Soft Base Station Technology in Wireless Communication Systems

1 Background
    Nowadays, mobile networks are rapidly migrating to all-IP networks. Many international standardization organizations, such as 3GPP, 3GPP2 and IEEE, have proposed all-IP network architectures. During the evolution towards 4G, the flat architecture is employed, that is, the aggregation node in Radio Access Networks (RAN) is removed from the network architecture and instead the NodeB directly connects to the core network. In order to support the coexistence of multiple standards and network convergence, the Iur-g interface is defined for the Radio Network Controller and Base Station Controller (RNC/BSC)[1]. The interfaces between NodeB and RNC and within the NodeB itself become standardized and open. The previous proprietary Abis, Iub, and baseband Radio Frequency (RF) interfaces are transformed into open standards.

    Moreover, RAN actively adopts the IP and technologies such as distributed database, Point to Point (P2P), virtualization, and cloud computing from IT industry. These technologies were originally designed for the load-balanced data storage, interaction, and processing in large servers or the Internet. Applying these technologies to the telecom field has resulted in the convergence of telecom and IT networks.

    Both the telecom and IT industries embrace the openness of standards. Therefore, many standards organizations have been founded to establish completely or partially open and standard architecture. Open Base Station Architecture Initiative (OBSAI)[2] , Micro Telecommunications Computing Architecture (MicroTCA)[3-4], and Common Public Radio Interface
(CPRI)[5] are three typical open architectures and standards.

    OBSAI is an open base station architecture, jointly developed by a number of manufacturers. It defines all sets of base station structure, architecture, and interface. However, from the perspective of implementation, the structure is not easy to scale down and the architecture is not compact. Therefore, it is not practically used by equipment manufacturers. Though the OBSAI RP03 interface[6] (the interface between radio frequency and the baseband) provides more flexibility for various systems and higher rate, only a few manufacturers currently use this interface due to its complexity in implementation and low bearing efficiency—the effective bandwidth is only 84%.

    MicroTCA is an open computing architecture defined by the PCI Industrial Computer Manufacturers Group (PICMG). The focus of MicroTCA is to define implementation technology and schemes in terms of structure size, power architecture, chassis management, and switch fabric. MicroTCA architecture can be used in high-performance embedded computing, communications, and physics. However, the standard is complicated and its implementation is difficult. There is still much room for improvement in applicability and configuration cost for the telecom field. ZTE’s soft base station system is based on the MicroTCA architecture with a lot of key technology improvements during implementation.

    CPRI is a specification defined for the baseband RF interface; most manufacturers use the CPRI specification. Based on CPRI, Next Generation Mobile Network (NGMN) defines the Open Baseband Radio Interface (OBRI), which further defines the frame format and realizes the unified software interface.

    All these architectures and standards target to realize unified architecture. However, there is still a gap between these architectures and standards and the implementation of multimode soft base stations.

    On the other hand, the rapid development of semiconductors and software technologies has made possible the commercial implementation of soft base station.Field Programmable Gate Array (FPGA) and Digital Signal Processing (DSP) make the concept of soft baseband feasible; the architecture design is no longer constrained by the processing power. The bus Serializer/Deserializer (SERDES) also provides high bandwidth in limited connections and simplifies system architecture; the unified clock system may benefit from the high-performance phase-locked loop technology.

    As to the software aspect, the open and standard protocols accelerate the convergence of architectures and interfaces that use different standards; virtualization technology separates equipment management from wireless services so that services from different systems easily coexist and isolated; distributed technologies such as cloud computing improve software configurability.

2 Architecture of Soft Base Stations
    In order to support multiple systems and the smooth evolution, various product implementations should be highly abstracted and summarized. The common parts are extracted for the design of universal architecture. The composition of a radio base station is shown in Figure 1.

    Generally, the soft base stations require the Baseband Unit (BBU) and RF Unit (RU) be compatible with the services from multiple systems with different standards. At the same time, the Iub (Abis) and Ir interface should be standardized to shield the differences of multiple systems.

