In just over ten years, mobile communication has developed 2G / 3G / 4G three-generation standards and more than 10 standard systems. At the same time, networks of multiple standards will coexist for a long time. For a long time, all equipment manufacturers have adopted a design mode corresponding to a base station, which has resulted in huge investment by operators and difficulty in operation and maintenance.
Operators need base stations to support 2G / 3G at the same time and then smoothly upgrade to 4G to protect equipment investment, and require base stations of various standards to behave as a network to reduce overall operating costs. Today's mobile communication market is becoming increasingly competitive, and achieving high-performance multi-mode soft base stations is of decisive significance to stand out in the global market competition; but due to the huge difference between the various standards, its implementation faces a lot of implementation difficulties and has been stuck on paper . Through years of research and development, ZTE Corporation has launched the world ’s first multi-mode soft base station and achieved industry-leading overall wireless performance through a large number of innovative technologies.
The article will introduce the architecture and implementation of soft base stations, mainly baseband units.
1 Industry efforts
Mobile networks are accelerating the evolution to ALL IP, and international standards organizations such as the 3rd Generation Partnership Project (3GPP), 3GPP2, the Institute of Electrical and Electronics Engineers (IEEE) have successively proposed network architectures based on ALL IP. In the evolution of 4G, the architecture of NodeB has become flat, and there is no longer a traditional convergence convergence point on the access side. Instead, it is connected to an open transmission network. With the development of multi-standard coexistence and network integration, the definition of Iur-g interface has appeared in the wireless network controller / base station controller (RNC / BSC) [1-2], and the communication between NodeB and RNC, and within NodeB are also trending. Standardization and openness. Abis interface, Iub interface and baseband radio frequency interface have also gradually changed from the private definitions of various manufacturers to open standards.
IP and IT on the wireless access side have also formed a trend. The ideas and technologies of the IT industry are widely used in communication equipment, such as distributed databases, peer-to-peer (P2P) technology, virtualization, cloud computing, etc. These technologies used to focus on data storage, interaction, and processing on large servers or the Internet to make the network load more balanced.
The Open Base Station Architecture Alliance (OBSAI) [3] was built by multiple vendors, with the goal of building an open base station architecture. The OBSAI architecture can basically describe the general form of the base station architecture, but from an implementation point of view, its structure is not small enough, the architecture is not compact enough, and it is not advanced enough, and it has not been actually adopted by equipment vendors. Although the OBSAI RP03 interface [4] (baseband radio frequency interface) provides higher flexibility for various standards and evolves to higher rates, it is complicated to implement and has low bearing efficiency (effective bandwidth is only 84%), The physical implementation is not economical enough, and is only used in a small number of manufacturers.
Micro Communications Computing Architecture (MicroTCA) [5-6] is an open computing architecture developed by the International PCI Industrial Computer Manufacturing Organization (PICMG) Association. MicroTCA focuses on the implementation technology, and defines a series of implementation solutions including structural size, power architecture, chassis management, and switching plane. The MicroTCA architecture can be used in many fields such as high-performance embedded computing, communications, physics, etc., but the standards are complex and there are difficulties in engineering implementation. In the application of the communications field, the architecture needs to be improved in terms of configuration cost and applicability. . ZTE's soft base station system is based on the MicroTCA standard, and has made many improvements and key technology implementations.
The Common Public Radio Interface (CPRI) [7] is a specification defined for the baseband radio frequency interface, and various equipment manufacturers basically use the CPRI specification. On the basis of CPRI, the next-generation mobile communication network (NGMN) composed of operators defines an open baseband radio frequency interface (OBRI), further defines the frame format, and strives to unify to the software interface.
In addition to the above open standards, there are other specifications such as the Ir interface developed by China Mobile for TD to standardize device interfaces.
The above standards have made some efforts to unify the architecture, but there is still a considerable distance between the soft base stations to achieve multi-mode coexistence. In recent years, the rapid development of semiconductor technology and software technology has enabled soft base stations to move from paper to reality. The development of field-programmable gate array (FPGA) and digital signal processing (DSP) technologies has made "soft baseband" gradually feasible; processor technology makes the processing power no longer severely restrict the architecture definition; bus series / deserializer technology can be used in Provides high bandwidth under limited connections and simplifies the system architecture; the large number of open and standardized protocols used in software accelerates the integration of multi-standards in architecture and interfaces; the application of distributed technology theories such as cloud computing is Software configurability provides direction.
