009 Interference Management in UMTS Femtocells

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Interference management in UMTS femtocells

Low-band

December 2013

009.06.02

SMALL CELL FORUM

RELEASE 6.0

Solving the HetNet puzzle

17 : 2 5

R U R A L & R E M O T E

U R B AN

E N T E R P R I S E

V I R T U AL I Z AT I O N

H O M E

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Report title: Interference management in UMTS femtocells Issue date: 01 December 2013Version: 009.06.02

Scope

This paper [3] provides detailed results of in-depth studies of interference between

femtocells and macrocells deployed in the UMTS ‘low’ bands around 850/900MHz [2].An accompanying study is also available for the UMTS ‘high’ bands around 2GHz. For ahigher level overview of the findings from both of these studies, we recommendreading our associated ‘topic brief’ [1].

Related SCF Publications

[1] “Topic brief: Interference Management in UMTS Femtocells”, Small Cell Forum,www.scf.io/doc/008

[2] “Interference Management in UMTS Femtocells ("High-band")”, Small Cell Forum,www.scf.io/doc/003

[3] “Interference Management in UMTS Femtocells” ("Low-band")”, Small Cell Forum,www.scf.io/doc/009


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Executive summary

Femtocells, by virtue of their simultaneous small size, low cost and high performance,

are a potentially industry-changing disruptive shift in technology for radio access incellular networks. Their small size means that the spectrum efficiency they can attainis much greater than that achievable using macrocells alone. Their low cost meansthey can be deployed as consumer equipment – reducing the capital load andoperating expenses of the host network. And their high performance means that allthis can be gained at no loss of service to the customer, and in many cases, owing tothe improved link budgets, improved service.

However, for these apparent benefits to translate into real advantage for networkoperator and consumer alike, we must answer serious questions about the interactionbetween the femtocell technology and the host macrocellular radio network into whichthey are deployed. If femtocells can only achieve their potential by disrupting themacro network, then they will be relegated to niche deployments, of little overall

relevance to next-generation networks. On the other hand, if the interactions betweenmacro and femto radio layers can be managed to the benefit of all, then theirproperties (in terms of lowered cost, improved spectrum efficiency and link budgetand general performance) can be fully realised, and femtocells will find themselves anessential component of all future radio access network designs.

So, what are these interactions? How can they be managed? What does that all meanfor the technology, to the operator and to the consumer? These are the questions thatthis paper is helping to answer. In doing so, it has deliberately maintained a tightfocus, according to the priorities of its authors. It is exclusively concerned with W-CDMA as an air interface technology (other teams within Small Cell Forum are lookingat other air interfaces). This paper is concerned primarily with the 850 MHz band in

the United States, but is equally applicable to the 900 MHz band in Europe andelsewhere. It should also be broadly applicable to similar bands (eg. 700 MHz).Another study has also been published with similar results for 2 GHz [2]. It isexclusively a theoretical treatment, using link level and system level simulations todraw its conclusions, although we expect to back these conclusions up in due coursewith trial campaign data. In view of the residential application that femtocells areaddressing, this paper is also concerned with femtocells operating with closed usergroups. Perhaps most importantly, this paper “stands on the shoulders of giants”,drawing on the great mass of study work that has already been undertaken by 3GPPRAN4 participants in analysing these issues, and referencing them for further reading.

The interacting components of the femto-enabled network include femtocells

themselves, which can be interacting in their downlinks with other nearby femtocellsand macrocells; macrocells, which interact with nearby femtocells; and users and userequipment (UEs), which, by virtue of intentional radio links to femtocells andmacrocells, may be causing unintentional interactions with both.

In approach, this paper has chosen to look at extreme cases, to complement as far aspossible the average – or typical – scenarios that RAN4 has already studied in 3GPP.In the main, the analysis has shown up internal contradictions in those extreme cases– meaning that they will never occur. For instance: analysing the case when the UE isoperating at full power in its uplink towards a femtocell is shown to occur only whenthe macrocell is nearby – in which case the macro downlink signal is so strong that theUE will never select the femtocell over the macrocell. This contradiction shows, forinstance, that the high noise rise that a UE could in principle cause will happily never


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occur. In other cases, the extreme cases are avoided by uplink powercapping, or byother techniques recommended in the paper.

With these extreme cases disarmed then, of the many potential interactions betweenUEs, femtocells and macrocells, the summary conclusion that we have reached, in

common with other studies, is that in order to be successful, femtocell technologymust manage three things:

• Femtocell downlink power – if femtocells transmit inappropriately loudly, thenthe cell may be large, but non-members of the closed user group willexperience a loss of service close to the femtocell. On the other hand, if thefemtocell transmits too softly, then non-group members will be unaffected,but the femtocell coverage area will be too small to give benefit to its users.

• Femtocell receiver gain – since UEs have a minimum transmit power belowwhich they cannot operate, and since they can approach the femtocell farmore closely than they can a normal macrocell, we must reduce the femtocellreceiver gain, so that nearby UEs do not overload it. This must be done

dynamically, so that distant UEs are not transmitting at high power, andcontributing to macro network noise rise on a permanent basis.

• UE uplink power – since UEs transmitting widely at high power can generateunacceptable noise rise interference in the macro network, we signal amaximum power to the UE (a power cap) to ensure that it hands off to themacro network in good time, rather than transmit at too high a power inclinging to the femtocell.

We have also shown that, with these issues addressed, the net effect of deployingfemtocells alongside a macro network is significantly to increase its capacity. Innumerical terms, and in terms of the simulated scenario, the available air interfacedata capacity is shown to increase by more than a hundredfold with the introduction of

femtocells.


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Contents

1. Femtocells, Femtocell Access Points and the Small Cell

Forum ...............................................................................1 1.1 What are Femtocell Access Points? ........................................ 1

1.2 What do femtocells offer? ..................................................... 1

1.3 What is the Small Cell Forum?............................................... 2

2. Introduction .....................................................................3

2.1 Objectives and Methods of this Paper ..................................... 3

3. Previous Work ..................................................................5

4. Simulation Scenarios and Definitions ...............................7

5. Abbreviations and Defined Terms ................................... 10

6. Scenario A: Macrocell Downlink Interference to theFemtocell UE Receiver .................................................... 12

6.1 Description ....................................................................... 12

6.2 Analysis ........................................................................... 12

6.3 Extended scenario: HSDPA coverage .................................... 15

6.4 Conclusions ...................................................................... 17

7. Scenario B: Macrocell UE Uplink Interference to theFemtocell Receiver ......................................................... 18

7.1 Description ....................................................................... 18 7.2 Analysis ........................................................................... 18

7.2.1 HSUPA ............................................................................. 21

7.3 Conclusions ...................................................................... 24

7.3.1 Customer (MUE) impact ..................................................... 25

7.3.2 Customer (FUE) Impact ...................................................... 25

7.3.3 Mitigation techniques ......................................................... 25

8. Scenario C: Femtocell Downlink Interference to theMacrocell UE Receiver .................................................... 26

8.1 Description ....................................................................... 26 8.2 Analysis ........................................................................... 28

8.3 Scenario analysis and conclusions........................................ 29

9. Scenario D: Femtocell Uplink Interference to theMacrocell NodeB Receiver .............................................. 31

9.1 Introduction ...................................................................... 31

9.2 Analysis of Scenario D - 12k2 Voice and HSUPA .................... 32

9.2.1 Assumptions ..................................................................... 32

9.2.2 Macro Node B Noise Rise .................................................... 34


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9.3 Conclusions ...................................................................... 36

9.4 Recommendations ............................................................. 36

10. Scenario E: Femtocell Downlink Interference to nearby

Femtocell UE Receiver. ................................................... 37 10.1 Description ....................................................................... 37

10.2 Capacity Analysis .............................................................. 38

10.3 Conclusions ...................................................................... 41

11. Scenario F: Femtocell UE Uplink Interference to NearbyFemtocell Receivers ........................................................ 42

11.1 Description ....................................................................... 42

11.2 Analysis ........................................................................... 42

11.2.1 Assumptions ..................................................................... 42

11.2.2 Analysis of Noise Rise received at the Victim AP .................... 43 11.3 Conclusions ...................................................................... 45

