5G communication "killer": large-scale antenna array technology perfectly matched with millimeter wave

Faced with strong demand, relying on scarce resources, how to meet the needs of the lowest rate of the era of science and technology, and set up a high-speed information pipeline for the society? "5g" is giving the answer. 5G, the fifth-generation wireless communication system, is another peak that communicators are climbing after passing the analog communication, the second generation, the third generation, and the fourth-generation LTE system that is being experienced.

This is the best era and the worst.

Living in the era of technological explosion, do you feel a little fortunate? Virtual reality, autonomous driving, and countless new and explosive applications are erupting, blurring the boundaries between virtual and reality, and profoundly changing the way we touch and recognize the world.

And this is a tough battle for the communicator.

It is well known that wireless communication relies on electromagnetic wave propagation, and the most precious resource is the frequency band. In order to prevent mutual interference of mobile communication networks, wireless television, broadcasting, military frequency bands, etc., each country has made a strict division of the use of wireless frequency bands. According to the characteristics of electromagnetic waves propagating in the air, the frequency band below 6G Hertz is regarded as a high-quality band resource because of its small attenuation in air and strong penetrating power. Many applications relying on radio are concentrated in this frequency band resource. Very crowded.

On the other hand, users' data demands on mobile communication networks are experiencing explosive growth, especially for wireless applications that need to transmit large amounts of data in real time, such as live video, high-definition teleconferencing, virtual reality games, etc., which is a severe test for network capacity. . The "mission-critical communication machine" (mission-criTIcal machine type communicaTIon) but also on reliability and latency communication made extremely stringent requirements, such applications include industrial automation, telecommunications and other vehicles. 1000 times the network capacity of the 4G LTE system and the extremely low latency of 1 millisecond have gradually become the consensus of the industry for the requirements of the next generation wireless communication network.

Faced with strong demand, relying on scarce resources, how to meet the needs of the lowest rate of the era of science and technology, and set up a high-speed information pipeline for the society? "5g" is giving the answer. 5g (Fifth GeneraTIon), i.e., the fifth-generation radio communication system, analog communication is passed, after the second generation, third generation and fourth generation are experiencing the LTE system, a communication with another person is climbing the highest peaks.

A system of innovation, which must contain countless innovations, 5g is also true. I will share with you a key technology, large-scale antenna array. Its application can not only greatly improve the network capacity and user experience, but also have a profound impact on the communications industry. The era of downloading a high-definition movie in 1 minute is not far from us.

Understanding large-scale antennas first requires an understanding of beamforming techniques. The traditional communication method is the electromagnetic wave propagation from single antenna to single antenna between the base station and the mobile phone. In the beamforming technology, the base station has multiple antennas, which can automatically adjust the phase of the signals transmitted by the respective antennas to form electromagnetic waves at the receiving point of the mobile phone. Superimposed to achieve the purpose of increasing the strength of the received signal.

From the perspective of the base station, the superposition effect produced by the digital signal processing is like the completion of the construction of the base antenna virtual antenna pattern, so it is called "Beamforming". Through this technology, the transmitted energy can be collected at the user's location without spreading in other directions, and the base station can track the user's signal and track it in real time, so that the optimal launch direction follows the user's movement, ensuring that at any time. The electromagnetic wave signals at the receiving point of the mobile phone are in a superimposed state.

For example, traditional communication is like a light bulb, illuminating the entire room, and the wave speed is shaped like a flashlight, and the light can be intelligently collected to the target position.

In practical applications, a multi-antenna base station can also target multiple users at the same time, construct different beams toward multiple target customers, and effectively reduce interference between beams. This multi-user beamforming effectively separates electromagnetic waves between different users in space and is the basis of large-scale antennas.

Large-scale antenna array

The large-scale antenna array is based on the principle of multi-user beamforming. Several hundred antennas are arranged at the base station, and the respective beams are modulated for dozens of target receivers. The spatial signals are isolated and transmitted simultaneously on the same frequency resource. signal. This full exploitation of space resources can effectively utilize valuable and scarce frequency band resources and increase network capacity dozens of times.

