LTE/LTE-A Interference Coordination for Femtocells
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LTE/LTE-A Interference Coordination for Femtocells

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Blog article with link to video is available at: http://3g4g.blogspot.com/2012/03/docomo-euro-labs-ltelte-interference.html

Blog article with link to video is available at: http://3g4g.blogspot.com/2012/03/docomo-euro-labs-ltelte-interference.html

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    LTE/LTE-A Interference Coordination for Femtocells LTE/LTE-A Interference Coordination for Femtocells Presentation Transcript

    • Copyright © 2012 DOCOMO Communications Laboratories  Europe GmbH  LTE/LTE‐A Interference Coordination for  Femtocells BeFEMTO Winter School February 6‐10 2012 Zubin Bharucha DOCOMO Euro‐Labs Munich, GermanyAcknowledgements: Serkan Uygungelen; Nobuhiko Miki (NTT DOCOMO) Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group
    • In a nutshell • Part 1: Refresh your memory! – LTE and LTE‐A – The road to the future – An overview of ICIC techniques • Part 2: Femto‐macro interference – Relevant details of the LTE air interface – Performance comparison of existing techniques – Introduction of a novel technique to protect non‐CSG users • Part 3: Femto‐femto interference – Network „densification“ and its effects – Centralized interference mitigation – Distributed interference mitigation • ConclusionCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 2
    • Part 1: Know your LTE‐ A  (B,Cs)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 3
    • What’s so great about LTE?• LTE System performance – Long‐term evolution of 3G using 3G  LTE‐Advanced spectrum Smooth introduction of  4G – Smooth introduction of 4G ~100 MHz bandwidth• LTE‐Advanced 5~20 MHz bandwidth – Evolution of LTE: Targets LTE achievement of sufficiently higher system performance than that for Long‐term  evolution of 3G LTE • Bandwidth: 100 MHz • Peak throughput: 1 Gbps HSUPA – Backward compatible with LTE to  HSDPA enable continuous enhancement  and deployment – Meet or exceed IMT‐Advanced  WCDMA Release 99 requirements within the ITU‐R time  plan 2000’s 2010’s Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 4
    • The old and the new • LTE‐Advanced shall be deployed as an evolution of LTE Rel. 8 with new  bands available • LTE‐Advanced shall be backwards compatible with LTE Rel. 8   Smooth and flexible system migration from LTE Rel. 8 to LTE‐Advanced  An LTE‐A UE works in an LTE cell  An LTE UE works in an LTE‐A cell • LTE‐Advanced contains all features of LTE Rel. 8&9 and additional features  for further evolutionCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 5
    • Target Performance for LTE‐Advanced LTE Rel. 8 LTE‐Advanced DL 300 Mbps 1 Gbps Peak data rate UL 75 Mbps 500 Mbps Peak spectrum efficiency  DL 15 30 [bps/Hz] UL 3.75 15 Ant. Config. LTE Rel. 8*1 LTE‐Advanced*2 2‐by‐2 1.69 2.4 DL 4‐by‐2 1.87 2.6 Capacity 4‐by‐4 2.67 3.7 [bps/Hz/cell] 1‐by‐2 0.74 1.2 UL 2‐by‐4 – 2.0 x 1.4‐1.7 2‐by‐2 0.05 0.07 Cell‐edge user  DL 4‐by‐2 0.06 0.09 throughput  4‐by‐4 0.08 0.12 [bps/Hz/cell /user] 1‐by‐2 0.024 0.04 UL 2‐by‐4 – 0.07* Target peak data rate of 1 Gbps for nomadic/local areas is specified in Circular Letter (CL)*1 See TR25.912 (Case 1 scenario)  *2 See TR36.913 (Case 1 scenario)  *3 See ITU‐R M.2135 (Base Coverage Urban scenario) Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 6
    • What’s new in LTE‐A? • Wider bandwidth (carrier aggregation) – Improves peak data rate and spectrum flexibility – Meets ITU‐R requirements for bandwidth (>=40  MHz) – Spectrum/carrier aggregation based on  component carrier (CC) concept to maintain  backward compatibility and allow smooth  network migration • Advanced MIMO techniques (covered yesterday) – Improves peak data rate and cell/cell‐edge  spectrum efficiency – Meets ITU‐R requirements for DL cell spectrum  efficiency – SU‐MIMO with up to 8‐layers for DL and 4‐layers  for UL – MU‐MIMO with enhanced CSI feedbackCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 7
    • What’s new in LTE‐A?• Enhanced inter‐cell interference coordination (eICIC) – Improves cell‐edge user throughput, coverage, and deployment flexibility – Interference coordination for layered cell deployment with different transmit power levels – Carrier aggregation can be used for frequency domain coordination – Time domain coordination and power control are also to be introduced• Relaying – Improves coverage and cost effective deployment – Type 1 relay node which can be seen as a Rel. 8 eNB from a Release 8 LTE terminal• Coordinated multipoint (CoMP) transmission and reception – Scope is limited to intra‐eNB CoMP (implementation issue) – LTE Self Optimizing Network (SON) enhancements – HNB and HeNB mobility enhancements Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 8
