This article focuses on the introduction of distributed base stations, conceptual content, advantages, and its existing problems. It has a positive reference significance for accelerating the process of TD network, improving network quality, and achieving indoor and outdoor integration coverage. Keywords: base station, baseband processing unit (BBU), radio frequency processing unit (RRU), TD-SCDMA The core concept of the distributed base station structure is to separate the traditional macro base station baseband processing unit (BBU) and radio frequency processing unit (RRU), and the two are connected by optical fibers. During network deployment, the baseband processing unit, core network, and wireless network control equipment are concentrated in the computer room, and are connected to the remote radio unit deployed on the planning site through optical fiber to complete network coverage, thereby reducing construction maintenance costs and improving efficiency. 1. The significance of distributed base stations for operators With the increase of voice service capacity and the development of high-speed data services, in order to provide greater voice capacity and higher data bandwidth, most traditional mobile communication operators face How to build a 3G network on a 2G network. 2. The concept and content of TD distributed base station Figure 2 TD-SCDMA distributed base station structure In the structure of a distributed base station, BBU is a baseband processing device, which mainly completes the baseband processing function of Uu interface (coding, multiplexing, modulation and spread spectrum, etc.), and INC interface of RNC Functions, signaling processing, local and remote operation and maintenance functions, and Node B system working status monitoring and alarm information reporting functions. All the baseband functional units serve as a baseband pool, and through configuration, each baseband processing module can process data of different load fans. In areas with large capacity requirements, the capacity can be increased by simply adding a baseband board to the BBU. In this way, on the one hand, it can reduce costs, on the other hand, it can provide sufficient flexibility for networking and solve the problem of large differences in 3G network capacity. 3. Advantages of distributed base stations Compared with traditional macro base stations, distributed base stations have the following obvious advantages. Fourth, the current problems restricting the construction of TD distributed base stations Although distributed base stations have become a cornerstone of TD development, but because its commercial use has just begun, there are still many problems that need to be solved in the future. V. Concluding remarks With the issuance of 3G licenses, China Mobile will invest 58.8 billion yuan in the third phase of bidding in 2009, and build about 60,000 TD-SCDMA base stations. Business hot spots covering 238 prefecture-level cities, accounting for more than 70% of the number of prefecture-level cities nationwide, of which the prefecture-level cities in the east will be fully covered. The advantages of TD-SCDMA distributed base stations in construction and network deployment will play a huge role in accelerating the process of TD-SCDMA networkization, improving network quality, and achieving indoor and outdoor integrated coverage.
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The base station equipment of the existing 2G network generally does not have the ability to directly upgrade to support 3G services. Therefore, building a 3G network requires additional new base station equipment. How to choose a site for these additional base station equipment and find a suitable installation space is an important issue facing operators. For emerging operators who have no site resources at all, this problem is more prominent due to new site planning and selection.
According to the survey, most operators are faced with the problems of difficult site selection and tight space in the equipment room. Whether it is co-stationed with other mobile communication systems or newly built, the available computer room reserves that meet the requirements in the city are very limited. How to save installation space as much as possible and adapt to various installation scenarios flexibly is an important requirement for 3G base station equipment. At the same time, environmental awareness is gradually gaining popularity, people are paying more and more attention to the problem of electromagnetic radiation, and finding jobs in the sky is becoming more and more difficult.
Therefore, how to reduce the requirements of the computer room and the sky, and reduce the supporting investment will be the focus of all circles in the network construction. According to the number of base stations and the number of base stations per square kilometer of China Mobile's first phase of TD-SCDMA network construction, TD needs to achieve good network coverage, and its total number of base stations will exceed 2G base stations. Not only is the number huge, but also because of the constraints of common sites, the choice and acquisition of 3G sites is more difficult than expected. Relevant information shows that China Mobile has adopted distributed base stations in 80% of its TD-SCDMA network construction, which largely resolves this sharp contradiction.
From the perspective of service development, data services will be the characteristic services of 3G, and the bandwidth occupied by data services is tens or even hundreds of times that of voice services. 3G provides many new services for end users, including video telephony, video streaming, games, MMS, E-mail, Web, etc. Compared with outdoor users, indoor stationary users are more likely to use 3G's rich and colorful data services. These new services are easier to apply indoors, so the system capacity required in indoor hotspots may be hundreds of times that of outdoor. This requires more flexible and diverse coverage solutions to adapt to the development of 3G services. For emerging services, due to high transmit power and high network density, operators can use distributed base stations to provide better hardware platforms for data services that require greater bandwidth. As China's 3G network construction has entered a period of rapid development, operators' requirements for rapid, flexible, and low-cost deployment of its networks have also made mobile distributed base stations a new base station model that has gradually become the trend and mainstream of network construction.
