Chapter 5: Third generation systems-Wideband Digital Modulation

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

Properties of PN codes PN codes are vectors with a combination of 1s and -1s: Example: [1 1 -1 -1] an [1 -1 1 -1] 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] = (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] = (1)(1) + (1)(1) + (-1)(-1) + (-1)(-1) = 4

PN code detection PN Coding User Data User A data: 12 User B data: 7 User C data: -10 PN codes A: [1 1 1 1] B: [1 -1 1 -1] C: [1 1 -1 -1] Transmission coding User 1 : User 1 Data x User 1 code = 12 x [1 1 1 1] = [12 12 12 12] User 2 : User 2 Data x User 2 code = 7 x [1 -1 1 -1] = [7 -7 7 -7] User 3 : User 3 Data x User 3 code = -10 x [1 1 -1 -1] = [-10 -10 10 10] Transmitted data = Sum of user data = [12 12 12 12] + [7 -7 7 -7] + [-10 -10 10 10] = [9 -5 29 15]

PN code detection PN Decoding Received User 1 data = Combined data . User 1 code Code Length = [9 -5 29 15] . [1 1 1 1] /4 = 48/4 = 12 Received User 2 data = Combined data . User 2 code = [9 -5 29 15] . [1 -1 1 -1]/4 = 28/4 =7 Received User 3 data = Combined data . User 3 code = [9 -5 29 15] . [1 1 -1 -1]/4   = -40/4 =-10

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

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

FHSS transmission and reception

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

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

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

DSSS transmission and reception

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

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;