    • HeteroGenius NetworksMotivation Characteristics•4G networks will be characterized by a high‐density • Wireless backhauldeployment of low‐power nodes • Open access• It is essential for these nodes to operate without negatively • Operator‐deployedaffecting the overall performance Major Issues • Effective backhaul design • Mitigating relay to macro‐ cell interferenceCharacteristics• Wired backhaul Characteristics• Open access • Wired backhaul• Operator‐deployed • Closed accessMajor Issues • User‐deployed• Effectively offloading Major Issuestraffic from macro‐cell • Mitigating femto‐to‐macro• Mitigating interference interferencecaused to macro‐cell • Mitigating interferenceusers between nearby femto‐cellsCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 9
    • HeteroGenius NetworksMotivation Characteristics•4G networks will be characterized by a high‐density • Wireless backhauldeployment of low‐power nodes • Open access• It is essential for these nodes to operate without negatively • Operator‐deployedaffecting the overall performance Major Issues • Effective backhaul design • Mitigating relay to macro‐ cell interferenceCharacteristics• Wired backhaul Characteristics• Open access • Wired backhaul• Operator‐deployed • Closed accessMajor Issues • User‐deployed• Effectively offloading Major Issuestraffic from macro‐cell • Mitigating femto‐to‐macro• Mitigating interference interferencecaused to macro‐cell • Mitigating interferenceusers between nearby femto‐cellsCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 10
    • Why do we need interference management with femtocell deployment? Significant femto interference for nearby macro UEs!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 11
    • Overview of ICIC in LTE/LTE‐A • LTE (Rel‐8/9) – Only one CC is available – Make do with what you have and devise interference management  techniques assuming that macro and femtocells use the same CC – Frequency‐domain ICIC ? – Time‐domain ICIC within one CC? • LTE‐Advanced (Rel‐10/11) – Multiple CCs available in the system – Frequency‐domain ICIC over multiple CCs is possible – Time‐domain ICIC within one CC is also possible – Much greater flexibility for interference managementCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 12
    • Sharing is caring • Fractional frequency reuse (FFR) improves the throughput for UEs close to  the cell boarder – Protecting UEs close to cell boarder employing frequency reuseCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 13
    • Sharing is caring, but keep us informed • Relative Narrowband Transmit Power (RNTP) is exchanged between macro eNBs via a backhaul interface (X2 interface) – The bitmap indicates whether transmission power of respective RB exceed the predetermined threshold or notCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 14
    • Rel‐10 ICIC in heterogeneous networks • To support femtocell deployment effectively, ICIC is necessary • Different from homogeneous network (macrocell deployments),  – Low power nodes (femto eNBs) must mute (or reduce transmission  power)  Named as “Protected resources” here – High power nodes (macro eNBs) need not mute  Named as “Non‐protected resources” here • Protected/Non‐protected resources are multiplexed in frequency or time‐ domain  Both ICIC techniques are effectively supported in Rel‐10 Frequency-domain ICIC Time-domain ICIC Frequency Frequency Carrier Carrier Carrier #1 #2 #1 Time Time Cell layer Macro layer Cell layer Femto layerCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 15
    • Frequency‐domain ICIC for LTE • Frequency‐domain ICIC for data channel is already supported from Rel‐8/9 employing RNTP, although frequency‐domain ICIC for control channel is not supported – Data channel is multiplexed in limited bandwidth, i.e., at RB‐level to obtain multi‐user diversity in the frequency‐domain – Control channel is multiplexed in the entire bandwidth to obtain frequency‐diversity • Here, control channel means downlink shared control channel (PDCCH) which sends the assignment information of UEs, and must be decoded correctly before decoding data channel (more on that later!)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 16
    • Frequency‐domain ICIC for LTE‐A • Multiple CCs are employed to perform ICIC for control channel • In order to indicate the assignment for different carriers, additional bits  (CIF: Carrier Indicator Field) is introducedCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 17
    • Time‐domain ICIC • In order to apply time‐domain ICIC, femto eNBs must mute specific  subframes to protect UEs connected to macro eNBs • However, cell‐specific reference signal (CRS) needs to be sent for  handover measurements, etc.  Known in the 3GPP community as “Almost blank subframes (ABSs)” • There are issues with CSI measurements on protected and non‐protected  subframes at the macro layerCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 18
    • What else? • Cell‐specific reference symbol (CRS) interference is a major issue • Additional mechanisms to cope with the CRS interference are under  discussion – Non‐zero transmit power ABS – CRS cancelation at UE – Transmitter side processing (sending interfering cell lists) – Etc.Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 19
