Cloud RAN for Mobile Networks—A Technology Overview

TLDR: This paper surveys the state-of-the-art literature on C-RAN and can serve as a starting point for anyone willing to understand C- RAN architecture and advance the research on the network.

Abstract: Cloud Radio Access Network (C-RAN) is a novel mobile network architecture which can address a number of challenges the operators face while trying to support growing end-user's needs. The main idea behind C-RAN is to pool the Baseband Units (BBUs) from multiple base stations into centralized BBU Pool for statistical multiplexing gain, while shifting the burden to the high-speed wireline transmission of In-phase and Quadrature (IQ) data. C-RAN enables energy efficient network operation and possible cost savings on baseband resources. Furthermore, it improves network capacity by performing load balancing and cooperative processing of signals originating from several base stations. This paper surveys the state-of-the-art literature on C-RAN. It can serve as a starting point for anyone willing to understand C-RAN architecture and advance the research on C-RAN.

Figures

Fig. 7: Interference handling in LTE network.

Fig. 6: Daily load on base stations varies depending on base station location.

Fig. 9: C-RAN architecture can be either fully or partially centralized depending on L1 baseband processing module location.

TABLE V: DSP and GPP processors

Fig. 1: Costs vs revenues in mobile networks.

TABLE II: IQ bit rates between a cell site and centralized BBU Pool

Full text

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Cloud RAN for Mobile Networks - a Technology Overview
Checko, Aleksandra; Christiansen, Henrik Lehrmann; Yan, Ying; Scolari, Lara; Kardaras, Georgios;
Berger, Michael Stübert; Dittmann, Lars
Published in:
I E E E Communications Surveys and Tutorials
Link to article, DOI:
10.1109/COMST.2014.2355255
Publication date:
2014
Link back to DTU Orbit
Citation (APA):
Checko, A., Christiansen, H. L., Yan, Y., Scolari, L., Kardaras, G., Berger, M. S., & Dittmann, L. (2014). Cloud
RAN for Mobile Networks - a Technology Overview. I E E E Communications Surveys and Tutorials, 17(1), 405-
426. https://doi.org/10.1109/COMST.2014.2355255

