White Paper Latency Analysis

© Tech Mahindra Limited 2010

LTE-Advanced: LatencyAnalysis for IMT-A Evaluation Deepti Singhal, Mythili Kunapareddy and Vijayalakshmi Chetlapalli

Abstract: Wireless networks have made dramatic advances inthe technology catering to multidimensional services. Most ofthese services viz. voice, gaming, interactive applications etc,are sensitive to latency. As such, latency has become a keyperformance indicator in wireless data networks, besidesthroughput and QoS. All these performance relatedrequirements drive innovations in wireless technologies. This isthe key for Third Generation Partnership Project (3GPP) towork on LTE-Advanced (LTE-A) in conjunction with theIMT-Advanced (IMTA) requirements. LTE-A is a technologyenhancement to Long Term Evolution (LTE). The demand forlower latency triggered studies on various means of achievingit. Optimization of latency may happen on control plane (C-Plane) as well as user plane (U-Plane). This tutorial presentscontrol plane and user plane latency calculations forFrequency Division Duplex (FDD) and Time Division Duplex(TDD) modes of operation of LTE-A, with the goal ofevaluating whether the latency complies with the guidelinesoutlined by International Telecommunication Union (ITU) for4G radio access technologies delivering high speed multimediaservices. The analysis concludes that both the control and userplane latencies of LTE-A are in compliance with IMT-Arequirements.

1 © Tech Mahindra Limited 2010

Table of Contents INTRODUCTION ......................................................................... 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


INTRODUCTION LTE-Advanced is the technological advancement proposed by 3GPP to meet the requirements set by ITU for IMTA standard. The LTE-A standard primarily lays down performance enhancements to LTE Release 8 in addition to being backward compatible with it. The performance improvement goals of LTE-Advanced are - flexible network deployment, improved spectral efficiency, better user experience and cost effectiveness. LTE-A is designed to support peak data rates of 1 Gbps in the downlink (DL) and 500 Mbps in the uplink (UL). Its spectral efficiency is targeted to be thrice as that of LTE Release 8, with the ability to support spectrum aggregation and scalable bandwidth. The idle to connected mode latency is less than 50ms and one-way packet transmission delay is less than 5ms. The cell-edge and average user throughput requirements of LTE-A are approximately 3 times that of LTE. Latency is defined as the 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 several applications that do not require very high data rate, but they do require low latency. Voice, real time gaming and interactive applications are some examples. In LTE-A, the requirements for better quality of experience are more stringent than LTE Release 8. To reduce latency, LTE employs smaller transmission time interval (TTI) of 1ms. On an average, in FDD mode, the packet needs to wait for 0.5ms for transmission. Amongst the suggested ways to improve latency performance of LTE-A are – simultaneous processing of Radio Resource Control (RRC) and Non Access Stratum (NAS) requests at the eNB (evolved NodeB), reduced message processing 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 Latency Control plane deals with signaling and control functions, while user plane deals with actual user data transmission. C-Plane latency is measured as the time required for the UE (User Equipment) to transit from idle state to active state. In idle state, the UE does not have an RRC connection. Once the RRC is setup, the UE transitions to connected state and then to the active state when it enters the dedicated mode. U-Plane latency is defined as one-way transmit time between a packet being available at the IP layer in the UE/E-UTRAN (Evolved UMTS Terrestrial Radio 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 to overall latency in state transition and packet transmission. Latency calculations are made for both FDD and TDD modes of operation. Technical details of C-Plane and U-Plane latency are cited in [1], [2]. This tutorial is organized as follows: Requirements and assumptions in Section II, C-Plane latency analysis in Section III and U-Plane latency analysis in Section IV. The conclusions are summarized in Section V. All the values indicated in the tables are in milli seconds (ms). The method of calculating these latencies is illustrated in the appendix.


REQUIREMENTS & ASSUMPTIONS

Requirements The requirements set by ITU for C-Plane and U-Plane latencies are given in table I. The analysis was carried out as per the IMT-A evaluation requirements and guidelines [3], [4]. C-Plane latency should be less than 100ms from idle to active states and U-Plane latency should be less than 10ms.

TABLE 1 IMT-A LATENCY REQUIREMENTS

Plane Max latency (ms)

Control Plane 100 User Plane 10

Assumptions All 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-C transfer delay + MME processing of NAS request. This is based on the S1-C transfer delay values 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/DL subframe locations in the respective frame configurations [5]. 4) In TDD mode analysis, UL/DL frame configuration 1 is considered. Figure 1 shows the TDD frame for UL/DL frame configuration 1.

