Suppose we have 3 bit sequences m1, m2 and m3 for 3 different sources

2. Suppose we have 3 bit sequences m1, m2 and m3 for 3 different sources These 3 bit streams are spreaded seperately by 3 orthogonal codes given by w1, w2 and w3 respectively,

3. 1 1 -1 -1 -1 Spread Process Digital Signal Spreading Signal 1 -1 -1 1 1 Spreading 1 -1 1 -1 Spreading Code

4. Waveforms of different stages

7. Output waveforms of integrators and decision threshold

9. PN code application in CDMA • PN code used in CDMA system • Long code：242 – 1 (r = 42) • Short code：215 -1 (r = 15) • Different purpose • ---Forward channel • long code : scramble • short code :orthogonal modulation and identify base station • ---Reverse channel • long code :spread spectrum and identify user • short code :orthogonal modulation

10. PN(Pseudo-random Noise) Short Code • Unique identifier for a cell or a sector in Forward • Consists of two PN Sequences, I and Q, each 32,768(215) chips long - Generated in similar but differently-tapped 15 bit shift registers - They’re always used together, modulating the two phase axes of a QPSK • Repeats every 26.67msec(75 repetition in 2 sec) • No Cross-correlation between PN Code Sequence ie orthogonal codes • Deterministic Code Shift Register • Seed register with 001(N registers) • Output(before repeating) will be 1001011(2N-1 bits) 0 1 0 Output Sequence Stage #3 Stage #1 Stage #2

11. Example of short PN generation – 4 shift register X1 X5 X2 X3 X4 Output C4=0 C5=1 C3=1 C1=0 C2=0 CP PN-Code Generation Mode (N=5) According to the chart, under the operation of CP we can know the state table should be: 00101 00010 00001 10000 01000 00100 10010 01001 10100 11010 01101 00110 10110 01100 11000 10001 00011 00111 01111 11111 11110 11100 11001 10011 11011 11101 01110 10111 01011 10101 01010 From the table ,the output sequence should be : 10100001001011001111100011011101010000…… The code sequence repetition period :P = 2n– 1 How to generate PN-code Assume the initial state is 00101.

12. Short PN Code Contd…. • The above two slides shows the description of short PN sequence and their example of generation. (from 3 shift register and 4 shift register respectively. • From above analysis, there could be a possibility of 215 -1 unique PN offsets. But, however, in practical implementation, there could only 512 practically usable short PN offsets, which is shown in the following example.

13. Distance to travel for 1chip to change

14. Short PN Code Contd…. • The above example shows that chip is equivalent to 244.14 m and if the sequential (continuous) short PN offsets are assigned to different sectors without any planning, an ambiguity may produce as shown in above fig. • In IS-95, the successive short PN offset must differ at least by 64 chips (26) in order to avoid such ambiguity. • Hence, the total practically possible short PN offsets is given by 215/26 = 512

16. Long Code Register (@ 1.2288 MCPS) AND Public Long Code Mask (STATIC) 1 1 0 0 0 1 1 0 0 0 P E R M U T E D E S N = User Long Code Sequence (@1.2288 MCPS) S U M Modulo-2 Addition The Long PN Sequence • Each mobile station uses a unique User Long Code Sequence generated by applying a mask, based on its 32-bit ESN and 10 bits from the ysytem, to the 42-bit Long Code Generator which was synchronized with the CDMA system during the mobile station initialization. • Generated at 1.2288 Mcps, this sequence requires 41 days, 10 hours, 12 minutes and 19.4 seconds to complete. • Portions of the User Long Codes generated by different mobile stations for the duration of a call are not exactly orthogonal but are sufficiently different to permit reliable decoding on the reverse link.

17. IS-95 CDMA Channels