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White paper latency_analysis
 

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    White paper latency_analysis White paper latency_analysis Document Transcript

    • LTE-Advanced: Latency Abstract: Wireless networks have made dramatic advances inAnalysis for IMT-A the technology catering to multidimensional services. Most of these services viz. voice, gaming, interactive applications etc, are sensitive to latency. As such, latency has become a keyEvaluation performance indicator in wireless data networks, besides throughput and QoS. All these performance related requirements drive innovations in wireless technologies. This isDeepti Singhal, Mythili Kunapareddy the key for Third Generation Partnership Project (3GPP) to work on LTE-Advanced (LTE-A) in conjunction with theand Vijayalakshmi Chetlapalli IMT-Advanced (IMTA) requirements. LTE-A is a technology enhancement to Long Term Evolution (LTE). The demand for lower latency triggered studies on various means of achieving it. Optimization of latency may happen on control plane (C- Plane) as well as user plane (U-Plane). This tutorial presents control plane and user plane latency calculations for Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes of operation of LTE-A, with the goal of evaluating whether the latency complies with the guidelines outlined by International Telecommunication Union (ITU) for 4G radio access technologies delivering high speed multimedia services. The analysis concludes that both the control and user plane latencies of LTE-A are in compliance with IMT-A requirements. © Tech Mahindra Limited 2010
    • Table of ContentsINTRODUCTION ......................................................................... 2  Control Plane and User Plane Latency .................................... 2 REQUIREMENTS & ASSUMPTIONS .......................................... 3  Requirements .......................................................................... 3  Assumptions ............................................................................ 3 CONTROL PLANE ANALYSIS .................................................... 3  Transition from Idle to Connected state ................................... 4  FDD Mode Analysis……………………………………………. 5  TDD Mode Analysis……………………………………………. 5  Transition from Dormant to Active state................................... 6  FDD Mode Analysis……………………………………………. 7  TDD Mode Analysis……………………………………………..7 USER PLANE ANALYSIS............................................................ 9  FDD Mode Analysis……………………………………………. 9  TDD Mode Analysis……………………………………………..9 CONCLUSION ........................................................................... 11 REFERENCE ............................................................................ 12 Appendix A: Method of calculating latencies for TDD ................ 13  1 © Tech Mahindra Limited 2010
    • INTRODUCTIONLTE-Advanced is the technological advancement proposed by 3GPP to meet the requirementsset by ITU for IMTA standard. The LTE-A standard primarily lays down performanceenhancements to LTE Release 8 in addition to being backward compatible with it. Theperformance improvement goals of LTE-Advanced are - flexible network deployment, improvedspectral efficiency, better user experience and cost effectiveness. LTE-A is designed to supportpeak data rates of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Its spectralefficiency is targeted to be thrice as that of LTE