    • Part 2: A comparison of  state‐of‐the‐art ICIC  techniquesCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 20
    • The almighty grid – the LTE frame structure • A lot of work has been done on data region interference mitigation • In this work, we focus on the control region because if it cannot be  decoded, the data region (and therefore the whole subframe) is anyway  lostCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 21
    • Introducing the control channels: PCFICH The control channel is  1/2/3 OFDM symbols  long!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 22
    • Introducing the control channels: PHICH OK Mr. UE, I’ve  received your UL  transmissions!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 23
    • Introducing the control channels: PHICH OK Mr. UE, I’ve  received your UL  transmissions!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 24
    • Introducing the control channels: PDCCH Hey you UE! Here are  your DL and UL  grants: x/y/z RBs!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 25
    • What the control region really looks like • The control region contains 3 control channels: – PCFICH: occurs only on first OFDM symbol; scattered in frequency  domain; indicates size of control region – PDCCH: spread in time and frequency; carries scheduling information – PHICH: spread in time and frequency; contains HARQ information • We focus on the performance of the first two because of differences in  their distribution patterns – the PCFICH has restricted positions in the time  domain, whereas the PDCCH is dispersed in the time and frequency  domainsCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 26
    • What is already done (a) (b) (c) •No coordination • Femto control •Almost blank subframe Heavy channel sparseness Only interference from interference on 2 Interference to reference symbol OFDM symbols first OFDM symbol Femto data transmission is lowered is not allowedCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 27
    • Enter my apartment at your own peril! • 5x5 grid model • Macro users uniformly distributed • Trapped macro UEs are the focus of attentionCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 28
    • System setup (simulation parameters) Parameter Value Avg. 5x5 blocks per sector 4 Avg. macro UEs per sector 10 Inter-site distance 500 m HeNB activation probability 10% System bandwidth 10 MHz eNB transmit power 46 dBm HeNB transmit power 20 dBm Wall penetration loss 20 dBCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 29
    • Results (1/3): PDCCH performance for trapped macro UEs • Significant improvement over benchmark • Sparseness also degrades femto‐to‐femto performance (not seen here)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 30
    • Results (2/3): PHICH performance for trapped macro UEs • Macro performance improves • Femto performance degrades (not seen here)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 31
    • Results (3/3): PCFICH performance for trapped macro UEs • Macro performance improves, but is still not good enough • Femto performance degrades, but is acceptable (not seen here)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 32
    • Discussion • The backward compatible macro‐to‐femto interference mitigation  techniques are good for PDCCH • However, their performance for the PCFICH is poor • The next section specifically deals with PCFICH protection for trapped  macro UEs • Once again, backward compatibility is key!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 33
    • Things others are doing • Closed Subscriber Group (CSG) ID manipulation [3GPP TR 36.921]. – The HeNB changes between a default CSG ID (assigned at deployment  time) and a dedicated (operator configured) CSG ID. – When there is a nearby macro UE, the HeNB uses the dedicated CSG ID  so that the UE can access the HeNB, otherwise it uses the default. The HeNB needs to be aware of when a macro UE is near it to trigger  CSG ID selection. Centralized controller is required to ensure that no HeNB uses either  CSG ID for a long time. Heavy signaling burden. • Physical Cell Identity (PCI) reservation – It is possible to reserve a subset of available PCIs for HeNB use No interference coordination through this approachWe actively change the PCI of the HeNB at startup so that it causes the lowest collision with the PCFICH of the trapped macro UE!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 34
    • Why is the PCFICH so important? • The PCFICH is important to protect because – Our past work has shown that it exhibits the worst SINR performance compared to the other control channels. – So far it has not been possible to satisfactorily protect the PCFICH from femto‐ cell interference. – If the PCFICH is incorrectly decoded by the trapped macro UE, the subframe is lost. • Further advantages: Since HeNBs serve a small number of users (with typically a low PDCCH aggregation level), the control channel is sparse enough to allow for the rearrangement of PCFICH, PHICH and PDCCH on the femto layer. This proposal can easily handle PCFICH protection for macro UEs trapped within the coverage of multiple HeNBs.Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 35
