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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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Main parameters of CDMA handoff
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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Main parameters of CDMA handoff
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.
Soft handoff/softer handoff
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Soft handoff/softer handoff
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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Soft handoff/softer handoff
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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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
Beacon pilot hard handoff
Handdown hard handoff
Direct hard handoff
Other characteristic handoff algorithms
Load balance of Inter-frequency pilots
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MS-assisted hard handoff
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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MS-assisted hard handoff
(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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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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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
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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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
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
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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