01 WCDMA RAN Fudamental

7/30/2019 01 WCDMA RAN Fudamental

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WCDMA RAN Fundamental

Confidential Information of Huawei. No Spreading Without Permission

N-4

Page4Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved.

Wireless Propagation

ReceivedSignal

Transmitted

Signal

Transmission Loss:Path Loss + Multi-path Fading

Time

Amplitude

l A mobile communication channel is a multi-path fading channel and

any transmitted signal reaches a receive end by means of multiple

transmission paths, such as direct transmission, reflection, scatter, etc.


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N-5


Propagation of Radio SignalSignal at Transmitter

Signal at Receiver

-40

-35

-30

-25

-20-15

-10

-5

dB

0

0dBm

-20

-15

-10

-5

5

10

15

20

Fading


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N-6


Fading Categories

l Fading Categories

p Slow Fading

p Fast Fading

Furthermore, with the moving of a mobile station, the signal amplitude, delay

and phase on various transmission paths vary with time and place. Therefore,

the levels of received signals are fluctuating and unstable and these multi-path

signals, if overlaid, will lead to fading. The mid-value field strength of Rayleigh

fading has relatively gentle change and is called Slow fading. And it conforms

to lognormal distribution.


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N-7


Multiple Access and Duplex Technology

l Multiple Access Technology

p Frequency division multiple access (FDMA)

p Time division multiple access (TDMA)

p Code division multiple access (CDMA)

l Duplex Technology

p

Time division duplex (TDD)p Frequency division duplex (FDD)

l In third generation mobile communication systems, TD-SCDMA adopts time

division duplex (TDD); WCDMA and cdma2000 adopt frequency division duplex

(FDD). WCDMA FDD mode has been consolidated with TD-SCDMA.


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N-8


Multiple Access Technology

frequ

ency

time

power

FDMA

frequ

encytime

power

TDMA

power

time

CDMA

frequency

l Frequency Division Multiple Access means dividing the whole available

spectrum into many single radio channels (transmit/receive carrier pair). Each

channel can transmit one-way voice or control information. Analog cellular

system is a typical example of FDMA structure.

l Time Division Multiple Access means that the wireless carrier of one bandwidth

is divided into multiple time division channels in terms of time (or called timeslot).

Each user occupies a timeslot and receives/transmits signals within this

specified timeslot. Therefore, it is called time division multiple access. This

multiple access mode is adopted in both digital cellular system and GSM.

l CDMA is a multiple access mode implemented by Spreading Modulation. Unlike

FDMA and TDMA, both of which separate the user information in terms of timeand frequency, CDMA can transmit the information of multiple users on a

channel at the same time. The key is that every information before transmission

should be modulated by different Spreading Code to broadband signal, then all

the signals should be mixed and send. The mixed signal would be demodulated

by different Spreading Code at the different receiver. Because all the Spreading

Code is orthogonal, only the information that was be demodulated by same

Spreading Code can be reverted in mixed signal.


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N-9


Duplex Technology

Time

Frequency

Power

TDD

USER 2

USER 1

DL

UL

DLDL

UL

FDD

Time

Frequency

Power

UL DL

USER 2

USER 1


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N-10


Contents

1. CDMA Principle

2. WCDMA Fundamental


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N-11


Processing Procedure of WCDMA System

SourceCoding

ChannelCoding

Spreading Modulation

Source

DecodingChannel

DecodingDespreading Demodulation

Transmission

Reception

chipmodulated

signalbit symbol

Service

Signal

RadioChannel

Service

Signal

Transmitter

Receiver

l Source coding can increase the transmitting efficiency.

l Channel coding can make the transmission more reliably.

l Spreading can increase the capability of overcoming interference.

l Scrambling can make transmission in security.

l Through the modulation, the signals will transfer to radio signals from digital

signals.

l Bit, Symbol, Chip

p Bit : data after source coding

p Symbol: data after channel coding and interleaving

p Chip: data after spreading


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N-12


Process Gain

l Process Gain

p Process gain differs for each service.

p If the service bit rate is greater, the process gain is smaller, UE

needs more power for this service, then the coverage of this

service will be smaller, vice versa.