Release 8, with the ability to support spectrumaggregation and scalable bandwidth. The idle to connected mode latency is less than 50ms andone-way packet transmission delay is less than 5ms. The cell-edge and average userthroughput requirements of LTE-A are approximately 3 times that of LTE. Latency is defined asthe average time between the transmission of packet and the reception of an acknowledgment.Low latency is a key performance criterion for better user experience. There are severalapplications that do not require very high data rate, but they do require low latency. Voice, realtime gaming and interactive applications are some examples. In LTE-A, the requirements forbetter quality of experience are more stringent than LTE Release 8. To reduce latency, LTEemploys smaller transmission time interval (TTI) of 1ms. On an average, in FDD mode, thepacket needs to wait for 0.5ms for transmission. Amongst the suggested ways to improvelatency performance of LTE-A are – simultaneous processing of Radio Resource Control (RRC)and Non Access Stratum (NAS) requests at the eNB (evolved NodeB), reduced messageprocessing delays at various nodes, reduced RACH (Random Access Channel) and PUCCH(Physical Uplink Control Channel) cycles and a contention based uplink [1].Control Plane and User Plane LatencyControl plane deals with signaling and control functions, while user plane deals with actual userdata transmission. C-Plane latency is measured as the time required for the UE (UserEquipment) to transit from idle state to active state. In idle state, the UE does not have an RRCconnection. Once the RRC is setup, the UE transitions to connected state and then to the activestate when it enters the dedicated mode. U-Plane latency is defined as one-way transmit timebetween a packet being available at the IP layer in the UE/E-UTRAN (Evolved UMTS TerrestrialRadio Access Network) edge node and the availability of this packet at the IP layer in the E-UTRAN/UE node. U-Plane latency is relevant for the performance of many applications.This tutorial presents in detail the delay budgets of C-Plane and U-Plane procedures that add tooverall latency in state transition and packet transmission. Latency calculations are made forboth FDD and TDD modes of operation. Technical details of C-Plane and U-Plane latency arecited in [1], [2]. This tutorial is organized as follows: Requirements and assumptions in SectionII, C-Plane latency analysis in Section III and U-Plane latency analysis in Section IV. Theconclusions are summarized in Section V. All the values indicated in the tables are in milliseconds (ms). The method of calculating these latencies is illustrated in the appendix.2 © Tech Mahindra Limited 2010
    • REQUIREMENTS & ASSUMPTIONSRequirementsThe requirements set by ITU for C-Plane and U-Plane latencies are given in table I. The analysiswas carried out as per the IMT-A evaluation requirements and guidelines [3], [4]. C-Planelatency should be less than 100ms from idle to active states and U-Plane latency should be lessthan 10ms. TABLE 1 IMT-A LATENCY REQUIREMENTS Plane Max latency (ms) Control Plane 100 User Plane 10AssumptionsAll latency calculations are done with the following assumptions:1) Ideally for best case scenario, the RRC and NAS requests are processed in parallel in LTE-A.However, in practical scenarios, it is reasonable to assume an additional delay of 10ms for S1-Ctransfer delay + MME processing of NAS request. This is based on the S1-C transfer delayvalues mentioned in [2].2) 1ms PRACH and PUCCH cycle in FDD.3) The RACH and PUCCH waiting times in TDD cases are calculated based on the UL/DLsubframe locations in the respective frame configurations [5].4) In TDD mode analysis, UL/DL frame configuration 1 is considered. Figure 1 shows the TDDframe for UL/DL frame configuration 1. Figure 1: TDD Frame Configuration 1CONTROL PLANE ANALYSISC-Plane latency is calculated for Idle-Connected and Dormant-Active states [1], [2]. Figure 2shows the transition states. 3 © Tech Mahindra Limited 2010
