Evolution of SK Telecom’s in-building LTE solutions

most of its base stations have become C-RAN

based, and small RRHs were introduced in some

regions last year.

For indoor, Distributed Antenna System (DAS)

solution has been pretty dominant. This solution,

originally designed for extending coverage, might

not work best today because what is sought after

most now is a solution that can extend an indoor/

in-building capacity for accommodating the ever-

increasing LTE traffic of these days. For this

reason, i) DAS solutions have been evolving to give

enhanced capacity, and ii) small cell-based

solutions have been introduced. Let's take a look at

SK Telecom's in-building solutions now.

I. SK Telecom's In-building SolutionsSK Telecom's highly scalable in-building solutions

(for medium and large-scaled buildings) can be

classified as follows:

• DAS Evolution

• 3G DAS + LTE small cell

• Indoor C-RAN: NEURON

DAS, traditionally focused on coverage expansion,

is now evolving to yield increased capacity. In the

meantime, small cells, capable of providing capacity

on their own, unlike DAS, are evolving to further

extend their coverage, beyond hotspots. This nature

of small cell inspired SK Telecom to come up with

new in-building solutions. First, in line with the

decision to convert some of the 2.1 GHz 3G band to

LTE band, it combined LTE small cell and 3G DAS

at 2.1 GHz in 2014. Second, it presented NEURON,

a solution designed to adopt C-RAN architecture

for indoor use (tested in early 2015).

II. DAS EvolutionConventional DAS: A popular coverage solution

since 2G/3G designed to extend the coverage of a

base station to areas with poor radio reception in a

building. It brings RF signals from a base station/

RRH into a building via optic/RF cables, and has

them transmitted across the building via antennas

distributed in the building. This solution lets an

As of April 2015, SK Telecom's mobile data traffic

accounts for 44% of the entire mobile traffic in

Korea (See Figure 1). Since the launch of LTE

service, the company has experienced rapid growth

of mobile traffic - more than tenfold from 5.7 PB in

June 2011 (right before LTE service launch) to 57.7

PB in April 2015. With the company's

announcement of unlimited LTE data policy in

May, LTE traffic is expected to increase even more

and faster.

Evolution of SK Telecom’s in-building LTE solutions | Dr. Michelle M. Do ([email protected])

Figure 1. SK Telecom's mobile traffic growth (source: MSIP and SK Telecom)

SK Telecom, with a 47% market share in Korean

LTE market, has the same amount of LTE

frequency bandwidth (in DL) as its competitors, but

is generating almost half of the entire LTE traffic in

the nation. Currently, the company has a total of 40

MHz commercial LTE frequency across three

bands: 10MHz at 850 MHz (Band 5), 20 MHz at 1.8

GHz (Band 3), and 10 MHz at 2.1 GHz (Band 1)

(FDD LTE and DL only). Since 3-band carrier

aggregation (CA) commercialization in last

December, SK Telecom has been supporting up to

300 Mbps.

For enhanced network capacity, SK Telecom made

LTE/LTE-A macro cells in urban areas smaller - as

small as several hundred meters in radius. And it

began to employ Centralized Radio Access Network

(C-RAN ) for reduced cell site construction costs

and efficient inter-cell coordination. As a result,

12 2 4 6 8 10 12 2 4 6 8 10 12 2 4 6 8 10 12 2 4 6 8 10 12 2 4

2011 2012 2013 2014 2015

Mobile data usage per month

Korea

SK Telecom

57.7 PB(44%)

131.6 PB

2011.06

5.7 PB

2011.07 LTE service launched

Evolution of SK Telecom’s in-building LTE solutionsDr. Michelle M. Do ([email protected]) | July 9, 2015

Korea Communication Review • Q3 2015

but also good capacity. In addition, installing

process should be fast and inexpensive. To satisfy

all these requirements, SK Telecom's DAS is

evolving to ensure:

• Cheaper and faster installation using UTP

cabling: To save costs of cabling, which often is

the No. 1 cost-increasing factor in DAS installa-

tion, economical and scalable standardized UTP

cables are used instead of RF cables.

• Remote management: A smart antenna unit

with both antennas and RF transmission control

modules is introduced. A smart antenna unit

reports radio signal measurements and status

information, and remotely performs tasks like

configuration, status check, fault management,

etc., allowing for remote optimization.

• Increased capacity: MIMO and CA are

supported for increased capacity. MIMO requires

more antennas because it allows different types of

radio signals to be sent within the same frequency

band. CA combines radio signals from different

frequency bands, and thus requires more

frequency bands. Although in need of more

frequency bands than MIMO, it can take

advantage of SISO mode, doubling the speeds at

devices.

outdoor cell work indoor as well, allowing its

coverage to extend indoors.