    • How are PCFICH elements physically mapped? • The 16 PCFICH resource elements are distributed over the entire frequency spectrum. • The PCFICH always occurs on the first OFDM symbol. • The location of the PCFICH resource elements undergoes an offset depending on the physical cell identity (PCI). x is an integer Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 36
    • And what about the PDCCH? • The PDCCH search space (which CCEs are used for the PDCCH) of a UE depends on the C‐RNTI assigned to that UE. • The order of the CCEs is interleaved – the interleaving pattern is fixed. • The CCE interleaved order is cyclically shifted, depending on the PCI of the H/eNB. • This leads to the PDCCH locations being randomized, depending on the PCI. Illustration only Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 37
    • So we propose… • The proposal advocates carefully selecting the PCI of HeNBs at start‐up, such that any interference caused by their control channels to the PCFICH of any trapped macro UEs is avoided. – In order for this to be possible, the HeNB needs to identify the eNB that it is closest to. • Identifying the eNB means that the HeNB must be aware of the PCI of the eNB (decoded using synchronization procedure). Illustration only Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 38
    • What needs to be done • HeNB identifies most Identify dominant macro eNB • HeNB decodes dominant Decode eNB’s PCI • HeNB adjusts its own PCI Adjust to reduce interference • This procedure can not only protect all the control channels but also the CRSs Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 39
    • Co‐channel deployment of macro and femto‐cells • Stripe model used • Not all UEs are allowed to connect to a HeNB For UEs having no access to HeNBs, downlink interference is  significant • Since the control channel is very important for proper functionality,  how do we protect the control channel of trapped macro UEs?Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 40
    • Overall macro UE performance Deceivingly small Improvement! • Compared to sparseness, this proposal results in an improvement of approximately  2 dB – especially at the low percentiles. This corresponds to the trapped macro  UEs. • Better performance than ABS configuration (due to better collision avoidance). Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 41
    • Overall macro UE performance with power control • All curves shift to the right due to power control • Femtocell performance is still acceptable (not seen here) Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 42
    • Improvements/advantages  Enables the aggressor HeNB to continue to transmit data. Not possible with almost  blank subframes  The proposed technology results in a significant improvement over introducing  sparseness to the control channel. • Therefore this technology incorporates the benefits of both sparseness and almost  blank subframes. • Multiple macro UEs can be protected simultaneously. • No additional hardware is needed. • No additional signaling is needed. • This procedure is backwards compliant with Rel.‐8/9 UEs. • Can be seamlessly combined with power control to boost performance even  further. Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 43
    • Lessons learned • First study dedicated to control channel performance for LTE • Impact on vulnerable trapped macro UEs assessed • Two backward compatible techniques analyzed • Results show significant performance improvements for PDCCH but not for  PCFICH • PCFICH protection is further analyzed • A novel technique employing only PCI manipulation is shown to  significantly improve PCFICH performance without losing the femto  subframe • A few topics for further work would involve data channel interference  mitigation, power consumption analysis and handover improvements for  legacy systems; new control channel designs for future releases.Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 44
    • Part 3: Femto‐to‐Femto  interferenceCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 45
    • Femtocells ‐ Overview FUE‐1 1 2 FBS‐1 MUE 3 macro‐BS FUE‐2 FBS‐2 1. Between FUE and MBS 2. Between MUE and FBS 3. Between FUE and FBS  Increase in coverage Increase in data rate Increase in interferenceCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 46
    • Femtocells ‐ Overview FUE‐1 1 2 FBS‐1 MUE 3 macro‐BS FUE‐2 FBS‐2 1. Between FUE and MBS 2. Between MUE and FBS 3. Between FUE and FBS  Increase in coverage Increase in data rate Increase in interference How can we maintain acceptable user experience in dense femtocell networks?Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 47