    • Figure 2: Control Plane State Transition DiagramTransition from Idle to Connected stateIn idle state the UE doesn’t have an RRC connection. UE monitors the paging channel to detectthe incoming calls and acquire system information. Once the UE transits to connected state, itestablishes an RRC connection and a radio bearer. So the UE is known at the cell level and canrespond to DL initiated transitions after the transition. Hence there is no case to consider for DLinitiated transitions for idle to connected mode latency calculation. The calculations are madeonly for UL case. Figure 3 shows the message flow in idle to connected transition. The stepsinvolved in idle to connected mode transition are: 1. UE waiting for RACH scheduling 2. RACH preamble to eNB 3. Preamble detection at eNB 4. Transmission of RA response (Time between the end RACH transmission and UEs reception of scheduling grant and timing adjustment) 5. Decoding of scheduling grant, timing alignment and CRNTI assignment at UE 6. Transmission of RRC connection request to eNB 7. L2 and RRC processing delay at eNB 8. Transmission of RRC Connection Setup to UE (Simultaneously NAS service is requested and processed) 9. L2 and RRC processing delay at UE 10. Transmission of RRC Connection Setup Complete to eNB. Steps 11-14 depict NAS service request and processing at MME. This happens in parallel with RRC Connection procedure in LTE-A and hence reduces latency. 11. Processing at eNB (Uu–S1-C) 12. NAS connection request, S1-C transfer delay 13. MME processing 14. NAS connection setup, S1-C transfer delay 15. Processing at eNB 16. Transmission of RRC Security Mode Command and Connection Reconfiguration (+TTI alignment) to UE 17. L2 and RRC processing at UE 18. Transmission of setup complete to eNB4 © Tech Mahindra Limited 2010
    •   Figure 3: Message Flow for Idle to Connected TransitionAll the calculations are based on 1ms RACH cycle for FDD mode and combined RRC and NASrequest in Idle to Connected transition.FDD Mode AnalysisIn FDD mode, UL and DL transmissions are done simultaneously on different frequency bands.RACH can be scheduled in UL once every subframe (1ms), so the average RACH waiting time tosend a RACH is 0.5ms (TTI/2). The delay in FDD mode for idle to connected transition is shownin table II.TDD Mode AnalysisIn TDD mode, the C-Plane latency calculations are dependent on UL/DL frame configuration [5]and the location of RACH trigger in the TDD frame [6]. The analysis for TDD is carried withUL/DL frame configuration 1 with the probability (P) of occurrence of RACH trigger. The reasonbehind the calculations being done only for one TDD frame configuration is that the TDD casehas multiple scenarios and covering all the cases is beyond the scope of this document. Forconvenience subframe is designated as SF in the tables. C-Plane latency analysis for TDD isshown in the table II. 5 © Tech Mahindra Limited 2010
