Soft HO Tutorial

8/20/2019 Soft HO Tutorial

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ofter handover is the situation +here one !ase station re'eives t+o user signa"s

from t+o ad9a'ent se'tors it serves( )n the 'ase of softer handover the !ase station

re'eives 2 se&arated signa"s through mu"ti4&ath &ro&agation( ue to re;e'tions on

!ui"dings or natura" !arriers the signa" sent from the mo!i"e stations rea'hes the

!ase station from t+o di*erent se'tors(

The signa"s re'eived during softer handover are treated simi"ar"# as mu"ti4&ath

signa"s( )n the u&"in dire'tion the signa"s re'eived at the !ase station are routed to

the same rae re'eiver and then 'om!ined fo""o+ing the maimum ratio 'om!ining

te'hniue( )n the do+n"in dire'tion the situation is s"ight"# di*erent as the !ase

station uses di*erent s'ram!"ing 'odes to se&arate the di*erent se'tors it serves(

o it is ne'essar# for the di*erent


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)n the 'ase of soft handover the mo!i"e station is in the over"a&&ing 'e"" 'overage

area of t+o se'tors !e"onging to di*erent !ase stations(

)n do+n"in dire'tion the signa"s re'eived from the t+o di*erent !ase stations are

'om!ined using M=C =ae &ro'essing in the mo!i"e station(

)n the u&"in dire'tion the re'eived signa"s 'an no "onger !e 'om!ined in the !ase

station !ut are routed to the =>C( The 'om!ining fo""o+s a di*erent &rin'i&"e8 in the

=>C the t+o signa"s are 'om&ared on a frame4!#4frame !asis and the !est

'andidate is se"e'ted after ea'h inter"eaving &eriod8 i(e( ever# 10, 20, ?0 or @0ms(

7s the outer "oo& &o+er 'ontro" a"gorithm measures the >= of re'eived u&"in

signa"s at a rate !et+een 10 and 100%A, this information is used to se"e't the frame

+ith the !est ua"it# during the soft handover(

The oft %andover &ro'edure is 'om&osed of a num!er of sing"e fun'tionsB

• Measurements

• -i"tering of Measurements


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• =e&orting of Measurement resu"ts

• The oft %andover 7"gorithm

• e'ution of %andover

Soft Handover Algorithm

oft handover in &ra'ti'e is a 'om&"e te'hniue( The fo""o+ing eam&"e +i""

des'ri!e soft handover for !est 'ase s'enario(

efore des'ri!ing the a!ove s'enario +e need to no+ three im&ortant terms(

Active Set

User information is sent a"" the 'e"" !e"ongs to 7'tive et( )n -, the 'e""s in the

a'tive set are invo"ved in soft handover( )n T the a'tive set a"+a#s 'om&rises one

'e"" on"#( The U sha"" on"# 'onsider a'tive set 'e""s in'"uded in the varia!"eCDDE)>-FED)T for measurement8 i(e( a'tive set 'e""s not in'"uded in the

CDDE)>-FED)T sha"" not !e 'onsidered in an# event eva"uation and measurement

re&orting(

Monitored Set


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Ce""s, +hi'h are not in'"uded in the a'tive set, !ut are in'"uded in the

CDDE)>-FED)T !e"ong to the monitored set(

Detected Set

Ce""s dete'ted !# the U, +hi'h are neither in the CDDE)>-FED)T nor in the a'tive

set !e"ong to the dete'ted set( =e&orting of measurements of the dete'ted set is

on"# a&&"i'a!"e to intra4freuen'# measurements made !# Us in CDDEC% state(

The a!ove s'enario 'an !e devided into three di*erent &arts(

Radio Link Addition (vent !A"

vent 17 is triggered +hen a &rimar# CP)C% enters the re&orting range( This means

+hen a 'e"" is strong enough to enter into 7'tive et(

Radio Link Deletion (vent !#"

vent 1 is triggered +hen a &rimar# CP)C% "eaves the re&orting range( This

ha&&ens +hen a 'e"" in a'tive set !e'ome +ea and needs to !e removed from

7'tive et(

$om%ined Radio Link Addition & Deletion (vent !$"

