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Heterogeneous LTE Networks and Inter-Cell Interference Coordination

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  • 1. Heterogeneous LTE Networksand Inter-Cell InterferenceCoordinationVolker Pauli, Juan Diego Naranjo, Eiko Seidel IntroductionNomor Research GmbH, Munich, Germany As explained a macro-cell deployment is carefullyDecember, 2010 planned prior to its roll-out and some optimization can be performed after roll-out to get the best of such a system. Nevertheless, future increase of performance in suchSummary homogenous networks is limited particularly inInitial deployments of LTE networks are based case of unequal traffic or user distribution.on so-called homogeneous networks consistingof base stations providing basic coverage, calledmacro base stations. The concept of An elementary and well-known strategy toheterogeneous networks has recently attracted increase the capacity of a cellular network is toconsiderable attention to optimize performance reduce the cell size. The underlying effect is toparticularly for unequal user or traffic further increase the frequency reuse, also knowndistribution. Here, the layer of planned high- as “cell-splitting gain”. For cost optimizationpower macro eNBs is overlaid with layers of different types of eNBs are used for differentlower-power pico or femto eNBs that are purposes, e.g. large-scale eNBs for basicdeployed in a less well planed or even entirely coverage, smaller eNBs to fill coverage holes oruncoordinated manner. Such deployments can to improve capacity in hot-zones or at theachieve significantly improved overall capacity boundaries between large-scale eNBs’ coverageand cell-edge performance and are often seen as areas, and possibly even smaller eNBs for indoorthe second phase in LTE network deployment. coverage.This paper discusses the concept of LTE is designed for a frequency reuse of 1,heterogeneous networks as compared to meaning that every base station uses the wholehomogeneous networks. It demonstrates the system bandwidth for transmission and there isneed for inter-cell interference coordination no frequency planning among cells to cope with(ICIC) and outlines some ICIC methods that are interference from neighbouring cells. Hence, LTEfeasible with release 8 /9 of the LTE standard. macro-cell deployments experience heavySystem-level simulation results illustrate the interference at the boundaries of the cells.benefits of the various features discussed in the Placing a new eNB between macro-cells wouldfollowing. boost the SINR levels for users located there, achieving a more uniform user satisfaction and overcoming link-budget constraints.Nomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 1/9
  • 2. Similarly, the deployment of eNBs inside always easy, although some function might bebuildings is a reasonable strategy, as it fills specific to different types, e.g. home eNBcoverage holes that typically occur due to the specific functions summarized in [1]. Basepenetration loss imposed by the walls, while it stations of different output power are defined incauses relatively little interference to the macro TS36.104 [5] with the following classification:network for the same reason. Hence, femto-celldeployments are being investigated vigorously in BS class Output power per Txindustry and in standardization bodies. antenna Wide Area BS no upper limitSince the backhaul connection of many small < + 24 dBm (1 antenna) Local Area BSsize base stations at remote locations can < + 21 dBm (2 antennas)increase operational expenditures significantly, < + 18 dBm (4 antennas)alternative backhaul techniques using the LTE air < + 20 dBm (1 antenna) Home BSinterface are a hot topic as well. The backhaul < + 17 dBm (2 antennas)connection to the base station, serving as relay < + 14dBm (4 antennas)node in this case, can be on the same frequency(in-band) or on different frequencies (out-band).In-band relays have been standardised in LTE The wide area or macro base station does notRelease 10 specification while the out-band have a limited output power. The local area baserelays can be used without Release 8/9 stations or pico base stations on the other handspecification. Another backhaul alternative is the are fully-featured eNBs with lower transmissionuse of remote radio heads (RRH). power, reduced size and cost that can be deployed easily for improving conditions in coverage holes and providing higher data-ratesLayers of Heterogeneous Networks in LTE at cell edge or in hot-spots. The home baseIn the previously described cases new types of station or femto-cell has a limited power that isnodes need to be installed in addition to the actually lower than a terminal output power. Themacro base stations resulting in a so-called restriction is applied to minimize the impact in“heterogeneous network” (HetNet). The new severe interference conditions fromnetwork elements might be pico- or