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Principles of LTE Protocol
Course objectives:
Understand the architecture of the LTE protocol
Understand the frame structure of the LTE system
Understand functions of each LTE system layer
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Contents
1 System Overview...........................................................................................................................................1
1.1 Overall Architecture of LTE ystem...................................................................................................1
1.! "rotocol Architecture of LTE ystem..................................................................................................!
2 Physical Layer Protocol................................................................................................................................7
!.1 #unctions of "hysical Layer................................................................................................................$
!.! tructure of %adio #rame &%#'............................................................................................................(
!.) "hysical Channel...............................................................................................................................1*
!.+ Transmission Channel........................................................................................................................1*
!., -appin /et0een Transmission Channel and "hysical Channel.....................................................11
3 Data Link Layer..........................................................................................................................................13
).1 Overvie0 of ata Lin2 Layer............................................................................................................1)
).! -AC ub3layer "rotocol...................................................................................................................1,
).!.1 #unctions of -AC ub3layer.................................................................................................1,
).!.! Loical Channel......................................................................................................................1,
).!.) -appin bet0een Loical Channel and Transmission Channel............................................14
).!.+ 5ey Technoloies of -AC Layer..........................................................................................1$
).) %LC ub3layer "rotocol....................................................................................................................1(
).).1 T- -ode................................................................................................................................!*
).).! U- -ode................................................................................................................................!1
).).) A- -ode................................................................................................................................!!
).).+ %LC "U tructure................................................................................................................!)
).+ "C" ub3layer "rotocol..................................................................................................................!+
).+.1 #unctions of "C" Layer.......................................................................................................!+
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).+.! "C" Entity............................................................................................................................!4
).+.) tructure of "C" "U.........................................................................................................!$
4 RRC Layer Protocol...................................................................................................................................29
+.1 #unctions of %%C Layer....................................................................................................................!(
+.! %%C tate..........................................................................................................................................)*
+.) inalin "rocess of %%C Layer.......................................................................................................)1
+.).1 /roadcast "rocess...................................................................................................................)1
+.).! "ain "rocess........................................................................................................................)!
+.).) %%C Connection etup...........................................................................................................)!
+.).+ %%C Connection %econfiuration..........................................................................................))
+.)., %%C Connection %eestablishment.........................................................................................))
+.).4 %%C Connection %elease.......................................................................................................)+
!"S Layer Protocol....................................................................................................................................3
,.1 A and 6A -odel...........................................................................................................................),
,.! 6A Layer "rotocol tates and tate Transition..............................................................................)$
,.) /asic "rocedures of 6A..................................................................................................................)(
,.).1 /asic "rocedures of 6A.......................................................................................................)(
,.).! -appin %elationship bet0een 6A Layer #unctions and /asic "rocedures......................)(
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1 System Overview
Knowledge points:
Overall architecture of the LTE system
Access stratum &A' and non3access stratum &6A'
User plane protocol architecture
Control plane protocol architecture
1.1 Overall Arcitect!re of LTE System
The fiure belo0 is the net0or2 topoloy of the LTE system.
#iure 1.131 Overall architecture of the LTE system
Compared 0ith the U-T system7 the 0ireless transmission technoloy7 air interface
protocol and system architecture of the LTE8AE net0or2 have been revolutioni9ed.
The correspondin 0ireless net0or2 and pac2et core net0or2 are called as E3UT%A6
and Evolved "ac2et Core &E"C' respectively. The 0hole net0or2 system is named as
Evolved "ac2et ystem &E"'.
n the E3UT%A67 " transmission is adopted in the bottom layer communication
bet0een e6ode/s 0hich are interconnected throuh the ;! interface loically. t is
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also often 2no0n as -esh type net0or2. uch net0or2 architecture desin can
effectively support the movability of UE in the 0hole net0or2 to ensure seamless
handover of the user. Each e6ode/ is connected to the --E83
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Chapter , 6A Layer "rotocol
#iure 1.!3! User plane protocol stac2 of the air interface
The follo0in fiure sho0s the control plane protocol stac2.
#iure 1.!3) Control plane protocol stac2 of the air interface
The control plane protocol stac2 contains the non3access stratum &6A'7 %%C7 "C"7
%LC7 -AC and "B layers. Amon them7 the "C" layer provides encryption and
interality protection functions. #unctions e@ecuted by the control plane in %LC and
-AC layers are consistent 0ith those of the user plane. The %%C layer protocol ends at
the e6ode/ and reali9es functions such as broadcastin7 pain7 %%C connection
manaement7 %adio /earer &%/' control7 mobility manaement7 UE measurementreport and control. The 6A layer ends at the --E and achieves functions such as
E" bearer manaement7 authentication7 mobility manaement in idle state7 pain
messae and security control.
The follo0in fiure briefly describes the LET protocol architecture of different layers7
functions and the interaction process bet0een layers. The follo0in fiure presents the
protocol architecture of the e6ode/ side7 0hich is similar to that of the UE side.
