Femtocell

Femtocells: Technology and DevelopmentsWireless Information Theory Summer School,Wireless Information Theory Summer School, Centre for Wireless Communications, Oulu, 29.7.2011Jyri Hmlinen, Comnet/Aalto UniversityOutline 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 materialFemtocell: BasicsBackground The recent explosive growth of the smartphone market has lead to mass 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 overall data rates. (Thinking exercise: is this claim really true?) Increasing the number of macrocell sites is costly and ineffective since around 50 % of voice calls and 70 % of data usage currently takes place indoors (*) where up to 20dB penetration loss(**) reduces the outdoor to indoor signal strength. (**) Default indoor penetration loss in 3GPP performance evaluation guidelines(*) G. Mansfield, Femtocells in the US Market Business Drivers and Consumer Propositions, FemtoCells Europe, ATT, London, U.K., June2008. Femto Forum, www.femtoforum.orgGrowth of data demandGrowth of transferred data in Western Europe (S. Liu et al: A 25 Gb/s(/km2) Urban Wireless Network Beyond IMT-Advanced, IEEE Comm. Magazine, Feb. 2011).What is femto base station? In general we can characterize femtocells as follows:The Femto Base Station is an inexpensive compact base station providing equal radio access interface as a common macrocellularbase station (MBS) towards User Equipments (UEs).The FBS devices are deployed autonomously by subscribers in residential 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 of deployment.Basic conceptThe standardization process of femtocells launched in August 2007 via the 3rd Generation Partnership Project (3GPP) is still under way. First products came to the market at 2008.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) Increased customer stickyness (?) Increased customer stickyness (?) 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)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 network manufacturers network manufacturers Femtocells are low cost high volume products => business concept 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 systemintegration.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. 3GPP Home (e)NodeB conceptService 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 HeNBAccess Control requirements Subject to operator and H(e)NB Hosting Party agreement, the operator 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, it shall be possible for the H(e)NB to provide services to subscribers of any PLMN, subject to roaming agreement. When the H(e)NB is configured for hybrid access mode, it When the H(e)NB is configured for hybrid access mode, it 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, only users that belong to its associated CSG shall be able to obtain services.Closed Subscriber Group (CSG) The CSG manager shall be able, under the operator supervision, to add, remove and view CSG membership. NOTE: the interaction of the user with the application that manages the Allowed CSG Lists is out of scope of 3GPP (e.g. Web interface). For each subscriber, the network maintains a single CSG list containing the CSG identities that the subscriber is allowed to use. The UE shall contain a list of allowed CSG identities (Allowed CSG The UE shall contain a list of allowed CSG identities (Allowed CSG List). It shall be possible to store the Allowed CSG List in the USIM. Each CSG identity shall be associated to a subscriber group which identifies the subscribers allowed to access the CSG. When the subscriber group is updated, the affected UE shall be informed accordingly.Closed Subscriber Group (CSG) For temporary members, it shall be possible to limit the period of time during which the subscriber is considered a member of a CSG (granted access rights). It shall be possible to configure a time period for each temporary member. The time period shall be configurable by the CSG manager and/or the operator operating the CSG. Unlimited membership to the CSG is allowed. In hybrid access mode when services cannot be provided to a CSG In hybrid access mode when services cannot be provided to a CSG member due to a shortage of H(e)NB resources it shall be possible to continue the established communication of non-CSG members in another cell. In hybrid access mode, to minimise the impact on CSG members from established communication of non-CSG members, it shall be possible for the network to allow the data rate of established PS communication of non-CSG members to be reduced.CSG: source for critical interference?Critical interferencebetween femtocellsmay occur since HO is not necessarilypossible in CSG. 22b 2a11HNB and HeNB Installation, identification and 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 until configured and authorised by the operator. When installing, provisioning, configuring or re-configuring an When installing, provisioning, configuring or re-configuring an H(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 difficultto reach. UE measurement reports can be used to detect adjacent (e)NBs.HNB and HeNB Installation, identification and 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 regulatory requirements. requirements. 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 the spectrum usage, the H(e)NB can be set to out-of-service by the operator. (**) Installation and activation of a new H(e)NB shall require no reconfiguration of the operators network.