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Qualcomm lte-performance-challenges-09-01-2011

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  • 1. 1 LTE Performance – Expectations & Challenges Engineering Services Group September 2011
  • 2. 2 Agenda Overview of ESG LTE Experience ESG – AT&T Engagements for LTE LTE Performance Expectations Factors Impacting LTE Performance Key Areas To Be Considered for LTE Launch 2
  • 3. 3 ESG LTE Experience Overview ESG EUTRA Vendor IOTs R&D 3GPP SA5 Participation Chipset Lab Testing • Technology trial participations • RFP development • LTE Protocols trainings & hands-on optimization workshops delivered to 2600+ engineers • LTE design guidelines • LTE capacity & dimensioning • Performance assessment & troubleshooting in commercial LTE networks • Performance studies & evaluations using ESG simulation platforms Early exposure to LTE through Qualcomm’s leadership position in technology 3
  • 4. 4 ESG-AT&T LTE Partnership Highlights Multiple engagements with NP&E and A&P teams LTE Technology Trial (2009) • ESG SME in Dallas for 6 months • Participation in Phase I & II Trial • SME support and technical oversight of execution by vendors • Review results and progress of the trial with the vendors RAN Architecture & Planning Team Field testing in BAWA & Dallas FOA clusters, lab testing in Redmond RAN Design Team LTE Design Optimization Guidelines LTE Design System Studies LTE Design & ACP Tool Studies Antenna Solutions Group • LTE capacity calculator for venues • IDAS/ODAS design & optimization guidelines CSFB Performance Assessment (starting next week) LTE Realization Group 4
  • 5. 5 World Wireless Academy – LTE Courses 5
  • 6. 6 Expected LTE Performance
  • 7. 7 Key Areas of LTE Performance LTE Call Setup and Registration LTE Single-user Throughput LTE Cell Throughput User Plane Latency Handover Success Rates and Data Interruption 7
  • 8. 8 Expected LTE Performance Dependencies  LTE System Bandwidth  1.4 -> 20 MHz  FDD/TDD  Throughput expectations  LTE UE Category – Current category 3 Devices  Deployment Considerations  Number of eNodeB Transmit Antennas  Backhaul Bandwidth  System Configuration  Transmission Modes used for DL (Diversity, MIMO schemes)  Control channel reservation for DL  Resource Reservation for UL  System Parameters 8
  • 9. 9 LTE Call Setup, Registration UE NW UE Power Up Initial acquisition PSS, SSS, PBCH, SIBs Idle, camped RRC Connection Setup Attach request incl. PDN connectivity request Attach response (accept) Incl. Activate Default Bearer Ctxt RequestAttach complete RRC connected RRC Connection Setup Duration, Success rates Attach and PDN Connectivity Duration, Success Rates RRC Connection Request (Msg3) RRC Conn. Setup Complete (Msg4) Idle, not camped RACH (Msg1, Msg2) Authentication, Integrity, Ciphering Security Procedures Number of RACH Attempt, RACH Power, Contention Procedure Success rates 9
  • 10. 10 Key LTE Call Setup Metrics 10 Metric Typical Expected Values Reasons for Variability Number of RACH and RACH Power RACH Attempts <3 RACH Power <23dBm Users at cell-edge, Improper Preamble Initial target Power, Power Ramping step RACH Contention Procedure Success Rate >90% Failed Msg3/Msg4, Delayed Msg4 delivery, Contention Timer RRC Connection Setup Success Rate >99% Poor RF conditions, Limited number of RRC Connected users allowed causing RRC Rejects, large RRC inactivity timers RRC Connection Setup Duration (Including RACH duration) 30-60ms Multiple RACH attempts, Msg3 retransmission, delayed contention procedure Attach and PDN Connectivity Success Rates >99% Failure of ATTACH procedure (EPC issues) or EPS Bearer setup, poor RF conditions, Integrity/Security failures Attach and PDN Connectivity Duration 250-550ms Multiple Attach Request, Authentication or Security related failures, EPC issues, delayed RRC Reconfiguration to setup Default RB
  • 11. 11 Peak Single User DL Throughput – 10 MHz 11 • “Ideal” case • 0% BLER, 100% scheduling • Near Cell field location • 5% BLER, 100% scheduling Scenario • LTE-FDD • Cat 3 UE • 2x2 MIMO • Max DL MCS 28 used with 50 RBs and Spatial Multiplexing
  • 12. 12 Peak Single User UL Throughput – 10 MHz 12 • “Ideal” case • 0% BLER, 100% UL scheduling • UL MCS 23 and 50 RBs • Near Cell field location • 5% BLER, 100% scheduling • UL MCS 24 and 45 RBs (some RBs reserved for PUCCH) Scenario • LTE-FDD • Cat 3 UE • Max UL MCS 23/24 depending on number of UL RBs
