02 G-LII 300 Handover-20081022-A-2.1

HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

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Internal

Handover

ISSUE 2.0


Handover is key technology of Mobile

communication system and make

continued conversation possible.

Handover algorithm in Huawei product

is flexible and powerful


� Upon completion this course, you will be

able to:

�Understand the type of handover.

�Master handover judgment flow

�Configure handover data

�Master handover signaling flow


Chapter 1 Chapter 1 Introduction of HandoverIntroduction of Handover

Chapter 2 Chapter 2 HO Algorithm processHO Algorithm process

Chapter 3 Chapter 3 HO Data ConfigurationHO Data Configuration

Chapter 4 Chapter 4 HO Signaling processHO Signaling process


Purposes of HO

� To keep a continuous communication with a moving MS

�To improve network service performance

− To reduce the call drop rate

− To reduce the congestion rate


� Quick Handover

� Emergency

� TA handover

� Interference handover

� Quick level drop handover

� Bad quality handover

� Quick handover

� Enhanced dual-band network handover

� Load handover

� General handover

� Edge handover

� Fast-moving micro cell handover

� Inter-layer (better cell) handover

� PBGT handover

� AMR handover

� Better 3G cell handover

� TIGHT BCCH handover algorithm

Classification by Reason


Classification by Synchronization

� Synchronous handover: source

and target cell belong the same

BTS

� Asynchronous handover: source and

target cell belong the different BTS



1.1 1.1 General HO processGeneral HO process

1.2 1.2 Measurement reportingMeasurement reporting

1.3 Measurement processing1.3 Measurement processing

1.4 1.4 HO judgmentHO judgment


General Process of HO Algorithm

MS entering

connection state

Measurement and

production of MRs

MR processing

Algorithms making

handover decision

Handover execution

MS entering new

connection state


Measurement Report

� Uplink MR includes uplink receiving level and quality.

� Downlink MR includes downlink receiving level, downlink

receiving quality of the serving cell and other downlink

receiving levels from the neighbor cells.

Serving cell Neighbour cell

The downlink measurement report of

the serving cell

The uplink measurement

report of MSThe downlink measurement report

of the neighbor cell (BCCH)


Measurement Report


Period of measurement report

� The downlink MR is sent to BTS in SACCH uplink

�The interval is 480ms/per time when MS is on TCH

�The interval is 470ms/per time when MS is on SDCCH

12TCH 12TCH1SACCH 1 Idle

480ms4 TCH multi-frames


MR interpolation

�Every time BSC receives a measurement report, there will be an

update to the basic queuing of the cells.

�BTS may fail to receive the measurement report from MS. Before the

rank-update, BSC needs to recover the lost measurement reports

according to Filter Table. If the lost MR amount is within the allowed

range, then recovers the lost MR according to the algorithm.


How to interpolate MR?

MR MR MR MR MR

Measurement report No. n

Measurement report No. n+4

Continuous MR

flow


Filter----Average several continuous MR.

Filtering

MRMR MRMR MR MR

Continuous MR flow


Handover judgment

� Penalty process

� Basic queuing

� 16bits queuing

� Handover judgment


Penalty Processing

1. An emergency handover caused by higher TA value succeeds;

2. An emergency handover caused by bad uplink quality succeeds;

3. An emergency handover caused by bad downlink quality succeeds;

4. Penalty after handover fails, including the handovers to 2G cells

and the handovers to 3G cells

5. Penalty on the microcell from which a fast-moving MS is handed

over to the macrocell;

6. A high-speed railway fast handover succeeds;

7. An overlaid-to-underlaid handover succeeds;

8. An overlaid-to-underlaid handover or underlaid-to-overlaid handover

fails;

The penalty scheme applies to the following situations:


BTS

TA HO

BSC

Cell A

Cell B

For example: TA HO succeeds

Punish the original serving cell when an emergency TA HO

occurs.


Procedure of Queuing

� Basic queuing and secondary queuing of cells are major parts of the HO

judgment. Queuing is made through 16bits-algorithm. The serving cell

and the neighbor cells will be listed in a cell list according to their 16bits

value. The queuing processes include:

�M criterion

�K criterion

�16-bit queuing


M criterion

� Only the cells with received signal level satisfy the following conditions can be put

into the candidate cell list, otherwise will be removed.