(*) Leads to operator specific H(e)NB products(**) Deciding this will create technical challengeOA&M Requirements H(e)NB shall support the automatic discovery of an operators management platform. It shall be possible to make use of the operators management platform to carry out OA&M functions for H(e)NB. The management connection between H(e)NB and the operator's management platform shall be end-to-end secure. H(e)NB shall support OA&M procedures which allow the operator to H(e)NB shall support OA&M procedures which allow the operator to remotely configure the H(e)NB, deploy software upgrades, detect and report changes in RF conditions and perform general OA&M tasks.(*) If the connection between H(e)NB and the rest of the operator network is out of service, then it shall be possible within an operators defined time period for the H(e)NB to deactivate the air-interface. (*) Remote configuration of HeNBs will be a great challengeonce mass deployment of femtocells has taken placeServices support Subject to availability of network resources there shall be no difference 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 from NB/eNBs.(*) NB/eNBs.(*) Deployment of H(e)NBs and NB/eNBs on the same spectrum should not degrade the NB/eNBs coverage and capacity. (*) H(e)NB shall support emergency calls for both CSG and non CSG members. It shall be possible for the operator to provide location information of the UE attempting an emergency call over a H(e)NB. (*) These requirements set a technical challenge for interworking between femto layer and macro layerLocal IP Access (LIPA)Mobile operatorscorenetworkUELocal IP trafficIP traffic to mobile operators CNscope of Local IP accesslogical connection for mobile operator IP trafficResidential/enterpriseIP NetworkLocal IP Access (LIPA) Local IP Access provides access for IP capable UEs connected via a 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 network components in the residential/enterprise premises.components in the residential/enterprise premises. Signaling traffic will continue to traverse the mobile operator networkThis maybe doesnt seem too exotic: it just states that user data can go directly to e.g. Public Internet. Yet, this is revolution and 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.Some other requirements The H(e)NB may support remote access for a CSG member to the home based network from a UE via a PLMN in order to provide access to IP capable devices connected to the home based network (*). It shall be possible to restrict the access to the home based network on per-subscriber basis (e.g. some subscribers may have managed access to their home network and others may not) (**) It shall be possible to support Television services (over e.g. MBMS). It shall be possible to support Television services (over e.g. MBMS). It shall be possible for the network to set different criteria for access control in a hybrid cell for CSG and non-CSG members. The H(e)NB shall provide a high level of security, equivalent or better than Rel-8 3GPP systems. Security policy shall be under the control of the H(e)NB network operator(*) You can use some devices at home remotely.(**) Your wife may use all devices at home remotely. Some use casesH(e)NB Guest UsersUser A and User B are subscribers of Operator 1 and Operator 2 respectively. User A visits User B in his home and User B allows User A to use H(e)NB in User Bs home. User A should be able to access all the services he is subscribed to from Operator 1 based on the policies set by User B and operator 2. Operator 1 and Operator 2 have roaming agreement.HNB/HeNB NB/eNB HandoversUser A subcribes to cellular services of Operator 1 and is authorised to access a HNB/HeNB from same or other operator. User A starts service in the H(e)NB coverage and continues moving into a cellular network. Similarly User A starts service in cellular network and continues moving into H(e)NB coverage. User A does not see any impact on services due to mobility in both cases.Some use casesHybrid access modeIn order to improve the coverage in a shopping mall, H(e)NBs are deployed. The shopping mall owner may have been provided a special deal by the network operator where the employees of the shopping mall will get preferential charging rates and priority access when accessing services via these H(e)NBs. In exchange, the shopping mall owner allows the public to use the H(e)NBs to access the normal network operator services. The H(e)NB Hosting Party should not need to manage the public access and the public shouldshould not need to manage the public access and the public should not need to do anything special in order to get services on the H(e)NB.Open access modeTypically to enhance coverage or capacity of an operators public network, for example in railway stations, airports, stadiums, etc, taking benefit of the H(e)NBs additional functionality (e.g. uncoordinated deployment).HeNB and HNB systems: the logical architectureReferences: 3GPP TR 23.830 V9.0.0 (2009-09; partly outdated) 3GPP TS 25.467 V10.0.0 (2010-12; focus on UTRAN)H(e)NB = HNB and HeNBLTE Rel.8 architectureEPC = Evolved Packet CoreMME = Mobility Management EntityP-GW, S-GW = Packet data network and Serving GateWayLTE Home eNB architectureLTE 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 be implemented either as a separate physical entity orbe implemented