Lecture 03 Spread SpectrumCDMA Code By Engr Muhammad

Lecture 03 Spread Spectrum(CDMA Code) By Engr. Muhammad Ashraf Bhutta

Codes in CDMA Codes Orthogonal Codes Walsh Pseudo-noise (PN) Codes Long PN Code Short PN PN Codes Short

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 Three Conditions for orthogonal 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)= ∑xix 1

Walsh Codes Generation

0 0 0 0 1 0 1 0 0 1 1 0

Walsh Codes Generation

Walsh Codes in CDMA 2000 1 x RC 1 & RC 2 S-95 A ( IS-95 A (cdmaone)

Walsh Codes

An Example of Spreading with 3 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 m 1=[+1 – 1 +1], m 2 =[+1 +1 -1], m 3 =[-1 +1 +1], Each of the three users is assigned a Walsh code respectively W 1=[-1 +1 – 1 +1], W 2=[-1 -1 +1 +1], W 3=[-1 +1 +1 -1], m 1(t), w 1(t), m 1(t)w 1(t), same for m 2 and m 3 C(t)= m 1(t)w 1(t)+ m 2(t)w 2(t)+ m 3(t)w 3(t) Composite signal is transmitted in RF band

RX multiplies C(t) by the assigned Wash code for each message C(t)w 1(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