Explain future antenna technology in 5G mobile communications

Over the past two decades, we have witnessed a shift in mobile communications from 1G to 4G LTE. During this period, the key technologies of communications are changing and the amount of information handled has multiplied. The antenna is an indispensable component to achieve this leap-forward promotion.

According to the definition of the industry, an antenna is an inverter that transforms a guided wave propagating on a transmission line into an electromagnetic wave that propagates in an unbounded medium (usually free space) or vice versa, that is, transmits or receives electromagnetic waves. It is popular to say that whether it is a base station or a mobile terminal, the antenna acts as a middleware for transmitting and receiving signals.

Now that the next-generation communications technology, 5G, has entered the end of the standard-setting stage, major carriers are also actively deploying 5G devices. Needless to say, 5G will bring new experiences to users. It has a transmission rate 10 times faster than 4G and puts forward new requirements for antenna systems. In 5G communication, the key to achieving high speed is millimeter wave and beam forming technology, but traditional antennas obviously cannot meet this demand.

What kind of antenna is required for 5G communication? This is a problem that project developers need to think about. To this end, Lei Feng network IoT Technology Review invited the National University of Singapore tenured professor, IEEE Fellow Chen Zhining to explain the future of antenna technology in 5G mobile communications.

Expert introduction

Chen Zhining, Ph.D., Ph.D., Tenured Professor, National University of Singapore, IEEE Fellow, International Society of Electrical and Electronics Engineers, Distinguished Speaker, Antennas and Communication Society, Presents Vice Chairman, IEEE Council on RFID (CRFID), Distinguished Lecture He has published more than 500 scientific papers, of which more than 100 IEEE Trans have published five monographs in English and have dozens of international antenna patents and successful technology transfer.

The following is organized from Professor Chen’s open class:

Evolution and Trend of Mobile Communication Base Station Antennas

Base station antennas are developed along with network communications. Engineers design different antennas based on network requirements. Therefore, in the past generations of mobile communication technologies, antenna technology has also been evolving.

The first-generation mobile communications used almost all omnidirectional antennas. At the time, there were few users and the transmission rate was low. This time it was also an analog system.

By the second generation of mobile communication technology, we have entered the era of cellular. This stage of the antenna gradually evolved into a directional antenna, the general lobe width contains 60 ° and 90 ° and 120 °. Take 120° as an example, it has three sectors.

In the 1980s, antennas were mainly based on single-polarized antennas, and the concept of arrays has begun to be introduced. Although the omnidirectional antenna also has an array, it is only an array in the vertical direction, and a single-polarized antenna shows plane and directional antennas. In terms of form, the current antenna is very similar to the second-generation antenna.

In 1997, dual-polarized antennas (±45° crossed dual-polarized antennas) began to take the stage. At this time, the antenna performance has been greatly improved compared to the previous generation. Whether it is 3G or 4G, the main trend is dual-polarized antennas.

In the 2.5G and 3G era, many multi-band antennas emerged. Because the system at this time is very complicated, for example, GSM, CDMA, etc. need to coexist, multi-band antenna is an inevitable trend. In order to reduce costs and space, multiple bands have become mainstream at this stage.

In 2013, we introduced the MIMO (Multiple-Input Multiple-Output) antenna system for the first time. It was originally a 4x4 MIMO antenna.

MIMO technology has increased the communication capacity. At this time, the antenna system has entered a new era, that is, from the initial single antenna to the array antenna and multiple antennas.

However, now we need to turn our attention to the distance, the deployment of 5G has started, what kind of role will antenna technology play in 5G, and what impact will 5G have on antenna design? This is a problem that we need to explore.

In the past, the design of the antenna was usually very passive: after the system design was completed, the indicator was used to customize the antenna. However, the concept of 5G is still not clear. Researchers working on antenna designs need to be prepared in advance to provide solutions for 5G communication systems and even influence the 5G standard customization and development through new antenna solutions or technologies.

From the past few years and the experience of cooperation and exchange of mobile communication companies, there are two major trends in base station antennas in the future.

The first is from passive antennas to active antenna systems.

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 customize the design according to the surrounding scenes. For example, in the urban area, the deployment will be more elaborate, rather than simply covering. 5G communication will use high frequency band, obstacles will have a great influence on communication, and customized antenna can provide better network quality.

The second trend is the systematization and complication of antenna design.

For example, beam arrays (realizing space division multiplexing), multiple beams, and multiple/high frequency bands. All of these put forward high requirements on the antenna. It will involve the entire system and compatibility issues. In this case, the antenna technology has gone beyond the concept of components and gradually entered the system design.

The evolution of antenna technology: the earliest from a single array of antennas, to multiple arrays to multiple units, from passive to active systems, from simple MIMO to massive MIMO systems, from simple fixed beams to multiple beams.

Design-level trends

For the base station, one of the major principles of antenna design is miniaturization.

Antennas of different systems are designed together. In order to reduce costs and save space, they must be small enough. Therefore, antennas are required for multi-band, wide-band, multi-beam, MIMO/Massive MIMO, and MIMO isolation for antennas. Massive MIMO has some special requirements for the mixed mutual coupling of antennas.

