RADIO PROPAGATION
Contents
A wireless channel is a physical medium that transports signals from a transmitter to a receiver. Unlike a wired channel, which is predictable and steady, a wireless channel is extremely dynamic. It is distinguished by phenomena such as path loss, fading, and multi-path propagation, which can substantially decrease signal quality and make reliable communication difficult. Radio wave propagation is the study of how electromagnetic waves travel from a transmitting antenna to a receiving antenna. This is a complex process influenced by the environment between the two points. The interaction of radio waves with their surroundings is categorized into three main mechanisms:Reflection, diffraction, and scattering1. Reflection
Reflection occurs when a radio wave strikes
a large, flat surface and bounces off it. For reflection to happen, the size of
the reflecting object must be significantly larger than the wavelength of the
radio wave. This is a common mechanism in urban environments where waves bounce
off large structures like buildings and the ground. The angle of the reflected
wave is equal to the angle of the incident wave, following the principle of
Snell's Law. This phenomenon can create multiple signal paths, which can lead
to constructive or destructive interference at the receiver.
Snell's Law
Snell's Law (Law of Refraction) describes the relationship between the angles of incidence and refraction for a wave, such as light or a radio wave, as it passes through the boundary between two different media.
Figure 2: Snell's Law
2. Diffraction
Diffraction is the bending of radio waves around an obstacle's sharp edges as shown in Figure 2 below. This method enables radio signals to areas where there are no
direct line-of-sight paths available. For example, diffraction allows a signal
to reach a receiver behind a building or a hill. The degree of diffraction is
determined by the wave's wavelength as well as the obstacle's size and form.
The "knife-edge" diffraction model is a standard method for analyzing
this effect. Without diffraction, mobile connectivity in cities with towering
buildings would be impossible
Figure 3: The Knife Edge
3. Scattering
Scattering happens when traveling waves encounter a change in the wave impedance and then reflect partially. And if the reflection is not total, it will also partially transmit into the new impedance. This is called scattering of the traveling wave. This is called when a radio wave strikes an item of the same size as its wavelength or when an object's surface is rough.
Reflection and diffraction are linked with
large-scale obstacles, while scattering affects smaller objects. When a wave
strikes these small objects, it scatters in multiple directions. The scattering
often reduces the main signal, which is necessary in complicated indoor and
outdoor contexts because it provides extra signals, although weaker, that help
the receiver pick up the signal.
4. Large Scale Path-Loss Models
Path loss is the reduction in power density of an electromagnetic wave as it
propagates through space. But large-scale path are used to predict the average signal strength over a large area, for distances measuring over 1 kilometer. Some of these loss models
- Free-Space Path Loss Models
The Free-Space Path Loss Model is considered the simplest loss model that assumes; direct line-of-sight without any any obstacles to cause reflection, diffraction, or scattering. The model is useful for scenarios like satellite communication or microwave links.
The path loss (Lfs) is given by the formula:
Lfs(dB)=10log10(PrPt)=20log10(λ4πd)
Here, Pt is the transmitted power, Pr is the received power, d is the distance between the antennas, and λ is the wavelength of the signal. The received power in this model decreases with the square of the distance (d2).
- Two-Ray Ground Reflection Model
The Two-Ray Ground Reflection Model considers two signal paths arriving at the receiver direct line-of-sight ray and a ray that reflects off the ground. It is considered more accurate. The received signal is the vector sum of the two rays. Due to the phase shift upon reflection and the difference in path length, these two rays can interfere. At distances much greater than the antenna heights, the received power (Pr) simplifies to:
Pr=d4PtGtGrht2hr2
where
Gt and Gr are the gains of the transmitting and receiving antennas, and ht and hr are their respective heights. This model shows that for large distances, the received power decreases with the fourth power of the distance (d4), which is a much steeper drop-off than in free space.
No comments:
Post a Comment