Sun Huarong, Datang Mobile RF Specialist: There are still many challenges in the implementation and commercialization of large-scale antenna arrays.

At present, China has built the world's largest mobile communication network. In the 4G to 5G transition and 5G era, the quality assurance and technical improvement of antennas become more and more important, especially in the case of increasingly scarce spectrum resources, antenna innovation will open wireless. The new world of spectrum resource utilization provides the basic premise for the implementation of 5G eMBB, mMTC, uRLLC and other scenarios.

Sun Huarong, an RF expert at Datang Mobile, believes that the use of Massive MIMO is an important means to ensure high speed of mobile communication networks. It can realize 3D multi-beam shaping and up to 16-24 streams of multi-stream data, which can greatly improve Spectrum efficiency, increasing user transmission rate. Therefore, the large-scale antenna array is the largest research direction of 5G. However, Sun Huarong also pointed out that the commercial and implementation technologies of large-scale antenna arrays will face a lot of challenges.

First of all, in terms of engineering installation, the volume, windward area, weight, and requirements for optical modules of large-scale antenna arrays need to be broken.

Recalling the 3G era, the smart antenna consisted of "a large door panel (antenna), a tumor (RF power amplifier TPA) and a dice (two sets of RF lines)", and later adopted a dual-polarized antenna array, which reduced the antenna bandwidth and RF. RRU initially solves the problem of tumors and scorpions, and smart antennas are actually used.

For large-scale antenna arrays, the current mainstream view in the industry is 128 antenna elements and 64 RF channels, each of which drives two vertical elements. The antenna array is arranged in an 8*8 dual-polarized antenna array with a horizontal antenna array spacing of 0.5λ and a vertical spacing of 0.5-0.75λ. Since there are 64 RF channels, plus one calibration channel, there are 65 RF connection interfaces. If the antenna array and the active radio remote unit (RRU) are independent of each other, there will be a large number of RF connection cables, and commercial engineering installation will face major challenges.

Now the RRU and antenna cables have been increased to 65, and the construction will be more complicated. Therefore, the integration of the antenna array and the RF remote unit to form an active antenna unit (AAU) will be an inevitable choice. However, since the AAU includes a 128-element antenna array and a 64-channel RF unit, the volume will be larger, and the windward area of ​​the AAU device will also become larger. These factors will affect the actual commercial deployment.

Secondly, reducing the weight of the AAU is a problem that must be solved in commercial applications, as this will directly affect the ability to mount the bracket and the ease of installation.

In addition, large-area arrays (AAUs) require up to 64 RF channels, each of which requires a bandwidth of 100 MHz. Thus, the baseband IQ data between the AAU and the BBU will be massive, several times that of the 4G era. According to the 100MHz bandwidth data sampling rate is 122.88MSPS, the data bit width is 16bit, the 64-channel baseband IQ data rate will be: 122.88M*2*16*64=251658.24M, the data rate exceeding 250Gbps; plus the transmission channel The encoding will be close to 300Gbps. If you use 10G fiber transmission, you need more than 20. This is unacceptable for both the optical module and the number of fibers, which will be the third challenge for large-scale antenna arrays.

To solve this problem, multiple measures can be taken: compress IQ data on one hand and reduce the transmission bandwidth requirement; at the same time, some baseband processing can be moved up to AAU to reduce the transmission bandwidth requirement; on the other hand, the optical fiber transmission rate can be improved. If it is increased to 25G or 100G, but this will bring great pressure on the optical module both in cost and technology.

In addition to the engineering installation problems mentioned above, large-scale antennas also present many challenges in implementing the technology.

The first is the issue of heat dissipation, which is the biggest challenge for AAU to commercialize. Since AAU is an outdoor unit, it needs to work around the clock and needs to meet the requirements of outdoor environment work. Outdoor equipment requires an operating temperature of -40 degrees to 55 degrees. Because AAU has a large number of channels, up to 64 RF channels, which is more than 8 times that of previous devices. In addition to power amplifiers and analog transceivers, FPGAs require a large number of FPGAs with digital IF processing. The channel base power consumption will increase significantly. The overall efficiency is much lower than that of traditional RRUs. A lot bigger. However, the weight and volume of the AAU must be as small as possible to meet the actual commercial engineering installation conditions. This will greatly increase the heat flux density of the whole machine, and it is the key to increase the heat dissipation capacity per unit volume. At the same time, the FPGA monolithic power consumption is large, and the high temperature resistance is poor, which is the biggest bottleneck of heat dissipation. Solving the problem of single point high heat dissipation heat has become a top priority. Solving the heat dissipation problem of AAU In addition to improving the heat dissipation efficiency in structural design, improving the efficiency of the whole machine is another important direction.

At the same time, in order to solve the miniaturization of the whole machine, the high integration of the device and the miniaturization of the device are the key points. Only by increasing the integration can the functions of multiple devices be completed in one chip, thereby reducing the size. The miniaturization of the device can reduce the size of a single device. The miniaturization of output filters in AAU is an important point. In order to filter out-of-band spurs in the RRU and meet the coexistence co-location spur requirements and blocking requirements of the base station, there must be an analog filter at the RRU output. The requirements of the analog filter are very high. Traditional base station output filtering uses a cavity filter, which is bulky. In large-scale antenna arrays, there are many RF channels, increasing to 64, and the number of antenna filters is increased to 64. If a cavity filter is also used, the volume will become very large. Therefore, the miniaturization of the antenna filter will be the direction that AAU will focus on. Of course, in addition to the miniaturization of the antenna filter, miniaturization of other devices is also important.

In addition to the two important technical challenges analyzed above, AAU's architectural design, inter-module connectivity, high-speed interconnects, high-speed routing, large bandwidth processing, power consumption, efficiency, production and installation, and production processes all require constant innovation. Only by solving these challenges one by one can the commercialization of large-scale antenna arrays be realized.

Antenna is a key element to support mobile communication systems. Promoting the innovation of large-scale antenna array technology and accelerating commercialization will directly affect the system capability of 5G. It is believed that Datang Mobile will continue to overcome difficulties and help with its leading technical strength in the field of antennas. 5G accelerates commercial use.