WCDMA Principles and Technical Features

Internal

Principles of
the WCDMA System

www.huawei.com

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential

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!

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential Page 2

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.


Contents

Chapter 1 WCDMA Principle

Chapter 2 Technical Features of WCDMA
FDD


Overview of CDMA Principles

 Radio Propagation Environment

 Multiple Access Technology and
Duplex Technology

 CDMA Principles and Rake Receiver


Multipath Environment

Transmitted

signal

Strength of the
received signal

Time


Fading
Transmitted data

Received data
0
-5
-10
-15
dB

-20
-25
-30
-35
-40


Fading
Rx power (dBm)

-20 Fast fading
Slow fading

-40

-60

Distance (m)
10 20 30


Frequency-Selective Fading

P(f) P(f)

Fading
Narrowband
System
f f
Transmit Signal Received Signal

P(f) P(f)

Fading

Broadband
System f f
Transmit Signal Received Signal


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)


Overview of CDMA Principles


Duplex Technology


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.


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.


Multiple Access Technology - Distinguish
Different Users
CDMA
Traffic channels: different
users are assigned unique
code and transmitted over
Power
the same frequency band,
for example, WCDMA and
Time ncy
Freque CDMA2000
TDMA

Power
Traffic channels: different time slots
cy are allocated to different users, for
en
Tim qu example, DAMPS and GSM
FDMA e Fre

Us
U U s er
e
Power U s ser r
U s er Traffic channels: different frequency bands
er cy are allocated to different users,for example,
en
u
Tim eq
e Fr AMPS and TACS


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


Overview of WCDMA Principles


Duplex Technology



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?


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.


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.


Basic Diagram of the WCDMA System

Channel RF
Source
coding and Spreading Scrambling Modulation transmission
coding
interleaving

Radio link

Channel
Source deinterleaving RF
decoding and De-spreading De-scrambling Demodulation
decoding reception
de-interleaving


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 multi-
mode 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.

Channel
Source RF
Interleaving Spreading
coding and Scrambling Modulation
coding transmission
interleaving


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).

Channel
Source RF
Interleaving
coding Spreading Scrambling Modulation
coding transmission
interleaving


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 ... ... 452 453 454 ……

Ist interleaving B0 B1 B2 B3 B4 B5 B6 B7
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16
. . . . . . . .
.... . . . . . . . .
....
. . . . . . . .
449 450 451 452 453 454 455 456

2nd interleaving

{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {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) >
1, when x1=x2 , and x = y
Note: <x1(x) ， x2(y)> =
0, others


Spreading Principle

 Users who need to send information: UE1, UE2 and UE3
 UE1 uses c1 for spreading: UE1 x c1
 c1, c2 and c3 are orthogonal to each other

 Information sent: UE1 x c1 + UE2 x c2 + UE3 x c3

Channel RF
Source Interleaving Scrambling
coding and Spreading Modulation transmission
coding
interleaving


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.


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: 0 -2 0 -2 0 +2 0 +2


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 -2
Integral: +4 -4
Decision: +4/4 = +1 -4/4 = -1

UE2 de-spreading with c2: +1 +1 +1 +1 +1 +1 +1 +1
De-spreading result: 0 -2 0 -2 0 +2 0 +2
Integral: -4 +4
Decision : -4/4 = -1 +4/4 = +1

Question: How to generate those orthogonal codes like c1 and c2?


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.


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.


Sample of code error
UE1xc1 + UE2xc2 ： 0 -2 0 -2 0 +2 0 +2
UE1xc1 + UE2xc2 error ： 2 -2 0 -2 1 +2 0 +2

c1: +1 -1 +1 +1 +1 -1 +1 -1
Result: 2 +2 0 +2 0 -2 0 -2
Integral: +6 -4
Decision: +6/4 = 1.5 -4/4=-1

C2 +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 +4/4=1


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.

Channel
Source RF
Interleaving
coding and Spreading Scrambling Modulation
coding transmission
interleaving


Spreading and Scrambling

Chips after
spreading
Symbols 3.84Mcps and
xx sps 3.84M
scrambling

OVSF code Scrambling
code
3.84Mcps


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.


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.


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.


Spreading/De-spreading Principle — Explanations
for Frequency Domain
Eb / No = Ec / Io × gain
Power spectrum

a2Tbit = Ebit

Demodulation
Allowed maximum interference threshold
level of the system

Gain Power that all the
users can share

Interference
signals from
other users
Echip


Spectrum Change in CDMA

Spreading code P(f)
P(f)

f
Broadband signal P (f)
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.


Rake Receiver

Correlator 1

The
Correlator 2 Combiner combined
signal
Receive set
Correlator 3

Searcher Calculate the
correlator time delay and
signal strength
s(t) s(t)

t t

RAKE receiver help to overcome on the multi-path fading and enhance the receive
performance of the system


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.


Contents

Chapter 1 WCDMA Principle

Chapter 2 Technical Features of WCDMA
FDD


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


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


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

Through network planning
QoS control Through the RRM algorithm
(frequency planning)
The 3.84 MHz bandwidth
enables the air interface to use
Frequency diversity Frequency hopping
the rake receiver for multipath
diversity.
Timeslot-based scheduling
Packet data Load-based packet scheduling
in GPRS
Supported by the protocol to Not supported by the
Downlink transmit diversity
improve downlink capacity standard but can be applied.


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.


WCDMA Principles and Technical Features

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