Femtocells Aalto University

7/29/2019 Femtocells Aalto University

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Femtocells: Technology andDevelopments

re ess n orma on eory ummer c oo ,Centre for Wireless Communications,

Oulu, 29.7.2011

Jyri Hmlinen, Comnet/Aalto University


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Outline

Femtocell: Basics

3GPP Home (e)NodeB concept Service requirements for Home Node B (HNB) and Home

eNode B (HeNB)

HeNB and HNB systems: the logical architecture

Home eNode B (HeNB) Radio Frequency (RF)radio and interference scenarios, and measurements

Femtocell Networks: Some Research problems

Some reference material


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Femtocell: Basics


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Background

The recent explosive growth of the smartphone market has lead tomass deployment of data-intensive wireless services, e.g.,

webbrowsing, emailing, streaming of multimedia content. Mobile networks are about to reach their capacity limits in terms of

the number of supported end-users as well as in terms of the overalldata rates. (Thinking exercise: is this claim really true?)

Increasing the number of macrocell sites is costly and ineffectivesince around 50 % of voice calls and 70 % of data usage currentlytakes place indoors (*) where up to 20dB penetration loss(**)reduces the outdoor to indoor signal strength.

(**) Default indoor penetration loss in3GPP performance evaluation guidelines

(*) G. Mansfield, Femtocells in the US Market Business Drivers andConsumer Propositions, FemtoCells Europe, ATT, London, U.K., June2008. Femto Forum, www.femtoforum.org


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Growth of data demand

Growth of transferred data in Western Europe (S. Liu et al: A 25 Gb/s(/km2) UrbanWireless Network Beyond IMT-Advanced, IEEE Comm. Magazine, Feb. 2011).


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What is femto base station?

In general we can characterize femtocells as follows:

The Femto Base Station is an inexpensive compact base stationproviding equal radio access interface as a common macrocellularbase station (MBS) towards User Equipments (UEs).

The FBS devices are deployed autonomously by subscribers inresidential or enterprise premises in a manner of plug-and play.

The user traffic in FBS is backhauled to the mobile operator core

network over IP via the residential broadband wireline connection(DSL, optical network etc.)which is available locally in the site ofdeployment.


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Basic concept

The standardization process of femtocells launched in August

2007 via the 3rd Generation Partnership Project (3GPP) is stillunder way. First products came to the market at 2008.


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Why femtocells?

For the mobile operator: Data offload from macrocell network

increased network capacitySlower growth in macrocell backhaul costs.

Expanded revenue opportunities (sometimes) Lower backhaul costs (less macrocell traffic)

For the user: Better indoor coverage, full speed data transfer at home and

ubiquitous mobility between home cell and overlaying macrocell. Lower terminal transmission power at home (who cares?)

Extended phone battery life (its short anyway) One phone number, phonebook & consolidated bill

(not so much of an issue in Finland)


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Market pull

Interestingly, the driving force behind femtocell concept havebeen the operators.

Usually new technologies are pushed by industry that has clearincentive to sell more HW. (market pull rather than technologypush)

Actually femtocell concept was initially not attractive to main

Femtocells are low cost high volume products => businessconcept is different than in case of macrocell networks.

There was a threat that femtocells could cannibalize operatorsmain business (macrocellular systems).

Currently femto BS production has been outsourced for

subcontractors. Big players like NSN and Ericsson carry out the system

integration.

Yet, it is understood that femtocell concept can become an important tool for

mobile operators to keep customers satisfied and to limit the increasing networkcosts. Survival of mobile operators will be crucial also for manufacturers.


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3GPP Home (e)NodeB concept


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Service requirements for Home Node B(HNB) and Home eNode B (HeNB)

Reference: 3GPP TS 22.220 V10.7.0 (2011-6)

H(e)NB = HNB and HeNB


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Access Control requirements

Subject to operator and H(e)NB Hosting Party agreement, theoperator shall be able to configure the H(e)NB with open,

hybrid or closed access mode. When the H(e)NB is configured for open access mode, itshall be possible for the H(e)NB to provide services tosubscribers of any PLMN, subject to roaming agreement.

,shall be possible for the H(e)NB to provide services to: its associated CSG members, and subscribers of any PLMN not belonging to its associated CSG,

subject to roaming agreement.

