"Over the past two decades, we have witnessed the transformation of mobile communication from 1g to 4G LTE. During this period, the key technologies of communication are changing, and the amount of information processed is increasing exponentially. The antenna is an indispensable component to realize this leap forward promotion.
According to the definition of the industry, an antenna is a converter, which transforms the guided traveling wave propagating on the transmission line into an electromagnetic wave propagating in an unbounded medium (usually free space), or performs the opposite transformation, that is, transmitting or receiving electromagnetic waves. Generally speaking, whether it is a base station or a mobile terminal, the antenna acts as a middleware for transmitting and receiving signals.
Now, the next generation communication technology - 5g has entered the end of the standard setting stage, and major operators are actively deploying 5g equipment. There is no doubt that 5g will bring users a new experience. It has a transmission rate ten times faster than 4G, which puts forward new requirements for the antenna system. In 5g communication, the key to achieve high rate is millimeter wave and beamforming technology, but the traditional antenna obviously can not meet this demand.
What kind of antenna does 5g communication need? This is a problem that engineering developers need to think about. Therefore, Lei Feng's IOT Technology Review invited lifelong professor of National University of Singapore and IEEE fellow Chen Zhining to explain the future antenna technology in 5g mobile communication.
Evolution and trend of mobile communication base station antenna
Base station antenna is developed with network communication. Engineers design different antennas according to network requirements. Therefore, antenna technology has been evolving in the past few generations of mobile communication technologies.
Almost all the first generation mobile communications used omni-directional antennas. At that time, the number of users was small and the transmission rate was low. At this time, it was still an analog system.
By the second generation mobile communication technology, we have entered the cellular era. At this stage, the antenna gradually evolved into a directional antenna. Generally, the lobe width includes 60 °, 90 ° and 120 °. Take 120 ° as an example, it has three sectors.
In the 1980s, antennas were mainly single polarization antennas, and the concept of array has been introduced. Although omnidirectional antennas also have arrays, they are only vertical arrays. Planar and directional antennas appear in single polarization antennas. In terms of form, the current antenna is very similar to the second generation antenna.
In 1997, dual polarization antenna (± 45 ° cross dual polarization antenna) began to enter the stage of history. At this time, the antenna performance has been greatly improved compared with the previous generation. Whether it is 3G or 4G, the main trend is dual polarization antenna.
In the 2.5G and 3G era, there are many multi band antennas. Because the system at this time is very complex, such as GSM, CDMA and so on, multi band antenna is an inevitable trend. In order to reduce cost and space, multi band has become the mainstream at this stage.
In 2013, we introduced MIMO (multiple input multiple output) antenna system for the first time. Originally 4 × 4 MIMO antenna.
MIMO technology improves the communication capacity. At this time, the antenna system has entered a new era, that is, from the initial single antenna to array antenna and multi antenna.
However, now we need to look far away. 5g deployment has been started. What role will antenna technology play in 5g and what impact will 5g have on antenna design? This is a problem we need to explore.
In the past, antenna design was usually passive: after the system design was completed, indicators were proposed to customize the antenna. However, the concept of 5g is still unclear. R & D personnel engaged in antenna design need to be prepared in advance to provide solutions for 5g communication system, and even affect the standard customization and development of 5g through new antenna schemes or technologies.
From the experience of cooperation and exchange with mobile communication companies in the past few years, there are two major trends in base station antenna in the future.
The first is from passive antenna to active antenna system.
This means that the antenna may be intelligent, miniaturized (co designed) and customized.
Because the future network will become more and more detailed, we need to carry out customized design according to the surrounding scenes. For example, the station layout in urban areas will be more detailed rather than simple coverage. 5g communication will use high frequency band, obstacles will have a great impact on communication, and customized antenna can provide better network quality.
The second trend is the systematization and complexity of antenna design.
For example, beam array (space division multiplexing), multi beam and multi / high frequency band. These put forward high requirements for the antenna, which will involve the whole system and mutual compatibility. In this case, the antenna technology has gone beyond the concept of components and gradually entered the design of the system.
The evolution process of antenna technology: from single array antenna to multi array and then to multi cell, from passive to active system, from simple MIMO to large-scale MIMO system, from simple fixed beam to multi beam.
Trends at the design level
For base stations, one of the principles of antenna design is miniaturization.
The antennas of different systems are designed together. In order to reduce cost and save space, they must be small enough. Therefore, the antenna needs to be multi band, broadband, multi beam, MIMO / massive MIMO, and the isolation of MIMO to the antenna. Massive MIMO has some special requirements for hybrid mutual coupling of antennas.
In addition, the antenna needs to be tunable.
The first generation antenna relies on machinery to realize the inclination, and the third generation realizes remote electrical tuning. If 5g can realize self tuning, it is very attractive.
For mobile terminals, the requirements for antennas are miniaturization, multi band, broadband and tunability. Although these features are available now, 5g requirements will be more stringent.
In addition, the antenna of 5g mobile communication also faces a new problem - coexistence.
To realize massive MIMO, multiple antennas are required for both transceiver and transmitter, that is, multiple antennas of the same frequency (8 antennas, 16 antennas...). Such a multi antenna system brings the biggest challenge to the terminal is the coexistence problem.
