C-RAN
The Road Towards Green RAN
White Paper
Version 3.0 (Dec, 2013)
China Mobile Research Institute
China Mobile Research Institute
i
Table of Contents C-RAN ............................................................................................................................................... i
The Road Towards Green RAN ..................................................................................................... i
1 Introduction ............................................................................................................................ 3
1.1 Background ......................................................................................................................... 3
1.2 Vision of C-RAN .................................................................................................................. 4
1.3 Objectives of this White Paper ....................................................................................... 4
1.4 Status of this White Paper ............................................................................................... 5
2 Challenges of Todays RAN ............................................................................................... 6
2.1 Large Number of BS and Associated High Power Consumption .............................. 6
2.2 Rapid Increasing CAPEX/OPEX of RAN.......................................................................... 7
2.3 Interference in LTE networks .......................................................................................... 9
2.3 Explosive Network Capacity Need with Falling ARPUs............................................. 13
2.4 Dynamic mobile network load and low BS utilization rate ..................................... 14
2.5 Growing Internet Service Pressure on Operators Core Network.......................... 14
3 Architecture of C-RAN ....................................................................................................... 16
3.1 Advantages of C-RAN ..................................................................................................... 18
3.2 Technical Challenges of C-RAN ..................................................................................... 20
4 C-RAN deployment scenarios ........................................................................................ 23
4.1 TD-SCDMA C-RAN deployment ..................................................................................... 23
4.2 TD-LTE C-RAN deployment ........................................................................................... 26
5 Technology Trends and Feasibility Analysis ...................................................................... 30
5.1 Wireless Signal Transmission on Optical Network.................................................... 30
5.2 Dynamic Radio Resource Allocation and Cooperative Transmission/Reception . 39
5.3 Large Scale Baseband Pool and Its Interconnection ........................................................... 42
5.4 Open Platform Based Base Station Virtualization ................................................................ 43
5.5 Distributed Service Network ................................................................................................. 47
6 Recent Progress .................................................................................................................. 49
6.1 C-RAN Field Trials ............................................................................................................... 49
6.1.1 TD-SCDMA and GSM Field Trial ................................................................................ 49
6.1.2 TD-LTE C-RAN Field Trial ........................................................................................... 55
6.2 Cooperative radio technologies under C-RAN ........................................................... 57
6.3 PoC development on C-RAN BBU pooling .................................................................. 60
6.4 Progress on C-RAN virtualization ................................................................................. 69
6.5 Edge Applications on C-RAN ......................................................................................... 74
Cover is for
position only
China Mobile Research Institute ii
7 Evolution Path ....................................................................................................................... 78
8 Global landscape of C-RAN activities .......................................................................... 81
9 Conclusions ........................................................................................................................... 82
Acknowledgements ............................................................................................................... 84
Terms and Definitions .......................................................................................................... 85
References ................................................................................................................................. 88
China Mobile Research Institute 3
1 Introduction
1.1 Background
Todays 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, BSs
utilization rate is low because the average network load is usually far lower than that in peak
load; while the BS processing power cant 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.
China Mobile Research Institute 4
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
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 sites 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 todays 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 Mobiles 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.
China Mobile Research Institute 5
1.4 Status of this White Paper
This document version 3.0 is an update on previous version 2.5 released in October 2011. 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.
China Mobile Research Institute 6
2 Challenges of Todays 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 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 stations 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.
China Mobile Research Institute 7
Channel, 6%
Transmission,
15%
Management
office, 7%
Cell site, 72%
Major
Equipment,
51%Air
Conditioners,
46%
Other Support
Equipment,
3%
Fig. 2-1 Power Consumption of Base Station
2.2 Rapid Increasing CAPEX/OPEX of RAN
Over recent years, mobile data consumption has experienced a record growth among the
worlds 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. 2-2: 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.
China Mobile Research Institute 8
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
2007-2012 forest are shown in Fig.2-2. 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.2-3, 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.
Fig. 2-3: 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. 2-4, 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.
China Mobile Research Institute 9
Fig. 2-4 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.
