2G-3G Cell Reselection & Handover

ZTE university

Learning goals

To know interworking between 2G-3G networks To master strategies of reselection & handover between

2G-3G networks To know algorithms of cell reselection & handover

between 2G-3G networks

Contents

Conditions for 2G-3G interworking 2G-3G reselection & handover strategies 2G-3G reselection algorithms 2G-3G handover algorithms 2G-3G load-balance handover algorithms

Conditions for 2G-3G interworkingDefinition of UTRAN adjacent cells

Define cells out of BSCDefine adjacent cell for cell reselection/ handover

2Quater system message

MS reads system messageMeasurement of UTRAN systems

Setting of 2G-3G interworking parameters

Handover & reselection parameters

2G covered area

3G covered area

Make measurement of 3G network at the its edge area

MS moves to 2G network after originating a call in 3G network

Conditions for 2G-3G interworking

2quater message The precondition for system to send message is that data

configuration must meet one of the following criteria: The cell is configured with 3G adjacent cells; The cell supports NC2 function (network controls cell reselection);

2quater content Description of FDD(TDD) at UTRAN side Description of 3G measurement parameters Description of GPRS 3G measurement parameters Network control measurement parameters Information of cells adjacent to UTRAN FDD(TDD)

Conditions for 2G-3G interworking (extended functions of inter-working)

IUR-G interface

To realize wireless integration of 2G and 3G networks To reach inter-system balance based on information of GSM cell

capacity and load To select handover target cell based on information of GSM cell

capacity and load To select target cell of RAB redirection based on information of

GSM cell capacity and load

Enhanced IUR-G-based 2G-3G handover flow (resource request)

SRNC BSCUE CN

MEASUREMENT REPORT

RADIO RESOURCE PREPARE

RADIO RESOURCE PREPARE READY

RELOCATION REQUIREDHANDOVER FROM UTRAN COMMAND

HANDOVER REQUEST

HANDOVER REQUEST ACKNOWLEDGE

RELOCATION COMMAND

HANDOVER COMPLETE

HANDOVER DETECT

HANDOVER COMPLETE

IU RELEASE COMMAND

IU RELEASE COMPLETE

NODE B

RADIO LINK DELETION REQUEST

RADIO LINK DELETION REPONSE

Conditions for 2G-3G interworking (extended functions of inter-working)

Selection of target cell of RAB redirection based on information of GSM

cell capacity and load SRNC BSCUE CNBSC

RAB ASSIGNMENT REQUEST

RNC Decides to perform Relocation

(Directed Retry)

RAB ASSIGNMENT RESPONSE

RELOCATION REQUIREDHANDOVER FROM UTRAN COMMAND

HANDOVER REQUEST

HANDOVER REQUEST ACKNOWLEDGE

RELOCATION COMMAND

HANDOVER COMPLETE

HANDOVER DETECT

HANDOVER COMPLETE

IU RELEASE COMMAND

IU RELEASE COMPLETE

RADIO RESOURCE PREPARE

RADIO RESOURCE PREPARE READY

Conditions for 2G-3G inter-working (extended functions of inter-working)

Contents

Conditions for 2G-3G interworking 2G-3G reselection & handover strategies 2G-3G reselection algorithms 2G-3G handover algorithms 2G-3G load-balance handover algorithms

Reselection

2G-3G interworking strategies

Handover

PLMN selection/cell reselection2G-3G interworking occurs when UE is in idle mode

CS handover/PS handoverWhen subscriber originates service, 2G-3G interworking occurs during the service connection between UE and mobile networks. The service falls into two types:

CS handover （ CS）

PS handover （ PS）

2G-3G interworking strategies

Areas of high valueAreas of high valueKey areasKey areas

WCDMA ： CS service/visual service/ medium-rate PS serviceWCDMA ： CS service/visual service/ medium-rate PS service

Whole network coverageWhole network coverage

GSM/GPRS ： CS service/ low-rate PS service

capacitycapacity

coveragecoverage GSM covers the whole area

WCDMA covers key areas

Actual capacity

Constructed capacity

Actual capacity

Constructed capacity

GSM faces heavy capacity load

WCDMA is idle; need to explore subscribers

Relation between GSM and WCDMA at the initial state:

WCDMA focuses on coverage and tries to form complete coverage with the help of GSM; GSM reduce its load pressure with the help of WCDMA.

