02 cpo_p02_e3 cdma basic priniple_24

7/27/2019 02 CPO_P02_E3 CDMA Basic Priniple_24

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CDMA Basic Principles

ZTE University

CDMA BSS T&D Team


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Content

Multiple Access Technology

Spread Spectrum Definition/Feature/Category

Spreading Codes Used in CDMA


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Since the invention of telephone and radio,system operators have tried to transmit themaximum of traffic over each circuit.


Transmission

Medium

Each pair of users enjoy adedicated and privatecircuit through thetransmission medium,unaware that the otherusers exist.

Multiple Access Technology: Multiple independent users

simultaneously use the transmission medium without any mutualinfluence.

The advantages of using the multiple accesstechnology are as follows:

Increases the system capacity to provide servicesfor more users.Reduces the system cost because of the lesseningtransmission mediums as required.Decreases the users charge.

The types of transmission medium are as follows:

Twisted pair

Coaxial cable

Optical cable

Air interface (radio signal).


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Channels

The physical transmission medium is a resource that can be subdivided into

individual channels according to the various technologies used.

Channel: An individually-assigned and dedicated pathway

transmitting one users information over the transmission medium.

CDMA

Each channel is a unique code sequence. Each user uses the same frequency and

coordinates a different code sequence.

TDMA

Each user uses a different time slot.

One channel is a specified time slot of a

specified frequency.

FDMA

Each user uses a different frequency.

One channel is a frequency.

Power

Power

Power

FDMA

TDMA

CDMA


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CDMA

45 MHz

CDMAReverse channel1.23 MHzCDMA

Forward channel1.23MHz

Code division channel

Frequency

CDMAfrequency

point and

channel


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Content


Spread Spectrum

Definition/Feature/Category



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Spread spectrum technology: At the transmitting

end, it adopts the spread spectrum code toincrease the bandwidthmaking it to be greatly

larger than the bandwidth required by the

transmitted data; at the receiving end, it adopts the

same spread spectrum code to demodulate the

signals to recover the transmitted data.

Spread

spectrum

Spread Spectrum Definition


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CDMA Spread Spectrum Principle

Transmitting end Receiving end

Spread spectrum

sequence

Spread spectrum

sequence

Input data(Baseband)

Demodulated data(Baseband)

Modulated data flow(Baseband signal + spread spectrum sequence)

1. The data flow at the transmitting end is combined with one spread

spectrum sequence.2. At the receiving end, only both at the correct timing and spread

spectrum sequences are available, the composite signals can becompressed and be demodulated to the original data.

3. After being demodulated by using the spread spectrum code, thedemodulated original data flow still keeps intact.


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Different Users Use Various Spread Spectrum

Codes


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The principle basis: Shanon formula

C=B*log2(1+S/N)

C: Channel capacity (unit: b/s)B: Bandwidth of signal (unit: Hz)S: Signal average power (unit: W)

N: Noise average power (unit: W)

Spread Spectrum Principle Basis

Conclusion Under the scenario of the fixed channel capacity C, the signal

bandwidth B and the S/N can be interchanged, which means

that the desired transmission quality can be obtained by

increasing the bandwidth of the transmission system in the

environment of lower S/N ratio.


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1. Good concealment and privacy

2. Anti-fading and anti-multipath interference

3. Strong anti-interference capacity

4. Multi-uses simultaneously occupy the same

bandwidth to realize the multiple access.

Features

Spread Spectrum Feature and Category

Categories 1. Direct Sequence Spread Spectrum (DSSS)

The CDMA adopts the DSSS, which combines the

signals needed to be transmitted with the PN code

(spread-spectrum code), the rate of which is greatly

larger than the information rate. In this way, the

spectrum width of the modulation signal is muchlarger than that of the original information.

2. Frequency hopping (FH)

3. Time hopping (TH)

4. Linear frequency modulation


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Content


Spread Spectrum Definition/Feature/Category



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Spreading code rate: 1.2288Mcps

Spreading code: The forward using the Walsh code

and short PN code; the reverse using the long PN

code.

