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 ID: 1040089
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1. Chapter 5:Third generation systems-Wideband Digital Modulation
2. 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 interferenceThis is possible through the use of Pseudo-Noise (PN) codesPrinciple of Spread Spectrum Communications
3. 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) = 0Dot 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) = 4Properties of PN codes
4. User Data User A data: 12User B data: 7User C data: -10PN codesA: [1 1 1 1]B: [1 -1 1 -1]C: [1 1 -1 -1]Transmission codingUser 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 detectionPN Coding
5. Received User 1 data = Combined data . User 1 code Code Length = [9 -5 29 15] . [1 1 1 1] /4 = 48/4 = 12Received User 2 data = Combined data . User 2 code Code Length = [9 -5 29 15] . [1 -1 1 -1]/4 = 28/4 =7Received User 3 data = Combined data . User 3 code Code Length = [9 -5 29 15] . [1 1 -1 -1]/4 = -40/4 =-10PN code detectionPN Decoding
6. 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 systemsSecond stage is PN modulation/demodulation which increases the bandwidth of the systemSpread Spectrum transmission and reception
7. FHSS evolved during World War II, primarily as a technique for naval ships to avoid detection by enemy submarinesShips would keep changing the radio signal frequency or hopping through a set of frequencies PN Codes would help design orthogonal frequency sets to avoid interferenceFrequency Hopping Spread Spectrum (FHSS)
8. FHSS transmission and reception
9. Processing Gain PG = System bandwidth/Channel bandwidth = W/BFHSS bandwidth BWFHSS = PG BFSKFHSS BER for M users FHSS bandwidth and BER performance
10. DSSS is simpler type of Spread Spectrum using PN codingDeveloped by Qualcomm in 1995 as CDMA (Code Division Multiple Access)Initial system was called IS-95 which led on to CDMA2000 and W-CDMA Direct Sequence Spread Spectrum (DSSS)
11. DSSS transmission and receptionDSSS coding multiplies the data with the PN codeData is random (bits) but PN code is repeating sequence (chips)Each user has a distinct PN code
12. DSSS transmission and reception
13. Processing Gain PG = Data period/Chip period = T/TcDSSS bandwidth BWDSSS = PG BPSKDSSS BER for M users DSSS bandwidth and BER performance
14. Spread spectrum systems have higher noise immunity and fading due to large bandwidthPseudo-random nature of the PN sequence provides increased signal security Lower jamming effects on system due to lower power densityHigh bandwidth requires wideband circuitry and wideband system modelsSpread Spectrum systems are more complex due to 2-stage modulation systems; Advantages and disadvantages of Spread Spectrum systems