    • Carrier Aggregation for LTE‐A CC1 CC2 CC3 CC4 CC5 100 MHz freq. • LTE-A makes use of carrier aggregation via the use of component carriers (CCs) • Improves peak data rate and spectrum flexibility • Meets ITU-R requirements for bandwidth (>=40 MHz) • Backward compatibility is maintained • Smooth network migration is possible with minimal loss of service for legacy terminalsCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 48
    • How should the cake be eaten? pow. B C 1 2 3 freq. Component Carrier A Interference Interference between femtocells is a severe problem in densely deployed networks  Desired quality of service cannot be achieved for cell edge users Resource partitioning is widely used to enhance the performance of cell edge users  interfering neighbors transmit data on different CCs  the drawback is that it decreases the network’s overall resource efficiency Vast variations of the interference conditions experienced by a BS during its operation  Dynamic environment BSs should use as many resources as possible depending on their interference environment  flexibility in the amount of assigned resources Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 49
    • How should the cake be eaten? pow. B C 1 2 3 freq. Component Carrier A Interference Interference between femtocells is a severe problem in densely deployed networks  Desired quality of service cannot be achieved for cell edge users Resource partitioning is widely used to enhance the performance of cell edge users  interfering neighbors transmit data on different CCs  the drawback is that it decreases the network’s overall resource efficiency Vast variations of the interference conditions experienced by a BS during its operation  Dynamic environment BSs should use as many resources as possible depending on their interference environment  flexibility in the amount of assigned resources Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 50
    • Aim pow. B C 1 2 3 freq. A Desired Signal InterferenceInterference mitigation techniques should:1. Be dynamic in nature  resource assignment should be updated according to changes in the radio environment2. Achieve high resource utilization Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 51
    • Aim pow. B 3 2 C 1 2 3 freq. 1 1 A 1 A Desired Signal BInterference mitigation techniques should: Interference 2 31. Be dynamic in nature C  resource assignment should be updated according to changes in the radio 3 environment2. Achieve high resource utilization3. Be suitable for multi‐user deployments PCC  Each user in the same cell experiences different interference conditions  CC allocation should be done according to the UE measurements  Primary CC (PCC) Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 52
    • Aim pow. B 3 2 C 1 2 3 freq. 1 1 3 A 1 3 SCC A Desired Signal BInterference mitigation techniques should: Interference 2 31. Be dynamic in nature C  resource assignment should be updated according to changes in the radio environment 32. Achieve high resource utilization3. Be suitable for multi‐user deployments  Each user in the same cell experiences different interference conditions PCC  CC allocation should be done according to the UE measurements  Primary CC (PCC)  Secondary CCs (SCC)4. Be applicable to the networks  with a central controller ‐ central approach  without a central controller ‐ distributed approach5. Be compatible with the LTE‐A systems Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 53
    • Two different approaches Dynamic interference mitigation by resource partitioningCentral Approach Distributed ApproachResources are assigned by a central  Resources are assigned autonomously bycontroller BSs More efficient resource utilization than  Less complexitythe distributed approach High signaling overhead Needs extra signaling between the BSs Requires long time period to reach a stableand the controller resource allocation High computational complexity at the Low resource efficiencycontrollerCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 54
    • Central brain • Interfering neighbor discovery: B C Interference Central  controller A  How does the controller assign resources to the BSs?Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 55
    • Central brain • Interfering neighbor discovery:  UE makes measurement  Identifies its interfering neighbors according to a predefined SINR threshold Feedback B C Interference A,C A Central  B controller A  How does the controller assign resources to the BSs?Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 56
    • A centrally controlled graph based scheme • Interfering neighbor discovery:  UE makes measurement  Identifies its interfering neighbors according to a predefined SINR threshold • BSs send cell IDs of the interfering neighbors to the central controller Feedback B C Interference A,C Backhaul A A A, C Central  B controller A B  How does the controller assign resources to the BSs?Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 57
