EE359 – Lecture 19 Outline

1. EE359 – Lecture 19 Outline • Announcements • Final Exam Announcements • HW 8 (last HW) due Sunday 5pm (no late HWs) • Bonus lecture today 6-8pm (pizza/cake); Hewlett 103 • 10 bonus points for course evaluations online • Projects due end of this week • Introduction to Spread Spectrum • Direct Sequence Spread Spectrum • ISI and Inteference Rejection • Spreading Codes and Maximal Linear Codes • Synchronization • RAKE Receivers • Multiuser Spread Spectrum

2. Final Exam Announcements • Final Wed., 12/14, 8:30-11:30, Gates B12 (here) • Covers Chapters 9, 10, 12, 13.1-13.2 (+ earlier chps) • Similar format to MT, but longer: open book, notes. • Practice finals posted by Wed (10 bonus points) • Turn in to Pat or Nima for solns, by exam for bonus pts • Bonus Lecture (Course review; advanced topics) today 6-8pm in Hewlett 103. • Review Session: Thu, Fri, Sun, or Mon? • Extra OHs in advance of the final • Me: 12/12 and 12/13 11:30-12:30 and by appt. • Nima: 12/12 and 12/13 5-6pm.

3. cos(2pfct) cos(2pfct) LPF A/D D/A Serial To Parallel Converter x x Review of Last LectureFFT Implementation of OFDM • Design Issues • PAPR, frequency offset, fading, complexity • MIMO-OFDM v X0 x0 TX Add cyclic prefix and Parallel To Serial Convert R bps QAM Modulator IFFT XN-1 xN-1 RX Y0 y0 Remove cyclic prefix and Serial to Parallel Convert R bps QAM Modulator Parallel To Serial Convert FFT yN-1 YN-1

4. Intro. to Spread Spectrum • Modulation that increases signal BW • Mitigates or coherently combines ISI • Mitigates narrowband interference/jamming • Hides signal below noise (DSSS) or makes it hard to track (FH) • Also used as a multiple access technique • Two types • Frequency Hopping: • Narrowband signal hopped over wide bandwidth • Direction Sequence: • Modulated signal multiplied by faster chip sequence

5. Tc Direct Sequence Spread Spectrum • Bit sequence modulated by chip sequence • Spreads bandwidth by large factor (G) • Despread by multiplying by sc(t) again (sc(t)=1) • Mitigates ISI and narrowband interference S(f) s(t) sc(t) Sc(f) S(f)*Sc(f) 1/Tb 1/Tc Tb=KTc 2

6. S(f) I(f) S(f) S(f)*Sc(f) I(f)*Sc(f) Despread Signal Receiver Input Info. Signal ISI and Interference Rejection • Narrowband Interference Rejection (1/K) • Multipath Rejection (Autocorrelation r(t)) aS(f) S(f)*Sc(f)[ad(t)+b(t-t)] S(f) brS’(f) Despread Signal Receiver Input Info. Signal

7. 1 -Tc Tc -1 N Maximal Linear Codes • Autocorrelation determines ISI rejection • Ideally equals delta function • Maximal Linear Codes • No DC component • Large period (2n-1)Tc • Linear autocorrelation • Recorrelates every period • Short code for acquisition, longer for transmission • In SS receiver, autocorrelation taken over Tb • Poor cross correlation (bad for MAC)

8. 1 -Tc Tc -1 2n-1 Synchronization • Adjusts delay of sc(t-t) to hit peak value of autocorrelation. • Typically synchronize to LOS component • Complicated by noise, interference, and MP • Synchronization offset of Dt leads to signal attenuation by r(Dt) r(Dt) Dt

9. x x x RAKE Receiver • Multibranch receiver • Branches synchronized to different MP components • These components can be coherently combined • Use SC, MRC, or EGC Demod sc(t) y(t) ^ dk Diversity Combiner Demod sc(t-iTc) Demod sc(t-NTc)

10. Multiuser DSSS • Each user assigned a unique spreading code; transmit simultaneously over same bandwidth • Interference between users mitigated by code cross correlation • In downlink, signal and interference have same received power • In uplink, “close” users drown out “far” users (a1>>a2: near-far problem) a a1 a2 a

11. Main Points • Spread spectrum increases signal bandwidth above that required for information transmission • Benefits include ISI and interference rejection, multiuser technique • DSSS rejects ISI by code autocorrelation • Maximal linear codes have good autocorrelation properties but poor cross correlation • Synchronization depends on autocorrelation properties of spreading code. • RAKE receivers combine energy of all MP • Use same diversity combining techniques as before