When the H(e)NB is configured for closed access mode, onlyusers that belong to its associated CSG shall be able toobtain services.


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Closed Subscriber Group (CSG)

The CSG manager shall be able, under the operator supervision, toadd, remove and view CSG membership. NOTE: the interaction of

the user with the application that manages the Allowed CSG Lists isout of scope of 3GPP (e.g. Web interface).

For each subscriber, the network maintains a single CSG listcontaining the CSG identities that the subscriber is allowed to use.

e s a conta n a st o a owe ent t es oweList). It shall be possible to store the Allowed CSG List in the USIM.

Each CSG identity shall be associated to a subscriber groupwhichidentifies the subscribers allowed to access the CSG.

When the subscriber group is updated, the affected UE shall be

informed accordingly.


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Closed Subscriber Group (CSG)

For temporary members, it shall be possible to limit the period oftime during which the subscriber is considered a member of a CSG(granted access rights). It shall be possible to configure a timeperiod for each temporary member.

The time period shall be configurable by the CSG manager and/orthe operator operating the CSG. Unlimited membership to the CSGis allowed.

In hybrid access mode when services cannot be provided to a CSGmember due to a shortage of H(e)NB resources it shall be possibleto continue the established communication of non-CSG members inanother cell.

In hybrid access mode, to minimise the impact on CSG membersfrom established communication of non-CSG members, it shall bepossible for the network to allow the data rate of established PScommunication of non-CSG members to be reduced.


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CSG: source for critical interference?

Critical interferencebetween femtocells

may occur since HOis not necessarily

possible in CSG. 2

2b2a

1 1


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HNB and HeNB Installation, identificationand location requirements H(e)NB shall have a unique equipment identity.

It shall be possible to support at least 125 million CSG Identities

within a PLMN of an operator.

The radio transmitter of a H(e)NB shall not be activated untilconfigured and authorised by the operator.

en ns a ng, prov s on ng, con gur ng or re-con gur ng anH(e)NB the operator shall be able to:

Verify the H(e)NB's identity.

Obtain the geographical location of the H(e)NB.(*)

(*) Macrocell level location can be easily found but accurate location is difficult

to reach. UE measurement reports can be used to detect adjacent (e)NBs.


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HNB and HeNB Installation, identificationand location requirements NOTE: The scenario where a H(e)NB is connected to one

operators network and later changed to another operators network

is not required (*). The operator shall be able to determine that the H(e)NB is installed

and operated in accordance with all relevant regulatoryre uirements.

The operator shall be able to configure the settings of the H(e)NB.In the case where the H(e)NB has detrimental impact on thespectrum usage, the H(e)NB can be set to out-of-service by theoperator. (**)

Installation and activation of a new H(e)NB shall require noreconfiguration of the operators network.

(*) Leads to operator specific H(e)NB products

(**) Deciding this will create technical challenge


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OA&M Requirements

H(e)NB shall support the automatic discovery of an operatorsmanagement platform.

It shall be possible to make use of the operators managementplatform to carry out OA&M functions for H(e)NB. The managementconnection between H(e)NB and the operator's managementplatform shall be end-to-end secure.

e s a support & proce ures w c a ow t e operator to remotely configure the H(e)NB, deploy software upgrades, detectand report changes in RF conditions and perform general OA&Mtasks.(*)

If the connection between H(e)NB and the rest of the operator

network is out of service, then it shall be possible within anoperators defined time period for the H(e)NB to deactivate the air-interface.

(*) Remote configuration of HeNBs will be a great challenge

once mass deployment of femtocells has taken place


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Services support

Subject to availability of network resources there shall be nodifference in the user experience when using the PLMN provided

services via H(e)NB or via NodeB/eNodeB (NB/eNB). Deployment of H(e)NBs and NB/eNBs on the same spectrum

should not degrade the performance of UEs receiving service fromNB/eNBs. *

Deployment of H(e)NBs and NB/eNBs on the same spectrumshould not degrade the NB/eNBs coverage and capacity. (*)

H(e)NB shall support emergency calls for both CSG and non CSGmembers.

It shall be possible for the operator to provide location information ofthe UE attempting an emergency call over a H(e)NB.