2G-3G interworking strategiesIn initial phase of network, 3G subscribers should choose 3G network preferably.

2G-3G interworking strategies: Only when UE leaves 3G-covered area , will it

perform 3G-2G cell reselection; once it returns to 3G-covered area, it will start 2G-3G cell reselection.

In order to ensure the continuity of speech, 3G-2G

handover should be supported. CS service which is being performed in 2G network

doesn’t need handover to 3G network, so as to

reduce call drops due to UTRAN handover (hard

handover). In order to guarantee PS subscribers’ satisfaction,

bi-directional handover of PS service is

recommended.

Contents

Conditions for 2G-3G interworking 2G-3G reselection & handover strategies 2G-3G reselection algorithms 2G-3G handover algorithms 2G-3G load-balance handover algorithms

2G-3G Reselection algorithms-PLMN selection/reselection

UE can select and roam between 2G and 3G networks through selecting PLMN （ MCC+MNC ） , and there is no need to upgrade 2G network.

Whether 3G and 2G adopt the same PLMN-ID, the strategy of ‘preferably selecting 3G network’ can be realized.

Set 3G network to be home network （ HPLMN ） ,UE searches and makes reselection periodically after it switches to 2G network （ VPLMN ） .

3G and 2G adopt different PLMN-ID

Set 3G network with the

preferred radio access

technology （ Access Technology

） , then UE will preferably adopt

3G to make radio access when it

selects PLMN.

3G and 2G adopt the same PLMN-ID

2G-3G Reselection algorithms-PLMN selection/reselection PLMN selection & reselection

3G and 2G networks are configured with different IDs during network planning;

HPLMN (home network) of USIM is set to be 3G when an account is opened;

Set USIM PLMN reselection interval when an account is opened; note there is an “hysteresis of 6mins”.

PLMN selection

RPLMN

HPLMN

“Operator Controlled PLMN Selector with Access Technology” in USIM

PLMN with good signal quality (in random order)

Other PLMN [in order of signal quality (high-low)]

PLMN selection after UE is powered

Change of PLMN （ ie. reselection,

which occurs during cell

reselection ）

+ “User Controlled PLMN Selector with Access Technology” in USIM

Histeresis of 6mins ： after UE resides in GSM network, it has to wait for at least 6mins before it can return to 3G network.

2G-3G Reselection algorithms-cell selection/reselection

Wireless controllers at 2G sites They should support the reselection

threshold parameters of cells configured with broadcast UTRAN system.

They should support configuration of 3G adjacent cells.

Requirements for 2G network equipment Cell reselection is dependent on subscribers’ satisfaction; as long as the

UE is within 3G network coverage, it can access 3G network

immediately.

This algorithm requires 2G-3G cell reselection in the whole network, so

2G network needs to be upgraded to some extent.

Requirements for 2G network equipment

2G-3G Reselection algorithms-cell selection/reselection

Cell selection: select the cell with the best signal in current network;

Cell reselection: as signals of the serving cell (in which UE resides) and its adjacent cells change/deteriorate, UE needs to select a new cell with better signal to reside.

Reside in original cell

Reside in cell at random（ call barring ）

Cell is non-HPLMN

Confirm primary scrambling code

of cell

S principle not satisfied, keep searching cells

Search cell S principleRead broadcast

channelNormal residence

Keep searching

cells

PLMA selectionY

N

N

2G-3G Reselection algorithms-cell selection/reselection-S principle

Formula: Squal = Qqualmeas – QqualminSrxlev = Qrxlevmeas – Qrxlevmin – Compansation

If For FDD: Squal > 0 & Srxlev > 0 For TDD: Srxlev > 0 then UE will take the cell as a suitable one and reside in it.

S-principle is used to judge whether a cell’s signal RxLev and

RxQual are suitable for UE to redside in the cell.

2G-3G Reselection algorithms-cell selection/reselection-reselection parameter 1

Cell reselection flow in idle mode & key parameters:

Parameter

Meaning Value range

Qsearch_I

Search for 3G cells if signal level is below (0‑7) or above (8‑15) threshold0 = - 98 dBm, 1 = - 94 dBm, … , 6 = - 74 dBm, 7 = (always) 8 = - 78 dBm, 9 = - 74 dBm, … , 14 = - 54 dBm, 15 = (never).Default value = (never).