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"rinciple of LTE "rotocol
#iure 1.!3+ "rotocol architecture of the LTE system &do0nlin2'
Li2e the U-T system and most mobile communication systems7 the data processin
of the LTE system is divided into different protocol layers. The fiure above describes
the overall protocol architecture of the LTE system in do0nlin2 transmission. The
do0nlin2 data is transmitted in " pac2et. /efore bein transmitted by the air interface7
the " pac2et 0ill be processed throuh multiple protocol layer entities. The follo0in
describes the process in detail.
? "C" layer: The "C" layer e@ecutes the header compression to reduce the bit
stream that must be transmitted by the radio interface. The header compression
mechanism is based on %OC 0hich is a standard header compression
alorithm and has been applied in the U-T and multiple mobile
communication specifications. The "C" layer is also responsible for
encryption and interity protection of transmit data. At the receivin end7 the
"C" protocol e@ecutes decryption and decompression. #or a terminal7 each
radio bearer has a "C" entity.
? %LC layer: The %LC layer is responsible for sementation and connection7
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Chapter , 6A Layer "rotocol
retransmission process and transmission of hih3level data in seuence.
ifferent from the U-T system7 the %LC protocol of the LTE system is at the
e6ode/7 because the architecture of the radio access net0or2 is flattened at the
LTE system and has only one layer of node>e6ode/. The %LC layer provides
service to the "C" layer in radio bearer mode. Amon them7 an %LC entity is
allocated for each radio bearer of a terminal.
? -AC layer: The -AC layer is responsible for A%D retransmission and
uplin28do0nlin2 schedulin. The -AC layer provides service to the %LC layer
in loical channel mode.
? "B layer: The "B layer is responsible for codin7 decodin7 modulation7demodulation7 multi3antenna mappin and other functions of the telecom
physical layer. The "B layer provides service to the -AC layer in
transmission channel mode.
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" Pysical Layer Protocol
Knowledge points:
#unctions of the physical layer
#rame structure of the physical layer
"hysical resource allocation
"hysical layer channel and mappin relations
".1 #!nctions of Pysical Layer
The physical layer of the air interface in the LTE system is mainly responsible for
providin the lo0er3layer data transmission service to the upper layer. To provide the
data transmission service7 the physical layer shall have the follo0in functions:
? etectin errors of the transmission channel and providin instructions to the
upper layer
? #or0ard error correction EC' and codin of the transmission channel
? ybrid Automatic %epeat %euest &A%D' soft combinin
? %ate matchin and mappin bet0een the transmission channel and the physical
channel
? "o0er30eihtin of the physical channel
? -odulation and demodulation of the physical channel
? Time and freuency synchronism
? -easurin %# characteristics and providin instructions to the upper layer
? --O antenna processin
? Transmit diversity
? /eamformin
? %# processin
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The follo0in briefly introduces the 2ey technical solutions of the physical layer in the
LTE system.
? ystem band0idth: The carrier spacin of the LTE system is 1, 29. The
minimum resource bloc2 of uplin2 and do0nlin2 is 1* 29F that is7 the
band0idth of 1! sub3carriers. The mappin from the data to the resource bloc2
can be in centrali9ed or distributed mode. Throuh reasonable number allocation
of sub3carriers7 the system can achieve fle@ible band0idth confiuration of
1.+G!* -9.
? O#-A and C3#-A: The basic do0nlin2 transmission of the LTE system
adopts the Orthoonal #reuency ivision -ultiple Access &O#-A' mode.The cyclic prefi@ &C"' in the O#- transmission mode is mainly used to
eliminate intersymbol interference effectively. ts lenth determines the anti3
multipath capability and coverae capability of the O#- system. To reach the
coverae reuirements of the cell 0ith the radius of 1** 2m7 the LTE adopts t0o
C" prorams>lon C" and short C" 0hich can be selected accordin to specific
scenario. The short C" proram is basic option7 and the lon C" proram is used
to support lare3rane cell coverae and multi3cell broadcast service. n the
uplin2 direction7 the LTE system adopts the C"3based inle3Carrier #reuency
ivision -ultiple Access &C3#-A' technoloy. The main reason of selectin
the C3#-A as the uplin2 sinal access mode of the LTE system is to reduce
the pea23to3averae po0er ratio&"A"%' of the transmittin terminal and then to
reduce the si9e and cost of the terminal.
? uple@ mode: The LTE system supports t0o basic 0or2 modes>#reuency
ivision uple@ ' and Time ivision uple@ &T' and t0o 2inds of
radio frame structure. The frame lenth is 1* ms in averae
? -odulation systems: The LTE system supports the follo0in modulation
systems: D"57 14DA- and 4+DA-.
? Channel codin: n the LTE system7 the channel codin proram used for the
transport bloc2 is Turbo codin 0ith the codin rate of %H18). t consists of t0o
3state sub3coders and one internal interleaver of Turbo codes. Amon them7 the
Trellis termination proram is adopted in Turbo codin.
