China Mobile: Successful C-RAN Trial in Changsha

With the rapid development of telecom services and constant changes in subscriber behavior, operators are faced with unprecedented challenges in their radio access networks (RANs): high-power consumption caused by large numbers of sites, increasing CAPEX and OPEX, and low wireless coverage quality caused by complex networks. Site resources are hard to acquire, and some base stations are being used inefficiently because of the tidal effect. In the midst of technology, cost, resource, and security challenges, a completely new platform is emerging.

In April 2010, China Mobile proposed the C-RAN architecture—a new-generation, green RAN architecture. C-RAN is another innovation based on distributed base stations. Using centralized baseband, collaborative radio technologies, and real-time cloud computing, C-RAN allows dynamic network load balancing, and it allows network resources to be shared. C-RAN is a clean and efficient RAN that allows smooth evolution and provides higher bandwidths. More flexible multiple operation standards are also supported.

Shortly after proposing the C-RAN architecture, China Mobile immediately conducted feasibility studies for commercial deployment in several provinces. In Changsha, Hunan province, China Mobile selected ZTE as its partner to trial the GSM C-RAN. To verify the C-RAN network functions and capability, China Mobile chose the college district near Yuelu Mount as the trial area. In a population of 40,000, there are 22,000 2G subscribers, so the location was suitable for verifying C-RAN capability in a densely populated metropolitan area with heavy traffic. The area contains many student dormitories and residential buildings. The radio environment is very complex, and network KPIs need further improvement. The trial area was also suitable for verifying dynamic sharing of baseband resources with tidal traffic from the college district.

After the test scheme was determined,   the network deployment and all tests were completed within 26 days. The test results show that C-RAN has great potential and show the highlights of ZTE’s GSM C-RAN solution.

Overcoming Tidal Effect Through Centralized Baseband

During the test, baseband units were placed in the central equipment room of Tianmashan. This saved equipment room. The monitoring data shows that the tidal effect was very strong between 21:45 and 22:45. When students return to their dormitories after evening study, the traffic in the teaching building declines and traffic in the dormitories increased markedly. When the tidal effect occurs, baseband resources are dynamically adjusted; that is, the baseband resources begin to migrate from the teaching buildings to the dormitories. The baseband resources are shared by two sites, so the resources are saved by 16% and BBUs are reduced by 14% in the baseband board.

Improving Service Through Radio Collaboration

Usually, the antenna feeder is mounted on the rooftop and covers two cells. In one coverage cell, signal loss is severe because of trees and plants. The other has a weak indoor area. In the C-RAN network, a multi-RRU and co-cell mode is used. Two RRUs are connected to two antennas. One antenna is mainly used to cover the remote end of the cell, and the other antenna is mainly used to cover the near end of the cell. Two RRUs are configured to one cell.

Drive test results show that the uplink obtains 3 to 7 dB gain, MOS increases by 11.4%, and intercell handover is reduced by 15.3%. The multi-RRU and co-cell mode enhances signal stability and increases cell coverage quality and handover success rate.

Saving Fiber Resources and Protecting Services Using RRU Cascaded Ring Networking

The test results showed that each of the 18 cascaded RRUs functioned normally. Delay compensation was normal, and QoS met the standards. Ring networking guarantees the reliability of cascaded RRU sites. If any fiber section or any RRU in the link is damaged, the current link is automatically switched to the reverse link so that upper-level RRU and lower-level RRU is not affected.

Reducing Network TCO and Shortening Deployment Time with C-RAN

Using C-RAN, China Mobile can reduce CAPEX by 27.1% and OPEX by 63.9% for 11 sites with 272 carriers. In Changsha, C-RAN can reduce China Mobile’s CAPEX by RMB 1.435 million and OPEX by RMB 666,000 a year. For a local network with 10,000 sites, C-RAN can reduce CAPEX by RMB 2 billion a year and OPEX by RMB 0.4 billion a year. If C-RAN is used to build networks on a large scale, OPEX is significantly reduced.

Using centralized baseband and collaborative radio, new-generation C-RAN enables dynamic network load balancing and network resources to be shared. C-RAN fully incorporates the concepts of “green, collaborative, and sharing.” At present, C-RAN is only in the initial stage, and with the application of new concepts and technologies, C-RAN will bring even greater benefits to networks.