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Deploying LTE Small Cells – Interference Mitigation Solutions


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Hans Kramer and Nagi Mahalingam or Radisys discuss real implementation challenges of LTE network equipment. In particular, this webinar explores the challenges that radio interference brings when …

Hans Kramer and Nagi Mahalingam or Radisys discuss real implementation challenges of LTE network equipment. In particular, this webinar explores the challenges that radio interference brings when small cells are deployed within a macro infrastructure.

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  • 1. Welcome! Real Implementation Challenges of LTE Network Equipment: April 12 Exploring the Trade-offs and Deploying LTE Small Cells: Solutions Interference Mitigation Solutions May 17 Building EPC Systems June 14 The LTE Service Layer July 12 End-to-end QoS in LTE To register for the next three webinars: Radisys Corporation Confidential 1
  • 2. Deploying LTE Small Cells:Interference Mitigation Solutions Hans Kramer and Nagi Mahlingam April 12, 2012
  • 3. Today’s Agenda Hans Kramer – Sr. Director, Business Development • Setting The Scene - The Radio Access Network in an LTE World Nagi Mahalingam – Chief Architect • Interference Mitigation Solutions Q&A Summary / Wrap-Up Radisys Corporation Confidential 3
  • 4. Embedded Wireless Infrastructure Solutions Radisys Corporation Confidential 4
  • 5. End-to-End LTE InfrastructureFrom Radio Access to Media ProcessingRadio Access Network Evolved Packet Core Policy Control IMS IP Policy & Multimedia User Mobility Charging Subsystem Equipment Management Routing Entity Function Hom e eNodeB Application Media Server Resource Function User Policy & Equipment Charging Enforcement Internet Function eNodeB LTE Security Serving Packet Gatew ay Gatew ay Gatew ay  Macro  Small Cells  10G  40G ATCA  Dumb  Smart Pipes  Audio  Video Conf  60+ Customer Wins ~40% ATCA Market Share  Traffic Management  ~65% Market Share Radisys Corporation Confidential 5
  • 6. Architecture for an LTE Small Cell SON RRM Interference / Power RACH Capacity Management Optimization SRB & DRB control DCAC OAM EARFCN, PCI ANR Selection Backhaul OSCDA TR- Mobility Admin Policy 069 / Mobility Robustness Optimization (MRO) QoS Control C Drift Lock DB [Idle, Connected Modes] Control Control 196 REM eNodeB Call Control FSM RRC S1AP X2AP Control Interfaces Security PDCP OTA GTP QoS RLC QoS SCTP QoS UDP MAC TCP Schedulers Transport Security PHY Convergence Layer Crypto & ROHC Accelerator PHY in HW RRM, TR-069/196, Call REM, SON Trillium Trillium Silicon-Specific 3GPP Compliant Control Trillium Applications Applications Convergence Layer Trillium Protocols Radisys Corporation Confidential 6
  • 7. Interference Mitigation Solutions Nagi Mahalingam
  • 8. Shared or Dedicated Spectrum? Macro Macro/ Macro Femto /Femto Femto Dedicated Spectrum Shared Spectrum  Where unused spectrum is  Where unused / spare available spectrum is not available  Suburban and rural areas  Urban areas  Not as spectrally-efficient  Most spectrally-efficient No matter how we quantify the problem, no single solution is satisfactory. We require multiple, concurrent techniques in operation to mitigate interference.Confidential and Radisys Corporation Confidential 8Proprietary
  • 9. Solutions in Carrier Sharing Macro Handset Inside / Near Femto Coverage HeNB Very Close to eNB Adaptive DL TX power setting Carrier redirection Macro Macro Small cell Small cell SINR SINR SINR SINR Small cell UE Small cell UE Macro UE Macro UE DL interference from HeNB to nearby macro UE DL interference from nearby eNB to small cell UE A macro UE far from eNB will be affected the most Interference from nearby eNB can lower SINR at small cell UE Adaptive UL TX power control / Transmission rate control and Carrier redirection Carrier redirection INT INT INT INT UL interference from Nearby macro UE to HeNB UL interference from small cell UE to nearby eNB A macro UE far from eNB can increase interference at HeNB Many active small UE near eNB can raise interference at eNB Radisys Corporation Confidential 9