 
    The Iub interface has already been standardized, and the channelized E1 is gradually replaced by IP transmission. In this way, 2G and 3G base stations can be unified at the
Iub/Abis interface. The Ir interface may also comply with CPRI and OBSAI standards. Therefore, from the macro view, these interfaces can be unified. The technical problem lies in the implementation of different scenarios for different systems.

    BBU can be divided into four parts, as shown in Figure 2.

    The architecture of BBU can be divided into three planes: public resource plane, service switching plane, and I/Q switching plane, as shown in Figure 3.

    The aforementioned plances are only roughly divided. The boundary between different planes may be vague. However, they are adequate for the BBU architecture analysis.

    According to the functional modules of BBU, transmission can be shared by multiple systems using different standards, and can therefore be regarded as unrelated to standards. The control, clock, power are independent of any specific standards. However, the baseband processing and RF interface involve different standards. For implementing soft base stations, the standard-involved function modules should be divided into finer granularity in order to find the standard independent part; for the standard dependent part, the encapsulation is at least required to shield the differences between standards.

    Classification of planes has the following features:
    (1) Different standards have different clock requirements. This is the major difference between standards.

    (2) A unified switching plane may be formed by using the GE/FE switching plane and internal protocols through software.

    (3) SERDES can be used to remove differences within standards from the architecture point of view. However, since the I/Q data rates vary according to different standard, the encapsulation is required to shield the differences in standards so as to achieve the multimode configuration.

3 Public Resources
    The key challenge in implementing soft base stations is how to isolate the public resource management from wireless service. The isolation allows base stations to support multiple standards and smoothly evolve through software configuration.

    In this paper, this challenge is addressed through the hardware and software design. As to the hardware, the major problem for base station is that different standard systems have different clocks. The chip rates differ from 1.2288 MHz to 3.84 MHz, 13 MHz, or 44.8 MHz (besides frequency multiplication or frequency dividing). In our design, the public frequency of 122.88 MHz is selected, which facilitate the usage of digital phase locked loop on each baseband unit and the generation of various frequencies. In this way, differences in standards on the public hardware resource can be shielded. As to the software, soft base stations require smooth evolution, coexistence, and non-interference among multiple systems using different standards. They should support the backup capability for transmission links and main control/clock. The software should be configurable and support flexible installation/uninstallation of systems for specific standard. ZTE applies pseudo-virtualization and operating system virtualization technology in embedded systems[7], and proposes a software-configurable soft base system.

    Virtualization is a popular technology in the IT industry, and one of the foundations of cloud computing.  It enables users to install multiple operating systems (virtual machines) on a computer and process multiple tasks. This saves IT budget and supports high-speed task processing. Virtualization supports dynamic resource deployment and re-configuration for the purpose of service expansion. Virtualization also helps in service isolation and division and provides controllable and secure access to data and services. In addition, virtualization could provide the interface of virtualized resources that are independent of physical resources and protocol compatibility.

    In traditional base stations, wireless service, database management, device management, alarm management, version management, and transmission management are mutually coupled with control modules. However, this leads to restrictions and conflicts where multiple systems coexist with different standards. By establishing a virtualized device management layer, wireless services, on the one hand, can be decoupled from device management, thus the public device management of the base station can be shielded from wireless services and the unified device management of the base station can be achieved. On the other hand, when services with different standards are running in independent virtual spaces, one service is not aware the existence of other services with different standards. Therefore, any standard can be flexibly added or removed. In this way, management of a multimode base station is unified, and services can be independently upgraded and maintained, which enable the flexible standard scalability.

 
4 Transmission
    Access and transmission networks are tending to be rapidly merged. Base stations are required to support L3 routing protocol and Ethernet management protocol. In wireless network clouds, access devices are gradually responsible for the transmission interface bearing, protocol termination, route conversion, internal node management, and even multi-node network management. The intelligent technologies, such as self-discovery and self-configuration, have also emerged.

    With the rapid growth of data service and the development of open networks, wireless base stations take on more roles than traditional base stations, that is, they provide users with voice and data services, but also serve as transmission routing nodes and aggregation nodes to provide transmission for multiple stations.

