C-RAN The Road Towards Green RAN

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1 C-RAN The Road Towards Green RAN White Paper Version 2.5 (Oct, 2011) China Mobile Research Institute


3 Table of Contents C-RAN... i The Road Towards Green RAN... i 1 Introduction Background Vision of C-RAN Objectives of this White Paper Status of this White Paper Challenges of Today s RAN Large Number of BS and Associated High Power Consumption Rapid Increasing CAPEX/OPEX of RAN Explosive Network Capacity Need with Falling ARPUs Dynamic mobile network load and low BS utilization rate Growing Internet Service Pressure on Operator s Core Network Architecture of C-RAN Advantages of C-RAN Technical Challenges of C-RAN Technology Trends and Feasibility Analysis Wireless Signal Transmission on Optical Network Dynamic Radio Resource Allocation and Cooperative Transmission/Reception Large Scale Baseband Pool and Its Interconnection Open Platform Based Base Station Virtualization Distributed Service Network Evolution Path C-RAN Centralized Base Station Deployment Multi-standard SDR and Joint Signal Processing Virtual BS on Real-time Cloud Infrastructure Recent Progress TD-SCDMA and GSM Field Trial Large Scale Baseband Pool Equipment Development C-RAN prototype based on General Purpose Processor Conclusions Acknowledgement Terms and Definitions Reference i China Mobile Research Institute


5 1 Introduction 1.1 Background Today s mobile operators are facing a strong competition environment. The cost to build, operate and upgrade the Radio Access Network (RAN) is becoming more and more expensive while the revenue is not growing at the same rate. The mobile internet traffic is surging, while the ARPU is flat or even decreasing slowly, which impacts the ability to build out the networks and offer services in a timely fashion.. To maintain profitability and growth, mobile operators must find solutions to reduce cost as well as to provide better services to the customers. On the other hand, the proliferation of mobile broadband internet also presents a unique opportunity for developing an evolved network architecture that will enable new applications and services, and become more energy efficient. The RAN is the most important asset for mobile operators to provide high data rate, high quality, and 24x7 services to mobile users. Traditional RAN architecture has the following characteristics: first, each Base Station (BS) only connects to a fixed number of sector antennas that cover a small area and only handle transmission/reception signals in its coverage area; second, the system capacity is limited by interference, making it difficult to improve spectrum capacity; and last but not least, BSs are built on proprietary platforms as a vertical solution. These characteristics have resulted in many challenges. For example, the large number of BSs requires corresponding initial investment, site support, site rental and management support. Building more BS sites means increasing CAPEX and OPEX. Usually, BS s utilization rate is low because the average network load is usually far lower than that in peak load; while the BS processing power can t be shared with other BSs. Isolated BSs prove costly and difficult to improve spectrum capacity. Lastly, a proprietary platform means mobile operators must manage multiple none-compatible platforms if service providers want to purchase systems from multiple vendors. Causing operators to have more complex and costly plan for network expansion and upgrading. To meet the fast increasing data services, mobile operators need to upgrade their network frequently and operate multiple-standard network, including GSM, WCDMA/TD-SCDMA and LTE. However, the proprietary platform means mobile operators lack the flexibility in network upgrade, or the ability to add services beyond simple upgrades. In summary, traditional RAN will become far too expensive for mobile operators to keep competitive in the future mobile internet world. It lacks the efficiency to support sophisticated centralized interference management required by future heterogeneous networks, the flexibility to migrate services to network edge for innovative applications and the ability to generate new revenue from revenue from new services. Mobile operators are faced with the challenge of architecting radio network that enable flexibility. In the following sections, we will explore ways to address these challenges. 1.2 Vision of C-RAN The future RAN should provide mobile broadband Internet access to wireless customers with low bit-cost, high spectral and energy efficiency. The RAN should meet the following requirements: Reduced cost (CAPEX and OPEX) Lower energy consumption China Mobile Research Institute 1

6 High spectral efficiency Based on open platform, support multiple standards, and smooth evolution Provide a platform for additional revenue generating services. Centralized base-band pool processing, Co-operative radio with distributed antenna equipped by Remote Ratio Head (RRH) and real-time Cloud infrastructures RAN (C-RAN) can address the challenges the operators are faced with and meet the requirements. Centralized signal processing greatly reduces the number of site s equipment room needed to cover the same areas; Co-operative radio with distributed antenna equipped by Remote Radio Head (RRH) provides higher spectrum efficiency; real-time Cloud infrastructure based on open platform and BS virtualization enables processing aggregation and dynamic allocation, reducing the power consumption and increasing the infrastructure utilization rate. These novel technologies provide an innovative approach to enabling the operators to not only meet the requirements but advance the network to provide coverage, new services, and lower support costs. C-RAN is not a replacement for 3G/B3G standards, only an alternative approach to current delivery. From a long term perspective, C-RAN provides low cost and high performance green network architecture to operators. In turn operators are able to deliver rich wireless services in a cost-effective manner for all concerned. C-RAN is not the only RAN deployment solution that will replace all today s macro cell station, micro cell station, pico cell station, indoor coverage system, and repeaters. Different deployment solutions have their respective advantages and disadvantages and are suitable for particular deployment scenarios. C-RAN is targeting to be applicable to most typical RAN deployment scenarios, like macro cell, micro cell, pico cell and indoor coverage. In addition, other RAN deployment solution can serve as complementary deployment of C-RAN for certain case. 1.3 Objectives of this White Paper The objective of this white paper is to present China Mobile s vision of C-RAN and provide a research framework by identifying the technical challenges of C-RAN architecture. We would like to invite both industry and academic research institutes to join the research to guide the vision into reality in the near future. 1.4 Status of this White Paper This document version 2.5 is a revised version of version 2.0 released in December It is not yet fully complete and there may still be some inconsistencies. However, it is considered to be useful for distribution at this stage. It is expected that new research challenges might be added in future versions. Comments and contributions to improve the quality of this white paper are welcome. 2 China Mobile Research Institute

7 2 Challenges of Today s RAN 2.1 Large Number of BS and Associated High Power Consumption As operators constantly introduce new air interface and increase the number of base stations to offer broadband wireless services, the power consumption gets a dramatic rise. For example: in the past 5 years, China Mobile has almost doubled its number of BS, to provide better network coverage and capacity. As a result, the total power consumption has also doubled. The higher power consumption is translated directly to the higher OPEX and a significant environmental impact, both of which are now increasingly unacceptable. The following figure 1 shows the components of the power consumption of China Mobile. It shows the majority of power consumption is from BS in the radio access network. Inside the BS, only half of the power is used by the RAN equipment; while the other half is consumed by air condition and other facilitate equipments. Obviously, the best way to save energy and decrease carbon-dioxide emissions is to decrease the number of BS. However, for traditional RAN, this will result in worse network coverage and lower capacity. Therefore, operators are seeking new technologies to reduce energy consumption without reducing the network coverage and capacity. Today, there are quite a number of amendment technologies that helps reduce BS power consumption, such as the software solutions which save power through turning off selected carriers on idle hours like midnight, the green energy solutions which offer solar, wind and other renewable energy for base station s power supply according to local natural conditions, and the energy-saving air conditioning technology which combined with the local climate and environment characteristics, reduce the energy consumption of the air conditioning equipment, etc. However, these technologies are supplementary methods and cannot address the fundamental problems of power consumption with the number of increasing BS. In the long run, mobile operators must plan for energy efficiency from the radio access network architecture planning. A change in infrastructure is the key to resolve the power consumption challenge of radio access network. Centralized BS would reduce the number of BS equipment rooms, reduce the A/C need, and use resource sharing mechanisms to improve the BS utilization rate efficiency under dynamic network load. Cell site, 72% Transmission, 15% Management office, 7% Channel, 6% Other Support Equipment, 3% Air Conditioners, 46% Major Equipment, 51% Fig.1 Power Consumption of Base Station China Mobile Research Institute 3

8 2.2 Rapid Increasing CAPEX/OPEX of RAN Over recent years, mobile data consumption has experienced a record growth among the world s operators as subscribers use more smart phones and mobile devices, like tablets. To satisfy this consumer usage growth, mobile operators must significantly increase their network capacity to provide mobile broadband to the masses. However, in an intensifying competitive marketplace, high saturation levels, rapid technological changes and declining voice revenue, operators are challenged with deployment of traditional BS as the cost is high, the return is not high enough. Average Revenue Per User (ARPU) are all affecting mobile operators profitability. They become more and more cautious about the Total Cost of Ownership (TCO) of their network in order to remain profitable and competitive. Fig. 2: Increasing CAPEX of 3G Network Construction and Evolution Analysis of the TCO The TCO including the CAPEX and the OPEX results from the network construction and operation. The CAPEX is mainly associated with network infrastructure build, while OPEX is mainly associated with network operation and management. In general, up to 80% CAPEX of a mobile operator is spent on the RAN. This means that most of the CAPEX is related to building up cell sites for the RAN. The historical CAPEX expenditure of forest are shown in Fig.2. Because 3G/B3G signals (deployed frequency 2GHz have higher path loss and penetration loss than 2G signals (deployed frequency 900MHz), multiple cell sites are needed for the similar level of 2G coverage. Thus, the dramatic increase was found in the CAPEX when building a 3G network. The CAPEX is mainly spent at the stage of cell site constructions and consists of purchase and construction expenditures. Purchase expenditures include the purchases of BS and supplementary equipments, such as power and air conditioning equipments etc. Construction expenditures include network planning, site acquisition, civil works and so on. As shown is Fig.3, it is noticeable that the cost of major wireless equipments makes up only 35% of CAPEX, while the cost of the site acquisition, civil works, and equipment installation is more than 50% of the total cost. Essentially, this means that more than half of CAPEX is not spent on productive wireless functionality. Therefore, ways to reduce the cost of the supplementary equipment and the expenditure on site installation and deployment is important to lower the CAPEX of mobile operators. 4 China Mobile Research Institute

