ECE 5221 Personal Communication Systems
Prepared by:
Dr. Ivica Kostanic
Lecture 24 – Basics of 3G – UMTS (3)
Spring 2011
OSI Communication model
Application Layer
Peer to peer protocols
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
Network Layer
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
Physical Layer
Physical Medium
Node A
•
•
Application Layer
Physical Medium
Node B
Node C
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
Note: WCDMA covers Layers 1-3 of OSI Model
Page 2
•
•
• 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
Intermediate communication nodes
require layers 1 through 3
Internal operation within each layer is
independent of the internal operation
in any other layer
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
Page 3
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
4
UMTS CS protocols – user plane
•
•
•
5
User plane –
caries user
data
Application –
end to end
protocol
Access stratum
the same for
both control
plane and user
plane
UMTS PS protocols – control plane
•
•
•
6
Control plane
for packet data
Very similar to
control plane
for PS
Identical
access stratum
UMTS PS protocols – user plane
•
•
•
7
Additional protocol
PDCP
PDCP –
compression of IP
headers
PDCP may or may
not be used
Layout of the Access Stratum
•
Two planes
–
–
•
•
•
•
8
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
Part 6
ELEMENTS OF PHY LAYER
PROCESSING
9
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)
720 ms
User Data
F0
Control
Information
F1
F71
Superframe = 72 Frames
10 ms
S0
S1
S14
Frame = 15 Slots
0.667 ms
Slot = 2560Chips
The number of bits per slot varies
Page 10
UMTS-FDD DL processing
•
I
X
Rb1 /2
Channel 1
Rb1
Rc
S/P
Rb1 /2
OVSF
Q
X
I
Rbn
SC1
G1
S
Rc
S/P
Rbn /2
Rc
OVSF
Q
X
X
Rc
SCn
X
P-SCH
Variable
Spreading
Scrambling
S
X
Real Signals
Page 11
Modulation
X
Gp
S-SCH
Complex Signals
I/Q separation
Variable spreading
Scrambling
Gain adjustment
Sync addition
Modulation
Gn
Gain
adjustment
Rc=3.84Mc/sec
I/Q
Separation
–
–
–
–
–
–
X
X
Rbn /2
Channel n
X
There are 6 steps in DL
PHY processing
Gs
Sync addition
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
Impulse response of the shaping filter
Frequency response of the shaping filter
1 .2
10
1
0
0 .8
-1 0
0 .6
gain [dB ]
shaping filter impulse response
5M Hz
0 .4
-2 0
-3 0
0 .2
-4 0
0
-5 0
-0 .2
-0 .4
-1
-0 .5
0
0 .5
1
tim e [m ic ro s e c ]
Analytical expression of the shaping filter
impulse response
 t



1  a   4a t cos t 1  a 
sin  
T
TC

 TC

RC0 t  =  C
2
t  
t  


1   4a
TC   TC  


Page 13
-6 0
-5
0
5
Note: only 30dBc on the sidebands –
may cause interference to GSM in
non 1-1 overlay scenarios
fre q ue nc y [M H z]
•
•
UMTS-FDD uses root-raised
cosine for the shaping filter
The roll-off is a = 0.22
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
UMTS-FDD available DL data rates
Spreading Factor
UMTS-FDD provides
high data rates through
• variable spreading
• code aggregation
512
256
128
64
32
16
8
4
4, with 3 parallel codes
User data rate
After coding
[Kb/ sec]
15
30
60
120
240
480
960
1920
5760
User data rates assume 1/2 convolutional encoding
Page 14
Approximate rate
before coding
[Kb/ sec]
1-3
6-12
42-52
~ 45
~ 105
~ 215
~ 450
~ 930
~ 2300
W-CDMA scrambling codes
•
•
•
Base
Station 1
W-CDMA
signals
Base
Station 2
Signal from BS1
Signal from BS2
Frequancy
Page 15
•
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
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
•
•
•
•
•
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
8192 Scrambling codes
512
SC0
SC16
SC32
SC8176
SC1
SC17
SC33
SC8177
SC2
SC18
SC34
SC8178
SC15
Page 16
SC31
SC47
SC8191
Primary Codes
Secondary
Codes
Scrambling
code tree
W-CDMA synchronization codes
•
Synchronization Codes
•
Primary
Secondary
PSC
SSC0
SSC1
SSC63
•
•
•
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
Page 17
•
•
•
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
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
512 Primary Scrambling Codes
Group 0
•
•
•
Group 1
SC0
SC128
SC8064
SC16
SC144
SC8080
SC32
SC160
SC8096
SC112
SC240
SC8176
There are 512 primary scrambling codes •
They are divided in 64 groups of 8 codes
•
Each cell is assigned one primary code
Page 18
Group 63
Note: after decoding
SSC, the mobile needs
to consider only 8 out
of 512 PrSC
Primary scrambling code is used to provide
orthogonality between different BS
Primary scrambling code is broadcast on
the Common Pilot Channel (CPICH)
W-CDMA code assignment example
A
pSC: SC16(1)
SSC: 0
pSC: SC256(16)
SSC: 2
•
B
pSC: SC80(5)
SSC: 0
pSC: SC32(2)
SSC: 0
pSC: SC128(8)
SSC: 1
C
pSC: SC4096(256)
SSC: 32
pSC: SC64(4)
SSC: 0
pSC: SC8064(504)
SSC: 63
pSC: SC5760(360)
SSC: 45
•
Primary sync code
is the same for all
cells
Secondary sync
code number is
the same as the
group of the
primary pSC
pSC - Primary Scrambling Code
SSC - Secondary Sync Code
Task: use previous two slides to verify code assignments for the above cells
Page 19
Note: in practice network operator assigns only PrSC. SSC is
assigned automatically on the basis of PrSC assignment
W-CDMA UL processing - dedicated
channels
•
DPDCH_1
DPDCH_3
DPDCH_5
DPDCH_2
Cd1
Gd
X
X
Cd3
Gd
X
X
Cd5
Gd
X
X
Cd2
Gd
X
Cd4
X
Gd
X
X
Cd6
Gd
X
X
Cc
Gd
X
X
S
I
R/C
X
There are 5 steps in the UL
DCHs processing
– Spreading
– Gain adjustment
– Complex addition
– Scrambling
– Modulation
Modulation
SC
DPDCH_4
DPDCH_6
DPCCH
Spreading
Page 20
Gain
Adjustment
S
Complex
Addition
Note: transmission from a
single mobile can aggregate
multiple codes to achieve
higher data rate
Q
Scrambling
DPDCH - Dedicated Physical Data Channel
DPCCH - Dedicated Physical Control Channel
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)
Spreading Factor
User data rate
[Kb/ sec]
256
128
64
32
16
8
4
4, with 6 parallel codes
15
30
60
120
240
480
960
5740
Approximate rate
before coding
[Kb/ sec]
1-3
6-12
42-52
~ 45
~ 105
~ 215
~ 450
~ 2300
User data rates assume 1/2 convolutional encoding
Page 21