At present, there is still a problem of system selection when constructing 3G networks. Currently, there are 3 international mainstream 3G systems (CDMA2000 / WCDMA / TD-SCDMA), and each 3G system has different stage versions (eg: CDMA2000 EV-DO Rel.0 / Rev.A / Rev.B). Most operators already have a clear choice for the 3G standard, but they are still on the sidelines for the evolution of 3G to the future 4G (UMB / LTE / WIMAX) and intermediate versions of evolution (such as EV-DO Rev.B). Among. In this case, operators need to consider how the network will evolve in the future when investing in the construction of a 3G network. It is hoped that once the subsequent 4G route is determined, the 3G network can be upgraded to a 4G network at a minimum cost without completely rebuilding. This requires that the base station equipment of the existing 3G network has the ability to upgrade to support the 4G standard selected by the operator, so as to better protect the operator's investment in the existing 3G network. LTE belongs to the post-3G development and evolution direction, and its maturity will take some time. When considering the evolution of the existing network to LTE, the requirements for BBU / RRU mainly focus on the protection of the operator's hardware investment, and platform sharing as much as possible. At present, the mainstream method in the industry is to smoothly evolve to LTE through SDR (software radio) technology and a common hardware platform by installing LTE software modules on existing 2G / 3G equipment. The use of distributed base station technology will help operators fully protect existing investments during the evolution of mobile communication technology.
The distributed base station divides the traditional macro base station equipment into two functional modules according to functions. Among them, the base station's baseband, main control, transmission, clock and other functions are integrated into a module called baseband unit BBU (Base Band Unit). The unit is small and the installation position is very flexible; integrate the radio frequency in the transceiver, power amplifier, etc. on another called remote radio module, and the radio unit RRU (Remote Radio Unit) is installed on the antenna side. The radio frequency unit and the baseband unit are connected by optical fiber to form a new distributed base station solution.
Taking TD-SCDMA as an example, the base station in the traditional way needs to be connected with multiple cables between the Node B and the smart antenna. The cable cost is very high, the transmission loss is large, the distance is short, and the cable itself is bulky, especially in the building, the construction is difficult, as shown in Figure 1. The distributed base station solution is an economical and fast wireless network construction solution under the situation that the machine room or the machine room is not ideal. It is emulated by the global equipment giants Ericsson and Nokia, and it has been applied to the scale of the world's famous mobile operators Vodafone, Orange, etc. . Distributed base stations first appeared in two other 3G systems-WCDMA and CDMA2000, but it was TD that really carried it forward. Because the TD base station uses smart antenna technology and is more sensitive to the power attenuation of the path, the TD base station is very suitable for a distributed architecture. The composition of the TD distributed base station is shown in FIG. 2. 
Figure 1 TD-SCDMA traditional base station composition
The remote radio unit (RRU) is divided into 4 large modules: intermediate frequency module, transceiver module, power amplifier and filter module. The digital intermediate frequency module is used for optical transmission modulation and demodulation, digital up-down conversion, A / D conversion, etc .; the transceiver module completes the conversion of the intermediate frequency signal to the radio frequency signal; and then passes through the power amplifier and filter module to transmit the radio frequency signal through the antenna port .
The BBU and RRU are connected by optical fiber according to the Ir interface protocol to complete the transmission of baseband data. Ir interface protocol supports network topology such as star connection, chain connection and ring connection, which makes BBU + RRU more flexible for networking. The Ir interface defines Layer 1 and Layer 2 protocols to support data transmission at the user layer, and synchronization and transmission of control information between BBU and RRU units.
In the operation of TD-SCDMA distributed base station, the RRU uplink converts the received BBU baseband signal into shape filter, peak cut, digital predistortion and other frequency conversion to intermediate frequency, and then further converts it to radio frequency through analog mode, through power amplifier After the transmission; the downlink of the RRU converts the received terminal signal to intermediate frequency and baseband after frequency selection and amplification, and then transmits it to the BBU through the optical interface. Through the use of digital signal processing techniques such as peak clipping, the efficiency of TD-SCDMA power amplifiers is greatly improved, and the cost of a single machine can be greatly reduced. At the same time, due to the small size and light weight of the RRU, it can be installed outdoors. The optical fiber can be laid directly on the top of the tower. The connection from the machine room to the sky surface is replaced by 1 optical cable instead of the original 28 feeders. The construction is simpler and faster; Unlimited, the computer room can be located on the ground floor of the building; the laying of optical fibers has less impact on the computer room and the surrounding environment. Therefore, the application of distributed base stations can effectively solve the problems of difficult location selection of the computer room and the sky and the difficulty of network construction.