9 Fig. 3: CAPEX and OPEX Analysis of Cell Site OPEX in network operation and the maintenance stage play a significant part in the TCO. Operational expenditure includes the expense of site rental, transmission network rental, operation /maintenance and bills from the power supplier. Given a 7-year depreciation period of BS equipment, as shown in Fig.4, an analysis of the TCO shows that OPEX accounts for over 60% of the TCO, while the CAPEX only accounts for about 40% of the TCO. The OPEX is a key factor that must be considered by operators in building the future RAN. The most effective way to reduce TCO is to decrease the number of sites. This will bring down the cost for the construction of the major equipment; and will minimize the expenditure on the installation and rental of the equipment incurred by their occupied space. Fewer sites means the corresponding cost of supplementary equipment will also be saved. This can significantly decrease the operators CAPEX and OPEX, but results in poorer network coverage and user experience in the traditional RAN. Therefore, a more cost-effective way must be found to minimize the non-productive part of the TCO while simultaneously maintaining good network coverage. Fig. 4 TCO Analysis of Cell Site Multi-standard environment It is understood that the large number of legacy terminals, 2G, 3G, and B3G infrastructure will coexist for a very long time to meet consumers demand. Most of the major mobile operators worldwide will thus have to use two or three networks (Table 1) [1]. In the new economic climate, operators must find ways to control CAPEX and OPEX while growing their businesses. The base station occupies the largest part of infrastructure investment in a mobile network. Multi-mode base station is expected as a cost efficient way for operators to alleviate the cost of network construction and O&M. In addition, sharing of hardware resources in a multi-mode base station is the key approach to lower cost. China Mobile Research Institute 5

10 Table 1. Multi-Network Operation of Major Mobile Service Providers Cellular Technologies Vodafone China Mobile TD-SCDMA WCDMA CDMA One & 2000 & EVDO GSM GPRS EDGE LTE France Telecom T- Mobile Verizon SK Telecom Telstra China Unicom 2.3 Explosive Network Capacity Need with Falling ARPUs Data rate of mobile broadband network grows significantly with the introduction of air-interface standards such as 3G and B3G; this in turn speeds up end user s mobile data consumption. Some forecasts indicated the number of people who access mobile broadband will triple in next several years, after LTE and LTE-A are deployed. These findings reflect the fact that the increasing bandwidth of wireless broadband triggers the increase in mobile traffic, because the mobile users can use a variety of high-bandwidth services, such as video-based applications. This new trend will become a serious challenge to future RAN. Based on the forecast data [2], global mobile traffic increases 66-fold with a compound annual growth rate (CAGR) of 131% between 2008 and The similar trend is observed in current CMCC network. On the contrary, the peak data rate from UMTS to LTE-A only increases with a CAGR of 55%. Clearly, as shown in Fig.5, there is a large gap between the CAGR of new air interface and the CAGR of customer s need. In order to fill this gap, new infrastructure technologies need to be developed to further improve the performance of LTE/LTE-A. Fig. 5 Mobile Broadband Data-rates/Traffic Growth On the other hand, the revenue of mobile operators is not increasing at the same pace as the network capacity they provide. Mobile operators voice volumes are steadily increasing and the data volume grows quickly, but revenues are not and ARPUs are even falling in some case. In order to face the slow growth in revenue, operators are forced to constantly hold down costs notably operating costs. That means mobile operators must find a low cost, high-capacity access network with novel techniques to meet the growth of mobile data traffic while keeping a healthy, profitable growth. 6 China Mobile Research Institute

11 2.4 Dynamic mobile network load and low BS utilization rate One characteristic of the mobile network is that subscribers are frequently moving from one place to another. From data based on real operation network, we noticed that the movement of subscribers shows a very strong time-geometry pattern. Around the beginning of working time, a large number of subscribers move from residential areas to central office areas for work; when the work hour ends, subscribers move back to their homes. Consequently, the network load moves in the mobile network with a similar pattern,so called "tidal effect". As shown in Fig.6, during working hours, the core office area s Base Stations are the busiest; in the nonwork hours, the residential or entertainment area s Base Stations are the busiest. Fig. 6 Mobile Network Load in Daytime Each Base Station s processing capability today can only be used by the active users in its cell range, causing idle BS in some areas/times and oversubscribed BS in other areas. When subscribers are moving to other areas, the Base Station just stays in idle with a large of its processing power wasted. Because operators must provide 7x24 coverage, these idle Base Stations consume almost the same level of energy as they do in busy hours. Even worse, the Base Stations are often dimensioned to be able to handle a maximum number of active subscribers in busy hours, thus they are designed to have much more capacity than the average needed, which means that most of the processing capacity is wasted in non-busy time. Sharing the processing and thus the power between different cell areas is a way to utilize these BS more effectively. 2.5 Growing Internet Service Pressure on Operator s Core Network With the hyper-growth of smart phones as well as emerging 3G embedded Internet Notebook, the mobile internet traffic has been grown exponentially in the last few years and will continue to grow more than 66x in the next 5-6 years. However because of increasingly competition between mobile operators, the projected revenue growth will be much lower than the traffic growth. There will be a huge gap between the cost associated with this mobile internet traffic and the revenue generated, let alone the mobile operators needing to spend billions of dollars to upgrade their back-haul and core network to keep up with the growing pace. This is a huge common challenge to all the mobile operators in the wireless industry. The exponential growth of mobile broadband data puts pressure on operators existing packet core elements such as SGSNs and GGSNs, increasing mobile Internet delivery cost and challenging the flat-rate data service models. The majority of this traffic is either Internet bound or sourced from the Internet. Catering to this exponential growth in mobile Internet traffic by using traditional 3G deployment models, the older 3G platform is resulting in huge China Mobile Research Institute 7

12 CAPEX and OPEX cost while adding little benefit to the ARPU. Additional issues are the continuous CAPEX spending on older SGSNs & GGSNs, the higher Internet distribution cost, the congestion on backhaul and the congestion on limited shared capacity of base stations. Therefore, offloading the Internet traffic, as close to the base stations as possible, can be an effective way to reduce the mobile Internet delivery cost. Fig. 7 Wireless traffic on a commercial 3G Meanwhile it is interesting to understand how people are using today s mobile internet. A recent research paper [3] published by one major TEM may give us a glimpse of the most popular mobile applications. It is surprising to see that people are gradually using mobile internet just like they use the fixed broadband network. Content services which include content delivered through web and P2P are actually dominating the network traffic. Fig.7 is an example of wireless traffic on a commercial 3G operator. Considering this usage pattern, do we have better choice than just blindly spending billions of dollars to upgrade back-haul and the core network? 8 China Mobile Research Institute

13 3 Architecture of C-RAN We believe Centralized processing, Cooperative radio, Cloud, and Clean (Green) infrastructure Radio Access Network (C-RAN) is the answer to solve the challenges mentioned above. It s a natural evolution of the distributed BTS, which is composed of the baseband Unit (BBU) and remote radio head (RRH). According to the different function splitting between BBU and RRH, there are two kinds of C-RAN solutions: one is called full centralization, where baseband (i.e. layer 1) and the layer 2, layer 3 BTS functions are located in BBU; the other is called partial centralization, where the RRH integrates not only the radio function but also the baseband function, while all other higher layer functions are still located in BBU. For the solution 2, although the BBU doesn t include the baseband function, it is still called BBU for the simplicity. The different function partition method is shown in Fig.8. Solution 2 Solution 1 Antenna GPS Main Control & Clock Core network Baseband processing Digital IF Transmitter /Receiver PA & LNA BBU RRU Fig. 8 Different Separation Method of BTS Functions Based on these two different function splitting methods, there are two C-RAN architectures. Both of them are composed of three main parts: first, the distributed radio units which can be referred to as Remote Radio Heads (RRHs) plus antennas which are located at the remote site; second, the high bandwidth low-latency optical transport network which connect the RRHs and BBU pool; and third, the BBU composed of high-performance programmable processors and real-time virtualization technology. Virtual BS Pool L1/L2/L3/O&M L1/L2/L3/O&M L1/L2/L3/O&M Fiber RRH RRH RRH RRH RRH RRH RRH Fig. 9 C-RAN Architecture 1: Fully Centralized Solution China Mobile Research Institute 9

