11.8.4 Why Do Radio Signals Sometimes Fade or Suddenly Become Much Stronger?
- What Is Fading?
- Why Doesn't a Radio Signal Follow Just One Path?
- What Is Multipath Propagation?
- Why Do Signals Sometimes Cancel One Another?
- Why Can Moving Only a Small Distance Make Such a Big Difference?
- Does the Ground Cause Fading?
- What Is Shadowing?
- Why Is Mobile Communication Particularly Affected?
- How Do Engineers Reduce Fading?
- Can Fading Ever Be Beneficial?
- Does Fading Affect Satellite Communication?
- Why Is Understanding Fading Important?
- What Should You Remember?
Short Answer
Radio signals rarely arrive at a receiver by only one path. Instead, they often reach the receiving antenna after travelling along several different routes, including direct paths, ground reflections, building reflections, and scattering from surrounding objects. Because these signals arrive with different phases, they may reinforce one another or partially cancel each other. The result is fading—the continual variation of received signal strength with time or location. Understanding fading is essential because it affects the reliability of virtually every wireless communication system, from mobile phones and Wi-Fi networks to satellite communications and radar.
What Is Fading?
Most people expect a radio signal to become steadily weaker as distance increases.
In reality, received signal strength often fluctuates significantly.
A receiver may indicate an excellent signal one moment and a much weaker signal only seconds later, even though neither the transmitter nor receiver has changed.
These variations are known collectively as fading.
Fading is one of the most common characteristics of radio communication and is encountered in almost every wireless system.
Fortunately, engineers understand the causes of fading and have developed numerous techniques to minimise its effects.
Why Doesn't a Radio Signal Follow Just One Path?
If communication occurred only in free space, a single radio wave would travel directly from the transmitting antenna to the receiver.
The real world is far more complicated.
As the transmitted wave propagates, part of its energy may:
- travel directly to the receiver;
- reflect from the ground;
- reflect from buildings;
- reflect from water;
- scatter from trees or vehicles;
- diffract around hills; or
- scatter from irregularities in the atmosphere.
Each of these paths has a different length.
Consequently, the various copies of the signal arrive at slightly different times.
The receiver therefore collects several versions of the same transmission simultaneously.
What Is Multipath Propagation?
The arrival of radio signals by multiple paths is known as multipath propagation.
Multipath is one of the defining characteristics of wireless communication.
Even in apparently open countryside, the receiver often detects several reflected signals in addition to the direct wave.
In cities, where numerous buildings, vehicles, and other structures produce reflections, dozens or even hundreds of propagation paths may exist simultaneously.
Modern communication systems are specifically designed to operate under these conditions.
Why Do Signals Sometimes Cancel One Another?
Radio waves possess both amplitude and phase.
When two waves arrive together, their phases determine the resulting signal strength.
If the waves arrive in phase, their amplitudes combine, producing a stronger received signal.
This is known as constructive interference.
If they arrive out of phase, one wave partially or completely cancels the other.
This is called destructive interference.
Even a very small change in antenna position may alter the phase relationship sufficiently to transform a strong signal into a weak one.
This continual interaction between multiple propagation paths is the principal cause of fading.
Why Can Moving Only a Small Distance Make Such a Big Difference?
At many frequencies, moving the receiving antenna only a fraction of a wavelength changes the relative phases of the arriving waves.
For example, at 900 MHz the wavelength is approximately 33 cm.
Moving the antenna only a few centimetres may completely alter the interference pattern.
This explains why:
- moving a mobile phone slightly may improve reception;
- Wi-Fi signal strength varies across a room;
- vehicle radios experience rapid fluctuations while driving; and
- handheld radios often work better when repositioned.
The phenomenon is particularly noticeable at microwave frequencies because the wavelengths are so short.
Does the Ground Cause Fading?
Yes.
One of the most important reflected signals is the ground-reflected wave.
In many communication links, particularly microwave and VHF systems, the receiver simultaneously receives:
- the direct signal; and
- a signal reflected from the Earth's surface.
Depending upon their relative phase, these signals may reinforce or cancel one another.