? -ulti3antenna technoloy: The LTE system has introduced the --O
technoloy. The simultaneous confiuration of multiple antennas at the
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Chapter , 6A Layer "rotocol
transmittin terminal and the receivin terminal can reatly enhance the 0hole
capacity of the system. The basic --O confiuration of the LTE system is !I!
antennas in do0nlin2 and 1I! antennas in uplin2. To confiure more antennas
can also be considered &up to +I+'. #or the do0nlin27 the --O technoloy
adopted by the LTE system includes transmit diversity7 spatial multiple@in7
space division multiple access &-A' and precodin. #or the uplin27 the LTE
system adopts the virtual --O technoloy to increase the capacity.
? "hysical layer process: The LTE system covers multiple physical layer
processes7 includin cell search7 po0er control7 uplin2 synchronism7 do0nlin2
timin control7 random access relevant process and A%D. Throuh physical
resource control in the time domain7 freuency domain and po0er domain7 the
LTE system also supports the interference coordination function implicitly.
? "hysical layer measurement: The LTE system supports the physical layer
measurement bet0een the UE and the e6ode/ and reports the correspondin
measurement results to the upper layer. The measurement inde@es are
measurement of co3pilot and inter3pilot handoff7 measurement of handoff
bet0een different radio access technoloies7 timin measurement and
measurement of radio resource manaement.
"." Str!ct!re of $adio #rame %$#&
The structure of the LTE system time domain frame is as sho0n in the fiure belo0.
The total lenth of each %# is T frame H 1* ms7 0hich can be further divided into 1* sub3
frames 0ith the lenth of Tsubframe H 1 ms each. To provide consistent and precise
definition of time7 the LTE system ta2es TsH 18)*$!**** s as the basic time unit. All
time slots in the system are inteer multiples of the basic unit. The time slot in the
follo0in fiure can be represented as T frame H )*$!** Ts and Tsubframe H )*$!* Ts.
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"rinciple of LTE "rotocol
#iure !.!3, tructure of the LTE system time domain frame
Each 1* ms %# contains t0o half3frames 0ith the lenth of , ms. Each half3frame
consists of four data sub3frames and one special sub3frame. The special sub3frame
contains three special slots: 0"T7
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Chapter , 6A Layer "rotocol
? o0nlin2 hared Channel &L3C': t supports transmittin data 0ithin the
0hole coverae area of the cell and A%D. Lin2 adaptation can be achieved
throuh multiple modulation modes7 encoded modes and transmit po0er. t
supports beamformin7 dynamic or semi3dynamic resource allocation7
discontinuous receivin &%;' of the UE to save po0er and transmission of the
-/- service.
? "ain Channel &"C': t supports transmittin data 0ithin the 0hole coverae
area of the cell. t can be mapped to the physical resources that for the use of
services and other dynamic channel. t supports discontinuous receivin &%;'
of the UE to save po0er and transmission of the -/- service.
? -ulticast Channel &-C': t supports transmittin data 0ithin the 0hole
coverae area of the cell. #or sinle freuency point net0or2 &-/#6'7 it
supports transport combination of multi3cell -/- and adopts semi3static
resource allocation.
? Correspondinly7 the uplin2 transmission channel of the LTE system has the
follo0in types:
? Uplin2 hared Channel &UL3C': t can achieve dynamic lin2 self3adaptation
by adjustin the transmittin po0er and modulation8codin format. t supports
beamformin7 A%D7 dynamic or semi3dynamic resource allocation.
? %andom Access Channel &%AC': t can bear limited control information and
supports the conflict8collision resolution mechanism.
".) *apping +etween Transmission Cannel and Pysical Cannel
The mappin relationship bet0een the transmission channel and physical channel in
the LTE system is as sho0n in the fiure belo0.
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"rinciple of LTE "rotocol
#iure !.,34 Uplin2 and do0nlin2 channel and mappin relationship
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' ,ata Lin- Layer
Knowledge points:
Uplin2 and do0nlin2 L! architecture of the LTE system
-AC sub3layer protocol
%LC sub3layer protocol
"C" sub3layer protocol
'.1 Overview of ,ata Lin- Layer
The data lin2 layer mainly consists of the -AC7 %LC and "C" sub3layers. The
follo0in fiure sho0s the uplin2 and do0nlin2 L! architecture of the LTE system.
#iure ).13$ o0nlin2 L! architecture of the LTE system
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#iure ).13 Uplin2 L! architecture of the LTE system
As sho0n in the fiure above7 the ervice Access "oints &A"' is adopted bet0een
layers as the end3to3end communication interface. The "C" layer provides radio
bearer service to the upper layer7 as 0ell as %obust eader Compression &%OC' and
security protection. The A" bet0een the physical layer and the -AC layer is the
transmission channel. The A" bet0een the -AC layer and the %LC layer is the
loical channel. Therefore7 the -AC layer is mainly responsible for mappin bet0een
the loical channel and the transmission channel and multiple@in multiple loical
channels &such as the radio bearer' to a unified transmission channel &such as the
transport bloc2'.
n the uplin2 and do0nlin2 architecture of the LTE system7 the functions of each sub3
layer are basically the same. The main difference is that the do0nlin2 reflects the
situation of the net0or2 side and processes multiple users 0hile the uplin2 reflects the
situation of the terminal side and only processes one user.
