Download - 01 WCDMA Principles
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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
� 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.
� After studying this course, you should be able to:
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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 EnvironmentRadio Propagation Environment
�� Multiple Access Technology and Multiple Access Technology and Duplex TechnologyDuplex Technology
�� CDMA Principles and Rake ReceiverCDMA Principles and Rake Receiver
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Multipath Environment
Time
Strength of the received signal
Transmitted signal
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FadingTransmitted data
-40
-35
-30
-25
-20
-15
-10
-5
0
dB
Received data
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Fading
Distance (m)
Rx power (dBm)
10 20 30
-20
-40
-60
Slow fading
Fast fading
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Frequency-Selective Fading
NarrowbandSystem
NarrowbandSystem
Fading
Transmit Signal Received Signalff
P(f) P(f)
Fading
Transmit Signal Received Signalff
P(f) P(f)
BroadbandSystem
BroadbandSystem
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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 EnvironmentRadio Propagation Environment
�� Multiple Access Technology and Multiple Access Technology and Duplex TechnologyDuplex Technology
�� CDMA Principles and Rake ReceiverCDMA Principles and Rake Receiver
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Duplex Technology (Distinguish UL and DL for one User) –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 UL and DL for one User) –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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Traffic channels: differentusers are assigned unique code and transmitted over the same frequency band, for example, WCDMA and CDMA2000
Traffic channels: different frequency bands are allocated to different users,for example, AMPS and TACS
Traffic channels: different time slots are allocated to different users, for example, DAMPS and GSM
Multiple Access Technology - Distinguish Different Users
FrequencyTime
Power
FDMA
User
User
User User
User
FrequencyTime
Power
TDMA
Power
Time Frequency
CDMA
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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 EnvironmentRadio Propagation Environment
�� Multiple Access Technology and Multiple Access Technology and Duplex TechnologyDuplex Technology
�� CDMA Principles and Rake ReceiverCDMA 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
ScramblingSpreading ModulationRF
transmission
Source decoding
deinterleavingChannel
decoding and de-interleaving
De-scramblingDe-spreading DemodulationRF
reception
Radio link
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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.
Source coding
InterleavingChannel coding and interleaving
ScramblingSpreading ModulationRF
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
InterleavingChannel coding interleaving
ScramblingSpreading ModulationRF
transmission
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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 ……1 2 3 4 5 6 7 8 ... ... 452 453 454 ……
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.... ....
B0 B1 B2 B3 B4 B5 B6 B7
{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {B7,C3}{A4,B0} {A5,B1} {A6,B2} {A7,B3} {B4,C0} {B5,C1} {B6,C2} {B7,C3}
Ist interleaving
2nd interleaving
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� 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
Spreading Principle
Source coding
InterleavingChannel coding and interleaving
ScramblingSpreading ModulationRF
transmission
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� 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.
De-spreading Principle
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____________
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
Spreading Principle
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UE1××××c1++++ UE2××××c2:::: 0 -2 0 -2 0 +2 0 +2
De-spreading Principle
Question: How to generate those orthogonal codes like c1 and c2?
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
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OVSF & Walsh
OVSF codes (Walsh) are completely orthogonal and their mutual correlation is zero.
SF = 1 SF = 2 SF = 4
Cch,1,0 = (1)
Cch,2,0 = (1,1)
Cch,2,1 = (1,-1)
Cch,4,0 =(1,1,1,1)
Cch,4,1 = (1,1,-1,-1)
Cch,4,2 = (1,-1,1,-1)
Cch,4,3 = (1,-1,-1,1)
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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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UE1xc1 + UE2xc2:::: 0 -2 0 -2 0 +2 0 +2
UE1xc1 + UE2xc2 error :::: 2 -2 0 -2 1 +2 0 +2
Sample of code error
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
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
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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
InterleavingChannel coding and interleaving
ScramblingSpreading ModulationRF
transmission
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Spreading and Scrambling
Symbols
OVSF code Scrambling code
Chips after spreading
and scrambling
3.84M3.84Mcps
3.84Mcps
xx sps
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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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Allowed maximum interference level of the system
Demodulation threshold
Spreading/De-spreading Principle — Explanations for Frequency Domain
Power spectrum
Power that all the users can share
a2Tbit = Ebit
Gain
Interference signals from other users Echip
Eb / No = Ec / Io × gain
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The CDMA broadband spreading technology effectively avoids frequency-selective fading of ra dio channels.
Spreading code
Spreading code
Signal combination
Spectrum Change in CDMA
Narrowband signal
f
P(f)
Broadband signal
P(f)
f
Noise
P (f)
f
Noise + broadband signal
P (f)
f
Separation of signals and noise
P (f)
f
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Rake ReceiverRake Receiver
Receive set
Correlator 1
Correlator 2
Correlator 3
Searcher correlator Calculate the time delay and signal strength
CombinerThe
combined signal
tt
s(t) s(t)
RAKE receiver help to overcome on the multi-path fa ding 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 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
Not supported by the standard but can be applied.
Supported by the protocol to improve downlink capacity
Downlink transmit diversity
Timeslot-based scheduling in GPRS
Load-based packet scheduling Packet data
Frequency hoppingThe 3.84 MHz bandwidth enables the air interface to use the rake receiver for multipath diversity.
Frequency diversity
Through network planning (frequency planning)
Through the RRM algorithmQoS control
2 Hz or lower1500 HzPower control frequency
1-181Frequency reuse coefficient
200 KHz5 MHzCarrier spacing
GSMWCDMA
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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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