� Formula:

� Downlink receive level of the cell RXLEV (the level after filtration and penalty)

>= Min DL Power on HO Candidate Cell + Min Access Level Offset

� Uplink receive level of the cell RXLEV (the level after filtration and penalty)

>= Min DL Power on HO Candidate Cell + Min Access Level Offset

− For serving cell: Min Access Level Offset is always 0; For neighbor cells: Min

Access Level Offset is configured toward the neighbor cell

− The uplink level of neighbor cells can be estimated in BSC.

Formula: SS_UPn = MS_PWRn - Ln

Path loss: Ln = BSTXPWRn – SS_DOWNn = Transmit power of the BCCH TRX of

the neighbor cell – Downlink level in the neighbor cell after interpolation, filtration,

and level rectification

MS_PWRn is min(the maximum allowable power of MS, or the maximum allowable

transmission power of MS in this neighbor cell)


K criterion

� After the M criterion , the serving cell and candidate neighbor cells are queued in

descending order according to the receiving level only.

� The K value of the serving cell is 0.

� For a neighbor 2G cell, K = BCCH signal level of the neighbor cell after filtering – TCH

signal level of the serving cell after filtering – KIAS;

� Both the serving cell and the neighbor cells have their own 16bits value. The

smaller the value is, the higher the priority and position the cell is in the cell list.

� The 1st-3rd bits: bit value is decided according to the cell signal level and the

penalty process taking place beforehand.

� The values come from max. 6 candidate cells and 1 serving cell according to

the level ranges from 000~110. The value for the cell with the strongest signal

level is 000.

141516 12345678910111213


16-bit queuing

DL RXLEV queuing1

DL RXLEV queuing2

Indicates the queuing result of seven cells (six neighbor cells + one serving cell)

based on RXLEV.

DL RXLEV queuing3

Set to 1 for a neighbor cell if RXLEV of the neighbor cell is lower than RXLEV of

the serving cell + Inter-cell HO Hysteresis.

Set to 0 for the serving cell.

Serving cell preferred4

HO Priority Hierarchy 15





Indicates the HCS (Hierarchical Cell Structure) priority. There are 64 levels in total

(4 layers with 16 levels on each layer).


Set to 1 if the current load of serving cell is higher than Load HO Threshold.

Set to 1 if the current load of neighbor cell is higher than Load Req. on Candidate Cell.

Current load of the cell higher

than load handover threshold

or not

11

Set to 1 in the case of not co-BSCCo-BSC or not12

Set to 1 in the case of not co-MSCCo-MSC or not13

Set to 1 for a neighbor cell if RXLEV is lower than Inter-layer HO Threshold +

Inter-layer HO Hysteresis.Set to 1 for the serving cell if RXLEV is lower than threshold - hysteresis.

DL RXLEV higher than Inter-

layer HO Threshold or not

14

ReservedRSVD15-16


GSM900

Cell

Micro Cell

Umbrella

Cell GSM 900

GSM1800 GSM1800GSM1800

GSM 900 GSM 900 GSM 900

GSM900GSM900

GSM1800GSM1800

GSM900 GSM900

GSM1800 GSM1800

GSM1800

Cell

Hierarchical cell structure

1

2

3

4


Handover judgment

� Each handover algorithm consists of two parts. One is to decide whether

the serving cell meets the triggering conditions, and the other is to select

the candidate cells.

� Key handover criterion

�Edge handover

�Hierarchical handover

�PBGT handover


Edge handover

� Measured value of signal strength on the uplink TCH after filtering < value of Edge

HO UL RX_LEV Threshold; Measured value of signal strength on the downlink TCH

after filtering < value of Edge HO DL RX_LEV Threshold

� According to the P/N criterion, if P reports out of the latest N MRs meet the previous

formulas, the uplink/downlink edge handover is triggered and the candidate cells are

selected.

� Remove the serving cell from the candidate cell list

� Remove the neighbor cell whose 16-bit sequence number is greater than the 16-

bit sequence number of the serving cell. If the sequence number is small, the

priority of the cell is high.

� Remove the neighbor cell whose BCCH receive level after filtering is less than

TCH receive level of serving cell after filtering + Inter-cell HO Hysteresis

� The neighbor cells should meet the P/N (Edge HO Valid Time (s)/Edge HO

Watch Time (s) ) criterion.