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Chapter , 6A Layer "rotocol
'." *AC S!/layer Protocol
'.".1 #!nctions of *AC S!/layer
The -AC protocol sub3layer of the LTE system mainly provides effective connection
bet0een the %LC layer service and physical layer. #rom this perspective7 the -AC
layer supports the follo0in functions:
? -appin bet0een the loical channel and transmission channel
? electin transport format7 for instance7 selectin the transport bloc2 si9e and
modulation scheme as the input parameters and providin them to the physical
layer
? "riority manaement of the loical channel in one UE or multiple UEs
? Error correction throuh the A%D mechanism
? "addin
? %LC "U multiple@in and demultiple@in
? -easurin and reportin the service volume
Compared 0ith the -AC layer in the e@istin U-T system7 the -AC layer of the
LTE system has ne0 feature. That is7 each cell has only one -AC entity to achieve all
functions relevant to the -AC layer.
n conclusion7 the service provided to the upper layer by the -AC layer mainly
includes data transmission and radio resource allocation. The service provided to the
-AC layer by the physical layer includes data transmission7 A%D ac2no0ledement
sinalin7 schedulin reuest sinalin and measurement.
'."." Logical Cannel
The -AC layer can provide data transmission service over the loical channel. n this
0ay7 a roup of loical channels are defined by the -AC layer for different types of
data service. Each loical channel has the type definition of transmitted messaes.
Accordin to the type of transmitted messae in each channel7 the loical channel is
divided into t0o cateories: control channel and traffic channel.
1. Control channel: The control channel is responsible for transmittin the control
plane messaes. The control channels provided by the -AC layer have the
follo0in types:
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"rinciple of LTE "rotocol
? /roadcast Control Channel &/CC': The /CC is a do0nlin2 channel used to
broadcast the system control messaes
? "ain Control Channel &"CC': The "CC is a do0nlin2 channel used to
transmit the pain messaes. This channel is used 0hen the net0or2 is una0are
of the cell position of the UE
? Common Control Channel &CCC': The CCC is an uplin2 and do0nlin2 t0o3
0ay channel used to transmit the control messaes bet0een the UE and net0or2.
This channel is used 0hen there is no %%C connection bet0een the UE and
net0or2
? -ulticast Control Channel &-CC': The -CC is a point3to3multipoint
&"T-"' do0nlin2 channel used to transmit the -/- schedulin and control
messaes from the net0or2 to the UE. This channel is only used for the UE to
receive the -/- service.
? edicated Control Channel &CC': The CC is a point3to3point &"T"' t0o3
0ay channel used to transmit the dedicated control messaes bet0een the UE
and net0or2. This channel is used 0hen there is %%C connection in the UE.
!. Traffic channel: The traffic channel is responsible for transmittin the user plane
messaes. The traffic channels provided by the -AC layer have the follo0in
types:
? edicated Traffic Channel &TC': The TC is a "T" channel and is used to
transmit the user plane messaes by a UE. t can be a conte@t t0o30ay channel.
? -ulticast Traffic Channel &-TC': The -TC is a "T-" do0nlin2 channel
used to transmit data from the net0or2 side to the UE. This channel is only used
for the UE to receive the -/- service.
'.".' *apping etween Logical Cannel and Transmission Cannel
Compared 0ith the U-T system7 types of the loical channel and transmission
channel in the LTE system decrease reatly. The mappin relationship is simpler.
The mappin relationship bet0een the loical channel and transmission channel is as
sho0n in the fiure belo0. #or the loical channel in uplin27 the CCC7 CC and
TC are all mapped to the UL3C in transmission channel. #or the loical channel
in do0nlin27 the "CC is mapped to the "C7 the /CC is mapped to the /C or
L3C7 and the CCC7 CC and TC are all mapped to the L3C in the
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Chapter , 6A Layer "rotocol
transmission channel.
#iure ).!3( -appin relationship bet0een the loical channel and transmission channel
'.".( Key Tecnologies of *AC Layer
'.".(.1 Priority Processing
"riority processin is a main function of the -AC layer. The process of priority
processin is to select one pac2et from different 0aitin ueues and transfer it to the
physical layer 0hich then transmits it throuh the radio interface. n vie0 of the
transmission of different information flo0s such as pure user data7 E3UT%A6 sinalin
&TC loical channel' and E"C sinalin &CC loical channel'7 the process is
very complicated. f the transmitted data is not correctly received7 0hether to
retransmit the data is related to priority processin. Therefore7 the process of priority
processin is closely relevant to the A%D.
/esides7 the -AC layer of the net0or2 side is responsible for priority processin of the
uplin2 since it needs to select from all uplin2 schedulin reuest messaes of multiple
terminals sharin the UL3C transmission channel7 as sho0n in the fiure belo0.
