wifi basic (a): radio waves and WIFI signal interference, attenuation

liwen01 2024.08.18

preamble

Whether in product development or in everyday life, when using a wireless network, you will often encounter some problems with poor signal and a lot of questions:

1. Why does the internet slow down when we are on a high speed moving high speed train?
2. Why does 5G WiFi not penetrate walls as well as 2.4G?
3. Why is it that when performing iperf pull tests on WiFi, the data varies from one test to the next?
4. Why does a WiFi network slow down in an environment with lots of routers?
5. Why is it that in some large stadiums, the internet signal is good when there are fewer people and poor when there are more?
6. Why would a router placed on the living room have a bad signal in a room away from the living room?
7. Why do Bluetooth, WiFi, and microwaves interfere with each other?

To go into more depth to answer these questions above, we need to review our secondary school physics knowledge before we can provide answers to these questions.

(i) The history of light recognition

The history of human understanding of natural light is a gradual process involving the development of philosophy, physics and human science and technology.

(1) B.C. and beyond

• Pythagorean school of thought (c. 500 B.C.) The idea that light is emitted from the eye and illuminates objects around it.
• Plato and Aristotle (4th century B.C.) Different theories of optics were proposed, with Aristotle suggesting that light was an effect emitted by an object that traveled through some medium to the eye.
• Ibn al-Haytham(Alhazen, 965-1040 A.D.) In his Book of Optics, he proposed that light travels in straight lines, and for the first time experimentally studied the phenomena of reflection and refraction of light, laying down the scientific foundation of optics.

(2) Debate on the nature of light

• René Descartes (1596-1650) French philosopher (René Descartes, 1637) in therefractiveThe theory of the fluctuation of light is presented in the book, which suggests that light travels through a hypothetical medium called the "Ether".
• Christiaan Huygens (1629-1695), Dutch mathematician and astronomer (Christiaan Huygens, 1678) formulated the famous fluctuation theory that light is fluctuating and proposed Huygens' principle to explain the phenomena of reflection, refraction and diffraction of light.
• Sir Isaac Newton (1642-1727), British mathematician and physicist (Isaac Newton, 1704) proposed the particle theory that light is composed of tiny particles and that the speed of light is faster in dense media. His theory successfully explained the phenomena of linear propagation, reflection and refraction, and discovered the dispersion of white light through prism experiments.

(3) Controversy between wave theory and particle theory

• Thomas Young (Thomas Young, 1801) proved the fluctuating nature of light through double-slit interference experiments, explaining the phenomenon of interference of light and providing strong support for the fluctuation theory.
• Fresnel (name) (Augustin-Jean Fresnel, 1818) further developed the fluctuation theory of light and proposed the theory of diffraction and polarization of light.
• Maxwell (name) (James Clerk Maxwell, 1864) predicted the existence of electromagnetic waves through Maxwell's system of equations and proved that light is an electromagnetic wave with a fluctuating nature. This theory unified optics and electromagnetism and completely confirmed the fluctuating nature of light.

(4) Duality of light

• Albert Einstein (1879-1955), famous Austrian physicist (Albert Einstein, 1905) By studying the photoelectric effect, he proposed the theory of photons, which suggests that light is both fluctuating and particulate (photons), and succeeded in explaining the photoelectric effect. This laid the foundation for the quantum theory of light and helped him win the Nobel Prize.
• De Bruyne (philosopher) (Louis de Broglie, 1924) proposed wave-particle duality, arguing that not only light, but all particles have both fluctuation and particle properties. This theory further deepened the understanding of light.

(5) Modern Optics

• Quantum electrodynamics (QED):: In the mid-20th century, the development of the theory of quantum electrodynamics (promoted by Richard Feynman, Julian Schwinger, Jinichiro Asunaga, and others) succeeded in explaining the interaction of light and matter, completely unifying the wave-particle duality of light.
• The invention of the laser:: In 1960, Theodore Maiman succeeded in inventing the first laser, which utilized the coherence of light and was a major milestone in the history of optics.
• Development of Optical Technology:: Modern optics are widely used in various fields such as communications (e.g., fiber optics), medicine (e.g., laser surgery), industry (e.g., precision cutting), and scientific research (e.g., astronomical observation).

We know from the above historical process of human understanding of light:Light is an electromagnetic wave, which has the properties of wave-particle duality, direct emission, reflection, refraction, interference, diffraction, polarization, dispersion, etc.