Benefits: DAS distributes RF signals to antennas,

and can deliver different Radio Access Technologies

(RAT) like CDMA, WCDMA, LTE, etc., all through a

single optic/RF cable (technology-neutral). Plus,

since a single cell covers both indoors and outdoors,

no quality degradation is caused by inter-cell

interference or handover.

Drawbacks: DAS has too many components

(attenuator, donor unit, remote unit, splitter,

coupler, booster, etc.), which means complicated

installation, longer construction period, and higher

construction costs. But, what is even worse is that it

has poor scalability which will make it hard to

increase capacity when needed in the future - a

serious drawback in this era of soaring data traffic.

Also, all antennas used in DAS require individual

power adjustment. Besides, if any of them fails, it is

hard to detect the one that failed. Because outdoor

cell resources are shared, high-speed data usage

can be limited.

In-building solution I: DAS EvolutionBecause LTE serves more data than voice, DAS

should be able to provide not only good coverage,


Figure 2. Architecture of DAS evolution

In-building

IP/MPLS

Macro RRH

BBUBBU...

Tens of BBUs(connecting 100s of RRHs)

CPRICPRI

EPC

SAEGW

Rooftop

# of CO » 400# of BBU Marco Cell » 300# of RRHs » 180,000

RT

RT

COT

C-RAN: Centralized/Cloud RANBBU: Baseband UnitRRH: Remote Radio HeadSCAN: Smart Cloud Access Network

GEMulti-10GE

BBU

SK Telecom’s CO

RURURRH

RURURRH

RURURRH

BBU

Fronthaul: Active WDM

Conventional DAS

DAS Evolution

LTE

RURURRH

MU: Master UnitRU: Remote UnitAtt.: Attenuator

Remote

UTP cable

MIMO

Smart antenna

LTE

RF cable

SISO

Passive antenna

LTE

MU Att.

MU Att. RT

RU

RU

RU

RU

RU

RU

outdoor base station are used. Small cells, on the

contrary, are low-powered base stations with their

own resources (i.e. with frequency re-use), and thus

can improve capacity by using the resources. Small

cell-based solutions have been actively adopted for

indoor, but it does not appear they can simply

replace DAS, yet. Then, what are key issues that SK

Telecom should consider to make small cell

solutions work as hotspot/in-building solutions in

urban areas?

• Interference: Operating with the same

frequency by indoor small cell and macro cell can

cause co-channel interference. To minimize this

interference, SK Telecom introduced multi-band

small cell in August 2014 and eICIC in January

2015. Multi-band small cell is a reconfigurable

public femto cell, and so it can use any of the

three LTE bands (Band 1, 3 and 5), eliminating

the chance of two cells' using the same band.

eICIC makes macro cell and small cell work at

different times, effectively avoiding co-channel

interference.

• Mobility: Handovers between macro and small

cells during hand-in (from outdoor to indoor) or

hand-out (from indoor to outdoor) may cause

QoE degradation. Other causes for QoE

degradation include co-channel interference, as

mentioned above, and technological disparity

among macro/small cell vendors in implementing

handovers. SK Telecom introduced eICIC to

minimize co-channel interference, and has

managed both macro and small cells through a

single solution, centralized SON, thereby

addressing multi-vendor interworking issues.

• Capacity: The success of a small cell solution in

III. Small Cell

How will they evolve?So far, SK Telecom's small cell evolution has been

focused on femto cell. Now the operator is

attempting to make small cell further evolve to

serve larger buildings as well. The evolution of

femto cell can be summarized as follows:

• In 2012, SK Telecom launched the world's first

LTE public femto cell for hotspots that serves up

to 32 active users. One public femto cell can

offload data traffic at a macro cell by 6.6%,

improving indoor capacity as well as macro user's

throughput.

• In 2013, the world's first C-RAN-typed (DU and

RU separated) LTE femto was introduced.

Although this Femto Remote Solution (FRS) has

not become very popular and ended up being

applied only in limited areas, R&D activities on

this type of indoor C-RAN went on. Then, at the

beginning of the year, NEURON, a new solution

featuring the ability to support fronthaul of any

kind and high scalability, was showcased, and is

getting ready for commercialization.

• In 2014, SK Telecom presented RF Reconfigurable

Femto, alleviating interference with outdoor cell

while increasing femto cell capacity to support up

to 64 users. In January 2015, it commercialized

eICIC, an interference coordination solution in

HetNet, for the first time in the world.