The spreading code usage is the key

technology of the spread spectrum communication.

CDMA Spreading Code Selection


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Walsh Code Definition

The Walsh code definition

The Walsh function is a non-sine wave orthogonal function system and isdefined byusing a Hadamard matrix H according to the recursive relation.

The Walsh function only gains the +1 and -1 or 0 and 1 values, which is

suitable to express and process the digital signal.

The Walsh function has a perfect correlation. In the Walsh function, the two

correlation functions are 0, which indicates they are orthogonal.


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WALSH CODES# --------------------- ------------- 64-Chi p 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 0110100101101001100101101001011001101001011010011001011010010110

24 000000001111111111111111000000000000000011111111111111110000000025 010101011010101010101010010101010101010110101010101010100101010126 001100111100110011001100001100110011001111001100110011000011001127 011001101001100110011001011001100110011010011001100110010110011028 000011111111000011110000000011110000111111110000111100000000111129 010110101010010110100101010110100101101010100101101001010101101030 001111001100001111000011001111000011110011000011110000110011110031 011010011001011010010110011010010110100110010110100101100110100132 000000000000000000000000000000001111111111111111111111111111111133 010101010101010101010101010101011010101010101010101010101010101034 001100110011001100110011001100111100110011001100110011001100110035 011001100110011001100110011001101001100110011001100110011001100136 000011110000111100001111000011111111000011110000111100001111000037 010110100101101001011010010110101010010110100101101001011010010138 001111000011110000111100001111001100001111000011110000111100001139 011010010110100101101001011010011001011010010110100101101001011040 000000001111111100000000111111111111111100000000111111110000000041 010101011010101001010101101010101010101001010101101010100101010142 001100111100110000110011110011001100110000110011110011000011001143 011001101001100101100110100110011001100101100110100110010110011044 000011111111000000001111111100001111000000001111111100000000111145 010110101010010101011010101001011010010101011010101001010101101046 0011110011000011001111001100001111000011001111001100001100111100

47 011010011001011001101001100101101001011001101001100101100110100148 000000000000000011111111111111111111111111111111000000000000000049 010101010101010110101010101010101010101010101010010101010101010150 001100110011001111001100110011001100110011001100001100110011001151 011001100110011010011001100110011001100110011001011001100110011052 000011110000111111110000111100001111000011110000000011110000111153 010110100101101010100101101001011010010110100101010110100101101054 001111000011110011000011110000111100001111000011001111000011110055 011010010110100110010110100101101001011010010110011010010110100156 000000001111111111111111000000001111111100000000000000001111111157 010101011010101010101010010101011010101001010101010101011010101058 001100111100110011001100001100111100110000110011001100111100110059 011001101001100110011001011001101001100101100110011001101001100160 000011111111000011110000000011111111000000001111000011111111000061 010110101010010110100101010110101010010101011010010110101010010162 001111001100001111000011001111001100001100111100001111001100001163 0110100110010110100101100110100110010110011010010110100110010110

Walsh code -------Walsh function,orthogonal, used in the forwardspread spectrum in the CDMA system

0110

1100

1010

0000

10

000

Hn Hn

H2n = ___

Hn Hn

For example:

The correlation of the 23rd Walsh code and the 59th Walsh code#23 0110100101101001100101101001011001101001011010011001011010010110

#59 0110011010011001100110010110011010011001011001100110011010011001

Sum 0000111111110000000011111111000011110000000011111111000000001111

Unique characteristic:

Mutually orthogonal

Walsh Code


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Code #23 0110100101101001100101101001011001101001011010011001011010010110(Code #23) 1001011010010110011010010110100110010110100101100110100101101001

Code #59 0110011010011001100110010110011010011001011001100110011010011001

PARALLEL

XOR: all 0s

Correlation: 100%

(100% match)

ORTHOGONAL

XOR: half 0s, half 1s

Correlation: 0%

(50% match, 50% no-match)

ANTI-PARALLEL

XOR: all 1s

Correlation: 100%

(100% no-match)

#23#23

(#23)

#23

#23

#59

Orthogonal and Correlated Features


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Walsh Code Application in CDMA

The 64-order Walsh function is used

for the forward spread spectrum,distinguishing the forward code

division channels in the sector and

orthogonally modulating the reverse

channels.

