Enhanced Mobile Broadband (eMBB) is the most important development direction of 5G at this stage, and it is also the tangible future for us.

Under the 5G base station, ITU's vision is that a single cell can reach the rate of 20Gbps. At the exhibition in Barcelona, China's 5G pioneer ZTE demonstrated an amazing peak download rate of 50Gbps!

▲ ZTE's 5G demonstration

Behind all this, who is sacred and can have such great energy?

This should be understood from two aspects: "Dao" and "Shu". "Tao" can be regarded as the theoretical basis for providing support, and "technique" is the realization method on this basis.

Two thousand five hundred years ago, Lao Tzu, a great thinker in China, once said, "The Tao gives birth to one, to two and to everything." . So what exactly is Tao? Laozi also said: "Tao can be Tao, but it is extraordinary."

Lao Tzu is right, but although "Tao" is hard to say clearly, some people are still making unremitting efforts for it. 70 years ago, Claude of the United States? Shannon published an epoch-making paper "Mathematical Theory of Communication", thus becoming the founder of wireless communication theory, and "Tao" finally became "Tao".

▲ Shannon

Shannon formula accurately describes several factors that determine the capacity of communication system and their relationships. As the "Tao" of mobile communication, 2G, 3G and 4G should be followed, and 5G is no exception.

▲ Shannon formula

This formula looks very ferocious and terrible. If we can get up the courage to watch it again, we will find that the system capacity is directly proportional to the channel bandwidth, or the greater the channel bandwidth, the greater the system capacity! If the capacity is large, isn't the network speed very fast?

The designers of 5G also think so: it is the first priority to greatly improve the network speed and increase the channel bandwidth. Then, increase the bandwidth from 20M of 4G to 100M or even 400M!

However, the commonly used frequency bands are all occupied by 2G/3G/4G, and even WiFi accounts for a large section, leaving little for 5G. You cannot make a silk purse out of a sow's ear. What can we do?

Second, the introduction of 5G millimeter wave.

As a result, 5G turned its attention to a new virgin land. Not only is there plenty of bandwidth here, but most of it is idle, just for the sake of 5 G. It is really a good place for honey milk to flow.

▲5G millimeter wave candidate spectrum

This virgin land is "millimeter wave", also called 5G high frequency, which generally refers to the spectrum in the range of 30GHz to 300GHz. Compared with the traditional Sub6G, the frequency is much higher, and the higher the frequency, the shorter the wavelength.

▲ The higher the frequency, the shorter the wavelength.

According to the formula of "wavelength λ = speed of light C ÷ frequency f" of electromagnetic waves, it can be concluded that the wavelength of this spectrum is between 1mm and 10mm, hence the name "millimeter wave", also known as millimeter wave. The lower limit of millimeter wave used in actual 5G is 24GHz.

The following figure shows the candidate frequency bands of 5G millimeter wave. It can be seen that compared with the crowded Sub6G spectrum (2G/3G/4G/WiFi are all in this narrow range), the millimeter wave spectrum resources are simply too rich! This is only a small part of the millimeter wave frequency band.

▲ The bandwidth of 5G millimeter wave is extremely rich.

At present, the 5G millimeter wave spectrum determined by the standard is called FR2 (Band 2), which is concentrated in the 5 G bandwidth from 24GHz to 29GHz, which can basically meet the needs of the initial deployment stage of 5 G.

▲ Standardized 5G millimeter wave spectrum

Since millimeter-wave resources are so abundant, why do 2G/3G/4G have to be squeezed into low-frequency Sub6G now, and even 5G has to be deployed in Sub6G first?

Third, the fatal weakness of millimeter wave.

This is because millimeter wave has a fatal weakness-poor coverage.

One characteristic of electromagnetic wave propagation in air is that the higher the frequency, the faster the loss and the worse the diffraction and penetration ability. Typical losses are classified as follows:

1. Free space path loss: Due to the propagation of signal energy in free space, after a certain distance, the signal energy will be attenuated, and the power loss is inversely proportional to the square of frequency. For example, if the frequency increases by 3 times, the loss will increase by 9 times!

▲ The higher the frequency, the greater the loss of free space.

2. Diffraction loss: additional propagation loss caused by obstacles during electromagnetic wave propagation. The higher the frequency, the worse the diffraction ability and the higher the diffraction loss.

▲ The higher the frequency, the greater the diffraction loss.

