Code Division Multiple Access(CDMA)

1. Code Division Multiple Access(CDMA) Presented by: Arash Mirzaei

2. history • Third type used in cellular systems after TDMA and CDMA. • At first it was used for military communication systems because detection of the signal is more difficult. • It is used in 2G systems like IS-95. • More wideband CDMA is used in 3G systems. • very useful in satellite communication.

3. Performance • We want to send the digital signal dk(t) with bit length of Tb . ak(t) is a chip sequence with bit length of Tc(Tc<<Tb). • Instead of dk(t) , dk(t).ak(t) is sent.

4. Performance • There are K users.Each of them send its data *its chip sequence. • In the receiver all of them are received. • All of them are received in the same time and same frequency.So dk(t) is detected by its chip sequence in the following way:

5. i≠k y(t) = dk(t).ak(t) + ∑di(t).ai(t) + w(t); y(t).ak(t) = dk(t) + ∑di(t).ai(t).ak(t) + w(t).ak(t); ∫y(t).ak(t)dt = dk(t).Tb + ∑di(t). ∫ai(t).ak(t)dt So ∫ai(t).ak(t)dt for i≠k should be zero or too small. (for reducing the interference). i≠k Tb Tb 0 i≠k 0 Tb 0

6. Receiver and Transmitter in CDMA

7. l • So each user has a distinct digital code. • The codes are orthogonal. xik=+1,-1 k=1,2,…,l ; ci.cj=∑xik.xjk=0 i≠j ; • Orthogonality is used for rejection of unwanted signals in the receiver. • Two types of code are often used:pseudo-random sequence and walsh codes. K=1

8. Advantage Frequency reuse is possible.The same user cde may be used in any cell of a given system. • Disadvantage: Its interference.

9. Pseudo-random sequence • Is generated by a shift register. • Chip (each bit of a pseudo-random sequence), is of length Tc . • Tc is chosen to be much shorter than the information bit length 1/R : Tc<<1/R;

10. Pseudo-random sequence • Information bit * Pseudo-random sequence 1.Is noise like sequence 2.Has much wider spectral width. So CDMA is named Spread Spectrum Communication. • Pseudo-random sequence often labled Pseudo-noise (PN).

11. Bandwidth of original information : R. Bandwidth of resultant wider-band binary sequence : 1/Tc=W. Spreading gain of the system : W/R. • P[0]=1/2-1/(2p) , P[1]=1/2+1/(2p); For n>10 P[0]=p[1]=1/2;

12. Maximum length shift register period is p=2^n-1 for all nonzero initial vectors. • A maximum length shift register of length n is used to produce mth binary output of PN am(t) in the following way: am=∑ci.am-i ci=0,1 , cn=1; n i=1

13. Shift Register

14. There are P sequences of length P. Walsh sequences are completely orthogonal(not like PN). w1=[0];w2=[0 0;0 1];w4=[0 0 0 0;0 1 0 1;0 0 1 1;0 1 1 0]; So w2p=[wp wp;wp wp ]; Walsh Code

15. Compare of PN & Walsh • Number of Walsh sequences of length P is too less than PN sequences with the same length. • For Walsh without delay there is no interference.But for PN without delay interference of each user is poorer than the original user with factor of G(spreading gain).

16. Compare of PN & Walsh • For Walsh, delay of K-1 other users interference is significant (orthogonality is canceled). But for PN, delay of other K-1 users is not important because a delayed PN sequence is also like the noise. • Result : In downlink Walsh sequences are used. But in uplink PN sequences are used (why?). Now you know why frequency reuse is possible in CDMA systems.

17. In uplink because of the reason that was said before the signal of the adjacent cell in addition of the path weaken with factor of G. • In downlink the signal of the adjacent cell in addition of the walsh sequence is multiplied by code of base station that is a PN code so interference of adjacent cell in downlink is like the uplink that was said before.

18. CDMA Capacity Single-Cell Case • K:Number of users • K-1:Number of interference for each user • Suppose that all of the users have the same power (PR) as received at the BS (It is possible with power control). So: S/I=PR/[(K-1)PR/G]=G/(K-1) so K=G/(S/I)+1

19. Increasing The Capacity 1.Soft hand-off (with factor of 2) 2. Sectorization (with factor of 3) 3.Voice inactivity (with factor of 2.5)

20. Increasing The Bit Rate 1.Variable spreading Gain 2.Multi-code 3.Increasing the transmission band 4.Multi-carrier

21. Duplexing • Two ways of duplexing is used in CDMA:FDD and TDD. • For FDD two different bands are assigned to uplink and downlink. • For TDD different slots are assigned to them.

22. Compare of FDD&TDD • In the way of TDD uplink and downlink are not possible at the same time but it is possible in FDD. • TDD is flexible and structure of assignment of slots can change with traffic. it is not possible for FDD. • Guard time is a disadvantage in TDD.

23. Compare of FDD&TDD

24. Receiver For CDMA • CDMA is a wide-band signal so delay spread happens. • Delay spread convert to a positive point by using rake receiver. • The table in the next page shows that:

25. Eb/I0 vs probability of bit error, different communicatin channels Required Eb/I0 (dB Required Eb/I0 (dB) Pe=0.01 Pe=.005 Pe=.001 Pe=10^-5

26. References • Digital communication fourth edition John G. Proakis McGraw-Hill Higher Education • Mobile wireless communication 2005Mischa Schwartz Cambridge University Press

27. THANKS FOR YOUR CONSIDERATION