Brief introduction to the basic composition of 5G base stations:
From the perspective of equipment architecture, 5G base station architecture can be divided into:
BBU-AAU
CU-DU-AAU
BBU-RRU-Antenna
CU-DU-RRU- Antenna
Integrated gNB, etc.
BBU
Full name Building Base band Unit, Chinese name: baseband processing unit. An optical fiber connection is required between the RRU (Remote Radio Unit) and the BBU (Baseband Processing Unit) . One BBU can support multiple RRUs . The BBU+RRU multi-channel solution can well solve the indoor coverage of large venues.
In the 5G era, the physical structure of the BBU has also evolved into CU (Centralized Unit) and DU (Distributed Unit) due to the changed network framework of 5G. Among them, the part with strong real-time performance of the BBU has become a DU (distributed unit), while the non-real-time function of the BBU has evolved into a CU (centralized unit). In addition, the 5G core network function has sunk to the edge, and the CU will also carry part Functions of the core network. The full name of CU is Centralized Unit, which means centralized unit; the full name of DU is Distributed Unit, which naturally means distributed unit. The segmentation of CU and DU is carried out according to the real-time requirements of different protocol layers. Under this principle, the physical bottom layer in the original BBU is lowered to the AAU for processing, the physical high layer, MAC, and RLC layers that require high real-time performance are placed in the DU for processing, and the PDCP and RLC layers that do not require high real-time performance are placed in the DU. The RRC layer is placed in the CU for processing. To put it another way: DU is responsible for independent functions with high real-time requirements; CU is responsible for functions that require information aggregation and low real-time requirements.
RRU
The remote radio unit RRU (Radio Remote Unit) brings a new type of distributed network coverage mode. It places large-capacity macrocell base stations centrally in the available central equipment room, and the baseband part is processed centrally. Optical fibers are used to connect the base stations The radio frequency module is pulled to the remote radio unit and placed on the site determined by the network planning, thus saving a large number of equipment rooms required by the conventional solution; at the same time, by using a large capacity macro base station to support a large number of remote optical fibers, the capacity can be realized Conversions to and from overlays.
The working principle of the RRU is: the downlink baseband signal is frequency-converted, filtered, radio-frequency filtered, linear power amplifier, and then transmitted to the antenna through transmission filtering. In the uplink, the received uplink signal of the mobile terminal is filtered, low-noise amplified, further amplified and filtered for small radio frequency signals, and down-converted, and then complete analog-to-digital conversion and digital intermediate frequency processing. There are two types of connection interfaces between the RRU and the base station interface: CPRI (Common Public Radio Interface) and OBSAI (Open Base Station Architecture Initiative). In terms of signal coverage, the RRU can be led out from the NodeB through the same frequency and different scrambling codes. It can also be derived from the RNC through different scrambling methods on the same frequency. These two coverage methods are conventional methods. In addition, for 3 sectors, base stations with redundant channel boards and redundant baseband processing equipment can use the baseband pool sharing technology to set the redundant baseband processing equipment as the second 4 plots.
The BBU and the RRU are connected by an optical fiber, and the RRU and the antenna are connected by a coaxial cable. That is, the backbone optical fiber and the branch coaxial. The BBU can be simply understood as the brain; and the RRU can be understood as the hand, just to get the information back to the brain for processing. So it is not difficult to understand that one BBU can connect to multiple RRUs . 
The reason why AAU appeared in 5G is actually because 5G introduced the technology of Massive MIMO (large-scale multiple input and multiple output) . MIMO is a multiple-input multiple-output technology. This is actually easier to understand. You can think of it as a road. If you let more cars run on the road, you need more lanes. Obviously, the traffic volume of eight lanes is higher than that of four lanes. Much bigger. The higher the level of MIMO, the more antennas are required, the more antennas, the more feeders, the more feeder interfaces on the RRU, and the complexity of the process is getting higher and higher. . In addition, the feeder itself has a certain attenuation, which will also affect some system performance. It is also for this reason that in 5G, the RRU and the original passive antenna are integrated to form the latest AAU (Active Antenna Processing Unit) . The antenna is integrated, so the size and weight of the AAU are larger than the RRU, the power consumption is also relatively large, and the price is much more expensive. 
1) Improve coverage and reduce interference through beamforming (Beamforming)
Beamforming is to adjust the amplitude and phase of multiple antennas to give the antenna radiation pattern a specific shape and direction, so that the wireless signal energy can be concentrated on a narrower beam, thereby enhancing coverage and reducing interference .
With beamforming, precise user-level ultra-narrow beams can be formed and moved with the user's location to direct energy to the user's location. Compared with traditional wide-beam antennas, it can improve signal coverage and reduce user interference in the area.
At the same time, an available dimension can be added in the vertical dimension through 3D beamforming, so that the vertical coverage of the area can be adjusted more flexibly, and the traditional 2D wireless design method can be changed.
2) Increase regional capacity through spatial multiplexing
Massive MIMO, a large-scale antenna array, can send multiple data streams multiplexed in space to multiple users at the same time through MU-MIMO, thereby increasing the regional capacity exponentially.