What to consider when using small cell in-building solutionDAS boosts RF signals from an outdoor base station

and distributes them. So, the resources of the


increasing capacity in

medium and large-scaled

building lies in the way

small cells are deployed.

Small cells are low-

powered base stations

designed specifically for

short range use. So, an

economical and scalable

in-building solution that

can extend coverage

across the entire building

is needed.

Figure 3. What to consider when deploying small cells

CPRI Fronthaul

BBU Pool

Small cell

EPC Core

Macro cell site

Hand-out

Co-channel Interference

Hand-in

Mobility

Capacity

PCI collision

PCI=100

PCI=100

Hanover Fail

Macro RRH

• Interference • Mobility• Capacity

Small Cell Deployment Issues

(Active WDM)

leveraging the established DAS infra. Currently,

SISO antennas are used in DAS systems, and the

capacity increase expected from an added LTE

small cell is 37.5 Mbps.

An LTE small cell is designed to be connected to a

Master Unit (MU) or Remote Unit (RU) at DAS.

Incoming LTE RF signals are received through a

MU (or RU), and then converted into optic signals

before being distributed by DAS. How many and

where to add small cells are determined according

to the amount of traffic. For example, just one at

MU will work for low traffic, but one at each RU is

required for high traffic. To an MU with sectoriza-

tion feature, multiple small cells can be connected,

allowing for dynamic control of in-building capacity

and the coverage of each small cell.

3G base stations are connected to the EPC core

network, through Packet Transport System (PTS)

and then through 3G mobile backhaul. However,

each LTE small cell can be connected to the core

network either through SK Telecom's mobile

backhaul or through SK Broadband (SKB) Internet

network depending on their locations and the

conditions of SK Telecom's infra. For example, an

LTE small cell installed at a 3G base stations

connects to the core network through PTS while

ones installed remotely (e.g. in other building)

connect through the SKB Internet network. A small

cell accessing through the Internet network must be

connected through security GW and small cell GW.

The goal of SK Telecom's small cell-based in-

building solution for medium and large-scaled

buildings is to find a cost-effective way to improve

both capacity and coverage. And SK Telecom has

been making efforts to achieve the goal through

introduction of new solutions like, 3G DAS-

combined LTE small cell solution in 2014, and

NEURON solution (deploying RUs of small cells as

in DAS), to be discussed in details below, in early

2015.

In-building solution II: 3G DAS + LTE Small Cell This solution can be useful when you want to

enhance in-building capacity in a building where

DAS system has already been installed. You can

simply add a small cell to the current DAS system -

fast and economical.

Last year, SK Telecom converted a part of its

2.1GHz 3G band to LTE band - 10 MHz out of 30

MHz (DL), and launched a new in-building solution

that adds a 2.1 GHz LTE small cell to the current 2.1

GHz DAS system. So, this solution allows for two

different RF signals, 3G signals from macro cells

and LTE signals from small cells, to be combined

on RF through an optic repeater. And as a result of

the combination, the DAS system, now with added

LTE small cell capacity, can benefit from the

resulting in-building capacity improvement while

the LTE small cell can extend its coverage by


Figure 4. Combining LTE small cell with 3G DAS (2.1GHz)

IP/MPLS

Macro RRH

BBUBBU...

CPRICPRI EPC

SAEGW

RT

RT

RT

COT

LTE

GEMulti-10GE

BBU

SK Telecom’s CO

RURURRH

RURURRH

RURURRH

BBU


Adding LTE Small Cells over Existing DAS

3G Network

Small Cell GW

SeGW

LTE traffic

In-building

3G traffic

RF cable

RU

RU

RU

sCell

sCell

sCell

LTE3G

SK Broadband

Ethe

rne

t

NEURON consists of DU, Optic Extendable Unit

(OEU) and RF Unit (RU). Between DU and OEU,

baseband signals are digitized, and transmitted

through optic cables. Then between OEU and RU,

IF signals are digitized, and transmitted through

UTP cables. Because distributed RUs are connected

through inexpensive UTP cables, it costs much less

and takes much shorter to build an in-building

network.

• DU: Cost-effective as a femto modem is used. Can

serve up to 64 active users when 10MHz used (up

to 128 when 20MHz used). Each DU can support

up to 80 RUs. Dynamic cell configuration made

possible as RUs can be grouped according to

traffic conditions.

• OEU: Connects to DU (or other OEU) through

optic cable, and to distributed RUs through UTP

cable. UTP cable connection works within up to

100m, but optic cable performs much better,

allowing RUs to be distributed pretty far.

