Building New-Generation GSM Networks with C-RAN

As 3G users become more habitual in their use of microblogging and mobile video, the era of mobile Internet is approaching at an ever-increasing pace. Competition facing mobile operators is fierce, and with the explosion of mobile Internet traffic, the costs of constructing, operating, and upgrading wireless access networks are increasing. However, in many cases, APRU is not increasing much or is even dropping. The development of mobile Internet is being hampered by traditional mobile networks, which cannot meet the demands of large amounts of mobile data.

Restrictions in Traditional Mobile Networks

Mobile Internet users with smart terminals and tablet PCs require ubiquitous signal coverage. High-order modulation, high spectrum, and high bandwidth are characteristics of LTE—a new wireless broadband technology designed to support mobile Internet access via cell phones and handheld devices. Microcell coverage is preferred in LTE; however, traditional mobile network design restricts the development of high-speed mobile Internet.      

• Construction and O&M costs for traditional mobile networks are high because auxiliary facilities such as equipment room, air conditioners, transmission and power supply are needed for base stations. Infrastructure equipment accounts for about 30% of TCO. Auxiliary facilities also consume much energy. 
• Cell signals in a traditional mobile network are separate and processed independently. Also, there can be much interference and little collaboration between base stations. Therefore, high bandwidth is not supported.
• The capacity of a mobile network varies from time to time and from place to place. This causes a tidal effect. Each cell in a traditional mobile network is designed with maximum capacity. If there is spare capacity, resources cannot be shared throughout the network. This is inefficient. The tidal effect is more pronounced in the mobile Internet era because of higher data throughput, and it is necessary to share network resources.
• Where multiple wireless systems coexist, wireless networks with different standards can be operated. However, multisystem convergence is difficult with a traditional network design.

To address these problems, China Mobile has put forward C-RAN—an innovative and environmentally friendly access network architecture for wireless networks. C-RAN has captured the attention of multinational operators.

The C-RAN Concept

C-RAN is a collaborative, real-time cloud radio access network. It is based on an open platform and consists of centralized baseband processing pools (virtual BTS pools), RRUs, and antennas.

Clean system, collaborative radio, and cloud computing is the essence of C-RAN. A complete industry study on C-RAN has already been carried out. ZTE was the first vendor to propose a “super baseband pool” solution. ZTE has worked with China Mobile to develop a baseband pool solution that supports more than a thousand carriers, to develop a prototype based on general-purpose processors, and to develop a future-proof C-RAN solution involving active antenna and LTE-A prototype.

To speed up network modernization, China Mobile has put higher requirements on its GSM network. More 3G (and even 4G) technologies will be applied. The introduction of C-RAN into such a large-scale GSM network will bring many benefits.

Energy saving

In a traditional mobile network, the cost of equipment rooms, transmission, and air conditioners chews up more than 60% of CAPEX. And when calculated over several years, OPEX accounts for more than 60% of TCO. A great deal of OPEX comes from power for air conditioners and from the rent of equipment rooms.

Using C-RAN, operators can focus their investment on infrastructure equipment, optical fibers, and communication pipes while greatly reducing the costs of auxiliary equipment. Baseband resource sharing and flexible dispatch can also be implemented. This significantly improves resource use. If an operator builds six sites in rural areas using C-RAN architecture, they can save equipment costs by around 9%, construction costs by around 30%, and O&M costs by around 76%.

Collaboration and sharing

China Mobile operates the world’s largest GSM network. Due to an upsurge in data traffic, the GSM network is heavily burdened, and resource use in the wireless network exceeds 75%. Each cell in urban sites has more than eight carriers, and the frequency reuse coefficient is less than 10. This is causing spectrum shortage. As more sites are built at intervals of less than 350 meters, network planning is becoming extremely complicated. The traditional way of expanding capacity by splitting cells and adding carriers has caused bottlenecks.

The tidal effect impacts wireless resource use, but C-RAN is designed to automatically adjust network resources as traffic migrates. Centralized baseband pools and multi-RRU cell combination technology solve problems caused by the tidal effect and also increase diversity gain through collaborative multi-RRU coverage.

GSM C-RAN Solution

ZTE has introduced C-RAN into its wireless network solution through technological innovation.

Sharing baseband resources

ZTE places the baseband parts of all wireless equipment on the BBU. The GSM BBU can dynamically share baseband resources of 108 carriers. By the end of 2011, ZTE will realize baseband resource sharing for over 1000 carriers. This will lay the foundation for collaboration and sharing between large-scale base stations.

Large capacity, low power consumption and flexible networking

ZTE has developed an innovative 2T4R RRU based on advanced multicarrier power amplification technology. A single RRU module supports 12 carriers and cross-cell configuration. Auxiliary facilities are unnecessary. The RRU is easy to install, has flexible networking (ring or chain topology), and is energy efficient (consumes only 460W peak power).

Multinetwork coexistence and smooth evolution

ZTE’s wireless networks are built on the unified SDR platform and open μTCA standard. This allows for smooth evolution of BBU and RRU. Baseband processing units of GSM, TD-SCDMA, and LTE networks based on the same platform architecture can be placed on the same BTS rack.  They can share the same backboard, power supply, main control, clock, and transmission facilities. In the future, GSM, TD-SCDMA and LTE networks will share the same BBU, and multiple RRUs will be networked and pooled together. This will simplify network architecture, reduce interactive latency between base stations, and provide basic architecture for wireless signal management and collaborative multicell radio management.

Complete transmission and O&M

China Mobile adopts SDH in its GSM bearer network. Usually, six to eight sites form a ring and share a pair of fiber optic cables. Using innovative technologies such as fast phase locking and high-quality clock, ZTE’s GSM C-RAN supports 18 chassis cascade. It employs 6G optical ports between BBU and RRU and consumes the same number of fiber optic resources as a conventional SDH bearer network. It also supports fiber connection and colored fiber connection and can be applied in various scenarios. ZTE uses the idle field of CPRI protocol between BBU and RRU to develop an array of functions for centralized O&M. This makes the network controllable and manageable.

The C-RAN architecture creates large capacity and uses a small number of nodes to dramatically reduce CAPEX. This helps operators build new-generation GSM networks. Today, GSM devices are being purchased in bulk to replace old devices. C-RAN-based GSM will become the equipment of choice for more operators.

Further research needs to be done on C-RAN. Mobile operators, equipment vendors, and traditional IT suppliers will jointly drive the growth of C-RAN and shorten the time to market for new-generation environmentally friendly RAN architecture.