10)cdma1x handoff algorithm

8/11/2019 10)CDMA1X Handoff Algorithm

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ORG000004CDMA1X Handoff AlgorithmISSUE 1.0

Mobile Network Curriculum

Development Section


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Introduction

Handoff of CDMA system not only has close relationship with

network quality (conversation quality, call-drop performance,

etc.), but also has complicated relationship with coverage,

capacity and interference. This document describes andanalyzes the handoff processes of CDMA 1X and the issues

related to handoff planning from the aspect of network

planning. No particular description will be given to the details

of handoff algorithm and the flows here. For details, please

refer to the protocol and the corresponding technical manual

or guide.


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Study objective

Have an understanding of the functions and classification of

handoff

Master the concept of handoff

Pilot set, search window, handoff parameter and handoffprocess

Master different handoff algorithms

Intra-frequency handoff/inter-frequency handoff

Master handoff signaling flow

Master the configuration of handoff data

It is required that after the course, you should


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Contents of course

Chapter 1 Basic Concept of CDMA Handoff

Chapter 2 CDMA Handoff Algorithm

Chapter 3 Brief Introduction to CDMA Handoff Flow

Parameter Setting of CDMA Handoff


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Reference

CDMA System Engineering Manual, Posts & Telecom Press

Guide to CDMA 1X Handoff Planning V1.0

Guide to CDMA 1X Network Planning Parameters

Configuration


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Meaning of handoff

The original connection between MS and original service

sector will be replaced by a connection between MS and new

service sector when MS moving to the edge of original service

sector and entering another sector. There are three kinds ofCDMA handoff in the traffic state:hard/soft/softer handoff.

Please refer to page 8 for details.


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Purpose of handoff

Purpose of handoffGuarantee continuous conversation, it is

a necessary requirement in any mobile communication system.

Proper handoff algorithm is advantageous to reduce system

call drop and increase network capacity.90% of radio call drop occurs in the process of handoff.


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CDMA handoff types

Idle handoff(idle state)

Access handoff(Access state)

Hard handoff(Traffic state)

Firstly break, then make

Soft handoff(Traffic state)

Firstly make, then break,the handoff

branches come from different BTS

Softer handoff(Traffic state)

Firstly make, then break, the handoff

branches come from the same BTS


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T1 Time

BTS2-PN2

channel2

BTS1-PN1

channel1

Access

Entry HO

T2

Page

Response

3Layer 2

Ack.

42

Page

Response

1Page

MessageExtended Channel

Assignment

Message

5

Access Handoff


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Original

Message

1 2Layer 2

Ack.

T1 Time

BTS2-PN2

channel2

BTS1-PN1

channel1

Access Handoff

Access

Probe HO

T2

Layer 2

Ack.

43Original

MessageExtended Channel

Assignment

Message

5


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Original

Message

1

Time

BTS2-PN2

channel2

BTS1-PN1

channel1

T1Access HO

T2

2Layer 2

Ack.

3

Extended Channel

Assignment

Message

Extended Channel

Assignment

Message

3

Access Handoff


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Soft handoff

Frame processing board of BSC

Abis Link

BTS BBTS A

Soft handoff branches are selected by frame processing board(FMR) of BS


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Softer handoff

Frame processing board of BSC

BTS

Softer handoff: Signal of different branches are combined in BTS


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Soft Handoff vs Softer Handoff


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Soft handoff and intra-frequency handoff

Note:

Soft handoff is an intra-frequency one

Not all the intra-frequency handoffs are soft

handoff

If the frequencies of the destination cell and the

original cell are the same, but:

Belonging to different BSC

Soft handoff path unavailable between

BSCs(A3/A7);

Frame offset unavailable in destination cell;

then hard handoff is required.


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Pilot set

Pilot channel: The Channel used in

the CDMA system to introduce access

and handoff. And the MS can

unceasingly measure the pilot signals

of the ambient cells.

Pilot set: Pilot (PN offset) sets with

the same CDMA frequency,to

distinguish the sectors.


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Classification of pilot sets

The pilot set includes active set, candidate set, neighbor pilot

set and remnain set.

Active set: Pilot set corresponding to the traffic channel

connecting to the MS.(In use in the traffic state)

Candidate set: The strength of the pilot signal is strong enough,

the MS can demodulate successfully and can access in any

moment. (not in use but the signal strength exceeds T-ADD)

Neighbor set: Set of the pilots, which are not in the active set or

candidate set, but may be sent into the candidate set(the system

tells the MS to search ).