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION 1
Cloud RAN for Mobile Networks - a Technology
Overview
Aleksandra Checko
∗†
, Henrik L. Christiansen
†
, Ying Yan
†
,
Lara Scolari
∗
, Georgios Kardaras
∗
, Michael S. Berger
†
and Lars Dittmann
†
∗
MTI Radiocomp, Hillerød, Denmark
†
DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark
Email: aleksandra.checko@mtigroup.com
Abstract—Cloud Radio Access Network (C-RAN) is a novel
mobile network architecture which can address a number of
challenges the operators face while trying to support growing
end-user’s needs. The main idea behind C-RAN is to pool
the Baseband Units (BBUs) from multiple base stations into
centralized BBU Pool for statistical multiplexing gain, while
shifting the burden to the high-speed wireline transmission of
In-phase and Quadrature (IQ) data. C-RAN enables energy
efficient network operation and possible cost savings on base-
band resources. Furthermore, it improves network capacity by
performing load balancing and cooperative processing of signals
originating from several base stations. This article surveys the
state-of-the-art literature on C-RAN. It can serve as a starting
point for anyone willing to understand C-RAN architecture and
advance the research on C-RAN.
Keywords—Cloud RAN; mobile networks; small cells; eICIC;
CoMP; Virtualization; IQ Compression; CPRI;
I. INTRODUCTION
Mobile data transmission volume is continuously rising. It
is forecasted to grow 13-fold from 2012 until 2017 according
to Cisco [1], with smart phones and tablet users driving the
growth. Therefore, to satisfy growing user demands, mobile
network operators have to increase network capacity. As spec-
tral efficiency for the Long Term Evolution (LTE) standard
is approaching the Shannon limit, the most prominent way
to increase network capacity is by either adding more cells,
creating a complex structure of Heterogeneous and Small
cell Networks (HetSNets) [2] or by implementing techniques
such as multiuser Multiple Input Multiple Output (MIMO)
[3] as well as Massive MIMO [4], where numerous antennas
simultaneously serve a number of users in the same time-
frequency resource. However, this results in growing inter-cell
interference levels and high costs.
Total Cost of Ownership (TCO) in mobile networks includes
CAPital EXpenditure (CAPEX) and OPerating EXpenditure
(OPEX). CAPEX mainly refers to expenditure relevant to
network construction which may span from network planning
to site acquisition, RF hardware, baseband hardware, software
licenses, leased line connections, installation, civil cost and site
support, like power and cooling. OPEX covers the cost needed
to operate the network, i.e., site rental, leased line, electricity,
operation and maintenance as well as upgrade [5]. CAPEX and
OPEX are increasing significantly when more base stations are
$-
$1,200.00
2009 2010 2011 2012 2013 2014 2015 2016 2017
bn $
Operator-Billed Revenues [JuniperResearch, 06 2011]
CAPEX/OPEX [JuniperResearch, 06 2011]
Fig. 1: Costs vs revenues in mobile networks.
deployed. More specifically, CAPEX increases as base stations
are the most expensive components of a wireless network
infrastructure, while OPEX increases as cell sites demand a
considerable amount of power to operate, e.g., China Mobile
estimates 72% of total power consumption originates from
the cell sites [6]. Mobile network operators need to cover the
expenses for network construction, operation, maintenance and
upgrade; meanwhile, the Average Revenue Per User (ARPU)
stays flat or even decreases over time, as the typical user
is more and more data-hungry but expects to pay less for
data usage. As presented in Figure 1 [7], mobile operators
are facing cases (2014-2015) where network cost may exceed
revenues if no remedial actions are taken [8]. Therefore,
novel architectures that optimize cost and energy consumption
become a necessity in the field of mobile network.
C-RAN is a novel mobile network architecture, which has
the potential to answer the above mentioned challenges. The
concept was first proposed in [9] and described in detail in
[6]. In C-RAN, baseband processing is centralized and shared
among sites in a virtualized BBU Pool. This means that it is
able to adapt to non-uniform traffic and utilizes the resources,
i.e., base stations, more efficiently. Due to that fact that fewer
BBUs are needed in C-RAN compared to the traditional
architecture, C-RAN has also the potential to decrease the cost
of network operation, because power and energy consumption
are reduced compared to the traditional RAN architecture. New
BBUs can be added and upgraded easily, thereby improving
scalability and easing network maintenance. Virtualized BBU
Pool can be shared by different network operators, allowing
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available athttp://dx.doi.org/10.1109/COMST.2014.2355255
Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