  • 10. Legacy Interference Management Problems  The small cell’s Network Listen Module performs measurement at start-up and at periodic intervals • Used to decide small cell’s carrier, PCI, Maximum DL TX power / UL TX power for the UE, etc. • Not enough for optimizing Interference Management • Two drawbacks are listed below 1. The measurement point • Measured at the specific point where the small cell is installed • Impossible to grasp the surroundings of the small cell very reliably 2. The measurement interval • Usually measured at start of Plug & Play / Auto Configuration procedure • Operational parameters require adaptive changes (updates); for example, the loading circumstances may swing drastically between day and night Radisys Corporation Confidential 10
  • 11. Interference from Small Cell to MacroSolution MethodLimit the femto UE maximum UL Tx Power Using Femto UE measurement’s calculating the path loss between femtoadaptively UE and eNBAggressor VictimSmall cell UE (UL) eNB Macro Macro L3 Measurment L3 Measurment Report Report Limit femto UE’s TX power adaptively (by scheduling) Radisys Corporation Confidential 11
  • 12. Interference from Small Cell to MacroSolution MethodLimit the Downlink (pilot / RS) TX PWR adaptively. Using macro UE measurement at eNBAdaptation aims to avoid dropping existing femto UE’s Sending the information to HeNB from neighbour with X2 interfaceconnections Using message (ex: RL failure) from Macro UE to small cell for triggerAggressor VictimSmall cell (DL) Macro UE Limit DL TX power Limit DL TX power adaptively adaptively X2 me n ntt ureeme assur t Neighbour Neighbour 3 Meea eporrt M R epo with X2 with X2 LL3 R Macro Macro Pathloss Pathloss RL RL measurement measurement Failure Failure Radisys Corporation Confidential 12
  • 13. Interference from Small Cell to Macro Solution Method Move Macro UE to another carrier Using Macro UE measurement from Macro UE to eNB (ex: S1 HO) This is possible (preferable) if there is an overlay carrier Using a message (ex: RL Failure) from Macro UE to small cell for trigger Aggressor Victim Small cell (DL) Macro UE Carrier redirection (for example via RRC release w/redirection IE) t en t n r em e m su re t u M ea s porrt ea e po L3 M R e L3 R Macro Macro Macro Macro Pathloss Pathloss RL RLmeasurementmeasurement Failure Failure Carrier redirection (for example via HO) Radisys Corporation Confidential 13
  • 14. POLL Question With what type of radio access technology will small cells be most widely deployed? A. 3G only B. LTE only C. Multi-mode D. Small cells won’t be widely deployed Radisys Corporation Confidential 14
  • 15. Approaches to SON  Centralised • SON solutions where SON algorithms are executed in a centralized entity – say, OAM • In such solutions SON functionality resides in a small number of locations • More control for operator (less scalable)  Distributed • SON solutions where SON algorithms are executed at network element (eNB / HeNB) • In such solutions SON functionality resides in many locations • Less control for operator (most scalable)  Hybrid • SON solutions where some of the SON algorithms are executed in the OAM system while others are executed at the network element level • Best option, most suited for adhoc networks (for femto / small cells) • Operator can “move” to Distributed option as and when they have more confidence on multi-vendor SON algorithms Radisys Corporation Confidential 15
  • 16. Three Main Topics of Concern Select a PCI that is  OAM configures a list of  Uplink and Downlink unique across all the EARFCNs power setting neighbors and  Perform Network Listen • Maximum DL TX power (preferably) neighbors’ Mode (NLM) procedure • Maximum UL TX power neighbors. If all the PCI and scan all valid • Reduced / almost NULL that are configured by frequencies power on some RB the OAM for the cell are  Calculate the RSSI on • Periodic modification of used by neighbors, each of the EARFCNs the above (adaptive & pronounce PCI conflict dynamic) and raise warning.  Typically, select the • Uplink power control PCI ‘conflict / collision’ channel with the lowest (closed loop) is controlled detection done based on RSSI for operation by MAC layer UE reports as well as X2 based eNB configuration updates PCI EARFCN TX powers Radisys Corporation Confidential 16