If the wireless network is likened to a highway, this is equivalent to expanding several roads without increasing the spectrum bandwidth.
The massive antenna array Massive MIMO technology has many benefits, but the problem is that to realize the massive antenna array Massive MIMO, the use of multiple antennas is a prerequisite.
The performance potential of beamforming technology will increase as the number of antennas increases. For this reason, 5G Massive MIMO uses dozens or even hundreds of antenna elements. Moreover, according to the Ministry of Industry and Information Technology on July 19, the number of 5G base stations in my country has accumulated to 1.854 million, and the total number of 5G base stations for the whole year will exceed 2 million. Therefore, the AAU device integrating the radio frequency unit and the antenna unit is an inevitable choice for the development of social information technology.
In the future, even if operators move towards SA networking, there will still be a large number of 5G base stations co-sited with 4G base stations, and there will be many 5G base stations and 4G base stations sharing BBU, and in many rural areas in the future, it is entirely possible that there will be 4G, 4G and 4G base stations. 5G has a BBU, RRU, and antenna mode.
What should I do if in some hotspot areas with high traffic demand, even if the spectrum is increased and the spectrum efficiency is improved, the traffic demand of users still cannot be met, and at the same time, new sites cannot be obtained and new base stations cannot be built?
Only the method of adding sectors/cells can be used.
Therefore, operators will upgrade the traditional three sectors to six sectors to double the capacity. 
By counting, the tower is equipped with a total of 18 5G AAU devices, 3 on each pole, occupying 6 poles. This is a communication tower from Japanese operator NTT DOCOMO.
Brief introduction of 5G base station hardware architecture:
The overall architecture of 5G base station equipment can be divided into two categories: BBU+AAU/RRU 2-layer architecture, CU+DU+AAU/RRU 3-layer architecture.
Among them, CU and DU are baseband devices, which jointly complete all functions of 5G baseband protocol processing. The CU is responsible for the processing of the high-level baseband protocol and provides the backhaul interface with the core network; the DU completes the processing of the underlying baseband protocol and provides the fronthaul interface with the 5G AAU/RRU; the CU and the DU interact through the F1 interface. The BBU integrates all baseband processing functions of the CU and DU. At present, 5G base station equipment mainly adopts BBU+AAU/RRU 2-layer architecture, so the following mainly analyzes the hardware architecture of 5G BBU and AAU/RRU. The DU device architecture is similar to the BBU, and is mainly implemented based on a dedicated hardware platform, while the CU device is generally implemented based on a general-purpose hardware platform.
(1) 5GBBU hardware architecture
The 5G BBU is a baseband device. The hardware architecture is shown in Figure 1, including functional modules such as a baseband processing unit, a main control transmission unit, a power module, and an interface unit. Among them, the baseband processing unit mainly completes the baseband protocol processing and provides an interface for communication with AAU/RRU; the main control transmission unit is responsible for configuration management, signaling processing, resource management, and data transmission of the base station, providing transmission, clock, LMT interface, power supply module It is mainly used for the management of the internal DC power supply of the equipment
In terms of hardware implementation, the 5G BBU integrates a variety of semiconductor devices and chips, and the core devices inside the main control transmission unit and baseband processing unit are shown in Figure 2. The processor (CPU) is mainly used for high-level baseband protocol and control signaling processing; the baseband chip (ASIC) is the key chip of the BBU, responsible for the underlying baseband protocol processing and the realization of software algorithms; the FPGA chip is used for hardware acceleration in baseband protocol processing , to realize special functions such as encryption/decryption or interface conversion; the optical module is responsible for completing the photoelectric signal conversion function, which is used for fronthaul interface processing; the switching chip is used for data exchange with the external interface; the high-precision crystal oscillator is used to support various functions inside the BBU Synchronization between modules.
(2) 5G AAU/RRU hardware architecture
The 5G AAU/RRU mainly completes the conversion between the baseband digital signal and the RF analog signal and the transceiver processing function of the RF signal .
For frequency bands below 6 GHz, AAU equipment is mainly divided into mainstream specifications such as 64T64R, 32T32R, and 16T16R, which support 64, 32, and 16 RF transceiver channels respectively. As the number of channels increases, the bandwidth requirements of the CPRI interface increase significantly. In order to reduce the bandwidth requirements of the fronthaul interface,
The 5G AAU uses the eCPRI interface to move some of the underlying baseband protocol processing functions of the BBU to the AAU. For 5G radio equipment with low channel counts such as 2 channels and 4 channels, the traditional RRU+antenna equipment form is still used, and there is no built-in antenna array inside the equipment.
The hardware architecture of 5G AAU and RRU is basically the same. As shown in Figure 3, the device contains main modules and devices such as interface, digital baseband, digital intermediate frequency, transceiver, power amplifier, and duplexer . Among them, the interface module is mainly used for front-haul interface signal processing, the digital baseband module is responsible for the underlying baseband signal processing, the digital intermediate frequency module realizes functions such as up-down conversion, pre-distortion and crest factor reduction, and the transceiver module completes digital-to-analog/analog-to-digital conversion (ADC /DAC) and analog signal receiving and transmitting signal processing functions, the power amplifier/low noise amplifier completes the amplification of downlink and uplink signals respectively, the filter is used for frequency selection and interference suppression of transmitting and receiving signals, and the duplexer is used for receiving and sending channels. Signal filtering and transceiver switching.