Figure 1: TDD Frame Configuration 1

CONTROL PLANE ANALYSIS C-Plane latency is calculated for Idle-Connected and Dormant-Active states [1], [2]. Figure 2 shows the transition states.


Figure 2: Control Plane State Transition Diagram

Transition from Idle to Connected state In idle state the UE doesn’t have an RRC connection. UE monitors the paging channel to detect the incoming calls and acquire system information. Once the UE transits to connected state, it establishes an RRC connection and a radio bearer. So the UE is known at the cell level and can respond to DL initiated transitions after the transition. Hence there is no case to consider for DL initiated transitions for idle to connected mode latency calculation. The calculations are made only for UL case. Figure 3 shows the message flow in idle to connected transition. The steps involved 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 eNB


Figure 3: Message Flow for Idle to Connected Transition All the calculations are based on 1ms RACH cycle for FDD mode and combined RRC and NAS request in Idle to Connected transition.

FDD Mode Analysis In 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 to send a RACH is 0.5ms (TTI/2). The delay in FDD mode for idle to connected transition is shown in table II.

TDD Mode Analysis In 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 with UL/DL frame configuration 1 with the probability (P) of occurrence of RACH trigger. The reason behind the calculations being done only for one TDD frame configuration is that the TDD case has multiple scenarios and covering all the cases is beyond the scope of this document. For convenience subframe is designated as SF in the tables. C-Plane latency analysis for TDD is shown in the table II.


TABLE II IDLE- CONNECTED TRANSITION

FDD TDD

Component Process Time[ms]

RACH in 2/7

(P=0.8)

RACH in 3/8

(P =0.2)

RACH in Special SF

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


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


Total delay 50 56 53.5 57.5

Average total delay 50 55.5 57.5

Transition from Dormant to Active state In dormant state, UE has an established RRC connection and radio bearers. UE may be in synchronized or unsynchronized state during the transition. The analysis has been carried on both the cases of UEs with UL initiated and DL initiated transmission. When UE is synchronized it waits for the PUCCH allocation for sending the Scheduling Request (SR). The steps involved in dormant to active transition are: 1) Average delay to next SR opportunity 2) UE transmits the SR the eNB 3) eNB decodes SR and generates the Scheduling Grant 4) Transmission of Scheduling Grant to eNB 5) Decoding of scheduling grant and L1 encoding of UL data at UE 6) Transmission of UL data Figure 4 shows the message flow in dormant to active transition for a synchronized UE. The main purpose of RACH is to obtain UL time synchronization and then to access the network. So for an UE without UL synchronization, RACH procedure must be followed. In the case of DL initiated transmission, the UE with UL synchronization monitors PDCCH (Physical Downlink Control Channel) during the on-duration of the DRX (Discontinuous Reception) cycle, and thus there is no additional delay component apart from the DRX cycle when compared to the case of the UL initiated for a synchronized UE. For the purpose of analysis, DRX cycle is not considered and the transmission is assumed to be error free.


Figure 4: Message Flow for Dormant to Active Transition

FDD Mode Analysis For Dormant to Active transition, all the calculations are based on 1ms PUCCH cycle for FDD mode. Table III shows the FDD mode calculations for synchronized UE (both UL initiated and DL initiated). Calculations for UL initiated unsynchronized UE and DL initiated unsynchronized UE are shown in tables IV, V.

TDD Mode Analysis TDD mode analysis is also carried out for dormant to active transition for synchronized and unsynchronized UEs. Table III shows the TDD mode calculations for synchronized UE (both UL initiated and DL initiated). Tables IV, VI show the calculations for UL initiated and DL initiated unsynchronized UEs respectively.