In the

CDMA,


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The definition of the m sequence

The m sequence is an important binary PN.

The m sequence is shortened from the longest linear

feedback shift-register sequence.

The detailed definition is as follows: If the n-level liner

shift register outputs the sequence period as P=2^n

1, this sequence is called the m sequence. The m

sequence generator consists of the shift register,

feedback tap and modulo-2 adder.

Pseudo noise (PN code): A signal similar to noise, seems

random but is actually the deterministic and periodic binarysequence.

The PN code

in the CDMA

system

PN Code in CDMA


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Short PN Sequence

The short PN sequence I and Q

are 32,768 chips long. The short PN sequence is a two

dimensional binary vectorincluding the I and Qsequences. The I and Q are32,768 chips long.

Each short PN sequence is

completely correlated with itself,which means,completelycorrelated with the short PNsequence whose time offset is 0.

The short PN sequence with the0 offset is orthogonal with anyshort PN sequences with the

non-0 offset engendered byitself.

64 chips offset is the offset unit(PN_OFFSET_INDEX). The PNcodes 0~511 are available.

IQ

32,768 chips long26 2/3 ms.

(75 repetitions in 2 sec.)

IQIQ

100% correlation: All bits are equal to 0

Short PN Sequence vs. Itself @ 0 Offset

IQIQ

Orthogonal:1 (Quantity:16,384) + 0 (Quantity:1,684)

Short PN Sequence vs. Itself @ Any Offset

Feature:


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Long PN Sequence

Each mobile station uses a unique user long code sequence generated byapplying a mask, based on its 32-bit ESN, and the 42-bit long code register

which was synchronized with the CDMA system during the mobile stationinitialization.

The rate reaches 1.2288 Mcps and the period is 41 d 10 h 12 m 19.4 s.

During calling, the user long PN codes generated by different mobile stationsare not strictly orthogonal, but they can be reliably demodulated to distinguishthe users in the reverse link because these long PN codes are greatly different.

Long code register

(@ 1.2288 MCPS)

Public long code maskSTATIC)

User long code sequence(@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

Modulo-2 add


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PN Code Application in CDMA

In the CDMA system, there are two types of PN

code sequences:Long PN code

Short PN code

2^42 1 (n = 42)

2^15 (n = 15)

Various usages

Forward channel

Reverse channel

The long PN code is used for scrambling and the

short PN code is used for orthogonal spreadspectrum (identifying a base station).

The long PN code is used for spread spectrum

(identifying a user) and the short PN code is used

for orthogonal modulation.


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Feature and Function Summary of Spread-spectrum

Codes

Cell

Walsh code

Short PNcode

Long PNcode

Type

Mutuallyorthogona

l

Orthogon

al exceptthe 0offset

Closelyorthogona

l

Feature

64

2

1

Quantity

64chips

1/19,200 sec.

32,768chips26-2/3

ms75x in 2

sec.

242

chips~41days

Length

Orthogonalmodulation

Quaternary

spreadspectrum (0

offset)

Distinguishthe user

Reverse linkfunction

Identifythe

channel

Distinguish thesector

Datascramblin

g

Forward

linkfunction

IQ

32,768 chips long

26-2/3 ms.(75 repetitions in 2 sec.)

64codes

64 chips long

AND=

SUMModulo-2 Addition

Each type of spread-spectrum sequence hasvarious functions respectively on the forward andreverse links. However, these sequences are usedto generate the users code division channel onthe both links.


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Summary

Spread-spectrum Theory

PN Code and Application

WALSH Code and

Application

Summary


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02 cpo_p02_e3 cdma basic priniple_24

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