)ratebit

ratechiplog(10GainocessPr =

l For common services, the bit rate of voice call is 12.2kbps, the bit rate of video

phone is 64kbps, and the highest packet service bit rate is 384kbps(R99). After

the spreading, the chip rate of different service all become 3.84Mcps.


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N-14


WCDMA Channel Coding

l Effect

p Enhance the correlation among symbols so as to recover the signal when

interference occurs

p Provides better error correction at receiver, but brings increment of the delay

l Types

p No Coding

p Convolutional Coding (1/2, 1/3)

p Turbo Coding (1/3)

Code Block

of N Bits

No Coding

1/2 ConvolutionalCoding

1/3 ConvolutionalCoding

1/3 Turbo Coding

Uncoded N bits

Coded 2N+16 bits

Coded 3N+24 bits

Coded 3N+12 bits

l During the transmission, there are many interferences and fading. To

guarantee reliable transmission, system should overcome these influence

through the channel coding which includes convolution and interleaving.

l The first is convolution that is used for anti-interference. Through the

technology, many redundant bits will be inserted in original information. When

error code is caused by interference, the redundant bits can be used to recover

the original information.

l In WCDMA network, both Convolution code and Turbo code are used.

Convolution code applies to voice service while Turbo code applies to high rate

data service.


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N-15


WCDMA Interleaving

l Effect

p Interleaving is used to reduce the probability of consecutive bits error

p Longer interleaving periods have better data protection with more delay

1110

1.........

............

...000

0100

0 0 1 0 0 0 0 . . . 1 0 1 1 1

1110

1.........

............

...000

00100 0 0 1 0 1 0 0 1 0 1 1

Inter-columnpermutation

Output bits

Input bits

Interleaving periods:

10, 20, 40, or 80 ms

l In channel coding , there is another technology named interleaving.

Communications over radio channel are characterized by fast fading that can

cause large numbers of consecutive errors. Most coding schemes perform

better on random data errors than on blocks of errors. By interleaving the data,

no two adjacent bits are transmitted near to each other, and the data errors are

randomized.


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N-16


Correlation (1)

l Correlation measures similarity between any two arbitrary signals.

l Identical and Orthogonal signals:

Correlation = 0Orthogonal signals

-1 1 -1 1

-1 1 -1 1

1 1 1 1

+1

-1

+1

-1

+1

-1

+1

-1

Correlation = 1Identical signals

-1 1 -1 11 1 1 1

-1 1 -1 1

C1

C2+1

+1

C1

C2

l Correlation is used to measure similarity of any two arbitrary signals. It is

computed by multiplying the two signals and then summing (integrating) the

result over a defined time windows. The two signals of figure (a) are identical

and therefore their correlation is 1 or 100 percent. In figure (b) , however, the

two signals are uncorrelated, and therefore knowing one of them does not

provide any information on the other.


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N-18


Orthogonal Code Usage - Decoding

UE1 C1 UE2 C2: 2 0 2 0 2 0 2 0

UE1 Dispreading by c1: 1 1 1 1 1 1 1 1

Dispreading result: 2 0 2 0 2 0 2 0

Integral judgment: 4 (means 1) 4 (means 1)




UE1 C1 UE2 C2: 2 0 2 0 2 0 2 0







l The receiver dispreads the chips by using the same code used in the transmitter.

Notice that under no-noise conditions, the symbols or digits are completely

recovered without any error. In reality, the channel is not noise-free, but CDMA

system employ Forward Error Correction techniques to combat the effects of

noise and enhance the performance of the system.

l When the wrong code is used for dispreading, the resulting correlation yields an

average of zero. This is a clear demonstration of the advantage of the

orthogonal property of the codes. Whether the wrong code is mistakenly used

by the target user or other users attempting to decode the received signal, the

resulting correlation is always zero because of the orthogonal property of codes.