In addition, the antenna needs to be tunable.

The first generation of antennas is based on mechanical inclination, and the third generation is remote ESC. If 5G can achieve self-tuning, it is very attractive.

For mobile terminals, the antenna requirements are also miniaturized, multiband, wideband, and tunable. Although these features are now available, the requirements of 5G will be more stringent.

In addition, the 5G mobile communication antenna also faces a new problem - coexistence.

To implement Massive MIMO, multiple antennas are required for transmission and reception, that is, multiple antennas with the same frequency (8 antennas, 16 antennas, ...). The biggest challenge that this multi-antenna system brings to the terminal is the coexistence problem.

How to reduce the influence of each other to couple, how to increase the isolation of the channel .... This puts forward new requirements for the 5G terminal antenna.

Specifically speaking, the following three points will be involved:

To reduce the mutual influence, especially the mutual interference between different functional modules and different frequency bands. Before the academic community believed that this situation would not exist, it did exist in the industry.

Decoupling, in the MIMO system, the mutual coupling of antennas will not only reduce the channel isolation, but also reduce the overall system's radiation efficiency. In addition, we cannot rely entirely on high-frequency millimeter waves to solve the performance increase. For example, 25GHz, 28GHz...60GHz all have system problems.

De-correlation, which can be solved from the antenna and circuit design, but the solution to the solution through the circuit bandwidth is very limited, it is difficult to meet the bandwidth of all frequency bands.

Antenna technology for 5G systems

This includes the design of a single antenna as well as system-level technologies, mentioned at the system level above, such as multi-beam, beamforming, active antenna array, Massive MIMO, etc.

From the perspective of specific antenna design, the technology developed based on the concept of metamaterials will be of great benefit. Currently, metamaterials have achieved success on 3G and 4G, such as achieving miniaturization, low profile, high gain, and frequency bands.

The second is a substrate or package integrated antenna. These antennas are mainly used in relatively high frequency bands, that is, millimeter wave bands. Although the antenna size of the high frequency band is small, the loss of the antenna itself is very large, so it is preferable to integrate the antenna with the substrate or the smaller package at the terminal.

The third is the electromagnetic lens. The lens is mainly used in high frequency bands. When the wavelength is very small, putting a medium can go to focus. The high frequency antenna is not very large, but the wavelength of the microwave is very long, which makes the lens difficult to use. It will be great.

The fourth is the application of MEMS. When the frequency is very low, MEMS can be used as a switch. In a mobile terminal, if an antenna can be effectively controlled and reconstructed, an antenna can be used for multiple purposes.

Taking an electromagnetic lens as an example, this design introduces the concept of placing an electromagnetic lens in front of a multi-element antenna array (herein a lens applied to the low-end frequency band of microwave or millimeter wave, unlike a conventional optical lens) when light When incident from a certain angle, a spot is created on a focal plane, and a large amount of power is concentrated on this spot, which means that the entire part of the capacity is received in a small area.

When the incident direction changes, the position of the spot on the focal plane also changes. As shown above, when the angle is projected, a black color energy distribution is generated. If it is incident at a certain angle θ (red color), the main energy deviates from the black color region.

This concept can be used to distinguish where the energy comes from. The direction of the incident and the energy on the array or the focal plane are in a one-to-one correspondence. Conversely, stimulating the antenna in different positions will cause the antenna to radiate in different directions. This is a one-to-one correspondence.

If multiple units are used to radiate in the focal plane, multiple carrier beam radiations can be generated, that is, so-called beamforming; if the switching between these beams occurs, beam scanning occurs; if these antennas are used simultaneously, Massive MIMO can be achieved. This array can be large, but high gain radiation can be achieved with very few arrays per beam.

If an ordinary array has the same size, every time the energy is received, all units must receive energy in this area. If only a single unit is received in a large area, the received energy is only a very small part; and The difference of the array is that the same caliber can receive all the energy with only a few units without any loss. Different angles come in. These energy can be received by different places at the same time.

This greatly simplifies the entire system. If there is only one direction for each job, only a partial antenna can work, which reduces the number of simultaneous antennas. The concept of the sub-array is different, it is to make the local multi-antenna form a sub-array, this time the number of channels decreases with the increase of the number of sub-array units. For example, a 10×10 array, if it becomes a subarray with 5×5, then it becomes only four independent channels, and the entire number of channels is reduced.

The figure on the right shows the effect of the lens on the system at baseband. The horizontal direction is the number of antennas. Suppose there is 20 units in the horizontal direction. In the case of a lens, only 5 units are used to accept the lens. The energy after being focused is better than all the 20 units without lenses. The former has higher communication quality and lower cost and power consumption. Even in the worst case, where the waves are incident from all directions, the effect of these 20 units on the latter is the same. So using the lens can improve the performance of the antenna - with a small number of antennas, to achieve the effect of large arrays in the past.

