What is the future antenna technology of 2G to 5G?

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

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

Now, the next-generation communication technology, 5G, has entered the end of the standard-setting phase, and major operators are actively deploying 5G devices. Undoubtedly, 5G will bring a new experience to users, it has a transmission rate ten times faster than 4G, and puts new requirements on the antenna system. In 5G communication, the key to achieving high speed is millimeter wave and beamforming technology, but the traditional antenna obviously cannot meet this demand.

Circuit characteristics and radiation characteristics are important indicators of base station antennas, such as gain, lobe width, front-to-back ratio, standing wave ratio, isolation, third-order intermodulation, and so on. As the antenna age increases and the intermittent high power input, the RF path temperature rises rapidly, accelerating the aging of the material, causing the attenuation of its radiation characteristics to affect the entire base station system.

What kind of antenna does 5G communication need? This is a problem that engineering developers need to think about.

A new round of technology and industrial transformation, represented by information technology, is gradually gestating and upgrading. With the proliferation of video traffic, the growth of user equipment and the popularity of new applications, there is an urgent need for the rapid maturity and application of the fifth-generation mobile communication system (5G), including mobile communications, Wi-Fi, high-speed wireless data transmission, without exception. The need for faster transfer rates, lower transfer latency, and higher reliability. In order to meet the high data rate requirements of mobile communications, one needs to introduce new technologies to improve spectrum efficiency and energy utilization efficiency, and the second is to expand new spectrum resources.

Broadband antenna miniaturization

Passive antenna activation

Fixed antenna reconfigurable

HF antenna integration

Military antenna civilization

Two new types of antenna technologies, including tightly coupled terminal antennas based on coupled resonator decoupling networks; MIMO based on metamaterials (super-surface), Massive MIMO antenna array coupling reduction and performance improvement techniques. Through the testing and evaluation of passive parameters, active parameters, and MIMO parameters, the obvious advantages and broad application scenarios of these two new types of antennas in 5G are confirmed.

In this context, large-scale multi-input and multi-output technology (Massive MIMO) has become an irreversible core technology for improving spectrum efficiency in the next generation of mobile communication systems. Multiple Input-Output Technology (MIMO) can effectively utilize multiple spatial channels existing between multiple antennas between transceiver systems to transmit multiple mutually orthogonal data streams, thereby improving data throughput without increasing the communication bandwidth. Rate and stability of communication. Massive MIMO technology goes one step further on this basis. Based on limited time and frequency resources, hundreds of antenna units are used to serve up to dozens of mobile terminals simultaneously, which further improves data throughput and energy usage. effectiveness

Evolution and trend of mobile communication base station antenna

The base station antenna is developed along with network communication, and engineers design different antennas according to network requirements. Therefore, in the past few generations of mobile communication technologies, antenna technology has also been evolving.

The first generation of mobile communications used almost all omnidirectional antennas. At that time, the number of users was small and the transmission rate was low. At this time, it was also an analog system.

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

The antennas of the 1980s were mainly dominated by single-polarized antennas, and the concept of arrays has begun to be introduced. Although omnidirectional antennas also have arrays, they are only vertical arrays, and single-polarized antennas have planar and directional antennas. In terms of form, the current antenna is very similar to the second generation antenna.

In 1997, dual-polarized antennas (±45° cross-multi-polarized antennas) began to enter the historical arena. At this time, the performance of the antenna has been greatly improved compared with the previous generation. Whether it is 3G or 4G, the main trend is dual-polarized antenna.

In the 2.5G and 3G eras, multi-band antennas have emerged. Because the system at this time is very complicated, such as GSM, CDMA, etc. need to coexist, multi-band antenna is an inevitable trend. In order to reduce costs and space, multi-band has become the mainstream at this stage.

By 2013, we introduced the MIMO (Multiple-Input Multiple-Output) antenna system for the first time. Originally a 4x4 MIMO antenna.

MIMO technology has increased communication capacity, and the antenna system has entered a new era, from the original single antenna to the array antenna and multiple antennas.

However, now we need to look into the distance, the deployment of 5G has started, what role does antenna technology play in 5G, and what effect will 5G have on antenna design? This is a problem 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 added to customize the antenna. However, the current concept of 5G is still unclear. R&D personnel who do antenna design need to be prepared in advance to provide solutions for 5G communication systems, and even influence the customization and development of 5G standards through new antenna solutions or technologies.

From another perspective, array antennas, multi-band antennas, and multi-beam antennas constitute the "magic triangle" for the development of base station antennas.