(*) These requirements set a technical challenge for

interworking between femto layer and macro layer


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Local IP Access (LIPA)

Mobileoperators

corenetwork

UE

Local IP traffic

IP traffic to mobile operators CN

scope of Local IP access

logical connection for mobile operator IPtraffic

Residential/enterpriseIP Network


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Local IP Access (LIPA)

Local IP Access provides access for IP capable UEs connected viaa H(e)NB (i.e. using H(e)NB radio access) to other IP capable

entities in the same residential/enterprise IP network. Data traffic for Local IP Access is expected to not traverse the

mobile operators network except mobile operator networkcom onents in the residential/enter rise remises.

Signaling traffic will continue to traverse the mobile operatornetwork

This maybe doesnt seem too exotic: it just states that user datacan go directly to e.g. Public Internet. Yet, this is revolutionand

may in long term imply that role of core networks is reduced.What this means for billing: operator cant necessarily any morecalculate volume of user data => flat rate or time based billing.


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Some other requirements

The H(e)NB may support remote access for a CSG member to thehome based network from a UE via a PLMN in order to provideaccess to IP capable devices connected to the home based network(*).

It shall be possible to restrict the access to the home based networkon per-subscriber basis(e.g. some subscribers may have managedaccess to their home network and others may not) (**)

It shall be possible to support Television services(over e.g. MBMS).

It shall be possible for the network to set different criteria for accesscontrol in a hybrid cell for CSG and non-CSG members.

The H(e)NB shall provide a high level of security, equivalent orbetter than Rel-8 3GPP systems.

Security policy shall be under the control of the H(e)NB networkoperator

(*) You can use some devices at home remotely.

(**) Your wife may use all devices at home remotely.


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Some use cases

H(e)NB Guest Users

User A and User B are subscribers of Operator 1 and Operator 2

respectively. User A visits User B in his home and User B allowsUser A to use H(e)NB in User Bs home. User A should be able toaccess all the services he is subscribed to from Operator 1 basedon the policies set by User B and operator 2. Operator 1 andOperator 2 have roaming agreement.

HNB/HeNB NB/eNB Handovers

User A subcribes to cellular services of Operator 1 and is authorisedto access a HNB/HeNB from same or other operator. User A startsservice in the H(e)NB coverage and continues moving into a cellular

network. Similarly User A starts service in cellular network andcontinues moving into H(e)NB coverage. User A does not see anyimpact on services due to mobility in both cases.


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Some use cases

Hybrid access mode

In order to improve the coverage in a shopping mall, H(e)NBs aredeployed. The shopping mall owner may have been provided aspecial deal by the network operator where the employees of theshopping mall will get preferential charging rates and priority accesswhen accessing services via these H(e)NBs. In exchange, theshopping mall owner allows the public to use the H(e)NBs to accessthe normal network operator services. The H(e)NB Hosting Party

not need to do anything special in order to get services on theH(e)NB.

Open access mode

Typically to enhance coverage or capacity of an operators publicnetwork, for example in railway stations, airports, stadiums, etc,taking benefit of the H(e)NBs additional functionality (e.g.uncoordinated deployment).


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HeNB and HNB systems: the logicalarchitecture

References:3GPP TR 23.830 V9.0.0 (2009-09; partly outdated)3GPP TS 25.467 V10.0.0 (2010-12; focus on UTRAN)



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LTE Rel.8 architecture

EPC = Evolved Packet Core

MME = Mobility Management EntityP-GW, S-GW = Packet data network and Serving GateWay


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LTE Home eNB architecture


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LTE Home eNB architecture

The HeNB Gateway concentrate a large number of HeNBsand appears as an MME to the HeNB and the EPC.

Amongst others it provides the Tracking Area Code (TAC)and network identification (PLMN ID) to the HeNB

The Security Gateway is a mandatory logical function. It may

integrated into the HeNB-GW. The SeGW secures thecommunication from/to the HNB.

HeNB architecture development is ongoing, furtherinformation can be found from 3GPP Feature and Study

Items list, see http://www.3gpp.org/ftp/Specs/html-info/FeatureListFrameSet.htm


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WCDMA/HSPA Architecture (until Rel.6)

RNC = Radio Network ControllerMCS/VLR = Mobile services switching centre/visitor location registerHLR = Home Location Register

SGSN/GGSN = Serving/Gateway GPRS Support NodeGMSC = Gateway MCS


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3G Home NB architecture


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The HNB-GW appears to the CN as an RNC and serves as aconcentrator of HNB connections.

