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transcript
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?