11.4 Recommendations ............................................................. 46

12. Scenario G: Macrocell Downlink Interference to anadjacent-channel Femtocell UE Receiver ........................ 47

12.1 Description ....................................................................... 47

12.2 Analysis ........................................................................... 47

12.2.1 Assumptions ..................................................................... 47

12.2.2 Simulation Analysis ............................................................ 48

12.2.3 Theoretical Analysis ........................................................... 48

12.3 Conclusions ...................................................................... 49

13. Scenario H: Macrocell UE Uplink Interference to theadjacent channel Femtocell Receiver .............................. 50

13.1 Description ....................................................................... 50

13.2 Analysis ........................................................................... 51

13.2.1 Parameter settings ............................................................ 51

13.2.2 Impact of MUE interference on AMR ..................................... 51

13.2.3 Impact of MUE interference on HSUPA ................................. 54 13.3 Conclusions ...................................................................... 57

13.4 Femto System Impact ........................................................ 58

13.5 Mitigation techniques ......................................................... 58

14. Scenario I: Femtocell Downlink Interference to theadjacent channel macrocell UE Receiver......................... 59

14.1 Description ....................................................................... 59

14.2 Analysis ........................................................................... 60

14.2.1 Parameter settings ............................................................ 60

14.2.2 Impact of Femtocell interference on AMR service ................... 61


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14.2.3 Impact of Femtocell interference on HSDPA .......................... 62

14.3 Conclusions ...................................................................... 65

14.4 Customer (MUE) Impact ..................................................... 65

14.5 Mitigation techniques ......................................................... 65 15. Scenario J: Femtocell UE Uplink Interference to the

adjacent channel Macrocell NodeB Receiver ................... 66

15.1 Introduction ...................................................................... 66

15.2 Analysis of Scenario J - 12k2 Voice and HSUPA ..................... 66

15.2.1 Assumptions ..................................................................... 66

15.2.2 Macro Node B Noise Rise .................................................... 69

15.3 Conclusions ...................................................................... 70

16. Downlink and Uplink Scenarios Modelling Power

Control Techniques for Interference Mitigation .............. 71 16.1 Modelling of Propagation loss .............................................. 71

16.2 HNB transmit power calibration for 850 MHz ......................... 71

16.3 Simulation results for Dense Urban Deployment .................... 72

16.3.1 Idle Cell Reselection Parameters .......................................... 72

16.3.2 Coverage Statistics at 850 MHz for Calibrated HNB TransmitPower .............................................................................. 73

16.3.3 Downlink Throughput Simulations ........................................ 74

16.3.4 Conclusions ...................................................................... 76

16.3.5 Uplink throughput simulations with adaptive attenuation ........ 76

16.3.6 Conclusions ...................................................................... 82

17. Summary of Findings ...................................................... 83

18. Overall Conclusions ........................................................ 93

19. Further Reading ............................................................. 94

19.1 Scenario A ........................................................................ 94

19.2 Scenario B ........................................................................ 94

19.3 Scenario C ........................................................................ 94

19.4 Scenario D ........................................................................ 94 19.5 Scenario E ........................................................................ 95

19.6 Scenario F ........................................................................ 95

19.7 Scenario G ........................................................................ 95

19.8 Scenario H ........................................................................ 95

19.9 Scenario I ......................................................................... 96

19.10 Scenario J ........................................................................ 96

19.11 Scenarios – Section 16 ....................................................... 96

20. Simulation Parameters and Path Loss Models ................ 97


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20.1 Simulation parameters ....................................................... 97

20.2 Path Loss Models ............................................................... 98

20.2.1 Okumura-Hata .................................................................. 98

20.2.2 ITU-R P.1238 .................................................................... 99 20.2.3 System Simulation (Section 16) Path Loss Models ................. 99

References .............................................................................. 101

Tables

Table 3-1 Scenarios ....................................................................................... 6

Table 4-1 Femtocell Deployments in Shared Spectrum ....................................... 7

Table 4-2 Femtocell Deployments in non-Shared Spectrum ................................ 8

Table 4-3 Scenario relationships ..................................................................... 9

Table 6-1 Macro Node B assumptions and transmit EIRP calculation ................... 13 Table 6-2 Link budget for the received power from macro Node B to UE ............. 13

Table 6-3 EIRP for the femtocell ..................................................................... 14

Table 6-4 Required Ec/No for voice connection ................................................ 15

Table 7-1 Assumptions for Scenario B ............................................................. 18

Table 7-2 MUE link budget at the femtocell receiver ......................................... 19

Table 7-3 FUE transmitter power requirements in order to hold a voice call ......... 19

Table 7-4 Maximum co-channel DL deadzone created by the femtocell for MUEs,based on [R4-070969] and assuming RSSI of -65dBm .......................21

Table 7-5 Link budget for HSUPA ................................................................... 22

Table 9-1 Macro Node B noise floor ................................................................ 32 Table 9-2 Femto UE TX power 1000 m from macro Node B ................................ 34

Table 9-3 Noise rise calculation for Scenario D (femto UE is transmitting at8.39dBm and 21dBm1000m from a macro Node B for a 12K2 serviceand 2Mbps HSUPA service) ............................................................. 35

Table 9-4 Macro UE Tx power 1,000m away from macro Node B receiver bywindow on a 12K2 voice and 2Mbps HSUPA data service ..................... 36

Table 11-1 Femtocell Sensitivity and Noise Rise at AP1 ....................................... 43

Table 12-1 Macrocell Downlink Interference to an adjacent channel Femtocell UEin this worst-case scenario .............................................................. 49

Table 13-1 Uplink radio link-budget for AMR 12.2 kbps RAB ................................ 53

Table 14-1 Maximum Macro NB – MUE separation for a given maximumFemtocell transmit power level, when the Femtocell – MUE separationis fixed at 5 m ............................................................................... 62

Table 14-2 UE receiver performance requirement (HSDPA), [TS25.101] ............... 64

Table 15-1 Macro Node B noise floor ................................................................ 68

Table 15-2 Femto UE TX power 1000 m from macro Node B ................................ 69

Table 15-3 Noise rise calculation for Scenario D1 (femto UE is transmitting at8.39dBm and 21dBm 1000m from a macro Node B for a 12K2 serviceand 2Mbps HSUPA service) ............................................................. 70

Table 16-1 Parameters for the co-channel idle cell reselection procedure .............. 72


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Table 16-2 Pilot acquisition statistics at 850 MHz for dense-urban model with 24active HNBs and calibrated HNB transmit power ................................ 73

Table 16-3 Coverage statistics for dense-urban model with 24 active HNBs andcalibrated HNB transmit power ........................................................ 74

Table 20-1 Recommended simulation parameters .............................................. 98

Figures

Figure 1-1 Typical femtocell deployment scenario. .............................................. 1

Figure 4-1 Small Cell Forum Scenarios A-J ........................................................ 9

Figure 6-1 Scenario A .................................................................................... 12

Figure 6-2 Received signal strengths at UE, from macrocell and femtocell. ........... 15

Figure 6-3 HSDPA throughput vs. UE to femtocell distance for various femtocellTx powers ..................................................................................... 16

Figure 7-1 Scenario B .................................................................................... 18

Figure 7-2 Interference Scenario B, voice call ................................................... 21 Figure 7-3 HSUPA simulation, Scenario B. E-DPDCH Ec/No compared to

throughput for RFC3....................................................................... 23

Figure 7-4 Throughput for HSUPA. 70% max bit rate for all FRCs ........................ 24

Figure 8-1 Illustration of the interference analysis for Scenario C ........................ 26

Figure 8-2 Path loss model ............................................................................. 27

Figure 8-3 TX power needed for 12.2 kbps for MUE (1000 metres away and 100metres away respectively) .............................................................. 28

Figure 8-4 MUE throughput with HSDPA for locations at 1,000 and 100 metresrespectively .................................................................................. 29

Figure 9-1 Interference Scenario D .................................................................. 31

Figure 10-1 Scenario E. Adjacent femto with UEs connected to each AP ................. 37