(ii) History of the understanding of electromagnetic waves

The understanding of electromagnetic waves can be broken down intoElectricity, magnetism, electromagnetic waves Three ways to look at it

(1) Magnetic

• The Chinese in 1000 B.C. learned of the iron absorbing properties of natural magnets and used them in the manufacture of compasses, which would have been the earliest recognition and use of magnetism.

(2) Electricity

• In the 18th century, Benjamin Franklin conducted experiments and research on the nature of electricity, and introduced the concepts of "positive" and "negative" electricity.
• In 1831 Michael Faraday experimentally discovered the phenomenon of electromagnetic induction, in which a changing magnetic field can produce an electric current. This discovery further unified the concepts of electricity and magnetism.

(3) Electromagnetic waves

• 1864James Clerk Maxwell(James Clerk Maxwell) proposed a system of Maxwell's equations, which theoretically and systematically described the nature of the electromagnetic field and predicted the existence of electromagnetic waves.
• 1887 German physicistHeinrich Hertz(Heinrich Hertz) experimentally generated and detected radio waves for the first time, verifying Maxwell's theory and confirming the existence of electromagnetic waves.

(iii) Properties of electromagnetic waves

Electromagnetic wave is a fluctuating phenomenon produced by changing electric and magnetic fields perpendicular to each other and interacting with each other. It has several important properties:

(1) Volatility

• Electromagnetic waves are transverse waves: The direction of propagation of electromagnetic waves is perpendicular to the direction of vibration of the electric and magnetic fields. In free space, the electric and magnetic fields are perpendicular to each other and both are perpendicular to the direction of propagation.

(2) Speed of light

• Speed of propagation in a vacuum: Electromagnetic waves propagate in a vacuum at a constant speed, i.e., the speed of light

(3) Energy Carrying

• Wave energy is frequency dependent:: Electromagnetic waves carry energy, and energy is proportional to frequency. High-frequency electromagnetic waves (e.g., X-rays, gamma rays) carry more energy, and low-frequency electromagnetic waves (e.g., radio waves, microwaves) carry less energy.
• wave intensity: The strength of an electromagnetic wave is proportional to the square of the amplitude of the electric and magnetic fields.

(4) No media required

• Vacuum propagation:: Electromagnetic waves do not require a medium to propagate, unlike mechanical waves such as sound waves. Therefore, electromagnetic waves can propagate in a vacuum, like light, radio waves, etc. are able to travel through space.

(5) Polarization

• Linear polarization, circular polarization and elliptical polarization: Electromagnetic waves can exhibit different forms of polarization, meaning that the direction of vibration of the electric field can be fixed in one direction (linear polarization) or rotated during propagation (circular or elliptical polarization).

(6) Reflection, refraction and diffraction

• reflex (i.e. automatic reaction of organism): When an electromagnetic wave encounters an interface of a different medium, part of the wave is reflected back. The angle of reflection is equal to the angle of incidence.
• reflect (in the figurative sense: to show the nature of): When an electromagnetic wave enters another medium from one medium, it undergoes a change in velocity, resulting in a change in the direction of propagation, a phenomenon known as refraction.
• diffraction: When an electromagnetic wave encounters an obstacle or passes through a slit, the wave bends, a phenomenon known as diffraction.

(7) Interference

• Superposition effect of electromagnetic waves:: Two or more electromagnetic waves can be superimposed on each other to form an interference pattern. Depending on the phase of the waves, the interference can be either phase length interference (enhancement) or phase cancellation interference (attenuation).

(8) Spectral breadth

• electromagnetic spectrum:: Electromagnetic waves cover a range of wavelengths and frequencies from low to high frequencies, forming the electromagnetic spectrum. These include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

(9). Quantum properties

• photon (particle physics):: At the microscopic level, electromagnetic waves exhibit a particle nature and can be viewed as a stream of photons. The energy of each photon

These properties make electromagnetic waves useful in a wide range of natural and technological applications, from wireless communications to medical imaging, from lighting to energy transmission, electromagnetic waves are everywhere.

(iv) Representation of electromagnetic waves

1. Mathematical representation of electromagnetic waves

Period. The period is the time required for a complete cycle of fluctuations in an electromagnetic wave and is usually denoted by the symbol 𝑇. The period is usually measured in seconds (s). The period reflects the frequency of the electromagnetic wave; the shorter the period, the higher the frequency.