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"rinciple of LTE "rotocol
#iure ).!31* "riority processin of the e6ode/ -AC layer
n uplin2 transmission7 the -AC layer at the terminal side just multiple@es its o0n
multiple uplin2 data flo0s and decides 0hether to transmit the uplin2 schedulin
reuest or the uplin2 data. o0ever7 in the shared channel of the do0nlin27 the e6ode/
shall consider flo0s &or loical channel' transmitted to all users in the cell.
'.".(." +asic Principle of 0A$
The basic principle of A%D is to buffer the data that are not correctly received and
combine the retransmission data and oriinal data. The soft combination mode adopted
in practice depends on the A%D Combinin mechanism. n the Chase Combinin
&CC' proram7 the data sent for the first time is the same as the retransmitted data and
the receivin end combines the 0hole data bloc2. n the ncremental %edundancy &%'
proram7 some ne0 verification information is added to the error bloc2 and the codedsymbols received by the terminal end contain the information different from the data
transmitted for the first time.
n the E3UT%A67 the A%D adopts multiple parallel channels in processin &63
"rocess'. =hen one process 0aits for the AC586AC5 messae7 other processes 2eep
transmittin data.
The A%D adopts the self3adaptation asynchronous retransmission mechanism for the
do0nlin2 and the synchronous retransmission mechanism for the uplin2. n the
synchronous mechanism7 the data can only be retransmitted accordin to the subframe
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Chapter , 6A Layer "rotocol
6o. transmitted for the first time. n the asynchronous retransmission mechanism7 the
retransmitted data can be transmitted at any time.
The adoption of the synchronous mode in uplin2 is to reduce the protocol overhead.
ince the receivin end can acuire the subframe 6o. 0hen the synchronous mode is
adopted7 there is no need to mar2 the channel 6o. for A%D processin.
The A%D of the E3UT%A6 and that of )< "A and E3C8U"A are similar.
'.' $LC S!/layer Protocol
The %adio Lin2 Control &%LC' protocol is to deliver the data to the peer3end %LC
entity. Therefore7 the %LC has three modes: Transparent -ode &T-'7 Unac2no0leded
-ode &U-' and Ac2no0leded -ode &A-'. The overall architecture of the %LC layer
in fiure belo0 reflects the three transmission modes.
TransmittingT* $LC entity
receivingT* $LC entity
receivingT* $LC entity
TransmittingT* $LC entity
Transmitting2* $LC entity
$eceiving 2*$LC entity
$eceiving 2*$LC entity
Transmitting2* $LC entity
A* $LC entity
A* $LC entity
e3+
2E
SAP.etween
!pperlaters
Logicalcanne
l
Logicalcanne
l
SAP.etween
!pperlaters
2pper layer
Lower layer
2pper layer
Lower layer
#iure ).)311 Overall architecture of the %LC layer
The T- mode is the simplest and does not chane any upper layer data. This mode is
typically applied in the transmission of the /CC or "CC loical channel. t needs
not to ma2e any special process on the %LC layer. The transparent mode entity of the
%LC layer receives the data from the upper layer and transmits it to the lo0er -AC
layer 0ithout any chane. There is no increase in %LC header7 data sementation or
concatenation.
The U- mode can detect the loss of data pac2et7 order and reassemble the data pac2et.
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"rinciple of LTE "rotocol
The U- mode can be used in any dedicated or multicast loical channel. The specific
usae depends on the type of application and e@pected Do. %eorderin the data pac2et
is to order the received disordered data.
The A- mode is the most complicated mode. /esides features supported by the U-
mode7 the A- %LC entity can reuire its peer entity to retransmit the data pac2et 0hen
detectin pac2et lossF that is7 the A%D mechanism. Therefore7 the A- mode is only
applied in the CC or TC loical channel.
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Chapter , 6A Layer "rotocol
=hen the transmittin T- %LC entity forms the %LC U to the T- &transparent
mode data' "U7 it 0ill not sement or concatenate the %LC U7 nor add any %LC
header. =hen the receivin T- %LC entity receives the T- "U7 it 0ill deliver the
T- "U &%LC U in fact' to the upper layer.
'.'." 2* *ode
The U- %LC entity can deliver8receive the %LC "U throuh the L8UL CC7
L8UL TC7 -CC or -TC loical channel7 as sho0n in the fiure belo0.
#iure ).)31) U- mode transmission
=hen the transmittin U- %LC entity forms the %LC U to the U-
&unac2no0leded mode data' "U7 it 0ill sement or concatenate the %LC U so
that the U- "U fits for the total si9e of %LC "U indicated by the lo0er layer. t
0ill indicate the total si9e of %LC "U 0hen the do0nlin2 notifies the specific
transmission opportunity and add %LC header to the U- "U.
=hen the receivin U- %LC entity receives the U- "U7 it detects if the U-
"U has been received as a duplicate and discards the duplicated U- "U. f the
received one is disordered7 the entity reorders the U- "U. At the same time7 it
detects the lo0er layer. f it finds that the U- "U is lost7 to avoid overmuch delay
in reorderin7 the entity reassembles the %LC U from the ordered U- "U and
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"rinciple of LTE "rotocol
delivers the %LC U in seuence to the upper layer. f it detects that a U- "U
pac2et of a specific %LC U is lost and the received U- "U cannot be
encapsulated to the %LC U7 the entity discards these U- "U pac2ets.
