Decentralized Femtocell Transmission Regulation in
Spectrum-Sharing Macro and Femto Networks
Xiaoli ChuKing’s College London, UK
OPTNet 2011, Sheffield, 14 September 2011
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Outline• Introduction
• Collocated spectrum-sharing macro and femto cells▫ Motivation
▫ Contribution
▫ System model
▫ Outage probability analysis
▫ Femtocell location and transmit power
• Simulation results▫ Analytical results verified by simulations
• Conclusion
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Technical challenges
Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2010–2015
• Current 2G and 3G networks will not be able to meet future mobile data traffic demands
• Most of the data traffic is performed indoors, where coverage is the worst
• As a result, vendors and operators are desperately looking for new solutions
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Solutions: Femtocells• Femtocells are low-power wireless access points (FAPs) that
operate in licensed spectrum to connect standard mobile devices to a mobile operator’s network using residential DSL or cable broadband connections [Source: Femto Forum].▫ Improve indoor coverage▫ Unload traffic from overburdened macrocells▫ Likely to be user-deployed
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Motivation• Spectrum-sharing macro and femto cells
▫ Benefits Spectrum-sharing allows for increased spectral efficiency and better spatial reuse
▫ Challenges Spectrum-sharing suffers from inter-cell interference and creates dead spots where UE QoS cannot be guaranteed.
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Contribution• Analysis of downlink (DL) outage probabilities (OPs)
▫ Closed-form macro and femto DL OP lower bounds embracing the randomness of transmit power employed by different interfering FAPs.
• Analysis includes both Rayleigh flat fading and shadowing ▫ Our work accounts for path loss, Rayleigh fading, lognormal (LN)
shadowing, and LN interfering FAP power, and allows different DL (SIR) targets and OP constraints for macro and femto cells.
• Decentralized resource allocation▫ Decentralized strategy to regulate FAP’s transmit power and
usage of radio resources to guarantee a satisfactory macro and femto DL coverage.
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• OFDMA downlink of collocated spectrum-sharing macrocell and closed-access femtocells
▫ A central MBS covers a disc area with radius rM
▫ Femtocells of radius rF are randomly distributed on R2 as a spatial Poisson point process (SPPP) with a density of F.
▫ NF femtocells per cell site on average
▫ UF indoor UEs per femtocell, each located on femtocell edge
▫ MBS transmit power PM,Tx is evenly distributed among RBs
▫ FAP transmit power PF,Tx is evenly distributed among RBs
▫ Each FAP transmits with a probability within an RB.
▫ Spatial intensity of co-channel FAPs is uF = F.
▫ Macro-to-macro interference and thermal noise are ignored.
System Model
MBS
rM
MUE
Femto coverage circle
Macro coveragecircle
FAP
rF
FUE
R2
PM,Tx
PF,Tx
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Channel Model• Path loss follows the IMT-2000 channel model
• fc is the carrier frequency in MHz, d is the distance of the link, and denotes the wall-penetration loss.
• Each frequency subchannel sees Rayleigh flat fading and lognormal shadowing
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Femtocell DL SIR• The received SIR of an indoor FUE at the femtocell edge
▫ PF = PF,TxGFAPGUE, PM = PM,TxGMBSGUE;
▫ DFM is the distance from the MBS to the FUE, DFFi is the distance from interfering FAP i to the FUE;
▫ HF, HFM and HFFi are unit-mean exponential channel power gains;
▫ QF ~ LN(F, 2F2), QFM ~ LN(FM, 2FM
2) and QFFi ~ LN(FF, 2FF2)
denote lognormal shadowing, = 0.1ln10;
▫ is the set of FAPs having access to the given RB, with intensity uF.
iiii DQHPDQHP
rQHPFFFM
F
FFFFFF1
FFFFMFMFM1
FMM
FFF1
FFFSIR
macro intef femto intef
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Femto Outage Probability• Outage probability of an indoor FUE w.r.t. the target SIR F
• For an indoor FUE at a distance dFM from the MBS
• Based on the stochastic geometry theory
FFM
FFFF
FM
FF
FFFFFF1
FFFFM
FFF ,SIRPPPSIRP
FF
I
S
I
S
DQHPI
S
iiii
N
n
M
mb
mn
bbamn
m
mmn
eba
eeuvw
dP
rPFdD
1 1~
2~2
F
~~22FF
2FM
2FFMF
FMFMF
FFFMFMFMFF
~2
exp1
~~,~~;SIRP
FFFMFMFF
FM
F
Prob of macro-to-femto interf. being strong enough to
create outage
Prob of femto-to-femto and macro-to-femto interf. causing outage
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Macrocell DL SIR• The received SIR of an outdoor MUE is
▫ DM is the distance from the MBS to the MUE, DMFi is the distance from FAP i to the MUE;
▫ HM and HMFi denote unit-mean exponential channel power gains;
▫ QM ~ LN(M, 2M2) and QMFi ~ LN(MF, 2MF
2) denote lognormal shadowing.
iiii DQHP
DQHPMF
M
MFMFMF1
MFF
MMM1
MMMSIR
femto intef
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Macro Outage Probability• Outage probability of an MUE w.r.t. the target SIR M
• For an MUE at a distance dM from the MBS
• Based on the stochastic geometry theory
MMFMFMF
1MFF
MMM MF
PSIRP
iiii DQHP
S
M
m
mm bu
vdD
1 MF
M
MF
MFMMMMM
2~22expexp1SIRP
2MF
2MF
MF
MF
2
MF
MFM
~2~2exp
MF
P
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Minimum MBS-to-FAP Distance
• P(SIRF < F) ≤ F and P(SIRM < M) ≤ M, where 0 ≤ F, M < 1
• P(SF/IFM < F|DFM = dFM) is a monotonically decreasing function of dFM.