Wavelength. Wavelength is the distance between two adjacent peaks (or troughs) in an electromagnetic wave, usually denoted by the symbol 𝜆. Wavelength is usually measured in meters (m). The wavelength determines the spatial scale of an electromagnetic wave, and the longer the wavelength, the greater the wave propagation range.

Amplitude. Amplitude is the maximum deviation from the electric or magnetic field in an electromagnetic wave, i.e. the maximum value of the fluctuation. Amplitude is related to the energy of the electromagnetic wave; the higher the amplitude, the higher the energy of the wave. The amplitude usually reflects the strength or brightness of the signal.

WiFi frequency versus wavelength.Wavelength = speed of light / frequency

According to the above formula, the wavelengths of 2.4Ghz and 5GHz radio waves are 12.5 centimeters and 6 centimeters, respectively.

2. Power units of electromagnetic waves

In signal processing, power is divided into absolute power and relative power, with absolute power expressed in dBm and relative power expressed in dB.

Absolute power unit (dBm)

dBm is an absolute unit used to express power relative to a level of 1 milliwatt (mW), and when the power is 1 mW, the power is 0 dBm. the relationship between them is given in the formula:

Some of the commonly used relationship values are listed in the table below:

The dBm (decibel milliwatt) range of WiFi signal strength can be used to assess the quality of the signal. In general, signal strength is categorized as follows:

• -30 dBm to -50 dBm: Excellent, almost no interference, very strong signal.
• -50 dBm to -60 dBm: Good, moderate signal strength for all network applications.
• -60 dBm to -70 dBm: General, signal strength may have some impact on performance, for basic network use.
• -70 dBm to -80 dBm: Weaker, low signal strength, may experience connection problems and slower network speeds.
• Below -80 dBm: Very poor, very weak signal, connection may be unstable or impossible.

In general, signal strengths between -50 dBm and -60 dBm are considered good and provide a stable and efficient network connection.

Relative power units (dB)

The dB (decibel) is a relative unit used to express the ratio of two powers or voltages. It is based on a logarithmic calculation, making ratios of different magnitudes easier to handle.

dB is used to indicate signal gain or attenuation, filter attenuation, antenna gain, and so on.

dB It provides a convenient way to express ratios over a wide range. In audio and wireless communications, some signal values vary very widely.

For example, a signal from1mW Attenuate to0.00000001mW Because0.00000001mW together with0.0000001mW cap (a poem)0.000000001mW It is difficult to distinguish them with the human eye, and accordingly, if one uses the-70dB、-80dB、-90dB to differentiate is much clearer.

(v) Answers to questions

1. Why is the ability of 5G WiFi to penetrate walls not as good as 2.4G?

The wavelength of 2.4G radio waves is about 12.5 centimeters, and the wavelength of 5G is about 6 centimeters. Why is it that the longer the wavelength, the greater the penetration ability at the same signal strength? It is mainly related to the diffraction of waves and the interaction of waves with matter.

diffraction effect: When electromagnetic waves encounter obstacles, electromagnetic waves with longer wavelengths are more likely to bypass them. This is because electromagnetic waves of longer wavelengths have a greater ability to diffract and continue to propagate around obstacles, thus making them more penetrating in complex environments.

Interactions with substances: Atoms and molecules within matter have an absorption or reflection effect on electromagnetic waves in a specific frequency range. Short-wavelength (high-frequency) electromagnetic waves are more likely to be absorbed or scattered by atoms and molecules because they are more energetic and have a greater probability of resonating with the microstructure of matter. In contrast, electromagnetic waves of longer wavelengths (e.g., radio waves) are less susceptible to such absorption and scattering and therefore penetrate matter more easily.

Let's look at one of the attenuation values (empirical) for a typical penetration of a router AP below

2.4G WiFi typical penetration loss empirical values

5G WiFi typical penetration loss empirical values

From the table above, it can be seen that the WiFi signal is attenuated by at least 10 dB after passing through a concrete wall , with reference to the power multiplier ratio.

Same principle.In some stadiums, its wireless signal AP is arranged around the seats, when there are many people, because the human body is mainly composed of water and organic matter, these substances have a strong absorption and scattering effect on electromagnetic waves, so the wireless signal can be searched for will be weaker。

High frequency radio waves, such as WiFi signals operating in the 2.4 GHz and 5 GHz bands, pass through the human body with significant attenuation, typical of the attenuation:

• 2.4G WiFi: typically 10-15dB attenuation.
• 5G WiFi : Attenuation is typically higher, possibly up to 20-30 dB.

