Internal
Principles of
the WCDMA System
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Preface
 Now, the most popular term in the mobile
communication field is WCDMA!
 What is WCDMA? And what is its benefit?
 Today, let’s go to know about WCDMA!
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Objectives
 After studying this course, you should be able to:
 Know the basic knowledge of the CDMA system.
 Master advantages of the CDMA technology
comparing
with
other
multiple
access
technologies.
 Know technical features of WCDMA FDD.
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Contents
Chapter 1
WCDMA Principle
Chapter 2
Technical Features of WCDMA
FDD
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Overview of CDMA Principles
 Radio Propagation Environment
 Multiple Access Technology and
Duplex Technology
 CDMA Principles and Rake Receiver
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Multipath Environment
Transmitted
signal
Strength of the
received signal
Time
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Fading
Transmitted data
dB
Received data
0
-5
-10
-15
-20
-25
-30
-35
-40
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Fading
Rx power (dBm)
Fast fading
-20
Slow fading
-40
-60
10
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20
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Distance (m)
30
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Frequency-Selective Fading
P(f)
P(f)
Fading
Narrowband
System
Transmit Signal
f
Received Signal
P(f)
f
P(f)
Fading
Broadband
System
Transmit Signal
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Received Signal
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f
Classification of Typical Radio Mobile Channels
 Static channel
 Pedestrian channel in typical urban areas (TU3)
 Vehicle mounted channel in typical urban areas (TU30)
 Vehicle mounted channel in rural areas (RA50)
 Vehicle mounted channel on the highway (HT120)
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Overview of CDMA Principles
 Radio Propagation Environment
 Multiple Access Technology and
Duplex Technology
 CDMA Principles and Rake Receiver
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Duplex Technology – Distinguish User’s UL and
DL Signal – FDD
 Frequency division duplex (FDD) : Distinguish the uplink and
downlink according to the frequencies.
 Adopted by the WCDMA, CDMA2000 and GSM
 Advantage: It can be easily implemented.
 Disadvantage: The spectrum utilization is low when the uplink and
downlink services (mainly the data services) are asymmetrical.
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Duplex Technology – Distinguish User’s UL and
DL Signal – TDD
 Time division duplex (TDD) : Distinguish the uplink and downlink
according to the timeslots.
 Adopted by the TD-SCDMA
 Advantage: The uplink and downlink can be allocated with different numbers
of timeslots when the uplink and downlink services are asymmetrical.
Therefore, the spectrum utilization is high.
 Disadvantage:
−
It cannot be easily implemented and needs precise synchronization. In the CDMA
system, GPS synchronization is needed.
−
When it is used with the CDMA technology, it is difficult to control interference
between the uplink and the downlink.
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Multiple Access Technology - Distinguish
Different Users
CDMA
Power
Time
y
nc
Freque
Traffic channels: different
users are assigned unique
code and transmitted over
the same frequency band,
for example, WCDMA and
CDMA2000
TDMA
Power
FDMA
e
Fre
Us
e
U
U s ser r
U s er
U s er
er
cy
en
Power
Tim
Tim
e
u
eq
Fr
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y
nc
e
qu
Traffic channels: different time slots
are allocated to different users, for
example, DAMPS and GSM
Traffic channels: different frequency bands
are allocated to different users,for example,
AMPS and TACS
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Characteristics of CDMA System
 High Spectral Efficiency
 Frequency multiplex coefficient is 1.
 Soft capacity
 Quality
 Coverage
 Interference
 Self-interference system
 A UE transmission power is interference for another UE.
 Wideband system
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Overview of WCDMA Principles
 Radio Propagation Environment
 Multiple Access Technology and
Duplex Technology
 CDMA Principles and Rake Receiver
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Questions
 Why does the WCDMA system have enhanced anti-
interference performance?
 Why is the WCDMA system more secure?
 Why are the WCDMA handsets more environment protective?
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Common Terms
 Bit, symbol and chip
 Bit (bps): the data that is obtained upon source coding and contains
information.
 Symbol (sps): the data obtained upon channel coding and interleaving.
 Chip (cps): the data obtained upon final spreading.
− The spreading rate of WCDMA is: 3.84 Mcps
 Processing gain
 It refers to the ratio of the final spreading rate to the bit rate (cps/bps).
 In the WCDMA system, the processing gain depends on the specific
service.
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Spreading Factor and Service Rate

Chip rate = symbol rate × spreading factor
 For WCDMA, if the chip rate is 3.84 MHz and the spreading factor is 4, the
symbol rate is 960 Kbps.
 For CDMA2000-1x, if the chip rate is 1.2288 MHz and the spreading factor is
64, the symbol rate is 19.2 Kbps.