3. Penetration loss: the loss caused by the penetration of obstacles such as buildings, flowers and trees in the process of electromagnetic wave propagation. The higher the frequency, the worse the penetration ability and the higher the penetration loss.

▲ The higher the frequency, the greater the penetration loss (penetrating trees).

▲ The higher the frequency, the greater the penetration loss (penetrating the building).

4. Rain attenuation loss: the phenomenon that electromagnetic wave signals are weakened by the absorption and scattering of rain, snow and ice in the atmosphere. Generally speaking, the higher the frequency, the greater the attenuation.

▲5G millimeter wave is seriously affected by the weather.

The signal propagation in space is the sum of the above attenuation methods. If the low-frequency 2.6GHz is compared with the high-frequency 28GHz, the attenuation experienced under the same signal propagation path is shown in the following figure.

▲ Layer-by-layer loss experienced by 5G millimeter wave

Since millimeter waves are at a high frequency of 28GHz, each step experiences more attenuation than at 2.6GHz:

① Free space loss: 20dB more;

② Diffraction loss:10dB; More;

③ Tree penetration loss: 8 dB more;

④ house penetration loss:14db; More;

⑤ Indoor propagation loss: 5dB more.

Adding these values together, we can draw the following conclusion: the signal received by the end user at 28GHz is one millionth of the signal intensity at 2.6GHz with the same transmission power and the same propagation path!

▲ Compared with 2.6GHz, the propagation loss of millimeter wave is very large.

Millimeter wave coverage is so poor that it seems that this "virgin land flowing with milk and honey" is really not very good to develop. However, the temptation of its large bandwidth and fast speed is irresistible, so we should foster strengths and avoid weaknesses.

Fourth, the deployment of 5G millimeter wave.

First of all, how to become stronger? The most important method is:

beam forming

Generally speaking, the antenna unit is most efficient when using half wavelength, so the shorter the wavelength of electromagnetic wave, the smaller the antenna unit required for transmission and reception.

▲ The higher the frequency, the shorter the wavelength of electromagnetic wave, and the smaller the required transmitting and receiving antenna elements.

Millimeter wave is characterized by short wave length, so the size of antenna can be very small, and more antennas can be accommodated in the same area. By adjusting the parameters of the basic unit of the antenna array, the signals at some angles can be enhanced and the signals at other angles can be cancelled, so that the signals can be enhanced in a specific direction, which is beamforming.

▲ The higher the frequency, the more antennas and the better the shaping effect.

Beamforming ability depends on the number of antenna elements. The more the number, the narrower the beam, and the more energy the beam can focus on users, so as to improve coverage and avoid interference, and the better the beam forming effect.

The 5G millimeter-wave equipment in the figure below contains 256 antenna elements, and each element is a group of 64. Narrow beams are generated by beamforming, so the device can provide four beams for high-speed service. This realization method is the mainstream of millimeter wave equipment at present.

▲ Real 5G Millimeter Wave Equipment

With the help of beamforming, the narrow beam of millimeter wave can concentrate energy, accurately align and track the user's movement, bring better user experience and reduce interference.

▲ Beamforming is working.

Let's talk about how to avoid the shortcomings of 5G millimeter waves.

1. Ultra-dense micro-station networking

First forget the macro coverage and ask the macro coverage to find the low frequency. Millimeter wave will be a micro-station, an indoor station with peace of mind, covering hot spots and indoors. After all, there are many people in these places, and the traffic demand is large, which requires 5G more.

▲ Millimeter wave is more suitable for micro-station coverage.

If the traffic demand in an area continues to rise, from the hot spot to the boiling point, even to the explosion point, although the coverage distance of millimeter wave is close, it can be densely distributed and then densely distributed, becoming an ultra-dense network.

▲5G millimeter wave ultra-dense networking

2. High and low frequency macro and micro collaborative networking

In order to make up for the coverage problem of millimeter wave, we can also use two frequency bands at the same time with low frequency Sub6G. The low frequency is responsible for the control plane, and the high frequency is responsible for the user plane, so that we can not only switch cells without perception, but also enjoy the extreme speed brought by the high frequency.

▲5G high and low frequency macro and micro collaborative networking

In short, millimeter wave is the killer of 5G to realize eMBB service. Millimeter wave has great disadvantages, but only outstanding advantages, which can be made up by various technical solutions.