  • 13. 13 LTE DL Cell Throughput – Multiple Devices Device- RUN Throughput [Mbps] Sched. Rate [%] BLER [%] MCS Num RB CQI RI RSRP [dBm] RSRQ [dB] FTP L1 Norm. L1** T2 13.90 14.44 46.71 30.91 5.74 23.31 49.4 14.18 2 -73.85 -9.06 P2 16.58 16.65 53.04 31.39 5.40 25.12 49.76 14.48 2 -71.01 -8.98 P2 17.34 17.87 60.0 29.68 1.52 26.47 49.80 14.87 2 -68.87 -9.06 Total (3 devices) 47.82 48.96 91.98 • All 3 devices are scheduled almost equally (~30% each) • Device with highest CQI reported receives highest MCS and low BLER and consequently highest DL L1 Throughput• Total L1 Cell Throughput ~49 Mbps • Total Scheduling rate ~92% (<100%) • Num of DL RB are ~50 for all devices Above data is from a commercial LTE network with all 3 devices in Near cell conditions • Peak DL Cell Throughput in close to Ideal Conditions* should be similar to Peak Single User DL Throughput • For a 10 Mhz system, Ideal DL Cell throughput at TCP should be ~67Mbps 13
  • 14. 14 User Plane Latency Ave (ms) Min (ms) Max (ms) STD (ms) 42.1 36 62 4.3 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 30 40 50 60 70 80 Distribution User Plane Latency (ms)pdf cdf  Stationary, Near cell conditions  Ping size = 32 Bytes  Ping Server: Internal server • Ping Round-Trip-Time distribution from one commercial network above is concentrated between 40 -50 ms • Lower Ping RTT ~25 ms have been observed in some networks • Ping RTT can be dependent on CN delays, backhaul, system parameters and device • Ping Round-Trip Time (RTT) in an unloaded system should be ~20-25ms • Such Ping tests are done to an internal server one hop away from LTE PGW (avoid internet delays)
  • 15. 15 LTE Intra-frequency Handover Success Rate DL Test Run Total HO HO Failure (case) Run 1 125 2 (A, B) Run 2 108 0 Run 3 95 1 (A) Total 328 3 UL Test Run Total HO HO Failure (case) Run 1 106 0 Run 2 118 0 Run 3 98 1 (A) Total 320 1 Some Handover failure cases: A) RACH attempt not successful and T304 expires B) HO command not received after Measurement Report HO Success Rate is high in both UL and DL 99.05 99.69 99.37 98.40 98.60 98.80 99.00 99.20 99.40 99.60 99.80 100.00 Percentage[%] HO Success rate HO Success Rate Download Upload Total 15
  • 16. 16 LTE Intra-frequency Handover/Data Interruption Ave (ms) Min (ms) Max (ms) STD (ms) 78 38 199 34 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 25 50 75 100 125 150 175 200 Distribution HO Interrupt Time (ms)pdf cdf HO Interrupt Time: Interval between Last DATA/CONTROL RLC PDU on source cell and First DATA/CONTROL RLC PDU on target cell Data Interruption Time: Interval between only DATA RLC PDUs becomes much higher than 199 ms Current LTE Networks have higher HO and Data Interruption Times – eNodeB buffer optimization and data forwarding support needed 16
  • 17. 17 Typical Factors Impacting LTE Performance
  • 18. 18 Factors Affecting LTE Performance Deployment Pilot Pollution, Interference Neighbor List Issues, ANR Parameters (Access, RRC Timers) EUTRAN, EPC Implementation and Software Bugs Unexpected RRC Connection Releases DL MCS and BLER, Control Channel impacts eNodeB Scheduler limitations Mobility Intra-LTE Reselection, HO Parameters – minimize Ping- pongs Inter-RAT HO Boundaries and Parameters Data Performance Backhaul Constraints TCP Segment losses in CN MTU Size settings on devices 18
  • 19. 19 RF Issues Impacting Call Setup Performance - 1 Sub-optimal RF optimization delays LTE call-setup • Mall served by PCI 367 • PCI 212 leaking in partly 19
  • 20. 20 RF Issues Impacting Call Setup Performance - 2 UE NW UE Power Up Initial acquisition (incl. attempt on PCI 367) Idle, camped: PCI 212 RRC Connection Request RRC Connection Setup RRC connected RRC Setup Duration: 60 ms RRC Conn. Setup Complete PSS, SSS, PBCH, SIBs Idle, not camped 1st Attach request incl. PDN connectivity request 2nd Attach request incl. PDN connectivity request Duration: 4.533 sec UL data to send RACH not successful RACH (Msg1, Msg2) RACH (Msg1-Msg4) UE Reselects to PCI 367 No attach response (accept) PCI 212: RSRP = -110 dBm PCI 367: RSRP = -104 dBm 3rd Attach request incl. PDN connectivity request Attach Accept is sent • Pilot Pollution can impact call-setup, causing intermediate failures impacting KPIs, reselections and higher call-setup time 20
  • 21. 21 RF Issues Causing LTE Radio Link Failure - 1 PCIs 426, 427,428 are not detected (site is missing) Lack of dominant server => Area of Pilot pollution PCI 376 PCI 42 & PCI 142 • Missing sites during initial deployment phase requires careful neighbor planning or optimal use of ANR 21