'.'.' A* *ode
The A- %LC entity can deliver8receive the %LC "U throuh the L8UL CC or
L8UL TC loical channel7 as sho0n in the fiure belo0.
#iure ).)31+ A- mode transmission
=hen the transmittin A- %LC entity forms the %LC U to the A-
&ac2no0leded mode data' "U7 it 0ill sement or concatenate the %LC U so that
the A- "U fits for the total si9e of %LC "U indicated by the lo0er layer in
specific occasion. =hen the transmittin A- %LC entity sements and retransmits the
A- "U formed from the upper layer %LC U or the A- "U formed from the
%LC "U7 it 0ill contain relevant %LC header in the %LC "U.
=hen the receivin A- %LC entity receives the %LC "U7 it detects if the %LC "U
has been received as a duplicate and discards the duplicated %LC "U. f the received
one is disordered7 the entity reorders the %LC "U. At the same time7 it detects the
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Chapter , 6A Layer "rotocol
lo0er layer. f it finds that the %LC "U is lost7 it reuires the peer3end A- %LC
entity to retransmit the %LC "U. ubseuently7 it assembles the ordered %LC "U to
the %LC U and delivers the %LC U in seuence to the upper layer.
'.'.( $LC P,2 Str!ct!re
The format and parameters of the %LC "U are as sho0n in the fiure belo0.
#iure ).)31, tructure of the %LC "U
The %LC header contains the 6. of the %LC "U 0hich is different from that of theU.
One %LC "U consists of the follo0in sements: The last sement of 6o. i U
concatenates n complete Us and then concatenates the first sement of 6o. iKnK1
U &n shall be an inteer reater than or eual to *'.
ifferent from the si9e of the "LC "U in the U-T system 0hich is uasi3static7 the
si9e of the %LC "U in the LTE system can vary dynamically. #or the hih3speed data7
to adopt a bi "U can acuire smaller overheadF 0hile for the lo03speed data7 adopt a
small "U. Therefore7 supportin dynamic "U in the LTE system can achieve the
variation of data rate from thousands bits per second to hundreds meabits per second.
ince the %LC schedulin and rate self3adaptation mechanism are at the e6ode/7 the
LTE system can easily support the dynamic "U mechanism.
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"rinciple of LTE "rotocol
'.( P,CP S!/layer Protocol
'.(.1 #!nctions of P,CP Layer
The "C" protocol layer of the LTE system mainly transmits or receives the pac2et
data of the peer "C" entity. This sub3layer mainly completes the follo0in functions:
" header compression and decompression7 data and sinalin encryption7 and interity
protection of sinalin. The fiure belo0 sho0s the main function model of the "C"
layer user plane and control plane.
#iure ).+314 "C" layer protocol model
On the control plane7 encryption and interity protection are reuired functions. On the
user plane7 robust header compression is reuired function. ata encryption is optional.
The data here can either be the user data or application layer sinalin7 such as " and
%TC".
The "C" provides service to the %%C at the UE side and the upper layer of user plane
or the relay at the e6ode/ side7 includin user plane data transmission7 control plane
data transmission7 header compression7 encryption and interity protection.
The service provided to the lo0er layer by the "C" layer includes transparent data
transmission service7 ac2no0leded data transmission service &includin indication of
successful "C" "U transmission'7 and unac2no0leded data transmission service
&transmission in seuence7 pac2et copyin or discardin'.
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Chapter , 6A Layer "rotocol
pecifically spea2in7 the user plane of the "C" layer has the follo0in functions:
? eader compression and decompression7 only supportin one compressionalorithm7 that is7 the %OC alorithm
? User plane data transmissionF that is7 transmittin the "C" U data received
from the 6A sub3layer to the %LC layer7 and vice versa
? eliverin the upper3layer "U in seuence in the "C" re3construction
process of the %LC A-
? etectin the lo0er3layer U repeatedly in the "C" re3construction process
of the %LC A-
? %etransmittin the "C" U durin %LC A- handover
? ata encryption
? iscardin U in uplin2 based on the timer
? The "C" layer control plane has the follo0in functions:
? Encryption and interity protection
? Control plane data transmissionF that is receivin the "C" U data from the
%%C layer and for0ardin it to the %LC layer7 and vice versa
? Compared 0ith the "C" layer in the U-T system7 the "C" layer in the LTE
system has the follo0in features:
? imple compression alorithm7 only supportin one compression alorithm
? 6ot supportin none3loss relocation
? upportin encryption
'.(." P,CP Entity
The "C" entity is at the "C" layer. #or a UE7 multiple "C" entities can be
defined. Each "C" entity carryin the user plane data can be confiured to use the
header compression technoloy. Each "C" entity carries the data of a radio bearer. n
the current protocol version7 only the %OC protocol is supported. Each "C" entity
can use one %OC instance at most.
=hether a "C" entity is relevant to the control plane or user plane depends on 0hat
2ind of radio bearer the entity carries the data for. The follo0in fiure sho0s the
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"rinciple of LTE "rotocol
functional schematic diaram of the "C" layer.