The Iu interface between the CN and the HNB-GW serves the samepurpose as the interface between the CN and a RNC.

The Local Gateway (L-GW) may be present only when the HNBoperates in LIPA mode. When present, it is co-located with the HNB, inwhich case the HNB has a Gn/S5 interface towards the SGSN/SGW.

Iuh is the interface between the HNB and HNB GW. For the control

plane, Iuh support HNB registration, UE registration and error handlingfunctions. For the user plane, Iuh support user plane transport bearerhandling

The Iurh interface between HNBs admit two options: Direct interface connectivity between HNBs

HNB-GW serves as a proxy between HNBs Gi is the interface towards the residential/IP network (in LIPA mode)

SGW = Serving GateWay


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The HNB management system (HMS): facilitates HNB-GW discovery.

provides configuration data to the HNB. performs location verification of HNB and assigns appropriateserving elements (HMS, Security Gateway and HNB-GW).

Security Gateway (SeGW): ,

certain deployment options. authentication of HNB. provides the HNB with access to the HMS and HNB-GW.

HNB Gateway (HNB-GW): terminates Iuh from HNB and appears as an RNC to the Core

network. supports HNB registration and UE registration over Iuh.

Details of functional split between HNB, HNB-GW and CN


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From 3GPP TS 25.467 one finds: Details of functional split between HNB, HNB-GW and CN.

UTRAN functions for HNB access (UE Registration, HNBRegistration, HNB-GW Discovery Function, HNB mobility issues,HNB Configuration Transfer, etc)

Interestingly, in 3GPP TS 25.467 just three interferencemiti ation scenarios has been mentioned:

In UL: Adaptively limiting the HNB UEs maximum UL Tx Power inconnected mode possibly using HNB UE measurement andcalculating the path loss between HNB UE and Macro NB.

In DL: (a) Redirecting unauthorized UE to another carrier possiblybased on uplink access attempts by unauthorised UE. (b) AdjustingHNBs DL CPICH Tx Power adaptively either temporarily or over

long term possibly based on uplink access attempts byunauthorised UE.

CPICH = Common Pilot CHannel


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Home eNode B (HeNB) Radio Frequency (RF)radio and interference scenarios,and measurements

References:3GPP TR 36.921 V10.0.0 (2011-04)3GPP TR 36.922 V10.0.0 (2011-04)3GPP TS 36.104 V10.3.0 (2011-06)3GPP TR 25.967 V10.0.0 (2011-04, omitted here)



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Radio Scenarios: Deployment

configurations Main deployment configurations for Home NodeB:

Open access or CSG (Closed Subscriber Group)

Dedicated channel or co-channel

Fixed or adaptive (DL) maximum transmit power

Also fixed or ada tive resource artitionin form a

deployment configuration. Specifically, the resource partitioning could be performed in

frequency, time or spatial dimensions for interferencecoordination.


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Radio Scenarios: Resource partitioning

Frequency partitioning

First example: softfrequency reuse.


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Frequency band for the network

Reuse onepartition

Orthogonal

Frequency partitioning Second example: partly or fully orthogonal partitioning

par on

Partially overlap

partition

Frequency allocated to macrocell

Frequency allocated to femtocell


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Frequency partitioning Third example: Configuration based spectrum partitioning


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Time partitioning

The resources used in Macro and Home eNBs can also be partitionedand coordinated in the time dimension. Different time zone or UL-DL

configurations between HeNBs and macro eNBs or among HeNBsunder specific conditions may provide some flexibility for interferencecoordination. However, it may also bring new interference risks. Furtherinterference mitigation methods based on the time partitioning needs to

.

Spatial partitioning

Due to uplink-downlink channel reciprocity, TDD HeNBs can use beamcoordination to improve interference conditions. For example, theHeNB can avoid beam collision with the Macro or other Home eNBs in

a proactive or reactive way. These mechanisms may require a certainamount of information exchange between the HeNBs.


