions on the ground. There are 2 + 1 steering positions, and each steering position has number of 1D array of laser RXs.
The number of laser beam RXs in each 1D array is .

The main advantage of a laser is spatial coherence result‑ 2.2 Laser mesh steering

ing in higher directivity compared to radio waves. A sin‑

gle laser beam has a small coverage area due to high di‑ The coverage area of the laser mesh, i.e., the size of the net,

rectivity. Therefore, to cover a large area, multiple laser in the azimuth plane depends on the number and separa‑

beams are required. The coverage area of a single laser tion distances of the laser RXs. We will assume that the

beam at a given distance from the TX depends on the di‑ laser RXs are placed on the ground. The top view of the
vergence of the beam. The divergence of the laser beam steered laser positions (RXs) in the ( , ) plane is given in

with distance results in the broadening of the beam, as Fig. 3. In this igure, a single two dimensional (2D) laser

shown in Fig. 2. We consider that the divergence of the mesh from an airborne UAV (e.g. UAV1 in Fig. 1) is com‑

th

laser beams is small (e.g. collimated laser beams). The posed of × laser RXs at steering position. The 2D
small divergence of laser beam results in approximately mesh contains = 1, 2, 3, … , , one dimensional (1D) ar‑

constant radius of the beam with distance. In Fig. 2, Ω and ray of laser RXs, and is the number of RX elements at
′
Ω , are the solid angles of the beams, and and ′ are each array, shown in Fig. 3.

0
0

the beam waists of the two sources. Given as the radius In Fig. 3, there are 2 + 1 azimuth positions due to

of the beam, and Δ as the separation between the two beam steering. The steering centers at the azimuth po‑

beams such that Δ ≫ , the area of a mesh element in sitions are = − , − + 1, … , 0, 1, 2, … , . The dis‑

2
Fig. 2 is approximated as = (Δ ) [18]. The area of tance between the consecutive RXs is represented as Δ ,

e
the laser mesh element, , is selected depending on the the distance between two consecutive 1D arrays is repre‑
e
type of the aerial threat. sented as Δ and the distance between the two consec‑

utive steering positions is Δ . The Δ and Δ should

In our proposed approach, the TXs (and RXs) of the laser
be carefully chosen to classify the aerial targets based on
beams can be either on a satellite, a HAP, or on a medium

their dimensions. From Fig. 3, a matrix of dimensions

altitude hovering UAV . The RXs (or TXs) can be on the

×
(2 + 1) is obtained over all the steering positions.
ground or over a hovering UAV . Multiple laser beams can
If ℎ is the height of the TX from the RXs at the central steer‑
be transmitted from a single TX. A blockage to the path

ing position, = 0, = /2, the slant range, from

of a laser beam by any aerial object (taken as target) is ,

the TX to t h steering position ( ≠ 0) and t h 1D array is
readily identi ied. Localizing a target in both azimuth and
2

elevation planes based on the blockage to the path of the , = √ ℎ + (( − /2)Δ + Δ ) . The total azimuth
2

laser beams by the target will require at least two sources distance covered during steering is 2 Δ Δ . In addi‑

of laser beams as shown in Fig. 1. The laser beams from tion, the speed of the steering between any two consecu‑

two different sources in a laser mesh can be overlapping tive steering positions is given as = Δ , where Δ is
s
s
Δ s

or non‑overlapping. Different wavelength lasers will be
needed to avoid co‑channel interference in case of over‑
lapping beams.
142 © International Telecommunication Union, 2021