Figure 10-2 Apartments Plan – Flats layout ........................................................ 38

Figure 10-3 Macrocell location relative to the house where the femtos are located .. 39

Figure 10-4 Dedicated carrier: CDF of HSDPA throughput .................................... 40

Figure 10-5 Shared carrier: CDF of HSDPA throughput ........................................ 41

Figure 11-1 Illustration of the Interference Scenario F ......................................... 42

Figure 12-1 Illustration of the Interference Scenario G ........................................ 47

Figure 12-2 CPICH Ec/Io for Femto ................................................................... 48

Figure 13-1 Illustration of the interference Scenario H ......................................... 50

Figure 13-2 Minimum separation between Femtocell and MUE to avoid blocking,for a given MUE ............................................................................. 54

Figure 13-3 E-DPDCH Ec/No variation as a function of MUE transmit power level .... 55

Figure 13-4 Required average FUE transmit power level to meet HSUPAthroughput requirements. ............................................................... 56

Figure 13-5 E-DPDCH Ec/No variation as a function of MUE transmit power level .... 57

Figure 14-1 Illustration of the Interference Scenario I ......................................... 59

Figure 14-2 Macro Node B signal strength relative to the interfering femtocellsignal strength measured at the MUE, required for successfuldecoding of AMR ............................................................................ 61

Figure 14-3 Maximum MNB - MUE separation as a function of femtocell – MUE

separation, assuming AMR voice service ........................................... 62


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Figure 14-4 Maximum macrocell-MUE separation as a function of femtocell-MUEseparation, for reception of HSDPA .................................................. 64

Figure 15-1 Interference Scenario J ................................................................... 66

Figure 16-1 In variance of HNB calibrated Tx Power in the two frequencies ............ 74

Figure 16-2 DL user throughput distribution under different minimum powers,User Throughput Distributions, 10 MUEs, 24 HUEs ............................. 75

Figure 16-3 Magnified version of Figure 1-2 showing outage statistics ................... 76

Figure 16-4 HUE uplink throughput distribution ................................................... 78

Figure 16-5 MUE uplink throughput distribution .................................................. 79

Figure 16-6 Transmit power distribution............................................................. 80

Figure 16-7 Transmit power distribution............................................................. 81

Figure 16-8 UE uplink throughput distributions in 850 MHz. There are, in total, 34UEs per macrocell, of which 24 UEs migrate to MNB in the ‘No HNBs’case. HNB deployment increases the system capacity significantly. ...... 82


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Report title: Interference management in UMTS femtocells Issue date: 01 December 2013Version: 009.06.02 1

1. Femtocells, Femtocell Access Points and the Small Cell Forum

1.1 What are Femtocell Access Points?

Femtocell Access Points (FAPs) are low-power radio access points, providing wirelessvoice and broadband services to customers primarily in the home environment. TheFAP provides cellular access in the home and connects this to the operator’s networkthrough the customer’s own broadband connection to the Internet.

FAPs usually have an output power less than 0.1 Watt, similar to other wireless homenetwork equipment, and they allow a small number (typically less than 10) ofsimultaneous calls and data sessions at any time. By making the access points smalland low-power, they can be deployed far more densely than macrocells (for instance,one per household). The high density of deployment means that the femtocellspectrum is re-used over and over again, far more often than the re-use that themacro network (with its comparatively large cells) can achieve. Trying to reach the

same levels of re-use with macrocellular technology would be prohibitively expensivein equipment and site acquisition costs. By using femtocells, the re-use, spectrumefficiency, and therefore the aggregate capacity of the network can be greatlyincreased at a fraction of the macrocellular cost.

A typical deployment scenario is shown in Figure 1-1.

Figure 1-1 Typical femtocell deployment scenario.

1.2 What do femtocells offer?

Zero-touch installation by end user: femtocells are installed by the end user withoutintervention from the operator. The devices will automatically configure themselves tothe network, typically using ‘Network Listen’ capabilities to select settings thatminimise interference with the macro network.

Moveability: The end user may move their femtocells – for example, to another room,or, subject to operator consent, to another location entirely.


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Backhaul via the end user’s fixed broadband connection: Femtocells will use thesubscriber’s broadband connection for backhaul, which typically will be shared withother devices in the home.

Access control – the “Closed User Group”: The operator and/or end user will be able to

control which mobile devices can access the femtocell. For example, subscribers maybe able to add guest phone numbers via a web page.

Supports a restricted number of simultaneous users: Femtocells will support a limitednumber (typically, fewerthan ten) of simultaneous calls and data sessions.

Femtozone (homezone) tariffs: Mobile services accessed through the femtocell may beoffered at a cheaper rate than the same services on the macro network. End users areadvised when services are accessed via the femtocell, either by an advisory tone, or adisplay icon or some other means, so they know when the femto-tariffs apply.

Ownership: Various ownership models are possible – for example, end users may own

their femtocells, just as they own their mobile phones, or the operator may retainownership, with end users renting the equipment (like a cable modem).

Small cell size/millions of cells in the network: The femtocell network can easilyextend to millions of devices.

Femto as a service platform: Novel mobile services can be made available on thefemtocell. For example, a femtocell-aware application on the mobile handset couldautomatically upload photos to a website when the user enters the home, anddownload podcasts.

1.3 What is the Small Cell Forum?

The Small Cell Forum is the only organisation devoted to promoting small celltechnology worldwide. It is a not-for-profit membership organisation, withmembership open to providers of small cell technology and to operators with spectrumlicences for providing mobile services. The Forum is international, representing around140 members from three continents and all parts of the femtocell industry, including:

• major operators,• major infrastructure vendors,•

specialist femtocell vendors, and• vendors of components, subsystems, silicon and software necessary to create

femtocells.

The Small Cell Forum has three main aims:

• to promote adoption of femtocells by making information available to theindustry and the general public,

•

to promote the rapid creation of appropriate open standards andinteroperability for femtocells, and

• to encourage the development of an active ecosystem of femtocell providers,to deliver ongoing innovation of commercially and technically efficientsolutions.

The Small Cell Forum is technology agnostic and independent. It is not a standardssetting body, but works with standards organisations and regulators worldwide toprovide an aggregated view of the small cell market. A full current list of Small CellForum members and further information is available at www.smallcellforum.org.

http://www.smallcellforum.org/http://www.smallcellforum.org/


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2. Introduction

2.1 Objectives and Methods of this Paper

The benefits of femtocells are not straightforward to realise. While network operatorswill see significant capacity gains, and end users can expect higher performance, toachieve this the radio layer must be carefully managed. The management of the radiointerference between the Macro and Femto Layers is a key industry concern addressedby this paper.

Interference adversely affects the capacity of a radio system and the quality of theindividual communication links on that system. Adding capacity is always based on atrade-off between interference, quality and capacity. Hence, there is a need forinterference management techniques to minimise interference that might otherwisecounteract the capacity gains and degrade the quality of the network.

1. The principal objectives of this study are:

• To develop an industry position on the interference risks from femtocelldeployments.

• To recommend mitigation techniques and any necessary associated RFparameters and performance requirements, to ensure minimal disruptionto the macro network or other femtocells.

2. To achieve these objectives, this paper develops detailed interferencescenarios for evaluation and inclusion in the interference managementassessment. The scenarios will cover worst-case deployment conditions andassess the respective system impact.

3. An immediate focus is to develop the assessment for W-CDMA, and in doing

so devise a process that should be consistent with alternative radiotechnologies.

4.

Two main steps were identified in order to accomplish the above goal:

• First, a baseline set of interference analysis conclusions for UMTSfemtocells, based on 3GPP RAN4 interference studies, was required. Thiswould be supplemented with specific analysis of identified microscenarios, their likelihood, and potential impact. Interference mitigationtechniques should also be considered on the understanding that vendorindependence be preserved wherever possible.

• Secondly, a recommendation for a common set of behaviours (RFparameters and/or test cases) that can be derived by any UMTS

femtocell was required. This is so that the femtocell can configure itselffor minimal disruption to either the macrocell layer or other deployedfemtocells.

