ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 24 – Basics of 3G...
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Transcript of ECE 5221 Personal Communication Systems Prepared by: Dr. Ivica Kostanic Lecture 24 – Basics of 3G...
ECE 5221 Personal Communication Systems
Prepared by:
Dr. Ivica Kostanic
Lecture 24 – Basics of 3G – UMTS (3)
Spring 2011
OSI Communication model
• Each layer communicates only with two adjacent layers and its peer on the other side
• Each layer receives services from the layer below and provides services to the layer above
Page 2
• Intermediate communication nodes require layers 1 through 3
• Internal operation within each layer is independent of the internal operation in any other layer
A pplica tion Layer
P resenta tion Layer
S ession Layer
T ransport Layer
N etwork Layer
D ata L ink Layer
P hysica l Layer
A pp lica tion Layer
P resenta tion Layer
S ession Layer
T ransport Layer
N etwork Layer
D ata L ink Layer
P hysica l LayerP hysica l Layer
D ata L ink Layer
N etwork Layer
Physical M edium Physical M edium
Node A Node B Node C
Peer to peer protocols • WCDMA interfaces described using OSI model
• OSI = Open System Interconnect
• Developed by ISO as a general model for computer communication
• Used as a framework for development and presentation of most contemporary communication standards
Note: WCDMA covers Layers 1-3 of OSI Model
Page 3
UMTS Protocol stack
• UMTS offers new Access stratum protocol stack
• Non-Access Stratum is largely inherited from GSM
• First three layers of the protocol stack are part of UTRAN
Note: SMS exists on both circuit switched and packet switched side
4
UMTS CS protocols – control plane
• Control plane – carries signaling
• RNC terminates the Access Stratum (AS)
• RRC, RLC and MAC terminate at RNC
• PHY terminates at Node B except for outer loop power control
• RAN (access stratum) acts as transport for NAS
Note: UTRAN protocols are layered in an architecture that follows OSI model
5
UMTS CS protocols – user plane
• User plane – caries user data
• Application – end to end protocol
• Access stratum the same for both control plane and user plane
6
UMTS PS protocols – control plane
• Control plane for packet data
• Very similar to control plane for PS
• Identical access stratum
7
UMTS PS protocols – user plane
• Additional protocol PDCP
• PDCP – compression of IP headers
• PDCP may or may not be used
8
Layout of the Access Stratum
• Two planes– User plane - user data– Control plane – signaling
• User data enters access through radio bearers (RABs)
• Signaling is handled by RRC• Upper layer signaling –
encapsulated through RRC messages (direct transfer)
• RRC has a capability of reconfiguring all lower layers
UMTS-FDD PHY frame structure
• UMTS-FDD PHY frame structure is based on 10ms frames
• Frames are broken in 15 time slots
• The number of bits/slot is variable
• Chip rate is always the same (3.84 Mchips/sec)
Page 10
F0 F1 F71
S0 S1 S14
S uperfram e = 72 F ram es
Fram e = 15 S lo ts
S lo t = 2560C hips
720 m s
10 m s
The num ber o f b its per s lo t varies
0 .667 m s
U ser D ata
C ontro lIn form ation
UMTS-FDD DL processing• There are 6 steps in DL
PHY processing– I/Q separation– Variable spreading– Scrambling– Gain adjustment– Sync addition– Modulation
Page 11
S
S /P O V S F
X
X
X X
S /P O V S F
X
X
X X
S
X
X
M odulation
V ariab leS pread ing
S cram bling
G ainad justm ent
S ync add ition
C hanne l 1
C hanne l n
R b1
R bn
R b1 /2
R b1 /2
R bn /2
R bn /2
R c=3.84M c/sec
R c
R c
R c
R c
S C 1
S C n
G 1
G n
G p
G sR ea l S igna ls
C om plex S igna ls
P -S C H
S -S C H
I/QS epara tion
I
Q
I
Q
Note: Number of channels depends on number of active users. P-SCH and S-SCH are always transmitted
W-CDMA DL Modulation
• UMTS-FDD uses simple QPSK modulation scheme• Complex code sequence is split into real and imaginary part and modulated
using carriers in quadrature
Page 12
W-CDMA Modulation
• UMTS-FDD uses root-raised cosine for the shaping filter
• The roll-off is = 0.22aPage 13
20
41
1cos41sin
CC
CCC
Tt
Tt
Tt
Tt
Tt
tRC
-1 -0.5 0 0.5 1-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
time [microsec]
sha
pin
g fi
lter
imp
ulse
re
spo
nse
-5 0 5-60
-50
-40
-30
-20
-10
0
10
frequency [MHz]
ga
in [d
B]
5MHz
Impulse response of the shaping filter Frequency response of the shaping filter
Analytical expression of the shaping filter impulse response
Note: only 30dBc on the sidebands – may cause interference to GSM in non 1-1 overlay scenarios
W-CDMA DL variable spreading
• Different data channels have different rates• The chip rate is always the same• W-CDMA supports variable spreading on the DL• Variable spreading is accomplished through use of orthogonal codes of
