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PDCCH LINK ADAPTATION tuning

PDCCH LINK ADAPTATION tuning. Summary. In the attempt to improve the performance (throughput) some parameters controlling the use of the PDCCH resources were tuned. The results is indicating that significant benefits are possible Highest gains can be expected in multi user scenarios

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PDCCH LINK ADAPTATION tuning

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  1. PDCCH LINK ADAPTATION tuning

  2. Summary • In the attempt to improve the performance (throughput) some parameters controlling the use of the PDCCH resources were tuned. • The results is indicating that significant benefits are possible • Highest gains can be expected in multi user scenarios • Exact gain will depend on traffic pattern and radio environment • The parameter changes and tests was done lab in a network without commercial traffic. • The following parameters were tuned: • pdcchCfiMode • pdcchLaGinrMargin (system constant) • nrOfTransmissionsSib1 • Some of the parameters might not be suitable for a live network deployment

  3. Lab Setup & ExecutionSingle cell – L12B ICP4 software – 5MHz Bw @2.1GHz • Spirent VR5 Channel Emulator (mainly EPA5 channel used) • Full buffer UDP or FTP traffic • Programmed Attenuation sweep • from ~100dB path loss  UE release (~150dB) (1dB steps, 30sec/step) • UE’s and Logging Tools • Qualcomm 8960 (FFA) & Quanta (Qualcomm chip) • All data shown from UE side (not network side data included)

  4. Definitions & Data Sources • Path Loss • As reported by Qualcomm UE (based on RS power) • SINR • As reported by Qualcomm UE (probably based on C-RS?) • UL & DL Scheduling Ratio • Ratio of subframes when the UE DRB data is scheduled to total subframes (ms) in averaging window • BLER • Ratio of successful Redundancy Version 0 (RV0) transmissions to all RV 0 transmissions

  5. Characteristics – Default parameters5MHz Bw – EPA5 – Single UE - UDP DL full buffer traffic UDP Downlink– Expected peak TP (35M) not reached Only RLC status and polling sent in uplink Uplink Limited  lowest MCS not reached No. of PRB reduced in good RF env. Sched. ratio decreased with decreasing RF env.

  6. Scheduling Ratio & PRBsDetailed View In good RF 25, 22, 19 and 5 PRBs are used In poor RF only 25 PRBs are used Sched. ratio reduced in poor RF especially for subframe 0 & 5 The PRB reduction in good RF is mainly in subframe 0 and 5 System Information (SI) is scheduled in subframe 0 and 5 with various repetition

  7. 0 1 2 3 4 5 6 7 PDCCH Control Chanel elements Control Channel Element 1 CCE – 8 spaces GOOD Radio Env. Low Robustness Low PDCCH Capacity Consumption  2 CCE – 4 spaces  High PDCCH Capacity Consumption POOR Radio Env. High Robustness 4 CCE – 2 spaces  8 CCE – 1 space Fewer CCE used for each PDCCH allow for more users scheduled in parallel

  8. PDCCH Link Adaptation • One PDCCH can be mapped to 1, 2, 4 or 8 CCEs • Selection of number of CCEs is done based on the same GINR estimate used for PDSCH link adaptation • A margin (back-off) is added to PDSCH GINR to compensate for different interference scenarios between the two channels • Degraded RF conditions  • Higher CCE Aggregation level • Lower MCS (for the same number of PRB’s) • If the required number of CCE’s are not available in the control region (1) the UE can not be scheduled on the PDSCH (2) in that subframe DL scheduler& Link Adaptation Resource allocation (PDCCH) Data (PDSCH) eNodeB UE 2CCE 8CCE 4CCE 1CCE

  9. PDCCH Candidates for 5MHzExample • Exact candidate positions depend on x-RNTI and subframe number • Number of candidates increasing with number of PDCCH OFDM symbols (CFI) • Number of candidates is dependent on number of PHICH groups configured (def. 3) • Both Uplink and Downlink Scheduling need the PDCCH resources • Uplink scheduling has priority over downlink scheduling • Common is always using 8 CCE to ensure full cell coverage (Ericsson) CFI = 1 No Common scheduling possible No Poor/Medium coverage UE Scheduling possible CFI = 2 One 8 CCE UE’s or common can be scheduled in the same Subframe CFI = 3 Two 8 CCE UE’s (or common + one 8 CCE UE) can be scheduled in the same subframe

  10. CCE Aggregation Level vs. SINR and MCS CCE Use increasing at low SINR PDSCH Link Adaptation (MCS) follow SINR PDCCH LA follow PDSCH LA

  11. Problem Summary Reduced Sched. ratio due to uplink scheduling (higher priority) Reduced Sched. ratio due to SI scheduling (higher priority) • In good radio environment (1 & 2 CCE required) • DRB (user) data can be scheduled on remaining PRB:s when SI is scheduled since there is enough space in the PDCCH • Possible to fit both uplink and downlink scheduling in the PDCCH • In poor radio environment (8 CCE required) • No DRB (user) data can be scheduled when the SI is schedule due to lack of PDCCH resources (only one 8 CCE candidate) • No downlink data can be scheduled when there is uplink data to schedule • Reduced downlink scheduling ratio

