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By Xu Fei

Telecommunication Standards Institute of Ministry of Information Industry

"Distributed" Represents the Direction of Base Station Evolution

The first generation of WCDMA base stations were launched in 2000. Since then, the increasing size of WCDMA networks has led to greater challenges in terms of network deployment, and 3G base stations are now required to balance high performance with low cost, possess flexible deployment capabilities and be future-oriented. To meet these demands, a new generation of base stations featuring digital power amplification, multi-carrier technology, HSDPA support and open architecture have come into being. In February 2005, Huawei was the first manufacturer to make available its new generation base station product. Currently, new generation base stations – typically distributed base stations - with the above mentioned features have become mainstream.

Additionally, the increased diversity in base station form is being witnessed. Traditional base stations provide macro and micro base station services. Large-sized, high capacity and powerful macro base stations are suitable in most cases, while smaller, less powerful micro base stations are applicable for densely-populated areas and indoor coverage. Distributed base stations are becoming increasingly prevalent given their great power capability and high capacity. For small areas such as a cluster of households, concept base station Pico cells are proving highly suitable. While an increasing number of base station types are emerging that boast an array of features, it is distributed base stations which no doubt represent "next-generation base station" development. RRH (Remote Radio Head) and baseband pool technologies are popularized in nascent products, and this in turn greatly promotes the development of distributed base stations.


Two Organizations Influence the Industry Development

Previously, base station manufacturers were accustomed to making their products in isolation. This resulted in an increased variety of modules and specifications for connecting them, and module suppliers had to make different processors or radio frequency devices to adapt to different base station products. Costs, therefore, rose. Not surprisingly, it was logical to move toward standardized base stations interfaces that allow interworking between a large number of third-party module manufacturers and digital interfaces, with possibilities for multiplexing modules. This gave rise to two organizations spontaneously formed by industry vendors.

Nokia, NEC, LG and Samsung established OBSAI (Open Base Station Architecture Initiative) in October 2002. The organization's remit was to standardize wireless base stations'architecture, internal interfaces, control modules, transport, baseband and radio frequencies. However, OBSAI was disadvantaged by possessing only one strong base station vendor: Nokia.

In June 2003, Ericsson, Huawei, NEC, Nortel Network and Siemens jointly set up an organization called CPRI (Common Public Radio Interface), which also aimed to identify a universal standard of key internal interfaces; specifically interfaces between baseband and RF modules. The organization's strength was increased by the fact that the five CPRI members are all large-scale base station vendors. NEC – a founding member of OBSAI – turned to support CPRI less than a year after its inception. Numerous manufacturers have continued to join the organization, which now has over 100 members.

"Telecommunication and mobile handset giants challenge internal interface standards of wireless base stations", CommsDesign.com stated in its online report regarding OBSAI and CPRI. While OBSAI founders Nokia, LG and Samsung are mobile handset giants, it remains unknown whether the base station standards they established will be supported by other base station manufacturers. It has been pointed-out that "the future standard of OBSAI is more likely to take the base station of Nokia as its prototype".


CPRI

According to CPRI, a base station can be divided into two units: BBU (Baseband Unit), which is also called as REC (Radio Equipment Control) and RRU (Radio Frequency Unit), which is also called RE (Radio Equipment). CPRI's interface mainly features baseband and radio frequency separation. Small in size, easy to install and offering all-round function provision with low power consumption, BBU can coexist with current sites and support expansion in the cascade mode. RRU is small, light, simple and can be directly mounted on a pole or a wall near the antenna thus maximizing its convenience. CPRI defines interfacing between BBU and RRU, thus making it possible for different manufacturers' BBU and RRU modules to be interconnected.

Fig.1 Interfaces defined by CPRI

OBSAI

OBSAI divides a base station into four functional modules: transport, processing, radio and control. The transport module provides base stations with standard external network interfaces including IP and ATM. The processing module is used to process baseband signals. The radio module receives, transmits and amplifies RF signals, and converts them from digital baseband signals to analog RF signals. The control module provides a system clock for timing, manages and configures base station devices, monitors the operational status of the other base station modules and submits a report to the NE management system. Fig.2 illustrates OBSAI-defined open interfaces.

Fig.2 Open interfaces defined by OBSAI

Inspiring Status of Industrialization

The superiority of a distributed base station is easily apparent. Equipment networking is more flexible due to the distributed base station's smaller size, and the equipment involved enhances the value of site resources given their current short supply. The construction of equipment rooms and their daily maintenance require numerous personnel and a high level of financial investment from operators. Apart from cost savings, RF module and baseband transport separation grants greater capacity allocation flexibility and base station expansion potential. Transport via optical fibers can be realized between different base stations or management modules, which will greatly improve an entire network's performance. These reasons underpin the trend for equipment manufacturers to produce this type of base station.

Particularly, Xilinx and PMC are further promoting the development of distributed base stations by providing a reference design scheme for baseband processing and RF module interworking in compliance with OBSAI reference point 3 (RP3) electric and protocol standard and the CPRI standard.

However, given that the CPRI and OBSAI standards were created by manufacturers, they remain non-mandatory. In terms of the standards themselves, CPRI only defines layers-1 and layer-2 of the CPRI interfaces between BBU and RRU, with nothing defined for layer-3. In layer-2, manufacturer-defined fields exist, but there is no definition for O&M. From a practical perspective, there is still a long way to go before interworking between RF and baseband modules and the openness of current repeater equipment can be achieved.