femto- base uncoordinated deployment. It is typicallystations, Remote Radio Heads (RHH) and/or operated in a closed mode, meaning that onlyrelays nodes. certain users that are part of a “closed subscriber group” (CSG) are allowed to connect to such an eNB. Interference Scenarios in HetNets Typically, pico-eNBs have a relatively small coverage area due to their low transmit power. If not placed specifically in a hot spot, only a Figure 1: Heterogeneous multilayer cellular small number of UEs will connect to the pico-eNB environment which will limit the gain from offloading theIn Figure 1, such a multi-layer deployment is traffic from the macro cells. Signalling thatillustrated, where a macro-cell deployment is supports load balancing between the macro andsupported by several low-power nodes, all of the pico-eNB has been standardized and iswhich solve specific issues as discussed above. commonly used. This can be achieved by biasingClassification into different types of nodes is not handover decisions between the different eNBsNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 2/9
  • 3. such that UEs are handed over to pico-eNBs Figure 2 shows an overview of a number ofearlier than usual, thus shifting load from the frequency partitioning methods. These ICICmacro-eNB to the pico-eNB. As this corresponds methods describe basic rules on how a systemto an expansion of the range of the pico-eNB, performance boost can be achieved by managingthis feature is typically referred to as “range the system bandwidth and transmit power. Theexpansion”. Similarly cell selection parameters following discussion will introduce generalcan be adapted for users that are idle. notions about frequency partitioning and the available options for LTE release 8/9.Range expansion is not only effective foroptimizing the use of resources in the system,but also for reducing the frequency of hand-overs, hence improving system throughput anduser experience [2].However, with range expansion the UEs handedover to the pico-eNB suffer from lower thanusual SINRs, a situation that other techniqueslike beamforming and power boosting would notbe able to completely resolve [2]. So, despite theabove benefits this feature presents a challenge Figure 2: Different inter-cell interferenceto interference management for HetNets, and coordination schemesthe deployment of appropriate ICIC algorithmscan further boost system performance. Full Frequency Reuse Full Frequency Reuse means that no frequencyFurthermore, when considering CSGs, yet partitioning is performed between eNBs of theanother challenge for interference management same network. eNBs in this configurationarises if UEs are in the coverage area of a home- transmit with uniform power over the entireeNB, typically well shielded from the macro-eNB, system bandwidth. This is the conventional waybut are not allowed access to it. This creates of operating an LTE network. The main pitfall tocomplex high-interference scenarios in both this configuration is that cell-edge userstransmission directions that cannot easily be experience heavy interference from neighbouringsolved. In the downlink such a “macro-UE” is the cells in the downlink and create heavyvictim being exposed to heavy interference from interference in the uplink, greatly degrading theirthe home-eNB, whereas in the uplink the macro- communication performance.UE is the aggressor severely disturbingtransmissions to the home-eNB, cf. e.g. [2]. Hard Frequency Reuse This ICIC method is typically seen in GSMICIC Methods networks, when it comes to distribution ofAs motivated in the previous section, Inter-Cell frequencies among the cells. When applied toInterference Coordination (ICIC) plays a vital LTE it means that the sub-carriers are dividedrole in heterogeneous networks. ICIC techniques into 3, 4 or 7 disjoint sets. These sets of sub-in LTE are mostly limited to the frequency carriers are assigned to the individual eNBs indomain, e.g. only partial use of resources in such a way that neighbouring cells dont use thefrequency direction and/or adaptation of power same set of frequencies. This reduces thelevels. interference at the cell edge of any pair of cellsNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 3/9