  12. Solution SummaryOptimized Parameter Setting • Increase max number of PDCCH symbols to 3 • pdcchCfiMode: 0  5 • 0: (Static by bandwidth – 2 for 5Mhz), 5: (Adaptive CFI, max 3) • Reduce SIB Scheduling • Sib 1 is scheduled every 80ms and repeated 4 times, i.e. every 20ms (default) • nrOfTransmissionsSib1: 4 1 (i.e. 1 repetition = every 80ms) • Reduce PDCCH Link Adaptation Margin (relative PDSCH LA) • Delaying the CCE increase to a lower SINR will make the PDCCH less robust • Analysis needed to find suitable tradeoff

  13. Tuning of PDCCH LA Margin • Low Margin  UE will fail to decode the PDCCH • UE will consider itself not scheduled  Reduced Scheduling Ratio as seen from the UE side • High Margin  UE will use unnecessarily many CCE’s • Increasing the probability of lack of PDCCH resources  Reduced Scheduling Ratio • In case of a single UE with DL UDP traffic this will only happen when SI is scheduled* • Strategy: • Reduce margin until a distinct decrease in Scheduling Ratio is seen* • Initially test in large steps, reduce later • Margin steps: 10dB, 6dB, 3dB, 0dB -3dB & -5dB

  14. DL Scheduling Ratio vs. MCS10- -5dB Margins - EVA70Evaluation Reference: 10dB (default) 6dB – no degradation 0dB – no obvious degradation 3dB – no degradation -5dB –severe degradation -3dB –obvious degradation

  15. analysisEVA70 Evaluation • Based on the EPA70 channel tests it was found that the Scheduling ratio was reduced with a PDCCH LA margin of between 3dB and -3dB • A focused test in the range of 4dB to -3dB margin was made • Step size: 0.5dB • Channel model was changed to EPA5 • Results are shown as • Downlink Scheduling Ratio vs. MCS • Distribution of PDCCH Aggregation level vs. MCS

  16. DL Scheduling Ratio & CCEs vs. MCS4- 3dB Margins – EPA5 Reference 4dB Margin 3.5dB Margin No degradation 3dB Margin No degradation

  17. DL Scheduling Ratio & CCEs vs. MCS3- 2dB Margins – EPA5 3dB Margin No degradation PDCCH decoding failures 2.5dB Margin PDCCH decoding failures 2dB Margin

  18. DL Scheduling Ratio & CCEs vs. MCS1.5- 0dB Margins – EPA5 PDCCH decoding failures 1.5dB Margin PDCCH decoding failures 1dB Margin PDCCH decoding failures 1dB Margin

  19. LA Margin Selection • Based on the results the PDCCH LA margin was selected as 3dB • Slightly lower margin might be possible without major degradation • Considering the limited test scenarios and channel models evaluated the chosen setting was considered reasonably safe for the non commercial network 3dB Margin No degradation relative to the reference case

  20. Evaluation of performance improvementmulti ue scenario • 2 UE’s in the same radio environment (EPA5 channel) • UE1: Full buffer TCP Downlink Traffic • UE2: Full buffer TCP Uplink Traffic • In this scenario the UE’s are competing on the PDCCH space rather than PDSCH • TCP Traffic is generating additional traffic in the reverse direction • TCP ACK’s • Programmed Attenuation Sweep of both UE’s together (splitter) • From ~100dB path loss  UE release (~150dB) (1dB steps, 30sec/step) • 3 Cases Evaluated: • Default Parameter Set • Flexible CFI with a max of 3 symbols • Fully Optimized Parameters • PDCCH LA margin – 3dB Data (PUSCH) Data (PDSCH) UE2 eNodeB UE1

  21. UE1: TCP Downlink UEDownlink & uplink Throughput Increased UL Thp. Is an effect of increased rate of TCP ACK’s Improved Thp. (~ 2MBps) from Improved scheduling ratio Increased sched. Ratio at low SINRdue to uplink PRB reduction

  22. UE1: TCP Downlink UEDownlink Scheduling ratio per subframe number Default Parameters Flexible CFI (max 3) Optimized Parameters

  23. UE2: TCP Uplink UEDownlink & uplink Throughput Increased DL Thp. Is an effect of increased rate of TCP ACK’s Improved Thp. (~ 1MBps) from Improved scheduling ratio Main improvement from Flexible CFI

  24. Field performance

  25. Filed performanceoptimized parameter setting • The optimized parameter setting was tested in the field • ~1 Hour drive route in suburban-rural area • Both unloaded and 100% OCNG downlink load was tested • Single UE FTP download • Limited file size causing idle periods during drive test • EPA5 Lab result curve included as reference

  26. Field: Throughput performanceSuburban-rural area The radio environment in field appears a bit less challenging for the UE than EPA5 in the Lab

  27. Field: Scheduling ratioSuburban-rural area • In general lower scheduling ratio during field test relative to lab • Handovers, quick variations in radio environment in connection to the TCP protocol etc. could account for this lowered scheduling ratio • Samples with very low scheduling ratio is due to idle times between filed downloads

  28. THNAK YOU

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