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Lecture 8

SFWRENG 4J03 Lecture 8: Lecture 2016-03-29
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6 Pages
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Department
Software Engineering
Course Code
SFWRENG 4J03
Professor
Rong Zheng

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Description
Lecture 2016-03-29 Author: Kemal Ahmed Instructor: Dr. Gowri Krishnasamy Course: SFWR ENG 4J03 Math objects made using MathType. Table of Contents Spread Spectrum Modulation ......................................................................................................... 1 Baseband..................................................................................................................................... 1 Transmission........................................................................................................................... 2 Receiving................................................................................................................................ 2 Product Modulator.................................................................................................................. 2 Passband...................................................................................................................................... 3 BPSK....................................................................................................................................... 3 Receiver.................................................................................................................................. 3 Channel Output....................................................................................................................... 4 Spread Spectrum Modulation  A type of digital modulation  Spreading the spectrum through Pseudo Noise sequence (PN) code  Despreading is achieved through the same code in R×1  Making the transmitted data appear like noise  Military/security purposes PN Code produced using feedback shift register Types:  Baseband  Passband o Phase Shifting … (PSK): o Frequency Shifting … (FSK): Baseband Transmission: binary data [b(t)] × PN Sequence [c(t)] → O/P m(t) Channel: m(t) + i(t) → r(t) Page 1 of 6 Tb v 1, v 0  r(t) (x) c(t) →0 dt  Decision device, i.e. τ = 0 →0, v 0    v ≠ 0 because you want your signal to appear as noise 1 → +A 0 → –A Integrator is a low-pass filter {bk} → Polar NRZ encoding → b(t) {ck} → Polar NRZ encoding → c(t) digital signal discrete in amplitude & time PN Seq Generated using 3 stage feedback shift register 0011101 time to send an individual binary bit [b ]: number of flip-flops [m]: Bit duration [T ]: c T b NT c N = 2 – 1 = 2 – 1 = 7 Transmission Output of channel [r(t)]: Output of transmission [m(t)]: Interference Signal [i(t)]: m(t) + i(t) → r(t) Receiving r(t) × c(t) → z(t) → bdt v Decision Device, τ = 0 → 1, v 0  0 0, v 0    Product Modulator z(t) = r(t) × c(t) r(t) = m(t) + i(t) = c(t) b(t) + i(t) z(t) = [c(t) b(t) + i(t)] c(t) = c (t) b(t) + i(t) c(t) Page 2 of 6 c(t) = ± 1 2  c (t) = 1  z(t) = b(t) + c(t) i(t) second part removed with LPF because it is high Passband BPSK Passband modulation with BPSK Direct Sequence Spread Spectrum (DSSS): Note: can use any encoding technique! {bk} → Polar NRZ level encoder → b(t) {ck} → Polar NRZ level encoder → c(t) b(t) × c(t) → m(t) m(t) + carrier signal → BPSK Modulator → x(t) Polarity of data sequence, b(t), at time, t + – Polarity of PN + 0 π sequence at time, t – π
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