Symbol rate = (service rate + check code) × channel code ×repetition
or punching rate
 For WCDMA, if the service rate is 384 Kbps and the channel code is 1/3
Turbo, the symbol rate is 960 Kbps.
 For CDMA2000-1x, if the service rate is 9.6 Kbps and the channel code is
1/3 convolutional code, the symbol rate is 19.2 Kbps.
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Basic Diagram of the WCDMA System
Source
coding
Channel
coding and
interleaving
Spreading
Scrambling
Modulation
RF
transmission
Radio link
Source
decoding
Channel
deinterleaving
decoding and
de-interleaving
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De-spreading
De-scrambling
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Demodulation
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RF
reception
Source Coding in WCDMA
 The WCDMA system adopts the adaptive multi-rate (AMR) speech coding.
 A total of eight coding modes are available. The coding rate ranges from 12.2
Kbps to 4.75 Kbps.
 Multiple voice rates are compatible with the coding modes used by current
mainstream mobile communication systems. This facilitates the design of multimode terminals.
 The system automatically adjusts the voice rate according to the distance
between the user and the NodeB, thus reducing the number of handovers and
call drop.
 The system automatically decreases the voice rate of some users according to
the cell load, thus saving power and containing more users.
Source
coding
Channel
coding
and
Interleaving
Spreading
interleaving
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Scrambling
Modulation
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RF
transmission
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Channel Coding in WCDMA
 Channel coding can enhance symbol correlation to recover
signals in the case of interference.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
 Code type
 Voice service: Convolutional code (1/2 and 1/3).
 Data service: Turbo code (1/3).
Source
coding
Channel
Interleaving
coding
interleaving
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Spreading
Scrambling
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Modulation
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RF
transmission
Interleaving
 Interleaving is used to damage symbol correlation and reduce the impact
caused by fast fading and interference of the channel.
1 2 3 4 5 6 7 8 ...
Ist interleaving
....
... 452 453 454 ……
B0
B1
B2
B3
B4
B5
B6
B7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
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.
.
....
449 450 451 452 453 454 455 456
2nd interleaving
{A4,B0}
{A5,B1} {A6,B2} {A7,B3} {B4,C0}
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{B5,C1}
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{B6,C2}
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{B7,C3}
Self Correlation and Mutual Correlation of
Code Words

Different users adopt different spreading code words,
such as x1(t) , x2(t) ….
 Self correlation determines multipath interference.
 Mutual correlation determines multiple access
interference.

Self-correlation function
 R(τ) =<x1(t) ， x1(t+τ) >

Mutual-correlation function
 V(τ) =<x1(t) ， x2(t+τ) >
Note: <x1(x) ， x2(y)> =
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1, when x1=x2 , and x = y
0, others
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Spreading Principle

Users who need to send information: UE1, UE2 and UE3
 UE1 uses c1 for spreading: UE1 x c1
 UE2 uses c2 for spreading: UE2 x c2
 UE3 uses c3 for spreading: UE3 x c3
 c1, c2 and c3 are orthogonal to each other

Information sent: UE1 x c1 + UE2 x c2 + UE3 x c3
Source
coding
Channel
Interleaving
coding and
interleaving
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Spreading
Scrambling
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Modulation
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RF
transmission
De-spreading Principle

UE1 uses c1 for de-spreading.
 (UE1 x c1 + UE2 x c2 + UE3 x c3) x c1
= UE1 x (c1 x c1) + UE2 x (c2 x c1) + UE3 x (c3 x c1)
= UE1 x 1 + UE2 x 0 + UE3 x 0
= UE1

