C RAN

Background

Traditional cellular, or Radio Access Networks (RAN), consist of many stand-alone base stations (BTS). Each BTS covers a small area, whereas a group BTS provides coverage over a continuous area. Each BTS processes and transmits its own signal to and from the mobile terminal, and forwards the data payload to and from the mobile terminal and out to the core network via the backhaul. Each BTS has its own cooling, back haul transportation, backup battery, monitoring system, and so on. Because of limited spectral resources, network operators 'reuse' the frequency among different base stations, which can cause interference between neighbouring cells.

There are several limitations in the traditional cellular architecture. First, each BTS is costly to build and operate. Moore's law helps reduce the size and power of an electrical system, but the supporting facilities of the BTS is not improved as well. Secondly, when more BTS are added to a system to improve its capacity, interference among BTS is more severe as BTS are closer to each other and more of them are using the same frequency. Thirdly, because users are mobile, the traffic of each BTS fluctuates (called 'tide effect'), and as a result, the average utilization rate of individual BTS is pretty low. However, these processing resources cannot be shared with other BTS. Therefore, all BTS are designed to handle the maximum traffic, not average traffic, resulting in a waste of processing resources and power at idle times.

Evolution of Base Station Architecture

All-in-One Macro Base Station

In the 1G and 2G cellular networks, base stations had an all-in-one architecture. Analog, digital, and power functions were housed in single cabinet as large as a refrigerator. Usually the base station cabinet was placed in a dedicated room along with all necessary supporting facilitates such as power, backup battery, air conditioning, environment surveillance, and backhaul transmission equipment. The RF signal is generated by the base station RF unit and propagates through pairs of RF cables up to the antennas on the top of a base station tower or other mounting points. This all-in-one architecture was mostly found in macro cell deployments.

Distributed Base Station

For 3G, a distributed base station architecture was introduced by Nokia, Huawei and other leading telecom equipment vendors. In this architecture the radio function unit, also known as the remote radio head (RRH), is separated from the digital function unit, or baseband unit (BBU) by fiber. Digital baseband signals are carried over fiber, using the OBSAI or CPRI standard. The RRH can be installed on the top of tower close to the antenna, reducing the 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 the tower. The fiber link between RRH and BBU also allows more flexibility in network planning and deployment as they can be placed a few hundreds meters or a few kilometres away. Most modern base stations now use this decoupled architecture.

C-RAN/Cloud-RAN

C-RAN may be viewed as an architectural evolution of the above distributed base station system. It takes advantage of many technological advances in wireless, optical and IT communications systems. For example, it uses the latest CPRI standard, low cost CWDM/DWDM technology, and mmWave to allow transmission of baseband signal over long distance thus achieving large scale centralised 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 utilises open platforms and real-time virtualisation technology rooted in cloud computing to achieve dynamic shared resource allocation and support of multi-vendor, multi-technology environments.

Architecture Overview

C-RAN architecture has the following characteristics that are distinct from other cellular 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 deployments of C-RAN systems with 1200 RRHs centralized 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 introduced a Cloud Computing Center (CCC) system in their 3G (WCDMA/HSPA) and 4G (LTE/LTE-A) network in 2011 and 2012. The concept of CCC is basically the same as C-RAN.

SK Telecom has also deployed Smart Cloud Access Network (SCAN) and Advanced-SCAN in their 4G (LTE/LTE-A) network in Korea no later than 2012.

In 2014 Airvana (now CommScope) introduced OneCell, a C-RAN-based small cell system designed for enterprises and public spaces.

Competing Architectures in Cellular Network Evolution

All-in-one BTS

One major alternative solution that is addressing similar challenges of RAN, is the small size, all-in-one out-door BTS. Thanks to the achievements of semiconductor industry, now the whole function of a BTS, including RF, baseband processing, MAC processing and package level processing can be implemented in a volume of <50 liters. Making the system small and weatherproof, can also reduce the difficulty of BTS site choice and construction, eliminate the air conditioning requirement and thus reduce the operational cost.

However, because each BTS is still working on its own, it cannot readily make use of the collaboration algorithms to reduce the interference between neighboring BTSs. It is also relatively hard to upgrade or repair because the all-in-one BTS are usually mounted near the antenna. More processing units in less protected environments also implies a 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

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

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 . C-RAN can also enable cell virtualization, a technique for virtualizing wireless spectrum resources.

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.

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 project.