Remain set: Set of all the rest pilots.

For MS ,there are four sets above ,but for BSS ,there are only

Active set and Neighbor set.


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Pilot set update

PilotStrength

Pilot Strength

exceeds T_ADD

EHODM received,

Pilot Dropped into Neighbor Set,

HOC mess age sent,

NLU mess age received

Time

Pilot strength drop s below T_DROP,

Drop Timer started

T_TDROP

T_ADD

T_DROP

EHODM received,

Pilot Add ed to Activ e Set,

HOC mess age sent,

NLU mess age received

N C NA

(1) (2) (3) (4) (5) (6) (7)

Drop Timer expires

PSMM sent (request to add) , Pilot

added to Candid ate Set

PSMM Sent (request to remov e)


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Main parameters of CDMA handoff

T_ADD: Pilot available threshold

When the condition Ec/Io>T_ADD is satisfied, MS sends

measurement message of pilot strength to move the pilot from

the neighbor set to the candidate set.If the Active set is not

full ,the pilot will be added into it .

Scope:-31.5~0dB recommended value:-14~-12 dB

T_DROP: pilot drop threshold (lowest available threshold of

pilot

When the pilot Ec/Io drops to T_Drop, timer T_TDrop will be

triggered

If the pilot Ec/Io exceeds T_Drop, the timer will be terminated.

And when the timer is full, the pilot will be wiped off from the

active set or the candidate set to the neighbor set.

Scope: -31.5~0dB recommended value: -16~ -13 dB


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T_TDROP:Active or Candidate Set drop timer

When the pilot dropped time in the pilot set and the candidate

set exceeds T_TDROP timer, then the pilot will be wiped off to

the neighbor set

If the candidate set is full and there is new pilot satisfying

T_ADD requirement, then the pilot closest to T_TDROP

threshold will be deleted so that the new pilot can be added

Scope: 0~319s recommended value: 2~4 s


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T_Comp:active vs.candidate Comparison threshold

When the pilot is added to the candidate set under the condition

Ec/Io>T_add, then the MS will send PSMM. If Ec/Io>Active Ec/Io

+ TComp*0.5, then the MS will send the supplemental PSMM to

tell BSS to add the pilot into active set .

Scope: 0~6 step: 0.5dB recommended value: 2~2.5dB


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Active versus Candidate Set

Comparison Threshold

Candidate Set: P0 Active Set: P1and P2

t0Pilot Strength Measurement Message sent, P0> T_ADD

t1Pilot Strength Measurement Message sent, P0> P1+ T_COMP x 0.5 dB

t2Pilot Strength Measurement Message sent, P0> P2+ T_COMP x 0.5 dB

(Typically 1 = 0.5 dB)

Pilot P0

Pilot P2

Pilot P1

T_ADD

t0

t1

t2

Pilot

Strength

(C / I)

Time

T_COMP x 0.5 dB

T_COMP x 0.5 dB


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Dynamic threshold of soft handoff

Static threshold (T_ADD, T_DROP) is used in IS95A. And dynamic

threshold needed for the active set has been added to IS95B andIS2000. In different cells or different noise environment, the

addition/deletion of the absolute threshold of the cell pilot in the

Active Set is related to the signal strengths of the strongest pilot and

the weakest pilot. If all the pilot signals in the Active Set are strong,

the requirement for other pilots being sent into the Active Set will be

higher. On the contrary, if the pilot signals in the Active Set are all

weak, the requirement for pilots being sent out from the Active Set

will be comparatively lower.


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Dynamic soft handoff


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Parameters of search window

During the MS detecting the pilot, the pilot required by the MS

may not arrive at the expected time, thus the system time

estimated by the MS should also include propagation delay of

the reference pilot. The same is true for other pilots. Because

the MS doesnt know the specific propagation delay of any

designated pilots, it should search by itself in the reasonable

delay window until it gets the actual sequence of the pilots.

The width for the MS searching the designated

pilot is called search window.


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Parameters of search window

The 3 kinds of search window parameters used by the MS to

search the pilots are as follows :

used for searching the pilots in the active set and the candidate

set

used for searching the pilots in the neighbor set

used for searching the pilots in the remain set


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Pilot searching

Search window (PN offset scope) has been set in the BTS, MS can

search the multi-path components of the available pilot signals within the

set .

SRCH_WIN_A

The MS can utilize the search window to search the pilots

of the active set and the candidate set based on the unit of PN code. The

MS fixes the center of the search window for each pilot signal of the

active set and the candidate set as nearby the pilot signal of the

available multi-path components that earliest reaches.Multi-path

components of the pilot that earliest reaches.