In the same way, UE2 uses c2 for de-spreading and UE3
uses c3 for de-spreading to get their own signals.
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Spreading Principle
____________
UE1:
+1
-1
1
_____________
UE2:
-1
+1
c1:
+1 -1 +1 -1
+1 -1 +1 -1
c2:
+1 +1 +1 +1
+1 +1 +1 +1
UE1xc1:
+1 -1 +1 -1
-1 +1 -1 +1
UE2xc2:
-1 -1 -1 -1
+1 +1 +1 +1
UE1xc1 ＋ UE2xc2:
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0
-2 0
-2
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0 +2 0 +2
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De-spreading Principle
UE1×c1 ＋ UE2×c2 ：
0
-2 0
-2
0
+2 0 +2
UE1 de-spreading with c1:
+1 -1 +1
-1
+1
-1 +1 -1
De-spreading result:
0
+2
0
-2 0
+2 0
Integral:
+4
-4
Decision:
+4/4 = +1
-4/4 = -1
UE2 de-spreading with c2:
De-spreading result:
Integral:
Decision :
-2
+1 +1 +1 +1 +1 +1 +1 +1
0
-2
0
-2
0 +2
0 +2
-4
+4
-4/4 = -1
+4/4 = +1
Question: How to generate those orthogonal codes like c1 and c2?
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OVSF & Walsh
Cch,4,0 =(1,1,1,1)
Cch,2,0 = (1,1)
Cch,4,1 = (1,1,-1,-1)
Cch,1,0 = (1)
Cch,4,2 = (1,-1,1,-1)
Cch,2,1 = (1,-1)
Cch,4,3 = (1,-1,-1,1)
SF = 1
SF = 2
SF = 4
OVSF codes (Walsh) are completely orthogonal
and their mutual correlation is zero.
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Why CDMA system is a self interference system?
 Since all the users use orthogonal code, where does the self
interference come from?
 Comes from the code error during the transfer.
 If there are some code error on one user’s information, it
misses some orthogonal to others. Then interference comes.
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Sample of code error
UE1xc1 + UE2xc2 ：
0 -2 0 -2
UE1xc1 + UE2xc2 error ：
c1:
2 -2 0 -2
0 +2 0 +2
1 +2 0 +2
+1 -1 +1 +1
+1 -1 +1 -1
Result:
2 +2 0 +2
0 -2 0 -2
Integral:
+6
-4
Decision:
C2
+6/4 = 1.5
-4/4=-1
+1 +1 +1 +1
+1 +1 +1 +1
Result:
2 -2 0 -2
0 +2 0 +2
Integral:
-2
+4
Decision:
-2/4=-0.5
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+4/4=1
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Scrambling in the WCDMA System

Downlink: Different cells have different downlink scrambling codes.
 Each cell is configured with a unique downlink scramble. The UE
identifies a cell based on the scramble.
 The OVSF code is used to distinguish different users in a cell.

Uplink: Scrambles are used to distinguish different users.
 In one cell, each user is configured with a unique uplink scrambling
code.
 The OVSF code is used to distinguish the services of a user.
Source
coding
Channel
Interleaving
coding and
Spreading
Scrambling
interleaving
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Modulation
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RF
transmission
Spreading and Scrambling
Symbols
xx sps
3.84Mcps
3.84M
OVSF code
Scrambling
code
3.84Mcps
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Chips after
spreading
and
scrambling
Code Generation Technologies in CDMA


Random sequence (Bernoulli sequence)

It consists of 0’s and 1’s only, with the number of 0’s equal to that of 1’s.

The probability of continuous 1 or 0 is as follows: 1/2 for one continuous 0 or 1,
1/4 for two continuous 0 or 1, 1/8 for three continuous 0 or 1….

One half of the shift sequence is the same as the original sequence and the other
half is different.
m sequence - scrambling code used in the CDMA2000 system

It is generated by the shift register.

As the longest linear shift register sequence, its period is 2n-1, where “n” stands
for the length of the shift register.

When the delay is 0, the self-correlation function has a maximum value. In other
cases, the function value is always -1.

It meets the Bernoulli sequence.

