FEMTOCELL PERFORMANCE STUDIES FOR LTESupervisor: Prof. Jyri HämäläinenInstructor: M.Sc Zhong Zheng

A part of NETS2020 project

Ying Yang18.05.2011

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OutlineIntroduction and motivationFemtocell challenges and problem statementResearch approach & Assessment

methodologyLTE Fractional Power Control (FPC)Stochastic Frequency Domain Scheduler

(SFDS)Simulation resultsConclusion

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Introduction & Motivation3GPP LTE project aiming to achieve a high quality

of services in packet switching optimized system with higher data rates and cheaper infrastructure.

- (3G UMTS ↑5.8↓14 <-> LTE↑50↓100 Mbps)

3G network drawback – Indoor coverage shortage Femtocell can solve this bottleneck and fulfill the

requirement of LTE development- Small BS with limited power and coverage area- Suitable for indoor communication environment- Decrease macrocell traffic load dramatically (50% call, 70%SMS

takes place indoor) and provide higher data rate by direct connecting to Internet through broadband or DSL

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Femtocell Challenges & Problem StatementInterference to existing macro cells- Inter-macro-femtocell interference suppressionInterference between femtocellsIntegration to LTE core networkMarketing and product promotionService stabilityPrice attraction

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Research Approach & Assessment methodology Adjust FUE PC parameters and find an optimal set to

compensate interference from neighboring MUEs while keeping minimum degradation of MUE performance

Apply a simple distributed frequency-domain scheduling strategy (SFDS) in FBS to roughly separate macro and femto spectrum in a probabilistic manner

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- User Throughput – The total throughput per PRB for FUE and/or MUE in bps. All the user throughputs are considered in link level.

- X%-tile User Throughput – The throughput at the X%-tile point of the Cumulative Distribution Function (CDF) of the user throughput in bps, which is the key indicator for coverage and capacity performance

- User SINR and Transmission Power – The experienced SINR and distributions of FUE/MUE total transmission power in dBm

- Spectral Efficiency – The data transmission rate for FUE/MUE normalized by the transmission bandwidth in bps/Hz

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LTE Fractional Power Control (FPC)In LTE UL, UE transmit power is determined

by open-loop FPC[1] according to formula (1):

(1)

- maximum allowed transmit power for UE- : a user-specific parameter to control SINR target ranged

from -126 to 24 dBm with step size of 1 dB- M: number of allocated PRB to UE- a cell-specific path loss compensation factor with value 0 or

from rage 0.4 to 1.0 with step size of 0.1- L: propagation path loss estimated by UE

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LTE FPC Cont.UL SINR per PRB of user seen at MBS or FBS

are given by:

(2)

(3)Where , , denotes serving BS of user

• Simulation begins by exhaustive search for and values for FUE over the parameter spaces

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Stochastic Frequency-Domain Scheduler (SFDS)

(4)

(5)

- is the available PRBs in indoor band after jth allocation- is a bias parameter

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In DL, and PRBs are allocated for each MUE and FUE, respectively

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SFDS Cont.MBS can identify indoor MUEs that potentially

create/receive critical interferences towards/from FBSs in UL/DL because MBS keep monitoring the measurement report from served MUEs[2], thus identifying close-by FBS since they have unique IDs.

SFDS rely on the MBS broadcasting signaling message and FBS made scheduling decision locally

- Fast Signaling ()- Slow Signaling ()

In LTE UL, single carrier FDMA constraints the allocated PRBs must continues in frequency domain

MBS monitors MUEs location according to measurement report when MUE enters CSG femtocell, and signaling the indoor PRB load to nearby FBSs

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Simulation Scenario

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Simulation Parameters

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System Parameters

Carrier Frequency 2 GHz

Inter-site Distance 500m (3GPP Case 1)

Bandwidth 10 MHz, 48 PRBs for data, + 2 PRBs for signaling

Thermal Noise PSD -174 dBm/Hz

Shadowing

Shadow Fading Log-normal

Standard Deviation 4 dB (FBS-serving UE), 8 dB (other links)