Massive MIMO

The base station is equipped with a large-scale antenna array, which utilizes spatial freedom formed by multiple antennas and effective multipath components to improve the spectrum utilization efficiency of the system.

Multi-beam antenna

Multi-beam antennas use multiple beams to split the sector, increasing capacity.

2G to 4G base station antenna development

In the 2G/3G era, the antenna is mostly 2 ports.

▲GSM antenna

▲CDMA antenna

▲ LTE-FDD independent 2-port antenna (2T2R)

In the 4G era, with the massive use of MIMO technology and multi-band antennas, we saw that the antenna on the tower is like a big beard.

▲ LTE-FDD independent 4-port antenna (2T4R)

▲CDMA (1T2R)/LTE-FDD (2T4R) 6-port dual-band antenna

▲LTE-TDD 8T8R 8-port antenna

Together with the RRU on the tower, the scene on the tower is quite spectacular...

What might the future base station antenna look like?

With the evolution of the C-RAN network structure, the RRU is far away, and various hidden antennas will appear...

From the experience of cooperation and exchange between mobile communication companies in the past few years, 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 of the network will become more and more detailed, we need to customize the design according to the surrounding scenes, for example, the station in the urban area will be more elaborate, rather than simple coverage. 5G communication will use high-frequency bands, obstacles will have a great impact on communication, and customized antennas can provide better network quality.

The second trend is the systematic and complex antenna design.

For example, beam array (implementing space division multiplexing), multi-beam and multi/high-frequency bands. These all place high demands on the antenna, which will involve the entire system and compatibility issues. In this case, the antenna technology has surpassed the concept of components and gradually entered the design of the system.

The evolution of antenna technology: from the 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 trend

For base stations, a major principle of antenna design is miniaturization.

The antennas of different systems are designed together. In order to reduce the cost and save space, the antennas are small enough. Therefore, the antennas need multi-band, wide-band, multi-beam, MIMO/Massive MIMO, and MIMO isolation. Massive MIMO has some special requirements for the hybrid coupling of antennas.

In addition, the antenna also needs to be tunable.

The first generation of antennas was mechanically used to achieve tilt angles, and the third generation achieved remote ESCs. 5G is very attractive if it can achieve self-tuning. For mobile terminals, the requirements for the antenna are also miniaturized, multi-band, wide frequency band, and tunable. Although these features are now available, the 5G requirements will be more demanding.

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

To implement Massive MIMO, multiple antennas are required for transmission and reception, that is, multi-antenna (8-antenna, 16-antenna...). The biggest challenge for such a multi-antenna system to the terminal is coexistence.

How to reduce the mutual influence to the couple, how to increase the isolation of the channel... This puts new requirements on the 5G terminal antenna.

Specifically, it will cover the following three points:

1. Reducing the mutual influence, especially the different functional modules, the mutual interference between different frequency bands, which was not considered by the academic community before, but this problem does exist in the industry;

2. Decoupling, in MIMO systems, the mutual coupling of the antenna not only reduces the isolation of the channel but also reduces the radiation efficiency of the entire system. In addition, we can't expect to rely entirely on high-band millimeter-waves to address performance gains, such as 25GHz, 28GHz...60GHz, all with system problems;

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

Antenna technology for 5G systems

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

From the perspective of specific antenna design, the technology developed by the metamaterial-based concept will be of great benefit. Metamaterials have been successful in 3G and 4G, such as miniaturization, low profile, high gain, and band.

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

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

The fourth is the application of MEMS. At very low frequencies, MEMS can be used as a switch. In mobile terminals, if the antenna can be effectively controlled and reconstructed, an antenna can be used.

If multiple cells are radiated on the focal plane, radiation of multiple carrier beams can be generated, which is called beamforming; if switching between these beams, beam scanning occurs; if these antennas are used simultaneously, Massive MIMO can be implemented. This array can be large, but high gain radiation can be achieved with very few arrays per beam.

Ordinary arrays, if they have the same size, each time the energy is received, all the cells 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; The difference in the array is that the same caliber can receive all the energy with only a few units without any loss. Different angles come in, and this energy can be received simultaneously in different places.

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

Millimeter wave antenna design

Another key technology of 5G is the high-frequency band (millimeter wave) transmission. Traditional mobile communication systems, including 3G and 4G mobile communication systems, whose operating frequencies are mainly concentrated below 3 GHz, and the spectrum resources have been extremely crowded. The communication system working in the high-frequency band has rich spectrum resources available and is more likely to occupy a wider continuous frequency band for communication, thereby meeting the requirements of 5G on channel capacity and transmission rate. Therefore, in November 2015, the World Radiocommunication Conference WRC-15, in addition to determining 470~694/698 MHz, 1427~1518 MHz, 3300~3700 MHz, and 4800~4990 MHz as important frequencies for 5G deployment, It has also been proposed to study several frequency bands within 24.25~86GHz in order to determine the frequency bands needed for future 5G development.