Table 1. Multi-Network Operation of Major Mobile Service Providers
Cellular Technologies Vodafone China
Mobile
France
Telecom
T-
Mobile
Verizon SK
Telecom
Telstra China
Unicom
TD-SCDMA
WCDMA
CDMA One & 2000 &
EVDO
GSM GPRS EDGE
LTE
2.3 Interference in LTE networks
LTE is designed to operate with frequency reuse factor (FRF) of one to improve spectrum
efficiency, which is different from both 2G and 3G network with FRF larger than one. OFDM and
SC-FDMA are the essential downlink and uplink transmission technologies for LTE. The
orthogonality among different sub-carriers eliminates the intra-cell interference. However, since
all the cells operate on the same frequency band, the inter-cell interference from and to the
China Mobile Research Institute 10
adjacent cells becomes unavoidable, which leads to low-throughput performance. How to avoid
and eliminate inter-cell interference becomes an important researching subject for LTE.
In the inter-cell interference tests in the trial networks, the comparison tests in terms of SINR
and single-user throughput have been done on the condition of different system loads. The
results are illustrated in Figure 2-5 and Figure 2-6. Comparing to 0% load case, the downlink
average SINR is decreased by 5.33dB and 8.28dB respectively, and the downlink throughput is
decreased by 40% and 55% respectively in case of 50% load and 100% load.
Fig. 2-5 SINR Changes under different loadings
China Mobile Research Institute 11
Fig. 2-6 DL Throughput Changes under different loadings
The co-channel interference in LTE are mainly attributed to two patterns: 3 or more adjacent
cells overlap and PCI mode 3 conflict.
For the interference induced by the PCI model 3 conflict pattern, the handover is not obviously
affected. It is observed that the handover success rate is decreased by 2 percent at most. The
reason is that the SINR of the target cell is too low which causes Radom Access Process to fail
when the UE receives the handover command. However, the pattern has much impact on the
traffic performance. In case of 0% load, the cell edge throughput is degraded by 4%~18%. In
case of non-zero loading, the CRS SINR and the cell edge throughput are little affected
(0.5~2dB decrease for CRS SINR and less than 10% decrease for cell edge throughput).
The interference due to 3 or more adjacent cell overlapping has much higher impact on cell
edge throughput. It is found that when the number of neighboring cells with 6dB less than the
serving cell decreases from 3 to 2, then there is a noticeable increase on user throughput with
30% improvement on average. It is also found that the interference from intra-cell has more
impact than neighboring cells. Switching off intra-cell can have a big increase on user
throughput (58% on average) while only 4% throughput improvement on average is observed
when switching off the neighboring cells. In addition, reducing the number of neighboring cells
or their transmission power can also help to improve the system performance.
In LTE networks, it is very common of coverage overlapping with neighboring cells. In our test,
we defined adjacent cell as the cell which RSRP is at most 10dB less than the serving cell and
made a statistical results on the number of adjacent cells. The result is shown in Figure 2-7. It
can be seen that in high-density urban area with inter-cell distance of from 300 to 500 meters,
China Mobile Research Institute 12
the probability for a UE to find one or more adjacent cells is as high as 71.8%. In some cases,
the UE can even find 6 adjacent cells.
Cell sectorization technology is usually used for 3 intra-site cells to set them to different
orientation. It is clear that on the cell edge, overlapping is unavoidable for coverage sake.
According to the statistics shown in figure 2-8, the probability is 30.1% for UEs to detect the
signals coming from the intra-site adjacent cells. At the same time, the probability is 1.4% for
UEs to simultaneously detect the signals coming from the intra-site 3 cells.
Fig. 2-7: The statistics of the number of adjacent cells in large-scale network
(RSRP is lower than the main cell within 10dB)
Fig. 2-8: The statistics of the number of adjacent cells loaded an eNB in large-scale network
(RSRP is lower than the main cell within 10dB)
Through the comparison tests, it can be seen that how to reduce the co-channel interference is
the major problem and challenge for large-scale LTE networks. At present, there are many
interference coordination technologies such as ICIC, CoMP etc. However the gain from those
technologies is limited under traditional distributed architecture. On the contrary, a centralized
China Mobile Research Institute 13
C-RAN architecture can facilitate their implementation and fully exploit their gain on system
performance.