• Minimum dFM required for P(SIRF < F|DFM = dFM) ≤ F
▫ = HFQF/(HFMQFM) approximately follows a LN
distribution
• Any UE located less than dFM,min from the MBS
should be associated with the macrocell.
FM
F
1
FFFM
2FM
2FFMFF
1FMF
minFM,
~~,~~;
rP
FPd
MBS
FAP
rF
FUE
dFM,min
rM
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FAP Transmit Power• Femtocells’ transmit power should be within the range
[PF,Tx,min, PF,Tx,max]
• PF,Tx,max is delimited by network standard.
• PF,Tx,min is chosen as the minimum PF,Tx that makes an FUE at the macrocell edge meet Pr(SF/IFM<F|DFM= rM) ≤F.
where is the inverse CDF of the LN RV evaluated at F.
2FM
2FFMFF
1MFMFAP
FFFMBSTxM,minTx,F, ~~,~~;FM
F
FrG
rGPP
2FM
2FFMFF
1 ~~,~~; F
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FAP Self-Regulation• FAP at a distance d (dFM,min ≤ d ≤ rM) from the MBS,
▫ For an RB, if P(LB)F,Tx(d) min{P(UB)
F,Tx(d), PF,Tx,max}, then the FAP can transmit in the RB with PF,Tx set in the range [P(LB)
F,Tx(d), min{P(UB)
F,Tx(d), PF,Tx,max}] for simultaneously meeting both the macro and femto DL OP constraints;
▫ otherwise, the FAP can only transmit in the RB with P(LB)F,Tx(d)
and at a reduced probability.
2
FM2FFMFF
1FMFAP
FFFMBSTxM,LBTxF, ~~,~~;FM
F
FdG
rGPdP
2MF
dBmmaxTx,F,dBmMUB
maxTx,F,dBmminTx,F,2
2MF
2
dBmmaxTx,F,2
dBmMUB
maxTx,F,2
MF
dBmmaxTx,F,dBmMUB
maxTx,F,
9
18exp
PrPP
PrPPrP
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Simulation Setup• FAPs and MUEs are randomly dropped within the macrocell
coverage, following two independent SPPPs.
Parameters Values Parameters Values
10 dB, 15 dB PM,Tx 43 dBm
M, FM 4 PF,Tx 23 dBm
F 3 GMBS 15 dBi
FF, MF 3.5 GFAP 2 dBi
M 8 dB GUE 0 dBi
F 4 dB rM 1000 m
FF 12 dB rF 30 m
MF, FM 10 dB UF 2
fc 2000 MHz M 5 dB
M, F 0.1 F 10 dB
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• Simulated DL OP vs. the distance from the MBS, when the femtocell regulation strategy is employed at each FAP.
Performance of Femto Self Reg
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Femto Self Reg• FAP transmit power and vs. the distance from the MBS, when using
the proposed femtocell regulation strategy.
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Conclusions• OFDMA downlink of collocated spectrum-sharing
macrocell and closed-access femtocells ▫ Closed-form analytical expressions for outage probabilities
▫ Analytical expression of minimum MBS-to-FAP distance
▫ Simulation results have verified the accuracy of analytical results.
• Interference caused by femtocells has to be limited by▫ regulating femtocell transmit power, which depends on the
distance from the MBS; or
▫ restricting the probability of each femtocell transmitting in each RB, which can be controlled in both frequency and time domains.
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Further Information• This research has been supported by the UK EPSRC Grants
EP/H020268/1, CASE/CNA/07/106, and the RCUK UK-China Science Bridges Project (EP/G042713/1): R&D on (B)4G Wireless Mobile Communications.
• Related publications and submissions: ▫ X. Chu, Y. Wu, D. López-Pérez and H. Wang, “Decentralized femtocell transmission
regulation in spectrum-sharing macro and femto networks,” IEEE VTC 2011-Fall, San Francisco, USA, Sep 2011.
▫ X. Chu, Y. Wu and H. Wang, “Outage probability analysis for collocated spectrum-sharing macrocell and femtocells,” IEEE ICC 2011, Kyoto, Japan, Jun 2011.
▫ X. Chu, Y. Wu, L. Benmesbah and W. K. Ling, “Resource allocation in hybrid macro/femto networks,” IEEE WCNC 2010 WS, Sydney, Australia, Apr 2010.
▫ X. Chu, Y. Wu, D. López-Pérez and X. Tao, “On providing downlink services in collocated spectrum-sharing macro and femto networks,” IEEE Trans. Wireless Commun., under review.
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