002 CBB_T01_E2 Principle and Implementation of CDMA Spread Spectrum-27
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CDMA Principle CBB_T01_E2 ZTE University CDMA-BSS Team
Transcript of 002 CBB_T01_E2 Principle and Implementation of CDMA Spread Spectrum-27
CDMA Principle
CBB_T01_E2
ZTE University
CDMA-BSS Team
Objectives
In this section, the student will learn:
Definitions of CDMA
Spread spectrum modulation
Spreading codes used in CDMA
Vocoding in CDMA
Multiple Access Technologies
CDMA Code Division Multiple Access
A channel is a unique code patternEach user uses the same frequency all the time, but mixed with different distinguishing code patterns
FDMA Frequency Division Multiple Access
Each user on a different frequencyA channel is a frequency
FrequencyTime
Power
FDMA
FrequencyTime
Power
TDMA
FrequencyTime
Power
CDMATDMA Time Division Multiple Access
Each user on a different window period in time (“time slot”)A channel is a specific time slot on a specific frequency
Defining Our Terms
CDMA Forward Channel1.25 MHz Forward Link
CDMA Reverse Channel1.25 MHz Reverse Link
CDMA Code ChannelCode channels in the forward link: Pilot, Sync, Paging and
Forward Traffic channelsCode channels in the reverse link: Access and Reverse
Traffic channels
45 or 80 MHz
CDMA CHANNELCDMA
ReverseChannel 1.25 MHz
CDMAForwardChannel 1.25 MHz
Main Content
Definitions of CDMA
Spread spectrum modulation
Spreading codes used in CDMA
Vocoding in CDMA
What is Spread Spectrum
ORIGINATING SITE DESTINATION
SpreadingSequence
SpreadingSequence
InputData
RecoveredData
Spread Data Stream
Spread Spectrum Principles
SHANON Formula
Where,C is capacity of channel, b/sB is signal bandwidth, HzS is average power for signal, WN is average power for noise, W
C=B*log2(1+S/N)
It is the basic principle and theoryfor spread spectrum communications.It is the basic principle and theoryfor spread spectrum communications.
CDMA Is a Spread-Spectrum System
Spread Spectrum Payoff:Processing Gain
Spread SpectrumTRADITIONAL COMMUNICATIONS SYSTEM
SlowInformation
Sent
TX
SlowInformationRecovered
RX
NarrowbandSignal
SPREAD-SPECTRUM SYSTEM
FastSpreadingSequence
SlowInformation
Sent
TX
SlowInformationRecovered
RX
FastSpreadingSequence
Wideband Signal
Spread Spectrum Principles
1.25 MHz30 KHz
Power is “Spread” Over a Larger BandwidthMATHHAMMER
MATHHAMMER
Spread Spectrum Principles
Many code channels are individually“spread” and then added together tocreate a “composite signal”
Spreading and De-spreading
ORIGINATING SITE DESTINATION
SpreadingSequence
SpreadingSequence
InputData
RecoveredData
Spread Data Stream
CDMA Spreading PrincipleUsing Multiple Codes
SpreadingSequence
ASpreadingSequence
BSpreadingSequence
CSpreadingSequence
CSpreadingSequence
BSpreadingSequence
A
InputDataX
RecoveredDataX
X+A X+A+B X+A+B+C X+A+B X+ASpread-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.
Main Content
Definitions of CDMA
Spread spectrum modulation
Spreading codes used in CDMA
Vocoding in CDMA
Spreading Codes in CDMA
Spreading codes selection is the key of spreadingSpectrum modulation!Spreading codes selection is the key of spreadingSpectrum modulation!
Spreading code chip speed：1.2288Mc/s；Spreading code：
forward link—Walsh code & Short PN Reverse link—Long PN
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).