  • 17. Downlink Transmit Power Setting Max allowed (set by OAM) Pmax  Simple power setting: (if not set, default = device’s rated tx limit) PTx • TX power = (RX power + Reqd coverage) Pmin Min allowed (set by OAM) If not set, default = 0dBm  Utility function can be used to Pathloss avoid power racing and take intoLinear average of power from all account diminishing returnsresource elements carrying pilot symbols (over operational BW) RSRP  RSRQ Global optimization involves multi- Power level at the antenna on dimensional utility function the whole carrier RSSI • Can be broken down into several Rx sensitivity incremental optimizations eNB cannot detect carriers weaker than this limit Radisys Corporation Confidential 17
  • 18. Downlink TX Power Setting – Link Budget  PTx = Q RxLevMin + 10Log 12 ∗ Number of RBs + 20Log ′d′in km + 20Log ′f ′in MHz + 32.45 + shadowing loss + ′extra′  Pmax     L  QRxLevMin  Calculate PTx as above.  K RB 12  dB • Set PUsed as parameter ‘RSPower’ in own cell’s SIB2 after calculating Pused such that {Pmin <= PUsed <= Pmax} and as shown in equation below. • Pused = MIN {PTx, RSPower IE of IE PDSCH-ConfigCommon from SIB2 of neighbour cell[i]}  Case of MIMO • If the transmission scheme is MIMO, Pused is divided by 2 and applied to each antenna port. Radisys Corporation Confidential 18
  • 19. Downlink: PL as Function of ‘Distancevs. Frequency’ Cell radius in (mtrs) 1 10m 2 25m 3 50m 4 75m 5 100m 6 125m 7 150m 8 175m 9 200m 10 225m 11 250m 12 275m 13 300m 80 85 90 95 700 Mhz Series1 Series2 800 MhzPathloss in (dB) 100 Series3 900 Mhz Series4 1500 Mhz 105 Series5 2100 Mhz 110 Series6 2300 Mhz Series7 2600 Mhz 115 120 125 130 Radisys Corporation Confidential 19
  • 20. Downlink: Cell Reselection as Functionof ‘QRxLevMin vs. Pathloss’ 57 52 47 700 Mhz Series1 42Transmit power in (dBm) Series2 800 Mhz Series3 900 Mhz 37 Series4 1500 Mhz 32 Series5 2100 Mhz Series6 2300 Mhz 27 Series7 2600 Mhz 22 17 12 10m 1 25m 2 50m 3 75m 4 100m 5 125m 6 150m 7 175m 8 200m 9 225m 10 250m 11 275m 12 300m 13 Cell radius in (mtrs) Radisys Corporation Confidential 20
  • 21. Max UL Tx Power Setting MACRO CELL #1 to #m SMALL CELL #1 to #n  Small cell UE interference at eNB must be kept below a target, e.g., -110 dBm DL RX mode (Radio Env Monitoring) • Target can be specified by TX PWR TX PWR TX PWR TX PWR Config:- operator and depends onUE #1 to macro_1, Rcvd PWR macro_m, Rcvd PWR femto_1, Rcvd PWR femto_n, Rcvd PWR - Max Interference to macro layer number of femto UEs in macro #i - Femto density under macro AVG / PL UE to M umbrella cell RSRP macro_1 macro_1 A X Pmax S AVG / C RSRP PL UE to Macro H  Small cell has following info: L3 macro_m macro_m M PMax E Measurement I UL D Reports UL RX Pmax N Grant • Macro TX Power from REM U RSRP Femto L femto_1 E / M • Macro RSRP at femto UE from AVG R RSRP PL UE to A femto_n femto_1 X AVG / PL UE to Sys UE measurement report Info femto_n • An estimate of UE Tx power Channel state information, Scheduling info such as power from UE measurement report headroom  Small cell controls the small cell UE’s power to limit eNB interference to desired level, through the scheduling grants • Small cell must monitor measurement reports / CSI and adjust grant Macro UE Interf erence @ HeNB can be 40 dB abov e its RX power at eNB! Radisys Corporation Confidential 21
  • 22. SON Cycle For Operational EARFCN, find known PCIs - if (strongest PCI found == strongest PCI from previous sniffing) AND if (RSRP of strongest PCI from previous reading == (RSRP of strongest PCI from current reading + TOLERANCE)) DO NOTHING. Resume eNB mode - else RUN calculate power algorithm Self Configure - select EARFCN - select PCI - select TAC If S1 setup is successful,Band Scan / Scan on Admin State = - Calculate Tx powers (DL & UL) Op State = True discrete EARFCNs TR-196 / data model Report / update True Start SON timer FAP puts out BCH (Tx on) Provisioning REM scans to (from TR-196 / _or_ delta updates OAM data model) S1 registration UE measurement Periodic reports SON 2 3 1, 4 5 6 7 8 9 10 11 X2 setup Periodic REM:- 2 