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 users 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 2013. 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. 2-9, there is a large gap between the CAGR of new air
interface and the CAGR of customers need. In order to fill this gap, new infrastructure
technologies need to be developed to further improve the performance of LTE/LTE-A.
Fig. 2-9 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.
China Mobile Research Institute 14
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 patternso called "tidal effect". As shown in
Fig.2-10, during working hours, the core office areas Base Stations are the busiest; in the non-
work hours, the residential or entertainment areas Base Stations are the busiest.
Fig. 2-10 Mobile Network Load in Daytime
Each Base Stations 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 Operators 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
China Mobile Research Institute 15
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
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. 2-11 Wireless traffic on a commercial 3G
Meanwhile it is interesting to understand how people are using todays 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.2-11 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?
China Mobile Research Institute 16
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. Its 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 doesnt include the baseband function, it is still called BBU for the simplicity.
The different function partition method is shown in Fig.3-1.
GPS
Main Control & Clock
Core net-work
Base-band
process-ing
Transmitter/Receiver
PA&
LNA
Antenna
DigitalIF
Solution 1Solution 2
BBU RRU
Fig. 3-1 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.
China Mobile Research Institute 17
RRH
RRH
RRH
RRH
RRH
RRH
RRH
Virtual BS Pool
L1/L2/L3/O&M L1/L2/L3/O&M L1/L2/L3/O&M
Fiber
Fig. 3-2 C-RAN Architecture 1: Fully Centralized Solution
RRH/L1
RRH/L1
RRH/L1
RRH/L1
RRH/L1
RRH/L1
RRH/L1
Virtual BS Pool
L2/L3/O&M L2/L3/O&M L2/L3/O&M
Fiber or
Microwave
Fig. 3-3 C-RAN Architecture 2: Partial Centralized Solution
The fully centralized C-RAN architecture, as shown in figure 3-2, has the advantages of easy
upgrading and network capacity expansion; it also has better capability for supporting multi-
standard operation, maximum resource sharing, and its 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.
China Mobile Research Institute 18
The other type of C-RAN is to centralize partial BBU functions which include collaborative
function, L2 and L3 scheduling, and wireless resource allocation. As shown in Figure 3-3, the
feature of this architecture is small centralization with partial BBU functions centralized into
one central point which is connected with the remained remote BBU via dark fiber or PTN
networks. With such architecture, the central point can schedule the wireless resource in each
cell on a global level and even realize the joint transmission or joint reception on PHY layer to
improve cell edge performance. The data bandwidth between the central point and remote sites
is small, which minimizes the change on existing transport networks. The major disadvantage
of this architecture is that it still requires remote equipment rooms. One-body type base station
is not preferred from the perspective of system management and future upgrade. In addition,
the delay on information exchange can have an impact on the system performance
improvement.
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 pools 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.
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 equipments 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.
China Mobile Research Institute 19
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 first-
mover advantage. Thus, operators can get large cost saving on site rental and O&M.
Capacity Improvement
In C-RAN, virtual BSs 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 UEs 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
service delivery quality for various applications. The service overlapping the core network
also supplies a better experience to users.
China Mobile Research Institute 20
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 BSs scheduling information. Therefore, the design of this interface must meet the real-
time 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 wont 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.
China Mobile Research Institute 21
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 edges integration with the RAN, intelligence
of DSN, and the deployment and management of distributed service.
China Mobile Research Institute 22
China Mobile Research Institute 23
4 C-RAN deployment scenarios The C-RAN deployment scenarios differ at different stages of 2G/3G/4G constructions. For GSM
network, the need for C-RAN deployment is limited and thus the main strategy is to maintain
the network reliability and stability. For TD-SCDMA, it has already provided country wide
coverage in most of the cities. Future network expansion will mainly focus on rural area and the
remaining few cities. The main construction strategy is to improve hot-spot and weak-spot
coverage.For 4G, CMCC just finished the large-scale field trials in the past few years and only a
few cities have the TD-LTE coverage. It can be foreseen that in the coming few years TD-LTE
deployment will be our main target.