#iure ).+31$ #unctional schematic diaram of the "C" layer
'.(.' Str!ct!re of P,CP P,2
The fiure belo0 sho0s the structure of the "C" "U.
#iure ).+31 tructure of the "C" "U
Amon them7 the lenth of the "C" "U is an inteer multiple of a byte and the
lenth of the "C" header can be one or t0o bytes.
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( $$C Layer Protocol
Knowledge points:
#unctions of the %%C layer
%%C layer protocol states and state transition
Typical sinalin process of the %%C layer
(.1 #!nctions of $$C Layer
The radio resource control &%%C' layer is the most important sinalin protocol
supportin multiple functions bet0een the terminal and e6ode/s. #unctions of the
%%C layer include:
? ystem messae bet0een the broadcast 6A layer and the A layer
? "ain function &e@ecuted throuh the "CC loical channel'
? %%C connection setup7 maintenance and release7 includin the allocation of
temporary s bet0een the UE and the E3UT%A67 and the radio bearin
confiuration of sinalin
? ecurity function7 includin 2ey manaement
? Establishment7 modification and release of end3to3end radio bearin
? -obility manaement7 includin UE measurement report7 report control for
inter3cell mobility and inter3%AT mobility7 inter3cell handover7 UE cell selection
and reselection7 and %%C conte@t transmission durin handover
? -/- service notification7 and radio bearin establishment7 modification and
release of the -/- service
? Do manaement function
? UE measurement report and measure control
? Transmission of the 6A messae
? nterality protection of the 6A messae
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(." $$C State
The main function of the %%C protocol is to manae the connection bet0een the
terminal and the E3UT%A6 access net0or2. The follo0in fiure sho0s the state of
the %%C protocol.
#iure +.!31( %%C tate
n fact7 each %%C protocol state represents a 2ind of connection status and describes
ho0 the net0or2 and terminals process the mobility of terminal7 pain messaes and
system information broadcast.
The follo0in table describes specific features of the idle and connected states of the
%%C layer.
Table +.!31 escription of LTE %%C layer state
#eatures of the idle state &%%C3LE state' #eatures of the connected state &%%C3
CO66ECTE'
"L-6 selectionF
ystem information broadcastF
iscontinuous receivin painF
The cell reselects the mobilityF
The UE has a uniue 0ithin the trac2in
area &TA' raneF
6o %%C communication conte@t is saved in
the e6ode/.
The UE has an %%C connection.
The UE has communication conte@t in the E3
UT%A6.
The E3UT%A6 2no0s 0hich cell the UE belons to.
ata can be transmitted and received bet0een the
net0or2 and terminal.
-obility manaement of net0or2 control7 includin
handover or net0or2 assisted cell chane &6ACC' to
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Chapter , 6A Layer "rotocol
#eatures of the idle state &%%C3LE state' #eatures of the connected state &%%C3
CO66ECTE'
The terminal can monitor and control the channel to
confirm 0hether the net0or2 has confiured shared
channel resources for it.
The e6ode/ can confiure the discontinuous
receivin &%;' period accordin to the active
situation of the terminal7 0hich saves the battery and
raises the utili9ation of radio resource.
n the U-T system7 the UE has five states: idle7 CELL3C7 CELL3#AC7 CELL3
"C7 and U%A3"C. Compared 0ith the %%C state in the U-T system7 the %%C
state in the LTE system is reatly reduced7 0hich indicates that the %%C state machine
in the LTE system is simpler and the comple@ity of the system is further reduced.
Li2e the U-T system7 after the startup7 the terminal selects a proper cell from the
desinated "L-6 to reside. =hen the UE resides in a cell7 it can receive system
information and cell broadcast messaes. 6ormally7 the UE needs to e@ecute the
reistration process for the startup for the first time7 0hich completes mutual
authentication and enables the net0or2 to acuire the basic information of the UE.
After that7 the UE can be in idle state all the time until the %%C connection is
established. The UE can enter into the connected state only after the %%C connection is
set up. ere7 the UE can interact data 0ith the net0or2. After the UE releases the %%C
connection7 the UE transits from the %%C3CO66ECTE state to the %%C3LE state.
(.' Signaling Process of $$C Layer
(.'.1 +roadcast Process
The UE acuires the A and 6A system information broadcasted by the E3UT%A6
throuh the system information reuest process.
E3UT%A6
MasterInformationBlock
UE
SystemInformationBlockType1
SystemInformation
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"rinciple of LTE "rotocol
=hen the e6/ cell is established successfully7 the e6/ sends the system information at
the air port: -asternformation/loc27 ystemnformation/loc2Type17
ystemnformation7 mainly includin the cell selection and reselection information of
the UE in idle state.
(.'." Paging Process
"ain is used to notify the UE in LE or CO66ECTE state of the chane of
system information.
After the pain information is submitted to the upper layer7 the %%C connection setup
process may be initiated subseuently.