2 The overall architecture of the soft base station
Soft base stations that support multiple standards and smooth evolution must macroscopically abstract and summarize the realization of various products, extract their common parts, and design a highly unified architecture. The composition of the wireless base station is shown in Figure 1.
From the perspective of the entire base station, the indoor baseband processing unit (BBU) and radio frequency unit (RU) must be compatible with multiple standard services, and at the same time standardize the Iub (Abis) and Ir interfaces to shield the differences in product form and standard. Meet the requirements of soft base stations. The Iub interface has been gradually standardized, and channelized E1 and other methods have been gradually replaced by IP, so that 2G and 3G base stations can be unified on the Iub / Abis port. Ir interface has CPRI, OBSAI and other standards to follow, which can be unified in macro. The technical problems mainly exist in the realization of various standards and application scenarios.
From within the BBU, the function can be divided into four parts as shown in Figure 2:
Through the decomposition and abstraction of functional modules, we decompose the BBU architecture into three planes: common resource plane, service switching plane, and I / Q switching plane. As shown in Figure 3.
The above two division methods are actually inaccurate, and the boundaries between the parts may be blurred, but it is easy to study the architecture.
From the perspective of functional module division, transmission can be shared by various standards and can be considered to be independent of the standard; the main control, clock, and power supply parts can also be considered to be independent of the standard (clock and standard related); baseband processing, RF interface part, and the standard related . For the realization of the soft base station, the parts related to the standard must be decomposed into finer particles, and as far as possible, there should be no standard difference on the finer particles; for the part that cannot eliminate the standard characteristics, it needs to be encapsulated, and the external characteristics shield the standard difference .
For flat surfaces:
(1) The main difference is that each system has different clock requirements.
(2) The mature Gigabit Ethernet (GE) or Fast Ethernet (FE) switching plane is adopted, and the software unifies internal protocols, which can easily form a unified switching plane.
(3) Using the Serdes method can eliminate the difference in standards from the architecture, but because the I / Q data rate of each standard is different, if multi-mode coexistence can be configured, it must be packaged in a certain way to shield the difference in standards.
3 Public resources
Realizing the irrelevance of public resource management and wireless services, so that the base station has the ability to support multi-standards, and the ability to smoothly evolve through software configuration, is a key issue for soft base stations.
In terms of hardware, the main problem of soft base stations is that the various standards have different clocks, and the chip rates used are 1.2288 MHz, 3.84 MHz, 44.8 MHz, and 13 MHz (and frequency multiplication or division). Choosing a common frequency of 122.88 MHz, it is easy to adopt a mature digital phase-locked loop on each baseband unit and generate each frequency point, thereby basically shielding the difference of the standard on the common resource plane.
In terms of software, the soft base station needs to realize smooth evolution of multiple systems, coexistence and ensure that the multiple systems do not interfere with each other, realize the transmission link and main control / clock backup capabilities, and realize the software configurable and free loading / unloading wireless standards . ZTE borrowed the idea of ​​paravirtualization and operating system virtualization in the embedded field [8], and optimized the system architecture to build a software-configurable soft base station system.
Virtualization is currently a popular technology in the IT industry and an important technical foundation for cloud computing. With the help of virtualization technology, users will be able to install multiple operating systems (virtual machines) in a single computer hardware, and achieve multi-tasking, so as to achieve IT savings and high-speed processing of computing tasks and other purposes [9]. Virtualization can realize dynamic resource deployment and reconfiguration to meet the needs of business expansion. It can also achieve more complete business isolation and division, controllable and secure access to data and services, and can provide virtual resources independent of physical resources. Interface and protocol compatibility.
In traditional base stations, wireless services, database management, device management, alarm management, version management, transmission management, and control are coupled with each other. When multiple systems coexist, there will be multiple restrictions and conflicts. By constructing a virtualized device management layer, you can decouple wireless services from device management, shield wireless services from common device management of base stations, and provide unified device management operations for base stations. At the same time, multiple services of different formats are running in independent virtual spaces, and there is no need to perceive the existence of other services, and the formats can be flexibly added and deleted. In this way, unified management of multi-mode base stations can be achieved, and independent upgrades and maintenance of multiple services can be provided, providing operators with flexible standard expansion capabilities.
4 Transmission
In recent years, the integration of the access network and the transmission network has shown an accelerating trend. The demand for the three-layer routing protocol and the Ethernet management protocol on the base station side has gradually increased. Access devices have increasingly assumed the role of transmission interface bearer, protocol termination, route conversion, internal node management, and even multi-node network management in the wireless network cloud. In recent years, comprehensive technologies related to intelligence such as self-discovery and self-configuration have emerged.