This chapter will describe different C-RAN deployment scenarios for 3G and 4G, respectively.
4.1 TD-SCDMA C-RAN deployment
A typical TD-SCDMA site has 3 sectors with 3 carriers per sector. The mainstream equipments
support three RRU cascade. The utilization efficiency of TD-SCDMA carriers is low due to the
severe network tidal effect. At the same time, there is still existing much area with weak
coverage in the current TD-SCDMA networks. On the other hand, as the number of subscribers
is increasing fast, the high-density area will require more sites to absorb the traffic, which in
turn increase the difficulty of site selection. In addition, there are also some other special area
such as expressway, railway, street and riverway in which the handover success rate is
relatively low due to a large number of fast handover. For these scenarios, the centralization of
BBU deployment can help to address the above-mentioned issues, i.e. to deal with tidal effect
effectively, to improve the utilization efficiency of carriers, to reduce the difficulty of site
selection and to improve handover success rate. .
4.1.1 Scenario 1: Capacity and coverage improvement using Pico-RRU for
weak-spot and hot-spots
In this scenarios, C-RAN is used to provide hot-spot coverage or improve weak coverage in
some area. The new BBUs can be installed in macro site room and connected with remote RRU
via fiber.
China Mobile Research Institute 24
BBU Pool
Fig. 4-1: Capacity and coverage improvement using Pico-RRU for weak-spot and hot-spots
With the increased difficulty on site acquisition and pressure on forced removal of existing
equipment rooms by proprietors, many area in high density urban cities are of weak coverage.
To address this issue, installation of BBU pool in the center equipment room and the small
RRUs will take more important roles . It is recommended that the centralization equipment
room should be owned by operators themselves to avoid impact by possible site relocation in
the future. At the same time, the so-called multi-RRU co-cell technology can be used to
improved the network quality. Generally there can be a vertical three-layer network
deployment mode: basic coverage by macro base stations, capacity and coverage supplement
by micro RRUs in the outdoor and traffic asorbion via indoor solution.
The characteristic of this scenario includes two key parts: BBU pool centralized in the existing
macro-site and 2-antennas Pico-RRU with low transmit power on the remote site. The scale of
centralized carriers is decided by area characteristics such as the traffic volumn. In addition,
the fiber from the last-mile pipeline can be utilized or it can be installed hanging over the
building.
4.1.2 Scenario 2: Area with tidal effect
The tidal effect in such area is evident. Examples include campus city, industrial parks,
dormitory area, commercial districts, residential area and so on.
China Mobile Research Institute 25
BBU pool
BTSBTS
BTSBTS
BTSBTS
BTSBTS
Residential area
Industrial area
Fig. 4-2 Tidal effect in residential and industrial area
Making use of construction of new area or re-construction of old area, the transport facilities
can be deployed to enable BBU centralization with dark fiber. Deployment of centralized BBU
pool can deal with tidal effect. In addition, the usage of carrier live migration can help to save
the overall number of carriers and improve the system performance-power ratio by dynamic
resource allocation.
4.1.3 Scenario 3: Region with massive fast handover
Such scenarios include the area such as the highway, railway, streets and riverway. For the
users moving fast through the regions, it is easy for a call to drop due to delay on mobile signal
measurement or fast handover. To address this, some technologies with optimization on fast
handover such as multi-carrier co-cell can be used in the centralized BBU pool.
BBU pool
RRU
Fig. 4-3 Frequent handover in railway coverage area
The scale of BBU pool is quite dependent on the available resource of fiber pipeline. The remote
RRU can be installed on the lampposts with power supply using either DC remote supply or
local supply. The BBU pool can be installed in the outdoor cabinet or simple equipment room in
an embellished way.
China Mobile Research Institute 26
4.2 TD-LTE C-RAN deployment
The construction of TD-LTE network is our current focus. From previous large-scale field trials,
we have accumulated a lot of experience and solved many key problems. However there are
still some issues left. On one hand, due to the co-site deployment of TD-LTE with 2G/3G
systems it is found that the TD-LTE antennas are usually either too high or too low and inter-
cell distance is very close. All these lead to severe interference by large overlap among cells
and as a result the system performance deteriorates a lot. Since LTE is more sensitive to the
interference than 2G/3G. Some 2G/3G sites are not suitable for TD-LTE deployment. This, in
other words, means that new sites are needed. In fact, it is estimated that around 30% and
5%~10% new sites are needed for TD-LTE D band and F band deployment respectively.
