SON in LTE

SELF OPTIMIZING NETWORK

Company Confidential | 2

Agenda

RLF

What is RLF ?

How/When/Why will it happen ?

Consequences of RLF

How to identify RLF from UE and X2 logs?

Observations

MRO

What are Too late/Too early and Ho to wrong cell

Test set up

Test case and its procedure

Observations

MLB

Concept of load balancing

MLB algorithm as per 3gpp spec and Samsung work

Test set up, test case and its procedure

Observations

RO

When RACH Occurs ?

PRACH procedures

RACH configuration parameters

Effect of improper RACH Parameters

RACH Optimization parameters

Test plans and Test set up

Observations

RLF - Radio Link Failure


Radio Link Failure ( RLF )

RLF ?

When/Why/How ?

PHY Layer

MAC Failure

RLC retransmissions reached


RLF Cont

Consequences:

UE loses its radio link

UE will send RRC Reestablishment Request eNB

Communication over X2

how to identify RLF from UE and X2 logs

UE eNB

RRC Reestablishment Request

Cause RRC Reconfig., Failure

HO Failure

Other

RRC Reestab., Response

X2 logs

RLF Indication

HO Report


RRC Reestablishment

When Reestablish request occurs , is it mean RLF happened ? No

To confirm ourselves we have started with creating scenarios which can produce Reestablishment message with all

its causes

1. HO Failure:

Set T304 to less value

Make UL to Target eNB to bad level using fader

Now try HO to target cell by making Source cell DL channel bad

UE keep on trying at PRACH for Target eNB; mean while T304 expires and leads to HO Failure

2. RRC Reconfiguration Failure:

Set up DRX or SPS in LSM which doesnt support by UE

3. Other ( RLF , Mobility from EUTRA, PDCP Integrity failure)

As other two are unlikely to come in our lab setup, if we see occurrence of cause other it signifies RLF

happening

We can easily identify RLF on X2 logs with RLF Indication message [explained in later slides]


Observations

RRC Restablishment : Cause - HO Failure [UE log - QCAT]


Observations Cont

RRC Reestablishment : Cause Other [UE log - QCAT]

MRO Mobility Robust Optimization


Mobility Robust Optimization ( MRO )

What is MRO ?

Why MRO ?

How will it help us ?

Too Late : UE is more aggressive than eNB HO parameter

Too Early: eNB is Reacting Slowly compared to UE speed

HO to Wrong Cell: Same as Too Early case with three cells


MRO Cont

Too Late HO to Wrong Cell

UE eNB A eNB B UE eNB A eNB C eNB B

HO

Re-establishment Request

Re-establishment Reject RRC Re-establishment Request

RLF Indication RRC Re-establishment Reject

RLF Indication

Too Early HO Report

UE eNB A eNB B

HO

Re-establishment Request

Re-establishment Reject

RLF Indication

HO Report


Test Set Up


Test Procedure: Too Late

LSM Configuration:

Add two eNBs (eNB A , eNB B) as neighbors to each other

Set N311 to high value and T310, N310 to less

Procedure:

Set RSRPs of both cells to good level and attach UE to eNB A.

Reduce RSRP of eNB A to very low value in very less time using fader.

Observations:

Check UE logs for RRC Reestablishment Request to eNB B with cause as other

RRC Reestablishment Response will be reject

Check X2 logs for RLF indication from eNB B to eNB A


Test Procedure: Too Early

LSM Configuration:

Add two eNBs as neighbors to each other

Set UE context release timer to high value in eNB A

Set N311 to high value and T310, N310 to less value in eNB B

Procedure:

Set RSRPs of both cells to good level and attach UE to eNB A.

Do successful HO from eNB A to eNB B

Create RLF in eNB B as specified in too late case within the UE context release timer value

Observations:

Check UE logs for

RRC Reconfiguration message from eNB A and Reconfiguration complete to enB B

Reestablishment Request to eNB B A and RRC Reestablishment response as rejected

Check X2 logs for

Successful HO from eNB A to eNB B

RLF INDICATION Message from eNB B A to eNB B

HO REPORT Message from eNB B B to eNB A


Test Procedure: HO To Wrong Cell

LSM Configuration:

Add two eNBs as neighbors to each other

Set UE context release timer to high value in eNB C

Set N311 to high value and T310, N310 to less value in eNB C

Procedure:

Set RSRPs of eNB A and C to good level and attach UE to eNB A.

Do successful HO from eNB A to eNB C; Reduce RSRP in both eNBs A and C worse than eNB B within UE

context release timer so that RLF will happen in eNB C and will send Reestablishment Request to eNB B

Observations:

Check UE logs for

RRC Reconfiguration from eNB A and Reconfiguration complete to eNB C

Reestablishment Request to eNB B and Reestablishment response as Rejected

Check X2 logs for

Successful HO from eNB A to eNB C

RLF INDICATION from eNB B to eNB C

HO REPORT from eNB C to eNB A


RLF Indication / HO Report


Observations

Too Late : RLF INDICATION


Observations Cont

Too Early: HO Report


Observations Cont..