Hn Hn
H2n = ___
Hn Hn0110
1100
1010
0000
10
000
H1 H2 H4
Walsh Codes64 Sequences, each 64 chips long
A chip is a binary digit (0 or 1)
Each Walsh Code is Orthogonal to all other Walsh Codes
This means that it is possible to recognize and therefore extract a particular Walsh code from a mixture of other Walsh codes which are “filtered out” in the process
Two same-length binary strings are orthogonal if the result of XORing them has the same number of 0s as 1s
WALSH CODES# ---------------------------------- 64-Chip Sequence ------------------------------------------0 00000000000000000000000000000000000000000000000000000000000000001 01010101010101010101010101010101010101010101010101010101010101012 00110011001100110011001100110011001100110011001100110011001100113 01100110011001100110011001100110011001100110011001100110011001104 00001111000011110000111100001111000011110000111100001111000011115 01011010010110100101101001011010010110100101101001011010010110106 00111100001111000011110000111100001111000011110000111100001111007 01101001011010010110100101101001011010010110100101101001011010018 00000000111111110000000011111111000000001111111100000000111111119 0101010110101010010101011010101001010101101010100101010110101010
10 001100111100110000110011110011000011001111001100001100111100110011 011001101001100101100110100110010110011010011001011001101001100112 000011111111000000001111111100000000111111110000000011111111000013 010110101010010101011010101001010101101010100101010110101010010114 001111001100001100111100110000110011110011000011001111001100001115 011010011001011001101001100101100110100110010110011010011001011016 000000000000000011111111111111110000000000000000111111111111111117 010101010101010110101010101010100101010101010101101010101010101018 001100110011001111001100110011000011001100110011110011001100110019 011001100110011010011001100110010110011001100110100110011001100120 000011110000111111110000111100000000111100001111111100001111000021 010110100101101010100101101001010101101001011010101001011010010122 001111000011110011000011110000110011110000111100110000111100001123 011010010110100110010110100101100110100101101001100101101001011024 000000001111111111111111000000000000000011111111111111110000000025 010101011010101010101010010101010101010110101010101010100101010126 001100111100110011001100001100110011001111001100110011000011001127 011001101001100110011001011001100110011010011001100110010110011028 000011111111000011110000000011110000111111110000111100000000111129 010110101010010110100101010110100101101010100101101001010101101030 001111001100001111000011001111000011110011000011110000110011110031 011010011001011010010110011010010110100110010110100101100110100132 000000000000000000000000000000001111111111111111111111111111111133 010101010101010101010101010101011010101010101010101010101010101034 001100110011001100110011001100111100110011001100110011001100110035 011001100110011001100110011001101001100110011001100110011001100136 000011110000111100001111000011111111000011110000111100001111000037 010110100101101001011010010110101010010110100101101001011010010138 001111000011110000111100001111001100001111000011110000111100001139 011010010110100101101001011010011001011010010110100101101001011040 000000001111111100000000111111111111111100000000111111110000000041 010101011010101001010101101010101010101001010101101010100101010142 001100111100110000110011110011001100110000110011110011000011001143 011001101001100101100110100110011001100101100110100110010110011044 000011111111000000001111111100001111000000001111111100000000111145 010110101010010101011010101001011010010101011010101001010101101046 001111001100001100111100110000111100001100111100110000110011110047 011010011001011001101001100101101001011001101001100101100110100148 000000000000000011111111111111111111111111111111000000000000000049 010101010101010110101010101010101010101010101010010101010101010150 001100110011001111001100110011001100110011001100001100110011001151 011001100110011010011001100110011001100110011001011001100110011052 000011110000111111110000111100001111000011110000000011110000111153 010110100101101010100101101001011010010110100101010110100101101054 001111000011110011000011110000111100001111000011001111000011110055 011010010110100110010110100101101001011010010110011010010110100156 000000001111111111111111000000001111111100000000000000001111111157 010101011010101010101010010101011010101001010101010101011010101058 001100111100110011001100001100111100110000110011001100111100110059 011001101001100110011001011001101001100101100110011001101001100160 000011111111000011110000000011111111000000001111000011111111000061 010110101010010110100101010110101010010101011010010110101010010162 001111001100001111000011001111001100001100111100001111001100001163 0110100110010110100101100110100110010110011010010110100110010110
EXAMPLE:Correlation of Walsh Code #23 with Walsh Code #59
#23 0110100101101001100101101001011001101001011010011001011010010110#59 0110011010011001100110010110011010011001011001100110011010011001XOR 0000111111110000000011111111000011110000000011111111000000001111
Correlation Results: 32 1’s, 32 0’s: Orthogonal!!
Walsh Code Function in Forward Link
SyncPilot
FW Traffic(for user #1)
Paging
FW Traffic(for user #2)
FW Traffic(for user #3)
Short PN: 4-bits register example
The PN sequences are deterministic and periodic.The length of the generated string is 2n-1, where “n” is the number of elements in the registerThe number of zeroes in the sequence is equal to the number of ones minus 1
1 0010 0110 1101 1011 0100 1011 0110 1111 1111 110
10 0 00 0100 100
1 1001 000
0 100p1 p2 p3 p4
p4 p5 p2 p3
p2 p3
p4
p5 = p1 + p4
p4
Short PN Functionin Forward Link
A B
Up to 64Code Channels
Up to 64Code Channels
The short PN Sequences are 32,768 chips long. Short PN is used to distinguish Different sectors.
Each Sector in each Base Station is transmitting a Forward Traffic Channel containing up to 64 forward code channels.
A Mobile Station must be able to discriminate between different Sectors of different 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 a particular sector.
Long PN:4-bits shift register example
(XOR)
mask
XOROriginal PNsequence
New PNsequence
AND AND AND AND
1 0010 0110 1101 1011 0100 1011 0110 1111 1111 110
10 0 00 0100 100
1 1001 000
0 100
Attention:different mask lead to different offset!Attention:different mask lead to different offset!
The Long PN Sequence
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.
Long Code Register (@ 1.2288 MCPS)
Public Long Code Mask(STATIC)
User Long CodeSequence
(@1.2288 MCPS)
1 1 0 0 0 1 1 0 0 0 P E R M U T E D E S N
AND
=S U M
Long PN Functionin Reverse Link
RV Trafficfrom M.S.
#1837732008RV Trafficfrom M.S.
#1997061104
RV Trafficfrom M.S.
#1994011508System AccessAttempt by M.S.
#2000071301(on access channel #1)
The CDMA system must be able to identify each Mobile Station that may attempt to communicate with a Base Station.
A very large number of Mobile Stations will be in the market.
Main Content
Definitions of CDMA and How to realize
Spread spectrum modulation
Spreading codes used in CDMA
Vocoding in CDMA
Vocoding (A1 & A2 interface)
BTS BSCBSC MSCMSC
Analog voiceVariable Rate PCM
