Concept and Challenges of Femtocells

EE 497AApril 21, 2010

Presentation by,Rachana Reddy

Overview

• Introduction▫Evolution of cellular systems▫Need for Femtocells▫Benefits▫ Industry Activities

•Design Issues▫Architecture▫Usage Model

•Radio Issues▫Coverage▫ Interference▫Spectrum Allocation

•Summary

Evolution of Cellular Systems

Single Base Station

Macro Stations

Evolution of Cellular Systems

Femto Stations

Macro Base Stations

2G3G

4G

As throughput demand and usage increases, cell size decreases

Current Traffic Trend

Future Traffic Trend

Outdoor

Home

Work

Need for Femtocells

• Macrocell cannot provide good signal strength for indoor coverage

• Higher data rate– Femtocells can enable higher capacity• Reduce the power on the macrocells

“Femtocells are low-power wireless access points that operate in licensed spectrum to connect

standard mobile devices to a mobile operator’s network using residential DSL or cable broadband

connections”

Reference: FemtoForum.org

BenefitsFor Operators

• Low cost solution

• Improve reliability

• Increases both coverage and capacity

• Reduce Macrocell backhaul capacity

requirements

• Works with all existing handsets no

need for expensive subsidizes on dual-

mode (3G + Wi-Fi)

• Increases 3G adoption

For Consumers

• Reduce cost. (Free calls at home!)

• No need for expensive new device.

• Superior indoor coverage and quality without change in phones.

• Simplicity: • One phone• One mode• One number.

Industry Activities

• Femto Forum (70 members)▫ Founded in 2007 to promote femtocell deployment▫ Members include mobile operators, telecom hardware, software

vendors, content providers, startups▫ Industrial partnerships with 3GPP, 3GPP2, GSM Assoc.

Manufacturer Partner/Operator Region Technology

Samsung(Ubicell) Sprint(Airave) North America a) IS-95, CDMA2000, 1xEV-DOb) WCDMA

AirWalk Communications Tatara Systems and Tango Networks

North America CDMA 1x RTT &1x-EVDO

Ericsson Europe GSM/3GPP UMTS

Airvana Nokia-Siemens 3GPP UMTS

Altacel-Lucent North America 3GPP UMTS

Axion Wireless PicoChip United Kingdom a) 3GPP UMTSb) WiMAX

Ubiquisys (Zonegate) Kineto Wireless, Google United Kingdom 3GPP UMTS/HSPA

Industry Activities• Ericsson, NEC, Samsung, Nokia-Siemems, Airvana, Qualcomm, et. al.

had launched 3Gfemtocellbase stations or femto-optimized handsets.

• FemtoForum has joint forces with Next Generation Mobile Network (NGMN) Alliance to look beyond the use of femtocells in 3G mobile networks.

• IEEE 802.16m and LTE-Advanced have taken the femtocell option into consideration.

• In WiMAX Forum, the Service Providers Working Group (SPWG) has started to draft system requirements for femtocells from perspective of network operators.

• Major WiMAX service providers such as Sprint-Nextel and KT (Korea

Telecom) are introducing femtocell into their networks.

Overview

• Introduction▫Evolution of cellular systems▫Need for Femtocells▫Benefits▫ Industry Activities

•Design Issues▫Architecture▫Usage Model

•Radio Issues▫Coverage▫Resource Allocation▫ Interference

•Summary

Architecture

• Current 2G/3G Network Architecture▫ RNC(Radio network controller):

Connected with tens to hundreds BS. RRM and handoff between BSs.

▫ MSC (Mobile switching center): Connected with several RNCs Roaming between RNCs.

▫ PSTN(Public switched telephone network)

▫ SGSN &GGSN(Serving GPRS support node & Gateway GPRS support node) Support mobile data services.