    To handle complicated transmission networking and protocols, soft base stations should have the following capabilities:

    (1) Diversified Embedded Transmission Capability
    The sites for base station deployment are generally limited and have to adapt to the legacy transmission modes of operator’s network. Soft base stations should be embedded with diverse transmission capabilities—such as microwave, Passive Optical Network (PON), E1/T1, Synchronous Transmission Module 1 (STM-1), and Ethernet—and also support flexible networking. In addition, base stations should divide transmission into multiple transmission media, for example, the Ethernet is used to carry data and E1 used to carry voice.

 
    (2) Standard Transmission Protocol Stacks
    The network protocol suites opened up by standards organizations, such as IETF, provide standardized high-level protocols that are independent of network hardware environments. They meet the requirements of co-existence and interconnection between multiple systems. ZTE initiated the research and development of all-IP wireless base stations in 2002, and has since proposed a transitive networking mode of IP over E1 as well as FE access to Multi-Service Transport Platform (MSTP) Resilient Packet Ring (RPR). In open network architecture, soft base station should pay more attention to transmission management and security, and provide the Internet Protocol Security (IPSec) (digital certificate management and deployment) and IPv6 solutions. Soft base stations should also provide Ethernet management protocols such as 802.3ah.

    (3) Unified Management of Transmission Resources
    In multimode base stations, transmission resource is shared by services using any standard. Soft base station should support the co-existence of multiple transmission modes and the interworking between them. The QoS scheduling and traffic control of different service flow should also be implemented for different services sharing the same transmission.

    (4) Convergence of Access and Transmission Networks
    With the development of 4G networks, base stations migrate from the L2 switching protocols towards L3 routing protocols in order to meet the increasingly complicated networking requirements. Base stations should not only serve as edge transmission nodes, but also integrate the route management and protocol termination nodes. In addition, the base station should partially replace Multiple Protocol Label Switching (MPLS) edge routers to reduce deployment costs across the network.

5 Soft Baseband
    Among various standard systems, the CDMA 2000 core technology is monopolized (by Qualcomm), which generally use the Application-Specific Integrated Circuit (ASIC) for baseband modulation/demodulation. For other standard systems, the baseband implementations of equipment manufactures are diverse, including ASIC, Digital Signal Processing (DSP), DSP+ASIC, and DSP+Field-Programmable Gate Array (FPGA). All these methods have their own strengths and weaknesses.

    With the development of FPGA, DSP, and baseband processing technology, the replacement of software (including the FPAG network table) and standard system in soft base station is not longer a dream. Hardware accelerators in FPAG, complicated algorithms in DSP arrays, and the high-speed Serial Rapid IO (SRIO) switching fabric to support the interworking between DSP array and FPGA all contribute to the powerful baseband processing capability for multi-standard systems. The main constraint for the application of soft baseband technology into various standard systems is cost. For example, GSM is a mature and cost-sensitive system with low baseband processing capability. Therefore, applying baseband hardware suitable for Long Term Evolution (LTE) services into a GSM system is a waste of resources. Other issues are how to flexibly allocate the processing power from a large, unified baseband processing resource pool to different standard systems and how to realize flexible resource expansion.
Driven by the cost pressure, equipment manufacturers may migrate from full softband to semi-softband and ASIC. However, the ASIC solution may lose the evolution capability with standards and flexibility of service migration.

6 Baseband RF Interface
    Among baseband RF interface standards, CPRI is widely adopted because of its simple implementation, cost-effectiveness and efficient use of bandwidth. Both the OBRI and ETSI Open Radio equipment Interface (ORI) borrows the bottom layer definition of the CPRI.

    CPRI protocol is divided into two layers, as shown in Figure 4. Layer 1 includes the physical-layer transmission and I/Q data Time Division Multiplexing (TDM) mapping, and Layer 2 includes specifications for control signaling. CPRI organization specifies the I/Q formats of Universal Mobile Telecommunications System (UMTS)/LTE, but does not define those of GSM and CDMA 2000—possibly because of the chip rate. In principle, this protocol division can support multiple services; however, the detailed definition in Layer 1 is not beneficial to fulfill the requirements of multimode soft base stations. Compared with CPRI, the four-layer structure of OBSAI RP03 is more practical. As shown in Table 1, the protocols for guaranteeing point-to-point transmission in OBSAI RP03 are independent of system standards.