14 Virtual BS Pool L2/L3/O&M L2/L3/O&M L2/L3/O&M Fiber or Microwave RRH/L1 RRH/L1 RRH/L1 RRH/L1 RRH/L1 RRH/L1 RRH/L1 Fig. 10 C-RAN Architecture 2: Partial Centralized Solution The fully centralized C-RAN architecture, as shown in figure 9, has the advantages of easy upgrading and network capacity expansion; it also has better capability for supporting multistandard operation, maximum resource sharing, and it s more convenient towards support of multi-cell collaborative signal processing. Its major disadvantage is the high bandwidth requirement between the BBU and to carry the baseband I/Q signal. In the extreme case, a TD- LTE 8 antenna with 20MHz bandwidth will need a 10Gpbs transmission rate. The partial centralized C-RAN architecture, as shown in figure 10, has the advantage of requiring much lower transmission bandwidth between BBU and RRH, by separating the baseband processing from BBU and integrating it into RRH. Compared with the full centralized one, the BBU-RRH connection only need to carry demodulated data, which is only 1/20~1/50 of the original baseband I/Q sample data. However, it also has its own shortcomings. Because the baseband processing is integrated into RRH, it has less flexibility in upgrading, and less convenience for multi-cell collaborative signal processing. With either one of these C-RAN architectures, mobile operators can quickly deploy and make upgrades to their network. The operator only needs to install new RRHs and connect them to the BBU pool to expand the network coverage or split the cell to improve capacity. If the network load grows, the operator only needs to upgrade the BBU pool s HW to accommodate the increased processing capacity. Moreover, the fully centralized solution, in combination with open platform and general purpose processors, will provide an easy way to develop and deploy software defined radio (SDR) which enables upgrading of air interface standards by software only, and makes it easier to upgrade RAN and support multi-standard operation. Different from traditional distributed BS architecture, C-RAN breaks up the static relationship between RRHs and BBUs. Each RRH does not belong to any specific physical BBU. The radio signals from /to a particular RRH can be processed by a virtual BS, which is part of the processing capacity allocated from the physical BBU pool by the real-time virtualization technology. The adoption of virtualization technology will maximize the flexibility in the C-RAN system. 10 China Mobile Research Institute

15 Both solutions described above are under development and evaluation. They could be properly deployed in different networks depending on the situation of the network. The following discussion will focus on the Fully Centralized Solution. 3.1 Advantages of C-RAN The benefits of the C-RAN architecture are listed as follows: Energy Efficient/Green Infrastructure C-RAN is an eco-friendly infrastructure. Firstly, with centralized processing of the C-RAN architecture, the number of BS sites can be reduced several folds. Thus the air conditioning and other site support equipment s power consumption can be largely reduced. Secondly, the distance from the RRHs to the UEs can be decreased since the cooperative radio technology can reduce the interference among RRHs and allow a higher density of RRHs. Smaller cells with lower transmission power can be deployed while the network coverage quality is not affected. The energy used for signal transmission will be reduced, which is especially helpful for the reduction of power consumption in the RAN and extend the UE battery stand-by time. Lastly, because the BBU pool is a shared resource among a large number of virtual BS, it means a much higher utilization rate of processing resources and lower power consumption can be achieved. When a virtual BS is idle at night and most of the processing power is not needed, they can be selectively turned off (or be taken to a lower power state) without affecting the 7x24 service commitment. Cost-saving on CAPEX &OPEX Because the BBUs and site support equipment are aggregated in a few big rooms, it is much easier for centralized management and operation, saving a lot of the O&M cost associated with the large number of BS sites in a traditional RAN network. Secondly, although the number of RRHs may not be reduced in a C-RAN architecture its functionality is simpler, size and power consumption are both reduced and they can sit on poles with minimum site support and management. The RRH only requires the installation of the auxiliary antenna feeder systems, enabling operators to speed up the network construction to gain a firstmover advantage. Thus, operators can get large cost saving on site rental and O&M. Capacity Improvement In C-RAN, virtual BS s can work together in a large physical BBU pool and they can easily share the signaling, traffic data and channel state information (CSI) of active UE s in the system. It is much easier to implement joint processing & scheduling to mitigate inter-cell interference (ICI) and improve spectral efficiency. For example, cooperative multi-point processing technology (CoMP in LTE-Advanced), can easily be implemented under the C- RAN infrastructure. Adaptability to Non-uniform Traffic C-RAN is also suitable for non-uniformly distributed traffic due to the load-balancing capability in the distributed BBU pool. Though the serving RRH changes dynamically according to the movement of UEs, the serving BBU is still in the same BBU pool. As the coverage of a BBU pool is larger than the traditional BS, non-uniformly distributed traffic generated from UEs can be distributed in a virtual BS which sits in the same BBU pool. Smart Internet Traffic Offload Through enabling the smart breakout technology in C-RAN, the growing internet traffic from smart phones and other portable devices, can be offloaded from the core network of operators. The benefits are as follows: reduced back-haul traffic and cost; reduced core network traffic and gateway upgrade cost; reduced latency to the users; differentiating China Mobile Research Institute 11

16 service delivery quality for various applications. The service overlapping the core network also supplies a better experience to users. 3.2 Technical Challenges of C-RAN The centralized C-RAN brings lots of benefits in cost, capacity and flexibility over traditional RAN, however, it also has some technical challenges that must be solved before deployment by mobile operators. Radio over Low Cost Optical Network In C-RAN architecture 1, the optical fiber between BBU pool and RRHs has to carry a large amount of baseband sampling data in real time. Due to the wideband requirement of LTE/LTE-A system and multi-antenna technology, the bandwidth of optical transport link to transmit multiple RRHs baseband sampling data is 10 gigabit level with strict requirements of transportation latency and latency jitter. Advanced Cooperative Transmission/Reception Joint processing is the key to achieve higher system spectrum efficiency. To mitigate interference of the cellular system, multi-point processing algorithms that can make use of special channel information and harness the cooperation among multiple antennas at different physical sites should be developed. Joint scheduling of radio resources is also necessary to reduce interference and increase capacity. To support the above Cooperative Multi-Point Joint processing algorithms, both end-user data and UL/DL channel information needs to be shared among virtual BSs. The interface between virtual BSs to carry this information should support high bandwidth and low latency to ensure real time cooperative processing. The information exchanged in this interface includes one or more of the following types: end-user data package, UE channel feedback information, and virtual BS s scheduling information. Therefore, the design of this interface must meet the realtime joint processing requirement with low backhaul transportation delay and overhead. Baseband Pool Interconnection The C-RAN architecture centralizes a large number of BBUs within one physical location, thus its security is crucial to the whole network. To achieve high reliability in case of unit failure, in order to recover from error, and to allow flexible resource allocation of BBU, there must be a high bandwidth, low latency, low cost switch network with flexible, extensible topology that interconnects the BBUs in the pool. Through this switch network, the digital baseband signal from any RRH can be routed to any BBU in the pool for processing. Thus, any individual BBU failure won t affect the functionality of the system. Base Station Virtualization Technology After the baseband processing units have been put in a centralized pool, it is essential to design virtualization technologies to distribute/group the processing units into virtual BS entities. The major challenges of virtualization are: real-time processing algorithm implementation, virtualization of the baseband processing pool, and dynamic processing capacity allocation to deal with the dynamic cell load in system. Service on Edge Unlike service in a data center, distributing services on the edge of the RAN has its unique challenges. In the following research framework part, we try to summarize these challenges into the following three categories: services on the edge s integration with the RAN, intelligence of DSN, and the deployment and management of distributed service. 12 China Mobile Research Institute

17 4 Technology Trends and Feasibility Analysis In order to solve the technical challenges of C-RAN architecture, based on current technical conditions and future development trends, we suggest to do further research in the following areas. The purpose is to solve the low cost high bandwidth wireless signal transmission problem based on an optical network, dynamic resource allocation and collaborative radio technology. It also comprehends the large scale BBU pool and associated interconnection problem, virtualized BS based on open platforms and distributed service network solutions. The following is a detailed analysis and discussion of these challenges. 4.1 Wireless Signal Transmission on Optical Network The C-RAN architecture, which consists of the distributed RRH and BBU, means that need to transport untreated wireless signals between BBU and RRH. The BBU-RRH connectivity requirements pose challenges to the optical transmission speed and capacity. Usually, optical fiber transmission must be used to carry the BBU-RRH signal to meet the strict bandwidth and delay requirements. BBU-RRH Bandwidth Requirement Air interface is upgrading rapidly, new technologies like multiple antenna technology (2 ~ 8 antenna in every sector), wide bandwidth (10 MHz ~ 20 MHz every carrier) has been widely adopted in LTE/LTE-A, thus the bandwidth of CPRI/Ir/OBRI (Open BBU-RRH Interface) link bandwidth is much higher than the 2G and 3G era. In general, the system bandwidth, the MIMO antenna configuration and the RRH concatenation levels are the main factors which have an impact on the OBRI bandwidth requirement. For example, the bandwidth for 200 khz GSM systems with 2Tx/2Rx antennas and 4xsampling rate is up to 25.6Mbps. The bandwidth for 1.6MHz TD-SCDMA systems with 8Tx/8Rx antennas and 4 times sampling rate is up to 330Mbps. The transmission of this level of bandwidth on fiber link is matured and economic. However, with the introducing of multi-hop RRH and high orders MIMO supporting 8Tx/8Rx antenna configuration, the wireless baseband signal bandwidth between BBU-RRH would rise to dozens of Gbps. Therefore, exploring different transport schemes for the BBU-RRH wireless baseband signal is very important for C-RAN. Transportation Latency, Jitter and Measurement Requirements There are also strict requirements in terms of latency, jitter and measurement. In CPRI/Ir/OBRI transmission latency, due to the strict requirements of LTE/LTE-A physical layer delay processing also improve the baseband wireless signal transmission delay jitter and requirements indirectly. Not including the transmission medium between the round-trip time (i.e., regardless of delays caused by the cable length), for the user plane data (IQ data) on the CPRI/Ir/OBRI links, the overall link round-trip delay may not exceed 5μs. The OBRI interface requires periodic measurement of each link or multi-hop cable length. In terms of calibration, the accuracy of round trip latency of each link or hop should satisfy ±16.276ns [4]. System Reliability For the reliability of the system, because the traditional optical transmission networks (SDH/PTN) in the access network links provide reliable loop protection, automatic replace and fiber optic link management function, C-RAN architecture in the access network must also provide comparative reliability and manageability. In traditional RAN architecture, each BBU on the access ring usually has access to the corresponding transmission equipment of the center transmission machine room through SDH/PTN. Through the SDH/PTN ring routing and China Mobile Research Institute 13

