C-RAN

C-RAN, i.e. Cloud-RAN, sometimes also referred as Centralized-RAN, is a new cellular network architecture for the future mobile network infrastructure. It was first introduced by China Mobile Research Institute in April 2010 in Beijing, China.^[1] Simply speaking, C-RAN is a centralized, cloud computing based new radio access network (commonly known as cellular network) architecture that can support 2G, 3G, 4G system and future wireless communication standards. Its name comes from the four 'C's in the main characters of C-RAN system, which are "Clean, Centralized processing, Collaborative radio, and real-time Cloud Radio Access Network".^[2]

Background

Traditional cellular network, or Radio Access Network (RAN), is built with many stand-alone base stations (BTS). Each BTS covers a small area, while a group BTS can provide coverage over a continuous area. Each BTS processes and transmits its own signal to and from the mobile terminal, and then forwards the data payload to/from mobile terminal to the core network via backhaul. Each BTS needs its own cooling, back haul transportation, backup battery, monitoring system, and so on. Because of the limited resource of spectrum, cellular network has to 'reuse' the frequency among different base stations. Due to the radio propagation, neighbor BTS that use the same frequency will experience interference in both downlink and uplink.

Traditional cellular systems have some limitations in its architecture. First, each BTS is costly to build and operate. Although Moore's law helps reduce the size and power of an electrical system, the supporting facilities of BTS are not improved as well. Secondly, when more BTS are added to the system to improve capacity, if the same frequency resource is reused, the interference among BTS is severe as more BTS are using the same frequency and BTS are closer to each other. Thirdly, because mobile users are moving from one place to another, the traffic of each BTS is fluctuating very much from time to time, which is called 'tide effect'. As a result, the average utilization rate of individual BTS is pretty low. However, these processing resources cannot be shared with other BTS. Thus all the BTS must be designed to handle the maximum traffic expected no matter the size of the average traffic. This means a lot of waste of processing resources and waste of powers at idle times.

Evolution of Base Station Architecture

All-in-One Macro Base Station

There has been evolution in base station architecture for cellular networks. In the 1G and 2G era, most base station were using the all-in-one architecture, having the analog, digital and power functions in one cabinet as large as a household refrigerator. Usually the base station cabinet is placed in a dedicated equipment room with all necessary site supporting facilitates like power, backup battery, air condition, environment surveillance system, backhaul transmission equipment etc. The RF signal from the base station RF unit goes through pairs of RF cable to the antennas on the top or base station tower or antenna mounting points. This all-in-one base station architecture is mostly used in macro cell deployment in 1G and 2G networks.

Distributed Base Station

When 3G was introduced, distributed base station architecture was introduced by Huawei and other leading telecom equipment vendors. In this architecture the radio function unit, also referred to as the remote radio head (RRH), is separated from the digital function unit, also referred to as the baseband unit (BBU) by fiber. Digital baseband signal are carried over the fibre, usually using OBSAI or CPRI standard. The RRH can be installed on the top of tower, close to the antenna. This reduces the cable loss compared to the traditional base station where the RF signal has to travel through a long cable from the base station cabinet to the antenna at the top of power. The fibre link between RRH and BBU also allows much more flexibility in network planning and deployment as they can be placed a few hundreds meters or a few kilometres away now. Now most modern base stations use this separated architecture.

C-RAN/Cloud-RAN

C-RAN can be viewed as an architectural evolution based on the above distributed base station architecture. It is based on many existing technology advances of wireless communication and optical technology as well as IT technology. For example, it makes use of the latest CPRI standard, low cost CWDM/DWDM technology, or mmWave to allow transmission of baseband signal over long distance thus achieving large scale centralized base station deployment. It applies recent Data Centre Network technology to allow a low cost, high reliability, low latency and high bandwidth interconnect network in the BBU pool. It utilizes open platform and real-time virtualization technology rooted in cloud computing to achieve dynamic shared resource allocation in BBU pool and support of multi-vendor, multi-technology environment.^[3]

Architecture Overview

C-RAN architecture has the following characteristics that are different from other cellular network architectures:

1. Large scale centralized deployment: Allows hundreds of thousands of remote RRH connect to a centralized BBU pool. The maximum distance can be 20 km in fiber link for 4G (LTE/LTE-A) system, even longer distance (40 km~80 km) for 3G (WCDMA/TD-SCDMA) and 2G (GSM/CDMA) systems. There are reports saying that some Asia operators have deployment C-RAN system which centralized 1200 of RRH to one central office.
2. Native support to Collaborative Radio technologies: Any BBU can talk with other BBU within the BBU pool with very high bandwidth (10Gbit/s and above) and low latency (10us level). This is enabled by the interconnect of BBU in the pool. This is one major difference from BBU Hoteling, or base station hoteling. In the later case, the BBU of different base stations are simple stacked together and has not direct link among them to allow physical layer co-ordination.
3. Real-time virtualization capability based on open platform: This is different from the traditional base station built on proprietary hardware, where the software and hardware are closed-sources and provided by one single vendor. C-RAN BBU pool is built on open hardware, like x86/ARM CPU based servers, plus interface cards to handle fiber link to RRH and inter-connection in the pool. Real-time virtualization make sure the resources in the pool can be allocated dynamically to base station software stacks, say 4G/3G/2G function modules from different vendors according to network load. However, to satisfy the strict timing requirement of wireless communication system, the real-time performance for C-RAN is at the level of 10s of micro-seconds, which is two magnitude higher than the milli-second level 'real-time' performance usually seen in Cloud Computing environment.