2 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
them to rent Radio Access Network (RAN) as a cloud service.
As BBUs from many sites are co-located in one pool, they can
interact with lower delays – therefore mechanisms introduced
for LTE-Advanced (LTE-A) to increase spectral efficiency
and throughput, such as enhanced ICIC (eICIC) and Coor-
dinated Multi-Point (CoMP) are greatly facilitated. Methods
for implementing load balancing between the cells are also
facilitated. Furthermore, network performance is improved,
e.g., by reducing delay during intra-BBU Pool handover.
C-RAN architecture is targeted by mobile network op-
erators, as envisioned by China Mobile Research Institute
[6], IBM [9], Alcatel-Lucent [10], Huawei [11], ZTE [12],
Nokia Siemens Networks [5], Intel [13] and Texas Instruments
[14]. Moreover, C-RAN is seen as typical realization of
mobile network supporting soft and green technologies in fifth
generation (5G) mobile network in year 2020 horizon [15].
However, C-RAN is not the only candidate architecture that
can answer the challenges faced by mobile network operators.
Other solutions include small cells, being part of HetSNets
and Massive MIMO. Small cells deployments are the main
competitors for outdoor hot spot as well as indoor coverage
scenarios. All-in-one small footprint solutions like Alcatel-
Lucent’s LightRadio can host all base station functionalities
in a few liters box. They can be placed outdoors reducing cost
of operation associated to cooling and cell site rental. However,
they will be underutilized during low-activity periods and can
not employ collaborative functionalities as well as C-RAN can
do. Moreover, they are more difficult to upgrade and repair than
C-RAN. Brief comparison between C-RAN, Massive MIMO
and HetSNets is outlined in [2]. Liu et al. in [16] prove
that energy efficiency of large scale Small Cell Networks
is higher compared with Massive MIMO. Furthermore, cost
evaluation on different options needs to be performed in order
for a mobile network operator to choose an optimal solution.
Comparison of TCO including CAPEX and OPEX over 8
years of traditional LTE macro base station, LTE C-RAN and
LTE small cell shows that the total transport cost per Mbps
is highest for macro cell deployment - 2200$, medium for
C-RAN - 1800$ and 3 times smaller for small cell - 600$ [17].
Therefore the author concludes that C-RAN needs to achieve
significant benefits to overcome such a high transportation cost.
Collaborative techniques such as CoMP and eICIC can be
implemented in small cells giving higher benefits in HetNet
configuration instead of C-RAN. The author envisions that
C-RAN might be considered for special cases like stadium
coverage. However, C-RAN is attractive for operators that have
free/cheap fiber resources available.
This article surveys the state-of-the-art literature published
on C-RAN and its implementation. Such input helps mobile
network operators to make an optimal choice on deployment
strategies. The paper is organized as follows. In Section II
we introduce the fundamental aspects of C-RAN architecture.
Moreover, in Section III we discuss in detail the advantages
of this architecture along with the challenges that need to be
overcome before fully exploiting its benefits in Section IV. In
Section V we also present a number of constraints in regards to
the transport network capacity imposed by C-RAN and discuss
possible solutions, such as the utilization of compression
schemes. In Sections VI, VII we give an overview of the
state-of-the-art hardware solutions that are needed to deliver
C-RAN from the radio, baseband and network sides. As the
BBU Pool needs to be treated as a single entity, in Section VIII
we present an overview of virtualization techniques that can
be deployed inside a BBU Pool. In Section IX we evaluate
possible deployment scenarios of C-RAN. In Section X we
summarize ongoing work on C-RAN and give examples of first
field trials and prototypes. Section XI concludes the paper.
II. WHAT IS C-RAN? BASE STATION ARCHITECTURE
EVOLUTION
C-RAN is a network architecture where baseband resources
are pooled, so that they can be shared between base stations.
Figure 2 gives an overview of the overall C-RAN architecture.
This section gives an introduction to base station evolution and
the basis of the C-RAN concept.
The area which a mobile network covers is divided into
cells, therefore mobile networks are often called cellular net-
works. Traditionally, in cellular networks, users communicate
with a base station that serves the cell under coverage of which
they are located. The main functions of a base station can
be divided into baseband processing and radio functionalities.
The main sub-functions of baseband processing module are
shown in left side of Figure 3. Among those we find coding,
modulation, Fast Fourier Transform (FFT), etc. The radio
module is responsible for digital processing, frequency filtering
and power amplification.
A. Traditional architecture
In the traditional architecture, radio and baseband processing
functionality is integrated inside a base station. The antenna
module is generally located in the proximity (few meters) of
the radio module as shown in Figure 4a as coaxial cables
employed to connect them exhibit high losses. X2 interface
is defined between base stations, S1 interface connects a
base station with mobile core network. This architecture was
popular for 1G and 2G mobile networks deployment.
B. Base station with RRH
In a base station with Remote Radio Head (RRH) archi-
tecture, the base station is separated into a radio unit and a
signal processing unit, as shown in Figure 4b. The radio unit
is called a RRH or Remote Radio Unit (RRU). RRH provides
the interface to the fiber and performs digital processing,
digital to analog conversion, analog to digital conversion,
power amplification and filtering [18]. The baseband signal
processing part is called a BBU or Data Unit (DU). More about
BBU can be found in Chapter 16 of [19]. Interconnection and
function split between BBU and RRH are depicted in Figure
3. This architecture was introduced when 3G networks were
being deployed and right now the majority of base stations use
it.
The distance between a RRH and a BBU can be extended up
to 40 km, where the limitation is coming from processing and
propagation delay. Optical fiber and microwave connections
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available athttp://dx.doi.org/10.1109/COMST.2014.2355255
Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