18 protection function, the system can quickly switch to the safe routing mode when any point on this loop experiences optical fiber failure, ensuring that business is not interrupted. Under the C-RAN architecture, it also should offer a similar optical fiber ring network protection function. Centralized BBU should support more than 10~1000 base station sites, and then the optical fiber connected OBRI link between distributed RRH and centralized BBU is long. If only point-2- point optical fiber transmission occurred between each distributed RRH and centralized BBU, then any fault on the optical fiber link will lead to the corresponding RRH loosing service. In order to ensure the normal operation of the whole system under the condition of any single point of failure in the optical fiber, the CPRI/Ir/OBRI link connecting the BBU-RRH should use fiber ring network protection technology, using the main/minor optical fiber of different channels to realize CPRI/Ir/OBRI link real-time backup. Operation and Management At the same time, under the traditional RAN architecture, the transmission network which consists of SDH/PTN also provides the unified optical fiber network management ability for the access ring. This includes unified management of the access ring fiber optic link of the entire network, supervisory control of the access ring optical fiber breakdown, etc. BBU-RRH wireless signal transport directly on the access ring, whose CPRI/Ir/OBRI interface should also, provides similar management ability and fit into unified optical fiber network management. Cost Requirements Finally, in terms of cost, the high speed optical module necessary for the CPRI/Ir/OBRI optical interface will be amongst the important factors affecting the C-RAN economic structure. Compared to traditional architecture, the wireless signal transmission data rate on C-RAN is more than times higher than the bearer service data rate after demodulation. Building the fiber transportation network in developed city is very hard. This is less of an issue for operators that already deploy optical fiber and particularly for operators own their own optical network. Although the cost of the optical fiber employing CPRI/Ir/OBRI for high speed wireless signal transmission doesn't need to increase, the high speed optic module or optical transmission equipment costs must compare to traditional SDH/PTN transmission equipment in order to make C-RAN architecture more attractive on the CAPEX and OPEX fronts.therefore, how to achieve a low cost, high bandwidth and low latency wireless signal optical fiber transmission will become a key challenge for realization of the future LTE and LTE network deployment by C-RAN. For the above problems and corresponding technical progress trend, we will analyze and put forward ideas for solving these problems Data Compression Techniques of CPRI/Ir/OBR Link In view of the above LTE/LTE-A BBU-RRH wireless signal transmission bandwidth problems, several data compression techniques that can reduce the burden on the OBRI interface are being investigated to deal with the inevitable bandwidth issue, including time domain schemes (e.g. reducing signal sampling, non-linear quantization, and IQ data compression) as well as frequency domain schemes (e.g. sub-carrier compression). For LTE system with 20MHz bandwidth, the BBU uses 2048 FFT / IFFT but the effective number of subcarriers is only 1,200, so if the FFT / IFFT is implemented in the RRH, then the Ir interface between BBU and the RRH only has to transmit effective data subcarriers, such that the Ir interface load can be reduced about 40%, However, frequency domain compression leads to an increase in IQ mapping complexity, which would increase the interface logic design and processing complexity. Meanwhile, the RRH needs to process 14 China Mobile Research Institute

19 parts of the RACH, Therefore, RRH cannot treat different RACH configurations transparently, instead RRH needs to process RACH based on configuration. Since there are hundreds of different configurations, each has to be controlled by different timing algorithms in the RRH, which could greatly increase the complexity of system design. Therefore, considering the implementation complexity and cost, such frequency domain compression is not feasible at the moment. DAGC time-domain based compression technology is a method used for IQ compression. The basic principle of DAGC is to select the average power reference based on the best baseband demodulation range, normalize the power of each symbol, and reduce the signal dynamic range. DAGC compression will adversely affect system performance. The receiver dynamic range of the uplink will be reduced, which leads to deterioration of the signal to noise ratio. At the same time, the EVM indicators will worsen on the downlink. With increased compression ratio, the system performance will deteriorate even more. Currently, we still need to investigate the impacts caused by different compression schemes. Table 2 lists the advantages and disadvantages of various compression schemes. As indicated, there is no ideal OBRI link data compression scheme. More studies in this area are required. Table 2. Comparison of Pros and Cons for Various Data Compression Techniques Bandwidth Compression Schemes Reducing signal sampling Non-linear quantization IQ data Compression Sub-carrier Compression Pros Low complexity; Efficient compression to 66.7%; Less impacts on protocols. Improve the QSNR; Mature algorithms available, e.g. A law and U law; High compression efficiency to 53%. Potential high compression efficiency; Only need extra decompression and compression modules. High compression efficiency to 40% ~58%; Easy to be performed in downlink. Cons Severe performance loss. Some impacts on the OBRI interface complexity. High complexity; Difficult to set up a relativity model; Real-time and compression distortion issues; No mature algorithm available. Increase the system complexity; Extra processing ability on optical chips and the thermal design; High device cost; Difficulty for maintenance; RACH processing is a big challenge; More storage, larger FPGA processing capacity Transmission delay and jitter of CPRI/Ir/OBRI link As mentioned previously, CPRI/Ir/OBRI link have strict demands on transmission delay, jitter and measurement. However, because the link round trip delay requirements (5 us) of the user plane data (IQ data) in CPRI/Ir/OBRI link do not include the transmission medium round-trip time (i.e. delay in optical transmission), this requirement can be satisfied by the existing technical conditions. At the same time, because CPRI/Ir/OBRI optical fiber routing generally does not change with time and delay jitter caused by transmission is relatively small, it is easy to meet the corresponding requirements. China Mobile Research Institute 15

20 On the other hand, because LTE/LTE-A has strict requirements about physical layer treatment delay, CPRI/Ir/OBRI total transmission delay on the link should not exceed a certain level. The physical layer HARQ process places the highest demand on processing delay. HARQ is an important technology to improve the performance of the physical layer, its essence is testing the physical layer on the receiving end of a sub-frame for correct or incorrect transmission, and rapid feedback ACK/NACK to the launching end physical layer, then let launching physical layer to make the decision whether or not to send again. If sent again, the receiver does combined processing for multi-launching signal in the physical layer, and then provides feedback to the upper protocol after demodulation success. According to the LTE/LTE-A standard, the ACK/NACK HARQ on uplink and downlink process should be finished in 3 ms after receiving the signals in the shortest case, which requires that sub-frame processing delay in the physical layer should be generally less than 1 ms. Because the physical layer processing itself takes us, then CPRI/Ir/OBRI optical transmission delay may be us at the most. According to the light speed(200,000 kilometers per hour) estimated in the fiber, CPRI/Ir/OBRI interface maximum transmission distance under the C-RAN framework is limited from 20 km to 40 km. Specific value is related to delay margin the physical layer treatment itself Optical Transmission Technology Progress and Cost Reduction As mentioned above, BBU-RRH wireless signal connection supporting LTE and LTE-Advance creates new challenges to optical transmission network rates and cost. The rapid development of the optical transmission technology provides more economic solutions to solve the problem. A single fiber capacity of current commercial WDM system can be up to 3.2 T.10 Gpbs optical transmission technology applies generally and become fundamental, 40 G system is mature and gradually being commercialized, 100 G technology is still not mature and costs too much, there is still 2-3 years until the telecommunication commercial level, but along with coherent technical breakthroughs, promoting of standardization has already become a now advantage. 10GE standardization and industrialization will greatly improve the relevant market capacity of the optical transmission module, which will help to reduce the cost of 10 Gbps optical modules. 40GE technology is still in the research process. On the other hand, at the access network level, 1.25 G,2.5 G EPON is already widely used in solving FTTX access, 10G PON technology can be commercial in one or two years, the future PON technological development have several directions like WDM-PON, Hybrid PON and 40G PON. Similar to what the Moore's Law is doing in the transformation of the semiconductor industry, the field of optical communication has a similar trend: Every year, the speed of optical transmission increases while the cost of the said module declines. Transceiver modules that are capable of supporting multi-wavelength WDM have emerged in the market place. Since commercial LTE deployment has just begun, we can safely predict that it will take about 5 years before the commercial LTE-A multi-carrier system deployment is needed. By then, if the optical module advancement and cost reduction has reached an acceptable level, then the RRH-BBU bottleneck will be effectively removed. Figure 11 shows the 2.5G SFP and 10G SFP / XFP / XENPAK optical modules pricing trends. We can deduce that optical modules pricing has dropped by 66% to 77% in nearly 3 years, and the trend will continue in the coming years, further reducing the cost of optical transmission network. If this price trend continues, it would greatly help to reduce CAPEX of a C-RAN network. 16 China Mobile Research Institute

