Lecture 03 Spread Spectrum(CDMA Code)

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

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

3. Walsh Codes are Orthogonal codes, as they have cross-correlation zero 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

4. 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)=∑xix1

5. Walsh Codes Generation Walsh codes are easily generated by starting with a seed of 0, repeating the 0 horizontally and vertically, & then complementing the 0 diagnoally

6. 0 0 0 0 0 1 0 1 0 0 0 0 1 0 0 1 1 0 1 1 0 Generation of Walsh Codes

7. The process is contiuned with the newly generated block until the desired no. of Walsh codes with the proper length are generated Walsh Codes Generation

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

9. CDMA2000 1x RC1 & RC2 IS-95A (cdmaone) use 64 by 64 Walsh Matrix i.e. There are 64 Walsh Codes Each of length 64 Chips

10. Walsh Codes of CDMA2000 1x RC1 &RC2 IS-95 A

11. On the Forward CDMA Link Walsh Codes are used to Separate users & Forward Channels Walsh Codes

12. 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.

13. Spreading of Three Users at the BTS C(t)

14. 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

15. 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

16. PN Codes

17. Sector Each Cell or is assigned a offset Short PN code Unique Short PN Code

18. Walsh Codes are used to separate users on the forward link Same Walsh Codes are used in every sector/cell Short PN codes are used to cover the Walsh Codes to make each sector unique Short PN Code

19. PNc PNb PNa

20. Short Code Repeats every 26.67 msec (at a clock rate of 1.2288 Mcps) Short PN Code

21. The minimum offset permitted is 64 PN chips maximum of This results in a 512 possible offsets 215 / 64 = 32768 / 64 = 512 Short PN Code Offsets

22. Short PN Code Offsets

23. prime polynomials using PN codes are generated from modulo 2 arithmetic The state machines generating these codes are very simple shift registers and XOR gates and consist of PN Code Generation & Offsets

24. 001 We have used the seed PN Code Generation

25. 1001011 Output is a 7-digit sequence that repeats continually PN Code Generation

26. In general, if there are N shift registers the length of the PN code is equal to 2N-1 In this example, the number of distinct states in the shift registers is 23-1=7 PN Code Generation

27. 3 Digit Mask ( 110 ) Masking will cause the generator to produce the same sequence but offset in time PN Code Offsets (Masking)

28. determines 3-digit mask The content of the the offset of the sequence

29. CDMA2000 1X Network Structure