From this PPT, it can be seen that using an electromagnetic lens can reduce costs, reduce complexity, increase radiation efficiency, and can also increase the filtering characteristics of the antenna array (shielding of interfering signals) and so on.

This PPT shows the antenna used in the 28 GHz millimeter-wave band and uses 7 unit antennas as the feed.

As shown on the left, the front lens is a screen lens made of metamaterials. The two layers of PCB are carved into different shapes to adjust the phase to achieve focusing in a specific direction. The right side shows the performance of seven radiating elements, the lobe width is 6.8°, the side lobe is 18 dB or less, and the gain is 24-25 dB.

This experiment verifies the application of the electromagnetic lens at the base station, and also verifies the role of the metamaterial technology in antenna miniaturization.

Millimeter wave antenna design

As we all know, 5G will have low frequency band and two millimeter wave frequency bands, and the wavelength of the millimeter wave is very short and very expensive. So in 5G communication, we must solve this problem.

The first solution is a substrate integrated antenna (SIA).

This type of antenna is mainly based on two techniques: When the empty waveguide transmits, the loss caused by the medium is very small, so the empty waveguide can be used for feed transmission. However, there are several problems. Because it is an air waveguide, which is very large in size and cannot be integrated with other circuits, it is suitable for high-power, large-volume application scenarios. The other is the microstrip line technology, which can be mass-produced, but It is a loss in itself as a transmission medium and it is difficult to construct a large-scale antenna array.

Substrate-integrated waveguide technology can be produced based on these two techniques. This technique was first proposed by the Japanese industrial community. In 1998, they published the first paper on the waveguide structure of dielectric integration. They mentioned that a waveguide is implemented on a very thin dielectric substrate and a small pillar is used to block electromagnetic waves and avoid Both sides expand. It is easy to understand that when the two small pillars are separated by a quarter of a wavelength of fish, energy will not leak out. This can result in high efficiency, high gain, low profile, low cost, easy integration, and low loss. Antenna.

The lower right of the figure above is a 60GHz antenna made using this technique on the LTCC, with a gain of 25dB and a size of 8x8 cells.

This scheme is suitable for the application of millimeter waves at the base station, and there is another scheme on the mobile terminal.

The second solution is to design the antenna in a package integrated antenna (PIA).

Because the biggest problem of the antenna on the chip is that the loss is too large, and the size of the chip itself is very small, the antenna design will increase the cost, so it can hardly get large-scale application in engineering. If the antenna is designed using a package (larger than the chip size) as a carrier, not only a single antenna but also an antenna array can be designed. This avoids the limitation of volume, loss and cost of the antenna directly on the silicon.

In fact, the antenna can not only be inside the package, but also at the top, bottom, and around the package.

Another point that needs attention is whether you can use the PCB board as an antenna. The answer is yes.

The key bottleneck is not the material itself, but the material design problems and processing problems. However, the PCB is only suitable for frequencies below 60GHz. LTCC is recommended after 60GHz, but after 200GHz, there is a bottleneck in LTCC.

to sum up

In the future, the antenna must be designed together with the system rather than a separate design. It can even be said that the antenna will become a bottleneck for 5G. If the bottleneck is not broken, the signal processing on the system cannot be achieved, so the antenna has become a 5G mobile communication system. Key technologies. The antenna is more than just a radiator. It has filtering characteristics, amplification, and suppression of interference signals. It does not require energy to achieve gain, so the antenna is more than just a device.

Wonderful question and answer

Q: What do domestic antenna companies do well? Is the 5G industry chain ready?

A: There are many leading antenna companies in China. The best base station antenna manufacturers in the world are located in China. There are several other foreign-funded enterprises in China. There are many scenarios for 5G. We are not sure which one will eventually be used. However, from now on, the existing devices can basically meet the requirements.

Q: In the future 5G terminal, what principles should be followed when designing the antenna position?

A: How many locations on the future 5G terminals can deploy antennas to us is a problem. At present, the design of the antenna still follows the system, and the system is well designed to take into account the position of the antenna. From a technical point of view, the farther away from the head of the device the better. At present, mobile phones are generally dual antenna, the main antenna is generally in the lower half, because the head has energy absorption and shielding; in addition, the antenna is shared as much as possible to reduce the antenna Occupied space; The third is a multi-antenna system, in principle, the farther away the better, but the area is limited, need to rely on space diversity, polarization diversity, to minimize the correlation between the antennas.

Q: There is a saying that the 5G antenna is an array patch. How does Professor Chen look?

A: If only the array is patched, then the entire 5G challenge will be greatly reduced, but this depends on the specific application. The lowest frequency band for 5G communications is 3GHz, which is almost the same as LTE, and it still uses array antennas. If it exceeds 5 GHz, it can be used as a patch or a patch, but after 28 GHz it is more suitable to use a patch, but a lens antenna or a waveguide slot antenna can also be used because the ohmic loss of the transmission of the high-frequency waveguide is relatively small, so that the entire system is In terms of efficiency, it is also possible to use a waveguide antenna. If it is limited to some form of antenna, it will limit the space used by the antenna.