This paper mainly briefly introduces the characteristics of electromagnetic waves in wireless communication and the requirements of 6G for electromagnetic waves.
What is the electromagnetic wave frequency band? What frequency band does 6G need? What are the advantages and disadvantages of this frequency band?
**Electromagnetic wave (electromagnetic wave)** refers to the electric field and magnetic field that oscillate in phase and are perpendicular to each other. It transmits energy and momentum in the form of waves in space, and its propagation direction is perpendicular to the oscillation direction of the electric field and magnetic field.
Wave Velocity, Wavelength and Frequency
Basic formula for waves: v = λ * f
This formula relates three fundamental properties of waves: wave velocity v, frequency f, and wavelength λ. In fact, this formula is more likely to be derived from the definitions of the above three attributes. For example, the wave velocity v can be defined as: the distance traveled by the wave crest per unit time. To put this definition in another way: the distance that the peak advances in one cycle is one wavelength. The cycle time here is actually the reciprocal of the frequency f.
The above definition of wave velocity is expressed by the formula: T * v = λ
A simple conversion gives: v = λ / T = λ * f
The electromagnetic wave in the physical sense refers to the wave generated by the mutual induction of the changing electric field and magnetic field, and still satisfies the above basic formula. The wave speed of electromagnetic waves is a very famous physical constant: the speed of light. Of course, this is actually because visible light is also an electromagnetic wave. The speed of light depends on the transmission medium of the electromagnetic wave. In general, the speed of light in vacuum and air can be estimated to be 3E+8 m/s; the speed of light in optical fibers is slower, approximately 2E+8 m/s.
In electromagnetic wave communication applications, the concept of wavelength is easier to apply. Common mobile communication signals have wavelengths on the order of decimeters . Specifically, the wavelength of a 900MHz signal is 0.333 meters, the wavelength of a 1.8GHz signal is 0.167 meters, and the wavelength of a 2.6GHz signal is 0.115 meters.
If the size of a specific object is compared with the wavelength of electromagnetic waves, several interesting conclusions can be drawn as follows.
Much larger than the wavelength : If the size of an object is on this scale, the object is equivalent to a huge obstacle in the direction of the electromagnetic wave, which will produce reflection, refraction, absorption and other effects on the electromagnetic wave. The "hugeness" of the object here is relative to the multiple of the wavelength. For example: the canopy of buildings and large trees, ranging from a few meters to hundreds of meters, will cause obstruction and loss to mobile communication signals.
Far smaller than the wavelength : If the size of an object is on this scale, it is difficult for the object to block the electromagnetic wave, and the electromagnetic wave will bypass the object and continue to propagate. This phenomenon is the diffraction or diffraction of the wave. For example: the size of raindrops is usually at the millimeter level, which has no effect on ordinary mobile communication signals, but it will cause obvious obstruction to millimeter waves (wavelengths at the millimeter level).
Close to wavelength : This scale is usually used in antenna design, for example, the length of a half-wave oscillator is half of the wavelength, and the element spacing of an array antenna is usually half of the wavelength. Large-wavelength signals must use large-sized antennas, and there is no way to use small-sized antennas for transmission and reception. For small-wavelength signals, a single small antenna can be used, and small antennas can also be combined into a super-sized array antenna. For this reason, long-wave communication has to use super huge antennas, and the antenna size of mobile communication is usually similar to the size of a mobile phone.
The higher the frequency, the shorter the wavelength, the weaker the penetrating ability, the smaller the antenna, the wider the available bandwidth of the signal, and the greater the amount of information transmission.
electromagnetic spectrum
Electromagnetic waves in the physical sense have a very wide frequency span, from very low alternating current frequencies to extremely high high-energy rays. **As the magnitude of the electromagnetic wave frequency (or wavelength) changes, its physical properties will also change greatly. **Electromagnetic waves can be arranged according to their wavelengths from large to small (equivalent to low to high frequency), called "electromagnetic spectrum", as shown in the figure below. Among them, the part of the electromagnetic spectrum above the infrared is the "radio wave" in the usual sense.
Long-wave or lower frequencies were actually used in dedicated communication systems very early on. Long waves can travel great distances and travel close to the Earth's surface (ground waves). Of course, long-wave transmission and reception are not easy, because the size of the antenna needs to be on the order of wavelength. Its enormity can be experienced by doing an image search for "longwave antenna". For this reason, long waves are basically not used in ordinary civilian communications.
The wavelength of the medium-wave frequency band is between 100 and 1000 meters, and the electromagnetic waves in this frequency band are not easily blocked by buildings. Although medium wave coverage is not as far as long wave, it can also be used for coverage in urban areas. The medium wave band is mainly used for radio. For example: AM990 of Shanghai Radio and Television Station actually uses amplitude modulation technology (AM) and works at 990kHz radio channel. This kind of AM radio is easy to make, has accompanied the growth of many contemporary people, and is still in use today.
The wavelength of the short wave band is between 10 and 100 meters. This wavelength has the unique property that it can be reflected back to the ground by the ionosphere of the atmosphere, enabling long-distance communication by transmitting back and forth (called "sky waves"). Therefore, short waves can be used for communication over longer distances and even around the world. The most common application of shortwave is various shortwave radio channels.
The VHF band with a higher frequency than shortwave has a wavelength of 1 to 10 meters, so it is called meter wave , and also called ultrashort wave. This frequency band is mainly used for FM radio and TV broadcasts. For example: Shanghai Radio and Television Station FM93.4 is a radio broadcast using frequency modulation technology (FM) with a frequency of 93.4MHz. For example: my country's wireless TV broadcasting, the frequency of channel DS-1 is 48.5 ~ 56.5MHz.
What frequency bands are used by different G?
In the 2G era, GSM mainly used the 900MHz frequency band, and later introduced 1.8GHz.
In the 3G era, UMTS is mainly 1.8GHz and 2.1GHz, and TD-SCDMA is mainly 2.3GHz and 2.6GHz.
In the 4G era, LTE continues to use the frequency bands of 2G and 3G, and the 3.5GHz frequency band is newly introduced.
In the 5G era, Sub6G in the low frequency band (FR1: 450 MHz - 6000 MHz); millimeter wave in the high frequency band (FR2: 24.25 GHz - 52.60 GHz)
In the 6G era, the terahertz (THz) frequency band will be used . The terahertz frequency band refers to 100GHz-10THz, which is a frequency band with a frequency much higher than that of 5G.
From communication 1G (0.9GHz) to the current 5G, and then to the future 6G, the frequency of the wireless electromagnetic waves we use is constantly increasing. Because the higher the frequency, the larger the bandwidth range allowed to be allocated, and the larger the amount of data that can be transmitted per unit time, which is what we usually say "the network speed has become faster".
However, another main reason for the development of frequency bands to high places is that the resources of low frequency bands are limited. Just like a highway, no matter how wide it is, the amount of vehicles it can accommodate is limited. When the road is not enough, the vehicle will be blocked and unable to pass, at this time, it is necessary to consider developing another road. The same is true for spectrum resources. With the increase in the number of users and smart devices, the limited spectrum bandwidth needs to serve more terminals, which will seriously degrade the service quality of each terminal. The feasible way to solve this problem is to develop new communication frequency bands and expand communication bandwidth.
Under the same power conditions, the propagation distance of high-frequency waves is shorter than that of low-frequency waves, and the penetration ability becomes weaker, which can be understood as being closer to the linear characteristics of light. Therefore, 5G and 6G require more dense base stations to achieve communication .