    • A centrally controlled graph based scheme • Interfering neighbor discovery:  UE makes measurement  Identifies its interfering neighbors according to a predefined SINR threshold • BSs send cell IDs of the interfering neighbors to the central controller • The central controller maps this information into an interference graph where  Each node corresponds a BS  An edge connecting two nodes represents the interference between two BSs Feedback B C Interference B C A,C Backhaul A A A, C Central  B controller A A B  How does the controller assign resources to the BSs?Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 58
    • So what is graph coloring? Graph coloring is a way of coloring the vertices of a graph such that no two adjacent vertices share the same color  here, Node  BS; color  CC 25 20 15 10 distance (m) 5 0 −5 −10 −15 −20 −25 −20 −10 0 10 20 distance (m)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 59
    • So what is graph coloring? Graph coloring is a way of coloring the vertices of a graph such that no two adjacent vertices share the same color  here, Node  BS; color  CC 25 25 1 20 20 3 15 15 6 10 10 2 5 distance (m) distance (m) 5 5 4 3 3 0 0 2 1 −5 −5 3 −10 −10 3 4 1 −15 −15 3 2 −20 −20 1 −25 −25 4 −20 −10 0 10 20 −20 −10 0 10 20 distance (m) distance (m)  Resources can be assigned dynamically One CC per BS is inefficient, as, when the number of CCs increases, a lot of bandwidth tends to be wasted Inefficiencies in terms of resource utilizationCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 60
    • How can we improve upon this? pow. E freq. D A B C FCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 61
    • The recursive step Applying the graph coloring algorithm multiple times Identify CCs that can be assigned to BSs without causing undue interference pow. E freq. E D A B D A B C C F FCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 62
    • Being clever helps too pow. freq. E D A B C Resource efficiency : 5/15Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 63
    • Being clever helps too  Identify the CC‐BS pairing which maximizes the resource efficiency pow. freq. E E D A B D A B C CResource efficiency : 6/15 Resource efficiency : 5/15 Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 64
    • Being clever helps too  Identify the CC‐BS pairing which maximizes the resource efficiency  CCs are assigned to BSs by using a cost function pow. freq. E E E D A B D A B D A B C C CResource efficiency: 6/15 Resource efficiency : 5/15 Resource efficiency : 9/15 Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 65
    • Graph based dynamic frequency reuse (GB‐DFR) • 1st Step:  Apply the graph coloring algorithm smin times – Where smin is the minimum number of CCs that must be allocated to each BS – Using the cost function, assign one CC to every BS in each iteration (gains seen especially when the number of available CCs is high) – doing so increases the reuse efficiency of the system • 2nd Step:  For each CC: – Using the cost function again, identify the combination of BSs which maximizes the utilization of this CC (example on slide 65) • Advantages:  Dynamic adaptation according to prevailing interference conditions  Number of assigned CCs per BS is automatically adjusted depending on the interference conditions  Very low wastage of resources  Low complexity and computational cost Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 66
    • Simulation Parameters • 5x5 grid case • Downlink only • Only femto‐femto interference is considered Parameter Value System bandwidth 20 MHz Traffic model  Full buffer max BS power 10 dBm Antenna gain 0 dBi Fading model No fast fading Activation ratio 0.5 Number of UEs per BS 1 Number of CCs  6 HeNB SINR threshold 5 dB UECopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 67
    • Performance Evaluation – CDF of SINR 1 0.9 0.8 0.7 0.6 CDF 0.5 0.4 0.3 0.2 Reuse-1 0.1 Conv. Graph Col. (S=6) GB-DFR (S=6) 0 -10 0 5 10 20 30 40 50 SINR [dB]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 68
    • Performance Evaluation – CDF of User Capacity 1 0.9 0.8 0.7 0.6 CDF 0.5 0.4 0.3 0.2 Reuse-1 0.1 Conv. Graph Col. (S=6) GB-DFR (S=6) 0 0 5 10 15 20 25 30 35 40 User capacity [Mbps]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 69
    • BS activation probability versus resource utilization Probability that an apartment contains an active femto BS 100 Conv. Graph Col. (S = 6) Percentage of Assigned Subbands 90 GB-DFR (S=6) 80 70 60 50 40 30 20 10 0 0.2 0.4 0.6 0.8 1 BS Activation Probability pCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 70
    • Effect of SINR threshold on performance 12 5th percentile user capacity 5th and 10th Percentile User Capacity 10th percentile user capacity 10 Sweet spot spots 8  th= 10dB 6  th= 5dB 4  th= 15dB  th= 0dB 2  th= 20dB 0 14 16 18 20 22 24 26 Average User Capacity [Mbps]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 71