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N-19


WCDMA Channelization Code

l OVSF Code (Orthogonal Variable Spreading Factor)

l SF = chip rate / symbol rate

p High data rates low SF code

p Low data rates high SF code

SF = 8SF = 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)

Cch,8,0 = (1,1,1,1,1,1,1,1)

Cch,8,1 = (1,1,1,1,-1,-1,-1,-1)

Cch,8,2 = (1,1,-1,-1,1,1,-1,-1)

Cch,8,3 = (1,1,-1,-1,-1,-1,1,1)

Cch,8,4 = (1,-1,1,-1,1,-1,1,-1)

Cch,8,5 = (1,-1,1,-1,-1,1,-1,1)

Cch,8,6 = (1,-1,-1,1,1,-1,-1,1)

Cch,8,7= (1,-1,-1,1,-1,1,1,-1)

l Orthogonal codes are easily generated by starting with a seed of 1, repeating

the 1 horizontally and vertically, and then complementing the -1 diagonally. This

process is to be continued with the newly generated block until the desired

codes with the proper length are generated. Sequences created in this way are

referred as Walsh code.

l Spreading code uses OVSF code, for keeping the orthogonality of different

subscriber physical channels. OVSF can be defined as the code tree illustrated

in the following diagram.

l Spreading code is defined as Cch SF, k,, where, SF is the spreading factor of

the code, and k is the sequence of code, 0kSF-1. Each level definition

length of code tree is SF spreading code, and the left most value of eachspreading code character is corresponding to the chip which is transmitted

earliest.


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N-20


Purpose of Channelization Code

l For uplink, Channelization code ( OVSF code ) is used to separate different

physical channels of one connection

l For downlink, Channelization code ( OVSF code ) is used to separate different

connections in a cell

16Data 128 kbps DL8Data 128 kbps UL


256Speech 4.75 DL128Speech 4.75 UL

32Speech 12.2 + Data 64 kbps DL16Speech 12.2 + Data 64 kbps UL




128Speech 12.2 DL64Speech 12.2 UL

SFRadio bearer**SFRadio bearer**

** With 3.4 kbps Signaling

l For voice service (AMR), downlink SF is 128, it means there are 128 voice

services maximum can be supported in one WCDMA carrier;

l For Video Phone (64k packet data) service, downlink SF is 32, it means there

are 32 voice services maximum can be supported in one WCDMA carrier.


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N-21


Correlation (2)

l Cross Correlation: related to the multi-user interference characteristic

l Auto Correlation: related to the multi-path interference characteristic

Time Delay

S1(t)

S1(t+t)

S1(t)

S2(t)


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N-22


WCDMA Scrambling Code Gold Sequence

l Scrambling codes Properties

p 38 400 chip long sequences

p Repeated every 10 ms

p Coming from Pseudo Noise sequences

l For uplink, Scrambling code (Gold sequence) is used to separate different

connection

l For downlink, Scrambling code (Gold sequence) is used to separate different cell

clong,1,n

clong,2,n

MSB LSB

scrambling sequence generator

l Gold sequence is made by two m sequence.

l m sequence is generate by a series of shift-registers, and the period is 2n-1,

here n is the length of the shift-registers.


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N-23


Code Multiplexing

l Downlink Transmission on a Cell Level

Scrambling code

Channelization code 1



User 1 signal

User 2 signal

User 3 signal

NodeB


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N-24


Code Multiplexing

l Uplink Transmission on a Cell Level

NodeB

Scrambling code 3

User 3 signal

Channelization code

Scrambling code 2

User 2 signal

Channelization code

Scrambling code 1

User 1 signal

Channelization code


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N-25


Spreading and Despreading

Spreading

Despreading

Chip

Symbol

Data

Spreading code

Spreading signal=Datacode

Data=Spreadingcode

1

-1

1

-1

1

-1

1

-1

1

-1

Known codeat receiver

Correlation at a CDMA receiver


l Suppose bit sequence modulated with BPSK is adopted for the subscriber data,

with a rate of R, then 1 value is adopted for the bit of subscriber data.

l The spreading here means to multiply each subscriber data bit with the

spreading code chip including N bits..Assume N=8,then data rate after

spreading will be 8R, with same random attribute as the spreading code.We

name its spreading factor as 8. And the broad band signal obtained after

spreading will be sent to the receiving end via the radio channel.

l As the product of signal rate and factor 8 equals to the bandwidth spreading of

subscriber data signal,CDMA system is also called the spreading system.

l During dispreading, the spread subscriber data will be multiplied, bit duration by

bit duration, with the same 8 code chips that are used during the spreading of

these bits.If only excellent synchronization can be realized between the spread

subscriber signal and the despreading code, can the subscriber bit sequence be

retrieved.The despreading operation restores the signal bandwidth to the

original value R.