You can see an antenna array consisting of 64 small antennas from the large-scale antenna array prototype of Rice University in the United States below, which is a good example of the prototype of a large-scale antenna system.

American University of Rice Argos Large-Scale Antenna Array Prototype

Why can a large-scale antenna array shake the underlying communication?

Large-scale antennas do not simply amplify the number of antennas because quantitative changes can cause qualitative changes. According to the large number theorem and the central limit theorem, the number of samples tends to infinity, the mean tends to the expected value, and the mean distribution of independent random variables tends to be normally distributed. Random variables tend to be stable, which is the beauty of "big".

In a single-antenna-to-single-antenna transmission system, due to the complexity of the environment, electromagnetic waves may propagate in multiple paths in the air and may be opposite in phase at the receiving point, weakening each other. At this time, the channel is likely to be trapped in strong fading, affecting The quality of the signal received by the user. When the number of base station antennas increases, there are hundreds of channels with respect to hundreds of antennas of the user, and they are independent of each other, and the probability of falling into fading is greatly reduced, which is simple and easy to handle for the communication system. .

What are the benefits of large-scale antennas?

First, of course, it is to greatly increase network capacity.

Second, because there are a bunch of antennas at the same time, the signal superposition gain formed by the wave velocity shaping will make each antenna only need to transmit signals with low power, thus avoiding the use of expensive large dynamic range power amplifiers and reducing hardware costs.

Third, the flat fading channel created by the law of large numbers makes low-latency communication possible. In order to counter the deep fading of the channel, the traditional communication system needs to use channel coding and interleaver to spread the continuous burst error caused by deep fading to different time periods (the purpose of the interleaver is to be noisy at different time periods, thus Disperse a continuous error for a short period of time), and this noisy process causes the receiver to accept all data in its entirety to obtain information, causing delays. Under large-scale antennas, the fading due to the large number theorem disappears, the channel becomes good, and the process of resisting deep debilitation can be greatly simplified, so the delay can be greatly reduced.

It is worth mentioning that the perfect match with the large-scale antenna is another key technology of 5g - millimeter wave. Millimeter waves have a lot of bandwidth, but the attenuation is strong, and the beamforming of large-scale antennas just complements the short board.

First of all, in order to realize the potential of all antennas, the base station needs precise channel information, and intuitive understanding requires that the location of different target customers be known in advance. How to accurately tell each antenna with this channel information between users is a very tricky thing.

The traditional communication system monitors the pilot (the pilot, which is a sequence known jointly by the base station and the mobile terminal) sent by the base station through the mobile phone, and estimating the channel and feeding back to the base station is not feasible in the large-scale antenna because the number of base station antennas is large. The uplink resources consumed by the mobile phone when feeding back to the base station are too large. At present, the most feasible solution is based on time-division duplex (TDD) uplink and downlink channel symmetry, that is, transmitting a pilot to a base station through a mobile phone, monitoring an uplink at a base station, and inferring a base station based on channel symmetry. Downlink information on the mobile side.

Secondly, in order to obtain uplink information, the mobile terminal needs to transmit pilots to the base station, but the number of pilots is always limited, which inevitably needs to be multiplexed in different cells, which may cause pilot interference. Theoretical derivation shows that pilot interference is the ultimate barrier to limiting large-scale antennas.

In addition, many large-scale antenna beamforming algorithms are based on matrix inversion operations, and their complexity increases rapidly with the number of antennas and the number of users simultaneously serving them, resulting in hardware not being able to complete the beamforming algorithm in real time. The fast matrix inversion algorithm is one way to overcome this problem.

In order to overcome these challenges, the world's top research institutions and major equipment manufacturers are stepping up the development of prototypes. In addition to the Argos prototype of Rice University in the United States mentioned above, there are Samsung MMW large-scale antenna prototypes, LumaMi prototype of Lund University in Sweden, and Eurecom Open Air Interface large-scale antenna prototype. The University of Bristol, UK, prototypes, etc.

I had the honor to be a challenge to the scientific and technological challenges involved in building the Open Air Interface 64 antenna model machine. The picture below is a miniature version (reduced to 16 antennas) designed to showcase our solution to the relevant technology bottlenecks and received good responses at the European Network and Communications Research Conference last month.