different length
Page 14
Spreading Factor User data rateAfter coding[Kb/ sec]
Approximate ratebefore coding
[Kb/ sec]
512 15 1-3256 30 6-12128 60 42-5264 120 ~ 4532 240 ~ 10516 480 ~ 2158 960 ~ 4504 1920 ~ 930
4, with 3 parallel codes 5760 ~ 2300
UMTS-FDD available DL data rates
UMTS-FDD provides high data rates through• variable spreading• code aggregation
User data rates assume 1/2 convolutional encoding
W-CDMA scrambling codes
• OVSF codes provide orthogonality between signals coming from the same BTS – form of channelization
• Scrambling codes allow mobile to distinguish signals coming from different base stations
• Scrambling codes do not change signal bandwidth
• Decoding a signal from a user is in 2 steps
– Descrambling the signal from the Node B
– De-spreading the signal from individual user
Page 15
Signal from BS2
BaseStation 1 Base
Station 2
S ignal from BS1
Frequancy
W -C D M Asigna ls
W-CDMA scrambling codes
• UMTS-FDD uses 8192 complex scrambling codes
• The codes are selected as parts of a 218 -1 long gold sequence (good correlation prperties)
• Each of the codes are associated with left and right alternative scrambling code
Page 16
8192 Scram bling codes
SC0
SC1
SC2
SC15
SC16
SC17
SC18
SC31
SC32
SC33
SC47
SC8176
SC8177
SC8178
SC8191
P rim ary C odes
S econdaryC odes
SC34
512
• Scrambling codes are 38400 chips long (10ms)
• Scrambling code repeats every frame• Organized in 512 groups of 16 codes• The first code in each group is
declared as the primary scrambling code (PrSC)
• PrSC are used for cell identification
Scrambling code tree
W-CDMA synchronization codes
• Synchronization codes are used for system detection
• They are 256 chips long complex codes
• One primary and 64 secondary codes
• Secondary codes consist of 15 code words
• Secondary codes remain unique under cyclic shifts smaller than 15
Page 17
• A cell is allocated one primary synchronization code
• The primary code is the same for all cells in the system
• Secondary code points to a group of primary scrambling codes
S ynchron iza tion C odes
P rim ary S econdary
P S C S S C 0
S S C 1
S S C 63
Note: PSC allows mobile to synchronize to the time slots. SSC allows mobile to synchronize with the beginning of frame.
W-CDMA primary scrambling codes
• There are 512 primary scrambling codes• They are divided in 64 groups of 8 codes• Each cell is assigned one primary code
• Primary scrambling code is used to provide orthogonality between different BS
• Primary scrambling code is broadcast on the Common Pilot Channel (CPICH)
Page 18
512 P rim ary S cram bling C odes
G roup 0 G roup 1 G roup 63
SC0
SC16
SC32
SC112
SC128
SC144
SC240
SC160
SC8064
SC8080
SC8096
SC8176
Note: after decoding SSC, the mobile needs to consider only 8 out of 512 PrSC
W-CDMA code assignment example
• Primary sync code is the same for all cells
• Secondary sync code number is the same as the group of the primary pSC
Page 19
pSC: SC16(1)SSC: 0
pSC: SC128(8)SSC: 1
pSC: SC256(16)SSC: 2
pSC: SC32(2)SSC: 0
pSC: SC64(4)SSC: 0
pSC: SC80(5)SSC: 0
pSC: SC8064(504)SSC: 63
pSC: SC5760(360)SSC: 45
pSC: SC4096(256)SSC: 32
A
B
C
pSC - Primary Scrambling CodeSSC - Secondary Sync Code
Task: use previous two slides to verify code assignments for the above cells
Note: in practice network operator assigns only PrSC. SSC is assigned automatically on the basis of PrSC assignment
W-CDMA UL processing - dedicated channels
• There are 5 steps in the UL DCHs processing
– Spreading– Gain adjustment– Complex addition– Scrambling – Modulation
Page 20
S
S
X X
X X
X X
X X
X X
X X
X X
R /C X
D P D C H _1
D P D C H _3
D P D C H _5
D P D C H _2
D P D C H _4
D P D C H _6
D P C C H
Cd1 Gd
Gd
Gd
Gd
Gd
Gd
Gd
I
Q
SC
S pread ingG ain
A d justm ent S cram bling
M odu la tion
C om plexA dd ition
Cd2
Cd3
Cd5
Cd4
Cd6
Cc
DPDCH - Dedicated Physical Data ChannelDPCCH - Dedicated Physical Control Channel
Note: transmission from a single mobile can aggregate multiple codes to achieve higher data rate
W-CDMA UL variable spreading • Variable data rates are allowed on U DPDCH• Variable data rate achieved through
– variable spreading 4 to 256– code aggregation - up to 6 parallel codes
• if code aggregation is used, spreading for all DPDCH is 4• UL DPCCH is a constant rate channel ~ 15kb/sec (assigned code C256,0)
Page 21
Spreading Factor User data rate[Kb/ sec]
Approximate ratebefore coding
[Kb/ sec]
256 15 1-3128 30 6-1264 60 42-5232 120 ~ 4516 240 ~ 1058 480 ~ 2154 960 ~ 450
4, with 6 parallel codes 5740 ~ 2300
User data rates assume 1/2 convolutional encoding