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Interference ScenariosNumber Aggressor Victim Priority

1 UE attached to Home eNode B Macro eNode B Uplink Yes

2 Home eNode B Macro eNode B Downlink Yes

3 UE attached to Macro eNode B Home eNode B Uplink Yes

4 Macro eNode B Home eNode B Downlink

6 Home eNode B Home eNode B Downlink Yes

7 UE attached to Home eNode Band/or Home eNode B

Other System

8 Other System UE attached to Home

eNode B and/or HomeeNode B


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Interference Scenarios

Downlink Uplink


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RF Aspects: HeNB output power

From HeNB coverage and capacity point of view, large output power could beattractive.

However, the maximum output power should be limited in order to control the

downlink interference from HeNB towards macrocell layer. So, the maximum HeNB output power should be a trade-off between the HeNB

performance and the interference towards close-by macrocell users, which donot have access to the HeNB.

Based on 3GPP studies the allowed out ut ower of the Home BS is limited to < + 20 dBm for 1 transmit antenna

< + 17 dBm for 2 transmit antennas < + 14 dBm for 4 transmit antennas

(< + 11 dBm for 8 transmit antennas, release 10)

Yet, aim is to use adaptive power setting rather than fixed output power.

In first HNB products the output power will be fixed and in most deployments

between 0dBm and 10dBm.


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HeNB measurements and adaptation

The objectives of the HeNB measurements are to provide sufficient information to the HeNB for the purpose of

interference mitigation to provide sufficient information to the HeNB such that the HeNBcoverage can be maintained.

According to the measurement type, there are two options tocollect measurements: From connected Mode UEs attached to the HeNB

Via a DL Receiver function within the HeNB itself. Such DL receiverfunction is also called Network Listen Mode (NLM), RadioEnvironment Measurement (REM) or "HeNB Sniffer".

These measurements can also be used during the HeNBself-configuration process


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LTE Rel.8 measurements in UE

RSSI, which is the total received wideband power on a givenfrequency (from all sources).

Reference Signal Received Power (RSRP), which for aparticular cell is the average of the power measured (and theaverage between receiver branches) of the resourcee emen s a con a n ce -spec c re erence s gna s.

Reference Signal Received Quality (RSRQ) is the ratio of theRSRP and the E-UTRA Carrier Received Signal StrengthIndicator (RSSI), for the reference signals.


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HeNB System Measurements (1/4):

Measurements from all cells

Measurement Type PurposeMeasurement

Source(s)

ReceivedInterference Power

Calculation of UL interferencetowards HeNB (from MUE)

HeNB UL Receiver

or examp e, a ece ve n er erence ower measuremen va ue arger an

a pre-defined threshold would mean that at least an MUE which is interferedby a HeNB is close to the HeNB and that the MUE's Tx power would causesignificant interference towards the HeNB. This measurement value may beused in calculating path loss between the HeNB and the MUE assuming thata single MUE dominates the interference.


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Measurements to identify surrounding celllayers

Measurement Type Purpose MeasurementSource(s)

Cell reselection priorityinformation

Distinction between cell types basedon frequency layer priority

HeNB DL Receiver

CSG status and ID Distinction between cell layersbased on CSG, and self-construction of neighbour list,

HeNB DL Receiver



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HeNB System Measurements (3/4):Measurements from macro cell layer


Source(s)

Co-channel RSRP

Calculation of co-channel DL interferencetowards macro UEs (from HeNB)Calculation of co-channel UL interferencetowards macro layer (from HUEs)

Calculation of co-channel UL interferencetowards HeNB (from MUEs) based onestimated MUE Tx powerDetermine coverage of macro cell (foro timization of h brid cell confi uration

HeNB DL Receiver

MUE (in case of hybridcell)

Co-channel RSRQDetermine quality of macro cell (foroptimization of hybrid cell configuration)

HeNB DL Receiver

MUE (in case of hybridcell)

Reference SignalTransmission Power

Estimation of path loss from to MeNB HeNB DL Receiver

Physical + Global Cell IDAllow HeNB to Instruct UEs to measurespecific cells.Allow UE to report discovered cells to HeNB.