With the rapid growth of data services and the gradual opening of the network, wireless base stations have taken on more roles than ever. They not only provide voice and data services to end users, but also need to act as transmission routing nodes and aggregation nodes to provide transmission for multiple sites. service.
In order to cope with complex transmission networking and transmission protocols, soft base stations need to have the following capabilities:
(1) Built-in diverse transmission capabilities. The base station deployment site is often limited, and it needs to adapt to the various transmission methods that the bureau has deployed. Soft base stations need built-in diversified transmission capabilities, such as microwave, passive optical network (PON), E1 / T1, synchronous transmission module 1 (STM-1), Ethernet and other transmission media, and can provide flexible groups Network mode. At the same time, the base station also needs to support simultaneous transmission of multiple transmission media, such as simultaneous access of Ethernet and E1, Ethernet carries data services, and E1 carries voice services.
(2) Adopt standardized transmission protocol stack. The Internet Engineering Task Force (IETF) and other standard organizations open network protocol clusters, which provide standardized high-level protocols independent of the network hardware environment, and can meet the needs of coexistence and interconnection of multiple standards of base stations. Since 2002, ZTE has started to research and develop all-IP wireless base stations, and has realized the transitional networking mode of flexible packet data ring technology (MSTP RPR) for IP over E1 and FE access multi-service transmission platform. For the gradually open network architecture, soft base stations need to pay more attention to transmission management and transmission security, provide solutions such as Internet Protocol Security (IPSec) (digital certificate management, deployment) and IPv6, and Ethernet management protocols such as 802.3ah.
(3) Unified management of transmission resources. Transmission resources are common resources for multi-standard services in multimode base stations. The soft base station needs to be able to realize the coexistence of multiple transmission modes through the transmission configuration, and the QoS scheduling and bandwidth flow control of the bandwidth of different services under the common transmission.
(4) Integration of access network and transmission network. With the development to 4G, the base station has gradually evolved from the first layer 2 switching protocol to the layer 3 routing protocol to meet the increasingly complex networking requirements. At the same time, the base station not only serves as a transmission leaf node, but also integrates a route management node and a protocol termination node, and even partially replaces a multi-protocol label switching (MPLS) edge router to reduce the deployment cost of the entire network.
5 Soft baseband
From the perspective of various systems, except that the core technology of CDMA2000 is basically monopolized by Qualcomm, and the baseband modulation and demodulation uses application specific integrated circuits (ASICs), the implementation of various other communication standard basebands has been diversified. Each equipment manufacturer has formed ASIC, digital signals Processing (DSP), DSP + ASIC, digital signal processing + field programmable gate array (DSP + FPGA) and other various implementation methods, these methods have their own advantages and disadvantages.
After years of development with FPGA, DSP technology, and baseband processing technology, it is no longer a fantasy to replace software (including field programmable gate array netlists) and replace standard soft base stations. Implementing a hard accelerator in an FPGA, implementing a complex algorithm in a DSP array, and using a high-speed SRIO switching plane to realize the interconnection between the DSP array and FPGA can provide powerful baseband processing capabilities and achieve multiple standard processing. The main constraint of the application of soft baseband technology in each standard is the implementation cost. For example, for a mature and very cost-sensitive market such as the Global System for Mobile Communications (GSM), the baseband processing capability is not high, and the baseband hardware capable of carrying long-term evolution (LTE) services will have enormous resources in the global mobile communication system. waste. In addition, how to allocate the processing capacity flexibly between different systems in the unified large baseband processing resource pool and achieve flexible resource expansion is also a subject that needs to be studied urgently.
With the solidification of services, driven by cost pressures, equipment manufacturers will move from all-soft baseband to semi-soft baseband and migrate to ASIC, while gradually losing the ability to evolve with standards and the flexibility of business migration.
6 Baseband RF interface
In the baseband radio frequency interface standard, CPRI has been widely recognized and applied due to a series of advantages such as simple implementation, good economy, and high bandwidth utilization. OBRI or ORI of the European Telecommunication Standardization Association ETSI also borrowed the underlying definition of CPRI.
The CPRI specification is divided into two levels, as shown in Figure 4. Layer 1 includes physical layer transmission, time division multiplexing (TDM) mapping of I / Q data, etc., and layer 2 includes definitions such as control signaling. The CPRI organization stipulates the I / Q format such as UMTS / LTE, but the I / Q data of GSM and CDMA2000 may be based on the chip rate and is not standardized. In principle, the two-layer division of the protocol can carry multiple services, but the over-detailed definition at the first layer is actually not conducive to the realization of multi-mode soft base stations. Relatively speaking, the four-layer structure of OBSAI RP03 is more adaptable. As shown in Table 1, in OBSAI RP03, the protocol layer that guarantees data point-to-point transmission can be independent of the standard.