either as a separate physical entity or integrated into the HeNB-GW. The SeGWsecures the communication from/to the HNB. HeNB architecture development is ongoing, further information can be found from 3GPP Feature and Study Items list, see http://www.3gpp.org/ftp/Specs/html-info/FeatureListFrameSet.htmWCDMA/HSPA Architecture (until Rel.6)RNC = Radio Network ControllerMCS/VLR = Mobile services switching centre/visitor location registerHLR = Home Location RegisterSGSN/GGSN = Serving/Gateway GPRS Support NodeGMSC = Gateway MCS3G Home NB architecture3G Home NB architecture The HNB-GW appears to the CN as an RNC and serves as a concentrator of HNB connections. The Iu interface between the CN and the HNB-GW serves the same purpose as the interface between the CN and a RNC. The Local Gateway (L-GW) may be present only when the HNB operates in LIPA mode. When present, it is co-located with the HNB, in which 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 Iuh is the interface between the HNB and HNB GW. For the control plane, Iuh support HNB registration, UE registration and error handling functions. For the user plane, Iuh support user plane transport bearer handling 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 GateWay3G Home NB architecture The HNB management system (HMS): facilitates HNB-GW discovery. provides configuration data to the HNB. performs location verification of HNB and assigns appropriate serving elements (HMS, Security Gateway and HNB-GW). Security Gateway (SeGW): terminates secure tunnelling for Iuh, and for Iurh and Gn/S5 for terminates secure tunnelling for Iuh, and for Iurh and Gn/S5 for 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 3G Home NB architecture From 3GPP TS 25.467 one finds: Details of functional split between HNB, HNB-GW and CN. UTRAN functions for HNB access (UE Registration, HNB Registration, HNB-GW Discovery Function, HNB mobility issues, HNB Configuration Transfer, etc) Interestingly, in 3GPP TS 25.467 just three interference mitigation scenarios has been mentioned: mitigation scenarios has been mentioned: In UL: Adaptively limiting the HNB UEs maximum UL Tx Power in connected mode possibly using HNB UE measurement and calculating the path loss between HNB UE and Macro NB. In DL: (a) Redirecting unauthorized UE to another carrier possibly based on uplink access attempts by unauthorised UE. (b) Adjusting HNBs DL CPICH Tx Power adaptively either temporarily or over long term possibly based on uplink access attempts by unauthorised UE.CPICH = Common Pilot CHannelHome 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)H(e)NB = HNB and HeNBRadio Scenarios: Deploymentconfigurations 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 adaptive resource partitioning form a Also fixed or adaptive resource partitioning form a deployment configuration. Specifically, the resource partitioning could be performed in frequency, time or spatial dimensions for interference coordination.Radio Scenarios: Resource partitioningFrequency partitioning First example: soft frequency reuse. Radio Scenarios: Resource partitioningFrequency band for the networkReuse one partitionOrthogonal partitionFrequency partitioning Second example: partly or fully orthogonal partitioning partitionPartially overlap partitionFrequency allocated to macrocellFrequency allocated to femtocellRadio Scenarios: Resource partitioningFrequency partitioning Third example: Configuration based spectrum partitioning Radio Scenarios: Resource partitioningTime partitioningThe resources used in Macro and Home eNBs can also be partitioned and 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 interference coordination. However, it may also bring new interference risks. Further interference mitigation methods based on the time partitioning needs to be studied. be studied.Spatial partitioningDue to uplink-downlink channel reciprocity, TDD HeNBs can use beam coordination to improve interference conditions. For example, the HeNB can avoid beam collision with the Macro or other Home eNBs in a proactive or reactive way. These mechanisms may require a certain amount of information exchange between the HeNBs.Interference ScenariosNumber Aggressor Victim Priority1 UE attached to Home eNode B Macro eNode B Uplink Yes2 Home eNode B Macro eNode B Downlink Yes3 UE attached to Macro eNode B Home eNode B Uplink Yes4 Macro eNode B Home eNode B Downlink5 UE attached to Home eNode B Home eNode B Uplink Yes 5 UE attached to Home eNode B Home eNode B Uplink Yes6 Home eNode B Home eNode B Downlink Yes7 UE attached to Home eNode B and/or Home eNode BOther System8 Other System UE attached to Home eNode B and/or Home eNode BInterference ScenariosDownlink UplinkRF Aspects: HeNB output power From HeNB coverage and capacity point of view, large output power could be attractive. 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 HeNBperformance and the interference towards close-by macrocell users, which do not have access to the HeNB. Based on 3GPP studies the allowed output power of the Home BS is limited to Based on 3GPP studies the allowed output power 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.