Doubtless, the addition of new sites adds the difficulty on site selection. C-RAN is deemed as
an efficient way to help network construction with the advantages of reducing interference,
saving cost, speeding up site construction and lowing down difficulty in site selection.
4.2.1 Scenario 1: HetNet with C-RAN
Similar to 3G, the need for improvement of weak-spot and hot-spot coverage still exists in TD-
LTE. There are three reasons for this.
1. The wall penetration ability of D-band is worse than F band. As a result, in the dense urban,
there will be more area with weak coverage caused by building shelter.
2. In TD-LTE data rate is one of the most important measurement to user experience. If we
use the minimum data rate to define the cell edge, then in order to provide high-quality service
the cell size will be smaller than 2G/3G networks.
3. In some urban area, there exist super hot spots which is of extremely high data traffic. To
absorb the traffic, multiple small cells can be deployed with seamless coverage.
The C-RAN deployment method in TD-LTE is similar to in 2G/3G networks. Considering the
relative abundance of the frequency resource at the initial stage, it is preferred that the small
cells use different frequency bands from the macro cells. After the introduction of the Carrier
Aggregation technology, it will be easy to implement the C/U split to further improve the overall
capacity. Reusing the same frequency bands between the macro and small cells can be
considered when the need for higher capacity becomes urgent. No mater what kind of
frequency scheme is used, the deployment of C-RAN can facilitate the cooperation between
macro and small cells.
At the same time, due to peoples more attention to the environment, the concern on radio
radiation has become the first reason that prohibits the deployment of wireless equipments.
Because of this recently in large cities such as BeiJing and ShangHai, we encountered many
obstacles when upgrading 2G/3G sites to 4G. Even more, some sites under construction were
forced to be removed because of residents complaint during site construction. On the other
hand, some 2G/3G sites do not have sufficient reserved space to accommodate TD-LTE .
China Mobile Research Institute 27
Installation of RRU and antennas needs reconstruction on the rooftop in original sites, which
instead, makes the civil work more difficult. As the result, in the predictable future, there will
appear large area of blind or weak coverage in the urban cities. To address this, small cells are
needed to provide continuous seamless coverage which imposes new requirements on
wireless equipments, including:
1. Smaller transmission power and miniaturization for RRU as well as smaller size for antennas.
RRU and antennas with smaller size can reduce the public concern on the radio radiation. And
RRUs of low power consumption will match the requirements of the environment-friendly
policies from government and save the time for installation permission. The current
transmission power is 5w per channel for an outdoor RRU. It is estimated through link budget
calculation that in case of typical inter-cell distance of 100 meters, the needed transmission
power can be smaller.
2. Collaborative radio support with BBU pool. Some technologies, such as multi-RRU co-cell and
generalized MIMO can help to reduce the interference and thus to improve system performance.
In this way, the network will consist of at least two layers. One is the macro cell for basic
coverage, and the other is the small cell to absorb the hot-spot traffic. It is estimated that the
ratio of macro to micro RRUs is between 1:3 and 1:6.
4.2.2 Scenario 2: Combination with the construction of integrated service
access zone
Integrated Service Access Zone (ISAZ) is a new method to plan and construct the transport
infrastructure with target at household wideband wireline customers, group wired customers as
well as BS access needs. The idea of ISAZ is to divide a city into several smaller zones with
each of area of 3~5 square kilometers. For each zone the transport resource will be planned
overally and comprehensively. Some good examples of ISAZ include university campus, hi-tech
science parks, residential area, exhibition parks and industrial parks.
According to our current planning, an ISAZ usually consist of 1~2 transport access ring ( may
have more rings in some big cities) with each ring of 6~8 mobile macro equipment rooms. In
some cases the maximum number of wireless macro equipment rooms can be 12. Considering
that current macro base stations typically have 3 sectors with each sector of one 20MHz TD-LTE
carrier, then the total number of TD-LTE carriers is between 24 and 36 in one access ring. It
could becomer higher to 50~70 in the future when sectors are upgraded with two carriers.
