Lecture 03 Spread Spectrum(CDMA Code)

By

Engr. Muhammad Ashraf Bhutta

Codes in CDMA

Walsh CodesWalsh Codes

Orthogonal Codes

Long PN Code Short PN CodesShort PN Codes

Pseudo-noise (PN) Codes

CDMA Codes

Walsh Codes

Two codes are orthogonal if the product of two signals (summed over a period of time) is zero

OR

Two codes are orthogonal if the process of “XORing” them results in an equal number of 1’s and 0’s

Walsh Codes

1-The Cross correlation should be zero or very small (Rxy(0)= ∑xiyi

2-each sequences in the set should have an equal nos. of I,s and 0,s or difference should be by at most one

3- The scaled dot product of each code should equal to 1((Rxx(0)= ∑xix1

Three Conditions for orthogonal codes

Walsh Codes Generation

0

0 1

0 0

0 1

0 0

0 11 1

1 0

00

0 0

0 1

Walsh Codes Generation

Walsh Codes in CDMA2000 1x RC1 & RC2S-95A (\ IS-95A (cdmaone)

Walsh Codes

An Ex An Example of Spreading with3 Users n

Example of Spreading w of Spreading i Users•In this example, three users, A, B, and C are assigned three orthogonal codes for spreading purposes

– User A signal = 00, Spreading Code = 0101

– User B signal = 10, Spreading Code = 0011

– User C signal = 11, Spreading Code = 0000

•The analog signal shown on the bottom of the figure is the composite signal when all of the spread symbols are summed together.

C(t)

Channelization Using Wash Codes

Example

The Separate three Messages

m1=[+1 –1 +1],m2 =[+1 +1 -1],m3 =[-1 +1 +1],

Each of the three users is assigned a Walsh code respectively

W1=[-1 +1 –1 +1], W2=[-1 -1 +1 +1], W3=[-1 +1 +1 -1],

m1(t),w1(t),m1(t)w1(t),same for m2 and m3

C(t)= m1(t)w1(t)+ m2(t)w2(t)+ m3(t)w3(t) Composite signal is transmitted in RF band

RX multiplies C(t) by the assigned Wash code for each message

C(t)w1(t) etc

The receiver integrates or adds up all values over each bit period and obtained M(t)

Decision Threshold: m(t)=1 if M(t)>1

m(t)=0 If M(t)<0

By applying original message is retrieved

PN Codes

PN Codes

Long PN Codes Short PN Codes

Short PN Code

Short PN Code

PNa

PNc

PNb

Short PN Code

Short PN Code Offsets

215 / 64 = 32768 / 64 = 512

Short PN Code Offsets

PN Code Generation & Offsets

PN Code Generation

PN Code Generation

PN Code Generation

2N-1

In this example, the number of distinct states in the shift registers is 23-1=7

PN Code Offsets (Masking)

3 Digit Mask ( 110 )

CDMA2000 1X Network Structure