If Fringe HO Allowed is set to Yes, the edge handover is allowed.


Hierarchical handover

� The priority level of the neighbor cell is higher than the serving cell.

� The receive level of the BCCH in neighbor cell i after filtering meets the following

formula:

Where,

SSi_f indicates the receive level of the BCCH in the neighbor cell after filtering and penalty.

Tlayer indicates Inter-layer HO Threshold.

Hlayer indicates Inter-layer HO Hysteresis.

� The 16-bit sequence number of the neighbor cell is smaller than that of the serving

cell. If the sequence number is small, the priority of the cell is high.

� If all these conditions are met during the period specified by Layer HO Valid

Time(s) within the latest Layer HO Watch Time(s), that is, if the P/N criterion is

met, then the hierarchical handover is triggered.

If Level HO Allowed is set to Yes, the inter-layer handover is allowed. To enable the

handover between different priorities of cells at the same layer, you also need to set

this parameter to Yes.

_f layer layeriSS T H≥ ＋


PBGT handover

� The neighbor cell (not the serving cell) has the same hierarchy as the serving cell at

the same layer.

� For the neighbor cell, the following formula is met within the period specified by PBGT

Valid Time(s) out of the period specified by PBGT Watch Time(s).

− SS_DLs_f indicates the receive level of the downlink TCH in the serving cell after filtering.

− SS_DLi_f indicates the receive level of the BCCH in neighbor cell i after filtering and penalty.

− Poff_DLs_f indicates the offset of the transmit power of the BTS on the downlink after the filtering in the

serving cell to the maximum transmit power on the TCH. The step is 2 dB.

− Pms_i indicates the maximum transmit power of the MS in neighbor cell i. It is related to the frequency

band for neighbor cell i. Generally, GSM900/850, GSM1800, and GSM1900 correspond to different

transmit power.

− Pms_s indicates the maximum transmit power of the MS in the serving cell. It is related to the frequency

band for the serving cell. Generally, GSM900/850, GSM1800, and GSM1900 correspond to different

transmit power.

� The PBGT handover is triggered if the previous conditions are met and if the number

of candidate 2G cells is not zero. The candidate cells are the neighbor cells with the

smallest 16-bit sequence number.

( ) ( )_ _ _ __ _ 2i f s f s_f ms i ms s

SS DL SS DL Poff_DL P P Margin − + × − − >


Handover data configuration

� All data configurations are related to

handover.


HO data lookup process

� BA2 table defines BCCH frequencies of all neighbor cells. It is sent to MS by

system message 5, 5-bis and 5ter on SACCH channel.

� MS reports the serving cell and BCCH, BSIC and signal levels of 6 strongest

neighbor cells to BSS through MRs. This is done through SACCH uplink.

� After MR pre-process is done, BSC gets all neighbor cells’ Cell Index and

CGI which are derived from Adjacent cell Relation Property table, and Cell

Common Data table (or 2G External Cell Data table) through BCCH and

BSIC in the MR.

� BSC performs HO judgment process, such as basic queuing of cells

(completed in LAPD board). When BSC finds suitable target, It sends HO

request messages containing the target CGI to MPU of BSC.

� MPU sends a HO command message to the target module and step up the

‘inter-cell/ intra-cell HO request’ counter by one.


For example: Intra-BSC Handover Signaling process

MS MSBTS1 BTS2BSC MSC

Measurement Report from MS

Channel_Active

Channel_Active ACK

HANDOVER COMMAND

Handover Access

Handover_DetectPHY INFO

First SABM

Establish_IND

PHY INFO

Handover Complete

Handover_Performed

UA


Intra-BSC Handover Signaling process

� Attention

� In asynchronous HO, if MS could not reach the new TCH channel

after the target cell has sent PHY INFO up to max times, the target

cell reports CONN FAIL IND to BSC with the reason: HO access

failure.

�After the above message is received, BSC release the assigned TCH

channel in the target cell .

�Max resend times of physical information*Radio link connection timer

> Time interval between EST IND and HO DETECT (120~180ms).

This is to make sure that the physical information reach MS.

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02 G-LII 300 Handover-20081022-A-2.1

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