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1 Basic concepts of electromagnetic waves
1.1 Generation of electromagnetic waves
Charge is the source of the electric field. Stationary charges produce a stationary electric field, and moving charges produce a changing electric field. The directional movement of charges creates an electric current, around which there is a changing electric field.
A changing electric field produces a magnetic field, a uniformly changing electric field produces a stable magnetic field, and a non-uniformly changing electric field produces a changing magnetic field. The changing magnetic field will produce a changing electric field. The two alternately change in space, couple with each other, and propagate forward to produce electromagnetic waves. Electromagnetic waves are changing electromagnetic fields.
In short, the relationship between electric field and magnetic field, in short, is: a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field.
1.2 Propagation characteristics of electromagnetic waves
Electromagnetic waves are oscillating particle waves emitted in space by electric fields and magnetic fields that oscillate in phase and are perpendicular to each other. They are electromagnetic fields that propagate in the form of waves and have wave-particle duality. Electromagnetic waves are actually divided into radio waves and magnetic waves, which are the general term for both. However, since electric fields and magnetic fields always appear, disappear at the same time, and transform into each other, they are usually collectively called electromagnetic waves, and sometimes they can be simply called radio waves.
Electromagnetic waves move in the form of waves in space, and their propagation direction is perpendicular to the plane formed by the electric field and the magnetic field. The direction of the electric field, the direction of the magnetic field, and the direction of propagation are perpendicular to each other, so the electromagnetic wave is a transverse wave. Electromagnetic waves do not propagate through media and have a fixed velocity in a vacuum, the speed of which is the speed of light.
2 Electromagnetic wave frequency characteristics and spectrum
2.1 Electromagnetic wave frequency characteristics
Several important properties of electromagnetic waves: frequency, period and wavelength.
- Frequency (f) : refers to the number of complete waveforms of electromagnetic wave propagation within unit time (1s). f is the derivative of the electromagnetic wave period (T). How many steps has the electromagnetic wave traveled? The higher the frequency, run fast in small steps, and the lower the frequency, run fast in long strides!
- Period (T) : refers to the time required to transmit an electromagnetic wave with a complete waveform.
- Wavelength (λ) : Because electromagnetic waves propagate forward in the form of waves, the physical distance traveled by a complete waveform is called wavelength, which is the "step length" of electromagnetic waves running forward.
Wavelength v=λf=λ/T. Since the transmission speed of electromagnetic waves is fixed, the wavelength is inversely proportional to the frequency. The longer the wavelength, the lower the frequency, and the shorter the wavelength, the higher the frequency.
The essence of wireless communication is to use electromagnetic waves of different frequencies to carry information.
- In wireless communications, when electromagnetic waves are used to transmit information, only a complete electromagnetic wave can carry the information.
- The higher the frequency, the more steps and complete waveforms in the same time, the more information can be carried, and the higher the data rate.
- The lower the frequency, the fewer steps and fewer complete waveforms in the same time, the less information that can be carried, and the lower the data rate.
2.1 Electromagnetic spectrum
In order to have a comprehensive understanding of various electromagnetic waves, people arrange these electromagnetic waves in order of their wavelength or frequency, wave number, and energy. This is the electromagnetic spectrum. As shown in the figure below:
Electromagnetic waves are energy waves composed of electronic parts and energy parts. Sound and light are two examples of electromagnetic waves. As can be seen from the above figure, whether it is radio waves or infrared rays, whether it is light or microwaves, they are essentially electromagnetic waves.
Radio spectrum is a subset of the electromagnetic spectrum that describes radio waves in the electromagnetic spectrum, which includes electromagnetic waves with frequencies between 9khz and 300,000Ghz. Waves in the wireless spectrum (that is, the waves used for broadcasting, cell phone calls, and satellite transmissions) are invisible and inaudible—at least until a receiver decodes them. All wireless signals travel by electromagnetic waves. Air transmitted.
The radio spectrum is a continuum of electromagnetic waves of varying frequencies and wavelengths used for long-distance communications. Each wireless service is associated with a certain area of wireless spectrum. For example, AM broadcasting involves frequencies at the lower end of the wireless communications spectrum, using frequencies between 535 and 1605khz.
There are also higher and lower frequency electromagnetic waves in nature, but they are not used for long-distance communications. Frequencies below 9kz are used for specialized applications such as wildlife tracking or garage door opening and closing. Electromagnetic waves with frequencies above 300,000Ghz are visible to humans and it is for this reason that they cannot be used to communicate over the air. For example, we identify electromagnetic waves with a frequency of 428570Ghz as red.
Signals traveling via electromagnetic waves do not necessarily remain within a country. Therefore, it is very important that countries around the world come to an agreement on wireless telecommunications standards. ITU is the management organization that determines the standards for international wireless services, including frequency allocation, signal transmission and protocols used by radio equipment, wireless transmission and reception equipment, satellite orbits, etc. If governments and companies don't adhere to ITU standards, wireless devices may not be usable outside the country where they are manufactured.
3 Wireless communication terminology
Frequency: It is the number of cyclic changes completed per unit time. It is a quantity that describes the frequency of periodic motion. The commonly used symbol is f.
Frequency band: People divide electromagnetic waves into several sections with continuous frequencies and a certain width according to frequency, called frequency bands, which are a frequency range with continuous electromagnetic wave frequencies.
Bandwidth: The width of the electromagnetic wave band. In wireless communications , data is not transmitted using electromagnetic waves of a single frequency (also known as "single tone" signals), but a period of continuous frequency electromagnetic waves is used to propagate information. Bandwidth is the highest value of the electromagnetic wave signal. The difference between the frequency and the lowest frequency, expressed in Hz. The larger the bandwidth, the greater the amount of information it can carry. It is a bit like a road. The wider the road, the greater the traffic volume it can carry.
Example: The evolution of the electromagnetic spectrum of 1G->6G cellular mobile communications
. Overall, from 1G-6G cellular mobile communications:
(1) Center frequency point: The center frequency of the carrier is getting higher and higher, from several hundred megahertz to several MHz. GHz, and then to electromagnetic waves of tens of GHz.
(2) Bandwidth: Bandwidth is getting higher and higher. The greater the bandwidth, the greater the amount of information carried.
Supplement: Understanding of frequency bands, channels, channel bandwidth, and transmission rates in WIFI