  • 4. significantly and can be considered the opposite SINR levels for cell-edge UEs in the high-extreme to Full Frequency Reuse in matters of transmit power regions, and the other regionsfrequency partitioning techniques. While user still have suitable SINR levels for cell-center UEs,interference at cell edge is maximally reduced, as it is depicted in Figure 3. It can nicely be seenthe spectrum efficiency drops by a factor equal that the high power region serves a largerto the reuse factor. coverage area. This scheme is particularly useful for ICIC in the downlink.Fractional Frequency ReuseThis is a hybrid frequency partitioning combiningthe concepts of the two previous schemes. Itconsists of dividing the spectrum into two partswhich will have different frequency reuse. Onesection of the system spectrum is used in allcells, while the other part of the spectrum isdivided among different eNBs as in hardfrequency reuse. The idea is that the eNB wouldassign the fully-reused frequency chunks tocenter-cell UEs and the other chunks to the cell-edge UEs. This scheme is particularly useful forICIC in the uplink, where severe interferencesituations can occur where the user is locatedclose to a strong interferer in the neighbor cell. Figure 3: SINR strength distribution over the cellSoft Frequency Reuse with soft frequency partitioning.In this frequency partitioning method an eNBtransmits in the whole system bandwidth, but Note that in the case of HetNets it may also beusing a non-uniform power spectrum. useful to block some of the resources available in frequency direction entirely at the pico or femtoFigure 2 (bottom) illustrates power spectrum eNBs but let the macro eNB use the entireassignments in the different cells of a system spectrum. It grants resources to the macro cellswith Soft Frequency Reuse and reuse 3. It can that are not interfered by the smaller eNBs, butbe noticed that in the spectrum there is a region comes at the price of reduced throughput for theof high-power transmissions and some regions of small eNBs. It does not necessarily result in alow-power transmissions. Using a similar reduced user throughput since the number ofstrategy as the previous method, resources in UEs connected to the small eNBs is usuallythe high-power region are preferably assigned to limited.UEs located at the cell edge, while cell-centerUEs are typically assigned resources in the low- X2 Signalling to support ICICpower regions.1 The already discussed partitioning schemes can in principle be easily implemented in the radioThis coordination scheme leads to improved resource management (RRM) of the eNB. They can be configured statically and run without1 Note, however that UEs can only be interaction between different eNBs. However,assigned to either of the regions and better performance can be achieved if the abovereassignments require RRC signaling. schemes are configured dynamically, based onNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 4/9
  • 5. active information exchange between eNBs in sent out proactively by eNBs, this indicator isorder to better adapt to the current state of the only triggered when high-interference in thenetwork. X2 signalling provides the mechanisms uplink direction is detected by an eNB. Thefor the exchange of this information, cf. [3] and overload indication will be sent to neighbourreferences therein. eNBs whose UEs are potentially the source of this high interference. The message contains aNote that the signalling and its information low, medium or high interference level indicationelements are well defined in 3GPP specifications, per PRB. The question, which cell is responsiblebut the specific reaction of an eNB in response to for the high interference is of course not a trivialthe reception of such a message is not. Hence, one.in a multi-vendor network, there is noguaranteed reaction of an eNB to an incoming It can be seen that specifications for Release 8/9ICIC-related message from another eNB. were defined having mostly homogeneous macro-cell deployments in mind. It provides onlyThe following sections describe a set of X2 relatively simple means for ICIC and fails formessages meant for ICIC in Release 8/9. instance to address mechanisms to improve inter-cell interference on control channels, e.g. PDCCH, PHICH and PCFICH. This limits forRelative Narrowband Transmit Power Indicator, example the aggressiveness in which rangeRNTP expansion can be configured and hence theThis information message is sent to neighbour gains that can be achieved using HetNets witheNBs. It contains 1 bit per physical resource range expansion. Release 10 will enhance the X2block (PRB) in the downlink, indicating if the signalling in order to support more sophisticatedtransmission power on that PRB will be greater ICIC algorithms, involving interferencethan a given threshold. Thus, neighbour eNBs coordination among eNBs in time domain [6].can anticipate which bands would suffer more Nomor Research will report in one of its nextsevere interference and take the right scheduling newsletter about progress in 3GPPdecisions immediately rather than relying on the standardisation.UEs CQI reports only. Simulation ResultsHigh Interference Indicator, HII In this section, we present some system-levelThis indicator for uplink transmissions works simulation results to illustrate the effects of thesimilarly to the previous RNTP message for the various features described above. For thisdownlink. There is one bit per PRB indicating if purpose 3 scenarios are evaluated. The firstneighbouring eNBs should expect high scenario corresponds to a macro-cellinterference power in the near