21 Price history of 2.5G modules (RMB). Price history of 10G modules (RMB) % % % 0 Aug-07 Feb-08 Aug-08 Feb-09 Aug-09 10Km 40Km 80Km % % % Aug-07 Feb-08 Aug-08 Feb-09 Aug m 10Km 40Km Fig. 11 Price history of Commercial 2.5G/10G Optical Modules BBU-RRH Optical Fiber Network Protection Although BBU-RRH direct transmission under C-RAN framework does not provide a ring network protection function like traditional SDH/PTN, the CPRI/Ir/OBRI interface rate standards provide a similar ring network protection function, and are supported by manufacturers. At the same time, in order to avoid having every RRH fully occupy two optical fibers on a physically routed pair the RRH s can be connected to each in a cascaded manner according to the CPRI/Ir/OBRI interface specification. This permits two different routing trunk cables to form a ring and be connected to the same BBU, as shown in Figure 10. As long as the CPRI/Ir/OBRI interface rate is high enough, the BBU-RRH ring network protection technology can save the use of many optical fibers and ensure a short round trip delay. Taking a TD-SCDMA system for example, a Gpbs CPRI/Ir/OBRI link can support 15 TD-SCDMA carriers of 8-antenna RRH and a typical TD-SCDMA macro station with 3 sectors, 5/5/5 configuration at most. The IQ data of a RRH with three sectors connected to the same BBU machine through two different physical routing backbone optical cables. When a trunk cable fails, three RRHs will connect to the BBU through another trunk cable under less than 40ms protection rotated time to guarantee that all business does not interrupt. For lower-rate GSM system, it is even simpler to connect six or more RRHs through such a CPRI/Ir/OBRI annular link and achieve the same functions. However, according to LTE/LTE-A system with higher wireless signal transmission rate, it is necessary to introduce WDM technology to realize a similar loop protection function. Radio remote head Optical switching box Trunk cable 2 Transmission ring Trunk cable 1 Fig. 12 RRH Ring Protection Loop Central apparatus room China Mobile Research Institute 17

22 4.1.5 Current Deployment Solutions In order to meet the high bandwidth transmission between RRH and BBU, operators can use different solutions based on their current transmission network resources. In China Mobile, the current backhaul is mainly an optical transport network with three layers of transmission network: the core transmission layer, the convergence transmission layer and the access transmission layer. All the layers are using ring topology to provide fail safe protection. The optical resources of different layers are similar to the following: at the core transmission layer, each optical route has 144 to 576 fibers; at the convergence transmission layer, each route has fibers; while at the access transmission layer, each route has fibers. If the Baseband pool is located in the transmission convergence equipment room, the optical fiber resource to and from the equipment room determines the coverage of the baseband pool. According to the resourcing of the optical transmission network, especially the fiber resource in the access transmission network, there are four different solutions to carry CPRI/Ir/OBRI over it: 1. Dark fiber; 2. WDM/OTN; 3. Unified Fixed and Mobile access like UniPON; 4. Passive WDM. These solutions have different advantages and disadvantages, and they are each suitable for different deployment scenarios. From the trials conducted, for a BBU pool with less than 10 macro BSs, it is preferred to use a dark fiber solution while other solutions still need more field tests and verification, because they may introduce new transmission devices and associated O&M issues. The first solution is Dark fiber. It is suitable when there is plenty of fiber resource. It is easy to deploy if there are a lot spare fiber resources. The benefits of this solution are: fast deployment and low cost because no additional optical transport network equipment is needed. The concerns of this solution are: it consumes significant fiber resource, thus the network extensibility will be a challenge; new protection mechanisms are required in case of fiber failure; and it is hard to implement O&M, therefore it will introduce some difficulties for optical network O&M. However, there are feasible solutions to address such challenges. For fiber resources, if there is already a channel route available, it is fairly inexpensive to add new fiber cables or upgrade existing fibers. To address fiber failure protection, there are CPRI/Ir/OBRI compliant products available now that have the 1+1 backup or ring topology protection features. If deployed with physical ring topology that provides alternative fiber route, it will be able to provide similar recoverability capability as SDH/PTN. For the O&M of the fiber in the access ring, we are considering introducing new O&M capabilities in the CPRI/Ir/OBRI standard to satisfy the fiber transport network management requirement. The second solution is WDM/OTN solution. It is suitable for Macro cellular base station systems when there is limited fiber resource, especially where the fiber resource in the access ring is very limited, or adding new fiber in existing route is too difficult or cost is too high. By upgrading the optical access transmission network to WDM/OTN, the bandwidth of transporting CPRI/Ir/OBRI interface on BBU-RRH link is largely improved. Through transmitting as many as 40 or even 80 wavelength with 10Gpbs in one fiber, it can support a large number of cascading RRH on one pair of optical fiber. This technology can reduce the demand of dark fiber, however, upgrading existing access ring into WDM/OTN transmission network means higher costs. On the other hand, because the access transport network is usually within a few tens of kilometers, the WDM/OTN equipment can be much cheaper than those used in long distant backbone networks. The third solution is based on CWDM technology. It combines the fixed broadband and mobile access network transmission at the same time for indoor coverage with passive 18 China Mobile Research Institute

23 optical technology, thus named as Unified PON. It can provide both PON services and CPRI/Ir/OBRI transmission on the same fiber [5]. In this solution, an optical fiber can support as many as 14 different wavelengths. In the UniPON standard, the uplink and downlink channel are transmitted on two difference wavelengths, thus other free wavelengths can be used for CPRI/Ir/OBRI data transmission between the BBU and RRH. Because of sharing the optical fiber resources, it can reduce the overall cost. It is suitable for C-RAN centralized baseband pool deployment of indoor coverage Summarize Based on the above analysis, fully centralized C-RAN architecture requires a high bandwidth, low latency, high reliability and low cost optical solution to transmit high speed baseband signal between BBU and RRH. It s promising to find feasible solutions emerging in the near future. However, there are still many challenges in the current solutions. For example, current data compression schemes fail to satisfy OBRI transmission in the LTE-A phase. The rapid development of high-speed optical modules and the associated cost reduction is heading in the right direction but we still need a breakthrough in optical devices. Failure protection schemes for BBU-RRH connection are able to provide similar functions to SDH/PTN in case of fiber cut, but we still need to find solutions for unified O&M with traditional transmission networks. UniPON based on passive WDM technology is a promising solution for certain deployment scenarios but it must be designed to be competitive in cost. In conclusion, we have various directions to solve the high-speed baseband signal transmission requirement of C-RAN but we still need to explore new technology or a combination of existing technology to find a more economical and effective solution. Considering the technical challenges as well as the limitation in current optical network resources, it is clear that C-RAN can be widely applied in a short time frame. Instead, a stepped plan should be used to gradually construct the centralized network: first, centralized deployment can be applied in some green field or replacement of old network in a small scale. Dark fiber can be used as the BBU-RRH transmission solution. One access ring that connects 8~12 macro sites can be centralized together, with a maximum ring range of 40km. In the future, a larger number of macro BS in various deployment scenarios can be further tested. China Mobile Research Institute 19

24 4.2 Dynamic Radio Resource Allocation and Cooperative Transmission/Reception One key target for C-RAN system is to significantly increase average spectrum efficiency and the cell edge user throughput efficiency. However, users at the cell boundary are known to experience large inter-cell interference (ICI) in a fully-loaded OFDM cellular environment, which will cause severe degradation of system performance and cannot be mitigated by increasing the transmit power of desired signals. At the same time, in view of the analysis, single cell wireless resources usage efficiency is low. To improve system spectrum efficiency, advanced multi-cell joint RRM and cooperative multi-point transmission schemes should be adopted in the C-RAN system. Cooperative Radio Resource Management for multi-cells The multi-cell RRM problem has been addressed in various academic studies. Many uses various optimization techniques in trying to determine the optimal resource scheduling and the power control solutions to maximize the total throughput of all cells with some specific constraints. To reduce the complexity incurred in the C-RAN network architecture and the scheduling process, the joint processing/scheduling should be limited to a number of cells within a cluster. The complexity of scheduling among the enbs clusters is determined by the velocity of mobile users and the number of UEs and RRHs in the cluster. Thus, choosing an optimal clustering approach will require balancing among the performance gain, the requirement of backhaul capacity and the complexity of scheduling. As shown in Fig.13, UEs will be served by one of the available clusters which are formed in a static or semi-static way based on the feedback or measurements reports of UEs. In this scenario, a subset of cells within a cluster will cooperate in transmission to the UEs associated with the cluster. To further reduce the complexity, it is possible to limit the number of cells cooperating in joint transmission to a UE at each scheduling instant. The cells in actual transmission to a UE are called active cells for the UE. The active cells can be defined from the UE perspective based on the signal strength (normally cells with strong signal strength are chosen among cells within the supercell). The activation/de-activation of a cell can be done by a super enb, which is the control entity in cell clustering and can adjust the sets scope based on the UE feedback. Cell cluster 1 Cell cluster 2 Cell cluster 3 Fig. 13 The UE assisted network controlled cell clustering 20 China Mobile Research Institute