5. 5. We focus exclusively on the Closed User Group model. This is the mostlikely residential deployment model, and restricts the pool of allowed users toa small group authorised by the operator or the owner of the femtocell. Non-authorised subscribers may suffer coverage and service impairment in thevicinity of a closed-access femtocell (the so-called “deadzone”), which isimportant to assess.

6. The study will also investigate methods of controlling the impact of deployinglarge numbers of femtocells on the macro network. For example, differentscrambling codes and adaptive power controls may be used to manage theinterference in the network.


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7. This paper has limited itself in scope, according to perceived priorities, asfollows:

• It is exclusively concerned with W-CDMA as an air interface technology(other teams within Small Cell Forum are looking at other air interfaces).

•

It is concerned primarily with the 850 MHz band in the United States, butis equally applicable to the 900 MHz band in Europe and elsewhere. Itshould also be broadly applicable to similar bands (eg. 700 MHz).

• It is exclusively a theoretical treatment, using link level and system levelsimulations to draw its conclusions, although we expect to back up theseconclusions in due course with experiment.

8. The femtocells have been modelled in terms of three power classes (10dBm,15dBm, 21dBm) or (10mW, 30mW, 125mW), although not all cases examineall three classes.

9.

In approach, this paper has chosen to look at extreme cases of generalindustry concern, to complement as far as possible the RAN4 scenarios

already studied in 3GPP. In the main, the analysis has shown up internalcontradictions in those extreme cases – meaning that they will never occur inpractice. Such contradictory analyses are then followed up with lessextreme, more realistic scenarios, where the interference effects and theirmitigation can be modelled and analysed.


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3. Previous Work

Analysis in this problem space has already been carried out as part of the 3GPP HomeNode B study item.

3GPP RAN4 concluded their study into the radio interface feasibility of Home Node B(aka femtocells) at RAN#39 in March 2008. Their results are presented in [TR25.820].Part of their study included the analysis of anticipated interference scenarios coveringa range of HNB deployments. A summary of their findings is presented in Table 4-1below.

The scenarios for this paper are defined in Section 4.

Scenario(thispaper)

25.820scenarioid

Summary of RAN4 conclusions

A 4 Macrocell DL interference can generally be overcome, as long as thefemtocell has sufficient transmit dynamic range.

B 3 The femtocell receiver must reach a compromise between protectingitself against uncoordinated interference from the macro UEs, andcontrolling the interference caused by its own UEs towards the MacroLayer. Adaptive uplink attenuation can improve performance, butconsideration must also be given to other system issues like theassociated reduction in UE battery life.

C 2 Downlink interference from a closed-access femtocell will result incoverage holes in the macro network. In co-channel deployments thecoverage holes are considerably more significant than when thefemtocell is deployed on a dedicated carrier. A number of models arepresented for controlling maximum femtocell transmission power, but itis acknowledged that no single mechanism alone provides a definitivesolution. Open access deployment should also be considered as amitigating option.

D 1 Noise rise on the Macro Layer will significantly reduce macroperformance; consequently, the transmit power of the femto UE shouldbe controlled. A number of mechanisms to achieve this are presented,generally providing a compromise between macro and femtocellperformance. Again, open access deployment should be seen as amitigating option in the co-channel case.

E 6 This scenario has received less coverage than the macro interference

cases, but it is noted that the performance of Closed Subscriber Group(CSG) femtocells is significantly degraded unless interference mitigationtechniques are used. This is generally a similar problem to macro DLinterference in the co-channel scenario.

F 5 It is difficult to avoid co-channel interference between CSG femtocells,and this limits the interference reductions achieved by deploying thefemtocell on a separate carrier from the macro network. Again,interference management techniques are required to manage femto-to-femto interference.

G 4 Macrocell DL interference can generally be overcome, as long as thefemtocell has sufficient transmit dynamic range.


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Scenario(thispaper)

25.820scenarioid

Summary of RAN4 conclusions

H 3 The femtocell receiver must reach a compromise between protecting

itself against uncoordinated interference from the macro UEs, andcontrolling the interference caused by its own UEs towards the MacroLayer. This is generally an easier compromise to arrive at with adjacent-channel deployments than it is with co-channel.

I 2 Downlink interference from a closed-access femtocell will result incoverage holes in the macro network. In adjacent-channel deploymentsthe coverage holes are considerably easier to minimise and control thanwhen the femtocell is deployed on the same carrier as the Macro Layer.A number of models are presented for controlling maximum femtocelltransmission power; all except the “fixed maximum power” approachare generally acceptable.

J 1 Noise rise on the Macro Layer will significantly reduce macroperformance; consequently, the transmit power of the Femto UE shouldbe controlled. A number of mechanisms to achieve this are presented,generally providing a compromise between macro and femtocellperformance. Adjacent-channel deployments can generally beaccommodated.

Table 3-1 Scenarios

In addition to the previous 3GPP analysis work, the Small Cell Forum conducted anearlier study covering the same scenarios at 2 GHz [FF08]. For this study at 850 MHz,several changes were made to the simulation parameters used in that earlier 2 GHzstudy:

• Wall loss was reduced from 20 to 10dB, to reflect greater building penetrationat 850 MHz.

• Macro basestation antenna height was increased from 25 to 30 metres, toreflect the higher antenna heights (larger cell size) typical in North Americandeployments.

• The minimum distance from a macro basestation was increased from 30 to1,000 meters, to again reflect typical North American deployment scenarioswhere cells are larger and basestations are not typically located in residentialareas. This also allowed us to eliminate the use of the ITU P.1411propagation model, and to use the Okumura-Hata model, simplifying theanalysis work.


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4. Simulation Scenarios and Definitions

The Small Cell Forum has identified 10 stretch scenarios that explore the limits ofoperation of femtocells and femtocell subscriber equipment.

The scenarios are summarised in the following tables and figure.

Scenario Description

Macrocell Downlink Interferenceto the Femtocell UE Receiver (A)

A femtocell UE receiver, located on a table next to theapartment window, is in the direct bore sight of a macrocell(1 km distance). The macrocell becomes fully loaded, whilea femtocell UE is connected to the femtocell at the edge ofits range.

Macrocell Uplink Interference tothe

Femtocell Receiver (B)

A femtocell is located on a table within the apartment.Weak coverage of the macro network is obtainedthroughout the apartment. A user UE1 (that does not have

access to the femtocell) is located next to the femtocelland has a call established at full power from the UE1device. Another device UE2 has an ongoing call at the edgeof femtocell coverage.

Femtocell Downlink Interferenceto the Macrocell UE Receiver (C)

UE1 is connected to the macro network at the edge ofmacro coverage. It is also located in the same room as afemtocell (to which it is not allowed to access). Thefemtocell is fully loaded in the downlink.

Femtocell Uplink Interference tothe

Macrocell Node B Receiver (D)

UE1 is located next to the apartment window, in direct boresight of a macrocell (1 km distance). UE1 is connected tothe femtocell at the edge of its range, and is transmittingat full power.

Femtocell Downlink Interferenceto

Nearby Femtocell UE Receivers(E)

Two apartments are adjacent to each other. Femtocells(AP1 and AP2) are located one within each apartment. Theowner of AP2 visits their neighbour’s apartment, and is onthe edge of coverage of their own femtocell (AP2) but veryclose (


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Scenario Description

Macrocell Downlink Interferenceto the adjacent-channel FemtocellUE Receiver (G)

A femtocell UE is located on a table next to the apartmentwindow, in direct bore sight of a macrocell (1 km distance).The macrocell becomes fully loaded, while a femtocell UE is

connected to the femtocell at the edge of its range.

Macrocell Uplink Interference tothe adjacent-channel FemtocellReceiver (H)

A femtocell is located on a table within the apartment.Weak coverage of the macro network is obtainedthroughout the apartment. A user (that does not haveaccess to the femtocell) is located next to the femtocell andhas a call established at full power from the UE1 device.Another device UE2 has an ongoing call at the edge offemtocell coverage.

Femtocell Downlink Interferenceto the adjacent-channel MacrocellUE Receiver (I)

Two users (UE1 and UE2) are within an apartment. UE1 isconnected to a femtocell at the edge of coverage. UE2 isconnected to the macrocell at the edge of coverage, and

located next to the femtocell transmitting at full power.