Estimated outage time: a few mins, default 180s. - periodicity configured by TR-196/data model via OAM (default = 24 hours) - if connected UE present at “periodic REM” timer - during periodic REM, FAP is idle expiry, FAP releases RRC (operator policy) or wait - no connected UE till call completion for immediate start - no attached UE - obtain PCI, RSRP, RSRQ and BCH contents : Intra and Inter Frequency cells - Tx off (BCH off) - S1 is still ON (unless MME / HeNB-GW FQDN has been changed) Periodic SON:- 11 Estimated outage time: a few seconds, default 4s. Whenever there are UE based measurement - periodicity is configured via vendor extensions (OAM) (default = 30 mins) 9 - if connected UE present at “periodic SON” timer reports, SON is used to control ANR and to set up - during periodic SON, FAP is IDLE expiry, FAP waits till call completion for immediate X2 links if needed. X2 is further used at this point - no connected UE start. for eNB configuration transfers - IDLE UE still present / attached - obtain power levels ONLY (RSRP, RSRQ) - S1 is still ON - BCH On - Measure ONLY “previously” known intra frequency PCI identified at “previous REM cycle”. - starting with strongest RSCP - Measure ONLY “newly” reported intra frequency PCI by UE in connected mode since “previous REM cycle” - Periodic SON also includes input of measurements obtained from UE. Radisys Corporation Confidential 22
  • 23. POLL Question What aspect of SON is most important? A. Plug and Play B. Auto / Dynamic reconfiguration C. Interference control D. SON is not important Radisys Corporation Confidential 23
  • 24. Interference Control: FractionalFrequency Reuse macro macro femtos femtos Full Band Full Band Band A Band B Band C Band D macro macro femtos femtos Full Band Full Band Band A Band B Band C Band D Band A Band B Band C Band D Radisys Corporation Confidential 24
  • 25. Interference Control: Soft FrequencyReuse  Soft frequency reuse (SFR) improves the throughput for UEs close to the cell boarder • Protecting UEs close to cell boarder employing frequency reuse freq Radisys Corporation Confidential 25
  • 26. SFR: Exchange of RNTP, LoadInformation  Relative Narrowband Transmit Power (RNTP) x2 is exchanged between eNBs via X2 interface x2 x2 x2 x2 • Bitmap indicates whether transmission power of x2 x2 respective RB exceeds the predetermined x2 threshold x2 x2 x2 x2 Radisys Corporation Confidential 26
  • 27. Time Domain Interference Coordination  In order to apply time‐domain ICIC, femtos mute (blank) freq specific subframes to protect UEs connected to macro eNBs time  However, cell‐specific reference symbols need to befreq sent for measurements • 3GPP terms these “Almost blank subframes (ABS)” time Radisys Corporation Confidential 27
  • 28. Shared Carrier Interference Mitigation  Data channel (PDSCH) femto-to-macro interference mitigation: • Can be mitigated by use of SFR / FFR • Requires advanced SON coupled with side information  Control channel femto-to-macro interference mitigation: • Very complex problem since PDCCH is always broadcasted in every subframe over the same set of between 1 and 3 OFDM symbols – Additionally, certain sub-frames contain other critical control information (e.g., Broadcast, Synchronisation, etc.) • Some mechanisms discussed in 3GPP include: – Frequency / Time-domain offsets between femto and macro – Frequency Partitioning (FFR) – Time-domain control channel blanking  Time-domain control channel blanking can be used to mitigate portions of this problem Radisys Corporation Confidential 28
  • 29. Control Channel Control channel PCFICH Scattered in the freq domain only PHICH (1st symbol) Scattered in containing freq & time format info containing PDCCH HARQ info Scattered in freq & time containing scheduling info PDCCH PCFICH RS (c) PHICH emptyTime Frequency Radisys Corporation Confidential 29
  • 30. Control Channel Protection PDCCH PCFICH RS (c) PHICH empty PDSCH PDCCH PCFICH RS (c) PHICH empty PDSCH macro femto No Coordination macro femto Control Channel Sparseness macro femto Almost Blank Subframe Radisys Corporation Confidential 30