HeNB DL Receiver

Detection of UL RS Detection of victim UE HeNB UL Receiver

RSRP = Reference Symbol Received Power

RSRQ = Reference Symbol Received QualityRS = Reference Signal


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Measurements of other HeNB cells


Source(s)

Co-channel RSRP

Calculation of co-channel DL interferencetowards neighbour HUEs (from HeNB)Calculation of co-channel UL interferencetowards neighbour HeNBs (from HUEs)

HeNB DL Receiver

Reference Signal

Transmission Power

Estimation of path loss from to HeNB HeNB DL Receiver

Physical + Global Cell ID Allow HeNB to Instruct UEs to measurespecific cellsAllow UE to report discovered cells toHeNB.

HeNB DL Receiver


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3GPP Interference Control Proposals

In LTE HeNB system interference control utilize above discussedmeasurements and other information exchanged through network.

There are different methods proposed for the protection of controlchannels and data channels, see TR 36.921 for details.

Proposed methods include both general approaches such as frequency partitioning and power control, and

LTE specific methods

Example of the latter methods is the control channel interferencemanagement based on fixed time-frequency location of controlinformation: If adjacent HeNBs apply time and/or frequency shift in DL transmissions =>

not all control channels are overlapping. If data channel transmission issuspended on radio resources that are used for control in adjacent cells,then control channel detection is clearly improved; see next slide.

Control region (36 subcarriers x 1

SF-1 SF-2 LegendSF-3SF-0


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Control region (36 subcarriers x 1

OFDM symbol)

DL resources available for scheduling(36 subcarriers x 1 OFDM symbol)

Macro-eNB (one

unit on x-axis is 1

OFDM symbol ~

71 us and one

unit on the y-axis

is 3 PRBs or 36

subcarriers)

SSCH

PBCH

PBCH

PBCH

PBCH

PSCH

PBCH = Physical Broadcast CHannel

Common macro-eNB and HeNB DL

bandwidth allocation

SF-2

SSCH

PSCH

PBCH

PBCH

PBCH

PBCH

SF-0 SF-1SF-9

Home-eNB, DLframe timing

offset by k = 16

OFDM symbols

PSCH, SSCH = Primary and Secondary

Synchronization Channels.


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Femtocell Networks: Some Researchproblems

S h 1/2


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Some research areas 1/2

Femtocell research classification: General level small cell/heterogeneous system research: Aim is

usually to find basic principles and new theoretical aspects. System specific research: existing specifications form the

framework for the research. Aim is usually to proposeenhancements/extensions to existing systems.

Interference: Management and avoidance (e.g. scheduling of transmissions,

Suppression (e.g. tranceiver algorithms like beamforming andadvanced receivers) Interference between macro and femto layers and within femto

layer.

Radio resource management

Scheduling of resources between macro and femto layers Static vs dynamic resource allocation. Load balancing between macrocells and (open access) femtocells

S h 2/2


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Some research areas 2/2

Power allocation/calibration for femtocells How to set TX powers in femtocells?

How to dynamically adjust TX powers in femtocells? Self-configuration and optimization

Femtocell networks can be unplanned (user deployed femtocells),or lanned o erator de lo ed femtocells .

Self-configuration and optimization algorithms needed to tune thenetwork.

Mobility Between femtocells and macrocells

Between femtocells

Mobility issues related to femtocells has not been widelyinvestigated.

C iti l i t f A Si l E l


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Critical interference: A Simple Example

2

Consider DL interference problem of the figure.For the rates there holds:

2

( )mmmmmfffff

BWARBWAR

+=

+=

1log1log

2

2(*)

11

2,2,121

222

1,1,212

111

/

/

/

fmN

m

fmN

f

IILPP

LP

IILPP

+++=

+++

=

In (*) A and Bare constants that can be used to fit rates with some practical systemslike LTE (P. Mogensen, W. Na, I. Z. Kovacs, et al., LTE capacity compared to theshannon bound, in Proceedings of the IEEE 65th Vehicular Technology Conference(VTC 07), pp. 12341238, April 2007.

(**)

Critical interference A Simple Example


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In general there holds:

Let us consider an illustrative example where we ignore terms If,kand Im,k related to other femtocell and macrocell interference. Then

2121122211 ,, PPLLLL


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Numerical values for illustration:

( )

=+===

==

==

==

)43.0)25.12/(11log88.0/(,4,3

indoor UE)(0),outdoor UE(20

indoor UE)(20),outdoor UE(0

10,46

2min

1212

2222

21

WRdB

dBLdBL

dBLdBL

dBmPdBmP

WW

WW

Assume that femtocell is 1km distance from macrocell eNB.