25 Cooperative Transmission / Reception Cooperative transmission / reception (CT/CR) is well accepted as a promising technique to increase cell average spectrum efficiency and cell-edge user throughput. Although CT/CR naturally increases system complexity, it has potentially significant performance benefits, making it worth a more detailed consideration. To be specific, the cooperative transmission / reception is characterized into two classes, as shown in Fig.14: Joint processing/transmission (JP) The JP scheme incurs a large system overhead: UE data distribution and joint processing across multiple transmission points (TPs); and channel state information (CSI) is required for all the TP-UE pairs. Coordinated scheduling and/or Coordinated Beam-Forming (CBF) With a minimum cooperation overhead, to improve the cell edge-user throughput via coordinated beam-forming: No need for UE data sharing across multiple TPs; Each TP only needs CSI between itself and the involved UEs (no need for CSI between other TPs and UEs). Fig. 14 JP scheme and CBF scheme In this section, the performance of the JP scheme with intra-cell collaboration, and performance with inter-cell collaboration in C-RAN architecture are evaluated in a TDD system. We assume that full DL channel state information (CSI) can be obtained ideally at the enb side. The downlink throughput and spectrum efficiency results with different schemes in both 2 antenna and 8 antenna configuration are shown in Fig.15. Detailed simulation parameters can be found in [6-9]. 3GPP Case 1 (TDD) Ave. cell spectrum efficiency (bps/hz) Tx (X)/2Rx SU-MIMO MU-MIMO 8Tx(XXXX)/2Rx Cell-edge spectrum efficiency (bps/hz) SU-MIMO MU-MIMO Tx (X)/2Rx 8Tx(XXXX)/2Rx China Mobile Research Institute 21

26 ITU UMi (TDD) 6 4 Ave. cell spectrum efficiency (bps/hz) SU-MIMO MU-MIMO Intra-site CoMP C-RAN CoMP Cell-edge spectrum efficiency (bps/hz) SU-MIMO MU-MIMO Tx (X)/2Rx 8Tx(XXXX)/2Rx 2Tx (X)/2Rx 8Tx(XXXX)/2Rx Fig. 15 Compare of Downlink Throughput and SE From the simulation results we can see, compared to the non-cooperative transmission mechanism (MU-BF in LTE-A), the spectrum efficiency of intra-cell collaboration and inter-cell collaboration under C-RAN architecture could achieve a 13% and 20% gain, respectively, while the cell edge user s spectrum efficiency, from the above two mechanisms can get 75% and 119% gain respectively. Technical Challenges Cooperative transmission / reception (CT/CR) has great potentials in reducing interference and improving spectrum efficiency of system. However, this technology has many problems that need to be further studied before it can be applied to the practical networks. There are many challenges listed as follows: Advanced joint processing schemes DL channel state information (CSI) feedback mechanism User pairing and joint scheduling algorithms for multi-cells Coordinated Radio resource allocation and power allocation schemes for multi-cells. 4.3 Large Scale Baseband Pool and Its Interconnection Centralized Baseband Pool There are many distributed BS products using RRH+BBU architecture in market. Some TEM s products have realized dynamic allocation of carrier processing within one BBU to adapt to dynamic workloads among different RRH connected to it. This architecture can be viewed as the first step of centralized baseband pool concept, but in general a single BBU has limited processing capability, typically only supporting about 10 macro BSs carriers. It s not yet capable of supporting dynamic resource allocation across different BBU, thus hard to resolve the dynamic network load in a larger area. In the current RRH+BBU architecture, the RRH is usually connected to a particular BBU by a fixed link, and it can only transmits its baseband signal and O&M signaling to the BBU it s connected to. This makes it difficult for another BBU to obtain any uplink baseband data from that RRH. Similarly, any other BBU has difficulty sending downlink baseband data to this RRH. Because of this limitation, the processing resources of different BBUs can hardly be shared: the idle BBU s processing resources are wasted and it cannot be used to help the BBU with a heavy workload. The centralized baseband pool should provide a high bandwidth, low latency switch matrix with an appropriate protocol to support the high speed, low latency and low cost interconnection among multiple BBUs. In a medium sized dense urban network coverage (approximately 25 sq. 22 China Mobile Research Institute

27 km in area), with an average distance between BS of 500m, a centralized baseband pool that can cover the whole area needs to support about 100 BS. For a typical TD-SCDMA system with 3 sectors per macro BS and 3 carriers/sectors, it means that the centralized baseband pool needs to support 900 TD-SCDMA carriers. Imagine if the centralized Baseband pool coverage is even larger, such as 15 km X 15 km, then the baseband pool would need to support up to 1000 macro BSs carriers. Because of the limitation in the high-speed differential signal transmission, the traditional BBU architecture cannot scale up to support such capacity by simply expanding the backplane dimensions. Infinite Band technology can provide significant switching bandwidth (20Gbps-40Gpbs/port) and very low switching latency. It is widely used in supercomputers. However, the cost per port is very high (20,000RMB) and as such does not meet the C-RAN cost requirement. Inspired by the data center network s distributed inter-connect architecture, the centralized BBU pool in C- RAN can also use a distributed optic interconnection to combine multiple BBU into a scalable baseband pool. Based on that, the RRHs signal can be routed to any one of BBUs in the pool. Thus load balance according to dynamic network load among BBUs can be achieved, and system power consumption can be reduced. It also makes the deployment of multi-point MIMO technology and interference mitigation algorithms easier, which can improve radio system capacity. Dynamic carrier scheduling The dynamic carrier scheduling of resources within baseband pools enhances redundancy of the BBU and increases overall operational reliability of the baseband pool. When a baseband card or a carrier processing unit fails, the work load can be promptly redistributed to other available resources within the pool, and restore the normal operation. In addition, for areas that have strong dynamic network load, the operator can deploy fewer baseband resources to meet the demands of different sites that have opposite peak loads at different times. For example, operator can use the same BBU pool with multiple RRHs to cover both residential areas and office areas. Then dynamically allocates baseband resources to ensure basic coverage for both areas. Remaining baseband resources can be dynamically allocated to cover the business area during working hours and the residential area during after working hours. This will increase the overall carrier resource utilization. Large-scale BBU Inter-connection A large scale baseband inter-connect solution should be able with the following requirements: to support macro BS, Inter-connection between BBUs must satisfy the wireless signal s requirements of low latency, high speed, and high reliability. The requirements are similar to the CPRI/Ir/OBRI interface, and should support real-time transmission of 2.5/6.144/10Gbps rate. Dynamic carrier scheduling among BBUs to achieve efficient load balance within the system and failure protection without service interruption. Support multipoint collaboration (CoMP). It needs to consider the data flow between different BBUs to support collaboration radio. Fault-tolerance. Fiber inter connection should support 1+1 failure protection, BBU frame and baseband processing board N +1 protection to achieve high system robustness. High scalability: it can extend the system capability smoothly without services interruption. China Mobile Research Institute 23

28 4.4 Open Platform Based Base Station Virtualization Current Multi-Standard BS Solutions Nowadays, most major mobile operators in the world have to operate multiple standards simultaneously. It is a natural choice to use multi-mode base stations for low cost operation. Therefore, SDR based on a common platform to support multi-standards has become the mainstream in TEM s products. The following are the two types of multi-mode base stations. Unified BBU system platform supporting multi-mode by plugging in different processing boards. The processing board which supports multi-standard (such as GSM, TD-SCDMA, TD-LTE) has a unified interface and can be plugged in the same BBU system platform. Operators can use one set of a BBU system platform to support multi-standard operation. In this case, some modules of BBU system such as control module, timing module and RRH I/O modules can be shared between BBU processing boards which support different standards. However, this structure can't share processing resources between different processing boards and usually need to replace or add new processing board hardware for upgrades. Unified BBU system platform and unified processing board hardware platform to support multi-mode through the software re-configuration. Through software upgrades or configuration, the same processing board can support different standards (e.g. LTE or TD- SCDMA). In some of the latest products, the RRH can also be SDR-enabled to support different standards in the same spectrum band. This solution allows the base station to be upgraded to a new standard without changing the hardware. However, current products usually require the BBU to restart in order to download new DSP / FPGA software for standards upgrade. This limits the sharing of hardware between different standards. In fact, this prevents the dynamic resources allocation according to real-time traffic load without interrupt of services. Current SDR base station products partially meets the requirements of multi standards support, however, it does not satisfy the operator flexible operation requirement of dynamically shared resources among multiple standards, load-balancing, etc. Evolution of Software Defined Radio Driven by Moore's law in semiconductor industry, Digital Signal Processor (DSP) and General Purpose Processors (GPP) have made a lot of progresses in the architecture, performance and power consumption in recent years. This provides more choices for SDR base stations. Multicore technology is widely used in DSP and 3 ~ 6 cores processors have been commercially available. At the same time, DSP floating-point processing capacity is also improving at a fast pace. The emergence of the DSP system based on SoC architecture combines traditional DSP core and communication accelerator together has improved the BBU processing density and improved the power efficiency. Moreover, real-time OS running on DSP pave the path to virtualization of DSP processing resources. On the other hand, DSP from different manufacturers and even a same manufacturer cannot guarantee backwards compatibility. The real-time operating systems are different from each other, and there is no de fact standard yet. Generally BBUs based on DSP platform are proprietary platforms. And it is still difficult to achieve smooth upgrading and resource virtualization. Meanwhile, General Purpose Processors have progressed rapidly, and they are now capable of efficiently processing wireless signals. Therefore, the telecom industry now has more choices for software defined radio. Technology evolution in areas such as multi-core, SIMD (singleinstruction multiple data), large on-chip caches, low latency off-chip system memory are facilitating the use of GPP in traditional signal processing applications such as baseband 24 China Mobile Research Institute