  • 22. 22 RF Issues Causing LTE Radio Link Failure - 2 1. UE is connected to PCI 411 2. UE reports event A3 twice for PCI 142 (Reporting int. = 480 ms) 3. UE reports event A3 for PCI 142 & 463 4. No Neighbor relation exists between PCI 411 and 142 (Clear need for ANR). UE does not receive handover command, RLF occurs 5. RRC Re-establishment is not successful, UE reselects to PCI 42 RLF  DL BLER increases to 70% UL power increases to 23 dBm RSRP & SINR decrease to -110 dBm & -8 dB MRM A3 RLF 22
  • 23. 23 Backhaul Limitations Reduce LTE DL Throughput 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 -10 -5 0 5 10 15 20 25 30 35 L1Throughput(kbps) SINR (dB) L1 Throughput vs SINR Throughput is always lower than 50 Mbps, even at high SINR Backhaul limitation negatively Impacts the allocation of radio resources Statistics are calculated by using metrics averaged at 1 sec intervals 23
  • 24. 24 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.12 0.18 0.64 0.00 0.00 0.00 0.04 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 MCS PDF CDF eNodeB Scheduler: MCS and BLER Relationship 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.43 0.56 0 0.2 0.4 0.6 0.8 1 0 0.1 0.2 0.3 0.4 0.5 0.6 CQI PDF CDF • Highest CQI is 15 and highest DL MCS is 28 • Although we see a significant number of CQI=15 reported, scheduler hardly assigns any MCS=28! • Whenever DL MCS 28 is scheduled BLER on 1st Tx is 100%, hence scheduler uses MCS 27 • Number of symbols for PDCCH is fixed at 2 and results in higher code-rate for MCS 28 • MCS=28: TBS = 36696 (@49&50 PRB) • MCS=27: TBS = 31704 (@49&50 PRB)  10 Mbps L1 throughput difference! (2x2 MIMO, 2 Code Words) PDF CDF PDF CDF • Lower than expected Peak DL throughput as eNodeB scheduler avoids MCS 28 due to high BLER and fixed control channel symbol assignment 24
  • 25. 25 RRC Releases UE DL Inactivity Timer has not not expired RSRP ~ -102 dBm PCI 465 PCI 237 • 10 RRC Connections are Released by PCI 465 Release Cause: other • UE logs do no show high UE Tx power or high DL BLER • DL FTP Stalls due to continuous RRC Releases Unexpected RRC Connection Releases • Unexpected eNodeB RRC Connection Releases impact user experience causing FTP time-outs. EUTRAN traces needed for investigation 25
  • 26. 26 Lower eNodeB Scheduling reduces DL Throughput P1_AvgL1Throughput P1_AvgScheduledRate P1_AvgMCS_DL P1_AvgL1BLER Time 19:13:1519:13:1019:13:0519:13:0019:12:5519:12:5019:12:4519:12:4019:12:3519:12:3019:12:2519:12:2019:12:1519:12:1019:12:0519:12:0019:11:55 kbps 50,000 40,000 30,000 20,000 10,000 0 percentage 100 90 80 70 60 50 40 30 20 10 0 N/A 26 24 22 20 18 16 14 12 percentage 6 5 4 3 2 1 0 • L1 thpt >50 Mbps • Following scheduling rate and DL MCS • Scheduling rate ~ 85-90% (< 100%) • Linked to lack of DL scheduling when SIB1 is transmitted and only 1 user/TTI support • MCS ~26-27 • Low BLER – negligible impact on throughput • Scheduling “dip” after ~78 sec L1TputSchedulingMCSBLER Internal Modem Time • eNodeB Scheduler implementation results in lower scheduling rate and lower DL throughput 26
  • 27. 27 Impact of MTU Size and TCP Segment Losses • TCP MSS: 1460, TCP MTU: 1500 • TCP packet stats: • Re-tx: 765 (0.2%) • ooOrder: 5380 (1.5%) • TCP graph shows quite some slow starts and irregularities • MTU of 1500 can also result in fragmentation of IP segments on backhaul given GTP-U headers => Negatively impacts DL throughput • TCP graph shows quite some slow starts and irregularities due to TCP segment losses in Core Network => Negatively impacts DL Application throughput • Setting device MTU sizes correctly and minimizing CN packet losses is important to avoid negative Application layer throughput impacts 27
  • 28. 28 Key Areas to be considered – LTE Initial Launch • Optimize pilot polluted areas • Verify neighbor list planning, use ANR if available • Optimization study of system parameters is critical for handling increased load Deployment • Insufficient backhaul can reduce DL throughput • Sporadic packet discards in Core Network • Correct MTU size enforcement on all devices Data Performance • •Optimize HO parameters to ensure high Handover Success rates and reduce handover ping-pongs • Unexpected Radio Link Failures can impact performance • Inter-RAT optimization to ensure suitable user-experience during Initial build-out Mobility • • Unexpected RRC related drops and RACH failures may need to be investigated • Several RAN limitations exist • Scheduler limitations must be addressed before demand increases Implementation 28
  • 29. 29 Thank you