If UE is outside and there is 20dB attenuation towards indoor femtocell,

then macrocell UE should be at least 33.5 meters away from femto eNB. If macrocell UE is indoors, then 335 meter separation is needed so that

minimum SINR requirement is fulfilled. This is impossible.

)indoorsUE(335.0outdoors)UE(0335.0

=

=

aa

Concluding remark: CSG femtocells may

create local holes on macrocellular coverage.

Interference management approaches


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(femto-macro layer interference) Dedicated frequency carrier for femtocells (not effective and costly

for operators).

Static split of the carrier frequency between macro and femto layers

Not feasible approach in single carrier systems like HSPA Brick wall separation of femto and macro spectrum (in e.g. LTE) is easy but

can be ineffective since load in femto layer and macro layer may vary a lotduring the day.

ynam c usage o requency resources:

Separation of femtocell and macrocell transmissions in frequency requireseither active information exchange between macro and femto layers orpredefined rules for spectrum usage. Femtocell backhaul do not supportreal time resource management (i.e. within fast fading coherence time)

Research question: How to design effective and simple(frequency) resource allocation methods when information

exchange between femto and macro layers is limited, load on layersvary and network topology may change (femtocell switched on andoff)? For some solutions and references, see e.g. TR 36.921



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(femto-macro layer interference) Multiantenna processing

Interference suppression by e.g. femto eNB null steering is

effective only if channel is directive and accurate channelinformation is available.

The number of antennas in femto eNBs is expected to be small=

of the interference. Femto eNB transmit power allocation/control

Tool for system optimization.

Not necessarily effective approach when solving instantaneous

interference problems.

Example: Dynamic usage of frequency


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resources Femtocell operation frequency can be limited since SNR is usually

high and number of users is small in femtocells.

Macrocell operation frequency cannot be limited since SNR can be

low and number of users maybe large. LTE example:

Let us reserve n frequency resource blocks exclusively for macrocell userswhile rest of the band can be used by both femtocells and macriocells.

Macrocell will schedule to (femtocell) interference free resources users that

are close to femtocells. Question: How macrocell know that some of its users heavily suffer from

femtocell interference?

Exclusive resources for

macrocell users

Resources used by both

macrocell and femtocells



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resources

UE measures strength of

the received signal fromHeNB and detect its ID

HeNB Broadcast UE send measurement

In LTE macrocell UE can measure the broadcasted reference signalfrom HeNB. Broadcast channels carry also HeNB identity (ID).

UE then send measurement results to macrocell eNB that knowsHeNB(s) that interfere UE. If there is critical interference, then eNB

can assign exclusive resources for UE. In this approach there is a trade-off between user rates on femtocell

and macrocell.

Interference problem


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Interference problem

within the femto layer:Illustration

Part of a row building including

3 closed femtocells and 3terminals.

Radio operations on the same

1

2

2

1

requency carr er.

Dedicated signal: Green arrows

Interference: Red arrows

3

3

Managing the interference within the


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Managing the interference within the

femto layer: Transmit power allocation Local interference problems between femtocells occur due to

unplanned nature of the femto network. Especially when network consists of user deployed femto eNBs

and Closed Subscriber Group configuration is used.

Power allocation can be used to optimize femtocell operations.

Two hases of ower allocation: When managing (roughly) the interference between macro and

femto layers it is assumed that femto eNBs close to macrocell eNBcan use higher TX power than femto eNBs on macrocell edge.

When managing the interference within femto layer the TX powersare adjusted such that local femto operations are optimized.

Power allocation approaches: Distributed and centralized.

Remark: Femto eNBs are turned on and switched off

by users => local femto network topology is dynamic

Distributed vs centralized approach


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Distributed vs centralized approach

In (extremely) distributed approach femtocells operateindependently: situation is similar like in WLAN

In (extremely) centralized approach the femto manager have allchannel information and can accurately adjust femto eNB TX

powers.

Measurement reports

Cell 1 Femto manager(connected to

Measurement reports

Cell K

All measurementinformation is forwardedto the femto manager.