In handoff state,BSS send the branchs SRCH_WIN_A to the MS,that the pilot strenght is the largest,

SRCH_WIN_A

The earlier arrived pilot

PILOT PHASE


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Pilot searching

The purpose of SRCH_WIN set by MS is to capture the multi-

path components which arrived before or after the current

signal. From the experience, the SRCH_WIN should be set wide

enough so that it can capture the signal which has the

maximum time delay due to multi-path.


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MS Searcher Working Progress


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Pilot searching

Delay budget and size of search windowSRCH_WIN_A

Width of

SRCH_WIN_A

chips)

Delay budget

ms

Availab-

le PN

0 4 T 1.64 512

1 6 1.64


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Summary

Soft handoff technology of the CDMA system fully utilizes the

character of direct-spread system. In comparison with the

hard handoff technology, it has the following advanced

performance:

Make-to-break of soft handoff may greatly reduce the probability

of communication interruption;

Diversity receive method is adopted to against fading;

Macro diversity receive of BTS provides communication

guarantee; Reduce the transmit power of MS as least as possible to lower

the interference to the system;

Reduce the congestion rate.


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Types of handoff algorithms

The handoff algorithm can be divided into the following types:

intra-frequency handoff

Soft handoff/softer handoff

Soft handoff of bad coverage

intra-frequency hard handoff

Inter-frequency handoff

MS-assisted hard handoff

Pseudo-pilot hard handoff

Handdown hard handoff

Direct hard handoff

Other characteristic handoff algorithms

Load balance of Inter-frequency pilots


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Beforehandoff After

handoff

The old connection will not be disconected during

handoff procession. There will be more than twoconnections between MS and BTS after handoff and nointeruption during handoff procession which greatlyreduced call drop rate.



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Flexible soft handoff branch of 1~6 channel(s)

In a network, the maximum allowable soft handoff branches

can be configured according to the actual condition.


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Special processing of data service

Causes:

The rate of the supplemental channel is comparatively high. If

the supplemental channel supports the same soft handoff

branches as the fundamental channel, then the forward capacity

of the system will be reduced.

There is certain valid period for the supplemental channel, that

is, the channel is not kept constantly.

Measures: Soft handoff branches of the supplemental traffic channel should

be adequately reduced than the basic channel. That is to say,

the active set of the supplemental channel should be the subset

of the active set of the basic channel.


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Adjustment of soft handoff of bad coverage

For those areas with pilot pollution and the sectors with

complex radio environment, soft handoff branch can be

intelligently added to utilize the advantage of multi-channel (6

channels at most) soft handoff to realize excellent coverage.


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Intra-frequency hard handoff

In some particular conditions, soft handoff/softer handoff

cannot be realized even the frequencies of the pilots are the

same. For example:

Inter-BSC pilot has no A3/A7 connection;

Destination sector cannot allocate the same frame offset;

In these cases, hard handoff can be adopted to complete the

handoff of the intra-frequency pilots


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Intra-frequency handoff



Intra-frequency hard handoff



Beacon pilot hard handoff


Direct hard handoff




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Trigger causes of MS-assisted hard handoff:

Triggered by pilot strength: The MS reports the pilot strength of

the BTS signal, and it will trigger hard handoff when it measures

that the current pilot strength cannot meet the setting conditions

of the system. And the destination sector cannot allocate the

same frame offset;

FER trigger: The system may obtain the FER of both of the

forward link and the reverse link, and it will trigger hard handoff

search when the quality of the forward link or the reverse link is

rather poor. If there is neighbor inter-frequency pilot reported by

the MS, which meets the setting conditions of the system, then

the system will trigger hard handoff.


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(Candidate Frequency Search Request Message)

(Candidate Frequency Search Response Message)(Candidate Frequency Search Control Message)

(Candidate Frequency Search Report Message)

(Universal Handoff Direction Message)


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Hard handoff of beacon pilot

Strength of inter-frequency pilot can bemeasured without

inter-frequency search.


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HANDDOWN hard handoff

It is applicable to the cells with the same frequency, but different

configurations.


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Direct hard handoff

It is applicable to the cells with simple network structure, but different

frequencies;


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intra-frequency handoff



intra-frequency hard handoff



Pseudo-pilot hard handoff


Direct hard handoff




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Inter-frequency load balance

Natural balance of intra-frequency traffic

The CDMA system is a self-interference one. The interference to

the carrier may increase along with the increase of the load,

while the strength of the carrier reduces. This may cause that

the calls handed off into the carrier reduce, while the ones

handed out increase. Judging from the aspect, it is obvious that

load balance between the intra-frequency pilots can be realized

via soft handoff in a certain degree.

