Lecture 03 Spread Spectrum(CDMA Code)
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Transcript of Lecture 03 Spread Spectrum(CDMA Code)
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