Its disadvantages are: Only one sequence is available and different users are
distinguished by different phases. So it has a high requirement for
synchronization.
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Gold Sequence — Scrambling Code in the
WCDMA System

Gold sequence
 It consists of two preferred m sequences on the Exclusive-OR
basis.
 Its self-correlation function has multiple values, which is worse
than the m sequence.
 It exceeds the m sequence in quantity.

The Gold sequence is used to distinguish the cells and users
in the WCDMA system owing to its good self correlation.
 Good self correlation determines the Gold sequences can be
used to distinguish users, thus the multiple access function is
realized.
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Advantages and Disadvantages of the
Gold Sequence

Advantages: The Gold sequence needs no GPS synchronization
and features high system flexibility and security.
 The NodeB can work in asynchronous mode.
 It is easy to realize indoor coverage.

Disadvantages: The Gold sequence has greater inter-code
interference than the m sequence.
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Spreading/De-spreading Principle — Explanations
for Frequency Domain
Eb / No = Ec / Io × gain
Power spectrum
a2Tbit = Ebit
Demodulation
threshold
Allowed maximum interference
level of the system
Gain
Interference
signals from
other users
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Power that all the
users can share
Echip
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Spectrum Change in CDMA
Spreading code
P(f)
P(f)
f
P (f)
Broadband signal
f
Narrowband signal
f
Noise
Separation of
signals and noise
P (f)
Signal
combination
Noise + broadband signal
P (f)
f
Spreading code
f
The CDMA broadband spreading technology
effectively avoids frequency-selective fading of radio channels.
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Rake Receiver
Correlator 1
Correlator 2
Combiner
Receive set
The
combined
signal
Correlator 3
Calculate the
time delay and
signal strength
Searcher
correlator
s(t)
s(t)
t
t
RAKE receiver help to overcome on the multi-path fading and enhance the receive
performance of the system
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Summary - Advantages of CDMA
 RAKE receiver is adopted
 The time diversity effect generated by channel
coherence time is efficiently used.
 Frequency diversity
 Wideband frequency spectrum
 Higher interference tolerance and security performance
 Low signal transmission power
 Great flexibility in carrying multiple services with largely
different bit rate and QoS requirement.
 Different spreading factors for different services with
different data rates
 High spectral efficiency
 All users can share the same frequency spectrum
simultaneously.
 Supporting soft handover and softer handover.
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Contents
Chapter 1
WCDMA Principle
Chapter 2
Technical Features of WCDMA
FDD
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Characteristics of WCDMA FDD
 Channel bandwidth: 5MHz
 Chip rate: 3.84Mcps
 Frame length: 10ms
 Voice coding: AMR (Adaptive Multi-Rate)
 Uplink and downlink modulation: QPSK/QPSK
 Coherence demodulation aided with pilot
 Fast closed loop power control: 1500Hz
 Handover: soft/hard handover
 Support synchronous and asynchronous NodeB
operation
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Characteristics of WCDMA FDD
 Satisfy the minimum performance requirement of IMT2000
 Compatible with GSM-MAP core network
 Comparatively steady version R99 has been released
 Support open loop and closed loop transmit diversity mode
 Support Common Packet Channel(CPCH) and Downlink
Share Channel, adapt to Internet data access mode
 Support macro diversity, selection diversity of NodeB
location
 Support different fast power control algorithms and open
loop, out loop power control
 Fully support UE locating services
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Differences Between the WCDMA and GSM on
the Air Interface
WCDMA
GSM
Carrier spacing
5 MHz
200 KHz
Frequency reuse coefficient
1
1-18
Power control frequency
1500 Hz
2 Hz or lower
QoS control
Through the RRM algorithm
Through network planning
(frequency planning)
Frequency diversity
The 3.84 MHz bandwidth
enables the air interface to use
the rake receiver for multipath
diversity.
Frequency hopping
Packet data
Load-based packet scheduling
Timeslot-based scheduling
in GPRS
Downlink transmit diversity
Supported by the protocol to
improve downlink capacity
Not supported by the
standard but can be applied.
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Conclusion
 This course describes the WCDMA system.
 This course first describes some key technologies. Then,
describes the basic principles of CDMA and the WCDMA
FDD mode.
 After studying this course, we can have a preliminary
understanding of WCDMA, thus laying a good foundation for
subsequent study.
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Thank You !
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