CHECKO et al.: CLOUD RAN FOR MOBILE NETWORKS - A TECHNOLOGY OVERVIEW 3
RRH 1
RRH 2
RRH n
...
Mobile
Backhaul Network
BBU 2
BBU n
BBU 1
(a) RAN with RRH
Cloud
RRH 1
RRH 2
RRH n
...
BBU Pool
Mobile
Backhaul Network
Aggregated
Traffic (h)
24 h
(b) C-RAN
Fig. 2: Statistical multiplexing gain in C-RAN architecture for mobile networks.
Control-RRC
Transport-MAC
L3 L2
CoMP
eICIC
Channel de-/
coding
De-/Quantization
Antenna
Mapping-MIMO
De-/Sampling
Resource-block
Mapping
De-/Modulation
IFFT/FFT
... ... ...
...
IQ DL
IQ UL
CPRI/
OBSAI
/ORI
CFR
/
DPD
DAC
ADC
Frequ
ency
filter
BBU
RRH
RRC Radio Resource Control SRC Sampling Rate Conversion DAC Digital-to-Analog Converter
MAC Media Access Control DUC/DDC Digital Up/Downconversion ADC Analog-to-Digital Converter
FFT Fast Fourier Transform CFR Crest Factor Reduction Power Amplifier
DPD Digital Predistortion
CPRI/OBSAI/ORI
L
1
S
R
C
D
U
C
S
R
C
D
D
C
Fig. 3: Base station functionalities. Exemplary baseband processing functionalities inside BBU are presented for LTE
implementation. Connection to RF part and sub modules of RRH are shown.
can be used. In this architecture, the BBU equipment can be
placed in a more convenient, easily accessible place, enabling
cost savings on site rental and maintenance compared to
the traditional RAN architecture, where a BBU needs to be
placed close to the antenna. RRHs can be placed up on poles
or rooftops, leveraging efficient cooling and saving on air-
conditioning in BBU housing. RRHs are statically assigned to
BBUs similarly to the traditional RAN. One BBU can serve
many RRHs. RRHs can be connected to each other in a so
called daisy chained architecture. An Ir interface is defined,
which connects RRH and BBU.
Common Public Radio Interface (CPRI) [20] is the radio in-
terface protocol widely used for IQ data transmission between
RRHs and BBUs - on Ir interface. It is a constant bit rate,
bidirectional protocol that requires accurate synchronization
and strict latency control. Other protocols that can be used are
Open Base Station Architecture Initiative (OBSAI) [21] and
Open Radio equipment Interface (ORI) [22], [23].
C. Centralized base station architecture - C-RAN
In C-RAN, in order to optimize BBU utilization between
heavily and lightly loaded base stations, the BBUs are cen-
tralized into one entity that is called a BBU/DU Pool/Hotel.
A BBU Pool is shared between cell sites and virtualized as
shown in Figure 4c. A BBU Pool is a virtualized cluster which
can consist of general purpose processors to perform baseband
(PHY/MAC) processing. X2 interface in a new form, often
referred to as X2+ organizes inter-cluster communication.
The concept of C-RAN was first introduced by IBM [9]
under the name Wireless Network Cloud (WNC) and builds
on the concept of Distributed Wireless Communication System
[24]. In [24] Zhou et al. propose a mobile network architecture
in which a user communicates with densely placed distributed
antennas and the signal is processed by Distributed Processing
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available athttp://dx.doi.org/10.1109/COMST.2014.2355255
Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

