UMTS Long Term Evolution–synchronisation and
cell search
Reiner Stuhlfauth
Reiner.Stuhlfauth@https://www.doczj.com/doc/852822224.html,
Training Centre
Rohde&Schwarz,Germany
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Base Station Base Station
Base Station
Base Station
Base Station
What does the mobile need to know?
1)Strongest base station4)Channel bandwidth
2)Slot and Frame timing
5)BCCH information 3)Physical layer cell identity
Initial Access
Power-up User Data RX/TX
Cell Search and Selection Derive System
Information
Random
Access
LTE cell acquisition process
l 1.carrier frequency detection
l 2.primary synchronisation signal ->5msec timing and l 3.secondary synchronisation signal ->10msec timing and l 4.Derive physical layer cell identity out of PSS and SSS
l 5.Blind detect cyclic prefix duration (extended or normal)and slot boarder
l https://www.doczj.com/doc/852822224.html,ing cell identity and channel bandwidth for reference symbol detection
l 7.PBCH detection,reading Master information block l 8.MIB ->channel bandwidth and system frame number l 9.PCFICH detection ->PDCCH ->SI-RNTI l 10.PDCCH ->PDSCH ->SIB1
l 11.SIB1scheduling information to acquire all other SIBs l
12.Reading necessary SIB information
(2)
ID
N (1)ID
N
LTE cell search–carrier frequency UE scans all
frequency bands
according to its
capabilities to find
carrier frequency of
the cell.No
priorisation between
bands,optionally
USIM information
will give priority
LTE Downlink
Cell search procedure -hierarchy
1.Primary synchronization signal:
3possible sequences to identify the cell’s physical layer identity (0,1,2)Transmitted every 5ms to identify 5ms timing
2.Secondary synchronization signal:
168different sequences to identify physical layer cell identity group Transmitted every 5ms to identify radio frame timing
Downlink
reference signal
3.Physical broadcast channel (PBCH):
Carrying broadcast channel with predefined information:system bandwidth,number of transmit antennas,reference signal transmit power,system frame number,…
Physical layer cell identity (1out of 504)
Initial synchronization
BCH and SCH always located at the center
20MHz bandwidth
P-SCH S-SCH
10MHz bandwidth
1.4MHz bandwidth
PBCH
DC subcarrier,or subcarrier 0=centre of channel bandwidth
Sent over 62subcarriers
Sent over 72subcarriers
LTE downlink
Hierarchical cell search scheme
1
…………..
167
Physical layer identity
012
012012Physical layer cell identity group identified by:
Physical layer cell identity (1out of 504)
(2)ID
(1)ID
cell ID
3N N N +
=
Physical layer cell identity
Physical layer cell identity =[0..503]
LTE cell search scheme
(2)ID
(1)ID cell ID 3N N N +=01 (167)
Physical layer identity
012
012012Physical layer cell identity group
Identified by P -S C H S -S C H
Primary Sync Channel P-SCH:3possible sequences
to identify physical layer identity =0,1,2
(2)
ID N Secondary Sync Channel S-SCH:168possible sequences
to identify physical layer cell identity group =0..167(1)ID
N …Peter“…Bob“…John“…Peter“…Bob“
…John“
…Peter“…Bob“
…John“
…Smith“
…Miller“
…Kennedy“
LTE Downlink:P-SCH and S-SCH
10ms radio frame
12345671234567
0.5ms slot
1ms subframe
Primary synchronization signal Secondary synchronization signal
Zadoff-Chu sequence,shows physical layer identity
binary sequence,2parts,showing physical layer cell identity group
f
t
DC subcarrier,No transmission
identical
Interleaved concatenation of 2binary sequences
LTE timing due to PSS and SSS
Secondary Synchronisation Signal:2concatenated
sequences interleaved over10msecs=1frame …tick“…tack“…tick“…tack“…tick“…tack“…tick“…tack“…tick“…tick“…tick“…tick“…tick“…tick“…tick“…tick“Primary Synchronisation Signal:1sequences
identically sent every5msecs=1halfframe
LTE TDD:PSS and SSS position
10ms radio frame
123456712345670.5ms slot
1ms subframe
PSS Primary synchronization signal SSS Secondary synchronization signal
PSS on 3rd symbol Subframe 1and 6
SSS on last symbol of slot 1
and 11
12345671234567
How does the UE know whether cell is TDD or FDD mode?PSS and SSS are on different positions!