    • Lessons learned • Femto‐femto interference is a severe problem in femtocell networks • Dynamic assignment of resources – Decreases coverage holes – Results in high resource utilization • GB‐DFR attains a significant capacity improvement for cell‐edge UEs, at the expense of a modest decrease for cell‐centre users • Next section: – Extending the GB‐DFR to the networks where BSs serve multiple UEs – Fully distributed/autonomous approachCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 72
    • Two different approaches (recap) Dynamic interference mitigation by resource partitioningCentral Approach Distributed ApproachResources are assigned by a central  Resources are assigned autonomously bycontroller BSs More efficient resource utilization than  Less complexitythe distributed approach High signaling overhead Needs extra signaling between the BSs Requires long time period to reach a stableand the controller resource allocation High computational complexity at the Low resource efficiencycontrollerCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 73
    • The decentralized technique – a summary • Aim: – Autonomously assign resources in unplanned wireless networks – Balance high spatial reuse of radio resources with interference protection for cell‐edge users • The proposed method relies on UE measurements – Dynamic adaptation to the interference conditions faced in random deployments • Less signaling overhead compared to existing LTE and LTE‐A signaling procedures • Can easily be adapted to work in either the time or the frequency domainCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 74
    • Resource assignment – who gets what? pow. 1 2 3 freq. A B CCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 75
    • Resource assignment – who gets what? pow. 1 2 3 freq. A B C Potential  interference path • Dynamic interference environment  Number and position of neighbors change during the operation  Fixed frequency planning is sub‐optimalCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 76
    • Resource assignment – who gets what? pow. 1 2 3 freq. 1 2 A B A 1 3 B C 2 Potential  3 interference path C • Dynamic interference environment 3  Number and position of neighbors change during the operation  Fixed frequency planning is sub‐optimal  Dynamic assignment of resources!Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 77
    • Resource assignment – who gets what? pow. 3 1 2 3 freq. 1 2 A B A 1 3 3 B C 2 Potential  3 interference path C • Dynamic interference environment 3  Number and position of neighbors change during the operation  Fixed frequency planning is sub‐optimal  Dynamic assignment of resources! • Multi‐user deployment  Users in the same cell experience different interference conditions  Resource assignment should depend on UE measurements to maximize resource utilization  Classify resources according to their foreseen usagesCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 78
    • Not all CCs are created equal• Reserved CC (RCC): 1 – Allocated to cell edge UEs 1 2 – Protected region A B Potential  3 interference path C A 1 B 2 C 3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 79
    • Not all CCs are created equal• Reserved CC (RCC): 1 – Allocated to cell edge UEs 1 2 – Protected region A B• Banned CC:  – Interfering neighbors are restricted to use  Potential  3 the RCC allocated to the victim UE interference path – This guarantees  desired SINR at cell edge  C UEs A 1X B X2 C XX3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 80
    • Not all CCs are created equal• Reserved CC (RCC): 13 – Allocated to cell edge UEs 1 2 – Protected region A B• Banned CC:  – Interfering neighbors are restricted to use  Potential  3 the RCC allocated to the victim UE interference path – This guarantees  desired SINR at cell edge  C UEs• Auxiliary CC (ACC): – Allocated to the UEs facing less interference – Neighbors are not restricted A 1X3 – Increases resource efficiency, especially, for  the multi‐user deployments B X2 C XX3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 81
    • What is needed to get this to work? 11. IDs of interfering BSs (UE  Serving BS) – Each UE can measure the received  1 2 A B power from the BSs in its vicinity – It identifies interfering BS IDs according  to the predefined SINR threshold 3 Potential  interference path C A 1 B 2 C 3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 82
    • What is needed to get this to work?1. IDs of interfering BSs (UE  Serving BS) – Each UE can measure the received  A B power from the BSs in its vicinity 2, 3 – It identifies interfering BS IDs according  1, 3 to the predefined SINR threshold Potential  interference path C Feedback from UE A 1 B 2 C 3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 83
    • What is needed to get this to work? 12. RCC Indicator (BS  Interfering BS) to B & C:  to A & C:  – Used for preventing interfering  Don’t use 1 1 2 Don’t use 2 A B BSs to use the RCC allocated to  the victim UE 3 Potential  interference path C RCC indicator A 1X B X2 C XX3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 84