  • 31. Time Domain Control Channel Blanking 40ms (4 frames) Problem: Macro 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 PBCH, Sync • Due to macro UE’s close proximity to the Femtocell, Paging, SIB#1 it may not be able to successfully decode its control GOOD DL HARQ UL channels TD CCH blanking overview: • Femtocell can periodically ‘blank’ sub-frames, thereby allowing the macro UE to achieve a higher Femto 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 PBCH, Sync control channel SNR in those ‘blanked’ frames Paging, SIB#1 FORBIDDEN MBSFN Issues that need to be heeded (for Rel 8/9 UEs): • Macro MAC scheduler needs to know which mobiles  Macro can use 1 UL process and 9 out of 40 DL SFs are close to femtocells (e.g., via measurement  Femto loses 9 out of 40 DL SFs reports) and also must know the PRECISE femtocell  But this is perfectly OK; per-user throughput is still high at blanking schedule so that: the femto – It can ensure UL transmissions are scheduled such  Those numbers can be traded-off further (e.g., in case that DL HARQ ACKs are received during blanked 4th subframe is also needed for Paging) subframes – It knows which CQI reports to ignore (e.g., CQIs measured during non-blanked subframes) • Femtocell UEs cannot receive downlink data in blanked subframes  must take care to schedule femtocell UE DL HARQ transmissions properly • Need to ensure that macro UE RSRQ averaging is done such that a benefit is obtained (this is implementation specific) Radisys Corporation Confidential 31
  • 32. PCFICH is King  PCFICH exhibits the worst SINR performance compared among control channels  It is not sufficiently possible to protect the PCFICH from femtocell interference  Decode the PCFICH incorrectly, your TTI is lost !!  What can we do? • Small cells serve a small number of users and hence carry a low PDCCH aggregation level • The control channel region is less dense; shuffle PCFICH, PHICH, and PDCCH at the small cell layer Radisys Corporation Confidential 32
  • 33. Autonomous PCI Reconfiguration– and as Often as Necessary !!!  CFI always occurs on the first OFDM symbol • 16 symbols distributed in frequency  Position of PCFICH symbols involve an offset depending on the PCI  PDCCH search space for given UE depends on C‐RNTI of that UE • Order of CCEs is interleaved though the interleaving pattern is predetermined; the interleaved order is cyclically altered depending on the PCI and hence the position of PDCCH symbols appear unsystematic  What can we do? • Select PCI at start‐up (and periodically), such that any interference caused by their control channels to the PCFICH of any confined macro UEs are avoided • Small cells monitor neighbors continually for this purpose • Monitoring the neighbor for this purpose involves the decode of the neighbors’ SCH Radisys Corporation Confidential 33
  • 34. Summary: Interference Mitigation Schemes– What We Implement in Our SW  Optimised Power Setting  UL power limiting • Small cells transmit at no higher • Small cells limit UE power such that their UL Downlink power than required to obtain the power received at nearby macros lower than the desired coverage noise floor  Optimised PCI selection  X2 Based Approaches • Small cells select a PCI such that • Macros transmit UL overload, DL RNTP, etc., RS collisions with nearby real-time indications to all small cells in their Desired macrocells are minimized sector signal • Small cells transmit UL overload, DL RNTP, etc.,  Soft Frequency Reuse real-time indications to each other Macro UE 1 subf rame • Small cells transmit at lower (or no) power in certain resource blocks Interfering  Frequency Partitioning Approaches Signal • Small cells occupy smaller piece of the available spectrum from macro • Small cells occupy orthogonal3 sy mbols 11 to 13 sy mbols (PDCCH) (PDSCH) pieces of spectrum from each other Uplink  Time Offset • Small cells operate synchronous to Interfering nearby macrocells, and offset Signal transmission by (for example) 3 Desired subframes + 3 symbols signal Femto UE Radisys Corporation Confidential 34
  • 35. Q&A  Please contact us! Hans Kramer Nagi Mahlingam For more information on our Trillium Solutions, visit: ̴ Please fill out our short survey ̴ THANK YOU FOR ATTENDING! Radisys Corporation Confidential 35