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N-26


Spreading and Despreading

Desired signal

Other userssignal

Desired spreadingsignal

Spreading code

Data afterdespreading

Other spreadingsignal

Other signal afterintegration

1

-1

1

-1

1

-1

8

-8

1

-1

8

-8

Data afterintegration

Other signal afterdespreading


Correlation with incorrect code

l During the process of receiving the expected correct signal that belongs to the

subscriber, complete synchronous despreading codes are adopted for the

despreading operation. After obtaining the despreading data, the correlation

receiver integrates the resulting products, then get the integration data.

l Signals of other subscribers using different spreading codes are actually the

interference signals to the first subscriber. In this case, multiply the signals and

the despreading code of the first subscriber to get the despreading signal, and

then perform integration. Finally, an interference signal with a signal value

fluctuating along with 0 will be got. It can be viewed that the signal amplitude of

the subscriber increases by 8 times than that of the other interference systems

in average. That is to say, the correlation detection increases the expected

subscriber signal by the multiple of spreading factor value within the

interference of CDMA system. This effect is called Processing Gain, and it is

the basic characteristic of the spreading system. Elementarily, this kind of

correlation receivers are adopted for the BTS and UE in the WCDMA system.

Because the existence of multi-path propagation and multi receiving antennas,

multiple correlation receivers are necessary for retrieving the signal energy from

all the paths or antennas. And the collection of these correlation receivers forms

the CDMA RAKE receiver.


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N-27


Modulation Overview

1 00 1

time

Basic steady radiowave:

carrier = A.cos(2Ft+)

Amplitude ShiftKeying:

A.cos(2Ft+)

Frequency ShiftKeying:

A.cos(2Ft+)

Phase Shift Keying:

A.cos(2Ft+)

Data to be transmitted:Digital Input


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N-28


Modulation Overview

l Digital Modulation - BPSK

1

t

1 10

1

t-1

NRZ coding

fo

BPSK

Modulated

BPSKsignal

Carrier

Informationsignal

=0 = =0

1 102 3 4 9875 6

1 102 3 4 9875 6

Digital Input

High Frequency

Carrier

BPSK Waveform


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N-29


Modulation Review

l Digital Modulation - QPSK

-1 -1

1 102 3 4 9875 6

1 102 3 4 9875 6

NRZ Input

I di-Bit Stream

Q di-Bit Stream

IComponent

QComponent

QPSK Waveform

1

1

-1

1

-1

1

1

-1

-1

-1

1 1 -1 1 -1 1 1 -1

l QPSK: Quadrature Phase Shift Keying . Phase shift keying in which four

different phase angles are used.


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N-31


Demodulation

l QPSK Constellation Diagram

1 102 3 4 9875 6

QPSK Waveform

1,1

-1,-1

-1,1

1,-1

1 -11 -1 1 -1-11-1 1

-1,1

NRZ Output


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N-33


Principle of RAKE Receiver

l The RAKE receiver take advantage of multi-path diversity

l The RAKE receiver processes the received signal

p Identify the time delay positions at which significant energy arrives

p Allocate correlation receivers (RAKE fingers) to those peaks

p Within each Rake finger, track the fast-changing phase and amplitude values

p Adjust the phase, remove the values originating from fast-fading

p Combine the demodulated and phase-adjusted symbols across all active

fingers

p

Present them to the decoder for further processingl This processing is called Maximal Ratio Combining


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N-34


Demodulation

Frequencytranslation

Matchedfilter

Fingercombination

Despreading Symboldecision

Fi

Delay (n)

Delay (1)

Delay (0)

Estimatedchips

-1,1

x

Rake receiver

1,1

1,-1-1,-1


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WCDMA RAN Fundamental N-35

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01 WCDMA RAN Fudamental

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