Similar Architecture and Systems

Korean Telecom's has introduced Cloud Computing Center (CCC) system in their 3G (WCDMA/HSPA) and 4G (LTE/LTE-A) network in 2011 and 2012.^[4] The concept of CCC is basically same to C-RAN.

SK Telecom's has also deployed Smart Cloud Access Network (SCAN) and Advanced-SCAN in their 4G (LTE/LTE-A) network in Korean no late than 2012.^[5]

Competing Architectures in Cellular Network Evolution

All-in-one BTS One major alternative solution that are addressing similar challenges of RAN, is the small size, all-in-one out-door BTS. Thanks to the achievement of semiconductor industry, now the whole function of BTS, include RF, baseband processing, MAC processing, package level processing can be implemented in a compact box of 50 liters. By make the system small and out door capable, it can also reduce the difficulty of BTS site construction, eliminate the air condition and thus reduce the operation cost.

However, because each BTS is still working on its own, it can not or it is hard to make use of the collaboration algorithms to reduce the interference between neighbor BTS. It is also relatively hard to upgrade or repair because the all-in-one BTS usually stands on roof-top. More processing unit in out-door also implies higher failure rate compared to C-RAN, which only has the RRU deployed out-door.

The advantage of Cloud RAN lies in its ability to implement LTE-Advanced features such as Coordinated MultiPoint (CoMP) with very low latency between multiple radio heads. However the economic benefit of improvements such as CoMP can be negated by the higher cost of backhaul for some operators.

Small Cell To be added.

The main competition between small cell and C-RAN are in two major deployment scenarios: out door hotspot coverage and indoor coverage.

Main Players and Activities

China Mobile started to promote the idea of C-RAN publicly since April 2010. It is also very active in various international standard organizations on promotion of R&D on C-RAN.

According to public information, the following companies have signed MoU on C-RAN collaboration with CMRI on April 23, 2010: ZTE, IBM, Huawei, Intel.

In the recent 4th International Mobile Internet Conference on December 13 and 14, 2010, 6 more companies joined: Orange, Chuanhua Telecom, Alcatel-Lucent, Datang Mobile, Ericsson, Nokia Siemens Networks.

In October, 2011, the 40th ITU Telecom Exhibition in Geneva, Switzerland, China Mobile exhibited four set of different Proof of Concept C-RAn system developed with partners including IBM, Huawei, ZTE, Orange Labs Beijing, Beijing University of Posts and Telecommunications. Over-the-air 2G/3G/4G communication with commercial terminals and test terminals have been demoed. Two of the four systems are based on open platform and two of them are based on proprietary platform.

NGMN has set up a working group named P-CRAN in April 2011 to study the requirement, solution suggestion and standardization of C-RAN. NGMN has published D3 Suggestions on Potential Solutions to CRAN and D4 Liaisons Contributions to 3GPP ETSI on Collaborative Radio MIMO ORI Interface as output of this project.

Green Touch has accepted an initial proposal of key technology research of C-RAN in June 2012.

Mr. Bill Huang, the GM of China Mobile Research Institute was invited to give a speech on World Mobile Congress in Feb. 2012^[6]

In the recent Linely conference [Linley Tech Carrier Conference 2012 ] pm June 5 and 6, 2012, ASOCS announced a Silicon IP product, the CR2100, accelerating Intel x86 based processing. ASOCS & China Mobile signed an MOU on 25 February 2013, announcing a joint development program to create commercially viable solutions for Cloud RAN.

In October, 2012, C-RAN has been extensively discussed in the 7th Base Station Conference in London, UK. Operators, vendors, analysts have given various views on C-RAN in the overall RAN evolution path.

While x86 platforms are considered by many as the prevailing platform. ARM based platforms may be available during 2017 and beyond . A hint towards that can be seen at a mutual Press release made by ARM and ASOCS at the ARMTeCon 2013.

Relation with Network Function Virtualization

Network Function Virtualization is treating C-RAN as one of its applications. The topic is defined in test case #6 . [ASOCS] has recently announced development of a full virtual base station as part of this test case .

Academic Research and Publications

As one of the promising evolution path for future cellular network architecture, C-RAN has attractive many academic research interest. Meanwhile, because the native support of cooperative radio capability built in the C-RAN architecture, it also enables many advanced algorithms that were hard to implement in cellular network, including Cooperative Multi Point Transmission / Receiving, Network Coding, etc.

In October, 2011, Wireless World Research Forum 27 was hosted in Germany, which China Mobile was invited to give a C-RAN presentation.

In August 2012, IEEE C-RAN 2012 workshop was hosted in Kunming, China.

CRC Press published book, Green Communications: Theoretical Fundamentals, Algorithms and Applications, has its 11th chapter: C-RAN: A Green RAN Framework.^[7]

in December 2012, a IEEE GlobalCom 2012 conference, International Workshop on Cloud Base-Station and Large-Scale Cooperative Communications, was hosted in California, USA.

European Committee Frame Project 7 has sponsors many problems related to cellular network architecture evolution research. Many of these projects have taken C-RAN as one of the future cellular network architecture, like Mobile Cloud Network^[8] project.

References

External links

• Official C-RAN website in China Mobile Research Institute
• the first C-RAN international workshop website, in Chinese
• ASOCS Asserts that C-RAN is a Superior Implementation
• ASOCS & ARM announce cloud-ran reference-design
• asocs and Windriver(INTC) popularize virtual Base Stations