In the cities to be deployed with TD-LTE, combination with ISAZ is a promising scenario for C-
RAN deployment. The basic idea is to make full use of the relatively rich transport resources
such as fiber, duct and pipeline. Then the BBUs within the same ISAZ can be centralized to the
aggregation site (which can be possibily the aggregation office in the transport network) with
remote site deployed with RRU. Dark fiber is now widely used in our C-RAN trials due to its
maturity. With CPRI compression and bi-direction single fiber technologies, one fiber core can
support one 20MHz TD-LTE carrier with 8 antenna. We then therefore suggested to reserve at
China Mobile Research Institute 28
least 48 fiber cores for C-RAN centralization in the ISAZs with sufficient fiber, taking into
account the potential centralization scale. In the future the usage of fiber can be further
reduced with the introduction of WDM equipments.
After the centralization of BBU, the collaborative radio technologies (e.g. JT/JR) can be further
adopted in the BBU pool to enhance the system performance.
There are three construction methods under this scenario.
A. Scenario a: If the TD-LTE equipements cant be installed in existing 2G/3G sites, then the
new BBUs can be centralized into aggregation office of ISAZ and a new remote site with
outdoor stand-by power supply is necessary for RRU installation.
B. Scenario b: If the TD-LTE equipments can be installed in existing 2G/3Gsites, then the new
BBUs can be centralized into aggregation office of ISAZ and the RRU can be installed in the
existing 2G/3G remote sites. Stand-by power resource for RRU is also required.
C. Scenario c: If the TD-SCDMA BBU can be upgraded to TD-LTE, then it is not necessary to
deploy C-RAN. However, if the network suffers from severe interference from neiboring cells,
then C-RAN centralization can be used for introduction of collaborative radio technologies to
address the issue.
4.2.3 Scenario 3: Comibination of the two scenarios above.
There is no conflict between the two above-mentioned scenarios, i.e. HetNet and ISAZ . In fact,
in the highly dense urban with ISAZ planning, there still exist many weak-spots and hot-spots.
For this scenario, the construction can be expanded as follows.
Fig. 4-4 Combination of HetNet and ISAZ
The BBUs are centralized into ISAZ aggregation rooms. When the fiber resource is limited,
WDM can be introduced to connect existing wireless equipment rooms into a ring. If WDM
equipments can be deployed outdoor, it can also act as an aggregation point to connect
together a few remote sites close to each other. The macro and micro cell BBUs are collocated
China Mobile Research Institute 29
in the same BBU pool which enables complex and fast collaborative radio technology to improve
wireless performance. With WDM solution, the typical length of a WDM ring is less than 20km.
China Mobile Research Institute 30
5 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.
5.1 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 5s. 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].
China Mobile Research Institute 31
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
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 100-200 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.
China Mobile Research Institute 32
For the above problems and corresponding technical progress trend, we will analyze and put
forward ideas for solving these problems.
5.1.1 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
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
Pros Cons
Reducing signal
sampling
Low complexity;
Efficient compression to 66.7%;
Less impacts on protocols.
Severe performance loss.
China Mobile Research Institute 33
Non-linear
quantization
Improve the QSNR;
Mature algorithms available, e.g. A law
and U law;
High compression efficiency to 53%.
Some impacts on the OBRI interface
complexity.
IQ data
Compression
Potential high compression efficiency;
Only need extra decompression and
compression modules.
High complexity;
Difficult to set up a relativity model;
Real-time and compression distortion
issues;
No mature algorithm available.
Sub-carrier
Compression
High compression efficiency to 40%
~58%;
Easy to be performed in downlink.
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.
5.1.2 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.
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 800-900 us, then CPRI/Ir/OBRI optical
transmission delay may be 100-200 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.
5.1.3 Optical Transmission Technology Progress and Cost Reduction
China Mobile Research Institute 34
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 5-1 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.