5G will have two bands of low-frequency band and millimeter wave, and the wavelength of millimeter wave is very short and the loss is very large, so in 5G communication, we must solve this problem.

5G low-frequency band: mainly refers to the frequency band below 6GHz.

Recently, the Ministry of Industry and Information Technology issued a draft opinion indicating:

The 3.3G-3.40GHz frequency band is basically confirmed as the 5G frequency band and is limited to indoor use in principle;

In the 4.8G-5.0GMHz frequency band, the specific frequency allocation is determined according to the needs of the operator.

The 4.4G-4.5GMHz band is added, but it cannot cause harmful interference to other related radio services.

5G high-frequency band: mainly refers to the frequency band above 20GHz.

China is mainly collecting opinions in the high-frequency bands of 24.75-27.5GHz and 37-42.5GHz, and the test is mainly carried out at 28GHz in the world.

Millimeter wave mobile communication also has shortcomings such as short transmission distance, poor penetration and diffraction capability, and vulnerability to the climatic environment. Therefore, high-gain antenna arrays with adaptive beamforming and beam steering capabilities are naturally the key technologies for 5G applications in the millimeter range.

However, considering the above-mentioned system, the actual application scenario and application environment of the antenna array, when the 5G base station with the Massive MIMO antenna array is built, the volume of the antenna array cannot be large due to the limited space. When the physical size of the antenna array is limited, the mutual coupling and interference between multiple antenna elements will inevitably lead to the degradation of the antenna performance, mainly in the following aspects:

(1) The antenna side lobes are relatively high, which has a great influence on the beam scanning capability of the array;

(2) The signal-to-noise ratio is deteriorated due to mutual interference between the antenna elements, which directly affects the data throughput rate;

(3) The energy that enables effective radiation is reduced, resulting in a decrease in antenna array gain and low energy utilization efficiency.

In summary, in the low-frequency band and high-frequency band applicable to 5G, it is urgent to find an effective theory and design method for improving the performance of the space-limited Massive MIMO antenna array, which can reduce the size of the antenna array and maintain the original Antenna array performance.

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

This kind of antenna is mainly based on two technologies: the loss caused by the medium when the air waveguide is transmitted is small, so the air waveguide can be used for feed transmission. However, there are several problems. Because it is an air waveguide, it is very large in size and cannot be integrated with other circuits, so it is suitable for high-power, large-volume applications. The other is microstrip technology, which can be mass-produced, but It is inherently a loss of transmission medium and it is difficult to construct a large-scale antenna array.

Based on these two technologies, a substrate integrated waveguide technology can be produced. This technique was first proposed by the Japanese industry. In 1998, they published the first paper on waveguide structure for dielectric integration. It was mentioned that the waveguide was realized on a very thin dielectric substrate, and the electromagnetic waves were blocked by small columns to avoid Expanded on both sides. It is not difficult to understand that when the distance between the two small columns is the one-quarter wavelength of the small fish, the energy will not leak out, which can form high efficiency, high gain, low profile, low cost, easy integration, low loss Antenna.

This scheme is suitable for the application of millimeter waves on 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 with the antenna on the chip is that the loss is too large, and the size of the chip itself is small, the design of the antenna will increase the cost, so it is almost impossible to obtain large-scale applications in engineering. If the antenna is designed with a package (larger than the chip) as a carrier, not only a single antenna but also an antenna array can be designed, which avoids the limitation in size, loss, and cost of the directional antenna on the silicon.

In fact, the antenna can be designed not only inside the package but also at the top, bottom and around the package. Another point to be aware of is whether the PCB can be used as an antenna. The answer is yes. The key bottleneck is not the material itself, but the design and processing problems that the material brings. However, the PCB is only suitable for the frequency band below 60 GHz, and LTCC is recommended after 60 GHz, but after 200 GHz, LTCC also has a bottleneck.

to sum up

In the future, the antenna must be designed together with the system instead of being designed separately. It can even be said that the antenna will become a bottleneck of 5G. If the bottleneck is not broken, the signal processing on the system cannot be realized, so the antenna has become a 5G mobile communication system key technology. An antenna is not 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.