29 processing in base stations. Traditional general processors usually have lower performance than DSP in power efficiency; however, in recent years the general processor has made a lot of improvements in this respect. Fig.14 shows the general processor technical progress in processing performance and power consumption in nearly 6-7 years. It is can be seen that the floating point computing capacity per watt improves very fast. These data points prove that the evolution in GPP has made it an attractive solution for various data processing tasks in the base station. The advantage of GPP is that they have a long history of backward compatibility, ensuring that software can run on each new generation of processor without any change, and this is beneficial for smooth upgrade of the BBU. On the operating system side, there are multiple OS s available on GPP that have real-time capability, and also allow the virtualization of BS baseband signal processing. Fig. 14: Compute performance evolution of GPP * (CPUs in watt power envelopes used as basis for comparison in graph) Technical progress in DSP and GPP has provided more powerful signal processing with less power consumption. This progress has made the SDR based BS solutions more attractive. Traditional DSP has become matured solution for product, and will continue to evolve. The advanced research on wireless signal processing on GPP has provided more choices for the base station, and has the potential to become part of the future open, unified multi-mode BS platform. Base Station Virtualization Once the large scale BBU pool with high-speed, low-latency interconnection, plus the common platform of DSP/GPP and open SDR solution could be realized, it has set the base for a a virtual BS. Virtualization is a term that refers to the abstraction of computer resources. It hides the physical characteristics of a computing platform from users, instead showing another abstract computing platform. If such a concept can be utilized in a base station system, the operator can dynamically allocate processing resources within a centralized baseband pool to different virtualized base stations and different air interface standards. This allows the operator to efficiently support the variety of air interfaces, and adjust to the tide effects in different areas and fluctuating demands. At the same time, the common hardware platform will provide cost effectiveness to manage, maintain, expand and upgrade the base station. Therefore, we believe China Mobile Research Institute 25

30 real time virtualized baseband pools will be part of the next generation wireless network, as shown in Fig. 15. Within in given centralized baseband pool, all the physical layer processing resources would be managed and allocated by a real time virtualized operating system. So, a base station instance can be easily built up through the flexible resource combination. The real time virtualized OS would adjust, allocate and re-allocate resources based on each virtualized base station requirements, in order to meet its demands. Physical Hardware Processors Processors Processors Processors Base station Virtualization PHY Layer (Signal processing) resource pool MAC/Trans. Layer (Packet processing) resource pool Accelerator (CODEC, cryto, etc.) resource pool Control & Manage (O&M processing) resource pool Base station Instances BS of standard 1 C A M P BS of standard 2 C A M P BS of standard 3 C A M P Fig. 17 Baseband Pool All the adjustments will be done by software only. With this mechanism, the base stations of different standards can be easily built up through resource reconfigure in software. Also, cooperative MIMO can get the required processing resources dynamically. In addition, the processing resources can be assigned in a global view, thus the resource utilization can be improved significantly. Technical Challenges Since wireless base stations have stringent real-time and high performance requirements, traditional virtualization technique is challenged to solve the latency requirements of wireless signal processing. In order to implement real time virtualized base station in a centralized base band pool, the following challenges have to be solved: High-performance low-power signal processing for wireless signals. General purpose processor and advanced processing algorithm for real time signal processing The high-bandwidth, low latency, low cost BBU inter-connection topology among physical processing resources in the baseband pool. It includes the interconnection among the chips in a BBU, among the BBUs in a physical rack, and among multiple racks. Efficient and flexible real-time virtualized operating system, to achieve virtualization of hardware processing resources management, and dynamic allocation of physical processing resources to each virtual base station, in order to ensure processing latency and jitter control HW level support on virtualization in order to minimize latency. 26 China Mobile Research Institute

31 4.5 Distributed Service Network DSN builds the elastic high-capacity switch system adopting P2P technology, which ensures high system reliability based on disaster tolerance and auto recovery technology in software implementation. By using self-organization and self-adapting technology, in conditions of capacity expansion, equipment failure or overload, the configuration can be completed automatically with little manual work, thus reducing OPEX. DSN can replace traditional carrier-class equipment with a general purpose server, and DSN introduces virtualization technology, the DSN nodes are encapsulated in VM(Virtual Machine), through VM live migration, when the traffic goes down, multi DSN nodes can aggregate to a few physical servers, and other servers can be turned off, thus implementing energy conservation and emission reduction. Distributed Service Network DSN element C-RAN element BBU pool BBU pool Fig. 18 C-RAN Integrated with DSN In a platform layer, DSN and C-RAN both encapsulate their network elements through virtualization technology on general servers, so, it is possible to run DSN and C-RAN on the same virtualized platform. But how to implement the resource management(including the dimension of time and the dimension of physical resource )is the key issue in the research of platform unification for DSN and C-RAN. China Mobile Research Institute 27

32 5 Evolution Path The novel C-RAN architecture is a revolution of the traditional RAN deployment. It is impossible to replace today s RAN overnight. Moreover, the technical challenges of C-RAN should be carefully developed and tested in labs and field environments to ensure its reliability. This naturally leads to a step-by-step evolution path of C-RAN to gradually replace traditional RAN. The following is our vision on how the evolution could take place: 5.1 C-RAN Centralized Base Station Deployment In the first step, Base Stations can be implemented by separating Remote Radio Heads (RRH) and Baseband Units (BBU), and baseband processing resources between multiple BBUs in a centralized Base Station can be scheduled in carrier level. The RRHs are small and light weight for easier deployment. They receive/transmit digital radio signals from/to the BBU via fiber links such as CPRI/Ir/OBRI. The BBU is the core of the radio signal processing. RRHs can be deployed in remote sites far from the physical location of the BBU (e.g. 1~10km). The optical fiber transmission network between RRH and BBU shall have the corresponding loop protection and management functions. The fiber link between RRHs and BBUs can be standardized like CPRI/Ir/OBRI so that RRHs and BBUs from multiple venders can be connected together. The centralized BS has a high bandwidth, low latency switch matrix and corresponding protocol to support the inter-connection of carrier processing units among multiple BBUs in order to constitute a large-scale baseband pool. The signals from distributed RRH can be switched to any BBU inside the centralized baseband pool. Thus, the centralized baseband pool can realize carrier load balance to avoid some BBUs overloaded while some BBUs idle, and realize faulttolerance to avoid that the fault of single BBU affect the overall functions and coverage of the wireless network. The above technologies can improve the usage efficiency of devices, reduce power consumption and improve system reliability. 5.2 Multi-standard SDR and Joint Signal Processing In the second step, on the basis of centralized BS deployment, the BBU baseband processing functions can be fully implemented by Soft Defined Radio (SDR) based on a unified, open platform. By moving the baseband processing to SDR, it is much easier to support multiple standards, upgrade the SW/HW, introduce new standard and increase system capacity. Meanwhile, with multiple RRHs attached to the centralized BBU pool, it is easier to implement coordinated beamforming (CBF) and cooperative multipoint processing (CoMP) in this platform. Multiple BBUs can coordinate with each other to share the scheduling information, channel status and user data efficiently to improve the system capacity as well as reduce interferences in system. 5.3 Virtual BS on Real-time Cloud Infrastructure Once centralized baseband pool consisted of a lot of standardized BBUs is built on a unified, open platform and the baseband processing is implemented by SDR, the virtual BS based on real-time cloud infrastructure is the next step of C-RAN evolution. The centralized baseband pool consisted of large-scale BBUs by a high bandwidth, low latency network, combined with some system software, can constitute a large real time baseband cloud, just like the cloud computing environment in IT industry. The difference is that the baseband processing tasks are real-time computing tasks in a real time baseband pool. 28 China Mobile Research Institute

33 Through the cooperation of BBU in the baseband pool and RRH to send and receive wireless signals, it can be achieved that multi-standard wireless network functions in the same platform. In the system software instructions of the baseband pool based on real-time cloud architecture, CPRI/Ir/OBRI optical fiber transmission network and optical Internet architecture in large-scale centralized baseband pool can send the baseband signal signals transmitted by RRH to the virtual base station running on the designated BBU. Then virtual base station uses the calculation resources of the designated BBU to finish the real-time processing of wireless baseband signals. Moreover, in a C-RAN system which has several baseband pools, CPRI/Ir/OBRI optical fiber transmission network should have the ability to forward the baseband signals from RRHs to other baseband pools in order to improve system reliability and realize load balance across different baseband pools. China Mobile Research Institute 29