Correlation Factor0.5 between sites

1 between sectors

MBS Parameters

Maximum Transmit Power 46 dBm

Maximum Antenna Gain 14 dBi

Noise Figure 7 dB

Antenna Pattern: where =25dB, =70 degrees

FBS Parameters

Maximum Transmit Power 20 dBm

Noise Figure 7 dB

Antenna Pattern Omni-directional

UE Parameters

Maximum Transmit Power 23 dBm

Maximum Antenna Gain 0 dBi

,,)(12min)( 2

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Channel Models

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MUE-MBSFUE-MBS FUE-MBS, same building

MUE-FBS

+FUE-BS, different buildings

+Where R is separation distance in meters, and are penetration losses due to inner wall between apartments and outer wall of the building, and are the number of penetrated walls and floors. The term accounts for the penetration loss within the apartment

RPLdB 10log6.373.15

lowdB LRPL 10log6.373.15

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indoorDdB qLndRPL )46.0)1(7)2((,210 3.187.0log2046.38

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)46.0)1/()2((,21010 3.187.0)log6.37log2046.38max( nnindoorDdB ndRRPL

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Optimal FUE PC parameters

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50%-tile SINR of FUE and MUE with

- decrease ( increase) -> average SINR for MUE deviates from FBS-free case due to increased ICI

- When all 10 dots are clearly

- Optimal PC parameter sets () max. FUE SINR while keeping minor MUE performance degradation

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UL Scheduling Gain with SFDS

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‘O’:= 4; ‘△’: =24; ‘▽’: =48

CDFs of FUE PRB throughput when and

- SFDS provides better FUE throughput than random PRB allocation (dash-dotted curves), especially when FBS traffic load is small (4)

- When , adjusting bias parameter can further improve the scheduling performance since it reaches higher probability in to avoid interference from indoor MUEs

- Slow signaling () results a few bps than fast signaling (), more suitable for scarce MBS signaling capacity

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UL Scheduling Gain with SFDS

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CDFs of FUE PRB throughput when and

‘O’:= 4; ‘△’: =24; ‘▽’: =48

- Similar simulation with 30 dB reduction compare with previous settings

- Same result- High-rate users

suffer significantly in terms of percentage

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FUE total transmission power comparison with 3 different settings:

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‘O’: dBmP f 630 8.0f ;‘△’: dBmP f 480 6.0f ;‘▽ ’: dBmP f 330

4.0f

- Smaller provides less mean power level, thus impacting the UE power consumption

- Selected PC sets ( and ) provide approximately 10 dB increase in PC dynamic range

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DL Scheduling Gain

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Solid curves: Fast signaling

Dashed curves: Slow signaling

50%-tile UE spectral efficiency by SFDS with fast and slow signaling

- When , non-bias, random PRB allocation when FBS has maximum freedom to allocate PRBs to FUE. FBS has smallest collision probability of PRB allocation with other FBS in DL -> highest FUE spectral efficiency

- ↑MUE performance↑ until when FBS avoid using , and FUE spectral efficiency decreases due to co-interference

- ↑ FUE and MUE spectral efficiency ↓ because FUE try to utilize the whole bandwidth, thus causing larger probabilities for PRB assignment between cells.

- Slow signaling provides few bps reduction especially with medium spectrum utilization (16 &24)

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ConclusionAn optimal PC configurations for FBS is

found by simulation which maximizes the FUE link quality while keeping minimum degradation to nearby MUEs

SFDS can roughly separates the spectrum usage for FUEs and MUEs in a probabilistic manner by performing biased resource selection in different part of the allocated spectrum, simulation results show significant performance gain when femtocell has low or moderate load

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THANK YOU!

Questions?

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References [1] C. U. Castellanos, D. L. Villa, C. Rosa, K. I. Pedersen, F.

D.Calabrese, P. H. Michaelsen, and J. Michel, “Performance of uplink fractional power control in UTRAN LTE,” in Proceedings of the IEEE Vehicular Technology Conference VTC′08, pp. 2517-2521, May 2008.

[2] 3GPP, TR 25.367, “Mobility procedures for Home Nod B(HNB), Overall description, Stage 2”, V9.4.0, June 2010.

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