0-15

Location: 3G adjacent cell list and cell reselection parameters are described in system broadcast message SI2quater （ and SI2ter ） which is sent on BCCH or extended BCCH.

Function: to control the adjacent cell measurement and reselection process under UE idle mode.

Location: 3G adjacent cell list and cell reselection parameters are described in system broadcast message SI2quater （ and SI2ter ） which is sent on BCCH or extended BCCH.

Function: to control the adjacent cell measurement and reselection process under UE idle mode.

2G-3G Reselection algorithms-cell selection/reselection-reselection parameter 2

Cell reselection flow in idle mode & key parameters:

Parameter Meaning Value range

FDD_Qoffset

Applies an offset to RLA_C for cell re‑selection to access technology/mode XXX (one or more) ： 0 = - (always select a cell if acceptable), 1 = -28 dB, 2 = -24 dB, … , 15 = 28 dB.Default value = 0 dB.

0-15

FDD_Qmin A minimum threshold for Ec/No for UTRAN FDD cell re-selection,0= -20dB,1=-6dB, 2= -18dB, 3= -8dB, 4= -16dB, 5= -10dB, 6= -14dB, 7= -12dB.Default

value= -12dB.

0-7

FDD_RSCPmin and FDD_Qmin_Offset are first added in version 3GPP R6, currently they are not supported in system.

2G-3G Reselection algorithms-cell selection/reselection-reselection parameter 2

Application of FDD_Qoffset and FDD_Qmin

这两个参数控制了跨系统的小区重选过程

These two parameters control inter-system cell reselection process. When a UE satisfies all the following conditions, it can make reselection to UTRAN adjacent cells.

RSCP >= FDD_Qoffset + RLA_C

RSCP >= FDD_Qoffset + RLA_n

Ec/No >= FDD_Qmin-FDD_Qmin_Offset

RSCP>= FDD_RSCP_threshold

Notes:

2G-3G cell reselection is not allowed within 5s after 3G-2G reselection; if more than one 3G cell satisfies the conditions, UE shall select the cell with the largest RSCP value.

Notes:

2G-3G cell reselection is not allowed within 5s after 3G-2G reselection; if more than one 3G cell satisfies the conditions, UE shall select the cell with the largest RSCP value.

2G-3G Reselection algorithms-suggestions

Principle of field strength Only when the field strength of 3G network is guaranteed, will

reselection to 3G occur. Normally, in 3G covered areas with weak field strength and the marginal areas overlapped by 3G and GSM networks,this principle shall be considered first.

Principle of time efficiency On condition that the principle of field strength is satisfied, selection

to WCDMA should be as soon as possible, so as to provide subscribers with high-end services. In areas continuouly covered by WCDMA network, this principle shall be considered first.

Principle of electricity efficiency Dual-mode terminal consumes more power in performing inter-

system signal measurement, so this principle shall be fully considered when we deploy networks with overlapped coverage areas.

这两个参数控制了跨系统的小区重选过程

2G-3G Reselection algorithms-cell reselection （ PS ） PBCCH supported in BSS

PSI3quarter sends parameters related to inter-system cell reselection (3G adjacent cell information, measurement and control parameters of cell reselection) to MS.

PBCCH not supported in BSS / PBCCH not configured Packet Cell Change Order or Packet Measurement Order send parameters

related to inter-system cell reselection (3G adjacent cell information, measurement and control parameters of cell reselection) to MS; these parameters will be used in PS cell reselection process.

parameter Meaning Value range

Qsearch_P Search for 3G cells if signal level below threshold (0-7): - 98, - 94, … , - 74 dBm, (always) or above threshold (8-15):- 78, - 74, … , - 54 dBm, (never). Default value = (never).

0-15

FDD_ GPRS_ Qoffset

Applies an offset to RLA_P for cell re‑selection to access technology/mode XXX (one or more), 0 = - (always select a cell if acceptable), 1 = -28 dB, 2 = -24 dB, … , 15 = 28 dB. Default value = 0 dBm.

0-15

2G-3G Reselection algorithms -NCCR

Explanation of （ NC2/NCCR ） It supports different cell reselection parameters in PS service to

realize cell reselection of “packet idle mode” and “packet transmission mode”.