future. Hence, deployment. The second scenario adds a layer oftypically only PRBs assigned to cell-edge UEs are pico-eNBs on top of the macro-cell deployment.indicated. RSRP measurements as part of The third scenario includes Range Expansion,handover measurement reports can identify cell- additionally to the HetNet layout. Rangeedge UEs. In a similar way this indicator can be expansion is achieved by setting the handoverused to identify the bands used in a frequency biases between pico-eNBs and macro-eNBs to 4partitioning scheme. dB.Interference Overload Indicator, OI These scenarios are configured according to theWhile the previously described X2 messages are guidelines for Case 1 described in the 3GPPNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 5/9
  • 6. technical reports for simulation of HetNets [4]. second improvement in throughput is achievedIn summary, the simulations are fully dynamic when range expansion (RE) is used. By thewith full RRM functionality such as scheduling handover of macro-UEs to pico-eNBs traffic is(simple proportional fair strategy in this case), offloaded from the macro-cell layer to a numberlink adaptation, hybrid ARQ, etc. Furthermore, of pico-eNBs.the users are moving with 3km/h, and they carry From Figure 6 it can be concluded that theout handovers at the cell boundaries as specified introduction of RE in the system lowers thein 3GPP (handover margin/hysteresis is set to effective SINRs.3dB). 30 UEs are dropped per macro-cell areaaccording to configuration 4a, where 2 UEclusters consisting of 2 UEs each are located ineach macro-cell and a pico-eNB is placed in themiddle of every UE cluster. The remaining UEsare dropped uniformly in the macro-cell area.Figure 4 illustrates an example of such adeployment. Figure 5: PDCP Throughput, Configuration 4a with 2 UE clusters per macro Figure 4: Cell layout for configuration 4a and 2 pico-eNBs per macro-cell scenario.Figure 5 and 6 show the CDFs of PDCP Figure 6: Effective SINR CDF, configuration 4a withthroughput and Effective SINR results obtained 2 UE clusters per macro.in this scenario, respectively. This happens because of the high interferenceThe first observation is the improvement of experienced by UEs in the extended pico-cellthroughput achieved by the introduction of a area. This is however more than compensatedpico-cell layer to the macro-layer scenario. A by the fact that the UEs connected to the pico- eNBs get significantly more resource than whenNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 6/9
  • 7. connected to the macro-eNB due to a lower load. ICIC technique is that the optimal configuration depends very much on the specific scenario, i.e.Furthermore, we evaluated a simple static ICIC the location of the pico-eNB in the macro-cellscheme for downlink. Here, frequency- and on the distribution of UEs and the traffic inpartitioning is performed in a way that macro- the vicinity of the pico-eNB. A semi-staticeNBs transmit in the full frequency spectrum (50 approach continuously adapting the frequencyPRBs) while pico-eNBs transmit in a reduced partitioning on a slow basis provides furtherband (30 PRBs), which means that the pico-eNB potential for performance enhancements.is not generating interference in the remainingunused downlink resources. Hence a frequency Configuration 4b [4] with 2 pico-eNBs was alsoselective scheduler at the macro-eNB, after simulated under the same RE+ICIC scenario.evaluating the measurement reports, would take The basic difference between configuration 4aadvantage of this effect. The macro-eNB would and 4b is the dropping of UEs. In Configurationassign the low-interference frequency segments 4b 10 UEs are dropped per UE cluster, but theto macro-UEs located close to the pico-cells, total number of UEs is still set to 30 per macro-protecting these macro-UEs from the cell. Figure 8 shows an example of such ainterference from the respective pico-eNB. In scenario, where the higher density of UEsthis scenario it can be observed that no around pico-eNBs is visible compared tosignificant variation is experienced at the PDCP configuration 4a in Figure 4.throughput CDF, although a slight increase in theSINR is seen, indicating a trade off betweenresources and transmission power that rendersin same throughput performance for pico-cellUEs. Figure 7 shows the TPs for the RE scenariowith and without this ICIC strategy. Figure 8: Cell layout for configuration 4b and 2 pico-eNBs per macro-cell scenario. Figure 7: PDCP Comparing effects on TP due to ICIC strategy, configuration 4a. Figure 9 shows the results for this configuration. In this case, the number of UEs connected to theIt can be concluded that severe interference pico-eNBs and hence the offloading gain fromimpact from pico- or femto-cells onto the macro- introducing pico-eNBs is already very significant.cells in HetNet deployments could be resolved Expanding the range of the pico-eNBs actuallyusing this technique. A problem with this static degrades the performance as the pico-eNBsNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 7/9