2. Why is there a difference in the data from each test when performing iperf pull tests on WiFi?

Electromagnetic waves can propagate in a vacuum at the speed of light, but the signal strength of radio waves propagating through the air gradually decays, and this is called path attenuation of electromagnetic waves in free space.

Path attenuation of electromagnetic waves in free space

Free Space Path Loss (FSPL) is the natural attenuation of signal strength due to the increase in distance during the propagation of electromagnetic waves.

Free space attenuation does not take into account the effect of any obstacles, reflections, refractions or scattering, and is simply a weakening of the signal strength due to the spreading of the wave front.

It is calculated by the formula:

2.4GHz signal with an attenuation value of approximately 74 dB at 50 meters from the RF source 2.4GHz signal with an attenuation value of approximately 80 dB at 100 meters from the RF source 2.4GHz signal with an attenuation value of approximately 86 dB at 200 meters from the RF source

Here's the 6dB rule:Doubling the transmission distance will result in a signal attenuation of 6dB

The reason why the data in the outdoor iperf pull-off test is different each time is related to the differences in obstacles, wireless interference, air temperature and air pressure in each test environment.

3. Why do Bluetooth, WiFi, and microwave interfere with each other?

The main reason Bluetooth, WiFi and microwave ovens interfere with each other is that they all operate in the same radio frequency band, the 2.4 GHz band.

WiFi： Most WiFi networks use the 2.4 GHz band, although there is also WiFi in the 5 GHz band, but generally 5 GHz WIFI is backward compatible with 2.4 Ghz and supports the 802.11b/g/n protocol.

Bluetooth: Bluetooth devices also operate in the 2.4 GHz band, using a technique called Frequency-Hopping Spread Spectrum (FHSS) to minimize interference with other wireless devices. Although Bluetooth switches channels frequently within the band, it may still cause interference with other devices operating in the same band.

Microwave: Microwave ovens generate strong electromagnetic radiation in the 2.4 GHz band when heating food. This radiation may interfere with wireless communication devices operating in the same frequency band, resulting in signal degradation or interruption.

Why do wireless signals operating in the same frequency band interfere with each other?

This is because radio waves interfere with each other

In the figure above, two waves f(x) and g(x) are interfering with each other to form a purple wave f(x)+g(x), change the phase of g(x) and you can see that the peak of the coherent wave is either doubled or canceled out to 0. This is used in radio signals to denote signal enhancement and attenuation.

Phase length interference (constructive interference): When the peaks and valleys of two beams of electromagnetic waves are aligned, they are either in phase or differ by 0 degrees, and the amplitude of the waves increases when superimposed, creating phase-length interference. This interference enhances the signal and may result in a stronger signal in wireless communications.

Phase cancellation interference (destructive interference): When the peaks of two beams of electromagnetic waves are aligned with the troughs of another beam, they are in opposite phase or 180 degrees apart, and the amplitude of the superimposed waves will decrease or even cancel out completely, forming phase cancellation interference. This interference leads to signal attenuation or loss, which is manifested as signal weakening or disconnection in wireless communication.

4. Why does the WiFi network slow down in environments with lots of routers?

channel overlap: In the 2.4 GHz band, there are a limited number of WiFi channels (usually only 11 or 13 channels), and the wide bandwidth of each channel usually covers a portion of an adjacent channel. If multiple routers operate on adjacent or identical channels, their signals will interfere with each other, resulting in slower data transmission.

channel congestion: Even if different routers use different channels, if there are too many devices on a particular channel, the channel will become congested, resulting in lower data transfer rates. This is because when a device sends a wireless signal, it will go to detect whether there is a device currently transmitting or not, and if there is a device transmitting, it will enter a time slice of waiting.

Limitations of the WiFi standard:In congested environments, if some devices or routers use older WiFi standards, such as 802.11b/g, their speeds can limit the performance of the entire network. This is because older devices tend to take more time to transmit data, affecting the efficiency of other devices.

wind up

The above is for the wireless network use in the process of signal attenuation, interference in the introduction and some simple answers to related questions. The actual cause of the problem is more complex, need to consider more factors affecting. For non-communication professional embedded application software development engineers, with these basics almost enough.

The above content, if there is any error, welcome to criticize and point out in the comment section, thank you.

 

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