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 to collect measurements: collect measurements: From connected Mode UEs attached to the HeNB Via a DL Receiver function within the HeNB itself. Such DL receiver function is also called Network Listen Mode (NLM), Radio Environment Measurement (REM) or "HeNB Sniffer". These measurements can also be used during the HeNBself-configuration processLTE Rel.8 measurements in UE RSSI, which is the total received wideband power on a given frequency (from all sources). Reference Signal Received Power (RSRP), which for a particular cell is the average of the power measured (and the average between receiver branches) of the resource elements that contain cell-specific reference signals. elements that contain cell-specific reference signals. Reference Signal Received Quality (RSRQ) is the ratio of the RSRP and the E-UTRA Carrier Received Signal Strength Indicator (RSSI), for the reference signals.HeNB System Measurements (1/4): Measurements from all cellsMeasurement Type PurposeMeasurement Source(s)Received Interference PowerCalculation of UL interference towards HeNB (from MUE)HeNB UL ReceiverFor example, a Received Interference Power measurement value larger thanFor example, a Received Interference Power measurement value larger than a pre-defined threshold would mean that at least an MUE which is interfered by a HeNB is close to the HeNB and that the MUE's Tx power would cause significant interference towards the HeNB. This measurement value may be used in calculating path loss between the HeNB and the MUE assuming that a single MUE dominates the interference. HeNB System Measurements (2/4): Measurements to identify surrounding cell layersMeasurement Type PurposeMeasurement Source(s)Cell reselection priority informationDistinction between cell types based on frequency layer priorityHeNB DL ReceiverCSG status and IDDistinction between cell layers based on CSG, and self-construction of neighbour list, HeNB DL ReceiverHeNB System Measurements (3/4): Measurements from macro cell layerMeasurement Type PurposeMeasurement Source(s)Co-channel RSRPCalculation of co-channel DL interference towards macro UEs (from HeNB)Calculation of co-channel UL interference towards macro layer (from HUEs)Calculation of co-channel UL interference towards HeNB (from MUEs) based on estimated MUE Tx powerDetermine coverage of macro cell (for optimization of hybrid cell configuration)HeNB DL ReceiverMUE (in case of hybrid cell)optimization of hybrid cell configuration)Co-channel RSRQDetermine quality of macro cell (for optimization of hybrid cell configuration)HeNB DL ReceiverMUE (in case of hybrid cell)Reference Signal Transmission PowerEstimation of path loss from to MeNB HeNB DL ReceiverPhysical + Global Cell IDAllow HeNB to Instruct UEs to measure specific cells. Allow UE to report discovered cells to HeNB.HeNB DL ReceiverDetection of UL RS Detection of victim UE HeNB UL ReceiverRSRP = Reference Symbol Received PowerRSRQ = Reference Symbol Received QualityRS = Reference SignalHeNB System Measurements (4/4): Measurements of other HeNB cellsMeasurement Type PurposeMeasurement Source(s)Co-channel RSRPCalculation of co-channel DL interference towards neighbour HUEs (from HeNB) Calculation of co-channel UL interference towards neighbour HeNBs (from HUEs)HeNB DL ReceiverReference Signal Transmission PowerEstimation of path loss from to HeNB HeNB DL ReceiverPhysical + Global Cell ID Allow HeNB to Instruct UEs to measure specific cellsAllow UE to report discovered cells to HeNB.HeNB DL Receiver3GPP 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 is suspended 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 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)SSCHSF-1 SF-2 Legend SF-3PBCHPBCHPBCHPBCHSF-0PSCHPBCH = Physical Broadcast CHannelPSCH, SSCH = Primary and SecondaryCommon macro-eNB and HeNB DL bandwidth allocationSF-2SSCHPSCHPBCHPBCHPBCHPBCHSF-0 SF-1 SF-9Home-eNB, DL frame timing offset by k = 16 OFDM symbolsPSCH, SSCH = Primary and SecondarySynchronization Channels.Femtocell Networks: Some ResearchproblemsSome 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, spectrum usage)spectrum usage) Suppression (e.g. tranceiver algorithms like beamforming and advanced receivers) Interference between macro and femto layers and within femtolayer. Radio resource management Scheduling of resources between macro and femto layers Static vs dynamic resource allocation. Load balancing between macrocells and (open access) femtocellsSome 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 planned (operator deployed femtocells). or planned (operator deployed femtocells). Self-configuration and optimization algorithms needed to tune the network. Mobility Between femtocells and macrocells Between femtocells Mobility issues related to femtocells has not been widelyinvestigated. Critical interference: A Simple Example2Consider DL interference problem of the figure. For the rates there holds:2( )( )m m m m mf f f f fB W A RB W A R + = + =1 log1 log2211 1/ L P= (*)112 , 2 , 12 122 21 , 1 , 21 211 1////f m Nmf m NfI I L P PL PI I L P PL P+ + += + + += In (*) A and B are 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 the shannon bound, in Proceedings of the IEEE 65th Vehicular Technology Conference (VTC 07), pp. 12341238, April 2007.