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CDMA PrincipleCBB_T01_E2
ZTE UniversityCDMA-BSS Team
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Objectives
In this section, the student will learn:
Definitions of CDMA
Spread spectrum modulation
Spreading codes used in CDMA
Vocoding in CDMA
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Multiple Access Technologies
Power
FDMA
Power
CDMATDMA Time Division
Multiple AccessEach user on adifferent windowperiod in time (timeslot)
CDMA Code DivisionMultiple AccessA channel is a uniquecode patternEach user uses the
same frequency allthe time, but mixedwith differentdistinguishing codepatterns
FDMA FrequencyDivision MultipleAccess
Each user on adifferent frequency
A channel is afrequency Power
TDMA
A channel is aspecific time slot ona specific frequency
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Defining Our Terms
CDMA Forward Channel
1.25 MHz Forward Link
CDMA Reverse Channel
1.25 MHz Reverse Link45 or 80 MHz
CDMA CHANNEL
CDMAReverseChannel1.25 MHz
CDMAForwardChannel1.25 MHz
CDMA Code Channel Code channels in the forward link: Pilot, Sync, Paging andForward Traffic channels
Code channels in the reverse link: Access and ReverseTraffic channels
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Main Content
Definitions of CDMA
Spread spectrum modulation
Spreading codes used in CDMA
Vocoding in CDMA
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ORIGINATING SITE DESTINATION
InputData
RecoveredData
Spread Data Stream
What is Spread Spectrum
SpreadingSequence
SpreadingSequence
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SHANON Formula
Where,
C=B*log2(1+S/N)
Spread Spectrum Principles
s capac y o c anne , sB is signal bandwidth, HzS is average power for signal, WN is average power for noise, W
It is the basic principle and theoryfor spread spectrum communications.It is the basic principle and theoryfor spread spectrum communications.
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CDMA Is a Spread-Spectrum System
Spread SpectrumTRADITIONAL COMMUNICATIONS SYSTEM
Slow
InformationSent
TX
Slow
InformationRecovered
RX
NarrowbandSignal
Spread Spectrum Payoff:
Processing Gain
-
FastSpreadingSequence
SlowInformation
Sent
TX
SlowInformationRecovered
RX
FastSpreadingSequence
Wideband Signal
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Spread Spectrum Principles
Power is Spread Over a Larger Bandwidth
1.25 MHz
30 KHz
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Spread Spectrum Principles
Many code channels are individuallyspreadand then added together tocreate a composite signal
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Spreading and De-spreading
ORIGINATING SITE DESTINATION
SpreadingSequence
SpreadingSequence
Input
Data
Recovered
Data
Spread Data Stream
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CDMA Spreading Principle
Using Multiple Codes
InputData
X
RecoveredData
X
X+A X+A+B X+A+B+C X+A+B X+A
Spread-Spectrum Chip StreamsORIGINATING SITE DESTINATION
Multiple spreading sequences can be applied in succession and then reapplied
in opposite order to recover the original data stream. The spreading sequences can have different desired properties.
All spreading sequences originally used must be available in proper
synchronization at the recovering destination.
SpreadingSequence
A
SpreadingSequence
B
SpreadingSequence
C
SpreadingSequence
C
SpreadingSequence
B
SpreadingSequence
A
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Main Content
Definitions of CDMA Spread spectrum modulation
Vocoding in CDMA
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Spreading Codes in CDMA
Spreading codes selection is the key of spreading
Spectrum modulation!
Spreading codes selection is the key of spreading
Spectrum modulation!
prea ng co e c p spee . c s
Spreading code forward linkWalsh code & Short PN
Reverse linkLong PN
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Walsh Code Definition
The Walsh function is named after Walsh, the mathematician who proved it an
orthogonal function in 1923. It is expressed as Walsh (n,t), n for the serial number.
The CDMA system of IS-95 is differentiated with the Walsh function.
Walsh code is an orthogonal square matrix. It is just composed of +1(0) and
1(1).
0110
1100
1010
0000
10
00
0
Hn Hn
H2n = ___Hn Hn
H1 H2 H4
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Walsh Code Function
in Forward Link
SyncPilot
FW Traffic FW Traffic
(for user #1)Paging
or user
FW Traffic(for user #3)
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Short PN: 4-bits register example
1 001
0 011
0 110
1 101
0 100p1 p2 p3 p4
p2 p3
p4
p4
The PN sequences are deterministic and periodic.
The length of the generated string is 2n-1, where n is thenumber of elements in the register
The number of zeroes in the sequence is equal to the number ofones minus 1
0 1011 011
0 111
1 111
1 110
10 0 0
0 010
0 100
1 100
1 000
p4 p5 p2 p3
= 1 4
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Short PN Function
in Forward Link
The short PN Sequences are 32,768 chips long. Short PN is used to distinguish
A B
Up to 64
Code Channels
Up to 64
Code Channels
.
Each Sector in each Base Station is transmitting a Forward Traffic Channelcontaining up to 64 forward code channels.
A Mobile Station must be able to discriminate between different Sectors ofdifferent Base Stations.
The Short PN Sequences are defined for the purpose of identifying sectors of
different base stations. The Short PN Sequences can be used in 512 different ways in a CDMA system.Each one of them constitutes a mathematical code which can be used to identify aparticular sector.
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Long PN:4-bits
shift register example
mask
XOROriginal PNsequence
1 001
0 011
0 110
1 101
0 100
XOR)
New PNsequence
AND AND AND AND
0 1011 011
0 111
1 111
1 110
10 0 0
0 010
0 100
1 100
1 000
Attention:different mask lead to different offset!Attention:different mask lead to different offset!
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The Long PN Sequence
Long Code Register(@ 1.2288 MCPS)
Public Long Code Mask(STATIC)
User Long Code
1 1 0 0 0 1 1 0 0 0 P E R M U T E D E S NAND
=S U M
Each mobile station uses a unique User Long Code Sequence
generated by applying a mask, based on its 32-bit ESN, to the 42-bit Long
Code Generator which was synchronized with the CDMA system during
the mobile station initialization.
(@1.2288 MCPS)
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Long PN Function
in Reverse Link The CDMA system must be able toidentify each Mobile Station that mayattempt to communicate with a Base
Station. A very large number of MobileStations will be in the market.
RV Trafficfrom M.S.
#1837732008RV Trafficfrom M.S.
#1997061104
RV Trafficfrom M.S.
#1994011508
System AccessAttempt by M.S.
#2000071301(on access channel #1)
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Main Content
Definitions of CDMA and How to realize
Spread spectrum modulation
Spreading codes used in CDMA
oco ng n
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Vocoding (A1 & A2 interface)
BTS BSCBSC MSCMSC
Analog voiceVariable Rate PCM
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TrFO (Transcoder Free Operation
AP interface)
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RTO (Remote Transcoder
Operation - AP interface)