4 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
BaseBand
a) Traditional macro base station
Antenna
b) Base station with RRH
RF
RF
c) C-RAN with RRHs
Virtual BBU Pool
RRH
RRH
RRH
RF
Fiber – Digital BaseBand Coax cable – RF
BS
cell
Transport
Control
Synch
BaseBand
Synch
Control
Transport
BaseBand
Synch
Control
Transport
BaseBand
Synch
Control
Transport
BaseBand
Transport
Control
Synch
S1/X2
RF
PA
Antenna
cell
S1
/
X2
RRH
RF
BBU
BaseBand
Transport
Control
Synch
Ir
Ir
S1
X2
Fig. 4: Base station architecture evolution.
Centers (DPCs). C-RAN is the term used now to describe this
architecture, where the letter C can be interpreted as: Cloud,
Centralized processing, Cooperative radio, Collaborative or
Clean.
Figure 5 shows an example of a C-RAN mobile LTE
network. The fronthaul part of the network spans from the
RRHs sites to the BBU Pool. The backhaul connects the BBU
Pool with the mobile core network. At a remote site, RRHs are
co-located with the antennas. RRHs are connected to the high
EPC
Base
band
Base
band
Base
band
Base
band
S1
S1
X2
BBU
pool
Access
network
MME
SGW
PGW
Fronthaul
Backhaul
Base
band
Base
band
Base
band
Base
band
BBU
pool
Aggregation
network
Mobile Core
Network
RRH
RRH
RRH
RRH
RRH
Ir
Fig. 5: C-RAN LTE mobile network.
performance processors in the BBU Pool through low latency,
high bandwidth optical transport links. Digital baseband, i.e.,
IQ samples, are sent between a RRH and a BBU.
Table I compares traditional base station, base station with
RRH and base station in C-RAN architecture.
TABLE I: Comparison between traditional base station, base
station with RRH and C-RAN
Architecture Radio and baseband
functionalities
Problem it
addresses
Problems it
causes
Traditional
base station
Co-located in one
unit
- High power con-
sumption
Resources are un-
derutilized
Base station
with RRH
Spitted between
RRH and BBU.
RRH is placed to-
gether with antenna
at the remote site.
BBU located within
20-40 km away.
Generally deployed
nowadays
Lower power con-
sumption.
More convenient
placement of
BBU
Resources are un-
derutilized
C-RAN Spitted into RRH
and BBU.
RRH is placed to-
gether with antenna
at the remote site.
BBUs from many
sites are co-located
in the pool within
20-40 km away.
Possibly deployed
in the future
Even lower power
consumption.
Lower number of
BBUs needed -
cost reduction
Considerable
transport
resources
between RRH
and BBU
III. ADVANTAGES OF C-RAN
Both macro and small cell can benefit from C-RAN ar-
chitecture. For macro base station deployments, a centralized
BBU Pool enables an efficient utilization of BBUs and reduces
the cost of base stations deployment and operation. It also
reduces power consumption and provides increased flexibility
in network upgrades and adaptability to non-uniform traffic.
Furthermore, advanced features of LTE-A, such as CoMP
and interference mitigation, can be efficiently supported by
C-RAN, which is essential especially for small cells deploy-
ments. Last but not least, having high computational processing
power shared by many users placed closer to them, mobile
This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.
The final version of record is available athttp://dx.doi.org/10.1109/COMST.2014.2355255
Copyright (c) 2014 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.

Frequently asked questions

Q1. What have the authors contributed in "Cloud ran for mobile networks - a technology overview" ?

This article surveys the state-of-the-art literature on C-RAN. 

Q2. What future works have the authors mentioned in the paper "Cloud ran for mobile networks - a technology overview" ?

Mobile network operators as well as telecommunication industry show a very high interest in C-RAN due to the fact that it offers potential cost savings, improved network performance and possibility to offer IaaS. 

Q3. What are the main contributors to the size of IQ data?

The main contributors to the size of IQ data are: turbocoding (e.g., in UMTS and LTE 1:3 turobocode is used resulting in three times overhead), chosen radio interface (e.g., CPRI) IQ sample width and oversampling of LTE signal. 

Q4. What are the prominent standards to be deployed as C-RANs?

as LTE is currently deployed all over the world, LTE and LTE-A are the most prominent standards to be deployed as C-RANs. 