    • TABLE II IDLE- CONNECTED TRANSITION FDD TDD RACH in RACH in Time RACH in Component Process 2/7 3/8 [ms] Special SF (P=0.8) (P =0.2) 1 RACH Waiting 0.5 2 0.5 2.5 2 Preamble 1 1 1 1 3-4 eNB processing and Grant 3 3 3 3 5 UE processing 5 6 5 7 6 RRC + NAS Request 1 1 1 1 7 eNB processing 4 6 6 6 8 RRC setup 1 1 1 1 9 UE processing 12 12 12 12 10 RRC setup complete 1 1 1 1 11-14 Parallel processing of NAS request at eNB+MME 0 0 0 0 15 eNB processing 4 4 4 4 16 NAS setup 1.5 3 3 3 17 UE processing 16 16 16 16 Total delay 50 56 53.5 57.5 Average total delay 50 55.5 57.5Transition from Dormant to Active stateIn dormant state, UE has an established RRC connection and radio bearers. UE may be insynchronized or unsynchronized state during the transition. The analysis has been carried onboth the cases of UEs with UL initiated and DL initiated transmission. When UE is synchronized itwaits for the PUCCH allocation for sending the Scheduling Request (SR). The steps involved indormant to active transition are:1) Average delay to next SR opportunity2) UE transmits the SR the eNB3) eNB decodes SR and generates the Scheduling Grant4) Transmission of Scheduling Grant to eNB5) Decoding of scheduling grant and L1 encoding of UL data at UE6) Transmission of UL dataFigure 4 shows the message flow in dormant to active transition for a synchronized UE. Themain purpose of RACH is to obtain UL time synchronization and then to access the network. Sofor an UE without UL synchronization, RACH procedure must be followed. In the case of DLinitiated transmission, the UE with UL synchronization monitors PDCCH (Physical DownlinkControl Channel) during the on-duration of the DRX (Discontinuous Reception) cycle, and thusthere is no additional delay component apart from the DRX cycle when compared to the case ofthe UL initiated for a synchronized UE. For the purpose of analysis, DRX cycle is not consideredand the transmission is assumed to be error free.6 © Tech Mahindra Limited 2010
    • Figure 4: Message Flow for Dormant to Active TransitionFDD Mode AnalysisFor Dormant to Active transition, all the calculations are based on 1ms PUCCH cycle for FDDmode. Table III shows the FDD mode calculations for synchronized UE (both UL initiated and DLinitiated). Calculations for UL initiated unsynchronized UE and DL initiated unsynchronized UEare shown in tables IV, V.TDD Mode AnalysisTDD mode analysis is also carried out for dormant to active transition for synchronized andunsynchronized UEs. Table III shows the TDD mode calculations for synchronized UE (both ULinitiated and DL initiated). Tables IV, VI show the calculations for UL initiated and DL initiatedunsynchronized UEs respectively. TABLE III DORMANT- ACTIVE TRANSITION SYNCHRONIZED UE FDD TDD PUCCH PUCCH Time Component Process in SF2/7 in SF3/8 [ms] (P=0.8) (P=0.2) 1  PUCCH Waiting 0.5 2 0.5 2  SR 1 1 1 3  eNB processing 3 3 5 4  Grant 1 1 1 5  UE processing 3 5 3 6  UL Data 1 1 1 Total delay 9.5 13 11.5 Average total delay 9.5 12.7 7 © Tech Mahindra Limited 2010
    • TABLE IV DORMANT- ACTIVE TRANSITION UL INITIATED UNSYNCHRONIZED UE FDD TDD RACH in RACH in Time RACH in Component Process SF2/7 SF3/8 [ms] Special SF (P=0.8) (P=0.2) 1 RACH Waiting 0.5 2 0.5 2.5 2 Preamble 1 1 1 1 eNB processing `3-4 3 3 3 3 and Grant 5 UE processing 5 6 5 7 6 UL Data 1 1 1 1 Total delay 10.5 13 10.5 14.5 Average total delay 10.5 12.5 14.5 TABLE V FDD: DORMANT- ACTIVE TRANSITION DL INITIATED UNSYNCHRONIZED UE FDD Time Component Process [ms] UE waiting for dedicated 0 6 PDCCH and UE processing 1 RACH Waiting 0.5 2 Preamble 1 `3-4 eNB processing and grant 3 5 UE