0
500
1000
1500
2000
2500
3000
Aug-07 Feb-08 Aug-08 Feb-09 Aug-09
Pri
ce h
isto
ry o
f 2
.5G
mo
dule
s (
RM
B).
10Km 40Km 80Km
66.7%
54.2%
62.2%
60%
61.5%
35.2%
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Aug-07 Feb-08 Aug-08 Feb-09 Aug-09Pri
ce h
isto
ry o
f 1
0G
mo
dule
s (
RM
B).
550m 10Km 40Km
Fig. 5-1 Price history of Commercial 2.5G/10G Optical Modules
China Mobile Research Institute 35
5.1.4 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 RRHs 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
5-2. 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 6.144 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.
Transmission ring
Trunk cable 2
Trunk cable 1
Optical
switching box
Central apparatus
room
Radio remote
head
Fig. 5-2 RRH Ring Protection Loop
5.1.5 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
China Mobile Research Institute 36
transmission layer, each optical route has 144 to 576 fibers; at the convergence
transmission layer, each route has 96-144 fibers; while at the access transmission layer,
each route has 24-48 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. OTN (Optical Transport
China Mobile Research Institute 37
Network) is another kind of WDM-based technology. ONT claims the advantages of
openness, good interoperability and scalability as well as powerful O&M functions. The main
issue for OTN solution lies on the high cost.
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
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.
5.1.6 Other consideration
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. Its 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.
5.1.7 Technology advancement
China Mobile Research Institute 38
In this and the subsequent sections in the White Paper, the transmission between the BBU
and the RRU in C-RAN is defined as fronthaul transmission (compared with traditional
backhaul transmission between the BBU and the core network).
The fronthaul transmission technology is of decisive significance to C-RAN large-scale
deployment. As more operators are paying importance to C-RAN, more resources are
committed to the issue. It is happy to see that many breakthroughs have been achieved
recently.
CPRI compression. With the maturity of CPRI compression, several vendors have
commercially realized 2:1 compression with lossless performance. It can help to save
half usage of fiber consumption. In addition, the Single Fiber Bi-direction (SFBD)
technology allows simultaneous UL and DL transmission on a single fiber, which further
halves fiber consumption. Combining CPRI compression and SFBD can save the fiber
consumption by 3 folds. CMCC has successfully verified the two technologies in C-RAN
TD-LTE field trials. More details and information can be found in Chapter 6.
WDM solution. Since WDM technology is sufficiently mature, vendors can develop WDM
equipments tailored to fronthaul transmisstion within a short period of time. Currently
a few operators have adopted this solution to enable the large-scale C-RAN
deployment. Some commercial products can support as many as 60 2.5Gbps CPRI
links in one pair of fiber, which significantly reduce fiber consumption. 1+1 or 1:1 ring
protection is also supported and several low data rate links can be multiplexed into one
link of high data rate. The main issue for the solution lies on the high cost, which
hinders its large-scale deployment by operators.
OTN solution. Compared with WDM solution, OTN provides more powerful O&M
capability, longer reach as well as flexible routing function. In addition, open interface
and standard protocol of OTN, in some sense, help to bring down the cost and drerease
the development difficulty. Some vendors suggested to integrate OTN functions into
optical modules rather than using active line cards, which can simplify network
deployment and maintenance to a large extent.
Millimeter microwave transmission. In some scenarios, it is too expensive, or even
impossible to deploy fiber. In that case, microwave transmission may come to play a
role as the last 100 meter fronthaul solution. 60GHz is currently the most common
frequency band for milli-meter microwave and can be implemented under loose
China Mobile Research Institute 39
regulation in many countries. The bandwidth in 60Ghz band is wide and thus it is easy
to get channels with 250MHz or wider bandwidth. With simple modulation technique, it
is easy to achieve over 1Gbps transmission rate within 100 ~ 400 meter range. For
LTE RRU with 20MHz bandwidth and 2 antennas, the data rate after 2:1 compression is
less than 1 Gbps and can be transmitted via millimeter microwave. 5GHz millimeter
microwave products just came into the market and can support the fronthaul
transmission of 20MHz LTE with 8 antennas.
CPRI redefinition. The basic idea of CPRI redefinition is to move a