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EE359 – Lecture 19 Outline

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  1. EE359 – Lecture 19Outline • Announcements • Final Exam Announcements • HW 8 (last HW) due Friday 5pm (no late HWs) • 10 bonus points for course evaluations online • Projects due Dec. 6 • 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 Friday, 12/13, 12:15-3:15 • Covers Chapters 9, 10, 12, 13.1-13.2 (+ earlier chps) • Similar format to MT, but longer: open book, notes. • Practice finals posted next week(10 bonus points) • Turn in to Pat or Mainakfor solns, by exam for bonus pts • Course summary Dec 6 12:30-2:05 here (pizza@noon) • Final review (Mainak) : Mon 12/9, 6 pm to 7 pm? • OHs next 3 weeks: • Me: next MTW by appt, 12/2 9-10am, 12/6 2-3pm, 12/9 by appt, 12/12 6:30-7:30pm (pls confirm), 12/13 9-10am, Skype • Mainak: Please respond to Piazza poll (see last slide)

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

  4. Tc Review of Last Lecture:Intro. to Spread Spectrum • Modulation that increases signal BW to combat narrowband interference and ISI • Also used as a multiple access technique • Two types: Frequency Hopping and DSSS • FH:Hops info. signals over a wide bandwidth • DSSS: modulates info signal by chip sequence S(f) s(t) sc(t) Sc(f) S(f)*Sc(f) 1/Tb 1/Tc Tb=KTc

  5. 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

  6. 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)

  7. 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

  8. 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)

  9. 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

  10. 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

  11. Proposed days/times for Mainak’s OHs, final review, and discussion Please respond to Piazza poll on this proposal 12/2 (8:30 am to 10:30 am): Discussion session for Homework 8 + Office hour (to replace Tuesday DH and Wednesday OH) 12/6 (11 am to 12 noon): OH (instead of Thursday OH) 12/9 (6 pm to 7 pm): Final review session 12/10 (7 pm to 8 pm): Extra 12/11 (7 pm to 8 pm): OH 12/12 (7 pm to 8 pm): OH