34 6 Recent Progress To accelerate the development and commercialization of C-RAN, China Mobile has been working actively with industry partners. We have made good progress in field trial, large scale BBU pool implementation, and baseband PoC based on IT platform. This chapter will first introduce the advantages and disadvantages observed in C-RAN field trial, followed by discussion of large scale BBU pool solution, up to 1000 carriers, based on current BBU device, and lastly the recent R&D result of multi-mode PoC based on IT platform. 6.1 TD-SCDMA and GSM Field Trial China Mobile conducted the first C-RAN trial with partners in It is a C-RAN centralized deployment field trial within the commercial TD-SCDMA system in Zhuhai city, Guangdong province. After that, there has been multiple GSM field trials conducted in multiple cities throughout China, include Changsha, Baoding, Jilin, Dongguan, Zhaotong, etc. Rest of the section discusses the pros and cons of the C-RAN centralized deployment solution s pros and cons in different scenarios. For the ease of discussion, two typical cases, TD-SCDMA trial in Zhuhai city and GSM trial in Changsha city are shown here. Overall situation The first trial in Zhuhai City only took 3 months to complete. The commercial trial has 18 TD- SCDMA macro sites covering about 30 square km area. This trial has verified some centralized deployment technologies feasibility. The construction and operation of a commercial clearly highlighted the C-RAN s advantage over tradition RAN in cost, flexibility and energy savings. At the same time, it also exposed challenges on fiber resource, as well as transmission construction. After that, there have been several trials on centralized deployment solutions of GSM system. The network layout is mainly consisted of replacing and upgrading existing sites. There are total15 sites covering 15 square km in the trial, where only 2 of them are new sites. Compared with TD-SCDMA network, GSM solutions have unique features, for example, it could support daisy-chain of 18 RRHs with only 1 pair of fiber. This could significantly reduce the number of fiber resources needed in C-RAN centralized deployment with dark fiber solution. The following sections will describe the network status before and after C-RAN deployment, key technology introduced, field test results and challenges observed.. Field Trial Area The trial area in Zhuhai city is mainly consisted of a national high tech development zone, a residential community, and a few college campuses. The data traffic in this area is growing rapidly, as the customers here are well-educated and early adapters of new services. Part of the trial areas has demonstrated tidal effect of traffic loading, with predictable traffic loading pattern associated with time, location or event. For example, the national high tech development zone has most people during working hours. The same group of customers usually returns to nearby community after work. Students in colleges tend to stay away from using wireless devices during school hours, while they tend to make a lot of calls in night. Traditionally, network planning must support the peak traffic load at each individual site, which is usually 10 times higher than the down time This results in a very low average utilization rate of the BTS devices. It also introduces difficulties in network planning, construction and optimization. It is suitable to adopt baseband pool with dynamic carrier allocation. In the trial field, there will be 9 sites co-located with existing GSM site, while another 9 sites is new. All 30 China Mobile Research Institute

35 these 9 sites have to be connected with new fiber channels and they are spread in 30 square km. This is a challenge for fiber construction. The trial area in Changsha city is consisted of a few campuses near Yuelu Mountains. The traffic load and traffic density is quite high here. In addition, there is a lot of dormitories, and local residential apartments. The propagation environment is very complex and the coverage KPI still has room to be improved. This makes it suitable to verify C-RAN s capacity in urban city environment. Finally, since most of the trial sites are reusing or upgrading existing ones, there is plenty of fiber resources. Overall Solution The solution starts with planning of system capacity in centralized deployment. In the Zhuhai trial, each TD-SCDMA site s configuration is 4/4/4, which means that there are 3 sectors in each site, and every sector has 4 carriers. Overall, the 18 trial sites need 216 carriers. When considering the BBU pool capacity, the total BBU pool can be planned to support the maximum co-current traffic for the same area. There are two kinds of TD-SCDMA carriers, R4 carrier is mainly used for voice traffic, and HSDPA carrier is mainly used for data traffic. Based on China Mobile s planning requirements, every site s traffic load should not exceed 75%. As a result, each R4 carrier supports up to 203 voice users, and each HSDPA carrier can support up to 93 users. There are total 17,000 effective users in the trial area. When BBU pool is deployed, 160 carriers will be able to support 20,000 effective users. This means the C-RAN centralized deployment can save the BBU capacity by roughly 25%, compared with traditional deployment method. Similarly, the trial in Changsha also has used the co-current capacity to decide the total capacity of the BBU pool. The second part of the solution involves dynamic carrier allocation. In TD-SCDMA system, each RRH/sector can support maximum 6 R4 and HSDPA carriers. In the idle situation, each RRH/sector has only one R4 carrier and one HSDPA carrier. There are different carrier allocation decision criteria whether more R4 and HSDPA carriers should be added. Whenever the existing R4 carrier s loading rate is above a threshold, there should be more R4 carriers allocated in this site. For HSDPA carrier, similar rule applies. Where there is not enough load in multiple R4 or HSDPA carrier, it is also possible to reduce the number of R4 and HSDPA carriers in one sector. For GSM system similar rule also applies but the criteria is the utilization rate of each GSM carrier. The third portion of the solution involves RRH daisy chain and fiber failure protection technologies. These technologies are derived from the distributed BBU-RRH deployment method which usually uses point-to-point dark fiber connections. When BBU-RRHs are separated by significant distance, it is important to consider the saving of fiber resource and protection against unpredictable fiber failure caused by external factors. In TD-SCDMA, each fiber link can handle up to 6.144Gpbs transmission, enough to support 15 TD-SCDMA carriers. Thus, one pair of fiber is able to support one site with 3 sectors and maximum carrier of 15. In the Zhuhai trial, each access ring has 9 sites and used 9 pair of fibers to support the 9 sites connected to the ring. On the other hand, GSM has far less baseband requirement due to its narrow band nature; therefore it can support more capacity in daisy-chain configuration. There are commercial products that can support 18 to 21 RRH daisy chained on one pair of dark fiber. We can calculate the fiber resource required per access ring as following: usually, each access ring has 8~ 12 physical sites and each site has 3 sectors, and has 900M and 1800M dual bands. This China Mobile Research Institute 31

36 means, each access ring may has up to 16~24 logical sites, which is 48 to 72 sectors/rrh. To connect all the RRH in daisy chain, we would need 4~5 pair of fibers in the ring. Lastly, the field trial has also verified key technology for outdoor deployment, like power supply for remote sites. In the Zhuhai Trial, there is no BTS equipment room in the 9 new sites. Thus the traditional DC power supply is not available. External power booth is used instead. Existing outdoor power solution met the need of network deployment: with sufficient operation temperature range, -40 ~+70, C-level anti-flash capacity and theft-proof solution to ensure the safety of device without on-site attendance. GSM and TD-SCDMA remote site both can apply this outdoor power solution. Technical Performance This section will outline the technical performance data from selected test cases in the trial, starting with the dynamic carrier allocation procedure. The following figure illustrates the total number of carriers allocated to one sector in a typical day on one site in Zhuhai trial. The blue curve represents this sector s total carrier capacity, while the purple curve represents the actual network load for this sector. It is clearly shown that the dynamic carrier allocation has adapted effectively to dynamic load in network. Fig. 19 Dynamic Carrier Allocation We also collected KPI of radio performance for both dynamic carrier allocation and static carrier allocation. We noted no KPI difference. In the Changsha trial, the C-RAN centralized deployment has shown better radio performance and improved user experience, due to the introduction of co-located multi-rrh per site technology. With this technology, multiple RRH transmit and receive signals for the same cell, just like fiber repeat does but provide additional receive combination gain. Multiple radio performance is improved, include uplink receive quality improved by 2%~3%, drop call rate was reduced and nearly eliminated in some sites. In addition, since inter-site handover has become an internal procedure in one BBU pool, the handover delay has been reduced. Finally, the fiber protection was in place when the access fiber ring was cut accidently, the BBU-RRH traffic will be automatically switched to another unaffected route in the ring. The switching delay during the failure protection is comparable to normal cross-bts or cross-msc. Thus the failure protection has very limited effects to network KPI. In summary, C-RAN centralized deployment does not have negative effect on radio performance. On the contrary, it may provide extra gains on radio performance. Moreover, RRH daisy chain could reduce the dark fiber resource needs, while out-door units meet the power requirement of out-door remote sites. Now dark fiber transportation solution has been well verified, and other transmission technologies are in testing. 32 China Mobile Research Institute

37 Economic analysis The trial in Zhuhai city shows that, compared with traditional RAN deployment method, C-RAN centralized deployment can reduce the TD-SCDMA network s CAPEX and OPEX significantly, especially for new TD-SCDMA site which is not reusing existing GSM site. In the following figure, it is shown that OPEX and CAPEX can be reduced by 53%, and 30% respectively for new cell sites. Fig. 20 Economic analysis for centralized deployment On OPEX, the savings are mainly come from A/C power consumption, site rental fee, regular on-site maintenance visit, and reduced human resource on repair and upgrade. The key factor is C-RAN has only RRH in remote site and no BTS equipment room, the site rental fee is much lower, and O&M cost is also lower. This is an important saving, as the site rental fee is a significant portion of the Zhuhai system TCO. On CAPEX consideration, the savings are mainly from: no new BTS room, reduced transmission devices on each remote site, and eliminating of various supporting devices in remote site. In addition, the adaption of BBU pool can reduce the BTS configuration and potentially lower the CAPEX on RAN. In GSM trial, similar CAPEX/OPEX savings have been observed. However, it is very clear that the savings achieved in these two cases are different, due to the different fiber resources, different deployment scenarios in different city. All-in-all, the economic analysis has shown the benefits in different areas. It is able to reduce RAN s O&M. however, it may be important to take account of each individual case to better calculate the saving of CAPEX and OPEX. In addition, RRH requires much less and power, it is easier to find new site, and easier to move to different place, which largely reduces the risk of cell sites being forced to relocate due to regulations or neighborhood complaints, and the cost and service disruptions associated with these. Construction Impact The centralized deployment of C-RAN greatly simplifies the remote site selection and construction requirements, construction time required for new base stations, which lead to faster network deployment. Table 3 shows the comparison of the construction process between traditional base station and C-RAN centralized approach in the China Mobile s TD-SCDMA network deployment in Zhuhai City, Guangdong Province. From figure 17, C-RAN showcases the advantage of deployment time. The savings are mainly from site selection/purchasing, base station equipment room construction and transmission system debug, etc. China Mobile Research Institute 33

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