processing 2 6 UL Data 1 Total delay 13.5 TABLE VI TDD: DORMANT- ACTIVE TRANSITION DL INITIATED UNSYNCHRONIZED UE TDD Component Process RACH in UL SF 2/3/7/8 RACH in Special SF1/6 PDCCH PDCCH PDCCH PDCCH PDCCH PDCCH in SF0/5 in SF1/6 in SF4/9 in SF0/5 in SF1/6 in SF4/9 (P=0.2) (P=0.2) (P=0.6) (P=0.2) (P=0.2) (P=0.6) UE waiting for 0 dedicated 0.5 0.5 1.5 0.5 0.5 1.5 PDCCH UE processing 1 and RACH 6 6 7 10 9 6 waiting 2 Preamble 1 1 1 1 1 1 eNB processing `3-4 3 3 3 3 3 3 and grant 5 UE processing 3 2 4 4 4 4 6 DL Data 1 1 1 1 1 1 Total delay 14.5 13.5 17.5 19.5 18.5 16.5 Average total delay 16.1 17.58 © Tech Mahindra Limited 2010
    • USER PLANE ANALYSISU-Plane latency is also calculated for both TDD and FDD cases. The steps involved in the U-Plane latency calculation are:1) UE processing time2) TTI duration3) HARQ retransmission4) eNB processing delayU-Plane latency for a scheduled UE consists of fixed node processing delays which include radioframe alignment, header compression delay, ciphering delay, RLC/MAC processing time. It alsoincludes eNB processing delay and HARQ retransmission delay.FDD Mode AnalysisFigure 5 shows the FDD analysis for U-Plane latency. In FDD mode HARQ process is fixed to8ms. So HARQ retransmission delay is n*8 ms, where n is number of retransmissions. Typicallythere would be 0 or 1 HARQ retransmission. For the given assumption, we consider n as theerror probability of the first HARQ retransmission. Minimum latency is achieved with 0% BLER.But we also considered 10% BLER for more reasonable calculations. Table VII shows the U-Planelatency calculations for FDD. Figure 5: U-Plane latency components for FDD TABLE VII U-PLANE LATENCY ANALYSIS FOR FDD FDD 0% HARQ 10% HARQ Component Process BLER BLER UE Processing Time (including Frame 1 1.5 1.5 Alignment) 2 TTI Duration (Fixed) 1 1 3 HARQ Retransmission 0 0.8 4 eNB Processing 1.5 1.5 Total one way delay (ms) 4 4.8TDD Mode AnalysisU-Plane latency analysis for TDD case is shown in figure 6 and 7. The analysis is done for allUL/DL frame configurations in TDD with 0% BLER and 10% BLER. Tables VIII and IX show the U-Plane analysis for DL with 0% BLER and 10% BLER respectively. Analysis for UL is shown in X and 9 © Tech Mahindra Limited 2010
    • XI for 0% BLER and 10% BLER respectively. The analysis is carried on different TDD UL/DL frameconfigurations. Figure 6: U-Plane latency components for TDD in DL Figure 7: U-Plane latency components for TDD in UL TABLE VIII U-PLANE LATENCY ANALYSIS FOR TDD IN DL WITH 0% BLER Component Process UL/DL configuration 0 1 2 3 4 5 6 1 eNB Processing 1 1 1 1 1 1 1 2.0 Frame Alignment 1.7 1.1 0.7 1.1 0.8 0.6 1.4 3 TTI duration 1 1 1 1 1 1 1 4.0 UE Processing 1.5 1.5 1.5 1.5 1.5 1.5 1.5 HARQ 5 0 0 0 0 0 0 0 Retransmission Total delay in ms 5.2 4.6 4.2 4.6 4.3 4.1 4.9 TABLE IX U-PLANE LATENCY ANALYSIS FOR TDD IN DL WITH 10% BLER Component Process UL/DL configuration 0 1 2 3 4 5 6 1 eNB Processing 1 1 1 1 1 1 1 Frame 2.0 1.7 1.1 0.7 1.1 0.8 0.6 1.4 Alignment 3 TTI duration 1 1 1 1 1 1 1 4.0 UE Processing 1.5 1.5 1.5 1.5 1.5 1.5 1.5 HARQ 5 1 1.02 0.98 1.05 1.16 1.24 1.12 Retransmission Total delay in ms 6.2 5.62 5.18 5.65 5.46 5.34 6.02 TABLE X U-PLANE LATENCY ANALYSIS FOR TDD IN UL WITH 0% BLER Component Process UL/DL configuration 0 1 2 3 4 5 6 1 UE Processing 1 1 1 1 1 1 1 2 Frame Alignment 1.1 1.7 2.5 3.3 4.1 5 1.4 3 TTI duration 1 1 1 1 1 1 110 © Tech Mahindra Limited 2010