The first is to increase the effective utilization of site resources and reduce construction and maintenance costs. In view of the very difficult site selection of the current base station, if the RRU is used, the space required is relatively small, and the RRU can be pulled anywhere, so it is a deployment scheme that is flexible and deployed according to local conditions. The distributed base station BBU and RRU are separated. The indoor BBU equipment is only responsible for baseband signal processing. There are no radio frequency devices, especially power amplifier modules, so it has the characteristics of small size, light weight, low power consumption, and easy installation. In the current situation where site selection for mobile communication networks is becoming more and more difficult, the characteristics of the occupied "0" equipment room of distributed base stations compared to macro base stations can save the space of the equipment room, reduce the cost of network construction, and accelerate the speed of network construction. At the same time, the distributed base station uses high-efficiency power amplifiers, which reduces the power consumption of air-conditioning and other supporting facilities, and has the characteristics of energy saving and emission reduction. The interface between the two ends is made of optical fiber, and the loss is small, and the power consumption can also be greatly reduced. This is especially precious today when the country vigorously promotes energy saving and emission reduction. According to the estimation of European operators, if this kind of base station is used in the entire network, the cost can be saved by more than 30%.
The second is to improve the coverage capability of the base station and facilitate network upgrades. Traditional macro base stations use feeders for both transmission and reception. The feeders will cause loss to the signal. The size of the loss depends on the type and length of the feeder. The optical fiber connection between BBU and RRU has almost no loss, so distributed base stations have higher receiving sensitivity and antenna-end transmit power compared to macro base stations. In terms of flexible resource allocation, because the distributed base station simplifies the cumbersome maintenance work to the baseband processing part, a baseband processing unit can connect multiple remote radio processing units in different ways to achieve resource scheduling and deployment between RRUs. It not only saves costs, but also improves the efficiency of networking. By making full use of this base station, it can coordinate the resources of the base station and flexibly allocate resources to the phenomenon of surge or dip in and out of work to achieve a flexible capacity and Override conversion. The use of distributed base stations also satisfies the needs of future network IP. Because of the new mobile communication standard, its evolution speed is very fast, and high-speed evolution will inevitably bring about the problem of base station upgrade. The modular design of the distributed base station is rich in functions, and it is more convenient to expand and upgrade the system, which meets the future-oriented needs of mobile communication networks.
The third is to facilitate the implementation of base station construction projects. Compared with macro base stations, locally distributed remote base stations use fiber optic transmission baseband signals instead of feeders to transmit radio frequency signals. Taking a three-sector station as an example, from the machine room to the sky, the engineering difficulty of laying three optical fibers is far less than laying six 7 / 8-inch feeders. Especially in the current, considering the urban landscape and other factors, more and more sites need to be concealed and disguised. In this respect, the advantage of optical fiber over feeder is more significant.
The fourth is to reduce the R & D costs of manufacturers. The standardization of the Ir interface within the distributed base station will allow many third-party module manufacturers to interconnect with the digital interface of the base station, which not only reduces R & D costs, but also enables the interconnection and interconnection of equipment from multiple manufacturers, which improves versatility and flexibility. It also reduces the procurement and networking costs of operators.
First, the maintenance and replacement of the distributed base station when the radio frequency unit fails is not as convenient as the traditional base station. Generally, it can only be replaced directly, and it is operated on the outdoor antenna site.
Second, related to the outdoor physical performance of the RRU, the bottleneck of the power amplifier process manufacturing affects the efficiency of the power amplifier and thus poses a problem for the heat dissipation of the outdoor unit.
Third, the optical fiber for transmitting intermediate frequency signals between the RRU and the base station baseband unit is currently equipped with bare optical fiber. The number of fiber cores is not reserved, and there is no reinforced core. It does not have resistance to compression and tension, and is prone to failure. Especially in indoor distribution, obstacle handling is more difficult.
Fourth, finally, regarding the flexibility of the RRU power supply method, especially for the poor site conditions and the shortage of funds, the AC power supply method may be directly adopted, and even the baseband unit also supports AC power supply. The potential problem with this "ideal" solution is that there is no reliability guarantee for power supply, and UPS should be configured for standard operation in AC power supply.
In general, through the construction and exploration in the past two years, mobile communication operating companies have accumulated experience and made progress. The implementation of the converged networking strategy and the research on the TD distributed base station will inevitably help us to further build the network, and any maturity of communication technology will go through a certain process. We must strengthen our confidence, strive for the support of the local government, continue to study and practice the construction of TD distributed base stations, embody the spirit of scientific development, and promote the good and fast development of TD networks.
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[2] Chen Huiling. 3G mobile communication base station construction and management strategy. Communication World Weekly.
[3] Peng Mugen, Wang Wenbo. Wireless resource management and 3G network planning optimization. People's Posts and Telecommunications Press. 