Femtocell Uplink Interference tothe adjacent-channel MacrocellNodeB Receiver (J)

A femtocell is located in an apartment, in direct bore sightof a macrocell (1 km distance). UE1 is connected to thefemtocell at the edge of coverage, but next to the widow –thus, in the direct bore sight of the macrocell antenna.

Table 4-2 Femtocell Deployments in non-Shared Spectrum

In addition to these extreme scenarios, we include shared-spectrum system levelsimulations specifically modelling the mitigation of downlink interference and uplinknoise rise by power control techniques (Section 15). These simulations also model the

effect of femtocells on the total throughput and capacity of the network.

The relationship between these scenarios and those already studied in RAN4 issummarised in the following table and figure.


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Victim

Femto UE

DL Rx

Femto AP

UL Rx

Macro UE

DL Rx

Macro

NodeB UL

Rx

Neighbour

Femto UE

DL Rx

A g g r e s s o r

Macro NodeB

DL Tx

A, G

4

Macro UE

UL Tx

B, H

3

Femto AP

DL Tx

C, I

2

E

6

Femto UE

UL Tx

D, J

1

Neighbour Femto UE

UL Tx

F

5

Table 4-3 Scenario relationships

A…F are the interference scenarios for co-channel deployments

G…J are the interference scenarios for adjacent-channel deployments

1…6 are the equivalent interference scenario IDs used in the 3GPP HNB analyses

[TR25.820]

The following diagram illustrates and summarises the Small Cell Forum Scenarios A-J:

Figure 4-1 Small Cell Forum Scenarios A-J


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5. Abbreviations and Defined Terms

Throughout this paper a number of abbreviations are used to identify various systemelements and parameters. The most frequently used are presented here for quick

reference. However, a more extensive list has been produced and is available underseparate cover.

AP Access Point

BER Bit Error Rate (or Bit Error Ratio) – the proportion of the total number ofbits received that are decoded wrongly

BS Base Station (assumed to be a wide-area BS, as defined in [TS25.104],unless otherwise stated)

EIRP Equivalent Isotropic Radiated Power – a measure of the transmitted powerin a particular direction that takes account of the antenna gain in that

direction

FAP Femto AP, also known as the femtocell

FUE Femto UE, also called the Home UE (HUE)

HUE Home UE, also called the femto UE (FUE)

HNB Home NodeB

MNB Macro NodeB

MUE Macro UE

QoS Quality of Service

UE User Equipment (handset, data terminal or other device)

RAN Radio Access Network

RAT Radio Access Technology

RSCP Received Signal Code Power

RTWP Received Total Wideband Power

LOS Line-Of-Sight

P-CPICH Primary Common Pilot Channel

Victim Is a radio node (macro node-B, or femto access point) whose receiverperformance is compromised by interference from one or more other radionodes (the Aggressor). Alternatively, the Victim may be a radio link, whosequality is degraded by unwanted interference from Aggressor nodes

Aggressor Is a radio node (either macro node-B, femto access point or UE) whosetransmissions are compromising the performance of another radio node (theVictim), or which are contributing to the degradation of quality of a (Victim)

radio link


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Deadzone Is an area where the quality of service is so poor as a result of interferencethat it is not possible to provide the demanded service. Deadzones are alsocharacterised by the fact that in the absence of any interference, a normalservice would be possible.

Deadzones are often specified in terms of the path loss to the Aggressor transmitter. A60dB deadzone in the femtocell is, therefore, a region around the femtocell where thepath loss to the FAP is less than 60dB.


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6. Scenario A: Macrocell Downlink Interference to the FemtocellUE Receiver

6.1 Description

A UE is located on a table next to the apartment window that is 1km distance awayfrom a macrocell. The macrocell is operating at 50% load, while the UE is connected tothe femtocell (ie. FUE) at the edge of its range. In this scenario the Victim link is thedownlink from the femtocell to the FUE, while the Aggressor transmitter is thedownlink from the macrocell. This interpretation of Scenario A is summarised in Figure6-1.

Figure 6-1 Scenario A

6.2 Analysis

The objective of the analysis of this scenario is to work out the services that can bedelivered to a femto UE when it is on the edge of the femtocell – the femtocell itselfbeing positioned, as required by the scenario, 1km from the macro. The analysisstrategy for this scenario is broken down as follows:

The first task is to determine the range of the femtocell as defined by the pilot power.This gives us the maximum range at which the UE can detect and decode the femto

beacon, and therefore camp on to it. Secondly, we work out the services that can beoffered by the femtocell at the edge of its coverage, given that interference level. Thefirst step is accomplished by the following sequence:

• Assume a given P-CPICH transmit power for both macro and femto; then• find the power due to the macro at the distance given by the scenario (1km);

then•

find the distance from the femto at which the ratio of femto power to macropower is sufficient for the UE to detect the femtocell. This distance is therange of the femtocell as defined by the pilot power – the maximum range atwhich a UE can detect the femtocell and camp on to it.


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The second step (to work out the services that can be offered at this range) isaccomplished as follows:

• For voice, work out how much dedicated channel power is required to sustaina voice call, given the interference level calculated in the first step, and

reconcile that with the total amount of power available to give the number ofvoice calls that may be sustained.

• For data, work out the Ec/Io that can be achieved by allocating all theremaining power to the HSDPA downlink shared channel, and derive athroughput from that, given an industry standard relationship between Ec/Ioand throughput.

Assumptions for the macrocell are as defined in [FF09] with variant values shown inTable 6-1, which shows the transmit EIRP of the macrocell. The link budget for themacrocell is defined in Table 6-2.

Value Units Comments

Macro Node B utilisation as percentage of totalpower

50 %

Macro Node B maximum Tx power 43 dBm Ptx_max

Macro Node B Tx power 40 dBm Ptx_m= Ptx_max +10*log(0.5)

Antenna gain 17 dBi Gm

Feeders and cable losses 3 dB Lc

Tx EIRP 54 dBm EIRP_m=Ptx_m+Gm-Lc

Table 6-1 Macro Node B assumptions and transmit EIRP calculation

Value Unit Comments

Distancemacro nodeBto UE

1000 m d_mu

Height macronodeB antenna

30 m hb

Height UE fromground

1.5 m hM

Path loss 125.75 dB PL_m is calculated from the Okumura-Hata Model, + 5dBwindow loss

UE antennagain

0 dBi Gue

UE connectorand body

losses

3 dBi Lc_u

Macro nodeBreceived powerat UE

-79.75 dBm Prx_m=eirp_m-PL_m+Gue-Lc_u

Table 6-2 Link budget for the received power from macro Node B to UE

The value Prx_m in Table 6-2 is the power due to the macrocell at the scenariodistance (1km), and takes account of the propagation, plus an allowance for thewindow loss (5dB).

The femtocell assumptions are presented in Table 6-3. Note that three types offemtocell are assumed with the defined femto transmit power classes (10dBm, 15dBm

and 21dBm).


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Value Unit Comments

Femtocell max transmit power 10 dBm Ptx_f for the three power classes modelled

15

21

Femtocell antenna gain 0 dBi Gf (same as UE)Femtocell feeders/connectorlosses

1 dB Lc_f

Maximum transmit EIRP 9 dBm eirp_f=Ptx_f+Gf-Lc_f, for the three powerclasses modelled14

20

P-CPICH power relative tomaximum power

10 % pcp_pctage

P-CPICH transmit EIRP -1 dBm Eirp_pcp_f = eirp_f * pcp_pctage

4

10

Table 6-3 EIRP for the femtocell

In order to complete the calculation of position of the cell edge according to P-CPICH,we calculate the P- CPICH power at the UE and compare it to the power at the UE dueto the macrocell. Note that in this scenario we are fixing the UE at the window andmoving the femtocell location – so the macrocell power is constant at the valuecalculated in Table 6-2. We use the indoor propagation model ITU-R P.1238, assuminga residential building and same floor operation, the femtocell characteristics fromTable 6-2 as well as the same UE characteristics as in Table 6-2. Figure 6-2 shows thefemtocell P-CPICH power received at the UE, and the power at the UE from themacrocell as taken from Table 6-2.