    • What is needed to get this to work?3. SINR over each CC (UE  Serving BS) – Each UE observes different SINR over each CC 1 – These measurements are used to find out which  1 2 CCs are available for transmission (as  a RCC or  A B ACC) depending on the predefined SINR threshold  value 3 Potential  interference path C A 1X B X2 C XX3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 85
    • What is needed to get this to work?3. SINR over each CC (UE  Serving BS) Received SINR on each CC (cell A): A B 1 2 3 + - - + = over threshold ‐ = below threshold + + + = banned  CC Potential  interference path Received SINR on each CC (cell B): C Feedback from UE 1 2 3 - + - Received SINR on each CC (cell C): 1 2 3 A 1X + + + B X2 C XX3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 86
    • What is needed to get this to work?3. SINR over each CC (UE  Serving BS) Received SINR on each CC (cell A): A B 1 2 3 + - - + = over threshold ‐ = below threshold + + + = banned  CC Potential  interference path Received SINR on each CC (cell B): C Feedback from UE 1 2 3 - + - next time slot Received SINR on each CC (cell C): 1 2 3 A 1X 1X3 + + + B X2 X2 C XX3 XX3Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 87
    • Our latest acronym: Dynamic Autonomous CC Assignment – DACCACopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 88
    • Our latest acronym: Dynamic Autonomous CC Assignment – DACCA  Event triggered CCs configuration is updated  only if there is a change in the  interference environment  All BSs are synchronized  with a time duration equal to  that of a so‐called ‘time slot’  Between the starting  instances of two time slots,  the CC configuration remains  undisturbedCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 89
    • Simulation parameters• 5x5 grid case and downlink direction is investigated 25• Only interference between femto BSs is considered 20• Statistics are taken at the end of 10th slot 15• Three methods are compared:  BS sniffing 1/4 and 2/4  10  DACCA 5 0 Parameter Value -5 System bandwidth 40 MHz (4 x 10 MHz) -10 Traffic model  Full buffer -15 Max. Tx Power per CC 20 dBm -20 Antenna gain 0 dBi -25 -20 -10 0 10 20 Shadowing std. dev. 10 dB Activation ratio 0.2 Femto BS Number of UEs per BS 4 (closed access) UE SINR threshold 5 dB Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 90
    • CDF of SINR  1 0.9 0.8 0.7 0.6 CDF 0.5 0.4 0.3 0.2 BS Sniffing (1/4) 0.1 BS Sniffing (2/4) DACCA 0 -20 -10 0 5 10 20 30 40 50 60 70 80 SINR [dB]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 91
    • CDF of user capacity 1 0.9 0.8 0.7 0.6 CDF 0.5 0.4 0.3 0.2 BS Sniffing (1/4) BS Sniffing (2/4) 0.1 DACCA 0 0 5 10 15 20 25 30 35 40 45 50 User capacity [Mbps]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 92
    • Mean cell capacity versus user capacity  9 20% BS Sniffing (1/4) 8 20% BS Sniffing (2/4) DACCA 7 User Capacity [Mbps] 6 20% 10% 5 4 5% 10% 3 2 10% 1 5% 5% 0 35 40 45 50 55 60 65 70 75 Mean Cell Capacity[Mbps]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 93
    • Convergence of the algorithm 80 Percentage of Assigned Resources 70 Percentage of Collisions (SINR<-10dB) 60 Allocated RBs / All RBs 50 Percentage 40 30 20 10 RBs Facing SINR below ‐10dB / Allocated RBs 0 1 2 3 4 5 6 7 8 9 10 Time SlotCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 94
    • Effect of SINR threshold 4 5 dB 10 dB 3.5 Cell Edge Capacity [Mbps] 0 dB 3 2.5 2 -5 dB 15 dB 1.5 50 55 60 65 70 75 80 Average Cell Capacity [Mbps]Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 95
    • Wrap up • We have had a look at some fairly simple and backward‐compatible femto‐ macro interference mitigation techniques and studied their performance • We have identified that the control channel is particularly susceptible to  interference – especially since it is so inflexible • In particular, the most important control channel exhibits the worst  performance • We have addressed this issue by proposing a clever interference mitigation  technique • We then consider the case of femto‐femto interference • We have had a look at an interference mitigation technique which relies  on a central controller • We have then attempted to remove the central controller and see if that  works (it does)Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 96
    • Where do we go from here? • Lots of interesting areas for further research • Femtocells are not going anywhere • Design of special air interfaces to deal especially with the interference  problem • New ways of handling handovers • Clever scheduling strategies with tight macro‐femto cooperation • Femtocells with cognitive radio? • MIMO? • Etc.Copyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group 97
    • Zubin Bharucha bharucha@docomolab‐euro.com  DOCOMO Communications Laboratories Europe GmbH Landsberger Strasse 312 – 80687 Munich, Germany Phone: +49 (89) 56824‐0 | www.docomolab‐euro.comCopyright © 2012 DOCOMO Communications Laboratories Europe GmbH  Infrastructure Research Group