What Is a Free-Space Display?

The free space propagation model is the simplest model of radio wave propagation. The loss of a radio wave is only related to the propagation distance and the frequency of the radio wave. When the frequency of a signal is given, it is only related to the distance. In the actual propagation environment, the environmental factor n must also be considered, and the formula is simplified as L = 38.45 + 10 * n * log (dBm). n Generally, it can be between 2 and 5 depending on the environment.

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When radio waves encounter smooth interfaces between two different media, reflections occur. If a plane wave is incident on the surface of a dielectric, part of the energy enters the second medium, and part of the energy is reflected back to the first medium without energy loss. If the second medium is an ideal reflector, all incident energy is reflected back to the first medium.

In wireless communication, because the earth and the atmosphere are different media, the incident wave will be reflected on the interface, as shown in Figure 1.

When the diffraction is such that the receiver is in the shadow area of the obstruction, the diffraction field still exists and often has sufficient strength.

The diffraction phenomenon can be explained by the Huygens principle, which states that all points on the wavefront can be used as a point source to generate secondary waves, which combine to form a new wavefront in the direction of propagation. Diffraction is formed by the propagation of secondary waves into the shadow area. The field strength of the diffracted wave in the shaded area is the vector sum of all secondary waves surrounding the obstacle.

The Fresnel zone represents a continuous area where the length of the secondary wave path from the transmitter to the receiver is greater than the total line-of-sight path length, as shown in Figure 2. The figure shows a Fresnel zone, which is a transparent plane located between the transmitter and the receiver. The concentric circles indicate that the secondary waves from adjacent circles reach the receiver. These rings are called Fei Niel.

In wireless communication systems, the strength of the wireless signal received at the receiving end is higher than that estimated using the diffraction and reflection models. The main reason is that the radio wave encountered a rough surface during the propagation process, causing the radio wave to scatter and spread In all directions.

In order to more accurately estimate the characteristics of radio wave propagation, it is necessary to determine the surface roughness. Given the incident angle i, the reference height hc defining the surface flatness is:

If the maximum protruding height h on the plane is less than hc, the surface is considered smooth, otherwise it is rough.

For rough surfaces, corrections need to be made to the reflection coefficient. The surface height h is a random variable with a Gaussian distribution of the local mean,

for:

In the formula:

Is the standard deviation of the surface height; I0 is the first kind of zero-order Bessel function.

application:

In the actual wireless environment, as long as the wireless signal is unblocked in the first Fresnel zone, it can be considered to propagate in free space. In this way, when estimating the propagation loss, it can be very simple.