In order for the FUE to detect the femtocell and camp onto it, the P-CPICH Ec/No must

be sufficient. It is assumed that a level of -18 dB will be adequate in this respect. Tofind the range of the femtocell we need to find the distance below which the P-CPICHpower is less than 18 dB below the power from the macrocell. By observing in Figure7-2 where the P-CPICH power exceeds the bounds on the macro interference powerminus 18 dB, it can be seen that even at the 10 dBm transmit power, the FAP has arange of more than 100 m. It is to be noted that this does not necessarily mean that aUE 100m away from the FAP will select the FAP in idle mode. Rather, it means that ifthe UE is already connected to this FAP, it can still sustain the connection at thisdistance


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Figure 6-2 Received signal strengths at UE, from macrocell and femtocell.

Further, it can be seen that, based on Table 6-4, voice services are readily achievable

at the edge of coverage, since they require about the same Ec/No as the minimumCPICH Ec/No assumed above.

Value Unit Comments

Chiprate 3.84e6 cps W

Bitrate of AMR voice call 12.2 kbps R

Eb/No requirement for voiceconnection

+7 dB Eb/No

Ec/No requirement for voiceconnection

-18 dB Ec/Io=Eb/No-10*log10(W/R)

Table 6-4 Required Ec/No for voice connection

Similarly for HSDPA, assuming that 80% of the femtocell power is reserved for HSDPAservices (9dB above P- CPICH), the HSDPA Ec/No will be at least -1.8 dB (@ 100mfrom HNB), which corresponds to > 1.5 Mbps, according to the translation equation in[R4-080149].

6.3 Extended scenario: HSDPA coverage

The HSDPA throughput at the UE as a function of the distance between the HNB andthe window is analysed by employing the rate mapping equation presented inreference [R4-080149]. The HSDPA max data rate is presented as a function ofaverage HS-DSCH SINR.

In this work, SINR is calculated using the formula in [Hol06]:


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Equation 6-1

where:

• SF16 is the spreading factor,• PHS-DSCH is the received power of the HS-DSCH, summing over all active

HS-PDSCH codes,• Pown is the received own-cell interference,•

α is the downlink orthogonality factor (assumed to be 1, fully orthogonal), • Pother is the received other-cell interference,• Pnoise is the received noise power (here it is assumed that the UE Noise

figure is 7dB).

Assuming:

• The femtocell transmit powers are 10dBm, 15 dBm and 21 dBm, with 80%allocated to HS-DSCH

•

And employing the path loss assumptions of the previous section• The UE is still assumed to be 1 km away from the macrocell.

The HSDPA throughput for the FUE at different distances from the femtocell is shownin Figure 6-3.

Figure 6-3 HSDPA throughput vs. UE to femtocell distance for various femtocell Tx powers


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It can be seen from Figure 6-3 that the maximum HSDPA throughput can be expectedup to 25 m away from the femto, even at the 10 dBm transmit power.

6.4 Conclusions

The scenario that has been analysed in this section examines the case of the UE beinglocated in front of a window overlooking a macrocell that is 1 km away. Assumingstandard models and parameters, it is shown that, even at 10 dBm transmit power,the femtocell is able to comfortably provide voice to the UE when the femtocell islocated as far as 100 m away, and maximum HSDPA throughput can be expected upto 25 m away.


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7. Scenario B: Macrocell UE Uplink Interference to the FemtocellReceiver

7.1 Description

A femtocell is located on a table within the apartment. Weak coverage of the macronetwork is obtained throughout the apartment. A user that does not have access tothe femtocell (MUE) is located next to the femtocell. Another user device (FUE) isconnected to the femtocell and has an ongoing call at the edge of femtocell coverage.The scenario is depicted in Figure 7-1. In this case the Victim receiver belongs to thefemtocell access point (FAP), and the Aggressor transmitter is that of the nearby MUE.

Figure 7-1 Scenario B

7.2

Analysis

The general assumptions for the analysis of this scenario are presented in Figure 7-1. The link budget for the MUE is shown in Table 7-2; note that three separationdistances between the MUE and the femtocell are taken into account (5, 10 and 15m).

Value Unit Comments

Voice call service rate 12.2 kbps R

Chip rate 3.84 Mbps W

Processing gain 24.98 dB PG=10*log10(W/R)

Required Eb/No for voicecall

8.3 dB Eb/No (performance requirement in[TS25.104] for AWGN channel, no

diversity)

Frequency 850 MHz Fc (Band V)

Table 7-1 Assumptions for Scenario B


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Value Unit Comments

MUE uplink transmittedpower

21 dBm Ptx_mue (power class 4)

UE antenna gain 0 dBi Gue

Connectors/body loss 3 dB Lue

MUE Tx EIRP 18 dBm eirp_mue=Ptx_mue+Gue-Lue

Distance MUE-femtocell 5, 10, 15 m d_mue

MUE-femtocell path loss 50.16 (@5m)

58.59(@10m)

63.52 (@15m)

dB PL_mue, Indoor to indoor pathloss model , where d=d_mue,f=fc

Femtocell antenna gain 0 dBi Gf

Femtocell feeders/connectorlosses

1 dB Lf

Uplink power received bythe femtocell from MUE atdifferent MUE-femtocellseparation distances

-33.16(@5m)

-41.59(@10m)

-46.52(@15m)

dBm Prx_mue=eirp_mue-PL_mue+Gf- Lf

Table 7-2 MUE link budget at the femtocell receiver

In Table 7-3, the FUE's minimum transmitted power requirement for holding a voicecall is calculated. Note that the power is well within the FUE's capabilities, even at thelargest separation distance.

Value Units Comments

Distance between FUEand femtocell

15 m d_fue

Path loss 63.51 dB PL_fue

Indoor to indoor path loss model

(d=d_fue, f=fc)Eb/N0 requirementsfor a voice call

8.3 dB Eb/No_fue [TS25.104]

Processing Gain 24.98 dB PG_fue

Noise power -103 dBm PN from [TS25.942]

FUE received power inorder to obtainrequired Eb/N0 fordifferent MUEdistances (d_mue)

-49.84 (@5m)

-58.27(@10m)

-63.20 (@15m)

dBm Prx_fue is calculated from equation[Hol06]:

FUE transmittedpower requirements

for different MUEdistances (d_mue)

17.68 (@5m)

9.25 (@10m)

4.32 (@15m)

dBm Ptx_fue=Prx_fue-Gue+Lue+PL_fue-Gf+Lf

Table 7-3 FUE transmitter power requirements in order to hold a voice call

The values calculated in Table 7-3 for the transmitted power of the FUE required arethe same as the one calculated for the 1900Mhz study. The reason for this is that thereduction on frequency affects both FUE and MUE in the same way. Moreover, as theMUE is near to the femtocell, the affect of Noise Power is small in the calculation ofPrx_fue.

In Figure 7-2, the results are interpolated for different UE distances and power levels.


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Note that the plot includes the downlink deadzones created by the femtocell, whichaffects the MUE. Downlink deadzone assumptions are summarised in Table 7-4.


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DL Tx power Maximum co-channelDLdeadzone

MUE-femtocell distance(using ITU-P.1238 indoor path loss model)

10dBm 60dB 11.3m

15dBm 65dB 17m

20dBm 70dB 25.7m

Table 7-4 Maximum co-channel DL deadzone created by the femtocell for MUEs, based on[R4-070969] and assuming RSSI of -65dBm

Within these zones, the MUE will be re-directed to another WCDMA frequency or RadioAccess Technology (RAT) by the macrocells, or the call may be dropped. In both casethe interference level in the femtocell reduces, and the uplink power requirements willrelax.

Figure 7-2 Interference Scenario B, voice call

7.2.1 HSUPA

In this section the affects of HSUPA are analysed. The link budget is shown in Table 7-5.