How to reduce the mutual influence to couple, how to increase the channel isolation... This puts forward new requirements for 5g terminal antenna.
Specifically, the following three points will be involved:
1. Reduce the mutual influence, especially the mutual interference between different functional modules and different frequency bands. Previously, the academic community thought that this situation would not exist, but this problem does exist in the industry;
2. Decoupling. In MIMO system, the mutual coupling of antennas will not only reduce the channel isolation, but also reduce the radiation efficiency of the whole system. In addition, we can't expect to rely entirely on high-frequency millimeter wave to solve the performance growth. For example, there are system problems at 25ghz, 28ghz... 60GHz;
3. Decorrelation can be solved by combining antenna and circuit design. However, the bandwidth of the solution through circuit is very limited and it is difficult to meet the bandwidth of all frequency bands.
Antenna technology of 5g system
This includes the design of a single antenna and technologies at the system level, which are mentioned above, such as multi beam, beamforming, active antenna array, massive MIMO, etc.
From the perspective of specific antenna design, the technology developed from the concept based on metamaterials will be of great benefit. At present, metamaterials have achieved success in 3G and 4G, such as miniaturization, low profile, high gain and frequency band.
The second is the substrate or package integrated antenna. These antennas are mainly used in the frequency band with relatively high frequency, that is, millimeter wave band. Although the size of the antenna in the high frequency band is very small, the loss of the antenna itself is very large, so it is best to integrate the antenna with the substrate or a smaller package on the terminal.
The third is electromagnetic lens. The lens is mainly used in the high-frequency band. When the wavelength is very small, a medium can be placed to focus. The volume of the high-frequency antenna is not large, but the wavelength of the microwave band is very long, which makes the lens difficult to use and large.
The fourth is the application of MEMS. When the frequency is very low, MEMS can be used as a switch. In the mobile phone terminal, if the antenna can be effectively controlled and reconstructed, one antenna can be multi-purpose.
Taking the electromagnetic lens as an example, this design introduces a concept: an electromagnetic lens is placed in front of the multi unit antenna array (here, it refers to the lens applied to the low-end band of microwave or millimeter wave, which is different from the traditional optical lens). When the light is incident from a certain angle, a spot will be generated on a focal plane, and a large number of capabilities will be concentrated on the spot, This means that the main part of the whole capability is received in a small area.
When the incident direction changes, the position of the spot on the focal plane will also change. As shown in the figure above, when the angle is projected, the energy distribution of black color is generated. If it is based on a certain angle θ When incident (red), the main energy deviates from the black region.
With this concept, we can distinguish where the energy comes from, and the incident direction corresponds to the position of the energy on the array or focal plane one by one. On the contrary, if the antenna is excited at different positions, the antenna will radiate in different directions, which is also one-to-one correspondence.
If multiple elements are used to radiate on the focal plane, the radiation of multiple carrier beams can be generated, that is, the so-called beamforming; If switching between these beams, beam scanning will occur; If these antennas are used at the same time, massive MIMO can be realized. This array can be large, but high gain radiation can be achieved with only a few arrays per beam.
If an ordinary array has an aperture of the same size, the energy received each time is required. All units must receive energy in this area. If only one unit is placed in a large area, the energy received is only a very small part; Different from ordinary arrays, the same aperture can receive all energy with only a few units without any loss. These energy can be received at different places at the same time from different angles.
This greatly simplifies the whole system. If there is only one direction at a time, only one local antenna can work, which reduces the number of antennas working at the same time. The concept of subarray is different. It allows local multiple antennas to form a subarray. At this time, the number of channels decreases with the increase of the number of subarray units. Example 10 × 10 array, if 5 × If 5 becomes a subarray, there will be only four independent channels, and the number of channels will be reduced.
The right side of the figure above shows the influence of the lens on the system calculated on the baseband. The horizontal direction is the number of antennas. It is assumed that a linear array in the horizontal direction has 20 units. When using the lens, only 5 units receive the focused energy, which is better than using all 20 units without the lens. The former has higher communication quality, lower cost and power consumption. Even in the worst case, when the wave is incident from all directions, these 20 units are used, and the effect of the latter is the same. Therefore, using a lens can improve the performance of the antenna - using a small number of antennas to achieve the effect of previous large arrays.
It can be seen from this ppt that the use of electromagnetic lens can reduce the cost, reduce the complexity, increase the radiation efficiency, and increase the filtering characteristics (shielding interference signals) of the antenna array.
This ppt shows the antenna used in the 28ghz millimeter wave band, and uses seven unit antennas as the feed.
As shown on the left, the front lens is a screen lens made of metamaterials. Two layers of PCBs are engraved into different shapes for phase adjustment to achieve focusing in a specific direction. The performance of 7 radiation units can be seen on the right. The lobe width is 6.8 °, the side lobe is less than 18db, and the gain is 24-25db.
This experiment verifies the application of electromagnetic lens in base station and the role of metamaterial technology in antenna miniaturization.
5g antenna
5g antenna circuit design“
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