    • 4 eNB Processing 1.5 1.5 1.5 1.5 1.5 1.5 1.5 HARQ 5 0 0 0 0 0 0 0 Retransmission Total delay in ms 4.6 5.2 6.0 6.8 7.6 8.5 4.9 TABLE XI U-PLANE LATENCY ANALYSIS FOR TDD IN UL WITH 10% BLER Component Process UL/DL configuration 0 1 2 3 4 5 6 1 UE Processing 1 1 1 1 1 1 1 Frame 2 1.1 1.7 2.5 3.3 4.1 5 1.4 Alignment 3 TTI duration 1 1 1 1 1 1 1 4 eNB Processing 1.5 1.5 1.5 1.5 1.5 1.5 1.5 HARQ 5 1.16 1 1 1 1 1 1.15 Retransmission Total delay in ms 5.76 6.2 7.0 7.8 8.6 9.5 6.05CONCLUSIONThe analysis is summarized in tables XII, XIII and XIV for C-Plane and U-Plane latencies. For C-Plane latency values, an additional average delay of 10ms is accounted for S1-C and S1-MMEprocessing delay for the NAS request as mentioned in section II.The minimum and maximum C-Plane latencies from idle to active mode in FDD are 69.5ms forUE in synchronization case and 73.5ms for UE in unsynchronized case respectively. In TDDUL/DL frame configuration 1, these values are 78ms and 85ms respectively.In U-Plane, the minimum and maximum latencies are 4ms with 0%HARQ BLER and 4.8ms with10% HARQ BLER for FDD case. In TDD case minimum and maximum latencies are 4.1ms and9.5ms for 0% and 10% BLER respectively.Similar analysis has been presented to ITU [7] considering 3GPP Release 8 latency calculations. TABLE XII C-PLANE LATENCY SUMMARY FOR FDD MODE S1-C Idle- transfer+MME Dormant- Idle- Mode Connected processing of NAS Active Active request UL Initiated, 50 10 9.5 69.5 UE Sync DL Initiated, 50 10 9.5 69.5 UE Sync 11 © Tech Mahindra Limited 2010
    • UL Initiated, 50 10 10.5 70.5 UE Unsync DL Initiated, 50 10 13.5 73.5 UE Unsync TABLE XIII C-PLANE LATENCY SUMMARY FOR TDD MODE S1-C RACH transfer+MME Dormant- Idle- Mode Idle-Connected Position processing of Active Active NAS request UL Initiated, UL SF 55.5 10 12.7 78.2 UE Sync UL Initiated, Special SF 57.5 10 12.7 80.2 UE Sync DL Initiated, UL SF 55.5 10 12.7 78.2 UE Sync DL Initiated, Special SF 57.5 10 12.7 80.2 UE Sync UL Initiated, UL SF 55.5 10 12.5 78 UE Unsync UL Initiated, Special SF 57.5 10 14.5 82 UE Unsync DL Initiated, UL SF 55.5 10 16.1 81.6 UE Unsync DL Initiated, Special SF 57.5 10 17.5 85 UE Unsync TABLE XIV U-PLANE LATENCY SUMMARY Case 0% BLER 10% BLER FDD 4 4.8 Min Max Min Max TDD DL 4.1 5.2 5.18 6.2 TDD UL 4.6 8.5 5.76 9.5REFERENCE[1] 3GPP TR 36.912 V9.1.0 (2009-12) Feasibility Study for Further Advancements for E- UTRA (LTE-Advanced) (Release 9). 3GPP, Technical Specification Group Radio Access Network.[2] 3GPP TR 25.912 V8.0.0 (2008-12) Feasibility Study for Evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN) (Release 8). 3GPP, Technical Specification Group Radio Access Network.[3] REPORT ITU-R M.2134, Requirements related to technical performance for IMT- Advanced radio interface(s).12 © Tech Mahindra Limited 2010
    • [4] REPORT ITU-R M.2135, Guidelines for evaluation of radio interface technologies for IMT-Advanced.[5] 3GPP TR 36.211 V9.1.0 (2009-12) Physical Channels and Modulation[6] R2-101596, 3GPP TSG RAN WG2 Meeting #69, San Francisco, US, Feb 22nd-26th, 2010.[7] Evaluation of IMT-Advanced candidate technology submissions in Documents IMT- ADV/4 and IMT-ADV/8 by TCOE India, Document IMTADV/16-E, July 2010.Appendix A: Method of calculating latencies for TDDThe latencies are calculated for TDD frame configuration1. The method of calculations isillustrated below - TABLE XV IDLE-CONNECTED TABLE XVI DORMANT-ACTIVE: UL/DL INITIATED, SYNC UE 13 © Tech Mahindra Limited 2010
    • TABLE XVII DORMANT-ACTIVE: UL INITIATED, UNSYNC UE TABLE XVIII DORMANT-ACTIVE: DL INITIATED, UNSYNC UE14 © Tech Mahindra Limited 2010