Advantage: it makes full use of available network information to make reasonable judgment and realize optimized distribution of network services; it reduces the useless cell reselection performed by MS, and improves TBF transmission efficiency.

Network_Control-Order

Meaning of value

MS operations

NC0 Controlled by normal MS

MS executes autonomous cell reselection

NC1 Controlled by MS which reports MRs

MS sends MRs to network; MS continues cell reselection.

NC2 Controlled by network

MS sends MRs to network, and realizes cell reselection only based on the reselection order from network. Only when cell reselection is triggered by failure of sending downlink signalling or random access, will MS execute autonomous cell reselection.

2G-3G Reselection algorithms -NACC

Explanation of （ NC2/NCCR ） 3GPP R5 stipulates NACC’s ability to support inter-BSC and

UTRAN services.

Realization of （ NC2/NCCR ） BSC and RNC can collect information of external cells through Gb

(lur-g) interface and RIM process. (Currently we can only realize external NACC through Gb interface and RIM process.)

Master BSC enables external cells to start RAN Information Send process through RAN Information Request, so as to transmit the system information of external cells to the serving cell.

BSC/RNC 1 BSC/RNC 2CN

RAN Information Request

Contents

Conditions for 2G-3G inter-working 2G-3G reselection & handover strategies 2G-3G reselection algorithms 2G-3G handover algorithms 2G-3G load-balance handover algorithms

2G-3G handover algorithms- （ 3G->GSM ） For 3G-GSM CS handover, main requirements stay with

3G network; for GSM network, attention should be paid to the following:

BSSAP signaling Handover Request at A interface should at least support R99 (version), which means it should adapt to the changes of coding mode of Cell Identifier (Serving) which carries 3G serving cell ID;

Ignore the RF value (6 , 7) in classmark2 which is carried in BSSAP signaling Handover Request;

For the encrypted algorithm sent by GSM MSC, even the encryption function is not enabled in BSS, it can also ignore the encrypted algorithm and perform the following handover process.

When BSS sends HANDOVER REQUEST ACK to GSM MSC, Chosen Encryption Algorithm IE must contain the specific encryption mode, whether encryption function is enabled or not.

Currently GSM-WCDMA and WCDMA-GSM PS handover is not supported in iBSC.

2G-3G handover algorithms-- （ 3G->GSM ） Signaling flow of 3G-GSM CS handover

2G MSC BSC3G MSCRNC

PREPARE HANDOVER

UE

RELOCATI ON REQUI RED

HANDOVER REQUEST

HANDOVER REQUESTACKPREPARE HANDOVER

RESPONSERELOCATI ON COMMAND

HANDOVER COMMANDHANDOVER DETECT

HANDOVER COMPLETE

HANDOVER COMPLETESEND END SI GNALREQUEST

RELEASE COMMAND

RELEASE COMPLETE SEND END SI GNALRESPONSE

2G-3G handover algorithms- （ GSM ->3G ） GSM cell collects information of 3G adjacent cells and measurement

parameters. BSC broadcasts SI 2quater on BCCH or extended BCCH; MS may receive Measurement Information in downlink SACCH sent by

BSC during a call.

3G initial stage

3G expansion stage

3G mature stage

HO due to

coverage

HO due to

coverage

HO due to

coverage

HO due to

coverage

HO due to

coverage

HO due to

coverage

HO due to serviceHO due to serviceHO due to serviceHO due to service

Network resource

optimization

Network resource

optimization

Network resource

optimization

Network resource

optimization

Messages to be collected by MS

Description of 3G adjacent cell parameters Configuration of 3G adjacent cellsParameters of adjacent cell measurementHandover parameters

2G-3G handover algorithms- （ GSM ->3G HO parameters ）

Parameter Meaning Value range

Qsearch_C Search for 3G cells if signal level below threshold (0-7): - 98, - 94, … , - 74 dBm, (always) or above threshold (8-15): - 78, - 74, … , - 54 dBm, (never)

0-15

Qsearch_C_Initial

Indicates the Qsearch value to be used in connected mode before Qsearch_C is received,0 = use Qsearch_I, 1 = (always).Default value = use Qsearch_I.

