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LTE Performance – Expectations & Challenges

Engineering Services Group

September 2011

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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

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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

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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

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World Wireless Academy – LTE Courses

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Expected LTE Performance

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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

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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

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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

Request Attach 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

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Key LTE Call Setup Metrics

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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

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Peak Single User DL Throughput – 10 MHz

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• “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

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Peak Single User UL Throughput – 10 MHz

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• “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

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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

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User Plane Latency

Ave (ms) Min (ms) Max (ms) STD (ms)

42.1 36 62 4.3

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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)

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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

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rce

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%]

HO Success rate

HO Success Rate

Download Upload Total

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LTE Intra-frequency Handover/Data Interruption

Ave (ms) Min (ms) Max (ms) STD (ms)

78 38 199 34

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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

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Typical Factors Impacting LTE Performance

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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

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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

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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

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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

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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

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Backhaul Limitations Reduce LTE DL Throughput

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L1 T

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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

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0.00 0.00 0.00 0.00 0.00 0.00 0.02

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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

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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)

PD

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• Lower than expected Peak DL throughput as eNodeB scheduler avoids MCS 28 due to high BLER and fixed control channel symbol assignment

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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

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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

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• 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

L1 T

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BLE

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Internal Modem Time

• eNodeB Scheduler implementation results in lower scheduling rate and lower DL throughput

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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

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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

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•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

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• 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

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Thank you