(**)Critical interference: A Simple Example In general there holds: Let us consider an illustrative example where we ignore terms If,kand Im,krelated to other femtocell and macrocell interference. Then2 1 21 12 22 11, , P P L L L L interference suppression can be used to suppress only part => interference suppression can be used to suppress only partof the interference. Femto eNB transmit power allocation/control Tool for system optimization. Not necessarily effective approach when solving instantaneousinterference problems.Example: Dynamic usage of frequencyresources Femtocell operation frequency can be limited since SNR is usuallyhigh and number of users is small in femtocells. Macrocell operation frequency cannot be limited since SNR can below 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 thatare close to femtocells. Question: How macrocell know that some of its users heavily suffer fromfemtocell interference?Exclusive resources for macrocell usersResources used by bothmacrocell and femtocellsExample: Dynamic usage of frequencyresourcesUE measures strength of the received signal fromHeNB and detect its IDHeNB BroadcasttransmissionUE send measurementresults to Macro eNB 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 eNBcan assign exclusive resources for UE. In this approach there is a trade-off between user rates on femtocelland macrocell.transmission results to Macro eNBInterference problemwithin the femto layer:Illustration Part of a row building including 3 closed femtocells and 3 terminals. Radio operations on the same frequency carrier.1221frequency carrier. Dedicated signal: Green arrows Interference: Red arrows33Managing 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 eNBsand Closed Subscriber Group configuration is used. Power allocation can be used to optimize femtocell operations. Two phases of power allocation: Two phases of power 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 offby users => local femto network topology is dynamicDistributed 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 reportsCell 1Femto manager(connected to macro network)Measurement reportsCell Kmacro network)All measurementinformation is forwardedto the femto manager.Practical femto networks are in between the extremes.Different manufacturers may apply different management conceptsFemto manager (or network OA&M) maycreate interference matrices etcInterference problem within the femto layer: Power calibration problem example Performance criteria example: 0, ,, ,, += k mm k j m Nk k j kk jL P PL PNB femto thand cell thinl th termina betweenloss Path NB femto thinpoweronTransmissi==km k j Lk P(*) The SINR requirement (*) aims to guarantee a certain level of service for femtocell users. We say that user is in outage if SINR is smaller than 0. power noise Whitereceiver in the SINR RequiredNB femto thand cell thinl th termina betweenloss Path 0, ,== =Nm k jPm k j LInterference problem within the femto layer: 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 criteria is widely used when mobile system performance is ( )out kPr Pr0= < (**)criteria is widely used when mobile system performance is evaluated. Criteria (**) assumes that a certain service level (e.g. in terms of bits/s/Hz) is achieved in kth cell with a given probability.Interference problem within the femto layer: Power calibration problem example The power allocation problem when goal is to guarantee a certain service level with minimum transmission powers:( ))` = < = =KkKkk k kout kkP P P PP1 1max0: minPr Pr:Find(***) Remark: The maximum transmission power may depend on the distance between femtocell cluster (e.g. building) and the macrocelleNB. Remark: Problem setting in (***) can be also extended to cover option for dynamic frequency resource allocations. Remark: due to small number of users within femtocells the problem (***) may suffer from scarce statistics. ) k k 1 1Femtocell Networks: The FutureGeneral future developments 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 a global success. Femtocell concept provides a natural playground for various flexible Femtocell concept provides a natural playground for various flexiblespectrumusage 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: Denseheterogeneous networks In addition to femtocellsvarious M2M communication may takeplace on mobile communication spectrum. The priorities of differentconnections may vary asconnections may vary as well as QoSrequirements. This may lead to localheterogeneous systemswhere femtocells need to share the spectrum withM2M communication.Some reference materialGeneral level publications 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 Management and M. Yavuz et al: Interference Management and Performance Analysis of UMTS/HSPA+ Femtocells, IEEE Communications Magazine, September 2009. V. Chandrasekhar, J. G. Andrews, and A. Gatherer: Femtocell networks: a survey, IEEE CommunicationsMagazine, vol. 46, no. 9, pp. 5967, 2008.3GPP Home (e)NodeB Concept 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 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 O. Simeone, E. Erkip, and S. Shamai Shitz: Robust Transmission and Interference Management For Femtocells with Unreliable Network Access, IEEE Journal on Selected Areas in Communications, vol. 28, no. 9, December 2010. S. Park, W. Seo, et al: Beam Subset Selection Strategy for Interference Reduction in Two-Tier Femtocell Networks, IEEE Transactions on Wireless Communications, 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, October 2010. October 2010. M. Husso et al: InterferenceMitigation by Practical Transmit BeamformingMethods in Closed Femtocells, EURASIP Journal on Wireless Communications 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.