Practical femto networks are in between the extremes.Different manufacturers may apply different managementconcepts

Femto manager (or network OA&M) maycreate interference matrices etc

Interference problem within the femto layer:


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Power calibration problem example Performance criteria example:

0,,

,,

,

+

=

km mkjmNkkjk

kj LPP

LP

NBfemtothinpoweronTransmissi=k kP

(*)

The SINR requirement (*) aims to guarantee a certain level of

service for femtocell users. We say that user is in outage if SINR issmaller than

0.

powernoiseWhite

receiverin theSINRRequired

emtotancetnt term naetweenossat

0

,,

=

=

=

N

mkj

P

m



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p b y

Power calibration problem example Let

kbe a random variable (SINR in the kth cell). Then, instead of

(1), we may define a statistical performance requirement

where right side defines the probability for outage. This kind of

( ) outk PrPr 0 =


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p y

Power calibration problem example The power allocation problem when goal is to guarantee a certain

service level with minimum transmission powers:

( )

=


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Femtocell Networks: The Future

General future developments


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p

Chipset for femto base stations will become cheaper due to massproduction => at some point femto NB will be an integral part of allDSL boxes (and desk computers too???).

This development takes place only if femtocell concept becomes aglobal success.

Femtocell conce t rovides a natural la round for various flexiblespectrum usage approaches

Example: joint femtocell spectrum for all operators.

Different cognitive radio applications may also become part of femtocellconcept (e.g. spectrum sensing).

Femto management systems will become more sophisticated.

Specifications will provide better means to control femtocelloperations.

One potential aspect: Dense


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p p

heterogeneous networks In addition to femtocells

various M2Mcommunication may take

place on mobilecommunication spectrum.

The priorities of different

well as QoSrequirements.

This may lead to localheterogeneous systemswhere femtocells need to

share the spectrum withM2M communication.


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Some reference material

General level publications


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p

P. Lin et al: Macro-femto heterogeneous networkdeployment and management: from business models to

technical solutions, IEEE Wireless Communications,June 2011.

M. Yavuz et al: Interference Mana ement andPerformance Analysis of UMTS/HSPA+ Femtocells,

IEEE Communications Magazine, September 2009.

V. Chandrasekhar, J. G. Andrews, and A. Gatherer:Femtocell networks: a survey, IEEE Communications

Magazine, vol. 46, no. 9, pp. 5967, 2008.

3GPP Home (e)NodeB Concept


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3GPP TS 22.220 V10.7.0 (2011-6)

3GPP TR 23.830 V9.0.0 (2009-09; partly outdated)

3GPP TS 25.467 V10.0.0 (2010-12; focus on UTRAN)

3GPP TR 36.921 V10.0.0 (2011-04)

3GPP TR 36.922 V10.0.0 (2011-04)

3GPP TS 36.104 V10.3.0 (2011-06)

3GPP TR 25.967 V10.0.0 (2011-04)

Some recent technical studies


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O. Simeone, E. Erkip, and S. Shamai Shitz: Robust Transmission andInterference Management For Femtocells with Unreliable Network Access,IEEE Journal on Selected Areas in Communications, vol. 28, no. 9, December2010.

S. Park, W. Seo, et al: Beam Subset Selection Strategy for Interference

Reduction in Two-Tier Femtocell Networks, IEEE Transactions on WirelessCommunications, vol. 9, no. 11, November 2010.

Han-Shin Jo et al: Self-Optimized Coverage Coordination in FemtocellNetworks, IEEE Transactions on Wireless Communications, vol. 9, no. 10,

c o er .

M. Husso et al: InterferenceMitigation by Practical Transmit BeamformingMethods in Closed Femtocells, EURASIP Journal on WirelessCommunications and Networking, Vol. 2010.

Vikram Chandrasekhar et al: Coverage in Multi-Antenna Two-Tier Networks,IEEE Transactions on Wireless Communications, vol. 8, no. 10, October 2009.

Vikram Chandrasekhar et al: Power Control in Two-Tier Femtocell Networks,

IEEE Transactions on Wireless Communications, vol. 8, no. 8, August 2009. Vikram Chandrasekhar et al: Spectrum Allocation in Tiered Cellular Networks,

IEEE Transactions on Communications, vol. 57 no. 10, October 2009.

Femtocells Aalto University

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