Autonomous Airborne Wireless Networks

Table 2.2 Measurement campaigns to characterize the path loss and large‐scale AG propagation fading.

References	Scenario		(dB)	(dB)
Yanmaz et al. [8]	Urban/Open field	2.2–2.6	—	—
Yanmaz et al. [9]	Open field	2.01	—	—
Ahmed et al. [10]	—	2.32	—	—
Khawaja et al. [11]	Suburban/Open field	2.54–3.037	21.9–34.9	2.79–5.3
Newhall et al. [12]	Urban/Rural	4.1	—	5.24
Tu and Shimamoto [13]	Near airports	2–2.25	—	—
Matolak and Sun [14]	Suburban	1.7 (L‐band)	98.2–99.4 (L‐band)	2.6–3.1 (L‐band)
		1.5–2 (C‐band)	110.4–116.7 (C‐band)	2.9–3.2 (C‐band)
Sun and Matolak [15]	Mountains	1–1.8	96.1–123.9	2.2–3.9
Meng and Lee [16]	Over sea	1.4–2.46	19–129	—

where and are the constant values related to the environment, is the elevation angle between the ground user and the UAV, is the altitude of the UAV, and is the distance between the ground projection of the UAV and the ground device. According to Eq. ( 2.6), as the elevation angle increases with the UAV altitude, the blockage effect decreases and the AG propagation becomes more LoS. An advantage of this model is that it is applicable for different environments and for different UAV altitudes. However, it is unable to capture the impact of path loss for AG propagation in mountainous regions and over water bodies due to the lack of information related to their statistical parameters.

Conventional well‐known channel models for cellular communications can be used for UAV communications for UAV altitude between 1.5 and 10 m. One such model for the macro‐cell network was designed for the rural environment by the 3rd Generation Partnership Project (3GPP) in [7,37].

Since LoS and NLoS links are treated separately, the probability of LoS propagation is expressed as

(2.7)

Path loss and large‐scale fading can be calculated once the LoS probability is known from Eq. ( 2.7). As the communication nodes change their position, path loss also changes and can be found as

(2.8)

(2.9)

where

(2.10)

(2.11)

(2.12)

(2.13)

with , , , and being the carrier frequency, height of ground BS, the average width of street, and the speed of light, respectively.

For the obstructed AG propagation with the UAV altitude between 10 and 40 m, the LoS probability in the rural environment for the macro‐cell network can be computed as [7]

(2.14)

where

(2.15)

(2.16)

The path loss for LoS and NLoS links can be computed as

(2.17)

(2.18)

For a high‐altitude AG channel with , the LoS probability is 1 and the path loss can be formulated as Eq. ( 2.17).

Small‐scale fading refers to the random fluctuations of amplitude and phase of the received signal over a short distance or a short period of time due to constructive or destructive interference of the MPC. For different propagation environments and wireless systems, different distribution models are suggested to analyze the random variations in the received signal envelop. The Rician and Rayleigh distributions are widely used models in the literature of wireless communications, where both are based on the central limit theorem. The Rician distribution provides better fit for the AA and AG channels, where the impact of LoS propagation is stronger. On the other hand, when the MPC impinges at the receiver with random amplitude and phase, the small‐scale fading effect can be captured by the Rayleigh distribution [6].