OEU uses radio signal digitization and

compression technologies for more efficient

transmission through UTP cables. After

terminating baseband signals, OEU converts them

into IF signal. Then it digitizes and compresses

the signal, and sends it to RU. OEU supplies the

RU with remote power using Power over Ethernet

(PoE)

• RU: Connected to OEU through UTP cables.

When digitized IF signal is received from OEU,

RU converts it into RF signal, and then transmits

it through antennas. There are two types of RU:

Each small cell can serve up to 64 active users at

once. According to the data released by SK Telecom

at the Small Cells Asia Summit 2015, the average

connected users at a 15-story building in Seoul was

around 60. In a test that applied Small Cell + DAS

solution in a 15-story building, the success rates of

E-RAB/RRC and hand-in/out were reported 100/

99% and 97/99%, respectively.

In-building solution III: NEURON - LTE small cell with C-RAN modified for indoors Another small cell-based in-building solution is

NEURON, which is a version of C-RAN modified

for indoor. This solution can be an excellent option

when DAS system cannot be leveraged because it

features small cells with C-RAN architecture where

DU and RU are separated. These types of small cells

can provide their own capacity (i.e. resources and

DU) unlike DAS, but their RUs are still distributed

across the building just like DAS. Because

NEURON allows small cell stations to group the

RUs the way they want, cells can be configured

dynamically and virtually. So, for example, you can

group all the RUs into one to create a single cell as

in DAS, or form multiple cells depending on the

traffic condition in the building.

NEURON (New Extendable eNB based on UTP, RF

and Optic for fronthaul) was demonstrated in the

beginning of the year, and is currently being tested

for commercialization in the second half of the year.


Figure 5. NEURON - LTE small cell with C-RAN modified for indoors

IP/MPLS

Macro RRH

BBUBBU...

CPRICPRI

EPC

SAEGW

RT

RT

RT

COT

LTE

GEMulti-10GE

BBU

SK Telecom’s CO

RURURRH

RURURRH

RURURRH

BBU


Small Cell GW

SK Broadband

SeGW

LTE traffic

DU: Digital UnitOEU: Optic Extendable UnitRU: RF Unit

NEURON

In-building

UTP cable

RU

LTE

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OEU

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i) LTE-only RU that can support only one LTE

band, and ii) WiFi-embedded RU that supports

two LTE bands and WiFi band. Because WiFi-

embedded RUs can operate with both 10MHz at

2.1GHz, and 20MHz at 1.8GHz, CA can be

supported, providing up to 225 Mbps throughput.

NEURON supports all types of fronthaul (optic, RF

and UTP cable), and thus can give fairly flexible

small cell coverage. It features DU with a femto

modem, LAN (UTP) cable-friendly RU, and OEU

with great scalability. Because of these properties,

the solution is known to extend coverage as well as

capacity in buildings of any scale, from small to

medium/large-scaled buildings, in a more cost-

effective, flexible way. In a test conducted by the

operator, NEURON was found to support 41~125

Mbps when using 20 MHz at 1.8 GHz, and 26 ~ 56

Mbps when using 10 MHz at 850 MHz.

IV. SummaryWe have so far explored the three in-building

solutions by SK Telecom: DAS evolution solution,

DAS + LTE small cell solution, and NEURON. We

have also learned how DAS and small cell,

respectively based on excellent scalability in

coverage and capacity, are evolving, independently

or dependently, for further enhancement of

coverage and capacity. Now with more options to

choose from, an operator can find a solution to use

to build, operate and extend a more economical and

reliable in-building network, according to its needs

and conditions, like the current network infra in the

building, the operator's infra in the area, etc. n


Table 1. Summary of SK Telecom's in-building solution

Target scenario

Capacity enhancement

Coverage enhancement

Dynamic cell configuration

SISO/MIMO

Multi-band

Carrier aggregation (CA)

Max. throughput

UTP cabling

WiFi support

Medium and large-scaled buildings

Support CA and MIMO

Originally designed for coverage

None (A single cell)

MIMO

Multi-band (1.8 GHz, 2.1 GHz)

CA

225 Mbps

UTP cable

-

I. DAS Evolution


Add small cell

Leverage DAS


SISO

Single-band (2.1 GHz)

-

37.5 Mbps

-

-

II. 3G DAS + LTE Small Cell


Add DU

Distribute RUs as DAS


MIMO

Milti-band (1.8GHz, 2.1GHz)

CA

225 Mbps

UTP cable

WiFi-embedded RU support

III. Indoor C-RAN: NEURON

Technology

Evolution of SK Telecom’s in-building LTE solutions