PSTN

Operator IP

Network

MSC

SGSN

RNC

GGSN

Macrocell Node B

Architecture

• With Femtocells▫ Home cell controller: interface

between IP internet and operators own IP Network

▫ Base station Router(BSR) concept▫ Flat:

Tens to thousands femtocells in a Macrocell

Current Architecture (e.g. RNC) may not be able to handle so many devices

Femtocell Architecture should be flat▫ IP Based: IP Backhaul provided by

ISP WiMAX Networks are suitable due to

flat all-IP network architecture

PSTN

Operator IP

Network

MSC

SGSN

RNC

GGSN

Macrocell Node B

Home cell

IP Intern

et

Home cell

controller/gatewa

y

Femtocell architecture reference model

Femto Acces

s Point (FAP)

Femto Gateway (FGW)

Mobile device

Femto Management

SystemFAP-MS FGW-MS

HPLMN Core Network

Subscriber

Databases

CS Core

PS Core

IMS Core

Femto AS

Fr

Fs

Fb-cs

Fb-ps

Fb-ims

Fa

Home GW

Broadband IP linkF

L

Fm Fg

Fas

HPLMN RAN

Radio i/f

• Open Access▫All users are allowed to connect to the FAP▫Hotspot type scenario: coffee shop or airport▫Femtocell becomes another part of the public mobile

network▫ Improves capacity, but increases handovers

• Closed Access▫Only authorized users are allowed to connect to a

privately accessible Femto-AP▫Home or enterprise environments▫Emergency calls bypass access restrictions

• Hybrid Access▫Nonsubscribers use only a limited amount of the

femtocell resources

Usage Model

OFDMA Femtocells

•OFDMA femtocells are more promising than CDMA ones

▫Inter-cell interference avoidance

▫Robustness to multipath

▫OFDMA FAPs can exploit channel variations in both frequency and time domains for avoidance of interference, while CDMA can only exploit the time domain

Overview

• Introduction▫Evolution of cellular systems▫Need for Femtocells▫Benefits▫ Industry Activities

•Design Issues▫Architecture▫Usage Model

•Radio Issues▫Coverage▫ Interference ▫Spectrum Allocation

•Summary

Radio Issues

• There are three respects in which radio issues differ for femtocells compared with macro base stations

Required coverage deliberately limited to the area of the house or small office associated with a given user group

Interference between macrocells and femtocells is controlled via entirely automated means rather than via manual planning

The cost of femtocells must be minimized, so radio specifications which drive excessive cost without significant performance benefits must be avoided

Pathloss model

• To analyze femtocell performance, it is important to study coverage

• Path loss model recommended by ITU known as ITU-R P.1238 is used▫ Assumes aggregate loss through furniture, internal walls and

doors represented by power loss exponent n that depends on type of building

Path Loss model is:

L50 ( r ) = 20 log (fc) +10n log r + Lf(nf) -28

n = Path loss exponentLf (nf)=Floor penetration

loss

Coverage

• Total Loss

• Coverage The maximum acceptable path loss required to deliver an adequate pilot

channel signal quality Ec/N0

LT=L50(r ) +LFM + LW

LFM = Shadow fade marginLW = Loss that represents outer wall of

building

Pmax =maximum power transmitted by the femtocell,NUE = user equipment receiver noise power

PCPICH = proportion of the femtocell power allocated to the pilot channel.

Coverage

-30 -25 -20 -15 -10 -5 0 5 10

20

40

60

80

100

120

140

160

180

200

Femtocell maximum transmit power (dBm)

Cov

erag

e ra

dius

(m

)

Coverage of femtocells

Coverage on same floor(m)

Coverage with 1 floor separation(m)

ParametersLFM = 7.7 dB

Lf(nf)=9 dB(for 1 floor)

Lf(nf)=19 dB(for 2 floors)

LW = 10 dB

Path loss exponent n=2

UE Noise figure = 7 dB

Fc = 1.5 GHz

Ec/N0 = -16 dB

PCPICH = 0.1

Interference Analysis

• Cross-layer Femto-Macro interference Aggressor (Femto Access

Point) and Victim(Macrocell user) belong to different network layers

• Co-layer Femto-Femto Interference Both aggressor(FAP) and

victim (neighboring femtocell user) belong to the same network layer

Femtocell

Macrocell

FemtocellFemtocel

l

Ways to overcome interference

•Use of Sectorial Antennas at the FAP

•Dynamic selection of pre-defined antenna patterns to reduce power leakage outdoors

•Hardware based approaches▫ increase in FAP cost

•Power control algorithms

•Efficient sub-channel allocation

Downlink Scenario

L2

26

L1

15

L4

4

L3

3

7L7

L5

L6

L1

L2

L3

L4

L5

L6

L7

OFDMA subchannels

Downlink Allocation of OFDMA subchannels in a macro/femtocells network with co-channel assignment