  • 8. become overloaded. Similarly, using statically in an upcoming newsletter from Nomorconfigured ICIC as described above will further Research.reduce the resources available for the pico-UEsleading to overall performance degradation. ReferencesHere, a dual ICIC scheme, leaving some [1] Nomor Newsletter, “LTE Home Node Bs andresources to the pico-UEs only would be more its enhancements in Release 9”.beneficial. [2] R1-100700, Interference conditions in Heterogeneous Networks. 3GPP TSG RAN 1 meeting #59 bis. [3] TS 36.420, E-UTRAN; X2 General Aspects and Principles; Release 9, December 2009. [4] TS 36.814, E-UTRAN; Further advancements for E-UTRA physical layer aspects; Release 9, March 2010. [5] TS 36.104 E-UTRA; Base Station (BS) radio transmission and reception; Release 9, Oct 2010 Figure 9: Comparing effects on TP due to ICIC strategy, configuration 4b. [6] RP-101229 RAN2 CRs on Core part:So again, it becomes clear that both range Enhanced ICIC for non-CA based deployments ofexpansion and ICIC must be configured heterogeneous networks for LTE RAN2; 3GPPdynamically depending on the scenario to TSG RAN meeting #50achieve best performance. LTE Discussion Forum http://www.linkedin.com/groups?gid=1180727ConclusionsHeterogeneous Networks constitute a promising Note: This white paper is provided to you by Nomorconcept to improve system performance, to Research GmbH. Similar documents can be obtained frommitigate coverage holes, to provide a uniform www.nomor.de. Feel free to forward this issue inuser experience over the entire cell area and last electronic format. Please contact us in case you arebut not least to satisfy high traffic demands in interested in collaboration on related subjects.hot-zones. Properly configured range expansionhas the potential to increase the gains as it leadsto better load balancing over the different Disclaimer: This information, partly obtained fromnetwork layers. The applicability of range official 3GPP meeting reports, is assumed to be reliable, but does not necessarily reflect the view ofexpansion is, however, limited by inter-cell Nomor Research GmbH. The report is provided forinterference calling for advanced ICIC methods. informational purpose only. We do not accept anyUnfortunately, support of ICIC is limited to data responsibility for the content of this newsletter. Nomorchannels in LTE release 8 / 9. Further Research GmbH has no obligation to update, modify or amend or to otherwise notify the reader thereof in theenhancements of ICIC support particularly for event that any matter stated herein, or any opinion,control channels are in the process of projection, forecast or estimate set forth herein,standardisation and will be included in Release changes or subsequently becomes inaccurate.10 specification. A related report will be providedNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 8/9
  • 9. System Level Simulation ServicesNomor Research has developed a comprehensive The simulator can be used on project basis or insimulation environment with fully compliance to customized simulation campaigns. Thestandards such as LTE, LTE Advanced and HSPA performance of the system level simulator hasand offers related services to support research, been calibrated to simulation results obtained indevelopment and standardisation. standardisation.Features of the dynamic multi-cell, multi-user Research on advanced algorithms include, butsystem level simulator include: are not limited to:• macro-cell and HetNet deployments (pico-, • advanced features as link adaptation, HARQ, femto-cell, relay nodes) power control, measurements• flexible base station and user configurations • scheduling and resource allocation and drop models algorithms considering channel and buffer• different transmitter and receiver chains incl. status, QoS etc. MIMO, ZF, MMSE • inter-cell interference coordination,• channel modeling with slow/fast fading, avoidance and cancellation pathloss, full user mobility • single user-, multi-user MIMO with open and• intra- and intercell interference modeling for closed loop feedback OFDMA, SC-FDMA and WCDMA • cooperative multi-point transmission and• simultaneous up- and downlink simulation reception• 2D and 3D antenna pattern & beam forming • functions for self-organising and self-• Extensive metrics and KPIs: capacity, optimizing networks (e.g. load balancing, throughput, spectral efficiency, user QoS etc mobility optimization, tilt optimisation, range extension, power saving etc. )• High-speed processing close to real-time using multi-core & graphic card processors • radio resource management for hetero- geneous networks If you are interested in our services please contact us at info@nomor.de or visit us at http://www.nomor-research.com/simulationNomor Research GmbH / info@nomor.de / www.nomor.de / T +49 89 9789 8000 9/9