l Primary synchronisation signal is a CAZAC sequence,
constant amplitude,zero autocorrelation
l Shows good autocorrelation(the 3selected root indices show best correlation results)
l Has good peak to average power ratio,PAPR
l based on Zadoff-Zhu sequence,sequence d u (n)given as
l Mapping onto physical ressources:62subcarriers around DC
subcarrier
===++ + 61
,...,32,3130,...,1,0)(63)
2)(1(63)
1(n e n e
n d n n u j n un j
u 34
2
291250(2)ID
N Root index
u
()2
3161
, 0
RB sc
DL RB ,N N n k n n d a l k +
===Alleviates search,UE can use
size 64FFT window
(2) ID =
N
l Interleaved concatenation of 2length-31binary sequences l BPSK modulated
l Scrambled based on physical layer identity,derived from
primary synchronisation signal PSS
l Identifies the physical layer cell identity group
l Transmitted on 62subcarriers around the DC subcarrier
() =+
===2
type structure frame for 11and 1slots in 11type structure frame for 10and 0slots in 22
3161
, 0
DL
symb DL symb RB sc
DL RB ,N N l N N n k n n d a l k
()()()()()() =+ =5
subframe in )(0subframe in )()12(5
subframe in )(0
subframe in )()2()(11)
(0)(1
1)(10)
(10)(010
0110n z n c n s n z n c n s n d n c n s n c n s n d m m m m m m Even resource element
Odd resource element
Indices m0and m1defines the physical layer cell identity group
(1)ID
N )
(21)(~i x i s =()25
0,
2mod )()2()5( ++=+i i x i x i x Sequence s()is a pseudo random sequence,given as:
()()31mod )(~)(31mod )(~)(1
)(1
0)
(010m n s n s m n s n s m m +=+=Sequence c()is a pseudo random sequence used as scrambling sequence
)31mod )3((~)()31mod )((~)()2(ID
1
)2(ID
0++=+=N n c
n c N n c n c )(21)(~i x i c =()25
0,
2mod )()3()5( ++=+i i x i x i x Sequence z()is a pseudo random sequence used as scrambling sequence
)
31mod ))8mod (((~)(0)
(10m n z n z m +=)
31mod ))8mod (((~)(1)(11m n z n z m +=)
(21)(~i x i z =()25
0,
2mod )()1()2()4()5( ++++++=+i i x i x i x i x i x identify physical
layer cell identity group
(1)ID
N identify cell,
within eNodeB
identify cell
at cell edge
1
)4(,0)3(,0)2(,0)1(,0)0(=====x x x x x
(1)ID
N 0
m 1
m (1)ID
N 0
m 1
m (1)ID
N 0
m 1
m (1)ID
N 0
m 1
m (1)ID
N 0
m 1
m 00134466891210215191362227112355769101310316201372328223366870111410417211382429334377971121510518221392530445388107213161061923140065563991173141710720241411766740101274151810821251422877841111375161910922261433988942121476172011023271444109910431315771821111242814551110101144141678192211225291466121111124515177920231132630147713121213461618802124114051488141313144717198122251151614991514141548182082232611627150101615151649192183242711738151111716161750202284252811849152121817171851212385262911951015313191818195222248627301206111541420191920532325870412171215515212020215424268815122813156162221212255252789261239141571723222223562628903712410151581824232324572729914812511161591925242425582830925912612171602026252526590393610127131816121272626276014947111281419162222827272861259581212915201632329282829623696913130162116424302929306347971014131172216507300264589811151321823166183113656999121613319241672932246671010013171342025---333
5
678
11
10114
18
13521
26
-
-
-
Indices m0and m1defines the physical layer cell identity group
(1)ID
N
Subcarrier -31
Subcarrier -31
Subcarrier +30
Subcarrier +30
Subframe #0
Subframe #5
DC subcarrier on Carrier frequency f c
d(2n)=s 0(m0)(n)c 0(n)
d(2n)=s 1(m1)(n)c 0(n)
d(2n+1)=s 1(m1)(n)c 1(n)z 1(m0)(n)
d(2n+1)=s 0(m0)(n)c 1(n)z 1(m1)(n)
LTE timing slot and symbol duration
How does the UE know the slot timing,OFDM symbol duration and the cyclic prefix length?
Cyclic Prefix
CP
CP CP
CP
:S ymbol OFDM Metrik
+
l l 1
l n
~Search
window -Reminder:
OFDM uses principle of cyclic prefix.UE can use search window to synchronise to the OFDM Symbol length
CP
CP
CP
CP
CP
CP
12345
6How does the UE know the slot timing,OFDM symbol duration and the cyclic prefix length?
UE has to blind
detect wether normal or extended CP length
Normal CP Extended CP
1.Symbol per slot:CP length =160*T s
Symbol 1-6per slot CP length =144*T s
1
2
3
4
5
Symbol 0-5per slot:CP length =512*Ts
OFDM symbol
duration =2048*Ts for both,extended and normal cyclic prefix