vent 1C is triggered +hen a non4a'tive &rimar# CP)C% !e'omes !etter than an

a'tive &rimar# CP)C%( When this event is triggered a &rimar# CP)C% is removed from

the 7'tive et and a ne+ stronger 'e"" is added into 7'tive et(

ou 'an get information on other events in another arti'"e(

Soft Handover Algorithm lo' Diagram

http://www.3glteinfo.com/umts/rrc/rrc-measurements-cheat-sheet-fddhttp://www.3glteinfo.com/umts/rrc/rrc-measurements-cheat-sheet-fdd


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rther Stdies

Why no Soft Handover in LTE

Prashant Panigrahi May 23, 2014 2 Comments

http://www.3glteinfo.com/author/prashant/http://www.3glteinfo.com/author/prashant/http://www.3glteinfo.com/soft-handover-lte/#disqus_threadhttp://www.3glteinfo.com/author/prashant/http://www.3glteinfo.com/soft-handover-lte/#disqus_thread


7/11

When LTE system was developed one of the big technical feature from UMTS Soft/Softer handover was missing from

the architecture There are many specific reasons why LTE dropped altogether soft handover !connect"before"brea#$

from the system design

%efore discussing about why LTE s#ipped soft handover and only implemented hard handover& let's chec# the basic

concept of soft/softer handover

Soft handover concept

Soft handover or connect"before"brea# handover is a category of handover procedure where radio lin#s are added or

removed in such a fashion that there is always one radio lin# connected to the UE

(ll the radio lin#s connected to the UE at a particular time are part of the ()T*+E SET and the number can vary from

, to - This means at least one and ma.imum of - radio lin#s are connected to the UE

Soft handover is a part of W)M( system which is basically same in concept as in )M( systems

*n the case of soft handover the mobile station is in the overlapping cell coverage area of two sectors belonging to

different base stations *n downlin# direction signals received from different base stations are combined at the ra#e

receiver of the UE *n the uplin# the scrambled signals from different base stations for a particular UE are compared

frame"by"frame basis at the 01) and the best candidate is selected after each interleaving period2 ie every ,3& 43&

53 or -3ms


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UMTS Soft Handover Architecture

Soft and softer handovers are only possible when UE is in connected state with dedicated channel !)H$


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Even with HSS)H !High"Speed ownlin# Shared )hannel$ there is no soft handover in UMTS *n cased HS6(

communication there is only hard handover which is brea# before connect Means UE is never connected to more

than one radio lin# at a time and when UE goes out of coverage of a particular cell that radio lin# is teared down

before adding the better radio lin#

0e7uirements for soft handover and what is missing in LTELet's discuss why LTE does not have soft handover as in W)M( and )M( systems Though there are various

reasons& * documented here three causes why soft handover was omitted from LTE

(d8acent )ell 9re7uency 0euse

Soft handover is possible in )M( because ad8acent cells can operate on the same fre7uencies as long as they use

different scrambling codes So& a UE can listen to two different cells by decoding the received signals twice& using a

scrambling code from each cell on each for each decoded signal This allows a UE to communicate with both cells

during handover& ma#ing it a soft handover

LTE is based on :9M(& which is fundamentally a fre7uency division method This means that a UE has to actually

resync to a different set of fre7uency subcarriers when it hands over between cells& removing the possibility for soft

handover *n fact& when a handover is beginning& an LTE UE has to go into a ;compressed mode' where it listens to its

current cell for part of the time and searches for a new cell the rest of the time With modern radio technology& this

retuning can happen fast enough to ma#e the inter"fre7uency retuning much more seamless than it was in older

technologies li#e


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This LTE video tutorial describes@

• (uthentication of UE in +oLTE call

• How control signalling is used to create default bearer in +oLTE

• How default internet bearer is established

•

)ontrol signalling to create default *MS bearer• efault *MS bearer establishment

• *MS registration via S*6

• 1otify change of state

• (ctual +oLTE call data pac#et transmission on U6

+oLTE Signalling )all 9low > +ideo Tutorial

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Soft HO Tutorial