With the rapid development of mobile communication, the development of low-band spectrum resources is very mature, and the remaining low-band spectrum resources can no longer meet the peak rate requirement of 10Gbps in the 5G era. Therefore, the future 5G system needs to find available spectrum resources in the millimeter-wave band. . As one of the key technologies of 5G, millimeter wave technology has become the focus of research and discussion among various standards organizations and industry chains. Millimeter wave will bring many technical challenges to the realization of 5G terminals in the future, and the test plan of millimeter wave terminals. It will also be different from the current terminal. This paper will introduce the current situation of millimeter wave spectrum division, and introduce and analyze the challenges and test schemes of millimeter wave terminal technology .
2, millimeter wave spectrum divisionIn 2015, ITU-R WP5D released a research report on IMT.ABOVE 6GHz, detailing the attenuation characteristics of radio waves in different frequency bands. In the same year, the World Radiocommunication Conference (WRC-15) proposed a number of 5G candidate millimeter wave bands, and the final 5G millimeter wave spectrum will be determined at WRC-19. After years of research and discussion, the division of millimeter-wave spectrum resources has progressed in various regions and regions. The following will focus on the current situation of the division of millimeter-wave bands in China, the United States and Europe.
China: In June 2017, the Ministry of Industry and Information Technology collected opinions on 24.5-27.5 GHz, 37-42.5 GHz or other millimeter wave bands for 5G systems, and included the millimeter wave band in the scope of 5G tests, aiming to promote 5G millimeter waves. Research and development of millimeter wave products.
United States: As early as 2014, the FCC (Federal Communications Commission) opened the distribution of the 5G millimeter wave band. In July 2016, it decided to allocate 27.5-28.35 GHz, 37-38.6 GHz, and 38.6-40 GHz as licensed spectrum. For 5G, another 5G is allocated 64-71 GHz as the unlicensed spectrum.
Europe: In November 2016, RSPG (European Commission's Wireless Spectrum Policy Group) released the EU 5G spectrum strategy, which identified 24.25-27.5 GHz as the leading band for 5G in Europe, 31.8-33.4 GHz and 40.5-43.5 GHz as potential bands for 5G.
3, millimeter wave terminal technology implementationThe high frequency and large bandwidth of the millimeter wave band will bring many challenges to the realization of the 5G terminal in the future. The influence of the millimeter wave on the terminal mainly lies in the antenna and the RF front-end device.
3.1 Terminal side large-scale antenna array
Due to the limitation of antenna size, large-scale antenna arrays can only be used on the base station side in the low frequency band. However, as the frequency increases, in the millimeter wave band, the size of a single antenna can be shortened to the millimeter level, and it is possible to arrange more antennas on the terminal side. As shown in Figure 1 below, most LTE terminals currently only have two antennas deployed, but the number of antennas in the future 5G millimeter wave terminals can reach 16 or more, and all antennas will be integrated into one millimeter wave antenna module. Due to the larger free-path path loss of millimeter waves, the characteristics of gas failure and rain attenuation are not as good as the low frequency band, and the coverage of millimeter waves will be seriously affected. The large-scale antenna array on the terminal side can obtain more diversity gain, improve the receiving and transmitting performance of the millimeter wave terminal, and can compensate for the shortcoming of the millimeter wave coverage to a certain extent. The large-scale antenna array on the terminal side will be commercially available in millimeter waves. One of the key factors.
Figure 1: LTE terminal and millimeter wave terminal antenna
The deployment of more antennas on the terminal means that the terminal design difficulty is increased. Unlike the large-scale antenna array deployed on the base station side, the large-scale antenna array on the terminal side is limited by the terminal size and terminal power consumption, and the implementation difficulty is greatly increased. The arrangement of large-scale antenna arrays can be implemented on fixed terminals. The large-scale antenna array design of mobile terminals faces many challenges, including antenna array calibration, mutual coupling between antenna elements, and power consumption control.
3.2 millimeter wave RF front end device
RF front-end devices include power amplifiers, switches, filters, duplexers, low-noise amplifiers, etc. Among them, power amplifiers are the most core devices, and their performance directly determines the communication distance, signal quality and standby time of the terminal. At present, the materials for manufacturing low-frequency RF front-end devices are mostly gallium arsenide, CMOS and silicon germanium. However, due to the large difference between the millimeter wave band and the low frequency band, the manufacturing material of the low frequency RF front end device will be difficult to meet the requirements of the millimeter wave RF front end device in physical characteristics.
Taking power amplifier as an example, the current mainstream power amplifier manufacturing material is gallium arsenide, but in the millimeter wave frequency band, the manufacturing process of gallium nitride and InP is stronger than gallium arsenide in performance index. The table below shows the development of the major RF front-end device manufacturing processes from the low frequency to the millimeter band.
In addition, the large bandwidth of the millimeter wave band puts forward higher requirements for the RF front-end device. In the future, the RF front-end device of the millimeter wave terminal will need to support continuous bandwidth above 1 GHz.
Although GaN is considered to be the mainstream manufacturing process for future millimeter-wave terminal RF, high-performance RF front-end devices based on GaN process are mostly used in special scenarios such as military and base stations due to cost and capacity. The development of millimeter-wave RF front-end technology will become the key to the realization of millimeter-wave terminals. It is expected that the technology and cost of millimeter-wave mobile terminal RF devices will reach large-scale commercial requirements after 2020.
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