0/1

3G_SEARCH_PRIO

Indicates if 3G cells may be searched when BSIC decoding is required,0 = no, 1 = yes

Default value = yes

0/1

Besides the important parameters listed above, some other parameters are also sent to MS in the system messages, such as:

FDD_REP_QUANT （ measurement of quantity, RSCP or Ec/No）；

REPORT_TYPE （ type of MR, MR or EMR ） ;…

2G-3G handover algorithms- （ GSM ->3G HO parameters ） Signaling flow of GSM-3G CS handover

2G MSC

HANDOVER COMMAND

HANDOVER REQUI RED

BSC MS

HANDOVER COMMAND

RELEASE COMPLETE

RELEASE

3G MSCRNC

PREPARE HANDOVERREQUEST

PREPARE HANDOVERRESPONSE

RELOCATI ON REQUEST

RELOCATI ONRESPONSE ack.

I AMACM

PROCESS SI GNALLI NGRELOCATI ON DETECT

ANM

SEND END SI GNALRELOCATI ONCOMPLETE

2G-3G handover algorithms-strategy of network selection priority

The strategy is to decide which network shall be prioritized for handover.

To decide target cell through processing of MRs and the strategy of network selection priority :

MSC controls strategy of network selection priority

HANDOVER REQUEST carries “Service Handover” field, which is used to indicate handover strategy for BSC in the following handover process.

BSC controls the strategy of network selection priority

Based on the configuration principles set in OMCR, BSC decides which network shall be prioritized for handover according to the strategy of network selection priority, which can be set in OMCR as well.

Both ways can be adopted:

Prioritize WCDMA cells； Prioritize GSM cells

What’s more, if both WCDMA and TD-SCDMA adjacent cells are available at the same time, the priority of WCDMA and TD-SCDMA can be decided through parameter “FDD/TDD handover” （ FDDTDDPrio ） at iBSC side.

Contents

Conditions for 2G-3G interworking 2G-3G reselection & handover strategies 2G-3G reselection algorithms 2G-3G handover algorithms 2G-3G load-balance handover algorithms

Load-balance handover algorithms- directed-retry handover process

UE UTRAN

Signaling connection establishment request

Signaling connection establishment reject

Signaling establishment fails due to lack of

resource in 3G network; 3G is redirected to be

2G network; UE starts access again in 2G

network without target cell defined.

To support directed-retry, the switch DrInd shall be open at iBSC OMCR.

Load-balance handover algorithms- traffic handover process

For 3G-GSM CS handover ， requirements stay with WCDMA network.

CN UTRAN

Assignment request

Success response to assignment request

When RNC receives RAB assignment message, if CN

requires handover of corresponding services to 2G

system, UTRAN handover process will be started,

which is 3G-2G handover, thus 3G network

performance will be optimized.

Handover request

Load-balance handover algorithms- handover due to serving cell load

CNUTRAN

Assignment request

Assignment failure

Service establishment fails due to lack of

resource in 3G network; if it’s 2G-supported

CS service or low-rate PS service, 3G will

appoint suitable 2G cell, and CN will start

handover to the appointed 2G cell.

iBSC sends LOAD INDICATION （ BSSAP ） to MSC periodically.

Load-balance handover algorithms- comparative analysis of handover algorithms

Methods Voice

histeresisSuccess

rateInfluence to 3G

networkApplication scenario

Directed-retry Long Low Light Signalling establishment

failure

Service handover

Short High Serious

CN requests handover of service to 2G network after

service establishment success.

Handover for load balance

Short High Serious Cell with light congestion

Others – avoidance of problems

Solution A+B of TDD When GSM cells are configured with TD adjacent cells, halt or restart may

occur to MS due to flaws in some terminal chips. There are two solutions to the above problem:

Solution A ： configure a GSM cell with an FDD adjacent cell and a TDD adjacent cell at the same time; when MS discovers there is FDD adjacent cell in SI2quater, it will not read the information of TDD adjacent cell, so that abnormal phenomena will not occur to MS when it receives information of TDD adjacent cell.

Solution B ： set parameter NR_OF_TDD_CELLS=31 in SI2quater to prevent MS entering endless-loop when it reads SI2quater message, so as to avoid abnormal phenomena of MS.

Solution A and solution B are directed at MS chips with different flaws, but they can exist mutually, so they can be used together to solve chip problems. We call this “solution A+B”

Method: change “TDD handover allowed” to “Not allowed” at iBSC.