Change in parameters :

CIO of eNB A towards eNB B has been incremented by 1 dB

MLB Mobility Load Balancing


Mobility Load Balancing ( MLB )

Concept: [TS 36.902]

To balance traffic load among eNBs

Minimizing number of HOs and redirections needed to achieve LB

Promising QoS; while taking LB decision

Cell A Cell B

LB Area


MLB Cont

MLB Procedure:

Resource Status Initiation

Request

Response

Resource Status Update

Resource Status Reporting

Periodicity

T_RESOURCE_STATUS_REPORTING: 1s/2s/5s/10s

Thresholds

PRB Usage or Available Capacity

Decisions

Requesting neighbor eNB for Mobility parameters change ( 3gpp Spec )

Force Handover ( Samsung Implementation )


Resource Status Request [TS 36.423]

IE/Group Name IE type and reference Semantics description

Message Type 9.2.13

eNB1 Measurement ID INTEGER (1..4095,...) Allocated by eNB1

eNB2 Measurement ID (C) INTEGER (1..4095,...) Allocated by eNB2

Registration Request ENUMERATED(start, stop,

) A value set to stop, indicates a request to stop all cells

measurements.

Report Characteristics (O) BITSTRING

(SIZE(32))

Each position in the bitmap

indicates measurement object

the eNB2 is requested to report.

First Bit = PRB Periodic,

Second Bit= TNL load Ind

Periodic,

Third Bit = HW Load Ind

Periodic,

Fourth Bit = Composite Available

Capacity Periodic.

Bits 5 to 32 shall be ignored by

the eNB2

Cell To Report Cell ID list for which

measurement is needed

>Cell To Report Item

>>Cell ID ECGI

9.2.14

Reporting Periodicity (O) ENUMERATED(1000ms,

2000ms, 5000ms,10000ms, )


Resource Status Response / Failure [ TS 36.423 ]

IE/Group Name Presence IE type and

reference

Message Type M 9.2.13

eNB1 Measurement ID M INTEGER

(1..4095,...)


(1..4095,...)

Criticality Diagnostics O 9.2.7


reference



(1..4095,...)


(1..4095,...)

Cause M 9.2.6



Resource Status Update [ TS 36.423 ]

IE/Group Name Presence IE type and reference


eNB1 Measurement ID M INTEGER (1..4095,...)

eNB2 Measurement ID M INTEGER (1..4095,...)

Cell Measurement Result

>Cell Measurement Result Item

>>Cell ID M ECGI

9.2.14

>>Hardware Load Indicator O 9.2.34

>>S1 TNL Load Indicator O 9.2.35

>>Radio Resource Status O 9.2.37

>>Composite Available Capacity Group O 9.2.44


Test Case

Test Setup

Procedure

LSM

Parameter Description Default Value Value Set

INTRAGROUP _LB_TRIGGER _THRESHOLD

Threshold of load distribution between two cells in the same group of same sector. Only when the radio resource load of source cell is greater than this threshold,

40 10

OVERLOAD_T HRESHOLD_P RB

Threshold of load distribution between two cells in the same group of same sector. Only when the radio resource load of target cell is smaller than this threshold

75 100

T_MEASUREM ENT_COLLECT ION_LB

Period of measurement report collection

1 min 2 min


Test Set Up

To test Samsung Implementation for MLB


MLB Cont

As Per 3gpp:

Mobility change request ( Source eNB Target eNB)

Mobility Change Acknowledge ( Target eNB Source eNB)

Change in HO Trigger ( A3-Offset )


reference


eNB1 Cell ID M ECGI

9.2.14

eNB2 Cell ID M ECGI

9.2.14

eNB1 Mobility Parameters O Mobility

Parameters

Information

9.2.48

eNB2 Proposed Mobility

Parameters

M Mobility

Parameters

Information

9.2.48

Cause M 9.2.6

IE/Group Name Presenc

e

IE type and

reference


eNB1 Cell ID M ECGI

9.2.14

eNB2 Cell ID M ECGI

9.2.14



MLB Cont


Observations

Resource Status Initiation : Request


Observations Cont

Resource Status Initiation: Failure


Observations Cont

Resource Status Update: Reporting


Observations Cont

HO: Cause MLB Optimization

RO RACH Optimization


PRACH

When RACH Occurs ?

Initial access from RRC_IDLE;

RRC Connection Re-establishment procedure;

Handover;

DL data arrival during RRC_CONNECTED requiring random access procedure;

UL data arrival during RRC_CONNECTED requiring random access procedure;

For positioning purpose during RRC_CONNECTED requiring random access procedure;

PRACH Procedure

Contention based Non Contention based

1. UE -> eNB: RA Preamble 2. eNB -> UE: RAR 3. UE -> eNB: L2/L3 Msg 4. eNB -> UE: Contention

Resolution

1. eNB -> UE : Dedicated Preamble assignment

2. UE -> eNB: RA Preamble 3. eNB -> UE: RAR


RACH / PRACH Configuration

Rach -Config Common [SIB2]

Number of RA-Preambles:

Size of RA-Preambles GroupA

Message Size GroupA

Message Power offset GroupB

Power Ramping Step

Preamble Initial Received Target Power

Preamble Trans Max

Ra-Response Window Size

Mac-Contention Resolution Timer

Prach Config Info

Prach-Config Index

High Speed Flag

zero Correlation Zone Config

Prach Frequency Offset


RACH Optimization

How RO is important ?

Effect of improper RACH Parameters

Call set up delays

Access Delays

HO delays

Wastage of Resources


Test Plans

Give your Suggestions

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