1. Chapter 5: Third generation systems-Wideband Digital Modulation

2. Principle of Spread Spectrum Communications
First and second generation communication systems divide the total available bandwidth into smaller bandwidth channels
However, third generation systems share the entire bandwidth without mutual interference
This is possible through the use of Pseudo- Noise (PN) codes

3. Properties of PN codes
PN codes are vectors with a combination of 1s and -1s:
Example: [ ] an [ ]
PN codes are orthogonal to each other, i.e. dot product of two different codes is zero
Example: [ ] . [ ]
= (1)(1) + (1)(-1) + (-1)(1) + (-1)(-1)
= 0
Dot product of two identical codes is equal to the length of the code
Example: [ ] . [ ]
= (1)(1) + (1)(1) + (-1)(-1) + (-1)(-1)
= 4

4. PN code detection PN Coding
User Data
User A data: 12
User B data: 7
User C data: -10
PN codes
A: [ ]
B: [ ]
C: [ ]
Transmission coding
User 1 : User 1 Data x User 1 code = 12 x [ ] = [ ]
User 2 : User 2 Data x User 2 code = 7 x [ ] = [ ]
User 3 : User 3 Data x User 3 code = -10 x [ ] = [ ]
Transmitted data = Sum of user data
= [ ] + [ ] + [ ]
= [ ]

5. PN code detection PN Decoding
Received User 1 data = Combined data . User 1 code
Code Length
= [ ] . [ ] /4
= 48/4 = 12
Received User 2 data = Combined data . User 2 code
= [ ] . [ ]/4
= 28/4 =7
Received User 3 data = Combined data . User 3 code
= [ ] . [ ]/4
= -40/4 =-10

6. Spread Spectrum transmission and reception
Spread spectrum systems have two stages of modulation (at transmitter) and demodulation (at receiver)
First stage is carrier modulation /demodulation as in 1G and 2G systems
Second stage is PN modulation/demodulation which increases the bandwidth of the system

7. Frequency Hopping Spread Spectrum (FHSS)
FHSS evolved during World War II, primarily as a technique for naval ships to avoid detection by enemy submarines
Ships would keep changing the radio signal frequency or hopping through a set of frequencies
PN Codes would help design orthogonal frequency sets to avoid interference

8. FHSS transmission and reception

9. FHSS bandwidth and BER performance
Processing Gain
PG = System bandwidth/Channel bandwidth
= W/B
FHSS bandwidth
BWFHSS = PG BFSK
FHSS BER for M users

10. Direct Sequence Spread Spectrum (DSSS)
DSSS is simpler type of Spread Spectrum using PN coding
Developed by Qualcomm in 1995 as CDMA (Code Division Multiple Access)
Initial system was called IS-95 which led on to CDMA2000 and W-CDMA

11. DSSS transmission and reception
DSSS coding multiplies the data with the PN code
Data is random (bits) but PN code is repeating sequence (chips)
Each user has a distinct PN code

12. DSSS transmission and reception

13. DSSS bandwidth and BER performance
Processing Gain
PG = Data period/Chip period
= T/Tc
DSSS bandwidth
BWDSSS = PG BPSK
DSSS BER for M users

14. Advantages and disadvantages of Spread Spectrum systems
Spread spectrum systems have higher noise immunity and fading due to large bandwidth
Pseudo-random nature of the PN sequence provides increased signal security
Lower jamming effects on system due to lower power density
High bandwidth requires wideband circuitry and wideband system models
Spread Spectrum systems are more complex due to 2-stage modulation systems;