    CPRI should adopt hierarchical architecture to support the transmission of multiple standard systems. Moreover, when defining the size of AxC (Antenna Carrier) at the bottom CPRI layer, the size should adapt to the I/Q data capacity and should be independent of the system standards. During transmission, only the AxC transmission independent of system standards is taken into consideration, other issues, such as the mapping mode, standard and sampling information when mapping the I/Q data to AxC are ignored. The difference between standards cannot be ignored in baseband modulation/demodulation and intermediate frequency processing, so the data from different standard systems are different.  This may sacrifice the efficiency and bring complexity. However, such costs may be worthwhile for flexibility and wireless product evolution.

7 Future Direction of Soft Base Stations
    Soft base station architecture is developing towards a flat, multimode structure. The multimode soft base stations should provide rich software services. Software services change from pure traditional base station services into integrated transmission, integrated controllers, and integrated routers. In addition, fixed services will turn to the configurable and customizable ones.

    The hardware architecture of soft base stations currently meets co-existence requirements of multi-services. Future soft base stations should support higher levels of integration, lower cost, and power efficiency. They will support more flexible baseband resource scheduling and more transmission modes.

    The futures software technology will be based on IP and IT, and base stations will use more open standards.  When base stations are connected to open networks, the security issue  in future IP networks will become a hot topic. Future soft base stations will support more intelligent, distributed, and virtualization technologies. These technologies will enable flexible combination of base station functions and balance the load of base stations.
Self Organizing Network (SON) technology is an innovation of the base station management mode. Self-discovery, self-download, and self-configuration enable the access network to add or delete network nodes smoothly and this allows automatic network optimization. With the improvement of related standards and technology implementation in base station equipment, intelligence of soft base stations will be improved greatly.

    Distributed data processing and technology can solve bottlenecks of storage space and processing resources that plague traditional base stations. With new technology, unbalanced services can be more reasonably distributed. A popular technology, cloud computing, is also evolved from distributed computing, parallel computing, and grid computing[8]. Cloud services are offered through centralized large-scale servers, and the concept has been put into commercial use. Although there is still a long way to go for cloud services, the basic theory of cloud computing can be incorporated into base station technology.

    Virtualization can further abstract the dividing of functions in a base station, creating a simplified dual layer structure consisting of processor resource pool and data processing pool. Each function can be dynamically assigned to a board that is in idle, and even distributed processing across base stations can be supported. Therefore, using virtualization technology, resource allocation can be optimized, power consumption can be reduced, and the goal of green base stations can be achieved.

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] BTS System Reference Document,V2.0 [R]. OBSAI, 2009.
[3] Micro Telecommunications Computer Architecture Base Specification, R1.0 [R]. PICMG, 2009.
[4] Advanced Mezzanine Card Base Specification, R2.0 [R]. PICMG, 2009.
[5] CPRI Specification, V4.0 [R]. CPRI, 2009.
[6] Reference Point 3 Specification,V4.2 [R]. OBSAI, 2009.
[7] 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.
[8] 中国云计算网. [EB/OL]. [2009-05-28]. http://www.cloudcomputing-china.cn/article/showarticle.asp?articleid=1.
      Cloudcomputing-China. [EB/OL]. [2009-05-28]. http://www.cloudcomputing-china.cn/article/showarticle.asp?articleid=1.

[Abstract] With the rapid development of wireless communications systems, different system standards are being merged. Operators take stringent measures to reduce Operational Expenditure (OPEX) and Capital Expenditure (CAPEX); and as a result, soft base stations supporting multiple standards become the evolutionary tend of wireless base stations. This paper introduces the background of soft base stations and analyzes their architecture design, system modules. The key technologies in system implementation and future directions are also presented.

[Keywords] wireless communications; soft base station; multimode base station