As antenna heights, operating frequency, or path length change, the phase relationship also changes.
Ground reflections therefore produce alternating regions of strong and weak reception along the propagation path.
This behaviour forms the basis of the two-ray propagation model, which is widely used when analysing terrestrial radio links.
What Is Shadowing?
Not all fading is caused by interference.
Sometimes large objects simply block part of the transmitted energy.
Buildings, hills, forests, and even large vehicles may reduce signal strength considerably.
This phenomenon is known as shadowing or slow fading.
Unlike multipath fading, which changes rapidly over short distances, shadowing usually varies over much larger distances.
A mobile phone travelling behind a large building, for example, may experience a noticeable reduction in signal strength until it emerges into a clearer propagation path.
Why Is Mobile Communication Particularly Affected?
Mobile communication systems continually experience changing propagation conditions.
As the user moves:
- propagation paths continually change;
- new reflections appear;
- existing reflections disappear;
- buildings create temporary shadowing;
- vehicles generate additional reflections; and
- the surrounding environment changes continuously.
Consequently, received signal strength may fluctuate many times each second.
Modern cellular systems constantly monitor these changes and automatically adapt transmitter power, modulation, coding, and data rate to maintain reliable communication.
How Do Engineers Reduce Fading?
Communication engineers employ numerous techniques to reduce the effects of fading.
These include:
- increasing antenna height;
- using directional antennas;
- providing additional fade margin;
- employing antenna diversity;
- frequency diversity;
- time diversity;
- adaptive modulation and coding;
- equalisation;
- error-correcting codes; and
- multiple-input multiple-output (MIMO) antenna systems.
Rather than attempting to eliminate multipath completely—which is usually impossible—modern systems are designed to exploit it wherever possible.
For example, MIMO technology uses multiple propagation paths to increase data throughput rather than treating them purely as a problem.
Can Fading Ever Be Beneficial?
Surprisingly, yes.
Although early communication systems regarded multipath solely as a source of interference, modern digital communication often turns it into an advantage.
Technologies such as:
- MIMO;
- beamforming;
- OFDM;
- spatial multiplexing; and
- diversity combining
allow receivers to process multiple propagation paths simultaneously.
Instead of selecting only the strongest signal, modern receivers combine information arriving along several different paths to improve both reliability and data throughput.
In this sense, today's wireless systems frequently exploit multipath rather than simply trying to avoid it.
Does Fading Affect Satellite Communication?
Satellite communication generally experiences much less multipath fading than terrestrial systems because most of the received energy follows a single direct path.
However, fading can still occur.
Signals may reflect from nearby buildings, water, or terrain before reaching the receiving antenna.
Rain, clouds, and atmospheric irregularities may also introduce additional attenuation.
For low-Earth-orbit satellites, changing geometry continually alters propagation conditions.
Consequently, satellite engineers also include fade margins and adaptive transmission techniques when designing communication links.
Why Is Understanding Fading Important?
Virtually every wireless communication system must operate in the presence of fading.
Without understanding its causes, engineers could neither predict communication reliability nor design systems capable of maintaining acceptable performance.
Modern communication technology succeeds largely because it anticipates fading rather than attempting to ignore it.
The ability to manage multipath propagation is one of the principal reasons why today's mobile phones, Wi-Fi networks, satellite links, and wireless broadband systems achieve such impressive reliability despite operating in extremely complex propagation environments.
What Should You Remember?
- Fading is the variation of received signal strength caused by changing propagation conditions.
- Most receivers collect several copies of the same signal arriving by different propagation paths.
- Multipath propagation results from reflections, diffraction, and scattering.
- Signals arriving in phase reinforce one another, while signals arriving out of phase produce destructive interference.
- Ground reflections are one of the most important causes of fading in terrestrial radio systems.
- Shadowing occurs when large objects partially block the transmitted signal.
- Modern communication systems use techniques such as MIMO, diversity, adaptive modulation, and error-correcting codes to minimise—or even exploit—the effects of fading.
- Understanding fading is essential for the design of reliable wireless communication systems.