If you want to make CPRI suitable for transmitting a variety of different standards, you need to consider fine-grained layering, and consider the capacity adaptation of I / Q data in the size definition of the underlying air interface data container AxC (Antenna Carrier), but in use It has nothing to do with the standard, only the transmission of non-standard AxC is considered in the transmission process, and it does not care about the way, standard, and sampling information of the I / Q mapping to AxC. Only at the two ends that cannot ignore the difference of the standard (that is, baseband modulation and demodulation and intermediate frequency processing) can the standard data be seen. In some cases, this may sacrifice some load-bearing efficiency and may increase some complexity, but from the perspective of wireless product evolution and flexibility, such a price is still very worthwhile.
7 Future trends of soft base stations
The architectural form of soft base stations will move towards multi-mode, flat architectures, especially multi-mode soft base stations, which put forward higher requirements for "soft" technology and need to provide more abundant software services. The content of software services has shifted from pure traditional base station services to integrated transmission, integrated controllers, integrated routers and other functional roles. It has shifted from fixed-function services to configurable and customized services.
The current hardware architecture of soft base stations can basically meet the needs of coexistence of multiple services. In the future, soft base stations will continue to develop in the direction of higher integration, more flexible baseband resource allocation, richer transmission methods, lower costs, and energy conservation and environmental protection.
In the future, software technology will firmly move towards IP and IT. Base stations will move towards open standards. After the base station accesses the open network, future IP network security will receive more attention; in the future, soft base stations will provide more intelligent, distributed, and virtualized related technologies, and flexibly combine base stations through such technologies Function, and gradually complete the load balancing of the base station:
(1) The emergence of technologies such as SON is an innovation for the management of base stations. Self-discovery, self-downloading, and self-configuration enable the access network to easily add or delete network nodes and automatically optimize the network. With the gradual improvement of standards in the next few years and the realization in base station equipment, the intelligent capability of soft base stations will be greatly improved.
(2) The application of distributed data processing model and technology can solve the bottleneck of storage space and processing resources in the previous base station model, distribute unbalanced business processing, and form balanced load processing. The recently popular "cloud computing" technology is also evolved from distributed computing, parallel computing, grid computing, etc. [10], providing cloud services through the centralized computing power of large servers, and commercializing these concepts. Although it will take time to finally move to cloud services, the basic theory of cloud computing can be used and applied in base stations.
(3) The application of virtualization technology will further abstract the functional division of the base station to form a two-layer simplified structure of the processor resource pool and the data processing pool. Various functional services can be dynamically allocated to boards with idle processing capabilities, and even provide technical support for distributed processing across base stations, optimize resource allocation and reduce energy consumption, and realize green base stations.
8 References
[1] ETSI TS 143 130 V5.0.0. Digital Cellular TelecommunicaTIons System (Phase 2+), Iur-g Interface, Stage 2 (3GPP TS 43.130 version 5.0.0 Release 5) [S]. 2002.
[2] Shen Changhu, Zhang Lishen. Iur-g, build a bridge for 2G / 3G convergence [J]. ZTE Technology (Newsletter),? 2009 (12): 12-13.
[3] BTS System Reference Document, V2.0 [R]. OBSAI, 2009.
[4] Reference Point 3 SpecificaTIon, V4.2 [R]. OBSAI, 2009.
[5] Micro Telecommunicaions Computer Architecture base SpecificaTIon, R1.0 [R]. PICMG, 2009.
[6] Advanced Mezzanine Card base SpecificaTIon, R2.0 [R]. PICMG, 2009.
[7] CPRI Specification, V4.0 [R]. CPRI, 2009.
[8] HEISER G. The Role of Virtualization in Embedded Systems [C] // Proceedings of the 1st Workshop on Isolation and Integration in Embedded Systems (IIES'08), Apr 1, 2008, Glasgow, UK. New York, NY, USA: ACM, 2008: 11-16.
[9] Gartner. Ten technologies that change the existing pattern of the IT industry [J]. Software Technology and Industry Dynamics, 2008 (59): 1-3.
[10] China Cloud Computing Network. [EB / OL]. [2009-05-28]. Http: //? Articleid = 1.
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