Value Unit Comments

FUE uplink transmitted power 21 dBm Ptx_fue

UE antenna gain 0 dBi Gue

Connectors/body loss 3 dB Lue

FUE Tx EIRP 18 dBm eirp_fue=Ptx_fue+Gue-Lue

Distance FUE-femtocell 5 m d_fue

FUE-femtocell path loss 50.16 dB PL_fueIndoor to indoor path lossmodel(d=d_fue, f=fc)

MUE distance from femtocell 21 dBm Ptx_mue

MUE-femtocell separation

MUE power at femtocell (see Table7-2 for d_mue=10)

Noise level

E-DPDCH Ec/No

10

-41.59

-103

mdBm

dBmdB

d_mue

Prx_mue

N0


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-2.57

Table 7-5 Link budget for HSUPA

The simulation results in Figure 8-3 show the E_DPDCH Ec/No for two cases:

• FUE is at 5m from the femtocell• FUE is at 15m from the femtocell.

In both cases, it is expected that the MUE is transmitting at maximum power(21dBm).

Figure 7-3 shows the fixed-reference channel (FRC) #3 (see [TS25.104], Pedestrian Achannel model) for the following requirements for E-DPDCH to be met:

• Ec/No of 2.4dB: provides R≥30% of max information bit rate •

Ec/No of 9.4dB: provides R≥70% of max information bit rate.

Note that DL deadzones are not taken into account. However, the grey area in thefigure represents the maximum extent (11.3m) of the DL deadzone for a femtocelltransmitting at +10dBm. This distance would reduce if the FAP was not loaded in thedownlink.

Note also that the indoor to indoor path loss model, ITU-R P.1238, may underestimatethe true path loss outside 15-20m range, as it is likely that other physical features(such as furniture, walls and buildings) will affect radio propagation (this is particularlytrue in dense urban areas.). A larger path loss reduces MUE interference, which, inturn, allows greater FUE throughput (linked to an increase in FUE-DPDCH Ec/No).


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Figure 7-3 HSUPA simulation, Scenario B. E-DPDCH Ec/No compared to throughput for RFC3

The results in Figure 7-3 are mapped to the TS 25.104 throughput model for

pedestrian A – no receiver diversity. The results are shown in Figure 7-4. Here, it isnoted how interference from the MUE has a strong affect on throughput; however, itshould be noted that the simulation assumes an MUE transmitting at maximum power(on the edge of the macrocell).


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Figure 7-4 Throughput for HSUPA. 70% max bit rate for all FRCs

7.3 Conclusions

Based on link budget calculations, the affects of uplink interference from one UE onthe macrocell and a UE on the femtocell have been analysed; in this work it isassumed that the same frequency is used by the Macro and Femto Layer.

In the analysis, it was assumed a femtocell serving an FUE on the physical edge of thecells (assumed to be 15m away) with a 12.2kbps AMR speech call; while a co-channelinterference MUE is in the proximity of the femtocell. The analysis results showed thatin order to be able to maintain the uplink connection between the FUE and femtocell,the transmitted power requirements are within the capability of the UE.

Additionally, the performance of HSUPA on the femto-FUE link has been analysed in

the presence of uplink interference from the Macro UE. By simulation, it has beenfound that in order to obtain HSUPA throughput of at least 2.8Mbps with a category 6UE, the FUE needs to be near to the femtocell (5m) and transmit at a power levelgreater than 15dBm if the MUE is within 15m of the femtocell.

However, such analysis must take into account the downlink deadzone created by thefemtocell. High power from the femtocell, in order to maintain the downlink, willinterfere with the macrocell signal at the MUE, and will force the macrocell tohandover the call to another WCDMA frequency or RAT; or, if none of these arepossible, the MUE call may be dropped.


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8. Scenario C: Femtocell Downlink Interference to the MacrocellUE Receiver

8.1 Description

In this scenario, MUE is connected to the macro network at the edge of coverage(RSCP


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Figure 8-2 Path loss model

The maximum indoor path loss is shown to be more than 90 dB in some locations. Theminimum outdoor path loss from an indoor Femto can be less than 60 dB. This will be

a challenge for operators to balance good indoor coverage while not causing excessiveoutdoor interference.

Studied in this section is a macrocell user (MUE) at cell edge, located in an apartmentwhere an active femtocell is operating with full capacity. Analysis is given for thefollowing case:

For the MUE to detect the macrocell and camp on it, or to maintain a call, the P-CPICHEc/No must be sufficient. We assume a -20 dB threshold – ie. the received P-CPICHRSCP from the macro must be no more than 20dB below the Rx P-CPICH RSCP of thefemto. It is assumed that cell-edge PCPICH RSCP for the macro is -103 dBm, and sowe can infer that the femto PCPICH RSCP must be lower than -83dBm for the MUE to

camp on the macrocell. (Note that techniques for facilitating cell re-selection, such asthe use of hysteresis, cell re-selection parameters, HCS, HPLMN, etc, are notdiscussed here, and are beyond the scope of this paper; the discussion in this paper ison the generic aspect of triggers for cell re-selection only.)

We have assumed two scenarios for the location of the femto relative to themacrocell: 100 metres and

1,000 metres away from the macro have been used. We have found that when thefemto is deployed in an area in close proximity to the macrocell (ie. 100 metresaway), the maximum output power of the femto should be increased beyond 100 mWin order to ensure operation in high coverage. Therefore, when we


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study the 100 metres case, we assume the femto is able to radiate up to 125 mW,while maximum output power is limited to 20 mW when the femto is deployed furtheraway (ie. 1,000 metres).

Figure 8-3 shows the statistics of the MUE performance when located near the femto

in the above mentioned two cases.

1. Femto being 100 metres away from macrocell10. Femto being 1,000 metres away from macrocell.

8.2 Analysis

Macrocell configuration:

• Macrocell site-to-site distance: 100 or 1,000 metres• Antenna height: 25 m•

Antenna gain: 18 dBi

•

Frequency carrier in 850 MHz band• Output power of the macro Node B: 20 Watts• Town size: 500m radius.

Femto location configuration:

• House size: 8.3X17.5 (m2)•

Houses cover 70% of the area• Wall penetration loss: 12 dB• CPICH power is 10% of max output power.

The following figures show the required power (as a proportion of the total macrocellpower) needed to support a voice call at 12.2 kbps within the house in the twodeployment scenarios.

Figure 8-3 TX power needed for 12.2 kbps for MUE (1000 metres away and 100 metres awayrespectively)

It is evident that the required power for a well-sustained call at 12.2 kbps is higher inthe following two cases:

• When the MUE is at the edge of the macrocell (i.e. 1,000 metres away) and isbehind the building where the femto is deployed. In this case the MUE

requires the macrocell to transmit the radio link at a higher power to


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compensate for the high path loss affecting the macro signal and theinterference from the femtocell.

•

When the MUE is in close proximity to the femtocell and the MUE is locatedinside the house. In this case the wall loss is adding additional attenuation tothe macro signal.

The following figures show the macro HSDPA throughput within the house in the twodeployment scenarios (based on how far the femto is from the macro).

Figure 8-4 MUE throughput with HSDPA for locations at 1,000 and 100 metres respectively

8.3 Scenario analysis and conclusions

In the scenario presented in this section, the performance of MUE attached to the

macrocell is shown to be affected by the femtocell in some locations. This can bemitigated by the use of adaptive power control on femto. Results show that in somecases the MUE might experience “deadzone” when in close proximity to the femto.One firm conclusion from this analysis is that adaptive power control is necessary forthe femtocells. Femtocells will require higher output power when the femtocell isdeployed in locations near the centre of the macrocell.

Adaptive power control on the femtocell mitigates interference by offering just therequired transmit power on the femto, based on the level of interference from macro.However, it is shown that a macrocell UE (MUE) might not receive an adequate signallevel from the macro to compensate for the femto interference. This is evident in allplaces in close proximity to the femto when the macro and femtocells share the same

carrier.

It is also concluded that there is no apparent and fundamental performance changewhether 850 MHz or 2100 MHz is used for the carrier.

In general, if a macro network is designed to provide fixed coverage in terms of cellsradius, then the macrocell requires lower output power when operating at 850 MHz.Therefore, the interference level seen by a femto is the same, regardless of the carrierfrequency.

