The fifth-generation mobile communication systems (5th generaTIon mobile networks, 5G for short) are getting closer and closer to formal commercial use (2020). These days, Huawei, Samsung and other major manufacturers have also released their own solutions, which can be described as "eight cents across the sea, each Show the magical powers."
A key indicator of 5G is the transmission rate: According to the communication industry's expectations, 5G should achieve a transmission rate that is more than ten times faster than 4G, that is, the transmission rate of 5G can achieve 1Gb/s. This means that it takes only 10 seconds to transfer a 1GB HD movie with 5G! In addition, such a high transmission speed will bring some other applications, such as cloud games (the game is executed in the cloud server, and the execution screen is directly transmitted back to the mobile phone, so that the mobile phone configuration is not high, and large games can be played), virtual reality (same reason) Put the operation in the cloud, the mobile phone is only responsible for the output screen) and so on.
How does 5G achieve such a high transmission rate?
There are basically two ways to increase the transmission rate of wireless transmission. One is to increase the spectrum utilization, and the other is to increase the spectrum bandwidth. In wireless transmission, data is transmitted in the form of symbols. In the case where the symbol transmission rate (code rate) is constant, the wireless bandwidth occupied by the signal does not change, and the amount of information data transmitted by each symbol is determined by the modulation method.
Modulation is how to signal information.
In ancient times, people used beacons to transmit information. In some cases, they ignited bonfires and extinguished bonfires whenever there were circumstances. From the perspective of modern communication theory, we have modulated the bonfire. Since the normal bonfire has only two states (igniting and extinguishing), the beacon can only transmit 1 bit of information at a time (0 = extinguish = no enemy, 1 = ignite = enemy). Can the beacon be improved to deliver more information at a time? We can achieve this by introducing more states. For example, in the improved beacon, we can control the fire of the bonfire, and divide the fire into four states: extinction, small fire, medium fire and big fire, so that we can pass two bits of information at a time (00=extinguished = no enemy, 01 = small fire = there are enemies and far away from us, 10 = medium fire = there are enemies and not far from us, 11 = fire = there are enemies and have been under the city).
However, the best of both worlds is that the introduction of more states will also increase the possibility of information transmission errors. If the weather is bad, the fire may be regarded as a small fire, so the transmission of information will go wrong. In contrast, if there are only two states (extinguished and ignited), the chance of error is relatively small.
The same is true for modulation in wireless communication, which can produce different states of the carrier by manipulating the amplitude and phase of the radio waves. When the modulation mode is changed from simple to complex, the number of carrier states increases, and the amount of information (number of bits) represented by one symbol also increases.
On the other hand, however, the spacing between each symbol state is also small, so that it is susceptible to noise interference such that the symbol deviates from the position it should be in, causing decoding errors. Therefore, complex modulation has higher requirements on the channel. When the channel noise is large, the complex modulation will result in high data transmission error rate, and the circuit required for decoding will be very complicated, resulting in large power consumption.
State diagram from simple (left) to complex (right) modulation
Compared to improving spectrum utilization, the method of increasing the spectrum bandwidth is simpler and more straightforward. In the case of constant spectrum utilization, the data transfer rate that can be achieved is doubled by doubling the available bandwidth. But the problem is that the commonly used frequency bands below 5 GHz are already very crowded. Where can I find new spectrum resources? The method that every major manufacturer thinks about is to use millimeter wave technology.
What is the millimeter wave? What are the characteristics of millimeter waves?
The millimeter wave refers to an electromagnetic wave having a wavelength of the order of millimeters, and its frequency is approximately between 30 GHz and 300 GHz.
According to the communication principle, the maximum signal bandwidth of wireless communication is about 5% of the carrier frequency, so the higher the carrier frequency, the larger the achievable signal bandwidth. In the millimeter wave band, the 28 GHz band and the 60 GHz band are the two most promising bands for use in 5G. The available spectrum bandwidth in the 28 GHz band is up to 1 GHz, while the available signal bandwidth per channel in the 60 GHz band is up to 2 GHz (the available spectrum for the entire 9 GHz is divided into four channels).
In contrast, the highest frequency carrier in the 4G-LTE band is around 2 GHz, and the available spectrum bandwidth is only 100 MHz. Therefore, if the millimeter wave band is used, the spectrum bandwidth can be easily reduced by 10 times, and the transmission rate can be greatly improved. In the 5G era, we can easily use the millimeter wave band to easily watch Blu-ray quality movies on the 5G mobile phone, as long as you are not afraid of running out of traffic!
Comparison of available spectrum bandwidths for each frequency band
Another characteristic of the millimeter wave band is that it has a large attenuation in air and a weak diffractive power. In other words, it is basically impossible to achieve signal through the wall with millimeter waves. However, the millimeter wave transmission attenuation in the air can also be used by us. The so-called "It's not a bug, it's a feature!": The millimeter wave signal attenuation used by your mobile phone is indeed relatively large, but similarly emitted by other terminals. The attenuation of the millimeter wave signal (interference signal for you) is also very large, so the millimeter wave system should be designed without special consideration of how to deal with the interference signal, as long as the different terminals are not too close. The choice of 60 GHz is to take advantage of this, because 60 GHz is exactly the resonant frequency of oxygen, so the 60 GHz electromagnetic wave signal decays very quickly in the air, so that interference between different terminals can be completely avoided.
Of course, the millimeter wave is very attenuated in the air. This feature is also destined to be less suitable for use in outdoor mobile terminals and base stations. The plan for the use of the 5G frequency band by major manufacturers is to use the more traditional 6 GHz frequency band to ensure signal coverage in outdoor open areas, and to use ultra-high-speed data transmission by using micro base stations plus millimeter wave technology indoors.
Millimeter waves must be used in conjunction with a micro base station (or access point)
Another feature of the millimeter wave compared to the traditional 6 GHz band is that the physical size of the antenna can be relatively small. This is because the physical size of the antenna is proportional to the wavelength of the band, and the wavelength of the millimeter wave band is much smaller than the conventional frequency band below 6 GHz, and the corresponding antenna size is also small. Therefore, we can easily equip the mobile device with a millimeter-wave antenna array to implement various MIMO (MulTIple-Input MulTIple-Output), which means that multiple transmitting and receiving antennas are used at the transmitting end and the receiving end respectively, so that the signal passes through the transmitting. Techniques for transmitting and receiving multiple antennas at the end and the receiver to improve communication quality, including beamforming (for beamforming, which we will cover in detail in the next article).
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