Q5. What are the challenges that need to be addressed before the commercial deployment of C-RAN architectures?

Before the commercial deployment of C-RAN architectures a number of challenges need to be addressed: A. High bandwidth, strict latency and jitter as well as low cost transportnetwork needs to be available, B. Techniques on BBU cooperation, interconnection and clustering need to be developed as well as C. Virtualization techniques for BBU Pool need to be proposed. 

Q6. What is the prominent solution for the physical medium?

As fiber is the most prominent solution for the physical medium, its availability for the network operator needs to be taken into account choosing the optimal transport network solution. 

Q7. What are the possible solutions to reduce signal sampling rate?

Potential solutions could be to reduce signal sampling rate, use non-linear quantization, frequency subcarrier compression or IQ data compression [6]. 

Q8. What is the advantage of a single entity doing signal processing?

If all the cells within a CoMP set are served by one BBU Pool, then a single entity doing signal processing enables tighter interaction between base stations. 

Q9. What is the way to reduce signal sampling rate?

Reducing signal sampling rate is a low complex solution having minimal impact on protocols, improves compression up to 66% with some performance degradation [6]. 

Q10. What are the main reasons why mobile network operators are interested in C-RAN?

Mobile network operators as well as telecommunication industry show a very high interest in C-RAN due to the fact that it offers potential cost savings, improved network performance and possibility to offer IaaS. 

Q11. How can the centralized architecture save compute resources?

Bhaumik et al. show that the centralized architecture can potentially result in savings of at least 22% in compute resources by exploiting the variations in the processing load across base stations. 

Q12. What are the challenges for real time virtualized base station in centralized BBU Pool?

There are also challenges for real time virtualized base station in centralized BBU Pool, like high performance lowpower signal processing, real time signal processing, BBU interconnection as well as between chips in a BBU, BBUs in a physical rack and between racks. 

Q13. What is the advantage of a virtualized BBU Pool?

C-RAN with a virtualized BBU Pool gives a smooth way for introducing new standards, as hardware needs to be placed in few centralized locations. 

Q14. What is the way to optimize the number of active RRHs/BBU units?

To achieve optimal energy savings of the C-RAN, base stations need to be chosen in a way that will optimize the number of active RRHs/BBU units within the BBU Pool. 

Q15. What are the challenges of sharing wireless interfaces among different virtual network operators?

Sharing wireless interfaces among different virtual wireless network operators faces challenges such as: switching between virtual network operators; different authentication and security; different usage of bandwidth resources. 

Q16. How can a centralized architecture maximize the gain of a statistical multiplexing?

Statistical multiplexing gain can be maximized by employing a flexible, reconfigurable mapping between RRH and BBU adjusting to different traffic profiles [33]. 

Q17. What is the path towards complete deployment of C-RAN?

The path towards complete deployment of C-RAN can be paved through following stages [117].1) Centralized RAN, where baseband units are deployed centrally supporting many RRHs. 

Q18. Why is dark fiber a preferred solution for a BBU Pool?

Various transport network solutions are discussed below [6].1) Dark fiber: Dark fiber is a preferred solution for a BBU Pool with less than 10 macro base stations [6], due to capacity requirements. 

Q19. How can a centralized architecture maximize the gain of a statistical multiplexing pool?

Statistical multiplexing gain depends on the traffic, therefore it can be maximized by connecting RRHs with particular traffic profiles to different BBU Pools [30]. 

Q20. How can The authorget permission for a CPRI2Ethernet gateway?

For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.map CPRI data to Ethernet packets, close to or at the interface of RRH towards BBU Pool. 

Q21. What are the two approaches to address the interference issue?

there are two approaches to address the interference issue: minimizing interference and exploiting interference paths constructively. 

Q22. What is the effect of RRHs on the air conditioning of radio modules?

Due to the usage of RRHs air conditioning of radio module can be decreased as RRHs are naturally cooled by air hanging on masts or building walls, as depicted in Figure 4.