Independence of pilot traffic

There is no such natural balance mechanism in the inter-

frequency pilots, thus extra control means are needed.


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inter-frequency load balance

According to the load of pilots, successive handoff mode is

adopted. To be specific, traffic will be transferred to the

carriers with lower load in the same sector to realize load

balance.

f1

f2


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Hard handoff of data service

For the data service, hard handoff doesnt support

supplemental channel, thus the required service negotiations

are all left for processing in the flow. The supplemental

channels will be firstly disconnected, and then be reallocated

after handoff.


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Question

How to plan the ratio of soft handoff?


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Answer

The soft handoff algorithm is performed on the basis of time

used for the soft handoff. Specific are as follows: Softhandoff ratio = [time of 2-channel soft handoff * (2-1) + time

of 3-channel soft handoff * (3-1)]/total call duration of

subscriber * 100%. For example, a subscriber makes a call

for an hour. Assume that there is no soft handoff in 0.5 hour,

but there is 2-channel soft handoff in 0.3 hour and 3-channel

soft handoff in 0.2 hour, then the soft handoff ratio would be:

[0.3*(2-1)+0.2*(3-1)]/(0.2*3+0.3*2+0.5)*100%=41%.Whether

softer handoff is also included can be found out according to

the actual condition and the check purpose. Generally, thecommon algorithm only includes the soft handoff. During

softer handoff, PA power and WALSH code resources are

used, but not the channel resources. During soft handoff, all

of the three are used.


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Summary

According to the factor that whether the frequency of the

original cell is the same as that of the destination cell, the

handoff algorithm is divided into two major categories, namely,

intra-frequency handoff and inter-frequency handoff. And theload balance is the handoff algorithm activated by load control.

All of these algorithms complete the control of CDMA soft

handoff, softer handoff, hard handoff and load balance.


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Contents of course






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Soft Handoff Process

ff f


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Hard handoff flow

Q i


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Question

Does Huawei the proprietary of inter-BSC soft

handoff?

A


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Answer

Inter-BSC soft handoff between the BSCs of

Huawei and those of other vendors can be

made possible if A3/A7 interfaces are

provided by the BSCs of these vendors. At

present, part of the equipment of vendors is

not provided with the A3/A7 interfaces.

S


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Summary

This chapter has introduced the CDMA handoff flow via the

illustration of BSC soft handoff and hard handoff flows.

C t t f


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Contents of course





D t fi ti t bl


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Data configuration table

Following configuration tables are involved in the

management of data configuration: Parameter table of module handoff: Handoff parameter

configured based on the unit of module, including maximum

branch number and a group of timer lengths

Configuration table of pilot handoff algorithms: Handoff algorithm

switch

Parameter table of pilot handoff: Configuration of common

handoff parameters such as T_Add

Various types of handoff algorithm parameters: Parameter table

of intra-frequency hard handoff, parameter table of pseudo pilot

hard handoff, parameter table of HANDDOWN hard handoff,

parameter table of direct hard handoff, destination table of

pseudo pilot hard handoff

D t fi ti t bl


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Data configuration table

Parameter table of inter-frequency search control, parameter

table of candidate frequency search control: Parameter

configurations used for handoff searching

External CDMA pilot table: Parameter configuration of external

BSC carrier

Intra-frequency neighborhood table of carrier: Parameter

configuration of intra-frequency neighborhood of carrier

Inter-frequency neighborhood table of carrier: Parameter

configuration of inter-frequency neighborhood of carrier

Q ti


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Question

What are the functions of the intra-frequency

neighborhood table of carrier?

A


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Answer

Intra-frequency neighborhood table of carrier

(SFNBRPILOT) is used for intra-frequency

handoff during conversation. And the service

pilot is finalized by the MODULE_ID,

PILOT_ID. One record is configured for each

intra-frequency neighborhood. For the

service pilot, there may be multiple neighbor

pilots, thus many of the recorded

MODULE_ID,PILOT_ID are the same. But

the module ID and the pilot ID of its

neighborhood pilot are not the same.

S mmar


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Summary

There is corresponding parameter table configuration for each

kind of handoff. For details of data configuration, please refer

to Guide to Data Configuration Specifications of CDMA 1X

BSS Network Planning. No more details will be given here.


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10)cdma1x handoff algorithm

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