Spectrum Allocation

Subchannels allocation in OFDMA Femto/Macrocells networks

Co-channel assignment

Orthogonal channel

assignment

Spectrum allocation

Subchannels sharing

Static

Non-cooperative

Cooperative

Dynamic

DistributedCentralized

Dependent on the

geographic area

Depends on

traffic demand, user

mobility

Automatic frequency planning

Distributed

frequency planning

Random AccessFragmented spectrum access

Throughput

0 0.5 1 1.5 2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Normalized throughput per user (b/s/Hz)

Cum

ulat

ive

Dis

trib

utio

n fu

nctio

n

Throughput CDF per user

Femtocell

Macrocell

ParametersMacrocell Radius = 400m

Femtocell Radius =40m

Transmit/Receive Antennas=1

Transmit Power(Macro) = 43dBm

Lognormal Std.Dev.(Macro) = 8dB

Macrocell Path Loss(dB)=34 + 40 log(d)

Transmit Power(Femto) = 23dBm

Lognormal Std.Dev.(Neighboring Femto) = 12dB

Lognormal Std.Dev.(Indoor) = 4dB

Indoor Path Loss(dB)=37+30log(d)

Penetration Loss = 10dB

α =3dB, W=100Hz

Number of Femtocells=50Users per Femtocell =2

Users per Macrocell =100

Median throughput gain ~ 1.2 b/s/Hz

C = W log 2(1+ SIR/10α[dB]/10)

Cellular OFDMA system with 100 active users. • Single Macrocell serving all 100 users • 50 femtocells, with 2 active users in each femtocell

Overview

• Introduction▫Evolution of cellular systems▫Need for Femtocells▫Benefits▫ Industry Activities

•Design Issues▫Architecture▫Usage Model

•Radio Issues▫Coverage▫ Interference ▫Spectrum Allocation

•Summary

Summary

“The wireless capacity has doubled every 30 months over the last 104 years”

-Martin Cooper of Arraycomm

• This translates into an approximately million-fold capacity increase since 1957. ▫ 25x improvement from wider spectrum▫ 5x improvement by dividing the spectrum into smaller slices▫ 5x improvement by designing better modulation schemes▫ 1600x gain through reduced cell sizes and transmit distance.

• The enormous gains reaped from smaller cell sizes arise from efficient spatial reuse of spectrum, or alternatively, a higher area spectral efficiency

Application of class lecturesIn the Project From Lecture

Concept of Femtocells, cell size, effect on throughput

Cellular concept

Industry activities, R&D Status of Femtocells

Current and future Wireless Standards

Architecture of femtocells, Usage Model Cellular Architecture, Process of telephone call, Move towards flat architecture

OFDMA Femtocells Multi-access schemes : CDMA, TDMA, FDMA, OFDMA

Interference Analysis, Allocation of spectrum, Throughput analysis

Inter, intra interference through calculation of SIR, channel assignment

References

• Femto Forum, http://www.femtoforum.org/ • S.R. Saunders, S. Carlaw, A. Giustina, Femtocells: Opportunities and

Challenges for Business and Technology, John Wiley & Sons. Ltd, 2009 

• H. Claussen, L.T. Ho, L.G. Samuel, An overview of the femtocell concept, Bell Labs Technical Journal, vol. 13, no. 1, pp. 221-246, 2008

• V. Chandrasekhar, J.G. Andrews, A. Gatherer, Femtocell Networks: A Survey, IEEE Communications Magazine, vol. 46, no.9, pp. 59-67, Sept. 2008

• S.P. Yeh, S. Talwar, S.-C. Lee, H. Kim, WiMAX Femtocells: A Perspective on Network architecture, Capacity and Coverage, IEEE Communications Magazine, vol. 46, no. 10, pp. 58-65, Oct. 2008

• L. Perez, D. Valcarce, A.D. Roche, G.J. Zhang, OFDMA femtocells: A roadmap on interference avoidance, IEEE Communications Magazine, vol. 47, no. 9, pp. 41-48, Sept.2009 

• Holger Claussen, Performance of macro- and co-channel femtocells in a hierarchical cell structure, IEEE PIMRC, 2007

Thank You

EE 497AApril 21, 2010

Questions?