(.'.' $$C Connection Set!p
This process is used to set up the %%C connection7 includin conflict resolution and
%/1 setup. At the same time7 the initial 6A messae is send from the UE to the E3
UT%A6.
(.'.( $$C Connection $econfig!ration
This process is used to modify the %%C connection and mainly achieves the follo0in
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Chapter , 6A Layer "rotocol
functions:
? %/ setup7 modification and release
? E@ecute s0itchover
? Transparent transmit the 6A messae from the E3UT%A6 to the UE.
(.'.) $$C Connection $eestalisment
This process is used to reestablish the %%C connection.
Application scenarios:
? %adio lin2 failure
? 0itchover failure
? nter3%AT failure
? nterity chec2in failure
? %%C connection reconfiuration failure
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"rinciple of LTE "rotocol
(.'.4 $$C Connection $elease
This process is used to release the %%C connection and mainly achieves the follo0in
functioins:
? %elease the radio resource
? %elease the established radio bearer
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) 3AS Layer Protocol
Knowledge points:
A model and 6A model
6A layer protocol states and state transition
-appin relationship bet0een 6A layer functions and basic procedures of the 6A
layer
).1 AS and 3AS *odel
The follo0in fiure sho0s the A and 6A model. The model is divided into t0o
layers>A and 6A and crosses multiple entities such as the terminal7 radio access
net0or2 and pac2et core net0or2.
#iure ,.13!* A and 6A -odel
The A layer is mainly responsible for connection 0ith the 0ireless interface. t is not
limited to the 0ireless part of the radio access net0or2 and terminal7 but also supports
some special functions relevant to the core net0or2.
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supports the follo0in functions:
? %adio bearer manaement: includin the allocation7 setup7 modification andrelease of radio bearer
? %adio channel processin: includin channel codin and modulation
? Encryption: t only refers to its o0n encryption process. The initiali9ation of
encryption and selection of security alorithm are responsible by the 6A layer.
/esides7 the end3to3end encryption dependin on the application may also be
used7 such as J"6.
? -obility manaement: such as handover7 cell selection and reselection
/y contrast7 the 6A layer is mainly responsible for access irrelevant or radio access
independence functions and process and achieves the follo0in functions:
? ession manaement: includin session setup7 modification and release7 as 0ell
as Do neotiation.
? User manaement: includin user data manaement7 adherence and de3
adherence
? ecurity manaement: includin the authentication bet0een the user and
net0or2 and encryption initiali9ation
? Charin
n the
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Chapter , 6A Layer "rotocol
&E--' 0hich supports
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"rinciple of LTE "rotocol
UT%A6 system and transition bet0een states.
#iure ,.!3!1 "rotocol states in the E3UT%A6
The above fiure sho0s that the LTE3LE state corresponds to the UE in the %%C3
LE state7 the LTE3ACTJE state corresponds to the UE in the %%C3CO66ECTE
state and the LTE3ETACE state corresponds to the UE 0ithout the %%C entity.
=hen po0er3on7 the terminal enters the LTE3ETACE state. Later7 the terminal
e@ecutes the reistration process and enters the LTE3ACTJE state. Throuh this
process7 the terminal can acuire the C3%6T7 TA3 and " address and set up security
contacts throuh the authentication process. f there is no other service7 the terminal
can release the C3%6T. t enters the LTE3LE state after acuirin the discontinuous
receivin &%;' period assined to the user to receive pain channel. =hen the user
has ne0 service reuirement7 it can acuire the C3%6T throuh %%C connection
reuest &random access process'. The terminal then transits from the LTE3LE state to
the LTE3ACTJE state. n the LTE3ACTJE state7 if the terminal moves to the
unac2no0leded "L-6 area or the de3reistration process is e@ecuted7 the C3%6T7
TA3 and " address of the user 0ill be 0ithdra0n and the terminal enters the LTE3
ETACE state. #or the user in the LTE3LE state7 if the TA rene0al process
e@ecuted by the user periodically times out7 the TA3 and " address 0ill be
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Chapter , 6A Layer "rotocol
0ithdra0n and the user terminal 0ill be s0itched to the LTE3ETACE state.
).' +asic Proced!res of 3AS
).'.1 +asic Proced!res of 3AS
The basic procedures of 6A is as sho0n in the table belo0.
Table ,.)31 /asic procedures of the 6A
/asic procedure nitial messae %esponse messae
uccessful Unsuccessful
Authentication
procedure
AUTE6TCATO6
%EDUET
AUTE6TCATO6
%E"O6E
AUTE6TCATO6
%EECT
Attach procedure for
E" services
ATTAC %EDUET ATTAC ACCE"T ATTAC %EECT
etach procedure ETAC %EDUET ETAC ACCE"T
"ain procedure %EDUET "A
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"rinciple of LTE "rotocol
etach procedure UE reuested "6 connectivity procedure
Trac2in area updatin procedure UE reuested "6 disconnect procedure
ervice reuest procedure UE reuested bearer resource allocation procedure
"ain procedure UE reuested bearer resource modification procedure
Transport of 6A messaes procedure E- information reuest procedure
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