It is shown that the femto is an effective vehicle for delivering a good carrier re-use.Furthermore, femtocells are an efficient technique for delivering the high-speed dataoffered by HSPA to femto users. This can be compared with the macrocell case, wherecell radius is larger, resulting in the distribution of the potential bandwidth of the


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HSDPA to a larger number of users. It is also well known that HSPA throughput isaffected by the location of the UE; the closer the UE to the centre of the cell, thehigher the throughput. This leads us to conclude that small cells like femtocells are anoptimum complementary technique for macrocells for addressing high-data usage.


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9. Scenario D: Femtocell Uplink Interference to the MacrocellNodeB Receiver

9.1 Introduction

This document provides an analysis of Femtocell Uplink Interference from femtocellmobiles (FUEs) to a Macrocell NodeB Receiver.

The scenario being investigated is as follows: An FUE is located next to the apartmentwindow that is in sight of a rooftop macrocell (approximately 1,000 m in distance), asshown in Figure 10-1. At the same time, the FUE is connected to the femtocell at theedge of its range, and is transmitting at full power.

Figure 9-1 Interference Scenario D

In this analysis the impact to the macro Node B is measured by the sensitivitydegradation, also referred to as noise rise (or relative increase in uplink Received TotalWide Band Power (RTWP)), experienced by the macro Node B, due to the femto UE.The impact is considered relative to the impact a macro UE will have on a macro Node

B from the same location as the femto UE. The rest of this document is structured asfollows:

• In Section 9.2, analysis of Scenario D described in [Law08] is presented,including the assumptions used. The analysis shows that the femto UEsimpact on the macro Node B is no worse that the impact a macro UE from thesame location would cause.

• In Section 9.4, a mitigation technique is suggested that would always ensurethere is minimal impact to macro Node Bs due to femtocell UEs.


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9.2 Analysis of Scenario D - 12k2 Voice and HSUPA

An analysis of this scenario is presented, based on link budget calculations. Theanalysis looks at the noise rise at the Macro Node B antenna connector due to thefemtocell UE in the described scenario.

9.2.1 Assumptions

A macro Node B with a noise floor based on the assumption that the sensitivity of theWide macro Node B for 12k2 voice service at the time is equal to -121 dBm (ie. the3GPP reference sensitivity level for a 12k2 voice service on a Wide Area Node B at theantenna connector [TS25.104]). This sensitivity captures both the loading and noisefigure of the macro Node B. The noise floor calculation is shown in Table 9-1.

Value Units Comment

Sensitivity @

antenna connector -121 dBm Pue_rec

3GPP reference sensitivity level for

Wide Area Node B

UE Service Rate 12.20 kbps R

Chip rate 3.84 MHz W

UE Processing Gain 24.98 dB PG = 10*log(W/R)

Required EbNo 8.30 dB EbNo

DCH performance without rx diversity

(see [FF09])

noise floor -104.32 dB nf_ant = Pue_rec +PG -EbNo

Table 9-1 Macro Node B noise floor

Next, the factors that could lead the femto UE to transmit at a power higher thanexpected are considered. This will occur if the femto UE is at the femto’s cell edge,and if the femtocell experiences a noise rise, or its receiver is experiencing a blockingeffect, caused by one of the following:

•

A co-channel macro UE.•

An adjacent channel macro UE.• Another femto UE located very close (~1m Free Space Loss) to the femtocell

– eg. a laptop with a 3G data card doing a data upload on the same desk asthe femtocell.

Subsequently, for the purposes of this scenario, the following assumptions are made:

• The femto is operating under extreme conditions, experiencing a total noiserise equivalent to 70% loading in the uplink.

• A 21 dBm class femto1 is used in the scenario that can provide a coveragepath loss of up to 120 dBs (path loss estimate based on minimum RSCPsensitivity of UE of -111 dBm and an 11 dBm CPICH transmit power andassumption of negligible downlink interference from surrounding Node Bs).

1

Under the same RF conditions a 21 dBm class femto cell will provide larger downlink coverage than a15dBm class or a 10dBm class femto


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Based on these assumptions, the link budget in Table 9-2 estimates the likely femtoUE uplink transmission power at the femtocell edge of coverage for a 12K2 voiceservice and a 2Mbps HSUPA service.


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Value

12K2Voice

2Mbps

HSUPA

Units Comments

Frequency 850.00 850.00 MHz F

Bandwidth 3.84 3.84 MHz BThermal NoiseDensity

-174.00 174.00 dBm/Hz tnd

Receiver NoiseFigure

8.00 8.00 dB NF

Receiver NoiseDensity

-166.00 -166.00 dBm/Hz rnd = tnd +NF

Receiver NoisePower

-100.16 -100.16 dBm rnp =rnd +10*log(B*1e6)

Loading 70.00 70.00 % L

Noise Rise due to

Loading 5.23 5.23 dB IM = -10*log(1-L/100)

Femto ReceiverNoise

Floor-94.93 -94.93 dBm trnp =rnp +IM

Femto UEService Rate

12.2 kbps R

Chip rate 3.84 MHz W

Femto UEProcessing

Gain24.98 dB PG = 10*log(W/R)

Required EbNo 8.30 dB EbNo

DCH performancewithout rx diversity

[FF09]

Required EcNo -16.68 0 dB

EbNo– PG for 12K2

Typical EcNo to achieveHSUPA rates of ~ 2Mbps[Hol06]

MinimumRequired

Signal Level forFemto

UE

-111.61 -94.93 dB Pfmin = trnp +EcNo

Femto UE Pathloss to

Femto120 120 dB DLcov

Femto UE TxPower

8.39 21 dBm Pfue = min(21, max ((Pfmin+ DLcov), -50)

Table 9-2 Femto UE TX power 1000 m from macro Node B

9.2.2 Macro Node B Noise Rise

The noise rise caused to the macro by a femto UE transmitting at 8.39dBm for a 12K2voice service and 21dBm for a 2Mbps HSUPA service was calculated, using the linkbudget in Table 10-3, as 1.44 dB and 9.12 dB respectively. Assuming that a macro UEis at the same location as the femto UE by the window (path loss of 130.77dB fromthe macro, see Ltot in Table 10-3), Table 10-4 shows that a macro UE operating fromthe same location as the femto UE will be transmitting at 9.94 dBm, and 21dBm if on


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a 12k2 voice service and 2Mbps HSUPA data service respectively and, hence, will leadto the same amount of noise rise as the femto UE.

Value

12K2

Voice

2Mbps

HSUPA

Units Comments

Node B Antenna Gain 17 17 dBi Gant [FF09]

Feeder/Connector Loss 3 3 dB Lf

Noise Floor at antenna

connector -104.32

-104.32 dBm nf_ant Table 9-1

Femto UE Tx Power 8.39 21 dBm Pfue

UE Antenna Gain 0 0 dBi Gmant

Femto UE Tx EIRP 8.39 21 dBmPfue_eirp =Pue – Gmant +m

Window/Wall Loss 5 5 dB Lw

Path loss to Macro NodeB

130.77 130.77

dB Ltot =1000m Okumura-Hata(Node B

at30m and mobile at 1.5m)

Femto UE Interference@

macro antenna-108.38

-95.77 dB Pfue_rec = Pfue_eirp – Ltot + Gant –Lf

Rise above noise floor -4.06 8.55 dB R Pfue_rec- nf_ant

Noise rise 1.44 9.12 dB NR =10*log( 1+ 100.1*R))

Table 9-3 Noise rise calculation for Scenario D (femto UE is transmitting at 8.39dBm and21dBm1000m from a macro Node B for a 12K2 service and 2Mbps HSUPA service)

Value Value Units Comments

12K2 HSUPA

Frequency 850 850 MHz

Bandwidth 3.84 3.84 MHz B

Thermal Noise Density -174.00 -174.00 dBm/Hz tnd

Receiver Noise Figure 5.00 5.00 dB NF

Receiver Noise Density -169.00 -169.00 dBm/Hz rnd = tnd + NF

Receiver Noise Power -103.16

Documents

009 Interference Management in UMTS Femtocells