TABLE III DORMANT- ACTIVE TRANSITION SYNCHRONIZED UE

FDD TDD

Component Process Time [ms]

PUCCH in SF2/7 (P=0.8)

PUCCH in SF3/8 (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


TABLE IV DORMANT- ACTIVE TRANSITION UL INITIATED UNSYNCHRONIZED UE

FDD TDD


RACH in SF2/7

(P=0.8)

RACH in SF3/8

(P=0.2)

RACH in Special SF

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


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


0 UE waiting for dedicated PDCCH and UE processing 6

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 in SF0/5 (P=0.2)

PDCCH in SF1/6(P=0.2)



PDCCH in SF1/6(P=0.2)


0 UE waiting for dedicated PDCCH

0.5 0.5 1.5 0.5 0.5 1.5

1 UE processing and RACH waiting

6 6 7 10 9 6

2 Preamble 1 1 1 1 1 1

`3-4 eNB processing and grant 3 3 3 3 3 3

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 17.5


USER PLANE ANALYSIS U-Plane latency is also calculated for both TDD and FDD cases. The steps involved in the U-Plane latency calculation are: 1) UE processing time 2) TTI duration 3) HARQ retransmission 4) eNB processing delay U-Plane latency for a scheduled UE consists of fixed node processing delays which include radio frame alignment, header compression delay, ciphering delay, RLC/MAC processing time. It also includes eNB processing delay and HARQ retransmission delay.

FDD Mode Analysis Figure 5 shows the FDD analysis for U-Plane latency. In FDD mode HARQ process is fixed to 8ms. So HARQ retransmission delay is n*8 ms, where n is number of retransmissions. Typically there would be 0 or 1 HARQ retransmission. For the given assumption, we consider n as the error 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-Plane latency calculations for FDD.

Figure 5: U-Plane latency components for FDD

TABLE VII U-PLANE LATENCY ANALYSIS FOR FDD

FDD

Component Process 0% HARQ BLER

10% HARQ BLER

1 UE Processing Time (including Frame Alignment) 1.5 1.5

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 4.8

TDD Mode Analysis U-Plane latency analysis for TDD case is shown in figure 6 and 7. The analysis is done for all UL/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


XI for 0% BLER and 10% BLER respectively. The analysis is carried on different TDD UL/DL frame configurations.

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

5 HARQ Retransmission 0 0 0 0 0 0 0

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


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

5 HARQ Retransmission 1 1.02 0.98 1.05 1.16 1.24 1.12

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


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 1


4 eNB Processing 1.5 1.5 1.5 1.5 1.5 1.5 1.5

5 HARQ Retransmission 0 0 0 0 0 0 0

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


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 1

4 eNB Processing 1.5 1.5 1.5 1.5 1.5 1.5 1.5

5 HARQ Retransmission 1.16 1 1 1 1 1 1.15

Total delay in ms 5.76 6.2 7.0 7.8 8.6 9.5 6.05

CONCLUSION The 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-MME processing 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 for UE in synchronization case and 73.5ms for UE in unsynchronized case respectively. In TDD UL/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 with 10% HARQ BLER for FDD case. In TDD case minimum and maximum latencies are 4.1ms and 9.5ms 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

Mode Idle-Connected

S1-C transfer+MME

processing of NAS request

Dormant-Active

Idle-Active

UL Initiated, UE Sync 50 10 9.5 69.5

DL Initiated, UE Sync 50 10 9.5 69.5


UL Initiated, UE Unsync 50 10 10.5 70.5

DL Initiated, UE Unsync 50 10 13.5 73.5

TABLE XIII C-PLANE LATENCY SUMMARY FOR TDD MODE

Mode RACH Position Idle-Connected

S1-C transfer+MME processing of NAS request

Dormant-Active

Idle-Active

UL Initiated, UE Sync

UL SF 55.5 10 12.7 78.2

UL Initiated, UE Sync

Special SF 57.5 10 12.7 80.2

DL Initiated, UE Sync

UL SF 55.5 10 12.7 78.2

DL Initiated, UE Sync

Special SF 57.5 10 12.7 80.2

UL Initiated, UE Unsync

UL SF 55.5 10 12.5 78

UL Initiated, UE Unsync

Special SF 57.5 10 14.5 82

DL Initiated, UE Unsync

UL SF 55.5 10 16.1 81.6

DL Initiated, UE Unsync

Special SF 57.5 10 17.5 85

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 9.5

REFERENCE [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).


[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 TDD The latencies are calculated for TDD frame configuration1. The method of calculations is illustrated below -

TABLE XV IDLE-CONNECTED

TABLE XVI DORMANT-ACTIVE: UL/DL INITIATED, SYNC UE


TABLE XVII DORMANT-ACTIVE: UL INITIATED, UNSYNC UE

TABLE XVIII DORMANT-ACTIVE: DL INITIATED, UNSYNC UE

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