5G Base Station Core Components and Industry Status
5G BBU core device
The 5G BBU is mainly implemented based on dedicated hardware, which integrates semiconductor devices such as ASIC, CPU, and FPGA. The industrial development status of core devices directly affects the performance of BBU equipment. On the one hand, the performance and process level of core devices determine the overall hardware processing capability and integration of BBU equipment; on the other hand, the development of the semiconductor industry can also promote the intergenerational replacement of dedicated hardware platforms, optimize the BBU hardware architecture, and improve equipment performance.
Inside the 5G BBU, the baseband chip is one of the most critical devices, which can reflect the performance differences of different devices. Baseband chips generally adopt the ASIC architecture self-developed by equipment manufacturers. The industry mainly adopts 14 nm or 7 nm technology, and 5 nm chips are in the technology introduction stage. TSMC and Samsung already have 5 nm mass production capabilities. The processors used by BBU are mainly based on ARM architecture and X86 architecture. High-performance processor chips are used to provide more powerful computing performance, lower power consumption, and support complex processing functions of 5G baseband. FPGA is Field Programmable Gate Array. Compared with AISC, it has the advantages of editability, more flexibility, and shorter time to market. 5G BBU uses FPGA to better support the backward upgrade of device software and hardware. Due to the high technical barriers in the industry, FPGA core technology is monopolized by leading companies such as Xilinx, Intel, and Lattice, and the three foreign giants occupy 90% of the global market share.
5G AAU/RRU core device
The core devices used by 5G AAU/RRU mainly include baseband chips, digital intermediate frequency chips, transceiver chips, ADC/DAC, power amplifiers, filters, etc. Among them, the power amplifiers used in 5G base stations mainly use LDMOS and gallium nitride technologies. Under the working conditions of high frequency, large bandwidth and high power, the performance of GaN power amplifier is better than that of LDMOS. Generally, 5G high-frequency devices use GaN power amplifiers, while low-frequency devices use both types of power amplifiers. LDMOS device technology is relatively mature, mainly using 8-inch 140 nm technology, and mainstream suppliers include NXP, Qorvo, etc. The cost of gallium nitride devices is relatively high, and the manufacturing process is more complicated. Major manufacturers include foreign manufacturers such as Sumitomo, Wolfspeed, and Qorvo, as well as domestic manufacturers such as Nengxun and Chuangyuanda. High-speed and high-precision ADC/DAC is an indispensable chip for 5G base stations. At present, the ADC/DAC market share is monopolized by foreign manufacturers such as ADI, TI, MAXIM, etc. Domestic manufacturers started late in the field of ADC/DAC chips, and there are few manufacturers capable of mass-producing high-precision, high-speed ADC/DAC, and their product lines are relatively single. . The capabilities of the baseband and digital IF chips need to meet the requirements of 100 MHz carrier bandwidth, 64 RF transceiver channels, and complex beamforming algorithm processing. It mainly adopts ASIC chips self-developed by main equipment manufacturers. Currently, it adopts 14nm or 7nm process, and the next-generation chip will support 5nm or 3nm technology. The transceiver chip is used for signal processing of the transceiver link, and can integrate functions such as digital frequency conversion, frequency mixing, multi-channel ADC/DAC, amplification and filtering. At present, the mainstream chip suppliers in the industry are ADI and TI. A single chip supports four RF channel processing. With the development of the process level, the processing capability of the single chip can be further improved, and the AAU volume and power consumption can be reduced. The filter used in 4G RRU is mainly a metal cavity filter, which has mature technology and low price, but the volume is relatively large due to the overall cutting of the metal. In the 5G era, the number of AAU antennas has increased significantly, and there are higher requirements for the size and heat generation performance of the filter, which limits the application of metal cavity filters. Ceramic dielectric filters are small in size and high in temperature stability, making them a better solution. plan. Therefore, in the early stage of 5G AAU, miniaturized metal filters with mature technology will be used, and ceramic dielectric filters will be mainly used in the later stage. At present, the large-scale ceramic dielectric filter manufacturers mainly include Canqin, Guohua, and Fangu.
grateful:
https://www.arrowsolution.com.cn/index.php/Home/Detail/articles/id/140.html
https://www.sohu.com/a/569583596_121337067
http://www.mojiax.com/dzsm/11056.html
https://baijiahao.baidu.com/s?id=1658312864299595416&wfr=spider&for=pc
https://3g.163.com/dy/article/H49SSJOG0531N0ME.html
https://baijiahao.baidu.com/s?id=1672892908759740796&wfr=spider&for=pc
https://blog.csdn.net/qq_63032461/article/details/127325363
https://blog.csdn.net/qq_43134477/article/details/122976621
http://finance.sina.com.cn/tech/2022-04-05/doc-imcwipii2521192.shtml